Oxygen Exchange between Pd and Oxide Supports
J. Phys. Chem. B, Vol. 106, No. 13, 2002 3441
favored for methane oxidation. This consumption generates
oxygen vacancies. The oxygen vacancies are refilled either from
the gas phase or from the bulk of the support, the isotopic
distribution of oxygen in the immediate vicinity of the metallic
particle being determined by the relative rates of the two vacancy
refilling processes. Because of the high oxygen mobility in
zirconia, vacancies created near the metallic particles are rapidly
refilled by migration of oxygen from the bulk and the gas phase,
and the oxygen distribution at the surface is equilibrated. By
contrast, the oxygen vacancies on the alumina-supported catalyst
whose oxygen mobility is lower are refilled from the gas phase
so that the concentration of 1 O in products increases more
rapidly, as seen for CO2. The lower 16O concentration in water
is most likely due to the higher mobility of surface hydroxyls
as compared with carbonyl or carboxyl species. This higher
mobility of surface hydroxyl makes it possible that the hydrogen
is actually oxidized at the support surface, far from the metallic
particle, where 16O is exchanged at a much slower rate.
In terms of reaction kinetics, based on the results observed
here one would expect different reaction rates for palladium
particles supported on carriers with different oxygen mobilities.
This is not the case simply because the rate-limiting step is the
dissociative methane adsorption on the metallic particle, which
competes with oxygen adsorption. This is consistent with the
negative first order kinetics with respect to oxygen reported by
many studies. However, oxygen at the support/metal interface
is likely kinetically favored for oxidation of the methane
decomposition products. This is consistent with earlier observa-
tions that support identity and contact with palladium can
influence the oxidation of palladium.
The oxygen from the support also participates in the methane
combustion reaction mechanism over supported metal particles.
It is proposed that the oxidation of methane involves both
oxygen adsorbed on palladium metal and surface oxygen from
the support. However, while adsorption of oxygen from the gas
phase competes with methane adsorption resulting in negative
first order kinetics with respect to oxygen, reaction intermediates
resulting from the activation of methane on the metal surface
are preferentially oxidized at the interface or spilled over the
support and oxidized by surface oxygen from the support,
generating oxygen vacancies. For supports with high oxygen
mobility, the vacancies are partially refilled with bulk oxygen,
8
16
which leads to O-rich reaction products. By contrast, the
oxygen vacancies created on the alumina surface are refilled
1
8
mainly from the gas phase, which generates O-rich reaction
products.
The oxygen mobility of the carriers appears of crucial
importance for real applications with fluctuating fuel composi-
tions; high mobility allows maintaining of reasonable oxidation
activities under fuel-rich conditions.
Acknowledgment. We acknowledge with pleasure the
support of this work by the Department of Energy, Division of
Chemical Sciences, Grant DE-FG02-96ER14679.
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