Role of MoOx/Ni(111) interfacial sites in direct deoxygenation of phenol toward benzene†
Abstract
Characterizing the active site structure and understanding the reaction mechanism at the metal/metal oxide interface are crucial for the design and optimization of many catalytic processes. Herein, the structural evolution of MoOx/Ni(111) accompanied by direct deoxygenation of phenol at the interfacial perimeter sites of MoOx/Ni(111) under hydrodeoxygenation conditions has been investigated based on density functional theory (DFT) calculations. The coordinatively unsaturated Mo site with oxygen vacancies (Moov) plays an essential role in the activation of the C–OH bond, with the Ni–MoOx site being the active site for C–OH bond breakage. The activation barrier for phenol dehydroxylation was reduced from 1.60 eV on Ni(111) to 1.35 eV on Mo3O7/Ni(111), and then to 0.95 eV on Mo3O5/Ni(111), indicating that the lower the oxidation state of the Moov site, the higher the activity toward the C–OH bond cleavage. Microkinetic parameter analysis showed that the dehydroxylation of phenol is the rate-limiting step. In addition, the reduction of Mo3O7/Ni(111) to Mo3O5/Ni(111) is energetically more favorable than the deoxygenation of phenol at the Moov site of Mo3O7/Ni(111). In contrast, the Moov site of Mo3O5/Ni(111) shows high activity for phenol deoxygenation and Mo3O5/Ni(111) cannot be further reduced under the reaction conditions, making the Mo3O5/Ni(111) interfacial sites the dominant active centers for deoxygenation. The nature of the active centers and the mechanism of deoxygenation revealed in this study are helpful for designing Ni-based catalysts for direct deoxygenation of phenolics to aromatics and optimizing the operating conditions.