POLYMERIZATION AND DECARBONYLATION REACTIONS OF ALDEHYDES ON THE PD(111) SURFACE
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The adsorption and reaction of formaldehyde, acetaldehyde, and propanal were compared in temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) studies on a clean Pd(111) surface. Formaldehyde reacted upon adsorption at 170 K via decomposition to CO(a) and H(a) and polymerization to paraformaldehyde. In contrast, decomposition of the higher aldehydes required significantly higher temperatures. Acetaldehyde adsorbed on the clean surface in an 2 configuration and reacted to form stable acetyl species (CH3C=O), which decomposed above 300 K to produce CO (a), H(a), and Q hydrocarbon fragments. Adsorbed acetyl groups were readily identified by the characteristic frequency of their CO stretch at 1565 cm-1. In competition with decarbonylation, desorption of acetaldehyde occurred at 220 and 325 K. The CO stretch of 2-acetaldehyde was observed at 1390 cm-1. Similarly, the adlayer produced by the adsorption of propanal on Pd(111) was a mixture of 2-propanaI and propanoyl species (CH3CH2C==O) produced by removal of the aldehyde hydrogen atom. No evidence was found for polymerization of either acetaldehyde or propanal on the clean Pd(111) surface. A kinetic isotope effect was observed in the decomposition of adsorbed acetyl intermediates, suggesting that C-H cleavage is rate determining for this reaction. These results illustrate the pathways, intermediates, and energetics of aldehyde decarbonylation reactions on group VIII metal surfaces and may provide insights into the mechanism of oxygenate synthesis from CO and H 2. 1989, American Chemical Society. All rights reserved.
