Aqueous phase electrocatalysis and thermal catalysis for the hydrogenation of phenol at mild conditions

The electrocatalytic hydrogenation (ECH) of phenol on Pt/C, Rh/C, and Pd/C was explored in an H-type two-compartment cell with respect to the impact of electrolyte, pH, current, and catalyst concentration. In all cases, the electric efficiencies increased with increasing phenol conversions. Rh/C exhibited the highest hydrogenation rate normalized to the concentration of accessible metal (TOF) followed by Pt/C in terms of mass of metal and intrinsic activities. Therefore, the effect of temperature on ECH and of mild thermal hydrogenation (TH) of phenol was explored on these catalysts. The activation energies for ECH were ca. 23kJ/mol and 29kJ/mol on Rh/C, and Pt/C, respectively. TH is much faster than ECH, although both pathways have the same activation energy. Cyclic voltammetry of bulk Pt and Pt/C in the presence of phenol indicated that phenol is adsorbed on the metal and reacted with hydrogen radicals. Hence, ECH was concluded to proceed via a Langmuir-type mechanism where the surface hydrogen is produced by reduction of protons (which occurs when the catalyst contacts the electrode) instead of H₂ dissociation as in TH. Although competitive reactions evolve H₂ during ECH, the involvement of this H₂ in phenol hydrogenation was minor. Thus, ECH and TH are independent processes and do not exhibit any synergy. In both pathways, the reaction path is phenol→cyclohexanone→cyclohexanol. C-O bond cleavage was not observed.