Enhancement of chlorine resistance in catalytic combustion of chlorobenzene over Nb-Ca/MnO<sub>2</sub> catalysts via acid sites modulation

Chang Wu; Yifei Yan; Jie Wu; Erhao Gao; Wei Wang; Jiali Zhu; Shuiliang Yao; Jing Li; Zuliang Wu

doi:10.4491/eer.2025.679

Environ Eng Res > Volume 31(5); 2026 > Article

Wu, Yan, Wu, Gao, Wang, Zhu, Yao, Li, and Wu: Enhancement of chlorine resistance in catalytic combustion of chlorobenzene over Nb-Ca/MnO2 catalysts via acid sites modulation

Research

Environmental Engineering Research 2026; 31(5): 250679.

Published online: February 28, 2026

DOI: https://doi.org/10.4491/eer.2025.679

Enhancement of chlorine resistance in catalytic combustion of chlorobenzene over Nb-Ca/MnO₂ catalysts via acid sites modulation

Chang Wu¹, Yifei Yan¹, Jie Wu³, Erhao Gao^1,^2,^†

, Wei Wang^1,², Jiali Zhu^1,², Shuiliang Yao^1,², Jing Li^1,², Zuliang Wu^1,^2,^†

¹School of Environmental Science and Engineering, Changzhou University, Changzhou 213164, China

²Key Laboratory of Advanced Plasma Catalysis Engineering for China Petrochemical Industry, Jiangsu 213164, China

³Wuxing District Environmental Emergency and Accident Investigation Center, Huzhou 313000, China

^†Corresponding author: E-mail: gaoerhao@cczu.edu.cn (E.H.G.), wuzuliang@cczu.edu.cn (Z.L.W.), Tel: +0086-0519-86330086 (E.H.G.), +0086-0519-86330086 (Z.L.W.), ORCID: 0000-0003-4742-7012 (E.H.G.), 0000-0003-0156-8696 (Z.L.W.)

Received November 13, 2025 Revised February 10, 2026 Accepted February 26, 2026

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Currently, manganese-based catalysts suffer from insufficient low-temperature activity and a tendency to chlorine poisoning-induced deactivation during the catalytic combustion of chlorobenzene. This study successfully developed Nb-Ca/MnO₂ catalysts, with the Nb(2)-Ca(2)/MnO₂ catalyst exhibiting superior performance: A T₉₀ of 340°C for CB conversion at 15,000 mL·g⁻¹·h⁻¹, over 85% conversion retention during a 30-hour test, and a significant reduction in polychlorinated by-products. Catalyst characterization results indicated that Nb existed in the form of Nb₂O₅. A small amount of Nb substantially increased the Mn³⁺/Mn⁴⁺ ratio and markedly raised the proportion of surface adsorbed oxygen. Furthermore, Nb loading enhanced the low-temperature redox capability of the catalyst. The introduction of Nb significantly altered the distribution of acidic sites, promoted the desorption of chlorine species, and effectively suppressed the formation of polychlorinated by-products. In situ DRIFTS results revealed the generation of abundant bidentate carbonates and maleates during the reaction, along with a significant reduction in chlorine species on the spent catalyst. The Nb-Ca/MnO₂ catalyst exhibited advantages in both catalytic activity and resistance to chlorine poisoning.

Keywords: Catalytic mechanism, Catalytic oxidation, Chlorobenzene, Chlorine poisoning resistance, Nb-Ca/MnO₂ catalyst

Graphical Abstract

Keywords: Catalytic mechanism, Catalytic oxidation, Chlorobenzene, Chlorine poisoning resistance, Nb-Ca/MnO₂ catalyst

1. Introduction

Chlorinated volatile organic compounds (CVOCs) have become a major focus in the management of emerging pollutants due to their high toxicity, persistence, and resistance to degradation [1–3]. Chlorobenzene (CB), as a representative aromatic CVOC, is widely used in the production of dyes, pharmaceuticals, pesticides, and organic synthesis [4]. CB not only poses risks to human health but also is associated with environmental issues such as secondary aerosol formation and ozone layer depletion. Catalytic combustion is considered one of the most promising technologies for treating CB-containing waste gases, owing to its high efficiency, low energy consumption, and minimal secondary pollution. However, catalysts for this process still face key challenges, including insufficient low-temperature activity, low CO₂ selectivity, and deactivation caused by chlorine poisoning.

Manganese-based catalysts possess a variable electronic structure, multiple oxidation states (Mn²⁺, Mn³⁺, and Mn⁴⁺), and abundant surface oxygen vacancies. Weng et al. [5] demonstrated that MnO₂, owing to its surface-rich oxygen vacancies, can activate O₂ and H₂O, exhibiting good catalytic activity for CB oxidation. However, polychlorinated by-products (C_xH_yCl_z) were generated during the reaction. These by-products adsorb onto the catalyst surface, leading to decreased catalytic activity and eventual chlorine poisoning. Similarly, Li et al. [6] reported that γ-MnO₂ possessed promising catalytic oxidation performance for CB, but also suffered from the formation of C_xH_yCl_z polychlorinated by-products, attributed to electrophilic substitution reactions during the process. Therefore, modifying MnO_x catalysts with additional active components to further enhance catalytic activity and suppress chlorine poisoning has become a key research focus in the catalytic oxidation of CB over Mn-based catalysts.

In our previous study [7], a Mn-based catalyst loaded with 2% Ca was synthesized. Experimental results demonstrated that compared to the MnO_x catalyst the Ca-MnO_x catalyst exhibited significant improvements in CB conversion (approximately 35% higher at 350°C), CO_x yield (about 20% higher between 250–300°C), and stability (maintaining over 75% CB conversion after 30 h). However, this catalyst still suffered from relatively high catalytic oxidation temperatures, insufficient long-term stability, and a tendency for Cl⁻ species to adsorb onto the active sites.

Niobium (Nb), a Group V element known for its notable resistance to chlorine poisoning, has attracted significant research attention. Some studies had indicated that Nb can increase oxygen vacancies, enhance surface acidity, and suppress chlorine poisoning [8–10]. For instance, Tao et al. [11] prepared WO₃-Nb₂O₅ mixed oxides via a citric acid sol-gel method and evaluated their activity in the oxidation of 1000 ppm CB. The catalyst with a W/Nb molar ratio of 2:1 exhibited the best performance (T₉₀ = 320 dechlorination), and during a 120-hour stability test, the CB conversion decreased only slightly from 91% to 86%. Yang et al. [12] synthesized a Ru/NbWO_x catalyst, revealing that the NbWO_x support possesses abundant oxygen vacancies and acid sites, which contribute to improved redox performance. In another study, Yang et al. [13] prepared Nb₂O₅ catalysts modified with different metals, demonstrating a synergistic catalytic effect between Nb and the other metals. They proposed that the mesoporous structure and surface acidity facilitated the adsorption, activation, and cleavage of the C-Cl bond in reactants, leading to HCl release and inhibition of chlorine poisoning. Furthermore, Yang et al. [14] developed a MnNb_0.4Ce_0.2O_x catalyst, confirming that the introduction of Nb significantly increased the number of Lewis acid sites and enhances the redox capability of the MnCe_0.2O_x system. Compared with recent manganese-based catalysts, current research predominantly focuses on optimizing redox performance through single or binary doping [15, 16]. However, while these strategies have made progress in enhancing oxidation activity, they often fail to effectively balance surface acidity and basicity. This leads to persistent issues of adsorption and accumulation of polychlorinated by-products during CVOCs oxidation, particularly due to the lack of an efficient pathway for chlorine species removal. In the case of Nb containing catalysts, recent studies have largely emphasized utilizing their surface acidity to promote C–Cl bond cleavage. Nevertheless, excessively high acidity inevitably impacts redox performance. The overall catalyst performance largely depends on finding an appropriate balance between acidic and redox characteristics [17, 18]. It is anticipated that Nb may play a role in modulating the surface acid-base balance in catalysts rich in alkaline Ca species. While modified Mn-based catalysts have been applied for the removal of CVOCs, further research is still needed regarding the combination of Nb and Ca supported on Mn-based catalysts, as well as aspects such as microstructure and loading amount of active components, in order to improve the catalytic oxidation performance and resistance to Cl poisoning. The effect of Nb modifying the Ca/MnO₂ catalyst on the catalytic behavior and mechanism of CB oxidation remains unknown.

Building upon our previous work on Ca/MnO₂ catalysts for CB oxidation, we introduced Nb as an additional active component. Different molar ratios of Nb were loaded onto the Ca(2)/MnO₂ catalyst to screen the optimal Nb loading level through activity evaluation. The catalysts were systematically characterized to analyze their physiochemical properties, and the catalytic oxidation mechanism of CB over the Nb(m)-Ca(2)/MnO₂ catalyst was elucidated.

2. Experimental Methods

2.1. Catalyst Preparation

In this experiment, a series of Ca(2)/MnO₂ catalysts with different Nb loadings were synthesized via the impregnation method, where 2 represents with 2% Ca molar percentage. The prepared catalysts were denoted as Nb(m)-Ca(2)/MnO₂, where m% represents the molar percentage of Nb. A certain amount of C₁₀H₅NbO₂₀·nH₂O and Ca(CH₃COO)₂·H₂O was weighed and dissolved in 10 mL of deionized water to prepare the Nb–Ca solution. A measured quantity of MnO₂ was then slowly added to the Nb–Ca solution, and the mixture was stirred for 3 h using a magnetic stirrer. After thorough mixing, the solution was placed in a blast drying oven and dried overnight at 80°C to obtain the solid material. Finally, the resulting solid was calcined at 450°C for 3 h in a muffle furnace to obtain the desired catalyst. The catalyst was sieved to 40–60 mesh prior to being used in the activity and stability tests.

2.2. Catalytic Performance Evaluation

The catalyst activity evaluation experiment was conducted in a fixed-bed reactor, and the test system is illustrated in Fig. S1. A total of 0.4 g of the 40–60 mesh catalyst was packed into a Φ6 × 400 mm quartz glass tube. This ensures the reproducibility of the catalytic performance data. Prior to the reaction, the catalyst was pretreated at 300°C for 1 h in a 20% O₂/N₂ atmosphere. The reaction gas mixture consisted of 1000 ppm CB and 20% O₂, with N₂ as the balance gas. The total flow rate was maintained at 100 mL/min, resulting in a weight hourly space velocity (WHSV) of 15,000 mL·g⁻¹·h⁻¹. The reaction temperature was varied from 150 to 450°C. The concentrations of chlorobenzene and the products CO/CO₂ were analyzed using a gas chromatograph (GC9790II, Fuli) equipped with an FID detector. During the testing, after the concentrations of CB and CO_x stabilized at each temperature point, five samples were collected and averaged.

The CB conversion and CO_x selectivity were calculated using the following equations:

(1)

η (%) = \frac{C_{C B \cdot i n} - C_{C B \cdot o u t}}{C_{C B \cdot i n}} \times 100 %

(2)

S_{C O_{X}} (%) = \frac{C_{C O_{X}}}{(C_{C B \cdot i n} - C_{C B \cdot o u t}) \times 6} \times 100 %

where C_{CB in} represents the inlet concentration of CB (ppm), C_{CB out} represents the outlet concentration of chlorobenzene after the reaction (ppm), and C_COx represents the concentration of CO_x in the effluent gas (ppm).

2.3. Catalyst Characterization

XRD analysis was performed using a powder diffractometer (Bruker D2 PHASER, Bruker AXS, Germany) with Cu K radiation (40 kV and 150 mA). The scanning range was from 10° to 80° at a speed of 2°/min. BET surface area and pore size distribution were measured using a static volumetric adsorption analyzer (AutosorbiQ-MP, Anton Paar, US). The specific surface area of the samples was evaluated by the standard Brunauer-Emmett-Teller (BET) method, while the pore size distribution was determined using the Barrett-Joyner-Halenda (BJH) method. SEM images were obtained using a Hitachi Regulus8100 scanning electron microscope (Hitachi, Japan) to observe the catalyst morphology. XPS characterization was conducted on a Thermo Fisher Nexsa spectrometer (Thermo Fisher, US) equipped with a 150 W Al-K X-ray source. The binding energies were calibrated using the C 1s peak at 286.4 eV. H₂-TPR, O₂-TPD, and NH₃-TPD measurements were carried out on an AutoChem 2920 chemisorption analyzer (Micromeritics, US) equipped with a thermal conductivity detector (TCD). Prior to each analysis, the catalysts were subjected to drying pretreatment. In situ DRIFTS experiments were performed using a Nicolet iS50 Fourier transform infrared spectrometer (Thermo Fisher, US) equipped with a liquid nitrogen-cooled MCT detector. The samples were pretreated at 400°C under a flow of 20% O₂/Ar for 2 h, then exposed to a gas stream containing 1000 ppm CB in 20% O₂/Ar at different temperatures. After purging with 20% O₂/Ar for 30 min, the corresponding spectra were recorded.

2.4. By-product Analysis

The tail gas was first absorbed using ethanol solvent at T₉₀ for 0.5 h. Qualitative analysis of the by-products in the solvent was then performed using a gas chromatography/mass spectrometry (GC-MS) system (GC-7890A; MS-5975C, Agilent Co., US) equipped with an HP-5MS UI capillary column (30 m × 0.250 mm × 0.25 μm). The temperature program was set as follows: The initial temperature was maintained at 30°C for 5 min, then increased to 60°C at a rate of 15°C/min and held for 1 min, and finally raised to 150°C at a rate of 5°C/min.

3. Results and Discussion

3.1. Activity Evaluation

The effect of different niobium loadings on the catalytic performance for chlorobenzene (CB) conversion is shown in Fig. 1(a) The temperatures required for 50% and 90% conversion of chlorobenzene (T₅₀ and T₉₀) and the temperature at which CO₂ selectivity reaches 70% are summarized in Table S1. The Ca(2)/MnO₂ catalyst exhibited relatively poor activity, with T₅₀ and T₉₀ values of 323°C and 390°C, respectively. After loading 2% Nb, the catalytic activity significantly improved, with T₅₀ decreasing to 275°C and T₉₀ to 340°C. However, when the Nb loading was further increased to 5%, the CB conversion efficiency decreased, resulting in a T₉₀ of 398°C. The catalytic activity of the manganese-based catalysts with different Nb loadings followed the order: Nb(2)-Ca(2)/MnO₂ (T₉₀=340°C) > Nb(3)-Ca(2)/MnO₂ (T₉₀=345°C) > Nb(1)-Ca(2)/MnO₂ (T₉₀=378°C) > Ca(2)/MnO₂ (T₉₀=390°C) > Nb(5)-Ca(2)/MnO₂ (T₉₀=398°C). This indicates that an optimal Nb loading can enhance the catalytic performance of the Nb(m)-Ca(2)/MnO₂ catalyst.

In the low-temperature stage (150°C–200°C), the CB conversion rate increased only slowly. The suppressed catalyst activity in this stage can be primarily attributed to two factors: Firstly, the lower reaction temperature is unfavorable for the activation of gaseous oxygen molecules and the migration rate of lattice oxygen on the catalyst surface; secondly, the decomposition rate of adsorbed CB on the catalyst surface is slow, leading to the occupation of active sites and thus reduced reaction efficiency. In the high-temperature stage (>200°C), Nb(2)-Ca(2)/MnO₂ demonstrated the best catalytic activity. XPS and H₂-TPR analyses confirmed that this catalyst possesses favorable redox properties.

The CO₂ and CO production yields during the CB conversion process over Ca(2)/MnO₂ catalysts with different Nb loadings are shown in Fig. 1(b) and Fig. 1(c), respectively. The influence of Nb loading on the CO_x yield followed a trend consistent with the catalytic activity. For the Nb(2)-Ca(2)/MnO₂ and Nb(3)-Ca(2)/MnO₂ catalysts, the temperatures required to achieve over 70% CO₂ selectivity were 327°C and 340°C, respectively, which are 48°C and 61°C higher than the 279°C observed for the Ca(2)/MnO₂ catalyst. The overall mineralization efficiency showed no significant change compared to Ca(2)/MnO₂. In contrast, the Nb(5)-Ca(2)/MnO₂ catalyst exhibited lower CO₂ selectivity and a significant increase in CO production, indicating a decreased capability for deep oxidation of CB. This is because excessive Nb loading alters the distribution of surface acid sites [19] and reduces the amount of highly active oxygen species, which was further confirmed by NH₃-TPD and O₂-TPD results.

Based on the comparative catalyst performance data presented in Table S2, the Nb(2)-Ca(2)/MnO₂ catalyst prepared in this study demonstrated a significant advantage in low-temperature activity for the catalytic oxidation of chlorobenzene. As shown in the table, the reported T₉₀ values for reference catalysts in the literature ranged from 350°C to 470°C. Under a chlorobenzene concentration of 1000 ppm, the T₉₀ of this catalyst was 340°C, which was lower than that of the reference catalyst systems. It is noteworthy that even under the high chlorobenzene concentration condition of 1000 ppm, the T₉₀ of Nb(2)-Ca(2)/MnO₂ remained lower than that of most reference catalysts tested at concentrations of 500 ppm and below. This indicates that the catalyst still exhibited superior low-temperature catalytic performance even under more stringent reaction conditions. These comparative results further confirmed the positive synergistic effect of Nb and Ca modification in enhancing the low-temperature activity and applicability of the MnO₂-based catalyst.

3.2. Texture Property

As shown in Fig. 2(a), the characteristic diffraction peaks of the MnO₂ catalyst appeared at 2θ = 28.7°, 37.3°, 41.1°, 42.8°, 46.081°, 56.7°, 59.4°, 64.8°, and 67.2°, which were indexed to the (1 1 0), (1 0 1), (2 0 0), (1 1 1), (2 1 0), (2 1 1), (2 2 0), (0 0 2), and (3 1 0) crystal planes of the β-MnO₂ phase (PDF#24-0375). No distinct peaks corresponding to calcium manganese compounds (PDF#50-1746). This could be attributed to the low loading amount of the Ca compound and its high dispersion on the catalyst surface, which resulted in the absence of diffraction peaks belonging to CaMnO₃ in the XRD pattern. This can be clearly observed in the EDS elemental mapping. For the sample with 1% Nb loading, no distinct diffraction peaks corresponding to Nb₂O₅ (PDF#26-0885) were detected, which could be attributed to the high dispersion of Nb species on the catalyst surface [20]. When the Nb loading exceeded 2%, diffraction peaks corresponding to Nb₂O₅ compounds were observed in the XRD patterns, indicating that Nb partially existed in the form of crystalline Nb₂O₅ in the MnO₂ catalyst.

The N₂ adsorption-desorption isotherm of the Nb(m)-Ca(2)/MnO₂ samples exhibited a typical H₃-type hysteresis loop and type III characteristics (Fig. 2(b)), with no distinct adsorption saturation plateau. This indicated the presence of slit-like pores on the catalyst surface that were not completely filled with condensate. Additionally, after Nb loading, the adsorption capacity increased rapidly in the high-pressure region, suggesting a predominantly mesoporous pore size distribution. The desorption branch analyzed by the BJH model revealed that the pore size of the Nb(m)-Ca(2)/MnO₂ samples particles was approximately 10 to 17 nm. The average pore size of Ca(2)/MnO₂ was 9.01 nm, which increased after Nb loading. The specific surface area (SSA) of the Nb(m)-Ca(2)/MnO₂ sample particles ranged from 12 to 16 m².g⁻¹, while that of Ca(2)/MnO₂ was 79.37 m².g⁻¹. This demonstrated that the loading of Nb altered the physical structure of Ca(2)/MnO₂ and significantly reduced its SSA. This significant decrease in SSA after Nb loading was attributed to the deposition of Nb species within the mesopores. Among the three catalysts compared in Table S3, Nb(2)-Ca(2)/MnO₂ exhibited the largest specific surface area and the smallest average pore size under similar pore volume conditions, indicating a more porous structure, which to some extent enhanced its catalytic oxidation performance. Furthermore, the surface morphology and elemental distribution of the Nb(2)-Ca(2)/MnO₂ catalyst were investigated using SEM and EDS analyses, respectively.

As shown in Fig. 3(a–f), the catalysts consisted of rough block-like particles stacked together, accompanied by a small number of fine particles, forming a porous structure. Specifically, the Ca(2)/MnO₂ catalyst exhibited relatively large and irregular block-like structures with relatively smooth surfaces in Fig. 3(a). In Fig. 3(b-c), the particles in Ca(2)/MnO₂ appeared densely packed, yet with non-uniform particle sizes and noticeable agglomeration. This indicated that some degree of agglomeration existed on the surface of the Ca(2)/MnO₂ catalyst, which was unfavorable for the exposure of active sites. Notably, compared with the block-like structure of the Ca(2)/MnO₂ catalyst, the surface morphology of the Nb(2)-Ca(2)/MnO₂ catalyst showed a reduction in particle size. Furthermore, the surface of the Nb(2)-Ca(2)/MnO₂ catalyst displayed finer and more uniformly dispersed particles, with mitigated agglomeration. These observations indicated that the introduction of Nb refined the catalyst particles and optimized their microstructure. Additionally, EDS elemental mapping (Fig. 3(g–i)) revealed that both Ca and Nb species were uniformly distributed on the surface of the Nb(2)-Ca(2)/MnO₂ catalyst. Manganese was densely distributed, covering almost the entire observed area, and was highly dispersed on the catalyst surface without large-scale aggregation, which facilitated the exposure of active sites.

3.3. Chemical Properties

Fig. 4(a-d) displays the XPS spectra of Mn 2p, Ca 2p, O 1s, and Nb 3d for the Nb(m)-Ca(2)/MnO₂ samples. The Mn 2p spectrum was deconvoluted into two Mn species, Mn³⁺ and Mn⁴⁺, with binding energies of 641.9 eV and 643.1 eV, respectively. The Mn³⁺/Mn⁴⁺ molar ratio is related to oxygen defects, and a higher Mn³⁺/Mn⁴⁺ ratio was beneficial for enhancing the redox performance of MnO₂ [21–23]. As shown in Table S4, the Nb(2)-Ca(2)/MnO₂ catalyst exhibited the highest Mn³⁺/Mn⁴⁺ ratio of 1.62, reflecting its higher ratio of surface oxygen vacancy, which was consistent with the activity test results. This was likely because Nb shared oxygen atoms with the surface MnO_x, leading to an increase in Mn³⁺ species. When the Nb loading was further increased, the Mn³⁺/Mn⁴⁺ ratio decreased to 1.21. This decrease can be attributed to the increased loading of Nb species. This occurred because insufficient oxygen atoms were available from the MnO_x support, causing Nb to react with surface-adsorbed oxygen (O_ads) to form Nb₂O₅, consequently reducing the O_ads concentration [18]. It indicates that the main form of calcium in Nb(m)-Ca(2)/MnO₂ catalysts is CaCO₃ [24, 25]. In the Nb 3d spectrum, the peaks observed at 209.5 eV and 206.8 eV were assigned to Nb⁵⁺. The intensity of these peaks for the three catalysts gradually increased with higher Nb loadings. In the O 1s spectrum, the peak at 529.4 eV was characteristic of surface lattice oxygen (denoted as O_latt). The peak centered at 530.2 eV was assigned to surface adsorbed oxygen (denoted as O_ads). The peak at 531.7 eV was associated with surface-adsorbed hydroxyl groups or molecular water (O_OH) [26, 27]. The O_ads/O_latt ratio partially reflects the quantity of oxygen vacancies in Nb(m)-Ca(2)/MnO₂, as O_ads preferentially occupies oxygen vacancies [28, 29]. The O_ads/O_latt ratios for the Mn catalysts with different Nb loadings were calculated and are listed in Table S2. As the Nb loading increased, the O_ads/O_latt ratio first increased from 1.50 to 2.45 and then decreased to 1.84. According to previously reported results, surface-adsorbed oxygen species (O_ads) were more reactive than lattice oxygen (O_latt) due to their higher mobility, which played a significant role in the complete degradation of CB and resistance to chlorine poisoning [30, 31]. Therefore, the formation of more oxygen vacancies after Nb loading resulted in the Nb(2)-Ca(2)/MnO₂ catalyst possessing the highest reactivity. This was consistent with the activity test results.

The influence of different Nb loadings on the redox properties of the catalysts was further investigated by H₂-TPR. A comparative analysis of the Nb(m)-Ca(2)/MnO₂ and Ca(2)/MnO₂ samples was conducted, and the results are presented in Fig. 4(e). All samples exhibited two reduction peaks. After loading Nb, a decrease in the areas of the reduction peaks was observed for Nb(m)-Ca(2)/ MnO₂ to varying degrees. After Nb loading, the areas of the reduction peaks decreased to varying degrees. The most pronounced decrease was observed for Nb(5)-Ca(2)/MnO₂, which was likely caused by excessive Nb loading. For Nb(1)-Ca(2)/MnO₂, the peaks centered at 397°C and 467°C were assigned to the reduction of MnO₂ to Mn₂O₃ and Mn₂O₃ to MnO, respectively [32–34]. When the Nb loading increased to 2%, the reduction peaks of the sample shifted significantly to lower temperatures, specifically to 359°C and 454°C. However, with a further increase in the Nb loading, the reduction peaks shifted back towards higher temperatures. This indicated that Nb(2)-Ca(2)/MnO₂ possessed the best reducibility among the Nb(m)-Ca(2)/MnO₂ catalysts. This finding was consistent with the XPS results. This phenomenon could be attributed to the enhanced synergy among Nb, Ca, and Mn induced by Nb incorporation. The strong interaction between Nb, Ca, and Mn species, primarily through electronic and structural effects, weakened the Mn–O bonds, created more surface defects and active sites, and consequently improved the low-temperature reducibility of the Mn species. This electronic modification effect reached its optimum at the Nb(2) loading level [35–38]. Previous studies had reported that synergistic effects in metal oxides can increase oxygen vacancies and lattice defects [39–42], which aligns with the XPS findings. The optimal low-temperature reducibility of Nb(2)-Ca(2)/MnO₂ contributes to promoting the catalytic oxidation of CB at lower temperatures. In contrast, the low Nb loading in Nb(1)-Ca(2)/MnO₂ may lead to an insignificant enhancement in low-temperature catalytic performance, while the excessive Nb loading in Nb(5)-Ca(2)/MnO₂ likely reduces the total number of active sites, thereby suppressing catalytic activity. These findings are consistent with the activity test results.

To investigate the distribution of oxygen species on the catalysts with different Nb loadings, O₂-TPD analysis was performed on the Nb(m)-Ca(2)/MnO₂ samples. Based on the desorption peak temperatures, the oxygen species on Mn-based catalysts could be broadly classified into three categories: The first desorption peak appearing between 100–400°C, which was typically attributed to surface-adsorbed oxygen (denoted as α-O), including oxygen physically or chemically adsorbed on the catalyst surface; the second desorption peak observed between 400–700°C, which was assigned to lattice oxygen near the surface (denoted as α′-oxygen); and the third desorption peak around 700°C, which was classified as bulk lattice oxygen (denoted as β-oxygen).

As shown in Fig. 4(f), three desorption peaks were observed for Nb(1)-Ca(2)/MnO₂ at 485°C, 552°C, and 738°C. The first two peaks corresponded to surface lattice oxygen, while the latter was attributed to bulk lattice oxygen. For the surface lattice oxygen species (α′-O), all three Nb-loaded samples exhibited a desorption peak near 480°C. A lower oxygen desorption temperature points to an increase in surface lattice oxygen activity, promoting catalytic performance. With increasing Nb loading, the intensity of this desorption peak around 480°C gradually weakened. The reduction peak near 550°C progressively decreased, and this phenomenon was more pronounced compared to Ca(2)/MnO₂. This indicated that the lattice oxygen near the surface on Nb(2)-Ca(2)/MnO₂ exhibited higher mobility compared to that on Ca(2)/MnO₂. Additionally, the introduction of Nb created highly reactive surface lattice oxygen (α′-O) on Nb(2)-Ca(2)/MnO₂. At low loadings (Nb (1), Nb (2)), Nb acts as a “structural promoter” to promote the dispersion of Mn species and enhance their catalytic activity. However, at a too high loading (Nb (5)), the “coverage effect” or “agglomeration effect” of Nb causes the active sites of Mn to be shielded or agglomerated, and the migrating surface lattice oxygen tends to form oxides with Nb, resulting in a decrease in the peak intensity around 550°C [10]. By calculating the oxygen desorption amount, the changes in oxygen species were quantitatively analyzed. The peak areas in the 400–700°C range (Table S5) follow the trend: Ca(2)/MnO₂ (3.7)>Nb(1)-Ca(2)/MnO₂ (3.0)>Nb(2)-Ca(2)/MnO₂ (2.8)>Nb(5)-Ca(2)/MnO₂ (2.1). These quantitative data further confirm the modulating effect of Nb loading on the distribution of oxygen species: At low loading levels, Nb promotes the activation and dispersion of oxygen species, whereas at high loading levels, the strong interaction among Nb, Mn, and Ca results in the covering of active sites, thereby inhibiting oxygen desorption. The incorporation of Nb caused the oxygen desorption peaks of the Ca(2)/MnO₂ catalyst to shift to lower temperatures and weaken in intensity. Particularly, the disappearance of the high-temperature oxygen peak (552°C) indicated that Nb suppressed the formation of strongly oxidative lattice oxygen, which corresponded to the decreased CO₂ selectivity observed in the activity tests.

The surface acidity of catalysts can be reflected by different desorption temperature ranges in NH₃-TPD curves, corresponding to weak acidity (< 280°C), medium-strong acidity (280–450°C), and strong acidity (> 450°C) [43, 44]. As observed in Fig. 4(g), NH₃ desorption on all four catalysts started at 300°C and was clearly concentrated in a relatively high-temperature NH₃ desorption peak (around 490°C). This indicated that these catalysts were predominantly characterized by strong acidity, with only a small number of medium-strong acid sites and virtually no weak acid sites. When the Nb loading was 1%, a desorption peak appeared around 559–573°C. With further increase in Nb loading, the peak near 559°C was obscured, while the area of the NH₃ desorption peak around 480°C increased, indicating a rise in the number of acid sites at this temperature. From the quantitative NH₃ data in Table S6, it can be observed that the strong acid site areas of the four catalysts are as follows: Ca(2)/MnO₂ (4.0) > Nb(1)-Ca(2)/MnO₂ (3.4) >Nb(2)-Ca(2)/MnO₂ (3.3) > Nb(5)-Ca(2)/MnO₂ (2.6). It is noteworthy that neither the Ca(2)/MnO₂ catalyst with the highest number of strong acid sites nor the Nb(5)-Ca(2)/MnO₂ catalyst with the fewest strong acid sites exhibited optimal catalytic performance. This phenomenon may originate from an imbalance between excessively strong acidity and compromised redox functionality in the Ca(2)/MnO₂ catalyst. Chlorinated intermediates generated via acid site-mediated dechlorination undergo incomplete oxidation due to insufficient redox capacity, leading to their accumulation as carbonaceous deposits. These deposits gradually poison the active sites, thereby resulting in degraded catalytic performance [45, 46]. The decline in the low-temperature redox capability of Nb(5)-Ca(2)/MnO₂ may be attributed to either the coverage of key manganese active sites by excessive Nb₂O₅ or the formation of phases that are unfavorable for electron transfer and oxygen migration. These results demonstrate that loading an appropriate amount of Nb enables the modulation of acid site distribution, thereby achieving an optimal balance between acidity and redox properties, which ultimately enhances the catalytic activity.

3.4. In Situ DRIFTS and By-product Analysis

To investigate the catalytic reaction process of the Nb(2)-Ca(2)/MnO₂ catalyst, in situ DRIFTS was employed to conduct an in-depth analysis of the changes in surface species and chemical groups during the catalytic oxidation of carbon black over the Nb(2)-Ca(2)/MnO₂ catalyst in the temperature range of 150–400°C.

The Ca(2)/MnO₂ (Fig. 5(a) and Nb(2)-Ca(2)/MnO₂ (Fig. 5(b)) catalysts were exposed to a 1000 ppm CB + 20% O₂/Ar atmosphere at different temperatures, and the corresponding in situ DRIFTS were collected. The surface species corresponding to different signal peaks on the catalysts are listed in Table S7.

The peak at 762 cm⁻¹ was attributed to the C–Cl stretching vibration of adsorbed CB. Peaks at 1136 and 1253 cm⁻¹ were associated with phenolate species [47, 48], while those near 1351 and 1390 cm⁻¹ were assigned to bidentate carbonate species [49, 50]. The peak around 1445 cm⁻¹ represented −CH₃. Additionally, the peak at 1534 cm⁻¹ was attributed to maleate species [51, 52], and the peak near 1583 cm⁻¹ was assigned to C=C vibrations of the benzene ring [53, 54]. The peak at 1715 cm⁻¹ was ascribed to −CO in formate species. Peaks at 3576, 3634, and 3740 cm⁻¹ indicated interactions between surface hydroxyl groups and CB molecules via weak hydrogen-bonded groups [55]. As shown in Fig. 5(b), CB was initially adsorbed on the catalyst surface. With increasing temperature, the peak at 762 cm⁻¹ gradually decreased, indicating C-Cl bond cleavage and progressive consumption of CB. This change became more pronounced at higher temperatures. The peaks at 1136 and 1253 cm⁻¹ were related to phenolate species formed via reactions between hydroxyl groups and the benzene ring. The peaks at 1351 and 1390 cm⁻¹, assigned to bidentate carbonates, intensified as the reaction proceeded, indicating the formation of substantial amounts of bidentate carbonate species on the catalyst surface. The variation in the maleate peak at 1534 cm⁻¹ was linked to benzene ring opening. The peak was observed at 100°C resulted from CB adsorption equilibrium. As the temperature increased, CB conversion rose, leading to a higher concentration of benzene ring intermediates and consequently an increase in maleate species as ring-opening products. The peak at 1715 cm⁻¹ was associated with −CO from formate. As shown in Fig. 5(b), the intensity of this peak progressively increased with rising temperature. This indicated that formic acid was scarcely formed at low temperatures (< 300°C), while the temperature exceeded T₉₀, certain formate species were generated, facilitated by carboxyl groups. This observation was consistent with the variation trends of the peaks at 3576, 3634, and 3740 cm⁻¹. In summary, in situ DRIFTS analysis revealed that the reaction pathway of CB primarily involved: Adsorption of CB on active sites composed of surface acid sites and lattice oxygen → formation of benzene and Cl species → generation of phenolate, maleate, and carbonate species → eventual oxidation to CO_x, H₂O, and HCl. The abundant acid sites and presence of Nb facilitated the removal of Cl species.

The gaseous intermediates from the Ca(2)/MnO₂ and Nb(2)-Ca(2)/MnO₂ catalysts were analyzed by GC-MS, with the results shown in Fig. 5(c) and Fig. 5(d). As expected, CO₂ constituted the dominant gaseous product. However, several incompletely oxidized intermediates, including carbon tetrachloride, polychloroethylene, and dichlorobenzene, were also detected by GC-MS. For the Ca(2)/MnO₂ catalyst, the GC-MS results confirmed the presence of by-products such as carbon tetrachloride, tetrachloroethylene, and dichlorobenzene. In the case of the Nb(2)-Ca(2)/MnO₂ catalyst, carbon tetrachloride was no longer detected, while a small amount of tetrachloroethylene was observed. This change could be attributed to the increased surface acidity following the introduction of Nb species. This enhancement promoted the desorption of chlorine species from the catalyst surface, suppressed the chlorination of MnO₂ active sites, and thereby improved the catalyst stability.

Compared to the Ca(2)/MnO₂ catalyst, the Nb(2)-Ca(2)/MnO₂ catalyst not only exhibited superior performance in CB decomposition (corresponding to activity tests), but also generated significantly fewer intermediate species (Fig. 5(c) and Fig. 5(d)). The main intermediates shifted from formates species to phenolates species. The combination of the phenolate-related peaks at 1136 and 1253 cm⁻¹, the intensity variation of the benzene ring peak at 1583 cm⁻¹(Fig. 5(b)), and the detection of trace amounts of dichlorobenzene byproducts by GC-MS(Fig. 5(d)) indicates that the benzene ring formed after the C–Cl bond cleavage in CB can undergo further bond breaking and react with oxygen species. Furthermore, the amounts of C=C bonds, −CH, and −CH₃ groups detected during the reaction over the Nb(2)-Ca(2)/MnO₂ catalyst were noticeably lower than those over the Ca(2)/MnO₂ catalyst. This indicated that the introduction of Nb optimized the acid sites, enhancing the adsorption and activation of chlorobenzene. In addition, it facilitated a more rapid supply of active oxygen species to the reactants, thereby accelerating the further oxidation of intermediate products. These findings were consistent with the GC-MS results, which showed a reduced intensity of tetrachloroethylene.

3.5. Catalyst Stability Analysis

As mentioned previously, chlorine poisoning is a critical issue in the catalytic oxidation of CB. To address this, we conducted stability tests for CB catalytic oxidation over Ca(2)/MnO₂ (at 400°C) and Nb(2)-Ca(2)/MnO₂ (at 340°C), respectively, under their respective T₉₀ conditions. As presented in Fig. 6, the CB conversion gradually decreased to 85% within the initial 18 h. After 30 h of continuous reaction, the CB conversion remained above 85%. In contrast, for the Ca(2)/MnO₂ catalyst (Fig. S2), tested at its T₉₀ (400°C), the CB conversion decreased from 90% to 85% during the first 6 h. After 30 h of reaction, the conversion stabilized at approximately 75%. The primary reason for this deactivation is likely catalyst chlorine poisoning. Chlorine species generated during the reaction adsorbed onto the catalyst surface and interacted with the active components, thereby reducing catalytic activity. This was corroborated by the measured chlorine species content on the spent catalyst. The introduction of Nb species mitigated this issue. The surface acidity and mesoporous structure of Nb₂O₅ facilitated the desorption of Cl species as HCl at elevated temperatures via acid sites. Furthermore, the redox properties of Nb₂O₅ promoted the complete oxidation of CB to H₂O, HCl, and CO₂ [56]. Consequently, the formation of polychlorinated byproducts was significantly suppressed, and the removal of Cl species as HCl was enhanced, leading to the improved stability of the Nb(2)-Ca(2)/MnO₂ catalyst.

The chlorine species content in the spent Ca(2)/MnO₂ and Nb(2)-Ca(2)/MnO₂ catalysts after activity and stability tests was determined by the silver nitrate titration method, and the results are listed in Table S4. As shown in Table S8, the chlorine species content on the spent catalyst increased by 228,270 mg/kg for the catalyst without Nb loading. In contrast, for the Nb-loaded catalyst, the chlorine species content increased by only 2,124 mg/kg after the reaction, representing a reduction of 99.1% compared to the catalyst without Nb. This indicated that the Nb-loaded catalyst exhibited a significantly enhanced ability to desorb chlorine species during both preparation and reaction. These results demonstrated that the Nb(2)-Ca(2)/MnO₂ catalyst possessed the best catalytic activity and stability. This was likely because the introduction of Nb provided additional Brønsted acid sites to the catalyst, which effectively promoted the desorption of chlorine species as HCl from the surface of the Nb(2)-Ca(2)/MnO₂ catalyst, thereby substantially mitigating the effects of chlorine poisoning.

3.6. Possible Reaction Mechanism

Fig. 7 illustrated the proposed reaction pathway over the Nb(2)-Ca(2)/MnO₂ catalyst. The active sites were composed of both surface acid sites and surface lattice oxygen. The primary reactions on the catalyst involved disproportionation and oxidation; the former led to the formation of methoxy species, while the latter generated formate species [57]. HCl was readily adsorbed on the surface lattice oxygen, resulting in the production of dichlorobenzene. The Nb⁵⁺ species on the catalyst surface functioned as Brønsted acid sites for the adsorption/activation of CB, while oxygen vacancies facilitated the adsorption/activation of oxygen species. In the presence of active oxygen species and various intermediates, reactions occurred more readily, thereby enhancing the catalytic activity. Simultaneously, the surface acid sites and oxygen vacancies reduced the availability of surface lattice oxygen, effectively suppressing the formation of by-products. Due to the surface acidity and mesoporous structure of Nb₂O₅, free Cl species generated from CB bond cleavage were desorbed as HCl at elevated temperatures on the acid sites. This process ultimately reduced the accumulation of Cl species on the catalyst, inhibited the formation of MnCl_x, MnO_xCl_y, and polychlorinated by-products, and consequently improved the catalytic activity and stability while mitigating catalyst deactivation.

4. Conclusions

A series of Nb(m)-Ca(2)/MnO₂ catalysts rich in oxygen vacancies were successfully synthesized, demonstrating excellent lowtemperature catalytic activity, high CO₂ yield, and resistance to chlorine poisoning. Specifically, the Nb(2)-Ca(2)/MnO₂ catalyst achieved T₅₀ and T₉₀ values of 280°C and 340°C, respectively, for CB conversion at a WHSV of 15000 mL·g⁻¹·h⁻¹, with a high CO_x yield (>90%). Furthermore, the catalyst maintained stable performance (CB conversion >85%) during a 30 h test at 340°C. Characterization results indicated that the Nb(2)-Ca(2)/MnO₂ catalyst possessed the highest O_ads/O_latt and Mn³⁺/Mn⁴⁺ ratios, optimal redox capability, and suitable surface acidity. In situ DRIFTS analysis revealed that benzene, phenol, and maleate species were the primary intermediates during CB oxidation over the Nb(2)-Ca(2)/MnO₂ catalyst. The presence of Nb introduced abundant acid sites, which promoted the removal of Cl species and suppressed the formation of polychlorinated by-products. Meanwhile, the strong electron transfer within the Nb-Ca/MnO₂ system created more surface defects and active sites, thereby enhancing the low-temperature reducibility of Mn species and accelerating the catalytic efficiency. This study elucidated the synergistic mechanism of the Nb-Ca/MnO₂ ternary catalyst in enhancing chlorobenzene oxidation activity and chlorine resistance, providing new insights for the design of efficient catalysts for CVOCs elimination.

Notes

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grand No.12475257).

Conflicts of Interest

The authors declare that they have no conflict of interest.

Author Contributions

C.W. (Master student) and Y.F.Y. (Master student) took charge of investigation, scientific experiments, data curation, formal analysis and writing. J.W. (Engineer) took charge of data supplement. E.H.G. (Associate Professor) and W.W. (Lecturer) did a formal analysis. J.L.Z. (Lecturer) and S.L.Y. (Professor) took charge of investigation. J.L. (Associate Professor) revised the manuscript. Z.L.W. (Professor) provided supervision, theoretical foundation, and experimental guidance.

Supplementary Materials

eer-2025-679-supplementary.pdf

References

1. Jin Q, Lu Y, Xu M, et al. Synergistic catalytic elimination of NO, mercury and chlorobenzene over WCeMnO_x/TiO₂-ZrO₂: Performance study of pore structure. Appl. Catal., A. 2022;643:118802. https://doi.org/10.1016/j.apcata.2022.118802

2. Yin R, Chen J, Mi J, et al. Breaking the Activity–Selectivity Trade-Off for Simultaneous Catalytic Elimination of Nitric Oxide and Chlorobenzene via FeVO₄–Fe₂O₃ Interfacial Charge Transfer. ACS Catal. 2022;12:3797–3806. https://doi.org/10.1021/acscatal.2c00161

3. Miao JQ, Qian L, Wu C, et al. Effect of morphology on the catalytic oxidation of dichloromethane over CeO₂ catalysts. Environ Eng Res. 2025;30. https://doi.org/10.4491/eer.2024.622

4. Shen Y, Hu X, Chen X, et al. Chlorine-Tolerant Chlorobenzene Combustion over Mullite Catalysts via In Situ Constructing Ru–O–Mn Sites. Environ. Sci. Technol. 2025;59:3826–3835. https://doi.org/10.1021/acs.est.4c12570

5. Weng X, Long Y, Wang W, Shao M, Wu Z. Structural effect and reaction mechanism of MnO₂ catalysts in the catalytic oxidation of chlorinated aromatics. Chin. J. Catal. 2019;40:638–646. https://doi.org/10.1016/S1872-2067(19)63322-X

6. Li L, Shi J-W, Tian M, et al. In situ fabrication of robust three dimensional ordered macroporous γ-MnO₂/LaMnO_3.15 catalyst for chlorobenzene efficient destruction. Appl. Catal., B. 2021;282:119565. https://doi.org/10.1016/j.apcatb.2020.119565

7. Li J, Feng J, Liu F, et al. Enhanced antichlorine poisoning of CaCO₃-modified MnO₂ catalyst for catalytic combustion of chlorobenzene. J. Environ. Chem. Eng. 2025;13:115038. https://doi.org/10.1016/j.jece.2024.115038

8. Liu J, Zuo S, Lin S, et al. Promoting the performance of Nb₂O₅ by doping transition metal oxide for catalytic degradation of monochlorobenzene and toluene. J. Mater. Res. Technol. 2023;25:3642–3653. https://doi.org/10.1016/j.jmrt.2023.06.178

9. Wang Z, Gao M, Chen X, et al. Boosting synergistic catalytic abatement of NOx and chlorobenzene via bidirectional promotion of Nb within asymmetrical Ce-O-Nb sites. Appl. Catal., A. 2025;697:120223. https://doi.org/10.1016/j.apcata.2025.120223

10. Zhao H, Han W, Dong F, Tang Z. Enhanced catalytic performance of Nb doping Ce supported on ordered mesoporous TiO₂-SiO₂ catalysts for catalytic elimination of 1,2-dichlorobenzene. Mol. Catal. 2019;479:110638. https://doi.org/10.1016/j.mcat.2019.110638

11. Tao H, Li J, Ma Q, et al. Synthesis of W-Nb-O solid acid for catalytic combustion of low-concentration monochlorobenzene. Chem. Eng. J. 2020;382:123045. https://doi.org/10.1016/j.cej.2019.123045

12. Yang S, Chen Y, Guo H, et al. Oxygen vacancies enriched Nb-WO_X supported Ru for polychlorinated aromatics oxidation: Reaction mechanism exploration. Fuel. 2022;324:124501. https://doi.org/10.1016/j.fuel.2022.124501

13. Yang P, Fan S, Chen Z, Bao G, Zuo S, Qi C. Synthesis of Nb₂O₅ based solid superacid materials for catalytic combustion of chlorinated VOCs. Appl. Catal., B. 2018;239:114–124. https://doi.org/10.1016/j.apcatb.2018.07.061

14. Yang B, Jin Q, Huang Q, et al. Synergetic catalytic removal of chlorobenzene and NO_x from waste incineration exhaust over MnNb_0.4Ce_0.2O_x catalysts: Performance and mechanism study. J.Rare Earths. 2020;38:1178–1189. https://doi.org/10.1016/j.jre.2020.06.013

15. Wang LF, Li HB, Dong H, Cheng Y, He WJ, Xu Y. Efficient phosphorus removal using the La-based perovskite oxides: Role of B-site metal for modulating the surface electronic structure. Environ Eng Res. 2025;30. https://doi.org/10.4491/eer.2024.377

16. Zhang ZM, Huang YX, Chen XL, Zheng HL, Li XY. In situ synthesis of Co-MOF@BC hybrid catalyst for enhanced BPA degradation via sulfite activation. Environ. Eng. Res. 2025;30:240572. https://doi.org/10.4491/eer.2024.572

17. Chen B, Wen Y, Gao S, et al. Mechanistic insights into the role of acidity to activity and anti-poisoning over Nb based catalysts for CVOCs combustion. Appl. Catal., A. 2022;636:118581. https://doi.org/10.1016/j.apcata.2022.118581

18. Cai Y, Li M, Gao M, et al. Effect of acid sites modification on the catalytic combustion of 1,2-dichloroethane: Revealing the synergy between NbO_x and CeO₂ . Sep. Purif. Technol. 2024;334:126069. https://doi.org/10.1016/j.seppur.2023.126069

19. Guisnet M, Magnoux P. Fundamental description of deactivation and regeneration of acid zeolites. Stud. Surf. Sci. Catal. 1994;88:53–68. https://doi.org/10.1016/S0167-2991(08)62729-9

20. Liu Z, Zhang X, Cai T, et al. Niobium Modification of Au/CeO₂ for Enhanced Catalytic Performance over Benzene Combustion. Nanomaterials. 2021;11:189. https://doi.org/10.3390/nano11010189

21. Gu W, Li C, Qiu J, Yao J. Facile fabrication of flower-like MnO₂ hollow microspheres as high-performance catalysts for toluene oxidation. J. Hazard. Mater. 2021;408:124458. https://doi.org/10.1016/j.jhazmat.2020.124458

22. Mo S, Zhang Q, Li J, et al. Highly efficient mesoporous MnO₂ catalysts for the total toluene oxidation: Oxygen-Vacancy defect engineering and involved intermediates using in situ DRIFTS. Appl. Catal., B. 2020;264:118464. https://doi.org/10.1016/j.apcatb.2019.118464

23. Jiang T, Wang X, Zhang J, Mai Y, Chen J. Highly efficient MnOx catalysts derived from Mn-MOFs for chlorobenzene oxidation: The influence of MOFs precursors, oxidant and doping of Ce metal. Mol. Catal. 2023;551:113653. https://doi.org/10.1016/j.mcat.2023.113653

24. Ma X, Sun Q, Feng X, et al. Catalytic oxidation of 1,2-dichlorobenzene over CaCO₃/α -Fe₂O₃ nanocomposite catalysts. Appl. Catal., A. 2013;450:143–151. https://doi.org/10.1016/j.apcata.2012.10.019

25. Zhang RY, Wang C, Li K, et al. Influence of Ca doping and calcination temperature on selective catalytic oxidation of NO over Mn-Ca-O_x-(CO₃)_y catalysts. New J. Chem. 2017;41:11742–11749. https://doi.org/10.1039/c7nj02230a

26. Cheng L, Li Y, Fan J, et al. High efficiency photothermal synergistic degradation of toluene achieved through the utilization of a nickel foam loaded Pt-CeO₂ monolithic catalyst. Sep. Purif. Technol. 2024;333:125742. https://doi.org/10.1016/j.seppur.2023.125742

27. Mo S, Li S, Li J, et al. Rich surface Co(III) ions-enhanced Co nanocatalyst benzene/toluene oxidation performance derived from CoIICoIII layered double hydroxide. Nanoscale. 2016;8:15763–15773. https://doi.org/10.1039/c6nr04902h

28. Liu B, Li C, Zhang G, Yao X, Chuang SSC, Li Z. Oxygen Vacancy Promoting Dimethyl Carbonate Synthesis from CO₂ and Methanol over Zr-Doped CeO₂ Nanorods. ACS Catal. 2018;8:10446–10456. https://doi.org/10.1021/acscatal.8b00415

29. Song W, Poyraz AS, Meng Y, Ren Z, Chen S-Y, Suib SL. Mesoporous Co₃O₄ with Controlled Porosity: Inverse Micelle Synthesis and High-Performance Catalytic CO Oxidation at −60 °C. Chem. Mater. 2014;26:4629–4639. https://doi.org/10.1021/cm502106v

30. Li X, Li X, Zeng X, Zhu T. Correlation between the physicochemical properties and catalytic performances of micro/mesoporous CoCeO_x mixed oxides for propane combustion. Appl. Catal., A. 2019;572:61–70. https://doi.org/10.1016/j.apcata.2018.12.026

31. Zhu L, Pan S, Liu Z, et al. Effect of Rare Earth Metal (RE = La, Pr, Nd, Y) Doping on Co–Ce Composite Oxide and Its Application in Catalytic Combustion of Chlorobenzene. Ind. Eng. Chem. Res. 2020;59:5686–5698. https://doi.org/10.1021/acs.iecr.9b07086

32. Guo R-t, Wang Q-s, Pan W-g, et al. The poisoning effect of heavy metals doping on Mn/TiO₂ catalyst for selective catalytic reduction of NO with NH₃ . J. Mol. Catal. A: Chem. 2015;407:1–7. https://doi.org/10.1016/j.molcata.2015.06.017

33. Zhang Q, Qiu C, Xu H, et al. Low-temperature selective catalytic reduction of NO with NH₃ over monolith catalyst of MnO_x/CeO₂–ZrO₂–Al₂O₃ . Catal.Today. 2011;175:171–176. https://doi.org/10.1016/j.cattod.2011.05.009

34. Wang J, Han Y, Zhang Y, Zheng J, Huang Z, Han X. Cu increases oxygen vacancies in MnO₂ to improve chlorine resistance in CO oxidation reaction. Mol. Catal. 2025;582:115103. https://doi.org/10.1016/j.mcat.2025.115103

35. Fu Y, Meng J, Luo J, et al. Oxidative decomposition of chlorobenzene on Mn_aVO_X catalysts: The critical roles of oxygen vacancies and hollow structure. Appl Surf Sci. 2023;613. https://doi.org/10.1016/j.apsusc.2022.155986

36. Zhou Z, Li Q, Su G, et al. Catalytic degradation of chlorinated volatile organic compounds (CVOCs) over Ce-Mn-Ti composite oxide catalysts. J. Environ. Sci. 2024;138:326–338. https://doi.org/10.1016/j.jes.2023.03.019

37. Cai Y, Li MQ, Gao MX, et al. Effect of acid sites modification on the catalytic combustion of 1,2-dichlo-roethane: Revealing the synergy between NbO_x and CeO₂ . Sep. Purif. Technol. 2024;334:126069. https://doi.org/10.1016/j.seppur.2023.126069

38. Yang B, Jin Q, Huang Q, et al. Synergetic catalytic removal of chlorobenzene and NO from waste incineration exhaust over MnNb_0.4Ce_0.2O_x catalysts: Performance and mechanism study. J.Rare Earths. 2020;38:1178–1189. https://doi.org/10.1016/j.jre.2020.06.013

39. Chen Z, Yang Q, Li H, Li X, Wang L, Chi Tsang S. Cr–MnOx mixed-oxide catalysts for selective catalytic reduction of NOx with NH₃ at low temperature. J. Catal. 2010;276:56–65. https://doi.org/10.1016/j.jcat.2010.08.016

40. Li HY, Song ZX, Chen X, Liu W, Yu HX, Zhang XJ. Bifunctional isolated Cu²⁺anchored on ZSM-5 enhances chlorobenzene oxidation via synergistic regulation of oxygen vacancies and acid sites. Appl. Catal. B-Environ. Energy. 2026. 382. https://doi.org/10.1016/j.apcatb.2025.125956

41. Wang J, Han YJ, Zhang YN, Zheng JF, Huang ZG, Han XJ. Cu increases oxygen vacancies in MnO₂ to improve chlorine resistance in CO oxidation reaction. Mol. Catal. 2025;582:115103. https://doi.org/10.1016/j.mcat.2025.115103

42. Mao LT, Song ZX, Fan J, et al. Regulating asymmetric oxygen vacancies in copper-ceria catalysts for achievement of excellent toluene catalytic oxidation. Sep. Purif. Technol. 2024;334:126035. https://doi.org/10.1016/j.seppur.2023.126035

43. Shao J, Lin F, Wang Z, et al. Low temperature catalytic ozonation of toluene in flue gas over Mn-based catalysts: Effect of support property and SO₂/water vapor addition. Appl. Catal., B. 2020;266:118662. https://doi.org/10.1016/j.apcatb.2020.118662

44. Zhang Z, Lin F, Xiang L, et al. Synergistic effect for simultaneously catalytic ozonation of chlorobenzene and NO over MnCoO_x catalysts: Byproducts formation under practical conditions. Chem. Eng. J. 2022;427:130929. https://doi.org/10.1016/j.cej.2021.130929

45. Huang Q, Li C, Zhang Y, et al. Co-modified rod-like α -MnO₂ with elevated activity, CO₂ yield and stability for efficient oxidation of chlorobenzene. Mol. Catal. 2025;583:115249. https://doi.org/10.1016/j.mcat.2025.115249

46. Lin F, Zhang Z, Li N, et al. How to achieve complete elimination of Cl-VOCs: A critical review on byproducts formation and inhibition strategies during catalytic oxidation. Chem. Eng. J. 2021;404:126534. https://doi.org/10.1016/j.cej.2020.126534

47. Long Y, Su Y, Xue Y, Wu Z, Weng X. V₂O₅–WO₃/TiO₂ Catalyst for Efficient Synergistic Control of NOx and Chlorinated Organics: Insights into the Arsenic Effect. Environ. Sci. Technol. 2021;55:9317–9325. https://doi.org/10.1021/acs.est.1c02636

48. Zhang C, Zhang J, Shen Y, et al. Synergistic Catalytic Elimination of NOx and Chlorinated Organics: Cooperation of Acid Sites. Environ. Sci. Technol. 2022;56:3719–3728. https://doi.org/10.1021/acs.est.1c08009

49. Sun Y, Xu S, Bai B, et al. Biotemplate Fabrication of Hollow Tubular Ce_xSr_1–xTiO₃ with Regulable Surface Acidity and Oxygen Mobility for Efficient Destruction of Chlorobenzene: Intrinsic Synergy Effect and Reaction Mechanism. Environ. Sci. Technol. 2022;56:5796–5807. https://doi.org/10.1021/acs.est.2c00270

50. Yang B, Ni M, Gu Q, et al. Catalytic Oxidation of Chlorobenzene over Ce-Mn-O_x/TiO₂: Performance Study of the Porous Structure. Catalysts. 2022;12. https://doi.org/10.3390/catal12050535

51. Wang J, Wang X, Liu X, Zhu T, Guo Y, Qi H. Catalytic oxidation of chlorinated benzenes over V₂O₅/TiO₂ catalysts: The effects of chlorine substituents. Catal.Today. 2015;241:92–99. https://doi.org/10.1016/j.cattod.2014.04.002

52. Aristizábal BH, de Correa CM, Serykh AI, Hetrick CE, Amiridis MD. In situ FTIR study of the adsorption and reaction of ortho-dichlorobenzene over Pd-promoted Co-HMOR. Micropor. Mesopor. Mat. 2008;112:432–440. https://doi.org/10.1016/j.micromeso.2007.10.020

53. Gu Y, Cai T, Gao X, et al. Catalytic combustion of chlorinated aromatics over WO_x/CeO₂ catalysts at low temperature. Appl. Catal., B. 2019;248:264–276. https://doi.org/10.1016/j.apcatb.2018.12.055

54. Hu Z, Chen J, Yan D, Li Y, Jia H, Lu C-Z. Enhanced catalytic activities of MnO_x/Co₃O₄ nanocomposites prepared via MOFs-templated approach for chlorobenzene oxidation. Appl. Surf. Sci. 2021;551:149453. https://doi.org/10.1016/j.apsusc.2021.149453

55. Song Z, Liu Z, Zhang X, et al. Influence of surface acidity and chemical valence on low temperature activity over Pt–HSiW/CeO₂ for catalytic combustion of chlorobenzene. J.Rare Earths. 2024;42:1693–1703. https://doi.org/10.1016/j.jre.2023.09.007

56. Yang P, Li J, Bao L, et al. Adsorption/catalytic combustion of toxic 1,2-dichloroethane on multifunctional Nb₂O₅-TiO₂ composite metal oxides. Chem. Eng. J. 2019;361:1400–1410. https://doi.org/10.1016/j.cej.2018.10.071

57. Zhang C, Gao F, Luo N, Shi J, Yi H, Tang X. Recent advances of chlorobenzene catalytic oxidation: influencing factors, roles of active sites and optimization. Sep. Purif. Technol. 2025;376:133962. https://doi.org/10.1016/j.seppur.2025.133962

Fig. 1

(a) Conversion of CB (b) CO₂ selectivity and (c) CO selectivity over the Nb(m)-Ca(2)/MnO₂ catalysts.

Fig. 2

(a) XRD patterns of the different Nb(m)-Ca(2)/MnO₂ catalysts and (b) N₂ adsorption-desorption isotherms and pore size distr7ibution of the different Nb(m)-Ca(2)/MnO₂ catalysts.

Fig. 3

(a–c) SEM images of the Ca(2)/MnO₂ catalyst (d–f) SEM images and (g–i) EDS mapping images of the Nb(2)-Ca(2)/MnO₂ catalyst.

Fig. 4

XPS spectra of the Nb(m)-Ca(2)/MnO₂ catalysts: (a) Mn 2p; (b) Ca 2p; (c) O 1s; (d) Nb 3d and (e) H₂-TPR; (f) O₂-TPD; (g) NH₃-TPD profiles of the Nb(m)-Ca(2)/MnO₂ catalysts.

Fig. 5

In situ DRIFTS of CB oxidation over (a) Ca(2)/MnO₂; (b) Nb(2)-Ca(2)/MnO₂ catalysts at different temperatures and Gas-phase intermediates over (c) Ca(2)/MnO₂; (d) Nb(2)-Ca(2)/MnO₂ catalysts.

Fig. 6

Long-term stability test results of the catalytic oxidation of CB over the Ca(2)/MnO₂ and Nb(2)-Ca(2)/MnO₂ catalyst.

Fig. 7

The proposed CB degradation mechanism over the Nb(2)-Ca(2)/ MnO₂ catalyst.