Negative Reaction Order for CO during CO₂ Electroreduction on Au

The potential-dependent negative fractional reaction orders with respect to the CO partial pressures were measured for CO₂ electroreduction (CO₂R) on Au under mass-transfer-controlled conditions using a rotating ring-disk electrode setup. At high overpotentials, the CO reaction order approaches -1 due to enhanced CO adsorption on Au, which is supported by kinetic analysis and density functional theory (DFT) simulations. This work illustrates that the CO site-blocking effect cannot be ignored, even on a weak CO-binding metal such as Au in the electrochemical environment. The CO site-blocking effect can greatly hamper the activity and the selectivity of the CO₂R to CO. This observation enriches the current mechanistic understanding of the CO₂R and could have significant implications not only in the theoretical modeling of the CO₂R but also in the evaluation of intrinsic CO₂R activity at practical current density and high conversion conditions.