A Review of Simultaneous Catalytic Removal of NO_x and VOCs: From Mechanism to Modification Strategy

Abstract

Simultaneous catalytic elimination of nitrogen oxides (NO_x) and volatile organic compounds (VOCs) represents a promising technology for addressing the synergistic pollution of fine particulate matters of <2.5 μm diameter (PM_2.5) and O₃. Nevertheless, it has been maintaining significant challenges in practical implementation, particularly the inherent mismatch in temperature windows between NO_x reduction and VOCs oxidation pathways, coupled with catalyst poisoning and deactivation phenomena. These limitations have hindered the industrial application of bifunctional catalysts for the removal of concurrent pollutant. This review systematically explored the fundamental mechanisms and functional roles of active sites in controlling synchronous catalytic processes. The mechanism of catalyst deactivation caused by multiple toxic substances has been comprehensively analyzed, including sulfur dioxide (SO₂), water vapor (H₂O), chlorine-containing species (Cl*), reaction by-products, and heavy metal contaminants. Furthermore, we critically evaluated the strategies of doping regulation, nanostructure engineering and morphology optimization to enhance the performance and toxicity resistance of catalysts. Meanwhile, emerging regeneration techniques and reactor design optimizations are discussed as potential solutions to improve the durability of catalysts. Based on the above critical aspects, this review aims to provide insights and guidelines for developing robust catalytic systems capable of controlling multi-pollutants in practical applications, and to offer theoretical guidance and technical solutions to bridge the gap between laboratory research and industrial environmental governance applications.

1. Introduction

NO_x and VOCs are the essential precursors of PM_2.5 and O₃, which could cause serious harm to the ecological environment and human health [1,2,3]. The Chinese government has set NO_x and VOCs as two of the five key atmospheric targets, and made the stringent emission standards to achieve at least a 10% reduction in NO_x and VOCs emissions during the 14th Five-Year Plan period (2021–2025) [4,5]. In recent years, waste incineration treatment technology has been vigorously developed due to its excellent capacity reduction effect and high energy efficiency [6,7]. However, the waste incineration process inevitably yields various pollutants, including NO_x and VOCs [8]. Among these, toluene and chlorobenzene (CB) are highly representative constituents. Toluene originates predominantly from biomass burning and solid waste incineration, whereas CB is generated mainly in municipal and medical waste incinerators [9,10,11]. Nowadays, NO_x and VOCs are generally removed separately in different units in the industrial application, which raises the cost of treatment. Thus, a simultaneous removal of NO_x and VOCs in the same unit may be a reasonable technical and economic option [12,13]. A large number of studies found that these two pollutants could be simultaneously removed by a bifunctional catalyst with NO_x reduction and VOCs oxidation [14]. Therefore, the simultaneous catalytic removal of NO_x and VOCs has attracted much attention recently.

In the past several years, great effort has been made to develop highly efficient and poison-resistant bifunctional catalysts. The existing catalysts are faced with the problems of a narrow operating temperature window and poor poisoning resistance in the process of simultaneous removal [15,16,17]. Moreover, flue gas usually contains SO₂, Cl species, and H₂O etc., which will poison the active site and inhibit the activity of the catalysts [18,19,20]. Therefore, many researchers tried to improve the activity of bifunctional catalysts by doping elements, designing core–shell structures and adopting morphologies. So far, some reviews have been reported on the simultaneous catalytic removal and described the mechanisms of NO_x reduction and VOCs oxidation, respectively. They focused on Mn-based catalysts, V-based catalysts, Cu-based catalysts, which mainly concentrated on the structure–performance correlation [21,22]. Nevertheless, there are very few comprehensive reviews that systematically report the performance and mechanism of the synergistic removal of NO_x and VOCs, as well as the related modification strategies. In this review, we systematically summarized the control technologies for multiple pollutants such as NO_x and VOCs, and mainly elaborated on the synergistic reaction mechanism, the role of acidity and active sites, as well as the mechanism of resistance to toxins. Figure 1 summarizes the roles of three modification techniques in simultaneous removal of NO_x and VOCs, such as increasing the catalytic activity by doping elements to increase the number of catalyst acidic sites and oxygen vacancies, enhancing the sulfur resistance of the catalyst by adopting a core–shell structure to protect the active sites, and altering the morphology of the catalysts to enlarging the number of active sites exposed. This enables the establishment of a connection between the active sites and the modification strategies, providing more ideas for the development and design of bifunctional catalysts.

2. Reaction and Poisoning Mechanism

In NO reduction, the active intermediates, such as NH_x species, adsorbed NH₃, NH₄⁺, and NH₂ species, have been demonstrated to be decisive factor for the low-temperature activity of Selective Catalytic Reduction (SCR) catalysts [23]. The kinetic model and mechanism of VOCs oxidation can be divided into Eley–Rideal (E-R), Langmuir–Hinshelwood (L-H) and Mars-van Krevelen (MvK) models. Due to the complexity of VOCs, the intermediate products of VOCs reactions vary greatly. Benzyl alcohol, benzaldehyde, benzoic acid and phenol are the intermediate products in the oxidation process of toluene. Benzyl alcohol, benzaldehyde, benzoic acid and phenol are the intermediate products in the oxidation process of toluene. CB produces some chlorine-containing by-products during the oxidation process [24,25,26]. However, the intermediates undergo significant changes during the simultaneous catalytic removal, resulting in a modification of the reaction pathway. In addition, the SO₂ and H₂O present in the feed gas can adversely affect the performance of bifunctional catalysts in the simultaneous removal process. Therefore, the influence of these components must be considered during catalyst design to mitigate or prevent their negative effects. A fundamental understanding of the interaction mechanism between NO_x reduction and VOCs oxidation, the role of active sites, and the poisoning mechanisms is essential for designing effective catalysts for the simultaneous removal of NO_x and VOCs.

2.1. Mechanism of Simultaneous Removal of NO_x and VOCs

2.1.1. Mechanism of Simultaneous Removal of NO_x and Toluene

In the process of simultaneous removal of NO_x and VOCs, the SCR reaction system will affect the toluene. Previous studies have shown that NH₃ can inhibit the adsorption of toluene. As shown in Figure 2a, the adsorption energy of toluene was reduced, and the electron transfer of toluene to the catalyst was markedly hindered when NH₃ existed on the catalyst surface [27]. Adsorbed NH₃ will react with reactive oxygen species, which will reduce the conversion of toluene [28]. The intermediate products formed during the oxidation of NH₃ and toluene are prone to generate by-products such as acetonitrile, depositing on the catalyst surface, which affected the deep oxidation of toluene and reduced the CO₂ yield [29].

NO_x can both promote and inhibit the oxidation process of volatile organic compounds. Zhao et al. [30] considered that the main reaction between toluene and NO is C₇H₈ + 18NO = 9N₂ + 7CO₂ + 4H₂O. In the absence of NO, the oxidation path of toluene will change, and the specific mechanism is shown in Figure 2b. The reaction of NO with reactive oxygen species can promote the oxidation of toluene. NO₂ has a certain oxidation ability and can promote the oxidation of toluene in the process of synchronous removal. The NO₂ gas molecule is firstly adsorbed on the catalyst, releasing an oxygen atom, and then the recovered lattice oxygen oxidizes toluene according to the MvK mechanism [31]. During the oxidation of NO to NO₂, NO₂ generated in situ assisted O₂ in enhancing the reoxidation capacity of metal, resulting in the higher conversion rate of VOCs [32]. The O atoms in NO₂ can readily provide additional surface oxygen (O_sur) and improved mobility and activity of lattice oxygen (O_lat), S thereby accelerating the reoxidation of reduced metal sites, which can promote the oxidation of toluene [33]. In certain scenarios, NO_x inhibits the oxidation of toluene. Lu et al. [34] found that NO would react with intermediates in the oxidation process of toluene to produce nitrogen-containing organic compounds, which would deactivate the catalyst. With oxygen, it is easy for NO to form nitrate, occupying the activation site, and hindering the oxidation of toluene [35]. NO can inhibit the removal of toluene at low temperature [27].

The oxidation process of toluene can have some effect on the SCR reaction. Many studies have shown that toluene can inhibit the reduction of NO_x. The competitive adsorption between toluene and NH₃ is the main reason for this phenomenon [36]. Compared with the single SCR reaction, the coordination with NH₃ on the Lewis acid site was weakened under synergistic catalysis, implying that toluene inhibited the adsorption of NH₃ and thus suppressed the NO_x reduction reaction [37]. Ye et al. [38] discovered that toluene readily reacts with NH₃ to form nitriles, which infringed on the active sites and decreased the available coordinated NH₃ on Lewis acid sites. Toluene strongly affected the conversion of NO on catalyst surface, directly inhibiting the oxidation of NO at low temperature. Shao et al. [39] found the negative impact of the interaction between toluene and nitric oxide, and revealed a more significant inhibitory effect. However, the higher the temperature, the weaker the inhibitory effect becomes, and the catalytic activity of NO gradually recovers to the equivalent value as in the absence of toluene [40]. However, some researchers believe that toluene has a certain promotion effect on NH₃-SCR. It was effectively co-removed with NO_x with the advantage of N₂O inhibition. Toluene suppressed the oxidation of NH₃ and the generation of N₂O in non-selective catalytic reduction (NSCR) [31]. At the same time, toluene can also act as a reducer of NO, which was beneficial to NH₃-SCR reaction. Based on the results, the main principle is that toluene consumes a limited amount of reactive oxygen in the reaction, thus promoting the reduction of NO_x [30]

In summary, there exist seemingly contradictory conclusions regarding the role of NO_x in toluene oxidation: some studies report a promoting effect, while others indicate inhibition. The origin of this apparent paradox may lie in the following factors: (1) Differences in the operating temperature window: at lower temperatures, NO tends to react with intermediate products to form toxic by-products, whereas at elevated temperatures, the oxidizing capacity of NO₂ becomes dominant; (2) Variations in the redox properties of catalysts: for metal oxides with facile valence cycling (e.g., CeO₂), NO may enhance the mobility of lattice oxygen, whereas in other catalytic systems, NO may preferentially form stable surface nitrates, thereby blocking active sites [41].

2.1.2. Mechanism of Simultaneous Removal of NO_x and CB

CB is a major challenge in air pollution control because of its attached chlorine-containing functional groups, and it is prone to generating polychlorinated by-products during the catalytic process, which complicates the catalytic conditions and causes significant damage to the environment [42]. The interaction between the oxidation of CB and reduction of NO_x is shown in Figure 3a. During the simultaneous removal of CB and NO_x, NO_x and NH₃ will promote the oxidation of CB, but this promotion condition is limited. NO can only promote the conversion of CB in the presence of O₂ [32,43]. The possible reason is that NO is oxidized to NO₂ by the metal oxides on the catalyst, and then NO₂ assists the reoxidation step of the metal phase, thus accelerating the oxidation cycle and increasing the conversion rate of CB on a macro level [44]. NH₃ inhibits the oxidation of CB during simultaneous removal process [45]. The competitive adsorption of NH₃ is the main reason. The strong adsorption of NH₃ by the catalyst leads to a decrease in CB oxidation efficiency. The certain NH₃ spent under the stabilizing procedure not for NO_x reduction but adsorbed on the catalyst surface [46,47]. This indicates that the adsorbed NH₃ participates in the reduction of NO_x to some extent, while the NH₃ that does not participate in the reduction reaction hinders the adsorption of CB. Reducing the strong adsorption of NH₃ is the key factor to improve the oxidation efficiency of CB [48].

Many studies have shown that CB can promote the reduction of NO_x. CB can promote the oxidation of NO_x, because the oxygen vacancy generated during the ring-opening process of CB is conducive to the adsorption of gas molecules, accelerating NH₃-SCR reaction [49]. Li et al. [50] found that CB inhibited the side effects of SCR. The specific mechanism is shown in Figure 3b. Gan et al. [51] discovered, through reaction kinetics decoding, that both KNSCR and δC-O decreased significantly when CB was introduced into the flue, especially above 200 °C. This direct evidence proves that CB has a significant inhibitory effect on NSCR and C-O reaction at high temperatures. Meanwhile, CB decomposition at higher temperatures will provide additional acidic sites for the catalyst. This will facilitate the adsorption of NH₃ and NO_x, thus promoting the reduction of NO_x [21]. At low temperatures, CB has a negative effect on NO_x reduction. CB significantly inhibited the SCR reaction below 250 °C [9]. During the oxidation of CB, the dissociated Cl will react with NH₃ to form NH₄Cl, which will block the active site of the catalyst and inhibit the reduction of NO_x [52]. Although the competitive adsorption between NH₃ and CB is the direct reason for the decreased CB oxidation efficiency, the root cause lies in the significantly stronger adsorption energy of NH₃ compared to CB on the catalyst surface. This insight suggests that future catalyst design should not solely focus on creating strong acidity. Instead, it should aim to precisely modulate the selectivity or spatial distribution of acid sites. For instance, spatially isolating SCR sites from oxidation sites could circumvent competitive adsorption at its fundamental level.

Figure 3. (a) A schematic illustration of the synergistic catalytic removal of NO_x and CB over MnO₂-CePO₄/TiO₂ catalysts [50]; (b) Simultaneous mechanism of the removal of NO and CB [53]. The ‘×’ symbol indicates an inhibited or ineffective reaction step.

Figure 3. (a) A schematic illustration of the synergistic catalytic removal of NO_x and CB over MnO₂-CePO₄/TiO₂ catalysts [50]; (b) Simultaneous mechanism of the removal of NO and CB [53]. The ‘×’ symbol indicates an inhibited or ineffective reaction step.

2.2. The Role of Active Site

2.2.1. Acid Sites

The acidic site plays an important role in simultaneous removal of NO_x and VOCs. Some studies have found that the acidic site can affect the reaction path of NO_x and VOCs, improving the reaction efficiency on the surface of the catalyst [54,55]. Firstly, in the SCR reaction, the acidic site can interact with NH₃ to form an active intermediate. These intermediates can effectively reduce NO_x to N₂, thereby improving the efficiency of NO_x removal [56]. Under certain conditions, the existence of an acid site can improve the durability of catalysts, enabling them to maintain better stability in complex flue gas environments. For example, under the condition of coke oven flue gas, the acid level can improve the sulfur resistance and water resistance of the catalyst [57]. Secondly, acidic sites can enhance the adsorption ability of catalyst for VOCs, making VOCs more easily adsorbed on the catalyst surface [49]. At the same time, the acidic site can activate the chemical bonds in VOCs molecules and reduce the activation energy of the reaction, thus promoting the oxidation reaction of VOCs [40]. Lewis and Brønsted acids are two important catalysts, which play an important role in simultaneous removal reaction, which played different roles in simultaneous removal [58]. Lewis acid focuses on the oxidation of VOCs in synchronous removal, and the main reason is that Lewis acid site polarizes or activates VOCs molecules by accepting electron pairs in VOCs molecules, thereby reducing their reaction activation energy and promoting their adsorption on the catalyst surface and further oxidation reactions [52,59]. The Brønsted acid focuses on the removal of NO_x, and NH₃ was mainly adsorbed at the Brønsted acid site [60]. In the SCR reaction, the Brønsted acidic site provides the proton (H⁺), which reacts with the NH₃ molecule by proton transfer, forming the NH₄⁺ of the adsorbed state. The NH₄⁺ of adsorbed state is able to react effectively with NO_x, reducing it to N₂ [61]. Su et al. [62] found that the Brønsted acid site helps stabilize the active intermediates in SCR reaction, such as N₂O and NH₂, etc. The stable existence of these intermediates on the catalyst surface is the key to achieving efficient NO_x removal.

2.2.2. Metal Sites and Oxygen Vacancy

Metal active sites refer to the metal atoms or ions in the catalyst that possess catalytic activity. These sites can interact with reactant molecules to promote chemical reactions [63]. In the process of simultaneous removal of NO_x and VOCs, metal sites can cooperate with acidic sites to achieve efficient collaborative removal of VOCs and NO_x. In addition, metal sites can also regulate the reaction path of pollutants and improve the removal efficiency of NO_x and VOCs [64]. It has been found that the interaction between the metal active site and the support can improve the thermal stability and anti-toxicity of the catalyst [65]. When metal or non-metal ions are doped in metal oxides, they will disturb the lattice structure of the catalyst and form oxygen vacancies [66], which is known as active defects and efficiently adsorb reactants. These vacancies also help to convert intermediates into CO₂ and H₂O, thereby accelerating the overall reaction process [67]. The reduction in metal cations from higher to lower valence states generates charge imbalance, vacancies, and unsaturated chemical bonds on the catalyst surface, which facilitates the formation of chemically adsorbed oxygen and/or weakly bonded oxygen species, and the generated oxygen species can migrate onto the metal oxide surface via oxygen vacancies [68,69,70]. For example, during the process from Ce⁴⁺ to Ce³⁺, more Ce³⁺ increases the number of oxygen vacancies and enhances the redox activity of the catalyst [40]. In addition, electron transfer can also occur between the metal active site and the oxygen vacancy, which can regulate the electronic structure of the catalyst, making it more conducive to the adsorption and activation of the reactants [71]. Furthermore, the metal active site and oxygen vacancy can synergistically promote catalyst regeneration. In the Mars-van Krevelen mechanism, the metal oxide catalyst forms an oxygen vacancy in the reduction reaction, and in the oxidation reaction, oxygen molecules can fill the oxygen vacancy, allowing the catalyst to be re-oxidize and regain its activity [72].

2.3. Poisoning Mechanisms

The catalyst is the core of the collaborative removal system, but in actual working conditions, flue gas contains trace pollutants (SO₂, H₂O, Cl and heavy metal, etc.), which seriously affect the structure and activity of the catalysts. The deactivation pathways include destroying the surface porous structures and Brønsted acid sites, inhibiting the redox cycle of active sites, competing adsorption between target and trace pollutants on the surface of catalysts, and blocking of by-products [73].

2.3.1. Sulfur Poisoning

The detrimental impact of SO₂ on bifunctional catalysts is a critical challenge, as extensively documented across various catalytic systems. For instance, Ye et al. [38] and Li et al. [49] reported the severe inhibition of NO and VOC (toluene or chlorobenzene) conversion upon SO₂ introduction, a trend corroborated by the performance summary of multiple catalysts in Table 1. Moreover, it can be observed from the table that Mn-based catalysts are highly susceptible to deactivation in the presence of SO₂. The poisoning mechanism is multifaceted. Conventionally, it is attributed to SO₂ competing with reactants for active sites and reacting with NH₃ to form ammonium sulfites/sulfates, leading to pore blockage [74]. However, a critical distinction must be made between reversible and irreversible poisoning. Sulfates like (NH₄)₂SO₄ and NH₄HSO₄ can be decomposed at elevated temperatures (e.g., 400 °C), allowing for activity recovery [75]. In contrast, the formation of stable metal sulfites and sulfates (e.g., on MnO_x-CeO₂) constitutes a more severe, often irreversible deactivation pathway by directly destroying redox-active sites [76]. Beyond mere activity suppression, a profound consequence of SO₂ poisoning is the potential alteration of reaction pathways, leading to undesirable by-product generation. As illustrated in Figure 4, the presence of SO₂ over a V₂O₅-WO₃/TiO₂ catalyst significantly increased the yield of toxic polychlorinated by-products during chlorobenzene oxidation, which was linked to catalyst sulfation [77]. This highlights that SO₂ not only deactivates catalysts but can also pervert their intrinsic selectivity, posing additional environmental risks.

The situation is further complicated in multi-pollutant atmospheres, where synergistic adsorption effects occur. Li et al. [78] demonstrated that, in a complex flue gas on activated carbon, the co-presence of pollutants paradoxically doubled the SO₂ adsorption capacity while drastically reducing the adsorption of CB and NO. This indicates that SO₂ can dominate the surface chemistry in complex mixtures, leading to a catastrophic performance deterioration. Strikingly, the role of SO₂ is not universally inhibitory and can be paradoxically promotional under specific conditions. A study on a V₂O₅/AC catalyst showed that SO₂ could enhance the conversion of dibenzofuran (DBF) and reduce its apparent activation energy, likely by participating in the reaction mechanism [79]. This critical observation reveals that the influence of SO₂ is highly context-dependent, determined by the catalyst’s chemical nature and the specific reaction. The overall negative impact, however, often prevails as the ensuing formation of stable species like VOSO₄ inevitably impairs the catalyst’s ultimate oxidation capacity [80].

Table 1. A summary of the effect of SO₂ on the catalyst.

Catalyst	Reactant Composition	Space Velocity (mL·g−1·h−1)	Conversion (Without SO2)	Conversion (with SO2)	Reference
Mn-Fe	Toluene 50 ppm, NO 500 ppm, SO2 300 ppm, 5% O2, 10% H2O	24,000	Toluene: 100% (230 °C) NO: 89% (230 °C)	Toluene: 56% (230 °C) NO: 0% (230 °C)	[40]
MnOx-CeO2	CB 50 ppm, NO 500 ppm, NH3 500 ppm, 10%O2, 5%H2O	60,000	CB: >90% (230 °C) NOx: >90% (250 °C)	Inactivation	[51]
MnO2	Toluene 100 ppm, NO 200 ppm, SO2 1000 ppm, 20%O2	24,000	Toluene: 100% (310 °C) NO: 89% (310 °C)	Toluene: 10% (310 °C) NO: 5% (310 °C)	[39]
MnCe/TNT	Toluene 50 ppm, NO 500 ppm, NH3 500 ppm, SO2 250 ppm, 10% O2, 10% H2O	60,000	Toluene: >90% (200 °C) NO: >90% (250 °C)	Toluene: 10% (200 °C) NO: 60% (250 °C)	[81]
PdV/TiO2	CB 600 ppm, NO 600 ppm, NH3 600 ppm, SO2 200 ppm, 10%O2, 5%H2O	30,000	CB: 90% (400 °C) NOx: 80% (300 °C)	CB: 80% (400 °C) NOx: 70% (300 °C)	[49]
VWT	CB 100 ppm, 600 ppm NH3,600 ppm NO, 100 ppm SO2, 5 vol % O2	40,000	CB: 90% (250 °C) NOx: /	CB: 80% (250 °C) NOx: /	[78]
MnOx	Toluene 100 ppm, NO 600 ppm, NH3 600 ppm, SO2, 200 ppm, 5% O2, 5% H2O	15,000	Toluene: / NO: 100% (200 °C)	Toluene: / NO: 60% (200 °C)	[82]

Table 1. A summary of the effect of SO₂ on the catalyst.

Catalyst	Reactant Composition	Space Velocity (mL·g−1·h−1)	Conversion (Without SO2)	Conversion (with SO2)	Reference
Mn-Fe	Toluene 50 ppm, NO 500 ppm, SO2 300 ppm, 5% O2, 10% H2O	24,000	Toluene: 100% (230 °C) NO: 89% (230 °C)	Toluene: 56% (230 °C) NO: 0% (230 °C)	[40]
MnOx-CeO2	CB 50 ppm, NO 500 ppm, NH3 500 ppm, 10%O2, 5%H2O	60,000	CB: >90% (230 °C) NOx: >90% (250 °C)	Inactivation	[51]
MnO2	Toluene 100 ppm, NO 200 ppm, SO2 1000 ppm, 20%O2	24,000	Toluene: 100% (310 °C) NO: 89% (310 °C)	Toluene: 10% (310 °C) NO: 5% (310 °C)	[39]
MnCe/TNT	Toluene 50 ppm, NO 500 ppm, NH3 500 ppm, SO2 250 ppm, 10% O2, 10% H2O	60,000	Toluene: >90% (200 °C) NO: >90% (250 °C)	Toluene: 10% (200 °C) NO: 60% (250 °C)	[81]
PdV/TiO2	CB 600 ppm, NO 600 ppm, NH3 600 ppm, SO2 200 ppm, 10%O2, 5%H2O	30,000	CB: 90% (400 °C) NOx: 80% (300 °C)	CB: 80% (400 °C) NOx: 70% (300 °C)	[49]
VWT	CB 100 ppm, 600 ppm NH3,600 ppm NO, 100 ppm SO2, 5 vol % O2	40,000	CB: 90% (250 °C) NOx: /	CB: 80% (250 °C) NOx: /	[78]
MnOx	Toluene 100 ppm, NO 600 ppm, NH3 600 ppm, SO2, 200 ppm, 5% O2, 5% H2O	15,000	Toluene: / NO: 100% (200 °C)	Toluene: / NO: 60% (200 °C)	[82]

2.3.2. Heavy Metal Poisoning

In the flue gas, lead (Pb) and arsenic (As) are the most common poisons [83,84]. There are many possible mechanisms for deactivation: heavy metals can clog the pores or block the active sites through irreversible reactions or change the adsorption capacity of other substances through electronic effects.

The deactivation of a catalyst by Pb is mainly chemical, and Pb not only accumulates on the catalyst surface and competes with gas phase reactants for adsorption at the active site, but also causes changes in the electronic properties of the catalyst surface, reducing surface acidity and redox capacity of the catalyst [85]. The accumulation of Pb will cause the loss of oxidation performance and surface acidity of catalyst and affect the selectivity of CB and CO₂. It also affects the oxidation pathway of CB, as illustrated in Figure 5a [86]. Pb²⁺ induces more toxic polychlorinated by-products [87]. The acid sites are located in SCR correlation, and the loss of surface acidity will affect the conversion efficiency of NO_x [59]. The existing literature has shown that the conversion rate of CB will fluctuate greatly, and the conversion rate of NO will decrease slightly under the condition of Pb poisoning [88].

In the case of As poisoning, the SCR reaction was severely deactivated [89]. The reasons for this phenomenon are as follows: Gaseous As₂O₃ reacts on the catalyst surface to form stable compounds, leading to a reduction in the active component of the catalyst and a decrease in the synergistic removal efficiency [90]. Zhao et al. [30] indicated that Al₂O₃ significantly reduces the oxygen vacancy formation energy at the V-O-V site, which leads to the production of more toxic polychlorinated by-products during synchronous removal. As₂O₃ is adsorbed and oxidized to solid As₂O₅, which can block the pores and diminish the specific surface area, leading to the physical deactivation of the catalyst [91]. As₂O₅ on a V-based catalyst attacks the adjacent V=O site, forming an As–O–V band, which inactivates the deNO_x reaction, as illustrated in Figure 5b [92].

Figure 5. (a) The elimination of chloroaromatic congeners on a commercial V₂O₅-WO₃/TiO₂ catalyst: The effect of heavy metal Pb [86]; (b) V₂O₅-WO₃/TiO₂ Catalyst for Efficient Synergistic Control of NO_x and Chlorinated Organics: Insights into the Arsenic Effect [92]. The ‘×’ symbol indicates an inhibited or ineffective reaction step.

Figure 5. (a) The elimination of chloroaromatic congeners on a commercial V₂O₅-WO₃/TiO₂ catalyst: The effect of heavy metal Pb [86]; (b) V₂O₅-WO₃/TiO₂ Catalyst for Efficient Synergistic Control of NO_x and Chlorinated Organics: Insights into the Arsenic Effect [92]. The ‘×’ symbol indicates an inhibited or ineffective reaction step.

2.3.3. Other Effects

Chlorine species also exert a profound influence on catalyst activity. According to the literature, the poisoning mechanism can be attributed to the following pathways: First, chlorine species compete with reactants for adsorption sites, covering active centers and consequently reducing catalytic activity. Second, they can form reactive intermediates that deposit on the catalyst surface, blocking pore channels. Finally, chlorine in the flue gas may react with the catalyst to generate volatile metal chlorides or oxychlorides, leading to the loss of active components and irreversible catalyst deactivation [93,94,95]. Wang et al. [96] found that dissociated Cl on the catalyst surface promoted the adsorption of NH₄⁺ and the formation of NH₄Cl, which was not involved in the SCR reaction, leading to a considerable blockage on the catalytic active sites.

The influence of H₂O on the simultaneous removal system is double-sided [63]. Wang et al. [97] found that when NO_x and CB are removed at the same time under humid conditions, Co-SM has a significant conversion rate of NO_x in the range of 160~300 °C, and the T₉₀ of CB is about 270 °C, which is better than the catalytic performance in the absence of water vapor. However, Kang et al. [98] found that the conversion rate of Co-Mn to NO_x and CB decreases at 250 °C when H₂O enters the reaction system. NO_x decreased slightly and CB decreased by 5%. Therefore, some scientists have started to research the above phenomenon. Long et al. [89] found that H₂O can promote the SCR reaction on as-supported VWT catalyst, because H₂O will spontaneously dissociate at the reduced V site to form a surface hydroxyl group (HO–V⁴⁺–O–V⁴⁺–OH), which accelerates the recovery of Brønsted acid. The influence of H₂O on the removal of NO_x and VOCs presents a dual role. As shown in Figure 6a,b, the impact of H₂O on NO_x and VOC abatement is of a bifunctional nature. Under high-temperature regimes, H₂O plays a facilitative role by enhancing NO conversion and N₂ selectivity through the suppression of excessive oxidation or NSCR. while simultaneously fostering the complete oxidation of VOCs, which elevates the selectivity for CO₂ and HCl and suppresses by-product generation. Conversely, at low temperatures, H₂O exerts a negative effect by competing with NO, NH₃, and VOCs for adsorption sites [99].

During the simultaneous removal of dioxins and NO_x, SCR and catalytic oxidation inevitably affect each other, and by-products can be generated, such as aniline, nitrobenzene, and benzonitrile. Among them, toluene reacts with NH₃ to form nitrile, destroying the active site and reducing the effective coordination of NH₃ at the Lewis acid site, thus affecting the SCR [100]. Excessive NH₃ adsorption and polychlorinated species accumulation on the surface are the primary issues of poison of catalysts and reduce of their lifetime [47]. The by-products produced from CB are another deactivation factor, for example, some thermally stable and chemically inert organics (such as polychlorinated compounds) can accumulate on the catalyst surface. CCl₄ and C₂Cl₄ were the two major deactivating species produced over V-Mo-Ti catalyst [9].

Figure 6. (a) Bimodal effect of water on V₂O₅/TiO₂ catalysts [89]; (b) A schematic diagram of the reaction mechanisms of H₂O in VOC oxidation [101].

Figure 6. (a) Bimodal effect of water on V₂O₅/TiO₂ catalysts [89]; (b) A schematic diagram of the reaction mechanisms of H₂O in VOC oxidation [101].

3. Strategies for Improving the Activity and Poisoning Resistance

Conventional SCR catalysts have some problems, such as low removal efficiency of VOCs and narrow temperature window [15,16,17]. Therefore, the development of efficient bifunctional catalysts for the simultaneous removal of NO_x and VOCs is of considerable importance. Leveraging the functions of the aforementioned active sites, the removal efficiency and poisoning resistance can be enhanced via strategies such as doping with metal elements, constructing core–shell structures, and engineering the catalyst morphology.

Table 2. Performance of catalyst at 300 °C before and after doping.

Catalyst	Reactant Composition	Space Velocity (mL·g−1·h−1)	Conversion	Conversion (After)	Reference
CuCeTi	Toluene 50 ppm, NO 500 ppm, NH3 500 ppm, SO2 0–500 ppm, 10% O2, 5% H2O	60,000	/	Toluene: >90% (300 °C) NO: >90% (300 °C)	[55]
	Benzene 100 ppm, Toluene 100 ppm, NO 500 ppm, NH3 500 ppm, SO2 1000 ppm, 3.3% O2	45,000	/	Benzene: >85% (260–420 °C) Toluene: >92% (260–420 °C) NO: >91% (260–420 °C)	[102]
SbPdV/TiO2	CB 600 ppm, NO 600 ppm, NH3 600 ppm, SO2 600 ppm, 10% O2	30,000	CB: >90% (325 °C) NO: >90% (200 °C)	CB: >90% (350 °C) NO: >90% (225 °C)	[103]
Sb/VWT	CB 100 ppm, NO 500 ppm, NH3 500 ppm, SO2 50 ppm, 10% O2	60,000	CB: 100% (325 °C) NO: ~95% (325 °C)	CB: 100% (325 °C) NO: ~90% (325 °C)	[47]
Cu0.1-VWT	Benzene 100 ppm, Toluene 100 ppm, NO 500 ppm, NH3 500 ppm, SO2 1000 ppm, 3.3% O2	45,000	/	Benzene: 98.65% (260–420 °C) Toluene: 99.89% (260–420 °C) NO: 86.5% (260–420 °C)	[104]
V-Mo/TiO2	Toluene 50 ppm, NO 50–500 ppm, NH3 50–1000, 600 ppmSO2 1000 ppm	120,000	Toluene: ~97% (350 °C) NO: /	Toluene: ~85% (350 °C) NO: /	[105]
MnCoOx	CB 50 ppm, NO 50 ppm, SO2 50 ppm	12,000	CB: ~97% (120 °C) NO: /	CB: ~90% (120 °C) NO: /	[106]
Cu-SSZ-13@Mn2Cu1Al1Ox	Toluene 800 ppm, NOx 100 ppm, NH3 100 ppm, SO2 50 ppm, 20% O2, 5% H2O	60,000	Toluene: ~99% (300 °C) NO: ~99% (300 °C)	Toluene: ~89% (300 °C) NO: ~95% (300 °C)	[107]
Sb-Mn	Toluene 100 ppm, NO 600 ppm, NH3 600 ppm, SO2 200 ppm, 5% O2, 5% H2O	15,000	Toluene: 100% (250) NO: 100% (200 °C)	Toluene:80% (250) NO: 80% (200 °C)	[82]

presents some modified anti-poisoning catalysts.

3.1. Doping of Catalyst

Doping could improve the dispersion of the active component and thus increase the surface area of the active component [82,108]. Moreover, metal could enhance the surface acidity, redox ability, chemically adsorbed oxygen content, and catalytic performance [109]. Conventional VWT and VMT catalysts often exhibit limited efficiency in the simultaneous removal of NO_x and VOCs. Metal doping has been demonstrated as an effective strategy to address this limitation.

Table 3. Summary of sulfur-resistant catalysts.

Catalyst	Doping Element	Reactant Composition	Space Velocity (mL·g−1·h−1)	Conversion	Conversion (After)	Reference
VWT	Cu	Toluene 50 ppm, NH3 500 ppm, NO 500 ppm, SO2 100 ppm, O2 5.5%, H2O 8%,	60,000	Toluene: ~50% NO: ~80%	Toluene: ~95% NO: 100%	[110]
	Fe	Benzene 100 ppm, Toluene 100 ppm, NH3 500 ppm, NO 500 ppm, SO2 1000 ppm, O2 3.33%	45,000	/	Benzene: ~99% Toluene: ~99% NO: ~76%	[111]
	Co	Benzene 100 ppm, Toluene 100 ppm, NH3 500 ppm, NO 500 ppm, SO2 1000 ppm, O2 3.33%			Benzene: ~94% Toluene: ~99% NO: ~74%	[111]
	Ce	Benzene 500 ppm, Toluene 500 ppm, NO 0.5% vol, NH3 0.5% mol, SO2 0.5% vol, O2 20% vol	45,000	Benzene: ~98% Toluene: ~85% NO: ~20%	Benzene: ~99% Toluene: ~99% NO: ~25%	[108]
	Mo	Benzene 500 ppm, Toluene 500 ppm, NO 0.5% vol, NH3 0.5% mol, SO2 0.5% vol, O2 20% vol			Benzene: ~98% Toluene: ~99% NO: ~15%	[108]
	Ni	NH3 500 ppm, NO 500 ppm, Benzene 100 ppm, Toluene 100 ppm, SO2 1000 ppm, 3.33% O2	45,000	Benzene: ~90% Toluene: ~99% NO: ~50%	Benzene: ~95% Toluene: ~99% NO: ~65%	[104]
						[104]
VMT	Ce	CB 50 ppm, NH3 300 ppm, NO 300 ppm, O2 10 vol%, H2O 5 vol %	21,000	CB: ~45% NO: ~90%	CB: ~66% NO: ~75%	[112]
	Fe	CB 50 ppm, NH3 300 ppm, NO 300 ppm, O2 10 vol%, H2O 5 vol %			CB: ~80% NO: ~82%	[112]
	Mn	CB 50 ppm, NH3 300 ppm, NO 300 ppm, O2 10 vol%, H2O 5 vol %			CB: ~65% NO: ~81%	[112]
	Cr	CB 50 ppm, NH3 300 ppm, NO 300 ppm, O2 10 vol%, H2O 5 vol %			CB: ~70% NO: ~89%	[112]

lists the activity of the doped catalysts.

3.1.1. Noble Metal

The doping of noble metal can improve the performance of catalyst effectively. Song et al. [113] reported that Ru-modified vanadia-based catalysts displayed a high bifunctional activity with high selectivity to N₂, CO₂, and HCl. The V–O–Ru chains strengthened the electron capability and narrowed down the band gap of vanadium, which weakened the Lewis acid sites to avoid the generation of polychlorinated species via electrophilic reactions (Figure 7). Zhang et al. [49] proposed that Pd doping created a multi-active center on PdV/TiO₂ catalyst, which exhibited excellent synergistic removal efficiency of CB and NO_x in the temperature range of 300–400 °C. The presence of Pd provided the adsorption center for each reaction gas and abundant oxygen vacancies with Brønsted acids, which promoted the formation of NO₂, enhancing the efficiency of NH₃-SCR and CB oxidation [20]. Additionally, Pd could provide the reactive sites on the catalyst surface, making it easy to redox cycle (2V⁴⁺ + Pd²⁺ ↔ 2V⁵⁺ +Pd⁰). This improves the catalytic performance for benzene oxidation and NO_x reduction [114]. Although noble metal doping (e.g., Pd, Ru) can markedly enhance catalytic activity, their high cost, alongside a pronounced propensity for sintering and sulfur poisoning, presents paramount obstacles to their practical application. Consequently, research on noble metal modification should place greater emphasis on: (1) maximizing atomic utilization efficiency (e.g., via single-atom catalysts); (2) exploiting synergistic effects with transition metals to reduce noble metal loading.

3.1.2. Transition Metals

Doping with transition metals (such as Fe, Cu, Co, Ce, etc.) is an efficient and low-cost method to increase the acid site and oxygen vacancy of the bifunctional catalysts. Du et al. [110] modified commercial VWT catalysts with Cu doping, which significantly increased the number of acidic sites and reactive oxygen species, and the conversion of NO and toluene reached 100% and 95.56% at 300 °C, respectively. The synergism between MnO_x and CeO₂ promoted the formation of Lewis acid and oxygen vacancy, which enhanced NH₃-SCR and toluene adsorption/activation, thus significantly reducing the operational temperature window. MnO_x-CeO₂ catalyst could achieve a simultaneous conversion of more than 80% of NO and toluene at the range of 182–300 °C [38]. Fan et al. [115] showed that the doping of Fe would cause lattice distortion, resulting in the (t_2g)³ (e_g)¹ configuration of Mn³⁺, anisotropic distortion of α-MnO₂ lattice, and a significant increase in local z direction Mn-O bond length, which causes the lattice oxygen bonds of manganese to be weakened and become more active in the oxidation reaction. The removal of NO and CB was promoted. Lin et al. [41] found that there is a strong interaction between V-Cu bimetallic oxide and TiO₂ support, which can improve the redox ability of the catalyst. In addition, V-Cu/TiO₂ has a higher total hydrogen consumption, which is conducive to the generation of more oxygen vacancies, accelerating oxygen migration, and further promoting the deep oxidation of toluene. Cu doping enables the catalyst to have uniform particle size and good dispersion [111]. The pore property of catalyst is the key to pollutant enrichment. The doping of Mo can inhibit the specific surface area loss of catalyst, promote the formation of porous structure, make a larger specific surface area, improve the activation point, and promote the decomposition of reactants. It is reported that the doping of FeO_x also showed a good activity [108]. The experimental results showed that the doping of FeO_x increased the specific surface area and pore volume of the catalyst, so that the active site of the catalyst was more dispersed, and the Fe and V positions would form a catalytic redox cycle. The adsorption and destruction efficiency of nitric oxide and dioxins were further improved [112]. By comparing the doping effects of different transition metals in Table 1, it can be found that there are significant differences in the degree of performance improvement. For instance, doping with Cu and Fe usually significantly enhances catalytic performance, while the improvement of NO conversion by Co and Mo is relatively limited. The underlying reason behind this lies in the fact that Cu²⁺/Cu⁺ and Fe³⁺/Fe²⁺ have excellent redox cycling capabilities, which can effectively activate oxygen and generate oxygen vacancies. At the same time, as Lewis acid sites, they enhance the adsorption of NH₃.

The doping of Al and Mo can reduce the interference of some harmful components (e.g., As, Pb, SO₂). As shown in Figure 8, arsenic species preferentially adsorb on both Al and Mo to form arsenate, thereby avoiding bonding to the catalytically active V sites, followed by tuning the polymetric vanadyl species and enriching the active V⁵⁺ sites to achieve excellent Multipollutant Control (MPC) performance and arsenic resistance [89]. Compared with other transition metal dopants, Co doping exhibits superior low-temperature activity. The Co-doped MnO_x catalyst has excellent activity at low temperature, achieving over 90% NO_x conversion within the temperature window of 105–275 °C, while CB conversion rates surpassed 90% above 180 °C. This phenomenon is attributed to the addition of Co, which increases the additional Lewis acid site, improves the adsorption of NH₃ and CB, and effectively reduces the energy barrier of N-H cleavage and oxygen vacancy formation [116]. Among the current modification strategies, transition metal doping stands out as the most promising direction for industrial application. However, a frequently overlooked yet critical issue is the “optimal dosage” of dopants. Excessive doping may lead to coverage of active phases or pore blockage, consequently compromising the overall performance. Future research should integrate theoretical calculations with advanced characterization techniques to precisely determine the optimal doping thresholds in different catalytic systems.

3.1.3. Other Metals

Pr shows a variety of stable oxidation states, which allow for more oxygen exchange at lower temperatures. Especially, Pr could maintain its oxygen storage capacity during the high-temperature sintering process, which is also appropriate for VOCs oxidation [117]. Yu et al. found that the addition of catalyst could promote activity of the low temperature NH₃-SCR [89]. After the addition of Pr and W in the vanadia-based catalyst, the redox capacity and surface active oxygen adsorption of the catalyst are significantly improved, and more abundant vanadia-based materials and crystals V₂O₅ are observed on the surface, which contributes to the oxidation activity of NH₃-SCR and CB [118]. Zhao et al. [30] found that the Al₂O₃-modified CuO-CeO₂ catalysts showed the excellent NH₃-SCR performance at a broad temperature range (200–400 °C) and good low-temperature activity for toluene oxidation. The modification of Al₂O₃ significantly tunes the physical and textural properties and induces the formation of oxygen vacancies via the interaction between Ce⁴⁺/Ce³⁺ and Cu²⁺/Cu⁺ (Ce⁴⁺ + Cu⁺ ↔ Ce³⁺ + Cu²⁺). Long et al. constructed the preferred adsorption sites for arsenic by Al loadings to protect active V sites, followed by tuning the polymetric vanadyl species and enriching the active V⁵⁺ sites to achieve excellent MPC performance and arsenic resistance [89].

The doping of Sb has been demonstrated as a particularly effective strategy for enhancing both the activity and SO₂ resistance of bifunctional catalysts. Its efficacy stems from a multifaceted mechanism that operates on electronic, structural, and acidic fronts. Li et al. [113] revealed that in PdV/TiO₂ systems, Sb acts as an electronic mediator, significantly promoting the charge transfer between V and Pd centers. This electronic synergy not only elevated the conversion of CB and NO_x to 90% at 250 °C but also conferred remarkable SO₂ tolerance, as evidenced by a minimal temperature shift of only 250 °C to maintain the T₉₀ target under sulfur-containing conditions. This sulfur resistance is further exemplified in Mn-based systems. The Sb-Mn catalyst showed superior NO and toluene conversions, and the NO conversion could reach 90% in 200–350 °C, while the toluene conversion could reach 100% in 200–400 °C. The conversion rates of NO_x and toluene change slightly in SO₂ gas environment. The primary role of Sb is to inhibit the oxidation of Mn by sulfur dioxide, thereby preventing the formation of inactive manganese sulfate and enhancing the catalyst’s durability. Furthermore, by modulating the surface adsorption of NH₃, Sb alleviates its inhibitory effect on the toluene oxidation pathway, mitigating a key competitive adsorption conflict [82]. Yuan et al. [47] found that the formation of V-O-Sb bonds on the catalyst surface serves as the cornerstone of its anti-poisoning capability. This structural configuration is crucial for two primary reasons: First, it electronically modifies the active vanadium sites, potentially rendering them less susceptible to sulfation. Second, the concomitant formation of Sb-O-Ti bonds facilitates the generation of oxygen vacancies, which promote the critical ring-opening step of benzoate intermediates during VOC oxidation. Simultaneously, this surface configuration effectively reduces the adsorption energy of NH₃, thereby optimizing the balance between SCR and oxidation cycles.

3.1.4. Nonmetallic Elements

The doping of nonmetallic elements is a novel modification method. Li et al. [63] found that the electronic structure of the catalyst was effectively modulated after N doping, resulting in effective charge flow between the catalyst surface and gas molecules. More importantly, the adsorption and deposition of sulfur species and reaction transient intermediates on active centers were restrained, while a new N adsorption center for NO_x was provided. Dai et al. [119] found that phosphoric-acid-modified CeO₂ (HP-CeO₂) can decompose CB without forming polychlorinated congeners under the industry-relevant reaction conditions. In the removal system, the phosphate group can react with CB and water to form phenol and HCl and then undergo deep oxidation. Moreover, the DFT calculation shows that the formation of oxygen vacancy in the nearest (VO-1) and the second closest (VO-2) surface of the phosphoric-acid-modified catalyst can be significantly reduced, which plays a very important role in the oxidation of VOCs. The doping of Si can effectively increase the number of acidic sites of the CeO₂ and regulate the adsorption interaction between NH₃ and toluene, contributing to a wider temperature window [120].

3.2. Regulating the Structure

3.2.1. Core–Shell Structure

The strategic design of core–shell architectures represents a sophisticated approach to engineering high-performance bifunctional catalysts. Beyond simply providing larger pore diameters for enhanced gas diffusion and adsorption compared to traditional catalysts [81,121], the core–shell structure fundamentally decouples and protects functional sites. Its primary advantage lies in the spatial confinement effect, where the shell acts as a molecular sieve or a protective barrier. This configuration can effectively shield the active sites in the core from direct exposure to SO₂, thereby mitigating irreversible poisoning [122]. Furthermore, the interface between the core and shell often creates synergistic sites that can act as sacrificial centers, preferentially interacting with SO₂ and preserving the intrinsic activity of the core. As illustrated in Figure 9, the core–shell structure can provide a shielding effect for manganese species and preferentially guide the reaction pathway. Simultaneously, Sn⁴⁺ species act as sacrificial sites that react with SO₂. This protective and synergistic mechanism is exemplified by Li et al., who synthesized a Cu-SSZ-13@Mn₂Cu₁Al₁O_x core–shell catalyst [107]. The architecture not only provided a greater number of oxygen vacancies and surface acid sites but also effectively minimized the competitive adsorption between H₂O and reactant molecules. As a result, it achieved nearly 100% conversion for both VOCs and NO_x at 300 °C with 5% H₂O present and notably maintained this high performance without significant degradation under SO₂-containing atmosphere. This underscores the critical role of the shell in managing complex gas compositions and poisoning agents. In addition, the core–shell structure can promote the further oxidation of NO_x and VOCs, thereby avoiding the production of NO_x and N-containing organic intermediates with high toxicity to the environment [123].

The choice of shell material is paramount and dictates the primary enhancement mechanism. Jiang et al. [124] demonstrated that a CeO₂ shell favors the exposure of active sites and augments the adsorption of key reactants like toluene, NH₃, and NO. The strong metal-support interaction at the core–shell interface also facilitates the generation of oxygen vacancies, boosting redox activity. Conversely, employing TiO₂ as a shell primarily enhances the catalyst’s surface acidity and improves oxygen mobility, which collectively elevates the SCR performance and N₂ selectivity [125]. The inherent hydrophobicity of TiO₂ also contributes to superior water resistance, ensuring stable conversion rates under humid conditions. Moreover, the hierarchy of the shell’s structure itself can be engineered for further optimization. Shi et al. [126] highlighted that using a hollow spherical TiO₂ (h-TiO₂) shell for CuO loading resulted in a catalyst (Cu@h-TiO₂) with stronger acidic sites and superior redox capability compared to its conventionally loaded counterpart (Cu/h-TiO₂). This structural advantage effectively lowered the toluene oxidation temperature, successfully aligning it with the operational window of the NH₃-SCR reaction, which is a pivotal step towards practical simultaneous removal.

Figure 9. Sulfur resistance mechanism of catalyst in simultaneous removal of NO_x and CB [127].

Figure 9. Sulfur resistance mechanism of catalyst in simultaneous removal of NO_x and CB [127].

3.2.2. Pore Structure

The pore size of the catalyst will limit the specific surface area of the catalyst, resulting in low catalyst activity, limiting its industrial applications to remove NO_x and VOCs at the same time. Tang et al. [128] found that porous CuZrCe catalyst, whose porous structure can provide a larger specific surface area, supply abundant surface reaction active sites, and facilitate the adsorption and mass transfer of the reactants. The catalyst with porous structure is more suitable for modification by doping. The doping of transition metals can improve their structural properties, enhance their redox properties, and produce abundant reactive oxygen species, including lattice oxygen with higher mobility, which significantly increases the conversion rates of NO and toluene at low and medium temperatures [129]. Different carrier pore sizes will affect the dispersion of the active components of the catalyst, which will affect the SCR performance, catalytic oxidation performance and related catalytic mechanisms [130]. The proper proportion of mesoporous pores in the catalyst can reduce the nitrogen-containing intermediates (benzonitrile and benzamide) generated by the reaction of toluene and oxidation intermediates with NH₃ and NO, and reduce the direct oxidation of sexual nitrate/nitrite to toluene to a certain extent, and promote the utilization of active nitrate/nitrite in NH₃-SCR reaction [131].

3.3. Regulating the Shape

The shape of the catalyst not only determines the catalytic efficiency of the catalyst, but also affects the diffusion of the reactants and the reaction rate [132]. Morphological changes may include changes in particle and crystal morphology, as well as changes in surface morphology. MnO_x nanorods catalyst possessed greater catalytic activity than the MnO_x nanocuboid catalyst. The content of Mn⁴⁺ on Mn nanorods is greater than that on Mn nanocuboid, because Mn⁴⁺ is conducive to NH₃-SCR reaction and catalytic oxidation of toluene at low temperatures (200–300 °C)[41]. A four-direction α-MnO₂-exposed surface (211) had better VOC oxidation performance and a better NO_x conversion rate. The experimental results showed that the catalyst on the exposed crystal surface contained more low-priced Mn active sites, and a higher surface adsorbed oxygen (O_ads)/lattice oxygen (O_latt) is retained in the outermost layer [133]. Fe induced a phase transition of MnO₂, resulting in Fe-MnO₂—F&R catalysts with a composite morphology of nanoflowers and nanorods that provided an abundant microporous structure to facilitate the diffusion of molecules of different sizes [134].

4. Summary and Perspective

This review has systematically examined the recent progress in bifunctional catalysts for the simultaneous removal of NO_x and VOCs. We have delved into the complex synergistic and competitive interactions between these pollutants, the critical roles of active sites (including acid sites, metal sites, and oxygen vacancies), and the multifaceted poisoning mechanisms induced by flue gas components such as SO₂, H₂O, chlorine species, and heavy metals. Furthermore, we have critically assessed advanced modification strategies, including elemental doping, structural engineering (e.g., core–shell architectures), and morphological control, which collectively enhance catalytic performance and durability. Despite significant advancements, this review identifies several persistent challenges and gaps in the mechanistic understanding that warrant further investigation:

(1)

From Model Systems to Realistic Multi-Pollutant: Most current studies focus on the interaction between NOₓ and a single model VOC (e.g., toluene or chlorobenzene). The reaction network, synergistic pathways, and competitive adsorption landscapes in complex flue gas containing multiple VOCs (e.g., aromatics, oxygenates, and chlorinated compounds) remain poorly understood. Future research must employ more sophisticated experimental and theoretical approaches to decipher these complex interactions.

(2)

Precision in Active Site Engineering and Poisoning Resistance: While doping and structural design have proven effective, a more precise strategy is needed. The optimal dosage of dopants and the spatial architecture of multi-functional sites (e.g., physically separating SCR-active sites from oxidation-active sites to mitigate competitive adsorption) require finer control. Furthermore, a clearer distinction between reversible and irreversible deactivation pathways for different poisons (SO₂, Cl, Pb, As) is crucial for developing more targeted, robust, and economically viable non-noble metal catalysts.

(3)

Bridging the Material–Reactor Gap for Industrial Translation: The inherent mismatch in optimal temperature windows for SCR and VOC oxidation remains a significant engineering hurdle. Future efforts should tightly integrate catalyst design with innovative reactor engineering, such as dynamic or multi-stage reactor systems (e.g., circulating fluidized beds, rotary reactors), to create spatially or temporally distinct zones for each reaction. Concurrently, establishing feasible in situ regeneration protocols and developing predictive catalyst lifetime models are indispensable steps toward successful industrial implementation.