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

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Progress in Chemistry >

2024 , Vol. 36 >Issue 1: 27 - 47

DOI: https://doi.org/10.7536/PC230604

Review

Design and Structure Regulation of VOCs Catalytic Oxidation Catalysts

  • Wenhao Yang 1 ,
  • Dongyue Zhao 1 ,
  • Haitao Song , 1, * ,
  • Junhua Li , 2, *
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  • 1 SINOPEC Research Institute of Petroleum Processing Co., Ltd., Beijing 100083, China
  • 2 State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China
* e-mail: (Haitao Song);
(Junhua Li)

Received date: 2023-06-08

  Revised date: 2023-08-30

  Online published: 2024-01-05

Supported by

National Natural Science Foundation Program of China(22306210)

National Key Research & Development Program of China(2022YFB3504004)

SINOPEC RIPP Project(PR20230115)

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Abstract

In recent years, with the improvement of the air quality in China, traditional pollutants such as NOx and SO2 have been effectively controlled. The emission control of volatile organic compounds (VOCs) has gradually become a key to further alleviating the regional composite air pollution so far. Catalytic oxidation is one of the most promising VOCs emission reduction technologies due to its high treatment efficiency, low energy consumption, and wide applicability. The development of high-performance catalysts is crucial for this technology. The design and structural regulation of catalysts associated with mechanism study is currently a research hotspot. This paper first outlines the catalytic oxidation mechanism of VOCs. Secondly, the research progress on the structural regulation of non-noble metal catalysts is reviewed from the perspectives of single transition metal oxides, mixed metal oxides, composite metal oxides, and interface structure regulation. Based on the dispersion state, the size effect and support effect of noble metal nanoparticles/clusters in noble metal catalysts are summarized. The regulation strategies based on the metal-support interaction for the emerging single-atom catalysts are also discussed. Finally, this paper provides a summary and prospects for future research trends. We believe that based on deeply clarifying the structure-activity relationship, developing simple and refined structure regulation methods of catalysts and adapting to actual operating conditions and industrial scale-up is the focus of future research.

Contents

1 Introduction

2 VOCs catalytic oxidation mechanisms

3 Structure regulation of non-noble metal catalysts

3.1 Single transition metal oxides

3.2 Mixed transition metal oxides

3.3 Composite transition metal oxides

3.4 Interface structure regulation

4 Regulation of metal dispersion state in noble metal catalysts

4.1 Noble metal nanoparticle/cluster catalysts

4.2 Noble metal single-atom catalysts

5 Conclusion and outlook

Key words： volatile organic compounds(VOCs); catalytic oxidation; structure regulation; noble metal dispersion state regulation; reaction mechanism

Cite this article

Wenhao Yang , Dongyue Zhao , Haitao Song , Junhua Li . Design and Structure Regulation of VOCs Catalytic Oxidation Catalysts[J]. Progress in Chemistry, 2024 , 36(1) : 27 -47 . DOI: 10.7536/PC230604

1 Introduction

Volatile Organic Compounds (VOCs) usually refer to Organic Compounds with boiling points below 250 ℃ at atmospheric pressure[1]. In the process of rapid industrialization and urbanization in China, anthropogenic VOCs emissions are growing rapidly. Most VOCs have biological toxicity and photochemical reactivity, which not only pose a threat to human health, but also have a significant impact on the atmospheric environment[2]. VOCs are one of the important precursors of tropospheric O3 and secondary organic aerosols in the process of atmospheric compound pollution[3]. By 2021, the proportion of days with O3 and PM2.5 as the primary pollutants exceeding the standard in 336 cities in China was still as high as 34.7% and 39.7%, respectively, which was significantly higher than that of NO2, SO2 and PM10[4]. In recent years, the sensitivity of VOCs emission reduction to the improvement of O3 pollution in Beijing-Tianjin-Hebei, Yangtze River Delta, Pearl River Delta and other regions in China has increased significantly[5]. Starting from the demand for pollution control and policy guidance, to achieve deep emission reduction of traditional pollutants such as NOx and SO2, and to control the emission of VOCs has become the top priority of air quality improvement.
The whole process of emission control is the general technical route for VOCs emission reduction, including: (1) controlling the source of VOCs and developing new alternative materials; (2) Strengthen the management of production and processing process and promote technological transformation and upgrading; (3) Adopt physical, chemical, biological and other means to control the end discharge. According to the current emission characteristics and actual demand of VOCs in China, the development and application of terminal emission control technology is one of the efficient ways to achieve rapid emission reduction of VOCs. According to whether VOCs are recycled or not, the end control technology can be divided into two categories: recycling and destruction. Adsorption, absorption, membrane separation and other recovery methods are mainly aimed at the recycling of VOCs, and the operating cost and the regeneration cost of adsorption and absorbent are relatively high, so they are suitable for the treatment of VOCs waste gas with recovery value or as a VOCs concentration unit combined with other technologies. Catalytic oxidation, thermal combustion, biodegradation, photocatalytic oxidation and other destruction methods mainly aim at the harmless elimination of VOCs, which completely oxidize the useless VOCs into CO2 and H2O. Among them, thermal combustion consumes more energy, biodegradation is limited by application scenarios, and photocatalytic oxidation is limited by the actual treatment efficiency. At present, catalytic oxidation method is widely used in VOCs emission control from stationary and mobile sources. Because of its high treatment efficiency, low energy consumption, low secondary pollution and strong equipment compatibility, it has become the frontier and focus of technology research and development at home and abroad[6,7].
Due to the variety of VOCs, the great difference of molecular properties and the complexity of emission conditions, the design and development of catalysts have been the focus of attention in academia and industry in recent years. According to the elemental composition, VOCs oxidation catalysts can be divided into non-precious metal catalysts and supported precious metal catalysts (referred to as precious metal catalysts)[1,2,8]. The development and application of catalysts usually need to consider factors such as cost, reactivity, operation stability and resistance, so the current research at home and abroad mostly focuses on the two technical routes of improving the activity of non-precious metal catalysts and controlling the cost of precious metal catalysts.There are also some studies focusing on the improvement of catalyst resistance to poisoning, sintering and deactivation, in order to solve the problems in practical application.
Transition metal oxides are the main components of non-noble metal catalysts. Due to the variable valence of transition metals, the crystal structure of catalysts has strong controllability and the surface structure has high plasticity. To this end, researchers have improved the redox, surface oxygen species migration, VOCs molecular activation and other properties of non-precious metal catalysts by controlling the exposed crystal plane, introducing doping elements, constructing composite oxides, designing phase interfaces and other special structures, and reduced the temperature of VOCs complete oxidation reaction to improve the performance of catalysts. For noble metal catalysts, the regulation of the dispersion state of noble metals on the surface of the carrier is a research frontier in recent years. It is an important direction for future research to study the size effect, support effect and atomic-level dispersion regulation of noble metal nanoparticles/clusters, and to clarify the internal relationship between noble metal dispersion state, metal-support interaction and VOCs oxidation activity, so as to optimize the loading form of noble metal, achieve efficient oxygen activation of catalysts and control the cost.
The research and development of VOCs oxidation catalysts are receiving increasing attention, involving many catalyst types and catalyst structure control methods, which need to be systematically reviewed to promote the development of the field. In this paper, the mechanism of VOCs catalytic oxidation is introduced, and the research progress in the structure control of non-precious metal catalysts and the dispersion state control of precious metal catalysts are reviewed.Finally, the research status in this field is summarized, and the existing challenges and future prospects are put forward.

2 Catalytic oxidation mechanism of VOCs

As shown in fig. 1A, the catalyst can significantly reduce the activation energy of the complete oxidation reaction of VOCs and reduce the reaction temperature, and the effect of the precious metal catalyst is more significant, so the precious metal catalyst usually shows a lower activity temperature range. As shown in Fig. 1b, the oxidation mechanism of VOCs molecules on the catalyst surface can be divided into: (1) Mars-van Krevelen (MvK) mechanism, in which VOCs molecules undergo redox cycles with the catalyst surface, VOCs extract lattice oxygen from the catalyst surface and are oxidized, the catalyst generates oxygen vacancies, and the oxygen vacancies are backfilled by O2 or supplemented by bulk lattice oxygen to restore the initial state; (2) Langmuir-Hinshelwood (L-H) mechanism, in which VOCs molecules react with O2 in the adsorbed state; (3) Eley-Rideal (E-R) mechanism, in which VOCs molecules react with O2 in adsorbed and gaseous States or vice versa[1,2,8].
图1 （a）催化剂降低VOCs氧化反应活化能示意图；（b）VOCs催化氧化机理

Fig. 1 (a) Schematic diagram of catalyst reducing the activation energy of VOCs oxidation reaction; (b) VOCs catalytic oxidation mechanisms

The oxidation mechanism of VOCs is closely related to the type of VOCs and the properties of the catalyst, among which the MvK mechanism is the most common in the transition metal oxide system, such as the oxidation of toluene catalyzed by Cu-Mn spinel and the oxidation of benzene series catalyzed by CuO/Mn2O3, but the oxidation of styrene catalyzed by MnO/Fe2O3 follows the L-H mechanism[9][10][11]. The L-H and E-R mechanisms are mostly applicable to catalysts with inert support （Al2O3, molecular sieve, activated carbon, etc.), such as the catalytic oxidation of toluene and propylene by Pd-Au/TiO2 and benzene by Pt/Al2O3, and the catalytic oxidation of cyclohexane by Co/AC follows the E-R mechanism[12][13][14]. In addition, the same catalyst may follow different mechanisms for different VOCs oxidation reactions, and vice versa, for example, the oxidation of cyclooctane catalyzed by Pt/γ-Al2O3 is E-R mechanism, while the oxidation of o-xylene is L-H mechanism[15]. For the same reaction system, different reaction temperatures can also lead to different mechanisms. At low temperature, toluene can react with the adsorbed oxygen species near the Ce site in the Pt@CeO2 to form benzoate by L-H mechanism.Benzoate is completely oxidized by lattice oxygen of CeO2 through MvK mechanism at high temperature, which shows that the oxidation mechanism of VOCs is complex and should be studied for specific reaction system[16]. The activation of O2 and the migration and conversion of oxygen species on the catalyst surface are important steps in the reaction process, regardless of the mechanism of VOCs oxidation on the catalyst surface. Especially in the MvK mechanism, the generation and consumption cycle of oxygen vacancies on the catalyst surface is the dominant factor affecting the catalyst activity, and the generation and consumption of oxygen vacancies are usually accompanied by the valence change of adjacent metals (Formula 1,2) The corresponding catalyst design and structure control methods focus on improving the oxygen vacancy formation ability, lattice oxygen storage/release ability and migration ability of transition metal oxides[17,18].
$\mathrm{M}^{(x+1)+}+\mathrm{O}_{\text {latt }}+\mathrm{VOCs} \rightarrow \mathrm{M}^{x+}+\mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{\mathrm{v}}$
$\mathrm{M}^{x+}+\mathrm{O}_{\mathrm{v}}+\frac{1}{2} \mathrm{O}_{2} \rightarrow \mathrm{M}^{(x+1)+}+\mathrm{O}_{\text {latt }}$
Where M is a transition metal, Olatt is a lattice oxygen species, and Ov is an oxygen vacancy.
At present, in-situ Diffuse Reflectance Fourier Transform Infrared Spectroscopy (In-situ DRIFTS) and Density Functional Theory (DFT) calculations are commonly used to characterize the adsorption and oxidation behavior of VOCs on the catalyst surface, which can directly or indirectly explore the interaction mechanism between VOCs and oxygen species on the catalyst surface. Using α-MnO2 as a model catalyst, Sun et al. Used in-situ DRIFTS to explore the adsorption and activation mechanism of toluene on the catalyst surface under the action of different oxygen species, and revealed the important role of gas-phase O2 and lattice oxygen in the activation and oxidation of methyl[19]. Ding et al. Used DFT to simulate the oxidation path of formaldehyde on Pt/TiO2, and found that the oxidation process of formaldehyde was more inclined to E-R mechanism, O2 was activated and dissociated into active O atoms at Pt sites, and formaldehyde was more likely to react with active O atoms[20]. Zhang et al. Combined in-situ DRIFTS and DFT to clarify the activation and oxidation process of propane and propylene on the surface of YMn2O5, and the results proved that the surface active oxygen was an important factor for the activation and dehydrogenation of propane end group and the activation of propylene C = C to acetate (Fig. 2)[21]. In addition to DRIFTS and DFT, kinetic experimental data fitting can also be used to assist in judging the reaction mechanism of VOCs oxidation. Under the exclusion of external diffusion, mass transfer, heat transfer and other factors, the relationship between VOCs conversion and reaction rate (R) was measured, and then the appropriate kinetic equation was selected to fit the relationship between R and reactant partial pressure （PVOCs, PO2） to determine the most likely mechanism of the reaction. In addition, the effects of product partial pressure （PH2O and PCO2） may be introduced according to different reaction systems and conditions[1,2,8].
图2 丙烷与丙烯在YMn2O5表面的活化和氧化过程[21]

Fig. 2 Activation and oxidation of propane and propene on YMn2O5[21], Copyright 2020, American Chemical Society

It can be seen that the mechanism study can deeply understand the influence of catalyst structural characteristics on its VOCs oxidation performance from another angle, which has an important guiding role in the design and structural regulation of catalysts.

3 Structure control of non-noble metal catalyst

Mn, Fe, Co, Cu, Ce and other single transition metal oxides or mixed/composite metal oxides and their mixtures are the main active components of non-precious metal catalysts. Most of the studies are based on single transition metal oxides, and by virtue of the characteristics of transition metal oxides such as crystal structure and variable exposed crystal faces, the physical and chemical properties of the catalyst bulk phase and surface are regulated by various modification means.Improve the migration and conversion ability of active oxygen species on the surface of the catalyst, increase the reaction active sites, improve the intrinsic activity or enhance the stability of the catalyst and other specific properties. The following is a systematic review of the research progress in the structural regulation of non-precious metal catalysts.

3.1 Ingle transition metal oxides

Single transition metal oxides are often used as model catalysts to study the catalytic mechanism of VOCs or to identify the important factors affecting the catalytic activity, and the research results can provide important theoretical basis and guidance for catalyst modification.

3.1.1 Oxidation state and crystal form

Transition metal oxides usually have a variety of oxidation States and abundant crystal forms, which often show different surface chemical properties, and the types and distribution of surface oxygen species are also significantly different. Taking MnOx as an example, such as Mn3O4, Mn2O3 and MnO2 showed different oxygen species migration ability in the catalytic oxidation of benzene and toluene[22]. Among many MnOx, the crystal structure of MnO2 is the most variable.Accord to that difference between the arrangement of the [MnO6] unit and the pore structure formed by the unit, the unit can be divide into a α-MnO2,β-MnO2, γ-MnO2, δ-MnO2, λ-MnO2, ε-MnO2, etc. (Figure 3A)[23,24]. These MnO2 have obvious differences in the catalytic oxidation of CO, formaldehyde, benzene, toluene and other reactions.Due to the rich surface active oxygen species, better oxygen vacancy formation ability and lattice oxygen activity, δ-MnO2 and α-MnO2 show the performance advantages[25][26,27][27][28][29]. In addition, δ-MnO2 and α-MnO2 often contain structure-associated heterocations (interlayer and channel), such as alkali/alkaline earth metal ions such as K+, Na+, or NH4+. The research of our group shows that the accompanying hetero-cations have a significant impact on the ability of oxygen vacancy formation and the ability of reactant adsorption and activation on the surface of MnO2, which has become an important research direction to control the performance of MnO2[28].
图3 （a）不同晶型MnO2的合成方法与晶体结构[29]；（b）不同形貌Co3O4与其主要暴露晶面的关系

Fig. 3 (a) Synthesis methods and crystal structures of MnO2 with different crystal phases[29], Copyright 2021, John Wiley and Sons; (b) Relationship between Co3O4 with different morphologies and main exposed crystal facets

3.1.2 Morphology and exposed crystal plane

The coordination environment and charge distribution of the surface atoms are affected by the morphology and exposed crystal face of the catalyst, which leads to the different interactions between VOCs molecules and O2 and the catalyst surface. The VOCs oxidation activity of α-MnO2 is related to the exposure of crystal planes, and the exposure of (310) high-index crystal plane is more conducive to the formation of oxygen vacancies, and its O2 adsorption and activation ability is stronger than that of (100) and (110) crystal planes, while the (110) crystal plane has stronger ability to activate and dissociate propane due to its lower electron energy[30][31]. The similar characteristics are also reflected in the α-Fe2O3, and the (110) crystal face has the strongest adsorption ability for ethane and O2, and has better catalytic performance than the (202) and (012) crystal faces[32]. The morphologies of Co and Ce oxides are significantly associated with the exposed crystal planes, as shown in Fig. 3 B, Co3O4-R（ nanorods), Co3O4-S（ nanosheets), Co3O4-C（ nanocubes) mainly expose the (110), (111), (100) crystal planes, respectively[33,34]. The (110) face is more open, has abundant unsaturated coordination atoms, the lowest oxygen vacancy formation energy and the polarization of electron distribution, and significantly promotes the adsorption activation process of O2 and the C — H activation of propane[34]. In addition, the Co2+ and Co3+ sites are exposed to the surface at the same time, and Wang et al. Clarified the difference between their roles through the inert metal ion substitution strategy, and found that the octahedral coordination Co3+Oh is the active center of benzene catalytic oxidation reaction.The Co3+Oh plays an important role in the process of ring-opening of benzene ring to form carboxylate, while the contribution of tetrahedral coordination Co2+Td is small, and the Co2+ will show significant catalytic activity only when it is in the octahedral coordination （Co2+Oh）[35]. The migration and transformation ability of oxygen species on the (110) plane with different CeO2 is different. The oxygen vacancy formation energy of the (110) plane is significantly lower than that of the (111) and (100) planes. The more (110) planes are exposed, the more favorable for the oxidation of toluene and polycyclic aromatic hydrocarbons[36][37][38].
To sum up, for single transition metal oxides, it is very important to further clarify the structure-activity relationship between morphology, exposed crystal plane, surface metal ion valence, coordination structure and catalytic activity for catalyst design and modification.

3.2 Mixed metal oxide

Single transition metal oxides generally can not meet the actual needs due to the limitations of redox and insufficient migration and transformation of oxygen species, and the introduction of doping elements into their structures can often induce the formation of crystal defects through the interaction between doping elements and oxides.The fluidity of the lattice oxygen of the catalyst is adjusted so as to improve the formation ability of oxygen vacancies on the surface of the catalyst and the adsorption and activation ability of VOCs. When a small amount of doping element is introduced, the catalyst usually maintains the original crystal structure to form stable doping, and when the compatibility between the parent oxide and the doping element is strong, a solid solution will be formed. When the concentration of the doping element is further increased or the compatibility between the parent oxide and the doping element is poor, the multiphase structure of phase separation tends to be formed, and the special structure of the phase interface can bring new properties to the catalyst.

3.2.1 Element doping

MnO2, Co3O4 and CeO2 are common element dopants. The MnO2 and CeO2 are mainly reviewed here, and the element doping and modulation in Co3O4 spinel are discussed in Section 3.3.1.
MnO2 has a special pore and lamellar structure, and impurity elements can replace the pore and lamellar associated ions to stabilize the [MnO6] structure gap, and can also replace Mn4+ to enter the [MnO6] structure. δ-MnO2 has a large interlayer heteroion capacity, and alkali metal ions and divalent cations such as Fe2+, Co2+, Ni2+, and Cu2+ can be stabilized in its interlayer[39⇓⇓~42]. Although the interlayer ions are not directly exposed to the surface, they have an effect on the properties of the [MnO6] layer. As shown in Figure 4A, compared with Na+ and K+, interlayer Cs+ can significantly reduce the oxygen vacancy formation energy of δ-MnO2, accelerating the reaction process of adsorbed oxygen and formaldehyde[43]. Similarly, the Sr2+ in the pore of α-MnO2 has a stronger ability to activate the lattice oxygen in [MnO6] than Mg2+ and Ca2+[44]. On the other hand, the catalytic activity of MnO2 can also be improved when the hybrid element is incorporated into the [MnO6] structure, and the redox performance of the catalyst can be improved through the Mx+/(x+1)+-Mn3+/4+ redox cycle. When the valence and coordination of the hetero element and Mn4+ are the same, such as the incorporation of Ti4+ into δ-MnO2, the coupling of [TiO6] and [MnO6] leads to the activation of oxygen species in the Ti-O-Mn unit of the octahedral common edge, and the activity of the catalyst for benzene oxidation is improved[45]. When the valence and coordination of impurity elements and Mn4+ are quite different, such as Fe, Co, Ni, Cu, Cr, etc., will have a significant impact on the structure of [MnO6], among which Cu has the best effect on the redox cycle of Mx+/(x+1)+-Mn3+/4+[46].
图4 （a）δ-MnO2不同层间离子掺杂对甲醛及其氧化中间物种吸附能的影响[43]，（b）MOF-71衍生Co-Ce混合氧化物及其丙酮氧化活性[73]

Fig. 4 (a) The effect of different interlayer ions in δ-MnO2 on the adsorption energy of formaldehyde and oxidation intermediate species[43], Copyright 2020, American Chemical Society; (b) MOF-71-derived Co-Ce mixed oxides and their acetone oxidation activity[73], Copyright 2020, American Chemical Society

The CeO2 has the advantages of excellent oxygen storage/release capacity, simple structure and relatively single substitution site of the impurity elements. Doping elements usually promote the generation of structural defects such as oxygen vacancies on the surface of CeO2. The DFT calculation results show that Mn3+, Zn2+ and Cu2+ doping have the most significant effect on the reduction of oxygen vacancy formation energy of CeO2（111）[47][48,49]. The Ce-O-M structure in the lattice can reduce the redox potential of doped elements and promote the redox cycle of Mx+/(x+1)+-Ce3+/4+. The introduction of Co into the lattice of CeO2 can increase the concentration of Co2+ and Ce3+, which is beneficial to the formation of oxygen vacancies and adsorbed oxygen species[50][51]. Another example is Cu-doped CeO2, which has significantly improved low-temperature redox performance and the best chlorobenzene oxidation activity[50]. The Cu+/2+ redox process promotes the transformation of Ce3+/4+ and accelerates the migration and transformation of oxygen species in CeO2[52]. When Cu is eluted from the middle of CeO2, the interaction between Ce3+ and Cu2+ is beneficial to the electron transfer between CuO and CeO2, which promotes the formation of oxygen vacancies and further improves the catalyst activity[53]. From the current research results, Cu is one of the most effective doping additives to improve the oxidation performance of single transition metal oxides VOCs.

3.2.2 Solid solution

Solid solution is another common form of mixed oxide. The doping element often uniformly replaces the parent metal element into the lattice without segregation or phase separation. Different parent oxides have different solid solution capacity for the doping element. Transition metals such as Ti, Mn, Fe, Zr and Ce usually form stable solid solutions due to their similar oxidation state, ionic radius and coordination state.
The three elements of Ti, Mn and Ce can maintain anatase TiO2, rutile TiO2, CeO2 and amorphous solid solution state respectively in a wide range of mixing ratio[54]. Among them, Mn-Ce solid solution has excellent oxygen storage/release ability, which has become a major research focus in the catalytic oxidation of VOCs, and it usually maintains a CeO2 crystal structure or a highly dispersed amorphous state. The surface Mn4+ content of Mn-Ce solid solution with CeO2structure is higher than that of MnOx, and the introduction of Mn promotes the oxygen vacancy formation ability of CeO2[55,56]. The DFT calculation also proves that the coordination number of O near the Mn site decreases, the activated C — H energy barrier decreases and the adsorption capacity of the catalyst for O2 is significantly enhanced[57]. Mn and Ce in the amorphous Mn-Ce solid solution are simultaneously in a disordered and highly dispersed state, have higher lattice oxygen activity and low-temperature oxidation-reduction property than MnOx and CeO2, and can stabilize benzene oxide VOCs under high humidity conditions[58]. Ce-Zr solid solution has higher thermal stability than Mn-Ce solid solution, and is often used as a carrier of precious metal active components in vehicle exhaust treatment[59]. Ce-Zr solid solution also performs well in the oxidation of Cl-containing VOCs (1,2-dichloroethane, trichloroethylene), and the introduction of Zr4+ increases the surface acidity and the number of active oxygen species of the catalyst, so that the oxidation ability of the catalyst for Cl-containing VOCs is higher than that of CeO2[60].
The properties of the solid solution can be further adjusted by continuously introducing doping elements into the solid solution. The introduction of Zr into the Mn-Ce solid solution can improve the specific surface area, the content of active oxygen species and the Cl resistance of the catalyst, and realize the improvement of the chlorobenzene oxidation ability[61].

3.2.3 Pecial precursor control

The synthesis of mixed metal oxides using specific precursor transformation is a new type of catalyst design method. Common precursors, such as Layered Double Hydroxides (LDH) and Metal-Organic Frameworks (MOF), can control the structure and properties of topologically transformed mixed Metal oxides by modulating the elemental composition and structure of the precursors.
(1) control of LDH precursor
LDH is a kind of anionic clay material (Mg-Al LDH), which is composed of layered metal hydroxides and interlayer anions.Zn2+ 、Ni2+ 、Cu2+ 、Co2+ 、Fe3+, Cr3+, Mn3+, etc. Can be stabilized in the LDH structure,NO3− with CO32− are common anions with the general structural formula [M1−x2+Mx3+(OH)2](An−)x/n·mH2O. LDH usually shows excellent performance in photocatalysis and electrocatalysis, but it is difficult to directly meet the needs of VOCs oxidation reaction because of its poor thermal stability[62]. Therefore, the stabilization of LDH precursor by heat treatment has become a common method to adapt to VOCs oxidation, and the obtained mixed metal oxides have high specific surface area, rich defect structure and high dispersion state of metal elements, which can be used as noble metal supports or as active components of catalysts alone[63]. Co-Mn-Mg-Al mixed oxides obtained by introducing Co2+, Cu2+, and Mn2+ into the M2+ site to partially replace Mg2+ have abundant phases, and the multiphase structure optimizes the redox properties of M-Al oxides, which makes them show better catalytic activity in butanol, ethanol, and toluene oxidation reactions[64]. When a transition metal is introduced into the M3+ site, the topologically transformed Co-Al oxide maintains a structure rich in Co3+, which increases the number of surface active sites and the surface acidity of the catalyst[65]. When the transition metal is introduced into the M2+ and M3+ sites at the same time, the structure and performance of the catalyst are further expanded. Jir Jirátová et al. Found that the Mn-containing ternary catalyst has outstanding activity for ethanol oxidation, and the Cu-Ni-Mn mixed oxide can promote the decomposition of acetaldehyde, which is the intermediate product of ethanol oxidation, and has the best catalytic performance[66]. In addition, the LDH precursor can also be used to synthesize spinel with a specific element composition by controlling the synthesis conditions.
(2) control of MOF precursor
MOF is a kind of polymeric porous material composed of metal ions and organic ligands.MOF is widely used in adsorption, energy storage, fine chemical industry and other fields because of its excellent specific surface area, （500~7000 m2/g）, pore structure and chemical properties[67]. The unique pore characteristics of MOF have become an efficient material for the adsorption and separation of VOCs molecules, and MOF-derived mixed metal oxides have also attracted attention in the field of VOCs catalytic oxidation[67]. Mn, Co and Ce-MOF derived oxides have excellent activity, and the introduction of doping elements is a new method to improve the performance of derived oxides[68][69,70][71]. When Ce is introduced into the Mn-MOF-74 precursor, the strong Mn-Ce interaction in the obtained MnOx-CeO2-MOF is stronger than that in the coprecipitation method, and the proportion of Mn4+ on the catalyst surface and the content of adsorbed oxygen species are increased[72]. Similarly, as shown in Figure 4B, the use of precursors such as MOF-71 can also achieve a high dispersion of Co and Ce in the mixed oxide, enhance the surface acidity and promote the formation of oxygen vacancies[73⇓~75]. Although the use of MOF precursor to synthesize catalysts has the advantages of large specific surface area and sufficient metal mixing state, the high preparation cost limits its practical application in VOCs emission control, so reducing the cost of MOF will become an important trend in future research.

3.3 Composite metal oxide

Spinel, perovskite, mullite and other composite metal oxides have the properties of high thermal stability and strong controllability of surface and bulk stoichiometry. Compared with mixed metal oxides, the synthesis temperature of perovskite and mullite is higher (> 600 ℃), and their characteristics of less element segregation and phase separation have attracted the attention of many researchers. In recent years, there have been many studies on the control methods of surface and bulk structures of composite metal oxides and the effects of modified structures on the catalytic oxidation of VOCs.

3.3.1 Spinel

The 5）ATd and BOh sites in the （AB2O4）（ diagram of spinel-type composite metal oxides are different metal elements or different valence States of the same element, and Mn, Co and Fe-based spinels are mainly used for VOCs catalytic oxidation. Due to the interaction between transition metals in the ATd-BOh site and the charge transfer process, spinels often show better oxygen species migration ability and catalytic performance than single oxides.
图5 尖晶石晶体结构示意图

Fig. 5 Schematic diagram of spinel crystal structure

The selection of ATd site elements has a significant effect on the low temperature redox and O2 activation ability of the catalyst, and the CuMn2O4 shows better toluene oxidation activity than the NiMn2O4 and ZnMn2O4.Cu+/2+ with high electronegativity and variable valence can promote the redox cycle between ATd-BOh sites relative to Ni2+ and Zn2+, which makes O2 more easily activated to active oxygen species[76]. The appropriate introduction of Cu2+ into the ATd site of Co3O4 spinel can also cause the lattice distortion of Co3O4, reduce the electronegativity of Co2+Td, and increase the reactivity of 3D orbital electrons. However, the appropriate introduction of Ga3+ at the BOh site can promote the reduction of Co3+Oh, accelerate the Co2+-Co3+ cycle of the catalyst, and enhance the reaction activity[77]. Chen et al. Designed to introduce Cu2+ into the ATd site of Co-Mn spinel to increase the proportion of Co/MnOh directionally, which intensified the Jahn-Teller effect of [Co/MnO6] and weakened the Co/MnOh—O bond energy to enhance the lattice oxygen mobility of the catalyst[78].
Adjusting the proportion of bulk or surface ATd and BOh site elements is also a common method to modify spinel catalysts. Faure et al. Found that the Co and propane oxidation performance was closely related to the specific surface area and Co concentration by optimizing the element ratio of Co-Mn spinel[79]. For another example, adjusting the element ratio in Mn-Fe spinel can optimize its toluene oxidation performance. The surface of Mn2.4Fe0.6O4 has the highest enrichment degree of Mn, which promotes the formation of oxygen vacancies and has the highest Turn-over Frequency (TOF)[80]. Enriching Mn on the surface of Fe3O4 spinel can enhance the oxidation of Fe3+, and the formaldehyde oxidation activity of the obtained Mn-Fe spinel is positively correlated with the surface Mn4+ reduction ability, and the surface substitution of Mn can improve the redox property and the amount of surface active lattice oxygen of the catalyst[81].

3.3.2 Perovskite

In the perovskite-type composite metal oxide （ABO3） (Fig. 6), the cuboctahedral coordination (tetrakaidecahedron, with the coordination number of 12）AC-Oh sites are usually alkaline earth metal or La-series metal elements with larger radius, and the BOh sites are usually trivalent or tetravalent transition metal elements, which are mainly Mn, Co and Fe-based perovskites used for VOCs catalytic oxidation. Perovskite is an ideal matrix for structural control because of its complex structure, wide variety of cations, high tolerance to changes in ionic charge distribution, （A+B5+O3, A2+B4+O3, and A3+B3+O3）, which make it easy to show non-stoichiometric properties (such as cation defects and oxygen vacancies)[82].
图6 钙钛矿晶体结构示意图

Fig. 6 Schematic diagram of perovskite crystal structure

The surface properties of the catalyst can be changed by adjusting the types and proportions of the elements at the AC-Oh and BOh sites in the perovskite structure. LaMnO3 and SmMnO3 are common Mn-based perovskites. In LaMnO3,The doping of Ag+, Sr2+, Sn4+ at the AC-Oh site can weaken the Mn — O bond, promote the surface lattice oxygen mobility and reduce the activation energy of O2 dissociation[83]. The redox process of BOh site elements is an important step in the oxidation of VOCs, and the direct change of the type of BOh site elements will have a greater impact on its catalytic activity. For example, in different BOh elements, the catalytic activity of LaFeO3 for hexane oxidation is better than that of LaMnO3 and LaCoO3.And that ability of LaFeO3 to activate C — H bond of hexane is even higher than that of PdO/Al2O3[84]. When the BOh bit introduces a different element,The formed double BOh site LaCo0.5Mn0.5O3 and the LaCu0.5Mn0.5O3 perovskite are compared with those of LaCoO3,LaMnO3 and LaCuO3 have lower apparent activation energy for the oxidation of toluene, isopropanol, ethanol and ethylene[85].
The AC-Oh site defect regulation is usually able to elevate the BOh site oxidation state and promote the generation of catalyst oxygen vacancies. Our group used urea-assisted synthesis to create some La vacancies on the surface of LaMnO3, which led to the shortening of Mn — O bonds, the reduction of orbital order and the weakening of Jahn-Teller effect, and the significant activation of lattice oxygen. After promotion and verification, this strategy can be effectively extended to LaFeO3 and LaCoO3 perovskites, which opens up an innovative way for perovskite defect control[86]. In addition, non-stoichiometric synthesis is another important method to modulate defects, and when the stoichiometric deviation is small, the perovskite host structure is usually maintained, for example, when La: Mn is between 0.9 and 1.11, the LaMnO3structure is maintained, and partial La deficiency will induce the transformation of Mn3+ to Mn4+[87]. When the deviation of the stoichiometric ratio is large, the AC-Oh site or BOh site elements will precipitate out of the perovskite parent in the form of oxides, such as La2O3/LaCoO3, Mn3O4/LaMnO3, etc[88][89].
On the other hand, the perovskite surface usually exposes AC-Oh sites, and the AC-Oh sites are mostly non-variable or difficult-to-variable metals, which are inert in the catalytic oxidation of VOCs. In order to remove the inert sites on the catalyst surface, researchers developed a selective dissolution method based on the common dealloying method, which fully exposed the transition metal active sites on the BOh of perovskite surface and greatly increased its specific surface area[90]. Taking LaMnO3 perovskite as an example, HNO3 is often used for the selective dissolution of AC-Oh site ions.The bond energy of the AC-Oh—O is weaker than that of the BOh—O, and the La3+ is easily dissolved to expose the Mn3+ at the BOh site. However, Mn3+ will undergo disproportionation reaction under acidic conditions, and Mn2+ will be leached while MnO2 is generated[91]. Our research group used this property to develop MnO2/LaMnO3 for toluene oxidation. After the surface La3+ was removed, the content of adsorbed oxygen species on the catalyst surface was significantly increased, the apparent activation energy of toluene oxidation was significantly decreased, and the catalyst also showed excellent stability in the reaction process[92]. Since MnO2 is insoluble in strong acids such as HNO3 (except concentrated HCl), Mn-based perovskites will not be completely dissolved even with high concentration of H+ and long dissolution time. However, for Co and Fe perovskites, Co3+ and Fe3+ are easily soluble in strong acid, which easily causes excessive loss of Co3+ and Fe3+ in the process of ion dissolution at the AC-Oh site. For this purpose, the contact time of the catalyst with the HNO3 can be shortened or a weak acid can be used for selective dissolution. Oxygen vacancies and active oxygen species on the La1-δFeO3 and La1-δCoO3 of the surface-removed La3+ are increased, and the CO and toluene oxidation ability of the surface-removed perovskite is significantly better than that of the original perovskite[93,94]. According to the difference of metal properties between the AC-Oh site and the BOh site, the selective dissolution method is not limited to acid treatment, and the removal of surface inert sites can also be achieved by using NaOH, ethylene glycol, etc[95,96]. Because the selective dissolution overcomes the defect of the enrichment of inert sites on the perovskite surface, the further improvement and development of this method is one of the important research directions for the regulation of perovskite catalysts in the future.

3.3.3 Mullite

The mullite-type composite metal oxide （AB2O5） group is relatively complex, and Mn-based mullite is the most common in VOCs oxidation (Fig. 7), with Mn4+ and Mn3+ occupying the BOh site and BTd site, respectively.The A-site ion is stabilized in the gap between [MnO6] and [MnO4] in the form of 8-coordination, which is usually a group IIIB element or a La-series metal element[97].
图7 Mn基莫来石晶体结构示意图

Fig. 7 Schematic diagram of Mn-based mullite crystal structure

The synthesis temperature of Mn-based mullite is higher than that of spinel and perovskite (usually more than 800 ℃), and its thermal stability is excellent, which is inversely proportional to the ionic radius of A-site[98]. Similar to perovskite, A-site ions in mullite are not redox sites, but their radius and electronic structure can affect the properties of Mn — O bond, and the change of A-site can affect the degree of p-d hybridization between MnOh—O, thus changing the interaction between external active oxygen and MnTd and affecting its redox properties[99]. The removal of surface A-site elements is also an effective way to improve its redox ability, due to the existence of Mn4+Oh in mullite, which can inhibit the deep etching of H+ to the bulk structure, while Mn-based perovskite will be completely etched to MnO2 under the action of excessive high concentration of H+, losing the main structure of perovskite. Our research group uses selective dissolution method to remove Sm3+ on the surface of SmMn2O5, and the surface of Sm1-δMn2O5 is enriched with Mn4+Oh.The oxidation-reduction property and the oxygen vacancy generation ability are improved, and the propylene oxidation activity is better than that of the SmMn2O5 and the 1 wt%Pt/Al2O3[100]. Due to the inhibition effect of Mn4+Oh on acid etching, all A-site ions can be dissolved out from the matrix by increasing the operating temperature of selective dissolution, and the selective dissolution of YMn2O5 at 90 ℃ can transform it into a α-MnO2 structure and release a large number of surface Mn4+Oh active sites, which shows more excellent performance than commercial Pt/Al2O3 in benzene catalytic oxidation[101].
It can be seen that the A-site and B-site regulation of spinel activity can efficiently optimize the redox capacity and surface properties of the catalyst. In contrast, the A-site of perovskite and mullite catalysts is inert in reaction.Its properties are more sensitive to the improvement and optimization of A-site, which is an important reason for the modification of perovskite and mullite catalysts. The controllable substitution and removal of A-site ions under the premise of maintaining the high stability of the matrix is still the focus of future research.
To sum up, there are many kinds of composite metal oxides, not limited to spinel, perovskite and mullite structures. Because of their excellent thermal stability, they have become one of the mainstreams in the research and development of high temperature VOCs catalysts. In addition, the wide range of stoichiometric control and the large capacity of doping elements also make the research and development of directional control methods of composite metal oxides become the focus of research in the future.

3.4 Phase interface structure control

The control of phase interface structure is common in the design of supported catalysts, and the selection and structure design of carriers and supported components are often more targeted to improve the performance of catalysts. In order to clarify the interaction mechanism between the support and the supported component at the phase interface and its effect on the catalytic performance, most of the current studies have selected materials with regular and uniform morphology as the support.

3.4.1 Supported heterostructure

When the content of the supported component is low, the supported component is mostly dispersed on the surface of the carrier in the form of nanoparticles or nanoclusters, forming an "island" supported heterostructure.
(1) Redox carrier
MnO2 and CeO2 with specific morphologies are often used as supports, and the oxide nanoparticles loaded on their surfaces can control the redox properties of the catalysts, and the abundant defects at the interface between the two phases can also enhance the reactivity of oxygen species. For example, CuO and CeO2 nanoparticles loaded on the surface of α-MnO2 nanorods can construct the phase interface of CuO-MnO2 and CeO2-MnO2 with strong interaction,The reduction of Mn4+ and the generation of oxygen vacancies are promoted, and the oxygen species migration ability and surface acidity of CuO and CeO2 nanoparticles are improved[102,103]. The toluene oxidation performance of CeO2 can be significantly improved by loading low content of CoOx on the surface of CeO2, and the dispersion state of CoOx on the surface of CeO2 with different morphologies/crystal planes is different.The highly dispersed CoOx species of the (111) facet exhibit better redox properties than the (100) and (110) facets, indicating that the CoOx-CeO2 interface changes the reactivity of the CeO2 facet[104].
(2) non-redox carrier
In addition to transition metal oxides, Al2O3, SiO2, zeolite molecular sieves, activated carbon and carbon nanotubes can also be used as supports and have outstanding potential for practical applications. For example, after 150 cycles of regeneration in mixed VOCs atmosphere, the catalytic activity and structural composition of MnOx/Al2O3 catalyst have no significant change, and the operation stability of actual working conditions is outstanding[105]. Zeolite molecular sieve support can bring the advantages of high specific surface area, high mechanical strength and stability to the catalyst. Mesoporous molecular sieves (MCM-41, MCM-48, BMS, KIT-6, SBA-15, SBA-16, HSM, etc.), Beta zeolite, Y-type zeolite, ZSM-5 zeolite, etc. Are often used as carriers in VOCs oxidation[106][107]. The pore structure of molecular sieve can inhibit the excessive agglomeration of its surface active components and improve the dispersion of oxides. FeOx, MnOx, Co3O4, CuO and CeO2, etc., are often used as active components[106]. He et al. Investigated the chlorobenzene oxidation performance of different transition metal supported KIT-6, and found that Mn/KIT-6 had the best activity, Mn was uniformly dispersed on the surface of KIT-6, and it could operate stably for more than 1000 min near 90% chlorobenzene conversion[108]. The spatial confinement of SBA-15 can also make the active components of Fe and Ce form Fe-Ce solid solution and highly dispersed active centers[109]. On the other hand, adjusting the acidity of the zeolite support can affect the oxidation performance of the catalyst for Cl-containing VOCs, and a suitable silica-alumina ratio of ZSM-5 can significantly improve the HCl and CO2 selectivity of the catalyst and reduce the formation of by-products[110]. Carbon materials have poor high temperature resistance, but they have large specific surface area and strong VOCs adsorption performance, so loading MnOx on their surface can achieve efficient decomposition of formaldehyde at room temperature, such as δ-MnOx/ activated carbon, MnO2/NCNT（N doped carbon nanotubes)[111][112].

3.4.2 Core-shell heterostructure

When the content of the support and the supported component in the supported catalyst is close, the support is wrapped by the supported component, and the density of the phase interface increases, the catalyst shows significant core-shell heterostructure properties, and the morphology, dimension and defect distribution of the heterointerface will significantly affect its catalytic performance[113].
(1) same element oxide interface control
MnOx is one of the preferred materials for heterostructure design because of its rich phase and structure, and different MnOx can form a variety of heterointerfaces. When Mn-based perovskite with high thermal stability is selected as the catalyst substrate, the heterogeneous catalyst can maintain excellent structural stability. Our group used H+/KMnO4 to treat LaMnO3 to construct LaMnO3@MnO2 heterostructure. Compared with the selective dissolution of LaMnO3 process, this method can better maintain the matrix structure of LaMnO3 and maintain excellent stability in toluene oxidation[114]. As shown in Fig. 8, based on this, our research group further selected Mn2O3 with better redox ability to replace LaMnO3 to design and construct the Mn2O3@δ-MnO2 heterogeneous catalyst. A transition region of mixed oxidation state exists at the heterointerface of the MnO2-Mn2O3, the abundant oxygen vacancy on the δ-MnO2 side can promote the activation of toluene methyl group, and the redundant coordination lattice oxygen on the Mn2O3 side is beneficial to the ring opening of benzene ring, so the catalyst not only shows excellent stability, but also has certain high temperature aging resistance[115]. In the follow-up study, a surface in-situ doping modification method was developed to construct a Cu-doped δ-MnO2 phase on the surface of the Mn2O3, and the interlayer Cuδ+ in the δ-MnO2 further increased the oxygen vacancy concentration of the catalyst, and significantly improved the C — H and C = C activation ability of the catalyst for toluene and propylene[116]. MnO2 with different crystal forms, such as α-MnO2 and δ-MnO2, can also form stable heterostructures, which provide new ideas for the design of Mn-based catalysts[117].
图8 Mn2O3@δ-MnO2核壳异质结构催化剂构建应用于甲苯氧化[115]

Fig. 8 Construction of Mn2O3@δ-MnO2 core-shell heterostructure catalysts for toluene oxidation[115], Copyright 2020, Elsevier

(2) Interface control of oxides of different elements
Morphologically controlled core-shell heterostructures can also be formed between MnOx, CoOx, CuOx, CeO2, etc., and the high-density heterointerface can enhance the intermetallic interaction and promote redox cycling through the M1-O-M2 structure. If the Co3O4 shell is wrapped outside the α-MnO2, the oxygen species migration process can be accelerated by promoting the Mn3+/4+-Co2+/3+ cycle through the heterogeneous interface[118,119]. In addition, the heterostructure can also improve the water resistance of the catalyst. For example, Zheng et al. Prepared CoCuOx@CuOx catalyst by in-situ growth of CoCu-MOF on foam Cu, and the abundant heterointerface formed fully activated the Co — O bond and promoted the formation of oxygen vacancies, which made it show excellent stability and water resistance in acetone oxidation[120]. The synthesis of heterostructure with SiO2 layered nanotubes as the carrier also provides an innovative idea for improving the water resistance of the catalyst, and the surface — OH group in the special "fiber" structure of the layered silicate can inhibit the competitive adsorption of H2O in the toluene oxidation reaction, which plays a hydrophobic role[121].
To sum up, there are many ways to design and regulate the structure of non-precious metal catalysts. In recent years, in addition to improving the activity of catalysts, the research on the improvement and enhancement of catalyst product selectivity, stability and water resistance has gradually increased.The research on the key factors affecting the catalytic performance is also more in-depth, and the modification of non-precious metal catalysts will remain the focus of future research.

4 Noble metal catalyst noble metal dispersion state control

Noble metal catalysts are usually composed of noble metal active components and carriers. In VOCs catalytic oxidation, the former is mostly the simple substance, alloy, oxide or mixture of Pt, Au, Pd, Ag and other noble metals, while the latter can be inert carriers (non-redox carriers, such as Al2O3, SiO2, zeolite molecular sieves, etc.) Or active carriers (redox carriers, such as various transition metal oxides in Section 3). The dispersion state of noble metal active components on the support surface can be divided into Nano Particles (NPs), Nano Clusters (NCs) and Single-atoms (SAs). Factors such as the nature of the support and the synthesis method of the catalyst will affect the dispersion state of the precious metal on its surface, resulting in differences in metal-support interaction, thus affecting the reaction performance of the catalyst[122,123]. Although noble metal catalysts have been widely used in the field of VOCs catalytic oxidation, their cost control is still a common concern of academia and industry. In recent years, it has been the focus of researchers to explore the dispersion mechanism of precious metals, and to improve the intrinsic activity and atom utilization of active sites by regulating the dispersion state of precious metals, so as to reduce the cost of catalysts. In the following, the design and regulation of noble metal nanoparticles/clusters and noble metal single-atom catalysts are reviewed.

4.1 Noble metal nanoparticle/cluster catalyst

When noble metal atoms are loaded on the surface of the support in the form of aggregates, the electronic structure of the aggregates is strongly dependent on the number and geometry of the atoms due to the overlap of the orbitals between the atoms. For clusters with particle size < 1 nm (usually less than 40 atoms), both the number of atoms and the geometric structure have a significant impact on their electronic structure and the surface charge state of the support; When the particle size increases to more than 1 nm, the geometry of the nanoparticle is relatively stable, although the geometry of the exposed surface atoms (facet, corner, edge, metal-support interface, etc.) May be different, but the size effect of the nanoparticle may play a dominant role in its properties[122]. In the catalytic oxidation of VOCs, noble metal nanoparticles are easy to synthesize, easy to control and have high catalytic activity. At present, most of the studies focus on the size effect and carrier effect. However, the synthesis conditions of nanoclusters are relatively harsh and more suitable for the selective oxidation of hydrocarbons. For example, Au, Ag and other NCs perform well in propylene epoxidation, benzyl alcohol selective oxidation and other reactions, but they are relatively rare in the complete oxidation of VOCs[124,125][126].

4.1.1 Size effect

Pt, Au, Pd, Ag and other nanoparticles are highly active components in the catalytic oxidation of VOCs. When facing different VOCs molecules, their catalytic activity is often different, but they all have significant size effect in the oxidation of VOCs.
(1) Pt, Au nanoparticles
Pt and Au NPs usually exist on the surface of the support in the form of simple substance, and Pt NPs have high catalytic oxidation activity for VOCs such as benzene series and formaldehyde. Kinetic experiments showed that the adsorption rate of O2 on Pt NPs was low, and the content of active Pt-O species on the catalyst surface increased with the increase of Pt particle size, and the Pt NPs with larger particle size was more conducive to the catalytic oxidation of benzene in the study range[127]. Within a certain range, the TOF of PtNPs/TiO2 for formaldehyde oxidation increases almost linearly with the particle size, and the ratio of (100) and (111) active crystal plane atoms increases, but when the particle size continues to increase, the specific surface area and reaction TOF of Pt NPs decrease significantly, which is not conducive to the reaction, so selecting the appropriate particle size range according to the molecular characteristics of VOCs is the core strategy to optimize the Pt atom utilization and reactivity[128]. As shown in Fig. 9, in the classical PtNPs/CeO2 system, the Pt-O-Ce species in the range of 1.3 ~ 2.5 nm increase with the increase of Pt particle size, which can promote the generation of Ce3+ and oxygen vacancies around them, while too large particle size also leads to poor toluene oxidation activity of the catalyst due to the decrease of Pt dispersion[129]. When facing different VOCs molecules, the optimal particle size of Pt may be different, and dichloroethane has higher reactivity when the particle size of Pt NPs is more than 2. 95 nm. The Pt2+ content is negatively correlated with the surface acidity of the catalyst, and the acid sites are important active sites for the adsorption and dechlorination of dichloroethane. With the increase of particle size, the electron transfer between Pt and carrier is weakened, the number of Pt2+ species is reduced, and a large number of surface acidic sites are retained, so the optimal Pt particle size is larger than that of VOCs such as benzene, n-hexane and ethyl acetate[130].
图9 Pt NPs粒径与CeO2表面氧空位和活性氧物种含量的关系

Fig. 9 The relationship between the particle size of Pt NPs and the surface oxygen vacancies/active oxygen species content of CeO2

Au NPs/NCs catalyst has been widely concerned due to its high catalytic activity for CO oxidation. As early as 1996, it was found that Au dispersed on the surface of supports such as α-Fe2O3 and TiO2 had good applicability for the oxidation of VOCs such as methanol, formic acid and formaldehyde[131]. Au on different types of support surfaces will also show similar size effects, which usually affect its oxygen species activation ability by changing its electronic interaction with the support Mn+. On the surface of MnO2, Au NPs with an average particle size of about 3.05 nm showed the highest activity for toluene oxidation, and the appropriate particle size of Au NPs optimized the interaction with the support and the degree of electron transfer, and promoted the formation of oxygen vacancies on the surface of MnO2[132]. On the surface of Co3O4, there is an electron transfer process between Au and Co3+, and the increase of Au NPs particle size leads to the partial reduction of surface Co3+ to Co2+, and the content of adsorbed oxygen species and redox properties on the surface of AuNPs/Co3O4 reach the optimum when the average particle size is 2. 8 nm[133]. Similar phenomena can also be observed in AuNPs/Fe2O3 and AuNPs/CeO2. Au NPs with moderate particle size can weaken the Fe — O and Ce — O bonds, activate the carrier lattice oxygen, and enhance the VOCs oxidation activity[134,135].
(2) Pd, Ag nanoparticles
Pd and Ag NPs can exist in the form of simple substance or oxide (such as PdO) and Ag2O）, depending on the synthesis method and support. Its oxidation state is often related to the particle size, which has an impact on the ability of the catalyst to activate oxygen species. Wang et al.Elemental Pd NPs in Al2O3, TiO2, SiO2, CeO2,The oxidation activity of o-xylene on MnO2 and Co3O4 supports was higher than that on PdOx[136]. More Pd0 species could be retained on the surface of PdNPs/CeO2, which balanced the benzene oxidation activity with the TOF of Pd species[137]. However, the VOCs oxidation activity of Pd and PdOx is still controversial. For example, on the surface of carbon materials, Pd NPs with larger particle size will be partially oxidized to PdO species during heat treatment, and the PdO species produced have higher toluene oxidation activity than Pd0[138].
The oxidation state of Ag NPs is more sensitive to the synthesis conditions and particle size of the catalyst. The elemental Ag NPs will be gradually oxidized in the air, and the Ag2O can be reduced back to the elemental state after calcination at 400 ℃[139][140]. However, in AgNPs/α-MnO2 and AgNPs/CeO2, due to the strong interaction of Ag-support, Ag2O can still be observed after calcination at 450 ℃[141,142]. The larger Ag NPs on the surface of α-MnO2 are more inclined to form abundant Ag2O species, and the appropriate Ag NPs particle size balances the ratio of Ag0 and Ag2O species, which is beneficial to the activation of oxygen species and the formation of oxygen vacancies at the Ag-support interface, thus enhancing the TOF of Ag[142]. It is not difficult to see that the size of nanoparticles not only affects their interaction with the support, but also is closely related to the oxidation state, which jointly affects the VOCs oxidation performance of Pd and Ag NPs catalysts. Therefore, it is still one of the challenges to clarify the relationship between the size, oxidation state of Pd and Ag NPs and the oxidation performance of VOCs.

4.1.2 Carrier effect

The VOCs oxidation activity of noble metal nanoparticles/clusters is not only related to the particle size, but also the dispersion state, electronic structure and interaction with the support, which are affected by the type, redox, morphology, exposed crystal face and special structure of the support.
(1) Effect of intrinsic properties of the carrier
According to the properties of noble metal and support, the type and structure of support, redox ability, surface defect concentration and so on will have a significant impact on its catalytic activity.
In terms of support types, Lu et al. Showed that Pt NPs dispersed on the surface of TiO2 exhibited higher toluene oxidation activity than SiO2 and CeO2, and the moderate redox ability of TiO2 was conducive to the transfer of active adsorption species to the surface of Pt NPs, which promoted the oxidation process of toluene and O2 at Pt sites[143]. In addition, the dispersion of noble metals is also affected by the difference of metal-support interaction, such as Au and Ag NPs have different dispersion on the surface of TiO2, ZnO, Al2O3 and CeO2[144,145]. The highly dispersed Au NPs on the surface of TiO2 can form stable Au+ species on the interface, which is more conducive to propylene oxidation, while the abundant defects on the surface of ZnO can stabilize Au species to prevent their sintering deactivation[145].
The crystal form of the support usually affects the dispersion state and oxidation state of the noble metal. The oxidation state and particle size distribution of Ag NPs on the surface of anatase phase (A) and rutile phase (R) are different, and their adsorption and activation abilities for O2 are also different. The abundant oxygen species and Ag2O on the surface of AgNPs/A-TiO2 promote the oxidation of formaldehyde in the low temperature range, while Ag0 plays a dominant role in the high temperature range, and AgNPs/R-TiO2 has poor activity at low temperature due to the lack of Ag2O[146].
The defect content of the support can change the oxygen activation performance by affecting the metal-support electron transfer, and the regulation of the Ti3+ defect content of the TiO2 surface can affect the dispersion of Pd and the formation of oxygen vacancies.The electron transfer from oxygen vacancies to Pd species leads to the decrease of the oxidation state of Pd NPs, and the abundant defects can enhance the electron contribution effect of Pd to the antibonding π * orbital of O2, which is beneficial to the activation of O2[147].
(2) The influence of the morphology of the support and the exposed crystal plane
The morphology and exposed crystal face of the support are important factors affecting the coordination environment and charge distribution of the surface atoms, and the morphology and crystal face effect is a hot topic in the study of noble metal catalysts. The typical morphology and crystal plane effect can be observed in PdNPs/CeO2, AgNPs/MnO2, AuNPs/Co3O4 and other systems[148][149][150].
On the surface of CeO2 with different morphologies, the electron transfer process from Pd NPs to Ce4+ significantly promoted the reduction of Ce4+ and the generation of oxygen vacancies, resulting in spindle-shaped nanoscale CeO2 with higher activity for ethyl acetate oxidation than nanospheres and nanocubes[148]. The morphology of MnO2 has a great influence on the particle size distribution of surface Ag NPs, and the electron transfer phenomenon between Ag NPs and the surface of MnO2 nanowires is the most significant, which fully activates the lattice oxygen of MnO2[149].
On the other hand, the exposed crystal face of the carrier also affects the dispersion and oxidation state of the noble metal, and Co3O4 and CeO2 with good morphology-exposed crystal face correlation are ideal carriers for studying the crystal face effect. In the Co3O4（112）, (001) and (111) exposed facets, Au NPs further increased the content of active oxygen species on the (112) surface, and the higher degree of coordination unsaturation of Co2+/3+ on the (112) facet and the synergistic effect of Au NPs promoted the catalytic oxidation of benzene[150]. The dispersion degree of Pt on the surface of CeO2（110） is higher, and there is a significant electron transfer process between Pt NPs and the surface of CeO2（110）, which can promote the adsorption and activation of O2 in the L-H mechanism of toluene oxidation[151]. The oxidation state of Pd NPs was also affected by the different crystal planes of CeO2. The strong Ce-O bond on the (111) crystal face is beneficial to the stabilization of the PdOxNPs, which can promote the cleavage of C-H bond and the decomposition of carboxylate during propane oxidation. However, on the (110) and (100) surfaces, the Pd2+ species tend to form the PdxCe1-xO2-δ partial solid solution structure, and the oxygen vacancy generation ability of the Pd2+-O2−-Ce4+ structure is stronger, which is more conducive to CO oxidation (Fig. 10a)[152].
图10 （a）Pd负载于不同形貌CeO2表面时其CO与丙烷氧化反应速率[152]，（b）MOF原位生长诱导Co3O4与Pt NPs间电子转移促进甲苯氧化机理示意图[153]

Fig. 10 (a) The reaction rate of CO and propane oxidation over Pd loaded on CeO2 with different morphologies[152], Copyright 2016, American Chemical Society; (b) Schematic diagram of electron transfer between Co3O4 and Pt NPs induced by MOF in situ growth promoting toluene oxidation[153], Copyright 2022, American Chemical Society

(3) Carrier structure optimization and new carrier research and development
At present, the modification of supports, such as element doping, improvement of synthesis methods, and structural regulation, has become an effective means to improve noble metal catalysts. For example, when Mn and Fe were introduced into the Co3O4, the surface of the Co3O4 was significantly reconstructed and the structural defects were increased, which enhanced the electron transfer from the Co3O4 to the Au NPs, and the negatively charged Au NPs had a higher TOFAu for benzene oxidation, which reduced the apparent activation energy of the reaction[150]. In terms of the improvement of the synthesis method, combined with the performance advantages of MOF, as shown in Fig. 10B, Xiao et al. Developed the MOF in situ growth method to synthesize PtNPs/Co3O4, which fully induced the electron transfer from Co3O4 to Pt NPs, and the generated Ptδ− species was beneficial to the electron transfer to it during the activation of O2, forming nucleophilic active oxygen species and improving the ability of catalyst to oxidize C — H[153]. In addition, the dispersion state and chemical environment of Pt on the surface of Al2O3 can be optimized by adjusting the solvent in the loading process. A Pt-Al(OH)x structure can be formed at the interface between Pt NPs and Al2O3. The strong interaction between Pt NPs and Al2O3 weakens the Pt — O bond and enhances the adsorption and activation of toluene. The Pt0 species play a key role in the deep oxidation of toluene[154]. In terms of carrier structure regulation, such as the tandem bifunctional ZSM-5-AgNPs/SBA-15 catalyst designed and developed by Li et al., which has two active sites with complementary functions[155]. Formaldehyde was converted to methyl formate by ZSM-5 acid sites, and then diffused to the AgNPs/SBA-15 surface for complete oxidation.Compared with the dioxomethyl-formate oxidation pathway on the AgNPs/SBA-15 surface alone, the tandem bifunctional catalyst significantly reduced the temperature required for formaldehyde oxidation by changing the reaction pathway.
In addition, new supports, such as non-oxide supports, often have special electronic and vacancy properties different from traditional oxides, and have become the research frontier of noble metal nanoparticle catalysts. Nitrogen vacancies containing excess electrons such as in PtNPs/C3N4 can effectively stabilize Pt NPs through strong p-d coupling. The Pt atom and the dangling C atom around the vacancy can cooperate to donate electrons to the antibonding orbital of the adsorbed O2, thus achieving O2 activation[156]. This study proves that the electron-rich oxygen-free carrier surface can also realize the complete oxidation of toluene and other macromolecular VOCs, which breaks the conventional understanding of the selection of VOCs oxide carriers and opens up new ideas for the design and development of VOCs oxidation noble metal catalysts.
To sum up, the size control of noble metal species and the structure control of support in noble metal nanoparticle/cluster catalysts will still be the focus of future research. With the rapid development of characterization technology, a lot of research efforts will be devoted to the metal-support interface structure, interface-reactant interaction, reaction mechanism, etc., to further clarify the influencing factors and mechanism of metal-support interaction, and to clarify the structure-activity relationship between catalyst structure and VOCs catalytic oxidation performance.

4.2 Noble metal single-atom catalyst

Single-atom catalyst is a new kind of noble metal catalyst in recent years. In 2011, Qiao et al. Supported Pt SAs on the surface of FeOx to achieve better low-temperature CO oxidation performance than AuNPs/Fe2O3, which makes the research and development of single-atom catalyst rapidly become the frontier and hot spot[157]. Monoatomic dispersion can maximize the utilization of precious metals (theoretically 100%), provide a feasible way for the reduction of precious metals, and also strengthen the metal-support interaction and promote the charge transfer between precious metals and the support surface. In addition, unlike particles/clusters, the degree of single-atom coordination unsaturation is higher, which can often affect the interaction between catalyst and reactant and reduce the activation energy, maintaining the advantages of homogeneous catalysts and heterogeneous catalysts[123,158,159]. These characteristics make single-atom catalysts widely used in photocatalysis, electrocatalysis, fine chemicals and other fields[160]. Its application and promotion in the field of environmental catalysis are in the stage of rapid development, such as selective catalytic reduction of NOx, CO oxidation, VOCs catalytic oxidation, diesel exhaust treatment and Fenton reaction[158,161,162].

4.2.1 Strategies for the preparation of single-atom catalysts

As shown in fig. 11, the preparation methods of single-atom catalysts are developing rapidly, and the loading controllability is improving day by day. At present, the appropriate synthesis method can be selected according to the target carrier, the nature of the noble metal and the target reaction. The strategies to achieve monatomic dispersion of noble metals can be summarized as follows: (1) "bottom-up" strategy (loading noble metals directly on the surface of carriers in monodisperse form), such as monodispersion of noble metal precursors, carrier modification to provide anchor sites, etc.; (2) "top-down" strategy (redispersion of metal colloids/nanoparticles/aggregates, etc., into single atoms by treatment), such as redispersion of heat-treated nanoparticles, etc[159]. Based on the above strategies, the commonly used preparation methods of single-atom catalysts include atomic layer deposition, mass selective soft deposition, special structure space confinement, electrostatic adsorption, carrier defect control, surface ligand modification, nanoparticle thermal dispersion and so on[162].
图11 单原子催化剂的制备方法[162]

Fig. 11 Synthesis strategies of single-atom catalysts[162], Copyright 2021, American Chemical Society

Atomic layer deposition and mass selective soft deposition synthesis have high accuracy, but the synthesis equipment is complex and the material recovery rate is low, so it is generally not suitable for large-scale industrial applications[163]. The special structure space confinement method is often used for periodic porous material supports such as zeolite molecular sieves and MOFs, which can usually provide specific space sites for anchoring noble metal atoms, but this method is limited by the space structure of the support, and it is difficult to completely remove the noble metal precursor ligand in the post-treatment process[164][165][162]. The electrostatic adsorption method is to achieve the dispersion of single atoms of precious metals by adjusting the pH value of the mixed solution of metal precursor and carrier to induce the electrostatic adsorption between the carrier and metal ions[166]. When the controllability of carrier defects is strong, noble metals can be anchored through their surface structural defects, such as metal oxide cation vacancies, anion vacancies (oxygen vacancies), step structures, etc[167,168][169,170][171]. For another example, in the C3N4 carrier, the defect site near the N atom can anchor the noble metal by forming a coordination bond[172]. Surface ligand modification does not require the introduction of uniform defect sites on the surface of the support, and can induce the monoatomic dispersion of noble metals with higher loading[173]. The thermal dispersion method of nanoparticles usually requires high thermal stability of the carrier, and Pd, Pt, Au NPs can be redispersed into SAs in an inert atmosphere above 900 ℃, which can be used to develop and apply single-atom catalysts with high thermal stability[174].

4.2.2 Metal-carrier interaction regulation

Transition metal oxides, such as TiO2, MnOx, Co3O4, and CeO2, can form strong interactions with noble metal single atoms through bridging oxygen bonds, and are often used as supports for regulating metal-support interactions in single-atom catalysts.
(1) Redox carrier
The interaction between TiO2 and noble metal SAs can be controlled by modification and introduction of promoters. For example, Pt SAs is intercalated into the lattice of TiO2 to replace the Ti4+ by the modification and removal of Mn, and Pt is stably coordinated by lattice oxygen. The carrier transfers electrons to Pt through Pt — O bond, so that the coordinated lattice oxygen is in an electron-deficient state. Electron-rich Pt can promote the adsorption and activation of O2 by providing additional electrons, while electron-deficient lattice oxygen is more likely to attract electrons from reducing VOCs molecules to promote their activation and oxidation[175]. The introduction of promoter Na+ into the Pt1/TiO2 can further promote the activation of O2 and formaldehyde, and the generated monodisperse Na-Pt-O(OH)x species promote the reaction between surface — OH and formate species at room temperature, thus changing the formaldehyde oxidation path and improving the formaldehyde oxidation activity of the catalyst[176].
The unique channel-heteroelement-associated sites of MnO2 may also provide a convenient route for monoatomic dispersion of noble metals. The strong interaction between Ag+ and [MnO6] channels leads to the depletion of Ag 4D electronic States and the increase of electron density near the Fermi level.The redox and formaldehyde oxidation activity of the catalyst were improved, but most of the Ag in this structure was confined to the inside of the [MnO6] pores, which was difficult to directly contact with VOCs of larger molecules[177,178]. In order to overcome this limitation, as shown in Fig. 12A, our research group developed a H2O2 assisted Ag monodispersion method to stabilize the Ag+ to the Mn vacancy on the surface of the δ-MnO2 for toluene oxidation. Compared with the Ag2O, the Ag SAs coordination environment has a longer and weaker Ag — O bond, which can additionally promote the formation of oxygen vacancies in the Ag-O-Mn unit and the activation of adjacent lattice oxygen[179].
图12 （a）H2O2辅助MnO2表面Mn空位捕获Ag单原子促进氧活化[179]；（b）Co3+位点稳定Ag单原子促进苯氧化[180]

Fig. 12 (a) H2O2-assisted Mn vacancy capture of Ag single atom on MnO2 surface promoting oxygen activation[179], Copyright 2022, Royal Society of Chemistry; (b) Co3+-site stabilized Ag single atom promoting benzene oxidation activity[180], Copyright 2022, American Chemical Society

The single-atom dispersion of noble metals on Co3O4 can have a significant impact on their surface properties, and both Co3+ and Co2+ defects on the surface can be used as anchoring sites for noble metals. As shown in Fig. 12b, when the surface Co3+ sites are occupied by Ag atoms, the Ag sites adsorb and activate a large amount of O2 to generate excess coordinated oxygen due to the Ag-O-Co interaction, and the Co — O bond is activated at the same time, which improves the reactivity of oxygen species on the catalyst surface[180]. When the surface Co2+ site is occupied by Pt atom, Pt has a higher occupied electronic state and a strong affinity for the 3D orbital of the adjacent Co, which promotes the electron transfer process from Pt to Co, increases the content of oxygen vacancies on the catalyst surface, and significantly reduces the activation energy barrier of methanol oxidation[181].
CeO2 support can provide excellent oxygen storage/release ability for single-atom catalysts, and Pt1/CeO2 has become a classical research system due to its rich coordination structure and special crystal face effect. The interaction between Pt SAs and different crystal planes of CeO2 is different due to the different surface electronic structures. The interaction between Pt and CeO2（100） is the strongest, which can lead to the spontaneous generation of structural distortion, and the electron transfer from Pt to CeO2 through Pt-O-Ce structure is more significant, which is conducive to the reduction of Ce4+ and the generation of oxygen vacancies, and promotes the adsorption process of O2 and methanol at the interface[182]. The Pt1/CeO2 not only has excellent activity, but also has outstanding high-temperature stability and hydrothermal stability, and the formation of a stable Pt2+-(OH)x(O)-Ce structure improves the reaction activity of lattice oxygen, which leads to the obvious improvement of the oxidation activity of CO, propylene and propane, and has great potential for practical application[183].
(2) non-redox carrier
On the other hand, for non-redox support, its surface coordination unsaturated sites can also stabilize noble metal single atoms, such as a large number of five-coordinated Al3+penta sites on the surface of γ-Al2O3 can achieve higher loading of noble metal single atoms[159,184]. In addition to the Al3+penta site, the — OH on the surface of γ-Al2O3（100） can also interact strongly with Ag to achieve the single-atom dispersion of Ag[185]. The introduction of promoter on the surface of Al2O3 can further regulate the interaction between noble metal and -Al2O3, and improve the catalytic activity or thermal stability of single atom. Pt1/Al2O3 has good CO and methanol oxidation activity, and the addition of WO3 and MgO to adjust the surface acidity of the catalyst can promote the formation of oxygen vacancies, thereby improving the catalytic performance of Pt1/Al2O3[186]. For another example, Pt SAs on the surface of La-modified Al2O3 has excellent activity in CO and propylene oxidation, but its Pt1/La-Al2O3 has poor anti-sintering ability. After adding Ba, Pt SAs is stabilized by Pt1-Ox-Ba structure, so that it still maintains single atom dispersion and high catalytic activity after hydrothermal aging at 650 ℃[187]. In addition to Al2O3, MgO is also a good support for Pt SAs. The Pt-MgO interaction changes the coordination environment of the Mg2+, resulting in electron transfer and the formation of oxygen vacancies. Pt1/MgO exhibits even higher toluene oxidation performance under aqueous atmosphere due to its strong · OH generation ability[188].
With the development of research, noble metal single-atom catalysts are no longer limited to the simple noble metal/single support structure, and nanoparticle/cluster-single-atom composite structures, multi-component composite structures such as PtNPs-Pt1/TiO2, M1/CeO2-rAl2O3（M=Pt, Pd, Rh), and Pt1-Co3O4/HZSM-5 have excellent catalytic oxidation performance for VOCs[189][190][191]. These studies have further opened up ideas for the regulation of metal-support interaction of single-atom catalysts.
It should be pointed out that the dispersion state of noble metals is not simply corresponding to their catalytic activity, and the optimal dispersion state of noble metals on the support surface is usually different in the face of different reactions. For example, Rh NPs, NCs and SAs on the surface of CeO2 show different catalytic performance in the diesel exhaust purification reaction system. Rh SAs has higher CO and NO oxidation activity, while Rh NCs has obvious advantages in propylene and propane oxidation. Rh SAs is almost inert, while Rh NPs is between them. Under the simulated actual conditions, the CO oxidation activity of Rh SAs decreased significantly due to the competitive adsorption of hydrocarbons, while Rh NCs showed better catalytic performance than commercial catalysts[192]. Therefore, when designing noble metal catalysts for VOCs oxidation, we should not unilaterally pursue the maximization of atom utilization, but should take into account the properties of noble metals, carriers, VOCs molecules and reaction conditions, and select the appropriate dispersion of noble metals, so as to achieve the goal of "reduction" and "high efficiency" at the same time.
With the rapid development of single-atom catalysts and the gradual maturity of synthesis technologies and methods, the future research focus will gradually shift from the research and development of synthesis methods to the fine structure control and performance optimization, so as to explore the application potential of single-atom catalysts and broaden their application fields. In addition, it is also a major challenge for future research to further clarify the dominant reaction of noble metal nanoparticles/clusters with single atoms and to clarify the optimal dispersion state of noble metals in different reactions.

5 Conclusion and prospect

With the continuous upgrading of environmental protection standards, VOCs emission control involves more comprehensive industries, and the pressure of emission reduction is increasing. Catalytic oxidation is one of the most promising methods to reduce VOCs emissions efficiently, and the design of catalysts and the fine modulation of catalyst structure are the core development directions of basic research.
In this paper, the important research progress at home and abroad is reviewed, focusing on the control of the structure of non-precious metal catalysts and the control of the dispersion state of precious metal catalysts. Non-precious metal catalysts focus on improving their intrinsic reactivity, and the research is usually based on their own structural characteristics to design and adjust, based on single transition metal oxides, based on doping, solid solution and special precursors (LDH, MOF, etc.) to optimize the surface structure and electronic properties of catalysts. Secondly, based on the wide stoichiometric modulation range of composite metal oxides (spinel, perovskite, mullite, etc.), the type, proportion and vacancy design of A-site and B-site ions are regulated, and the exposure tendency of surface active sites is optimized and the number of active sites is increased on the basis of maintaining the stability of the matrix. Optimizing and combining different oxides to control the phase interface is another idea of structural design. By modulating the interface defects, the local electronic properties of the two phases can be improved, and the advantages of coupling the two phases can be provided with new active sites. Based on this strategy, it can also be endowed with additional properties such as water resistance and high stability. For noble metal catalysts, the research focuses on the optimization of noble metal dispersion preparation strategy and the metal-support interaction mechanism. The size effect and support effect of aggregated noble metals are important factors to be considered in catalyst design. The interface contact form, configuration and oxidation state of noble metals will affect their VOCs adsorption, activation and oxidation properties. On the other hand, the preparation methods of single-atom catalysts based on "bottom-up" and "top-down" strategies are becoming more and more mature, and their suitability for VOCs oxidation is constantly improving. The monatomic dispersion of noble metals has a significant impact on their oxidation state, local coordination structure and electronic properties, and usually shows different reactant adsorption activation properties from those of aggregated noble metals.
At present, although great progress has been made in the design and structural regulation of VOCs oxidation catalysts, there are still many challenges in future research and even in the process of industrial application: (1) to clarify the structure-activity relationship and establish an efficient catalyst design method system. There are many kinds of VOCs and the catalytic system is complex, so it is necessary to fully combine theoretical calculation simulation, structural micro-characterization and in-situ characterization methods to deeply analyze the geometric and electronic structure of catalyst active sites, clarify the interaction mechanism between reactants and active sites, and clarify the important factors affecting the oxidation process of target VOCs from the molecular and atomic perspectives. Specifically, for non-precious metal catalysts, it is necessary to further explore the role of different oxygen species on the surface of oxides in the activation and oxidation of VOCs, especially to focus on the migration path of oxygen species in the dynamic reaction process and the dynamic interaction mechanism between oxygen species and VOCs molecules; For noble metal catalysts, it is necessary to continue to cultivate the metal-support interaction mechanism, further clarify the atomic coordination and electronic state of the metal-support interface, and clarify the influence of the interface structure on the activation and transformation of oxygen species.Determine the optimal noble metal loading form, and then gradually establish a catalyst design system to further enhance the pertinence in the selection of catalyst types and structural design. (2) Develop more refined, accurate and concise means of structural control. At present, there are some shortcomings in the structural control methods, such as complex steps, low yield, lack of controllability, and easy to produce secondary pollution. A simple method is developed to realize the directional and precise control of the crystal structure of transition metal oxides and the dispersion behavior of noble metals, to realize the hierarchical and efficient regulation of the bulk phase and surface structure of catalysts for optimization objectives, to improve the operability of synthetic means, and to lay a foundation for their industrial application. And (3) improve that reaction resistance of the catalyst under complex work conditions. Poisoning and sintering deactivation are the problems that must be overcome in the application of catalysts. When the flue gas contains inevitable water vapor, S, Cl, heavy metals and other components, the competitive adsorption and active site poisoning process is deeply analyzed from the microscopic point of view by combining theoretical calculation and experimental characterization.It is an important research direction to develop anti-poisoning catalysts by weakening the interaction between active sites and poisons, shielding the contact between poisons and active sites, and adding sacrificial components. In addition, in order to cope with the structural instability of catalysts caused by temperature fluctuations, it is particularly urgent to further reveal the mechanism of structural instability and develop universal methods to improve the structural stability of oxides and the dispersion stability of precious metals. (4) To clarify the oxidation reaction characteristics of mixed VOCs. The actual flue gas usually contains a variety of VOCs, and the research on the mechanism of competitive adsorption and oxidation is still relatively scarce. The mechanism of the effect of the difference in the microscopic interaction between different VOCs and active sites on the macroscopic activity of the catalyst needs to be further studied. (5) Development of scale-up production methods for supporting industrial monolithic catalysts. Most of the industrial VOCs oxidation catalysts are monolithic honeycomb catalysts. On the one hand, efforts should be made to develop new industrial catalyst carriers, which can optimize the reaction mass transfer and heat transfer efficiency from the perspective of carriers, or improve the surface properties of carriers to enhance the utilization efficiency of active components. On the other hand, the catalyst design and structure control methods suitable for the laboratory are difficult to adapt to the monolithic catalyst production process, so it is necessary to invest efforts to improve the matching degree between catalyst microstructure control and industrial production, and develop supporting industrial production technologies, so as to realize the industrial application of new catalysts.
In a word, the design and structure control of catalysts will still be the focus of future research in the field of VOCs catalytic oxidation, and the continuous optimization of catalyst structure oriented to improve activity, reduce cost, enhance stress resistance and stability is the common development trend of the two types of catalysts. With the gradual clarification of structure-activity relationship, the fine control means of catalyst structure are becoming more and more mature, which will strongly support the follow-up basic research and industrial application to achieve major breakthroughs.

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