Metal-catalyzed hydrogenations are regarded as non-sensitive reactions in the gas phase. However, varying proportions of high- and low-coordination surface atoms may lead to differences in the selectivity when hydrogenation is performed in aqueous phase especially under electrochemical condition where H₂ evolution reaction (HER) appeared to be a major competitive route. The aim of this work was to test this hypothesis by performing the electrocatalytic hydrogenation (ECH) of aromatic hydrocarbons on Pt/C and Pd/C catalysts with varying metal particle sizes. Phenol and benzaldehyde, representative model compounds for bio-oils derived from lignocellulosic biomass, were chosen as target compounds. ECH Experiments were performed at ambient conditions using H-cells wherein cathodic potential was varied systematically. The results of these experiments were compared with catalytic thermal hydrogenation, where the reduction equivalents are provided by H₂ dissociation.

In the studied range (from -0.6 V to -0.8 V vs Ag/AgCl), the rates of both ECH and HER increase with increasing cathodic potentials. These increases, however, depended on the metal loading of the cathode. As an exemplary result, the figure shows that the rates of phenol ECH reached a maximum at the Pt dispersion of 26%, which corresponded to a Pt loading of 5 wt.% on carbon. However, the rates normalized per active site showed a different picture. TOF values increased as the dispersion of the metal decreased. In stark contrast, the rates of the H₂ evolution reaction (HER) were proportional to the dispersion. Hence, the main conclusion of this work is that the intrinsic rates of ECH of polar molecules are higher on extended metal surfaces than in the corners and edges exhibited by small metal particles. The intrinsic rates of HER, on the other hand, are favored by small particles under the applied reaction conditions. The combination of these two opposite trends rendered the catalyst with Pt particles with the average size of 4.5 nm (5 wt.% Pt on C in this work) as the most active catalyst per gram. However, the catalyst with average Pt particle size of 10 nm shows the highest intrinsic activity as well as faradaic efficiency (30%)

Thermal catalytic hydrogenation (TCH) also followed the negative particle size effect to the activity trend, thus, coincide with ECH experiments. This strongly suggests the particle size effect noted in the electrochemical experiments is primarily related to the aqueous environment of the reaction instead of presence of electric potential or competing HER. It is worth highlighting that ECH rates were higher than TCH rates at cathodic potentials higher than -0.7 V vs Ag/AgCl for all tested substrates, which is ascribed to higher coverages of reactive H.

This work contributes to the understanding of metal catalysis at low temperatures in condensed phases. It also shows directions for the development of highly active electrocatalysts for the conversion of polar molecules. In specific, we show that as the dispersion of the metal phase increases, the compromise between increasing concentrations of available active sites and their decreasing reactivity leads to optimum particle sizes.

Figure caption. Left: Rates of phenol hydrogenation on Pt/C along with dispersion at varying cathodic potentials (vs Ag/AgCl). Right: Turnover frequency (TOF) for phenol hydrogenation on Pt/C along with dispersion at varying cathodic potentials (vs Ag/AgCl). The points labeled as "thermal" correspond to hydrogenation performed with H₂ gas.