Infrared spectroscopic study of catalytic activity of Rh/TiO2 for the CO + H2 reaction

Abstract

The catalytic hydrogenation of CO with hydrogen over Rh/TiO₂ reduced at 200, 400 and 500 °C (denoted Cat-200,-400 and-500) was studied with in situ F.t.i.r. by monitoring the behaviour of adsorbed CO species to explain their activity orders of Cat-400 > Cat-200 > Cat-500. The very high reactivity of irreversibly adsorbed CO with hydrogen was retarded by reversibly adsorbed CO and hydrocarbon intermediates in the steady state of the catalytic reaction. A competitive adsorption study of H₂ and CO on the catalyst revealed that their adsorption varied delicately according to the reduction conditions of the catalyst to influence the catalytic activity. Adsorption of CO monitored by F.t.i.r. was fully consistent with the reaction kinetics. Based on these results, the highest catalytic activity of Cat-400 may be ascribed to the following factors: (i) very high reactivity of irreversibly adsorbed CO with H₂; (ii) the largest number of adsorption sites under the reaction conditions; (iii) the balanced adsorption of both substrates, which is very sensitive to their pressure, leading to the minimum retardation of H₂ adsorption by reversibly adsorbed CO and larger reaction orders in both substrates.