Abstract
Combining molecular beam techniques and time-resolved infrared reflection
absorption spectroscopy (TR-IRAS) we have studied the kinetics of
the CO oxidation reaction on an alumina-supported Pd model catalyst.
The Pd particles are deposited by metal evaporation under ultrahigh
vacuum (UHV) conditions onto a well-ordered alumina film, prepared
on a NiAl(110) single crystal. Particle size, density and structure
of the Pd deposits have been characterized in previous studies. In
the low temperature region, transient and steady-state experiments
have been performed over a wide range of CO and oxygen fluxes by
crossing two effusive molecular beams on the sample surface. We determine
the steady-state CO2 production rate as a function of the CO fraction
in the impinging gas flux. Simultaneously, the occupation of CO adsorption
sites under steady-state conditions is monitored by in situ IR spectroscopy.
The origin of different types of CO2 transients is discussed. In
particular we focus on the transient CO2 production after switching
off the CO beam. For the model catalyst investigated, detailed reaction
rate measurements in combination with time-resolved IRAS show that
the origin of the particular transient behavior of the supported
model system is not due to the presence of specific adsorption sites
on small particles, as has been proposed previously. Instead, we
show that the transient behavior can be semiquantitatively simulated
on the basis of a simple kinetic model considering a homogeneous
surface, and accounting for the inhibition of the dissociative adsorption
of O2 at high CO coverage. Moreover, it is discussed how the inherent
heterogeneity of the supported particle system can additionally enhance
the observed effect.
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