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Role of Dissociation of Phenol in Its Selective Hydrogenation on Pt(111) and Pd(111)
journal contribution
posted on 2015-03-06, 00:00 authored by Gaofeng Li, Jinyu Han, Hua Wang, Xinli Zhu, Qingfeng GeThe
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.