A kinetic study on the adsorption and reaction of hydrogen over silica-supported ruthenium and silver-ruthenium catalysts during the hydrogenation of carbon monoxide
An interesting phenomenon in bimetallic catalysis is the synergistic effect between two metals during CO hydrogenation. For example, addition of Ag to Ru catalysts results in a dramatic decrease in the specific activity of the catalyst, even though Ag alone does not appreciably adsorb H2 or CO. A series of Ag-Ru/SiO2 catalysts containing 0, 3, 10, 20 and 30 atomic percent silver was used to examine the kinetics of the adsorption and reaction of H2 on Ru during CO hydrogenation. CO was saturatively adsorbed at room temperature on each catalyst, followed by subsequent reaction with ambient H2 at 460 torr and 400--500K. The kinetics of CH4 formation for each system was monitored with mass spectrometry, and the coverage of surface H was monitored via in situ proton-NMR. The apparent activation energy for methanation decreased from about 23 kcal/mol for Ru/SiO2 to about 18 kcal/mol for the Ag-Ru/SiO2 series. These results show good correlation with previous hydrogen microcalorimetry measurements on the same catalysts by our group. The mechanism of the synergistic effect of Ag on Ru is presented in terms of "portal site mediated adsorption", whereby hydrogen is supplied to the catalyst surface by rapid dissociative adsorption occurring at low coordination sites which are preferentially blocked by Ag in bimetallic systems.