Deciphering the Electrocatalytic Reactivity of Glucose Anomers at Bare Gold Electrocatalysts for Biomass-Fueled Electrosynthesis

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.

Electrification of glucose valorization over NiO/Ni foam

“…In the presence of glucose, the onset shifted to 1.2 V vs. RHE, a potential less anodic than the Ni 2+ /Ni 3+ oxidation, while the current density in the high potential region was lowered. These results suggested, respectively, that the substrate oxidation could occur without the mediation of NiOOH species, as recently reported for gold, 55 and that glucose lowered the OER contribution. An anodic current was recorded by reversing the potential, though the peak intensity was lower than in the forward going scan.…”

“…An anodic current was recorded by reversing the potential, though the peak intensity was lower than in the forward going scan. 32 The Tafel slope for the glucose oxidation was 130 mV dec −1 , in a range similar to that reported by Holade et al for a glucose dehydrogenation limiting step over Au, 55 nevertheless it was much higher than the value previously reported for Ni foam (0.10 M glucose and 1.0 M KOH). 26 To confirm the advantage of using a 3D support, the CV was also recorded in a calcined Ni plate (Fig.…”

“…63 In electrocatalysis it is well established that the glucose molecule in the pyranose configuration adsorbs at the C1 position (anomeric carbon) either through the hydrogen directly bound to the C (β-anomer) or to the O (α-anomer), nevertheless under our reaction conditions (basic media) the β-anomer is agreed to be the most reactive. 55 The gluconolactone formed is then hydrolyzed, opening the ring and forming gluconate, which could be adsorbed on the electrocatalyst surface and further reacted; the existence of vicinal OH groups favored the C–C cleavage. The consecutive reactions involving the C2–C4 molecules could be both chemically and electrochemically promoted.…”

Nano‐Impact Single‐Entity Electrochemistry Enables Plasmon‐Enhanced Electrocatalysis**

“…Based on a combination of electrochemical measurements, surface-enhanced resonance Raman (SERS) and DFT calculations, it has been previously shown that glucose electrooxidation starts at a potential region, where the gold surface is not yet fully oxidized and is dominated by the dehydrogenating adsorption of glucose followed by its oxidation into gluconate. [33,34] Thus, the first peak can be ascribed to the electrooxidation of electrochemically adsorbed glucose molecules into gluconate, and the latter represents a competitive or parallel oxidation of glucose and gluconate into glucaric acid. [35,36] Upon illumination of the electrode by the 532 nm laser, an increase of the Faradaic anodic current by 39 � 7 % for the current peak at 0.2 V vs. Ag/AgCl is evident.…”

“…Two well‐defined peaks are observed at ≈−0.4 and 0.2 V vs. Ag/AgCl on the LSVs, which are not present on bare HOPG electrodes in the absence of AuNPs (Figure S1). Based on a combination of electrochemical measurements, surface‐enhanced resonance Raman (SERS) and DFT calculations, it has been previously shown that glucose electrooxidation starts at a potential region, where the gold surface is not yet fully oxidized and is dominated by the dehydrogenating adsorption of glucose followed by its oxidation into gluconate [33, 34] . Thus, the first peak can be ascribed to the electrooxidation of electrochemically adsorbed glucose molecules into gluconate, and the latter represents a competitive or parallel oxidation of glucose and gluconate into glucaric acid [35, 36] …”