Available online at www.sciencedirect.com
Catalysis Communications 9 (2008) 1704–1708
www.elsevier.com/locate/catcom
Methane oxidation by NO and O2 from reverse spillover
on alumina supported palladium catalysts
Rui Marques a, Sandra Capela b, Stéphanie Da Costa c, Franck Delacroix d,
Gérald Djéga-Mariadassou a, Patrick Da Costa a,*
a
Laboratoire Réactivité de Surface, CNRS UMR 7609, Case 178, U.P.M.C. Paris 6, 4 Place Jussieu, 75252 Paris Cedex 05, France
b
Departamento de Engenharia Quı́mica – Instituto Superior Técnico, Avenue Rovisco Pais, 1049-001 Lisboa, Portugal
c
Gaz de France, Direction de la recherche, 361 Avenue du Président Wilson, B.P. 33, 93211 La Plaine Saint-Denis Cedex, France
d
ADEME, 2 Square La Fayette, 49004 Angers, France
Received 19 November 2007; received in revised form 24 January 2008; accepted 26 January 2008
Available online 1 February 2008
Abstract
Pd/Al2O3 was tested for the selective reduction of NO and simultaneous oxidation of methane by NO and O2 from reverse spillover
on alumina supported palladium. The results of CH4/Ar and CH4/NO/Ar experiments clearly demonstrated the two sources of surface
oxygen species able to oxidize the methane. When oxygen from the reverse spillover is totally consumed, the reaction NO/CH4 is stoichiometric, all oxygen come from the NO dissociation. The role of methane is similar to CO in TWC since methane removes the oxygen,
from NO dissociation on the cationic sites of Pd atoms, and permits the reaction to proceed further.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Methane; Oxidation; NO; Oxygen; Palladium
1. Introduction
Environment concerns of NOx reduction and removal
of hydrocarbons are a major challenge with strong interest in the academic and industrial communities. The use
of hydrocarbons as reducing agents for NOx was first
reported in the 1970s [1,2]. Depending on the application,
three-way catalysis (TWC) in lean burn conditions or
removal of NO in the presence of an excess of oxygen
(deNOx), various solutions have been proposed. The
SCR of NOx by methane is very attractive in the stationary sources fuelled by natural gas. Furthermore, natural
gas vehicles (GNV) are highly demanded by the consumers and removing NOx and methane from exhaust gases is
a concern. CH4/NO reaction have been studied over various catalysts. It was found that increasing the carbon
*
Corresponding author. Tel.: +33 1 44273630; fax: +33 1 44276033.
E-mail address: [email protected] (P. Da Costa).
1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2008.01.027
number resulted in a temperature decrease for NO conversion [2]. Over Pt/SiO2, it was observed [3] that NH3
was the major product at 350 °C for CH4/NO reaction.
On the contrary, less ammonia is obtained over Ru/
SiO2. A comparative study of the reduction of NO by
CH4 on Pt, Pd and Rh catalysts was performed by Burch
and Ramli [4]. Under fuel-rich conditions from natural
gas engine, Pt was the most active catalyst for the CH4/
NO reaction. Over platinum group metal-based catalysts,
in a zero valent oxidation state, the mechanism of NOx
reduction is understood and is well established [5]. On
other catalytic materials, other mechanisms have been
suggested [5,6]. More recently, on cationic species, a general three-function model for deNOx catalysis was proposed [7–9]. On this model, the last overall reaction is
the dissociation of NO and the subsequent oxidation of
oxidized species by the atomic oxygen left by NO during
the dissociation [9]. This manuscript deals with the
reaction between atomic oxygen and methane, the atomic
R. Marques et al. / Catalysis Communications 9 (2008) 1704–1708
oxygen coming from NO or O2 present on the catalyst
surface.
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3. Results and discussion
3.1. Highly dispersed palladium species
2. Experimental
The alumina (c-Al2O3, SBET = 190 m2 g 1, Pore
volume = 0.7 cm3 g 1) was provided by Procatalyse.
Pd(0.5 wt%)/Al2O3 was prepared by impregnation of the
alumina by an aqueous solution of Pd(NH3)4(NO3)2. The
suspension was maintained under stirring at 50 °C for
3 h, after evaporation of water, the catalysts were dried
overnight at 120 °C.
2.2. Characterization
The catalysts were characterized by Transmission
Electron microscopy (TEM) and UV–visible–near-infrared
(NIR). Metal contents were determined by chemical
analyses (CNRS–Vernaison). High-resolution transmission
electron microscopy was performed to determine the particle size of palladium particles and to check their dispersion.
HRTEM studies were performed on a JEOL-JEM 100 CXII
apparatus. EDS analysis was performed with the same
apparatus using a LINK AN 10000 system. EDS analyses
were obtained on large domains of samples (400 nm 533 nm). Diffuse reflectance spectra were recorded at room
temperature between 190 nm and 2500 nm on a Varian
Cary 5E spectrometer equipped with a double monochromator and an integrating sphere coated with polytetrafluoroethylene (PTFE). PTFE was the reference.
Prior to catalytic runs, the samples were calcined in situ
in dry air at 500 °C (5 °C min 1) for 2 h with a flow rate of
500 mLNTP min 1 g 1 catalysts. For the sake of comparison, another catalyst was prepared and reduced in 5%
H2/Ar at 500 °C (5 °C min 1) for 2 h, the final oxidation
state of palladium is then zero.
TPSR and steady-state experiments were performed
using a U-type quartz reactor. The total flow was 250
mLNTP min 1 (GHSV = 40,000 h 1). The composition of
the CH4–O2 reacting mixture was 1500 ppm CH4 and 7%
O2, CH4–NO reacting mixture was 1500 ppm CH4 and
150 ppm NO and 0% O2 or CH4 reacting mixture was
1500 ppm CH4 using Ar as carrier gas. These reactants
were fed from independent gas cylinders (Air Liquide) of
Ar diluted gas mixtures. The reactor outflow was
continuously analysed using the combination of four different detectors. An Eco Physics CLD 700 AL chemiluminescence NOx analyser allowed the simultaneous detection of
both NO and NOx. Two Ultramat 6 IR analysers were
used to monitor N2O, CO and CO2. A FID detector
(Fidamat 5A) was used to follow the concentration of
hydrocarbons and a micro chromatograph Variant CP4900 was used for N2 detection. The effluents streams were
routed to the mass spectrometer (Hiden Analytical) for
analysis.
Pd(0.5 wt%)/Al2O3 was characterized by XRD, no diffraction peaks of Pd was detected. Furthermore no crystallized phases were detected by TEM, although EDS analyses
showed that Pd/Al ratio is constant. One can conclude that
the palladium species are then highly dispersed on the support. Furthermore, UV–visible–NIR diffuse reflectance
was performed to characterized theses latter species. The
catalyst Pd/Al2O3 presents a band at 420 nm, this band is
characteristic of isolated PdII+ in an oxygen environment
[9]. No band characteristic of bulk PdO was detected.
As conclusion, by characterization methods one can
conclude that the high dispersed palladium species are
PdII+ particles surrounded by oxygen.
3.2. Methane oxidation by O2
TPSR of CH4/O2 as reacting mixture was performed
over Pd/Al2O3 (Fig. 1). One can see that methane oxidation, leading to CO2 starts at 230 °C, no CO is observed.
The maximal conversion (100%) of methane is obtained
at 490 °C.
3.3. Methane oxidation by NO (CH4/NO/Ar reaction)
Fig. 2. reports the results of CH4/NO TPSR on (a) oxidized Pd/Al2O3; (b) reduced Pd/Al2O3. In Fig. 2a, at room
temperature NO chemisorbs without any dissociation. NO
adsorption is not possible since after the pretreatment over
dry air the surface of the catalyst is saturated by oxygen.
Furthermore, no N2O and N2 were detected during the
adsorption process. The NO adsorbed at RT, desorbs as
NO between 240 °C and 275 °C. At 275 °C methane oxidation begins. The methane oxidation under rich conditions
1500
CH4(FID)
1200
Concentration (ppm)
2.1. Catalyst preparation
900
600
300
CO2
0
50
100 150 200 250 300 350 400 450 500
Temperature (°C)
Fig. 1. Evolution of methane and CO2 during the methane oxidation on
Pd/Al2O3 catalyst.
1706
a
R. Marques et al. / Catalysis Communications 9 (2008) 1704–1708
1500
300
CH4(FID)
1200
200
900
NOx
150
NO
CO
100
600
CO2
300
N2 O
50
CH4(FID), CO, CO2 (ppm)
N2O,NO, NO2, NOx (ppm)
250
NO2
0
50
0
100 150 200 250 300 350 400 450 500
Temperature (°C)
b
300
1500
CH4(FID)
3.4. Methane oxidation by O2 from reverse spillover
(CH4/Ar)
250
200
900
NOx
150
600
NO
100
CO
CH4(FID), CO2, CO (ppm)
1200
N2O,NO, NO2, NOx (ppm)
dissociation and methane oxidation (Figs. 1 and 2). The
methane consumption increases with the temperature. At
500 °C, 550 ppm of methane is consumed. During the
TPSR (CH4/NO/Ar), the products of methane oxidation
are CO2 and CO. CO2 is detected between 275 °C and
425 °C. CO production starts at 325 °C and increases with
the temperature. At 500 °C, 550 ppm of CO is detected.
During the TPSR, the methane oxidation reaction is not
stoichiometric, between NO and methane. The same results
are obtained on both reduced and oxidized catalysts. This
is evidence of another oxygen source during the reaction.
Similar results had also been reported for partial oxidation
of methane, in which an oxygen spillover is included in the
reaction mechanism [12–15]. Thus, the additional oxygen
source is O2 and OH spillover from the Al2O3. On metal
supported catalysts, isotopic exchange mechanism occurs
and was described over Rh supported alumina by Martin
and Duprez [13]. Furthermore the authors showed that
the rate of oxygen surface diffusion increases with the oxide
surface basicity (OH).
300
50
N2 O
NO2
0
50
CO2
0
100 150 200 250 300 350 400 450 500
Temperature (°C)
CH4/Ar reaction was performed over Pd/Al2O3 (Fig. 3).
Prior TPSR, the catalyst was calcined in dry air. During the
TPSR, the methane oxidation starts at 225 °C and leads to
CO2 and CO formation. The CO2 is detected between
225 °C and 425 °C. The CO is detected at 310 °C and
increases during the TPSR (325 ppm at 500 °C). Similar
profiles of CO and CO2 were obtained in presence of NO
in the feed (CH4/NO/Ar). When the methane oxidation
starts, the catalyst surface is saturated by oxygen that
becomes from the calcination. Thus, the amount of O2
available allows the total oxidation of methane in CO2.
Since the oxygen adsorbed on surface decreases the concentration of COx(x=1or2) changes. Total oxidation in CO2
stops and CO becomes the major product of methane
Fig. 2. Evolution of reactants in the course of NO dissociation in presence
of methane over (a) oxidized Pd/Al2O3; (b) reduced Pd/Al2O3.
1500
Concentration (ppm)
CH4(FID)
should induce, under dynamic reaction conditions, differences in the oxidation state of the PdOx due to interactions
with the support [4]. Furthermore, from this moment NO
can now adsorb and dissociate due to the reactivity
between methane and oxygen surface species (*O). In this
transient experiment, the NO dissociation leads to the formation of N2O, between 275 °C and 325 °C. This means
that nitrogen monoxide adsorbed on an isolated free site
reacts with an adjacent adsorbed nitrogen atom to yield
N2O [9–11]. At 325 °C, 100% NO conversion is achieved
and the catalyst is 100% selective in N2 until 450 °C. At this
temperature, the PdII+ species are reduced in Pd0.
At higher temperature, as already proposed by Burch
et al. [11], the NO reduction leads to NH3. From this experiment, it is clear that the temperature is the same for NO
1200
900
600
CO
300
CO2
0
50
100 150 200 250 300 350 400 450 500
Temperature (°C)
Fig. 3. Evolution of reactants during the Methane oxidation by O2 from
reverse spillover (CH4 1500 ppm/Ar) on oxidized Pd/Al2O3.
R. Marques et al. / Catalysis Communications 9 (2008) 1704–1708
oxidation. The CH4/NO/Ar and CH4/Ar runs lead us to
conclude that during the TPSR, methane can be oxidized
by NO and/or by O2 from reverse spillover. Since in the
reaction the concentration in methane is 10 times more
than those of NO, enough reductant is available to achieve
100% of NO decomposition. However, only with these
results with CH4/Ar reaction, we are not able to conclude
on a real reverse oxygen spillover, because palladium can
be reduced during the reaction. To conclude isothermal
reaction was studied on the reduced catalyst.
3.5. Isothermal steady-state reaction (CH4/NO/Ar reaction)
Fig. 4 shows the TPSR from RT to 400 °C, i.e. 1.9 h of
run, and subsequent steady-state reaction at 400 °C. From
RT to 400 °C, the results obtained are equal to those presented in Fig. 2b. At 400 °C we can observe that 100% of
NO dissociation is already achieved and this conversion
remains constant during the all experiment. The maximum
of CO concentration is obtained at the beginning of isothermal reaction at 400 °C and then decreases to 150 ppm
at steady-state reaction. CO2 is observed during the TPSR.
In the subsequent steady-state reaction no more CO2 is
detected. After 10 h of run, the methane oxidation is stoichiometric between NO and methane. There is no other
source of O2, the oxygen provided from the reverse spillover is ended. All oxygen available to oxidize CH4 comes
from the NO. In steady-state reaction the amount of methane consumed is equal to the atomic O of the NO decomposition (150 ppm). The methane oxidation leads only to
CO. At 400 °C the selectivity of NO decomposition to N2
is 100%. N2O is only detected up to the temperature at
which NO conversion is complete. The amount of N2
detected in isothermal steady-state reaction is 75 ppm.
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Water formation is never detected, however, hydrogen
(m/z = 2) is detected during the reaction.
3.6. Overall mechanism
The formation of NH3 results from the combination of
N and H adsorbed, this recombination occurs at only at
high temperature [4], at 400 °C no ammoniac was detected.
At this temperature, the rate of formation of Hads must be
less than the rate of removal by Oads to form H2O. Since
water is not detected and hydrogen is, we can suppose that
methane reacts with water formed from the methane
oxidation and produce CO and hydrogen. In this reaction
methane is reformed with water as follows: CH4 + H2O =
CO + 3 H2. Consequently, a sequence of 3 equations can
be proposed.
σ
CH4 + 3 NO = CO +3/2 N2 + 2 H2O
1
CH4 + H2O = CO + 3 H2
2
3 CH4 + 3 NO = 3 CO +3/2 N2 +6 H2
In which, r is the stoichiometric number according to the
net reaction, and (') is net equation. To verify the second
overall step, the methane (1500 ppm) steam reforming reaction, in presence of 3% of water was performed. The products obtained are CO, CO2 and H2. In the methane
oxidation by NO in presence of water, CO2 is observed,
since CO can react with H2O to form CO2 and H2.
4. Conclusions
300
1500
CH4(FID)
1200
200
900
NOx
150
NO
NO
600
100
N2O
CH4(FID), CO, CO2 (ppm)
NO, NO2, NOx (ppm)
250
300
50
CO
CO
CO2
0
0
0
2
4
6
8
10
12
14
Time (h)
Fig. 4. Evolution of reactants during the methane oxidation by NO on
reduced Pd/Al2O3 in isothermal conditions (400 °C).
Pd(0.5 wt%)/Al2O3 was studied for the oxidation of
methane by NO and O2 from reverse spillover on alumina
supported palladium. The results of CH4/Ar and CH4/NO/
Ar experiments clearly demonstrated the two sources of
surface oxygen species *O able to oxidize the methane on
reduced or oxidized Pd/Al2O3 catalysts. When *O from
the reverse spillover is totally consumed, the methane oxidation is stoichiometric between NO and methane, each
oxygen comes from the NO dissociation.
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