In situ reduction of CO₂ forms surface redox mediators on nanoparticle catalysts

Pathways and selectivities for oxygen reduction (ORR) on nanoparticle catalysts respond sensitively to the addition of exogenous redox mediators, and recent findings indicate organic surface redox mediators influence formation rates for hydrogen peroxide (H₂O₂) and water. Here, the introduction of carbon dioxide (CO₂) increases both steady-state rates and selectivities to H₂O₂ during thermocatalytic oxygen reduction by H₂ on nanoparticle catalysts in pure water. Contact with CO₂ influences catalysis over Pd long after removal of gaseous CO₂, whereas differences for Au nanoparticles revert immediately. These observations reflect the formation of carbonic acid, which increases proton activity and surface coverage of H*-atoms formed by the Volmer reaction. Furthermore, interpretation of kinetic isotope effects, operando spectroscopy, and open circuit potentiometry measurements indicate that persistent organic residues (e.g., formate and methyl groups) form by CO₂ hydrogenation on Pd nanoparticles and give rise to an increase in H₂O₂ formation rates (two-fold) and selectivities (55% to 90%). These surface redox mediators introduce pathways with distinct kinetic barriers for H₂ activation and suppress transitions between metallic and hydridic phases. These findings provide new insights, revealing previously unrecognized roles of CO₂ in modulating ORR pathways toward H₂O₂ formation and establish CO₂-derived surface species as benign redox mediators.