The impact of alkali and alkaline earth metals on green ammonia synthesis

Abstract

Challenges and opportunities:•The demand for decarbonizing the ammonia industry and the potential use of ammonia for renewable energy storage stimulated tremendous research efforts to pursue green ammonia production.•A prominent feature of recent progress is the active participation of unconventional and reactive alkali or alkaline earth metal (AM)-based materials, such as hydrides, oxyhydrides, electrides, nitrides, and amides, in mediating dinitrogen reduction and ammonia synthesis via organometallic, thermocatalytic, electrochemical, and chemical looping processes.•The elucidation of the unique chemistry of these AM-based materials with N2, H2, and NH3 will deepen fundamental understanding of the multifunctionalities of AM and therefore afford new avenues for the rational development of materials and processes to tackle mild-condition ammonia synthesis, the grand challenge in chemistry. Alkali and alkaline earth metals (AMs) play indispensable roles in dinitrogen activation and ammonia synthesis. AM oxides have been used as catalyst promoters for industrial ammonia synthesis, while AM metals are common reducing agents in dinitrogen fixation by organometallic complexes. With the strong push toward green ammonia synthesis, AM is now on a greater platform, playing a more prominent role in mediating dinitrogen reduction via a variety of processes that can be coupled with renewable energy harvest and storage. This perspective discusses the rich chemistry among AM, N2, H2, and NH3; summarizes the classic understandings and new exciting progress in the exploration of AM for both heterogeneous and homogeneous nitrogen fixation; and highlights the multifunctional roles of unconventional and reactive AM-containing materials for future greener ammonia synthesis. Alkali and alkaline earth metals (AMs) play indispensable roles in dinitrogen activation and ammonia synthesis. AM oxides have been used as catalyst promoters for industrial ammonia synthesis, while AM metals are common reducing agents in dinitrogen fixation by organometallic complexes. With the strong push toward green ammonia synthesis, AM is now on a greater platform, playing a more prominent role in mediating dinitrogen reduction via a variety of processes that can be coupled with renewable energy harvest and storage. This perspective discusses the rich chemistry among AM, N2, H2, and NH3; summarizes the classic understandings and new exciting progress in the exploration of AM for both heterogeneous and homogeneous nitrogen fixation; and highlights the multifunctional roles of unconventional and reactive AM-containing materials for future greener ammonia synthesis. The alkali and alkaline earth metal (AM) elements, located in Groups IA and IIA of the periodic table, respectively, have close parallelism and exhibit some physical properties that are different from other metals, i.e., they have relatively low first ionization energies, electronegativities, and densities. Chemically, AMs, having an ns1 or ns2 configuration of the outermost electron, are highly reactive and may act as strong electron donors because of the ease of losing the valence electrons. They can readily react with a wide range of elements, forming compounds with a rich and varied coordination chemistry.1Greenwood N.N. Earnshaw A. Lithium, sodium, potassium, rubidium, caesium and francium.in: Greenwood N.N. Earnshaw A. Chemistry of the Elements. Elsevier, 1997: 68-102Google Scholar These characteristics endow AMs with the power to be actively involved in versatile applications such as chemical transformations (e.g., organic synthesis, catalysis), energy storage, and utilization (e.g., batteries, hydrogen storage).2Gentner T.X. Mulvey R.E. Alkali-metal mediation: diversity of applications in main-group organometallic chemistry.Angew. Chem. Int. Ed. Engl. 2020; 60: 9247-9262Google Scholar, 3Li Y. Lu Y. Adelhelm P. Titirici M.M. Hu Y.S. Intercalation chemistry of graphite: alkali metal ions and beyond.Chem. Soc. Rev. 2019; 48: 4655-4687Google Scholar, 4He T. Cao H. Chen P. Complex hydrides for energy storage, conversion, and utilization.Adv. Mater. 2019; 31: e1902757Google Scholar, 5Hutchings G.J. Promotion in heterogeneous catalysis: A topic requiring a new approach?.Catal. Lett. 2001; 75: 1-12Google Scholar The importance of AM in heterogeneous catalysis was recognized in 1845.6Doebereiner J.W. Neue Beiträge zur Geschichte der chemischen Dynamik des platins.Ann. Phys. Chem. 1845; 140: 94-96Google Scholar,7Mross W.D. Alkali doping in heterogeneous catalysis.Catal. Rev. 1983; 25: 591-637Google Scholar In particular, AM additives have been widely applied as catalyst promoters for many important chemical processes, such as ammonia synthesis, Fisher-Tropsch synthesis, and water-gas shift reaction.8King D.A. Woodruff D.P. Coadsorption, Promoters and Poisons. Elsevier, 1993Google Scholar Their promotion effects are reflected by the enhancement of activity, selectivity, stability, or structural integrity of related catalysts. The industrial Haber-Bosch process that produces NH3 from N2 and H2 may serve as a typical example benefiting from AM promoters, i.e., the rate of ammonia production increases substantially with adding only a few percent of K2O and CaO to the Fe catalyst. Because of the practical importance, intensive investigations have been performed to unravel the specific chemistry of AM in ammonia synthesis over the 20th century (Figure 1).9Jennings J.R. Catalytic Ammonia Synthesis: Fundamentals and Practice. Springer Science & Business Media, 1991Google Scholar In contrast to its solid promotion effect, the chemical form and function mechanism of AM, however, are controversial, owing to the difficulties in identifying atomic-level details of the interactions of AM with transition metal surfaces and reacting species. In recent years, there have been increasing research efforts for mild-condition “green” NH3 synthesis driven by renewable energy sources. This is because the traditional Haber-Bosch process is energy and emission intensive, and thus decarbonizing the ammonia industry is imperative.17Wang Q. Guo J. Chen P. Recent progress towards mild-condition ammonia synthesis.J. Energy Chem. 2019; 36: 25-36Google Scholar,18The Royal Society Ammonia: Zero-carbon fertilizer, fuel and energy store.https://royalsociety.org/topics-policy/projects/low-carbon-energy-programme/green-ammonia/Date: 2020Google Scholar Moreover, ammonia is a promising carbon-free energy carrier for the storage and transport of renewable energy.19Guo J. Chen P. Catalyst: NH3 as an energy carrier.Chem (Catalyst). 2017; 3: 709-712Google Scholar The most prominent feature of recent progress lies in the applications of a variety of reactive AM-containing compounds as new catalysts or functional materials that encouragingly facilitate effective ammonia synthesis under environmentally benign conditions (Figure 1). In homogeneous nitrogen fixation, AM-based reductants (K or Na metal, KC8, Na/Hg, etc.) are commonly employed to promote N2 reduction or even NH3 formation over a range of organometallic complexes20Connor G.P. Holland P.L. Coordination chemistry insights into the role of alkali metal promoters in dinitrogen reduction.Catal. Today. 2017; 286: 21-40Google Scholar; in heterogeneous ammonia synthesis, various new types of AM-containing materials, such as electrides, hydrides, nitrides, amides, and imides, were found exceptionally effective in facilitating N2 reduction through thermocatalytic and chemical looping processes.17Wang Q. Guo J. Chen P. Recent progress towards mild-condition ammonia synthesis.J. Energy Chem. 2019; 36: 25-36Google Scholar A few electrochemical systems that have been verified to produce genuine NH3 have the action of Li.16Lazouski N. Schiffer Z.J. Williams K. Manthiram K. Understanding continuous lithium-mediated electrochemical nitrogen reduction.Joule. 2019; 3: 1127-1139Google Scholar,21Andersen S.Z. Čolić V. Yang S. Schwalbe J.A. Nielander A.C. McEnaney J.M. Enemark-Rasmussen K. Baker J.G. Singh A.R. Rohr B.A. et al.A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements.Nature. 2019; 570: 504-508Google Scholar, 22Andersen S.Z. Statt M.J. Bukas V.J. Shapel S.G. Pedersen J.B. Krempl K. Saccoccio M. Chakraborty D. Kibsgaard J. Vesborg P.C.K. et al.Increasing stability, efficiency, and fundamental understanding of lithium-mediated electrochemical nitrogen reduction.Energy Environ. Sci. 2020; 13: 4291-4300Google Scholar, 23Lazouski N. Chung M. Williams K. Gala M.L. Manthiram K. Non-aqueous gas diffusion electrodes for rapid ammonia synthesis from nitrogen and water-splitting-derived hydrogen.Nat. Catal. 2020; 3: 463-469Google Scholar These recent progresses indicate that AMs, the indispensable catalyst promoters in the current ammonia industry, have the potency to exert an even stronger impact on future “green” ammonia synthesis processes. In this context, a retrospective overview of knowledge accumulated in the past and analysis of new findings generated recently would be beneficial for deepening our understanding of the fundamentals of AM in ammonia synthesis, which, in turn, would stimulate new ideas for the design and development of catalysts or processes toward green ammonia production. This perspective begins with an analysis of possible scenarios for AM in mediating N2 reduction, and a description of sets of chemical reactions among AM, N2, H2, and NH3 that have yet to be widely aware of but are important for ammonia synthesis. Next, a brief summary of the early investigations on AM with respect to industrial ammonia synthesis, and a survey/analysis of the recent observations on the effects of AM on homogeneous and heterogeneous nitrogen fixation are provided, followed by a discussion and outlook of AM for future ammonia synthesis. N2 is a highly inert molecule with tightly bound σ and π electrons. The key factors for effectively converting N2 to NH3 lie in I. weakening N≡N triple bond by filling electrons to its antibonding orbitals and/or withdrawing electrons from its bonding orbitals, II. supplying reactive H to bind with the activated nitrogen species, and III. moderately stabilizing the NxHy (x = 1, 2; y = 0–5) intermediates via forming chemical interactions with their chemical environment. The latter is of particular importance for catalytic NH3 synthesis, as indicated by the Brønsted-Evans-Polanyi (BEP) and scaling relations.24Vojvodic A. Medford A.J. Studt F. Abild-Pedersen F. Khan T.S. Bligaard T. Nørskov J.K. Exploring the limits: A low-pressure, low-temperature Haber–Bosch process.Chem. Phys. Lett. 2014; 598: 108-112Google Scholar These facts point out the necessity of having an electron-rich mediator to disrupt the N2 bonding system and a chemical environment to modulate the reactivity of H and NxHy intermediates toward NH3 formation via an energy favorable pathway. N2-to-NH3 is generally mediated by transition metals (TMs) owing to their right combination of occupied and unoccupied d orbitals that can backdonate/accept electrons to/from the N2 molecule. AMs play an indispensable role in such a TM-mediated process. On the one hand, AM metals are strong electron donors and some of them have been employed as reducing reagents that can facilitate molecular TM complexes for nitrogen fixation. On the other hand, AM cations are hard Lewis acids that can exert electrostatic interactions with NxHy species, which may be one of the reasons why AM compounds are purposely added into heterogeneous catalysts and a set of molecular TM–N2(N) complexes are found to bond with AM cations. However, the story does not stop here. There is a rich and unique chemistry among AM, N2, H2, H2O, and NH3, as shown in Figure 2.25Gao W. Guo J. Chen P. Hydrides, amides and imides mediated ammonia synthesis and decomposition.Chin. J. Chem. 2019; 37: 442-451Google Scholar,26Guo J. Chen P. Interplay of alkali, transition metals, nitrogen, and hydrogen in ammonia synthesis and decomposition reactions.Acc. Chem. Res. 2021; 54: 2434-2444Google Scholar It is well known that Li and most AMs can react with N2 to form nitrides (AMN), but other reactions were overlooked in the past. As a matter of fact, AM metals can be hydrogenated to AM hydrides (AMH); AMH are strong reductants and reactive H carriers, and some of them can directly reduce N2 to form AM amides/imides (AMNH) and H2. A few AMN can be hydrogenated to AMH and AMNH; AMNH can split H2 heterolytically, producing AMH and NH3. Furthermore, AMN and AMNH undergo hydrolysis producing NH3 and AM oxides/hydroxides (AMO/AMOH), and the latter can be converted back to AM metals via electro- or photo-reduction. Such beautiful chemistry discloses that AM, despite having a different electron configuration from TM, can exert sophisticated functions in mediating N2-to-NH3, even in the absence of TM! More discussion can be found in the last two sections. In the early 20th century, Mittasch et al. noted that the addition of foreign substances into magnetite (Fe3O4) resulted in a strong influence on its catalytic behavior, a finding that stimulated systematic research on the effect of thousands of additives on the Fe catalyst for ammonia synthesis.27Mittasch A. Geschichte der Ammoniaksynthese. 55. Verlag. Chem, Berlin-Weinheim1951: 258Google Scholar It was finally recognized that adding alumina and potash (a mixture of KOH and K2CO3) into magnetite could achieve catalytic NH3 production in an efficient and stable manner. Following such an important finding, the industrial catalyst for the Haber-Bosch process was successfully developed by fusing Fe3O4 with small amounts of K2O, Al2O3, CaO, MgO, and SiO2. The oxides of Al, Ca, Mg, and Si are generally considered as structural promoters that improve the activity by increasing the surface area and structural stability of the activated Fe catalyst. The oxide of K, on the other hand, tends to stay on the catalyst surface and significantly enhances the intrinsic activity of Fe, and thus is often referred to as an electronic promoter.9Jennings J.R. Catalytic Ammonia Synthesis: Fundamentals and Practice. Springer Science & Business Media, 1991Google Scholar The surface potassium promoter lacks crystalline information. Nevertheless, well-defined surface science studies have significantly advanced our understanding of the role of potassium promoter. Ertl et al. conducted a series of ultra-high vacuum (UHV) studies28Ertl G. Weiss M. Lee S.B. The role of potassium in the catalytic synthesis of ammonia.Chem. Phys. Lett. 1979; 60: 391-394Google Scholar,29Paál Z. Ertl G. Lee S.B. Interactions of potassium, oxygen and nitrogen with polycrystalline iron surfaces.Applications of Surface Science. 1981; 8: 231-249Google Scholar and found that the surface potassium metal may act as an electronic donor, transferring its electrons to the Fe surface and thus increasing the Fe-to-N2 backdonation, which significantly accelerates the N2 dissociation rate over the Fe surface (e.g., by more than a factor of 200 for Fe(100)). The authors also observed that, at low potassium coverage, the potassium atoms arrange into an ordered (3 × 3) structure on the Fe(111) surface.30Lee S.B. Weiss M. Ertl G. Adsorption of potassium on iron.Surf. Sci. 1981; 108: 357-367Google Scholar Considering that potassium in technical Fe catalyst would bind with oxygen under operating conditions, the authors further investigated the influence of K+O adlayers on N2 dissociation over Fe surfaces and realized that the promotion effect was still observable but at a reduced level, and meanwhile, the thermal stability of surface potassium was significantly improved.29Paál Z. Ertl G. Lee S.B. Interactions of potassium, oxygen and nitrogen with polycrystalline iron surfaces.Applications of Surface Science. 1981; 8: 231-249Google Scholar Therefore, the authors proposed that, under technical conditions, a Fe-O-K layer might be the actual surface phase of the industrial catalyst. However, the nature of the K+O adlayer is uncertain, in which K may either sit on the top of the adsorbed O layer or combine with O to form a uniform single layer over the Fe surface. The function of potassium promoter under high pressure was investigated by Somorjai et al.31Strongin D.R. Somorjai G.A. The effects of potassium on ammonia synthesis over iron single-crystal surfaces.J. Catal. 1988; 109: 51-60Google Scholar Under 20 MPa of N2–3H2, they found that the surface potassium species affected the kinetic behaviors of the Fe catalyst, resulting in an increase in ammonia reaction order and a decrease in hydrogen reaction order, respectively. They concluded that the presence of potassium did not change the activation energy but facilitated the desorption of NH3 product, which in turn opened more active sites for dissociative N2 adsorption and thus increased the rate of ammonia synthesis. Such interpretations are consistent with the earlier observations that potassium promotion is pronounced under high pressures with high ammonia concentrations.32Altenburg K. Bosch H. Van Ommen J. Gellings P. The role of potassium as a promoter in iron catalysts for ammonia synthesis.J. Catal. 1980; 66: 326-334Google Scholar On the other hand, a recent work by Huo and Jiao et al., through a combined theoretical and experimental investigation, suggested that the potassium promoter could stabilize high index and more active Fe facets, and thus favor ammonia synthesis.33Huo C.F. Wu B.S. Gao P. Yang Y. Li Y.W. Jiao H. The mechanism of potassium promoter: enhancing the stability of active surfaces.Angew. Chem. Int. Ed. Engl. 2011; 50: 7403-7406Google Scholar Sixty years after the birth of the promoted Fe catalyst, Tamaru et al. and Aika et al. found that the promotion effect of potassium was not only specific to Fe but also to other transition metals, especially Ru.34Aika K.-i. Role of alkali promoter in ammonia synthesis over ruthenium catalysts—effect on reaction mechanism.Catal. Today. 2017; 286: 14-20Google Scholar, 35Ozaki A. Aika K.-i. Hori H. A new catalyst system for ammonia synthesis.Bull. Chem. Soc. Jpn. 1971; 44: 3216Google Scholar, 36Ichikawa M. Kondo T. Kawase K. Sudo M. Onishi T. Tamaru K. Catalytic synthesis of ammonia by graphite–alkali metal complexes containing transition–metal chloride.J. Chem. Soc. Chem. Commun. 1972; : 176-177Google Scholar Moreover, AM promotion on the Ru catalyst is electronegativity-dependent, i.e., Cs > K > Na, no matter if AM is in metallic or oxide form. The Cs2O-promoted and MgO-supported Ru (Cs-Ru/MgO) catalyst is thus developed and often serves as a benchmark catalyst in the current investigations. In the early 1990s, with the joint efforts of British Petroleum (BP) and Kellogg, the catalyst made of ruthenium, graphitized carbon support, cesium and barium oxides was commercialized for ammonia production through the Kellogg Advanced Ammonia Process (KAAP).37Bielawa H. Hinrichsen O. Birkner A. Muhler M. The ammonia-synthesis catalyst of the next generation: barium-promoted oxide-supported ruthenium.Angew. Chem. Int. Ed. Engl. 2001; 40: 1061-1063Google Scholar Afterward, many studies attempted to disclose the promotion effect of barium. In particular, Muhler et al. showed that the promotion effect of BaO on Ru/MgO was even stronger than that of Cs2O.37Bielawa H. Hinrichsen O. Birkner A. Muhler M. The ammonia-synthesis catalyst of the next generation: barium-promoted oxide-supported ruthenium.Angew. Chem. Int. Ed. Engl. 2001; 40: 1061-1063Google Scholar However, there is much debate about whether barium serves as an electronic promoter or a structural promoter. With the aid of in situ atomic-resolution transmission electron microscopy, Jacobsen et al. observed that Ba atoms were quite mobile under NH3 synthesis conditions and presented in oxide form. In addition, Ba promoter did not alter the morphology of Ru crystals but distributed in the vicinity of the crystal edges (B5 sites).38Hansen T.W. Wagner J.B. Hansen P.L. Dahl S. Topsøe H. Jacobsen C.J. Atomic-resolution in situ transmission electron microscopy of a promoter of a heterogeneous catalyst.Science. 2001; 294: 1508-1510Google Scholar Nørskov et al. simulated the reaction pathway of NH3 synthesis over an alkali metal promoted Ru single-crystal surface by using DFT calculations and disclosed that the electrostatic interaction between the adsorbed alkali atoms and the reaction intermediates plays an important role.39Mortensen J.J. Hammer B. Nørskov J.K. Alkali promotion of N2 dissociation over Ru (0001).Phys. Rev. Lett. 1998; 80: 4333-4336Google Scholar,40Dahl S. Logadottir A. Jacobsen C.J.H. Nørskov J.K. Electronic factors in catalysis: the volcano curve and the effect of promotion in catalytic ammonia synthesis.Appl. Catal. A. Gen. 2001; 222: 19-29Google Scholar An electric dipole is created when an electron is transferred from AM to the Ru surface, which exerts an attraction to the dipole induced by the transition state of the dissociating N2 molecule, but a repulsion to the adsorbed NHx (x = 1, 2, 3) intermediate species. As a consequence, the N2 dissociation barrier is lowered, and at the same time, the absorbed NHx intermediates are destabilized. Both effects lead to a significant increase in ammonia synthesis turnover frequency. Notably, this electrostatic model is applicable to explaining both the promoting effects of AM observed in the above-mentioned surface science studies, i.e., enhancing N2 dissociation and weakening NH3 adsorption. These important findings and fundamental understandings form a strong foundation for today’s commonsense of AM promotion not only to ammonia synthesis but also to other chemical processes that employ AM as promoters. In contrast to the forcing conditions (>623 K, >10 MPa) required in the industrial Haber-Bosch process, nitrogen fixation in nature, which is catalyzed by the nitrogenase enzyme and driven by the hydrolysis of ATP, occurs at ambient conditions. To mimic biological systems and to pursue artificial energy-saving methods, great efforts have been devoted to the nitrogen fixation process mediated by molecular TM complexes.41Chalkley M.J. Drover M.W. Peters J.C. Catalytic N2-to-NH3 (or -N2H4) conversion by well-defined molecular coordination complexes.Chem. Rev. 2020; 120: 5582-5636Google Scholar The first dinitrogen complex [Ru(NH3)5(N2)]2+ was synthesized in 1965.42Allen A.D. Senoff C.V. Nitrogenopentammineruthenium (II) complexes.Chem. Commun. (London). 1965; : 621-622Google Scholar Thereafter, TM-dinitrogen (TM–N2) and TM-nitride (TM–N) complexes that encompass a wide range of TMs, supporting ligands and binding modes have been prepared and investigated, especially in recent years.43Burford R.J. Fryzuk M.D. Examining the relationship between coordination mode and reactivity of dinitrogen.Nat. Rev. Chem. 2017; 1Google Scholar AM-based reductants have been commonly employed as effective electron carriers to reduce TM precursors into electron-rich TM complexes that would facilely interact with N2, forming TM–N2 or TM–N complexes, as well as to drive the stepwise reduction of the TM–N2 complexes to produce NH3. As a matter of fact, KC8, Na/Hg, K, or Na metal, etc. can reduce the number of d-block (Cr, Mo, W, V, Y, Nb, Zr, Fe, Co, Ru, and Ni) and f-block (U and Th) TM precursors to form corresponding dinitrogen complexes under N2 atmosphere. Several multinuclear TM (Fe, V, and Nb) complexes, on the other hand, can split N2 into bridging nitrides with the aid of AM-based reductants.20Connor G.P. Holland P.L. Coordination chemistry insights into the role of alkali metal promoters in dinitrogen reduction.Catal. Today. 2017; 286: 21-40Google Scholar,44Yin J. Li J. Wang G.X. Yin Z.B. Zhang W.X. Xi Z. Dinitrogen functionalization affording chromium hydrazido complex.J. Am. Chem. Soc. 2019; 141: 4241-4247Google Scholar, 45Falcone M. Chatelain L. Scopelliti R. Živković I. Mazzanti M. Nitrogen reduction and functionalization by a multimetallic uranium nitride complex.Nature. 2017; 547: 332-335Google Scholar, 46Arnold P.L. Ochiai T. Lam F.Y.T. Kelly R.P. Seymour M.L. Maron L. Metallacyclic actinide catalysts for dinitrogen conversion to ammonia and secondary amines.Nat. Chem. 2020; 12: 654-659Google Scholar In the 1970s, Chatt and co-workers successfully prepared a series of [TM(N2)2(PR3)4] (TM=Mo or W; PR3=PMe2Ph or PPh2Me) complexes by directly reducing Mo or W chloride precursor with Na/Hg or Mg under N2 atmosphere.47Chatt J. Dilworth J.R. Richards R.L. Recent advances in the chemistry of nitrogen fixation.Chem. Rev. 1978; 78: 589-625Google Scholar These TM–N2 complexes can be further protonated to release NH3 in variable yields. Peters et al. recently reported that an Fe–N2 complex [(TPB)Fe(N2)][Na(12-crown-4)2], which was prepared by reducing the Fe precursor with Na/Hg under N2, could catalyze N2 to NH3 using excess KC8 as the electron donor (Figure 3A).48Anderson J.S. Rittle J. Peters J.C. Catalytic conversion of nitrogen to ammonia by an iron model complex.Nature. 2013; 501: 84-87Google Scholar More recently, a handful of TM–N2 complexes (TM = Ti, V, Fe, Ru, Os, Co) have also been reported to rely on KC8 as the electron source for catalytic ammonia synthesis.41Chalkley M.J. Drover M.W. Peters J.C. Catalytic N2-to-NH3 (or -N2H4) conversion by well-defined molecular coordination complexes.Chem. Rev. 2020; 120: 5582-5636Google Scholar It is worth noting that KC8 is a strong reducing agent and not suitable for the Mo-based catalytic systems that often require relatively mild potential to drive N2 fixation. After reducing the TM precursor into TM–N2(N) complex, AM cations, on the other hand, were often found to engage in noncovalent interactions with the coordinated N2(N) and/or supporting ligands of TM–N2(N) complex, as reflected by the crystallographic information of isolated products and/or intermediates. Connor and Holland presented a comprehensive review on the effects of AM cations during the binding and cleavage of N2 in homogeneous systems, which the readers may like to refer to.20Connor G.P. Holland P.L. Coordination chemistry insights into the role of alkali metal promoters in dinitrogen reduction.Catal. Today. 2017; 286: 21-40Google Scholar A brief summary is given hereinafter. A few TM–N2 complexes have been identified to contain an end-on or side-on interaction between the N2 ligand and AM cations, and several multinuclear TM–N complexes have also been observed to incorporate AM cation interactions with one or both of the bridging nitrides (Figure 3B). Usually, complexes with an end-on interaction contain a linear TM–N≡N–AM motif, where AM cations interact with electron density from the N2 σ-orbital and induce polarization of the coordinated N–N bond. Such an effect leads to greater activated N2 that would be easily attacked by electrophiles (such as acids, carbon-, or silyl-electrophiles) toward functionalization reactions. While in complexes with side-on interaction between AM cations and N2 ligand, AM cations interact with electron density from N2 π-orbitals. Typically, removing the side-on coordinated AM cations results in significant structural change of the complexes. For multinuclear TM complexes that bind and cleave N2, such as the intensively studied Fe-diketiminate system (Figure 3C), potassium cations could interact with supporting ligands to structurally assemble multiple Fe fragments into the necessary configuration for N2 binding, as well as stabilize the reduced bis-nitride product to thermodynamically promote N2 reduction.20Connor G.P. Holland P.L. Coordination chemistry insights into the role of alkali metal promoters in dinitrogen reduction.Catal. Today. 2017; 286: 21-40Google Scholar,13Rodriguez M.M. Bill E. Brennessel W.W. Holland P.L. N2 reduction and hydrogenation to ammonia by a molecular iron-potassium complex.Science. 2011; 334: 780-783Google Scholar Unlike the TM-dependent molecular systems mentioned above, Harder et al. demonstrated the activation of N2 on a low-valent CaI complex, (BDI)Ca–Ca(BDI) (BDI=HC{C(Me)N[2,6-(3-pentyl)-phenyl]}2), very recently.64Rösch B. Gentner T.X. Langer J. Färber C. Eyselein J. Zhao L. Ding C. Frenking G. Harder S. Dinitrogen complexation and reduction at low-valent calcium.Science. 2021; 371: 1125-1128Google Scholar As shown in Figure 3D, reacting (BDI)Ca–Ca(BDI) with the reducing agent K/KI under N2 atmosphere leads to the formation of (BDI)Ca(N2)Ca(BDI), which can be isolated by adding tetrahydrofuran (THF) or tetrahydropyran (THP). In both products, the N22− anion adopts a side-on bridging motif between two Ca centers. Meanwhile, the involvement of Ca d orbitals in N2 activation has been discussed. This finding shows the potential of low-valent AM complexes to achieve N2-to-NH3 without resorting to transition metals. These flexible interactions shown above manifest the multiple functions of AM on the activation and reduction of N2 in homogeneous systems. The direct interaction between AM and the N2(N) ligand observed here (Figure 3), on the other hand, may provide useful clues on the interpretation of the chemical form and local environment of surface AM promoters as well as their interactions with NxHy intermediates in heterogeneous ammonia synthesis, as discussed in AM in recently developed heterogeneous ammonia synthesis processes. In recent years, various unconventional AM-containing compounds, such as electrides, hydrides, oxyhydrides, nitrides, (oxy)nitride-hydride, and amides, have been employed to facilitate thermocatalytic NH3 production. Meanwhile, some alkali or alkaline metals and their compounds have been found to play noticeable roles in mediating ammonia formation under mild or even ambient conditions via chemical looping and electrochemical processes. With these new findings, insights into AM function are substantially extende