Alumina and titania overlayers on rhodium: A comparison of the chemisorption catalytic properties

Abstract

The effects of submonolayer deposits AlO x on the adsorption and hydrogenation of CO on a Rh foil have been investigated and compared with the earlier studied effects of TiO x deposits. AlO x is found to suppress CO chemisorption in direct proportion to AlO x coverage without affecting the energy of adsorption, indicating that the only function of AlO x is site blockage. CO hydrogenation activity of Rh decreases in proportion to AlO x coverage, but no change is observed in product selectivity or rate parameters for methane synthesis. By contrast, TiO x suppresses CO chemisorption to an extent greater than that expected for simple site blockage. With increasing TiO x coverage, the CO hydrogenation activity of Rh passes through a maximum at θ TiO x = 0.15 ML. The rate parameters for methane synthesis are affected by TiO x coverage: relative to clean Rh, the activation energy is lower and the partial dependences on H 2 and CO are both larger. The unusual effects of TiO x are attributed to the formation of Ti 3+ cationic centers at the perimeter of TiO x islands present on the Rh surface.