Mechanism for the Direct Synthesis of H₂O₂ on Pd Clusters: Heterolytic Reaction Pathways at the Liquid-Solid Interface

Direct synthesis (H₂ + O₂ → H₂O₂) is a promising reaction for producing H₂O₂, which can replace chlorinated oxidants in industrial processes. The mechanism of this reaction and the reasons for the importance of seemingly unrelated factors (e.g., Pd cluster size and solvent pH) remain unclear despite significant research. We propose a mechanism for H₂O₂ formation on Pd clusters consistent with steady-state H₂O₂ and H₂O formation rates measured as functions of reactant pressures and temperature and the interpretations of proton concentration effects. H₂O₂ forms by sequential proton-electron transfer to O₂ and OOH surface intermediates, whereas H₂O forms by O-O bond rupture within OOH surface species. Direct synthesis, therefore, does not proceed by the Langmuir-Hinshelwood mechanism often invoked. Rather, H₂O₂ forms by heterolytic reaction pathways resembling the two-electron oxygen reduction reaction (ORR); however, the chemical potential of H₂ replaces an external electrical potential as the thermodynamic driving force. Activation enthalpies (ΔH^‡) for H₂O formation increase by 14 kJ mol^-1 when Pd cluster diameters increase from 0.7 to 7 nm because changes in the electronic structure of Pd surface atoms decrease their propensity to cleave O-O bonds. ΔH^‡ values for H₂O₂ remain nearly constant because barriers for proton-electron transfer depend weakly on the coordinative saturation of Pd surface atoms. Collectively, these results provide a self-consistent mechanism, which clarifies many studies in which H₂O₂ rates and selectivities were shown to depend on the concentration of acid/halide additives and Pd cluster size. These findings will guide the rational design of selective catalysts for direct synthesis.