Emission Control of Toluene in Iron Ore Sintering Using Catalytic Oxidation Technology: A Critical Review

Abstract

Iron ore sintering flue gas containing large amounts of volatile organic compounds (VOCs) can form secondary photochemical smog and organic aerosols, thus posing a serious threat to human health and the ecological environment. Catalytic combustion technology has been considered as one of the most prospective strategies for VOC elimination. This paper focuses on a review of studies on catalytic removal of typical VOCs (toluene) on transition metal oxide catalysts in recent years, with advances in single metal oxides, multi-oxide composites, and supported metal oxide catalysts. Firstly, the catalytic activities of a series of catalysts for toluene degradation are evaluated and compared, leading to an analysis of the key catalytic indicators that significantly affect the efficiency of toluene degradation. Secondly, the reaction pathway and mechanism of toluene degradation are systematically introduced. Considering the site space and investment cost, the conversion of VOC pollutants to harmless substances using existing selective catalytic reduction (SCR) systems has been studied with considerable effort. Based on the current development of simultaneous multi-pollutant elimination technology, the interaction mechanism between the NH₃-SCR reaction and toluene catalytic oxidation on the surface is discussed in detail. Finally, views on the key scientific issues and the challenges faced, as well as an outlook for the future, are presented. This overview is expected to provide a guide for the design and industrial application of NO/VOC simultaneous removal catalysts.

1. Introduction

In recent years, the pattern of air pollution in China has undergone profound changes, and the combination of PM 2.5 and ozone (O₃) pollution has become an important air pollutant affecting urban and regional air quality in China [1]. PM 2.5 and O₃ can result in respiratory/cardiovascular diseases and pose a great risk to human health. Since the implementation of China’s air pollution prevention and control action plan, PM 2.5 pollution has been improved, but O₃ pollution is still deteriorating and has become an important exceedance pollutant. It has been reported that ozone concentrations in China continued to increase during 2013–2019 [2]. VOCs are a variety of organic compounds with boiling points of 50 °C to 260 °C at room temperature. Many VOCs are highly toxic, diffusible, and volatile and are released into the environmental matrix as important precursors of PM 2.5 and O₃. This leads to urban haze and photochemical smog, and therefore it is considered as a key factor in PM 2.5 and O₃ pollution.

Given the worsening environmental risks, the Chinese government implemented a multi-pronged approach to monitor VOC emissions from many enterprises and put in place many control measures. In the 14th Five-Year Plan, China has committed to decreasing VOC emissions by at least 10% in 2025 compared to 2020 and to achieving an emission control level that is basically at the international advanced level. In 2020, the Ministry of Ecology and Environment of the People’s Republic of China revised the air pollution evaluation index from “SO₂ and NO_x” to “VOC and NO_x”. VOCs are primarily released from industrial production processes, solvent use, transportation, and fuel combustion [3]. As various industrial pollutant abatement measures are being implemented, the contribution of energy-intensive industries to VOC emissions should not be underestimated. In China, VOC emissions from combustion sources (excluding biomass combustion) are estimated to account for 7.4–10.4% of VOC emissions from all anthropogenic sources [4,5,6]. With government efforts, most primary pollutants concentrations have been significantly decreased [2]. These include the use of new energy vehicles to control mobile source pollutant emissions [7], the use of biomass or carbonized biomass with a lower environmental burden to partially replace coal to produce coke [8], and the development of catalytic oxidation technology to reduce VOC emissions in coal-fired power plants [9]. For the iron and steel industry, the VOC emissions from iron sintering process have also raised some special concerns. In China, the annual production of sintered ore can reach 1 billion tons/year, and more than 60 million tons of fossil fuels (coal, coke, natural gas, and oily sludge, etc.) are consumed in these steel plants [10]. The iron ore sintering process results in significant VOC emissions owing to the mixed combustion of fossil fuels, iron ore, and coke [10]. Especially in typical iron- and steel-producing cities such as Tangshan and Handan, iron ore sintering processes account for 18.7–30.0% of the national VOC emissions [11,12]. In response to the large VOC emissions from iron ore sintering plants, legislation has been introduced in China to regulate VOC emissions in the iron and steel industry. However, the actual industrial control technology is still in the initial stage. Li et al. [13] reported the VOC emission profiles in the iron ore sintering process, comparing sintering tanks and sintering simulation tests to determine VOC species and their concentrations. Wang et al. [10] explored the mechanisms and patterns of VOC emissions in iron ore sintering flue gas and critically discussed the feasibility of source control, process control, end-of-pipe control, and industrial VOC emission control technologies to treat sintering flue gas. Qian et al. [14] also systematically investigated the mechanism of PCDD/Fs generation in the iron ore sintering process and summarized new advances and technologies for PCDD/Fs emission reduction in terms of source, process, and end-of-pipe treatment. In these reviews, it is widely agreed that catalytic oxidation-based technology is one of the most effective methods to control emissions of VOCs, which can be converted to harmless CO₂ and H₂O as end products at low temperatures. However, a detailed exploration of catalytic oxidation control technologies and a review of key catalysts are still lacking based on the available iron ore emission characteristics.

NO as a precursor to ozone is also widely present in iron sintering flue gases. Selective catalytic reduction (SCR), a well-established reaction for NO elimination from such flue gases, has a process temperature range similar to that of catalytic oxidation technology. In recent years, the Chinese government has promoted the development of multi-pollutant synergistic control technologies to reduce the emissions of VOCs and NO_x [1]. However, in iron sintering flue gas emission control, SCR technology is usually used for single NO_x pollutant removal from flue gas. Installing separate end-of-pipe treatment units for VOCs in the same stationary emission source would occupy a large amount of site space, resulting in a complex process with large and costly systems [15]. Therefore, instead of building redundant facilities, the existing equipment can be fully utilized to remove both VOCs and NO in one catalytic system, which is a reasonable and cost-effective option. It can be seen that studying the synergistic removal of multiple pollutants under complex flue gas conditions is an inevitable trend in the development of industrial flue gas cleaning technology. However, feasible ways aiming at simultaneous multi-pollutant removal by SCR reactors in existing sinter flue gas cleaning technologies have not been identified. The recent research about the oxidation process of VOCs alone and its oxidation path and the synergistic mechanism of SCR + VOC catalytic oxidation still lacks a systematic review.

Therefore, this paper reviews the catalytic oxidation of VOCs over various transition metal oxides based on the emission characteristics of iron ore sintering processes to better understand the removal of VOCs and to determine whether they can be applied in practical areas. In addition, we focus on an overview of the oxidation path of toluene degradation over non-homogeneous catalysts and the key factors affecting catalyst performance. In summary, this comprehensive review focuses on the relationships between catalyst performance, behavior, and the principles of catalytic oxidation of VOCs. The tasks of the review are to guide the design of more effective catalytic materials for VOC oxidation, to support the development of new technologies for the synergistic removal of multiple pollutants, and to provide a reference point to benefit designers, researchers, and managers by providing a useful resource for advancing VOC removal methods.

2. Emission Characteristics and Terminal Treatment

In a conventional iron ore sintering system (Figure 1a), sintering is carried out with iron ore fines and white ash fines as uncooked substances and coal fines and coke as fuels. After ignition, the sintered material layers undergo different treatment processes in sequence. The granular sintered material is transported into the sintering machine, where the surface of the sintered material is ignited by the vertical airflow and sintered from top to bottom [16]. The VOCs are generated in the low-temperature region without passing through the high-temperature region and are all volatilized into the sintering flue gas, thus resulting in VOC emissions. In addition, another significant contributor to VOC emissions is the presence of a micro-reducing environment surrounding some of the fuel inside the dense “sintered raw material pellet” during the sintering process. Hebei Province and the neighboring areas have the highest concentration of VOC emissions from the iron and steel industries. There are significant differences in VOC emission types and concentrations from distinct steel plants owing to coal type, raw materials, operating parameters of the sintering process, and significant differences in end-of-pipe treatment. Wang et al. [17] reported that samples were collected from sintering and pollution source processes in 2020 for high-energy-consumption industries in Hebei Province. It was found that in the steel industry (Figure 1b), alkanes were the most prevalent VOC species in the steel industry’s sinter flue gas with SCR denitrification equipment installed, making up 45.9% of the total VOCs. Olefins were in second place with 27.3%, followed by aromatics (17.4%), halogenated hydrocarbons (6.8%), and oxygenated VOCs (2.5%). The best way to reduce VOC emissions without compromising iron ore sinter production goals is flue gas end-treatment technology, which can also handle the possible risk of sinter air pollutants. The attractive emission reduction efficiency achieved by flue gas end-treatment technology is highly tied to the emission characteristics of sinter flue gas, which include a high flow rate, low temperature, and low concentration. The VOCs generally have no recovery value, and catalytic oxidation in the presence of oxygen can decompose them completely, which has the advantages of high capacity and good real-time performance for VOC removal [18]. An important consideration in catalytic combustion technology is the choice of an appropriate catalyst. Catalytic combustion is a deep oxidation process in which reactive oxygen species break down chemical bonds, including the C-H and C-C bonds in VOCs [19]. The ideal active element in the catalyst must have strong redox properties to accelerate oxidative VOC degradation. It has been demonstrated that there is a relationship between the oxidizability of VOCs and their molecular weights. VOCs with higher molecular weights are expected to have more challenging degradation processes and higher activation energies [20]. Therefore, in this paper, toluene, as the most common aromatic and a pollutant difficult to degrade in iron sintering flue gas, is the subject of the pollutant review.

3. Transition Metal Oxide Catalysts for Toluene Purification

Many catalysts have been shown to be reactive for toluene oxidation. Noble metal oxides (Pt, Ag, Pb, etc.) and transition metal oxides (Mn, Co, Fe, Cu, Ce, etc.) are the most common catalysts. Among them, noble metal oxide catalysts have remarkable catalytic activity and stability, while the high price of precious metals leads to high industrial application costs and improper use can easily lead to catalyst poisoning [21]. In contrast, transition metal oxide catalysts are regarded as the best for catalyzing the removal of toluene due to their affordability, high catalytic activity, high selectivity, and robust tolerance to poisoning. The use of single [22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38] or multiple combinations of transition metal oxides has been widely reported. Here, the catalytic characteristics of the catalysts are summarized by dividing them into three categories: single metal oxides, multi-oxide composites, and supported metal oxide catalysts.

3.1. Single Metal Oxides

3.1.1. Manganese Oxides

MnO_x catalysts, with their low cost, high catalytic activity, and stability, show great potential for catalytic oxidation of VOCs and are recognized as the most competitive alternative to noble metal catalysts in the low-temperature region [39]. In general, the catalytic ability of MnO_x prepared by different methods is mainly influenced by numerous factors such as valence changes, crystal structure, exposed crystal surfaces, and microscopic morphology. These factors affect the catalytic activity of VOC abatement to some extent, as shown in

Table 1. Catalytic activity for toluene combustion over single metal oxides reported in the recent literature.

Catalysts	Synthesis Method	Toluene Concentration	WHSV or GHSV (mL·g−1·h−1 or h−1)	Activity T90 (°C)	Ref.
Mn3O4	Solution combustion synthesis	1000 ppm	19,100 h−1	250	[22]
Mn2O3				268
MnxOy				291
δ-MnO2	Hydrothermal process	1000 mg·m−3	36,000 mL·g−1·h−1	233	[23]
γ-MnO2				283
α-MnO2				309
β-MnO2				325
α-MnO2 (1 1 0)	Hydrothermal method	500 ppm	15,000 mL·g−1·h−1	109	[24]
α-MnO2 (3 1 0)				170
α-MnO2 (2 1 0)				170
α-MnO2-60	Redox precipitation	1000 ppm	30,000 h−1	203	[25]
Rod-like α-MnO2	Hydrothermal or solution method	1000 ppm	20,000 mL·g−1·h−1	225	[26]
Tube-like α-MnO2				233
Flower-like Mn2O3				238
Wire-like α-MnO2				245
MnO2-manganese acetate	Redox reaction method	1000 ppm	90,000 mL·g−1·h−1	200	[27]
MnO2-manganese nitrate				210
MnO2-manganese chloride				225
MnO2-manganese sulfate				236
MnOx-(NH4)2CO3	Co-precipitation	500 ppm	60,000 mL·g−1·h−1	260	[28]
Mn-Na2CO3				281
Mn-NaOH				326
Mn-NH3·H2O				331
CeO2 microspheres	Polymer-modified hydrothermal method	1000 ppm	60,000 mL·g−1·h−1	210	[29]
CeO2 hollow spheres	Hydrothermal method	1000 ppm	48,000 mL·g−1·h−1	207	[30]
CeO2 nanorods				239
CeO2 cubes				296
CeO2 nanopolyhedra	Hydrothermal method	1000 ppm	24,000 mL·g−1·h−1	394	[31]
CeO2 nanorods				400
CeO2 nanocubes				>400
CeO2 nanobelt	Conducted electrospinning	100 ppm	60,000 mL·g−1·h−1	230	[32]
CeO2 nanotube				255
CeO2 wire-in-nanotube				345
CeO2-MOF	Sacrificial precursor	1000 ppm	20,000 mL·g−1·h−1	223	[33]
1D-Co3O4-nanoneedle	Hydrothermal method	1000 ppm	48,000 mL·g−1·h−1	257	[34]
2D-Co3O4-nanoplate				249
3D-Co3O4-nanoflower				238
Cube-stacked Co3O4 microspheres	Hydrothermal method	1000 ppm	48,000 mL·g−1·h−1	248	[35]
Plate-stacked Co3O4 flower				254
Needle-stacked Co3O4 two-spheres				259
Sheet-stacked fan-shaped Co3O4				>260
NH/Am-Co3O4	ZIF template induction	3000 ppm	30,000 mL·g−1·h−1	220	[36]
Hollow Co3O4 polyhedron	ZIF-67 template	12,000 ppm	21,000 mL·g−1·h−1	258	[37]
ZSA-1 Co3O4	Co-MOFs template	-	20,000 mL·g−1·h−1	240	[38]

T₉₀: the temperature at which conversion efficiency reaches 90%.

. Mn has various oxidation states, such as +2, +3, and +4, and MnO_x has different values of x due to the different oxidation states of manganese. The formation of various kinds of manganese oxides reflects the various valence states. Due to the different intrinsic properties, the catalytic oxidation activity of these MnO_x towards toluene is also different. Piumetti et al. [22] prepared mesoporous Mn₂O₃, Mn₃O_4, and Mn_xO_y (mixture of Mn₂O₃ and MnO₂) catalysts by solution combustion synthesis and evaluated their catalytic abilities for toluene oxidation. The results indicated that Mn₃O₄ exhibited excellent catalytic activity with 90% conversion at 248 °C because of the highest surface electrophilic oxygen content. Different crystal structures of MnO₂ in the same fraction (Figure 2) can affect the degree of active site exposure, resulting in different catalytic performances. Li et al. [23] explored the catalytic degradation of toluene over different phase structures of MnO₂, including β-, α-, γ-, and δ-MnO₂, showing that the catalytic activity followed the sequence of β- < α- < γ- < δ-MnO₂. The different crystal facets (Figure 2) of the exposed metal oxides confer special physicochemical properties to the functional materials, which are also closely related to their catalytic activity. Huang et al. [24] compared the toluene oxidation performance over α-MnO₂ catalysts with (110), (210), and (310) crystal facets. The α-MnO₂ with exposed (210) crystal facet catalysts exhibited the best catalytic performance and achieved complete toluene conversion at 140 °C. Chen et al. [25] prepared a series of α-MnO₂ catalysts by tuning the precipitation temperature in the redox precipitation process. The α-MnO₂-60 greatly enhanced the catalytic efficiency (T₉₀ = 203 °C) and provided excellent H₂O resistance in toluene oxidation, which was mainly attributed to a large number of surface oxygen vacancies. The various morphologies of the MnO_x materials, including cubes, sheets, tubes, rods, and spheres, provide catalysts with unique characteristics that have an impact on the toluene degradation. Wang et al. [26] prepared nanorod, nanowire, and nanotube α-MnO₂ and nanoflower spherical Mn₂O₃ by hydrothermal and CCl₄ solution methods, respectively. The catalytic activities of these materials for toluene degradation were in the following order: nanorod α-MnO₂ > nanotube α-MnO₂ > nanoflower Mn₂O₃ > nanowire α-MnO₂, which was mainly caused by the differences in low-temperature reducibility, oxygen species concentration, and specific surface area. Moreover, the preparation conditions such as different types of Mn precursors and precipitating agents during the preparation process also affect the catalytic performance of toluene degradation to some extent. Lyu et al. [27] explored the catalytic activity of MnO₂ prepared from four Mn (II) precursors. The catalytic activity of these materials for toluene degradation followed the order of MnO₂-manganese acetate > MnO₂-manganese nitrate > MnO₂-manganese chloride > MnO₂-manganese sulfate. Zhang et al. [28] synthesized a series of MnO_x for the catalytic oxidation of toluene using four different precipitants. The results showed that Mn-(NH₄)₂CO₃ (T₉₀ = 260 °C) had better catalytic efficiency than Mn-Na₂CO₃ (T₉₀ = 281 °C), Mn-NaOH (T₉₀ = 326 °C), and Mn-NH₃·H₂O (T₉₀ = 331 °C).

3.1.2. Cerium Oxides

Ce has great oxygen storage capacity, abundant acidic sites, and unique valence change capability (Ce⁴⁺ to Ce³⁺), which makes the CeO₂ materials rich in defects and oxygen vacancies in the crystal, thus giving CeO₂ excellent redox properties in the catalytic combustion of toluene [40,41]. However, due to the limitation of its own structure and physicochemical properties, a pure CeO₂ catalyst can only enable the complete oxidation of toluene in the high-temperature region. Therefore, enhancing the low-temperature performance of CeO₂ catalysts has been widely reported in the literature (Table 1).

Hu et al. [29] synthesized CeO₂ microspheres with layered structures self-assembled by nanowires using a hydrothermally driven assembly method. The catalytic activity of the novel layered CeO₂ catalysts was significantly better than that of the similar non-porous catalysts prepared by the conventional hydrothermal method. Compared with the bulk cerium dioxide, the nanowire self-assembled layered CeO₂ exhibited better combustion activity at low temperatures, with toluene conversion of more than 90% even at 210 °C. The high catalytic activity could be due to the large surface area and the layered porous structure, which contributed to the exposure of more surface oxygen vacancies. Feng et al. [30] prepared CeO₂ catalysts in rod, hollow spherical, and cubic forms via hydrothermal methods. The results indicated that CeO₂ hollow spheres exhibited the best catalytic activity for the degradation of toluene. This was because CeO₂ hollow sphere catalysts have the largest specific surface area and the most oxygen vacancies. Mi et al. [31] prepared CeO₂ catalysts with different morphologies (nanopolyhedra, nanorods, and nanocubes) using a hydrothermal method. CeO₂ nanopolyhedra have better catalytic activity than CeO₂ nanorods and nanocubes. The results demonstrated that CeO₂ with distinctive morphologies has different oxygen distributions, especially the surface lattice oxygen distribution. The surface lattice oxygen concentration showed a linear relationship with the catalytic activity, indicating that the surface lattice oxygen participated in the oxidation process of toluene. Yan et al. [32] prepared CeO₂ materials with nanotube, nanobelt, and wire-in-nanotube morphologies using the electrostatic spinning technique. The high catalytic activity of CeO₂-nanobelt catalysts could be attributed to their porous nanoribbon morphology, large specific surface area, and abundant surface oxygen vacancies. Chen et al. [33] successfully prepared CeO₂ catalysts with mesoporous structures by the in situ pyrolysis of Ce-MOF precursors. In comparison with commercial and co-precipitation-prepared CeO₂ catalysts, CeO₂-MOF/350 catalysts exhibited a stronger ability to achieve 100% conversion, whereas CeO₂-C and CeO₂-P catalysts usually require higher temperature regions to complete the oxidation of toluene. This high catalytic activity could be attributed to the large specific surface area, rich three-dimensional pore channels, small average grain size, higher reactive oxygen species and oxygen vacancy concentrations, higher relative percentages of Ce³⁺/Ce⁴⁺ and O_Sur/O_Latt, better low-temperature reducibility, and more acidic sites. In conclusion, the main method for improving the performance of CeO₂-catalyzed toluene degradation is to promote the adsorption and activation of molecular oxygen through the formation of oxygen vacancies, thus further improving the oxygen storage and release capacity of CeO₂ catalysts. In this process, surface vacancies tend to adsorb and activate gaseous O₂ to form adsorbed oxygen species, while bulk vacancies increase the activity and mobility of lattice oxygen species through transport effects [42]. Adsorbed oxygen is mainly involved in the chemisorption and partial oxidation of toluene. As the temperature increases, the lattice oxygen of the catalyst promotes the decomposition of the aromatic ring, which further accelerates the conversion of intermediates to CO₂ and H₂O.

3.1.3. Cobalt Oxides

Co₃O₄, a typical spinel metal oxide, has received wide attention in the catalytic combustion of VOCs. It is considered as one of the most active inexpensive metal catalysts, and the high activity is mainly related to the mobile oxygen species and high oxygen-binding rate in its spinel structure [41,43]. The catalyst has high reduction, more oxygen vacancies, and a high concentration of electrophilic oxides. The catalytic activity of the Co₃O₄ catalyst depends mainly on the microscopic morphology, preparation method, processing conditions, oxidation state, and surface area, and the Co₃O₄ catalyst is the most efficient catalyst for the complete oxidation of toluene. Therefore, efforts have been devoted to the development of Co₃O₄-based catalysts with various structures and morphologies (Table 1).

Ren et al. [34] reported a series of 1D-Co₃O₄, 2D-Co₃O₄, and 3D-Co₃O₄ materials and investigated their applications in the catalytic combustion of toluene. The results showed that 3D-Co₃O₄ had the best activity with 90% conversion of toluene at about 238 °C. This can be attributed to the excellent properties of 3D-Co₃O₄ nanoflowers with low reduction temperature, large specific surface area, abundant surface reactive oxygen species, and rich defect structure. The successful synthesis of 3D Co₃O₄ nanocatalysts by a hydrothermal method can also result in different morphologies, including 3D hierarchical flake-stacked fan-shaped Co₃O₄ (S), 3D hierarchical needle-stacked begonia-like Co₃O₄ doublets (N), 3D hierarchical flat-stacked Co₃O₄ flowers (P), and 3D hierarchical cubic-stacked Co₃O₄ microspheres (C) [35]. As confirmed by HRTEM analysis, they mainly had exposed (112), (110), (110), and (111) crystal faces, respectively. The catalytic activity for toluene oxidation decreased as follows: C > P > N > S. However, in the 3D hierarchical Co₃O₄ catalysts with distinct different morphologies and exposed crystal faces synthesized by Liu et al. [44], the S-160 catalyst with exposed (110) crystal faces had a higher oxidation activity with toluene conversion up to 50% at 234 °C, which was 17 °C lower than the material with predominantly exposed (111) faces. This was mainly due to its abundant adsorbed oxygen, good redox properties, and highly defective structure.

A metal–organic framework (MOF) is a kind of hybrid material with a high specific surface area, high porosity, and tunable structure formed by an organic ligand and a metal ion through coordination bond assembly [45]. Templating is a commonly used method for preparing nanomaterials and allows the structure and morphology of the material to be designed according to the requirements. MOFs have received much attention as novel precursors for the synthesis of porous inorganic materials. Among them, zeolitic imidazolate frameworks (ZIFs) are a subfamily of metal–organic frameworks (MOFs) composed of Zn²⁺ or Co²⁺ and imidazole or imidazole derivatives; they combine the high stability of inorganic zeolites with the high porosity of MOFs [38]. The versatility of Co-ZIFs and the low cost and simple synthesis methods make them the preferred precursors for Co₃O₄ [46]. It is known that in the spinel structure of Co₃O₄, Co³⁺ is usually present in octahedral coordination, while Co²⁺ is present in tetrahedral coordination [47]. Co³⁺ is usually regarded as the active site for VOC combustion, as evidenced by previous studies [37,48].

Han et al. [36] accurately regulated the growth of ZIF structures by varying the distribution of N elements on the nanofiber surface. The N-element microenvironment on the nanofiber surface affects not only the growth mechanism of ZIFs, but also the molecular space configuration (tetrahedral and octahedral structures of Co₃O₄). In order to expose more Co³⁺, further octahedral structure formation was induced by using the triethylamine (TEA) induction method. The results demonstrated that the prepared Co₃O₄ materials possessed more Co³⁺ and exhibited the best catalytic performance (90% catalytic efficiency at 220 °C). Zhao et al. [37] successfully synthesized a series of Co₃O₄ hollow polymorphs with different sizes by the pyrolysis of ZIF-67. The samples still maintained the nanosize and shape of the MOF precursors. The Co₃O₄-400 catalyst with a particle size of 400 nm showed good catalytic performance with a complete conversion temperature of 280 °C for toluene (T₁₀₀). Co₃O₄ catalysts prepared with distinct shapes and ligands of Co-MOF as precursors also exhibited different physicochemical properties [38]. A series of mesoporous Co₃O₄ samples prepared by pyrolysis of dodecahedral ZIF-67, rod-shaped MOF-74, and octahedral ZSA-1 exhibited toluene catalytic activity in the following sequence: ZSA-1-Co₃O₄ (octahedral-shaped N-O-ligand) > MOF-74-Co₃O₄ (rod-shaped N-O-ligand) > ZIF-67-Co₃O₄ (dodecahedral-shaped N-O-ligand). This could be attributed to the differences in different exposure surfaces, low-temperature reducibility, specific surface area, and ratio of Co³⁺/Co²⁺ and O_ads/O_latt.

3.2. Multi-Oxide Composites

The poor chemical and thermal stability of single metal oxides usually leads to particle aggregation and ultimately to a decrease in catalytic performance [49]. Multiple transition metal oxide composites can further contribute to thermodynamic stability and modulate the electronic structure and surface properties compared to single metal oxides [46,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70]. Multi-oxide composites, such as MnO_x doped, loaded, or forming solid solutions with other transition metals, can exhibit altered physicochemical properties and create synergistic effects in the VOC conversion process [21]. This synergistic property may lead to better surface mobility of reactive oxygen species and promote the electron transport between toluene and the material, thus enhancing the reaction between toluene and reactive oxygen species. Therefore, multi-oxide composites are of interest. The toluene catalytic degradation performance and reaction conditions of the binary composites are summarized in

Table 2. Catalytic activity for toluene combustion over the binary composites reported in the recent literature.

Catalyst	Synthesis Method	Toluene Conc.	WHSV or GHSV (mL·g−1·h−1 or h−1)	Activity T90 (°C)	Ref.
1Cu1Mn	MOF template	1000 ppm	60,000 mL·g−1·h−1	208	[50]
CuO-MnOx-500	Sol–gel	600 ppm	19,800 mL·g−1·h−1	228	[51]
CuMnOx-HS	Oxidation method	500 ppm	60,000 mL·g−1·h−1	212	[52]
CuO/MnO2-R-10	Solid-state redox strategy	500 ppm	60,000 mL·g−1·h−1	234	[53]
MnCu0.5	Hydrothermal–redox	1000 ppm	20,000 mL·g−1·h−1	210	[54]
Tunneled Cu-Mn	Hydrothermal method	1000 ppm	10,000 h−1	169	[55]
L-to-T Cu-Mn				199
Layered Cu-Mn				221
Mn0.6Ce0.4O2	Redox–precipitation and hydrothermal	500 ppm	22,500 h−1	207	[56]
Ce-Mn-Ox	Hydrothermal method	500 ppm	60,000 h−1	246	[57]
	Citrate sol–gel			249
	Co-precipitation			259
	Impregnation			261
Mn0.85Ce0.15	Co-precipitation and hydrothermal	1000 ppm	32,000 mL·g−1·h−1	216	[58]
Mn12Ce1-SW	SPT process	1000 ppm	15,000 mL·g−1·h−1	277	[59]
CoMn2O4	Sol–gel	500 ppm	22,500 mL·g−1·h−1	210	[60]
Nanocrystal-like Mn1Co2	Hydrothermal method	1000 ppm	30,000 mL·g−1·h−1	240	[61]
Co3O4/α-MnO2	Solvothermal method	500 ppm	60,000 h−1	248	[62]
MnO2@ Co3O4	ZIF-derived Co3O4 template	1000 ppm	48,000 mL·g−1·h−1	229	[63]
Ni-MnO2	EG reduction strategy	1000 ppm	20,000 mL·g−1·h−1	199	[64]
Cu-MnO2				217
Co-MnO2				218
Mn0.3Zr0.7O2	Impregnation	1000 ppm	60,000 mL·g−1·h−1	235	[65]
MnOx@ZrO2-NA	MOF template	1000 ppm	60,000 mL·g−1·h−1	260	[66]
CeCu-HT	Hard template	10,000 ppm	66,000 mL·g−1·h−1	225	[67]
CeCu-CA	Complex method			270
Cu1Ce3	Co-precipitation	1000 ppm	30,000 h−1	180	[68]
Ce1Co2	Carbon xerogel template	1000 ppm	60,000 h−1	241	[69]
Co0.2Ce0.8O2-δ	Hydrothermal	1000 ppm	20,000 mL·g−1·h−1	230	[70]
Co1Cu1Ox	Co-MOF template	1000 ppm	20,000 mL·g−1·h−1	208	[46]
Co1Mn1Ox				227
Co3O4				239
Co1Fe1Ox				234
Co1Ni1Ox				246

. The results indicate that different transition metal oxide blends also exhibit different toluene catalytic behaviors under the synergistic effect.

3.2.1. Binary Composites

The integration of MnO_x with other metal oxides such as CuO, CeO₂, Co₃O₄, NiO, and ZrO₂ is an effective strategy for improving catalytic performance. The introduction of secondary metals can effectively reduce the crystallinity of MnO_x and produce intermetallic synergistic effects, which can introduce a large number of oxygen vacancies and improve the catalytic activity. The potential of CuMnO_x oxides for the oxidation reaction of VOCs is well known in industry [41]. For instance, the 1Cu1Mn reported by Hu et al. [50] exhibited great toluene oxidation performance and corrosion resistance. The experimental results indicated that the catalytic activity of both binary composites for toluene was higher than that of single MnO_x and CuO_x. During the preparation of the materials, the toluene conversion first increased and then decreased with the annealing temperature increasing, and the best toluene oxidation activity was acquired for the catalyst annealed at 500 °C [51]. Liu et al. [52] prepared hollow spherical CuMnO_x-HS catalysts by an oxidation method using Cu₂O as a template; this special oxidation method enhanced the synergistic interaction between Mn and Cu, which resulted in CuMnO_x-HS with large specific surface area, good reduction, oxygen mobility, and rich adsorbed oxygen species, thus promoting the catalytic efficiency of toluene oxidation. Liu et al. [53] modified the surface defect concentration of α-MnO₂ using a solid-state redox strategy. Surface reduction of α-MnO₂ by Cu₂O greatly improved the oxygen vacancy concentration for CuO/MnO₂-R catalysts. The T₉₀ of Cu₂O-reduced CuO/MnO₂-R-10 catalysts with increased structural defects was 36 and 32 °C lower than that of the α-MnO₂ and CuO-reduced CuO/MnO₂-10 catalysts, respectively. The lattice defects and oxygen vacancy concentration of the prepared catalysts can also be regulated by controlling the Mn/Cu molar ratio. Spinel Mn-Cu oxides synthesized by hydrothermal–redox method with Mn/Cu molar ratios of 2/1 had abundant adsorbed oxygen, high Cu⁺ and Mn³⁺ contents, and good low-temperature reduction. They exhibited the highest catalytic activity, excellent durability, and water resistance in the total oxidation of toluene [54]. In addition, Luo et al. [55] designed a series of Cu-modified MnO catalysts with tunneling, layered, and transition structures using a hydrothermal method. The tunnel structure catalysts showed the best performance and excellent water resistance among the three Mn-Cu oxides. This could be attributed to the catalyst having abundant oxygen species, good low-temperature reducibility, a lower average Mn oxidation state, and a large specific surface area.

CeO₂ has attracted attention for its abundant oxygen defects and high oxygen storage ability for rapid reversible redox cycling (Ce⁴⁺/Ce³⁺). The doping of Ce into Mn-based catalysts could form abundant Ce³⁺ and Mn³⁺, which facilitates the generation of oxygen vacancies and enhances the migration of active oxygen in the catalyst. The synthesized Mn-Ce mixed oxides can outperform the single metal oxides in the oxidation of toluene [56]. Zhang et al. [57] prepared Ce-Mn-O_x catalysts using four methods: co-precipitation (CP), hydrothermal (HT), impregnation (IM), and citrate sol–gel (SG). CM-HT prepared by HT methods had more crystallization defects, oxygen vacancies, and surface adsorbed oxygen and a normalized conversion rate, thus showing the best catalytic oxidation performance for toluene. Liao et al. [58] successfully synthesized Mn-Ce nanorod materials with different Mn contents, which will affect the catalytic activity and result in higher stability. The right ratio of Mn and Ce oxides could induce the formation of a solid solution in catalysts and more Mn⁴⁺ and oxygen vacancies, which were key to maintaining high activity and stability. Li et al. [59] found that trace CeO₂-doped δ-MnO₂ catalysts with a high Mn⁴⁺/Mn³⁺ ratio, at lower reduction temperatures, could convert toluene with an efficiency of 90% at 277 °C. The incredible oxidation activity of the Mn₁₂Ce₁O_x catalyst might be due to the synergistic interaction of highly dispersed CeO₂ nanoparticles with the δ-MnO₂ substrate at the interface.

Researchers also found that Mn-Co spinel catalysts have high catalytic performance and stability in catalytic oxidation. In the spinel (AB₂O₄) structure, A and B sites are in tetrahedral and octahedral positions, respectively. Adjacent cations (A and B) can optimize catalytic performance through electron transfer. Dong et al. [60] studied the catalytic oxidation of toluene by nanoflower-shaped CoMn₂O₄ synthesized by the sol–gel method. The results indicated that the activation energy (35.5 kJ/mol) for toluene oxidation catalyzed by spinel-type CoMn₂O₄ was lower than that of using metal oxides (Co₃O₄, MnO_x, and Co₃O₄/MnO_x). Among the numerous Mn-Co solid solutions, strong interactions were formed between MnO_x and CoO_x, which promoted the toluene oxidation activity. At a Mn/Co molar ratio of 1:2, the Mn-Co material prepared by a two-step hydrothermal method could achieve complete conversion of toluene at 250 °C [61]. The unique high porosity, large surface area, and high concentration of reducible oxygen are the main reasons for the significantly enhanced catalytic activity of these spinel binary materials. The construction of heterogeneous structures by interfacial engineering is another effective way to improve catalytic performance. Liu et al. [62] successfully immobilized Co₃O₄ on the surface of different phases of MnO₂ and investigated the effect of different interfaces of Co₃O₄-MnO₂ for the catalytic oxidation of toluene. The temperature order of T₉₀ was as follows: Co₃O₄/α-MnO₂ (248 °C) < Co₃O₄/γ-MnO₂ (266 °C) < α-MnO₂ (270 °C) < γ-MnO₂ (280 °C) < Co₃O₄/β-MnO₂ (282 °C) < β-MnO₂ (288 °C). Ren et al. [63] successfully grew ZIF-derived Co₃O₄ in situ on a one-dimensional α-MnO₂ material (α-MnO₂@Co₃O₄). The synthesized α-MnO₂@Co₃O₄ catalysts exhibited excellent catalytic performance at about 229 °C (T₉₀), which was 47 and 28 °C lower than that of pure α-MnO₂ nanowires and Co₃O₄-b, respectively. The interfacial effect resulting from the coupling between α-MnO₂ and Co₃O₄ was the main reason for the enhanced activity of these catalysts. The construction of the Co₃O₄-MnO₂ interface significantly increased the vacancy concentration, accelerated lattice oxygen mobility, improved the redox cycle of Mn⁴⁺/Mn³⁺ and Co²⁺/Co³⁺, and greatly enhanced the catalytic toluene oxidation.

The interaction of Mn and Ni with Zr ions can likewise enhance the redox ability of Mn-based binary catalysts. Importantly, promoting the uptake and activation of molecular oxygen at defective sites can significantly improve the oxidation activity of the catalyst. Dong et al. [64] synthesized a series of Ni-, Cu-, and Co-doped α-MnO₂ catalysts (M-MnO₂) for the catalytic oxidation of toluene using the reduction of KMnO₄ by ethylene glycol (EG) for the first time. Significant low-temperature activity (T₉₀ = 199 °C) was obtained over Ni-MnO₂ catalysts. This could be attributed to the Ni²⁺ substitution doping into the octahedral MnO₆ backbone which improved the formation of active lattice oxygen and active oxygen mobility. Yang et al. [65] prepared oxygen vacancy-containing Mn_xZr_1-xO₂ catalysts by partial substitution of Zr⁴⁺ for low-valent manganese (Mn²⁺). Their performance with better toluene conversion and specific reaction rate could be due to the increase in oxygen vacancy concentration. Li et al. [66] proposed a double-constrained strategy to reduce the agglomeration of Mn and prepared well-dispersed MnOx@ZrO₂ catalysts using metal–organic frameworks. With a grain size of 17.65 nm, both Mn and Zr elements were uniformly dispersed, obtaining higher toluene oxidation efficiency than MnO_x.

Binary mixtures of Ce, Co, and Cu formed with each other are also common catalysts for toluene oxidation. By compounding, the catalytic performance can be substantially increased. Zhou et al. [67] prepared Ce-Cu composite oxides using the complex method (CeCu-CA) and hard template method (CeCu-HT). The results showed that the catalytic activity of the CeCu-HT catalyst was higher than that of CeCu-CA in air, which could be attributed to the difference in composition and structure. Song et al. [68] reported a series of Cu-Ce binary oxides for the catalytic oxidation of toluene using the co-precipitation method. The Cu₁Ce₃ catalysts exhibited superior toluene oxidation activity due to higher specific surface area, better low-temperature reduction, and stronger Cu-Ce synergy. Carabineiro et al. [69] synthesized binary oxides of Ce-Co and La-Co using an external template method and evaporation. The mixed oxides were superior in toluene oxidation compared to single oxides, and the Ce-Co binary oxides were more active than the La-Co catalyst. The results indicated that the strong interaction between Ce/La and Co led to a sufficient dispersion in the binary system, which improved the specific surface area and reduction of the catalyst. Ismail et al. [70] prepared a series of Co_xCe_1-xO₂-δ (x = 0.05 ~ 0.6) oxides using a hydrothermal method, and the Co_xCe_1-xO₂-δ had higher catalytic activity than pure Co₃O₄ and CeO₂. This good performance was mainly due to the strong interaction between Ce and Co in a solid solution resulting in an abundance of surface reactive oxygen species and an increase in surface Co³⁺ and Ce³⁺ species. For Co₁Y₁O_x (Y = Cu, Mn, Fe, Ni) catalysts, the introduction of different dopants had a significant effect on the physicochemical properties of the catalysts, but the best catalytic activity was obtained for Co₁Cu₁O_x [46]. It was due to the improved catalytic properties of Co₁Cu₁O_x with larger specific surface area, smaller grain size, more surface defect sites, higher Co³⁺ and O_ads, and better reduction. Xu et al. [71] prepared bimetallic CuO/Co₃O₄ oxides by in situ pyrolysis using the MOF template method for partially replacing the Cu ions with Co ions. Compared with CuO, Co₃O₄, and Mix-CuO/Co₃O₄, CuO/Co₃O₄ had the best catalytic activity, accomplishing 90% conversion efficiency at 229 ◦C. This could be attributed in general to the lower temperature reducibility, high degree of dispersion of CuO/Co₃O₄, more reactive oxygen species, abundant lattice defects, and higher molar ratios of Co³⁺/Co²⁺ and O_latt/O_ads.

3.2.2. Perovskite Oxides

Perovskite oxides (ABO₃) have become a common choice for catalysts of aromatic VOCs due to their special structure and properties [72]. In the ABO₃ chemical formula, A is a large cation with 12 times the coordination number (A-site) and B is a small cation with 6 times the coordination number (B-site), coordinated to the oxygen anion [10]. Perovskite catalysts are characterized by good thermal stability due to high-temperature calcination conditions, the unique valence of transition metal ions, easy-to-adjust redox performance, and good low-temperature reduction performance [10,72]. Currently, Mn-based perovskites are the most common catalysts [39]. The Mn site in the structure can provide more oxidation states, and the rare earth elements at the A site have strong oxygen storage capacity, which can promote the generation of active oxygen in the reaction. For example, Zhang et al. [73] synthesized LaMnO₃ perovskite oxides using three methods: citric acid sol–gel (SG), co-precipitation (CP), and glycine combustion (GC). The high specific surface area, the concentration of adsorbed oxygen species, and the low reduction temperature on the surface were in the order of LMO-SG > LMO-CP > LMO-GC, which matched well with the order of catalytic performance. Liu et al. [74] synthesized highly reactive SmMnO₃ oxides by self-fusion polymerization, co-precipitation, impregnation, and sol–gel methods. The SmMnO₃ prepared by self-fusion polymerization had the highest catalytic performance. This was associated with its higher lattice oxygen content, optimal low-temperature reduction, and higher surface Mn⁴⁺/Mn³⁺ ratio. In addition, Chen et al. [75] successfully prepared multi-shell spherical PrMnO₃ oxides for the first time and applied them to the catalytic combustion of CO and toluene. Compared with the catalysts prepared by the conventional co-precipitation method (PrMnO₃-CP), PrMnO₃-HoMSs showed higher catalytic activity for the oxidation of both CO and toluene. The high Mn⁴⁺/Mn³⁺ molar ratio, the improved low-temperature reduction, and the increased reactive oxygen species were the main reasons for the good catalytic performance of PrMnO₃-HoMSs.

The catalytic performance of perovskite oxides depends mainly on the nature of the A and B elements and their valence states, and the A-site ions are usually considered to be catalytically inactive. Therefore, the catalytic performance of perovskite is usually limited by its small surface area and the relatively small amount of surface B-site cations that participate in the catalytic oxidation of aromatic VOCs. To improve the redox performance of perovskite, perovskite oxides are often loaded on carriers such as Al₂O₃, CeO₂, zeolite, and cordierite [10,76]. Among various carriers, CeO₂ is one of the most popular carriers due to its high oxygen storage capacity with abundant oxygen vacancy properties. Wang et al. [76] prepared different morphologies of CeO₂ carriers (rods, cubes, and polyhedra) using a hydrothermal method. The La_0.8Ce_0.2MnO₃/CeO₂ nanopolyhedra exhibited catalytic properties such as small particle size, high specific surface area, more oxygen vacancies, and active oxygen species. Moreover, the establishment of LaMnO₃/MnO₂ interface is beneficial for acquiring a great catalytic capacity for toluene oxidation, while the conventional solid-phase and sol–gel methods have problems in regulating the formation of LaMnO₃ and immobilizing LaMnO₃ on a specific crystalline phase, δ-MnO₂. Yang et al. [77] first assembled LaMnO₃ on δ-MnO₂ using a simple gunpowder-like combustion method. Compared with the conventional solid-state method and sol–gel method, the catalyst synthesized by this strategy can achieve complete oxidation of toluene at 275 °C. This was because the interaction between LaMnO₃ and δ-MnO₂ improved the redox ability. Moreover, a typical strategy for enhancing the catalytic activity of perovskite is the loading of MO_x (metal oxides) on ABO₃, where perovskite can be used not only as an active component but also as a carrier. Suárez-Vázquez et al. [78] focused on the effect of B-site cations in the dendritic SrTiO₃ perovskite on the toluene catalytic behavior. Unlike the role of Cu on the catalyst surface, the addition of Mn leads to Mn⁴⁺ binding at the Ti⁴⁺ site in the perovskite structure. This interaction resulted in the highest catalytic performance of the Mn-doped catalyst for the complete conversion of toluene to CO₂ at less than 350 °C.

3.2.3. Multiple Composites

Multiple composites are multi-element complex oxides composed of three or more transition metal oxides. The tight mixing of these components in the multiple composites can further enhance the interaction and thus the catalytic activity. Clarifying the role of each element and the interactions between these composites is beneficial for designing catalysts for applications under actual working conditions. Dula et al. [79] prepared layer-doped MgMnAlNO₃ and interlayer-doped MgAlMnO₄ by co-precipitation and anionic precipitation, respectively. The MgMnAlNO₃ had higher Mn⁴⁺ phase reduction compared to MgAlMnO₄, which was considered to be the main reason for the higher oxidation performance of the sample in the total oxidation of toluene. Lu et al. [80] prepared Cu-Mn, Cu-Ce, and Cu-Mn-Ce mixed oxides by the sol–gel method. The Cu-Mn-Ce ternary oxide showed the highest catalytic activity. The Cu and Mn atoms were doped into the CeO₂ lattice to form a cerium-based solid solution, which resulted in better interactions of Cu-Mn-Ce, which enhanced the catalyst reduction and oxygen mobility. Hu et al. [81] synthesized a series of spongy Cu-Mn-Ce ternary oxides by a surfactant-modified co-precipitation method, and the catalytic performance was optimized at the mixing ratio of 1:5:5. The results indicated that the appropriate amount of Cu facilitated the combination of highly reducing Mn⁴⁺ with Ce lattice to form a MnCeO_x solid solution, which improved the redox behavior and produced a large amount of active oxygen at the CuO_x/MnCeO_x interface, thus facilitating the combustion of toluene at lower temperatures. Wang et al. [82] doped Cu into Co-Ga layered spinel catalysts using layered double hydroxides as precursors. The difference in the radii of Cu(II) and Co(II) ions resulted in lattice distortion and increased surface oxygen defects, adsorbed oxygen species, and reducibility. This greatly enhanced the catalytic efficiency of toluene oxidation. In conclusion, the results of all the above studies demonstrate the importance of Cu in the preparation of multiplex composite catalysts. Wang et al. [83] synthesized Cu_xCe_1-xFe₂O₄ (x = 0, 0.2, 0.5, 0.8, 1) catalysts using the sol–gel method. The suitable Ce ratio doped Cu_0.8Ce_0.2Fe₂O₄ showed the highest oxidation performance for toluene. The excessive increase in Ce doping leads to the gradual loss of CuFe₂O₄ spinel structure, and the catalytic combustion activity is significantly suppressed. Therefore, it can be assumed that suitable Ce doping into CuFe₂O₄ can increase its oxygen storage capacity and enhance the catalytic activity of toluene oxidation. Liu et al. [84] grew γ-MnO₂ in situ on SmMnO₃ perovskite prepared by a one-step roasting strategy. The prepared γ-MnO₂/SmMnO₃ showed better performance in the catalytic activity of aromatic VOCs in wet air compared to SmMnO₃ and γ-MnO₂. This might be due to its higher surface molar ratio of O_latt/O_ads and better reducibility at low-temperature.

3.3. Supported Metal Oxide Catalysts

A supported metal oxide catalyst is a catalyst whose active components and cocatalysts are uniformly dispersed and loaded on a specially selected carrier. The selection of a suitable carrier can provide reaction space for the catalytic reaction [21]. A catalyst carrier with a high specific surface area and porous characteristics can improve the mass transfer efficiency and adsorption capacity of toluene. In addition, the degradation behavior is closely related to the nature of the carrier. Some high-acidity carriers can also directly participate in the catalytic reaction and contribute to better activity and selectivity. In addition, the strong active phase–carrier interaction effect in the supported metal catalysts has a significant positive impact on their catalytic performance. As shown in

Table 3. Catalytic activity for toluene combustion over supported metal oxide catalysts reported in the recent literature.

Catalyst	Synthesis Method	Toluene Conc. (ppm)	WHSV or GHSV (mL·g−1·h−1 or h−1)	Activity T90 (°C)	Ref.
9.5MnO2/α-Al2O3	Impregnation	1000	15,000 h−1	289	[87]
Cu0.5Mn0.75/α-Al2O3	Impregnation	1000	120,000 h−1	332	[88]
CuMn(y)Ox/γ-Al2O3	Impregnation	1200	15,000 h−1	229	[89]
CuO/Al2O3	Impregnation	1000	200,000 mL·g−1·h−1	320	[90]
CuV/ZSM-5	Hydrothermal method	800	15,000 h−1	315	[93]
10%MnOx/HZSM-5	Impregnation	1000	15,000 mL·g−1·h−1	261	[95]
9.5MnO2/clinoptilolite-type zeolite	Impregnation	1000	15,000 h−1	297	[97]
Fe-Ti-PILC	Impregnation	1000	60,000 mL·g−1·h−1	347	[100]
10%Co-5%Ce/SPC	Soft template	1000	20,000 h−1	175	[101]
10%Co-5%Ce/MMT				208

, the common conventional carriers are TiO₂ [85,86], Al₂O₃ [87,88,89,90,91,92], ZSM-5 [93,94], HZSM-5 [95], beta (BEA) zeolite [96], clinoptilolite-type zeolite [97], MCM-41 [98], cordierite [99] and pillared clay [100,101].

Different carrier materials have a crucial influence on catalytic performance. Pozan et al. [87] compared the effects of a series of carriers loaded with MnO₂ components (α-Al₂O₃ and γ-Al₂O₃ obtained from boehmite, commercial γ-Al₂O₃, SiO₂, TiO₂, and ZrO₂) on the toluene degradation efficiency. Among them, the 9.5MnO₂/α-Al₂O₃(B) catalyst had the highest catalytic performance with 90% toluene conversion at 289 °C. Wang et al. [88] synthesized Cu/Mn-loaded γ-Al₂O₃ catalysts by the impregnation method. The results indicated that CuO and MnO could be highly dispersed on the γ-Al₂O₃ carrier. In practical applications, it was found that the activity of catalytic oxidation of VOCs is negatively affected by the water vapor of the feed material. However, catalysts with different carriers can weaken the inhibition of water vapor. For the catalysts loaded with CuMn, the water durability followed the order CuMn₍₁₎O_x/cordierite > CuMn₍₁₎O_x/TiO₂ > CuMn₍₁₎O_x/γ-Al₂O₃ [89]. In addition, the catalytic performance largely depended on the reducibility of the active component. The catalytic performance of reducible materials is usually much higher than that of irreducible materials [90]. Under the same metal oxide load (10 wt% M_xO_y/Al₂O₃), CuO/Al₂O₃ was by far the most active catalyst in the series, followed by catalysts loaded with MnO, V₂O₅, Cr₂O_3, and CeO₂, and finally, Al₂O₃-loaded MgO, Nd₂O₃, CsO, and ZrO₂ catalysts were practically inactive [90]. Saqer et al. [92] used the impregnation method to load Cu-Mn, Mn-Ce, and Cu-Ce on Al₂O₃ for the catalytic oxidation of toluene. The results demonstrated that the catalytic activity of mixed metal oxides was also significantly improved by optimizing the M_xO_y loading and the composition of the multi-metal oxides.

Zeolite is an excellent carrier of transition metal oxides because of its unique porous structure, thermal stability, acid resistance, and catalytic properties. Its catalytic properties are determined by the acidic distribution, pore size, and cation exchange properties. Different experimental conditions for the preparation of supported transition metals affect this catalytic property. Palacio et al. [93] deposited copper vanadate precursors on two types of carriers, ZSM-5 and amorphous SiO₂, by a hydrothermal method and a mechanical mixing method, respectively. The CuV₂O₆ deposited on ZSM-5 using the hydrothermal method showed the best oxidation efficiency. This might be related to the smaller particle size and higher dispersion of the active component on the carrier. Zhang et al. [95] synthesized a series of 10% MnO_x/HZSM-5 catalysts with different ratios of Si/Al (Si/Al = 25, 50, and 75) using the primary impregnation method. The results showed that all samples had similar structures and redox properties but differed in surface acidity. The catalytic activities of these catalysts were 10%MnO_x/HZSM-5-25 > 10% MnO_x/HZSM-5-50 > 10% MnO_x/HZSM-5-75 in descending order. The 10%MnO_x/HZSM-5-25 had excellent catalytic stability under dry conditions and high water resistance under wet conditions. In particular, beta (BEA) zeolite is an industrially produced high-silica zeolite with a 3D structure and three perpendicular 12-ring channel systems [96]. The pore size is generally larger than that of ZSM-5. The activity of MnO₂-loaded BEA zeolite catalysts was significantly higher than that of unloaded and MnO₂-loaded ZSM-5 [96].

Natural clay minerals have received great attention owing to their low price, environmental compatibility, high selectivity, and plasticity [100,101]. Montmorillonite (MMT) is a class of clay minerals with a special layered structure. In general, pillared montmorillonite catalysts (PILC) are synthesized by ion exchange (TiO₂, SiO₂, etc.) of Na⁺ and Ca²⁺ adsorbed in the montmorillonite layer with organic or inorganic ions [102]. Compared with montmorillonite, PILC has the advantages of large specific surface area, uniform pore size distribution, and good thermal stability, creating quality conditions as catalyst carriers. Liang et al. [100] synthesized Ti-pillared montmorillonite (Ti-PILC) catalysts loaded with Fe and used them successfully for the catalytic oxidation of toluene. Cheng et al. [101] prepared mesoporous Si-pillared montmorillonite (SPC) using a soft template strategy. Compared with the original montmorillonite, the thermal stability was improved, the specific surface area increased from 30 to 453 m²/g, and the layer spacing of SPC increased from 0.96 nm to 2.61 nm. SPC nanocatalysts loaded with Co₃O₄ and rare earth (Y, La, Ce, Pr, and Nd) were synthesized using the deposition–precipitation strategy. Among them, 10%Co-5%Ce/SPC had the highest performance and achieved 98% conversion for catalytic combustion of toluene at 200 °C.

4. Oxidation Kinetics and Catalytic Mechanism

Determining the oxidation mechanism of toluene on a non-homogeneous surface is important for the design of catalysts. Three models, Langmuir–Hinshelwood (L-H), Eley–Rideal (E-R), and Mars–van Krevelen (MVK), are generally used to elucidate the deep oxidation kinetics of VOCs [41]. However, the validity of each model depends strongly on the essential nature of catalysts and the type of VOCs. The L-H mechanism means that the reactants are chemically adsorbed on the catalyst surface, and the reaction is carried out through the interaction of molecules or atoms adsorbed on the surface. The E-R model refers to the chemical adsorption of a component in the reactant on the surface of the catalyst, and the reaction is carried out through the interaction of chemisorption atoms or molecules with molecules in the gas phase (or physical adsorption). For the MVK model, the reaction occurs between the adsorbed VOC and the lattice oxygen on the catalyst. First, the VOC reacts with the active oxygen in the catalyst, causing the reduction of the metal oxides in the catalyst, and then the reduced metal oxides are oxidized by the gas phase oxygen. Among them, the MVK model has been widely used for the simulation of the kinetics of toluene oxidation reactions, especially for transition metal oxide materials [103].

In order to match the most appropriate kinetic model, Mi et al. [31] subjected experimental data to linear regression analysis with computational data. By analyzing the reaction rate equation of these models, it was demonstrated that the catalytic oxidation of toluene followed the MVK mechanism. Based on the MVK mechanism, it is widely believed that the catalytic combustion of VOC molecules by transition metal oxides mainly takes place with the participation of lattice oxygen. Furthermore, Liao et al. [104] and Liu et al. [62] reported the main reaction pathway for the oxidation of toluene on the catalyst surface. It was first oxidized to benzyl alcohol, which was immediately converted to benzaldehyde and benzoic acid. Then upon an increase in the reaction temperature, the benzene ring opened to form maleic anhydride. Finally, maleic anhydride was oxidized to CO₂ and H₂O. In detail, the toluene degradation pathway (Figure 3a) can be interpreted as follows: toluene → benzyl alcohol → benzaldehyde → benzoate → phenols → benzoquinone → acid anhydride → CO₂/H₂O [36]. Mi et al. [31] also suggested that toluene first adsorbed rapidly on the CeO₂ surface to form adsorbed state, which reacted with surface -OH groups to form benzyl species. Benzyl species were further oxidized to benzyloxy, benzaldehyde, and benzoic acid species and finally completely oxidized to H₂O and CO₂. Weak acids can provide more adsorption sites for toluene [36]. As shown in Figure 3b, most toluene molecules after adsorption were attached to reactive oxygen species near oxygen vacancies, and the catalytic degradation process occurred through the abstraction of H atoms from the weakest side chain of the methyl group, then producing benzyl alcohol [68] or benzoyl oxides [27]. In these works, lattice oxygen was consumed to form new oxygen vacancies while surface reactive oxygen was replenished by gas-phase oxygen, thus leading to the regeneration of lattice oxygen and oxygen vacancies. Similar reaction pathways have been found in other studies [42,105,106].

5. Mechanism of Synergistic Removal of Toluene and NO

NH₃-SCR and toluene catalytic oxidation both utilize the redox properties of the catalyst, providing a well-established foundation for achieving simultaneous reduction reaction of NO and toluene. However, the synergistic promotion/inhibition between SCR and toluene has proven to be the key to the feasibility of the reaction. In the synergistic elimination of toluene and NO, NH₃-SCR and toluene catalytic oxidation inevitably interact with each other. The interaction between various pollutants is complex and not a simple facilitative or inhibitory effect; it is associated with multiple factors such as reactant concentration, reaction temperature, and catalyst surface characteristics (adsorption properties, surface acidity, redox properties). In order to gain a deeper understanding of the interaction mechanisms of various reactants, clarifying the effects of reaction conditions on the simultaneous elimination performance can help guide the design and regulation of catalysts for the synergistic purification of toluene and NO.

There is no unified understanding of whether there is a synergistic effect between toluene catalytic oxidation and NH₃-SCR. For example, Liu et al. [107] investigated the interaction of simultaneous NO_x and toluene removal over a CeO₂-TiO₂ catalyst which exhibited excellent removal efficiency for both pollutants. The results indicated that NH₃ inhibited toluene oxidation through competitive adsorption, while NO_x promoted its oxidation. Ultimately, the NH₃-SCR process as a whole promoted toluene oxidation. In addition, toluene inhibited NO_x reaction. As shown in Figure 4a, during the simultaneous removal process, the -CH₃ on toluene was first activated by O₂ to produce electrophilic oxygen and the electrophilic oxygen then underwent the toluene catalytic oxidation process, generating a benzoic acid intermediate. In the meantime, gaseous NO was adsorbed on the material surface to form NO_ad+, and NO_ad+ rapidly attacked the offset of the -COOH in the above product to produce m-nitrobenzoic acid. M-nitrobenzoic acid is an electron-rich benzene series that reacts more readily with electrophilic oxygen than benzoic acid. Subsequently, various carboxylic acids were formed under the action of lattice oxygen due to the opening reaction of the benzene ring. The benzene ring reacted with nitrates adsorbed on the surface to produce organic R-NO_x (containing R-NO, R-ONO, R-NO₂, etc.) and dehydrate the R-NO_x to -CN and -NCO. Finally, these intermediates reacted with nitrates or gas-phase NO/O₂ to form N₂ and CO₂/H₂O. In addition, the reaction temperature affected the redox process of the catalyst. As shown in Figure 4b,c, at lower temperatures (≤200 °C), the O₂ adsorbed on the oxide surface was activated to O₂^-, followed by the oxidation of toluene by O₂^- generating CO₂ and reducing the CeO₂-TiO₂. As temperature increases to 275 °C, the O₂^- is easily converted to O₂^2- anion, and Ti³⁺ transfers electrons to Ce⁴⁺, eventually forming Ti⁴⁺-O₂^2--Ce³⁺ group. It has a strong oxidizing property, which was more favorable for the oxidation of toluene. In addition, Lu et al. [108] found that the existence of NO and NH₃ enhanced the conversion of toluene from 76.6% to 91.8% at 250 °C on MnCe/HZSM-5 catalysts, while the CO₂ selectivity decreased from 60.5% to 34.1%. This was because the NO and NH₃ reacted with intermediates such as aldehydes, carboxylic acids, or phenols to produce nitrile or nitrobenzene during the oxidation of toluene. Nitrile and nitrobenzene were mainly deposited as byproducts on the catalyst surface, which deactivated the catalyst. Specifically, NO competed with toluene on the catalyst for adsorption but promoted toluene oxidation. Conversely, NH₃ promoted toluene adsorption while inhibiting toluene oxidation.

In conclusion, the negative influence of NH₃ on the oxidation of toluene has been confirmed. Reactive oxygen reacts preferentially with NH₃ rather than with the oxidation intermediates of toluene [109]. NH₃ was also thought to promote the formation of nitrile compounds and indirectly inhibit the formation of benzoic acid, maleic acid, and nitrobenzene, which were key factors in reducing CO_x selectivity in the catalytic oxidation of toluene [110]. Moreover, NO had a facilitative effect on toluene oxidation through the process C₇H₈ + 18NO = 9N₂ + 7CO₂ + 4H₂O. Toluene could serve as a reducing agent for NO, facilitating the NH₃-SCR reaction [109]. As shown in Figure 5, Liu et al. [111] found that NO has a facilitating effect on the removal of toluene in the presence of O₂ over the Mn2Fe1 catalyst. However, below 200 °C, toluene had an inhibitory influence on the NO removal. Above 200 °C, the inhibiting effect of toluene on NO diminished. Below 140 °C, NO had a negative influence on toluene oxidation, but this negative effect disappeared as the temperature increased. To some extent, the toluene oxidation reaction was dominant in the process of simultaneous elimination of NO and toluene. O₂ significantly improved the oxidation of toluene by NO, and the presence of strongly oxidizing NO₂ played a positive role in the deep oxidation of toluene. Conversely, the presence of toluene had a negative influence on the deep degradation of NO.

From the above study, it is clear that there is both a facilitative and an inhibitory influence of toluene on the NH₃-SCR process. The interactions may be related to the reaction temperature or active oxygen. In addition, it has been suggested that porous CuZrCe materials modified with various transition metals (Mn, Fe, Co) affect the reaction mechanism for the synergistic elimination of toluene and NO at low-to-medium temperatures [112]. For CuFeZrCe and CuMnZrCe, toluene showed a slight inhibition of NO conversion owing to the competing adsorption of toluene with NO on active sites. In contrast, the SCR atmosphere promoted the oxidation of toluene owing to the consumption of NO and the generation of emerging Brønsted acid sites during the adsorption of toluene. For CuCoZrCe, there was a mutual promotion effect under toluene and an SCR atmosphere. The addition of Co provided Co³⁺ as toluene adsorption sites, which counteracted the competitive adsorption of NO and toluene. In particular, Zhao et al. [113] found that there was also a mutual promotion between NO and toluene in the La_1-xCo_xFeO₃ system. However, for four tunneling structure crystal types (α-MnO₂, β-MnO₂, γ-MnO_2, and δ-MnO₂), there was a mutual inhibition of the synergistic elimination of NO and toluene [114]. Toluene had a significant inhibitory effect on NO conversion, and the existence of NO also increased the conversion temperature of toluene [114]. Furthermore, one of the main problems of Mn-based catalysts was the formation of unfavorable N₂O byproducts in NH₃-SCR, which could destroy the ozone layer [115]. N₂O was primarily generated by NH₃ oxidation and non-catalytic selective reduction (NSCR), which was interpreted as the E-R mechanism and/or the L-H mechanism [115,116,117]. For the E-R process, the reaction of intermediate NH produced by over-dehydration of adsorbed NH₃ with gaseous NO can generate NHNO, which subsequently decomposes further to N₂O. In the L-H process, adsorbed NH⁴⁺ reacts with adsorbed NO₃^- to generate NH₄NO₃, which could decompose to N₂O. Toluene affected the redox performance of SCR catalysts and also altered the pathway for N₂O formation in NH₃-SCR. Lu et al. [118] evaluated the effect of toluene on the mechanism of N₂O formation. It was concluded that NSCR dominated below 250 °C and the E-R mechanism dominated at 200 °C. Toluene had a significant inhibition of N₂O formation along with a slight inhibition of NO reaction. This was because toluene inhibited NH₃ peroxidation and N₂O production by NSCR. Both gaseous O₂ and oxygen oxidation of NH₃ on the surface were limited by toluene, resulting in less NH adsorption. However, Ye et al. [119] found that toluene led to more N₂O production in NSCR, which could be due to the generation of oxygen vacancies in the MnCe catalyst.

6. Conclusions and Prospect

This review presented the recent advances and advantages of single metal oxides, multi-oxide composites, and supported metal oxide catalysts for the catalytic elimination of a typical VOC (toluene). The activities of different catalysts were summarized and compared, and the critical factors influencing the degradation of toluene were revealed. The kinetic model, toluene oxidation mechanism, and synergistic reaction mechanism between toluene oxidation and NH₃-SCR were also investigated. Among various catalysts, Mn-based catalysts usually have excellent oxidation activity, while supported metal oxide catalysts have high stability. The catalytic activity is influenced by the type of active phase and carrier, the type of active metal precursor, the loading amount, the preparation method, and the intrinsic properties of metal oxides (dispersion, crystal structure, microscopic morphology, valence, reduction, reactive oxygen species, etc.). The multi-metal oxides in general have higher catalytic activity than similar individual metal oxides. The enhanced interaction between the active component and the loaded/doped component becomes a bottleneck limiting catalyst development. The synergistic promotion/inhibition between toluene and NO, NH₃, heavy metals, etc., was shown to be the key to the synergistic reaction control technology.

Despite the remarkable progress, there are still some outstanding issues that need attention in future research:

(1)

Industrial monolithic catalysts with improved VOC reduction should be further developed. Surface modification methods should have easy preparation processes, large scale, and precisely controlled conditions.

(2)

Catalyst deactivation is a major defect in industrial applications. The iron sintering flue gas environment is often very complex, and trace contaminants such as water vapor, SO₂, chlorine-containing compounds, and heavy metals may be present simultaneously. Therefore, catalysts with high activity, stability, and resistance to poisoning need to be designed. In addition, further understanding of the sources of poisoning or deactivation of different catalysts can enable industrially viable regeneration techniques by establishing correlations between the surface properties of the parent body and its catalytic activity.

(3)

In the simultaneous elimination of VOCs and NO, the reactions between NH₃/NO, oxygen, and toluene over catalysts are still intricate and in their infancy. Theoretical calculations can be used to understand the reaction pathways, intermediate species migration, and conversion mechanisms for the simultaneous elimination of VOCs and NO at the molecular level.