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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2023 , Vol. 35 >Issue 6: 904 - 917

DOI: https://doi.org/10.7536/PC221224

Review

Condensed Matter Chemistry in Nitrogen Fixation

  • Xueli Wang 1 ,
  • Qianru Wang 2 ,
  • Di Li 2 ,
  • Junnian Wei 1 ,
  • Jianping Guo 2 ,
  • Liang Yu 2 ,
  • Dehui Deng , 2, * ,
  • Ping Chen , 2, * ,
  • Zhenfeng Xi , 1, *
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  • 1 College of Chemistry, Peking University,Beijing 100871, China
  • 2 Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
*Corresponding authore-mail: (Dehui Deng);
(Ping Chen);
(Zhenfeng Xi)

†These authors contributed equally to this work.

Received date: 2022-12-28

  Revised date: 2023-02-24

  Online published: 2023-06-12

Supported by

The National Natural Science Foundation of China(21988101)

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Abstract

Nitrogen is an indispensable element for life and the material world. The development of efficient conversion strategies to transform dinitrogen gas into various valuable nitrogen-containing compounds is of great economic and scientific importance. The activation and transformation of dinitrogen molecule is an eternal topic in chemistry, and it is of profound significance to understand nitrogen fixation from the level of condensed matter chemistry. Several related examples have been illustrated here to discuss the effects of condensed matter phenomena in nitrogen fixation chemistry. Some critical scientific problems in the field are discussed from three aspects: nitrogen fixation in homogeneous solution, heterogeneous ammonia synthesis, and coupling multiple energy for N2/O2 conversion. We hope this review will inspire more chemists to think about the fundamental nature of nitrogen fixation chemistry from the perspective of condensed matter chemistry, offering more ideas to solve the related problems.

Contents

1 Introduction

2 Condensed matter chemistry in nitrogen fixation in homogeneous solution systems

3 Condensed matter chemistry in heterogeneous ammonia synthesis

4 Condensed matter chemistry in the coupling multiple energy for N2/O2conversion

4.1 N2/O2conversion by non-thermal plasmas

4.2 N2/O2conversion by electrochemistry

4.3 N2/O2conversion by ultra sound

4.4 N2/O2conversion by photochemistry

5 Conclusion and outlook

Key words： condensed state; dinitrogen gas; nitrogen fixation; ammonia synthesis

Cite this article

Xueli Wang , Qianru Wang , Di Li , Junnian Wei , Jianping Guo , Liang Yu , Dehui Deng , Ping Chen , Zhenfeng Xi . Condensed Matter Chemistry in Nitrogen Fixation[J]. Progress in Chemistry, 2023 , 35(6) : 904 -917 . DOI: 10.7536/PC221224

1 Introduction

Nitrogen molecule is the most important source of nitrogen in the material world and living matter. Nitrogen is an inexhaustible source of nitrogen because of its abundant reserves, low price and easy availability. However, the free nitrogen in the air can not be directly absorbed and utilized by human beings or animals and plants. Only by converting the free nitrogen into combined nitrogen (that is, the fixation of nitrogen) can it be used by most organisms. The development of efficient conversion methods to fix nitrogen in nitrogen molecules and turn them into nitrogen-containing compounds with various functions has long been the "holy grail" of academia and industry. However, as we all know, the chemical properties of nitrogen molecules are very stable, and it is difficult to react with other substances at room temperature. Therefore, the efficient activation and conversion of nitrogen molecules is a scientific problem that has lasted for a century. Fortunately, more than a hundred years ago, human beings established the process of synthesizing ammonia from nitrogen and hydrogen (Haber-Bosch ammonia synthesis process), which realized the large-scale production of ammonia. Based on synthetic ammonia technology, human beings not only deal with the food problem, but also provide "available nitrogen source" for various functional compound molecules. In other words, at present, ammonia is the source of nitrogen for almost all nitrogen-containing compounds, and industrial ammonia synthesis is the only way to convert nitrogen on a large scale in the history of human synthesis. However, due to the high energy consumption, high pollution and other problems of the traditional ammonia synthesis industry, as well as the difficulties in converting NH3 into some important nitrogen compounds, chemists have been exploring ways to improve or even subvert the industrial ammonia synthesis technology, while seeking to develop other ways to directly use nitrogen.
Over the past century, researchers have systematically and deeply studied the nitrogen conversion process from different perspectives, such as homogeneous chemistry, heterogeneous chemistry and enzymatic chemistry, and have obtained many new understandings, formed many new concepts and methods, synthesized a series of new compounds and new materials, and greatly enriched the knowledge system of nitrogen fixation. Despite these important research advances, there is still a long way to go to achieve the ambitious goal of "controlled conversion of nitrogen". Part of the reason is attributed to the fact that the understanding of the scientific problem of nitrogen conversion is disciplinary and one-sided, and lacks a holistic and comprehensive understanding.
Recently, Mr. Xu Ruren of Jilin University put forward the concept of "condensed matter chemistry". Condensed state is composed of atoms, ions and molecules bonded by chemical bonds (ionic, covalent, metallic, coordination, hydrogen bonds) which have specific electronic structures and are traditionally regarded as the main body of reaction.It is formed by establishing "stable adhesion relationship" through electrical interaction (Coulomb interaction, hybridization and superposition of electronic orbitals, etc.), and has specific composition, multi-level structure, properties and functions[1⇓~3]. Taking the solid state of crystal as an example, the structure of condensed matter can be summarized into three levels: the first level is the electronic structure and geometric structure of the basic particles that make up the solid state; The second level is the crystal structure of the solid state; The third level is the solid state macrostructure. Mr. Xu and others pointed out that in actual reactions, condensed matter is the real subject of all chemical reactions. When a chemical reaction occurs, the condensed state structure of a substance will also change. Therefore, it is not deep and comprehensive enough to understand the chemistry of nitrogen fixation only from the traditional molecular and ideal crystal models. Condensed matter chemistry provides a direction and basis for further understanding of nitrogen fixation reaction chemistry. In this paper, some scientific problems in the field of nitrogen fixation chemistry are discussed from the perspective of condensed matter chemistry, including nitrogen fixation in homogeneous solution system, nitrogen/oxygen multi-energy coupling conversion and heterogeneous ammonia synthesis, and it is expected that this review can provide a useful reference for the study of nitrogen fixation chemistry.

2 Condensed Matter Chemistry in Nitrogen Fixation in Homogeneous Solution System

In order to realize the activation and conversion of nitrogen in homogeneous solution system under mild conditions, scientists have made a lot of explorations. In solution system, an important way to achieve nitrogen activation is to use transition metals to form complexes with nitrogen[4⇓⇓⇓⇓⇓⇓⇓~12]. In 1965, Senoff et al. Synthesized the first transition metal-nitrogen complex [Ru(NH3)5N2]X2(X=Br-,I-,B F 4 -, P F 6 -); in 1968, Ibers et al. First reported the crystal structure of the transition metal-nitrogen complex CoH(N2)(PPh3)3[13][14]. Through the unremitting efforts of chemists at home and abroad, as of March 2020, more than 800 metal-nitrogen complexes with definite structures have been included in the Cambridge Crystal Data Center (Cambridge Crystal lographic Data Centre, CCDC), UK, covering a rich variety of metal-nitrogen coordination modes (Fig. 1). However, in general, the number of well-characterized structures is limited, and only a few metal-nitrogen complexes can undergo nitrogen derivatization[15⇓⇓⇓⇓~20].
图1 过渡金属-氮气配合物常见的配位模式

Fig.1 Typical coordination modes of transition metal-nitrogen complexes

At present, the successful examples of nitrogen derivatization by metal nitrogen complexes are mainly focused on the reaction of activated nitrogen with protons (ammonia formation reaction). In 1975, Chatt reported that the nitrogen complexes of zero-valent Mo and W reacted with acid to form ammonia equivalently, and proposed a mechanism of gradual protonation of metal-terminal nitrogen complexes to release ammonia, namely the "Chatt cycle"[21]. In 2003, Yandulov and Schrock et al. Reported the first homogeneous ammonia synthesis catalyst Mo[(HIPTN)3N](N2), which can realize the catalytic conversion of N2 at normal temperature and pressure under the condition of external proton ({2,6-lutidinium}{BAr'4},Ar'=3,5-(CF3)2C6H3)) and electron donor (CrC p 2 *), namely the "Schrock cycle"[22]. In 2010, Nishibayashi et al. Reported a dinuclear Mo-based complex supported by PNP ligand [Mo(N2)2(PNP)]2(μ-N2), which realized the catalytic synthesis of ammonia at normal temperature and pressure by using CoCp2 as a reducing agent, and greatly improved the efficiency of ammonia production[23]. In 2013, Peters et al. Successfully expanded the catalyst from the Mo system to the Fe system, and reported the Fe-N2[24].
The discovery of these landmark ammonia synthesis systems is inseparable from the detailed characterization of key intermediates and the in-depth exploration of the reaction mechanism. In this process, scientists have come to realize that the activation of nitrogen is strongly affected by factors such as the oxidation state of the central metal, the coordination environment, and the coordination mode, and that small changes may lead to very different results. The Dewar-Chatt-Duncanson (DCD) theoretical model can partially explain the activation mode of nitrogen in this coordination mode. As shown in Fig. 2, the occupied d electron of the transition metal can form a feedback π bond with the π* orbital of the nitrogen molecule, while the nitrogen lone pair electron forms a σ coordination bond with the empty d orbital of the transition metal. The non-polar nitrogen molecule is polarized to a certain extent, and the terminal Nβ atom will carry some negative charges. In this process, nitrogen gets electrons from the metal center and is reduced to a certain extent.
图2 过渡金属氮气配合物的DCD模型

Fig.2 DCD model for transition metal-nitrogen complexes

The DCD theoretical model reveals the basic principle of nitrogen activation, but it still has great limitations. Because in solution, metal complexes often do not exist in the form of monomers, but form complex clusters. The complex condensed state process is involved in the process of nitrogen entering the liquid phase from the gas phase, then coordinating with the metal cluster, and finally forming a stable metal nitride. At the same time, solid reducing agents insoluble in solvents, such as graphite potassium, sodium amalgam, metal magnesium, metal potassium, etc., are often used in the process of nitrogen fixation in homogeneous solution system, and the reduction reaction is often accompanied by the precipitation of solid salt products. These metal clusters (solution phase), nitrogen (gas phase) and reducing agent (solid phase) are in three different phases, and the interaction between them cannot be simply analyzed by the single-center metal-nitrogen coordination mode. In addition, the reaction solvent, reductant, additive and even the metal salt generated in situ in the system may in turn affect the state of the metal cluster in the solution, which greatly affects the reaction results. There are many corresponding examples, and due to the limitation of space, this article only gives a few examples to illustrate.
Take the cyclopentadienyl metal titanium-nitrogen complex as an example (Scheme 1). Bercaw et al. In 1971 reported that pentamethyl-substituted titanocene solutions can react with nitrogen at atmospheric pressure to form metal complexes in a molar ratio of 2:1[25]. In a series of subsequent works, it was found that the speciation of Ti-N2 species in solution was strongly affected by the solvent, nitrogen pressure, temperature, as well as the concentration of Ti complex. For example, the linear Ti-NN-Ti complex is a blue-black crystal, which can be separated by crystallization in toluene at 0 ℃[26]. However, if the temperature of the system is further lowered (< -10 ℃) under a nitrogen pressure, an additional nitrogen molecule will be coordinated to each Ti atom, forming a blue-purple crystal[27]. However, if the concentration of titanocene in the solution system is reduced and the solvent is changed to n-pentane, so that relatively more nitrogen can be dissolved, the monocyclopentadienyl nitrogen complex can be separated[28].
图式1 五甲基取代的茂基金属钛-氮气配合物[26⇓~28]

Scheme 1 Pentamethyl-substituted metallocene titanium-nitrogen complexes[26⇓-28]

Chirik's group has systematically explored the cyclopentadienyl titanium nitrogen complexes, and the results show that different kinds of Ti-N2 clusters can be obtained by adjusting the steric hindrance of the substituent on the cyclopentadienyl ring (Scheme 2)[29]. For example, when the substituent of the cyclopentadienyl ring is reduced from pentamethyl to trimethyl, the resulting Ti-NN-Ti complex changes from linear terminal coordination to pendant coordination. When the substituent on the cyclopentadienyl is further reduced to dimethyl, an unstable blue-green crystal can be separated and crystallized at a low temperature, and the X-ray single crystal diffraction characterization result shows that:The two cyclopentadienyl rings in the Ti-N2 cluster underwent dehydrogenative coupling with the reductant graphite potassium, resulting in the reduction of steric hindrance, thus enabling the use of three Ti centers to cooperatively participate in nitrogen activation[30].
图式2 三甲基与二甲基取代的茂基金属钛-氮气配合物[29,30]

Scheme 2 Trimethyl- and dimethyl-substituted metallocene titanium-nitrogen complexes[29,30]

Not only do Ti complexes show such abundant condensed States when reacted with nitrogen, but similar phenomena can also be observed in other transition metal-nitrogen complex systems. In an earlier study of nitrogen activation by transition metal salts, Shilova et al. Found that if a suitable mixed metal salt was used as a catalyst, the efficiency of ammonia or hydrazine formation was significantly improved compared to that of a single metal salt[31]. For example, a variety of Mo-Mg clusters with very complex structures can be observed when MoCl5 and MgCl2 It can catalyze the conversion of nitrogen to hydrazine in methanol solution in the presence of sodium amalgam and phospholipids, and has good activity. It is worth mentioning that there are many other Mo-Mg condensed structures with different nuclear numbers in solution, and these complex structures that have not been explored in depth are likely to show different nitrogen activation effects[32]. How does the structure of the condensed state of the cluster in solution change when different combinations of metal salts are used, or when the bridging group methanol (methoxy) is replaced by other groups? This is a question worth exploring in depth.
图3 Mo-Mg簇氮气配合物的晶体结构[31]

Fig.3 Crystal structure of the [Mg2Mo8O22(OMe)6(MeOH)4]2- anion[31]

Another example is the nitrogen derivatization reaction of metal chromium nitrogen complexes reported by Xi Zhenfeng's group (Scheme 3)[33]. Xi Zhenfeng's group used the cyclopentadiene-organophosphine ligand (L1 ligand) developed by Xi Zhenfeng to effectively stabilize the chromium metal center and obtained the corresponding L1-CrCl complex. The monovalent chromium nitrogen complex can be obtained by reducing this divalent chromium chloride using potassium triethylborohydride (KHBEt3) or potassium graphite (KC8) under nitrogen at room temperature and atmospheric pressure. And replace that substituent on the cyclopentadiene-organophosphine ligand to obtain a complex dinuclear or trinuclear univalent chromium nitrogen complex, wherein the number of nitrogen coordinated by the metal chromium center of the univalent chromium nitrogen complex can be different to form a symmetric structure or an asymmetric structure. In the condensed state structures of these clusters, nitrogen at different positions has different binding modes with the metal center, so the degree of activation is also different. However, nitrogen in this coordination mode is not easy to undergo further nitrogen derivatization.
图式3 环戊二烯-有机膦配体支持的一价铬氮气配合物[33]

Scheme 3 Cp-phosphine ligand-supported Cr(I)-N2 complexes[33]

To achieve derivatization with coordinated nitrogen, the oxidation state of the metal center needs to be further lowered (Scheme 4). Monovalent chromium-nitrogen complexes can be reduced by excess alkali metals (potassium, rubidium, cesium) to obtain electrons and form the corresponding zero-valent chromium-nitrogen complexes. The crystal structure shows that the alkali metal is complexed by the cryptand- [2.2.2], with two nitrogen molecules coordinated to each zero-valent chromium center. However, depending on the type of metal counterion, potassium, rubidium and cesium, which are also Lewis acids, have different distances and binding modes with nitrogen. Its effect on the degree of nitrogen activation can be confirmed by infrared spectroscopy.
图式4 环戊二烯-有机膦配体支持的零价铬氮气配合物[33]

Scheme 4 Cp-phosphine ligand-supported Cr(0)-N2 complexes[33]

Although the theoretical models of nitrogen activation by transition metals have been established for a long time and the crystal structures of the corresponding complexes have been accumulated, the understanding of the effects of different aggregation States of metal complexes on nitrogen activation is still very limited, and the detailed characterization data are very scarce. In particular, at present, the study of nitrogen activation in homogeneous system is mainly carried out at one atmosphere, and at relatively high pressure (10 ~ 200 atmospheres), the number of metal clusters combined with nitrogen in real solution system, the binding mode and the activation mode are unknown. In addition, the single crystal structure does not correspond to the real active species in solution in all cases, especially in the presence of additives, salts and coordinated solvents, the condensed state structure of the real metal-nitrogen active species may be very complex, which is far from the highly simplified theoretical model. Finally, the transformation from nitrogen to nitrogen-containing organic compounds is a multi-step process, especially the complex electron transfer process from the reducing agent (solid phase) to the nitrogen complex (solution phase).As the derivatization reaction of nitrogen proceeds, the condensed state structure of the key intermediate formed in solution is likely to be unable to remain unchanged and will undergo complex changes. Chemists in related fields need to develop more means to monitor and characterize the real structure of the condensed state of reactive intermediates in solution. Through the systematic exploration of these condensed States, it is helpful to deeply understand the real activation mode of nitrogen in the actual system, and provide useful guidance for the design of subsequent nitrogen derivatization reactions.

3 Condensed Matter Chemistry in Multiphase Ammonia Synthesis

The reaction of nitrogen (N2) and hydrogen (H2) in the presence of a solid catalyst to form ammonia (NH3) is a typical condensed matter reaction process. The multi-level structure of condensed matter has a significant effect on the reaction mechanism and kinetics of ammonia synthesis. Like other heterogeneous catalytic reaction processes, ammonia synthesis also needs to go through the steps of reactant (gaseous nitrogen and hydrogen) adsorption, surface reaction and product (gaseous ammonia) desorption. The active sites formed by several atoms on the surface of the solid catalyst dominate the reaction rate of this chemical process. Transition metal is an important component of ammonia synthesis catalyst. It is generally believed that the direct dissociation mechanism is dominant in the heterogeneous catalyst system of transition metals such as Fe and Ru, especially at high temperatures[34⇓~36]. Adsorption dissociation of N2 and H2 on transition metal surface to generate N atom (Nad) and H atom (Had), respectively,The subsequent stepwise hydrogenation of N atoms generates N H x , a d (X = 0 ∼ 3) species, and finally NH3 desorbs from the catalyst surface (Fig. 4A). The overall reaction rate of ammonia synthesis is related to the activation energy of each elementary step and the pre-exponential factor. Among them, the dissociative chemisorption of N2 molecules is generally considered to be the rate-determining step for ammonia synthesis on transition metal surfaces. Ertl used surface science and technology to investigate the adsorption of N2, H2 and NH3 on the surface of metal Fe, as well as the formation and transformation of intermediate species, and constructed the potential energy diagram of ammonia synthesis reaction on the surface of Fe, which clearly revealed the microscopic process of the reaction at the atomic level (Fig. 4B)[37,38]. Through a large number of scientific studies, it has been found that there is a volcano-type curve relationship between the reaction rate of ammonia synthesis and the nitrogen adsorption energy on the surface of transition metals. This phenomenon can be understood from the classical Sabatier principle or the Scaling relations recently proposed by Nørskov et al. (Fig. 5)[39].
图4 (A)固体催化剂表面合成氨反应的直接解离式机理示意图;(B)金属Fe表面上合成氨反应势能图,能量单位为kJ·mol-1[37]

Fig.4 (A) Schematic representation of ammonia synthesis on a solid surface via the dissociative mechanism; (B) potential energy diagram of ammonia synthesis on Fe surface. The energies are given in kJ·mol-1. Reprinted with permission[37]. Copyright John Wiley and Sons

图5 各种过渡金属表面台阶位上的N吸附能与合成氨活性之间呈火山型曲线关系[39]

Fig.5 Volcano plot of ammonia synthesis rates with the nitrogen adsorption energy at stepped metal surface[39]

Sabatier's principle qualitatively describes the ideal catalyst: the catalyst with better performance should have moderate bonding strength with the reaction species, which is conducive to the activation of reactants and allows the desorption of products. Nørskov et al. Found that there is an energy limiting relationship between different adsorption species on the transition metal surface and the energy barrier of the transition state[39,40]. Among them, Fe, Ru and CoMo have moderate N adsorption energy, which can not only effectively implement nitrogen activation and dissociation, but also hydrogenate to produce ammonia and complete the catalytic cycle. Therefore, for a long time, these active metals are also the most concerned elements in the research of ammonia synthesis catalysts. It should be pointed out that in the ammonia synthesis system with mild reaction conditions such as nitrogenase, organometallic complexes, electrochemistry/photochemistry, and low-temperature thermal catalysis, the hydrogen-assisted dissociation mechanism may be dominant, and the N ≡ N bond cleavage occurs after hydrogenation[41].
Solid surfaces are usually very complex and inhomogeneous. Up to now, we still lack sufficient understanding of the composition, structure and microscopic reaction process of the gas-solid interface. How to understand the nature of active sites is the most basic scientific problem in heterogeneous catalysis. Based on a large number of surface science experiments and theoretical calculations, researchers have proposed possible models for the active center structure of ammonia synthesis on transition metal surfaces. At present, it is generally believed that the activation site of N2 on transition metal is composed of multiple adjacent atoms on its surface, for example, the active site of Fe-based catalyst is composed of seven adjacent Fe atoms (C7 site, Fig. 6a), and the active site of Ru is composed of five adjacent Ru atoms located at the step (B5 site, Fig. 6B)[42,43][44,45]. According to the Wulff rule, Ru nanoparticles with a size of 2 nm have the most number of B5 sites, and thus will have a higher catalytic activity for ammonia synthesis. However, with the rapid development of materials synthesis and characterization technology, this traditional understanding has been challenged. For example, Li et al. Recently prepared Ru sub-nanocluster catalysts (Ru/MIL-101, Ru particle size is about 1 nm) supported on metal-organic framework materials, and their ammonia synthesis activity is better than that of supported Ru-based catalysts (such as Ru/MgO) with Ru particle size of 2 ~ 4 nm[46]. The authors believe that this is due to the fact that under the actual reaction conditions, the Ru sub-nanometer cluster undergoes surface reconstruction to produce more B5 active sites. Jiang et al. Investigated the effect of Ru particle size on the performance of ammonia synthesis, and found that smaller Ru clusters showed higher TOF values than Ru nanoparticles[47]. Transition metal cluster species may be difficult to break the N ≡ N bond, in which case the hydrogen-assisted dissociative ammonia synthesis pathway is more feasible. Li et al. Used DFT calculations to construct the Fe3 and Rh1Co3 surface cluster active centers, and found that the energy barrier for direct dissociation of N2 was higher, while it was more inclined to hydrogenation to generate N2H species[48,49].
图6 (A)Fe催化剂的C7活性位点示意图;(B)Ru催化剂的B5活性位点示意图[50]

Fig.6 Schematic representation of the C7 active site on Fe catalyst (A), and B5 active site on Ru catalyst (B)[50]

In order to improve the atom utilization of transition metals, it is usually necessary to support the active metal on some kind of support. For some functional supports (such as electronic compounds, reducible rare earth oxides, etc.), the support not only plays a role in dispersing the active metal, but also affects the geometric structure and electronic properties of the transition metal through the metal-support interaction, thereby increasing the reaction rate of ammonia synthesis. For example, Hosono et al. Reported a class of inorganic electron compound carriers with low work function (such as [Ca24Al28O64]4+:4e- and [Y5Si3]0.79+:0.79e-), and the strong electron-donating ability of these compounds enhanced the feedback effect of Ru's d electron on the antibonding orbital of N2, which further weakened the N ≡ N bond and accelerated the dissociative adsorption of N2[51⇓~53]. Nagaoka et al. Found that rare earth oxides (such as La0.5Ce0.5O1.75, La0.5Pr0.5O1.75, etc.) pre-reduced at high temperature could generate strong metal-support interaction with metal Ru under ammonia synthesis reaction conditions, thus enhancing the electron cloud density of Ru and promoting the dissociative adsorption of N2[54,55]. Chen et al. Found that the active Sm-H species generated in situ on the surface of rare earth oxide support such as Sm2O3 can effectively increase the surface electron density of Ru cluster and promote the activation and dissociation of N2 under ammonia synthesis reaction conditions[56]. Moreover, as an active hydrogen species, it can directly participate in the formation of ammonia in the reaction process, and significantly improve the activity of ammonia synthesis.
The promoter is usually the key component of the catalyst in the ammonia synthesis reaction. In the industrial practice of ammonia synthesis, it has long been recognized that the addition of promoters such as alkali (earth) metals can significantly improve the catalytic performance of transition metals. However, the chemical state of the promoter and its mechanism of action have been controversial. Ozaki, Ertl, Aika et al considered that alkali (earth) metals have strong electron-donating ability, which can act as electron donors to transfer electrons to transition metals, reduce the surface work function of transition metals, enhance the electron transfer between transition metals and N2, thus weakening the N ≡ N bond and promoting the activation and dissociation of N2[57⇓~59]. However, Somorjai et al believed that the presence of alkali metal could accelerate the desorption of product NH3, thus exposing more sites of transition metal for nitrogen activation[60]; According to Nørskov et al., the promotion effect of alkali metals is mainly due to the electrostatic interaction between alkali metals and intermediate or transition species[61]. Recently, Nørskov et al also proposed that for magnetic metal (such as Co, Ni) catalysts, the addition of alkali (earth) metal promoters can also effectively reduce the spin polarization effect of the magnetic metal surface, thereby further reducing the dissociation energy barrier of the N2[62]. In addition to being used as electron promoters, alkali (earth) metals can also be used as key active components of catalysts to participate in ammonia synthesis. Chen et al. Found that the catalytic behavior of active metals could be strongly changed by introducing alkali (earth) metal hydrides into transition metals[63]. For example, the introduction of lithium hydride (LiH) makes 3D transition metals with very low activity, such as Cr, Mn, Co, Ni, etc., show good catalytic activity, even better than some Ru-based catalysts. Further studies have found that a coordinated hydride species can be formed at the interface between hydride and transition metal, which may be the active site for nitrogen activation[64]. Through the synthesis of complex hydrides Li4RuH6 and Ba2RuH6 with well-defined structures, the authors confirmed that complex hydrides can be used as a new type of catalyst for ammonia synthesis[65].
In the catalytic process of ammonia synthesis, nitrogen and hydrogen are fed together, and the competitive adsorption of nitrogen and hydrogen is inevitable due to the different adsorption strength of nitrogen and hydrogen species on the surface of solid catalyst. This problem is particularly prominent on metallic ruthenium surfaces. For example, on the traditional Ru/MgO catalyst, the order of H2 is negative (-0.7), indicating that the H2 is preferentially adsorbed and dissociated on the Ru surface, thus hindering the adsorption and activation of nitrogen[66]. The competitive adsorption of reactants can be changed to a certain extent by adjusting the catalyst support or adding additives. Another strategy is to adopt the chemical looping approach, that is, to adopt the mode of feeding nitrogen and hydrogen in steps. The concept of chemical looping was proposed as early as the 19th century, such as the formation of ammonia by alternating N2 and H2 on heated titanium nitride[67]. Chen et al. Have recently proposed a low-temperature chemical looping ammonia synthesis technology using alkali (earth) metal imides as nitrogen carriers, in which alkali (earth) metal hydrides (such as LiH,BaH2) first generate the corresponding imides (such as Li2NH, BaNH) through "fixed ”N2", and then the reaction atmosphere is switched to hydrogen to hydrogenate the imides to release NH3[68]. It is worth mentioning that for some systems, the chemical looping ammonia production rate is significantly higher than the catalytic rate. For example, under the same reaction conditions, the chemical looping ammonia production rate of BaH2/BaNH is 20 times that of its catalysis[69]. With the assistance of transition metal catalysts, the ammonia production rate of the BaH2/BaNH system at 250 ℃ is about one order of magnitude higher than that of the highly active Cs-Ru/MgO catalyst[68]. Another significant difference between the chemical looping process and the catalytic process is that the chemical looping process is a non-equilibrium process, and the phase structure of the material changes with time and reaction conditions. For example, in the process of nitrogen fixation, alkali (earth) metal hydrides can form corresponding alkali (earth) metal imides. In the catalytic process, the phase structure of the catalyst can remain unchanged in a certain time scale.
Over the past century, a great deal of knowledge has been accumulated in understanding the heterogeneous catalytic process of ammonia synthesis, which has played an important guiding role in the design and development of industrial catalysts. However, it is regrettable that the goal of "low temperature, low pressure and high efficiency ammonia synthesis" has not yet been achieved. With the help of external fields such as electricity, light and plasma, it may be an effective way to achieve efficient conversion of N2 to NH3 under mild conditions, which is more in line with the development trend of green ammonia synthesis. The mechanism of electron transfer and energy conversion between solid surface and N2 molecules will change under the external field driving, which may help to construct a new mode of nitrogen activation conversion to implement low-temperature, low-pressure and efficient ammonia synthesis. Moreover, we need new ideas in the face of this classical research topic. With the help of the perspective and research methods of condensed matter chemistry, the systematic study of the multi-level condensed matter structure, properties and functions of ammonia synthesis reaction process may help us to understand this scientific problem more deeply and form a new concept of catalyst research and development.

4 Condensed Matter Chemistry in Nitrogen/Oxygen Multienergy Coupling Conversion

Nitrogen oxides (NOx, such as NO, NO2, etc.), as derivatives of the main components of air (N2 and O2), are very important raw materials for the chemical industry. Downstream products of NOx, such as nitric acid (HNO3) and trinitrotoluene (TNT), are essential for fertilizer and military fields, and are in great industrial demand[70,71]. At present, the synthesis of NOx and its downstream products requires industrial ammonia synthesis first, and then NOx is obtained by NH3 oxidation. The pure H2 consumed in this process is usually obtained from hydrogen production from fossil energy (including water-gas shift, natural gas reforming, etc.), so the energy consumption of the whole process is extremely high, accompanied by huge CO2 emissions[72,73]. It will be of great significance if NOx can be synthesized directly and efficiently from air, or even high value-added organic compounds containing C-N-O can be synthesized directly. The direct conversion of N2 and O2 by traditional thermochemical methods is very difficult due to the weak polarity of N2 molecules and the high N ≡ N bond energy. From the perspective of condensed matter chemistry, it is of great significance to explore the interaction mode and mechanism between different energy fields (such as light, electricity, sound) or multi-energy coupling fields and condensed matter (reaction molecules, catalysts) for the direct activation and conversion of N2 and O2 to NOx[71]. In the process of different energy field input, chemical reactions often show different forms from traditional thermochemistry, such as different intermolecular reaction pathways under different energy field input conditions, and different physicochemical properties and catalytic characteristics of condensed materials under different energy fields. Studying such problems from the perspective of condensed matter chemistry can better deepen researchers' understanding of these processes, provide a theoretical basis for more rational design of direct and efficient air conversion, and also extend the breadth of condensed matter chemistry to the interaction between matter and energy fields. This chapter will discuss this kind of problem involving the interaction between matter and energy field in combination with the research work of air transformation under different energy fields, and provide some reference for its future development direction.

4.1 Low temperature plasma process

Non-thermal plasmas (NTPs) can be used as a feasible method for efficient air conversion under mild conditions due to their unique Non-equilibrium characteristics[74]. NTPs are composed of atmosphere molecules, high-energy electrons, excited molecules (such as vibrationally excited States and electronically excited States) and ions, in which the temperature of high-energy electrons can exceed thousands of K, while the temperature of macro-atmosphere molecules can be kept at or slightly above room temperature[75]. Therefore, reactions that cannot be driven by thermochemistry under mild conditions can usually occur under NTPs conditions, and efficient plasma-catalytic reaction processes can be achieved by means of the coupling of NTPs with the electronic States of condensed structures. Because the N2 molecule has a single vibrational mode, an extensive vibrational level distribution, and a relatively large vibrational level spacing (about 0.3 eV), it is possible to promote the activation and dissociation of N2 by vibrational excitation[75]. On the one hand, the promotion of the vibrational quantum number of the vibrationally excited N2 molecule can make the N2 molecule dissociate directly in the gas phase through the continuous accumulation of energy. On the other hand, as shown in fig. 7, the vibrationally excited state N2 may undergo a dissociation reaction more easily on some catalyst surfaces than the ground state N2[76].
图7 从基态(蓝色)或振动激发态(绿色)开始的N2离解活化能的反应坐标图,红色和黄色虚线曲线对应于不同的振动效率(α= E t E α ( v ) + E v )[76]

Fig.7 Reaction coordinate of activation energies for N2 dissociation starting from ground (blue) or vibrationally excited states. The dashed red and yellow curves correspond to different vibrational efficiency(α= E t E α ( v ) + E v )[76]

Based on the above advantages, there have been many reports on the synthesis of NOx by air inversion under mild conditions using NTPs in recent years[77⇓⇓⇓⇓⇓⇓~84]. Bogaerts et al. Developed a new type of rotary arc plasma reactor.Can efficiently convert the N2 and the O2 to obtain the NO and the NO2 with the generation energy consumption of 2.5 MJ·mol-1NOx under the condition of normal pressure and high flow rate (2 L·min-1),The total concentration of NO and NO2 in the outlet can reach 5.5%[79]. In addition, the coupling of NTPs to catalysis is also an important pathway. In general, researchers believe that free radical reactions between N2 and O2 occur in the gas phase under the conditions of NTPs in the presence of catalysts:[85,86]
e-+N2→e-+N2(v)
e-+O2→e-+2O·
N2(v)+O·→NO+N(v)·
N(v)·+O2→NO+O·
On the surface of the catalyst, the adsorption and dissociation of the vibrationally excited N2 are mainly experienced, and the O · radicals are combined to form NOx:[86].
N2(v) $\mathop{ }_{\rightleftarrows}^{surface}$2N*
O·+N*$\stackrel{surface}{\longrightarrow}$NO*$\stackrel{desorption}{\longrightarrow}$NO
The process of air conversion by NTPs coupled with catalysis is very complex, involving many physicochemical processes and catalyst-adsorbate condensed state interactions, which is directly related to the non-equilibrium and multi-level structure of the "state" in NTPs. At the same time, because NTPs are excited by electromagnetic field, the characteristics of these condensed structures in electromagnetic field will also have an important impact on chemical reactions. Firstly, the particle energy in NTPs is in a non-equilibrium state (electronic energy > molecular excited state energy > molecular translational energy), and the multi-level structure of particle energy allows the exchange and accumulation of energy between particles to drive the direct activation of N2 molecules. In addition, the catalytic properties of solid catalytic materials are often different under plasma atmosphere and thermochemical reaction conditions. Therefore, the catalyst-adsorbate condensed state interaction in the plasmon-catalytic coupling, greatly affects the catalytic reaction process. For example, researchers have shown that the appearance of the inflection point of the volcano-type curve of plasma-catalyzed ammonia synthesis lags behind the thermocatalytic process, and the rate of ammonia synthesis is faster and can break through the thermodynamic equilibrium limit under thermocatalytic conditions[76,87]. This is due to the fact that the condensed structure formed by the adsorption of vibrationally excited N2 on the catalyst surface has a higher dissociation reaction efficiency than that of the N2 adsorbed under the same translational energy, which makes the rate-determining step of ammonia synthesis change from the activation and dissociation of N2 molecules to hydrogenation and desorption of products[87]. In addition, the existence of condensed matter can also affect the formation of plasma, such as the morphology and dielectric properties of the catalyst, which can affect the polarization of condensed matter under high frequency electric field, and then affect the local plasma discharge. Condensed structure can produce different local thermal effects due to different local impedance, which has a certain influence on the reaction of N2 and O2.

4.2 Electrochemical catalytic process

The electrochemical catalytic reaction includes the steps of reactant dissolution, mass transfer, adsorption, activation, reaction and desorption, and the reaction involves the complex changes of gas, liquid and solid three-phase condensed structure. To design electrochemical reactions, first of all, factors such as the nature of the electrolyte, the electrode potential and the catalytic activity of the electrode materials should be fully considered. The electrochemical oxidation of N2 is very difficult because the N2 molecule itself is very inert, has low solubility, and has weak adsorption strength on the surface of the catalyst[88]. For the electrode reaction, according to the Nernst equation, the standard potentials of oxygen evolution reaction (OER) and nitrogen oxidation reaction (NOR) are very close to the standard hydrogen electrode (vs SHE) at room temperature and pressure, which are + 1.23 V and + 1.24 V, respectively, so the competition of OER is unavoidable in the NOR system with water as electrolyte[89,90]. Therefore, the regulation of solution pH and electrode potential is essential to control the competitive reaction of OER. Based on the N2-H2O Pourbaix diagram established in pure water or aqueous solutions of 1 mol·L-1 solutes at 298 K and 1 bar, the NOR process with 10 electron transfer (7) is more favorable than the OER process with 4 electron transfer (8) at solution pH higher than 1.3, especially under neutral and alkaline conditions[91][89,90].
N2(g)+6H2O(lig) ⇌ 2NO 3 ( a g ) -+12 H ( a g ) ++10e-
O2(g)+4H++4e-⇌2H2O
NOR requires not only the sufficient activity of the catalytic material, but also the accurate balance of the competitive adsorption, activation, dissociation of N2 and H2O/OH-, as well as the desorption of products. Therefore, the design of NOR catalytic materials is more demanding than OER, oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). Theoretically, the reaction rate of NOR depends on the dissociation energy barrier of N2 molecules on the active sites of the catalyst surface, and also depends on the number of active sites. For the reduction of N2 to NH3(NRR), it is theoretically and experimentally confirmed that the dissociation of N2 is the rate-determining step on the catalyst surface[92⇓~94]. The NOR process is similar to this, which requires the catalyst to have a high activity for the adsorption and dissociation of N2 molecules, while considering the oxidative etching effect of the electrochemical environment, the catalyst is required to have both high stability. Noble metals are excellent catalysts in the field of electrochemical catalysis because of their incompletely filled d orbitals, easy adsorption of reactants, and moderate adsorption strength. In the early stage, Zhang et al. Successfully realized the oxidation of V vs RHE to HNO3 by using Pt electrode under the potential condition of 2.19 V vs RHE, however, the faradic efficiency was only about 1.23%, and it was difficult to effectively solve the efficiency and selectivity problems of NOR process by using noble metal alone[95]. Yan et al. Successfully constructed two kinds of Ru-containing active centers by doping Ru on TiO2 (Fig. 8)[96]. Wherein the Ruδ+ doped in the crystal lattice of the TiO2 can effectively activate the N2 and oxidize the N2 into an intermediate NO*,While the RuO2 species on TiO2 is the active site for electrocatalytic OER, which can promote the further oxidation of the intermediate NO* to HNO3. By balancing the relative number of the two kinds of active sites, the faradaic efficiency of NOR can reach 26.1%. Other catalysts, such as doped bifunctional catalysts, spinel oxide catalysts and supported two-dimensional material catalysts, can achieve higher Faradaic efficiency than pure noble metal catalysts[97][98][99].
图8 Ru/TiO2复合电催化剂上的NOR反应示意图[96]

Fig.8 The scheme of NOR on Ru/TiO2 composite electrocatalysts[96]

At present, the electrocatalytic NOR process is still in the basic research stage. From the point of view of condensed state chemistry, it is very important to study the condensed state interaction mechanism between the surface and interface of catalyst and reactants, active intermediates and products. At the anode, most of the catalyst surfaces prefer to adsorb OH-. Even if N2 molecules can be preferentially adsorbed on some catalyst surfaces, they are usually difficult to be effectively activated, and may be desorbed due to the competitive adsorption of OH-, and finally the OER process occurs. However, the NOR process can be accelerated by maintaining a proper OER activity. It is one of the feasible ways to separate the OER and NOR active sites by reasonably designing the catalyst, and to construct two adjacent active sites for OER and NOR, respectively, and to balance their number. It should also be noted that not all active oxygen species can combine with adsorbed N2 to form NO, and most active oxygen species will combine with other oxygen species (such as OH-) to form O2. Therefore, it is very important to clarify the preferential adsorption mode and condensed state interaction between N2 and OH- and other species at different potentials. This also requires researchers to fully consider the various effects under electrochemical conditions (such as electronic effect, size effect, strain effect, ligand effect, boundary effect, diffusion effect, etc.) brought about by the complexity and diversity of condensed state structures of catalysts in the process of catalyst design, rather than simply considering the active site.

4.3 Ultrasonic conversion process

Ultrasound is a sound wave with a frequency above the audible range (above about 20 kHz). When ultrasonic wave propagates in liquid medium, the interaction between them will produce a series of physical and chemical effects, namely ultrasonic effect, including mechanical effect, thermal effect, cavitation effect and chemical effect. Among them, ultrasonic cavitation can locally produce extremely high instantaneous temperature, pressure, and sonoluminescence[100,101]. In addition, free radicals generated during cavitation can initiate chemical reactions, the so-called sonochemical effect. This process involving complex condensed structure changes can be described as that inside the cavitation bubble filled with liquid medium vapor and gas molecules, the molecules are activated and further dissociated by the high temperature and high pressure environment[102⇓⇓~105]; Inside the bubble or at the two-phase interface, the generated radicals can combine with reactant molecules to form products. Therefore, N2 can be directly oxidized to nitrous acid and nitric acid in air-saturated aqueous medium under ultrasonic field.
As early as 1936, Gohr et al. Found that H2O2, HNO2 and HNO3 could be generated in air-dissolved saturated aqueous solution under the action of ultrasonic field with a frequency of 540 kHz[106]. The subsequent related work explored the effects of different conditions (frequency, pH, etc.) On the synthesis of HNO2 and HNO3 by ultrasonic air conversion, as well as the related reaction mechanism, in which the activation mode of N2 was controversial[107⇓⇓⇓⇓⇓~113]. David et al. Believed that both · OH and · O produced in the high temperature and high pressure microenvironment of ultrasonic cavitation bubbles could oxidize N2 molecules, while some studies believed that · OH could not directly oxidize N 2[110][108]. In any case, researchers generally believe that the high temperature and high pressure (about 5000 K, about 500 atm) environment inside the cavitation bubble generated under the action of ultrasonic field is enough to make molecules such as O2 and H2O produce free radicals, thus driving the N2 conversion reaction. As a result, the solution in the ultrasonic field also presents the characteristics similar to the low-temperature plasma, that is, the overall solution temperature may be slightly higher than the room temperature, but the local solution microenvironment is in a state of high temperature and high pressure, which changes the condensed structure of the reactants. At the same time, the nature and state of the solution, such as solvent characteristics, pH, anion and cation types, and the addition of solid catalyst, can also affect the nucleation and fragmentation of cavitation bubbles, the type and quantity of free radicals, and thus directly affect the reaction process of N2 oxidation.
At present, the direct conversion of air under the action of ultrasonic field is limited by the reaction rate. To improve the conversion efficiency of N2 in air-saturated aqueous solution under ultrasonic field, on the one hand, it is necessary to enhance the cavitation effect in order to improve the yield of active groups; On the other hand, it is necessary to increase the solubility of the N2, such as using high pressure conditions to increase the number of activated N2 molecules. In addition, when homogeneous or heterogeneous catalysts are introduced, the change of the condensed structure of the active intermediate in the process, as well as the dissolved fraction and the number of active sites of the catalyst and their effects on cavitation bubble nucleation should be fully considered. Finally, the ultrasonic field can also produce polarization effect and piezoelectric effect on some solid catalysts such as pyroelectric materials and piezoelectric materials, which can not only affect the generation efficiency of cavitation bubbles, but also introduce additional local micro-electric field, thus regulating the conversion efficiency of N2[114,115].

4.4 Photocatalytic conversion process

Photocatalytic conversion of N2 and O2 with sunlight as the energy source provides a very ideal alternative to the traditional air conversion based on fossil energy. As a typical condensed matter reaction process, the photocatalytic N2 oxidation process can be divided into the following steps: first, electrons are excited from the valence band (VB) to the conduction band (CB) by the absorption of light by the photocatalytic material, leaving holes (h+) in the VB; Then,Photogenerated hole h+ oxidizes N2 to NO,O2 with H2O as the oxygen sourceElectrons are reduced to H2O;NO by photoexcitation, which can be further oxidized to nitric acid or nitrate by O2 and H2O. Early studies on photocatalytic nitrogen fixation mainly focused on the process of photocatalytic conversion of N2 to NH3. While in 2013, Yu et al. First reported that air could directly form nitrate on the surface of nano TiO2 under ultraviolet or sunlight (UV/Vis) irradiation, and the FTIR differential spectroscopy and theoretical calculation results showed that the nitrate formation was a stepwise oxidation process, in which NO* was the intermediate[116,117]. Subsequently, Zhang et al. Used Z-type heterojunction TiO2/WO3 nanorods as photocatalyst to also achieve NO generation from N2 oxidation (yield 0.16 mmol·g-1·h-1) with a quantum efficiency of 0.31% at 365 nm under photothermal synergism, which also proved that NO is an intermediate product of photocatalytic N2 oxidation[118].
Photocatalytic N2 oxidation needs to be carried out on the photogenerated hole h+ of semiconductor, and the reaction also has a high energy barrier. Therefore, in the process of designing related catalysts, in addition to the electronic properties of the material itself, the overall structure design of the catalyst needs to be considered, such as doping, construction defects, morphology modification and other means to promote the rapid contact and combination of N2 molecules and photogenerated hole h+ of the catalyst, forming a key adsorption condensed state structure. Xie et al. Designed and synthesized a WO3 nanosheet with a rich pit structure, which not only improves the contact area between N2 and the catalyst, but also has more dangling bonds than monovacancy WO3, which can effectively activate N2 molecules.At the same time, it is easier to excite high-energy electrons, which can effectively provide energy for the subsequent oxidation of activated N2 molecules, that is, by modifying the morphology of WO3, it can overcome the two difficulties of photocatalytic oxidation of N2: the activation of :N2 molecules and the high energy barrier involved in the reaction[119]. The pit-rich WO3 nanosheets can generate nitrate at a rate of 1.92 mg·g-1·h-1 with an apparent quantum efficiency (AQE) of 0.11% under UV/Vis irradiation at 380 nm at room temperature, which is better than that of non-pitted nanosheets and bulk WO3. Zhang et al. Found that dispersing Cu single atoms in the lattice of TiO2 can effectively improve the separation and transfer efficiency of photogenerated carriers, and promote the adsorption and activation of N2 on Cu sites, as well as subsequent oxidation[120]. A nitrate formation rate of 0.93μmol·h-1 can be achieved under light irradiation at 365 nm, while the spectroscopic characterization verifies the NOx (including NO2+, NO+, or NOδ+) intermediate species adsorbed on the single-atom Cu surface (Fig. 9).
图9 单原子Cu/TiO2上光催化空气转化到硝酸盐过程示意图[103]

Fig.9 The scheme of NOR on Ru/TiO2 composite electrocatalysts[103]

At present, the production of nitrate by photocatalytic conversion of air under mild conditions is still limited by the product formation rate and apparent quantum efficiency. In the process of designing catalysts, it is necessary to fully consider not only the intrinsic condensed state properties of catalytic materials, but also how to construct the structure of catalysts, because materials with the same elements may show different photocatalytic properties when their microstructures are different. It can also modify the condensed electronic structure by doping, improve the separation and transfer efficiency of photogenerated carriers, increase the number of photocatalytic active sites, and promote the formation of condensed active intermediates and their structural changes in the subsequent oxidation process. In addition, it has been proved that photon energy and heat energy also have coupling and synergistic effects, and photothermal coupling catalysis is also one of the effective ways to convert air[121].

5 Conclusion and prospect

For more than a century, human beings have made great achievements in the field of nitrogen fixation, which objectively enriches the academic understanding of condensed matter chemistry. However, there are still many questions that need to be answered in this field. For example, in a real system containing additives, salts and coordinating solvents, what is the condensed structure formed by nitrogen and active metal clusters? The transition from nitrogen to nitrogen-containing species is a multistep process, and how does the condensed state structure of the key intermediate change as the nitrogen derivatization reaction proceeds? At the third level of condensed matter chemistry, especially at the level of solid macrostructure, can we use the principle of self-assembly to develop new ammonia synthesis catalysts?So that the adsorption energy of the reaction species can be dynamically optimized and adjusted to avoid the widespread energy limitation in catalysis and achieve efficient conversion of nitrogen under mild conditions[3]? Conversely, the development of condensed matter chemistry will also bring new ideas and understanding to the study of nitrogen fixation. From the perspective of condensed matter chemistry, can we more reasonably design the direct and efficient conversion of air through multi-energy coupling, and extend the scope of condensed matter chemistry to the interaction between matter and energy field?
This paper is not a complete summary, but a superficial analysis of some condensed matter chemical phenomena in the field of nitrogen fixation chemistry based on the author's own understanding. We believe that with the improvement of characterization technology and the enhancement of theoretical computing power, the concept of condensed matter chemistry will receive more and more attention in the field of nitrogen fixation in the future, and ultimately promote the solution of some key basic scientific problems, providing ideas for human development of new ways of nitrogen utilization.

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