Adsorption and reaction of acrolein on titanium oxide single crystal surfaces: coupling versus condensation

The reactions of acrolein have been investigated on TiO2 (0 0 1) single crystal surfaces by temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS). Two carbon-carbon bond-forming reactions were observed. The first, on defect-containing surfaces, is reductive coupling to form olefins. The high reaction yield of ca. 80% shows the high activity of such surfaces for carbon-oxygen bond dissociation (needed for surface oxygen restoration) and carbon-carbon bond formation to make olefins. The second reaction, observed on the stoichiometric surface, is condensation of two acrolein molecules to give a C6H8O product tentatively identified as 2-methyl-2,4-pentadienal. Condensation reactions of carbonyls are characteristic of TiO2 surfaces; for acrolein, this reaction is proposed to involve initial hydrogen addition followed by nucleophilic attack on a second molecule of acrolein. This results in an aldol condensation followed by dehydration.

NEXAFS analyses were conducted in order to differentiate the states of molecularly adsorbed acrolein on the two distinctly different surfaces. The C=O bond of adsorbed acrolein is maintained in the case of the stoichiometric surface (evidenced by a pi(C)(*) = O transition at 286.6 eV), while it is absent on the reduced surface. The absence of this NEXAFS transition on the reduced surface suggests that the O atom of the C=O bond has reacted with the oxygen-deficient lattice. The restoration of these oxygen deficiencies is concomitant with the formation of the reductive coupling products (as observed by TPD and XPS experiments). (C) 2003 Elsevier B.V. All rights reserved.