Homolytic versus Heterolytic Hydrogen Evolution Reaction Steered by a Steric Effect

Abstract

Several H−H bond forming pathways have been proposed for the hydrogen evolution reaction (HER). Revealing these HER mechanisms is of fundamental importance for the rational design of catalysts and is also extremely challenging. Now, an unparalleled example of switching between homolytic and heterolytic HER mechanisms is reported. Three nickel(II) porphyrins were designed and synthesized with distinct steric effects by introducing bulky amido moieties to ortho‐ or para‐positions of the meso‐phenyl groups. These porphyrins exhibited different catalytic HER behaviors. For these Ni porphyrins, although their 1e‐reduced forms are active to reduce trifluoroacetic acid, the resulting Ni hydrides (depending on the steric effects of porphyrin rings) have different pathways to make H2. Understanding HER processes, especially controllable switching between homolytic and heterolytic H−H bond formation pathways through molecular engineering, is unprecedented in electrocatalysis.

Hydrogen evolution catalysis by terminal molybdenum-oxo complexes

“…The non-sustainability of platinum, by far the best catalyst for reduction of protons to hydrogen (the hydrogen evolution reaction [HER]), and also inspiration from biology (e.g., Fe-hydrogenases) continues to drive focus on developing catalysts based on cheap, non-toxic, and earth-abundant metal complexes (Abbas and Bang, 2015;Bullock et al, 2014;Fukuzumi et al, 2018;Guo et al, 2020;Mondal et al, 2013;Roger et al, 2017;Xie et al, 2020). Molybdenum may safely be considered the only non-precious heavy transition metal, and it is also essential for human life due to its presence and roles in more than 30 enzymes (e.g., DMSO reductase, sulfite oxidase, xanthine oxidase) (Schwarz et al, 2009).…”

Electrocatalytic Hydrogen Evolution of the Cobalt Triaryl Corroles Bearing Hydroxyl Groups

“…The reason may be that in weak acids, Co III ‐H of complexes 1 and 2 requires further 1e − ‐reduction to produce H 2 . In contrast, due to the small spatial barrier and the excellent activity of the phenyl para‐hydroxyl group, [35] the Co III ‐H of complex 3 can generate H 2 in a different way.…”

Water‐Soluble Polymers with Appending Porphyrins as Bioinspired Catalysts for the Hydrogen Evolution Reaction

“…For those water‐soluble catalysts, systemic modification of molecular structures is challenging from both design and synthesis points of view, making further activity improvements very difficult. This dilemma is especially true for metal porphyrins, which have been found to be highly active for HER . Porphyrins are usually not soluble in water, which is an environmental benign and ideal solvent to provide protons.…”