Sub-Nano Copper Sites on SiO2 with Downshifted d-Band Centers Dominate Non-Radical Selective Oxidation of Benzene to Phenol.

The identification of dominant reactive oxygen species (ROS) in the direct oxidation of benzene with H2O2 holds crucial significance for understanding catalytic mechanisms and guiding catalyst design. Herein, we developed a sub-nanoscale Cu cluster supported on SiO2 (Cun/SiO2) catalyst, which achieved comparable benzene conversion to its nanoparticle counterpart (46.6% vs 55.4%) while exhibiting significantly enhanced phenol selectivity (96.8% vs 88.1%). Systematic studies reveal distinct reaction pathways between the two catalytic systems. In the Cun/SiO2-catalyzed system, benzene oxidation predominantly proceeds through a nonradical pathway mediated by Cu=O* species, while the nanoparticle Cup/SiO2 catalyst follows a free radical mechanism dominated by •OH radicals. Density functional theory (DFT) calculations elucidate the sub-nanoscale Cun sites in Cun/SiO2 exhibit a downshifted d-band center relative to CuO nanoparticles in Cup/SiO2, which weakens substrate adsorption strength and redirects H2O2 dissociation pathways. The fundamental insights gained from this comparative study elucidate structure-activity relationships in copper-based catalytic systems.