The adsorption and reaction of ethylene glycol and 1,2-propanediol on Pd(111): A TPD and HREELS study

The reactions of ethylene glycol and 1,2-propanediol have been studied on Pd(111) using temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). Both molecules initially decompose through O-H activation, forming ethylenedioxy (-OCH2CH2O-) and 1,2-propanedioxy (-OCH2CH(CH3)O-) surface intermediates. For ethylene glycol, increases in thermal energy lead to dehydrogenation and formation of carbonyl species at both oxygen atoms. The resulting glyoxal (O═CHCH═O) either desorbs molecularly or reacts through one of two competing pathways. The favored pathway proceeds via C-C bond scission, dehydrogenation, and decarbonylation to form carbon monoxide and hydrogen. In a minor pathway, small amounts of glyoxal undergo C-O bond scission and recombination with surface hydrogen to form ethylene and water. The same reaction mechanism occurs for 1,2-propanediol after methyl elimination and formation of glyoxal. However, this is accompanied by a minor pathway involving a methylglyoxal (O=CHC(CH3)=O) intermediate. The prevalence of the dehydrogenation/decarbonylation pathway in the current work is consistent with the high selectivity for C-C scission in the aqueous phase reforming of polyols on supported Pd catalysts.