Impact of solvated hydronium ions and local pH on H2O2 direct synthesis over Pd catalysts supported on Brønsted acid solids

Abstract

Supported Pd nanoparticles exhibit insufficient rates and selectivities for H₂O₂ direct synthesis (H₂ + O₂ → H₂O₂) in pure aqueous solvents, limiting cost-effective production. Water soluble mineral acids increase H₂O₂ formation rates and selectivities but corrode solid catalysts and process equipment. Here, we synthesize Pd catalysts supported on aluminosilicate zeolites (CHA, MFI, BEA, FAU), and other Brønsted acid materials (sulfonic acid resin, Al-MCM-41) and examine how acid properties and zeolite confinement influence H₂O₂ kinetics. Infrared spectra of adsorbed CO reveal Pd exists as both single atoms and nanoparticles. Comparisons of apparent activation enthalpies for H₂O₂ and H₂O formation demonstrate Pd nanoparticles contribute to observed rates, but ion-exchanged Pd atoms at Brønsted acid sites remain inactive. Pd supported on medium- and large-pore zeolites (BEA, FAU) give ∼10-fold higher H₂O₂ rates than small-pore zeolites (MFI, CHA) and provide similar selectivities (60–70%). Other Brønsted acid supports yield H₂O₂ formation rates comparable to the zeolites, and all solid acid supports deliver greater H₂O₂ selectivities than SiO₂ and Al₂O₃, which suggests that zeolite topology alone does not account for kinetic differences. Combined assessment of rate measurements on physical mixtures and measured points of zero charge indicate that the greater H₂O₂ selectivities on Brønsted acid supports stems largely from decreases in the fluid-phase pH, where higher concentrations of H₃O⁺ enthalpically destabilize transition states for H₂O₂ formation (by 5–10 kJ mol⁻¹) and more significantly impact H₂O formation (25–50 kJ mol⁻¹). These findings demonstrate that local pH influences apparent enthalpies for catalysis and offers an alternative to soluble acid promoters for H₂O₂ synthesis.