Chemisorption and disproportionation of carbon monoxide on palladium/silica catalysts of differing percentage metal exposed

Abstract

Silica-supported palladium catalysts ranging from 5 to 80% metal exposed have been characterized by pulse adsorption/flow desorption of hydrogen and/or carbon monoxide. Measured gas uptakes were used to calculate an average CO to Pd_s ratio to determine whether CO chemisorption exhibits variation with metal particle size. Average values of CO/Pd_s range from 0.50 to 0.79, indicating a structure-sensitive adsorption. However, in view of experimental uncertainties only the lowest percentage exposed sample has a unique CO/Pd_s ratio of 0.5, indicative of bridge-bonded CO. All catalysts have adsorption stoichiometries less than unity. A structure-sensitive CO surface reaction occurs, in the vicinity of 575 K, during CO adsorption/desorption cycles between 300 and 725 K in the presence of either He or H₂. Overall, the results suggest that CO dissociates into surface carbon either by disproportionation or by a hydrogen-assisted bond dissociation on the high-percentage exposed samples, but not on lower percentage exposed materials. A small amount of surface reconstruction may also occur.