A theoretical study of the effect of a non-aqueous proton donor on electrochemical ammonia synthesis

Abstract

Ammonia synthesis is one of the most studied reactions in heterogeneous catalysis. To date, however, electrochemical N reduction in aqueous systems has proven to be extremely difficult, mainly due to the competing hydrogen evolution reaction (HER). Recently, it has been shown that transition metal complexes based on molybdenum can reduce N to ammonia at room temperature and ambient pressure in a non-aqueous system, with a relatively small amount of hydrogen output. We demonstrate that the non-aqueous proton donor they have chosen, 2,6-lutidinium (LutH), is a viable substitute for hydronium in the electrochemical process at a solid surface, since this donor can suppress the HER rate. We also show that the presence of LutH can selectively stabilize the *NNH intermediate relative to *NH or *NHvia the formation of hydrogen bonds, indicating that the use of non-aqueous solvents can break the scaling relationship between limiting potential and binding energies.

Biomimetic Nitrogen Fixation Catalyzed by Transition Metal Sulfide Surfaces in an Electrolytic Cell

“…However, RuS 2 is also expected to contribute to some hydrogen evolution that can decrease the yield of ammonia if used in aqueous electrolytes. However, using non‐aqueous electrolytes like 2,6‐lutidinium (LutH + ) or titanocene dichloride [(η5‐C 5 H 5 ) 2 TiCl 2 ] might attenuate the HER, and thus not affect the yield of ammonia considerably.…”

“…Interestingly,i tw as found that RuS 2 is the mosta ctives ulfide with an overpotential of only 0.29 V. Although this sulfide can contributet of ormation of hydrogen as well and, thus, lower the yield of ammonia formation in experiments,t his should not diminish the importance of RuS 2 for further experimental investigations.F or example, RuS 2 can be tested experimentally with the use of non-aqueous electrolytes like 2,6-lutidinium (LutH + ) [53] or titanocene dichloride ((h5-C 5 H 5 ) 2 TiCl 2 ) [54] to attenuate the HER. Another interesting observation is that on all the NiAs-type structures as wella st he pyrite RuS 2 ,t he step associated with the NNH 3 formation does thermodynamically lead to dissociation of the NÀNb ond and formation of Na nd NH 3 .T hus, dinitrogen dissociation should be relativelyf acile on these sulfide surfaces.…”

“…[88][89][90][91] However, RuS 2 is also expected to contribute to some hydrogen evolution that can decrease the yield of ammonia if used in aqueous electrolytes. However, using nonaqueous electrolytes like 2,6-lutidinium (LutH + ) [53] or titanocene dichloride [(h5-C 5 H 5 ) 2 TiCl 2 ] [54] might attenuate the HER, and thus not affect the yield of ammonia considerably. For the dissociative mechanism, the PDS for all these sulfides is the reduction of NH to NH 2 ,l ocated on the green line in the volcanoc urve.…”

“…[48][49][50][51] In addition to nitrides, there are some transition metal oxides (TMOs) such as IrO 2 and NbO 2 in their rutile structuret hat are predicted to show considerable activity towards electrochemical ammonia formation by associative mechanism. [52] To help suppress the HER for more efficient ammonia synthesis, there are also some recent works investigating the effect of the electrolyte in which 2,6-lutidinium (LutH +) [53] and titanocene dichloride( ( h5-C 5 H 5 ) 2 TiCl 2 ) [54] were reported to be promising. Furthermore, single atom catalysts and nitride monolayers were also studied with the aim of suppression of the HER.…”

Atomic Modulation, Structural Design, and Systematic Optimization for Efficient Electrochemical Nitrogen Reduction

“…Moreover, to increase the hydrogen evolution energy barrier, 2,6‐lutidinium (LutH + ) can be utilized instead of hydronium, as suggested by theoretical calculations. This led to a higher barrier for HER on the electrocatalyst surface via the Volmer process . In addition to regulating the electrolyte for controlling the catalyst/electrolyte interface, engineering the electrocatalyst surface such that it is hydrophobic and repels hydrogen, is also a useful strategy for promoting NRR.…”