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).