On the Switch Between Selective Oxidation and Selective Hydrogenation of a Terminal Alkene on Well-Defined Titania Surfaces

Abstract

Stoichiometric, hydroxylated titania surfaces are effective for the partial oxidation of styrene to acetophenone. The initial product of reaction is styrene epoxide which undergoes isomerisation to acetophenone, the thermodynamically favoured product. The oxygen in the product derives exclusively from the water used for hydroxylation, and not from the titania lattice. Surface hydroxyl groups are the active sites for this 100% selective, partial oxidation. Controlled reduction of titania to TiO_x leads to the appearance of bandgap photoemission associated with the formation of Tiⁿ⁺ (n = 0--3) at the surface. This results in complete suppression of all oxidation activity and a switch to selective hydrogenation: the reaction product is now ethylbenzene, again produced with 100% selectivity. The important implications of these findings for an understanding of the properties of metal/titania catalysts used for alkene conversion are discussed.