Reaction kinetics on supported model catalysts: Molecular beam/in situ time-resolved infrared reflection absorption spectroscopy study of the CO oxidation on alumina supported Pd particles

By combining molecular beam techniques and time-resolved infrared reflectio n absorption spectroscopy (TR-IRAS) we have studied the kinetics of the CO oxidation reaction on an alumina-supported Pd model catalyst. The Pd partic les are deposited by metal evaporation under ultrahigh vacuum conditions on to a well-ordered alumina film, prepared on a NiAl(110) single crystal. Par ticle size, density, and structure of the Pd deposits have been characteriz ed in previous studies. In the low temperature region, transient and steady -state experiments have been performed over a wide range of CO and oxygen f luxes by crossing two effusive molecular beams on the sample surface. We de termine the steady-state CO2 production rate as a function of the CO fracti on in the impinging gas flux. Simultaneously, the occupation of CO adsorpti on sites under steady-state conditions is monitored by in situ infrared spe ctroscopy. The origin of different types of CO2 transients is discussed. In particular we focus on the transient CO2 production after switching off th e CO beam. For the model catalyst investigated, detailed reaction rate meas urements in combination with TR-IRAS show that the origin of the particular transient behavior of the supported model system is not due to the presenc e of specific adsorption sites on small particles, as has been proposed pre viously. Instead, we suggest that the transient behavior is a consequence o f the inhibition of the dissociative adsorption of O-2 at high CO coverage. Additionally, the inherent heterogeneity of the supported particle system can enhance the observed effect. (C) 2001 American Vacuum Society.