Mechanistic Origins of the pH Dependency in Au-Catalyzed Glycerol Electro-oxidation: Insight from First-Principles Calculations

Abstract

Electrocatalytic oxidation of glycerol (EOG) is an attractive approach to convert surplus glycerol to value-added products. Experiments have shown that EOG activity and selectivity depend not only on the electrocatalyst but also on the electrode potential, the pH, and the electrolyte. For broadly employed gold (Au) electrocatalysts, experiments have demonstrated high EOG activity under alkaline conditions with glyceric acid as a primary product, whereas under acidic and neutral conditions Au is almost inactive producing only small amounts of dihydroxyacetone. In the present computational work, we have performed an extensive mechanistic study to understand the pH and potential dependency of Au-catalyzed EOG. Our results show that activity and selectivity are controlled by the presence of surface-bound hydroxyl groups. Under alkaline conditions and close to the experimental onset potential, modest OH coverage is preferred according to our constant potential calculations. This indicates that both Au­(OH)ads and Au can be active sites and they cooperatively facilitate the thermodynamically and kinetically feasible formation of glyceric acid thus explaining the experimentally observed high activity and selectivity. Under acidic conditions, hydroxide coverage is negligible and the dihydroxyacetone emerges as the favored product. Calculations predict slow reaction kinetics, however, which explains the low activity and selectivity toward dihydroxyacetone reported in experiments. Overall, our findings highlight that computational studies should explicitly account for pH and coverage effects under alkaline conditions for electrocatalytic oxidation reactions to reliably predict electrocatalytic behavior.

Grand Canonical DFT Investigation of the CO2RR and HER Reaction Mechanisms on MoTe2 Edges

“…Figure shows the grand free energies of six different terminations (shown as insets in Figure ), each calculated at seven different potentials. The data have been fitted to second-order polynomials in accordance with the expected potential dependence. ,, The data generally follow the expected trend; only for the OH terminated edges is some deviation observed. The grand free energy curvatures and maxima correspond to the capacitance and potential of zero charge (PZC), respectively.…”

“…Furthermore, charge-transfer reactions not involving a simultaneous proton transfer cannot be described with CHE. The electrode potential effects can be explicitly described with grand canonical ensemble (GCE)-based methods, such as GCE-DFT, where the electrochemical potential of an electron, i.e., the electrode potential, rather than the number of electrons, is kept fixed. , GCE-DFT -based approaches have previously provided detailed descriptions of various electrochemical reaction pathways, such as the NRR/HER on Ru–N 4 , Au-catalyzed glycerol electro-oxidation, and the HER at MoS 2 and MoTe 2 basal planes . Also, the CO2RR has been the subject of GCE-DFT investigations.…”

“…The electrode potential effects can be explicitly described with grand canonical ensemble (GCE)-based methods, such as GCE-DFT, where the electrochemical potential of an electron, i.e., the electrode potential, rather than the number of electrons, is kept fixed. 38,40 41 Au-catalyzed glycerol electro-oxidation, 42 and the HER at MoS 2 43 and MoTe 2 basal planes. 33 Also, the CO2RR has been the subject of GCE-DFT investigations.…”

Effect of pore mesostructure on the electrooxidation of glycerol on Pt mesoporous catalysts

“…12,13 Various synthesis and catalysis techniques such as thermo chemical 14,15 , photochemical 16,17 , and biocatalytic [18][19][20] techniques have been used to oxidize glycerol and thereby synthesize value-added products. Electrocatalytic conversion of glycerol is an alternative production method that provides a high electrode-to-product efficiency 8,21 , the flexibility of product selectivity 22,23 , a co-production of H2 at the cathode with a lower operating cell potential compared to water splitting, and moderate reaction conditions. 24 Several studies have shown that noble metal catalysts such as Pd, Pt, Au, and Ag and their alloys are excellent for glycerol electrooxidation reaction (GEOR) with a great advantage of having lower electrode potential and high fraction percentage of C3 products (where the economic value of C3-chemicals is larger than C1chemicals).…”

Photochemical Diodes for Simultaneous Bias-Free Glycerol Valorization and Hydrogen Evolution

“…Compared to OWS, GOR coupled with HER reduces the energy requirement by 700–1000 mV, allowing for smaller band gap materials with larger photocurrents to be used (Figure b). , Additionally, selectivity can be steered by choosing a semiconductor material such as Si where the valence band maximum (VBM) lies between the potentials for GOR and the potentials for the OER, preventing any holes from being used for the OER. In this design framework, a selective bias-free device can be achieved by coupling a medium band gap material such as silicon with a low onset potential catalyst for GOR.…”