SPECTROSCOPIC IDENTIFICATION OF ALKOXIDE, ALDEHYDE, AND ACYL INTERMEDIATES IN ALCOHOL DECOMPOSITION ON PD(111)

The reactions of primary and secondary alcohols were studied on clean Pd(111) surfaces with High Resolution Electron Energy Loss Spectroscopy (HREELS). Emphasis was placed on elucidating the reaction pathways involved in alcohol decomposition and upon determining the influence of different substituents on the stability of adsorbed intermediates. On the clean Pd(111) surface, both primary and secondary alcohols were found to react via decarbonylation pathways. Although the initial step in alcohol activation was alkoxide formation via O-H bond cleavage, the stabilities of the intermediates involved in subsequent reactions differed among the alcohols studied. Methoxides generated by the reaction of methanol underwent selective dehydrogenation to η2(C,O)-formaldehyde species which ultimately decomposed to adsorbed carbon monoxide and hydrogen atoms. Analogous behavior was observed for ethanol and 1-propanol, with the exception that partial dehydrogenation of the η2-aldehyde species derived from these alcohols produced stable acyl intermediates. Decomposition of these higher acyls required temperatures above 260 K and was the rate-limiting step in higher alcohol decomposition. The secondary alcohol 2-propanol reacted via similar pathways, although some desorption of the intermediate dehydrogenation product (acetone) occurred. © 1990.