CH bond activation and radical–surface reactions for propylene and methane over α-Bi2O3

Abstract

A molecular-orbital study has been made of CH activation in methane and the methyl group of propylene by an α-Bi₂O₃ surface. Three oxidation states of the surface bismuth cations were modelled: II(reduced), III(normal) and V(oxidized). It was found that barriers to the formation of surface OH^– and gas-phase methyl and allyl radicals decrease as the bismuth oxidation state increases. This is because the radical electron from hydrogen is promoted by σ-bond formation to successively lower-energy bismuth 6s, 6p lone-pair or surface-state dangling bond orbitals as the state of bismuth oxidation increases. Experimental results in the literature also show this trend for forming gas-phase allyl radicals: in an oxidizing environment with O₂ in the gas stream the barrier is lower than in a reducing environment with no oxygen. We found a similar trend for stabilities of heterolytically adsorbed dissociation products and for homolytic adsorption on oxygen. The increased stability with increasing bismuth oxidation state is the result of reducing lower-lying surface orbitals. We found heterolytic adsorption, H on O^2– and CH₃ or C₃H₅ on Bi, most stable for the Bi^III and Bi^V surfaces and we predict that the Bi^II surface is unreactive.