CARBON-MONOXIDE ADSORPTION AND HYDROGENATION ON CU-RH SIO2 CATALYSTS

The effects of copper on carbon monoxide adsorption and carbon monoxid e hydrogenation over Rh/SiO2 have been studied. Copper decreases the a ctivity for carbon monoxide hydrogenation and the rate of formation of methane, C2 and C3+, hydrocarbons. The selectivity for C2 oxygenate f ormation shows a slight variation with addition of copper. Cu-Rh/SiO2 catalyst with copper-to-rhodium ratio of 1.0 exhibits a high initial m ethanol selectivity which decreases with reaction time. The use of eth ylene hydroformylation as a probe reaction shows that addition of copp er causes little variation in carbon monoxide insertion activity; the propionaldehyde selectivity at copper-to-rhodium ratio of 0.05 is high er than that for the other catalysts studied. In situ infrared study r eveals that copper blocks the formation of bridged CO. However, blocka ge of bridged CO site does not lead to enhancement in carbon monoxide insertion as observed with Ag-Rh/SiO2 and sulfided Rh/SiO2 catalysts. The lack of effect of copper promotion on carbon monoxide insertion ca n be attributed to a high hydrogenation activity of copper and a possi ble electronic interaction between copper and rhodium leading to a red uced rhodium surface that is less active for carbon monoxide insertion than the oxidized rhodium. The difference observed in the infrared sp ectra of adsorbed carbon monoxide at 303 K and 513 K in the presence a nd absence of reactants shows that carbon monoxide induces surface rec onstruction at 513 K. Results of catalyst characterization at 303 K ca n not be extrapolated to elucidate the state of catalyst surface under reaction conditions.