TEMPERATURE-PROGRAMMED REACTION AND IN-SITU INFRARED STUDY OF CO ADSORBED ON RH SIO2 AND AG-RH/SIO2 CATALYSTS/

The temperature-programmed reaction (TPR) technique coupled with infra red (IR) spectroscopy has been employed to study the reactivity of ads orbed CO towards hydrogen. The combination of the two techniques provi des information on the structure and reactivity of adsorbates, activat ion energy and kinetic data for the CO hydrogenation reaction. CO adso rption on Rh/SiO2 at 298 K produced mainly linear and bridged CO on th e surface. Linear CO appears to be more reactive than bridged CO in th e formation of methane during TPR over Rh/SiO2. The addition of silver to Rh/SiO2 promotes the formation of gem-dicarbonyl CO and suppresses the formation of bridged CO during CO adsorption at 298 K. The dispro portionation of gem-dicarbonyl CO and the reduction of Rh+ by oxidatio n of the CO ligand of Rh+(CO)(2) produce CO, CO2, and surface carbon o n the Ag-Rh/SiO2 catalyst at temperatures above 310 K. Hydrogenation o f surface carbon begins at 390 K leading to methane formation. The met hane peak temperature was observed at 442 K on the Ag-Rh/SiO2 and at 4 20 K on Rh/SiO2. The higher hydrogenation temperature of adsorbed CO a nd surface carbon over Ag-Rh/SiO, as compared to Rh/SiO, is attributed to the suppression of hydrogenation by Ag.