The effects of titania and alumina overlayers on the hydrogenation of CO over rhodium

Submonolayer quantities of TiOx and AlOx were deposited on a polycrystalline Rh foil. The samples were characterized by Auger electron spectroscopy and then enclosed in an atmospheric-pressure chamber to evaluate their activity for CO hydrogenation. AlOx deposition suppresses the rate of CO hydrogenation in direct proportion to the AlOx coverage, but the product distribution and kinetics are unaffected. These observations indicate that the only effect of AlOx is to block Rh sites. TiOxdeposition has a more complex effect. With increasing TiOx coverage, the methanation rate passes through a maximum at a TiOx coverage of 0.15 monolayer. The maximum rate is three times as large as the rate of clean Rh. The deposited TiOx also affects the catalyst selectivity as well as the kinetics of methane formation. The selectivity for methane reaches a minimum for a TiOx coverage of ca. 0.20 monolayer and in the same range the activation energy for methanation reaches a minimum and the H2 and CO partial pressure dependences reach a maximum. The unusual behaviour of TiOx-promoted Rh is attributed to the occurrence of highly active catalytic sites near the perimeter of the TiOx islands making up the overlayer.