Adsorption and catalytic studies on model mono- and bimetallic catalysts

Abstract

The adsorption and oxidation of carbon monoxide were studied on model mono- and bimetallic catalysts of Pd(111), Pd(100), Cu(100), Rh(100) and Cu/Rh(100), using combined elevated pressure reactor/IR cell-UHV surface analysis systems. Adsorption studies were carried out in wide pressure and temperature ranges of 10^-7-10^1 Torr and 90-1000K, respectively, employing techniques of RAIRS and TPD. The adsorption of CO on Pd(111) was found to be very sensitive for the preparation conditions of the adsorbed CO layer. In contrast, no effect of preparation conditions was seen on the adsorption of CO on Pd(100). By carrying out CO adsorption measurements under isobaric conditions, isosteric heats of adsorption were determined for Pd(111), Pd(100) and Cu/Rh(100) as a function of CO coverage. Discontinuities in the isobaric plots were observed in the range of the phase transition of adsorbed CO layers due to the different IR absorption cross sections of CO molecules in different adsorption positions. For Pd(111) an equilibrium phase diagram was constructed which shows the continuity of CO adsorption phases with increasing CO pressure and surface temperature. Excellent agreement was found between IR spectra of equilibrated CO layers on Pd(111) and Pd(100) single crystals and a Pd/SiO2 catalyst which suggests that the small metal particles on a support have crystal faces with mostly (111) and (100) orientations. Results of elevated pressure kinetic and IR measurements for the oxidation of CO on Pd(111) and Pd(100) strongly support the generally accepted correspondence between the heat of CO adsorption and the activation energy. Therefore, we believe that as far as the adsorption of CO is structure sensitive, the oxidation of carbon monoxide is structure sensitive, as well. Deposition of Cu on the Rh(100) surface results in an increase in catalytic activity for CO oxidation. Maximum activity is seen at θ[Cu]=1.3ML and the activity of Cu/Rh(100) catalysts are higher than that of Rh(100) even at θ[Cu]>3ML...