ADSORPTION AND REACTION OF ALDEHYDES ON PD SURFACES

Aldehydes have been proposed as important intermediates during alcohol synthesis on supported transition metal catalysts, To develop insight s into higher oxygenate syntheses, adsorption and reaction of aldehyde s 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 wer e investigated with temperature-programmed desorption (TPD) and high-r esolution electron energy loss spectroscopy techniques. The slate of d esorption products observed in TPD experiments following acetaldehyde adsorption on the clean Pd(lll) and Pd(110) surfaces was the same: CO, H-2, CH4, and CH3CHO were observed in both cases. Likewise, propional dehyde decomposition gave rise to CO, H-2, C2H4, and C2H6 On both surf aces, However, acetaldehyde isotope-labeling experiments indicated tha t methyl groups were released following decarbonylation reactions of a cetaldehyde on Pd(110), in contrast with earlier suggestions of methyl ene release on the clean Pd(lll) 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 spe cies released after decarbonylation of adsorbed aldehydes on Pd(110). These results demonstrate that a variety of surface hydrocarbon ligand s can be produced by aldehyde decarbonylation on palladium surfaces an d imply that higher oxygenate synthesis by carbonylation of surface hy drocarbon fragments may involve an equally large variety of hydrocarbo n intermediates on supported metal catalysts.