Synthesis Of Atomically Precise Gold Nanoclusters On Ternary LDH/rGO Multilevel Structure With Highly Efficient Solvent-free Oxidation Of Benzyl Alcohol

Au-based catalysts,especially atomically precise Au_n nanocluster（NCs）catalysts with definite structure have attracted considerable attention due to their mild reaction conditions and low peroxidation resistance compared with other noble metal catalysts.In order to solve the problems of relatively single support composition and large-sized gold nanoparticles synthesized by traditional coprecipitation/impregnation reduction method in the study of liquid phase oxidation over gold catalysts,the present paper proposed the synthesis of multistage Au NCs catalyst with ternary hydrotalcite/graphene hierarchical structure as the support and atomically precise gold clusters as the precursor.The effect of molar ratio of metal elements in support and the number of Au atoms in the obtained catalysts on the catalytic performance for solvent-free molecular oxygen oxidation of benzyl alcohol were systematically investigated to reveal the structure-activity relationship.The main results and innovations are as follows:（1）Based on the enhanced hydrophobicity of Ni Mg Al-LDH/r GO hybrid support compared with pure LDH,a double p H-controlled electrostatic adsorption method was developed to prepare a series of multistage catalysts Au_n/Ni_xMg_3-xAl-LDH/r GO-T（x=0,1,1.5,2 and 3,T represents calcination temperature,the ligand was completely removed at280 ^oC）by using captopril protected atomically precise Au_n（n=25,38 and127）as precursor.The obtained catalysts present nanosheet array-like morphology with LDH nanosheets grown vertically and interleaved on both sides of r GO lamellae and ultrafine Au_n NCs（1.0-1.9 nm）highly dispersed at the edge sites of LDH and the interface between LDH and r GO.（2）All the hierarchically structured Au_n NCs catalysts exhibit excellent catalytic activity for the solvent-free oxidation of benzyl alcohol without additional base.For the catalysts Au₂₅/Ni_xMg_3-xAl-LDH/r GO-280,the reaction activity increases first,then decreases with the increase of Ni/Mg ratio,while for the catalysts Au_n/Ni_1.5Mg_1.5Al-LDH/r GO-280,the reaction activity decreases gradually with increasing Au atomic number.The Au₂₅/Ni_1.5Mg_1.5Al-LDH/r GO-280 displays the highest activity(TOF=5118 h^-1).XPS and CO-DRIFTS results reveal that the high activity of this catalyst is closely related to the small size of Au₂₅ NCs,the Au⁰/Au^δ+-Ni²⁺-OH interface with the highest Au^δ+/(Au⁰+Au^δ+)ratio,the strongest synergy between Au₂₅ NCs and LDH/r GO,and the improved transport and diffusion of the reactants upon the multilevel structure.