Exploring Hydrogen Evolution Accompanying Nitrogen Reduction on Biomimetic Nitrogenase Analogs: Can Fe–N_xH_yIntermediates Be Active Under Turnover Conditions?

Abstract

Nitrogen reduction reaction (N2RR) carried out on biomimetic catalytic systems is considered to be a promising alternative for the traditional Haber–Bosch ammonia synthesis. Unfortunately, the selectivity of the currently known biomimetic catalysts is poor, as they also catalyze the unproductive hydrogen evolution reaction (HER). In the present computational study, we examine the HER activity of early N2RR intermediates in EP3 (E = B, Si) ligated single-site biomimetic iron complexes by calculating and comparing the activation Gibbs free energies of HER and N2RR elementary steps. We find that, in contrast to previous suggestions, early N2RR intermediates are not likely sources of HER under turnover conditions, as the barriers of the competing N2RR steps are significantly lower. Consequently, future research should focus on preventing other potential HER mechanisms, e.g., hydride formation, rather than accelerating the consumption of early N2RR intermediates as proposed earlier to design more efficient biomimetic catalysts.

Heterojunction-based photocatalytic nitrogen fixation: principles and current progress

“…Photocatalytic NRR is initiated when N2 molecules are trapped and activated on the photocatalyst surface and react with the photogenerated electrons and protons in the electrolyte to generate NH3 and other nitrates. One critical issue in current photocatalytic NRR is the low selectivity of NRR due to the competing hydrogen evolution reaction (HER) 31 . The standard equilibrium redox potential for NRR and HER is +0.092 V and 0 V, respectively, on the reference scale of NHE 59 .…”

“…Despite the substantial progress that has been made in photocatalytic nitrogen fixation, several issues must be addressed before any practical significance can be achieved. Firstly, nitrogen reduction reaction (NRR) and hydrogen evolution reaction (HER) possess similar reduction potentials, and thus the selectivity for NRR is reduced in favour of HER in aqueous electrolyte 30,31 . Secondly, the lack of suitable adsorption and activation sites for stable N≡N molecules onto the surface of photocatalysts results in sluggish kinetics of NRR 32 .…”

Understanding Potential-dependent Competition Between Electrocatalytic Dinitrogen and Proton Reduction Reactions

“…The control of reaction selectivity between NRR and HER has been extensively studied in fields of biochemistry, bio-electrochemistry, molecular catalysis, and electrochemistry [46][47][48] . It has also been known that natural N 2 -fixation catalyst, e.g.…”

“…As the fine-tunning of NRR selectivity over HER has been demonstrated in metal-complex catalysis, more delicate design of binary, tertiary, or multi-component electrocatalysts can regulate the *H formation over the *N 2 or *NNH production 47 . More practically, controlling the concentration of N 2 and H + at the electrode-electrolyte interface with increasing the *H formation barrier could be helpful, such as by the coatings with a hydrophobic layer 51 , utilizing polar aprotic ionic solvent (high N 2 solubility) 52,53 , using gas diffusion electrode (high N 2 concentration) 54 or using bulky proton donor in nonaqueous electrolytes 55,56 .…”

Relating N–H Bond Strengths to the Overpotential for Catalytic Nitrogen Fixation

“…Indeed, whereas N-H bonds in amines typically have BDFEs between 90 and 100 kcal•mol −1 , [8] density functional theory (DFT) studies on the BDFE N-H of several diazenido species have consistently predicted values less than 50 kcal•mol −1 . [18][19][20][21] This is an important value to keep in mind, because below 50 kcal•mol −1 N-H (and other X-H) bonds are thermodynamically prone to bimolecular reactions that release H 2 (BDFE(H 2 ) = 102.3 kcal•mol −1 in MeCN). [8] Thus, although a transition metal can significantly stabilize an NNH species via coordination compared to its free state (calculated gas-phase BDE N-H (NNH) = −4 kcal•mol −1 ), [8] M-NNH species remain susceptible to the hydrogen evolution reaction (HER, vide infra).…”