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.