Indian Journal of Chemistry
Vol. 49A, April 2010, pp. 401-406
CuO, K2O and V2O5 supported on ceria-titania: Synthesis, characterization and
application for diesel soot combustion
K Joseph Antony Raj & B Viswanathan*
National Centre for Catalysis Research, Indian Institute of Technology Madras, Chennai 600 036, India
Email: [email protected]
Received12 January 2010; revised and accepted 16 March 2010
Diesel soot oxidation with CuO/V2O5/K2O/CeO2-TiO2 catalyst samples has been studied. Mixtures of the catalyst with
diesel soot lower the oxidation temperature from 601 to 427°C. TG/DTA studies of soot oxidation show that K-Cu-V
modification improves the low temperature activity in air. Sample B with 12.5 % K2O has been found to be most active with
peak activity at 427 °C. The activity further improved on incorporation of K-Cu-V, due to the mobility of potassium on
melting. Chemical interactions between the K2O and titania result in formation of potassium titanate. In addition, the XRD
patterns obtained for samples C and D loaded with higher concentrations of potassium show the formation of solid solutions
with the oxides of Ce, K and V. Nevertheless, the variation of potassium content has a limited effect on soot ignition
temperature. The catalyst with 12.5 wt. % of K2O, 7.5 wt. % of V2O5 and 2 wt.% of CuO exhibits complete combustion at
Tmax = 427 °C.
Keywords: Diesel soot oxidation, Catalytic combustion, Ceria, Titania, Mixed oxides, Catalysts
IPC Code: Int. Cl.9 B01J21/00
The emission of the various pollutants from the
exhaust gases of the different energy sources leads to
serious atmospheric pollution and climate change1,2.
Diesel engines emit carbonaceous particulates in
addition to other harmful substances such as
hydrocarbons, nitrogen oxides and CO. These
carbonaceous particulates, referred to as "diesel soot"
are particularly rich in carcinogenic polynuclear
hydrocarbons, namely, benzopyrene and nitropyrene.
The Cu-based catalysts are active for oxidation of
graphite3-5 and its catalytic role has been explained by
a redox mechanism in which CuO oxidizes the
graphite and is then regenerated by oxygen.
Clambelli et al.6 have used potassium as a promoter to
enhance the activities of Cu-V catalysts. Peralta et al.7
have suggested that potassium in K/CeO2 catalyst
may form a carbonate-type intermediate with the
partially oxidized soot and the high volatility of
K may improve the effective contact between
active sites and soot. Potassium compounds show
considerable
activity
towards
diesel
soot
combustion8-11.
CeO2 is used in a well-known three-way catalyst
for CO, HC, and NOx abatement and in catalysed soot
filters for elimination of the soot particulates12,13. The
redox properties of CeO2 will act as active centers for
the oxidation of the soot particulates and lead to an
efficient oxidation. CeO2 alone as a catalyst or as a
support in passive regeneration of particulate filter is
probably of little interest because of its low textural
stability for the high-temperature reactions usually
encountered in the exhaust gases. When exposed to
high temperature, the surface area of CeO2 decreases
drastically and at the same time loses its redox
properties and oxygen storage ability. Modification of
CeO2 with various ions is known to improve the
stability towards sintering and oxygen storage
activity12.
Modification by doping with a transition metal and
rare earth oxides will stabilize the surface area and
improve the redox and/or oxygen storage properties of
CeO2. Recently, it has been shown that the La3+
modified CeO2 catalysts can improve the soot
oxidation activity through the participation of the
lattice oxygen14,15. It has been reported that
Ce0.5Zr0.5O2 presents better catalytic activity for soot
oxidation than pure CeO216. Some other studies on
soot oxidation over PbOx/CoOx17, CsNO3/ ZrO218,
Pt20,
molten
salts21,
Mo/Al2O3,
V/Al2O319,
22
14,15,23
perovskites and different metal oxides
have
also been reported. Some studies suggest that titania
as support shows poor performance for diesel soot
402
INDIAN J CHEM, SEC A, APRIL 2010
oxidation24,25. van Doorn et al.26 have studied the
catalytic role of several metal oxides in soot
combustion by O2 and concluded that Al2O3 and SiO2
have no catalytic effect while TiO2 and ZrO2 have
moderate activity, and, CeO2, La2O2CO3 and V2O5
exhibit substantial activity for soot combustion. Many
catalytic systems developed so far are sensitive to
sulfur and are irreversibly deactivated by sulfur
compounds in diesel exhaust, even at low
concentrations27,28. Titania is thermally stable and
highly resistant to sulphur compounds.
The aim of the present study is to explore the
catalytic activity of high surface area titania-ceria and
the dispersion of Cu/K/V on titania-ceria carrier for
diesel soot oxidation with air. The reason for selecting
titania as the first catalytic support is to ascertain if
the performance of titania support is as poor as it has
been claimed and also whether it is resistant to
sulphur compounds. Ceria serves as a second catalyst
support and oxygen storage device, while vanadium
and copper serve as catalytic promoters. Mixed metal
catalysts are effective in lowering the light-off
temperature of diesel soot, and hence, a series of
Cu/K/V/CeO2/TiO2
catalysts
with
different
concentrations of K2O and CuO have been examined.
The samples have been characterized by XRD, XRF
and BET-specific surface area. Their activity on
diesel soot oxidation has been studied as a function of
amount of K2O and CuO.
Materials and Methods
Carbon
CDX-975
(Columbian
Chemicals
Company),
Carbon
VulvanXC-72
(Cabot
Corporation, USA), potassium hydroxide (Qualigens),
cuprous chloride (Merck), ammonium metavanadate
(Qualigens), and cerium nitrate hexahydrate (Aldrich)
were used without further purification. Doubly
distilled water was used as a solvent.
Preparation and characterisation of catalyst samples
The catalyst support hydrated titania was prepared
by literature method29. Hydrated titania (1 g) was
dispersed in a beaker containing 15 ml of doubly
distilled water and stirred for 10 minutes with a
magnetic stirrer. To this, was added 0.626 g of cerium
nitrate hexahydrate and mixed well for 15 minutes.
Thereafter, ammonium metavanadate (0.13 g),
cuprous chloride (0.045 g) and potassium hydroxide
(0.1 g) were added one after the other at an interval of
10 minutes to ensure appropriate mixing of each salt.
The temperature of the slurry was increased from
room temperature to 80°C while stirring and allowed
to dry at this temperature. Furthermore, the sample
was dried at 105°C for 12 hours and subsequently
calcined at 700°C for 2 hours. Similarly, samples B,
C, and D were prepared by varying the quantity of
KOH and sample E and F were prepared by varying
the quantity of cuprous chloride. The composition of
the catalyst samples are presented in Table 1. The
obtained samples were evaluated for the activity of
diesel soot oxidation.
Wide-angle XRD patterns for the calcined
materials were obtained on a Rigaku Miniflex II,
using Cu Kα irradiation. The composition of these
catalysts was analyzed using Rigaku XRF-Primini
spectrometer. The BET-specific surface area was
measured on Micromeritics ASAP-2020 surface area
and porosity analyzer.
Preparation of soot-catalyst samples for soot oxidation study
The soot was collected from the exhaust of the
diesel vehicles and was dried in an oven for 24 h at
120°C. The obtained soot was found to have a surface
area of 47 m2/g and 0.4 wt.% sulfur content. This soot
was used for preparing mixtures of soot and catalyst.
The surface area30 of CDX-975 (240 m2/g), VXC-72
(250 m2/g) and composition of model soot31 are
reported elsewhere.
Each catalyst sample was mixed with diesel soot at
a weight ratio of 1:0.1 for catalyst:soot. The sootcatalyst samples with lower soot content were also
prepared and studied. The catalyst and soot were
mixed in a vial with a spatula before being transferred
to a crucible. This mixing ensured a loose sootcatalyst contact. The soot-catalyst with tight contact
mixtures were obtained by careful grinding in an
agate mortar. Although the tight contact combustion
is too rigorous to compare with what is essentially
Table 1 – Composition and BET surface area of the samples
Compositiona (wt.%)
Sample
A
B
C
D
E
F
a
BET surface
area (m2/g)
TiO2
CeO2
V2O5
CuO
K2O
63.5
59.5
52.9
43.4
61.9
55.9
19.7
18.5
16.4
13.4
19.1
17.2
8.0
7.5
6.7
5.5
7.8
7.1
2.1
2.0
1.8
1.4
10.2
18.6
6.7
12.5
22.2
36.3
1.3
1.2
Composition determined by XRF.
19
16
7
4
30
27
RAJ & VISWANATHAN: DIESEL SOOT OXIDATION WITH CuO/V2O5/K2O/CeO2-TiO2 CATALYSTS
possible in soot trap, this study forms a basis for
activity screening of the catalyst.
Catalytic activity test
The loose and tight contact catalyst-diesel soot
mixtures were evaluated by TG/DTA studies carried
out on a Perkin-Elmer thermal analyzer for
combustion of the soot. The samples were heated
from 40 to 650°C at a rate of 10°C/min using
50 ml/min. of air as the combustion gas to identify the
soot oxidation potential of the catalyst samples. The
stability of the catalysts was tested by running
repeated experiments where the used catalyst was
further mixed with fresh soot and re-tested until eight
successive experiments. The activity of a catalyst is
defined by the combustion peak maximum, which is
the temperature of the TGA curve where 100% (Tmax)
of the soot was oxidized. To verify the reproducibility
of results, all TGA experiments were repeated twice.
The combustion onset temperature, T50, and Tmax
values showed a variation of less than 5°C for
repeated measurements.
Results and Discussion
Characterization of the catalysts
The XRD patterns of the catalyst samples calcined
at 700°C for 2 hours in air are shown in Fig. 1 and
have been indexed and compared with standard
JCPDS cards. The samples were calcined at 700 °C as
the maximum application temperature is 650°C. The
XRD patterns obtained for samples A and B showed
the presence of anatase titania. The peaks observed at
403
2θ of 25.3° 37.9°, 38.78° and 48.1° are characteristic
of anatase phase32. The samples A and B showed a
peak at 2θ of 24°, which may be due to the formation
of titanium vanadium oxide (PDF#: 850383). This
peak is absent in samples C, D, E and F, and may be
due to the presence of higher concentration of K2O or
CuO. The broad peak obtained at 28.5° is due to the
presence of ceria (PDF#: 810792) in the samples. The
peaks observed at 2θ = 28.3° and 40.5° for the
samples C and D are due to the formation of
potassium titanate (PDF#: 470690). Concentrations
lower than 22.2 wt.% of K2O did not affect the
formation of potassium titanate. The increase in
concentration of K2O from 22.2 to 36.3 wt.%
decreased the intensity for the anatase phase peak at
2θ of 25.3° and enhanced the intensity of the peak at
28.3°. The shoulder peak appeared at 2θ of 28.3° may
be due to the presence of CeO2 and formation of
KVO3 (PDF#: 700677). The samples C and D with
higher concentrations of K2O showed additional peaks
at 2θ of 29.74° and 30.8°, which may be essentially
due to the formation of potassium titanium vanadium
oxide (PDF#: 480713). The increase in CuO
concentration from 2 wt.% to 10 – 18.6 wt.% for the
samples E and F enhanced the peak intensity at 2θ of
28.5° and converted the twin peaks into a broad peak
at 2θ of 33°, 37.8° and 47.8°. In addition to these
observations, the absence of any new peaks for the
increase in concentration of CuO for the samples E
and F reveals the formation of solid solution with
ceria, vanadia, and titania. The presence of low
melting compounds such as vanadia and potassium
did not lead to the formation of rutile phase which is
evident from the results of XRD. The composition
and BET-specific surface area of the catalyst samples
are presented in Table 1. The surface area values
showed a significant drop with the concentration of
potassium and attained a minimum for sample D with
a surface area of 4 m2/g. This may be due to the
formation of potassium titanate in the samples C and
D. The increase in CuO concentration for the samples
E and F showed a significant enhancement in surface
area.
Soot oxidation on TiO2 and CeO2-TiO2 mixture
Fig. 1—XRD patterns of samples A-F calcined at 700 oC for 2 h.
The soot ignition temperature was measured on a
TGA instrument. The soot collected from a vehicle
diesel engine was dry-mixed with the catalyst samples
in a ratio of 0.1:1 (by wt). The mixture was ground
with a pestle in a mortar for maximum blending.
Thereafter, 5 mg of the mixture was loaded on a
INDIAN J CHEM, SEC A, APRIL 2010
404
TG/DTA sample crucible for analysis in a stream of
air. The onset temperature for beginning of weight
loss was used as the soot ignition temperature. The
loading of 10% diesel soot in sample B by loose
contact mixing showed an elevation of about 120°C
on the combustion onset temperature, T50 and Tmax.
This ensures the physical contact between soot and
catalyst, which is essential for lowering the
combustion temperature. The hydrated titania having
a BET-surface area of 275 m2/g was used as a support
for the preparation of mixed metal oxide catalysts and
showed a Tmax for diesel soot at 600°C. Similarly, the
hydrated titania modified with 20 wt.% ceria showed
a Tmax of 598°C, which is not significantly different
from the result obtained for hydrated titania. Hence,
the support alone is not sufficiently active to be a
suitable catalyst for soot oxidation. This observation
demonstrates the significance of addition of K, V and
Cu in CeO2-TiO2.
Effect of composition and loadings on soot combustion
The effect of diesel soot combustion obtained on
samples A – F is given in Table 2. The results show
that soot ignites in the range of 276 - 372°C for the
samples. Half of the soot present in the soot-catalyst
mixture (T50) is combusted in the temperature range of
359 - 442°C, while complete combustion of soot is
attained in the temperature range of 427 – 504°C. The
surface area of the samples is not found to show any
specific effect on the combustion of the soot. Among
the samples, sample E showed higher surface area
although the sample demonstrated a greater light-off
and complete combustion temperature than the other
samples. Increase in quantity of K2O initially showed a
lowering of light-off temperature for sample A and B;
nevertheless, further increase in K2O for samples C
and D led to increase in onset combustion temperature,
T50 and Tmax. This shows that the optimum quantity of
K2O for the mixed metal oxide catalyst is between 12
and 13 wt.%. If the potassium content is too high, then
ceria/titania is fully covered, and its well-known redox
Table 2 – Effect of diesel soot combustion on samples A – F
Sample
Onset temp.
(oC)
T50 (oC)
Tmax (oC)
A
B
C
D
E
F
307
276
315
332
370
358
396
359
411
420
442
418
461
427
470
479
504
487
capacity is greatly suppressed. This is observed when
the potassium content is increased to above 12.5 wt.%.
Similar results were previously found on K/La2O333
and K/CuFe2O434 catalysts. They also found an
optimum composition of potassium above which the
activity decreased. It has been reported7 that potassium
may act to form a carbonate-type intermediate with the
partially oxidized soot, which subsequently
decomposes to CO2. Therefore, if the CeO2 redox
capacity and/or the potassium carbonate formation
capacity are lost, then a decrease in activity can be
anticipated.
The quantity of CuO was increased from 2 wt. % to
18.6 wt.% in the catalyst samples to study its effect on
onset combustion temperature, T50 and Tmax. A CuO
content of 2 wt.% (samples A and B) showed
effective soot oxidation than the samples with greater
CuO content of 10-18.6 wt.% (samples E and F). The
results shown in Table 2 demonstrate the advantage of
using mixed oxide catalysts for diesel soot
combustion. The reason for using mixed metal oxides
is that each metal oxide acts on the soot in a different
temperature region and consequently the metal oxide
that acts in the first temperature region conditioning
the soot particulate for an effective reaction with the
second metal oxide. For example, the catalyst
containing oxides of K and V, K2O interacts with the
soot and keeps it dispersed in the oxidizing fuel/air
charge. As the temperature begins to fall from peak,
vanadium oxide becomes the dominant oxidation
catalyst leading to lowering the light-off temperature,
thereby catalyzing oxidation at lower temperatures. If
K2O did not interact with the soot before it aggregated
to larger particle sizes, then the activity of other
metals present in the catalyst is anticipated to be
greatly lowered thus decreasing the efficiency of the
catalyst.
The significance of using the mixed metal oxide
catalysts in the present study to burn out diesel soot is
that the alkali metal is employed to inhibit the
particulate agglomeration and to enhance the surface
activity of V and Cu by bringing the catalyst into
tight-contact with the soot particulates. The potassium
present in the samples causes the catalyst to partially
melt7 when the soot burns. This partial melting results
in more effective contact with the soot and thereby
efficient combustion, resulting in a significant
lowering of the soot ignition temperature. The Cu and
V have been added to increase the rate of catalytic
oxidation by lowering the particulate light-off
RAJ & VISWANATHAN: DIESEL SOOT OXIDATION WITH CuO/V2O5/K2O/CeO2-TiO2 CATALYSTS
temperature. The catalytically active particles
comprise Group I-B metals such as Au, Ag, and Cu.
The present study employed Cu in the catalytic
system as the other two metals are expensive. The
Group V-B metal salt, V was included in the mixed
metal oxide system as it apparently covers the surface
of the CuO particles. The other advantage of
including V in the catalyst is that it prevents the
oxidation of SO2. The refractory metal oxide, titania
was used as a primary support to make the catalyst as
sulphur resistant. Hence, the catalytic material
employed in the present study is specially formulated
to form multi-metal oxide based catalytic system.
The effect of combustion of various loadings of
diesel soot on sample B was studied and the results are
given in Table 3. The results demonstrate a significant
lowering of onset combustion temperature, T50 and
Tmax, with decrease in wt.% of soot loading on the
catalyst sample B. The T50 and combustion onset
temperature showed only 2.8 – 5% elevation of
temperature for the 100% soot loading as compared to
the loading of 10%. Nevertheless, the lower loading of
soot is anticipated to improve the life of the catalyst.
Combustion of commercial carbon and diesel soot
The diesel soot oxidation performance was studied
on sample B using the composite diesel soot collected
from a few vehicles running on diesel. In order to
evaluate the oxidation performance of diesel soot on
sample B, the oxidation of commercial carbons such
as CDX-975 and VXC-72 was studied on sample B at
10% loading. The Tmax, T50 and combustion onset
temperature show the following trend: Carbon
CDX-975 > Carbon VXC-72 > diesel soot on
sample B. The CDX-975 showed a complete
combustion at 464°C, VXC-72 at 452°C and diesel
soot at 427°C on sample B.
Reusability of the catalyst
The recyclability of sample B on the combustion of
diesel soot was studied for eight continuous runs. The
catalyst sample obtained after the first run of soot
oxidation was mixed again with 10% of diesel soot
and subsequently tight-contact mixture was prepared
by grinding in an agate mortar and subsequently soot
oxidation was evaluated. This procedure was repeated
for eight times to study the effect of re-usability of the
catalyst. The Tmax result obtained for eight runs on
sample B showed the shifting of combustion peak
maximum from 427 to 499°C, which, accounts about
17% deactivation of the catalyst. The loss in activity
405
Table 3 – Effect of combustion on various loadings of diesel soot
on sample B
Diesel soot
loading (%)
Onset temp.
(oC)
T50
(oC)
Tmax
(oC)
10
25
50
100
276
278
280
284
359
366
370
378
427
434
441
448
of the catalyst on repeated runs may be attributed to
the partial removal of potassium from the catalyst and
formation of sulfates of potassium and cerium.
Although there was a change in catalyst composition
due to the removal of about 1 wt.% potassium, the
surface area showed no significant change. These
results demonstrate that there is a deactivation of the
catalyst however the activity retention of about 83%
shows that the catalyst may be effectively used for
longer time under lower soot loadings.
Conclusions
Diesel soot oxidation with CuO/V2O5/K2O/CeO2TiO2 catalyst samples at a ratio of 1:0.1 for catalyst
and soot was studied. Mixtures of the catalyst with
diesel soot lowered the oxidation temperature from
601 to 427°C. TG/DTA studies on soot oxidation
showed that the K-Cu-V modification improved the
low temperature activity in air. Sample B was found
to show greater activity than the other catalyst
samples with a peak activity at 427°C. Further, the
activity improvement was accomplished with the
incorporation of K-Cu-V, attributed to the mobility of
potassium upon melting7. Chemical interactions
between the K2O and titania resulted in potassium
titanate formation. In addition, the XRD patterns
obtained for samples C and D loaded with higher
concentrations of K showed the formation of solid
solutions with the oxides of Ce and V. Nevertheless,
the variation of K content had a limited effect on soot
ignition temperature. The catalyst with a 12.5 wt.% of
K2O, 7.5 wt.% of V2O5 and 2 wt.% of CuO exhibited
a complete combustion at Tmax = 427°C.
Acknowledgement
The authors acknowledge the Department of
Science and Technology, Government of India for
funding the National Centre for Catalysis Research
(NCCR) at IIT Madras, Chennai, India. Thanks are
also due to M/s. Shell India (P) Limited for a
fellowship to one of the authors (KJAR).
406
INDIAN J CHEM, SEC A, APRIL 2010
References
1 Finlayson B J & Pitts J N, Science, 276 (1997) 1045.
2 Somers C M, McCarry B E, Malek F & Quinn J S, Science,
304 (2004) 1008.
3 Heintz E A, & Parker W E, Carbon, 4 (1966) 473.
4 McKee D W, Carbon, 8 (1970) 131.
5 McKee D W, Carbon, 8 (1970) 623.
6 Ciambelli P, Corbo P, Parrella P, Scmlo M & Vaccaro S,
Thermochim Acta, 162 (1990) 83.
7 Peralta M A, Milt V G, Cornaglia L M & Querini C A,
J Catal, 242 (2006) 118.
8 Jelles S J, van Setten B, Makkee B M & Moulijn J A, Appl
Catal B, 21 (1999) 35.
9 Fino D, Russo N, Saracco G & Speechia V, J Catal, 217
(2003) 367.
10 McKee D W, Fuel, 62 (1983) 170.
11 Shangguan W F, Teraoka Y & Kagawa S, Appl Catal B, 16
(1998) 149.
12 Matsumoto S I, Catal Today, 90 (2004) 183.
13 Miro E E, Ravelli F, Ulla M A, Cornaglia L M & Querini C
A, Catal Today, 53 (1999) 631.
14 Bueno Lopez A, Krishna K, Makkee M & Moulijn J A,
J Catal, 230 (2005) 237.
15 Bueno Lopez A, Krishna K & Makkee M & Moulijn J A,
Catal Lett, 99 (2005) 203.
16 Wu X, Liu D, Li K & Weng D, Catal Commun, 8 (2007)
1274.
17 Genc V E, Altay F E & Uner D, Catal Today, 105 (2005)
537.
18 Galdeano F N, Ponzi M I & Lick I D, Thermochim Acta, 421
(2004) 117.
19 Leocardio I C L, Braun S & Schmal M, J Catal, 223 (2004)
114.
20 Oi Uchisawa J, Wang S, Nanbaa T, Ohi A & Obuchi A, Appl
Catal B, 44 (2003) 207.
21 van Setten B A A L, Bremmer J, Jelles S J, Makke M &
Moulijn J A, Catal Today, 53 (1999) 613.
22 Biamino S, Fino P, Fino D, Russo N & Badini C, Appl Catal
B, 61 (2005) 297.
23 Oi Uchisawa J, Obuchi A, Enomoto R, Liu S, Nanba T &
Kushiyama S, Appl Catal B, 26 (2000) 17.
24 Neeft J P A, van Pruissen O P, Makkee M & Moulijn J A,
Appl Catal B, 12 (1997) 21.
25 Braun S, Appel L G & Schmal M, Appl Surf Sci, 201 (2002)
227.
26 van Doorn J, Varloud J, Meriaudeau P, Perrichon V,
Chevrier M & Gauthier C, Appl Catal B, 1 (1992) 117.
27 Farrauto R J & Wedding B, J Catal, 33 (1973) 249.
28 Iwamoto M, Yahiro H, Shundo S, Yu Y & Mizuno N, Appl
Catal, 69 (1991) L15.
29 Joseph Antony Raj K, Ramaswamy A V & Viswanathan B,
J Phys Chem C, 113 (2009) 13750.
30 Himakumar L, Viswanathan B & Srinivasa Murthy S, Bull
Catal Soc India, 5 (2006) 45.
31 Dhakad M, Rayalu S S, Rakesh Kumar, Pradip Doggali,
Bakardjieva S, Subrt J, Mitsuhashi T, Haneda H, & Nitin
Labhsetwar, Catal Lett, 121 (2008) 137.
32 Joseph Antony Raj K & Viswanathan B, Indian J Chem, 48A
(2009) 1378.
33 Milt V G, Pisarello M L, Miro E E & Querini C A, Appl
Catal B, 41 (2003) 397.
34 Shangguan W F, Teraoka Y & Kagawa S, Appl Catal B, 16
(1998) 149.