Catalytic Activities for CO-H2 Reaction and CO Activation over Noble Metal Catalysts Supported on TiO2

Abstract

Catalytic activities of three kinds of noble metal catalysts (Rh, Pd and Ru) supported on TiO2 for CO-hydrogenation reaction were studied to find factors which may define their activities. Although the activities of these catalysts at 250°C were found to be in the order of Rh>Ru>Pd, those depended significantly on the reduction temperature: Rh and P d catalysts exhibited higher activities by the reduction at 400°C than those at 200 and 500°C, whereas reduction at 200°C provided the maximum activity for Ru catalyst. Near the reaction temperatures, adsorption amounts of H2 and CO on the catalysts which were reduced at 400°C decreased in the order of Rh>Pd>Ru which was the same for the amount of irreversively adsorbed CO. The order was very different from that of the catalytic activity. The reactivity of irreversibly adsorbed CO with H2 at 200°C observed by using FT-IR revealed. that its conversion rate into CH, decreased in the following order, Rh>Ru>Pd, corresponding to that of catalytic activity. The rate over Rh catalyst was much larger than that of the catalytic reaction, whereas those over Ru and Pd catalysts were similar to those of the latter reactions. Major portion of the irreversibly adsorbed CO on Ru and Pd catalysts were found to be converted into surface carbon, confirming the acceleration effects of H2on the production of the carbon from adsorbed CO. Such carbon- on Ru and Pd catalysts hindered the adsorption of CO. Catalytic activities of these catalysts for the hydrogenation of acetone are not necessarily related to CO-H, reaction. Based on such results, activation abilities for irreversibly adsorbed CO were suggested to be the important factor for the catalytic activity. The poisoning effect of carbon produced on the surface may not be ignored in the catalytic reactions.