Electrochemical Ammonia Synthesis Mediated by Titanocene Dichloride in Aqueous Electrolytes under Ambient Conditions

Abstract

Under ambient conditions, the catalytic and electrocatalytic syntheses of ammonia from nitrogen and various proton sources including wet tetrahydrofuran (THF) and the protic solvents methanol and water were performed using titanocene dichloride ((η5-C5H5)2TiCl2, commonly abbreviated to CP2TiCl2) in a two-electrode cell containing 1.0 M LiCl as the electrolyte. The highest rate of ammonia synthesis, 9.5 × 10–10 mol·cm–2·sec–1·M CP2TiCl2 –1, was achieved at −1 V in water, whereas the highest faradaic efficiency (0.95%) was achieved at −2 V in THF. On account of its lower Gibbs free energy, density functional theory calculations suggest that the nitrogen-reduction reaction catalyzed by CP2TiCl2 in the presence of THF, methanol, or water preferably occurs via the Cp2TiClN2 intermediate rather than Cp2TiN2N2. Future strategies to improve both the rate of ammonia synthesis and its faradaic efficiency must consider ways of maximizing nitrogen selectivity to the catalytic active sites by controlling the transfer rates of protons and/or nitrogen.

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

“…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.…”

Enhanced Electrocatalytic N2 Reduction via Partial Anion Substitution in Titanium Oxide–Carbon Composites

“…[3] In aqueous solution, the increase of the electric potential to enhance the activation of N 2 is an option of limited applicability as NRR under these conditions is largely inhibited by the competitive hydrogen evolution reaction (HER). [5] Until now, various promising catalysts have been investigated for electrochemical NRR, including metals, [6] metal oxides, [7] metal nitrides, [8] metal carbides, [9] metal complexes [10] and carbon-based materials. Theu ltimate requirement to such types of catalysts is to bind and polarize N 2 molecules in away that the electron density within the molecule is shifted to facilitate the reaction with protons and/or electrons.…”

“…Theu ltimate requirement to such types of catalysts is to bind and polarize N 2 molecules in away that the electron density within the molecule is shifted to facilitate the reaction with protons and/or electrons. [5] Until now, various promising catalysts have been investigated for electrochemical NRR, including metals, [6] metal oxides, [7] metal nitrides, [8] metal carbides, [9] metal complexes [10] and carbon-based materials. [11] Among them, noble metals exhibit favorable NRR performance, [6] but considering the cost, they are not suitable for large-scale implementation of NRR.…”