STRUCTURE AND COMPOSITION REQUIREMENTS FOR DEOXYGENATION, DEHYDRATION, AND KETONIZATION REACTIONS OF CARBOXYLIC-ACIDS ON TIO2(001) SINGLE-CRYSTAL SURFACES
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The reactivities of TiO2(001) single-crystal surfaces for acetic acid decomposition were studied by TPD and XPS. Surface structure and composition were varied by a combination of argon ion bombardment and annealing in order to investigate reaction selectivity and to identify the active sites responsible for adsorption and decomposition. Acetic acid adsorbed both molecularly and dissociatively at 200 K, but only dissociatively at 300 K. The molecular species desorbed readily below 300 K. Approximately 15% of the dissociatively adsorbed species (acetates) desorbed as acetic acid at 390 K via recombination with surface hydroxyls. The remaining acetates decomposed at higher temperatures via three different reaction pathways with selectivities dependent on surface composition and structure. Reduced surfaces containing Ti cations of lower valences favored direct deoxygenation of the acetates to fill the oxygen vacancies of the surface. This reaction occurred even at room temperature and deposited atomic carbon on the surface. Oxidized surfaces containing only Ti4+ cations favored reactions to form volatile molecular products. On the {011}-faceted surface, surface acetates decomposed via net unimolecular dehydration to ketene. On the {114}-faceted surface, the bimolecular reaction of surface acetates to form acetone also took place. The reactions of propionates formed by dissociation of propionic acid were directly analogous to those of acetates: methyl ketene was produced by net unimolecular dehydration on the {011}-faceted surface and 3-pentanone was the bimolecular ketonization product on the {114}-faceted surface. Since the {011}-faceted surface exposes only Ti4+ cations of fivefold oxygen coordination, these sites were determined to be responsible for the net unimolecular dehydration. Bimolecular ketonization required the coordination of two acetates to a common Ti4+ cation on a stoichiometric surface. Of the single-crystal surfaces examined, only the {114}-faceted surface presented the necessary fourfold oxygen-coordinated Ti4+ sites for this reaction. 1990.
