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

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


Xie, T. - Presenter, Penn State University
Rioux, R., Pennsylvania State University
Dasgupta, A., Pennsylvania State University
Ruthardt, L., Pennsylvania State University
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.