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The hydrogenation of but-2-enal over supported Au catalysts is discussed, together with a detailed characterisation study using X-ray di †raction, infrared spectroscopy and transmission electron microscopy. Au/ZnO catalysts are found to be selective for the formation of the unsaturated alcohol, but-2-en-1-ol rather than the saturated aldehyde, butanal. In general, the addition of thiophene is found to enhance the yield of the unsaturated alcohol. Detailed transmission electron microscopy and infrared spectroscopy studies show that thiophene modiÐcation of Au/ZnO catalysts does not a †ect the Au-particle size or morphology ; rather, thiophene undergoes irreversible dissociative adsorption giving a surface in which the Au sites are electronically promoted by sulfur. It is observed that thiophene modiÐcation does not give any marked e †ect on catalyst performance for the catalysts that contain large Au-particles (P10 nm) and, hence, it is considered that the sulfur promotion observed is associated with smaller Au nanoparticles. The highest but-2-en-1-ol selectivities (D80%) are observed for 5 wt.% Au/ZnO catalysts reduced at 400 ¡C prior to reaction. It is proposed that the origin of high selectivity is associated with large Au particles (10È20 nm in diameter) that are present in this catalyst.

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The associative adsorption of acetone onto Lewis acidic Ti4+ ion sites on rutile is followed by the formation of a surface enolate complex which reacts with a further acetone molecule to give mesityl oxide. The enolate is also an intermediate in the exchange of hydrogen atoms between acetone and hydroxyl groups on rutile. The effects of dehydroxylation and dehydration of the rutile surface on the associative adsorption of acetone and on the formation of the enolate complex and mesityl oxide have been studied using infrared spectroscopy. The reversibility of the surface reactions has been established, in part by a study of the adsorption of mesityl oxide, which not only associatively adsorbs onto Ti4+ ion sites but also decomposes to enoIate complex and coordinatively liganded acetone molecules. Reduction of rutile in hydrogen decreases the Lewis acidity of the oxide surface but promotes the formation of surface acetate ions by the chemisorption of acetone.

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