Isolated Pd₁ Sites and Surface Hydrogen Cooperativity Enable Selective Direct H₂O₂ Synthesis in PdSn-Based Catalysts

Direct synthesis of hydrogen peroxide (H₂O₂) from H₂ and O₂ has been pursued as an alternative to the anthraquinone process. Pd-based catalysts offer high activity by enabling H₂ and O₂ activation, but unselective O─O bond cleavage on Pd limits H₂O₂ selectivity. Using density functional theory calculations and molecular dynamics (MD) simulations, we show that pristine Pd, Pd₃Sn, and O/H-covered Pd₃Sn surfaces intrinsically favor O─O bond cleavage and fail to account for the high selectivity reported for PdSn catalysts. MD simulations under working conditions show that the PdSn alloy/SnO₂ interface evolves toward atomically dispersed Pd species, with isolated Pd₁ thermodynamically preferred over contiguous Pd₂ and Pd₃ motifs. On Pd₁/SnO₂, hydrogen readily dissociates and stabilizes at surface O sites. These surface hydrogens reshape the reaction landscape by tuning the surface electronic structure, thereby promoting O₂ hydrogenation while kinetically suppressing competing O─O bond cleavage. These results reveal cooperative roles of isolated Pd sites and surface hydrogen in governing selectivity and provide mechanistic insight for designing Pd-based catalysts for H₂O₂ synthesis.