Promotional effect of C2-C4 hydrocarbon on CH4 oxidation on sulfated Pt/γ-Al2O3 catalysts

Article abstract of DOI:10.1016/j.molcata.2007.11.003

The addition of the alkanes C₂H₆, C₃H₈, or n-C₄H₁₀ in the CH₄-O₂-SO₂ reaction feed resulted in an improvement of pre-sulfated 1% Pt/γ-Al₂O₃ activity on CH₄ oxidation in lean conditions. These results are explained assuming that (i) the energy generated during the oxidation of the alkanes may be transferred to activate the rate determining step of CH₄ oxidation, over the catalyst surface, increasing the reaction rate at lower temperatures and (ii) alkanes oxidation may lower the coverage of oxygen on the Pt surface, leaving surface active sites free for CH₄ adsorption, thus increasing its oxidation rate, since, oxygen species act as an inhibitor for the reaction at full coverage. n-C₄H₁₀ addition in the reaction feed generated the highest promotion on CH₄ complete oxidation. This promotional effect was further increased on pre-sulfated 2% Pt/γ-Al₂O₃. Results presented in this investigation suggest that small amounts of n-butane in the gas feed during CH₄-O₂ reaction over a pre-sulfated Pt/γ-Al₂O₃ catalyst may eliminate completely very low concentrations of methane emissions at low temperatures from lean-burn NGV exhausts.

Full text of DOI:10.1016/j.molcata.2007.11.003

Available online at www.sciencedirect.com
Journal of Molecular Catalysis A: Chemical 281 (2008) 179–183
Promotional effect of C2–C4 hydrocarbon on
CH4 oxidation on sulfated Pt/␥-Al2O3 catalysts
Griselda Corroa,∗, Carlos Canoa, J.L.Garcia Fierrob
a Instituto de Ciencias, Beneme´rita Universidad Auto´noma de Puebla,
14 Sur 6301, Puebla, Puebla 72570, Mexico
b Instituto de Cata´lisis y Petroleoqu´ımica, CSIC, Cantoblanco, 28049 Madrid, Spain
Available online 13 November 2007
Abstract
The addition of the alkanes C2H6, C3H8, or n-C4H10 in the CH4–O2–SO2 reaction feed resulted in an improvement of pre-sulfated 1% Pt/␥-Al2O3
activity on CH4 oxidation in lean conditions. These results are explained assuming that (i) the energy generated during the oxidation of the alkanes
may be transferred to activate the rate determining step of CH4 oxidation, over the catalyst surface, increasing the reaction rate at lower temperatures
and (ii) alkanes oxidation may lower the coverage of oxygen on the Pt surface, leaving surface active sites free for CH4 adsorption, thus increasing
its oxidation rate, since, oxygen species act as an inhibitor for the reaction at full coverage. n-C4H10 addition in the reaction feed generated the
highest promotion on CH4 complete oxidation. This promotional effect was further increased on pre-sulfated 2% Pt/␥-Al2O3. Results presented
in this investigation suggest that small amounts of n-butane in the gas feed during CH4–O2 reaction over a pre-sulfated Pt/␥-Al2O3 catalyst may
eliminate completely very low concentrations of methane emissions at low temperatures from lean-burn NGV exhausts.
© 2007 Published by Elsevier B.V.
Keywords: Methane oxidation; NGVs emissions; Sulfated Pt catalysts; Sulfur deactivation resistance
1. Introduction
designed to achieve methane emission abatement by catalytic
combustion. However, difficulties arise due to the high stability
of the CH4 molecule and from the reaction conditions specific
to lean-burn engines. Typical exhaust gases are characterized by
low temperatures (less than 500–550 ◦C), low concentrations of
CH4, large amounts of water vapor, large excess of oxygen and
presence of SO2 and NOx.
The complete oxidation of methane can be performed over
either noble metals or transition metal oxides [2–5]. Pd/␥-Al2O3
catalysts are widely recognized as the most active in combus-
tion of methane [1,6–14], but are deactivated by water and sulfur
containing compounds present in the exhaust. Platinum catalysts
properties in the complete oxidation of methane at low temper-
ature have been less studied. It has been shown that Pt has a
very poor activity for methane oxidation, but would be more
resistant to sulfur poisoning [6,10]. By adding small amounts of
platinum into the palladium catalyst, relative improved activity
was obtained in comparison with the monometallic palladium
catalyst for complete oxidation of methane [15,16].
Natural gas represents one promising alternative energy
source for the future in automotive and heavy-duty vehicles. In
comparison with traditional fuelled vehicles, lean-burn natural
gas vehicles (NGV) generate a lower impact on environment,
due to smokeless exhausts and reduced nitrogen oxide emis-
sions due to the lower combustion temperatures associated with
the high air to fuel mass ratios (20 or greater) at which the lean
engine operates. Also tailpipe CO2 emissions in NGV light-duty
vehicles can be lower than in conventional fuelled vehicles due
to the higher hydrogen-to-carbon ratio of natural gas. Natural gas
engines can operate under lean conditions so that the fuel effi-
ciency can be increased compared to stoichiometric conditions.
Under these conditions, unburned methane, a potent greenhouse
gas, is emitted in the exhaust gases. Accordingly, severe limi-
tations on CH4 emissions have been enforced [1]. In order to
meet these severe regulations, a catalytic exhaust converter has
to be installed on lean-burn heavy-duty NGVs. This device is
Now, in recent investigations, we found a strong promotional
effect of the presence of 1000 ppmV C3H8 in the reaction feed
on CH4–SO2–O2 reaction over pre-sulfated Pt/␥-Al2O3 [17]
and pre-sulfated Pt–Sn/␥-Al2O3 [18] catalysts. Moreover, the
∗
Corresponding author.
E-mail address: cs001380@siu.buap.mx (G. Corro).
1381-1169/$ – see front matter © 2007 Published by Elsevier B.V.
doi:10.1016/j.molcata.2007.11.003

180
G. Corro et al. / Journal of Molecular Catalysis A: Chemical 281 (2008) 179–183
catalysts showed no deactivation due to the SO2 present in the
gas feed. These results were obtained using a reaction feed con-
taining a concentration of 2 vol% CH4, which is higher relative
to the concentration of methane in a real NGV exhaust (as low
as 1000 ppmV) [9,19]. This promotional effect of propane was
explained in terms of (i) the propane combustion heat which may
be transferred, to activate the abstraction, of the first hydrogen
of the adsorbed methane molecule and (ii) to propane oxidation
that may lower the high coverage of oxygen on the Pt surface,
leaving surface active sites free for CH4 adsorption. In order
to confirm these explanations, in this study, we investigated the
effect of the presence of other alkanes with different combus-
tion heats [20] and different number of carbon atoms. Therefore,
we studied the effect of the presence of ethane, propane or n-
butane on the activity of pre-sulfated Pt/␥-Al2O3 catalysts on
CH4–O2–SO2 reaction using a reaction feed containing a low
concentration of methane (2000 ppmV) in excess oxygen and in
the presence of SO2.
of interest were scanned a number of times in order to get good
signal-to-noise ratios. The intensities of the peaks were esti-
mated by determining the integral of each peak after subtracting
an S-shaped background and fitting the experimental peak to
Lorentzian/Gaussian lines (80% L/20% G). The binding ener-
gies (BE) were referenced to the Al 2p peak, the BE of which was
fixed at 74.5 eV. Using this reference, BE values of C 1s peak
coming from adventitious carbon appeared at 284.9 0.2 eV.
The catalysts tests were performed in a continuous flow
tubular quartz reactor (inner diameter 7 mm) placed in a pro-
grammable furnace. The catalyst temperature was controlled by
a thermocouple mounted internally in direct contact with the
sample in order to read the true catalyst temperature. Measure-
ments over the catalysts were performed using a feed volume
flow rate of 100 cm3 min−1 consisting of 2000 ppmV CH4,
20 vol% of O2, 50 ppmV SO2, without or with C2H6 or C3H8,
or n-C4H10 (2000 ppmV, 1333 ppmV, 666 ppmV, or 333 ppmV)
and balance N2. The catalyst loading was of 200 mg. The reac-
tor out flow was analyzed using a gas chromatograph (GC-17
Shimadzu) equipped with a thermal conductivity detector. Two
consecutive heating and cooling cycles at temperatures from 25
to 600 ◦C were conducted; the results from the second cycle were
used in this study. The conversions of CH4, C2H6, C3H8, or n-
C4H10 to all products were calculated based on the following
equation:
2. Experimental
The support used was ␥-Al2O3 Merck with a grain size of
0.063–0.200 mm (70–230 mesh ASTM). Before use, the support
was calcined for 6 h at 600 ◦C in air. Pt catalysts supported on
alumina were prepared by impregnation using acidic aqueous
solution (0.1 M HCl) of H2PtCl6·6H2O (Merck, minimum 98%
purity). After impregnation, the catalysts were dried at 120 ◦C
overnight, and then, calcined in flowing air for 6 h at 500 ◦C.
Finally, the catalysts were reduced in pure hydrogen flow for
8 h at 500 ◦C. A reference alumina support was prepared in the
same way using only diluted hydrochloric acid.
Xin − Xout
conversion (%) =
× 100
Xin
Xin is the concentration of X introduced in the experiment and
Xout is the concentration of X at the reactor outlet.
A sample of the reduced catalyst was then sulfated. Sulfa-
tion was performed on samples heating to 500 ◦C in flowing air
(100 cm3 min−1). The feed was then changed at 500 ◦C for a
3. Results
3.1. Catalysts characterization
nitrogen flow containing 50 ppm SO2, 5% O2 (100 cm3 min−1
for 10 h. The sample was then cooled to 25 ◦C.
)
The catalyst characterization data are summarized in Table 1.
Pt dispersion values of 0.35 and 0.30 were obtained for unsul-
fated 1% Pt/␥-Al2O3 and 2% Pt/␥-Al2O3, respectively.
Pt dispersion values for the catalysts after reaction or after
sulfation were not determined because the samples needed to be
reduced again under a H2 atmosphere at 100 ◦C to reduce the
Pt. This reduction might have led to the reduction of sulfate to
H2S which poisons Pt sites.
Platinumaccessibilitymeasurementsonthereducedcatalysts
were obtained by the H2–O2 titration method at room tempera-
ture in a static volumetric apparatus and using the stoichiometric
(H/Pt)surf = 1. The chemical microanalysis of the catalyst was
determined by energy dispersive X-ray spectroscopy (EDS),
(NORAN) performed in conjunction with a scanning electron
microscope (JEOL, model JSM-6300). Catalysts metal contents
are reported in Table 1.
Photoelectron spectra were recorded over pre-sulfated fresh
sample using a VG Escalab 200R electron spectrometer
equipped with a hemispherical analyzer, operating in a con-
stant pass energy mode, and a non-monochromatic Mg K␣
(hν = 1253.6 eV, 1 eV = 1.603 × 10−19 J) X-ray source operated
at 10 mA and 12 kV. The energy regions of the photoelectrons
3.2. Methane oxidation
Pre-sulfated ␥-Al2O3 was not active for methane oxidation
at the temperatures studied in this work, in the presence and in
the absence of C2H6, C3H8, or n-C4H10.
Table 1
Catalysts characterization data
Catalyst
Pt (wt%)
Pt dispersion
Pt surface atoms (×10−18 g−1
)
Cl (wt%)
S (wt%)
Pre-sulfated 1% Pt/␥-Al2O3
Pre-sulfated 2% Pt/␥-Al2O3
1.02
2.01
0.35
0.30
10.80
18.52
1.1
1.1
1.65
1.60

G. Corro et al. / Journal of Molecular Catalysis A: Chemical 281 (2008) 179–183
181
Table 2
CH4 light-off50 values obtained for methane oxidation in CH4–O2–SO2 reaction in the presence of alkanes in the feed
Catalyst (pre-sulfated)
Alkane
Alkane combustion heat (−ꢀHC◦ ) kcal mol−1
Alkane concentration (ppmV)
Light-off50 CH4 (◦C)
1% Pt/␥-Al2O3
–
–
–
333
666
1333
2000
570
550
550
540
534
C2H6
372.8
C3H8
530.6
687.9
333
666
1333
2000
546
535
511
500
n-C4H10
333
666
1333
2000
520
513
480
465
2% Pt/␥-Al2O3
n-C4H10
687.9
2000
430
Table 2 reports the temperatures at which 50% CH4 con-
version (light-off50) for methane oxidation in CH4–O2–SO2
reaction are reached in the presence of different concentrations
of the alkanes in the feed for pre-sulfated 1% Pt/␥-Al2O3 cat-
alyst. The evolution of methane conversion in CH4–O2–SO2
reaction over pre-sulfated 1% Pt/␥-Al2O3 catalyst in the pres-
ence and in the absence of 2000 ppmV C2H6, C3H8, or n-C4H10
is shown in Fig. 1. In this figure and in Table 2 it can be observed
that the presence of C2H6, C3H8, or n-C4H10, results in a lower-
ing of the CH4 light-off50 values on pre-sulfated 1% Pt/␥-Al2O3
catalyst. This decrease is stronger as the alkane concentration
in the feed increases and as the number of carbon atoms in the
alkane molecule increases.
ther strong enhancement on CH4–O2 reaction resulted from
increasing Pt content in the catalyst. In Fig. 2 the evolution
of CH4 conversion in CH4–n–C4H10–O2–SO2 reactions over
pre-sulfated 1% Pt/␥-Al2O3 and pre-sulfated 2% Pt/␥-Al2O3
catalyst is shown. It is evident that the increase in platinum con-
tent in the catalyst resulted in an increase of the catalyst activity
on methane oxidation.
3.3. X-ray photoelectron spectroscopy
Binding energies of core electrons of pre-sulfated 1% Pt/␥-
Al2O3 and pre-sulfated 2% Pt/␥-Al2O3 catalyst are reported in
Table 3. Two components were observed; one at the lower bind-
ing energies (315.4 eV) associated with metallic platinum. The
binding energy of the Pt 4d5/2, revealed a higher binding energy
component (318.5 eV) that can be associated to a more oxidized
species of Pt (Pt4+) [21–24]. The binding energies of Pt species
In Table 2, CH4 light-off50 values obtained for methane oxi-
dation in CH4–O2–SO2 reaction in the presence of 2000 ppmV
n-C4H10 are also reported for 2% Pt/␥-Al2O3 catalyst. A fur-
Fig. 1. Methane conversion as a function of temperature on the CH4–O2 reaction
(ꢀ) over pre-sulfated 1% Pt/␥-Al2O3. Effect of C2H6 (ꢁ), C3H8 (᭹) and n-
C4H10 (ꢂ) addition in the reaction feed: 2000 ppmV CH4, 20% O2, 50 ppmV
SO2, balance N2.
Fig. 2. Methane conversion as a function of temperature on the CH4–O2 reac-
tion over pre-sulfated 1% Pt/␥-Al2O3 (ꢂ) and pre-sulfated 2% Pt/␥-Al2O3 (᭹).
Reaction feed: 2000 ppmV CH4, 2000 ppmV n-C4H10, 20% O2, 50 ppmV SO2,
balance N2.

182
G. Corro et al. / Journal of Molecular Catalysis A: Chemical 281 (2008) 179–183
Table 3
Thus, C2H6, C3H8, or n-C4H10 combustion heat may be
largely transferred to reach the activation energy needed to
abstract this first hydrogen. This assumption is in agreement
with the results presented in Table 2. In this table it can be
observed that the promotional effect of the alkane added on
CH4 oxidation increases as a function of the alkane combus-
tion heat values [20]. Moreover, Table 2 also shows that this
promotional effect increased as a function of the alkane con-
centration. The energy generated during alkane combustion
increased as a function of the alkane molecules concentration.
• On the other hand, C2H6, C3H8, or n-C4H10 oxidation may
lower the coverage of oxygen on the Pt surface, leaving sur-
face active sites L free for CH4 adsorption, thus increasing
its oxidation rate, since, oxygen species inhibit the reaction
at full coverage [26]. It is worth noting that the promotional
effect of the presence of the alkane added on CH4 oxidation
increases as a function of the number of moles of oxygen nec-
essary for the oxidation per-mole of the alkane (according to
reactions 1, 2, and 3).
Bindingenergies(eV)of core electronsof Pt4d5/2 in pre-sulfated1% Pt/␥-Al2O3
and pre-sulfated 2% Pt/␥-Al2O3 catalysts
a
Catalyst (pre-sulfated)
Al 2p
74.5
Pt 4d5/2
S 2p
1% Pt/␥-Al2O3
315.4 (49)
318.5 (51)
168.9
2% Pt/␥-Al2O3
74.5
315.4 (51)
318.5 (49)
168.9
a
In parenthesis are peak percentages.
were constant for both catalysts. The binding energy of the S
2p3/2 (168.9 eV) revealed that S is only present as S6+, thus only
sulfate species are present in the catalyst [25].
In Table 4 the atomic surface ratios of the pre-sulfated cat-
alysts determined by XPS are indicated. In this table, it can be
seen that the Pt/Al XPS signal ratio increases with Pt content in
the catalyst.
4. Discussion
It is well known that the oxidation of SO2 to SO3 is catalyzed
by Pt at temperatures greater than 200 ◦C [28–30] according to
the following reaction:
Figs. 1 and 2 and Table 2 indicate the effect of C2H6, C3H8, or
n-C4H10 on CH4–O2 reaction over pre-sulfated 1% Pt/␥-Al2O3
and pre-sulfated 2% Pt/␥-Al2O3.
The increase of methane oxidation rate due to the presence
of C2H6, C3H8, or n-C4H10 is explained considering two facts:
SO2 + O·L → SO3
(5)
This reaction over oxygen-covered Pt may cause the removal
of surface oxygen, thus may also facilitate adsorption of CH4,
resulting in a further increase in the CH4 conversion. The pro-
moting effect of the oxidation of SO2 to SO3 on methane
oxidation may be the same in the presence or in the absence
of alkanes.
• The exothermic combustions of C2H6, C3H8, or n-C4H10
[20], which initiate at lower temperatures, relative to methane
combustion generate large amounts of energy according to the
following reactions:
C2H6 + 3.5O2 → 2CO2 + 3H2O
C3H8 + 5O2 → 3CO2 + 4H2O
n-C4H10 + 6.5O2 → 4CO2 + 5H2O
(1)
(2)
(3)
Now, results presented in Fig. 2 and Table 2 reveal that
increasing Pt content in the catalyst resulted in a further strong
increase of the catalytic activity for methane combustion in
the presence of 2000 ppmV n-butane in the reaction feed. This
enhancement cannot be due to a change in platinum disper-
sion, since both catalysts have similar dispersion values. This
enhancement cannot be due to a change of the oxidation state of
Pt surface atoms, since both catalysts revealed the same binding
energy values of Pt species (Table 3). This enhancement cannot
be due to a change in sulfates concentration, since both catalysts
have similar amounts of sulfates (Table 1). However, in Table 1
it can be seen that the number of Pt surface sites is higher in 2%
Pt/␥-Al2O3 relative to 1% Pt/␥-Al2O3. This result is in agree-
ment with the values of the atomic surface ratios determined by
XPS of the catalysts (Table 4), showing an increase in Pt surface
atoms as the Pt content increased. The increase in Pt surface
sites results in a higher number of Pt surface active sites L for
CH4 adsorption according to reaction (4). Thus the probability
of CH4 adsorption increases and may result in an increase in the
oxidation rate for CH4–O2 reaction.
The energy generated during the combustion of C2H6,
C3H8, or n-C4H10 may be transferred to activate the rate
determining step of CH4 oxidation, over the catalyst surface,
increasing the reaction rate at lower temperatures, accord-
ing to the mechanism proposed by Burch et al. [26] and
Garetto and Apesteguia [27]. These authors interpreted the
methane oxidation mechanism in terms of Mars–van Krave-
lenreduction–oxidationpathways, wheretheratedetermining
step is the abstraction of the first hydrogen on the adsorbed
methane molecule:
k
CH4 · L + O · L−→ CH3 · L + OH · L
(4)
L is an active site.
Table 4
Atomic surface ratios of pre-sulfated 1% Pt/␥-Al2O3 and pre-sulfated 2% Pt/␥-
Al2O3 catalysts determined by XPS
5. Conclusions
Catalyst (pre-sulfated)
Pt/Al
1% Pt/␥-Al2O3
2% Pt/␥-Al2O3
0.0014
0.0030
Results obtained in this investigation demonstrated the pro-
moting effect of the presence of C2H6, C3H8, or n-C4H10 in the

G. Corro et al. / Journal of Molecular Catalysis A: Chemical 281 (2008) 179–183
[5] S. Park, H.J. Hwang, J. Moon, Catal. Lett. 87 (2003) 219.
183
activation of methane oxidation over pre-sulfated Pt/␥-Al2O3
catalysts in CH4–O2–SO2 reactions. This promoting effect is
explained on basis of the following facts: (i) C2H6, C3H8, or
n-C4H10 combustion heat may be transferred to activate the
CH4 dissociative adsorption over the catalyst surface increas-
ing methane oxidation rate at lower temperatures; (ii) C2H6,
C3H8, or n-C4H10 oxidation may lower the coverage of oxy-
gen on the Pt surface, leaving surface active sites free for CH4
adsorption; (iii) the increase in Pt content in the catalyst results
in an increase in Pt surface active sites for CH4 adsorption, thus
the probability of CH4 adsorption increases and may result in an
increase in surface Pt sites oxidation rate for CH4–O2 reaction.
The results presented in this investigation suggest that the
combination of both promoting effects described (i) addition of
C2H6, C3H8, or n-C4H10 in the gas feed and (ii) increase of
the Pt content (thus increase in the number of Pt surface active
sites) may lead to a catalytic system having a very high activity
for complete methane oxidation at low temperature. Such new
catalytic process may eliminate low concentrations of methane
emissions from lean-burn NGV exhausts.
[6] J.A. Lampert, M.S. Kazi, R.J. Farrauto, Appl. Catal. B: Environ. 14 (1997)
211.
[7] D.L. Mowery, M. Grabosky, T.R. Ahno, R. McCormick, Appl. Catal. B:
Environ. 21 (1999) 157.
[8] D.L. Mowery, R.L. McCormick, Appl. Catal. B: Environ. 34 (2001) 287.
[9] J.K. Lampbert, M.S. Kazi, R.J. Farrauto, SAE 961971 (1996) 15.
[10] P. Gelin, L. Urfels, M. Primet, E. Tena, Catal. Today 83 (2003) 45.
[11] R. Burch, D.J. Crittle, B.W.L. Southward, J.A. Sullivan, Catal. Lett. 72
(2001) 153.
[12] J.M. Jones, A. Dupont, R. Brydson, D.J. Fullerton, N.S. Nasri, A.B. Ross,
A.V.K. Westwood, Catal. Today 81 (2003) 589.
[13] V. Meeyoo, D.L. Trimm, N.W. Cant, Appl. Catal. B: Environ. 16 (1998)
L101.
[14] S. Ordonez, P. Hurtado, H. Sastre, F.V. Diez, Appl. Catal. A: Gen. 259
(2004) 41.
[15] K. Persson, A. Ersson, K. Jansson, N. Iverlund, S. Jaras, J. Catal. 231 (2005)
139.
[16] K. Persson, A. Ersson, K. Jansson, J.L.G. Fierro, S.C. Jaras, J. Catal. 243
(2006) 14.
[17] G. Corro, J.L.G. Fierro, O. Vazquez, Catal. Commun. 6 (2005) 287.
[18] G. Corro, J.L.G. Fierro, O. Vazquez, Catal. Commun. 7 (2006) 436.
[19] F. Arosio, S. Colussi, G. Groppi, A. Trovarelli, Catal. Today 117 (2006)
569.
[20] J.M. Smith, H.C. Van Ness, Thermodynamics, McGraw-Hill, Mexico,
1990.
[21] B. Pawelee, R. Mariscal, R.M. Navarro, S. van Bokhorst, S. Rojas, J.L.G.
Fierro, Appl. Catal. A 225 (2002) 223.
[22] E. Czaran, J. Finster, K.H. Schnabel, Z. Anorg. Allg. Chem. 443 (1978)
175.
[23] Z. Zsoldos, L. Guczi, J. Phys. Chem. 96 (1992) 23.
[24] L. Guczi, A. Sarkany, Z. Koppany, Appl. Catal. 120 (1994) L1.
[25] D. Briggs, M.P. Seah (Eds.), Practical Surface Analysis by Auger
and X-ray Photoelectron Spectroscopy, 2nd ed., Wiley Chichester, UK,
1990.
Acknowledgements
The authors are pleased to acknowledge valuable sup-
port for this research from CONACYT-SEMARNAT (project
2002-COL-0212), DEGUSSA CATALYST S.A. de C.V. and
´
Vicerrectoria de Investigacion y Estudios de Posgrado (BUAP).
References
[26] R. Burch, D.J. Crittle, M.J. Hayes, Catal. Today 47 (1999) 229.
[27] T.F. Garetto, C.R. Apesteguia, Catal. Today 62 (2000) 189.
[28] G.C. Bond, Catalysis by Metals, Academic Press, London, 1962.
[29] G.I. Godolets, Stud. Surf. Sci. Catal. 15 (1983) 365.
[30] D.D. Beck, M.H. Krueger, D.R. Monroe, SAE, 1991 (Paper no. 910844).
[1] P. Gelin, M. Primet, Appl. Catal. B: Environ. 39 (2002) 1.
[2] L. Ning, A. Wang, X. Wang, M. Zheng, R. Cheng, T. Zhang, Appl. Catal.
48 (2004) 259.
[3] D. Roth, P. Gelin, E. Tena, M. Primet, Top. Catal. 16 (2001) 77.
[4] C.F. Cullis, B.M. Willatt, J. Catal. 83 (1983) 267.