So far, humans have discovered and created more than 230 million substances, and it is worth noting that most substances exist in the form of condensed matter with multi-level structure, except for a small number of gaseous species. However, the understanding of the chemical reaction between substances, which is the core problem in chemical science, still stays at the level of isolated atoms, molecules and ideal crystals. In 2018, our research group published a short paper (Editorial) in the National Science Review, and published an outlook in 2019, putting forward the view that "the function, nature and transformation of matter depend on the composition and multi-level structure of the condensed state of matter, and the condensed state should be the main body of chemical reaction"^[1][2]. He has organized and published two "special issues" on Chemical Progress, which focus on condensed matter chemistry in chemical reactions of solid and liquid substances. Condensed matter chemistry in the reaction of gaseous molecular substances is a scientific problem of great interest and plays an important role in the construction of condensed matter chemistry. So we invited a number of chemical researchers to organize this special issue entitled "Condensed Matter Chemistry in Gas Molecular Reactions".

H and He in the first period of the periodic table of elements, some elements in the second period, their simple substances and compounds formed between them, such as a large number of inorganic and organic compounds with small gaseous molecules (such as low carbon hydrocarbons and their derivatives),As well as the inert elements of Group VIIIA, are the main gas species in chemistry and exist in the state of dispersed gas molecules under normal conditions. These small gaseous molecules, which exist in dispersed form, often lead to the inertness of chemical reactions due to structural stability (high strength of chemical bonds, low electron affinity and polarizability). Therefore, from the point of view of thermodynamics, these small molecules can react spontaneously, which is of great significance in industry and economy, such as the oxidation of SO₂, the synthesis and oxidation of NH₃ to produce nitric acid, the reaction of H₂ and Cl₂ to produce hydrochloric acid, the conversion of CO, CO₂ and a large number of low-carbon hydrocarbons, and the polymerization of ethylene, propylene and other small gas molecules.Most of them can only be carried out through homogeneous and heterogeneous catalytic reactions by means of the "activation" of the catalyst active center to the reaction gas molecules in the condensed state (such as solid, molten, liquid solution, mesoscopic and biological condensed state) with specific composition and multi-level structure, which can greatly activate the molecular structure of the reaction and improve the reaction performance. The mechanism of the catalytic reaction between these small gas molecules, the type and structure of the main products, the yield and side reactions, as well as the catalytic efficiency and lifetime, are all determined by the characteristics, composition and multi-level structure of the specific condensed catalyst under the reaction conditions. We must pay attention to the reaction conditions, the components in the condensed state, the different structures,In particular, the interactions between the active component and the support (SMSI) and their effects on the composition, structure (including geometric and electronic structures) of the catalytic active component and the properties related to the catalytic reaction (such as adsorption, overflow diffusion, catalytic activity and route, etc.), as well as the various effects and results on the catalytic reaction. There is a rich and complex condensed matter chemistry in condensed matter catalytic reactions between such gas molecules.

The chemical reaction between gas molecules is carried out under the action of a condensed state catalyst with a specific composition and structure,A small number of gas molecules can also promote the change of electronic structure, geometric structure and "state" of basic particles such as molecules and atoms under extreme reaction conditions such as ultra-high pressure, ultra-low temperature, laser, plasma and supercritical, resulting in the production of specific condensed chemical reactions.

Here are five simple examples.

Homogeneous Hydrogenation of Olefins

Homogeneous catalysis is generally characterized by high efficiency because the catalyst molecules can fully contact the reactant molecules in principle.

Homogeneous catalysis includes many reactions between small gas molecules such as hydrogenation. For the hydrogenation of olefins, complexes of d-electron group metal atoms often show catalytic activity in liquid solution. However, in general, complexes of 4D metal elements or organometallic compounds have higher catalytic activity and substitution activity, so they are often used as homogeneous hydrogenation catalysts for olefins. One of the most studied catalysts for homogeneous hydrogenation of olefins is the complex [RhCl(PPh₃)₃] with the 4d⁸structure Rh (I), see Fig. 1A for the structural formula^[3]. This compound is commonly referred to as Wilkinson catalyst (Wilkinson's catalyst)^[4]. The catalyst can hydrogenate a variety of olefins and alkynes at room temperature and hydrogen pressure close to or less than 1 atm. The main cycle of terminal olefin hydrogenation catalyzed by Wilkinson catalyst is shown in Fig. 1,H₂ undergoes hydrogenation addition with 16-electron complex [RhCl(PPh₃)₃](A) to produce 18-electron dihydro complex (B). One of the phosphine ligands in B undergoes dissociation to form the coordinatively unsaturated complex (C), which in turn forms the alkene complex (D). The hydrogen atom in D is transferred from Rh to the coordinating olefin, forming a transient 16-electron alkyl complex (E). E coordinates with the phosphine ligand to form F, and the hydrogen atom of the latter migrates to the carbon, resulting in the reduction of the alkyl group and the regeneration of A. Catalyst A begins to repeat the next cycle. Another parallel but slower catalytic cycle (in which the order of H₂ and olefin coordination to the metal is reversed) is known to exist, but is not shown in fig. 1. A catalytic cycle based on the 14-electron intermediate [RhCl(PPh₃)₂] is also known to exist. Although very little of this intermediate is present in the reaction, it contributes significantly to the catalytic cycle. This is because it reacts with H₂ at a much faster rate than [RhCl(PPh₃)₃]. In this cycle, E would directly eliminate alkane to regenerate [RhCl(PPh₃)₂], which rapidly reacts with H₂ to produce C.

Wilkinson catalysts are highly sensitive to the structure and properties of the phosphine ligand and the substrate olefin. Similar alkylphosphine ligand complexes have no catalytic activity, mostly due to the strong binding force between the ligand and the metal atom, which makes it difficult to dissociate. Similarly, the size of the olefin should be appropriate: Wilkinson catalysts cannot catalyze the hydrogenation of highly hindered olefins and ethylene without steric hindrance, probably because the sterically hindered olefins cannot coordinate with the metal, while the coordination ability of ethylene is too strong to further reaction. These phenomena confirm the earlier conclusion that the catalytic cycle is a series of reactions that maintain a fragile balance, and any factor that disrupts this normal balance will hinder the catalytic reaction or change the mechanism of the reaction.

Enantioselective reaction with Wilkinson catalyst is a reaction for the synthesis of specific chiral products, in which related Rh (I) catalysts containing chiral phosphine ligands are used to synthesize optically active products. The olefin to be hydrogenated must be a Prochiral olefin, which means that the olefin must have a structure capable of producing R or S chirality when coordinated to the metal. Two diastereomeric complexes are formed depending on the fact that the alkene is coordinated to the metal atom in different faces. In general, diastereomers have different stability and instability. In favorable cases, one or the other effect will lead to the enantioselectivity of the product.

Therefore, in homogeneous catalytic reactions, the progress of the reaction and the results of the reaction are also subject to or even determined by the composition and structure of the catalyst under the reaction conditions.

In addition to the above small gas molecules, which can react through homogeneous catalysis, most of the small gas molecules react through heterogeneous catalysis. Of the 20 synthetic chemical products with the largest output in the United States in recent years, 16 are produced by heterogeneous catalytic reactions under the action of high-specific surface area condensed state (solid state, mesoscopic state, molten state, etc.) materials with catalytic properties and multi-level structure. According to the different sites of the catalytic active surface, these catalysts can be generally divided into two categories: a large number of heterogeneous catalysts are highly dispersed solid, and the catalytic active sites are located on the surface of the particles; The other is solid state with pore structure (such as microporous zeolites and mesoporous materials, MOFs and COFs), in which the catalytic active site is located on the inner surface of the pore or cavity. In this special edition, experts will be invited to give in-depth introductions. Examples 2 and 3 are heterogeneous catalytic reactions of small gas molecules. The progress, mechanism and results of the reactions are mainly controlled and determined by the composition, multi-level structure and interaction between the components of the heterogeneous solid catalytic material.

Example 2 Hydrogenation of crotonaldehyde over Co/SiO₂ with different surface structures

Silica SiO₂ is a common catalyst support, which can efficiently catalyze a variety of chemical reactions by supporting metal and clusters as small as atomic level and nanometer on SiO₂. Although the active sites are metal atoms, nano-metals, clusters and so on, the interaction between them and the surface microstructure environment of the support will have different effects on their catalytic activity and selectivity under reaction conditions. For the SiO₂ support, the rough surface structure is rich in types, including conventional defect-free bridging oxygen ≡ Si — O — Si ≡, Si dangling bond (E 'center), non-bridging oxygen center ≡ Si — O ·, and oxygen vacancy ≡ Si — Si ≡ structures, as shown in fig. 2^[6]. In 1999, Pacchioni and his collaborators used Hartree-Fock density functional theory to study the interaction between Cu, Pd, and Cs atoms and these structures (Figure 2), and found that the interaction between metal atoms and non-defect sites on the surface (Figure 2a) was very weak, with an interaction energy of less than 0.2 eV (Figure 2a '), and they could only adhere to defect sites (with an adsorption energy of between 1 and 3 eV, see Figure 2b ~ e')^[6].

In 2002, Bueno and his collaborators studied the effect of the surface structure of Co in SiO₂ supported nano-Co catalyst on the selectivity of crotonaldehyde (CH₃—CH=CH—CHO) hydrogenation^[7]. They investigated the surface structure of Co using diffuse reflectance FTIR spectroscopy as well as H₂-TPD of Co-adsorbed Co catalyst. They found that there were obvious peaks at 50, 95, 120 ~ 160 and 160 ~ 300 ℃ in the TPD-H₂ curve of the sample, and they attributed these peaks to the existence of four different Co cluster surface sites α, β, γ and σ, which carried different charges (Co^δ+). The Co loading and preparation method (calcination temperature) can affect the distribution of these sites, and the enhancement of CoO_x-SiO₂ interaction favors the formation of β sites over γ and σ sites. The products of crotonaldehyde hydrogenation include Butylaldehyde (BAL, hydrogenation to C = C double bond), crotyl alcohol (CrOH, hydrogenation to C = O double bond), and n-butanol (BOL, hydrogenation of both C = C and C = O double bonds). The results of the reaction show that the ratio of site β to the sum of site γ and σ does not affect the selectivity of butyraldehyde (that is, it has no effect on the hydrogenation of C = C double bond), but there is a good correlation between the ratio and the selectivity of butenol (CrOH) and n-butanol (BOL).An increase in the value of β/ (γ + σ) favors the formation of butenol (i.e., favorable for the hydrogenation to the C = O double bond) and disfavors the formation of n-butanol (i.e., unfavorable for the hydrogenation to the C = C double bond), whereas an increase in the value of β/ (γ + σ) favors the production of n-butanol (i.e., favorable for the hydrogena- tion to the C- = C double bond) and disfavors the production of butenol (i.e, unfavorable for the hydrogena- tion to the C- =. In the Co-catalyzed hydrogenation of crotonaldehyde, there are three adsorption modes of crotonaldehyde molecules on Co: C = O double bond adsorption, C = C double bond adsorption, and C = O double bond and C = C double bond simultaneous adsorption. The product of the first adsorption mode is butenol, the product of the second adsorption mode is butyraldehyde, and the product of the third adsorption mode is butanol. The probability of the three adsorption modes occurring on the surface sites α, β, γ and σ of the Co cluster is not the same, which leads to a certain distribution of the products of crotonaldehyde hydrogenation. Changing the preparation conditions (loading and calcination temperature) will change the surface structure of the support and the interaction strength between the support and the metal cluster, thus changing the surface structure of the metal cluster with catalytic function, and then affecting the conversion and selectivity of the catalytic reaction.

Example 3 Catalytic Dehydrogenation of Propane

The reaction of propane C₃H₈ dehydrogenation to propene C₃H₆ must be carried out under the action of solid catalyst Pt/SiO₂ with specific composition and multi-level structure. Deng et al. Studied in detail the effects of the preparation method and process of the catalyst on the structure and performance of the catalyst^[8]. They immersed amorphous SiO₂ in an aqueous solution of H₂PtCl₆ for several hours, filtered, dried, and then treated with different atmospheres (O₂, N₂ or H₂) at a certain temperature.It was found that only the Pt/SiO₂ sample treated by H₂ reduction under certain conditions had a catalytic effect on the above propane dehydrogenation reaction (Fig. 3) and had a certain adsorption effect on CO (Table 1).

The Pt/SiO₂ treated by H₂ reduction at different temperatures was used for propane dehydrogenation reaction to study its composition and structure (the size and specific surface area of nano-Pt, see Table 1), the crystal phase and structure of support SiO₂ (the phase transition and crystal structure of SiO₂ at different temperatures, see Fig. 4 and related descriptions), and the SMSI (strong metal-support interaction) between structures, so as to understand the catalytic dehydrogenation reaction of C₃H₈.

At different temperatures, the carrier SiO₂ will undergo phase change, and its phase change and stability at atmospheric pressure are shown in Figure 4. It can be seen from the figure that 1713 ℃ is the melting point of crystalline cristobalite SiO₂, and the SiO₂(l) is $\mathop{\rightleftharpoons}\limits^{1713 \ ℃}$ cristobalite (tetragonal system).Both cristobalite (tetragonal) $\mathop{\rightleftharpoons}\limits^{1470 \ ℃}$ and tridymite (tetragonal) $\mathop{\rightleftharpoons}\limits^{870 \ ℃}$ β-quartz (hexagonal) are slow reaction transformation, while β-quartz $\mathop{\rightleftharpoons}\limits^{575 \ ℃}$ α-quartz is rapid reaction transformation.

Under the action of catalyst Pt/SiO₂, Deng et al. Studied the catalytic dehydrogenation of C₃H₈^[8]. It can be clearly seen from fig. 3 that the dehydrogenation conversion rate of the C₃H₈ is closely related to the composition and structure (including the crystalline surface structure of nano-Pt and SiO₂ and the result of interaction (SMSI)) of the Pt/SiO₂ catalyst obtained after H₂ reduction treatment at different temperatures.Taking Pt/SiO₂-773K, 1073K and 1273K H₂ as examples, the relationship between the catalytic dehydrogenation of C₃H₈ and the composition and structure of the Pt/SiO₂ of solid catalyst (especially the influence of SMSI on the structure) was discussed^[10⇓~12]. On the basis of this work, Deng et al. Put forward a further understanding that, as shown in Fig. 4, with the increase of temperature, the crystalline state of catalyst Pt/SiO₂, 773K H₂ and carrier SiO₂ in Pt/SiO₂-1073K H₂ changes phase.It changed from α-SiO₂ to β-SiO₂, and its crystal structure changed from a long columnar trigonal system to a hexagonal bipyramidal (short cylindrical) hexagonal system (Fig. 5).

The change of the structure of the support SiO₂ leads to the change of the morphology and electronic structure of the interface between SiO₂ and nano-Pt, thus affecting the catalytic dehydrogenation function for C₃H₈. Deng et al. Have characterized catalysts supported on α and β-SiO₂ with different structures by transmission electron microscopy (TEM). For the former, the measured lattice spacing of 0.23 nm is consistent with the lattice spacing of Pt < 111 >, and there is only a very weak interaction between Pt and the support interface, which is analyzed by Necking and large contact angle. However, for the latter, the nanoscale Pt clusters on the β-SiO₂ surface are trapped in the surface structure, indicating that there is a considerable interaction between the nanoscale Pt and the β-SiO₂structure at the interface. Deng et al. Suggested that it might be due to the formation of alloy-like Pt₃Si species between Si dangling bonds in the surface structure of the support β-SiO₂ and nano-Pt, and electron transfer occurred between the surface and nano-Pt clusters.Not only the interface morphology was changed, but also the catalytic function of dehydrogenation was greatly improved due to the change of electronic structure (the conversion and selectivity of C₃H₈ were increased to 21.3% and 94.4%, respectively), as shown in Fig. 6^[14].

Deng et al. Further studied the structure of propane dehydrogenation catalyst (Pt/SiO₂-1273K,H₂) obtained by H₂ reduction treatment at higher temperature (1273 K)^[8]. According to the TEM results, Deng et al believed that the increase of temperature led to the aggregation of some surface Pt nanoclusters and the formation of larger clusters (Table 1).In his review of "particle size effect of strong metal-support interaction", Martin pointed out that Du et al had proposed a "thermodynamic equilibrium model dependent on surface tension", and the study showed that the larger the particle size of the metal, the greater the surface tension.Therefore, the larger the size of the nano-Pt cluster on the surface of the carrier SiO₂ is, the higher the surface energy is, and the SMSI is more likely to occur. The "coating" by the "wetting" carrier SiO₂ (as shown in the figure above) not only changes the surface structure, but also makes some nano-Pt clusters fall into the surface of the carrier SiO₂ and be covered by the SiO₂ film, resulting in CO adsorption and loss of catalytic function for propane^[15][16][8]. In addition to the above reasons, we also put forward the experimental results that during the crystallization of tridymite (β₂-Tr), a large number of nano-Pt clusters are often trapped between the thin layer assemblies (Fig. 6) due to the stacking crystallization of layered structural units (Fig. 7), resulting in the complete loss of CO adsorption and propane catalytic dehydrogenation function.

Most of the catalytic reactions between gas molecules are generally carried out under the action of solid catalysts as described in Examples 2 and 3 above. However, some reactions between specific gas molecules must be carried out under the action of molten, nano-state or Biological condensed matter (BCM) catalysts with specific properties, functions, compositions and structures. It is found that the progress, mechanism and results of these catalytic reactions have their own advantages and characteristics.

Let's take the molten catalytic system as an example. Catalytic media are mainly composed of monobasic or multicomponent molten salts, metals and molecular melts, and there are more than 3000 systems so far. The special valence, composition and catalytic properties of anions, cations, complex ions and molecules dissolved in the medium molten salt at high temperature, as well as the multi-level structures such as "lattice defects", "ion vacancies", "holes and dislocations" in the condensed structure of the molten salt, will affect the progress and results of the molten catalytic reaction^[18]. Secondly, the molten catalytic system has good thermal stability, good electrical conductivity, large heat capacity, stable chemical properties and other good properties, and has large solubility for a variety of catalytic active substances.They can be characterized by the integration of medium (solvent), heat carrier and catalyst, so that some catalytic reactions including specific gas molecules can only be or are more suitable for the catalytic action of molten catalysts. in 1972, with the title of "Catalysis in Molten salt Media", Parshall reviewed and introduced the synthesis of benzene from acetylene, the reaction of acetylene and ammonia to form pyridine, and the development of Molten catalysts under the action of molten salt catalysts with Catalysis in Molten salt Media "as the main body since the 1940s^[19]. In recent years, Tang Yongliang and Yu Qingkai in China have found that methane can be decomposed into carbon atoms under the surface catalysis of molten copper and can be further self-assembled into graphene, which opens up a new way for the preparation of high-quality graphene^[20].

Example 4 Synthesis of CO/H₂ in the Presence of Ruthenium Molten Salt Catalyst

One of these molten catalysts contains one or more ruthenium-containing compounds, such as ruthenium carbonyl, ruthenium oxide (RuO₂), and the complex ruthenium is dissolved and uniformly dispersed in a low-melting four-coordinate phosphonium salt, such as Bu₄PBr or Bu₄PI, and an ammonium salt, to become a molten salt catalyst. This type of molten ruthenium-containing catalyst has the inherent advantages of homogeneous and heterogeneous catalysts, that is, under the conditions of the usual CO plus H₂ reaction, it has the advantages of high selectivity and repeatability of liquid homogeneous catalysts and easy separation of products of heterogeneous catalysts. Of course, halides of Rh, Co, Mn, etc. Or other compounds can also be added to the molten transition metal ruthenium catalyst, that is, the composition of the ruthenium containing molten catalyst can be adjusted to improve the selectivity of specific reactions. Uch as the following reaction:^[21]

CO+H₂ $\stackrel{Ru+Rh}{\longrightarrow}$ HO-CH₂-CH₂-OH

CO+H₂ $\stackrel{Ru+Co}{\longrightarrow}$ C_nOH+C_nH_2n+1OAc

CO+H₂ $\stackrel{Ru+Mn}{\longrightarrow}$ CH₃OH

CO+H₂ $\stackrel{Ru+Co}{\longrightarrow}$ CH₃COOH

The synthesis of (CH₂OH)₂ from CO+H₂ in the presence of a molten catalyst containing Ru and Rh will be described in more detail below.

The direct synthesis of ethylene glycol over heterogeneous solid catalysts Ru, Rh and Co is limited by high pressure, which often affects the conversion of the reaction. Under the action of Ru or Ru-Rh catalyst dispersed in molten tetra-coordinated phosphonium salt or ammonium salt, the above limitation can be overcome, and ethylene glycol and single-chain alkyl ester can be obtained.

Pruett has already proposed that the reaction for the synthesis of (CH₂OH)₂ by CO/H₂ over Rh homogeneous catalyst includes hydroxymethyl propagation reaction^[22]. Knifton found that the product distribution on the molten catalyst containing Ru can also follow the law of chain growth:^[23]

Ru(H)C≡O+H₂→≡RuCH₂OH $\stackrel{Co}{\longrightarrow}$

≡RuC(=O)CH₂OH $\stackrel{H_{2}}{\longrightarrow}$ ≡RuCH(OH)CH₂OH

$\stackrel{H_{2}}{\longrightarrow}$ (CH₂OH)₂

Both Rh and Ru species dispersed in tetrabutylphosphonium bromide (Bu₄PBr) catalyze the synthesis of (CH₂OH)₂. With the increase of Rh in the binary composite molten catalyst, the yield of ethylene glycol increases. When Ru/Rh = 1/1, the yield is the largest, and it is interesting that there is only a very small amount of CO+H₂→(CH₂OH)₂ in the product when there is no Ru component or the four-coordinated phosphonium salt is absent, that is, the presence of ruthenium in the molten salt is the core of the CO+H₂→(CH₂OH)₂ catalytic reaction.

As for the nanoscale catalytic reaction between gas molecules, the condensate composed of nanoparticles is a very important mesoscopic condensate. The size of nanoparticles is small, the proportion of interface atoms is large, and the interface contains a large number of defects and surface atoms with dangling bonds. In fact, the nano-condensed state is generally a three-dimensional combination of order to disorder, a two-dimensional layered structure, a one-dimensional fibrous structure, and a zero-dimensional nano-condensed material with a zero-dimensional structure. Due to the special structure of mesoscopic nanoparticles, they have some unique and important characteristics of nanoparticles (such as interface and surface effects, quantum size effects, small size effects and quantum tunneling effects), which lead to their unique physical functions, chemical activities and catalytic properties. This is a feature that other condensates do not have. In this special edition, relevant experts will be invited to give a more in-depth introduction.

Let's look at one last example. The catalysis of biocondensed catalysts, represented by many enzymes, on the intermolecular reactions of specific gases^[24]. Such as zinc, magnesium and iron enzymes involved in acid-base catalysis, peroxidases, oxidases and oxygenases involved in H₂O₂ and O₂ reactions, molybdoenzymes and tungstoenzymes catalyzing oxygen atom transfer, hydrogenases and Co-containing enzymes involved in the interconversion of H₂ and H⁺. Because of its unique composition and complex multi-level structure of enzyme catalytic active center, it has a specific catalytic environment and catalytic effect on the reaction between some gas molecules^[24]. In this paper, Example 5 is taken as the main example.

Example 5 Ammonia Synthesis from N₂+H₂ Catalyzed by Nitrogenase

The great inertness of N₂ (and the somewhat lesser inertness of H₂) necessitates the use of a catalyst for the ammonia synthesis reaction. The catalyst used in industry is metallic iron containing a small amount of alumina, potassium salt and other promoters. A large number of studies on the mechanism of ammonia synthesis have shown that the rate-determining step under normal operating conditions is the dissociation of N₂ molecules coordinated to the surface of the catalyst. The dissociation of the other reactant (H₂) on the metal surface is much easier. A series of dissociative insertion reactions between the adsorbed species lead to the generation of NH₃.

Because of the high bonding enthalpy of N₂, the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the N₂ molecule is large, and the dissociation is quite difficult, so the reaction of ammonia synthesis must be carried out at high temperature (usually 400 ℃). However, due to the exothermic reaction, increasing the temperature will reduce the equilibrium constant of the reaction. To recover some of the yield loss, the pressure is usually increased to about 100 atm to promote product formation. People have been looking for biomass condensed state catalysts like Nitrogenase, which can give good equilibrium yield at room temperature.

"Nitrogen fixing" bacteria found in soil and some plant root nodules contain an enzyme called nitrogenase, which catalyzes the reduction of N₂ to NH₃, which couples 16 ATP molecules through hydrolysis and produces H₂:

N₂+8 H⁺+8 e^-+16 ATP→2 NH₃+H₂+16 ADP+16 P_i(P_i for inorganic phosphate)

"Fixed" nitrogen is needed for the synthesis of amino acids and nucleic acids, so nitrogen fixation is an important problem in agricultural production. Industrial production of ammonia by the Haber process involves the reaction of N₂ and H₂ at high temperature and pressure; In contrast, nitrogenase produces NH₃ under mild conditions. It is not surprising that it has attracted so much attention. In fact, coordination chemistry-organometallic chemists have been studying the mechanism of nitrogenase activation of N₂ molecules for decades. For a reaction that is thermodynamically unfavorable, this study is valuable from an energy perspective. N₂ is an inactive molecule, and its reduction requires energy to overcome a high activation energy barrier.

Nitrogenase is a complex enzyme composed of two types of proteins, the larger of which is called "MoFe" protein and the smaller of which is called "Fe protein" (fig. 8)^[24]. The Fe protein (A) contains a [4Fe-4S] cluster, which is coordinated by two cysteine residues from two subunits. The role of the Fe protein in the reaction is to transfer electrons to the MoFe protein. However, the reaction involved is far from being understood, and in particular, it is not understood why each electron transferred is always accompanied by the hydrolysis of two ATP molecules (which were originally bound to the Fe protein).

The MoFe protein is a α₂β₂ tetramer in which each αβ-pair contains two types of superclusters. The [8Fe-7S] cluster is called "P cluster" (B), and the P cluster is considered to be an electron transfer center. However, scientists believe that another cluster is the site where the N₂ is reduced to the NH₃. The cluster called "FeMo-co" (FeMo coenzyme), denoted [Mo7Fe-9S, C], is shown in fig. 8C. Coordinated to the Mo atom in the cluster are one imidazole N atom (from histidine) and two O atoms (from the exogenous molecule R-homocitrate). One negatively charged C atom is in the center of six Fe ion cages (fig. 9). The crystal structure of the enzyme was completed nearly 10 years ago, and with improved resolution, the small volume of the central negatively charged C atom was first identified. The C atom was identified by X-ray emission spectroscopy and detailed modeling of X-ray interference features, which took almost 10 years. One role of the central C atom may be to stabilize the 6Fe cage, which would otherwise collapse towards the center. However, the catalytic mechanism of nitrogenase is not yet fully understood.

Furthermore, a large number of catalytic reactions between gas molecules are summarized from the level of condensed matter chemistry, and the following important scientific problems worthy of study are put forward, such as:

1) The composition, structure and existing state (such as monatomic, cluster, nanoscale, etc.) Of the catalytic active sites in different condensed catalysts and their effects on the reaction;

2) microenvironment of catalytic active site and local catalysis;

3) The effect of the hierarchical structure of the catalyst support and the strong interaction of the active component (SMSI) on the catalytic reaction;

4) Directional synthesis and precise preparation of specific multi-level structures in catalytic materials with specific functions, as well as interfacial defects and dynamic catalysis, heterogeneous cluster catalysis, etc.

To systematically and thoroughly study and summarize the laws of function-structure-construction (synthesis, preparation and self-assembly), and gradually provide a basis for the construction of condensed matter engineering in chemistry and materials science.

The chemical reaction between gas molecules is carried out under the action of condensed state catalysts with specific composition and structure, and a small part of gas molecules mainly include VIII a group inert gases with monatomic molecular structure.The chemical reactions of these gas molecules often have to be carried out under extreme conditions such as high and ultra-high pressure, ultra-low temperature, laser, plasma and supercritical. This is because under the above extreme conditions, the structure (electronic structure and geometric structure), state and phase structure of gas molecules change significantly, thus changing or even improving the reactivity of condensed gases with different structures. The phase transition of H₂ at low temperature, ultralow temperature (several K) and high pressure (GPa) is taken as an example (Fig. 10). It can be seen from the phase diagram that under extreme conditions, H₂ gas can be changed into solid hydrogen with many different structures, including liquid metal H and solid metal hydrogen (solid MH, with metallic electronic structure)^[27].

Next, Xe, which is the most studied inert gas under high pressure and ultra-high pressure, is taken as an example.

Since 1960, Xe has been reacted under extreme conditions such as high pressure to synthesize a series of chemical species including oxidation States of + 2, + 4, + 6 and + 8, mainly containing O and F, which has greatly enriched the chemistry of inert gases^[28]. Under the condition of high pressure and ultra-high pressure, with the increase of pressure, the reaction gas molecules will be affected by both the distance between molecules and the electronic structure, and at the same time, the condensation between molecules will occur. Taking Xe as an example, when the pressure increases, the gas Xe molecules gradually condense into liquid and amorphous solid, crystallize into face-centered cubic at 3 GPa, and then increase the pressure to 80 GPa, the crystal phase becomes hexagonal close packing.At the same time, the change of electronic structure state, including the change of energy level, the activation of outer electrons and the interaction between orbitals (including the radial range and shape of orbitals, thus affecting the crossing, overlapping and transfer of intermolecular electronic orbitals). When the pressure rises to 135 GPa, Xe has the properties of a metal due to the change of its electronic structure^[29]. The influence of high pressure on the above two aspects will make the reaction between gas molecules become the chemical reaction between condensed States due to "state change", and with the change of pressure, it will affect the reaction mechanism and process, the type and structure of products, and even the generation of side reactions. For example, the reaction of Xe with H₂O or O₂ under high pressure, at 1 GPa, Xe with H₂O, at 3 GPa,Xe and {{\mathrm{O}}_2}^{} can form corresponding hydrates and oxides with weak bonding. When the pressure rises to 50 GPa, Xe and O₂ and H₂ can form compounds with definite composition^[30][31][32]. These chemical species and compounds are essentially obtained by chemical reactions between condensed States under high pressure.

The condensation reaction of this kind of gas molecules under high pressure is caused by the influence of high pressure on the gas molecules in two aspects: the condensation of intermolecular distance and the influence of high pressure on the electronic structure of reactants, as well as the characteristics of the reaction mechanism and the formation of the reaction product structure. A more detailed example: in 2016, Dewaele synthesized two xenon oxides, Xe₂O₅ and Xe₃O₂, under high pressure of 100 GPa^[33]. Due to the different synthesis conditions, the structure and stability of the solid products of the two crystals are different, as shown in Figure 11. The experimentally determined structure of the two oxides shown in fig. 11 is Xe₂O₅ (fig. 11A), a crystal with oxidation States of Xe of + 4 and + 6 and consisting of alternating layers of XeO₄ in a square structure and XeO₅ in a pyramidal structure, and Xe₃O₂ (fig. 11B), a crystalline solid consisting of alternating layers of XeO₄ in oxidation States of 0 and + 4.

They also theoretically calculated a series of enthalpies of formation ΔH of xenon oxides at different high pressures (83 GPa, green point, 150 GPa, red point and 200 GPa, blue point) (Fig.A comparison of the stability of the following xenon oxides Xe₂O₅, XeO₂, Xe₃O₂ and Xe₂O,At the same time, it is proposed that oxides such as Xe₂O₅ and Xe₃O₂ are easy to form stably under the synthesis conditions of a certain high pressure range.

A condensed state reaction between He and Ne in the first and second periods under high pressure is also introduced. In 1993, Loubeyre et al. Synthesized the only known stable van der Waals compound NeHe₂^[34]. Looking at the reaction of non-inert gas molecules such as CO, CO₂ and inorganic gas small molecules such as N₂ under high pressure, not only will they condense, but also take CO as an example. At low pressure, CO liquefies into liquid state and condenses into disordered hexagonal phase solid state. At room temperature, CO can crystallize into β phase at about 2 GPa. When the pressure rises to more than 5 GPa, it irreversibly transforms into colored non-molecular phase^[35]. Recent studies have shown that CO molecules can be polymerized into ordered P-CO structures from one-dimensional [─C═O─]_n under high pressure^[36]. Under different pressure conditions, CO decomposes into a polymer of CO₂ and another low carbon oxygen compound C₃ {{\mathrm{O}}_2}^{}, and changes the electronic structure state in different solid States, so that it can react with other gas molecules in different condensed States under different pressure conditions^[37]. Secondly, under high pressure, inorganic small molecules will show many novel physical and chemical properties, resulting in a variety of chemical reactions. At present, it has become one of the hot research directions in condensed matter theory and materials science^[38].

In inert rare gases, except for Xe, which can produce condensed state reactions under high pressure to form a number of compounds, other rare gas molecules are more chemically inert due to the stability of their electronic structure, especially with the reduction of the radius of the element atom. Therefore, it was not until 2000 that the first stable argon-containing compound (HArF) was synthesized in an ultra-low temperature (7.5 K) argon medium by photolysis of solid HF under external field (such as optical radiation, laser, etc.), and by the interaction of nascent F with Ar^[39]. R Räsänen et al. Have studied the above reaction route and reaction mechanism of HXY compounds containing rare gases in detail^[40]. First of all, because these compounds (including compounds containing almost all rare gases) have very low thermal stability, that is, they are very easy to decompose at slightly higher temperatures, the synthesis of these compounds must be carried out at low or even ultra-low temperatures, generally at 5 ~ 50 K (that is, the synthesis reaction is carried out under the condition that the rare gas is solid).The reaction precursor is usually prepared by mixing gaseous HY (hydrogen halide, HCN, H₂S, etc.) With an excess of rare gas (1 ∶ 1000) at room temperature, and the solid precursor is placed in an optically transparent reaction device at 5 ~ 50 K. In order to study the photolysis conditions of HY, they studied the Dissociation energy of HY. For example, HCl, HBr and HI are 4.43, 3.75 and 3.05 eV respectively, which can be excited and dissociated by ultraviolet (UV) radiation. It is also found that 193 nm (ArF laser) can decompose H₂S and HCN (in the matrix of solid Kr and Ar). After the solid HY-Xe precursor is irradiated by the above external field light source, most of the HY molecules are photolyzed into H atoms and excited state Y. After the system is slightly heated (for example, the synthesized precursor of HY-Xe after irradiation is heated to 45 K), the H atoms after photolysis can diffuse to the vicinity of X ← ○ Y as shown in Figure 13 to form HXY molecules. R Räsänen et al. Further studied their formation mechanism, bonding model, and the properties, structures, and spectral characteristics of these compounds obtained by computer simulation. This synthesis reaction is generally applicable to the synthesis of general noble gas-containing compounds. In recent years, some theoretical work combined with spectroscopic experiments have proved that under certain conditions, weak covalent chemical bonds between rare gases and metals can be formed, such as Ar-AuCl, Ar-CuX (X = F, Cl, Br) and so on^[41,42]. This provides a direction and hope for the construction of condensed matter chemistry in which Ar, even Ne, He and other rare gases with smaller atomic radii are activated to form bonds and react at low temperatures by external fields, that is, these inert gases react under the action of low temperatures and external fields.

Finally, a direction for studying the reaction of gas molecules is proposed, that is, in the liquid state of supercritical water or supercritical fluids such as CO₂,Because of its good solubility in many small inorganic and organic gas molecules and its ability to stimulate and activate their structures and reactions, many gas molecules can produce unique reactions in it, showing rich condensed matter chemistry.