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.

Nanoparticle metal oxide photocatalysts are attractive because of their increased reactivity and ease of processing into versatile electrode formats; however, their preparation is cumbersome. We report on the rapid bulk synthesis of photocatalytic nanoparticles with homogeneous shape and size via the cathodic corrosion method, a simple electrochemical approach applied for the first time to the versatile preparation of complex metal oxides. Nanoparticles consisting of tungsten oxide (HWO) nanoplates, titanium oxide (TiO) nanowires, and symmetric star-shaped bismuth vanadate (BiVO) were prepared conveniently using tungsten, titanium, and vanadium wires as a starting material. Each of the particles were extremely rapid to produce, taking only 2-3 min to etch 2.5 mm of metal wire into a colloidal dispersion of photoactive materials. All crystalline HWO and BiVO particles and amorphous TiO were photoelectrochemically active toward the water oxidation reaction. Additionally, the BiVO particles showed enhanced photocurrent in the visible region toward the oxidation of a sacrificial sulfite reagent. This synthetic method provides an inexpensive alternative to conventional fabrication techniques and is potentially applicable to a wide variety of metal oxides, making the rapid fabrication of active photocatalysts with controlled crystallinity more efficient.

Electrochemical analysis provides a convenient way to screen active materials for thermocatalytic processes involving implicit electrochemical redox mechanisms. Here, we show how a combinatorial method based on scanning electrochemical microscopy (SECM) rapidly screens multiple catalysts for hydrogen peroxide direct synthesis reaction by independently analyzing their hydrogen oxidation and oxygen reduction reactivities on the same chip. We present a reproducible and quantifiable procedure to fabricate catalyst spot array samples using photolithography and microdispensing. This procedure enables the exploration of up to 10 catalysts with three replicates each on one chip, allowing us to study 30 compositions to identify reactive trends in a vast compositional space. SECM imaging with linear sweep voltammetry improved the accuracy and efficiency of data collection. Kinetic parameters of both half-reactions for each catalyst were extracted from experimental data with the help of established analytical theory and finite element analysis simulation. A library of Au x Pt y catalysts with a range of Au/Pt ratios from 1200:1 to 120:12 were examined. For the synthesized Au x Pt y catalysts, the rate constants of oxygen reduction and hydrogen reduction increase as Pt content increases and then level off beyond Au120Pt7. Therefore, to achieve the highest activity while keeping the cost low, Au120Pt7 would be a promising composition to further investigate. We believe this SECM-based technique will expedite the catalyst design and discovery process for classes of thermochemical reactions involving underlying heterolytic electrochemical mechanisms, thus assisting in the synthesis and utilization of sustainable fuels and commodity chemicals.

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