Improvement of the catalytic performance of Pd/WO3/ZrO2 in the selective NO-CH4-O2 reaction by the addition of water vapor

Article abstract of DOI:10.1246/cl.2001.1018

The Pd/WO₃/ZrO₂ catalyst exhibited activity for the NO-CH₄-O₂ reaction in the presence of water vapor when WO₃ monolayer covered the ZrO₂ surface. The selectivity of methane consumption for

Full text of DOI:10.1246/cl.2001.1018

1018
Chemistry Letters 2001
Improvement of the Catalytic Performance of Pd/WO3/ZrO2 in the Selective NO–CH4–O2
Reaction by the Addition of Water Vapor
Kazu Okumura,* Tetsuji Kusakabe, Naoko Yasunobu, and Miki Niwa
Department of Materials Science, Faculty of Engineering, Tottori University, Koyama-cho, Tottori 680-8552
(Received June 8, 2001; CL-010536)
The Pd/WO3/ZrO2 catalyst exhibited activity for the
NO–CH4–O2 reaction in the presence of water vapor when
WO3 monolayer covered the ZrO2 surface. The selectivity of
methane consumption for NO reduction was considerably
improved by the addition of 10% of water vapor.
Recently, much attention has been directed to the metal
loaded catalyst for catalytic reduction of NOx using hydrocar-
bons as reductants. Among various hydrocarbons, methane is
the most preferable because it is the main component of natural
gas. Many kinds of catalysts were found to be efficient for the
catalytic reduction of NO. However, the activity is sometimes
severely suppressed by water vapor. Therefore, the enhance-
ment of tolerance to the water vapor is an important subject. Pd
is promising at this point because it is relatively tolerant to
water vapor.1 On the other hand, as for the support compo-
nents, zeolites such as ZSM-5 and mordenite have been used to
keep the active Pd species, where the Brønsted acid sites played
an important role to generate the active Pd center.2,3 However,
zeolite has a disadvantage when it is used in the presence of
water vapor. This is because water vapor causes the elimina-
tion of Al from the zeolite framework, which leads to the irre-
versible suppression of the catalytic activity of Pd. WO3 sup-
ported on ZrO2 exhibits strong Brønsted acidity, when WO3
covers the ZrO2 surface as a monolayer form.4,5 The monolay-
er-WO3/ZrO2 is promising to be used as a support for Pd,
because it is expected to be tolerant to water vapor.6 In the
present study, catalytic NO–CH4–O2 reaction was carried out
over Pd loaded on WO3/ZrO2. Especially, the attention was
focused on the influence of water vapor on the catalytic per-
formance and acid properties of Pd/WO3/ZrO2 catalyst.
ZrO2 was prepared by the hydrolysis of zirconium oxyni-
trate solution using aqueous ammonia, followed by calcination
in air at 573 K. The obtained ZrO2 was immersed in an ammo-
nium tungstate solution. After drying, the obtained solid was
calcined at 923 K for 4 h under an atmospheric condition to
obtain WO3/ZrO2. Pd was loaded on WO3/ZrO2 by an ion
exchange method using Pd(CH3)4Cl2 solution. It was then
treated with N2 at 773 K for 4 h. The loaded amount of Pd was
measured by an ICP method. Before the reaction, the catalyst
was treated in flowing O2 at 773 K for 1 h. A mixture of NO,
O2 and methane (NO, 1000 ppm; CH4, 1000 ppm; O2, 1%; He
balance; total flow rate, 150 mL min–1) was fed over 2 g of the
catalyst at 623 K, and the outlet gas was analyzed by a gas
chromatograph and a NOx meter. Water was fed into the reac-
tor with a microfeeder. Water was vaporized at the inlet of the
reactor and then mixed with the reactant gas mixture.
loading was fixed at 0.07 wt%. The reaction was carried out
under the dry conditions. The maximum conversion of NO was
obtained when the WO3 loading reached 20 wt%. Considering
the surface area of the catalyst (96 m2 g–1), the value corre-
sponded to the formation of WO3 monolayer over the ZrO2 sur-
face.5 Therefore, it was inferred that the Brønsted acid sites
generated on the WO3 monolayer effectively anchored the
active Pd species.
In the next step, the catalytic performance was studied over
the Pd/WO3/ZrO2 with various Pd loadings. The loading of
WO3 was fixed at 20 wt%. When the reaction was carried out
under the dry conditions, the NO conversion first increased
with Pd content (Figure 2(a)). The maximum value was
obtained when Pd loading reached 0.17 wt%. However, the
further increase in the Pd loading lowered the value.
Alternatively, the conversion of methane to carbon dioxide
Firstly, the influence of the WO3 loading on the catalytic
reaction of Pd was examined. Figure 1 shows the dependence
of the conversion of NO to N2 on the WO3 loading. The Pd
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
1019
reached 100% when the Pd loading exceeded 0.48 wt%.
Probably, this is because of the formation of agglomerated PdO,
which was favorable for the simple methane combustion.7
However, the feature changed significantly in the presence of
10% of water vapor (Figure 2(b)). The conversion of methane
to carbon dioxide was considerably suppressed. On the other
hand, the change in the NO conversion was small. As a result,
the selectivity of CH4 to CO2 consumed for the NO–CH4–O2
reaction increased under the water vapor. Especially when the
Pd loading was 0.17 wt%, the conversion of methane was half
that of NO. Considering the reaction stoicheometry and the gas
composition, the fact meant that the oxidation of methane was
completely suppressed. In other words, the selectivity for NO
reduction reached 100% in the presence of water vapor. The
improvement of the selectivity is an important phenomenon,
taking the fact into account that the water vapor usually hin-
dered the NO reduction. However, a promoting effect of water
vapor on the selective reduction of NO has been reported by
using C3H6, C3H8, iso-C4H10 and n-octane as a reductant.8–10
In the present case, the effect of water vapor was attributed to
the enhancement of Brønsted acid acidity over the WO3/ZrO2
as proved by IR spectra of adsorbed pyridine.
Finally, in order to examine the influence of water vapor
on the acid properties of Pd/WO3/ZrO2 catalyst, temperature
programmed desorption (TPD) of ammonia was measured
before and after the reaction. This is because it was considered
that the acidity played an important role to keep the active dis-
persed PdO species.11,12 TPD spectrum measured after the
reaction completely agreed with that measured before the reac-
tion (Figure 3). Therefore, the acid site of WO3/ZrO2 was not
affected by the exposure to water vapor during the catalytic
reaction. The fact was contrasted with the result obtained for
the zeolite support.
References
1
2
3
Y. Nishizaka and M. Misono, Chem. Lett., 1993, 1295.
Y. Nishizaka and M. Misono, Chem. Lett., 1994, 2237.
C. J. Loughran and D.E. Resasco, Appl. Catal. B, 7, 113
(1995).
4
5
6
7
8
9
M. Hino and K. Arata, J. Chem. Soc., Chem. Commun.,
1987, 1259.
N. Naito, N. Katada, and M. Niwa, J. Phys. Chem., 103,
7206 (1999).
Y.-H. Chin, W. E. Alvarez, and D. E. Resasco, Catal.
Today, 62, 159 (2000).
C. J. Hicks, H. Qi, M. L. Young, and R. G. Lee, J. Catal.,
280, 122 (1990).
Y. Hirao, C. Yokoyama, and M. Misono, Chem. Lett.,
1996, 597.
A. Shichi, A. Satsuma, and T. Hattori, Chem. Lett., 2001,
44.
10 H.-Y. Chen and W. M. H. Sachtler, Catal. Lett., 50, 125
(1998).
11 K. Okumura, J. Amano, N. Yasunobu, and M. Niwa, J.
Phys. Chem. B, 104, 1050 (2000).
12 K. Okumura and M. Niwa, J. Phys. Chem. B, 104, 9670
(2000).