(541g) Selective Reduction of O2 and H2 Via Hydroxyquinone Mediators in H2O2 Synthesis

The direct reduction of O₂ to H₂O₂, which involves the transfer of two protons and two electrons, is a highly desirable transformation over traditional chemical transformations for H₂O₂ synthesis. We have shown recently that the reduction of O₂ with H₂ on Pd-based catalysts is co-catalyzed by surface redox mediators. Hydroxymethyl surface intermediates form in methanol solvents and act as redox mediators that enhance reactivity and selectivity towards H₂O₂ production. Herein, we explore a family of quinone substrates, including 1,4-benzoquinone, and their influence on O₂ reduction to H₂O₂. Ab-initio molecular dynamics simulations and density functional theory calculations are employed to elucidate the reaction environments and understand the experimentally observed activity and selectivity trends. In conjunction with Fourier transform infrared spectroscopy and temperature-programmed oxidation measurements, first-principles calculations show that these quinone substrates adsorb strongly on the Pd surface. These surface-bound quinones then sequentially catalyze H₂ oxidation and O₂ reduction via. proton-coupled electron transfer steps by forming hydroxyquinone intermediates as mediators. H₂ oxidation results in the formation of protons and electrons, which hydrogenate the carbonyl bond of the quinone to generate the hydroxyquinone mediator. The bound-hydroxyquinone intermediate mediates the subsequent proton and electron transfer to the adsorbed molecular oxygen to form the *OOH intermediate and regenerate the quinone species. The two-proton and two-electron steps leading to either H₂O₂ or water formation are investigated. The results show a higher intrinsic barrier for water formation due to the penalty involved in O-O bond cleavage. The electrophilicity of these surface-bound species influences the selectivity towards H₂O₂ production by selectively manipulating the surface electron density. The insights from this work can be further extended to develop improved transformation involving novel pathways with redox-active mediators for the selective production of H₂O₂.