We analyze relationships between the electrocatalytic and thermocatalytic reduction of O2 to H2O2 and H2O on metal nanoparticle catalysts, known as the oxygen reduction reaction (ORR) and the direct synthesis of H2O2 in their respective fields. These reactions show mechanistic similarities in protic solvents, indicating that metal nanoparticles should produce similar formation rates and selectivities at equivalent electrochemical potentials, determined by reactant activities, electrode potential, and temperature. We test this hypothesis by comparing the kinetic parameters measured independently within a fixed-bed thermocatalytic reactor and with electrochemical methods upon twelve mono- and bimetallic nanoparticle catalysts. Butler-Volmer analysis yields electrochemical rate constants and transfer coefficients for these materials, providing a basis to model each catalyst as a short-circuited electrochemical cell. These models assume that metal nanoparticles couple the steady-state rates of the hydrogen oxidation reaction (HOR) and the ORR to at any given combination of H2 and O2 pressure to conserve charge. Analysis of the elementary steps of these reactions indicates that the resulting operating potential depends on the ratio of the intrinsic rate constants for the HOR and ORR, the activities of H2 and O2, the H2O2 selectivity, and the magnitude of charge transfer coefficients. We find that H2O2 selectivities under thermocatalytic and electrocatalytic conditions generally correlate across all catalysts when operating at equivalent electrochemical potentials; however, small deviations from parity reflect differences in the coverage of surface H*-atoms. These comparisons corroborate a mechanism of the ostensibly thermochemical production of H2O2 that closely resembles the electrochemical ORR. Such findings and interpretations provide quantitative relationships that inform the optimization of H2O2 formation rates and selectivities by controlling the electrode potential, reactant activity, and temperature. Generally, catalysts produce the greatest H2O2 selectivities at low temperatures, high hydrogen pressures, and electrical potentials that maximize H2O2 relative to H2O formation paths. This understanding guides the design, operation, and screening of catalysts with desirable rates and selectivities for H2O2 and H2O under either thermocatalytic or electrocatalytic conditions, and these concepts may extend to other chemistries at solid-liquid interfaces.

Unifying concepts in electro- and thermocatalysis towards hydrogen peroxide production

ACS Spring Meeting 2021

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