Condensed Matter Chemistry in Single-Atom Catalysis

Qinghe Li; Botao Qiao; Tao Zhang

doi:10.7536/PC230310

Progress in Chemistry >

2023 , Vol. 35 >Issue 6: 821 - 838

DOI: https://doi.org/10.7536/PC230310

Review

Condensed Matter Chemistry in Single-Atom Catalysis

Qinghe Li ,
Botao Qiao ^,^* ,
Tao Zhang

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CAS Key Laboratory of Science and Technology on Applied Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

*Corresponding authore-mail: bqiao@dicp.ac.cn

Received date: 2023-03-13

Revised date: 2023-05-18

Online published: 2023-05-25

Supported by

The National Key Research and Development Program of China(2021YFA1500503)

The National Natural Science Foundation of China(21961142006)

The National Natural Science Foundation of China(21972135)

The CAS Project for Young Scientists in Basic Research(YSBR-022)

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Abstract

Single-atom catalysis (SAC), the catalysis by single-atom catalysts (SACs), has been developed as one of the most active research frontiers in the field of heterogeneous catalysis. SACs are multilevel atomic aggregates with relatively clear active center consisting of single metal atoms stabilized on support atoms through covalent or coordination interaction. Their composition, structure and properties are typical research objects of condensed matter chemistry. This review paper starts from the view of condensed matter chemistry and the main contents are as follows: briefly describing the historical basis and development status of the concept of SAC; systematically summarizing the condensed matter phenomena involved in the field of SAC, that's the aggregate of the surrounding atoms and the metal center; elaborating the influence of coordination environment on the structure and properties of aggregates and the dynamic evolution of aggregate structure under real reaction condition. Finally, the application and future development trend of condensed matter effect of single atom in heterogeneous catalytic reactions are summarized and prospected.

Contents

1 Introduction

2 The concept of"single atom catalysis"

3 The development of"single atom catalysis"

3.1 Preparation of single atom catalyst

3.2 Characterization of single atom catalyst

3.3 Application of single atom catalyst

4 Condensation effect between metal center and coordination atoms

4.1 Interaction form between metal and support

4.2 Aggregates structure modulating via coordination atoms

4.3 Effect of metal aggregation form on catalytic performance

5 Dynamic evolution and characterization of aggregates under reactive conditions

6 Conclusion and outlook

Key words： single-atom catalysis; aggregation; condensed matter chemistry; structural evolution; in-situ characterization

Cite this article

Qinghe Li , Botao Qiao , Tao Zhang . Condensed Matter Chemistry in Single-Atom Catalysis[J]. Progress in Chemistry, 2023 , 35(6) : 821 -838 . DOI: 10.7536/PC230310

1 Introduction

Condensed state refers to the formation of aggregates with specific composition, multi-level structure, properties and functions through the "stable adhesion relationship" established by atoms, ions and molecules bonded by chemical bonds, which have specific electronic structures and are regarded as the main body of reaction in traditional chemistry, and is the main body of all chemical reactions^[1,2]. Chemical reactions depend on the composition, multi-level structure and properties of the condensate. The chemical industry is an important part of the national economic development, and its direct process and related indirect processes together can contribute a quarter of the total economic output of developed countries^[3]. Catalysis is the soul of chemistry and the engine of chemical industry. At present, more than 80% of chemical industry reaction processes are catalytic processes, which require the use of heterogeneous catalysts. About half of them are supported noble metal catalysts, and the metal active centers exist in different forms of condensed States, such as nanometer, sub-nanometer and even single atom. With the rapid development of the chemical industry, the demand for catalysts is increasing year by year, and the global market turnover of heterogeneous catalysts continues to grow. Therefore, improving the metal utilization efficiency has long been one of the core issues in catalyst preparation science, and its essence is the transformation between different condensed States, so it is an important part and research direction of condensed matter chemistry.

Single-atom catalyst is a kind of heterogeneous catalyst in which the active center metal is dispersed on the carrier in the form of a single atom, which is the minimum limit of the active center of catalytic reaction in the space scale and maximizes the utilization efficiency of the metal atom^[4]. Compared with traditional nano/sub-nano catalysts, single-atom catalysts have a clear composition of active atoms, which form a well-defined aggregate with surrounding atoms through coordination interaction, and are ideal research objects in condensed matter chemistry. Although the concept of single-atom catalysis has only been proposed for about a decade, the research on "single-atom catalysis" has made rapid progress. At present, the preparation of high-density single-atom catalysts and the regulation of the interaction between metal single atoms and supports have been realized, which enriches the types of condensed matter. Its research scope has also expanded rapidly from thermal catalysis to electrocatalysis, photocatalysis and enzymatic catalysis^[5].

In this paper, from the point of view of condensed matter chemistry, the background, development status and future prospects of the concept of monatomic catalysis are briefly introduced, and the aggregation effect between metal centers and coordinating atoms is summarized in detail.Including the interaction form between the metal center and the surrounding coordinating atoms, the geometric and electronic structure of the aggregate modulated by the coordinating atoms, the catalytic properties of metals with different sizes, and the characterization methods of the dynamic evolution of the aggregate structure under reaction conditions and its effect on the catalytic activity. It is hoped that this paper can provide some thoughts and help for the further development and application of condensed matter chemistry concepts.

2 Proposal of the Concept of Monatomic Catalysis

Heterogeneous catalytic reactions usually take place on the surface of the active component of the catalyst, and the reactant molecules adsorb and interact on the active site atoms to form intermediate species and then convert to the target product^[6,7]. In this process, the bulk atoms of the active component of the catalyst are likely to be ineffective, thereby reducing the efficiency of the catalyst. In order to improve the efficiency of the catalyst, a simple and effective way is to increase the surface atom ratio of the metal component of the catalyst, that is, to disperse the metal center into fine particles (which can be as small as a few nanometers or even about one nanometer), which is called improving the dispersion or high dispersion of the catalyst. The limit of high metal dispersion is that the metal active component is dispersed on the support in the form of a single atom, forming a so-called single-atom catalyst. Although this concept is very simple, it is limited by catalyst preparation and characterization technology, and this idea has remained in the conceptual stage in the past, making it difficult to realize it experimentally^[8].

Since the late 1990s, with the development and progress of catalyst preparation, especially characterization technology, people have begun to explore highly dispersed supported metal catalysts and made a series of important progress (Figure 1)^{[9⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~20]}. At the same time, based on the long-term focus on the preparation of highly dispersed metal catalysts, our research group successfully prepared oxide-supported single-atom catalysts in 2009, and worked closely with Liu Jingyue of Arizona State University and Li Jun of Tsinghua University.The monoatomic dispersion state of the metal in the catalyst and the catalytic reaction mechanism were systematically studied by spherical aberration corrected electron microscopy (AC-STEM), in situ infrared spectroscopy, X-ray absorption spectroscopy (XAS) and theoretical calculation. After more than two years of in-depth research, the new concept of "Single-atom catalysis (SAC)" was first proposed in the world in 2011, which aroused the research upsurge of single-atom catalysis, rapidly developed into a new research frontier in the field of heterogeneous catalysis, and widely affected other fields such as physics, biology, medicine and electronics^[4]^{[21⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~34]}. Single-atom catalysis can not only understand complex heterogeneous catalytic reactions at the atomic level, but also has great potential in the field of industrial catalysis due to its excellent catalytic performance. The detailed development of Single-atom catalysis and its differences and connections with other similar concepts (Surface org anometallic chemistry, SOMC; single-site heterogeneous catalyst, SSHC; Atomically dispersed catalyst, ADC) are detailed in the relevant review articles^{[17⇓⇓~20]}.

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图1 “单原子催化”概念提出

Fig.1 Proposing the concept of "single atom catalysis"

3 Current Status of Monatomic Catalysis

After more than 10 years of intensive research, "single-atom catalysis" has been rapidly developed. At present, the research of single-atom catalysis has been gradually extended to materials, energy, environment, biomedicine (biodiagnosis, tumor therapy), sensors, semiconductors, single-atom manufacturing, etc. (Fig. 2), showing potentially important application prospects in interdisciplinary science^{[35⇓⇓⇓⇓⇓⇓~42]}.

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图2 “单原子催化”发展历程^[27]

3.1 Preparation of single atom catalyst

In terms of catalyst preparation, almost all precious metals, most transition metals, as well as many alkali metals, rare earth metals and even non-metals in the periodic table of elements have achieved single-atom dispersion^[42]. The development of single-atom catalysis has led to a rapid expansion of single-atom catalyst systems (Figure 3): from the earliest reported oxide-supported noble metal single-atom catalysts to single-atom alloy catalysts, nitrogen-modified carbon-supported metal single-atom catalysts (M-N-C), and various two-dimensional material-supported single-atom catalysts^[4]^{[43⇓⇓⇓~47]}. The preparation methods of single-atom catalysts have also developed from the initial wet chemical methods to a variety of new methods, which can be divided into bottom-up strategy and top-down strategy, and their characteristics are shown in Table 1^[48⇓~50].

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图3 制备SACs的多种宿主材料^[71]

表1 单原子催化剂不同制备方法对比

Table 1 Comparison of the different preparation methods for SACs

分类	方法名称	优点	缺点
自下而上	质量分离软着陆法	原子分布均匀	需要特殊设备、反应条件苛刻、成本高
	原子层沉积法
	球磨法	成本低、操作简便	小球或添加剂的污染
	湿化学法	制备过程简便、无需特殊复杂设备
	电沉积法	精确控制原子分散	电解质溶液可能引入杂质,金属单原子与载体的作用力不可控,对金属物种还原电位有特殊要求
自上而下	高温热解法	热稳定性高	成本高

The bottom-up strategy starts from the metal precursor monomer, and the metal atoms are anchored on the surface of the support through adsorption, reduction and confinement. Mass separation soft landing method, which can precisely control the size of metal species by mass selective molecular or atomic beam; The combination of gas cluster ion source, mass spectrometry and soft landing technology can precisely control the number of atoms in the deposited clusters, thus controlling the preparation of single-atom catalysts^[51]. Most support materials, such as metals, oxides, carbon/diamond, can use this method to support clusters or atoms without the limitation of nucleation and growth, but need to provide sufficient surface holes or defect sites. Atomic layer deposition (ALD) is a vapor phase synthesis method, which is based on the surface reaction between the vapor phase metal precursor and the support, and the number and size of deposited metal atoms can be precisely controlled by adjusting the number of cycles^[52,53]. The above two methods have uniform atomic distribution, but require special equipment, harsh reaction conditions and high cost, which limits the large-scale development of such preparation technology. Electrodeposition is also considered to be an effective method for the preparation of single-atom catalysts. The target metal species in the electrolyte solution are deposited on the electrode, so that the isolated metal atoms remain stable on the support. Adjusting the anode voltage and deposition time helps to accurately control the atomic dispersion^[54,55]. However, the main problems of the electrodeposition method are that the electrolyte solution may introduce impurities, the force between the metal single atom and the support is uncontrollable, and there is a special requirement for the reduction potential of the metal species. Single-atom catalysts prepared by ball milling can effectively break and reconstruct chemical bonds due to the input of high mechanical energy^[56,57]. This method is low cost and easy to operate, but the single-atom catalyst produced by ball milling may be contaminated by small balls or additives in the ball milling tank. Wet chemical method is considered to be the most promising method for the large-scale production of single-atom catalysts because of its simple preparation process and no need of special and complex equipment.

Top-down preparation strategies are usually based on the decomposition of ordered nanostructured materials or organic polymer precursors into smaller, more complex monomers, achieving precise control of the atomic structure^[58]. High temperature pyrolysis and high temperature vapor migration trapping are the most common top-down synthesis methods of monatomic catalysts in recent years, which have broad application prospects in industry^[59]. High-temperature pyrolysis, in which a suitable metal precursor is selected for decomposition at an appropriate temperature and atmosphere, has been widely used to prepare various carbon-based single-atom catalysts, especially atomically dispersed catalysts supported on nitrogen-doped porous carbon materials. According to the different precursors of high temperature pyrolysis, it can be divided into two categories. The first category is the regular porous material-metal-organic framework (MOF), which is pyrolyzed at high temperature to obtain single-atom catalysts with high specific surface area^[60⇓~62]. The second type is to pyrolyze the mixture of metal salts and carbon-nitrogen precursors at high temperature to obtain atomically dispersed catalysts^[63,64]. During the pyrolysis process, the migration and agglomeration of metal species can be controlled by adjusting the temperature, heating rate and gas flow rate. Pyrolytically ordered MOF-based precursors can precisely control the size, metal coordination number, dispersion tendency, and metal atom bonding form of the derived carbon support. In contrast, the second type of high temperature pyrolysis has more metal sources, carbon sources and heteroatom sources to choose from, but it is difficult to accurately control the morphology and atomic structure of the product catalyst.

During the high temperature catalytic process, the small size metal nanoparticles have a tendency to aggregate to form large size particles due to their high surface free energy, which leads to the loss of catalyst activity. However, if the migrating metal atoms are captured by the support in this process and form a strong metal-support interaction, the surface energy of the metal atoms can be effectively reduced, and the tendency of the surface small nanoparticles to migrate to form large particles can be slowed down. Taking advantage of this phenomenon, the high temperature atom migration trapping method has gradually developed into an effective method for the preparation of single-atom catalysts^{[65⇓⇓~68]}. Nanoparticles can migrate through space and then be captured by the carrier, anchoring individual atoms to the target carrier; Cheap and readily available bulk metals can also be converted into single atoms by this method, but in the actual process, bulk metals are relatively difficult to volatilize into corresponding atoms and migrate to the carrier, and there are not many successful examples^[69,70]. Due to the particularity of the preparation method, the single-atom catalyst prepared by the high-temperature atom migration trapping method has high thermal stability. In addition, the conversion of nanoparticles into single atoms is of great potential significance in industry for the regeneration and reuse of sintered nanometal catalysts.

3.2 Characterization of single-atom catalyst

In terms of catalyst characterization, the research paradigm of single-atom catalyst characterization was determined when the concept of "single-atom catalysis" was proposed. It is found that the coordination structure and electronic properties of the monoatomic active center dynamically evolve with the adsorption, transformation and desorption of the reaction species on the monoatomic active center during the reaction process. Monitoring the dynamic change of the coordination environment of the monatomic center in the chemical reaction state is the key to realize the precise construction of the catalytic center at the atomic scale and the precise understanding of the catalytic mechanism. The development of the theory and practice of single-atom catalysis is strongly dependent on the development of relevant characterization techniques. In recent years, a variety of ex situ, in situ and working condition characterization methods have emerged, which provide technical support for the characterization of the whole catalytic process from multiple perspectives, and the comprehensive understanding of the structure and properties of catalysts, especially the behavior under real reaction conditions^[72⇓~74]. In this paper, the characteristics of various characterization methods and the structural information obtained are systematically summarized, as shown in Table 2.

表2 单原子催化剂表征方法

Table 2 Characterization methods for SACs

	表征方法	简写	特点	结构信息
1	透射电子显微镜	TEM	直观、可视性;检测区域具有限制性,无法反映样品的整体信息	催化剂原子尺度信息
2	扫描透射显微镜	STEM	通过机械操作导电尖端,记录隧穿电流,对表面原子位置进行常规成像	催化剂原子尺度信息
3	X 射线光电子能谱	XPS	表面信息	揭示单原子催化剂表面化学组成和原子价态信息
4	红外光谱技术	IR	仪器和操作简单;能够方便、快速且经济地提供位点特异性信息	催化剂金属原子分散性质,推断出活性中心及其局部结构特征
5	X射线吸收光谱	XAS	分辨率高、可在原位条件下操作	提供高灵敏度的宏观平均结构特征和配位信息
6	电子自旋共振	EPR	用于探测含有未配对电子的顺磁性物种	可提供顺磁中心的性质:对称性、电子结构、价态变化以及与反应物的相互作用等
7	核磁共振	NMR	确定金属原子的锚定位点、跟踪有机金属前驱体的吸附情况	提供单原子催化剂的结构信息
8	低能离子散射谱	LEIS	对被测元素最外层原子敏感	有助于定性分析目标原子表面分布,或进一步对其浓度定量

Through the combination of AC-STEM, probe molecule adsorption in situ infrared spectroscopy (IR) and XAS characterization techniques, the researchers have explored the dispersion nature, chemical state and coordination structure of metal atoms in single-atom catalysts. AC-STEM is an integrated electron microscopy technology, which can directly observe metal active sites at the atomic scale, but inevitably the detection area is limited and can not reflect the overall information of the sample. This deficiency is well compensated by XAS, which provides macroscopically averaged structural features and coordination information with high sensitivity. Because most metal oxides have excellent infrared photoresponse performance, and the IR technique is very sensitive to surface probe molecules, the instrument and operation requirements are simple, so it is one of the most effective tools for the structural study of supported single-atom catalysts. In addition, scanning tunneling microscopy (STM), nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), low-energy ion scattering spectroscopy (LEIS) and X-ray photoelectron spectroscopy (XPS) have also played an important role in the study and understanding of the structure-activity relationship of single-atom catalysts^[75,76].

3.3 Application of single-atom catalyst

In terms of catalyst application, single-atom catalysts have shown unique catalytic properties and potential application prospects in many important industrial reactions^[17]. In recent years, the application of single-atom catalysts in traditional homogeneous catalytic reactions such as hydroformylation, carbonylation, Suzuki coupling and hydrosilylation has been reported one after another, which verifies the prediction that single-atom catalysis is expected to be homogeneous catalysis from the perspective of basic research^[77,78]^[79]^[80]^[81,82]^[17]. Especially in 2020, Ding Yunjie, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, independently developed a heterogeneous single-atom catalyst for ethylene hydroformylation and hydrogenation technology to produce n-propanol industrial plant, which achieved a successful start-up and proved the great potential of single-atom catalyst in the field of traditional homogeneous catalysis from the perspective of industrial application. In addition, single-atom catalysts have a wide range of applications in the field of photocatalysis and electrocatalysis (such as electrochemical oxygen reduction reaction, electrochemical nitrogen fixation, Photoelectrochemical CO₂ reduction, etc.) Because of their high atom utilization and controllable aggregate structural units. For details, please refer to the related review articles^[36].

4 Condensation effect of metal center and coordination atom

An aggregate formed by a metal center and a coordinating atom with an exact structure. Its basic coordination configurations are mainly (Fig. 4): regular tetrahedron, plane quadrilateral, and regular octahedron^[83]. In this paper, taking metal oxides, carbon nitrogen materials and monatomic alloys as examples, the interaction forms between the metal center atom and the above three supports were discussed, and the influence of the condensation effect between the metal center and the surrounding coordinating atoms in the monatomic catalyst on the structure and performance of the catalyst was described in detail.

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图4 单原子催化剂聚集体构型(a)正四面体(b)平面四边形(c和d)正八面体

Fig.4 Aggregate configurations of SACs (a) tetrahedron, (b) Plane quadrilateral, (c) and (d) regular octahedron

4.1 Metal-support interaction form

The interaction between metal and support in supported catalysts has an important influence on the structure and performance of catalysts. The support microenvironment of single-atom catalyst, such as defect sites, surface functional groups (O or OH), pore structure and lattice, interacts with the metal center to form an aggregate with a defined structure. With the charge transfer between the support and the metal, the difference of the surrounding coordination environment affects the structure and properties of the aggregate.

4.1.1 Defect site anchoring

It has been shown that single-atom sites can be anchored by defect sites through a charge transfer mechanism. However, the oxide support has a limited concentration of defect sites, which can limit the preparation of high-loading single-atom catalysts. Therefore, it is necessary to construct abundant defect sites to anchor single atoms by improving the preparation process and post-treatment conditions of the support. The common defect sites of oxide support are metal vacancy (M_v), oxygen vacancy (O_v), boundary, step and dislocation, which can trap metal atoms, so it is difficult to distinguish the precise trapping sites. In recent years, with the development of advanced characterization techniques and theoretical calculations, there has been a deeper understanding of the identification of single-atom sites. Depending on the different chemical properties of the oxides, the stable forms of the metal atoms are diverse (see Table 3 for details). Generally, for non-reducing supports (such as Al₂O₃ and SiO₂), the interaction with metal species is weak due to the lack of surface defects. However, there are a large number of six-coordinate Al³⁺ unsaturated sites on the surface of Al₂O₃ support, which is beneficial to stabilize the noble metal atoms with high loading. Conversely, on reducible supports, M_v and O_v are common metal anchor sites.

表3 金属单原子(M)在载体(Sup)上锚定位置列表^[19]


M	CeO₂	FeO_x	Al₂O₃	TiO₂	WO_x	MgO	ZnO	MnO₂
Co		D,M_v
Ni		M_v
Cu	O_v			M_v
Ru			Al³⁺					M_v
Pd			D	D		D,O_v
Ag								M_v
Pt	D,O_v	O_v,M_v	Al³⁺	D,O_v, M_v,Ti³⁺	O_v		M_v
Au	O_v, M_v	M_v		O_v			M_v	M_v
Mo		M_v

D: defect. O_v: oxygen vacancy. M_v: metal vacancy.

Atomic resolution imaging and X-ray absorption fine structure (EXAFS) characterization techniques are important tools to study the coordination structure of single-atom active sites^{[17⇓⇓~20,84⇓⇓~87]}. For example, when a small amount of Pt precursor was deposited on the Fe₂O₃, it could be identified by spherical aberration corrected scanning transmission electron microscopy that Pt occupied the Fe sites on the surface of the support, proving that Pt was trapped by Fe vacancies. Similarly, Mo^5+/6+ and W^5+/6+, which have an ionic radius of roughly 0.66 Å, can be equally precisely anchored at the Fe site^[88]. When the metal ion is captured by the metal vacancy, it will coordinate with the surrounding oxygen species to form a MO_n aggregate with catalytic activity. The average M-O coordination number can be fitted by EXAFS curve, and sometimes different configurations of monatomic metal sites may be obtained. For example, when Pt atoms are anchored at the TiO₂ nanowire, density functional theory (DFT) calculations show that Pt is mainly located at the Ti vacancy, forming a stable PtO_n configuration. However, the coordination number of the exact O ion is uncertain. When the coordination number is 6, the configuration is Pt⁴⁺ in the fully oxidized state, with six Pt — O bonds and one O atom directly above Pt (Pt—O_top); However, the Pt—O_top bond is easily broken, and the Pt⁴⁺ is converted into a 5-coordinated Pt²⁺. X-ray absorption near edge structure (EANES) and XPS analysis show that Pt in the oxidized state has a valence between 2 + and 4 +, indicating that both Pt⁴⁺ and Pt²⁺ configurations exist simultaneously^[89].

In addition, EPR technique can be used to identify the interaction between single atoms and oxygen vacancies on oxide supports (WO₃ and TiO₂). For example, when Pt⁴⁺ is deposited on the WO₃ support, the O_v signal (G = 2.005) of the defect state WO₃ will decrease slightly, indicating that the unpaired electrons of O_v will form a static electron adsorption interaction with Pt atoms^[90]. The reduction of TiO₂ by H₂ atmosphere will form a large number of O_v.Some Ti⁴⁺ are reduced to Ti³⁺ by unpaired electrons from neighboring O_v. Therefore, Ti³⁺-O_v(g=1.93) is considered to be the preferred site for single-atom anchoring^[91]. The Au atom is anchored at the O_v of the defect state TiO₂, forming a three-center Ti-Au-Ti structure^[92,93]. Researchers believe that this metal-support interaction formed by oxygen vacancies is beneficial to activate catalytic sites, and its catalytic activity is better than that of single-atom catalysts formed by oxide supports without defects.

4.1.2 Interaction between metal center and O (OH)

On the surface of the oxide support, the excess oxygen ion can coordinate with the transition metal through M-O (OH) interaction. The above process is common and is used to explain the thermal stability of metal clusters on oxyhydroxide-rich supports against sintering. For example, the hydroxyl-rich TiO₂(110) facet loaded Au clusters do not form larger Au clusters due to agglomeration compared with the O_v containing surface^[94]. The migration of supported metal atoms can be effectively reduced by increasing the concentration of hydroxyl groups on the oxide surface. First, nanoscale support materials are selected. For example, at least two hydroxyl groups are required for each Ag atom to form a stable structure. Therefore, compared with micro γ-Al₂O₃, the surface hydroxyl concentration is limited, and the dispersion of Ag is much larger than that of nano γ-Al₂O₃^[95]. Secondly, the addition of alkali metal ions (such as Na⁺, K⁺, etc.) to form oxygen-coordinated species (M₁-O_x) is beneficial for noble metal species to achieve atomic-scale dispersion. The method is equally applicable to reducible and non-reducible supports^[21]. In the catalytic hydrogenation reaction, when H₂ is heterolytically cleaved at the metal-oxide interface to form hydride (M-H^δ-) and proton (O-H^δ+) species, the interface structure may be destroyed by hydrogen species. However, Zheng et al. Reported that the strong Na⁺…H^δ- interaction can effectively inhibit the transfer of hydrogen to the adjacent oxygen, and then prevent the sintering of isolated Ru³⁺ species in Ru/Al₂O₃ single atoms. Therefore, the oxide-type single-atom catalyst rich in alkali metal ions can maintain high catalytic activity and intrinsic structure under reductive reaction conditions^[96]. In addition, steam treatment of CeO₂ can also produce hydroxyl groups on the surface. Under certain high temperature conditions, metal nanoparticles can become isolated metal species. The hydroxyl group formed in situ on the oxide surface can form a strong interaction with the metal atom to stabilize the metal to form a single-atom catalyst^[97]. The stabilization effect of surface oxygen species can be used to prepare high-loading single-atom catalysts, and can maintain the durability of isolated metal sites. At the same time, regulating the concentration of surface hydroxyl groups can change the catalytic reaction path and improve the catalytic performance. For example, in the CO oxidation reaction, the addition of water to the reaction gas can effectively improve the reactivity of the Au₁/CeO₂ single-atom catalyst^[98].

4.1.3 Spatial confinement effect

Confinement of isolated metal species within the pores of nano-oxide supports, such as Fe₂O₃,

W O x

, and

T i O 2

and industrially widely used molecular sieves, is another strategy to keep metal species highly dispersed by preventing aggregation^[99]^[100]^[101]^[102⇓~104]. The most commonly used preparation method is to capture the metal precursor directly on the support. For example, 12CaO·7Al₂O₃(C₁₂A₇) has a unique interconnected cage structure, each cage exhibiting a + 1/3 valence and an inner diameter of 0.4 nm. With matching size and positive charge, isolated metal ions are confined within C₁₂A₇ nanocavities. Pt atoms are very stable in the C₁₂A₇ even if the reduction temperature is up to 600 ℃^[105]. In addition, the Pt single-atom catalyst prepared by the colloidal method can effectively inhibit the sintering of Pt, so it has good catalytic cycle stability^[106]. On the contrary, the activity of Pt catalyst prepared by commercial Al₂O₃ decreased gradually. Mesoporous oxides can effectively improve the metal-support interaction and catalytic activity by virtue of the confinement effect, even under harsh reduction conditions.

4.1.4 Metal-support strong interaction (SMSI).

Strong metal-support interaction (SMSI) means that the adsorption ability of oxide-supported small nanoparticles to small molecules will change after high temperature reduction/oxidation treatment. However, whether SMSI occurs with single-atom catalysts is not known. Recently, our research group used the photodeposition method to deposit 1 wt% Pt on the TiO₂ nanosheets, and the Pt/TiO₂ single-atom catalyst was prepared by calcination at 350 ° C to remove the protective agent. After reduction at 250 ° C, the linear and bridged CO adsorption peaks on the Pt nanoparticles completely disappeared, which indicated that the Pt nanoparticles underwent SMSI (Fig. 5)^[107]. However, the SMSI of Pt single atoms occurs only at a higher reduction temperature (600 ℃), and the CO adsorption on Pt nanoparticles and Pt single atoms can be restored after oxidation at 300 ℃, which proves the reversibility of the SMSI adsorption characteristics on noble metal species. When reduced at 500 ℃, the CO—Pt₁ bond still exists, and the absorption peak is red-shifted to the 2080 cm^-1, indicating that there is a charge transfer from the carrier TiO₂ to the metal Pt. The CO adsorption on Pt atom is suppressed under high temperature reduction conditions, which is due to the 18-electron saturated coordination configuration of Pt at this time, combined with theoretical calculations. In the optimal structure, Pt atoms are in the outermost layer of the support, and the SMSI of Pt atoms is mainly derived from the strong electronic effect, which is completely different from the encapsulation of Pt nanoparticles by TiO₂ under reducing conditions. The above differences were confirmed by micro-area electrochemical impedance spectroscopy (LEIS), and the signal of Pt decreased significantly after reduction at 250 ℃, indicating the intercalation of Pt nanoparticles. However, the signal of Pt does not decrease after reduction at 600 ℃, which is consistent with the calculation result that the single atom of Pt will not be encapsulated by the carrier.

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图5 Pt₁/TiO₂催化剂原位CO-DRIFT谱图^[107]

4.1.5 Covalent metal-support interaction (CMSI)

In the traditional supported Au catalyst, the nano-Au particles are easy to sinter in the reaction; The Au₁/FeO_x single-atom catalyst is more stable and maintains good thermal stability for CO oxidation at 400 ℃. DFT calculations show that the Au single atom with positive core charges interacts with the oxygen species on the surface of iron oxides, and the Au — O bond is more covalent. Therefore, the interaction between Au single atom and FeO_x support is defined as Covalent metal-support interaction (CMSI)^[108]. Iron oxide-supported Pt, Ir, and Ru single-atom catalysts also exhibit CMSI, and the effect is more pronounced after high temperature treatment (fig. 6)^[109]^[110]^[111]. PtO₂ can be obtained from Pt nanoparticles after calcination at 800 ℃, and reducible iron oxides can capture Pt atoms and form Pt — O — Fe covalent bonds. However, only larger Pt nanoparticles can be obtained by non-reducible Al₂O₃. In addition, CMSI can be used to prepare high-loading, thermally stable single-atom catalysts on reducible supports, such as CeO₂, NiO, and MnO_x. For example, an Ir/NiO single-atom catalyst with 18 wt% loading can be prepared by the formation of an Ir — O covalent bond^[112]. Metal sulfides and carbon materials also have the property of covalent bonding with metal centers^[113,114]^[115⇓~117]. For example, a Pt atom is anchored to the Mo vacancy of MXene by forming a Pt — C bond with the support^[118].

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图6 Pt在不同载体上的烧结与分散^[109]

4.1.6 Electron metal-support interaction (EMSI).

Electronic metal-support interaction (EMSI), defined as the Electronic perturbation at the interface between Pt cluster and support, can significantly regulate the electron density of d-band center of active site and tune the electronic state structure of isolated metal center when Pt cluster is dispersed in CeO₂ support^[119]. The direction of charge transfer is closely related to the distribution of charge. When a single atom is located at a defect site, electrons will be transferred from the carrier to the metal atom. For example, the surface of Mg-Al hydrotalcite support has abundant — OH groups, which can improve the electronegativity of metal atoms^[120⇓~122]. When the isolated metal species is anchored to the reducible oxide support through M — O bonds, electrons can be transferred from the metal to the support through the hybridization of the O (2p) orbital with the metal (d/p) orbital.

4.2 Coordination atom modulates the structure of aggregate

In nano and cluster catalysts, multi-metal atoms interact with surrounding coordination atoms to form a metal support interface; However, the active metal center of monatomic catalyst is an aggregate with a certain structure formed by the interaction of a single atom with the surrounding coordinating atoms. Therefore, compared with nano and cluster catalysts, the geometric and electronic structures of single-atom catalysts have obvious differences, which are summarized as follows^[24].

4.2.1 Coordination atom modulates the geometry of the aggregate

Compared with traditional heterogeneous catalysts, single-atom catalysts not only have significantly improved dispersion and maximum utilization efficiency of metal atoms, but also change the adsorption mode and adsorption configuration of reactant molecules when isolated single-metal atom centers interact with coordinating atoms. Metal nanocatalysts or cluster catalysts contain metal-metal bonds, and reactant molecules can be simultaneously adsorbed on multiple metal atoms for activation. Monatomic catalysts, on the other hand, consist of atomically dispersed metal centers, with the active metal coordinated to the surrounding atoms without metal-metal bonds (Figure 7 a), and the metal atoms bond to the support surface through hetero-atoms to form aggregates with exact structures, resulting in changes in the configuration of the adsorbed molecules, thereby changing the activity and/or selectivity of the reaction. For example, ethylene is preferentially adsorbed on polyatomic active sites in the form of δ-adsorption with greater adsorption strength, but only on monatomic active sites in the form of π-adsorption with weaker adsorption strength, resulting in a huge difference in acetylene hydrogenation selectivity.

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图7 纳米颗粒、团簇和单原子的(a)几何结构与(b)电子结构^[24]

4.2.2 Electronic structure of aggregates modulated by coordinating atoms

In addition to active site geometry differences, single-atom catalysts also bring about changes in electronic effects. In bulk metals, the distribution of electronic energy levels is continuous. When the size is small to a certain extent (such as less than tens of nanometers), the energy level splitting begins to appear, resulting in quantum effects. When the metal is as small as a single atom, the energy levels are more quantized (Fig. 7 B). However, in single-atom catalysts, the metal atoms are deposited on a certain support, and there is an interaction between the metal atoms and the support, which can change the electronic properties of the single atom, thereby modulating the catalytic activity and selectivity. Therefore, the catalytic performance of single-atom catalysts can be adjusted by the modulation of the support and the microenvironment around a single metal atom. In addition, in many catalytic reactions, the interface between nanoparticles and carriers is considered to be the real active center. In single-atom catalysts, there is no such interface, or to some extent, the single atom itself is the interface atom in traditional nanocatalysts.

In addition, the electronic structure of the aggregate can be significantly changed by controlling the crystal form of the support, which can affect its catalytic performance. Hong et al. Compared the nitro/vinyl selective hydrogenation performance of crystalline TiO₂ and amorphous TiO₂ supported Ru single-atom catalysts^[123]. The results show that the former mainly occurs in the hydrogenation of vinyl site, while the latter mainly occurs in the hydrogenation of nitro site. It was found that compared with the crystal phase support, the metal center Ru of the amorphous TiO₂ support is more electropositive when it is coordinated with a single metal atom, and the adsorption configuration of the reaction molecule will be reversed, thus showing the selective hydrogenation of different sites. In addition, they found that amorphous FeO_x can reduce the energy barrier of photogenerated charge transfer, promote the transfer of electrons from carrier FeO_x to metal Ru, and effectively regulate the density of electronic States around Ru, thus showing excellent catalytic performance^[124].

4.3 Effect of Metal Aggregation on Catalytic Performance

The active metal in the supported catalyst exists in the form of single atom, cluster or nanoparticle, and the size effect has an important influence on the catalytic activity and selectivity. The above three forms of metal aggregates can not only participate in chemical reactions as active centers alone, but also achieve synergistic catalysis. In terms of geometric structure, with the decrease of metal size, the low-coordinated atoms are gradually exposed and the proportion is gradually increased, which significantly changes the structure and proportion of active centers of catalytic materials. From the point of view of electronic structure, the electronic energy level of metal is also significantly changed by quantum size effect, which greatly affects the orbital hybridization and charge transfer between catalytic materials and reactants.

4.3.1 Catalytic properties of aggregates such as single atoms, clusters and nanoparticles

Guo et al. compared the CO₂ methanation activity of CeO₂ supported Ru single atom, sub-nanometer cluster (about 1.2 nm) and nanoparticle (about 4 nm), and found that the sub-nanometer Ru cluster showed the best low-temperature activity and selectivity (Fig. 8)^[125]. Further study showed that the metal carbonyl formed by metal activation of CO₂ was the key intermediate in the reaction process, and the carbonyl hydrogenation reaction was also the key step, and the corresponding activation and reaction sites were Ce³⁺-OH and Ru near the interface, respectively. On Ru single atoms and larger Ru nanoparticles, too strong interfacial charge transport in the former is not conducive to carbonyl activation, and too strong hydrogen flooding in the latter is not conducive to the carbonyl hydrogenation step, while on Ru sub-nanoclusters, the two competing effects reach a balance, thus having the optimal low temperature performance. This study confirms that sub-nanometer clusters have more unique reaction properties than single atoms and nanocatalysts.

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图8 CeO₂负载的Ru单原子、团簇以及纳米颗粒的甲烷化活性对比^[125]

In 2022, Tan Li et al. Of Fuzhou University reported a Cu/ZrO₂ catalyst with Cu isolated active sites for CO₂ hydrogenation to methanol^[126]. The results show that the single-atom Cu-Zr catalyst with Cu₁-O₃structure can synthesize methanol singly at about 180 ℃, while the Cu clusters or nanoparticles with Cu-Cu structure tend to form CO by-products. In addition, it was observed that the Cu₁-O₃ unit with quasi-planar structure would gradually migrate to the surface of the catalyst during the catalytic process, which promoted the hydrogenation process of CO₂.

Tianjin University Liu Changjun et al. Reported the hydrogenation of CO₂ to methanol over indium oxide-supported rhenium catalyst (Re/In₂O₃) (Fig. 9)^[127]. Studies have shown that the hydrogenation activity of Re/In₂O₃ catalysts is highly dependent on the nanostructure and particle size of the catalyst. When the loading of Re is low, the (≤1 wt%),Re/In₂O₃ catalyst has obvious promotion effect on methanol synthesis. The methanol selectivity at 300 ° C exceeds 70%, and the space-time yield of methanol also reaches 0.54 g_MeOH·

g c a t - 1

·h^-1. However, the selectivity of methane in the products of CO₂ hydrogenation increased sharply when the loading of Re was further increased.

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图9 In₂O₃负载的Re单原子、团簇以及纳米颗粒的CO₂加氢产物选择性^[127]

The characterization results show that when the Re loading is low, the Re species is mainly doped in the lattice of In₂O₃ in the form of single atom. When the Re loading is higher, Re is loaded on the surface of In₂O₃ in the form of larger clusters or particles. The DFT calculation results further confirmed that the size effect of the supported Re particles would induce changes in the electronic structure of the Re/In₂O₃ catalyst, thereby affecting the selectivity of the final product.

When the metal single atom and the nanoparticle coexist, the selectively encapsulated nanoparticle only exposes the single atom as the active center, which can realize the selective regulation of the product. Semi-hydrogenation of acetylene is an important chemical process, which is of great significance in both basic research and industrial applications. Pd single-atom catalysts are often used in acetylene semi-hydrogenation because of their advantages such as high atom utilization, isolated and homogeneous active sites. It was found that the product ethylene was adsorbed by a weak π-bond, which could significantly improve the reaction selectivity. However, the reactivity needs to be improved due to the poor ability of the monoatomic active center to crack H₂. In addition, it is still a challenge to prepare a stable Pd single-atom catalyst without agglomeration in a reducing atmosphere. Guo et al. Constructed Pd₁/TiO₂ single-atom catalyst by controlling the conditions of strong metal-support interaction between single atoms and nanoparticles to expose single atoms while coating nanoparticles, which can significantly improve the selectivity of acetylene semi-hydrogenation reaction (Fig. 10)^[128]. Hydrogenation of CO₂ into fuels and chemicals is an important strategy to achieve CO₂ emission reduction and sustainable utilization of carbon resources. At atmospheric pressure, CO₂ hydrogenation mainly produces CO and CH₄, but these two products are usually concomitant, and how to produce a single product with high selectivity is still a challenge. Han et al. Used the Rh/TiO₂ catalyst prepared by incipient wetness impregnation method, in which Rh single atoms and nanoparticles coexist^[129]. According to the difference of the conditions of strong metal-support interaction between single atoms and nanoparticles, the catalyst was reduced/oxidized at different temperatures, and the products of atmospheric CO₂ hydrogenation could be reversibly switched between CO and CH₄.

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图10 包裹纳米颗粒暴露单原子调控加氢产物选择性。乙炔半加氢反应中不同还原温度处理的催化剂(Pd/TiO₂-H200,Pd单原子和Pd纳米颗粒共存;Pd/TiO₂-H600,Pd纳米颗粒被包裹,Pd单原子为活性中心)的(a)C₂H₂转化率和(b)C₂H₄选择性^[128]

Fig.10 Encapsulation nanoparticles while exposing single atoms to regulate hydrogenation product selectivity. Semi-hydrogenation of acetylene (a) The C₂H₂ conversion and (b) C₂H₄ selectivity of catalysts treated at different reduction temperatures (Pd/TiO₂-H200: contains Pd single-atoms and Pd nanoparticles; Pd/TiO₂-H600: Pd nanoparticles are encapsulated while exposing Pd single-atoms as the active center)^[128]. Copyright 2022, Springer

4.3.2 Cooperative catalysis between aggregates

In addition, metal single atoms and nanoparticles can achieve synergistic catalysis. The selective photoreduction of CO₂ into carbon-neutral fuels (e.g., CH₄) is of great significance, however, it remains extremely challenging due to the slow multiple proton-electron coupled transfer and various C1 intermediates involved. In 2022, Zheng et al. Of China University of Science and Technology constructed a synergistic Pd₁ and Pd_NPs bifunctional site on the C₃N₄ for the photoreduction of CO₂ to CH₄ in pure water.A high selectivity of 97.8% and a yield of 20.3 μmol ·

g c a t - 1

·h^-1 were achieved^[130]. In situ diffuse reflectance infrared Fourier transform spectroscopy and near atmospheric pressure X-ray photoelectron spectroscopy showed that the Pd₁ in the catalyst was the active site for CO₂ activation and reduction. At the same time, Pd_NPs promotes the dissociation of H₂O and increases the H^* coverage. The Pd_NPs generated H^* migrates to the Pd₁ site via hydrogen spillover, promoting the photocatalytic CO₂ methanation electron-proton coupling kinetics while suppressing the competing hydrogen evolution side reaction. DFT calculations further show that the adjacent Pd₁ and Pd_NPs lower the barrier from ^*CO to ^*CHO, stabilizing the intermediate species such as ^*CHO. Yang et al. Reported the research work on the cooperative catalysis of iridium single atoms and nanoparticles for the decomposition reaction of N₂O^[131]. The Ir single-atom catalyst prepared by the high-temperature trapping method has low catalytic activity in the decomposition reaction of nitrogen oxides, but after continuing to support the nanoparticles, the single-atom and the nanoparticles show significant synergistic catalytic effect. X-ray photoelectron spectroscopy and in situ diffuse reflectance infrared spectroscopy characterization of CO adsorption combined with reaction kinetics analysis revealed that the active center of the reaction was metallic Ir nanoparticles. Although the oxidized Ir single atom can not directly activate N₂O molecules, it can change the electronic structure and adsorption properties of Ir nanoparticles, promote the desorption of O₂ from Ir nanoparticles, and thus improve the reactivity of the catalyst.

The structure and properties of the aggregates formed by metal oxide supported single atoms are mainly affected by the coordination between the metal center and the surrounding oxygen atoms. In addition to oxygen atoms, the metal center can also form aggregates with other non-metal atoms through coordination or with a second metal through "metal-metal bonding" (such as carbon-nitrogen supported single-atom catalysts and single-atom alloy catalysts).The aggregate has unique geometric structure and electronic properties, and has important applications in heterogeneous catalysis and other fields.

Carbon nitrogen supported single-atom catalysts (M-N-C) have been widely used in electrocatalysis for O₂ reduction (ORR), H₂O decomposition, CO₂ reduction, and N₂ reduction. Li Yadong et al. Constructed a class of Co-N-C single-atom catalysts with 4 wt% Co loading by pyrolysis of Co/Zn metal-organic frameworks, and their ORR performance was superior to that of commercial Pt/C catalysts^[132]. As shown in Fig. 11, Co-N-C maintains the original rhombic dodecahedron structure and Co is completely monodispersed after high temperature calcination, and the coordination structure of the aggregate metal center Co is confirmed to be Co-N₄ by EXAFS. In addition, they developed a series of M-N-C single-atom catalysts (M = Pt, Ir, Pd, Ru, Mo, Ga, Cu, Ni, Mn) for formic acid oxidation with the help of host-guest strategy^[133]. Compared with the nanoparticle catalyst, Ir-M-C has stronger resistance to CO poisoning, and its excellent catalytic performance comes from the spatially separated Ir sites and the modulated electronic structure. The single-atom FeN₄ site in the Fe-N-C structure is considered to be the most active non-Pt-family catalyst material for the study of the ORR reaction in PEMFC. However, the durability of the existing Fe-N-C catalyst is poor, and it does not have practical application prospects. Wu et al. Designed and developed a class of Fe-N-C with good durability, and found that the main reason for their excellent catalytic performance was that the FeN₄ with rich defect pyrrole N coordination would be transformed into the FeN₄ with pyridine N coordination^[134].

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图11 Co-N-C单原子催化剂形貌、组成及结构分析^[132]

Single-atom alloy (SAA) catalyst refers to an aggregate formed by one atomically dispersed metal through "metal-metal bonding" of another metal in the whole catalyst^{[135⇓⇓~138]}. The aggregate combines the traditional advantages of alloy catalysts with the performance customization of single-atom catalysts, and the catalytic properties of its active sites are caused by the strong metal interaction formed between isolated single atoms and metals at the nanoscale. At present, the research on this kind of materials mainly focuses on alloying a small amount of group 8 ~ 10 transition metals (such as Pd, Pt, Rh, Ni, Ru, etc.) onto the surface of group 11 metals (Cu, Ag, Au). HAADF-STEM can clearly distinguish the single atom sites in the SAA aggregate. As shown in Fig. 12, the interplanar spacing of 0.21 nm corresponds to the Cu (111) crystal plane, and the white bright spot indicated by the red arrow is the Pt single atom, which indicates that Pt is embedded into the Cu lattice and is completely dispersed on the Cu nanoparticles^[139].

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图12 SAA表征方法:(a,b)Pt₁Cu SAA催化剂的 HAADF-STEM图像,Pt₁Sn SAA催化剂的(c)Pt-L3 edge XANES 和 (d) Pt 4f XPS谱^[139]

The electronic structure of single atoms in SAA aggregates is affected by the second metal and is complex, which requires a combination of (in situ) characterization methods and theoretical calculations to reveal their electronic properties. Kim et al. Studied the electronic properties of Pt atoms in Pt₁Sn monatomic alloy by means of XANES and XPS. The white line shifts to high energy and the Pt 4F XPS peak shifts to high binding energy, which indicates that electron transfer occurs between Pt and the support^[140]. The unique electronic structure of this kind of aggregate determines its excellent performance in a variety of catalytic reactions, such as Pt₁Au for formic acid oxidation, Au₁Pd for glucose oxidation, Pt₁Cu for selective hydrogenation, Ni₁Cu and Pd₁Cu for selective dehydrogenation. In addition, compared with the bulk metal, the single-atom dispersion of metal leads to the narrowing of its valence band, which significantly affects the adsorption behavior of SAA aggregates. In the reaction system of methanol reforming catalyzed by Ag₁Cu, the activation energy of Ag₁Cu single-atom alloy is lower than that of bulk Cu, which is due to the narrowing of metal d-band, which can enhance the interaction strength with hydrogen adsorbate in methoxy group, thus increasing its catalytic activity^[141].

5 Dynamic evolution and characterization of aggregates under reaction conditions

The properties and functions of catalysts are closely related to their multi-level condensed structure. Many heterogeneous chemical reactions involve the catalytic conversion of gas molecules on the surface of solid catalysts during heating, and the condensed structure of catalysts will change. In the reaction process, the reactant molecules will adsorb, transform and desorb on the monatomic active center, and the coordination structure and electronic properties of the aggregate formed by the monatomic active center and the surrounding atoms will dynamically evolve accordingly, thus affecting the overall chemical reaction and its results. Monitoring the dynamic change of the coordination environment of the monatomic center in the chemical reaction state is the key to realize the precise construction of the catalytic center at the atomic scale and the precise understanding of the catalytic mechanism. To develop new characterization methods with high sensitivity, high spatial resolution, high energy resolution and high time resolution, not only to statically characterize single atomic sites in catalysts,It is also necessary to obtain the aggregate structure of the surrounding environment of the single atom (such as ligands, coordinating atoms in different layers, etc.) And its synergistic effect and interaction with the single atom center, and to investigate the dynamic changes of the single atom catalyst in the reaction.

In recent years, synchrotron radiation X-ray spectroscopy (SRXS), including X-ray absorption fine structure (XAFS) and X-ray emission spectroscopy (XES), can provide information on the local atomic and electronic structure of catalytic materials, especially XAFS has become an indispensable characterization tool for single-atom structure analysis. Molecular spectroscopy such as (enhanced) infrared/Raman spectroscopy and solid-state nuclear magnetic resonance spectroscopy can provide fingerprint structural information of the substance to be determined, and play an important role in non-destructive in-situ study of catalyst surface structure and reaction process, especially in characterizing the molecular structural information of key reaction intermediate species. In addition, environmental transmission electron microscopy (ETEM) with atomic resolution is widely used to visualize the gas-solid catalytic reaction process and monitor the dynamic change of catalyst structure under real reaction conditions^[142]. In this paper, nitrogen-carbon materials and metal oxide materials supported single-atom catalysts are mainly used to illustrate the effect of aggregate structure change on catalytic activity under reaction conditions.

Metal ion doping into porous nitrogen-doped carbon (M-N-C), also known as M-N-C single-atom catalysts, is a promising class of electrocatalysts as an alternative to precious metal-related important transformations (e.g., reduction reactions of oxygen and carbon dioxide) that are scarce and unaffordable in various energy sources. The high activity of M-N-C is generally attributed to the M-N_x site, but its exact structure remains elusive. In particular, under operating conditions, prefabricated M-N_x may undergo structural transformations driven by the applied potential and/or the interaction with reactants or electrolytes, which not only complicates the understanding of structure-property relationships, but also seriously hinders the rational design of efficient catalyst structures. Therefore, revealing the dynamic evolution of the M-N_xstructure during electrolysis is essential for identifying the true active site. During alkaline HER reaction, in situ XAS detected high oxidation state HO-Co₁-N₂ on graphitic phase carbon nitride (g-C₃N₄) substrate at open circuit state; When the voltage is − 0.4 V, H₂O-(OH-Co₁-N₂) is the dominant species^[143]. In addition, the configuration of Ru₁-N₄ anchored at g-C₃N₄ is O-Ru₁-N₄^[144]. Theoretical studies have shown that excess ^*OH or ^*O species can reduce the overpotential and then improve the catalytic activity. In the photocatalytic reaction, XPS was used to characterize the Pt — N bond cleavage and C = N bond reconstruction, confirming the presence of Pt⁰ in the g-C₃N₄ supported Pt₁ single atom^[145]. In situ EXAFS showed that the Pt coordination number decreased in the Pt₁ single-atom catalyst supported on nitrogencarbon materials during the alkaline electrochemical reaction, confirming the dynamic near-free state of single atoms.

Yang et al. Designed and constructed a uniformly dispersed copper single-atom catalyst (Cu-N-C SAC) on a nitrogen-carbon support, applied it to alkaline electrocatalytic oxygen reduction reaction, and revealed its reversible dynamic evolution during the reaction by in situ X-ray absorption spectroscopy (Operando-XAS)^[146]. The "structure-activity relationship" based on static structure analysis does not fully reflect the real catalytic mechanism in the reaction process. Based on this, a uniformly dispersed static Cu-N₄ single-atom catalyst was constructed, which showed comparable catalytic activity to noble metal Pt in the electrocatalytic alkaline oxygen reduction reaction. XANES, which is sensitive to the coordination environment of the metal center, was used to observe the reversible structural evolution of the static Cu-N₄ site during the reaction. Further through the FDMNES calculation method, it was determined that Cu-N₄ was first driven by the applied potential to generate a new catalytic active site Cu-N₃ during the reaction, and further reconstructed into a HO-Cu-N₂ structure during the oxygen reduction reaction (Fig. 13).

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图13 铜单原子催化剂在N₂电化学还原反应中结构变化^[146]

Furthermore, Yang et al., with the help of in situ X-ray absorption spectroscopy and advanced electron microscopy, revealed a green ammonia production route in which the synthesized Cu-N₄ single-atom sites were reconstructed into nanoparticles of about 5 nm during the electrochemical reduction of nitrate to ammonia, combined with plasma-assisted oxidation of nitrogen^[147]. The reduction of V vs RHE to Cu⁺ and Cu⁰ and the subsequent Cu⁰ single-atom aggregation during the switching of the applied potential from 0.00 V to − 1.00 V vs RHE occurred simultaneously with the enhancement of the NH₃ yield. The maximum productivity of ammonia reached V vs RHE

g C u - 1

·h^-1) with a FE of 84.7% at − 1.00 V vs RHE, which is better than most other copper catalysts reported before. After electrolysis, aggregated Cu nanoparticles reversibly disintegrate into individual atoms and then revert to the Cu-N₄structure upon exposure to ambient atmosphere, which masks the potential-induced reorganization during the reaction. The simultaneous changes in the percentage of Cu⁰ and ammonia FE and applied potential indicate that Cu nanoparticles are the true active sites for the reduction of nitrate to ammonia (Fig. 14).

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图14 铜单原子催化剂在O₂电化学还原反应中结构变化^[147]

Taking a single copper atom embedded in N-doped graphene as an example, Wang Jinlan et al. Of Southeast University used the "potentiostatic hybridization-solvation kinetics model" to evaluate the reversible transformation between copper single atom and cluster under real reaction conditions (Fig. 15)^[148]. It is revealed that the adsorption of H is an important driving force for the leaching of Cu single atoms from the catalyst surface. The more negative the electrode potential, the stronger the H adsorption. Therefore, the competitive hydrogen evolution reaction is inhibited and the Cu — N bond is weakened, resulting in some Cu atoms bound to the catalyst surface and some dissolved in the aqueous solution. The collision of the copper atoms in the two States forms a transient copper cluster structure, which acts as a true catalytic active site to promote the reduction of CO₂ to ethanol. When the applied potential is released or switched to a positive value, the hydroxyl radical plays a dominant role in the oxidation process of the Cu cluster, and then Cu returns to the initial atomically dispersed state by redeposition, completing the reconstitution cycle of the copper catalyst.

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图15 Cu单原子和团簇之间的可逆转变^[148]

The results show that the oxide support itself has phase transformation and structural reconstruction under reaction conditions, so the surface defect sites will be generated and disappeared in situ, resulting in the migration or aggregation of metal species. For example, under mild reaction conditions, the redox properties of cobalt oxidation favor the formation of O_v, forming isolated Rh₁Co_n bimetallic active sites in situ^[149]. In addition, in the CO oxidation reaction, the heating of the O_v can lead to the sintering and deactivation of the noble metal atoms anchored by the O_v^[150]. Therefore, the formation energy of O_v on oxide support under different treatment conditions plays an important role in predicting the bond energy of metal atoms in O_v. In addition, reducible oxide supports such as CeO₂ can trap metal atoms to form single-atom catalysts. To understand the properties and stability of single-atom catalysts under reducing conditions, Xiong et al. Prepared Rh/CeO₂ catalysts with different Rh loadings (0.2 – 4 wt%), treated them under different atmospheres (H₂ and CO) and at different temperatures, and studied the structural evolution and catalytic performance of these Rh/CeO₂ catalysts under syngas conversion (260 ° C, 2 MPa)^[151]. Combining CO-DRIFTS, AC-HADDF-STEM and XPS studies, it was found that the Rh loading on the Rh/CeO₂ catalyst was up to 3 wt%, and single-atom Rh was dispersed on the catalyst. The 0.5Rh/CeO₂ catalyst was stable in H₂ at 350 ° C, whereas Rh nanoclusters were formed in CO at 200 ° C. Rh single atoms on the 0.5Rh/CeO₂ slowly agglomerated to form Rh nanoclusters during the reaction, and the Rh_iso/Rh_NC ratio was about 1 ∶ 2.5 and remained unchanged in the syngas reaction atmosphere. With the increase of Rh loading in the Rh/CeO₂ catalyst, when the catalyst reaches a steady state (~ 10 H), the Rh_iso/Rh_NC ratio decreases, and the reaction activity first increases and then decreases. On the 1Rh/CeO₂ catalyst, the ratio of Rh_iso/Rh_NC is 1 ∶ 3, and the reaction rate (r_CO) is 85.8μmol·s^-1·

g R h - 1

.The ethanol selectivity (S_EtOH) is 26.2%, indicating that the combination of Rh single atoms and Rh nanoclusters is beneficial to improve the ethanol selectivity and reaction rate.

In addition, some heterogeneous catalytic reactions require different catalytic reaction sites, that is, multiple aggregate units. For such reactions, single-atom catalysts are not omnipotent, and they are really powerless for some reactions or their performance is not as good as that of traditional nanocatalysts or cluster catalysts. Faced with this situation, the solutions we can think of or have been used in the field are roughly as follows: (1) Use some special carriers, so that the carrier (or the site with direct interaction with the single atom) can also participate in the activation of the substrate or reactant molecules, thereby improving the performance of the single atom catalyst. For example, for some oxidation reactions, a reducible support can be selected, so that the reaction proceeds through a redox cycle at the support-metal interface (MvK mechanism)^[152]; (2) By coupling with a second component metal, cooperative catalysis is formed. For example, monatomic alloy catalysts, which have been widely used at present, can maximize the utilization efficiency of precious metals and minimize the amount of precious metals used^[136⇓~138]. One of the most typical examples is the nano-copper supported palladium single atom prepared by Zheng Nanfeng of Xiamen University, which achieves the effect of "point copper into palladium" through hydrogen overflow effect^[153]. (3) Polyatomic active sites can be further developed on the basis of single-atom catalysts. For example, the "single cluster catalyst" proposed by Li Jun of Tsinghua University and the "fully exposed cluster catalyst" proposed by Martin of Peking University belong to this^[154,155]^[156,157]. (4) Synergistic catalysis of monoatomic cluster/nanoparticle. At present, some reports have found that the synergistic catalysis of a specific ratio of single atoms and nanoclusters has higher catalytic performance than that of pure single atoms and pure nanoparticles/clusters. (5) Finally, the author reiterates that single-atom catalysts are not omnipotent, and not all reactions must use single-atom catalysts. One of the important contents of single-atom catalysis research is to explore the reactions suitable for single-atom catalysts, and then promote the application of single-atom catalyst in these reactions.

6 Conclusion and prospect

The proposal and development of the concept of monatomic catalysis provides a new opportunity for the precise construction of catalyst active sites at the atomic scale, which has an important impact on the field of heterogeneous catalysis and is an important part and research object of condensed matter chemistry. In the study of single-atom catalysis, some traditional concepts of heterogeneous catalysis, such as dispersion, metal-support interface, size effect, morphology effect, etc., are no longer meaningful. The research focus of heterogeneous catalysis has also changed from the traditional morphology and size control to the regulation of active site electronic structure and coordination environment. Corresponding to nanoscience, which has promoted the understanding of catalysis to the nanometer level, the development of single-atom catalysis will also further deepen the understanding of heterogeneous catalysis to the (single) atom level.

Monatomic catalysis provides new historical opportunities for the development of heterogeneous catalysis and other fields, but also faces major challenges. The study of single-atom catalysis has brought new opportunities for the rational design and precise preparation of catalysts, especially for the precise construction of active sites containing several to dozens of atoms at the sub-nanometer scale^[158⇓~160].

Monatomic catalysis is in the ascendant, and the understanding of monatomic catalysis needs to be further deepened. How to manipulate the coordination microenvironment to obtain high-density single-atom catalysts and how to achieve the balance between stability and reactivity are the current challenges. In addition, real-time monitoring of the dynamic evolution of single atoms in the catalytic reaction process is also very challenging. The development of single-atom catalysts has also brought very high requirements for characterization, and the development of advanced characterization techniques with high energy resolution and high spatial resolution is a major challenge. Industrial applications require the development of highly stable, highly loaded, and highly active single-atom catalysts. Although studies have shown that monatomic dispersed metal atoms on some specific supports have high stability, the interaction between most metal single atoms and supports is relatively weak, so further improving the stability of monatomic catalysts and making them practical is a challenge for industrial application.

There are still many key scientific problems to be solved in the field of characterization of monatomic catalysts, such as: (1) developing spectroscopic techniques with higher spatial resolution to directly obtain the interaction between the central atom and the surrounding coordination structure and local microenvironment; (2) further improve the sensitivity of spectral detection, and realize the in situ/Operando characterization of key reaction intermediate species with low concentration, short life and weak signal; (3) Develop high energy resolution and high time resolution working condition characterization technology, track the dynamic evolution of catalyst active site structure and reaction intermediate species under working conditions in real time, establish a more realistic structure-activity relationship, and provide direct spectroscopic evidence for revealing the mechanism of monatomic catalytic reaction at the atomic and molecular levels.

The success of the concept of monatomic catalysis in the field of heterogeneous catalysis is likely to give birth to "monatomic science", that is, scientific research based on the thinking of individual atoms. For example, "single-atom enzyme", "single-atom manufacturing", sensors and semiconductors based on single atom have appeared, and there may be more research directions and fields involving the concept of "single-atom science" in the future.

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Outlines

模态框（Modal）标题

Abstract

Cite this article

1 Introduction

2 Proposal of the Concept of Monatomic Catalysis

图1 “单原子催化”概念提出

3 Current Status of Monatomic Catalysis

图2 “单原子催化”发展历程[27]

3.1 Preparation of single atom catalyst

图3 制备SACs的多种宿主材料[71]

表1 单原子催化剂不同制备方法对比

3.2 Characterization of single-atom catalyst

表2 单原子催化剂表征方法

3.3 Application of single-atom catalyst

4 Condensation effect of metal center and coordination atom

图4 单原子催化剂聚集体构型(a)正四面体(b)平面四边形(c和d)正八面体

4.1 Metal-support interaction form

4.1.1 Defect site anchoring

表3 金属单原子(M)在载体(Sup)上锚定位置列表[19]

4.1.2 Interaction between metal center and O (OH)

4.1.3 Spatial confinement effect

4.1.4 Metal-support strong interaction (SMSI).

图5 Pt1/TiO2催化剂原位CO-DRIFT谱图[107]

4.1.5 Covalent metal-support interaction (CMSI)

图6 Pt在不同载体上的烧结与分散[109]

4.1.6 Electron metal-support interaction (EMSI).

4.2 Coordination atom modulates the structure of aggregate

4.2.1 Coordination atom modulates the geometry of the aggregate

图7 纳米颗粒、团簇和单原子的(a)几何结构与(b)电子结构[24]

4.2.2 Electronic structure of aggregates modulated by coordinating atoms

4.3 Effect of Metal Aggregation on Catalytic Performance

4.3.1 Catalytic properties of aggregates such as single atoms, clusters and nanoparticles

图8 CeO2负载的Ru单原子、团簇以及纳米颗粒的甲烷化活性对比[125]

图9 In2O3负载的Re单原子、团簇以及纳米颗粒的CO2加氢产物选择性[127]

图10 包裹纳米颗粒暴露单原子调控加氢产物选择性。乙炔半加氢反应中不同还原温度处理的催化剂(Pd/TiO2-H200,Pd单原子和Pd纳米颗粒共存;Pd/TiO2-H600,Pd纳米颗粒被包裹,Pd单原子为活性中心)的(a)C2H2转化率和(b)C2H4选择性[128]

4.3.2 Cooperative catalysis between aggregates

图11 Co-N-C单原子催化剂形貌、组成及结构分析[132]

图12 SAA表征方法:(a,b)Pt1Cu SAA催化剂的 HAADF-STEM图像,Pt1Sn SAA催化剂的(c)Pt-L3 edge XANES 和 (d) Pt 4f XPS谱[139]

5 Dynamic evolution and characterization of aggregates under reaction conditions

图13 铜单原子催化剂在N2电化学还原反应中结构变化[146]

图14 铜单原子催化剂在O2电化学还原反应中结构变化[147]

图15 Cu单原子和团簇之间的可逆转变[148]

6 Conclusion and prospect

References

图2 “单原子催化”发展历程^[27]

图3 制备SACs的多种宿主材料^[71]

表3 金属单原子(M)在载体(Sup)上锚定位置列表^[19]

图5 Pt₁/TiO₂催化剂原位CO-DRIFT谱图^[107]

图6 Pt在不同载体上的烧结与分散^[109]

图7 纳米颗粒、团簇和单原子的(a)几何结构与(b)电子结构^[24]

图8 CeO₂负载的Ru单原子、团簇以及纳米颗粒的甲烷化活性对比^[125]

图9 In₂O₃负载的Re单原子、团簇以及纳米颗粒的CO₂加氢产物选择性^[127]

图10 包裹纳米颗粒暴露单原子调控加氢产物选择性。乙炔半加氢反应中不同还原温度处理的催化剂(Pd/TiO₂-H200,Pd单原子和Pd纳米颗粒共存;Pd/TiO₂-H600,Pd纳米颗粒被包裹,Pd单原子为活性中心)的(a)C₂H₂转化率和(b)C₂H₄选择性^[128]

图11 Co-N-C单原子催化剂形貌、组成及结构分析^[132]

图12 SAA表征方法:(a,b)Pt₁Cu SAA催化剂的 HAADF-STEM图像,Pt₁Sn SAA催化剂的(c)Pt-L3 edge XANES 和 (d) Pt 4f XPS谱^[139]

图13 铜单原子催化剂在N₂电化学还原反应中结构变化^[146]

图14 铜单原子催化剂在O₂电化学还原反应中结构变化^[147]

图15 Cu单原子和团簇之间的可逆转变^[148]