Understanding the effect of metal particle size on the reactions during hydrodeoxygenation of phenolics is of great importance for rational design of a catalyst for selective control of a desirable reaction. To this end, vapor phase hydrodeoxygenation of m-cresol was studied over 5% Ni/SiO 2 catalysts with varying Ni particle sizes (2−22 nm) at 300 °C and 1 atm H 2 . The Ni particle sizes were confirmed by several characterization techniques, and the varying surface concentration of terrace, step, and corner sites with Ni particle sizes was verified by H 2 temperature-programmed desorption. Decreasing the Ni particle size from 22 to 2 nm improves the intrinsic reaction rate by 24 times and the turnover frequency (TOF) by 3 times. The TOFs for toluene and methylcyclohexanone/methylcyclohexanol formation increase by 6 and 4 times, respectively, while the TOF for CH 4 formation decreases by 3/4, indicating that smaller particles with more defect sites (step and corner) favor deoxygenation and hydrogenation while larger particles with more terrace sites favor C−C hydrogenolysis. Density functional theory study shows that the barrier for direct dehydroxylation of phenol on Ni(111), Ni(211), and defected Ni(211) decreases from 175.6 to 145.6 and then to 120.5 kJ/mol. The results indicate that a highly coordinatively unsaturated surface Ni site is responsible for C−O cleavage through facile adsorption and stabilization of −OH in the transition state, thus facilitating deoxygenation toward toluene. Our results indicate that tuning the metal particle size is an effective approach to control reactions during hydrodeoxygenation.

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Catalysts based on earth-abundant elements, such as Ni and Mo, that can be used for the conversion of lignin-derived compounds are desirable. However, they usually exhibit low activity and/or selectivity toward the target reaction, hydrodeoxygenation (HDO). For example, conversion of m-cresol in H 2 over a typical Ni/SiO 2 leads to ring hydrogenation at low temperatures and C−C hydrogenolysis to CH 4 at high temperatures. Here, we report that a bimetallic Ni−Mo/SiO 2 catalyst with Ni:Mo ratio ≈ 1 reduced at an optimized temperature can be very active and selective for HDO of m-cresol to toluene over a wide range of reaction temperatures (250−350 °C) and 1 atm of H 2 . This behavior is explained in terms of the surface structure of Mo oxides on the surface of Ni nanoparticles. Detailed characterization (XRD, Raman, TPR, EXAFS, and XPS) indicates that, after calcination, NiMoO 4 is the predominant phase. However, after subsequent reduction, metallic Ni nanoparticles segregate out of the partially reduced MoO x . Interestingly, while no significant structural/electronic modifications are detected for the bulk of the metallic Ni particles, the surface chemistry is clearly altered (i.e., no hydrogenolysis/hydrogenation, weak CO/H 2 adsorption, and lower electron density in the d band of Ni). These results suggest that after reduction, in contrast to the formation of NiMo alloy, the Ni surface gets decorated by reduced MoO x moieties, a phenomenon similar to that previously observed on reducible oxides (so-called SMSI), which is essential for maximizing HDO and inhibiting hydrogenolysis.