Reaction of Acrylic Acid on TiO₂(001) Single Crystal Surfaces. Evidence of Different Pathways for Vinyl and Carboxyl Groups

The TiO 2 (001) single crystal surface is unique in that surface structure, coordination number of titanium cations, and degree of reduction may be easily controlled by varying the annealing temperature in ultrahigh vacuum. In an effort to understand the chemical pathways of conjugated molecules over the surfaces of semiconductors, the complementary techniques of temperature-programmed desorption, X-ray photoelectron spectroscopy, and scanning kinetic spectroscopy have been applied to study the interaction and surface chemistry of CH 2 CHCOOH with the TiO 2 (001) single crystal surface in detail. A clear difference is observed between the reaction of this unsaturated carboxylic acid and that of saturated analogues. Two different coupling pathways were observed. The first, involving the H 2 C CH moiety, generated C 4 and C 6 hydrocarbon products (butene, butadiene, and benzene). The second results in the production of divinyl ketone, formed by ketonization of two adsorbed acrylate molecules at a doubly coordinatively unsaturated titanium center on the {114}-faceted TiO 2 (001) surface only.