Hydrogenation of hydrocarbons in gas phase is commonly accepted to be a structure-insensitive reaction in wide ranges of conditions and metal dispersion. However, varying proportions of high- and low-coordinated surface atoms may lead to differences in the selectivity when hydrogenation is performed in aqueous phase, especially under electrochemical condition where the H2 evolution reaction (HER) is the prevalent side reaction. The aim of this study was to test this hypothesis by performing the electrocatalytic hydrogenation (ECH) of aromatic hydrocarbons on Pt/C, Rh/C and Pd/C with varying metal particle sizes. Phenol and benzaldehyde, representative model compounds for bio-oils derived from lignocellulosic biomass, were chosen as target compounds. ECH of phenol and benzaldehyde using Pt/C as catalysts were studied with Pt particle size varying from 3-10 nm. The intrinsic hydrogenation activity and Faradaic efficiency (i.e. selectivity towards hydrogenation over HER) was found to increase with increasing Pt particle size. A lineal correlation found between hydrogenation TOFs with the fraction of metal atoms in (100) and (111) planes (terrace sites) suggests a preferential adsorption of aromatic compounds on extended Pt surface which results in higher activity and selectivity. This was further confirmed by in-situ XANES characterization which showed that presence of higher fraction of surface site capable of phenol/benzaldehyde adsorption on larger Pt particles compared to the smaller ones. A very similar activity trend (i.e. increase in hydrogenation TOF with increasing Rh particle size) in case of phenol hydrogenation was obtained when Rh/C was used as catalyst. This suggest both Pt and Rh possess similar structural requirement for the hydrogenation. In stark contrast, smaller Pd particles were more active towards ECH of benzaldehyde when a systematic study was carried out on series of Pd/C catalysts with varying Pd particle size (2-10 nm). The Faradaic efficiency remained almost insensitive (~95%) when the Pd particles were within 2-5 nm size range but decreased continuously by further increasing Pd particle size. The results of the detail kinetic studies, and the corresponding reaction mechanisms will be addressed in this presentation.

Electrocatalytic hydrogenation of oxygenates exhibits particle size effects and they are metal specific

2019 Fall National Meeting

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