Surface studies of oxide single crystals. Structure-activity and structure-selectivity relations
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The authors consider the selectivity of facetted TiO2 surfaces for reaction of carboxylic acids; in this example there is a clear-cut relationship between reaction selectivity and surface structure which permits the identification of the active sites required by the different reactions observed. It is found that (1) Dissociative adsorption of carboxylic acids on TiO2 is not sensitive to surface structure, provided that all surface cations have coordination vacancies. (2) Carboxylates decompose on-sites with a single degree of coordinative unsaturation (five-fold coordinated Ti4+ sites) to form net unimolecular dehydration products. (3) Bimolecular ketonization of carboxylates requires two coordination vacancies on a common cation (four-fold coordinated Ti4+ sites).