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 MoO_x/Ni(111) accompanied by direct deoxygenation of phenol at the interfacial perimeter sites of MoO_x/Ni(111) under hydrodeoxygenation conditions has been investigated based on density functional theory (DFT) calculations. The coordinatively unsaturated Mo site with oxygen vacancies (Mo_ov) plays an essential role in the activation of the C–OH bond, with the Ni–MoO_x 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 Mo_ov 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 Mo_ov site of Mo₃O₇/Ni(111). In contrast, the Mo_ov 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.