Buffering Donor Shuttles\nin Proton-Coupled Electron\nTransfer Kinetics for Electrochemical Hydrogenation of Hydroxyacetone\nto Propylene Glycol

Electrochemical hydrogenation reactions demand rapid\nproton-coupled\nelectron transfer at the electrode surface, the kinetics of which\ndepend closely on pH. Buffer electrolytes are extensively employed\nto regulate pH over a wide range. However, the specific role of buffer\nspecies should be taken into account when interpreting the intrinsic\npH dependence, which is easily overlooked in the current research.\nHerein, we report the electrochemical hydrogenation of hydroxyacetone,\nderived from glycerol feedstock, to propylene glycol with a faradaic\nefficiency of 56 ± 5% on a polycrystalline Cu electrode. The\nreaction activities are comparable in citrate, phosphate, and borate\nbuffer electrolytes, encompassing different buffer identities and\npH. The electrokinetic profile reveals that citrate is a site-blocking\nadsorbate on the Cu surface, thereby decreasing buffer concentration\nand increasing pH will enhance the reaction rate; phosphate is an\nexplicit proton donor, which promotes the interfacial rate by increasing\nbuffer concentration and decreasing pH, while borate is an innocent\nbuffer, which can be used to investigate the intrinsic pH effect.\nCombined with <i>in situ</i> SEIRAS, we demonstrate that\nwater is the primary proton source in citrate and borate electrolytes,\nreiterating the rationality of the proposed mechanism based on the\nmicrokinetic modeling. Our results emphasize the intrinsic complexity\nof the buffer system on the kinetic activity for electrocatalysis.\nIt calls for special care when we diagnose the mechanistic pathway\nin buffer electrolytes convoluted by different buffer identities and\npH.