Elucidating the Role of Surface Metal Geometry at the Metal–Oxide Interface for C–O Bond Activation of Oxygenates

The precise function of metal sites at the bifunctional metal–oxide interface in catalytic hydrodeoxygenation (HDO) of biomass-derived oxygenates remains a subject of debate: do they serve merely as hydrogen dissociation centers, or do they also directly participate in activating the oxygenate substrate? To elucidate this, we fabricated two model Ni/TiO 2 catalysts with distinct surface Ni geometries by leveraging the crystal phase-dependent strong metal-support interaction. Ni/r-TiO 2 features exposed surface Ni ensembles, while Ni/a-TiO 2 presents predominantly isolated Ni sites due to partial encapsulation by a TiO x overlayer. Their HDO performance was evaluated using a suite of model compounds, including m -cresol, anisole, benzyl alcohol, and furfuryl alcohol, representing key C–O bond types in bio-oils. The results demonstrate that Ni/r-TiO 2 exhibits broad activity for all substrates, whereas Ni/a-TiO 2 selectively cleaves only C–O bonds connected to sp 3 -hybridized carbon atoms. This fundamental difference originates from the bond dissociation energy. The cleavage of high-energy C Aromatic –O bonds necessitates a synergistic activation mode at the Ni ensemble-oxygen vacancy interface, which, while effective, concomitantly promotes undesirable ring-related side reactions such as C–C hydrogenolysis, thereby compromising deoxygenation selectivity. Conversely, lower-energy C sp 3 –O bonds can be directly activated on oxygen vacancies assisted by spillover hydrogen from the isolated Ni sites, enabling precise and selective deoxygenation. This study clarifies the structure-dependent role of interfacial metal geometry and provides a mechanistic guideline for the rational design of selective HDO catalysts tailored to specific feedstock platforms.