| Gas-phase hydrogenation of crotonaldehyde (CH3CH=CHCHO) was studied over Pt(111), the (2 x 2)-Sn/Pt(111), (√3 x √3)R30°-Sn/Pt(111), Fe/Pt(111), and (√19 x √19)R 23.4°-Ge/Pt(111) ordered surface alloys which were used as a model catalysts. The influence of alloy structure, hydrogen pressure, and temperature on the activity and selectivity toward 2-butenol (CH3CH=CHCH2OH) formation was investigated. The results were compared with those obtained for the pure Pt(111) surface. All of these surfaces were characterized prior to use as catalysts by XPS and LEED to insure proper alloy formation. The hydrogenation activity was increased by a factor of two for the bimetallic Pt-Sn catalysts, ten for the Fe/Pt(111) surface alloy, and one to two for the Ge-Pt surface alloy compared to that for Pt(111). Little change in the selectivity was observed for the Sn-Pt or Fe-Pt alloys. However, the Ge-Pt alloy displayed a significant selectivity improvement over Pt(111). Butyraldehyde was formed as the main product in all cases. Alloy formation unselectively improves the hydrogenation activity toward both C=C and C=O functional groups, and thus alloy formation can not solely explain the improvement in selectivity toward formation of the unsaturated alcohol that has often been reported for supported bimetallic Pt-Sn and Fe-Pt catalysts versus pure-Pt supported catalysts. An attempt was made to create model catalysts containing oxidic Sn, Fe, and Ge species on the Pt(111) surface, because these oxidic species have been suggested to improve the selectivity to the unsaturated alcohol product. However, the oxidic species formed by oxidizing the admetal with O2 or NO2 were either not stable under reaction conditions and were reduced to form a surface with low catalytic activity (Sn and Ge) or did not show any positive effect on the selectivity (Fe). |
