Mesoporous Ce-Ti Catalysts Modified by Phosphotungstic Acid and Chitosan for the Synergistic Catalysis of CVOCs and NOx

Abstract

Nitrogen oxides (NOx) and chlorinated volatile organic compounds (CVOCs) are major environmental pollutants, posing severe risks to human health and ecosystems. Traditional single-component catalysts often fail to remove both pollutants efficiently, making synergistic catalytic technologies a critical research focus. In this study, a mesoporous HPW-CS-Ce-Ti oxide catalyst, modified with H₃PW₁₂O₄₀ (HPW) and chitosan (CS), was synthesized via self-assembly. The optimized 10HPW-CS-Ce_0.3-Ti catalyst achieved nearly 100% NO conversion at 167–288 °C and a T₉₀ of 291 °C for CVOC conversion, demonstrating superior dual-pollutant removal. HPW and chitosan facilitated mesoporous structure formation, enhancing mass transfer and active site availability. HPW doping also modulated the Ce⁴⁺/Ce³⁺ ratio, boosting redox capacity and surface-active oxygen species, while increasing acidity to promote NH₃ and CVOC adsorption. This study presents a novel catalyst and synthesis method with significant potential for environmental protection and human health.

1. Introduction

Nitrogen oxides (NOx) and chlorinated volatile organic compounds (CVOCs), commonly emitted from metal smelting industries and municipal solid waste incineration, contribute to photochemical smog and ozone layer depletion, and pose significant toxicity to the environment and living organisms [1,2,3,4]. Consequently, the efficient elimination of NOx and CVOCs is crucial for mitigating ecological and human health hazards associated with air pollution, making it a central focus in environmental catalysis. Traditional pollution control methods have primarily targeted the removal of individual pollutants. For instance, selective catalytic reduction (SCR) technology is widely used for NOx abatement, converting it into harmless nitrogen gas using ammonia (NH₃) as a reducing agent in the presence of specific catalysts [5,6]. On the other hand, CVOCs are typically decomposed into carbon dioxide, water, and hydrochloric acid through adsorption, combustion, or catalytic oxidation processes [7,8,9]. However, industrial exhaust gases often contain a mixture of NOx and CVOCs, making single-treatment approaches inefficient. This not only results in high capital and operational costs, but also increases the risk of prolonged treatment processes and secondary pollution [10]. Therefore, synergistic catalysis technology has emerged as a promising solution. It aims to achieve the simultaneous conversion and removal of NOx and CVOCs within a single catalytic system. This integrated treatment approach aligns with the principles of green and efficient environmental protection, demonstrating high innovation potential. It is expected to become a key strategy for addressing the challenges of mixed pollutant treatment.

As a critical component of catalytic oxidation technology, catalyst systems and their compositions have undergone significant advancements over the years, including noble metals [7,9,11], transition metals [12,13], and zeolite catalysts [14,15,16]. In earlier studies, Jiang and colleagues [13] demonstrated that vanadium–tungsten–titanium catalysts, widely used in commercial applications, exhibited good catalytic activity for both NOx and CVOCs. The oxygen vacancies and Brønsted acid sites within the catalysts facilitated the cleavage of the C-Cl bond in chlorobenzene (CB), initiating the degradation process. However, these catalysts suffered from poor selectivity and a tendency to generate polychlorinated by-products. Concurrently, the Song team [14] reported progress in the simultaneous removal of NOx and chlorobenzene (CB) using a Ru-modified V₂O₅-WO₃/TiO₂ catalyst. The introduction of Ru enhanced C-Cl bond cleavage, ensuring high conversion rates for both NOx and CB. However, the use of the precious metal Ru significantly increased the catalyst cost, limiting its large-scale industrial application. Additionally, research has focused on V-Cu/ZSM-5 composite catalysts based on zeolite materials. These catalysts, with their unique pore structures and abundant acid sites, exhibit excellent NH₃ adsorption properties, favoring the SCR reaction and demonstrating effectiveness in the catalytic oxidation of CVOCs. However, compared to TiO₂-based carriers, they face challenges such as structural collapse, insufficient resistance to poisoning, and stability issues under high-temperature and high-humidity conditions [15]. In another study, Yin et al. [16] investigated the synergistic catalysis of NOx and CVOCs using FeVO₄-Fe₂O₃. Their results demonstrated that charge transfer through interfacial oxygen atoms from Fe to V cations in FeVO₄ enhanced its redox capability, increasing HCl selectivity and suppressing the formation of polychlorinated by-products. Similarly, Kang et al. [17] reported a Co-MnO_x catalyst for the simultaneous removal of NOx and CB at low temperatures. They found that Co doping promoted the formation of MnCo₂O₄, which enhanced the catalyst’s redox performance and surface acidity by stabilizing higher Mn oxidation states and improving reactant adsorption. Meanwhile, our team [18] aimed to improve the catalytic activity of MnO_x for the simultaneous removal of NOx and CVOCs. Compared to pure MnO_x, the SO₄²⁻/Fe₂O₃-modified MnO_x catalyst exhibited significantly enhanced activity, with improved NO conversion and CB oxidation rates. These studies highlight the vast potential of transition metal oxide catalysts for the synergistic catalytic purification of NOx and CVOCs.

Mesoporous materials, with pore diameters ranging from 2 to 50 nm, have emerged as prominent players in catalysis due to their unique advantages [19,20,21]. Their well-ordered and tunable pore structures, coupled with high specific surface areas, provide ample adsorption and diffusion spaces for reactant molecules, significantly enhancing catalytic efficiency [22,23]. Ce-Ti composite oxide catalysts are highly promising due to the synergistic effects of Ce and Ti: Ce provides abundant oxygen vacancies and redox properties for efficient oxygen storage/release, while Ti enhances structural stability and surface acidity, optimizing reactant adsorption. These features make Ce-Ti composites ideal for catalytic oxidation and SCR reactions, particularly in air pollutant treatment [24,25]. Phosphotungstic acid (HPW), with its Keggin structure, strong acidity, and proton conductivity, efficiently activates reactants and weakens C-Cl bonds in chlorinated organics, mitigating chlorine poisoning. In SCR systems, HPW regulates catalyst acidity, synergistically enhancing catalytic activity [26,27,28,29,30]. Chitosan, derived from chitin, is biodegradable and biocompatible, with reactive amino groups enabling chemical modifications and biological functionality [31,32]. Its polar hydroxyl and amino groups interact with metal oxides, inhibiting particle aggregation and enhancing metal dispersion in carriers or solutions [33,34]. Additionally, these groups serve as active sites, facilitating NOx and CVOC adsorption. Considering the characteristics of these materials, the introduction of HPW into the mesoporous Ce-Ti catalyst system for the synergistic catalysis of CVOCs and NOx is expected to combine their strengths and achieve a performance breakthrough.

In this study, chitosan was used as a templating agent to synthesize mesoporous Ce-Ti composite oxides. Through a self-assembly process, a stable complex was formed by combining the SDBS surfactant with phosphotungstic acid (H₃PW₁₂O₄₀, HPW), which was then successfully modified onto the surface of the mesoporous Ce-Ti composite oxides. The mesoporous structure of the Ce-Ti composite oxides not only increased the specific surface area and pore volume of the catalyst but also facilitated the effective binding of HPW to the catalyst surface. Additionally, it provided abundant active sites and an optimal pore structure for the adsorption and catalytic reaction of NOx and CVOCs. Furthermore, HPW modification enhanced the acidic site content and redox capacity of the Ce-Ti catalyst, resulting in excellent synergistic catalytic performance for the removal of NOx and CVOCs. This study introduces a novel material for industrial waste gas purification, demonstrating promising application prospects.

2. Results and Discussion

2.1. Structural Property and Morphology

In this study, a mesoporous Ce-Ti composite catalyst modified with chitosan-coupled phosphotungstic acid (H₃PW₁₂O₄₀, HPW) was prepared for the synergistic catalysis of NO and chlorobenzene (CB). The X-ray diffraction (XRD) patterns of the catalyst samples are shown in Figure 1. The results indicate that the Ce_0.1-Ti catalyst exhibits peaks at 25.3°, 38.6°, 48.0°, and 55.1°, corresponding to the (101), (112), (200), and (211) planes of TiO₂ (PDF #21-1272), respectively [33]. Notably, no diffraction peaks attributable to CeO₂ are observed, likely due to the low Ce doping amount or its uniform dispersion. Additionally, the intensity of the derivative peak for the CS-Ce_0.1-Ti catalyst, prepared using a chitosan template, is significantly reduced. This reduction may result from the coordination between chitosan and metal oxides, which reduces particle size and improves dispersion. As the Ce content increases, the diffraction peaks at 25.3°, 38.6°, 48.0°, and 55.1° associated with TiO₂ gradually weaken. When the Ce/Ti molar ratio reaches 0.3:1, all diffraction peaks disappear, suggesting that the interaction between Ce and Ti promotes the amorphous nature of the catalyst. Further increasing the Ce content results in a slight diffraction peak at 28.5°, corresponding to the (111) plane of CeO₂ (PDF #43-1002) [34]. The Ce_0.3-Ti catalyst was selected for further characterization due to its amorphous structure, as confirmed by XRD analysis. Amorphous structures are typically less stable and tend to expose a higher density of active sites, which are crucial for catalytic performance. Therefore, further characterization of the Ce_0.3-Ti catalyst modified with HPW was conducted. The XRD results (Figure 1b) show no diffraction peaks attributable to HPW, indicating that the catalyst retains its amorphous structure. This suggests that HPW modification does not alter the crystal structure of the Ce-Ti catalyst and is uniformly dispersed within it. The amorphous structure of the composite oxide catalyst, characterized by abundant oxygen vacancies and defects, is advantageous for catalytic reactions [35,36].

To further elucidate the morphology of the Ce-Ti catalyst modified by phosphotungstic acid, SEM + EDS and HRTEM characterizations were conducted on the CS-Ce_0.3-Ti and 10%HPW-CS-Ce_0.3-Ti samples, and the results are presented in Figure 2. Figure 2a demonstrates that the CS-Ce_0.3-Ti catalyst presents a morphology of nanoparticles. After being modified by phosphotungstic acid, no significant changes occurred in the microscopic morphology, and the EDS images (Figure 3) indicate that the HPW is evenly distributed throughout the catalyst, which is crucial for achieving consistent catalytic performance. The good dispersion is partly attributed to the interaction between polar groups such as hydroxyl and amino groups in chitosan molecules with the surface of metal oxides, which inhibits the aggregation and growth of metal particles to a certain extent. Additionally, the interaction between HPW and Ce-Ti contributes to enhancing the dispersion of metals, resulting in a more uniform distribution of metal oxides on the carrier or in the solution. From the HRTEM results (Figure 2c), it can be observed that the 10HPW-CS-Ce_0.3-Ti nanoparticles are relatively uniform in size, with a size ranging from approximately 5 to 10 nm. Meanwhile, abundant pores can be noticed among the nanoparticles. This will lead to more exposure of the active sites of the catalyst to the reactants and enhance the excellent catalytic performance of the catalyst. Furthermore, upon further magnification of the results, no obvious lattice fringes were detected, suggesting that the catalyst samples possess an amorphous structure, which is in line with the XRD results.

2.2. Pore Structure Property

Figure 4 and

Table 1. Textural properties of different catalysts.

Catalysts	SBET (m2/g)	Pore Volume (cm3/g)	Pore Size (nm)
CS-Ce0.3-Ti	217	0.43	8.5
5HPW-CS-Ce0.3-Ti	210	0.41	8.3
10HPW-CS-Ce0.3-Ti	204	0.39	8.3
15HPW-CS-Ce0.3-Ti	178	0.35	7.9
20HPW-CS-Ce0.3-Ti	157	0.33	7.6

present the N₂ adsorption–desorption isotherms and pore structural characteristics of the catalysts. As shown in Figure 4a, all catalysts exhibit typical Type IV isotherms with H1 hysteresis loops, indicative of mesoporous materials. Figure 4b further confirms the mesoporous structure, with pore size distribution curves in the range of 5–25 nm. The BET specific surface area (S_BET) results in Table 1 reveal that CS-Ce_0.3-Ti has the largest S_BET of 217 m²/g. After introducing phosphotungstic acid (HPW), the S_BET gradually decreases, likely due to pore occupation by HPW. However, the decrease in S_BET for 10HPW-CS-Ce_0.3-Ti is relatively small, and its impact on catalytic performance is minimal. In contrast, when higher amounts of HPW (15% and 20%) are added, both the specific surface area and pore volume decrease significantly (e.g., 157 m²/g for 20HPW-CS-Ce_0.3-Ti), which cannot be ignored in terms of catalytic activity

2.3. Surface Chemistry Analysis

XPS analysis was used to further investigate the chemical species and surface atomic composition of the catalysts. The adventitious carbon located at 284.8 eV was used to calibrate samples, and the outcomes are depicted in Figure 5 [37].

Figure 5a displays the Ce 3d XPS spectra. According to the peak fitting results, all catalysts exhibit eight distinct energy peaks. Among them, v′ and u′ belong to Ce³⁺, while v, v″, v‴, u, u′, and u‴ are attributed to Ce⁴⁺ [38,39]. The surface atomic concentrations are also summarized in

Table 2. XPS results of Mn/ZSM-5 and 10%Ce-Mn/ZSM-5 catalysts.

Samples	Ce4+/(Ce3+ + Ce4+) (%)	Oα/(Oα + Oβ) (%)
CS-Ce0.3-Ti	72.6	21.4
10HPW-CS-Ce0.3-Ti	78.3	25.5

. The results indicate a slight increase in the Ce⁴⁺ content for the 10HPW-CS-Ce_0.3-Ti sample. This decrease can be attributed to two factors: firstly, the interaction between HPW and Ce, where the electron interaction between the W⁶⁺ species introduced from HPW leads to an increase in the presence of surface Ce⁴⁺; and secondly, the excellent oxidizing properties of HPW oxidize a portion of Ce³⁺ to Ce⁴⁺. As is well-known, Ce⁴⁺ is the primary active species in cerium-based catalysts [40], and an increase in Ce⁴⁺ content enhances the adsorption and activation of gases.

Figure 5b shows the O 1s spectra of 10HPW-CS-Ce_0.3-Ti and CS-Ce_0.3-Ti catalysts, and the asymmetric signal can be decomposed into three peaks: the peak (530.0 eV) labeled as O_α was attributed to the adsorbed oxygen species, and the peak (531.2 eV) labeled as O_β corresponded to the surface lattice oxygens, while the peak at 532.6 eV were assigned to O in −OH [40,41,42]. It is well known that surface adsorbed oxygen has higher activity and good mobility. In addition, the oxygen vacancy concentration is typically represented by the amount of oxygen adsorbed. Importantly, oxygen that is adsorbed has the ability to participate in CB deep catalytic oxidation. As a result, excellent CB and NOx degradation efficiency will result from abundant surface-active oxygen. According to the O_α/(O_α + O_β) calculation results in Table 2, the 10HPW-CS-Ce_0.3-Ti showed higher adsorbed oxygen species compared to CS-Ce_0.3-Ti, and the content of adsorbed oxygen species (O_α) increased from 21.4% to 25.5%, implying that HPW modification increased the oxygen vacancies of the catalyst. Additionally, it can also be observed that the content of oxygen species belonging to -OH in the 10HPW-CS-Ce_0.3-Ti sample increases significantly. This increase mainly originates from the hydroxyl functional groups of chitosan and the incorporation of HPW. In catalytic processes, -OH often acts as a Brønsted acid site. An abundant presence of Brønsted acid can enhance the SCR (selective catalytic reduction) activity by promoting the adsorption of ammonia. Additionally, Brønsted acid sites can inhibit the formation of polychlorinated by-products and improve the selectivity towards HCl [43]. In summary, the modification of the catalyst with phosphotungstic acid results in the generation of abundant surface oxygen species. It is foreseeable that this will lead to an enhancement in its synergistic catalytic activity.

2.4. Redox and Acid Sites Properties Analysis

As shown in Figure 6a, the reducibility of various catalysts was investigated using hydrogen temperature-programmed reduction (H₂-TPR) experiments. CS-Ce_0.3-Ti exhibits a prominent reduction peak at 528 °C, which corresponds to the reduction of Ce⁴⁺ to Ce³⁺, representing the surface lattice oxygen species [41,44]. For the 10HPW-CS-Ce0.3-Ti sample, the shift of the reduction peak to a higher temperature (556 °C) indicates a weakened redox capability, likely due to the interaction between HPW and the Ce-Ti catalyst, which increases the energy barrier for Ce⁴⁺ reduction. This is likely due to the coverage of the CeO₂ surface by HPW. However, it is worth noting that the reduction peak of active oxygen species of 10HPW-CS-Ce_0.3-Ti is relatively lower, suggesting its excellent surface-active oxygen species and superior oxygen mobility. Additionally, a weak reduction peak is observed at 738 °C, which belongs to the reduction of W⁶⁺, confirming the presence of W species [42], consistent with the EDS results. Furthermore, the number of acid sites in the catalysts was investigated through NH₃-TPD (temperature-programmed desorption of ammonia) experiments, with the results shown in Figure 6b. All catalysts exhibit a distinct NH₃ adsorption–desorption peak between 100 and 500 °C, with weak acid sites at <150 °C, medium acid sites at 150–350 °C, and strong acid sites at >350 °C [45,46]. Previous reports have indicated that medium to strong acid sites on catalysts are more crucial for NH₃-SCR performance and catalytic oxidation of CB. The NH₃-TPD results suggest that the 10HPW-CS-Ce0.3-Ti catalyst exhibits enhanced acid strength, which can effectively adsorb and activate NH₃ and CB molecules, thereby further promoting the synergistic catalytic reaction.

2.5. Catalyst Performance

Figure 7a showed the NO conversion performance of mesoporous Ce-Ti catalysts at various temperatures. It is evident that the NO conversion rate for all catalysts exhibits an increasing trend followed by a decrease. However, compared to the CS-Ce_0.3-Ti catalyst, the enhancement in catalytic performance for NO due to phosphotungstic acid modification is not pronounced at low temperatures, with the primary improvement observed at medium to high temperatures. Notably, the 10HPW-CS-Ce_0.3-Ti catalyst achieves nearly 100% NO conversion over a broad temperature range of 167–288 °C, whereas the CS-Ce_0.3-Ti catalyst only reaches this level within the narrower temperature range of 170–252 °C.

Table 3. Comparison of the catalytic activity of different catalysts.

Catalysts	NO Conversion (°C)	CB Conversion (°C)	Space Velocity (h−1)
MnOx/TiO2	90% at 120	80% at 330	80,000 [47]
MnCoOx	90% at 104	90% at 181	60,000 [48]
MnCeTiOx	80% at 250	96% at 300	60,000 [49]
CoSmMn2O5	90% at 160	90% at 270	50,000 [50]
MnCeSmSnOx	90% at 120	90% at 238	24,000 [51]
This study	90% at 167	90% at 291	60,000

compares the catalytic performance of various catalysts for NO and CB (chlorobenzene) conversion. Our HPW-CS-Ce-Ti catalyst achieves 90% NO conversion at 167 °C and 90% CB conversion at 291 °C under a space velocity of 60,000 h⁻¹. This performance is superior to that of previously reported catalysts, such as MnOx/TiO₂, MnCoO_x, MnCeTiO_x, CoSmMn₂O₅, and MnCeSmSnO_x, which require higher temperatures or lower space velocities for similar conversion rates. The incorporation of phosphotungstic acid (HPW) into the ceria-based catalyst often leads to the formation of an abundant mesoporous structure. This mesoporous structure facilitates the diffusion of reactants and products, enhancing mass transport efficiency. Additionally, the high surface area provided by the mesoporous structure increases the number of active sites available for catalytic reactions, thereby promoting the catalytic activity of the catalyst towards CVOCs and NO. Figure 7b depicts the catalytic conversion curves of CB for the catalysts, indicating that the conversion rates for all catalysts show an upward trend. Below 200 °C, the change is relatively minor, but above 200 °C, the conversion efficiency rapidly increases. The CS-Ce_0.3-Ti catalyst exhibits a T₉₀ (the temperature required to achieve 90% conversion) of only 332 °C, whereas the introduction of phosphotungstic acid shifts the T₉₀ progressively towards lower temperatures, with the 10HPW-CS-Ce_0.3-Ti catalyst achieves a T₉₀ of 291 °C. This enhancement in conversion performance can be attributed not only to the facilitative effect of the aforementioned pore structure but also to the superior redox properties and the abundance of surface-active oxygen, as inferred from previous tests. Furthermore, the increase in the content of medium-strong acids plays a crucial role in expanding its high-temperature activity.

2.6. Promotional Effects of Phosphotungstic Acid and Chitosan

The modification with phosphotungstic acid (HPW) and chitosan results in excellent dispersion properties. This good dispersion is partly attributed to the interaction between the polar groups, such as hydroxyl and amino groups in the chitosan molecules, and the surface of the metal oxide, which inhibits the aggregation and growth of metal particles to a certain extent. Additionally, the catalyst can form a mesoporous structure, which facilitates the diffusion of reactants and products, thereby enhancing the transport efficiency of CVOc and NO. This is because the mesoporous structure ensures that reactants can easily penetrate into the interior of the catalyst particles, increasing the chances of interaction with active sites. This is particularly significant in the context of rapid reactions, as it minimizes the diffusion limitations that might otherwise hinder the reaction kinetics. The enhanced transport efficiency of CVOCs and NO through the mesoporous channels allows for more efficient utilization of the catalyst’s surface area, thereby promoting higher reaction rates. Furthermore, the high surface area provided by the mesoporous structure increases the number of active sites available for catalytic reactions, thereby promoting the catalytic activity of the catalyst towards CVOCs and NO. In addition, the doping of HPW into the Ce-Ti catalyst can affect the Ce⁴⁺/Ce³⁺ ratio, which can shift the equilibrium towards a higher Ce⁴⁺ concentration, and enhance the reducing ability. This shift in the Ce⁴⁺/(Ce³⁺ + Ce⁴⁺) ratio is beneficial for the reduction of NO and the oxidation of CVOCs, as it provides more active electrons for these reactions. Surface-active oxygen is another key factor in the catalytic activity of ceria-based catalysts. The doping of HPW can increase the number of surface-active oxygen species, which are crucial for the oxidation of CVOCs. These active oxygen species can react with CVOCs to form intermediates that are more easily degraded. Additionally, the presence of HPW may stabilize the surface-active oxygen, preventing its deactivation during the catalytic process. Furthermore, the doping of HPW into the ceria-based catalyst is expected to increase the number of acid sites, particularly medium to strong acid sites, which is essential for the NH₃-SCR process. The increased acidity of the catalyst promotes the adsorption of NH₃, leading to higher NO conversion efficiencies during the NH₃-SCR reaction. In conclusion, the doping and modification of phosphotungstic acid into ceria-based catalysts can significantly enhance their synergistic catalytic activity towards CVOCs and NO. This enhancement is attributed to the formation of a mesoporous structure, changes in the Ce⁴⁺/Ce³⁺ ratio, an increase in surface-active oxygen species, and an increase in acid sites. These promotional effects, combined with the redox properties of cerium and the acidity of the catalyst, contribute to the high catalytic performance of the doped and modified ceria-based catalysts for the simultaneous removal of CVOCs and NO.

3. Experimental Procedure

3.1. Catalyst Preparation

Preparation of Mesoporous CS-Ce-Ti composite oxide: Firstly, A certain amount of cerium nitrate and tetraethyltitanate were dissolved in 60 mL of ethanol and stirred uniformly to obtain a mixed solution. Subsequently, some chitosan (CS) was dissolved in 60 mL of 2% acetic acid solution, with the molar ratio of Ce:Ti:CS = 0.1–0.5:1:1. Then, the cerium–titanium mixed solution was slowly added dropwise to the chitosan solution while stirring and maintaining an 80 °C water bath, heating to form a uniform gel system. Next, the gel system was placed in a freeze dryer and freeze dried at −20 °C for 48 h to obtain a dried chitosan-modified Ce-Ti precursor. This precursor was then placed in a tubular furnace and heated to 600 °C at a heating rate of 5 °C/min under nitrogen protection, with a dwell time of 2 h. After the heat treatment, it was naturally cooled to room temperature to obtain chitosan-modified mesoporous Ce-Ti composite oxide (CS-Cex-Ti, X = 0.1–0.5).

In addition, the Ce_0.1-Ti catalyst without chitosan treatment was also synthesized for comparison. In addition to not introducing chitosan, the other steps were consistent with the above.

Preparation of Mesoporous HPW-CS-Ce-Ti composite oxide: A certain amount of phosphotungstic acid (HPW) was dissolved in a mixed solvent containing ethanol and water to obtain a heteropolyacid solution. Subsequently, the surfactant sodium dodecylbenzenesulfonate (SDBS) was added, and the mixture was stirred to form an HPW-SDBS complex solution with a molar ratio of HPW to SDBS of 1:2. A quantity of 1 g of the pre-prepared CS-Ce-Ti composite oxide was added to the mixed solution and stirred for 6 h, followed by drying at 80 °C for 12 h. Subsequently, the catalyst was calcined in air at 500 °C for 4 h to obtain the final catalyst, which is named mHPW-CS-Ce-Ti (where “m” indicates the mass percentage of HPW in the Ce-Ti catalyst, m = 5–20%).

3.2. Characterization of Catalyst

A D8 Advance diffractometer (Bruker, Bremen, Germany) with Cu Kα (λ = 0.154 06 nm) at an angle of 2θ from 10° to 80° was used to analyze the X-ray diffraction (XRD) patterns. The BET-BJH and the BET surface areas of the catalysts were tested by N₂ adsorption and desorption using a TriStar II 3020 gas sorption analyzer (Mike, MO, USA). The instrument can provide microporous, mesoporous, macroporous, and BET tests. The pore size test range is 3.5–5000 Å, the minimum detection amount of pore volume is 0.0001 cc·g⁻¹, and the minimum detection of specific surface area is 0.0001 m²·g⁻¹. In this test, all samples were degassed at 300 °C for 3 h, and then analyzed under liquid nitrogen. XPS was implemented on a surface analysis system (VG Multilab2000, Thermo Fisher Scientific, East Grinstead, UK) operating at 10^–9 Pa with Al Kα radiation (1486.6 eV). Meanwhile, the observed spectra were corrected using the C 1s line at 284.6 eV as the standard. The morphology and structure of the catalysts were observed by field emission scanning electron microscopy (FESEM, Ultra plus, Zeiss, Oberkochen, Germany). In addition, the dispersion of the elemental composition and semiquantitative determination were verified by energy dispersive spectroscopy (EDS) analysis. Temperature-programmed reduction (TPR) and temperature-programmed desorption (TPD) tests were both performed on the automated catalyst characterization apparatus (Chemisorb 2720, Micromeritics, Norcross, GA, USA) with a thermal conductivity detector (TCD). For temperature-programmed H₂ reduction (H₂-TPR), the samples were reduced by increasing the temperature from 50 °C to 800 °C at 10 °C/min. A 150 mg sample were placed in a reaction tube. The sample was programmatically heated from room temperature to 300 °C at a rate of 10 °C/min for drying pretreatment and purged with a helium gas flow (30–50 mL/min) for 1 h. The sample was cooled to 50 °C, and then a 10% NH₃/He mixture (30–50 mL/min) was introduced for 1 h until saturation was reached. A helium gas flow (30–50 mL/min) was then used to purge for another 1 h to remove weakly physically adsorbed NH₃ on the surface. Finally, the sample was heated to 800 °C under a helium atmosphere at a heating rate of 10 °C/min for desorption, and the desorbed gases were detected using a TCD (thermal conductivity detector).

3.3. Catalytic Activity Measurement

The typical CVOC chlorobenzene (CB) and NO were selected as the subjects of catalytic investigation. The catalyst activity tests were conducted in a fixed-bed reactor with a space velocity of 60,000 h⁻¹. The catalyst (100 mg) was pelletized to a particle size of 40–60 mesh, and the simulated gas composition consisted of 500 ppm NO, 500 ppm ammonia, 50 ppm CB, 5 vol.% O₂, with N₂ serving as the balance gas. The concentration of NO was analyzed using a flue gas analyzer, while the concentration of CB was determined using an online gas chromatograph (GC9790II, Fuli, Wenling, China) equipped with an FID detector. Additionally, to avoid gas adsorption and condensation, all tubing was maintained in a heated state. The test temperature ranged from 100 to 340 °C, with measurements taken at intervals of every 40 °C. The calculation process for toluene oxidation conversion is as follows:

$$\mathrm{N}{\mathrm{O}}_{\mathrm{conversion}}(\%)={\displaystyle \frac{\mathrm{N}{\mathrm{O}}_{\mathrm{in}}-\mathrm{N}{\mathrm{O}}_{\mathrm{out}}}{\mathrm{N}{\mathrm{O}}_{\mathrm{in}}}}\times 100\%$$

(1)

$$\mathrm{CB}(\%)={\displaystyle \frac{\mathrm{C}{\mathrm{B}}_{\mathrm{in}}-\mathrm{C}{\mathrm{B}}_{\mathrm{out}}}{\mathrm{C}{\mathrm{B}}_{\mathrm{in}}}}\times 100\%$$

(2)

where NO_in, CB_in and NO_out, CB_out are marked as inlet NO/CB concentration and outlet NO/CB concentration, respectively.

4. Conclusions

NO and CVOCs are major pollutants that pose significant threats to the environment and human health, necessitating the development of efficient and eco-friendly catalytic materials for their simultaneous removal. In this study, we synthesized mesoporous HPW-CS-Ce-Ti oxide catalysts using a chitosan template method and H₃PW₁₂O₄₀ self-assembly. The 10HPW-CS-Ce_0.3-Ti catalyst demonstrated exceptional performance, achieving nearly 100% NO conversion between 167 and 288 °C and a T₉₀ for CVOCs as low as 291 °C. The introduction of HPW enhanced the catalyst’s mesoporous structure, improving mass transfer and active site accessibility. It also optimized the Ce⁴⁺/Ce³⁺ ratio, boosting redox capacity and surface-active oxygen species. Additionally, HPW increased catalyst acidity, promoting NH₃ adsorption for NH₃-SCR and enhancing CVOC degradation while suppressing polychlorinated by-products. These synergistic effects significantly improved the catalytic efficiency for CVOCs/NOx removal.