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

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 H-2 molecules originated from the noncatalytic reaction of acid and reductant reagents, which can coordinate to a vacant binding site of an Fe complex. Thus, H-2 coordination competes with the N-2 coordination step of the desired N2RR catalytic cycle, and the resulting Fe-H-2 complex can be protonated at both hydrogen atoms to release two H-2 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-H-2 complexes, (ii) suppressing the noncatalytic H-2 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.