Hydrogenation of Crotonaldehyde over Sn/Pt(111) Alloy Model Catalysts

Gas-phase hydrogenation of crotonaldehyde (CH3CHCHCHO) was studied over the (2×2) Sn/Pt(111) and (□3×□3) R30° Sn/Pt(111) surface alloys which were used as model catalysts. The influence of the alloy structure, hydrogen pressure, and temperature on activity and selectivity toward 2-butenol (CH3CHCHCH2OH) formation was investigated. The results were compared to those obtained for the pure Pt(111) surface. All catalysts were characterized prior to use by X-ray photoelectron spectroscopy and low-energy electron diffraction to ensure proper alloy formation. The hydrogenation activity was about two times higher for the bimetallic Pt–Sn catalysts compared to that for Pt(111); however, little change in selectivity was observed. Butyraldehyde was formed as the main product in all cases. Therefore, alloy formation improves hydrogenation activity toward CC and CO functional groups and cannot solely explain the improvement in selectivity toward the formation of the unsaturated alcohol that has often been reported for supported Pt–Sn catalysts versus pure Pt catalysts. Both alloys investigated had similar catalytic activity and selectivity. An attempt was made to create oxidic tin species on the Pt(111) surface, because these species have been suggested to improve selectivity to the unsaturated alcohol product. However, the oxidic species formed by heating the alloys in 5 Torr O2 for 5 min at 600 K were not stable under reaction conditions and were reduced to form a surface with low catalytic activity, probably consisting of metallic Sn rafts over the Pt(111) surface.