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

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Progress in Chemistry >

2023 , Vol. 35 >Issue 6: 928 - 939

DOI: https://doi.org/10.7536/PC221226

Review

Strong Metal-Support Interactions of Metal/Meatal Oxide Catalysts

  • Xuetao Qin ,
  • Ziqiao Zhou ,
  • Ding Ma , *
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  • College of Chemistry and Molecular Engineering, Peking University,Beijing 100871, China
*Corresponding authore-mail:

Revised date: 2023-05-17

  Online published: 2023-05-25

Supported by

The National Natural Science Foundation of China(22102007)

The National Natural Science Foundation of China(21991150)

The National Natural Science Foundation of China(22172150)

The National Natural Science Foundation of China(21821004)

The National Natural Science Foundation of China(22072090)

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Abstract

Catalysis plays an important role in the modern chemical industry, and developing catalyst with high efficiency is one of the important targets in catalysis research. Due to the outstanding activity of those catalysts with strong metal-support interactions (SMSI), SMSI has become an important scientific topic in catalysis research. The SMSI phenomenon involves the encapsulation of the metal nanoparticles (NPs) by support, resulting in the improved stabilization of NPs, and the different catalytic performances due to the new interaction between NPs and the support. Currently, a great number of catalysts with SMSI have been designed and partially applied, also there are considerable literatures focusing on SMSI of supported catalysts, especially those using metal oxide for support. However due to the complexity, the nature of SMSI and the catalytic mechanism of SMSI deserve further study, and the argument about the driving force of SMSI formation still exists. This review summarizes the recent progress, effect, and the regulation of SMSI, hopefully providing the understanding of SMSI from the perspective of condensed matter chemistry, and a new strategy of catalyst design.

Contents

1 Introduction

2 Research progress in SMSI

2.1 Research history of SMSI

2.2 New types of SMSI

3 Influence of SMSI on catalytic performance

3.1 Activity and stability enhancement

3.2 Selectivity tuning

4 Modulating of SMSI

4.1 Pre-treatment conditions

4.2 Supports

4.3 Metal nanoparticles

5 Conclusion and outlook

Key words： condensed matter chemistry; catalysis chemistry; catalysts; strong metal-support interactions

Cite this article

Xuetao Qin , Ziqiao Zhou , Ding Ma . Strong Metal-Support Interactions of Metal/Meatal Oxide Catalysts[J]. Progress in Chemistry, 2023 , 35(6) : 928 -939 . DOI: 10.7536/PC221226

1 Introduction

So far, human beings have discovered or created billions of substances, most of which exist in the form of condensed matter with multi-level structure. Chemistry plays an important role in the transformation of substances, and the core of chemistry is "chemical reaction". The function and properties of matter depend on its condensed structure, so chemical reactions depend on the composition, multi-level structure and characteristics of the condensed state. Rethinking chemical reactions from the perspective of condensed matter is of great significance for us to understand chemical reactions deeply and comprehensively. Condensed matter chemistry focuses on solid state, mesoscopic nanometer state, molten state and liquid state, which are closely related to chemical science.It is a new scientific field to study the composition, multi-level structure, properties and functions of "state", the reactions between States and in "state" media, the chemical reaction laws and the correlation between multi-level structures of catalytic reactions in solid and molten States with specific composition, the construction and creation of condensed matter, and the related laws and theories[1⇓~3].
From personal clothing, food, housing, transportation and medical care to national energy, environment, economy and security, chemical reaction and chemical industry have already penetrated into every corner of human life. Catalytic chemistry is the basis of modern chemical industry. Most chemical production processes involve the application of catalysts, so catalysis plays a very important role in national construction and economic growth. The development of new catalysts or new catalytic processes may greatly improve production efficiency, bring innovation to the chemical industry and enrich the material world of mankind. Or provide new materials to greatly improve people's quality of life. In addition, many challenges facing modern society, such as energy shortage, efficient use of fossil resources (such as coal, oil, etc.) And prevention and control of environmental pollution, are also inseparable from catalytic chemistry.
The development and research of catalysts and catalytic reactions are the focus in the field of catalytic chemistry. Whether it is the optimization of catalyst structure, the study of structure-activity relationship between catalyst and chemical reaction, or the analysis of catalytic reaction mechanism, the ultimate goal is to develop efficient catalysts and apply them to practical production. Therefore, catalyst is the core of catalytic chemistry research. To date, a catalyst has been recognized as a substance that transforms reactants into products through an uninterrupted repetitive cycle of elementary reaction steps, at the end of which the catalyst is regenerated to its original state. More simply, a catalyst is a substance that changes the rate of a chemical reaction without itself being consumed in the process. Understanding the working principle of catalysts and developing efficient catalysts are the core of catalytic chemistry. The change of catalyst structure and its effect on reaction performance (activity, selectivity, stability, etc.) In catalytic reactions provide an opportunity to examine the structure-activity relationship between catalysts and reaction performance from the perspective of condensed matter chemistry. Therefore, the condensed structure change involved in the catalyst is an important object in the field of modern catalytic chemistry.
In the field of catalysis, Strong metal-support interactions (SMSI) can effectively regulate and even change the properties of catalysts by changing the structure of the condensed state of catalysts, thus having a significant impact on catalytic reactions[4]. SMSI has become an important scientific problem in the field of catalysts and has attracted wide attention of researchers. Since the concept of strong metal-support interaction was proposed, the SMSI effect in supported catalysts, especially in metal/metal oxide catalysts, has attracted great interest of researchers. Due to the lack of early synthetic methods and characterization methods, researchers have not yet fully understood the nature of the strong metal-support interaction and its impact on the catalytic reaction mechanism, especially the driving factors for the formation of the strong metal-support interaction. Therefore, this paper will focus on the SMSI effect of catalysts, and introduce the research progress of SMSI, the effect of SMSI on the condensed state of catalysts, and the methods of regulating SMSI in catalysts, hoping to understand the SMSI effect of catalysts from the perspective of condensed state chemistry through this review.

2 Research Progress of SMSI

Since the discovery of SMSI, it has been widely concerned by researchers in the field of catalysis, and great progress has been made in the study of SMSI and the methods of adjusting the interaction between metal and support. On the basis of classical SMSI,The researchers further designed and discovered a series of new metal-support interaction forms (EMSI, CMSI, A-SMSI, O-SMSI, R-SMSI, etc.), and studied the relationship between these interactions and catalyst performance in detail (Fig. 1)[5,6][7][8][9][10]. Due to the diversity and complexity of SMSI regulation methods, catalyst systems, and catalytic reactions, a systematic overview of the research history and development of SMSI will help guide the determination of optimal SMSI regulation strategies for real catalyst reaction systems.
图1 SMSI研究发展历程

Fig.1 Research progress in SMSI

2.1 Classical SMSI

In 1978, Tauster et al. First proposed the SMSI effect[4]. In this study, it was found that the adsorption performance of platinum group metals supported on reducible titanium dioxide for small molecules such as CO or H2 decreased significantly after reduction at high temperature (500 ℃). The study also found that the high temperature reduction did not lead to the agglomeration of metal nanoparticles, and still maintained a good dispersion. Subsequently, based on the experimental results, Tauster et al speculated that a new intermetallic compound was formed at the interface between the supported metal and titanium after the catalyst was reduced at high temperature, and proposed the concept of strong metal-support interaction.
Vannice et al. Also found that after reduction at high temperature, the chemisorption of Ni/TiO2 on H2 and CO was weakened, but its CO methanation activity was one order of magnitude higher than that of Ni catalyst supported on inert support, and the selectivity of high-carbon alkanes was also significantly improved[11,12].
Advances in catalyst characterization techniques provide support for the study of metal-support strong interactions. The researchers then confirmed the existence of strong metal-support interaction by characterization means, and the noble metal/titania catalyst, after being reduced at high temperature, triggered a strong support-noble metal electron transfer and generated low valence metal ions (Ti3+).At the same time, part of the titanium dioxide carrier migrates to the surface of the precious metal to form a stable coating structure, which prevents the sintering of the supported metal particles at high temperature and greatly improves the stability of the catalyst[13,14].
Classical SMSI catalyst systems generally have the following characteristics: 1) the chemisorption capacity of the supported metal to small molecules is reduced; 2) that carry can migrate to the loaded metal particle; 3) there is electron transfer between the supported metal and the carrier; 4) The SMSI effect can be reversed after the catalyst is oxidized or reduced. Early results suggested that the classical SMSI effect is limited to the group VIII metal and metal oxides with reducing properties (TiO2, V2O3, Nb2O5, Ta2O5, etc.)[4,13]. Although some researchers believe that the SMSI effect can lead to the passivation and deactivation of catalysts, more studies have shown that SMSI can greatly improve the activity and stability of catalysts at the same time.
In recent years, SMSI has played an important role in selective hydrogenation, catalytic oxidation, reforming and water gas shift reactions, which has revived the research boom of SMSI in the field of heterogeneous catalysis[15⇓⇓⇓⇓⇓⇓~22][23⇓⇓⇓⇓⇓⇓~30][31][5,32⇓⇓⇓⇓~37]. With the development and improvement of SMSI effect research, SMSI catalytic system is no longer limited to the classical SMSI catalyst induced by reduction treatment.New SMSI effect systems (metal and non-reducing oxide, metal and composite oxide, nano-alloy and oxide support, metal and carbide support, metal and carbon support, metal andhydroxide support etc.) And forms (EMSI, CMSI, A-SMSI, O-SMSI, R-SMSI etc.) Have been continuously discovered and explored.It has been widely used in the field of catalysis, showing excellent catalytic performance and stability, which further broadens the research scope of SMSI effect[26][5,27,32][38][31,35,37][39][40].

2.2 New SMSI

2.2.1 Metal-support electron interaction (EMSI).

For metal catalysts supported on reducible oxides, the interaction between metal and support will lead to a certain degree of electron transfer between metal nanoparticles and support. Rodriguez et al. First reported a metal-support interaction and described the electron transfer between the metal and the support (Figure 2A), that is, the electron transfer from the CeO2 support to the metal Pt occurs, which increases the ability of Pt to dissociate O — H bonds, resulting in an increase in the activity of the water-gas shift reaction[5]. Subsequently, Campbell also proposed to use the electronic interaction between metal and support to describe the strong interaction between Pt nanoparticles and CeO2 support, and carried out a more detailed study of this phenomenon (Figure 2B)[6]. It was found that the metal-support interaction could promote the strong electron transfer from the CeO2 support to the Pt atom at the interface, and then the Electronic metal-support interaction was proposed.EMSI) concept, and the existence of EMSI is further verified by theoretical calculations. The results also show that EMSI leads to strong electronic oscillation at the interface between metal and support, thus weakening the energy barrier for adsorption and activation of reactive molecules[5,6].
图2 (a)模型Pt/CeO2(111)催化剂WGS活性随金属覆盖度的变化规律[5];(b)氧化铈负载的铂簇上吸附H2O解离的反应路径计算结果[6]

Fig.2 (a) WGS activities of model Pt/CeO2(111) catalysts as a function of admetal coverage[5]; (b) Calculated reaction path for the dissociation of adsorbed H2O on a ceria-supported Pt cluster[6]

Zeng et al. Found that the energy change of the highest occupied state of the electron in Rh monatomic was induced by the carrier VO2 nanorods when studying the hydrolysis of ammonia borane to hydrogen. Because the electronic state of supported metal Rh depends on the energy band structure of the carrier,However, the highest electronic occupied state of Rh single atom determines the catalytic performance, so the interaction between VO2 support and Rh single atom improves the reaction performance of the catalyst[41]. Lu et al. Used Atomic layer deposition (ALD) to prepare Pt single-atom catalysts supported on four different supports, and studied the role of EMSI in catalytic reactions (Fig. 3)[42]. Compared with other supports (Graphene,ZrO2,CeO2), there is a strong EMSI between Pt atoms and supports in Pt1/Co3O4, which greatly improves the ability of ammonia borane hydrolysis, and its reactivity is 68 times higher than that of Pt single-atom catalysts on other supports, while showing good hydrothermal stability.
图3 (a)Pt1/Co3O4、Pt1/CeO2、Pt1/ZrO2、Pt1/石墨烯、Pt箔以及PtO2的样品在Pt L3边的XANES谱;(b)傅里叶变换EXAFS谱;(c)Pt1/Co3O4、Pt1/CeO2、Pt1/ZrO2三个样品的漫反射红外CO吸收谱;(d)Pt1/Co3O4、Pt1/CeO2、Pt1/ZrO2、Pt1/石墨烯和PtO2样品在Pt 4f区域内的XPS谱图[42]

Fig.3 (a) XANES spectra of Pt1/Co3O4, Pt1/CeO2, Pt1/ZrO2, and Pt1/graphene SACs as well as the Pt foil and PtO2 reference at the Pt L3-edge; (b) the corresponding K2-weighted Fourier transform spectra; (c) DRIFTS of CO chemisorption on Pt1/Co3O4, Pt1/CeO2, and Pt1/ZrO2 at the saturation coverage; (d) XPS spectra of Pt1/Co3O4, Pt1/CeO2, Pt1/ZrO2, Pt1/graphene, and PtO2 in the Pt 4f region[42]

EMSI is a new concept that can be used to explain the relationship between the electronic properties of metals and catalytic activity. From a broader perspective, EMSI refers to the charge transfer from the support to the supported metal in the catalyst.The d-electron center of the transition metal supported on the catalyst increases, which affects the adsorption of the supported metal on the reactant and the desorption behavior of the product, thus significantly improving the reaction performance of the catalyst. In addition to the properties of the support and the metal, the pretreatment conditions or reaction conditions of the catalyst can be used to change the chemical bonding and coordination environment of the metal, optimize the geometric and electronic structure of the catalyst, and thus improve the catalytic performance of the catalyst. Therefore, EMSI can be used to adjust the electronic state of the supported metal, design catalysts with high activity and stability, and use in-situ characterization to understand the structure of the catalyst metal, so as to obtain more effective information for understanding the dynamic interaction between the metal and the support.

2.2.2 Covalent metal-support interaction (CMSI)

Zhang et al. Proposed a Covalent metal-support interaction (CMSI) to elucidate the excellent stability of Au1/FeOx single-atom catalysts[7]. The traditional supported gold catalyst is very prone to gold agglomeration during the reaction, while the Au1/FeOx single-atom catalyst constructed by Zhang et al. Still shows extremely high stability even in the CO oxidation reaction at 400 ° C. The experimental and theoretical results show that the Au — O bonding is strong covalent bonding, the highly dispersed positive valence Au species are located at the defect sites of Fe atoms on the FeOx, and there is a strong CMSI between Au atoms and lattice oxygen atoms.
When the defect sites of supports are no longer used only as single-atom loading sites, high-loading, thermostable single-atom catalysts can be prepared on some reducible supports (such as CeO2, NiO, and MnOx) by using strong CMSI. Gu et al. Used the Ir — O covalent bond to obtain an ultra-high loading single-atom catalyst with Ir1/NiO loading up to 18 wt%, and showed excellent catalytic activity and stability in the electrochemical oxygen evolution reaction (OER)[43]. Lu et al. Prepared a Pt single-atom catalyst supported on a Mn3O4, which was transformed into a Pt1/Mn2O3 after heat treatment. Due to the strong CMSI between Pt and the support, the single atom could still exist stably even after calcination in humid air at 800 ℃ for 5 days, and then showed strong sintering resistance[44].
In addition to metal oxides, metal sulfides, metal carbides, and carbon-based materials also exhibit the property of forming strong covalent bonds with metal single atoms[19]. Li et al. Found that Pt single atom can stably exist in the Mo vacancy of the MXene(Mo2TiC2Tx) by forming Pt-C covalent bond with the surrounding carbon atoms[45]. Therefore, the CMSI between metal-metal oxide catalysts can be extended to the CMSI between metals and various supports, which further broadens the scope of C-SMSI.

2.2.3 Adsorbate-induced metal-support interaction (A-SMSI)

In 2017, Christopher et al. Proposed Adsorbate-induced metal-support interaction (Adsorbate-mediated SMSI) (Fig. 4)[8]. In the study of CO2 hydrogenation catalyzed by Rh/TiO2, our group found that the catalyst treated by 20 CO2∶2 H2 at 150 ~ 300 ℃ had higher CO selectivity. Further investigation showed that during the pretreatment process, HCOx was formed on the surface of the catalyst, which was tightly adsorbed on the surface of the support and induced the formation of oxygen vacancies on the support. Then the partially reduced support modified by HCOx moves to Rh and coats it, forming a new type of SMSI effect involving adsorbed species, which significantly affects the selectivity of the catalyst and is subsequently named A-SMSI. Compared with the classical SMSI effect, the A-SMSI effect has a lower formation temperature, and can be formed and stabilized in situ during the pretreatment and reaction of the catalyst, so it can continuously affect the performance of the catalyst. However, the classical SMSI effect will be oxidized by the generated species such as H2O or CO2 during the reaction and disappear. The discovery of A-SMSI effect enables researchers to reasonably adjust the selectivity of hydrogenation products of CO2 according to actual needs.
图4 SMSI及A-SMSI的包覆结构及催化行为[8]

Fig.4 Structure and catalytic behavior of SMSI and A-SMSI[8]

In 2018, Wang et al. Used the same conditions as above to pretreat the Cu/CeO2 catalyst and used it for the water gas shift reaction[46]. Although the sintering of Cu is effectively inhibited under the A-SMSI effect, the activity of the catalyst does not show significant change. This result indicates that the influence of the A-SMSI effect on the catalytic performance of the catalyst is closely related to the specific type of catalytic reaction.

2.2.4 Oxidative metal-support interaction (O-SMSI)

In the past few decades, supported gold catalysts have attracted much attention due to their unique catalytic properties for a series of important chemical reactions. In related reports, some researchers believe that Au nanoparticles can not have SMSI effect with the carrier, and some results show that the reason why Au nanoparticles can not have SMSI effect is that Au has smaller surface energy and work function than platinum group metals. However, in 2012, Liu et al. Found that Au nanoparticles were coated by ZnO species after the Au/ZnO catalyst was treated in an oxygen atmosphere at 300 ℃, and observed that the electrons on the Au surface were transferred to the ZnO carrier[9]. When the oxidized Au/ZnO was reduced by Oxidative strong metal, the coating layer disappeared, which was opposite to the electron transfer in the classical SMSI induced by the treatment environment. Liu et al later called the new SMSI induced by this high temperature oxidation condition as the Oxidative strong metal-support interaction effect[9]. The discovery of O-SMSI shows that the SMSI effect can also occur under high temperature oxidation conditions. This study is another case that can show the SMSI effect in supported gold catalysts, breaking through the previous understanding of the initiation conditions and metal systems of SMSI.
In 2016, Wang et al. And Zhang et al. Jointly found that the O-SMSI effect also exists between Au nanoparticles and nonmetallic oxide carrier hydroxyapatite (HAP), and the effect can also exist on Au catalysts supported by other phosphates (such as LaPO4)[47]. This study extends the support range of SMSI catalysts to phosphate, a non-metallic oxide, and further broadens the understanding of SMSI.
In 2018, Wang et al. Again collaborated with Zhang et al. To discover and confirm the O-SMSI effect between Pt and Pd and HAP in VIII group metals[48]. Under this effect, the metal nanoparticles are stably coated on the surface of the HAP support, which significantly improves the thermal stability of the catalyst in the gas phase reaction and the cycle stability in the liquid phase reaction. However, the current research on O-SMSI focuses on the expansion and application of catalytic systems, and the driving force of O-SMSI effect needs to be further studied[49].

2.2.5 Reaction-induced metal-support interaction (R-SMSI)

In the process of studying SMSI, the researchers found that the reaction atmosphere can also induce the catalyst to form a strong metal-support interaction. In 2013, Armbruster et al. Found that the structure of ZnPd/ZnO catalyst changed during Methanol steam reforming (MSR), and observed the migration of ZnO island species on the surface of ZnPd alloy particles by HRTEM (High resolution transmission electron microscope)[10]. Recently, Bao et al. Found that in the Dry reforming of methane (DRM) reaction catalyzed by Ni/h-BN, Dry reforming of methane can oxidize h-BN near Ni particles into BOx species and coat them on the surface of Ni particles, and the coating disappears after H2 treatment (Fig. 5)[50]. The results indicate that the Ni/h-BN catalyst has Reaction-induced strong metal-support interaction (R-SMSI) effect in DRM reaction. In addition, the authors extended the R-SMSI catalytic system and found that H2O molecules could also induce the formation of R-SMSI from Ni/h-BN. At the same time, metal Fe, Co, Ru, etc. Can also form reaction-induced metal-support strong interactions with inert h-BN[50].
图5 Ni/h-BN催化剂中反应诱导的金属-载体强相互作用[50]

Fig.5 Reaction-induced strong metal-support interactions in Ni/h-BN catalyst[50]

2.2.6 Other generalized metal-support interactions

With the further study of metal-support interaction, the SMSI effect is no longer limited to the formation of supported metal coating, and more and more generalized SMSI catalytic systems have been developed.
Ma et al. And Bao et al. Reported the SMSI effect between Au and MoCx in 2017 and 2018, respectively[37][51]. After the Au/MoO3 is carbonized at high temperature, the MoO3 is transformed into MoCx, and the loaded Au is attached to the MoCx carrier in a highly dispersed form; When the formed Au/MoCx was oxidized, the Au species aggregated, and the MoCx was reoxidized to MoO3, which was restored to the original Au/MoO3. When the catalyst was subjected to the CH4/H2 carbonization -O2 calcination cycle treatment, MoO3 and MoCx were alternately formed, Au species continuously underwent the dispersion-aggregation process, and the electron transfer between Au and Mo at the interface was reversibly changed accordingly. Therefore, the authors believe that there is a generalized SMSI effect in the Au/MoCx catalytic system (Fig. 6).
图6 Au与碳化物间的SMSI效应[51]

Fig.6 Schematic illustration of the SMSI effect between Au overlayers and carbide supports[51]

Xiao et al. Successfully constructed the SMSI effect by using the "hydroxide-oxide" conversion process of the support through the high-temperature inert atmosphere treatment of layered bimetallic hydroxides[24]. The construction of this SMSI effect occurs with the coating of the metal by the in situ formed support oxide (although the coating is not reversible) in a high temperature inert atmosphere (different from the previously reported reducing or oxidizing atmospheres). The occurrence of the SMSI effect significantly improves the thermal stability of the catalyst, so that the gold catalyst shows excellent anti-sintering performance for both CO oxidation and ethanol dehydrogenation under high temperature conditions. Further studies have found that the SMSI has certain universality and can be used to prepare other sintering-resistant noble metal (Pt and Rh) catalysts[24].
In order to solve the problem that general metal-support interactions usually require high temperature treatment, Xiao et al. Reported a Wet-chemistry strong metal-support interactions (wcSMSI) method, which can construct catalysts in aqueous solution at room temperature[52]. The redox interaction between Ti3+ and Auδ+ facilitates the electron transfer from Ti3+ to Auδ+, resulting in the formation of an oxide coating on the Au nanoparticles. The wcSMSI strategy avoids the sintering of Au species caused by high temperature treatment and realizes the metal-support interaction on Au nanoparticles with an average size of about 2 nm.
Recently, Qiao et al. Found that the Pt single-atom catalyst supported on TiO2 support can also form SMSI effect after high-temperature H2 reduction treatment. Unlike the classical SMSI, the Pt single-atom is not coated by the TiO2 support at this time. The reason why the adsorption of CO on the catalyst is significantly reduced is that the Pt atom forms a saturated coordination structure with H and Ti3+ species, and meets the 18-electron rule[53].
Metal nanoparticles and the support can form a new condensed state due to the existence of interaction. The occurrence of this phenomenon can have a two-sided consequence, that is, the substances produced can either improve the catalytic performance or the opposite. On the one hand, the generation may be inactive phase species formed at the expense of active metal sites, such as mixed metal oxides (such as metal aluminates), and this change has long been considered as a deactivation process of catalysts[4,13,54]. On the other hand, metal or metalloid ions obtained by reducing the support can combine with the supported metal nanoparticles to form highly Reactive intermetallic nanoparticles, which has recently attracted great attention and is sometimes referred to as Reactive metal-support interactions (RMSI)[10].
Recently, Bert et al. First observed the reconstruction of metal oxide overlayer under in-situ reaction conditions during the hydrogenation of COx catalyzed by Ni/TiO2 catalyst.The activity and selectivity of Ni species in the hydrogenation of COx were reasonably explained by in situ electron microscopy and spectroscopy[55]. The results provide a new perspective of SMSI, which is different from the traditional cognition, and suggest that researchers should conduct more detailed studies on catalysts at the level of single metal particles to re-examine the "structure-activity relationship" between catalyst structure and reaction performance.
Through a brief overview of the above different metal-support interactions, it can be found that the metal-support interactions can affect the properties of the condensed state of the catalyst to a certain extent, and then affect the reaction performance of the catalyst. However, different metal-support interactions occur under different conditions, which can be simply classified as follows: metal-support interaction induced by pretreatment (SMSI, EMSI, CMSI, O-SMSI), metal-support interaction induced by reaction conditions (R-SMSI), and metal-support interactions induced by both pretreatment and reaction conditions (A-SMSI). At present, much attention has been paid to the metal-support interaction induced by pretreatment in order to better control the geometric and electronic properties of the catalyst to obtain the desired catalytic activity, selectivity and stability. In order to better understand the underlying causes of metal-support interactions and their effects on reaction performance, the mutation of reaction performance during the reaction process and the induction of corresponding metal-support interactions should be paid more attention and further studied.

3 Effect of SMSI on Catalytic Performance

As described in the above sections, SMSI can significantly affect the geometric and electronic structure of the condensed state of the catalyst, and even affect its chemical composition to create a new condensed interface. Furthermore, the changes in the composition, structure and properties of the condensed state of the catalyst caused by the above effects will affect the interaction between the catalyst and the reactant molecules.Therefore, the correlation between catalytic reaction performance (activity, selectivity, stability, etc.) And the multi-level structure of catalyst condensed state can be studied from the perspective of condensed state, and the inherent nature of "structure-activity relationship" can be understood from the perspective of condensed state.

3.1 Improving the stability of catalytic activity

SMSI usually causes changes in the morphology of the metal or the crystal structure of the oxide support in the condensed state of the catalyst (such as encapsulation behavior), while affecting the electronic properties of the condensed state of the catalyst (metal and support) and creating more condensed interface sites. The regulation of condensed electronic properties and the increase of interface site density can promote the occurrence of some basic steps, which ultimately lead to the enhancement of catalytic activity. Stabilizing metal species and preventing metal aggregation is another non-negligible effect of SMSI on the catalyst, which can significantly improve the thermal stability of the catalyst.
The SMSI effect of metal nanoparticles and supports can trigger electron transfer in the condensed state of the catalyst, regulate the arrangement of electrons between the supported metal and the support, and then improve the catalytic performance of the catalyst. The Cu/CeO2 has very high low-temperature water gas shift reaction catalytic activity, the research result shows that a new chemical bond can be formed at the interface between Cu and the CeO2, and the newly formed Cu+-O-Ce3+ is essential for improving the water gas shift reaction activity of the catalyst, and the :Cu+ is mainly responsible for adsorbing and activating CO,On the adjacent O-Ce3+, the dissociation activation of H2O is mainly carried out, so the Cu+ and O-Ce3+ at the metal-oxide interface achieve synergistic catalysis, and finally the efficient conversion of water and carbon monoxide is realized[56].
Wei et al. And Ma et al. Reported that the TOF of TiO2-x/Ni with SMSI effect in water gas shift reaction was as high as 3.8 s-1, which was 7 times higher than that of Ni/SiO2 without SMSI effect.The SMSI between the Ni particle and the TiO2 support can form a rich metal-support interface, and the SMSI can effectively modify the electronic structure characteristics of the Ni particle, so the catalyst can efficiently activate water molecules and promote the water-gas reaction[57].
In recent years, the observation and description of electron transfer phenomena at the metal-support interface have also attracted extensive attention from researchers in the field of catalysis. The electron transfer process between metal and support was quantitatively analyzed by using Pt/CeO2 as a model catalyst[58]. The size of Pt nanoparticles loaded on the CeO2 was controlled by depositing different amounts of Pt on the surface of CeO2(111), and the valence state of Pt was analyzed by photoelectron spectroscopy of synchrotron radiation light source. The results show that when the number of Pt nanoparticles is about 50, the charge transfer between Pt and the CeO2 reaches the maximum, and 10 Pt atoms transfer one electron to the CeO2 on average. For larger Pt particles, the charge transfer reaches the upper limit of charge accommodation determined by the carrier properties. For smaller Pt particles, the charge transfer process is inhibited by particle nucleation at the CeO2 defects. For the Pt/CeO2 catalyst, the origin of the strong metal-support interaction is that the energy level of the 4F electron orbital of Ce is close to that of the valence electron of Pt, so there is a dynamic equilibrium of electron transfer between them. Similar to it, the energy level of the 3F electron orbital of Ti in the Ni/TiO2 catalyst is close to that of the valence electron of Ni, so it also shows the property of strong metal-support interaction[57].
In 2015, Huber et al. Designed and prepared a stable Co/TiO2 catalyst by using the SMSI effect. After 105 hours of liquid phase hydrogenation of furfuryl alcohol, the Co species of the catalyst neither aggregated nor lost, while the loss of Co species of the Co/TiO2 catalyst without SMSI effect (that is, without TiOx coating) as a comparison was as high as 44.6% after 35 hours of reaction[59]. The TiO2 coating layer generated by the SMSI effect can bind metal species on the surface of the support, thereby effectively inhibiting the aggregation, growth and loss of metal nanoparticles in the pretreatment and reaction process, and improving the hydrothermal stability of the catalyst.

3.2 Regulatory reaction selectivity

In 2017, Ma et al. Found that the SMSI effect between Ir species and cerium oxide could be regulated only by adjusting the loading of iridium during the study of carbon dioxide hydrogenation catalyzed by Ir/CeO2, thus successfully changing the selectivity of carbon dioxide hydrogenation products (Fig. 7)[60]. The results show that different Ir loading (5% ~ 20%) does not significantly affect the particle size of Ir nanoparticles in the catalyst, but leads to significant differences in the valence of Ir species after reduction under the same conditions: the catalyst with lower Ir loading contains more Ir — O bonds, showing weaker CO adsorption performance, thus showing higher CO selectivity; However, the catalyst with higher Ir loading contains more Ir — Ir bonds and shows more obvious metallic state, which has a strong adsorption effect on CO and is not conducive to the desorption of CO products, resulting in a stronger methanation tendency of the catalyst.
图7 (a)Ir—Ir和Ir—O配位数和反应选择性关系;(b)Ir/Ce-used催化剂Ir L3 EXAFS图;(c)Ir/Ce-used催化剂的XPS图[60]

Fig.7 (a) The coordination number (CN) of Ir—Ir and Ir—O shells (data, right axis) relative to catalytic selectivity (bars, left axis) with Ir/Ce catalysts with different Ir loadings; (b) Ir L3-edge EXAFS of the Ir/Ce-used catalysts; (c) XPS analysis of the Ir/Ce-used catalysts[60]

In 2008, Corma et al. Found that the activity of Pt/TiO2, Ni/TiO2 and Ru/TiO2 in the hydrogenation of 3-nitrostyrene was significantly improved after reduction at 450 ℃ compared with that of the catalyst reduced at 200 ℃, and the selectivity of 3-aminostyrene was also improved[21]. Through structural characterization of the catalyst and infrared study of CO adsorption, they attributed the improvement of catalyst activity and selectivity to the change of surface charge density of Pt nanoclusters caused by SMSI and the geometric modification of Pt nanoclusters surface by TiOx coating.

4 Regulation of catalyst SMSI

In the field of heterogeneous catalysis, the design and preparation of supported metal catalysts with high efficiency, high stability and low cost are the problems and challenges faced by catalytic researchers and industries.At the same time, researchers have been devoting themselves to the fine control of catalyst active site structure and the study of structure-activity relationship of catalytic reaction, in order to effectively construct high-performance catalysts, so as to improve the activity, selectivity and stability of catalysts. As the core of carrier structure regulation, the modulation of metal-carrier strong interaction is widely used in the field of heterogeneous catalysis, and has made brilliant achievements.
In recent years, researchers have synthesized a variety of highly dispersed supported metal catalysts by controlling the strong interaction between metal and support, and effectively controlling the strength of the interaction (such as the strength of the interfacial bond, the degree of electron transfer between metal and support, etc.)[37,51,57]. In addition, the obtained catalyst also exhibits excellent catalytic activity, extremely high selectivity and stability. Through the combination of in situ characterization and DFT calculation, researchers have not only revealed the structure-activity relationship of catalytic reactions, but also gradually discovered the inherent law of strong interaction regulation between metal and support, which provides a new means for the fine regulation of catalyst active site structure. With the development of various in-situ characterization techniques and fine catalyst synthesis methods, the construction of new metal-support strong interaction systems has attracted wide attention of researchers. At present, the key points of the regulation of the strong interaction between the metal and the support are: 1) effectively controlling the dispersion of the supported metal while improving its stability; 2) modulating the degree of electron transfer between the metal and the support; 3) regulating the defect structure of the metal surface, and reasonably controlling the coverage of the metal surface (the degree of carrier migration to the metal surface); and 4) finely regulating the interface microstructure between the metal and the carrier, that is, controlling the composition and strength of the interface bond and the abundance of the interface defect structure. The key points of SMSI regulation can be regulated from the aspects of catalyst pretreatment conditions, metal oxide supports and metal particles.

4.1 Preconditioning condition

In the heterogeneous catalytic reaction, the catalyst is not active until it is pretreated. Changing the pretreatment conditions can effectively control the dispersion, geometric structure and electronic structure of the supported metal on the catalyst, thus greatly affecting the activity and selectivity of the catalyst. After a large number of previous studies and explorations, researchers have found that the control of catalyst pretreatment conditions is closely related to the strength of metal-support interaction, which can be controlled by changing the calcination or reduction temperature of the catalyst and the pretreatment atmosphere.
In 1978, Tauster et al. Found that for platinum group noble metal catalysts supported on TiO2, increasing the activation temperature of the catalyst can effectively enhance the interaction strength between the metal and the support[4,13]. According to this method, researchers have further carried out detailed and in-depth exploration and research, and tried to expand the scope of application of this kind of catalyst. A large number of subsequent studies have shown that for supported metal catalysts supported on TiO2, the change of the interaction between the metal and the support brought about by the increase of reduction temperature can significantly promote the activity of the catalyst and the selectivity of the target product. Corma et al. Applied Pt/TiO2, Ru/TiO2 and Ni/TiO2 to the selective hydrogenation of 3-nitrostyrene and carried out a detailed study[21]. The experimental results show that the hydrogenation activity of the catalyst increases by two orders of magnitude with the increase of the activation temperature (from 473 K to 723 K), and the selectivity of hydrogenation products increases from < 1% to about 95%, which is surprising. The HRTEM images show that a large number of disordered TiOx cross layers partially coat the surface of Pt nanoparticles after high temperature activation, which is consistent with the classical phenomenon of strong metal-support interaction found by Tauster et al.
There is a strong interaction between the gold nanoparticles and the TiO2 support, and after the Au/TiO2 catalyst is reduced at high temperature, the TiOx cross-layer can migrate to the surface of the Au nanoparticles (Fig. 8), accompanied by a strong electron transfer from the TiO2 support to the gold nanoparticles[61]. Due to the existence of SMSI effect, the Au/TiO2 used in the CO oxidation reaction shows extremely strong thermal stability. The SMSI effect was found to be universal and could be extended to gold catalysts supported on other oxides (such as Fe3O4 or CeO2) or other metals (such as Cu or Ag) supported on TiO2[61]. This new discovery further fills the gap of the classical SMSI research system, and provides an effective means for the development and preparation of high thermal stability catalysts, that is, through the fine control of the interaction strength between metal nanoparticles and reducible supports. Therefore, adjusting the pretreatment temperature is an effective way to create catalyst SMSI. In addition, changing the pretreatment atmosphere of the catalyst can also effectively regulate the interaction between the metal and the support. In 2019, Bao et al. Investigated the activity of carbon dioxide hydrogenation catalyzed by Ni/TiO2, and by changing the pretreatment atmosphere of the catalyst (ammonia or hydrogen), they successfully adjusted the intensity of SMSI effect between Ni and the carrier TiO2 (that is, the degree of metal coating by the carrier), and achieved high methane selectivity conversion of carbon dioxide hydrogenation while significantly improving the conversion rate of CO2[62].
图8 HRTEM和EELS谱图(A~F):RR2Ti-fresh, RR2Ti-H200, RR2Ti-H300, RR2Ti-H400, RR2Ti-H500, RR2Ti-(H500+O400); (G) RR2Ti-H500样品的EELS谱图[61]

Fig.8 HRTEM images and EELS spectra. (A-F) HRTEM images of RR2Ti-fresh, RR2Ti-H200, RR2Ti-H300, RR2Ti-H400, RR2Ti-H500, and RR2Ti-(H500+O400); (G) EELS spectra of the RR2Ti-H500 sample[61]

In 2017, Christopher et al. Found that novel A-SMSI (adsorbate-induced metal-support strong interaction) catalysts could be constructed by treating Rh/TiO2 or Rh/Nb2O5 with CO2-H2[8]. The results showed that the ability of Rh nanoparticles to form C — H bonds on the surface was greatly inhibited by the modification of the support containing HCOx species, which effectively inhibited the methanation side reaction and led to the formation of a large number of target products CO, thus greatly improving the reaction performance of the reverse water gas shift catalyst. The results also show that the Rh catalyst supported on TiO2 or Nb2O5 after reduction by hydrogen at high temperature will form the classical SMSI effect, that is, a large number of active sites of the reverse water gas shift reaction on the surface of Rh are covered by the generated TiOx cross layer, which eventually leads to the methanation of CO2. Chandler then made further comments on this major breakthrough work in Nature Chemistry, and emphasized the importance of the design idea of A-SMSI catalyst to the construction of new high-performance catalysts[63].

4.2 Metal oxide support

As one of the components of supported metal catalysts, the support has an important influence on the activity, selectivity and stability of the catalyst. Current research on SMSI catalyst systems is also focused on metal oxide supports.
Early studies have found that group VIII metal catalysts supported on TiO2 can produce significant SMSI effect, which has been widely used in the field of heterogeneous catalysis, but it is still an important challenge in this field to effectively control the intensity of SMSI only through simple methods. Recently, Ramani et al. Found that the doping of TiO2 support could effectively adjust the microstructure of TiO2, and then regulate the SMSI intensity between the modified TiO2 support and metal Pt nanoparticles[64]. Wherein a certain amount of Ta element is doped into TiO2 to form Ta0.3Ti0.7O2, and then Pt nanoparticles are loaded on the Ta0.3Ti0.7O2 composite carrier. The results of XANES show that the doping of Ta element can increase the electron transfer from the support to the Pt nanoparticles, thus enhancing the interaction between Pt and the support. In addition, the absorption spectroscopy study also shows that the increase of SMSI intensity reduces the number of unoccupied d-orbitals of Pt metal, and the number of unfilled d-orbitals decreases from ~ 1.60 to ~ 1.47 compared with Pt/C catalyst. The activity of the 20%Pt/Ta0.3Ti0.7O2 catalyst is higher than that of the commercial 20% Pt/C catalyst due to the increased electron density on the Pt surface[64]. Therefore, carrier doping can also effectively regulate the structure of the carrier, and then achieve the control of SMSI intensity.

4.3 Metal particle

For supported metal catalysts, the particle size, electronic structure and composition of the supported metal have a great influence on the activity and selectivity of the catalyst. In the field of heterogeneous catalysis, researchers generally agree that the effect of supported metal particle size on the catalytic reaction performance is mainly reflected in the difference of geometric sites exposed by the metal on the catalyst surface. Recently, Liù and Neyman et al. Quantitatively studied the degree of electron transfer between Pt nanoparticles with different particle sizes and CeO2 carriers by combining experiments with theoretical calculations, and revealed the relationship between the particle size of the supported metal and the degree of charge transfer[58]. Therefore, adjusting the particle size of the supported metal on the catalyst surface can effectively control the electronic interaction between the metal and the support.
In addition to controlling the particle size of the supported metal, another way to control the interaction between the metal and the support is to construct supported nanoalloy catalysts. Zhong et al. Reported the interaction between a nanoalloy and a metal oxide, which effectively regulates the catalytic active site, especially the structural composition and arrangement of surface atoms[38]. The results of XPS, XANES, XRD, etc. Indicate that there is a strong interaction between the nanoalloy (Pt-Ni-Co alloy) and the TiO2 support. In view of the complexity of the interaction between alloy and metal oxide supports, the mechanism needs to be further studied, but it provides a new idea for the construction of new catalyst systems.

5 Conclusion and prospect

Since the discovery of SMSI effect, the development of catalysts with SMSI effect and the study of its nature have attracted the attention and great interest of researchers. Up to now, catalysts with SMSI effect have been widely used in heterogeneous catalysis, such as Fischer-Tropsch synthesis, water gas shift reaction, reforming hydrogen production, alkane dehydrogenation, selective hydrogenation, catalytic oxidation and other important chemical reaction processes. Catalysts with SMSI effect have many advantages in the field of heterogeneous catalysis: 1) Catalysts with SMSI effect constructed by specific atmosphere will trigger strong electron transfer from support to metal nanoparticles,The metal nanoparticles with increased electron density can promote the adsorption and activation of reactant molecules under relatively mild conditions (such as low temperature and low pressure), effectively improving the activity of the catalyst; 2) The existence of SMSI effect can selectively and preferentially activate specific groups of reactant molecules or stabilize specific reaction intermediates, thus effectively improving the selectivity of the catalyst; 3) The existence of SMSI effect will promote the support to be coated on the surface of metal nanoparticles, and the construction of this core-shell structure will greatly enhance the stability of the catalyst. The presence of SMSI enables the catalyst to effectively construct an interface with specific geometric and electronic structures, thus greatly promoting the interfacial synergistic catalysis between the metal and the support.
The occurrence of SMSI effect can significantly affect the catalytic activity, selectivity and reaction stability of the catalyst. To some extent, the proposal of SMSI effect is helpful for us to give a reasonable explanation for some reaction phenomena, especially for heterogeneous catalytic reactions, and for researchers to prepare new and efficient catalysts by adjusting SMSI effect. Therefore, the new understanding or discovery of SMSI system is of great significance to the supported catalyst system, and may also lead to innovations in the preparation and application of catalysts. At the same time, we should also realize that the current understanding and understanding of SMSI and the impact of SMSI on catalysis are not deep enough. At this stage, the impact of SMSI on catalytic performance and the mechanism of the formation of SMSI need further study.

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