Role of Dissociation of Phenol in Its Selective Hydrogenation on Pt(111) and Pd(111)

The adsorption, dissociation, and hydrogenation of phenol on the Pt(111) and Pd(111) surfaces have been studied using density functional theory slab calculations. The results show that phenol favors adsorption through a mixed σ–π interaction on both surfaces through its phenyl ring, with the hydrogen atoms and hydroxyl tilted away from the surface. The dissociation of phenol to phenoxy is both thermodynamically and kinetically favored on Pd but not on Pt. The phenoxy adsorbs on Pd through both the phenyl ring and the oxygen atom, whereas the O atom points away from the surface on Pt. On Pt, the barrier for adding one hydrogen atom to the adsorbed phenol is 0.49 eV lower than the overall barrier for phenol dissociation to phenoxy followed by adding the hydrogen atom to its phenyl ring, resulting in direct hydrogenation of the adsorbed phenol to cyclohexanol as the dominant reaction pathway. In contrast, on Pd, the barrier for direct hydrogenation (1.22 eV) is higher than the overall barrier of dissociation followed by the hydrogenation process (0.85 eV), resulting in hydrogenation of the adsorbed phenoxy to cyclohexanone as the major reaction pathway. Microkinetics analysis confirms that hydrogenation of the adsorbed phenol is the dominant pathway on Pt, whereas phenoxy hydrogenation drives the turnover on Pd. These results are consistent with the experimentally observed selectivity of phenol hydrogenation on Pd and Pt catalysts.