Hydrogenation of benzaldehyde via electrocatalysis and thermal catalysis on carbon-supported metals

Selective reduction of benzaldehyde to benzyl alcohol (model reaction for low-temperature stabilization of bio-oil) on C-supported Pt, Rh, Pd, and Ni in aqueous phase was conducted using either H₂ (thermal catalytic hydrogenation, TCH) or hydrogen generated in situ electrocatalytically (electrocatalytic hydrogenation, ECH). In TCH, the intrinsic activity of the metals at room temperature and 1 bar H₂ increased in the sequence Pt/C < Rh/C ≤ Pd/C, while Ni/C is inactive. At these conditions, the coverage of benzaldehyde is high while the coverage of adsorbed H is low and the reaction follows a Langmuir-Hinshelwood mechanism. All tested metals were active in ECH of benzaldehyde above the onset potentials of the H₂ evolution reaction (HER). Thus, hydrogenation competes with HER. The relative rates of H reacting to H₂ and H addition to benzaldehyde determines the selectivity to ECH and HER. Accordingly, the selectivity of the metals towards ECH increases in the order as follows: Ni/C < Pt/C < Rh/C < Pd/C. The latter having ECH selectivity around 99%. In ECH, the intrinsic activities of all tested metals were higher and the activation energies of benzaldehyde hydrogenation were lower than in TCH.