Abstract
The reactions of carboxylic acids were investigated on a TiO2 (001) single crystal surface which exhibits two stable faceted structures upon annealing at different temperatures. The {011}-faceted structure contains only Ti+4 cations which are five-fold coordinated, and the {114}-faceted structure exposes Ti+4 cations which are four-, five-, and six-fold coordinated. It was observed that the dissociation, reduction, and dehydration of carboxylic acids require only one degree of coordinative unsaturation of the surface cations and are therefore relatively insensitive to surface structural transformations, since both faceted structures of the TiO2 (001) surface contain five-fold coordinated cations. However, the bimolecular ketonization of acetates to acetone and of acrylates to divinylketone was only observed on the {114}-faceted structure and is, therefore, a structure-sensitive reaction. This reaction requires two coordination vacancies on a common cation to accommodate the pair of carboxylate species to be coupled. This latter result suggests that the active site for bimolecular ketonization is analogous to that on Ziegler-Natta catalysts for the oligomerization and polymerization of olefins.