Challenges in reduction of dinitrogen by proton and electron transfer

Abstract

Ammonia is an important nutrient for the growth of plants. In industry, ammonia is produced by the energy expensive Haber-Bosch process where dihydrogen and dinitrogen form ammonia at a very high pressure and temperature. In principle one could also reduce dinitrogen upon addition of protons and electrons similar to the mechanism of ammonia production by nitrogenases. Recently, major breakthroughs have taken place in our understanding of biological fixation of dinitrogen, of molecular model systems that can reduce dinitrogen, and in the electrochemical reduction of dinitrogen at heterogeneous surfaces. Yet for efficient reduction of dinitrogen with protons and electrons major hurdles still have to be overcome. In this tutorial review we give an overview of the different catalytic systems, highlight the recent breakthroughs, pinpoint common grounds and discuss the bottlenecks and challenges in catalytic reduction of dinitrogen.

Recent progress towards the electrosynthesis of ammonia from sustainable resources

“…21 The optimum rate of ammonia production of 0.19 mol h -1 m -2 was achieved at a cell bias of 0.8 V and a temperature of 450 ⁰C. An overall electrical energy efficiency of 7700 kJ mol -1 of NH 3 produced has been estimated for this system by Hetterscheid and co-workers, 6 which is an energy requirement around 15 times greater than that required by a Haber Bosch-type system even before the costs of obtaining H 2 for this electrosynthesis are considered. This in many ways indicates the scale of the challenge facing electrocatalytic methods of nitrogen reduction if they are ever to approach the efficiency of existing routes to NH 3 production.…”

“…This is the highest Faradaic yield for electrolytic ammonia production using water as the proton source yet reported (see Table 2) and crucially this yield was not significantly affected by replacing the N 2 feed with air. However, using the methods of Hetterscheid and co-workers, 6 the overall efficiency of this system is still sub-optimal, requiring around 1000 kJ per mole of NH 3 that is produced (at a current density of 2 mA cm -2 ). The remaining electrons not used in ammonia production were consumed in hydrogen production.…”

“…36 However, these yields again fell off on account of nitride reoxidation to nitrogen. The electrical energy consumption for this process has been calculated as 3100 kJ mol -1 of ammonia produced, 6 making this system around six times less efficient overall than the Haber Bosch process. In terms of efficiency and overall rate of NH 3 production, this represents a high-water mark for NH 3 electrosynthesis from N 2 and H 2 O.…”

Silylation of Dinitrogen Catalyzed by Hydridodinitrogentris(Triphenylphosphine)Cobalt(I)

“…Although such a transformation is feasible, as evidenced by the biosynthesis of ammonia enabled by nitrogenase enzymes [1][2][3][4], the activation of dinitrogen on transition metal complexes is not trivial and the best man-made homogeneous catalysts for ammonia synthesis can perform only several dozens of catalytic turnovers [5][6][7][8][9][10][11][12][13].…”

Molecular Pnictogen Activation by Rare Earth and Actinide Complexes

“…There are reviews on transition metal N2 activation which cover N2 binding modes [27][28][29], multimetallic N2 activation [30,31], the relevance of metal hydride complexes to N2 activation [32,33], N2 cleavage and functionalisation [34,35] (including electrochemical [36] and photolytic N2 cleavage [37]), and N2 activation at bare metal atoms [38] and using surface organometallic chemistry [39]. Specific reviews have also focused on activation by group 4 metals [40][41][42], iron [31,43,44], molybdenum [24,[45][46][47], and the mid-to-late transition metal centres [48].…”