Search

The hydrodynamics of electrochemical CO₂ reduction (CO₂R) systems is an insufficiently investigated area of research that has broad implications on catalyst activity and selectivity. While most previous reports are limited to laminar and CO₂-sparged systems, herein we address a wide range of hydrodynamics via electrolyte recirculation systems. We find that increased hydrodynamics at the electrode surface results directly in changes to the ethylene and methane Tafel slopes, demonstrating that mass transport is on equal footing with catalyst active sites in determining reaction mechanisms and the ensuing product distribution. Mass transport is traditionally considered to be in the purview of systems-level engineering, yet the present work shows that CO₂R mechanistic work must be considered in the context of the mass transport conditions. We extend our analysis to organic coatings, demonstrating that the films shield the active sites from variability in hydrodynamics and increase the residence time of CO so that it may be further reduced to desirable products.

Whereas synthetically catalyzed nitrogen reduction (N₂R) to produce ammonia is widely studied, catalysis to instead produce hydrazine (N₂H₄) has received less attention despite its considerable mechanistic interest. Herein, we disclose that irradiation of a tris(phosphine)borane (P₃ᴮ) Fe catalyst, P₃ᴮFe⁺, significantly alters its product profile to increase N₂H₄ versus NH₃; P₃ᴮFe⁺ is otherwise known to be highly selective for NH₃. We posit a key terminal hydrazido intermediate, P₃ᴮFe=NNH₂, as selectivity-determining. Whereas its singlet ground state undergoes protonation to liberate NH₃, a low-lying triplet excited state leads to reactivity at N_α and formation of N₂H₄. Associated electrochemical and spectroscopic studies establish that N₂H₄ lies along a unique product pathway; NH3 is not produced from N₂H₄. Our findings are distinct from the canonical mechanism for hydrazine formation, which proceeds via a diazene (HN=NH) intermediate and showcase light as a tool to tailor selectivity.

Inspired by momentum in applications of reductive photoredox catalysis to organic synthesis, photodriven transfer hydrogenations toward deep (>2 e⁻) reductions of small molecules are attractive compared to using harsh chemical reagents. Noteworthy in this context is the nitrogen reduction reaction (N₂RR), where a synthetic photocatalyst system had yet to be developed. Noting that a reduced Hantzsch ester (HEH₂) and related organic structures can behave as 2 e⁻/2 H⁺ photoreductants, we show here that, when partnered with a suitable catalyst (Mo) under blue light irradiation, HEH₂ facilitates delivery of successive H₂ equivalents for the 6 e⁻/6 H⁺ catalytic reduction of N₂ to NH₃; this catalysis is enhanced by addition of a photoredox catalyst (Ir). Reductions of additional substrates (nitrate and acetylene) are also described.

The quest for solar-driven conversion of carbon dioxide to chemicals and fuels hinges upon the identification of an efficient, durable, and selective photocathode. Chalcogenide p-type semiconductors exemplified by chalcopyrite Cu(In,Ga)Se₂ (CIGS) have been effectively deployed as photocathodes. However, selectivity toward CO₂ reduction and durability of the commonly used CdS adlayer remain primary challenges. Here, we demonstrate that for the wide band gap CuGa₃Se₅ chalcopyrite absorber these challenges are well addressed by an organic coating generated in situ from an N,N′-(1,4-phenylene)bispyridinium ditriflate salt in the electrolyte. The molecular additive provides a 30-fold increase in selectivity toward CO₂R products compared to the unmodified system and lowers Cd corrosion at least 10-fold. This dual functionality highlights the promise of hybrid solid-state-molecular photocathodes for enabling durable and efficient solar fuel systems.

Boundary conditions for catalyst performance in the conversion of common precursors such as N₂, O₂, H₂O, and CO₂ are governed by linear free energy and scaling relationships. Knowledge of these limits offers an impetus for designing strategies to alter reaction mechanisms to improve performance. Typically, experimental demonstrations of linear trends and deviations from them are composed of a small number of data points constrained by inherent experimental limitations. Herein, high-throughput experimentation on 14 bulk copper bimetallic alloys allowed for data-driven identification of a scaling relationship between the partial current densities of methane and C₂₊ products. This strict dependence represents an intrinsic limit to the Faradaic efficiency for C–C coupling. We have furthermore demonstrated that coating the electrodes with a molecular film breaks the scaling relationship to promote C₂₊ product formation.

Electrochemical carbon dioxide reduction to multicarbon products provides the storage of renewable energy in the form of chemical bonds, as well as a means to displace fossil sources of chemical feedstocks. However, the accompanying anodic oxygen evolution reaction (OER) reduces the energy efficiency of the process without providing a salable product. Replacing OER with alternative organic oxidation reactions (OORs) is an emerging strategy to reduce the full-cell potential and generate valuable products on both sides of the cell. We pursue carbon monoxide reduction that avoids carbonate formation and benefits from highly alkaline anode conditions favorable for OOR. This coelectrolysis strategy achieves a cathodic C₂₊ product stream (71% FE) and an anodic C₃ product stream (75% FE) at 180 mA cm⁻² with a full-cell potential of 1.34 V. The integrated system reduces the CO-to-C₂H₄ energy requirement by 55% (to ∼72 GJ/ton_(C₂H₄)), halving the projected energy cost of ethylene production from CO₂.

The carbon dioxide reduction reaction (CO₂RR) in aqueous electrolytes suffers from efficiency loss due to the competitive hydrogen evolution reaction (HER). Developing efficient methods to suppress HER is a crucial step toward CO₂ utilization. Herein we report the selective conversion of CO₂ to CO on planar silver electrodes with Faradaic efficiencies >99% using simple pyridinium-based additives. Similar to our previous studies on copper electrodes, the additives form an organic film which alters CO₂RR selectivity. We report electrochemical kinetic and other mechanistic data to shed light on the role of these organic layers in suppressing HER. These data suggest that hydrogen production is selectively inhibited by the growth of a hydrophobic organic layer on the silver surface that limits proton but not CO₂ mass transport at certain applied potentials. The data also point to the involvement of a proton-transfer within the rate-determining step of the catalysis, instead of the commonly observed electron-transfer step for the case of planar Ag electrodes.

M(NHₓ) intermediates involved in N−N bond formation are central to ammonia oxidation (AO) catalysis, an enabling technology to ultimately exploit ammonia (NH₃) as an alternative fuel source. While homocoupling of a terminal amide species (M‐NH₂) to form hydrazine (N₂H₄) has been proposed, well‐defined examples are without precedent. Herein, we discuss the generation and electronic structure of a NiIII‐NH₂ species that undergoes bimolecular coupling to generate a Ni^(II)₂(N₂H₄) complex. This hydrazine adduct can be further oxidized to a structurally unusual Ni₂(N₂H₂) species; this releases N₂ in the presence of NH₃, thus establishing a synthetic cycle for Ni‐mediated AO. Distribution of the redox load for H₂N‐NH₂ formation via NH₂ coupling between two metal centers presents an attractive strategy for AO catalysis using Earth‐abundant, late first‐row metals.

The prototypical reactivity profiles of transition metal dihydrogen complexes (M‐H₂) are well‐characterized with respect to oxidative addition (to afford dihydrides, M(H)₂) and as acids, heterolytically delivering H⁺ to a base and H⁻ to the metal. In the course of this study we explored plausible alternative pathways for H₂ activation, namely direct activation through H‐atom or hydride transfer from the σ‐H₂ adducts. To this end, we describe herein the reactivity of an isostructural pair of a neutral S = ½ and an anionic S = 0 Co‐H₂ adduct, both supported by a trisphosphine borane ligand (P₃^B). The thermally stable metalloradical, (P₃^B)Co(H₂), serves as a competent precursor for hydrogen atom transfer to ᵗBu₃ArO·. What is more, its anionic derivative, the dihydrogen complex [(P₃^B)Co(H₂)]¹⁻, is a competent precursor for hydride transfer to BEt₃, establishing its remarkable hydricity. The latter finding is essentially without precedent among the vast number of M‐H₂ complexes known.

Nitrogen fixation, the six-electron/six-proton reduction of N₂, to give NH₃, is one of the most challenging and important chemical transformations. Notwithstanding the barriers associated with this reaction, significant progress has been made in developing molecular complexes that reduce N₂ into its bioavailable form, NH₃. This progress is driven by the dual aims of better understanding biological nitrogenases and improving upon industrial nitrogen fixation. In this review, we highlight both mechanistic understanding of nitrogen fixation that has been developed, as well as advances in yields, efficiencies, and rates that make molecular alternatives to nitrogen fixation increasingly appealing. We begin with a historical discussion of N₂ functionalization chemistry that traverses a timeline of events leading up to the discovery of the first bona fide molecular catalyst system and follow with a comprehensive overview of d-block compounds that have been targeted as catalysts up to and including 2019. We end with a summary of lessons learned from this significant research effort and last offer a discussion of key remaining challenges in the field.

Terminal Ni^(III) hydrides are proposed intermediates in proton reduction catalyzed by both molecular electrocatalysts and metalloenzymes, but well-defined examples of paramagnetic nickel hydride complexes are largely limited to bridging hydrides. Herein, we report the synthesis of an S = 1/2, terminally bound thiolate–Ni^(III)–H complex. This species and its terminal hydride ligand in particular have been thoroughly characterized by vibrational and EPR techniques, including pulse EPR studies. Corresponding DFT calculations suggest appreciable spin leakage onto the thiolate ligand. The hyperfine coupling to the terminal hydride ligand of the thiolate–Ni^(III)–H species is comparable to that of the hydride ligand proposed for the Ni–C hydrogenase intermediate (Ni^(III)–H–Fe^(II)). Upon warming, the featured thiolate–Ni^(III)–H species undergoes bimolecular reductive elimination of H₂. Associated kinetic studies are discussed and compared with a structurally related Fe^(III)–H species that has also recently been reported to undergo bimolecular H–H coupling.