Effects of Altervalent Cation Doping of TiO₂ on Kinetic Parameters of CO Hydrogenation and CO Oxidation on Supported Rh

The reactions of CO hydrogenation and CO oxidation over Rh are used to determine the influence of altervalent cation doping of a TiO₂ support on kinetic parameters. Rh crystallites are dispersed on TiO₂ doped with W⁶⁺ cations at concentration levels up to 0.67 at%. It is shown that turnover frequencies of CO hydrogenation increase monotonically with increasing dopant content in the TiO₂ matrix, apparent activation energy is reduced, and selectivity shifts towards smaller-chain saturated hydrocarbons. Turnover frequencies of CO oxidation at low CO/O₂ ratios are shown to pass through a maximum with increasing W⁶⁺ dopant content of TiO₂. A small activity enhancement is observed at higher CO pressures. The apparent activation energy is found to vary with W⁶⁺ content. It is proposed that charge transferred from the doped semiconducting carrier to the metal crystallites alters the chemical potential of the crystallites and, therefore, the work function of surface metal atoms, while a strong electrostatic field is caused by the dipole developed at the metal-semiconductor interface. As a result, the mode of H₂ and CO adsorption is altered, as well as the intrinsic kinetic parameters of the reactions investigated.