Much effort has been devoted in the development of efficient catalysts for electrochemical reduction of CO. Molecular level understanding of electrode-mediated process, particularly the role of bicarbonate in increasing CO reduction rates, is still lacking due to the difficulty of directly probing the electrochemical interface. We developed a protocol to observe normally invisible reaction intermediates with a surface enhanced spectroscopy by applying square-wave potential profiles. Further, we demonstrate that bicarbonate, through equilibrium exchange with dissolved CO, rather than the supplied CO, is the primary source of carbon in the CO formed at the Au electrode by a combination of in situ spectroscopic, isotopic labeling, and mass spectroscopic investigations. We propose that bicarbonate enhances the rate of CO production on Au by increasing the effective concentration of dissolved CO near the electrode surface through rapid equilibrium between bicarbonate and dissolved CO.

Understanding how pH affects the activity of hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) is key to developing active, stable, and affordable HOR/HER catalysts for hydroxide exchange membrane fuel cells and electrolyzers. A common linear correlation between hydrogen binding energy (HBE) and pH is observed for four supported platinum-group metal catalysts (Pt/C, Ir/C, Pd/C, and Rh/C) over a broad pH range (0 to 13), suggesting that the pH dependence of HBE is metal-independent. A universal correlation between exchange current density and HBE is also observed on the four metals, indicating that they may share the same elementary steps and rate-determining steps and that the HBE is the dominant descriptor for HOR/HER activities. The onset potential of CO stripping on the four metals decreases with pH, indicating a stronger OH adsorption, which provides evidence against the promoting effect of adsorbed OH on HOR/HER.

Reducing oxygen content in biomass-derived feedstocks via hydrodeoxygenation (HDO) is a key step in their upgrading to fuels and valuable chemicals. Organic molecules, e.g., alcohols and formic acid, can donate hydrogen to reduce the substrate in a process called catalytic transfer hydrogenation (CTH). Although it is practiced far less frequently than molecularhydrogen-based HDO processes, CTH has been proven to be an efficient and selective strategy in biomass upgrading in the last two decades. In this paper, we present a selective review of recent progress made in the upgrade of biomass-derived feedstocks through heterogeneous CTH, with a focus on the mechanistic interpretation. Hydrogenation and cleavage of CO and C−O bonds, respectively, are the two main categories of reactions discussed, owing to their importance in the HDO of biomass-derived feedstocks. On acid−base catalysts, Lewis acid−base pair sites, rather than a single acid or base site, mediate hydrogenation of carbonyl groups with alcohols as the hydrogen donor. While acid−base catalysts typically only catalyze the hydrogenation of carbonyl groups with alcohols as the hydrogen donor, metal-based catalysts are able to mediate both hydrogenation and hydrogenolysis reactions with either alcohols or formic acid. Several model reactions involving platform chemicals in biomass upgrading, e.g., 5-hydroxymethylfurfural, levulinic acid, and glycerol, are used in the discussion to illustrate general trends. Because alcohols are typically both the hydrogen donor and the solvent, the donor and solvent effects are intertwined. Therefore, solvent effects are discussed primarily in the context of sugar isomerization and reactions with formic acid as the hydrogen donor, in which the solvent and hydrogen donor are two separate species. Current challenges and opportunities of future research to develop CTH into a competitive and complementary strategy of the conventional molecular-hydrogen-based processes are also discussed.

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