(735b) Water Assisted Pathway during Pd-Catalyzed H2O2 Decomposition

Direct synthesis of hydrogen peroxide (DSHP) from H₂ and O₂ is a promising alternative to the current industrial method for the manufacture of H₂O₂, the Riedl-Pfleiderer process. H₂O₂ does not accumulate to the required industrial-grade concentration with current catalytic technology because of significant secondary reactions (H₂O₂ decomposition and hydrogenation). Most studies focus on increasing H₂O₂ selectivity by alloying Pd with another metals (e.g., Au, Sn, Zn, Pt)¹. Plauck et al.² used DFT and found O-O bond break is rate determining for Pd-catalyzed H₂O₂ decomposition, suggesting a secondary KIE (1~2) should be observed. However, a large primary kinetic isotope effect (KIE) of ~7 was measured for H₂O₂ decomposition. A step involving O-O bond breaking would at a minimum require a pair of sites, leading to an inhibitory effect of H₂O₂ at a high H₂O₂ concentration, but our experimental work demonstrates Pd-catalyzed H₂O₂ decomposition follows saturation kinetics. The inconsistency between experiment and theory may be attributed to a solvent-assisted pathway. Proton electron transfer, a reaction involving the concerted transfer of electrons and protons, typically has a large KIE, and is identified as a kinetically relevant step. Larger particles with smaller work function have increased reactivity towards H₂O₂ decomposition due to increased electron transfer to H₂O₂ relative to smaller Pd particles, supporting a proposed mechanism involving kinetically relevant electron transfer steps. The addition of a Pd catalyst to Fenton system (Fe³⁺/Fe²⁺ catalyzed H₂O₂ decomposition with HO• formation) accelerates the formation rate of hydroxybenzoic acid even though Pd catalyst does not catalyze hydroxylation of benzoic acid with H₂O₂. Through the kinetic coupling of Pd-catalyzed H₂O₂ decomposition and Fenton probe chemistry, we demonstrated the most abundant reactive intermediate are *HOO and *H₂O₂, their relative concentrations vary with particle size during reaction.

(1) ACS Catal. 2018, 8, 1520–1527.(2) Proc. Natl. Acad. Sci. 2016, E1973–E1982.