The enhancement of CO hydrogenation on rhodium by TiO x overlayers

Abstract

The kinetics of CO hydrogenation at atmospheric pressure over a polycrystalline Rh foil with titania deposits were characterized with respect to the surface composition. The titania overlayers (TiO x , x ~ 1.9) were prepared by titanium evaporation under ultrahigh vacuum with subsequent oxidation. Coverages were determined by Auger electron spectroscopy (AES). A sharp maximum in methanation activity, relative to the clean Rh surface, was observed with coverage. A threefold enhancement in activity was found for a TiO x coverage of 0.15 of a monolayer. The increase in rate at low coverages was accompanied by a higher selectivity toward olefins (10% → 31%), a lower activation energy (24.4 → 16.8 kcal/mol), a higher H 2 reaction order (1.0 → 2.6), and a higher CO reaction order (−1.0 → −0.3). The kinetic parameters for olefin production were noticeably different than those for paraffin production as well. The dependence of the activity on TiO x coverage has been modeled through a Monte Carlo simulation assuming the existence of sites of high activity at the oxide-metal interface. This model is consistent with the premise of Ti 3+ participation in the dissociation of CO.