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

摘要

Combining molecular beam techniques and time-resolvecl infrared reflection absorptior spectroscopy (TR-lRAS) we have studied the kinetics of the CO oxidation reaction on ar alumina-supportecl Pd model catalyst. The Pd particles are deposited by metal evaporation uncle] ultrahigh v~cuum (UHV) ~onditions onto a well-orderecl alumina film, prepared on a NiAl(llO: single crystal. Pafticle size, density and structure of the Pd deposits have been characterized ir previous studies. In the low temperature region, transient and steady-state experiments have beer performed over a wide range of CO and oxygen fluxes by crossing two effusive molecular beam1 on the sample surface.. We determine the steady-state C0_2 production rate as a function of the CC fraction in the impinging gas flux. Simultaneously, the occupation of CO adsorption sites J.lnclel steady-state conditions is monitored by in situ IR spectroscopy. The origin of different types of CO: transients is discussed. In particular we focus on the transient C0_2 productiollafterswitching offthf CO beam. For the model catalyst investigated, detailed reaction rate measurements in combinatior with time-resolvecl lRAS show that the origin of the particular transient behavior of the supportec model system is not due to the presence of specific adsorption sites on small particles, as has beer proposed previously. Instead, we show that the transient behavior can be semiquantitativel) simulated on the basis of a simple kinetic model considering a homogeneous surface, and accounting for the inhibition of the dissociative adsorption of 0_2 at high CO coverage. Moreover it is discussed how the inherent heterogeneity of the supported particle system can additionall) enhance the observed effect.