STRUCTURE SENSITIVITY OF THE HYDROGENATION OF CROTONALDEHYDE OVER PT SIO2 AND PT/TIO2/

Hydrogenation of crotonaldehyde has been studied over SiO2- and TiO2-s upported Pt catalysts. Over Pt/SiO2, the selectivity to the primary pr oducts butyraldehyde and crotylalcohol depends critically on the Pt pa rticle size; i.e., the selectivity to the unsaturated alcohol increase s with increasing particle size. For large metal particles, the high f raction of Pt(111) surfaces is concluded to favor the adsorption of cr otonaldehyde via the carbonyl bond. On small Pt particles, the high ab undance of metal atoms in low coordination allows unconstrained adsorp tion of both double bonds. In this case, the hydrogenation of the C=C bond is kinetically favored. Activity and selectivity of Pt/TiO2 catal ysts after low-temperature reduction are similar to those of Pt/SiO2. After high-temperature reduction the selectivity to crotylalcohol is g enerally enhanced. The selectivity of Pt/TiO2 catalysts is then determ ined by the metal particle size and the extent of decoration of Pt wit h TiOx particles. The presence of coordinatively unsaturated Ti cation s in these oxide particles enhances the sorption strength of the C=O b ond resulting in an enhanced selectivity to crotylalcohol. The effects of metal particle size and promotion by TiOx are additive. TiOx promo tion of catalysts with small particles and the presence of unpromoted large particles allow reaching of selectivities to crotylalcohol of ap proximately 45%. Promotion of large Pt particles with TiOx yields 64% selectivity to crotylalcohol. (C) 1997 Academic Press.