NITROGEN DIOXIDE REDUCTION WITH METHANE OVER PALLADIUMBASED SULFATED ZIRCONIA CATALYSTS: A COMPONANT OF A LEAN
EXHAUST AFTERTREATMENT SYSTEM
DISSERTATION
Presented in Partial Fulfillment of the Requirements for the Degree of
Doctor of Philosophy
In the Graduate School of The Ohio State University
By
Erik Michael Holmgreen, B.S.
2006
*****
Dissertation Committee:
Approved by
Professor Umit S. Ozkan, Advisor
Professor Jeffery J. Chalmers
Professor Stuart L. Cooper
Professor Yann G. Guezennec
__________________________
Advisor
Department of Chemical
Engineering
ABSTRACT
Nitrogen oxides emitted from the combustion of fossil fuels represent a
significant environmental risk. These compounds directly contribute to global
warming, acid rain deposition, and the formation of both smog and ground level
ozone.
Catalytic aftertreatment is a proven method for removal of these
pollutants from certain exhaust streams, but current technologies are unsuitable
for use with next-generation lean-burn engines.
This work examines the
development of catalysts active for the reduction of NO2 with CH4, and evaluates
their performance as a component of a two-catalyst aftertreatment system. In
this proposed system NO is first oxidized to NO2 on an oxidation catalyst. NO2 is
a more easily reduced species. The reduction reaction is then carried out over a
reduction catalyst.
Highly selective NOx reduction catalysts must be used for hydrocarbonbased reduction under lean conditions, as in excess oxygen the hydrocarbon
tends to unselectively combust. Two novel Pd-sulfated zirconia catalysts were
prepared and demonstrated to be active for the reaction. Pd-sulfated zirconia
was prepared through incipient wetness techniques on a monoclinic zirconia
support. Characterization showed the presence of surface sulfate groups stable
under reaction conditions.
Activity testing demonstrated N2 yields of 60%
ii
during the reduction of NO2.
These results were significantly better than
observed during the reduction of NO, validating the two-stage concept. In-situ
DRIFTS investigations comparing the reduction of NO and NO2 identified
potential reaction intermediates. These intermediate species formed to a greater
extent during the reduction of NO2, possibly explaining the improved results.
Pd/SZ was also prepared through a single-step sol-gel procedure.
Addition of Pd to the sol-gel preparation was shown to directly affect the
formation of crystallinity in the zirconia support. Catalyst calcination temperature
directly affected the activity of the catalysts through the formation of different
surface sulfate species associated with catalyst acidity. Catalysts calcined at
700°C reached N2 yields of 70%
Testing of a mixed catalyst bed, containing both an active oxidation
catalyst and Pd/SZ prepared by incipient wetness, showed very good activity for
the reduction of NOx with CH4 under lean conditions. N2 yields of 80% were
achieved under simulated exhaust conditions.
Additionally the two-catalyst
system was shown to be active for the removal of unburned hydrocarbons and
carbon monoxide. Testing of the system in the presence of water vapor showed
a loss of NOx reduction activity through a competitive adsorption phenomenon
on the Pd/SZ catalyst. Modification of the catalyst through an increase in sulfate
content was shown to improve resistance to this deactivation effect.
iii
DEDICATION
For Lucie.
Your turn.
iv
ACKNOWLEDGMENTS
The pursuit of my doctorate has undoubtedly been the most challenging
undertaking of my life. I am deeply grateful of all the support I have received
from friends and family through encouragement, inquiry, and inspiration. Most of
all I remain indebted to their continual blind faith in my ability to do this, even
when I doubted. My wife Lucie has kept by driven, happy, and sane more than
any other.
The hard work and help of the Heterogeneous Catalysis Research Group
was indispensable, and any success of mine is partially theirs.
Thanks in
particular to Matthew Maurice Yung, my partner on the dual catalyst project. This
work was done in collaboration with him and the NO oxidation results contained
here are the product of his work.
Throughout my graduate studies Dr. Umit Ozkan has always led from the
front, setting the highest standards in commitment and quality of research. It has
been an honor to work with her.
v
VITA
November 22, 1977………..……Born – Black River Falls, WI, United States
May, 2001...………………………B.S. Chemical Engineering,
Iowa State University
September 2001 – Present…….Graduate Research Associate,
The Ohio State University
PUBLICATIONS
1. Holmgreen, E.M., Yung, M. and Ozkan, U.S., “Two-Stage Catalytic NOx
Reduction with Hydrocarbons for Lean Burn Gas Fired Reciprocating
Engines” Proceedings ICEF04 ASME 2004 Fall Technical Conference 893, 17 (2004).
FIELDS OF STUDY
Major Field: Chemical Engineering
vi
TABLE OF CONTENTS
Abstract………..……………………………………………………………………….. ii
Dedication…………………………………………………………………………..…. iv
Acknowledgments………..………………………………………………………..…. v
Vita…………………………………………………………………………….……….. vi
List of Tables…………………………………………………………………………… xi
List of Figures…………………………………………………………………………..xii
Chapters:
1. Introduction………………………………………………………………………… 1
2. Literature Review…………………………………………………………………. 7
2.1 NOx Reduction with Higher Hydrocarbons……………………………. 7
2.2 Reduction of NOx with CH4…………………………………………….. 14
2.3 NOx Reduction Over Solid Acid Supports……………………………. 21
3. Experimental Methods…………………………………………………………… 25
3.1 Catalyst Synthesis……………………………………………………….. 25
3.2 Steady-State Reaction Studies…………………………………………. 26
3.3 Physisorption for BET Surface Area Analysis………………………… 29
3.4 X-ray Photoelectron Spectroscopy (XPS)……………………………... 30
3.5 X-ray Diffraction (XRD)………………………………………………….. 31
vii
3.6 Diffuse Reflectance Infrared Fourier Transform Spectroscopy
(DRIFTS)………………………………………………………………….. 33
3.7 Temperature Programmed Techniques……………………………….. 33
3.8 Thermogravimetric Analysis/Differential Scanning Calorimetry
(TGA/DSC)………………………………………………………………... 34
4. NO2 Reduction Over Ag-Based Catalysts with Higher Hydrocarbons………. 37
4.1 Steady-State Reaction Experiments………………………………...… 37
4.2 XRD of 1%Ag and 3%Ag/Al2O3………………………………………… 46
5.3 X-ray Photoelectron Spectroscopy of 1%Ag and
3%Ag/Al2O3 Catalysts……………………………………………….. 47
5. Pd-Supported on Sulfated Monoclinic Zirconia for the Reduction of NO2
with CH4 Under Lean Conditions.………………………………………………. 52
5.1 Experimental Details…………………………………………………… 52
5.2 Catalyst Preparation……………………………………………………. 54
5.3 Steady-State Reaction Experiments………………………………….. 63
5.4 Temperature Programmed Desorption……………………………….. 70
5.5 Diffuse Reflectance Fourier Transform Spectroscopy………………. 72
5.6 Conclusions……………………………………………………………… 74
6. Pd Doped Sulfated Zirconia Prepared by a Single Step Sol-Gel Procedure
for Lean NOx Reduction.………………………………………………………… 76
6.1 Experimental Details……………………………………………………. 76
6.2 BET Surface Area………………………………………………………. 78
viii
6.3 Steady-State Reaction Results……………………………………….. 79
6.4 Thermogravimetric/Differential Scanning Calorimetry and Mass
Spectrometry………………………………………………………… 86
6.4 In-situ X-ray Diffraction…………………………………………………. 98
6.5 IR Spectroscopy………………………………………………………… 104
6.6 Conclusions……………………………………………………………... 107
7. In-Situ DRIFTS Investigation of NO and NO2 Reduction Over Pd-Supported
on Sulfated Monoclinic Zirconia………………………………………………… 109
7.1 Experimental Details…………………………………………………… 109
7.2 Temperature Programmed Desorption………………………………. 110
7.3 Temperature Programmed Desorption Under CH4…………………. 116
7.4 Temperature Programmed Desorption Under CH4 and O2………… 122
7.5 In-situ DRIFTS under reaction conditions……………………………. 128
8. Dual Catalyst Aftertreatment of Lean-Burn Natural Gas Engine Exhaust…. 142
8.1 Experimental Details…………………………………………………… 142
8.2 Comparison of Direct NO2 and Dual-Catalyst NO Reduction……… 145
8.3 NO Reduction Using Simulated Natural Gas………………………... 149
8.4 Simulated Exhaust Containing CO and CO2………………………… 158
8.5 Effect of Water………………………………………………………….. 162
8.6 Interaction of H2O with Pd/SZ…………………………………………. 167
8.7 Conclusions……………………………………………………………... 172
9. Conclusions and Recommendations…………………………………………... 174
ix
Appendix A: List of Acronyms………………………………………………………180
Appendix B: Sample Calculations………………………………………………… 181
References…………………………………………………………………………...183
x
LIST OF TABLES
Table 4.1: BET surface areas of Ag-based NO2 reduction catalysts…………… 37
Table 5.1: BET surface areas of the unloaded support and the prepared Pd/SZ
catalysts……………………………………………………………………….. 54
Table 6.1: Catalyst surface areas and pore volumes for catalysts prepared
with different Pd loadings and prepared at different calcination
temperatures………………………………………………………………….. 79
Table 6.2: Zirconia crystallite sizes observed during in-situ calcination. Effect of
temperature and Pd loading……………………………………………….. 103
xi
LIST OF FIGURES
Figure 3.1: Schematic of the steady-state reaction system……………………… 27
Figure 3.2: Conceptual Drawing of X-ray Photoelectron Spectroscopy………... 30
Figure 3.3: Schematic of the TGA/DSC-111 system………………………………35
Figure 4.1: Reduction of NO2 with C3H6………………………………………….... 40
Figure 4.2: NO2 reduction with C3H8……………………………………………….. 41
Figure 4.3: Comparison of NO2 reduction over 1%Ag/Al2O3 with propane and
propene………………………………………………………………………... 43
Figure 4.4: Comparison of NO2 reduction over 3%Ag/Al2O3 with propane and
propene………………………………………………………………………... 44
Figure 4.5: NO2 reduction with propane over 1%Ag/Al2O3 in 5% and 10% O2... 45
Figure 4.6: XRD of 1% and 3% Ag/Al2O3………………………………………….. 46
Figure 4.7: XPS spectra of the Al 2p region of 1% and 3%Ag/Al2O3…………… 48
Figure 4.8: XPS spectra of the O 1s region of 1% and 3%Ag/Al2O3……………. 49
Figure 4.9: XPS spectra of Ag 3d region of 1% and 3%Ag/Al2O3………………. 50
Figure 5.1: TPD of SO2 (m/e=64) during the heat treatment of impregnated and
dried sulfated zirconia. Samples were treated under He (♦) and
10%O2/He („)………………………………………………………………… 56
xii
Figure 5.2: Zr3d region of the X-ray photoelectron spectra of catalyst samples
taken at each step in the synthesis procedure. From bottom to top:
commercial monoclinic ZrO2, dried but uncalcined SZ, SZ support,
impregnated but uncalcined 0.5%Pd/SZ, the final 0.5%Pd/SZ………….. 59
Figure 5.3: S2p region of the X-ray photoelectron spectra of catalyst samples
taken at each step in the synthesis procedure. From bottom to top:
commercial monoclinic ZrO2, dried but uncalcined SZ, SZ support,
impregnated but uncalcined 0.5%Pd/SZ, the final 0.5%Pd/SZ………….. 60
Figure 5.4: O1s region of the X-ray photoelectron spectra of catalyst samples
taken at each step in the synthesis procedure. From bottom to top:
commercial monoclinic ZrO2, dried but uncalcined SZ, SZ support,
impregnated but uncalcined 0.5%Pd/SZ, the final 0.5%Pd/SZ………….. 61
Figure 5.5: In-situ XRD patterns taken during the calcination in air of sulfated
zirconia………………………………………………………………………… 62
Figure 5.6: Steady-state reaction results from the reduction of NO (empty) and
NO2 (filled) with CH4 over 0.1%Pd/SZ. N2 yield („) and CH4 conversion
(c). Reaction conditions: 1000ppm NOx, 3000ppm CH4, 10% O2 in He,
GHSV=20,000hr-1…………………………………………………………….. 64
Figure 5.7: Steady-state N2 yields in the reduction of NO2 with CH4 over Pd/SZ
catalysts. 0.5%Pd/SZ (c), 0.3%Pd/SZ („), and 0.1%Pd/SZ (‹).
Reaction conditions: 1000ppm NO, 3000ppm CH4, 10% O2 in He,
GHSV=20,000hr-1…………………………………………………………….. 67
xiii
Figure 5.8: Steady-state NO2 conversions in the reduction of NO2 with CH4 over
Pd/SZ catalysts. 0.5%Pd/SZ (c), 0.3%Pd/SZ („), and 0.1%Pd/SZ (‹).
Reaction conditions: 1000ppm NO, 3000ppm CH4, 10% O2 in He,
GHSV=20,000hr-1…………………………………………………………….. 68
Figure 5.9: Steady-state CH4 conversions in the reduction of NO2 with CH4 over
Pd/SZ catalysts. 0.5%Pd/SZ (c), 0.3%Pd/SZ („), and 0.1%Pd/SZ (‹).
Reaction conditions: 1000ppm NO, 3000ppm CH4, 10% O2 in He,
GHSV=20,000hr-1…………………………………………………………….. 69
Figure 5.10: NO (♦) and NO2 („) TPD experiments from 0.5%Pd/SZ. Ions
corresponding to m/e=46, 44(x5), and 30 are shown…………………….. 71
Figure 5.11: DRIFTS spectra of the Pd/SZ and KBr mixture. Spectra shown
correspond to the sample, and following adsorption of NO or NO2…….. 73
Figure 6.1: N2 yield during the reduction of NO2 with CH4 over 0.1%Pd/SZ (c),
0.3%Pd/SZ („), and 0.5%Pd/SZ (z) calcined at 600°C…………………. 80
Figure 6.2: CH4 conversion during the reduction of NO2 with CH4 over
0.1%Pd/SZ (c), 0.3%Pd/SZ („), and 0.5%Pd/SZ (z)
calcined at 600°C…………………………………………………………..… 82
Figure 6.3: N2 yield during the reduction of NO2 with CH4 over 0.1%Pd/SZ (c),
0.3%Pd/SZ („), and 0.5%Pd/SZ (z) calcined at 700°C…………………. 83
xiv
Figure 6.4: CH4 conversion during the reduction of NO2 with CH4 over
0.1%Pd/SZ (c), 0.3%Pd/SZ („), and 0.5%Pd/SZ (z)
calcined at 700°C…………………………………………………………….. 85
Figure 6.5: Thermogravimetric measurement of the calcination of SZ (c) and
0.5% Pd/SZ („)……………………………………………………………….... 87
Figure 6.6: DSC and dTG profiles collected during the calcination of sol-gel
prepared SZ…………………………………………………………………... 88
Figure 6.7: Calcination of SZ monitored by mass spectrometer. Shown are ions
associated with the desorption of H2O (m/z=18,17 - S,U), CO2 (m/z=
44,28 - „,…), and SO2 (m/z=64,48 – z,{)……………………………….. 89
Figure 6.8: DSC and dTG profiles collected during the calcination of sol-gel
prepared 0.1%Pd/SZ……………………………………………………….... 91
Figure 6.9: Calcination of 0.1%Pd/SZ monitored by mass spectrometer. Shown
are ions associated with the desorption of H2O (m/z=18,17 - S,U), CO2
(m/z= 44,28 - „,…), and SO2 (m/z=64,48 – z,{)……………………….. 92
Figure 6.10: DSC and dTG profiles collected during the calcination of sol-gel
prepared 0.3%Pd/SZ……………………………………………………….... 94
Figure 6.11: Calcination of 0.3%Pd/SZ monitored by mass spectrometer. Shown
are ions associated with the desorption of H2O (m/z=18,17 - S,U), CO2
(m/z= 44,28 - „,…), and SO2 (m/z=64,48 – z,{)………………………... 95
Figure 6.12: DSC and dTG profiles collected during the calcination of sol-gel
prepared 0.5%Pd/SZ……………………………………………………….... 96
xv
Figure 6.13: Calcination of 0.5%Pd/SZ monitored by mass spectrometer. Shown
are ions associated with the desorption of H2O (m/z=18,17 - S,U), CO2
(m/z= 44,28 - „,…), and SO2 (m/z=64,48 – z,{)………………………... 97
Figure 6.13: XRD patterns taken during the calcination of sol-gel prepared SZ..99
Figure 6.14: XRD patterns taken during the calcination of sol-gel prepared
0.1%Pd/SZ…………………………………………………………………... 100
Figure 6.15: XRD patterns taken during the calcination of sol-gel prepared
0.3%Pd/SZ…………………………………………………………..………. 101
Figure 6.16: XRD patterns taken during the calcination of sol-gel prepared
0.5%Pd/SZ…………………………………………………………….…….. 102
Figure 6.17: DRIFTS spectra of 0.5%Pd/SZ calcined at 500, 600, and 700°C. 105
Figure 7.1: NO temperature programmed desorption by DRIFTS…………….. 111
Figure 7.2: NO temperature programmed desorption by DRIFTS. High
wavenumber region………………………………………………………… 112
Figure 7.3: NO2 temperature programmed desorption by DRIFTS…………… 114
Figure 7.4: NO2 temperature programmed desorption by DRIFTS. High
wavenumber region……………………………………………………….... 115
Figure 7.5: NO temperature programmed desorption by DRIFTS. TPD
performed under CH4 flow…………………………………………………. 117
Figure 7.6: NO temperature programmed desorption by DRIFTS. TPD
performed under CH4 flow. High wavenumber region………………..… 118
xvi
Figure 7.7: NO2 temperature programmed desorption by DRIFTS. TPD
performed under CH4 flow…………………………………….…………… 120
Figure 7.8: NO2 temperature programmed desorption by DRIFTS. TPD
performed under CH4 flow. High wavenumber region…………………... 121
Figure 7.9: NO temperature programmed desorption by DRIFTS. TPD
performed under CH4 and O2 flow……………………………………..….. 124
Figure 7.10: NO temperature programmed desorption by DRIFTS. TPD
performed under CH4 and O2 flow. High wavenumber region…… 125
Figure 7.11: NO2 temperature programmed desorption by DRIFTS. TPD
performed under CH4 and O2……………………………………………… 126
Figure 7.12: NO2 temperature programmed desorption by DRIFTS. TPD
performed under CH4 and O2. High wavenumber region…………….… 127
Figure 7.13: 0.3%Pd/SZ under a constant flow of 2%O2 at 375°C.
Evolution of surface species after the introduction of NO2 and the
further introduction of CH4. Low wavenumber region………………...… 129
Figure 7.14: 0.3%Pd/SZ under a constant flow of 2%O2 at 375°C. Evolution of
surface species after the introduction of NO2 and the further introduction
of CH4. High wavenumber region……………………………………….... 130
Figure 7.15: 0.3%Pd/SZ under a constant flow of 2%O2 at 375°C. Evolution
of surface species after the introduction of NO and the further introduction
of CH4. Low wavenumber region……………...………………………….. 133
xvii
Figure 7.16: 0.3%Pd/SZ under a constant flow of 2%O2 at 375°C. Evolution of
surface species after the introduction of NO and the further introduction
of CH4. High wavenumber region………………………………………… 134
Figure 7.17: 0.3%Pd/SZ under a constant flow of 2%O2 at 375°C. Evolution of
surface species after the introduction of CH4 and the further introduction
of NO2. Low wavenumber region…………………………………………. 137
Figure 7.18: 0.3%Pd/SZ under a constant flow of 2%O2 at 375°C. Evolution of
surface species after the introduction of CH4 and the further introduction
of NO2. High wavenumber region………………………………………… 138
Figure 7.19: 0.3%Pd/SZ under a constant flow of 2%O2 at 375°C. Evolution of
surface species during flushing of the catalyst after steady-state reaction.
Low wavenumber region…………………………………………………… 140
Figure 7.20: 0.3%Pd/SZ under a constant flow of 2%O2 at 375°C. Evolution of
surface species during flushing of the catalyst after steady-state reaction.
High wavenumber region…………………………………………………... 141
Figure 8.1: Comparison of NO2 reduction over 0.5%Pd/SZ and NO reduction over
a mixed bed containing 10%Co/TiO2 and 0.5%Pd/SZ in a 0.5:1 ratio.
Reduction catalyst: (♦) N2 yield and (◊) CH4 conversion. Mixed bed:
(■) N2 yield and (□) CH4 conversion…………………………………….… 146
xviii
Figure 8.2: Comparison of NO2 reduction over 0.3%Pd/SZ and NO reduction over
a mixed bed containing 10%Co/ZrO2 and 0.3%Pd/SZ in a 0.5:1 ratio.
Reduction catalyst: (♦) N2 yield and (◊) CH4 conversion. Mixed bed: (■) N2
yield and (□) CH4 conversion………………………………………………. 148
Figure 8.3: N2 yield during the reduction of NO over a mixed-bed of 10%Co/ZrO2
and 0.3%Pd/SZ with a simulated natural gas exhaust. Effect of catalyst
ratio, with amount of reduction catalyst held constant.
Reduction:Oxidation catalyst ratios of 1:1 (♦), 1:0.5 (ƒ), and 1:0.25
(c)……………………………………………………………………………. 151
Figure 8.4: NO2 yield during the reduction of NO over a mixed-bed of
10%Co/ZrO2 and 0.3%Pd/SZ with a simulated natural gas. Effect of
catalyst ratio, with amount of reduction catalyst held constant.
Reduction:Oxidation catalyst ratios of 1:1 (♦), 1:0.5 (ƒ), and 1:0.25
(c)……………………………………………………………………………. 152
Figure 8.5: N2O yield during the reduction of NO over a mixed-bed of
10%Co/ZrO2 and 0.3%Pd/SZ with a simulated natural gas.
Effect of
catalyst ratio, with
constant.
amount
of
reduction
catalyst
held
Reduction:Oxidation catalyst ratios of 1:1 (♦), 1:0.5 (ƒ),
and 1:0.25 (c)………………………………………………………………. 153
xix
Figure 8.6: C3H8 conversion during the reduction of NO over a mixed-bed of
10%Co/ZrO2 and 0.3%Pd/SZ with a simulated natural gas. Effect of
catalyst ratio, with amount of reduction catalyst held constant.
Reduction:Oxidation catalyst ratios of 1:1 (♦), 1:0.5 (ƒ),
and 1:0.25 (c)………………………………………………………………. 155
Figure 8.7: C2H6 conversion during the reduction of NO over a mixed-bed of
10%Co/ZrO2 and 0.3%Pd/SZ with a simulated natural gas. Effect of
catalyst ratio, with amount of reduction catalyst held constant.
Reduction:Oxidation catalyst ratios of 1:1 (♦), 1:0.5 (ƒ),
and 1:0.25 (c)………………………………………………………………. 156
Figure 8.8: CH4 conversion during the reduction of NO over a mixed-bed of
10%Co/ZrO2 and 0.3%Pd/SZ with a simulated natural gas.
Effect of
catalyst ratio, with
constant.
amount
of
reduction
catalyst
held
Reduction:Oxidation catalyst ratios of 1:1 (♦), 1:0.5 (ƒ),
and 1:0.25 (c)……………………………………………………………… 157
Figure 8.9: Yield of N-containing species during the reduction of NO over a
mixed-bed of 10%Co/ZrO2 and 0.3%Pd/SZ in simulated lean exhaust.
N2 (♦), NO2 (ƒ), and N2O (c)……………………………………………… 159
Figure 8.10: Hydrocarbon and CO conversion during the reduction of NO over a
mixed-bed of 10%Co/ZrO2 and 0.3%Pd/SZ in simulated lean exhaust.
CH4 (♦), C2H6 (ƒ), C3H8 (c), and CO (•)…………………………………. 161
xx
Figure 8.11: N2 yields from the reduction of NO over a mixed bed containing
0.3%Pd/SZ and 10%Co/ZrO2 in a 1:0.25 ratio. Reduction performed
with a feed containing 1% (♦), 3% (ƒ), and 10%(c) water…………….. 164
Figure 8.12: CH4 conversions during the reduction of NO over a mixed bed
containing 0.3%Pd/SZ and 10%Co/ZrO2 in a 1:0.25 ratio.
Reduction performed with a feed containing 1% (♦), 3% (ƒ),
and 10%(c) water………………………………………………………….. 165
Figure 8.13: NO2 yields during the reduction of NO over a mixed bed containing
0.3%Pd/SZ and 10%Co/ZrO2 in a 1:0.25 ratio. Reduction performed with
a feed containing 1% (♦), 3% (ƒ), and 10%(c) water………………….. 166
Figure 8.14: Desorption of SO2 from Pd/SZ reduction catalyst with or without 2%
water………………………………………………………………………….. 168
Figure 8.15: DRIFTS investigation of H2O desorption from 0.3%Pd/SZ. Low
wavenumber region……………………………………………………….... 171
Figure 8.16: DRIFTS investigation of H2O desorption from 0.3%Pd/SZ. High
wavenumber region……………………………………………………….... 172
xxi
CHAPTER 1
INTRODUCTION
Worldwide energy generation is overwhelmingly supplied by the
combustion of fossil fuels. In 2004 energy consumption in the United States from
natural gas, petroleum, and coal accounted for 85.7% of total usage [1]. NOx
species are common in the exhaust streams of high temperature combustion
processes, thus their effective removal from power generation exhausts has long
been recognized as an environmental necessity. The health effects of NOx on
both plants [2] and humans [3] have been known since the late 1960’s. More
recent research further indicated that even low levels of NO2 increased the risk of
human respiratory infections [4]. NOx emissions also lead to acid deposition [5]
and the formation of smog and ground level ozone through a photochemical
reaction with volatile organic compounds [6]. Additionally, N2O is a significant
greenhouse gas. Although emissions of N2O are relatively low, it has 270 times
the Greenhouse Warming Potential of CO2 and represented 4.6% of US
greenhouse emissions in 2003 [7]. There are three recognized sources of NOx in
combustion exhaust.
Fuel NOx is the result of the oxidation of nitrogen
containing species in the fuel itself. Prompt NOx is formed through the reaction
1
of radicals within the combustion flame. By far the most significant source is
thermal NOx, which results from the reaction of molecular nitrogen and oxygen in
the combustion air [8].
The use of lower hydrocarbons for power generation is an extremely
active field of research. Investigated approaches include use in fuel cells and in
combustion applications. As a fuel source methane is significantly cleaner than
gasoline, diesel, or coal, as it lacks organically bound nitrogen and sulfur.
Methane is also abundant as the principle component of natural gas, and is a
relatively low cost resource.
Natural gas reciprocating engines represent an
increasingly popular technology for use in a distributed energy context.
Reciprocating engines are a well-understood technology from both a use and
business model standpoint, and as such they represent both the best-selling and
lowest-cost distributed energy technology in the world [9]. Although automotive
applications are the most commonly known use of reciprocating engines they are
also used in a stationary capacity as back-up generators, in industrial service, as
pipeline pump drivers, and at local heat and power cogeneration sites. With
slight modifications reciprocating engines can run using light hydrocarbons such
as landfill gas, as well as across a wide range of natural gas compositions.
While natural gas fired reciprocating engines can be operated under both
lean and rich combustion conditions, lean operation is preferred for several
reasons.
Operating under lean conditions increases engine efficiency while
reducing engine out exhaust pollutants.
Specifically, emissions of NOx are
reduced out of the engine due to a lower
2
combustion
temperature.
However, NOx emissions remain a concern and are a barrier to the wider
application of these engine systems. The United States Department of Energy,
through the Advanced Reciprocating Engine Systems (ARES) Program, has
outlined goals for efficiency improvements and emissions reduction from leanburn natural gas fired reciprocating engines.
Included in these goal is a
reduction by 90% of current state-of-the-art NOx emissions.
Current technologies to control NOx emissions from reciprocating engines
include both combustion control and catalyst based schemes.
Combustion
control approaches include modifying combustion chamber design or reducing
cylinder temperatures through exhaust gas recirculation (EGR). Engine design is
a mature field, and it is generally accepted that optimization of cylinder design
has been reached.
EGR, while used in some systems to meet current NOx
restrictions, results in a significant efficiency loss and is not considered a feasible
final solution. Catalytic aftertreatment approaches have significant advantages,
including ease of retrofit and no associated loss of efficiency.
Currently
automobile emissions are controlled with Three Way Catalysts (TWC). These
catalysts reduce nitrogen oxides and oxidize carbon monoxide and waste
hydrocarbons. TWCs are ineffective under lean-burn conditions, limiting their
usefulness as manufactures develop more efficient lean burn engines [10]. The
second commonly used catalytic control technique is Selective Catalytic
Reduction (SCR) of NOx with NH3.
SCR is a well-established technology,
offering the highest potential NOx removal efficiencies, up to 90% depending on
the application, of any technology commercially available. In SCR systems
3
NH3 is vaporized and injected into the exhaust, which then flows over a catalyst
bed.
Current SCR catalysts are generally metal oxide or zeolite based,
incorporating vanadium pentoxide, titanium dioxide, or tungsten trioxide.
Although effective, NH3 SCR has a number of drawbacks. Storage of ammonia
makes installation on mobile sources impractical, while safety and delivery issues
complicate use in fixed applications. Ammonia must be completely vaporized
and evenly dispersed across the face of the catalyst. Monitoring and control
units must be employed to ensure that ammonia is fed to the exhaust in the
proper ratio.
This is especially difficult in conjunction with variable load
applications, such as pipeline service. Depending on the engine and application,
relatively small changes in load can cause large shifts in NOx production by the
engine.
Current feedback SCR systems have a sensor lag of around five
minutes during which time either excess ammonia or NOx will be exhausted to
the environment.
The low cost and wide availability of natural gas make it a good option as
a NOx reducing agent in addition to it being an attractive fuel. This is especially
true for natural gas fueled power generation where the gas is already present,
and is in fact typically found to some extent in the exhaust. One proposed use of
hydrocarbon reducing agents in a lean-NOx trap system. Under this approach
the system selectively adsorbs NOx species under lean conditions until an
adsorbent material is saturated. Upon saturation the exhaust flow is directed
around the trap, while excess fuel is injected to it.
conditions
the
adsorbed
NOx
is reduced.
4
Under the created rich
A full system is typically
comprised of an oxidation, a reformer, and a trap/reduction catalyst. To further
increase NOx reduction efficiencies a pair of catalyst beds are used and operated
on an alternating trap/regeneration cycle. The switching of the exhaust stream
between two catalyst beds and the injection of controlled amounts of
hydrocarbon fuel require the inclusion of a control system and injected fuel to
create rich conditions during regeneration. Fuel penalties of 1-5% are typically
required to accomplish the desired NOx reduction.
Ideally these unburned hydrocarbons in the engine exhaust would be used
as the reducing agent, as this scheme would eliminate both fuel penalty costs
and injection control issues. Despite significant research towards these ends
inherent problems in performing lean hydrocarbon SCR have prevented the
commercialization of such a system. The principle challenge is that under lean
combustion conditions the hydrocarbon tends to react with oxygen rather than
selectively with the NOx species.
The present work seeks to develop a catalyst system active for the
reduction of NOx under high excess oxygen conditions using natural gas as the
reducing agent. To address the challenge of lean NOx reduction a novel twostage reduction scheme has been proposed. The central idea of this approach is
that NO2 is a more easily reduced species than NO. As such it should be better
able to compete with oxygen for reaction with the hydrocarbon reducing agent.
Additionally NO2 has been proposed as a reduction reaction intermediate for
several hydrocarbon based reduction systems. Development of the two-stage
system
is
based
upon
the development
5
separate
catalysts
to
perform separate functions. One active for the oxidation of NO to NO2, and a
second active for the reduction of NO2 to N2.
The work presented here is
focused on the development of novel NO2 reduction catalysts based on Pd
sulfated zirconia preparations. Two series of catalysts have been developed,
one prepared by a simple incipient wetness synthesis, and the second by a
modified ‘one-pot’ sol-gel method. Characterization of these catalysts has been
performed, along with a mechanistic IR study to explore the improved
performance observed during the reduction of NO2. Additionally, performance of
the two-stage concept under simulated lean exhaust conditions was examined.
6
CHAPTER 2
LITERATURE REVIEW
2.1 NOx reduction with Higher Hydrocarbons
To replace ammonia as a reducing agent, hydrocarbons for NOx reduction
would offer obvious practical benefits that include distinct advantages such as no
ammonia slip, no direct ammonia oxidation, lower cost and relative ease of
handling. Various catalysts and reductants have been studied for the reduction
of NOx with hydrocarbons. Hamada and coworkers have studied H-form zeolites
for the reduction of NOx with propane and propene. In oxygen rich atmospheres
and with propane as the reductant, N2 formation reached a maximum of 673 K.
NO reduction activity was related to the catalytic activity for hydrocarbon
oxidation. Propene was observed to result in higher N2 yields [11]. Further
contributions over H-ZSM-5 demonstrated that the reduction of NO requires the
presence of O2 in the feed stream, and that the reaction proceeds through an
NO2 intermediate [12,13].
The reduction of NO with propane was also
investigated on metal-doped oxide supports [14] and sulfur treated TiO2, ZrO2,
and Fe2O3 [15].
Ce and Pr exchanged ZSM-5 catalysts were examined for NO reduction
with propene. Maximum conversion of NO to N2 was seen at the temperature
7
corresponding to 80-90% propene conversion. Ce-ZSM-5 was the most effective
catalyst studied [16]. Further work revealed a promotional effect of doping CeZSM-5 with alkaline earth metals [17].
In a study on the role of cobalt species in the reduction of NOx with
propane over Co-Beta, the authors conclude that highly dispersed cobalt is
necessary for high activity. They also state that Co3O4 promotes NOx reduction at
low temperatures but favors propane combustion at high temperatures [18].
Bethke et al. showed that NOx was reduced by propane and propene over CuZrO2 and Cu-ZSM-5 catalysts. The postulated reaction scheme involves
hydrocarbon activation by adsorbed NO2 to form an adsorbed nitrogen-containing
hydrocarbon intermediate. The authors propose that the reaction of this
intermediate with NO is the route to nitrogen formation under lean conditions [19,
20].
Burch and coworkers have examined several catalytic systems for the
reduction of NO with hydrocarbons, including Cu/ZSM-5, Co/ZSM-5, Rh/ZSM-5,
and Pt/ZSM-5 [21,22]. They also extensively studied propane and propene NO
reduction along with the selection of silica or alumina supported platinum.
Propene was shown to be a more effective reductant over both catalysts [23-25].
Bethke and Kung investigated the use of supported Ag catalysts for the
lean reduction of NO with propene. They compared 2% and 6% wt. Ag/Al2O3
catalysts. Interestingly, the 2% catalyst achieved much higher NO conversions to
N2 than the 6% catalyst, which formed a substantial amount of nitrous oxide, an
8
undesirable by-product. The difference in reactivity was attributed to the oxidation
state of silver, which is Ag+1 in the 2% catalyst and Ag0 in the 6% catalyst. The
reason is that Ag0 leads to a high rate of propene combustion at the expense of
nitrogen formation. A synergistic effect was observed when Al2O3 was mixed with
silver [26,27].
The use of zirconia as a support for copper oxide was studied in the
reduction of NOx with propane. The material showed good low temperature
conversion in excess oxygen. The high activity is suggested to be the result of
surface stability of copper species [28].
A comparison of Au, Rh, and Pt metals supported on alumina and silica
was examined. The catalysts were examined for the reduction of NO and NO2
with propene in 5% O2. The metals appear to catalyze the reduction through
different routes. The Rh and Pt catalysts showed no dependence on the support
and very similar behaviors toward reduction of both NO and NO2. Observations
on the Au based catalysts showed that Au/SiO2 was inactive for the reduction of
NO2 while Au/Al2O3 performed worse than bare alumina. Only Au/Al2O3 was
active for the reduction of NO. The authors conclude that over Pt and Rh the
reduction of NOx occurs exclusively over the metal site, while over Au based
catalysts the support plays an active role in the reduction reaction [29,30].
Tanaka et al. investigated the addition of Mo and Na to Pt-silica catalysts.
Under a simulated feed stream of 4.3% O2, 1200ppm NO, 800ppm propene, and
3% H2O they observed that the addition of Na and Mo resulted in an
9
active temperature window that was both wider and shifted to higher temperature
than for Pt-silica only. Mo and Na alone were inactive for the NO reduction. The
improved activity of the trimetallic catalyst is ascribed to the Mo and Na
improving the resistance of Pt sites to oxidation [30,31].
Pt-USY zeolite was shown to be active for NOx reduction with propene,
though not propane. 1%Pt-USY was prepared by an exchange method and
tested in a reactant stream of 0.1 kPa NOx, 1.5 kPa propene, and 0-10 kPa O2.
XRD analysis demonstrated that Pt existed in the metallic phase, and TEM
imaging revealed that large Pt clusters (>15nm) are present on the outside
surface of the catalyst while smaller clusters are present in the pores. While the
catalyst was active at low temperatures and high space velocities, selectivity to
N2 was poor at only 30% [33].
The effects of preparation method and calcination temperatures on the deNOx activity of Co/Al2O3 catalysts for lean NOx reduction with propene were
examined. Reactants were varied over values of 200-1000 ppm NO, 200-3000
ppm propene, and 4-10% O2. Catalysts prepared by a sol-gel method possessed
higher surface area and better activity than catalysts prepared by wet
impregnation. The optimal loading and calcination temperatures to form the
2+
active Co phase are discussed [34].
SnO2/Al2O3 catalysts were prepared by incipient wetness, and the effect of
loading and preparation methods were examined for the lean SCR of NO with
propene.
The
maximum
NO
10
conversion
was
attained
at
lower
temperatures with increasing Sn loading, but the activity per mole of Sn
decreased with loading due to poorer dispersion. Similar activities were observed
for the catalysts when NO was replaced by NO2 in the feed. XPS analysis
demonstrated the presence of the Sn4+ oxidation state for all samples [35].
Ag/Al2O3 catalyst prepared by sol-gel and impregnation methods were
tested for the reduction of NO using the higher hydrocarbons n-hexane and noctane, similar to what is found in diesel engine exhaust. The feed contained
1000 ppm NO, 6000 ppm hydrocarbon, and 10% O2. UV-VIS and XAFS were
used to demonstrate that at silver loadings of 2% or below the metal was in the
Ag1+ state, while at higher loadings metallic silver was formed. Confirming results
from previous groups, only the low loading samples were active for the reduction
of NO. Up to a maximum loading the activity of the catalysts increased with Ag
loading. The 2% Ag sol-gel prepared catalyst was the most active [36]. A study
over the same catalyst on the effect of hydrocarbon reducing agent revealed
higher activities corresponded to the choice of higher over lower alkanes,
unbranched over branched alkanes, and alkenes over alkanes [37].
Chen and coworkers studied the reduction of NO by C4 hydrocarbons on
platinum in oxygen, as well as the effect of SO2. The reduction occurs directly on
the metal surface, and no catalyst support is needed. Oxygen is believed to two
roles, the formation of active intermediates and deactivation of the surface. Low
concentrations of sulfur dioxide promoted the reduction of NO, but higher levels
poisoned the reaction because of a surface site blocking effects [38].
11
Yang and Li have studied NO reduction with ethylene over ion-exchanged
2+
pillared clays. It was observed by the authors that Cu
exchanged TiO2-pillared
inter-layered clays were substantially more active than Cu-ZSM-5 catalysts in
presence of 5% oxygen [39].
H-mordenite loaded with Ce and/or Pd by incipient wetness methods were
tested for the selective reduction of NO and NO2 by dodecane. The monometallic
catalysts were shown to have low activity for the reaction, but the bimetallic
catalyst attained 70% NOx conversion at 350°C at a GHSV of 30,000 hr-1. The
feed in these experiments contained 6% oxygen. The addition of 40 ppm SO2 to
the feed deactivates the catalyst, but the simultaneous addition of 40 ppm SO2
and 15% water has little effect. The use of NO2 as the NOx species to be reduced
improves catalyst activity, and also broadens the activity window. XPS results
indicate that Pd2+, but no explanation is made for the promotional effects of Pd
on the reaction [40].
Meunier et al. performed mechanistic investigations into the role of silver
loading on the performance of Ag-alumina catalysts. The reducing agent was
propene. In-situ DRIFTS and thermogravimetric analysis revealed that a 1.2%
loaded sample promoted the formation of ad-NOx species that after conversion to
organo-NOx species perhaps react with NO to form N2. Enhanced performance
was observed over the 1.2% sample when NO2 was fed instead of NO. A 10%
loaded sample, while relatively active at low temperatures, was mainly selective
12
to N2O. Reaction studies also demonstrated that the highly loaded sample was
active for the oxidation of NO to NO2, reaching the thermodynamic equilibrium at
around 350°C [41].
Further studies on these catalysts were performed examining their
performance for the oxidation of NO to NO2. Metallic silver could be formed on
the catalyst by reduction in H2 or by exposure to NOx SCR reaction conditions.
Results confirmed that metallic Ag is responsible for the NO oxidation, and that
Large Ag clusters could be reduced during the SCR reaction even in net
oxidizing conditions [42].
Dry impregnated 1.2 wt% Ag/γ-alumina was studied for the SCR of NO
and NO2 with propene in the presence of sulfur containing feeds.
The NO
reduction activity of the catalyst was shown to disappear after the exposure to
100 ppm SO2.
The activity of the Ag/alumina for the reduction of NO2 was
superior to the activity for NO reduction. Sulfidation resulted in a much smaller
loss of activity for NO2 reduction than for NO reduction. The relative activities for
NO and NO2 reduction are explained by the proposal that silver oxides are
responsible for the formation of adsorbed NOx species, which are then reduced
to N2 by propane over alumina sites [43]. Further studies on the resistance of
Ag, Ni, and Ir supported on alumina to SO2 and H2O deactivation were also
performed [44].
13
2.2 Reduction of NOx with CH4
Co-ZSM-5 catalyst has been extensively studied for NO reduction by CH4
in the presence of excess O2 by several groups. Some of the original work in the
field was performed by Li and Armor, who reported conversion of NO at
CH4=2000ppm, NO=820ppm, and O2=2.5% with GHSV=7500h-1 at 400°C on
Co/ZSM-5. In this contribution they also demonstrated that Cu/ZSM-5, which is
effective for NO decomposition, was not active for the CH4 reduction [45]. In
subsequent work they reported that the NO reduction activity was proportional to
Co2+ exchange level; however an excess amount did not contribute to better
activity [46]. The simultaneous reduction of N2O and NO were tested over this
catalyst. While NO reduction activity was unchanged in the presence of N2O,
N2O decomposition was suppressed until NO conversion has decreased [47].
The presence of 2% water in the reaction feed significantly reduced NO
conversions below 450°C, but was less severe at 500°C.
Temperature
programmed desorption studies indicated that this effect was likely due to
competitive adsorption between NO and H2O [48]. With 10% water in the
reaction feed, a dramatic loss in NO conversion to N2 occurred, although some of
the effect is reversible when water is removed from the feed. When the support is
replaced by a ferrierite zeolite, the NO conversion increased two-fold at 500°C.
Reaction conditions were 30,000hr-1 GHSV, CH4=1015ppm, NO=1600ppm,
O2=2.5%. In the ferrierite environment, CH4 is more selectively used for NO
reduction. Water severely retarded the
activity of the catalyst. The presence of
14
water increased the apparent activation energies for NO reduction and CH4
oxidation [49]. The group also compared the activity of Co-ZSM-5 to Ga-H-ZSM5 and found the latter to have higher methane selectivity toward NO reduction
than combustion with O2. However, the NO conversion is only 40% over Ga-HZSM-5, with CH4=2000ppm, NO=820ppm, and O2=2.5% at a GHSV=7500hr-1 at
400°C under which Co-ZSM-5 gave complete conversion. The presence of H+
acid sites was suggested to be an important factor in reduction activity [50].
Nishizaka and Misono studied the SCR reaction over Pd loaded zeolite
catalysts and found Pd-H-ZSM-5 and Pd-Ce-H-ZSM-5 to be effective catalysts
for NO reduction. At 500°C, with NO=1000ppm, CH4=2000ppm, and O2=2%,
conversion was 70%. Comparing the NO reduction activity over non-proton
exchanged catalysts, they concluded that the presence of Pd and protonic acidity
were essential for reduction activity [51]. It was also found that the rate of the
NO2 reduction reaction was higher than that of the NO reduction reaction [52].
The primary role of protonic acidity is speculated to be in the reaction between
NO2 and CH4 [53].
Ogura and coworkers have examined bimetallic H-ZSM-5 catalysts for the
reduction of NO with CH4. The addition of precious metals (Pt, Rh, Ir) was shown
to increase NOx reduction in the presence of water vapor through the promotion
of NO oxidation [54-56]. Exploration of other promoters also showed that Ga
was active for the oxidation of NO to NO2 [57]. Further work by these authors
also examined bifunctional Pd/Co/H-ZSM-5 [58, 59]. The active site in ZSM-5
15
supported palladium was determined by NaCl titration, which can be used to
quantify Pd2+. The results showed that with an increase in the amount of the
isolated Pd2+, the catalytic activity for NO2 reduction with CH4 increased up to a
maximum loading of 0.6 wt.%. Above this loading an increase in PdO decreased
the selectivity toward NO2 reduction [60].
Bell et al. showed that the state of Pd in Pd-H-ZSM-5 during the reaction
is found to be dependent on the reaction temperature and the presence of O2 in
the gas phase. In the absence of O2, Pd remains highly dispersed as Pd2+ up to
610K. In the presence of O2, most of the Pd remains dispersed as Pd2+ up to
873K. The activity toward NO reduction over Pd-H-ZSM-5 is much higher than
that of H-ZSM-5, showing that Pd2+ is the active site [61,62].
Cerium and silver ion-exchanged ZSM-5 catalysts formulated by Li et al.
reached 80% NO conversion at 500°C with NO=CH4=0.5% and O2=2.5% at
7500hr-1 GHSV. In the presence of water, NO conversion dropped to
approximately 20%, although the effect was reversible. The Ce ions were
suggested to be the sites for NO oxidation, and also to play a role in suppressing
the direct CH4 combustion [62,64]. In a later study, characterization of this
catalyst was performed using STEM/EDS, HRTEM/EDS, XPS, and UV-VIS DRS
techniques. They found that the dispersed Ag+ state was more active for the SCR
reaction. The oxidation state of cerium was a combination of Ce3+ and Ce4+ [65].
The function of zeolite type on the activity of Pd based catalysts for the
reduction of NOx with methane was studied by Montes de Correa et al. 1 wt% Pd
16
was loaded by wet impregnation on HZSM-5, ferrierite, and mordenite, and a
bimetallic Pd-Pt-HMOR. H-ZSM-5 was shown to result in the highest activity,
around 40% conversion. The catalysts were also tested in 30ppm SO2 and 10%
water. All catalysts showed a slight deactivation with the addition of SO2, and a
stronger deactivation when water was simultaneously added. The authors
propose that the sulfided catalysts are more capable of adsorbing water, thus
causing further deactivation [66]. Bimetallic Pd/Co/mordenite was shown to be
active in the presence of water vapor, and that the addition of Pd resulted in a
reduction in the amount of NO2 produced when compared to the Co-only
catalyst. Pd was shown to increase the reducibility of exchanged Co [67]. The
authors have also examined Pd/Co supported on sulfated zirconia for NO
reduction with CH4. This catalyst showed high sulfur retention, likely through the
formation of cobalt sulfide. IR experiments indicated that NO could be converted
to adsorbed NO2 on the catalyst under both lean and rich conditions.
This
occurred through oxidation, or disproportionation, respectively [68].
Bustamante et al. investigated the reduction of NOx under lean conditions
over Pd doped Co exchanged mordenite. Pd loading was 0.15% by mass.
Typical reaction conditions were 1000 ppm NOx, 2000-3000 ppm CH4, 6% O2,
and 8% H2O with a GHSV of 30,000 hr-1. At these conditions NOx conversions of
60% were reported, and little CH4 oxidation was observed. XPS studies reveal
that Co2+ and Pd0 are present on the catalyst surface. The authors conclude that
17
CH4 oxidation over Pd sites leads to more favorable conditions for NO reduction
over Co2+.
Also observed was NO oxidation to NO2 by over-exchanged Co
zeolites [69].
Praserthdam et al. studied the effect of crystal size on the hydrothermal
stability of Co/H-ZSM-5 towards the catalytic reduction of NO with CH4. Reactant
flow was composed of 1000 ppm NO, 1% CH4, and 10% O2 in He at a GHSV of
10,000 hr-1. Catalysts were deactivated by exposure to 10% steam at 600°C for
24 hours. Results indicated that smaller Co crystal size led to both a higher
activity toward NO reduction with methane; as well as increased hydrothermal
stability of the catalysts. The authors report a critical crystal size for improved
stability of 2.0 nm for these reaction conditions [70].
Although the studies over zeolite-based catalyst are quite extensive, the
common shortcoming of these catalysts is poor hydrothermal stability, making
their application to actual combustion exhausts unlikely. Non-zeolitic oxide
supports are known to be more hydrothermally stable. Studies of these materials
include extensive work by Vannice and co-workers who reported on NO
reduction by CH4 over Li/MgO [71], La2O3 [72], CeO2, Nd2O3, Sm2O3, Tm2O3, and
Lu2O2 [73]. In the presence of excess O2, the rate-determining step was reported
to be methyl radical formation by reaction between adsorbed NO2 and CH4.
When Lu2O3 was dispersed over an Al2O3 surface, the rate of NO reduction by
CH4 in the presence of O2 was significantly enhanced. The activity of this catalyst
than that of a Co-ZSM-5 catalyst [74].
at 700°C was reported to be higher
18
Manganese based porous oxides containing Ce, Sr, and La were studied
for the lean de-NOx reaction at low temperatures, 200-300°C under 0.67% CH4,
0.2% NO, 5% O2. These catalysts showed good stability even under 4% H2O.
They also showed high activities and N2 selectivities [75].
Balint et al. examined alumina supported Ru nanoparticles for the reaction
of CH4 with NO under a flow of 1% NO and 0.55% CH4 with a GHSV of 60,000 hr1
. Their goal was to both reduce NO and partially oxidize CH4 to syngas (CO, H2).
The reduction of NO was observed beginning at 450°C. The catalysts with larger
particles (>5nm) completely oxidized CH4, while the catalysts with a smaller
distribution (<5nm) were more active toward the partial oxidation. Their studies
also demonstrated that the larger particle size distribution improved oxygen
resistance [76,77]. Alumina supported Pt nanoparticles were also examined and
shown to be less selective for the formation of CO and NH3 than more
conventionally prepared catalysts [78,79].
The role of TiO2-ZrO2 mixed oxide support in the reduction of NO with
methane over Pt loaded catalysts was investigated by Mariscal et al. Supports
were prepared by a sol-gel method, and then impregnated with Pt. BET surface
area measurements show that the mixed oxide supports have significantly better
surface area than the single oxides. A 1:1 ratio of TiO2-ZrO2 resulted in the
highest surface for the prepared catalysts, 465 m2/g. DRIFT spectra reveal the
formation of nitrite and nitrate species to a larger extent over the mixed oxide
supports. The authors propose that this is due to the increased acidity
19
associated with the mixed support. They further call attention to the conclusions
of other researchers stating that acid sites are important for the reduction of NOx.
The highest activity reported was for the catalyst with a 1:9 TiO2-ZrO2 ratio
[80,81].
The role of catalyst support was studied for Co based de-NOx catalyst with
natural gas lean-burn exhaust from 200-700°C. The activities of the catalysts
from highest to lowest were Co-ferrierite, Co-beta, and Co-ZrO2. Addition of
water to the feed gas (0.1% NO, 0.01% NO2, 0.1% CH4, 2.5% O2) decreased the
activities toward NO reduction. Selectivity and activity could be recovered with
the removal of water. The highest conversion reported was 65% [82].
We have previously reported the effective use of Pd-based catalysts
supported on titania for the NO reduction with CH4 in the presence of O2 [83-94].
The extent of the reaction was shown to be highly dependent on the palladium
oxidation state, with the metallic phase of palladium necessary for the reduction
of NO to N2. We extensively studied the mechanistic aspects of NO/CH4
reactions using isotopic labeling techniques under both steady-state and
transient conditions. From a series of unsteady-state and steady-state isotopic
labeling studies using labeled species such as 15N16O, 15N18O, 13CH4, 18O2, it was
concluded that N2 is formed through direct participation of CH4, possibly through
a methyl-nitrosyl type intermediate, whereas N2O formation was mainly a result
of the NO decomposition reaction [84].
We also used steady-state oscillations
as a probe to gain insight into the reaction network and found that the oxidation
20
state of the active metal was responsible for three competing reactions; NO
reduction with CH4, direct CH4 oxidation, and NO decomposition [85]. In-situ
DRIFTS was used to identify adsorbed species under NO, NO+CH4, and
NO+CH4+O2 flow, and the formation of various nitrogen-oxo adspecies such as
bridged/bidentate nitrate, monodentate nitrate, nitro, and linear NO was
confirmed. It was found that linearly adsorbed NO species on Pd, with a IR
absorption band at 1780 cm-1, was dominant on reduced Gd-Pd/TiO2 at high
temperatures under NO flow. However, the intensity of nitrate species was
increased on an oxidized surface relative to the linear NO. Based on these
results, the key surface species under reaction conditions were determined to be
CHx, NHx, monodentate nitrate, nitro, and linear NO species [91-93].
2.3 NOx Reduction Over Solid Acid Supports
Important work by Li and Armor [49], as well as work by Nishizaka and
Misono [51-53] identified surface acidity in H-ZSM-5 zeolite as being key to
activity in the reduction of NO with CH4. Several groups identified surface acidity
as being particularly key for Pd-based systems.
Adelman and Sachtler studied Pd species supported on H-ZSM-5 and NaZSM-5 supports. In the NO2+CH4+O2 reaction Pd/H-ZSM-5 was observed to be
active for NO2 reduction. Furthermore activity of a reduced catalyst was shown
to increase with time-on-stream do to the redispersion of PdO clusters as Pd2+
ions. Activity was not observed over the Pd/Na-ZSM-5 support even with the
21
presence of Pd2+. The authors conclude that acidic protons are necessary for
the redispersion of Pd and play a role in the reduction reaction [94].
Loughran and Resasco presented results indicating that both Pd and acid
sites were necessary for catalytic activity.
Over Pd/H-ZSM-5, activity was
observed to pass through a maximum with Pd loading. Other solid acid supports,
including sulfated zirconia (SZ), were shown to have reduction activity.
Additionally, physical mixtures of SZ and inactive Pd/SiO2 were active for the
reaction. A bifuntional mechanism making use of both acid sites and Pd sites
was proposed [95]. Further examination of these supports by EXAFS/XANES
indicated that surface acidity led to the formation of Pd2+ ions on the catalysts
surface. These species were selective for the reduction, while PdO favored the
combustion reaction. Examination of Pd loading level showed that at high levels
formation of the unselective PdO was favored. The observation of Pd2+ species
on inactive H-Y zeolites showed that these species are necessary but not
sufficient to predict activity [96]. Further studies demonstrated a reduction in CH4
combustion over Pd catalysts supported on acidic supports, due to redispersion
of Pd [97].
Although quite active under dry conditions, zeolitic supports a subject to
permanent deactivation under high temperatures and water vapor containing
streams. It is therefore desirable to use metal oxide supports that are much
more stable to hydrothermal deactivation.
With the identification of the
importance of surface acidity for the NOx reduction with methane, the use of solid
superacids
has
been
explored. Superacids are defined as having a
22
Hammett acidity function less than that of 100% H2SO4 [99]. The most well
known example of a solid superacid is sulfated zirconia. Sulfated zirconias are
commonly used in acid-catalyzed organic reactions, including the difficult
isomerization of n-butane [100].
Reduction of NO using n-decane in 9% O2 gave N2 yields of around 40%
over Cu/SZ.
Activity increased with sulfur loading, and the corresponding
increase in acidity. Through comparison to Cu/ZrO2 the authors conclude that
acidity plays a significant role only at high temperatures, where a bifunctional
mechanism governs the reduction [101,102].
Temperature programmed
desorption studies showed the presence of only CuO on the sulfated zirconia
catalyst, resulting in the formation of only mononitrosyl species after exposure to
NO [103].
For the reduction of NO with CH4 Pd/SZ catalysts went through an activity
maximum with Pd loading, although the maximum occurred at 0.1 wt%, lower
than on Pd/H-ZSM-5. Activity was improved by increasing the sulfate loading of
the catalyst. Comparison to Pd/H-SZM-5 showed that the SZ catalyst lost less
activity upon exposure to water vapor, and was better able to recover activity
after its removal from the reaction feed [104]. Deactivation of SZ in the presence
of water is due to attack on the Lewis acid sites. Under water vapor these sites
are protonated to Brønsted sites [105].
Ohtsuka examined SZ supports impregnated with various precious metals,
and found Pd to be the most active in the NOx reduction with CH4. Pt was
observed to have good activity for the oxidation of NO to NO2. Reaction tests
23
comparing the reduction of NO and NO2 showed increased N2 yields when using
NO2 [106]. Further work led to the development of a highly active bimetallic
catalyst. Pd-Pt/SZ was shown to maintain activity even in the presence of 10%
O2 and 9% H2O [107]. Long-term activity tests of these catalysts showed a
gradual deactivation with time. The cause of deactivation was the conversion of
the tetragonal to the monoclinic zirconia phase.
The catalysts as prepared
contained the tetragonal phase, but the monoclinic is stable below 1000°C. The
conversion results in sulfate loss and decreased activity. Addition of Fe to the
support was shown to stabilize the tetragonal phase and improve stable activity
[108,109].
.
24
CHAPTER 3
EXPERIMENTAL METHODS
3.1 Catalyst Synthesis
Catalysts were prepared for this study using both incipient wetness
impregnation and sol-gel techniques.
The impregnation to incipient wetness
technique consists of the physical mixing of unloaded support and a solution
containing the dissolved metal salt.
Water was used as the solvent for all
catalysts presented here. The appropriate amount of metal salt is first dissolved
in a volume of water equal to the pore volume of the support material. The
solution is then added to the support powder and thoroughly mixed.
The
impregnated support is dried and then calcined in air. The sol-gel preparation
methods allow for greater control of support characteristics such as pore volume,
pore diameter, and catalyst surface areas. In addition, for metal-doped catalysts,
addition of the metal before gelation can result in higher dispersions on the
surface of the catalyst. The catalyst structure is built by the controlled hydrolysis
of metal alkoxide precursors using several possible agents.
The alkoxide
precursor is placed in an organic solvent, and under stirring the hydrolysis agent
is added. After hydrolysis the sample gel is allowed to dry overnight before
calcination.
25
Detailed descriptions of the preparations of each catalyst prepared in this
work are included at the beginning of the appropriate chapters.
3.2 Steady-State Reaction Studies
A schematic of the reaction system is given in Figure 3.1. The system is
constructed of stainless steel tubing connected by Swagelok fittings. Up to six
feed gasses can be introduced to the reactor through Brooks 5850 and Tylan FC280 mass flow controllers. In addition water can be added to the feed by routing
a portion of the stream through a heated bubbler. Feed gases are mixed and
then sent through a four-port valve. This valve allows gas to bypass the reactor
for feed analysis measurements.
The reactor is a 0.635-cm (¼-inch) outer
diameter stainless steel tube placed in a resistively heated furnace capable of
temperatures up to 700°C. Catalyst samples are packed in the reactor between
plugs of quartz wool, and the sample is centered in the furnace. Temperature
monitoring and control is done using an Omega CN4400 temperature controller,
and an Omega 0.159-cm (1/16-inch) k-type thermocouple. The thermocouple is
inserted into the reactor tubing and is in contact with the quartz wool upstream of
the catalyst sample. Downstream of the reactor the product gasses pass through
a chilled condenser and a packed tube of Drierite desiccant to remove water
before the analysis portion of the system.
26
Figure 3.1: Schematic of the steady-state reaction system.
Analysis of O2, N2, CH4, C2H6, C3H6, C3H8, CO2, CO, and N2O
concentrations is performed by a Varian CP-4900 micro gas chromatograph.
The micro-GC draws sample for 30 seconds to ensure flushing of the injection
ports. The gas sample is then injected a Molecular Sieve 5A and a Porapak Q
GC column. The two columns are arranged in parallel to reduce analysis time to
less than 70 seconds. Gas concentrations are calculated from response factors
determined using gases of known concentration. Gas flow not pulled into the GC
sample loop passes through two further analytical instruments where continuous
monitoring of NH3, NO, and NO2 occurs. Ammonia analysis is performed by a
27
Siemens Ultramat 5F Gas analyzer.
The instrument operates through IR
adsorption measurements. Half of a split IR beam is passed through a reference
cell containing a non-absorbing gas (N2), the second half is passed through the
sample cell where partial absorption occurs. The radiation passing through the
sample and reference cells then strike a sealed receiving chamber filled with
ammonia.
The two different intensities of radiation reaching the receiving
chamber cause differences in pressure. These differences are measured by a
microflow detector and correlated to sample ammonia concentration.
NO and NO2 concentrations are measured by a Thermo Environmental
Model 42H Chemiluminescence NOx analyzer.
Sample is drawn into the
instrument by an external pump. As sample enters the detection chamber it is
mixed with O3 generated by the instrument. The reaction of NO and O3 results in
an excited NO2 molecule which decays to a lower energy state by light emission.
A photomultiplier tube measures this emission. For the measurement of NO2, the
sample is passed through a molybdenum converter before entering the detection
chamber. The converter reduces NO2 to NO for detection. When the samples is
passed through the converter a total NOx measurement is taken, and NO2
concentration is calculated as the difference of NOx and NO.
Experimental details on component gas concentrations, catalysts loading,
reactant flow rates, and examined temperature ranges are all included in the
appropriate chapters before the discussion of results.
28
3.3 Physisorption for BET Surface Area Analysis
Physisorption experiments were performed on a Micromeritrics ASAP
2010 accelerated surface area and porosimetry system. Sample tubes were
sealed, evacuated and weighed before catalyst was added. The samples were
degassed overnight at 130°C, and the tube containing the sample was weighed
again.
Sample weight was then calculated from the difference of these two
measurements. The sample tube was immersed in liquid N2 and exposed to N2
gas.
Pressure measurements are logged by the ASAP 2010 software to
calculate pore volume and surface area of the samples using the BET method.
Developed by Brunauer, Emmett, and Teller; the BET method is the most
commonly used method for measuring catalyst surface areas. With knowledge
of the cross sectional area of an adsorbing gas, pressure measurements may be
used to quantify the amount of gas adsorbed by the surface and condensed in
the pores. The BET isotherm is:
P
1
(C − 1) P
=
+
V ( P0 − P) VM C VM CP0
in which V is the volume of gas adsorbed at pressure P. P0 is the saturation
pressure of the gas at the experimental temperature, and VM is the volume of an
adsorbed monolayer. C is a constant. By plotting P/(V(P0-P)) against P/P0, the
slope and intercept of this linear equation yields VM and thus surface area.
29
3.4 X-ray Photoelectron Spectroscopy (XPS)
XPS is a highly surface sensitive technique of particular importance to
catalysis research.
A monochromatic X-ray source, operating under high
vacuum, is used to bombard the sample resulting in photoelectrons being ejected
from surface atoms.
Figure 3.2: Conceptual Drawing of X-ray Photoelectron Spectroscopy
The kinetic energy of the ejected core electrons is measured, and the
binding energy of this electron calculated through the following equation:
KE = hν − BE − φ SP
in which KE is the kinetic energy of the electron, hν is the energy of the incident
X-ray, BE is the binding energy of the photoelectron, and φsp is the spectrometer
work function. hν and φsp are known for the spectrometer, and thus BE can be
30
calculated. The binding energy of the emitted photoelectron is characteristic of
the chemical state of the source atom. Information on surface concentration,
oxidation state, and chemical environment can be determined through XPS
measurements.
XPS experiments were performed on an AXIS Ultra instrument using a
magnesium anode.
Before analysis, samples are loaded in a high-vacuum
preparation chamber where they are degassed overnight to a pressure of
10-6
torr. Samples are then transferred to the analysis chamber, which operates at an
ultra-high vacuum of 10-9 torr. A survey of the complete binding energy range is
performed to confirm expected peaks, then a more detailed scan of the binding
energy range characteristic of each species of interest is performed. Charge
shift correction was performed based on the location of the carbon 1s peak.
3.5 X-ray Diffraction (XRD)
XRD was performed to determine the crystalline phase of catalyst
samples. In XRD a sample is exposed to a monochromatic X-ray source. As Xrays scatter from two atomic planes in a crystalline sample they constructively
interfere at certain scattering angles, producing diffraction. The angles at which
constructive interference occurs is determined by the atomic lattice spacing of
the material under investigation, and thus appears in a characteristic pattern
allowing for phase determination. Angles of diffraction are determined by the
Bragg equation:
31
nλ = 2d sin θ
in which n is an integer multiple, λ is the wavelength of the X-ray source, d is the
distance between atomic lattices, and θ is the angle at which the X-rays strike the
flat surface of the sample. Measurements of peak width can also be used to
determine crystallite sizes through the Scherrer equation:
τ=
Kλ
( B − b) cos θ
where τ is the mean crystallite dimension, and K is the shape factor which is
typically given a value of 0.9. B is the measured full width at half max for the
peak, and b is the inherent instrument broadening.
The ability to determine
crystallite size arises from the fact that at small deviations from θ destructive
interference between diffracted X-rays is not complete.
XRD
experiments
were
performed
for
phase
identification
and
observations of supported metal crystal size. Diffraction patterns were taken
using a Bruker D8 Advance X-ray diffractometer equipped with a Cu Kα source
with
wavelength
1.54
Å.
The
unit
is
equipped
with
a
controlled
atmosphere/controlled temperature chamber, and a position sensitive detector
capable of simultaneously scanning across an 8° angle. The temperature can be
varied from cryogenic temperatures to 1200°C.
Samples are placed in shallow
sample holders and pressed flat with a glass microscope slide.
32
3.6 Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)
DRIFT spectroscopy is an IR absorption technique particularly suited to
the analysis of powdered samples. Signal is based on the diffuse reflectance
portion of the light coming off of the sample, which has penetrated particles and
interacted with surface adsorbed species. Specular reflectance, which has not
interacted with the sample in this way, is blocked. Control of sample temperature
and surrounding atmosphere allow for the monitoring of surface species under
temperature ramps and feed concentration changes. In this work two DRIFTS
instruments were used, a Bruker IFS66 and a Nicolet 6700. Both were equipped
with DTGS detectors and KBr beamsplitters. In-situ measurements were carried
out with the use of a controlled atmosphere diffuse reflectance cell. The cell was
equipped with ZnSe windows and contained a built in thermocouple mount which
allowed for direct measurement of sample temperature. Spectra were averaged
over 1000 scans in the mid-IR range (650-4000cm-1) to a nominal 4 cm-1
resolution.
3.7 Temperature Programmed Techniques
Temperature programmed techniques can be used to evaluate the
reducibility catalyst surface sites, as well as relative abundances and adsorption
strengths of catalytic sites. Catalyst samples were packed between quartz wool
plugs in a quartz U-tube reactor. The reactor was placed in an electrically heated
furnace controlled by an Omega CSC32 temperature controller and an Omega k-
33
type thermocouple. Brooks 5850e mass flow controllers were used to control
gas flows.
Prior to temperature programmed experiments catalysts were
calcined in 10%O2/He at 400°C for 30 minutes. Samples were then flushed with
He during cooling back to room temperature. All temperature programs were
performed at a 5-10°C/min ramp rate to a maximum temperature of 600-800°C.
Product monitoring was performed using a Thermo-Finnegan Trace DSQ mass
spectrometer.
3.8 Thermogravimetric Analysis/Differential Scanning Calorimetry
Thermogravimetric and differential scanning calorimetry measurements
were taken using a Setaram TGA/DSC-111. Thermogravimetric analysis (TGA)
is a method of measuring mass changes in a sample under various experimental
conditions, including gas adsorption and desorption.
Physical changes in
catalyst samples, including metal oxidation/reduction and support phase changes
can be monitored by exposure to controlled temperature ramps. Samples are
placed inside a sample cup hung in a sealed heating furnace. The sample cup is
suspended from a microbalance with 3 μg sensitivity. Suspended to the opposite
side of the TGA balance is an empty sample cup, which undergoes the same
environmental changes as the sample, and serves as a reference.
Mass
changes are calculated as sample side minus reference side. To eliminate TGA
signals that may be associated with slight differences in the two sides, a blank
run is performed with no sample. Both inert purge gases and reactive gases can
be flowed over the sample. Two gas
inlet streams are used to protect the
34
microbalance components from exposure to reactive gas streams. A schematic
diagram of the TGA/DSC-111 system in shown in Figure 3.2.
Figure 3.3: Schematic of the TGA/DSC-111 system.
The TGA/DSC-111 system also allows for the collection of differential
scanning calorimetry (DSC) simultaneously with TGA data. DSC allows for the
collection of heat flow data associated with adsorption, desorption, and phase
change processes. When used in conjunction with TGA measurements specific
heats of these phenomena may be calculated. The TGA/DSC-111 instrument
collects heat flow data through a pair of Calvet type sensors.
35
A cluster of
thermocouples, which detect temperature changes in the sample, surrounds
each sample cup. As with TGA measurements heat flow data is calculated as
sample signal minus reference signal. Additionally the subtraction of a blank run
is performed to remove the effect of small heat flow differences between the two
sides.
For this study TGA/DSC was used to monitor the calcination of catalyst
samples. Samples were placed in the TGA/DSC-111 and exposed to a flow of
5%O2/He. After a stable mass signal was reached the samples was treated
under a 5°C/min temperature ramp from 30 to 700°C.
36
CHAPTER 4
NO2 REDUCTION OVER AG-BASED CATALYSTS WITH HIGHER
HYDROCARBONS
4.1 Steady-State Reaction Experiments
Low loadings of transition metals, particularly silver and cobalt, have been
examined in the literature for the hydrocarbon reduction of NO. Several Agbased catalysts were synthesized and tested for the reduction of NO2 with
propene, propane, and methane. Catalysts were prepared by both sol-gel and
wet impregnation methods. Table 4.1 presents synthesis parameters and BET
surface area measurements for these catalysts.
Catalyst
1%Ag/TiO2-ZrO2
3%Ag/TiO2-ZrO2
1%Ag/Al2O3
3%Ag/Al2O3
2%Pd/Al2O3
Calcination SA m2/g
104.4
500°C 4hr Air
65.1
500°C 4hr Air
161.1
500°C 4hr Air
171.2
500°C 4hr Air
294.5
500°C 4hr Air
Table 4.1: BET surface areas of Ag-based NO2 reduction catalysts.
A mixed oxide support of titania-zirconia in equal molar ratios was synthesized
based on reports of beneficial effects on
37
the
reduction
of
NO
[52,70].
Additionally, palladium was explored based on our group’s previous experience
with CH4 NO reduction under rich conditions [71,72]. As with other precious
metals. Pd is able to activate methane, but is an expensive catalytic material.
Figure 4.1 presents the NO2 reduction data for several Ag-based catalysts
using propene as a reducing agent. Reactant concentrations were 5% O2, 1000
ppm NO2, and 5000 ppm C3H6. NO2 conversions are quite high even at 300C.
The 1% and 3%Ag/TiO2-ZrO2 catalysts are slightly above 80%, while the 1%Ag
and 3%Ag/Al2O3 are above 90%. All four catalysts show a slight decrease in
conversion at higher temperatures. This behavior is most pronounced over the
3%Ag/Al2O3. N2 yields are seen to differ significantly over the four catalysts. The
3%Ag/TiO2-ZrO3 shows the highest low temperature yield, nearly 70% at 300°C.
The 1%Ag/TiO2-ZrO2 reaches a maximum conversion at 400°C.
These two
catalysts both suffer a drop in yield at 500°C. The 3%Ag/Al2O3 has a higher N2
yield than the 1%Ag/Al2O3 at 300°C, but both then achieve above 90% yield at
400°C and 500°C. The loss of conversion in all catalysts, and yield in the TiO2ZrO2 catalysts at higher temperatures may be a function of competition for
hydrocarbon as combustion begins to occur.
NO2 reduction with propane is presented in Figure 4.2 over 1%Ag and
3%Ag/Al2O3, the bare Al2O3 support, and 2%Pd/Al2O3. Reactant concentrations
were 5% O2, 1000 ppm NO2, and 5000 ppm C3H8. Ag supported on the TiO2ZrO2 mixed oxide was not active for the reduction with C3H8. The 3%Ag/Al2O3
and 2%Pd/Al2O3 achieve above 80% conversion by 300°C, and all four samples
38
are at this level by 350°C. 2%Pd/Al2O3 loses conversion at higher temperatures,
dropping to 65% conversion by 500°C.
Nitrogen yields over the Ag/Al2O3
catalysts increases with reaction temperature, with the other product being NO.
Superior yield is seen with 3%Ag over 1%Ag loading at low temperatures, 300°C
and 350°C. At 400°C the yields over the two catalysts equalize at just above
90%. 2%Pd/Al2O3 gives the best low temperature yields reaching 47% and 67%
at 300°C and 350°C respectively. After 350°C nitrogen yield on Pd decreases,
dropping to 42% by 500°C. Nitrogen yield over the alumina support reaches a
maximum of 47% at 400°C. The low yield demonstrates that the supported Ag is
playing an important role in nitrogen selectivity, especially at high temperatures.
The high conversion and low yield may also indicate that the support is largely
responsible for the partial reduction to NO.
The catalyst testing discussed above demonstrates that the 1% and
3%Ag/Al2O3 catalysts are the most active in this series for the reduction and the
most selective to nitrogen, particularly at 400°C and above. With these catalysts
being identified as the most promising, further testing was performed. Figures
4.3-4.5 present comparisons of different reduction conditions over the two
Ag/Al2O3 catalysts. Comparisons of catalyst activity for 1% and 3% Ag/Al2O3
using propane and propene as reducing agents are shown in Figures 4.3 and
4.4. Reactant concentrations were 5% O2, 1000 ppm NO2, and 5000 ppm of the
hydrocarbon. At 350°C, over both catalysts and with both hydrocarbons, above
95% NO2 conversions are attained. Over both the 1% and 3% Ag, NO2
39
Figure 4.1: Reduction of NO2 with C3H6.
40
Figure 4.2: NO2 reduction with C3H8.
conversion at 300°C is higher with propene. This matches expectations, as the
double-bond containing alkene should be easier to activate than the alkane.
Nitrogen yields all reach above 90% by 400°C. At both 300°C and 350°C the
yields with propene are greater than with propane. Over 1%Ag at 350°C nitrogen
yield with propene is 87% and with propane only 17%. Over 3%Ag the difference
is 91% to32%.
Nitrogen yields below 400°C are higher for both hydrocarbons
over the 3%Ag/Al2O3.
41
To examine catalyst activity changes with increased O2 concentration,
further tests were performed on the 1%Ag/Al2O3 catalyst in 10% O2, 1000 ppm
NO2, and 5000 ppm C3H8. Comparisons with the 5% O2 tests using propane are
presented in Figure 4.5. In 10% O2, 82% N2 yield is attained at 350°C. N2 yield
is improved at this temperature in by the increase in oxygen concentration. At
400°C and above N2 yield is above 90% under both O2 concentrations.
Testing of the 1% and 3%Ag/TiO2-ZrO2, and the 1% and 3%Ag/Al2O3 for
the reduction of NO2 with CH4 showed that none of these catalysts were active
for the reaction. Reaction conditions were: 5% O2, 1000 ppm NO2, and 5000
ppm CH4. The 2%Pd/Al2O3 catalyst showed some activity, but N2 yields were
below 15%.
42
Figure 4.3: Comparison of NO2 reduction over 1%Ag/Al2O3 with propane and
propene.
43
Figure 4.4: Comparison of NO2 reduction over 3%Ag/Al2O3 with propane and
propene.
44
Figure 4.5: NO2 reduction with propane over 1%Ag/Al2O3 in 5% and 10% O2.
45
4.2 XRD of 1%Ag and 3%Ag/Al2O3
The x-ray diffraction patterns of 1% and 3%Ag/Al2O3 in Figure 4.6
demonstrate that the incipient wetness preparation results in well-dispersed
surface silver species. No peaks characteristic of silver species are observed.
The labeled diffraction peaks are all characteristic of the alumina support.
Figure 4.6: XRD of 1% and 3% Ag/Al2O3.
46
4.3 X-ray Photoelectron Spectroscopy of 1%Ag and 3%Ag/ Al2O3 Catalysts
X-ray photoelectron spectroscopy was performed on the 1% and
3%Ag/Al2O3 to determine the oxidation state and the chemical environment of the
surface species. Figure 4.7 shows the Al 2p region for both catalysts. The Al 2p
peak occurs at 74.2 eV, characteristic of Al2O3. The O 1s peak presented in
Figure 4.8 is centered at 530.9 eV. This binding energy corresponds to oxygen
in γ-Al2O3, which is in agreement with the Al 2p results. Figure 4.9 contains data
for the Ag 3d region of each catalysts. The peak location of 368.1 eV is in the
region of several silver oxide species. The greater intensity in the 3%Ag/Al2O3
sample is consistent with the higher loading.
47
Figure 4.7: XPS spectra of the Al 2p region of 1% and 3%Ag/Al2O3.
48
Figure 4.8: XPS spectra of the O 1s region of 1% and 3%Ag/Al2O3.
49
Figure 4.9: XPS spectra of Ag 3d region of 1% and 3%Ag/Al2O3.
50
The most significant challenge to catalytic aftertreatment of NO from lean
burn exhaust is maintaining activity in the high excess oxygen atmosphere. In
response to this challenge the two-stage reduction concept was proposed, where
NO is first oxidized to NO2 and NO2 is then reduced to N2 with a hydrocarbon.
NO2 is a more easily reduced species than NO, and thus should be better able to
compete with the combustion reaction for hydrocarbon. For the reduction stage
1%Ag/Al2O3 gave nitrogen yields around 90% in the 350°C to 400°C range in
10% O2. The reducing agent used was propane. It was also observed that both
of these catalysts demonstrated a beneficial effect from increased O2
concentration, leaving open the possibility of operation in even leaner conditions.
Despite high N2 yields obtained from the reduction of NO2 with higher
hydrocarbons, the Ag-based catalysts discussed here were not active for the
reduction with CH4.
51
CHAPTER 5
Pd-SUPPORTED ON SULFATED MONOCLINIC ZIRCONIA FOR THE
REDUCTION OF NO2 WITH CH4 UNDER LEAN CONDITIONS
5.1 Experimental Details
Palladium-based sulfated zirconia catalysts were prepared over a
commercial monoclinic zirconia support, supplied by Saint-Gobain. The zirconia
was received in pelletized form, and the first preparation step was to grind and
screen the support. The 100-140 mesh cut (0.149-0.105 mm) was saved, then
calcined in air for 3 hours at 500°C.
The calcined powder was screened a
second time and the 100-140 mesh cut was used for catalyst preparation.
Sulfated zirconia supports were prepared to a nominal 5wt% loading of
sulfate through a standard incipient wetness technique. Incipient wetness was
performed using an aqueous solution of ammonium sulfate (99+%, SigmaAldrich). A solution volume equal to the pore volume of the catalyst was added
drop wise, and the powder thoroughly mixed. After addition of ammonium sulfate
the samples were dried at 110°C overnight. After drying the sulfated zirconia
samples were calcined in air for 3 hours at 500°C.
performed through the same incipient
Palladium addition was
wetness technique using a solution of
52
palladium chloride (59% Pd, MCB Reagents) or palladium nitrate (Aldrich). After
addition of the palladium the samples were again dried at 110°C overnight, and
then were calcined in air at 500°C for 3 hours. Catalysts were prepared using
5%SZ at palladium loadings of 0.1%, 0.3%, and 0.5% by weight.
Catalyst activity measurements were performed in a packed bed reactor
made of 1/4” o.d. stainless-steel tubing. Catalyst samples (0.2g) were packed
into the reactor between two plugs of quartz wool.
purchased from Praxair.
All reactant gases were
Reaction conditions for the steady-state reaction
studies were 1000 ppm NO or NO2, 3000 ppm CH4, and 10% O2 in balance He.
The total flow rate corresponded to a GHSV of 20,000 hr-1.
Reactant
conversions were calculated as (1-outlet/inlet)*100%. N2 yields were calculated
as (2xN2 produced/NOx fed)*100%.
The stability of the zirconia crystalline phase was confirmed using a
Bruker D8 Advanced X-ray diffractometer equipped with a Cu-Kα source and a
position sensitive detector. In-situ diffraction patterns of the sulfated zirconia
sample were taken during calcinations under controlled atmosphere and
controlled temperature conditions using a HTK 1200 sample holder. The sample
was maintained under a flow of air at approximately 15 mL/min. The sample
temperature was raised at 3ºC/min in 50ºC increments with a 10-minute pause to
allow for temperature equilibration. After the 10-minute hold a diffraction pattern
was taken. Diffraction data were taken between 50-750ºC. TPD studies were
performed using a Finnigan Trace DSQ mass spectrometer.
experiment 0.1 grams of catalyst
For each
sample was placed in a quartz U-tube
53
reactor between plugs of quartz wool.
Samples were pretreated under
10%O2/He, held at 400°C for 30 minutes. The catalyst was then cooled to room
temperature in He. The samples were then exposed to either He or 10%O2/He,
and the desorption of sulfur species was monitored by mass spectrometer.
Experimental conditions were 30 cm3/min flow of gas, and a temperature ramp
rate of 10°C/min.
5.2 Catalyst Preparation
Table 1 shows BET surface are results for the commercial monoclinic
ZrO2 support, as well as the three Pd/SZ catalysts with different Pd loadings.
The loading levels were kept very low, with the maximum being 0.5wt%. The
surface area for the commercial support was taken after the sieving and
calcination procedure used to prepare it. The low metal and sulfate loadings
resulted in very little surface area difference among the catalysts.
Catalyst
BET SA
(m2/g)
Commercial ZrO2
48.4
0.1%Pd/SZ
48.0
0.3%Pd/SZ
47.9
0.5%Pd/SZ
46.3
Table 5.1: BET surface areas of the unloaded support and the prepared Pd/SZ
catalysts.
54
The addition of surface acidity through the presence of sulfate groups is
known to be key in determining activity for NOx reduction with methane. Several
experiments were performed to examine the effect of preparation steps and
preparation parameters of the sulfated zirconia catalysts, specifically the
behavior of surface sulfur species. These species are known to be thermally
unstable; so the effect of calcination temperature was examined through
temperature-programmed techniques. Dried, but uncalcined samples of sulfated
zirconia were treated in flows of He and 10%O2/He while the effluent was
analyzed by on-line mass spectrometry. Figure 5.1 shows the data for m/e=64,
corresponding to SO2 taken under O2/He and He-only atmospheres.
The
desorption profiles for under each atmosphere revealed a strong maximum
around 525°C.
This was the only desorption feature observed under O2/He
atmosphere below 650°C. During the treatment with He, however, desorption of
SO2 was initially observed just above 250°C. Two additional small features were
also observed, a peak at 350°C and a shoulder at 440°C. There was also a
shoulder seen around 600°C. The final desorption feature which begins around
650°C is due to thermal decomposition and is seen under both atmospheres. In
both experiments the ion for SO (m/e=48) was observed to match that of SO2,
indicating that it was present due to fragmentation.
The most significant
difference between the two pretreatment atmospheres however is the large
intensity difference. When treated under He the loss of sulfur species is several
times greater across the entire temperature range.
desorption of SO2 and the higher
The low temperature
intensity under He suggests that O2 can
55
be stabilizing the surface sulfate groups. We therefore conclude that the majority
of the added sulfate is thermally stable under O2 pretreatment up to 650°C.
Figure 5.1: TPD of SO2 (m/e=64) during the heat treatment of impregnated and
dried sulfated zirconia. Samples were treated under He (♦) and 10%O2/He („).
56
Confirmation of this conclusion was provided by XPS spectra taken of the Pd/SZ
catalyst at each of the five separate synthesis steps. Spectra of the Zr3d, S2p,
and O1s regions were taken of the untreated commercial ZrO2 sample, the
sulfated but uncalcined sample, calcined sulfated zirconia, loaded but uncalcined
0.5%Pd/SZ, and the final 0.5%Pd/SZ catalyst.
Figures 5.2-5.4, respectively.
These results are shown in
In Figure 5.2 the Zr3d5/2 peak is located at a
binding energy of 182.1 eV, typical for ZrO2. After treatment with ammonium
sulfate and drying, a shift to higher binding energy was observed. The 5/2 peak
was shifted to 182.9 eV. This increase in binding energy is due to a decrease in
electron density as a result of increased surface acidity [110]. The shift was
stable through the subsequent catalyst preparation steps, indicating stable
surface conditions. Examination of the S2p region in Figure 5.3 shows direct
evidence of stable sulfate species. As expected, no peak was observed in this
region on the commercial ZrO2 sample. Upon the addition of ammonium sulfate,
a relatively broad peak centered on 169.3 eV is visible. Such a peak falls in the
standard binding energy range of sulfur in metal sulfates. Through calcination of
the sample and the addition of Pd, the intensity and location of this peak do not
change. Splitting between the 2p3/2 and 2p1/2 was not observed due to relatively
low counts in this region, and the close spacing of these peaks for sulfur. The
O1s region presented in Figure 5.4 shows an increase in binding energy from
530.0 to 530.7 eV after sulfation. Such a shift is consistent with the observations
made in the Zr3d region.
Additionally, the growth of a shoulder at 531.9
corresponds to oxygen present in sulfate species [111].
57
The observed
peak shift and intensities are again constant with additional preparation steps.
In-situ XRD during the calcination of sulfated zirconia was performed, and
showed no evidence of a bulk crystalline change in the support. Beginning with
an uncalcined sulfated zirconia sample, the calcination was performed under
flowing air with diffraction patterns taken every 50ºC.
portion of this data, patterns taken every 100ºC.
Figure 5.5 contains a
The range 20-70° 2θ is
presented, and clearly show the presence of only monoclinic zirconia with peaks
at 24, 28, and 31°.
58
Figure 5.2: Zr3d region of the X-ray photoelectron spectra of catalyst samples
taken at each step in the synthesis procedure. From bottom to top: commercial
monoclinic ZrO2, dried but uncalcined SZ, SZ support, impregnated but
uncalcined 0.5%Pd/SZ, the final 0.5%Pd/SZ.
59
Figure 5.3: S2p region of the X-ray photoelectron spectra of catalyst samples
taken at each step in the synthesis procedure. From bottom to top: commercial
monoclinic ZrO2, dried but uncalcined SZ, SZ support, impregnated but
uncalcined 0.5%Pd/SZ, the final 0.5%Pd/SZ.
60
Figure 5.4: O1s region of the X-ray photoelectron spectra of catalyst samples
taken at each step in the synthesis procedure. From bottom to top: commercial
monoclinic ZrO2, dried but uncalcined SZ, SZ support, impregnated but
uncalcined 0.5%Pd/SZ, the final 0.5%Pd/SZ.
61
Figure 5.5: In-situ XRD patterns taken during the calcination in air of sulfated
zirconia.
62
5.3 Steady-State Reaction Experiments
The NOx reduction activity of the Pd/SZ catalysts were tested in a
feed containing 1000 ppm NOx, 3000 ppm CH4, and 10% O2 in balance He.
Figures 6 and 7 show results from these tests. Figure 6 shows a comparison of
the activities for NO versus NO2 reduction with CH4 over 0.1%Pd/SZ catalyst.
The N2 yield and CH4 conversion are plotted on the same graph for two different
feed molecules. As shown in Figure 5.6, the catalyst has little activity for NO
reduction until 325°C. The maximum N2 yield of 35% was observed at 400°C,
after which N2 production decreased. NO conversion was very close to N2 yield,
although a small amount of N2O was observed.
For NO2 conversion, on the
other hand, there was reduction to N2 at temperatures even as low as 200°C,
with a broad maximum over 325-425°C range. The maximum N2 yields achieved
were much higher than those observed for NO reduction, reaching 57-61%. CH4
conversions in the two experiments showed very similar trends, increasing with
temperature, with a sharper light-off observed above 375°C and reaching
conversion levels over 40% by 450°C.
The corresponding increase in CH4
conversion with a decrease in N2 yield is common in reactions performed with
hydrocarbon reducing agents. Especially under lean conditions CH4 combustion
with O2 becomes favored at higher temperatures.
NO2 conversion was also monitored and revealed a significant side
reaction. We observed the partial reduction of NO2 to NO. Reaction tests (not
shown) in which the CH4 flow was stopped during the reaction confirmed that this
was due to a reduction reaction, not
due to NO2 decomposition or NO-NO2
63
Figure 5.6: Steady-state reaction results from the reduction of NO (empty) and
NO2 (filled) with CH4 over 0.1%Pd/SZ. N2 yield („) and CH4 conversion (c).
Reaction
conditions:
1000ppm
NOx,
GHSV=20,000hr-1.
64
3000ppm
CH4,
10%
O2
in
He,
equilibrium driven reactions. Almost complete NO2 conversion occurred in the
temperature range of the maximum in N2 yield. In light of the improved activity
for the reduction of NO2 it seems that N2 production may be capped by the
complete removal of NO2 by this side reaction.
A large amount of NO is
produced, which the catalyst is much less effective at reducing. Reaction with
NO2 also showed small quantities of N2O formation. There was no NH3 formation
in either reaction.
The large difference observed in the N2 yields in NO versus NO2 reactions
clearly confirms that NO2 is much easier to reduce than NO.
This result is
important in validating the two-stage concept, which has been the basis for this
study.
The effect of Pd loading on catalyst activity for the reduction of NO2 with
CH4 was also investigated.
Figures 5.7, 5.8, and 5.9 show the steady-state
reaction results obtained over catalysts with three different Pd loading levels.
With both N2 yield and NO2 conversion, shown in 5.7 and 5.8 respectively, an
increase in the catalyst Pd loading resulted in increased activity at low reaction
temperatures. Over all three catalysts a broad maximum was observed in N2
yield. For each of the three catalysts the maximum corresponded to high NO2
conversions. This maximum on the 0.5%Pd/SZ occurred between temperatures
of 300-375°C, giving 47-53% N2 yield. On 0.3%Pd/SZ yield reached a maximum
in the same range, however it was higher, reaching 55-58%. In comparison to
the higher loadings, the entire curve of N2 yield with temperature was shifted to
approximately
25°C
temperatures
higher
65
over
the
0.1%Pd/SZ
catalyst. The maximum N2 yield was observed between 325-425C, reaching 5560%. For all three catalysts, N2 production dropped at high temperatures due to
competition from the combustion reaction.
Figure 5.9 shows the CH4
conversions during NO2 reduction. Below 400 °C conversion with all three Pd
loadings was similar.
At 400 °C a significant increase was observed on
0.5%Pd/SZ, which reached a conversion of 70% at 450°C. With lower loading
maximum CH4 conversion dropped to 42% on 0.3%Pd/SZ and 39% on
0.1%Pd/SZ at the same temperature.
Comparisons of the results from these Pd loading levels indicate that with
higher Pd content the two competitive reactions that bracket the maximum N2
yield window are increased.
The 0.5%Pd/SZ catalyst showed higher NO2
conversion at low temperatures, much of which was converted to NO.
Additionally at high temperatures the higher loading increased the rate of CH4
combustion and decreased N2 yield. Lowering the metal loading resulted in an
increase in N2 and a broadening of the activity temperature range.
66
Figure 5.7: Steady-state N2 yields in the reduction of NO2 with CH4 over Pd/SZ
catalysts.
0.5%Pd/SZ (c), 0.3%Pd/SZ („), and 0.1%Pd/SZ (‹).
Reaction
conditions: 1000ppm NO, 3000ppm CH4, 10% O2 in He, GHSV=20,000hr-1.
67
Figure 5.8: Steady-state NO2 conversions in the reduction of NO2 with CH4 over
Pd/SZ catalysts.
Reaction
0.5%Pd/SZ (c), 0.3%Pd/SZ („), and 0.1%Pd/SZ (‹).
conditions:
1000ppm
NO,
GHSV=20,000hr-1.
68
3000ppm
CH4,
10%
O2
in
He,
Figure 5.9: Steady-state CH4 conversions in the reduction of NO2 with CH4 over
Pd/SZ catalysts.
Reaction
0.5%Pd/SZ (c), 0.3%Pd/SZ („), and 0.1%Pd/SZ (‹).
conditions:
1000ppm
NO,
GHSV=20,000hr-1.
69
3000ppm
CH4,
10%
O2
in
He,
5.4 Temperature Programmed Desorption
The adsorption behavior and interactions of reactants with the catalyst
surface
were
investigated
through
temperature
programmed
desorption
techniques. Comparison of the desorption of NO and NO2 from the 0.5%Pd/SZ
catalyst is presented in Figure 5.10. The desorption profiles for m/e=46 (NO2),
m/e=44 (N2O) and m/e=30 (NO) are shown. Before the desorption experiments
were performed, fragmentation of NO2 by the mass spectrometer was
determined and the NO2:NO ratio was found to be 1.25:1.
Following NO
adsorption, neither NO2 nor N2O is observed, indicating that NO does not
disproportionate on the catalyst surface.
Three peaks are observed for the
desorption of NO located at 75, 130, and 250°C.
TPD performed with NO2
showed four desorption peaks for m/e=46 at 85, 140, 190, and 320°C, indicating
significant levels of reversible adsorption for NO2. Additionally NO2 appears to
undergo some rearrangement on the catalyst surface. Small amounts of both
NO (m/e=30) and N2O (m/e=44) were observed and the temperature of
desorption for both of these species correspond closely with peaks for NO2. The
relative intensity of the NO signal is slightly higher than predicted by NO2
fragmentation, indicating that, however small, some decomposition of NO does
occur on the catalyst surface. The N2O signal was small, and is multiplied by a
factor of 5 in Figure 9 to make the peaks visible.
70
Figure 5.10: NO (♦) and NO2 („) TPD experiments from 0.5%Pd/SZ.
corresponding to m/e=46, 44(x5), and 30 are shown.
71
Ions
5.5 Diffuse Reflectance Fourier Transform Spectroscopy
Figure 5.11 contains DRIFTS spectra for the 0.5%Pd/SZ catalyst after
pretreatment and after adsorption of either NO or NO2. In these experiments the
catalyst was mixed to 15 wt% in KBr. The spectrum taken of the Pd/SZ catalyst
shows a large absorption band at 1378 cm-1. This band is due to S=O stretching
of sulfate groups on the catalyst surface [112]. Additionally the feature in the
range of 950-1200 cm-1 can be assigned to S-O stretch [113]. These bands are
typical for sulfated zirconia synthesized through other preparation routes, and
confirm the presence of sulfate groups on the monoclinic support.
After exposure to NO several changes in the spectrum appear. The S=O
stretch band at 1378 cm-1 decreases in intensity. This decrease is due to a shift
to lower wavenumber upon interaction with adsorbed NO. The formation of a
strong band and shoulder at 1621 and 1578 cm-1, respectively, are also
observed. The band at 1621 cm-1 is assigned to the formation of nitric acid [114]
and corresponds to an intensity decrease in the hydroxyl region (not shown).
The shoulder at 1578 cm-1 is associated with the formation of bidentate nitrate
species. Upon adsorption of NO2, the band at 1378 cm-1 decreases in intensity
more sharply than in the NO experiment. Additionally, the bands at 1621 and
1578 cm-1 are more intense. New bands 1271 and 1128 cm-1 are observed and
can be assigned to bidentate and bridged nitrates, respectively. In the region
between these bands an increase in absorbance is also seen. A wavenumber of
1225 cm-1 is assigned to the N-O stretch in N2O [115]. Comparison of the NO
and NO2 adsorption experiments seem
to indicate a stronger interaction of NO2
72
Figure 5.11: DRIFTS spectra of the Pd/SZ and KBr mixture. Spectra shown
correspond to the sample, and following adsorption of NO or NO2.
73
with the sulfated zirconia, through a larger decrease in sulfate band intensity and
a larger absorbance associated with surface nitrate species. These observations
are consistent with the much larger NO2 adsorption observed through
temperature programmed desorption.
5.6 Conclusions
A simple incipient wetness impregnation method used with a commercial
monoclinic support was demonstrated to produce a sulfated zirconia catalyst with
good activity for the reduction of NO2 with CH4 under lean conditions.
Characterization of this catalyst showed the formation of surface sulfate groups,
and the corresponding surface acidity, that were thermally stable below the
calcination temperature of 500°C. N2 yields of over 60% were achieved under a
10% O2 feed (O2:CH4>30). Additionally the catalysts achieved maximum activity
over a broad temperature range of 300-425°C, depending on Pd content. This
was observed to occur simultaneously within a temperature range of relatively
low CH4 and nearly complete NO2 conversion. We believe that the activity is
therefore limited at the low end by the competitive partial reduction of NO2 to NO,
and on the high end by the light off of CH4 combustion with O2. If the formation
of NO is indeed limiting the yield of N2, then incorporation into a system including
a NO oxidation catalyst could be expected to increase performance.
NO
produced through the partial reduction could be quickly reoxidized. DRIFTS and
TPD experiments examining the interaction of NOx species with the Pd/SZ
74
catalyst indicate a stronger surface interaction with NO2, which may be significant
in the improved reduction performance.
75
CHAPTER 6
Pd DOPED SULFATED ZIRCONIA PREPARED BY A SINGLE STEP SOL-GEL
PROCEDURE FOR LEAN NOx REDUCTION
6.1 Experimental Details
Pd-based sulfated zirconia catalysts were prepared using a ‘one-pot’ solgel technique in which all the catalysts components were added to solution
before gelation.
This preparation was similar to that used by Hamouda and
Ghorbel [116], making use of acetic acid as a controlled hydrolysis agent. Solgel catalysts were prepared using n-propanol as a solvent. In the Pd-containing
samples Pd acetate was first dissolved in the propanol under constant stirring.
The amount of Pd added was set to deliver the desired final catalyst loadings of
0.1, 0.3, and 0.5 wt% Pd.
Zirconium propoxide (Aldrich) and sulfuric acid
precursors were then added to the solution to achieve final concentrations of 2 M
and 0.5 M, respectively. After 5 minutes of stirring acetic acid was added as the
hydrolyzing agent in an acid/zirconia ratio of 4:1. This solution was gently stirred,
with gel formation occurring over approximately 3 hours. Upon gel formation the
sample was transferred to a drying oven and dried overnight at 100°C. After
drying the resulting powder was lightly ground, and portions of the sample
calcined at 500, 600, and 700°C. Calcination was performed in air with
76
the sample held at temperature for 4 hours. A sample of unloaded sulfated
zirconia was also prepared. This material was prepared by the same method,
lacking only the addition of Pd acetate to the sol.
Catalyst activity measurements were taken for 0.1, 0.3, and 0.5% Pd/SZ
samples calcined at 600 and 700°C. Reactions were performed on an equal
surface area basis. A catalyst sample of 12.5 m2 surface area was loaded into a
1/4” O.D. stainless steel reactor tube and held in place between two plugs of
quartz wool. Conditions used for the steady-state reaction studies were 1000
ppm NO2, 3000 ppm CH4, and 10% O2 in balance He. Total gas flow was 45
cm3/min. Before beginning each steady-state reaction experiment the catalysts
samples were pretreated in 10% O2/He for 30 minutes at 400°C.
Simultaneous
thermogravimetric/differential
scanning
measurements were performed on a Setaram TGA/DSC-111.
calorimetry
25 mg of
uncalcined sample was loaded for each experiment. A flow of 5% O2/He at 25
cm3/min was introduced, and the sample held at 30°C for 20 minutes to achieve
a stable mass signal. A temperature ramp from 30 to 700°C, at a rate of 5°C/min
was then begun.
The temperature-programmed calcination of sol-gel prepared catalysts
was monitored by a Finnigan Trace DSQ mass spectrometer to examine evolved
species during treatment. For these experiments 0.1 g of uncalcined catalyst
sample was loading into a quartz U-tube reactor. The sample was held in place
between two plugs of quartz wool. 5%O2/He was introduced at a flow rate of 30
cm3/min, and the sample was flushed at room temperature for 15 minutes.
77
After flushing the temperature program was begun. Sample temperature was
raised from 25°C to 700°C at a ramp rate of 5°C/min.
The development of support crystallinity was monitored using in-situ XRD.
Experiments were performed using a Bruker D8 Advance diffractometer.
Samples precalcined at 500°C were placed in a HTK 1200 controlled
atmosphere/controlled temperature sample holder. Under a flow of 10 cm3/min
air, diffraction patterns were taken at 25°C steps between 500 and 700°C.
Samples were held at each temperature for 10 minutes before beginning the
scans.
Infrared spectra were taken of 0.5%Pd/SZ samples which had been
calcined at 500, 600, and 700°C. Spectra were recorded on a Thermo Nicolet
6700 spectrometer. Catalysts samples were mixed to 10 wt% in KBr. Samples
were loaded into an environmental chamber and treated at 200°C in He for 30
minutes to remove adsorbed water. For each sample a background spectra was
taken of pure KBr exposed to the same pretreatment.
6.2 BET Surface Area
BET surface areas of the prepared catalysts are shown in Table 6.1. The
addition of Pd to the sol-gel prepared SZ results in an increase in both the
surface area and pore volume of the catalysts at a given calcination temperature.
No trend was observed between surface area and Pd content of the catalysts.
Additionally, both properties reach a maximum for each catalyst when it was
calcined at 600°C.
78
Catalyst
0.5%Pd/SZ
0.3%Pd/SZ
0.1%Pd/SZ
SZ
SA (m2/g)
PV (cm3/g)
SA (m2/g)
PV (cm3/g)
SA (m2/g)
PV (cm3/g)
SA (m2/g)
PV (cm3/g)
Calcination Temperature
500°C
600°C
700°C
55.4
100.6
68.5
0.20
0.27
0.23
45.9
101.9
64.7
0.18
0.21
0.19
66.4
103.2
80.2
0.23
0.27
0.25
44.3
85.6
61.8
0.18
0.22
0.22
Table 6.1: Catalyst surface areas and pore volumes for catalysts prepared with
different Pd loadings and prepared at different calcination temperatures.
6.3 Steady-State Reaction Results
Figure 6.1 shows the N2 yields obtained during the reduction of NO2 with
CH4 over 0.1Pd/SZ, 0.3%Pd/SZ, and 0.5%Pd/SZ catalysts calcined at 600°C.
Observed N2 yields are relatively low. Yields over all three samples increased
with reaction temperature up to a maximum, and then were seen to decrease.
This trend is typical of the hydrocarbon reduction of NOx under lean conditions,
and is due to the depletion of the reducing agent due to combustion at high
temperatures. In the temperature range of 450 to 500°C the 0.3%Pd/SZ and
0.5%Pd/SZ catalysts reached N2 yields of 32% and 24% respectively. Below
450C the 0.1%Pd/SZ catalysts showed lower activity, but yield increased with
reaction temperature reaching a maximum of 45% at 550°C. Yield was then
observed to fall at 600°C. The corresponding CH4 conversions are shown in
79
Figure 6.1: N2 yield during the reduction of NO2 with CH4 over 0.1%Pd/SZ (c),
0.3%Pd/SZ („), and 0.5%Pd/SZ (z) calcined at 600°C.
80
Figure 6.2. CH4 conversion over the 0.3%Pd/SZ and 0.5%Pd/SZ catalysts were
very similar across the examined temperature range, although conversion of the
0.3%Pd/SZ sample was generally 5-10% lower at a give reaction temperature.
CH4 conversion took off sharply above 450°C, corresponding to the maximum in
N2 yield over these catalysts. Complete conversion of CH4 was observed for
both at 600°C.
CH4 conversion over 0.1%Pd/SZ was similar to the other
samples at low temperatures, but was not observed to take off until above 500°C.
Lower CH4 conversion at high reaction temperatures could explain the observed
shift and higher maximums observed in the N2 yields over the 0.1%Pd/SZ
sample. A maximum of 80% conversion was observed at 600°C.
Significantly higher N2 yields were observed at each of the three Pd
loadings when the catalysts were calcined at 700°C. These results are shown in
Figure 6.3. Again, all three catalysts reached a maximum N2 yield after which a
decrease was observed. Similar N2 yields were observed on the 0.3%Pd/SZ and
0.5%Pd/SZ catalysts. Maximum conversions of 65% and 69% were reached at
450°C. The increase in Pd loading of 0.5% seems to have resulted in a shift in
activity to slightly lower temperature. Although these two catalysts had similar
activity curves, yields below 450°C were higher on the 0.5%Pd/SZ sample, while
they were lower above this temperature. Such a temperature shift is much more
obvious when comparing the activity of the 0.1%Pd/SZ catalyst.
Over this
sample N2 yield was observed to increase with temperature, reaching 66% at
500°C. In fact across the tested temperature range, a shift in activity of 50 to
81
Figure 6.2: CH4 conversion during the reduction of NO2 with CH4 over
0.1%Pd/SZ (c), 0.3%Pd/SZ („), and 0.5%Pd/SZ (z) calcined at 600°C.
82
Figure 6.3: N2 yield during the reduction of NO2 with CH4 over 0.1%Pd/SZ (c),
0.3%Pd/SZ („), and 0.5%Pd/SZ (z) calcined at 700°C.
83
100°C higher temperatures was observed. CH4 conversions shown in Figure 6.4
display a similar trend. CH4 conversion at a given reaction temperature generally
decreases with a reduction of Pd loading.
This difference is small when
comparing the 0.3%Pd/SZ and 0.5%Pd/SZ samples. Conversions are similar,
and both reach 100% at 550°C. In comparison CH4 conversion is again shifted
to higher temperatures on the 0.1%Pd/SZ sample.
An increase in reaction
temperature of at least 50°C for a given conversion was seen across most of the
range.
Comparison of CH4 conversion between catalysts of the same Pd loading,
but calcined at different temperatures also reveals a trend. Conversions over the
three catalysts calcined at 700°C are shifted to lower reaction temperatures than
observed over the samples calcined at 600°C.
Clearly both Pd content and
preparation calcination temperature play a role in the activity of the catalysts for
both the NO2 reduction reaction, and the combustion of CH4 at high
temperatures. Calcination at 700°C significantly improves reduction activity and
results in a lowering of the temperature at which maximum conversion is
reached. When calcined at 700°C changes in the Pd loading did not result in
significantly different N2 yields, however with increased Pd content the maximum
yields are reached at lower reaction temperatures. This effect is significant when
increasing loading from 0.1%Pd to 0.3%Pd, but less so with further addition. We
can therefore conclude that a maximum Pd loading exists to minimize precious
metal content while giving high N2 yield in the desired temperature window.
84
Figure 6.4: CH4 conversion during the reduction of NO2 with CH4 over
0.1%Pd/SZ (c), 0.3%Pd/SZ („), and 0.5%Pd/SZ (z) calcined at 700°C.
85
6.4 Thermogravimetric/Differential Scanning Calorimetry and Mass Spectrometry
The calcinations of the Pd-containing samples, as well as unloaded SZ,
were examined using both TGA/DSC and mass spectrometry.
With the
combination of these techniques specific mass losses can be attributed to the
evolution of gas species from the catalyst sample. Samples were treated under
the same conditions in each set of experiments.
The calcinations were
performed in 5%O2/He, and the samples were ramped from 30 to 700°C at a rate
of 5°C/min. The TGA signals taken during the calcination of SZ and 0.5%Pd/SZ
are shown in Figure 6.5. Between 100 and 550°C the percentage mass loss is
larger for the 0.5%Pd/SZ sample. The largest observed difference occurs at
265°C where a sharp mass loss of 10% was observed. Above 600°C the curves
meet and overlap, indicating identical mass losses of 39% at 700°C. The region
of overlap included a high temperature mass loss of approximately 7%, which
began around 650°C.
This large observed mass loss is typical of sol-gel
preparations due to the removal of organic precursors at elevated temperatures.
To more effectively compare the TGA/DSC and MS data, the differential
TGA signals (dTG) are shown in further figures. Figure 6.6 contains both the
dTG and DSC data from the calcination of the unloaded SZ catalyst. Figure 6.7
shows the MS calcination data for the same catalyst. During the calcination of
sol-gel prepared SZ dTG peaks were observed at 118, 190, 372, and 460°C. A
sharp mass loss is also observed starting at 650°C.
The two lowest dTG
features correspond to a broad endothermic signal in the DSC data, and the
86
Figure 6.5: Thermogravimetric measurement of the calcination of SZ (c) and
0.5% Pd/SZ („).
87
Figure 6.6: DSC and dTG profiles collected during the calcination of sol-gel
prepared SZ.
88
Figure 6.7: Calcination of SZ monitored by mass spectrometer. Shown are ions
associated with the desorption of H2O (m/z=18,17 - S,U), CO2 (m/z= 44,28 „,…), and SO2 (m/z=64,48 – z,{).
89
removal of H2O (m/z=18,17) from the catalyst. H2O evolution stops by 300°C,
and a broad CO2 (m/z=44,28) peak is observed then between 300 and 550°C.
This peak is centered at 380°C with a shoulder at 460°C.
at 380°C coincides with the production of CO2.
An exothermic peak
The loss of sulfur from the
catalysts is observed through the ions of SO2 (m/z=64,48). The evolution of SO2
was first observed above 350°C, and increased slowly with temperature until a
sharp peak at 645°C. The corresponding DSC and dTG show an endothermic
mass loss.
Data from the calcination of 0.1%Pd/SZ are shown in Figures 6.8 and 6.9.
As in the calcination of the unloaded SZ dTG peaks at 117 and 175°C
corresponded to the desorption of H2O from the catalyst. Additionally, above
650°C and endothermic peak and mass loss occur with loss of SO2. Although
the main peak in the MS SO2 data occurs at the same temperature as in SZ, it
was noted that sulfur loss did not start until above 400°C, a shift to slightly higher
temperature. The main observed difference was in the evolution of CO2 from the
sample. The intensity and shape of both the dTG and DSC peaks were close to
those observed for SZ, however a shift to lower temperature was observed. The
DSC peak began close to 250°C and was centered at 330°C, with a shoulder at
380°C. In the MS the CO2 was observed as a sharp peak at 300°C and shoulder
at 375°C. A second small peak was also seen at 440°C.
90
Figure 6.8: DSC and dTG profiles collected during the calcination of sol-gel
prepared 0.1%Pd/SZ.
91
Figure 6.9: Calcination of 0.1%Pd/SZ monitored by mass spectrometer. Shown
are ions associated with the desorption of H2O (m/z=18,17 - S,U), CO2 (m/z=
44,28 - „,…), and SO2 (m/z=64,48 – z,{).
92
Figures 6.10 and 6.11 show the dTG/DSC and MS data taken during the
calcination of 0.3%Pd/SZ. The desorption of H2O was again observed below
250°C in all three signals. A sharp SO2 desorption peak was observed at 645°C,
however the gradual loss of SO2 at temperatures below this was less intense
than for the SZ and 0.1%Pd/SZ catalysts. Additionally, the evolution of CO2 from
the catalyst samples occurred over a significantly narrowed temperature range.
The observed exothermic feature occurs between 250 and 350°C. Along with
narrowing, the heat flow reached 46.5 mW.
Two distinct DSC peaks were
observed at 270 and 315°C, which were matched in both the dTG and CO2
signals. As with the DSC these peaks both narrowed and increased in intensity.
With the further increase in Pd content to 0.5% the trends observed in the
previous samples were observed to continue. Figures 6.12 and 6.13 contain the
data on the calcination of 0.5%Pd/SZ.
As with the 0.3%Pd/SZ sample the
desorption of SO2 began near 450°C and was relatively low below 600°C. The
main SO2 desorption peak again occurs near 650°C, but was broadened in
comparison to the other catalyst samples.
680°C.
A distinct shoulder was seen at
The evolution of CO2 from the sample was again shifted to lower
temperature with the MS peak centered at 280°C.
Both the DSC and dTG
signals showed corresponding peaks, which were narrowed and of greater
intensity than in the 0.3%Pd/SZ sample. The exothermic DSC peak reached a
maximum heat flow of 135 mW while the dTG reached –0.7 mg/min. A smaller
second peak at 325°C was visible in the DSC and dTG data but was not resolved
93
Figure 6.10: DSC and dTG profiles collected during the calcination of sol-gel
prepared 0.3%Pd/SZ.
94
Figure 6.11: Calcination of 0.3%Pd/SZ monitored by mass spectrometer. Shown
are ions associated with the desorption of H2O (m/z=18,17 - S,U), CO2 (m/z=
44,28 - „,…), and SO2 (m/z=64,48 – z,{).
95
Figure 6.12: DSC and dTG profiles collected during the calcination of sol-gel
prepared 0.5%Pd/SZ.
96
Figure 6.13: Calcination of 0.5%Pd/SZ monitored by mass spectrometer. Shown
are ions associated with the desorption of H2O (m/z=18,17 - S,U), CO2 (m/z=
44,28 - „,…), and SO2 (m/z=64,48 – z,{).
97
in the MS signal. The MS did show a second small peak at 450°C, as was
observed in the other samples.
With the addition of increasing amounts of Pd to the sol-gel SZ catalyst,
the evolution of CO2 during calcination occurs at much lower temperatures. As
evidenced by the narrowed and more intense DSC peaks, the rate is also seen to
increase.
CO2 produced during calcination is due to the combustion of the
organic sol-gel precursors. It is likely therefore that the observed change is due
to Pd catalyzing the combustion reaction. With increased Pd content, the loss of
SO2 during calcination is somewhat reduced below 600°C. It does not however
seems to have much of an effect at higher temperatures as the main loss of
sulfur occurs near 650°C for all four tested catalysts.
6.5 In-situ X-ray Diffraction
The effect of Pd-content on the formation of zirconia crystallites was
examined through in-situ calcination by XRD. Diffraction patterns were taken in
25°C steps between 500 and 700°C. Samples were held at each temperature for
10 minutes before scans were begun.
Diffraction patterns from these
experiments are shown in Figure 6.13, 6.14, 6.15, and 6.16. All four catalyst
samples showed the presence of only the tetragonal zirconia phase, as
evidenced by diffraction peaks at 30.2, 50.2, and 60.2° 2θ. In the unloaded SZ
sample peaks are first observed at 600°C, and were seen to sharpen with
increasing calcination temperature. Formation of the monoclinic zirconia phase,
98
Figure 6.13: XRD patterns taken during the calcination of sol-gel prepared SZ.
99
Figure 6.14: XRD patterns taken during the calcination of sol-gel prepared
0.1%Pd/SZ.
100
Figure 6.15: XRD patterns taken during the calcination of sol-gel prepared
0.3%Pd/SZ.
101
Figure 6.16: XRD patterns taken during the calcination of sol-gel prepared
0.5%Pd/SZ.
102
which could be identified by a peak at 28° 2θ, was not observed.
This is
consistent with other one-step sol-gel preparations in the literature [117,118].
With the addition of 0.1%Pd very low intensity peaks were observed beginning at
575°C. With further increases in Pd loading tetragonal zirconia peaks formed at
575°C.
The highest peak intensities were seen in the 0.5%Pd/SZ sample.
These results indicate that the addition of Pd prior to the gelation step results in
the formation of zirconia crystallites at lower temperatures.
Calculation of
crystallite size using the Scherrer equation demonstrated that increased Pd
loading also resulted in increased zirconia crystallite size at a give calcination
temperature.
These complete results are presented in Table 6.2. Observed
crystallite sizes were in the range of 13-19 nm.
Calcination Temperature
Catalyst 575°C 600°C 625°C 650°C
SZ
13.3
13.7
13.9
0.1%Pd/SZ
13.9
14.2
14.4
0.3%Pd/SZ 13.5
15.8
15.5
16.1
0.5%Pd/SZ 14.7
16.8
17.5
17.5
675°C
14.4
15.5
16.5
18.7
700°C
14.7
16.1
17.9
18.7
Table 6.2: Zirconia crystallite sizes observed during in-situ calcination. Effect of
temperature and Pd loading.
103
6.6 IR Spectroscopy
DRIFT spectra of 0.5%Pd/SZ calcined at 500, 600, and 700°C are shown
in Figure 6.17. The region between 800 and 1800 cm-1 contains information of
sulfate groups and their interaction with the zirconia surface. The spectra of the
0.5%Pd/SZ sample calcined at 500°C contains a peak at 1620 cm-1. This band
is assigned to a bending vibration of OH groups on the surface. Other distinct
peaks were not well resolved, but broad absorption was observed in the sulfate
region between 1000 and 1400 cm-1. Several changes were observed in the
sample calcined at 600°C.
The OH band was seen to decrease slightly in
intensity and to shift to 1616 cm-1.
Li and Gonzalez [119] observed similar
behavior during dehydration of a sol-gel prepared SZ sample. The observed
change would therefore indicate less remaining H2O after treatment at 200°C.
Bands at 1045 and 1149 cm-1 were also observed to appear. These species
were assigned to stretching frequencies of S-O [120] and bidentate sulfate
coordinated with Zr4+ [121]. The OH band in the 0.5%Pd/SZ catalyst calcined at
700°C showed a further decrease in intensity and shift to lower wavenumber. In
the sulfate regions an overall decrease in absorbance was observed. Such a
decrease is expected based upon the previously discussed MS data. Despite
the loss of sulfate species during calcination at 700°C the
S-O bands at 1045
and 1149 cm-1 did not disappear, and an additional band associated with
bidentate sulfate was observed as a shoulder at 1022 cm-1. The formation of a
band at 1370 cm-1 was also observed. This band is commonly assigned as the
104
Figure 6.17: DRIFTS spectra of 0.5%Pd/SZ calcined at 500, 600, and 700°C.
105
S=O stretch on sulfated zirconia samples, and represents the Lewis acid site of
the catalyst.
Consideration
of
the
DRIFTS
data
and
previously
discussed
characterization may be used to shed light on the observed NO2 reduction
reaction results. Crystallinity was not observed in the sample calcined at 500°C.
This corresponded to a lack of IR bands showing sulfates attached to the zirconia
support. Calcination at 600°C results in the formation of both the tetragonal
zirconia phase, as well as the development of bidentate sulfate species that are
coordinated with zirconia. While these samples were active for the reduction of
NO2 with CH4, they were significantly less so than catalysts calcined at 700°C.
Although a significant loss of sulfates occurred near 650°C during sample
calcination, sulfate species clearly remained on the catalyst surface, and the
formation of S=O species associated with Lewis acid sites was observed in the
sample treated at 700°C.
The development of these sulfate species is in
agreement with both Ward and Ko [122], and Armendariz et al. [117]. The later
group demonstrated higher activities for the hydroconversion of n-hexane on
sulfated zirconia samples treated at temperatures above 600C, corresponding to
monolayer sulfate coverage. Here we have shown a similar trend in activity for
the reduction of NO2 with CH4, with sol-gel SZ catalysts calcined at 700°C being
capable of high N2 yields.
The remaining question, with regard to catalytic activity for NO2 reduction,
is the state of Pd in the various catalysts. Due to low concentration and overlap
106
of characteristic XPS peaks determining the surface state of Pd in sulfated
zirconia is difficult. Resasco [97] had made use of EXAFS to show that the state
of Pd in the active reduction catalyst is dispersed Pd2+ ions. In comparison PdO
is highly selective for the combustion of CH4. Activity of these catalysts for the
selective reduction reaction would therefore argue for the presence of dispersed
Pd species in these sol-gel samples. Shifts in activity to lower temperatures with
an increase in Pd content may simply be due to a resulting increase Pd on the
surface. We have previously observed such a shift on samples prepared by
incipient wetness techniques [123].
6.7 Conclusions
The preparation of Pd/SZ catalysts through a ‘one-pot’ sol-gel technique
has been shown to result in catalysts active for the reduction of NO2 with CH4
under lean conditions. Catalysts calcined at 700°C were more active than those
prepared at lower temperatures, and were capable of N2 yields near 70%. This
activity at higher calcination temperature was shown to correspond to both the
formation crystallinity in the zirconia support and to the appearance of Lewis acid
sites associated with surface sulfate groups.
Pd content was also shown to
affect activity. Higher Pd loadings resulted in maximum N2 yields being shifted to
lower reaction temperatures.
Characterization of the catalysts during calcination revealed the increasing
Pd content resulted in the evolution of CO2 at lower temperatures and across a
narrower range.
Such behavior is
believed to be due to Pd catalyzing the
107
combustion of organic precursors during calcination. Increased Pd loading was
also seen to reduce SO2 loss from the catalyst at temperatures below 600°C,
although it appeared to have little effect on the principle sulfate loss at 650°C that
was observed on all of the prepared catalysts.
In-situ XRD revealed that
increased Pd loading results in the formation of the tetragonal zirconia phase at
lower temperatures and results in larger crystallite sizes.
108
CHAPTER 7
IN-SITU DRIFTS INVESTIGATION OF NO AND NO2 REDUCTION OVER PD
SUPPORTED ON SULFATED MONOCLINIC ZIRCONIA
7.1 Experimental Details
Mechanistic studies focused on understanding the differences in the CH4
reduction of NO and NO2 were performed by in-situ diffuse reflectance infrared
Fourier transform spectroscopy.
DRIFTS studies were performed using
0.3%Pd/SZ prepared by incipient wetness. Several types of experiments were
carried out. Temperature programmed desorption experiments were performed
to observe the thermal transformations of adsorbed NO and NO2. Additional
experiments repeated the TPD-DRIFTS of NO or NO2, but did so under a flow of
either CH4 or CH4+O2. These allowed for the monitoring of the transformations of
adsorbed NOx species in the presence of the other reactant gases to identify
reaction intermediates. In all of these experiments NOx species were adsorbed
for 30 minutes at a concentration of 1000ppm. The TPD runs under reactant
gases the concentrations were 2000ppm CH4 and 5000ppm O2. To assist in the
identification of surface species observed under reaction conditions a series of
experiments were also performed in which reactants were introduced
sequentially. Changes in the observed
109
species were monitored with time until
steady-state was reached, typically 45 minutes.
For each experiment
background measurements were taken under the appropriate conditions. The
presented spectra show changes in absorbance from these backgrounds.
7.2 Temperature Programmed Desorption
NO TPD-DRIFTS results are presented in figures 7.1 and 7.2. Upon NO
adsorption, bands at 1688, 1628, 1579, and a negative band at 1398 cm-1 are
observed. The negative band at 1398 cm-1 is due to interaction of NO with
sulfate species and decreases with increasing temperature, indicating the
desorption of NO.
The feature at 1688 cm-1 appears as a shoulder that
decreases with temperature, disappearing by 300°C. The appearance of this
band is due to a linearly adsorbed NO species [124]. The 1628/1579 cm-1 pair
appears together and grow with increasing temperature, but disappear at 350°C.
These two bands are assigned to bridging and bidentate nitrates, respectively.
They are commonly observed with NO2 adsorption, and indicates that NO is
transformed to adsorbed NO2 species on the surface [125,126]. At 300°C bands
at 1385 and 1287 cm-1 appear and grow with temperature. These bands may be
associated with sulfate species and may also be due to free or bridged nitrates
[127,128]. At 350°C a band at 1617 cm-1 appears which changes intensity with
110
Figure 7.1: NO temperature programmed desorption by DRIFTS.
111
Figure 7.2: NO temperature programmed desorption by DRIFTS. High
wavenumber region.
112
temperature, reaching a maximum at 400°C. This band is also in the bridged
nitrate region. The pair of room temperature peaks centered on 1850 cm-1 are
due to gas phase NO remaining from the adsorption step.
In the high wavenumber region (Figure 7.2) the evolution of several bands
in the hydroxyl region around 3600 cm-1 is observed. Clear band assignment in
this region is difficult because of the wide range of hydroxyl interactions that can
occur with reactant adsorption. There is some evidence in the literature however
that the bands at 3680 and 3550 cm-1 are associated with sulfate interactions
with surface hydroxyls. Specifically they are associated with sulfite-like species
on basic sites [129,130].
Figures 7.3 and 7.4 contain spectra from the NO2 TPD-DRIFTS
experiment. Three bands appear on adsorption of NO2, and are the only bands
observed over the temperature range. The large negative band at 1398 cm-1 is
again associated with NO2 interaction with the sulfate species. This band is of
greater intensity than for NO adsorption, and persists up to the maximum
temperature of 450°C. These observations indicate that NO2 interacts with the
surface sulfates more strongly than does NO. A pair at 1628 and 1579 cm-1 is
present as previously observed due to bridged and bidentate nitrates. Compared
with the NO adsorption observations these bands are larger in intensity, persist
to a higher temperature (350°C), and do not evolve to the bridged nitrate at 1617
cm-1.
113
Figure 7.3: NO2 temperature programmed desorption by DRIFTS.
114
Figure 7.4: NO2 temperature programmed desorption by DRIFTS. High
wavenumber region.
115
Figure 7.4 contains the high wavenumber region of the NO2 TPD experiment.
The band at 3680 cm-1 may indicate interaction with similar hydroxyls as after
NO adsorption, but differences in the other bands, such as the lack of the 3550
cm-1 species, are observed. The band near 2235 cm-1 that is present for only the
low temperature spectra is in the region assigned to NO2+ or NO+ acidic
hydroxyls [130,131,132]. Formation of this species was not observed under NO
adsorption.
7.3 Temperature Programmed Desorption Under CH4
Spectra taken of the NO TPD-DRIFTS performed under CH4 are
presented in figures 7.5 and 7.6. After NO adsorption bands at 1688, 1628, 1579
and the negative at 1398 cm-1 are again present. After the introduction of CH4
the linear NO species at 1688 cm-1 does not persist above room temperature.
The 1628/1579 cm-1 nitrate pair intensity initially increases with temperature but
then appears to diminish above 200°C. This loss of intensity was not observed in
the NO TPD experiment. As in the NO TPD run however, the pair evolves into
the 1617 cm-1 bridging nitrate species at 350°C. The negative 1398 cm-1 band
from sulfate interactions remains until after 350°C, a full 100°C higher than in the
NO only case. Its disappearance is again accompanied by the observation of the
1385 and 1287 cm-1 species. The most significant difference observed is the
appearance of a strong band at 300°C located at 1836 cm-1.
remains until 400°C.
116
This species
Figure 7.5: NO temperature programmed desorption by DRIFTS. TPD
performed under CH4 flow.
117
Figure 7.6: NO temperature programmed desorption by DRIFTS. TPD
performed under CH4 flow. High wavenumber region.
118
In the high wavenumber region the hydroxyl bands appear similar to those
previously observed, but under CH4 flow the band at 3550 cm-1 does not form at
high temperatures.
After the introduction of CH4 flow a band at 3012 cm-1
appears. This band is due to gas phase CH4. At 350°C, and growing with
temperature, a pair appears centered on 2335 cm-1. These bands are due to gas
phase CO2, and indicate that a reaction with CH4 is occurring.
Figures 7.7 and 7.8 show the results of the NO2 TPD-DRIFTS performed
under CH4 flow. Upon NO2 adsorption the bands 1628, 1579, and the negative
1398 cm-1 appear. Under CH4 flow the sulfate interaction band at 1398 cm-1
disappears by 350°C, while during NO2 TPD it was never completely removed.
Additionally, its disappearance is accompanied by the formation of species at
1385 and 1287 cm-1 as in the previous NO adsorption experiments.
The
bridged/bidentate nitrate species at 1628 and 1579 cm-1 grow until 200°C, then
decrease and evolve into the 1617 cm-1 bridged nitrate by 300°C. The band at
1836 cm-1 appears at 300°C as in the NO under CH4 case, but is stronger than
observed in that experiment.
The 1836 cm-1 species is gone at 400°C,
disappearing at a lower temperature than in the NO under CH4 case.
Bands observed above 2000 cm-1 are similar to those seen in the NO TPD
under CH4. With the introduction of CH4 the 3012 cm-1 band characteristic of the
gas phase appears. The CO2 peaks centered on 2335 cm-1 appear in the high
temperature range.
119
Figure 7.7: NO2 temperature programmed desorption by DRIFTS. TPD
performed under CH4 flow.
120
Figure 7.8: NO2 temperature programmed desorption by DRIFTS. TPD
performed under CH4 flow. High wavenumber region.
121
The band appearing at 1836 cm-1 is of interest as it is seen to appear with
the addition of CH4 to the system. Resasco has assigned a band here to NO
adsorbed on Pd2+ ions over various catalysts [133]. In those experiments
however a second band at 1883 cm-1 that maintained its relative intensity to 1836
cm-1 was observed. Additionally, this band was not observed after only NOx
adsorption, but after exposure to CH4 at elevated temperatures. Alternately this
band may be assigned to bridged carbonyl species from the activation of CH4.
Similar species have been seen during the CH4 treatment of Co/SZ [134,135]. If
this species is reactive with adsorbed NOx it may explain some of the
observations made in the above TPD-DRIFTS experiments. From steady-state
reaction experiments it is known that NO2 is significantly more reactive in the
reduction with methane, which may correlate to the earlier disappearance of the
1836 cm-1 band in the NO2 experiment. Also it was observed in reactions that
some NO2 is reduced to NO with methane. The formation of NO during the NO2
reduction may serve to explain the formation of the higher temperature features
at 1617, 1385, and 1287 cm-1, which in NOx adsorption TPD-DRIFTS were only
observed for NO.
7.4 Temperature Programmed Desorption Under CH4 and O2
TPD-DRIFTS experiments were also performed for both NO and NO2
under a flow of CH4+O2. The results of the NO TPD-DRIFTS under CH4+O2 are
presented in Figures 7.9 and 7.10.
negative band at 1398 cm-1 persists
The sulfate interaction indicated by the
across
122
the
entire
investigated
temperature range. Unlike in the NO under CH4 case it does not disappear, and
the 1385/1287 cm-1 pair to not form. The 1628 and 1579 cm-1 species again
grow with temperature until 200°C, then decrease and evolve into the 1617 cm-1
bridged nitrate at 350°C as in the CH4 only experiment. Under CH4+O2 the 1836
cm-1 band does not appear until 350°C, and remains present until 450°C. This
represents a 50°C shift to higher temperature for the formation and presence of
this species.
In the high wavenumber range the formation of CO2 bands at
350°C are larger than in the CH4 only TPD.
In the presence of O2 the
combustion of CH4 is expected to occur.
Results for NO2 TPD-DRIFTS under CH4+O2 flow, presented in Figure
7.11 and 7.12 are similar to those observed in the NO experiment.
The
conversion of the 1628/1579 cm-1 bridged/bidentate nitrate pair to the 1617 cm-1
bridged nitrate occurs at 350°C, which is 50°C higher than in the CH4 only run.
The negative 1398 cm-1 band remains up to 450C. As in the NO experiment the
formation of the 1836 cm-1 species occurs at 350°C and remains until 450°C. In
this temperature range however the 1836 cm-1 band is more intense with NO2
adsorption. Above 2000 cm-1 the greater formation of CO2 is again observed as
compared to the TPD under only CH4. The CO2 bands are present at 300°C,
possible further evidence of the improved reducibility of NO2.
The hydroxyl
region appears similar to the NO experiment, with the formation of a negative at
3750 cm-1 again observed. For both experiments performed under CH4+O2 flow,
an increase in sulfate interaction and in the temperature window for the
123
1898 cm-1 species is observed. This may indicate that the presence of O2 in the
feed stream stabilizes surface nitrate species.
Figure 7.9: NO temperature programmed desorption by DRIFTS. TPD
performed under CH4 and O2 flow.
124
Figure 7.10: NO temperature programmed desorption by DRIFTS. TPD
performed under CH4 and O2 flow. High wavenumber region.
125
Figure 7.11: NO2 temperature programmed desorption by DRIFTS. TPD
performed under CH4 and O2.
126
Figure 7.12: NO2 temperature programmed desorption by DRIFTS. TPD
performed under CH4 and O2. High wavenumber region.
127
7.5 In-situ DRIFTS under reaction conditions.
In-situ experiments were performed with the catalysts maintained a flow of
1000 ppm NOx, 3000 ppm CH4, and 2%O2 in balance He. Examination of the
catalyst surface under reaction conditions allows direct observation of surface
species during the reaction. To aid in the identification of surface species a
series of experiments was performed in which reactants were introduced
sequentially and surface species were monitored with time. Temperature for
these experiments was 375°C, corresponding to the maximum observed N2 yield
from steady-state reaction experiments.
Figures 7.13 and 7.14 show the results from the sequential introduction
experiment in which O2 was first introduced, followed by NO2, and finally CH4.
This will be designated at O2+NO2+CH4. Blank spectra were taken under a flow
of 2%O2, and upon introduction of NO2 several bands appeared. Bands at 1630
and 1599 cm-1 are assigned to bridging and bidentate nitrate species,
respectively [136]. The negative band at 1405 cm-1 corresponds to the S=O
stretch in surface sulfate groups. The observed negative intensity indicates an
interaction with adsorbed species. In the high wavenumber region a negative
band at 3640 cm-1 indicates interaction with surface hydroxyl groups.
Upon
introduction of CH4 to the sample, the immediate appearance of several new
bands was observed.
The sharp peak and 1305 cm-1, and the peak at 3016
cm-1 with corresponding rotational bands are due to gas phase CH4. A strong
band at 1835 cm-1 is observed, similar to that seen during NO and NO2 TPDs
performed under CH4. Again, a similar
band has been observed of Co/SZ
128
Figure 7.13: 0.3%Pd/SZ under a constant flow of 2%O2 at 375°C. Evolution of
surface species after the introduction of NO2 and the further introduction of CH4.
Low wavenumber region.
129
Figure 7.14: 0.3%Pd/SZ under a constant flow of 2%O2 at 375°C. Evolution of
surface species after the introduction of NO2 and the further introduction of CH4.
High wavenumber region.
130
catalysts as being due to bridging carbonyls [135]. Slight growth in this band was
observed between 5 and 25 minutes on stream, but was relatively steady with
time. The nitrate species observed under O2+NO2 are not were no longer visible
due to a broad absorption just above 1600 cm-1. This band, along with the very
broad absorption region centered around 3400 cm-1 are characteristic of
adsorbed H2O species.
With the appearance of H2O there is also a
corresponding decrease in the sulfate band at 1405 cm-1, although this decrease
may also correspond to interaction with adsorbed C-containing species such as
the observed carbonyl group.
The appearance of H2O is evidence of
combustion, or a NO2 and CH4 reaction occurring on the catalyst. Confirmation
of this is the strong doublet centered of 2350 cm-1, corresponding to gas phase
CO2. Evidence of a NO2 and CH4 interaction include the formation of new nitrate
species. Monodentate nitrate at 1265 cm-1, and bidentate species at 1170 cm-1
were observed [136]. Additionally, several species attributed to NO2 reduction
intermediates were observed. In the high wavenumber region three peaks were
observed between 2200-2300 cm-1. Peaks were observed at 2288, 2237, and
2201 cm-1.
Although difficult to assign directly, peaks in this region are
characteristic of isocyanate species. Aylor et al. have identified NCO species at
2260 and 2270 cm-1 during the reduction of NO with CH4 over Mn-ZSM-5 and
Co-ZSM-5, respectively [137,138]. The band at 2237 cm-1 may also be due to
adsorbed N2O [16], which was observed to a small extent during steady-state
reaction studies on these catalysts. Additional bands at 1524 and 1453 cm-1
were
also
observed
upon
the
introduction of CH4 and were observed
131
to grow much stronger with time. Several authors have assigned bands in this
region to nitro-hydrocarbon species believed to play a key from in hydrocarbon
NOx reduction. IR bands attributed to such species have been observed on CoZSM-5 at 1530 and 1476 cm-1 [139], and on Co/ZrO2 at 1632 and 1508 cm-1
[140].
Direct reduction of NO2 was shown in steady-state reaction studies to give
much higher N2 yields than the reduction of NO. To compare surface species
observed during these two reactions the sequential introduction experiment was
also performed with NO. The results from O2+NO+CH4 are presented in Figures
7.15 and 7.16. With the introduction of NO negative bands corresponding to
sulfate and hydroxyl interactions were observed. With time under the NO flow
bands at 1270, 1168, and 1840 cm-1 were observed to grow larger. The bands at
1270 and 1168 cm-1 were observed during the O2+NO2+CH4 experiment after the
introduction of CH4, and were assigned to monodentate and bidentate nitrates,
respectively. The species observed at 1840 cm-1 can only be assigned to NO
adsorbed on Pd2+ ions. At room temperature a similar band has been observed
over both Pd/SZ [134] and Pd/H-ZSM-5 [139], but is typically accompanied by a
second Pd-NO band at approximately 40 cm-1 higher wavenumber. If the two
bands
previously
observed
correspond
to
Pd-NO
species
in
different
environments [139] then it can be concluded that the species observed here is
the only stable one under elevated temperatures.
All three of the observed
nitrate bands grow with time, and a corresponding decrease in the sulfate band
was observed.
132
Figure 7.15: 0.3%Pd/SZ under a constant flow of 2%O2 at 375°C. Evolution of
surface species after the introduction of NO and the further introduction of CH4.
Low wavenumber region.
133
Figure 7.16: 0.3%Pd/SZ under a constant flow of 2%O2 at 375°C. Evolution of
surface species after the introduction of NO and the further introduction of CH4.
High wavenumber region.
134
With the introduction of CH4 the expected gas phase bands at 3016 and
1305 cm-1 immediately appeared.
Evidence of CH4 combustion through the
appearance of CO2 and 2350 cm-1 and adsorbed H2O at 1625 cm-1 were seen.
As with the O2+NO2+CH4 experiment, bands at 2288, 2337, and 2201 cm-1
associated with NCO and N2O species, formed upon the introduction of CH4.
The formation of such species is not unexpected since N2O formation in the NO
and NO2 reduction reactions were similar.
Two significant differences were
observed in comparison to the previous experiment. The band at 1840 cm-1 was
observed to grow in intensity with the addition of CH4, and the bands at 1524 and
1453 cm-1 grew much more slowly and were less intense at steady-state.
The continued growth of the 1840 cm-1 band with the addition of CH4
would seem to indicate the presence of two bands at this location. As previously
discussed, bridging carbonyl may give rise to a band here. During the previously
discussed DRIFTS-TPD experiments this band was only observed during NO
and NO2 TPD under CH4, and not during NO TPD in He. Since most of the
observed nitrate species were desorbed at temperatures corresponding to the
formation of the 1840 cm-1 band, it is possible that Pd-NO species never formed
during the He TPD. Therefore the band was only observed after hydrocarbon
activation in the CH4 TPD. In the sequential introduction experiment a constant
flow of NO is maintained over the catalyst, allowing Pd-NO to form. Formation of
this band in the O2+NO2+CH4 experiment could be due to both species, since the
partial reduction of NO2 to NO was observed during the reaction studies.
135
The difference in the observed formation and band intensities at 1524 and
1453 cm-1 may be the most significant. These bands were assigned to nitrohydrocarbon type reaction intermediates, and a decrease in these species may
be directly related to the lower reduction activity observed with NO.
It is
commonly proposed that NO reduction with hydrocarbons under lean conditions
proceeds first through the oxidation of NO at adsorbed NO2 species, followed by
the formation of nitro-hydrocarbon intermediates, and finally the reaction of these
intermediates with gas phase NO or NO2.
Comparison of these DRIFTS
experiments would therefore indicate that higher N2 yields are due to an increase
in the rate of formation of these reactive intermediate species.
To provide additional evidence for band assignments made in the previous
discussion, the sequential introduction of reactants was also performed reversing
the order of introduction. Figures 7-17 and 7-18 contain the DRIFTS spectra
from this experiment. The background was again taken under 2% O2, followed
by the introduction of 3000 ppm CH4. Gas phase CH4 is immediately observed at
3016 and 1305 cm-1, and combustion is observed through the formation of CO2
and adsorbed H2O. A negative band at 1405 cm-1 indicates interaction of surface
sulfate groups with adsorbed species, but was much smaller than during the
introduction of NO or NO2.
Upon introduction of NO2 the 1840 cm-1 band
increased with the formation of NO. The intermediate species with bands at
1524 and 1453 cm-1 also appeared, and reached similar intensity as in the
O2+NO2+CH4 experiment. Their formation in this experiment confirms that these
species
are
nitro-hydrocarbon
intermediates, and not due to CH4.
136
Figure 7.17: 0.3%Pd/SZ under a constant flow of 2%O2 at 375°C. Evolution of
surface species after the introduction of CH4 and the further introduction of NO2.
Low wavenumber region.
137
Figure 7.18: 0.3%Pd/SZ under a constant flow of 2%O2 at 375°C. Evolution of
surface species after the introduction of CH4 and the further introduction of NO2.
High wavenumber region.
138
Surface species stability was examined by flushing the catalyst after
exposure to steady-state reaction conditions. The catalyst sample was exposed
to 1000 ppm NO2, 3000 ppm CH4, and 2% O2 in He at 375°C for 1.5 hours. After
this exposure the DRIFTS sample chamber was flushed with 2% O2/He and the
surface species were monitored with time.
Results of this experiment are
presented in Figures 7.19 and 7.20. Upon flushing the sample, gas phase CH4
and CO2 both immediately disappear. The NCO/N2O species observed between
2200-2300 cm-1 are visible for approximately 25 minutes before being completely
removed.
The band at 1840 cm-1 associated with both Pd-NO and bridging
carbonyl species also quickly fades under flushing.
Surprisingly, the bands at 1524 and 1453 cm-1 assigned to nitrohydrocarbon intermediates are the most stable.
While these species initially
decrease slightly, their intensity does not continue to drop with further flushing.
The stability of these species indicates a strong interaction with the catalyst
surface. Although intermediates would not typically be expected to be stable,
accepted NOx reduction mechanisms should proceed through the interaction of
these species with gas phase NO or NO2. However the removal of CH4 and CO2
upon flushing of the sample indicate a quick depletion in gas phase reactants,
which may allow the intermediate species to remain on the surface.
139
Figure 7.19: 0.3%Pd/SZ under a constant flow of 2%O2 at 375°C. Evolution of
surface species during flushing of the catalyst after steady-state reaction. Low
wavenumber region.
140
Figure 7.20: 0.3%Pd/SZ under a constant flow of 2%O2 at 375°C. Evolution of
surface species during flushing of the catalyst after steady-state reaction. High
wavenumber region.
141
CHAPTER 8
DUAL CATALYST AFTERTREATMENT OF LEAN-BURN NATURAL GAS
ENGINE EXHAUST
8.1 Experimental Details
Both NO oxidation and NO2 reduction catalysts were prepared for this
work. Commercial TiO2 and ZrO2 supports were supplied by Saint-Gobain. The
support materials were received in pelletized form, which were first ground and
sieved. The 100-140 mesh cut (0.149-0.105 mm) was saved, then calcined in air
for 3 hours at 500°C. The calcined support was then screened a second time.
Co/TiO2 and Co/ZrO2 NO oxidation catalysts were prepared through
incipient wetness techniques. Cobalt nitrate (Aldrich) was dissolved in water and
the solution was added to produce a calculated Co loading of 10-wt%. Solubility
of the nitrate precursor required the use of two impregnation steps. After each
impregnation the catalyst was dried at 110°C overnight. After the second drying
the catalysts were calcined in air at 300°C for 3 hours.
For the reduction of NO2 Pd supported on sulfated zirconia (SZ) was
prepared through a two-step incipient wetness technique. An aqueous solution
of ammonium sulfate (Aldrich) was added to the commercial monoclinic zirconia
142
support, which was then dried at 110°C. After drying, the sulfated zirconia was
calcined in air at 500°C for 3 hours.
Pd was then added through incipient
wetness impregnation, and the catalysts was again dried and calcined under the
previously used conditions. In this work Pd loadings of both 0.5 and 0.3-wt%
were used.
Steady state reaction experiments were performed on the dual catalyst
bed and on samples of the 10%Co/ZrO2 NO oxidation catalyst. Reactions were
performed in a ¼” o.d. stainless steel tube. In testing the two-catalyst system
sample of the oxidation and reduction catalysts were thoroughly mixed before
being placed into the reactor.
In testing the effect of catalyst ratio on NO
reduction activity the amount of Pd/SZ reduction catalyst was kept constant,
while the amount of NO oxidation catalyst was changed. Prior to each reaction
experiment the packed catalyst bed was pretreated in 10%O2/He at 300°C for 30
minutes. After pretreatment the bed was flushed with He and cooled to 250°C
while a feed analysis was performed. Initial tests were performed in 1000 ppm
NO, 3000 ppm CH4, and 10% O2 in balance He. At this composition the twocatalyst bed was also tested in the presence of 1%, 3%, and 10% H2O. Feeds
that more closely approximated the composition of lean natural gas engine
exhaust were also used. Examination of the catalyst ratio was performed in 400
ppm NO, 1700 ppm CH4, 200 ppm C2H6, 100 ppm C3H8, and 10% O2 in balance
He. The catalyst mixture was also tested under these concentrations, with the
further addition of 600 ppm CO and 6% CO2.
143
Steady-state reactions testing the activity of 10%Co/ZrO2 for the oxidation
of NO to NO2 were conducted with and without water in the feed stream. Prior to
the activity studies the catalyst sample was pretreated in 10%O2 at 300°C for 30
minutes, then cooled to the first reaction temperature. The reaction feed was
then introduced, which contained 1000 ppm NO, 10% O2, and either 0 or 10%
H2O in balance He. Activity measurements were made in the range of 200500°C.
The interaction of water with the surface of the Pd/SZ reduction catalysts
was examined with DRIFTS. A sample of 0.3%Pd/SZ contained in a heated
sample cup was placed in a Spectratech high temperature diffuse reflectance
cell, equipped with ZnSe windows. The sample was pretreated at 400°C in 10%
O2 for 30 minutes. The cell was then flushed with He for 30 minutes and the
temperature raised to 450°C. Under He flow background spectra were taken
every 50°C, down to room temperature. The catalyst was then exposed to a
stream containing 5000 ppm H2O for 30 minutes. Following adsorption the cell
was again flushed with He for 30 minutes. Spectra were then taken in 50°C
increments up to 450°C.
The thermal stability of surface sulfate groups on the Pd/SZ NO2 reduction
catalyst was tested through temperature programmed desorption performed with
and without H2O in the carrier. 100 mg of 0.3%Pd/SZ was loaded in a quartz Utube reactor between plugs of quartz wool. The sample was pretreated in 10%
O2 by ramping to 400°C at 10°C/min, then holding for 30 minutes. The sample
144
was then cooled to room temperature. Sulfate desorption was measured under
two flows: 10% O2 in He, and 10% O2 and 2%H2O in He. In both experiments
the sample temperature was ramped from 30 to 700°C at a rate of 10°C/min.
8.2 Comparison of Direct NO2 and Dual-Catalyst NO Reduction
The development of NO oxidation and NO2 reduction catalysts was initially
pursued separately, and these results have been described elsewhere [123,141].
Initial tests of the dual-catalyst system were performed using 10%Co/TiO2 and
0.5%Pd/SZ in a ratio of 0.5:1. These formulations were later observed to not be
the most active samples. However these initial tests demonstrated the merit of
the dual-catalyst approach, and in comparison to later results illustrate the
importance of each catalyst in achieving high N2 yields. Figure 8.1 compares the
N2 yields and CH4 conversions obtained during the reduction of NO2 over
0.5%Pd/SZ and during the reduction of NO over the mixed bed. Below 350°C N2
yields in the NO2 reduction are 10-20% higher than observed over the mixed
bed. At 375°C and above however, N2 yields are similar, indicating that the dual
catalyst system gives improved performance over the direct reduction of NO. At
these higher reaction temperatures N2 yield is observed to decrease due to loss
of the CH4 reducing agent.
The CH4 conversion is similar during the two
reactions until 350°C. At higher temperatures the CH4 conversion over the mixed
bed increases more quickly.
It was observed that at the higher reaction
temperatures N2 yields were very similar despite the higher conversion of CH4
145
Figure 8.1: Comparison of NO2 reduction over 0.5%Pd/SZ and NO reduction over
a mixed bed containing 10%Co/TiO2 and 0.5%Pd/SZ in a 0.5:1 ratio. Reduction
catalyst: (♦) N2 yield and (◊) CH4 conversion. Mixed bed: (■) N2 yield and (□) CH4
conversion.
146
over the mixed bed. Figure 8.2 shows steady-state reaction results comparing
the reduction of NO2 over 0.3%Pd/SZ with the reduction of NO over a mixed bed
containing 10%Co/ZrO2 and 0.3%Pd/SZ in a ratio of 0.5:1.
In our previous
publications we have discussed the higher NO oxidation activity of the Co/ZrO2
catalyst as compared with Co/TiO2. Additionally the 0.3%Pd/SZ was shown to
give higher N2 yields than the 0.5%Pd catalyst, and to do so at relatively low
temperatures. Below 300°C the N2 yields for the two reduction reactions were
within 10%, with the mixed bed slightly lower. Above 300°C a broad maximum
was observed, reaching between 57-60% N2 yield.
We have shown this
maximum to coincide with complete NO2 conversion, with the other major product
being NO. In contrast, N2 yield continues to increase at temperatures above
300°C over the mixed bed. The maximum of 80% is reached at 400°C. Above
400°C a decrease in N2 yield was observed in both systems. Across the entire
temperature range CH4 conversion was much higher over the mixed bed than
over only the reduction catalyst.
In the dual-catalyst reduction of NO, CH4
conversion reached 94% at 450°C.
Comparison of the two mixed bed reactions demonstrates that the choice
of catalysts will significantly affect the performance of the system for NO
reduction. Particularly important is the NO oxidation activity of the Co-based
catalysts. The oxidation of NO to NO2 is thermodynamically limited within these
temperatures ranges. However the system is capable of bypassing this limit as
NO2 is removed from the system through the reduction with CH4. We
147
Figure 8.2: Comparison of NO2 reduction over 0.3%Pd/SZ and NO reduction over
a mixed bed containing 10%Co/ZrO2 and 0.3%Pd/SZ in a 0.5:1 ratio. Reduction
catalyst: (♦) N2 yield and (◊) CH4 conversion. Mixed bed: (■) N2 yield and (□) CH4
conversion.
148
have shown 10%Co/ZrO2 to be a much more effective NO oxidation catalyst than
10%Co/TiO2 [141]. Steady-state experiments have shown it to reach equilibrium
conversions at significantly lower temperatures, and higher rates in the kinetically
controlled regime. It is not surprising therefore that the mixed bed containing
10%Co/ZrO2 reaches higher N2 yields across the reaction temperature range.
Even at low temperatures the ZrO2-based oxidation catalyst reaches a high NO2
yield, explaining the similar N2 yields in the direct reduction of NO2 and the mixed
bed reduction of NO. In the direct reduction of NO2 the reaction seems to be
limited by a side reduction reaction, which yields NO, not N2. However, in the
Co/ZrO2 containing mixed bed this NO can be quickly reoxidized to NO2. Such a
route explains the higher N2 yields observed, and emphasizes the importance the
two-catalyst approach.
8.3 NO Reduction Using Simulated Natural Gas
The exhaust composition from lean-burn natural gas engines can vary
significantly based upon the equivalence ratio at which the engine is operating.
Within typical operating ranges of current engines NO concentrations may vary
from between 90-900 ppm and unburned hydrocarbons can be present between
1500-2500 ppm. From natural gas engines the unburned hydrocarbons may
consist of a mixture of species, primarily CH4, C2H6, and C3H8 typically found in
natural gas. Actual concentrations in the exhaust can vary with the fuel source.
Steady-state reactions were performed
149
to examine the dual-catalyst system
under more closely simulated exhaust conditions. In these experiments the ratio
of oxidation and reduction catalyst in the mixed bed was also varied.
This
change was accomplished by changing the amount of 10%Co/ZrO2 oxidation
catalyst in the bed, while keeping the amount of 0.3%Pd/SZ reduction catalyst
constant. Reduction catalyst to oxidation catalyst ratios of 1:1, 1:0.5, and 1:0.25
were used. Feed concentrations in these reactions were 400 ppm NO, 1700
ppm CH4, 200 ppm C2H6, 100 ppm C3H8, and 10% O2 in balance He.
Figures 8.3, 8.4, and 8.5 show the yields of the N-containing species N2,
NO2, and N2O, respectively. At 250°C N2 yields are similar over each of three
bed compositions, but differences appear at higher temperatures.
At a
composition ratio of 1:1 yields are the lowest, reaching a maximum of around
60% in the 350-400°C temperature region. Reactions performed at ratios of
1:0.5 and 1:0.25 reached similar yields at temperatures below 375°C, with
maximums above 75%. Above these temperatures N2 yields decreased over all
three bed compositions. Such a decrease is typical at high temperatures during
the hydrocarbon reduction of NOx due to large conversions of the reducing agent
through combustion.
It was observed that at elevated reaction temperatures
larger N2 yields were achieved over the beds containing less oxidation catalyst.
The NO2 yields as a function of reaction temperature are presented in Figure 8.4.
Similar trends were observed for each of the three catalyst beds. NO2 yield was
relatively high at low temperatures before dipping between 300 and 375°C.
Above 375°C NO2 yields again increased. Across the temperature range higher
NO2 yields corresponded with an
increase of oxidation catalyst in the
150
Figure 8.3: N2 yield during the reduction of NO over a mixed-bed of 10%Co/ZrO2
and 0.3%Pd/SZ with a simulated natural gas exhaust. Effect of catalyst ratio,
with amount of reduction catalyst held constant. Reduction:Oxidation catalyst
ratios of 1:1 (♦), 1:0.5 (ƒ), and 1:0.25 (c).
151
Figure 8.4: NO2 yield during the reduction of NO over a mixed-bed of
10%Co/ZrO2 and 0.3%Pd/SZ with a simulated natural gas. Effect of catalyst
ratio, with amount of reduction catalyst held constant.
catalyst ratios of 1:1 (♦), 1:0.5 (ƒ), and 1:0.25 (c).
152
Reduction:Oxidation
Figure 8.5: N2O yield during the reduction of NO over a mixed-bed of
10%Co/ZrO2 and 0.3%Pd/SZ with a simulated natural gas. Effect of catalyst
ratio, with amount of reduction catalyst held constant.
catalyst ratios of 1:1 (♦), 1:0.5 (ƒ), and 1:0.25 (c).
153
Reduction:Oxidation
bed. The temperature region of decreased NO2 yields corresponded with that of
maximum N2 yields. NO2 is present across the entire investigated temperature
range for each ratio, indicating that even small amounts of Co/ZrO2 are able to
oxidize NO at faster rates than NO2 can be reduced over the Pd/SZ catalyst.
The production of N2O, an undesirable side product, was also observed. Steadystate N2O yields are presented in Figure 8.5.
When the two catalysts that
comprise the system were studied individually, N2O was observed during the
reduction of NO2 with CH4 over Pd/SZ, but not during the oxidation of NO.
However in the direct reduction of NO2 the observed yields were lower, less than
5%. In the reduction of NO over the mixed beds N2O yields were between 515%. Between 350 and 400°C N2O yield was approximately 10% over each of
the three bed compositions.
The conversions of C3H8, C2H6, and CH4 as a function of temperature are
shown in Figures 8.6, 8.7, and 8.8 for the three catalyst ratios. Both C2H6 and
C3H8 conversions were very high, even at temperatures as low as 250°C. At this
temperature C3H8 conversion was 83, 90, and 100% for the 1:0.25, 1:0.5, and 1:1
ratios, respectively. C2H6 conversions were lower but still large, 58, 76, and
83%.
Conversion of both these hydrocarbons was essentially complete by
300°C. Steady-state CH4 conversions, shown in Figure 8.8, increased steadily
with temperature, reaching complete conversion at 450°C over all three
compositions. As with the higher hydrocarbons, CH4 conversion was increased
at a given temperature by an increase in the amount of oxidation catalyst present
in the mixed bed. Through most of the temperature range the conversion
154
Figure 8.6: C3H8 conversion during the reduction of NO over a mixed-bed of
10%Co/ZrO2 and 0.3%Pd/SZ with a simulated natural gas. Effect of catalyst
ratio, with amount of reduction catalyst held constant.
catalyst ratios of 1:1 (♦), 1:0.5 (ƒ), and 1:0.25 (c).
155
Reduction:Oxidation
Figure 8.7: C2H6 conversion during the reduction of NO over a mixed-bed of
10%Co/ZrO2 and 0.3%Pd/SZ with a simulated natural gas. Effect of catalyst
ratio, with amount of reduction catalyst held constant.
catalyst ratios of 1:1 (♦), 1:0.5 (ƒ), and 1:0.25 (c).
156
Reduction:Oxidation
Figure 8.8: CH4 conversion during the reduction of NO over a mixed-bed of
10%Co/ZrO2 and 0.3%Pd/SZ with a simulated natural gas. Effect of catalyst
ratio, with amount of reduction catalyst held constant.
catalyst ratios of 1:1 (♦), 1:0.5 (ƒ), and 1:0.25 (c).
157
Reduction:Oxidation
difference between the 1:1 and 1:0.25 bed was 10% or greater. Examination of
these NO reduction results show that selection of a catalyst ratio for the dualcatalyst approach can significantly affect the bed activity. Increasing the amount
of the Co/ZrO2 oxidation catalyst results in higher hydrocarbon conversions.
High hydrocarbon conversions do, however, result in lower N2 yields due to loss
of the reducing agent.
Additionally, although C2H6 and C3H8 are very easily
activated, even at low temperatures, their presence was not observed to increase
the N2 yield. It seems therefore that these species are poor NO2 reducing agents
under these conditions.
8.4 Simulated Exhaust Containing CO and CO2
Steady-state reaction tests in simulated exhaust were also performed with
the further addition of CO and CO2. NO, hydrocarbon, and O2 concentrations
were maintained at the same levels as in the previous reactions. To this stream
600 ppm CO and 6% CO2 were added. These levels again represent typical
exhaust concentrations. The mixed bed was composed of a 1:0.25 reduction to
oxidation catalyst ratio. The yields of the N-containing species are shown in
Figure 8.9. N2O yields were similar to those obtained during the reaction without
CO and CO2, around 10% in the temperature range of maximum N2 yield. NO2
yield was observed to follow a similar pattern are previously, increasing up to
300°C and then decreasing as N2 yield reaches its maximum. At low reaction
temperatures the NO2 yield was significantly higher than in the CO/CO2 free
experiment.
This increase in NO2
corresponded to lower N2 yields below
158
Figure 8.9: Yield of N-containing species during the reduction of NO over a
mixed-bed of 10%Co/ZrO2 and 0.3%Pd/SZ in simulated lean exhaust. N2 (♦),
NO2 (ƒ), and N2O (c).
159
325°C. Although initially lower with the addition of CO and CO2, the N2 yield
increased with temperature reached a broad maximum of 80-82% between 375425°C.
Above these temperatures yield decreased.
Activity was therefore
comparable to the earlier experiments, but the maximum was shifted to slightly
higher temperatures. Conversions for the three hydrocarbons and CO are shown
in Figure 10. With the addition of CO and CO2 to the reaction feed stream the
conversion of all three hydrocarbons was lowered for a given reaction
At 250°C conversions of C2H6 and C3H8 were 37% and 76%,
temperature.
respectively. Both quickly increased with temperature and complete conversion
was reached by 300°C. CH4 conversion was initially low, taking off above 300°C
and steadily increasing with temperature. With the addition of CO and CO2, CH4
conversion was decreased by approximately 10% across the temperature range.
A maximum of 94% was reached at 450°C. Analysis of these results indicates
that the presence of CO and CO2 inhibit the conversion of the hydrocarbons in
the feed. Again the results show that the higher hydrocarbon species do not
improve the N2 yield over the mixed bed. In fact N2 yield and CH4 conversion
were observed to increase sharply above 300°C, corresponding with complete
conversion of C2H6 and C3H8.
CO conversion was also monitored during the NO reduction reaction.
Complete CO oxidation to CO2 was achieved across the investigated
temperature range. Reaction testing of the individual catalysts showed that over
0.3%Pd/SZ CO was not an effective NOx reducing agent. CO oxidation was
observed
to
occur
over
the
10%Co/ZrO2 oxidation catalyst, even
160
Figure 8.10: Hydrocarbon and CO conversion during the reduction of NO over a
mixed-bed of 10%Co/ZrO2 and 0.3%Pd/SZ in simulated lean exhaust. CH4 (♦),
C2H6 (ƒ), C3H8 (c), and CO (•).
161
at very low reaction temperatures. Further investigations of this functionality will
be presented in forthcoming publications.
The tests performed using typical pollutant concentrations found in lean
natural gas engine exhaust demonstrate the potential advantages of this dualcatalyst system. High N2 yields were achieved with hydrocarbon concentrations
typical of engine exhaust.
Effective use of unburned hydrocarbons in the
exhaust would remove the need for injection of a reducing agent, which is
attractive because the fuel penalties associated with hydrocarbon injection are a
barrier to commercialization.
In addition to effectively using unburned
hydrocarbons as the NOx reducing agent, the dual-catalyst system is capable of
removing them to a high degree. As these species are smog producers and
green house emissions their removal is a benefit. Additionally the system can
completely remove CO from the simulated exhaust.
8.5 Effect of Water
Water vapor also comprises a significant portion of combustion exhaust
streams.
Examination of the effect of water on the dual-catalyst system
performance was performed using three different water concentrations. Figures
8.11, 8.12, and 8.13 show, respectively, the N2 yields, CH4 conversions, and NO2
yields observed during the reduction of NO with CH4 over the mixed catalyst bed.
Figure 8.11 shows that N2 yields are reduced with the addition of increasing
amounts of water, and that with the addition of water the window of maximum
activity
was
narrowed.
In
the
presence of 1% water vapor yields
162
increased with reaction temperature up to a maximum of 73%, achieved at
400°C.
Increasing the water further decreased the N2 yields across the
temperature range. In 3% water vapor a maximum of 56% was observed, and
with 10% water a maximum of 40%. In all three reactions N2 production fell at
reaction temperatures above 400°C. With the addition of H2O to the reaction
feed stream, CH4 conversion was suppressed. The effect was stronger with
increased H2O concentration. These results are shown in Figure 8.12. At 325°C
and below, conversion was less than 10% at all three H2O concentrations. With
increasing reaction temperature CH4 conversions in 1 and 3% H2O took off more
sharply and were generally within 5% of each other. At 400°C, corresponding
with maximum N2 yield, conversions in 1 and 3% H2O were 55 and 51%,
respectively. With 1 and 3% H2O, CH4 conversion was complete at 500°C. The
reaction performed in 10% H2O showed much lower CH4 conversion. At 400°C
conversion was only 36%, and was not yet complete at 500°C. Figure 8.13
shows the NO2 yields achieved under increasing water concentrations.
NO2
yields were less than 30%, tending to decrease with temperature and with
increased H2O concentration. The decrease with temperature is similar to the
result that was observed the dry reaction experiments, where NO2 yield
decreased as high conversion in the reduction reaction was reached.
Even in a 10% H2O containing stream, NO2 yield was observed. The
presence of excess NO2 over the dual catalyst bed during the NO reduction
reaction indicates that N2 yield in the system is not limited by the oxidation step.
163
Figure 8.11: N2 yields from the reduction of NO over a mixed bed containing
0.3%Pd/SZ and 10%Co/ZrO2 in a 1:0.25 ratio. Reduction performed with a feed
containing 1% (♦), 3% (ƒ), and 10%(c) water.
164
Figure 8.12: CH4 conversions during the reduction of NO over a mixed bed
containing 0.3%Pd/SZ and 10%Co/ZrO2 in a 1:0.25 ratio. Reduction performed
with a feed containing 1% (♦), 3% (ƒ), and 10%(c) water.
165
Figure 8.13: NO2 yields during the reduction of NO over a mixed bed containing
0.3%Pd/SZ and 10%Co/ZrO2 in a 1:0.25 ratio. Reduction performed with a feed
containing 1% (♦), 3% (ƒ), and 10%(c) water.
166
8.6 Interaction of H2O with Pd/SZ
Direct reaction testing of the effect of H2O on the reduction of NO2 over
Pd/SZ was not performed in order to protect the analytical equipment. NO2 is
quite soluble in water and some initial testing indicated that with large amounts of
both species in the stream nitric acid was formed, which would damage
instruments.
The interaction of H2O with the reduction catalyst, in particular the acid
sites associated with surface sulfates, was examined through two experiments.
Of primary interest is the stability of sulfate groups in the presence of a H2O
containing feed stream.
0.3%Pd/SZ was exposed to a temperature program
under 10% O2 and 0 or 2% H2O to examine relative thermal stability of the
surface sulfate groups.
The mass spectrometer signal for ion m/z=64 is
presented in Figure 8.15.
m/z=64 corresponds to SO2, which was the
predominant sulfur containing species observed.
The other major ion was
m/z=48 (SO), a fragment of SO2 that closely followed its desorption pattern.
Under the dry feed sulfates were observed to desorb from the catalyst starting at
around 550°C. The desorption is broad and did not reach a maximum by 700°C.
In the presence of 2% H2O, SO2 desorption begins at around 475°C and also
increases with temperature. Above the initial desorption temperature sulfate loss
is higher in the presence of H2O.
However the desorption below 500°C is
minimal, and no difference was observed within the temperature range of high N2
yields.
167
Figure 8.14: Desorption of SO2 from Pd/SZ reduction catalyst with or without 2%
water.
168
Temperature programmed desorption of H2O was performed over
0.3%Pd/SZ using in-situ DRIFTS to examine its interaction with surface sulfates.
DRIFTS spectra are shown in Figures 8.16 and 8.17. Spectra shown were taken
during H2O adsorption, after flushing at room temperature, and in 50°C steps
from 50°C to 450°C. Figure 8.16 contains the spectra of the low wavenumber
region, 1000-2000 cm-1. Upon exposure to water, five features are observed at
1054, 1140, 1205, 1322, and 1640 cm-1. The peaks at 1054, 1140 and 1205 are
all assigned to bidentate SO42- species resulting from the hydration of adsorbed
surface sulfates. With increasing temperature these peaks are observed to shift
position at higher temperatures. Above 100°C absorption bands are observed at
1040 and 1250 cm-1, which are assigned to adsorbed H2SO4. This evolution of
surface species is similar to that observed by Babou, Coudurier, and Vedrine
during the dehydration of a sulfated zirconia prepared using ZrOH as a precursor
[142].
The intensity of species in this region decrease with increasing
temperature so that they are no longer observed at 350°C and above. The band
initially observed at 1640 cm-1 was also observed to decrease with sample
temperature, and to shift toward lower wavenumbers.
A peak in this region
would be typical of adsorbed H2O on the catalyst surface. The peak observed at
1322 cm-1 is strongly negative.
With increasing temperature this peak was
observed to both decrease in intensity, and to shift toward higher wavenumbers.
By 400°C the peak is much less intense and is located at 1398 cm-1. 1398 cm-1
corresponds to the IR absorption band of S=O in the surface sulfate species.
169
Figure 8.15: DRIFTS investigation of H2O desorption from 0.3%Pd/SZ.
wavenumber region.
170
Low
Figure 8.16: DRIFTS investigation of H2O desorption from 0.3%Pd/SZ.
wavenumber region.
171
High
The negative intensity and wavenumber shift are a clear indication of H2O
interacting with these groups. In the high wavenumber region a broad band
centered on 3400 cm-1 appears due to H2O adsorption.
At 450°C the IR
spectrum contains no absorption bands, indicating the removal of adsorbed
water. Importantly the band at 1398 cm-1 recovers fully, showing that no loss of
sulfate species occurs during the desorption of water. The IR data indicates that
the sulfate groups may be easily hydrated in the presence of H2O. Although the
effect is reversible, under a constant H2O containing feed several sulfur
containing species are likely present. Such behavior may explain the increased
loss of sulfur at high temperatures during the temperature programmed
desorption experiment.
Combining the DRIFTS and TPD data indicate that
around 400°C, the temperature of maximum NOx reduction activity, sulfate loss
is not significant. It would therefore seem that the loss of activity for the reaction
in the presence of H2O is due to a competitive adsorption and transformation of
acid sites.
8.7 Conclusions
The investigated dual-catalyst system has shown potential as an approach
to achieving high NOx conversions from the reduction with CH4 under lean
conditions. Improved rates of NO2 reduction have been previously observed,
however during this reaction performed over Pd/SZ, NO is produced through a
competitive reduction reaction. By incorporating an active NO oxidation catalyst
in a mixed catalyst bed this challenge
is overcome by the replenishment of
172
NO2 in the system. The dual-catalyst bed is capable of achieving N2 yields of
above 80%. Additionally, when the dual-catalyst system was tested in simulated
lean natural gas exhaust, N2 yields were not inhibited and the system was very
active for the oxidation of CO, C3H8, C2H6 and CH4. Elimination of these species
is an additional benefit to an aftertreatment system. Activity in the simulated
exhaust also shows that NOx reduction could be accomplished without the
injection of additional hydrocarbons reducing agents, and without the associated
fuel penalties. Reaction testing indicated that designing a practical mixed bed
will require further study to balance the NO reduction and hydrocarbon oxidation
functions, as high hydrocarbon conversions can reduce the maximum N2 yield.
Addition of H2O to the feed system causes a reduction in both NOx reduction and
hydrocarbon oxidation activities. Investigation of this effect reveals that it is due
to a competitive adsorption phenomena on the sulfated zirconia NO2 reduction
catalyst. The loss of activity is not due to the permanent removal acid sites
related to sulfate species.
173
CHAPTER 9
CONCLUSIONS AND RECOMMENDATIONS
9.1 Conclusions
One of the principle challenges to the effective and commercial use of
hydrocarbons as NOx reducing agents for exhaust streams is the competitive
combustion in excess O2. Since NO2 is a stronger oxidizing agent it can better
compete with O2 for reaction with the hydrocarbon. Examination of the reduction
of NO2 has shown that Ag-based catalysts are very active for the reaction using
C3H6 or C3H8 as the reducing agent. 1% and 3%Ag/Al2O3 catalysts were able to
achieve above 90% N2 yields by 400°C using both species.
However, the
developed Ag-based catalysts where shown to be inactive for the reduction
reaction using CH4. Due to its availability in natural gas CH4 would be the most
practical choice as reducing agent.
For activity with CH4 two different series of NO2 reduction catalysts were
formulated. Low levels of Pd supported on sulfated zirconia where shown to be
active in the reduction of both NO and NO2 using CH4. Catalysts prepared by
both incipient wetness and sol-gel techniques were shown to be active. The
preparation of sulfated zirconia through incipient wetness demonstrated that a
monoclinic support could be used to
prepare a NO2 reduction catalyst. The
174
use of the monoclinic support is a potential advantage, as this is the stable
zirconia phase at typical reaction temperatures.
Testing the Pd/SZ prepared
using sol-gel techniques as demonstrated good activity for NO2 reduction with
CH4. Activity for this reaction indicates that the single-step sol-gel procedure
results in dispersed Pd species on the surface of the catalyst. The addition
Comparison of activities in these reactions demonstrates higher N2 yields during
the reaction with NO2 than with NO. Both sulfate loading and Pd content have
been observed to significantly affect catalyst activity. Without surface acidity
imparted by sulfation a Pd/ZrO2 catalyst was shown to have little activity for either
NO2 reduction of CH4 activation. Increased Pd loading corresponded to higher
CH4 combustion and lower NO2 reduction rates.
The stability of sulfate groups on the SZ catalysts was examined during
both preparation of the catalyst and under reaction conditions. In the Pd/SZ
catalysts prepared by incipient wetness sulfate species were stable during
calcination at 500°C, as evidenced by XPS analysis. Temperature programmed
desorption of reactant species showed no evidence of sulfate loss below the
catalyst calcination temperature.
Heat treatment in the presence of water
increased the loss of sulfate groups at high temperatures, but not in the
temperature region of NOx reduction activity. During the preparation of sol-gel
catalysts increasing Pd content was shown to decrease sulfate loss below
625°C, although a large desorption of these species was observed near 650°C
for all catalysts. Despite this loss, reaction data indicated that samples calcined
reduction.
at 700°C were the most active for NO2
175
IR spectroscopy revealed
the formation of acidic sulfate sites at this temperature.
Mechanistic studies using DRIFTS have been performed to examine the
difference in the NO and NO2 reduction reactions over incipient wetness
prepared Pd/SZ.
Results indicate that upon adsorption NO2 interacts more
strongly with acid sites than does NO.
Additionally, the two species are
adsorbed differently on the catalyst surface. During desorption in the presence
of CH4 and O2, the formation of the reaction product CO2 is observed at lower
temperatures for NO2.
Spectra taken in-situ during the reduction reaction
showed the formation of intermediate species to a greater extent during the
reduction of NO2 than of NO.
The development of NO2 reduction catalysts is part of a novel approach to
lean NOx reduction.
It is proposed that these catalysts will form an active
reduction system when combined with catalysts active for the oxidation of NO to
NO2. Testing of the proposed system in the form of mixed catalyst beds has
proven to have several potential benefits. N2 yields of 80% from the reduction of
NO with CH4 have been observed.
These activities are dependant on both
catalyst formulations and ratio of oxidation and reduction catalyst in the bed. The
combined system is also active for the reduction of NO using a simulated natural
gas that contained C2H6 and C3H8. These species appear to be selective for the
partial reduction of NO2 to NO. Over the mixed bed however reoxidation of NO
maintains high N2 yield. This effect also results in high conversion of excess
hydrocarbons, which would be an added benefit of the system in application to
real exhaust.
176
Pd/SZ catalysts are subject to deactivation by water vapor in the feed
stream. Examination of H2O temperature programmed desorption of DRIFTS
indicates that this effect is due to a competitive adsorption.
H2O interacts
strongly with surface acid sites, but appears to fully desorb at 450°C. Because
H2O deactivation does not appear to be due to an irreversible structural change
in the catalyst it may be possible to modify surface properties to overcome it.
Initial examination of SZ supports with higher surface acidity appears promising.
Improved activity in the presence of H2O was observed.
9.2 Recommendations
Further work of incipient wetness prepared Pd/SZ catalysts should be
focused on the improvement of tolerance to water vapor and the examination of
long term catalyst stability.
1. Initial results have shown that an increase in sulfur loading results in
improved NOx reduction performance. Further characterization of high
sulfur loading catalysts should be performed. IR analysis of surface
sulfate groups should be performed and compared to results
previously obtained.
2. The thermal stability of the high sulfur loading samples should also be
tested through temperature programmed techniques.
177
3. Alternative methods of surface modification may also be possible.
Future catalyst testing should incorporate methods to increase surface
hydrophobicity, and decrease competitive adsorption phenomena.
4. The preparation of Pd/SZ based upon a monoclinic support may
improve long-term stability. Time-on-stream activity tests should be
performed for both the direct reduction of NO2 on Pd/SZ, and for the
reduction of NO in the dual catalyst system.
The investigated Pd/SZ catalysts prepared by sol-gel techniques were
active for the reduction of NO2 with CH4. Steady-state reaction studies of these
catalysts as a component of the dual catalyst system are still required. Testing of
Pd/SZ prepared by single step sol-gel techniques as part of the dual catalyst
aftertreatment system should be performed.
1. Evaluation under both test and simulated exhaust streams is required.
2. Activity in the presence of water vapor should be evaluated.
3. Time-on-stream studies will be required to evaluate the stability of the
tetragonal zirconia phase.
If sol-gel prepared catalysts continue to show promise in the dual catalyst
system then extensive opportunities exist to made improvements in their physical
properties.
1. Stability of the tetragonal zirconia phase may be improved through
doping with yttrium or iron.
178
2. Modification
of
sol-gel
preparation
parameters
such
as
pH,
temperature, hydrolysis agent, and hydrolysis ratio are known to
directly affect physical catalyst properties such as surface area and
pore size.
Catalyst preparation studies of these variables, in
combination with the addition of stabilizers, may be used to develop
more active and stable catalysts.
179
APPENDIX A
LIST OF ACRONYMNS
DRIFTS – Diffuse Reflectance Infrared Fourier Transform Spectroscopy
DSC – Differential Scanning Calorimetry
NOx – Nitrogen Oxides
TGA – Thermo Gravimetric Analysis
TPD – Temperature Programmed Desorption
TPR – Temperature Programmed Reduction
TPRxn – Temperature Programmed Reaction
XRD – X-ray Diffraction
XPS – X-ray Photoelectron Spectroscopy
180
APPENDIX B
SAMPLE CALCULATIONS
Calculating Reaction Results
NO and NO2 concentrations were read directly from the NOx analyzer.
Concentrations of O2, N2, N2O, CH4, C2H6, C3H8, CO, and CO2 were
calculated based on gas chromatograph response factors.
GC R.F. =
Known Gas Concentration
⎛ 7
⎞
⎜ ∑ Integrated Peak Area n ⎟ 7
⎝ n
⎠
Gas concentrations were then calculated as:
Concentration = (Difference in GC Area Between Feed and Outlet) × (GC R.F.)
181
Calculating Conversion
Conversion = 1 −
(Feed Gas Concentration ) − (Outlet Gas Concentration )
(Feed Gas Concentration )
Calculating Selectivity
Selectivity =
(Concentration of Species in Desired Product )
(Concentration of Species in All Products )
Calculating Yield
Yields were calculated for N2, N2O, and NO based on nitrogen.
Yield =
(Concentration of Species as Desired Product )
(Concentration of Species as Feed Componant )
Calculating Component Balances
Species balances were calculated based on atomic nitrogen,
oxygen and carbon. Balances should equal 1.
Balance =
(∑ Amount of species in all outlet gases)
(∑ Amount of Species in all inlet gases)
182
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