Unifying Concepts in Electro- and Thermocatalysis toward Hydrogen Peroxide Production

Abstract

We examine relationships between H 2 O 2 and H 2 O formation on metal nanoparticles by the electrochemical oxygen reduction reaction (ORR) and the thermochemical direct synthesis of H 2 O 2 . The similar mechanisms of such reactions suggest that these catalysts should exhibit similar reaction rates and selectivities at equivalent electrochemical potentials (μ ̅ 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 (H 2 , O 2 ), H 2 O 2 selectivity, and rate constants for the HOR and ORR, as confirmed by measurements of the operating potential during the direct synthesis of H 2 O 2 . The selectivities and rates of H 2 O 2 formation during thermocatalysis and electrocatalysis correlate across all catalysts when operating at equivalent μ ̅ i values. This analysis provides quantitative relationships that guide the optimization of H 2 O 2 formation rates and selectivities. Catalysts achieve the greatest H 2 O 2 selectivities when they operate at high H atom coverages, low temperatures, and potentials that maximize electron transfer toward stable OOH* and H 2 O 2 * while preventing excessive occupation of O−O antibonding states that lead to H 2 O formation. These findings guide the design and operation of catalysts that maximize H 2 O 2 formation, and these concepts may inform other liquid-phase chemistries.

Analysing the relationship between the fields of thermo- and electrocatalysis taking hydrogen peroxide as a case study

“…Therefore, it seems plausible that catalyst performance could be rationalized by studying the HOR/ORR for thermocatalytic materials electrochemically in isolation. Supporting this hypothesis, Flaherty and co-workers investigated Au-, Pd- and Pt-based catalysts and quantitatively demonstrated the mechanistic similarities between the direct synthesis of H 2 O 2 and synchronous ORR and HOR electrochemistry 41 .…”

“…Intriguingly, materials such as Au or Ag possess relatively low catalytic current densities as the half-cell kinetics for the HOR are poor (hypothetically shown in Fig. 2b ) 41 . However, due to low H 2 O 2 residence time and the subsequent hindrance of oxygen adsorption, they tend to demonstrate high selectivity.…”

Electron transfer rather than direct hydrogen atom transfer can drive catalytic hydrogenation on supported metal catalysts

“…[15] More thorough analyses pointed out that protic solvent at solid-liquid interfaces assists the transfer of protons and electrons to reduce oxygen. [16] Further, electro-and thermo-catalysis toward hydrogen peroxide production share an identical proton-electron transfer pathway. [17] Along the same line, Surendranath et al…”

Demonstrating the electron-proton transfer mechanism of aqueous phase 4-nitrophenol hydrogenation on supported metal catalyst using unbiased electrochemical cells

“…27 Further, electro-and thermo-catalysis toward hydrogen peroxide production share an identical proton-electron transfer pathway. 28 By measuring the catalyst potential, Surendranath et al showed that aerobic formic acid oxidation reactions should be viewed as two electrochemical half reactions: 1) HCOOH → CO2 + 2H + + 2e -; 2) 0.5O2 + 2H + + 2e -→ H2O, in the same way as the electro-catalytic oxidation of formic acid. 29 These findings provide a quantitative and predictive link between thermo-and electro-catalytic aerobic oxidation.…”