CO Oxidation on Rh/SiO2/Mo(112) Model Catalysts at Elevated Pressures

Well-defined model catalyst samples, prepared under ultra-high-vacuum (UHV) conditions, hold the potential of bridging the gap between traditional surface science and technical catalysis studies. However, work remains to understand and characterize reaction kinetics of such samples under more realistic pressure conditions, specifically, how reaction kinetics of a well-studied reaction (such as CO oxidation) over model surfaces compares to single-crystal surfaces studied under identical conditions. Here, the CO oxidation reaction under elevated-pressure conditions (P ) 8.0 Torr) and CO-rich reaction conditions is used as a probe reaction to characterize Rh/SiO2 model catalyst samples, prepared on a Mo(112) substrate under UHV conditions, in a contiguous high-pressure reactor cell-UHV surface analysis chamber. CO oxidation reaction kinetics are studied as a function of O2/CO partial pressures (from 1/1 to 1/10 O2/CO) and Rh coverage (θRh ) 0.25-10 ML), along with measurements on a Rh(111) single crystal for direct comparison. CO desorption measurements on the Rh/SiO2 films and STM measurements of Rh particles on ultrathin SiO2 films at T ) 300 K have been obtained at various Rh coverages to provide two additional methods to estimate reactive sites. Results demonstrate that, under the reaction conditions employed, CO oxidation reaction on Rh/SiO2/Mo(112) exhibits O2/CO dependencies and reaction activation energies similar to those observed on single-crystal samples. These data show that model catalyst samples can be used to gain qualitative and quantitative reactivity data over a range of elevated-pressure reaction conditions. Measurements of CO oxidation under CO-rich conditions can provide a reasonable estimate of the number of active sites on Rh model catalyst surfaces, thus providing a useful starting point for future studies of particleand structure-dependent reactions on model catalyst surfaces.