Promotional effect of Au on reduction of Ni(II) to form Au–Ni alloy catalysts for hydrogenolysis of benzylic alcohols

Gold–nickel bimetallic catalysts were prepared from Au/NiO and Au(OH)3–Ni(OH)2–NiCO3 coprecipitates by treatment with hydrogen. Gold promoted the reduction of Ni(II) to Ni(0) at relatively low temperatures in the range of 100–150 °C, which was confirmed by H2-TPR and in situ XAFS measurements, whereas NiO without Au was not fully reduced even at 300 °C. The obtained catalysts were characterized by XRD, HAADF-STEM, XAFS, and 197Au Mössbauer, and these analyses revealed the formation of Au–Ni alloy components in the obtained catalysts. Au existed as Au nanoparticles together with Au–Ni alloy components in Au–Ni-1 prepared from Au/NiO by H2 treatment. When Au(OH)3–Ni(OH)2–NiCO3 was treated in a flow of H2 to produce Au–Ni-2, the formation of Au NPs was not clearly observed, thereby meaning that Au atoms were highly dispersed as a single atom and/or small clusters in the obtained catalysts. Moreover, most of the Au atoms were alloyed with Ni atoms for Au–Ni-2. The obtained Au–Ni-1 and Au–Ni-2 exhibited superior catalytic activities for the selective hydrogenolysis of benzylic alcohols into alkylbenzene derivatives in terms of reaction rates normalized by catalyst surface area. Accordingly, Au–Ni-1 and Au–Ni-2 recorded the reaction rates of 4.79 and 9.79 mmol L−1 h−1 m−2, respectively. These values were greater than that obtained for Raney Ni (0.14 mmol L−1 h−1 m−2). In addition, Au–Ni-2, which contains higher Au–Ni alloy content, showed greater reaction rates when compared to Au–Ni-1. Since Au/TiO2 showed poor catalytic activity for the hydrogenolysis, Au–Ni alloy enhanced the catalytic activities of Ni(0).