Electronic vs. Geometric effects of Al2O3-supported Ru species on the adsorption of H2 and substrate for aromatic LOHC hydrogenation

Abstract

The intrinsic rate of aromatic hydrogenation is tuned by electronic and geometric characters of active metals. Herein, Ru/Al2O3 catalysts were prepared by varying the Ru loading from 0.25 to 12 wt% and tested in the hydrogenation of toluene and (di)benzyltoluene well-known as hydrogen carriers. At lower Ru loadings (<5 wt%), Ru nanoclusters smaller than 0.82 nm with strong Ru–Al2O3 interaction were formed, enriching electron-deficient Ru atoms for activity-unfavorable H2 adsorption. In contrast, Ru nanoparticles larger than 0.82 nm with weak Ru–Al2O3 interaction were produced at higher Ru loadings (>5 wt%), making larger Ru0 slabs for activity-unfavorable substrate adsorption. The kinetic barrier was higher for strong Ru–Al2O3 interaction or for the Ru species similar to close-packed Ru. The identified electronic and geometric properties of Ru species guaranteed the structure-sensitivity of Ru/Al2O3-catalyzed aromatic hydrogenation, recommending small-sized Ru species close to zero valence for accelerated aromatic hydrogenation.