Demonstrating the Direct Relationship between Hydrogen Evolution Reaction and Catalyst Deactivation in Synthetic Fe Nitrogenases

Abstract

Synthetic Fe nitrogenases are promising catalysts for atmospheric pressure ammonia synthesis. However, their catalytic efficiency is severely limited by the accompanying hydrogen evolution reaction (HER) and fast catalyst deactivation. In order to reveal the origin of these undesired transformations, we study potential reaction routes of HER, catalyst deactivation, and nitrogen reduction reaction (N2RR) by density functional theory in combination with microkinetic modeling, using a triphosphino-silyl ligated iron complex as model system. Our results show that the most favorable HER cycle is initiated by H2 molecules originated from the noncatalytic reaction of acid and reductant reagents, which can coordinate to a vacant binding site of an Fe complex. Thus, H2 coordination competes with the N2 coordination step of the desired N2RR catalytic cycle, and the resulting Fe–H2 complex can be protonated at both hydrogen atoms to release two H2 molecules. The proposed mechanism, called autocatalytic hydrogen evolution reaction (aHER), explains all experimentally observed results including catalyst deactivation, as aHER intermediates can be easily converted into thermodynamically stable, catalytically inactive monohydrides. Our results suggest that improved efficacy of synthetic Fe nitrogenases can be achieved by several ways: (i) proper ligand modifications hindering the formation of Fe–H2 complexes, (ii) suppressing the noncatalytic H2 formation in the catalytic mixture by different reagent choice, and (iii) using flow or semiflow reactor setup instead of batch reactors and keep the proton and electron reagent excess low.

H2 and N2 Binding Affinities Are Coupled in Synthetic Fe Nitrogenases Limiting N2 Fixation

“…Similar to our previous DFT studies on nitrogenases, ,, we performed the geometry optimization at the BP86/def2-SVP level of theory. − The RI density fitting approximation was applied with the def2/J auxiliary basis set . To take relativistic effects and dispersion interactions into account, the zeroth-order relativistic approximation (ZORA) , and Grimme’s Becke–Johnson damped dispersion correction (D3­(BJ)) , were used in all calculations.…”

“…The selection of conformation and spin state of EP 3 Fe–N 2 , EP 3 Fe–H 2 , and EP 3 Fe complexes was based on recent experimental and computational studies. ,, …”

“…This problem, which is manifested in the relative position of the isopropyl substituents of the phosphine arms in the case of EP 3 Fe derivatives, was overcome in the present study by initiating optimizations from the conformations of the X-ray structures published by the Peters group. Since all intermediates characterized by XRD had the same spatial position of the isopropyl groups, ,, it presumably does not introduce a major error into our calculations.…”

“…In the present computational paper, we examine whether the undesired coordination of H 2 to Fe can be avoided by the proper redesign of the catalyst complex. In particular, since N 2 /H 2 exchanges presumably proceed as consecutive coordination/de-coordination steps , ( Fe–N 2 ⇌ Fe ⇌ Fe–H 2 or through N 2 RR: Fe–N 2 → (N 2 RR) → Fe–NH 3 ⇌ Fe ⇌ Fe–H 2 ), we compute the chemical affinity of variably substituted EP 3 Fe complexes toward N 2 and H 2 . We explore how these two affinities are related to each other and to the electronic properties of the modified binding sites and determine whether a direct relationship exists between A N 2 and A H 2 .…”

The Reactivity of Human and Equine Estrogen Quinones towards Purine Nucleosides

“…Such an analysis is difficult to achieve experimentally, as only macroscopic quantities (such as the time course of adduct formation) can be determined in this way; moreover, precise measurements can only be performed on simplified model systems [9,19,25] where the reaction conditions (e.g., pH, distribution of protonated/deprotonated species, reactant concentrations) might significantly deviate from those peculiar to an actual breast cell nucleus. Computational chemistry, on the contrary, enables the detailed examination of any reaction sequence at the level of elementary steps, which has been proven to be especially useful in studying complex reaction mechanisms [26][27][28][29], including biochemical transformations [30,31].…”