Designed Nanomaterials for Electrocatalytic Organic Hydrogenation Using Water as the Hydrogen Source

ConspectusThe hydrogenation reaction is one of the most frequently used transformations in organic synthesis. Electrocatalytic hydrogenation by using water (H2O) as the hydrogen source offers an efficient and sustainable approach to synthesize hydrogenated products under ambient conditions. Such a technique can avoid the use of high-pressure and flammable hydrogen gas or other toxic/expensive hydrogen donors, which usually cause environmental, safety, and cost concerns. Interestingly, utilizing easily available heavy water (D2O) for deuterated syntheses is also attractive due to the widespread applications of deuterated molecules in organic synthesis and the pharmaceutical industry. Despite impressive achievements, electrode selection mainly relies on trial-and-error modes, and how electrodes dictate reaction outcomes remains elusive. Therefore, the rational design of nanostructured electrodes for driving the electrocatalytic hydrogenation of a series of organics via H2O electrolysis is developed.In this Account, we review recent advances in the electrocatalytic hydrogenation of different types of organic functional groups, including C≡C, C≡N, C═C, C═O, and C-Br/I bonds, -NO2, and N-heterocycles, with H2O over nanostructured cathodes. First, the general reaction steps (reactant/intermediate adsorption, active atomic hydrogen (H*) formation, surface hydrogenation reaction, product desorption) are analyzed, and key factors are proposed to optimize hydrogenation performance (e.g., selectivity, activity, Faradaic efficiency (FE), reaction rate, and productivity) and inhibit side reactions. Then, ex situ and in situ spectroscopic tools to study key intermediates and interpret mechanisms are introduced. Third, based on the knowledge of key reaction steps and mechanisms, we introduce catalyst design principles in detail on how to optimize the adoption of reactants and key intermediates, promote the formation of H* from water electrolysis, inhibit hydrogen evolution and side reactions, and improve the selectivity, reaction rate, FEs, and space-time productivity of products. We then introduce some typical examples. (i) P- and S-modified Pd can decrease C═C adsorption and promote H* formation, enabling semihydrogenation of alkynes with high selectivity and FEs at lower potentials. Then, creating high-curvature nanotips to concentrate the substrates further speeds up the hydrogenation process. (ii) By introducing low-coordination sites into Fe and combining low-coordination sites and surface fluorine to modify Co to optimize the adsorption of intermediates and facilitate H* formation, hydrogenation of nitriles and N-heterocycles with high activity and selectivity is obtained. (iii) By forming isolated Pd sites to induce a specific σ-alkynyl adsorption of alkynes and steering S vacancies of Co3S4-x to preferentially adsorb -NO2, hydrogenation of easily reduced group-decorated alkynes and nitroarenes with high chemoselectivity is realized. (iv) For gas reactant participated reactions, by designing hydrophobic gas diffusion layer-supported ultrasmall Cu nanoparticles to enhance mass transfer, improve H2O activation, inhibit H2 formation, and decrease ethylene adsorption, ampere-level ethylene production with a 97.7% FE is accomplished. Finally, we provide an outlook on the current challenges and promising opportunities in this area. We believe that the electrode selection principles summarized here provide a paradigm for designing highly active and selective nanomaterials to achieve electrocatalytic hydrogenation and other organic transformations with fascinating performances.