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.
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