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

Abstract

Submonolayer quantities of TiO_x and AlO_x 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. AlO_x deposition suppresses the rate of CO hydrogenation in direct proportion to the AlO_x coverage, but the product distribution and kinetics are unaffected. These observations indicate that the only effect of AlO_x is to block Rh sites. TiO_xdeposition has a more complex effect. With increasing TiO_x coverage, the methanation rate passes through a maximum at a TiO_x coverage of 0.15 monolayer. The maximum rate is three times as large as the rate of clean Rh. The deposited TiO_x also affects the catalyst selectivity as well as the kinetics of methane formation. The selectivity for methane reaches a minimum for a TiO_x 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 TiO_x-promoted Rh is attributed to the occurrence of highly active catalytic sites near the perimeter of the TiO_x islands making up the overlayer.