Unifying Concepts in Electro- and Thermocatalysis toward Hydrogen Peroxide Production

We examine relationships between H₂O₂ and H₂O formation on metal nanoparticles by the electrochemical oxygen reduction reaction (ORR) and the thermochemical direct synthesis of H₂O₂. The similar mechanisms of such reactions suggest that these catalysts should exhibit similar reaction rates and selectivities at equivalent electrochemical potentials (μ̅_i), determined by reactant activities, electrode potential, and temperature. We quantitatively compare the kinetic parameters for 12 nanoparticle catalysts obtained in a thermocatalytic fixed-bed reactor and a ring-disk electrode cell. Koutecky-Levich and Butler-Volmer analyses yield electrochemical rate constants and transfer coefficients, which informed mixed-potential models that treat each nanoparticle as a short-circuited electrochemical cell. These models require that the hydrogen oxidation reaction (HOR) and ORR occur at equal rates to conserve the charge on nanoparticles. These kinetic relationships predict that nanoparticle catalysts operate at potentials that depend on reactant activities (H₂, O₂), H₂O₂ selectivity, and rate constants for the HOR and ORR, as confirmed by measurements of the operating potential during the direct synthesis of H₂O₂. The selectivities and rates of H₂O₂ formation during thermocatalysis and electrocatalysis correlate across all catalysts when operating at equivalent μ̅_i values. This analysis provides quantitative relationships that guide the optimization of H₂O₂ formation rates and selectivities. Catalysts achieve the greatest H₂O₂ selectivities when they operate at high H atom coverages, low temperatures, and potentials that maximize electron transfer toward stable OOH* and H₂O₂* while preventing excessive occupation of O-O antibonding states that lead to H₂O formation. These findings guide the design and operation of catalysts that maximize H₂O₂ formation, and these concepts may inform other liquid-phase chemistries.