Structure sensitivity of the hydrogenation of crotonaldehyde over Pt/SiO₂ and Pt/TiO₂

Hydrogenation of crotonaldehyde has been studied over SiO₂-and TiO₂-supported Pt catalysts. Over Pt/SiO₂, the selectivity to the primary products butyraldehyde and crotylalcohol depends critically on the Pt particle size; i.e., the selectivity to the unsaturated alcohol increases with increasing particle size. For large metal particles, the high fraction of Pt(111) surfaces is concluded to favor the adsorption of crotonaldehyde via the carbonyl bond. On small Pt particles, the high abundance of metal atoms in low coordination allows unconstrained adsorption of both double bonds. In this case, the hydrogenation of the C=C bond is kinetically favored. Activity and selectivity of Pt/TiO₂ catalysts after low-temperature reduction are similar to those of Pt/SiO₂. After high-temperature reduction the selectivity to crotylalcohol is generally enhanced. The selectivity of Pt/TiO₂ catalysts is then determined by the metal particle size and the extent of decoration of Pt with TiO_x particles. The presence of coordinatively unsaturated Ti cations in these oxide particles enhances the sorption strength of the C=O bond resulting in an enhanced selectivity to crotylalcohol. The effects of metal particle size and promotion by TiO_x are additive. TiO_x promotion of catalysts with small particles and the presence of unpromoted large particles allow reaching of selectivities to crotylalcohol of approximately 45%. Promotion of large Pt particles with TiO_x yields 64% selectivity to crotylalcohol.