(8c) Promotional Effects of CO2 on Pd Catalysts for Direct Synthesis of Hydrogen Peroxide

Direct catalytic reduction of O₂ with H₂ offers a sustainable and cost-effective route for hydrogen peroxide (H₂O₂) production, yet process safety and catalyst design to achieve high H₂O₂ rates and selectivities remain key hurdles. The introduction of promoters (e.g., acids, halides, organics) to this reaction results in greater rates and selectivities at the cost of leaching and stability challenges. Furthermore, the origins of the promotional effects remain unexplored. Here, we investigate the promotional effects of adding CO₂, a benign diluent gas typically used to avoid explosive O₂/H₂ mixtures. Through a combination of kinetics, in situ spectroscopy, and potentiometric measurements, we examine the influence of CO₂ on the rates, selectivities, mechanism, and structure of silica-supported Pd catalysts for H₂O₂ direct synthesis.

Kinetic measurements reveal CO₂ acts as an in situ acid promoter, lowering the reactant solution pH through carbonic acid formation among other potential species and providing additional protons. These changes increase hydrogen coverage on Pd catalysts, doubling H₂O₂ formation rates (Figure 1a). In situ X-ray absorption spectroscopy evidences the suppression of the selective Pd-hydride phase in the presence of CO₂ (Figure 1b), yet kinetic measurements reveal an increase in H₂O₂ selectivity from 55% to 90%. Kinetic isotope effects, combined with in situ infrared spectroscopy and open circuit potentiometry measurements (Figure 1c) confirm that CO₂ reduction forms organic residues, which act as surface redox mediators and facilitate proton-electron transfer steps responsible for H₂O₂ formation. Concurrently, these residues block unselective sites and increase selectivity. Prior studies claim improved H₂O₂ rates and selectivities with CO₂ reflect greater reactant gas solubilities, decreased H₂O₂ decomposition, and site blocking, however, this work demonstrates CO₂ additionally modifies the catalyst structure, surface coverages, and mechanism to collectively result in higher rates and selectivities towards H₂O₂ on Pd.