Abstract
One of the questions not yet elucidated in the electrocatalytic oxidation of glucose is whether the first step of dehydrogenating proton-coupled electron transfer (PCET) concerns the hydrogen directly bound to an anomeric carbon (β-anomer) or that bound to oxygen of the anomeric carbon (α-anomer). The knowledge is necessary for renewable-energy-powered electrosynthesis of chemicals/fuels. To decipher that, we have used α-d-, β-d-, and d-glucose models to interrogate the electrocatalysis of the glucose anomers in neutral and alkaline pHs. We have also optimized a pulse methodology to directly grow surfactant- and binder-free gold particles onto the gas diffusion electrode (GDE) as free-standing electrocatalysts to bridge the scales between fundamental and applied research in fuel cells and electrolysis. Cyclic voltammetry measurements show that the electrooxidation of all of the glucose anomers starts at a potential region, where the gold surface is not yet fully oxidized and is dominated by the dehydrogenating adsorption of glucose, which rules out the hypothesis that glucose first adsorbs on the hydroxylated gold surface. The results in neutral pHs highlight the better electrocatalytic reactivity of the α-anomer over the β-anomer and the opposite in alkaline pHs, which invalidates the traditional thoughts that the β-anomer would always be the most reactive. Potential-dependent energy profiles computed by density functional theory (DFT) mainly confirm the promoted approach by the OH of the anomeric carbon (α-anomer). The deciphering of the electrocatalytic reactivity of glucose anomers at GDE-Au electrocatalysts, where gluconate is the main oxidation product at high selectivity and faradaic efficiency (>80%), opens opportunities to stimulate the electrosynthesis of renewable platform chemicals from the cellulosic biomass. The high selectivity and faradaic efficiency toward gluconate, a commodity renewable chemical, open opportunities to stimulate the biomass-fueled electrosynthesis.