Adsorption and reaction of aldehydes on Pd surfaces
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Aldehydes have been proposed as important intermediates during alcohol synthesis on supported transition metal catalysts. To develop insights into higher oxygenate syntheses, adsorption and reaction of aldehydes on transition metal surfaces and the surface structure dependence of these processes are considered here. In this work, the adsorption and reactions of acetaldehyde and propionaldehyde on Pd(110) surfaces were investigated with temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy techniques. The slate of desorption products observed in TPD experiments following acetaldehyde adsorption on the clean Pd(111) and Pd(110) surfaces was the same: CO, H2, CH4, and CH3CHO were observed in both cases. Likewise, propionaldehyde decomposition gave rise to CO, H2, C2H4, and C2H6 on both surfaces. However, acetaldehyde isotope-labeling experiments indicated that methyl groups were released following decarbonylation reactions of acetaldehyde on Pd(110), in contrast with earlier suggestions of methylene release on the clean Pd(111) surface. Further studies on H- and D-precovered Pd(110) surfaces elucidated the competing decomposition and hydrogenation pathways as well as the distribution of hydrocarbon species released after decarbonylation of adsorbed aldehydes on Pd(110). These results demonstrate that a variety of surface hydrocarbon ligands can be produced by aldehyde decarbonylation on palladium surfaces and imply that higher oxygenate synthesis by carbonylation of surface hydrocarbon fragments may involve an equally large variety of hydrocarbon intermediates on supported metal catalysts.
