H-D kinetic isotope effects of alcohol electrooxidation on Au, Pd and Pt electrodes in alkaline solutions

The identification of rate-determining step (RDS) is vital for understanding reaction mechanisms and designing advanced catalysts. Here, we systemically studied the H-D kinetic isotope effects (KIEs) on electrooxidation of ethanol, isopropanol, and ethylene glycol (EG) on Au, Pd and Pt electrodes in alkaline solution by combining cyclic voltammetry and in situ FTIR spectroscopy. Large primary KIEs are observed on Au electrode: 8.4 for ethanol, 6.2 for EG and 7.9 for isopropanol when α-H of alcohols are substituted by deuterium, but much lower KIEs (< 3.2) were observed on Pt, and the values on Pd are between Au and Pt. According to the criteria (KIEs = 6-10) for CH (D) breaking/forming as RDS, we conclude that the cleavage of the α-H from CH bond is the RDS for alcohol electrooxidation on Au, but not on Pt. On Pd, the RDS depends on the structure of alcohols. The difference can be rationally correlated with the binding affinity of hydrogen on precious metal surfaces.