The CO oxidation kinetics on supported Pd model catalysts: A molecular beam/in situ time-resolved infrared reflection absorption spectroscopy study

Combining molecular beam techniques and time-resolved infrared reflection a bsorption spectroscopy (TR-IRAS) we have studied the kinetics of the CO oxi dation 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 character ized in previous studies. In the low temperature region, transient and stea dy-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 frac tion in the impinging gas flux. Simultaneously, the occupation of CO adsorp tion sites under steady-state conditions is monitored by in situ IR spectro scopy. The origin of different types of CO2 transients is discussed. In par ticular we focus on the transient CO2 production after switching off the CO beam. For the model catalyst investigated, detailed reaction rate measurem ents in combination with time-resolved IRAS show that the origin of the par ticular transient behavior of the supported model system is not due to the presence of specific adsorption sites on small particles, as has been propo sed previously. Instead, we show that the transient behavior can be semiqua ntitatively simulated on the basis of a simple kinetic model considering a homogeneous surface, and accounting for the inhibition of the dissociative adsorption of O-2 at high CO coverage. Moreover, it is discussed how the in herent heterogeneity of the supported particle system can additionally enha nce the observed effect. (C) 2001 American Institute of Physics.