Ammonia formation by means of electrochemistry

It is an ongoing effort to replace harsh or energy-consuming reducing agents and perform catalytic ammonia formation by means of electrochemistry. There are roughly two approaches that both try to reconsider the production route or usage of H2. As mentioned, the HB process is associated with a large energy consume, mostly because of the fossil-fuel based production of hydrogen. As alternative strategies, either the production of H2 is envisioned to be electrified via water electrolysis to maintain the conventional HB industry. Alternative, N2 is directly electrochemically reduced in presence of protons. A benefit of the latter is the possibility to set up this new industry de-centralised close to sources of renewed electricity. The direct reductive

approach could be coupled to e.g. water or hydrogen oxidation. It was recently estimated in detail that without e.g. carbon taxes, both methods are still 2-2.5 times more expensive per ton NH3 as conventional HB.^103,104 All the examples discussed herein and in Section I.2.1.2 of electrochemical ammonia formation are far from a commercial application, yet they show the state of the field and general challenges encountered when transforming reactions from chemical reagents towards electrochemistry.

Electrochemical ammonia formation from N2 mediated by molecular complexes is still very limited.¹⁰⁵ In general, the use of a molecular catalyst is associated with a higher selectivity and therefore this is a highly interesting field. A first well-defined stoichiometric molecular example is from Pickett in 1985, where under reductive conditions at a Hg pool electrode, circa 0.25 eq.

of NH3 was released from hydrazido [W(NNH2)(OTs)(PMe2Ph)4]⁺ (OTs = 4-CH3(C6H4)OS(O)2−) (Scheme 20). The source of the required protons is intramolecular from the hydrazido compound that reforms a neutral bis (dinitrogen) compound. Likely, no external acid was introduced, as it would exclusively form H2 under these severe reducing conditions. In subsequent protonation and reduction cycles, 0.75 eq. of ammonia could be collected.

Electrocatalysis on molecular complexes was realised only many years later.¹⁰⁶

Scheme 20. Examples of electrochemical ammonia release on molecular complexes. Left: first stoichiometric example by Pickett.¹⁰⁶ Right: First electrocatalytic example by Peters.¹⁰⁷

While examining the mechanism of catalytic ammonia formation with Fe-catalyst [Fe(P3B)]⁺ , the group of Peters measured CV of this complex in presence of acid to identify the redox state that initiates catalysis. A current increase of the Fe^I/0 reduction upon addition of acid triggered to perform initial electrochemical studies using HBAr^F24·(Et2O)2, that however formed quantitatively 2.0 eq. of NH3 from coordinated N2.¹⁰⁸ In a next attempt using the weaker acid (Ph2PH2)OTf, they applied the PCET knowledge from the chemical catalysis by adding equivalent(s) of [Co(Cp*)2]BAr^F24. That proved a promising strategy: electrolysis at Eappl. = −2.1 V vs. Fc^+/0 in Et2O at −35˚C yielded 4.0 equivalents of NH3 per iron, indicating a catalytic process with a faradaic efficiency (FE) of 28 % (Scheme 20).¹⁰⁷ A glassy carbon working

electrode was applied, which has a large overpotential against competing HER.¹⁰⁹ As counter electrode, a solid sodium rod as sacrificial reductant was used, as Na⁺ reduction is stable against electrolysis conditions, and it was made sure that this hypothetical strong reductant is not the origin of ammonia formation.

Although not a catalytic example, Berben applied an NNN-pincer as coordinated to Al for electrochemical NH3 formation as alternative strategy to transition metals. The starting complex [Al(PDI)Cl] ((PDI = 2,6-bis[1-(2,6-diisopropylphenylimino)methyl]pyridine)) can be protonated twice on one of the pincer arms (Scheme 21). This doubly protonated species shows two reductions by CV that increase in current upon titration of 20 eq. of 4-(1,1-dimethylamide)pyridinium. Notably, this behaviour is more prominent under Ar, and it was shown that reductive electrolysis under Ar forms H2 with regeneration of the starting material.

Under N2, the HER activity decreases and is accompanied by formation of sub-stoichiometric amounts of NH3 (circa 25 %).¹¹⁰ A higher yield cannot be obtained, due to catalysts inhibition via ammonia coordination. Likely, a hydride transfer mechanism takes place, without direct interaction between dinitrogen and aluminium.

Scheme 21. Sub stoichiometric NH3 formation driven by electrochemistry.¹¹⁰

Besides organometallic molecular examples, several examples are known for non-metallic (catalytic) ammonia formation from for instance organic polymers or carbon based nanomaterials.^111,112 More prominent is the huge body of work on electrochemical ammonia formation for heterogeneous based electrocatalysts. Most of these systems suffer from low selectively of NH3 formation over competing HER: faradaic efficiencies for most systems are below or around 10 %.^113,114 A recent example of relatively high FE (≈ 20 %) is given by the group of Zhang. Fe-doped SnO2 layers catalyses electrochemical ammonia formation in a HCl solution at Eappl. = −0.3 V vs. NHE with an overall yield rate of 14 nmol s^-1 cm^-2. Although amongst the highest results within the field, it is still far from the formulated goals for commercial interest

(≈ 1 mmol s^-1 cm^-2).¹⁰⁴ Interestingly, this material also catalyses ammonia oxidation to nitrates, although the selectively for this latter reaction is very low.¹¹⁵

Hand in hand with numerous heterogeneous examples appearing in quick pace, critical notes towards the true origin of ammonia come up, as recently addressed by various authors.^116–118 Many possibilities for (unexpected) background ammonia presence are now identified, i.e. from separator (membrane) material, nitrile gloves, or a non-negligible air concentration.

Chorkendorff emphasises the importance of appropriate background studies and presents an extended flow chart that can be followed to ensure that the ammonia originates from N2.¹¹⁷ Simonov categorises the established research based on three criteria: 1) if the NH3 formation is rate sufficiently high, 2) if isotopically labelled ammonia has been formed, and 3) whether the background NOx impurities in the N2 feed have been quantified. In their opinion, as none of the reviewed studies accounts for the third criterion, successful electrochemical dinitrogen reduction should be approached carefully.¹¹⁶