Chlorinated volatile organic compounds (Cl-VOCs) such as 1, 2-dichloroethane (DCE) are widely used and released to the atmosphere through several industrial operations [1, 2]. These compounds are well known for their high volatility, toxicity, and stability, and can be greatly harmful to both human health and the environment [1-4]. Therefore, the elimination of gas pollutants containing low concentration CVOCs is of significance. Compared to other techniques (such as adsorption, absorption condensation, and burning), deep catalytic oxidation [5-7] has been deemed as one of the most effective methods because of its high efficiency, low energy consumption, and the absence of secondary pollution.

Up to now, two types of catalysts have been commercially applied to the deep catalytic oxidation process: supported noble metal [2, 3, 6] and metal oxide catalysts [2, 3, 8]. Although supported noble metal catalysts generally exhibit higher oxidation activities, they react more easily with chlorine species, leading to the poisoning of the catalysts and the formation of polychlorinated byproducts during the oxidation process. Therefore, metal oxide catalysts, especially CeO₂-based mixed oxides (such as CeO₂-MnO_x, CeO₂-ZrO₂, and Ce_xPr_1−xO₂) [9-12] have been investigated and used extensively, owing to their lower cost and better resistance to chlorine poisoning. Among them, (Ce, Cr)_xO₂ exhibits outstanding catalytic performance for the deep oxidation of Cl-VOCs owing to its high oxidizing ability, but it is still necessary to increase its selectivity to HCl rather than towards Cl₂, in order to reduce the toxicity of the final products [11, 13-15]. On the other hand, various researchers [16-19] have reported that H-type zeolites exhibit high selectivities to HCl formation for the degradation of Cl-VOCs owing to their proper strong/weak acid properties. However, notable formations of CO and byproducts, as well as obvious deactivation by coking, are observed at the same time, caused by the lack of sufficient oxidation sites [20, 21]. Thus, both the acid and oxidation properties need to be improved to further realize a desirable catalytic performance. High-valence metal oxides (TiO₂, WO₃, MoO₃, and Nb₂O₅) [22-27] with abundant acid sites are well known as "solid-acid" catalysts and are widely used in acid-catalyzed and redox reactions. Therefore, a combination of (Ce, Cr)_xO₂ mixed oxides with solid-acid metal oxides would be beneficial, rendering such materials promising for the catalytic oxidation of CVOCs.

Based on the previous investigations mentioned above, in this study, Nb₂O₅ was chosen as a prospective solid-acid support material, and a series of (Ce, Cr)_xO₂/Nb₂O₅ catalysts with different (Ce, Cr)_xO₂ to Nb₂O₅ mass ratios were synthesized by a deposition-precipitation method and their performance for the deep catalytic oxidation of DCE was evaluated. The catalysts were further characterized by means of X-ray diffraction (XRD), N₂ adsorption-desorption, UV-Raman, scanning electron microscopy (SEM), ammonia temperature-programmed desorption (NH₃-TPD), and H₂ temperature-programmed reduction (H₂-TPR) techniques. The objective of this study was to obtain a satisfactory activity and selectivity for DCE oxidation by regulating the active components, while obtaining some insight into the synergy between the acid sites and the oxidation sites of the (Ce, Cr)_xO₂/Nb₂O₅ catalyst system.

The support material Nb₂O₅ was purchased from Guoyao Company and calcined in air (500 ℃, 2 h), followed by mechanical milling for 0.5 h. (Ce, Cr)_xO₂/Nb₂O₅ mixed oxides with different (Ce, Cr)_xO₂ to Nb₂O₅ mass ratios were synthesized by the traditional deposition-precipitation method. First, a certain amount of Nb₂O₅ was added into a solution mixed with quantified amounts of Ce(NO₃)₃ and Cr(NO₃)₃ (molar ratio of Ce/Cr was 4:1), and then, ultrasonic concussion was performed for 1 h. Afterwards, 0.50 mol L⁻¹ (NH₄)₂CO₃ was added dropwise to the above mixed solution under vigorous stirring under vigorous stirring until the final pH of the solution reached 9.0. Upon aging at 25 ℃ for 12 h, the precipitated solids were filtered, washed with distilled water, and then dried in ethanol under supercritical conditions (265 ℃, 2 h, 7.5 MPa). The solids were subsequently calcined in air at 500 ℃ for 2 h. The obtained samples were named as y(Ce, Cr)_xO₂/Nb₂O₅, with the mass ratio y of (Ce, Cr)_xO₂ to Nb₂O₅ being 4, 2, 1, 0.75, 0.5, 0.25, 0.1, and 0.05. For comparison, a (Ce, Cr)_xO₂ mixed oxide was also prepared by following a similar procedure. All the samples were finally pressed into pellets and sieved to 40–60 mesh (0.3–0.45 mm) before use.

The XRD patterns of the samples were recorded on a D/Max-2550pc powder diffractometer by using Cu K_α radiation. The X-ray tube was operated at 40 kV and 250 mA, and the powder diffractogram was recorded at 0.02° intervals in the 2θ range 10°–90°, with the scanning rate being 5°/min.

The N₂ adsorption/desorption isotherms of the samples were obtained at –196 ℃ by using a Micromeritics TrisStarII automatic adsorption instrument. The samples were pretreated under high vacuum at 200 ℃ for 4 h before the measurements.

The UV-Raman spectra of the samples were obtained on a LabRam-HR800 instrument using 325 nm He-Gd as the laser source; the spectral resolution was 4 cm^–1. The scanning range was 150–2000 cm^–1.

The SEM images were recorded on a SU1510 apparatus.

NH₃-TPD was performed in a quartz fixed-bed microreactor equipped with a TCD detector. Prior to the adsorption of ammonia, the catalyst (about 100 mg) was pretreated in a N₂ stream (99.99%, 35 mL min^–1) at 500 ℃ for 0.5 h. After cooling to 100 ℃, the catalyst was exposed to a flow (30 mL min^–1) of 20 vol% NH₃/N₂ mixture for 0.5 h, and then treated in pure N₂ flow for 1 h in order to remove the physically adsorbed NH₃. Finally, desorption experiment was carried out in pure N₂ flow (40 mL min^–1) from 100 to 600 ℃ at the heating rate of 10 ℃ min^–1. All these profiles were simulated by Gaussian functions.

The redox properties of the catalysts were examined by H₂-TPR experiments. The catalyst (100 mg, 40–60 mesh) was placed in a U quartz reaction tube and pretreated in N₂ flow (99.999%, 30 mL min^–1) at 200 ℃ for 30 min, before being cooled to 100 ℃. The reduction process was performed in 5 vol% H₂/Ar (40 mL min^–1) between 50 and 900 ℃, with the heating rate being 10 ℃ min^–1. The H₂ consumption was detected on-line by a gas chromatograph (GC 1690, China) that was equipped with a TCD.

The catalytic performance tests of the catalysts were conducted in a microreactor (quartz tube, 6 mm i.d.) at atmospheric pressure, with the gas hourly space velocity being 15000 h^–1. The feed flowing through the microreactor consisted of 1000 ppmv DCE, with dry air as the balance gas, and the total flow was 75 mL min^–1. A mass of 500 mg of the catalyst (40–60 mesh) was loaded. The concentrations of DCE, any possible organic byproducts, and CO_x in the effluent gas were analyzed on-line by using the gas chromatograph that was equipped with an FID and a TCD. The concentrations of HCl and Cl₂ in the effluent gas were measured by bubbling the waste gases through 0.0125 mol L^–1 NaOH solution for 20 min when DCE was completely converted. The Cl^– concentration was measured by a Cl^– selective electrode, whereas the Cl₂ concentration was determined through chemical titration with ferrous ammonium sulfate by using N, N-diethyl-p-phenylenediamine sulfate as the indicator. The HCl selectivity was calculated as S_HCl = [HCl]/([HCl] + [Cl₂]).

The performance of the obtained catalysts for the degradation of DCE is presented in Fig. 1(A). Previous tests have revealed that the mass transfer limitation is absent under this test condition, and no obvious DCE conversion is observed below 420 ℃ in the blank experiment. As shown in Fig. 1(A), the T_50% (the temperature needed to achieve 50% conversion) of Nb₂O₅ is about 304 ℃, and DCE can be completely converted at 360 ℃. Obvious improvements in the catalytic activities can be observed by introducing (Ce, Cr)_xO₂, as all the y(Ce, Cr)_xO₂/Nb₂O₅ catalysts are able to fully convert DCE below 300 ℃. The T_50% values of y(Ce, Cr)_xO₂/Nb₂O₅ catalysts tend to decrease first and then increase with increasing (Ce, Cr)_xO₂ to Nb₂O₅ mass ratio, although all the mixed oxide catalysts show slightly higher T_50% compared to single (Ce, Cr)_xO₂. To better verify the intrinsic catalytic activity, the TOF values were obtained at 150 ℃, and the results are shown in Fig. 1(B). All the mixed catalysts show larger TOF values than the single (Ce, Cr)_xO₂ catalyst. Moreover, 0.25(Ce, Cr)_xO₂/Nb₂O₅ catalyst exhibits a noticeable intrinsic catalytic activity owing to its largest TOF value, and its T_90% is almost similar to that of (Ce, Cr)_xO₂. These results indicate that there exists a synergistic effect between (Ce, Cr)_xO₂ and Nb₂O₅. Such an effect can contribute to improving the intrinsic activities of the mixed oxide catalysts.

For the catalytic elimination of Cl-VOCs, any possible byproducts should be detected carefully, since they may be more toxic and more difficult to destroy. As shown in Fig. 1(C), within the detection limits of the TCD and FID used, C₂H₃Cl is the sole byproduct observed during the DCE oxidation, and no other possible byproducts (such as phosgene, chloroform, CCl₄, and dioxins) are detected over these investigated catalysts. A noticeable amount of C₂H₃Cl is detected over Nb₂O₅, as the concentration (38 ppm) of C₂H₃Cl is maximum at 400 ℃; C₂H₃Cl is not eliminated even at 480 ℃. For the y(Ce, Cr)_xO₂/Nb₂O₅ catalysts, both the maximum C₂H₃Cl concentration and the peak temperature decrease obviously with increasing (Ce, Cr)_xO₂ to Nb₂O₅ mass ratio. This result indicates that the synergistic effect between (Ce, Cr)_xO₂ and Nb₂O₅ not only is beneficial for degrading DCE into C₂H₃Cl, but also facilitates C₂H₃Cl oxidation.

For the complete oxidation of DCE, the final products are CO/CO₂, HCl/Cl₂, and H₂O. The S_HCl (selectivity to HCl) data of all these catalysts is presented in Table 1. All the y(Ce, Cr)_xO₂/Nb₂O₅ catalysts reveal high selectivities toward the desired HCl and CO₂ (more than 98%, not listed). Nb₂O₅ alone shows the highest selectivity to HCl. As the (Ce, Cr)_xO₂ to Nb₂O₅ mass ratio increases, S_HCl gradually decreases. This can be explained in terms of the partial oxidation of HCl into Cl₂ [21, 28], due to the strong oxidation ability of (Ce, Cr)_xO₂.

Fig. 2 displays the XRD patterns of all the catalysts, and Table 1 lists the corresponding crystallite sizes. The characteristic diffraction peaks of TT-phase Nb₂O₅ (at 2θ = 22.6°, 28.5°, 36.7°, 46.2°, and 50.6°) [29] and cubic CeO₂ (at 2θ = 28.6°, 33.0°, 47.6°, and 56.4°) [21] appear for y(Ce, Cr)_xO₂/Nb₂O₅ catalysts. On the other hand, the characteristic diffraction peaks of Cr₂O₃ (at 2θ = 33.6°, 36.3°, and 54.9°) [28] are not detected, which can be attributed either to the low content of the Cr species or the high dispersion of the Cr species in the CeO₂ matrix. As the (Ce, Cr)_xO₂ to Nb₂O₅ mass ratio increases, the diffraction peaks of CeO₂ become sharper and the peak intensities increase, revealing the formation of large CeO₂ crystallites. The CeO₂ crystallite size in the y(Ce, Cr)_xO₂/Nb₂O₅ catalysts varies between 4.5 and 6.3 nm, which is smaller than that of single (Ce, Cr)_xO₂ (6.5 nm) (a TEM image and the particle size counts of (Ce, Cr)_xO₂ are shown in Fig. S1, Supporting Information); this is because the large surface area of Nb₂O₅ favors the dispersion of (Ce, Cr)_xO₂. Together with the results of the catalytic performance evaluation, we can see that (Ce, Cr)_xO₂ is highly dispersed on the Nb₂O₅ surface when the (Ce, Cr)_xO₂ to Nb₂O₅ mass ratio is less than 0.25, thus, the synergistic effect between (Ce, Cr)_xO₂ and Nb₂O₅ is enhanced, and the catalytic activity is further promoted. When the (Ce, Cr)_xO₂ to Nb₂O₅ mass ratio exceeds 0.25, the catalytic activity slightly decreases, possibly due to a worse surface dispersion of (Ce, Cr)_xO₂ or the less exposed surface acid centers of Nb₂O₅.

Raman spectroscopy is considered to be a useful tool for obtaining additional structural information related to lattice vibrations that is sensitive to crystal symmetry. The UV-Raman profiles of all the y(Ce, Cr)_xO₂/Nb₂O₅ catalysts are presented in Fig. 3. All the catalysts show a characteristic peak at 325 cm⁻¹, which is attributed to the LO vibration mode that is related to the defect spaces of the O vacancies present in the metal oxide lattice. For Nb₂O₅ support, two peaks at 687 and 810 cm^–1 can be observed, which can be assigned to the symmetrical stretching vibrations of the Nb‒O bonds in both the surface N₂O₅[NbO₆] and the [NbO₄] structure [30, 31]. For (Ce, Cr)_xO₂ catalysts, three peaks appear at 460, 587, and 1176 cm^–1, which can be assigned to the symmetrical stretching vibrations of the Ce‒O bonds in the [CeO₄] structure [14, 30, 31]. For the y(Ce, Cr)_xO₂/Nb₂O₅ catalysts, increasing the (Ce, Cr)_xO₂ content leads to decreases in the intensities of the peaks observed at 325 and 687 cm^–1. When the (Ce, Cr)_xO₂ to Nb₂O₅ mass ratio exceeds 0.25, the characteristic peak of CeO₂ can be clearly observed, and the peak intensity increases with increasing mass ratio. No Raman signal corresponding to Cr₂O₃ is observed, which agrees well with the XRD result. Moreover, the peak at 687 cm^–1 shifts to a lower wavenumber over mixed oxide catalysts, compared to that observed over Nb₂O₅, which indicates the existence of a strong interaction between (Ce, Cr)_xO₂ and Nb₂O₅ [30, 31].

The results of the N₂ adsorption-desorption characterizations are displayed in Fig. 4 and Table 1. It can be seen that the y(Ce, Cr)_xO₂/Nb₂O₅ catalysts are mesoporous materials, as the pore diameters are mainly distributed around 4 and 8 nm. Increasing the (Ce, Cr)_xO₂ content results in larger specific surface areas and pore volumes (see Table 1), as well as broader pore size distributions. In combination with the results of the catalytic performance tests, it can be seen that the activities of the y(Ce, Cr)_xO₂/Nb₂O₅ mixed oxide catalysts are not directly linked to their pore diameters or pore size distributions. Moreover, the BET specific surface areas and the pore volumes of the materials are not consistent with the trend of their catalytic activities. Such results indicate that these textural properties may not be the main factors responsible for the improved catalytic activity of this system.

The SEM images of the samples are displayed in Fig. 5. As shown in Fig. 5(A)–(F), mono-component (Ce, Cr)_xO2 and N₂O₅ presents distinctively different morphologies, as Nb₂O₅ exhibits typical sheet structure with various particle sizes, while (Ce, Cr)_xO2 exhibits irregular spherical morphology with relatively smaller particle sizes of approximately 4 to 7 nm (see Fig. S1.). For y(Ce, Cr)_xO2/Nb₂O₅ catalysts, (Ce, Cr)_xO₂ is well dispersed on the matrix of the Nb2O5 support. As (Ce, Cr)_xO₂ content increases, more agglomerated (Ce, Cr)_xO₂ particles are observed. For the 4(Ce, Cr)_xO₂/Nb₂O₅ catalyst, instead of fully covering the Nb₂O₅ surface, most of the (Ce, Cr)_xO₂ particles still remain dispersed in the mixed oxide system. As shown in Fig. 5(A)–(F), Nb₂O₅ exhibits the typical sheet structure with various particle sizes, whereas (Ce, Cr)_xO₂ exhibits a rice-particle-like structure with relatively smaller particle sizes. For the y(Ce, Cr)_xO₂/Nb₂O₅ catalysts, (Ce, Cr)_xO₂ is "snow-like" dispersed on the matrix of the Nb₂O₅ support. As the (Ce, Cr)_xO₂ content increases, more agglomerated (Ce, Cr)_xO₂ particles are observed. For 4(Ce, Cr)_xO₂/Nb₂O₅ catalyst, instead of fully covering the Nb₂O₅ surface, most of the (Ce, Cr)_xO₂ particles remain dispersed in the mixed oxide system. Therefore, for y(Ce, Cr)_xO₂/Nb₂O₅ catalysts, appropriate contact of (Ce, Cr)_xO₂ and Nb₂O₅ is beneficial for enhancing their interaction. Meanwhile, proper exposure of the Nb₂O₅ particle surface can facilitate the synergistic effect between (Ce, Cr)_xO₂ and Nb₂O₅, which can result in better catalytic performance.

The surface acid properties of the catalysts were characterized by NH₃-TPD measurements. The obtained profiles are shown in Fig. 6. As can be seen in Fig. 6(A), three distinct desorption peaks can be observed for all the y(Ce, Cr)_xO₂/Nb₂O₅ catalysts. The peaks below 150 ℃ are considered to correspond to the desorption of the physically adsorbed NH₃. Two peaks are observed in the range 150–450 ℃. The peak α appears at a lower temperature and is ascribed to the weak acid sites, whereas the peak β is observed at a higher temperature, and is assigned to the strong acid sites [28]. Compared to (Ce, Cr)_xO₂, the peak β obtained over Nb₂O₅ support shifts to a higher temperature, along with an increase in its area, indicating that Nb₂O₅ contains more strong acid sites and exhibits stronger acidity than (Ce, Cr)_xO₂. For the y(Ce, Cr)_xO₂/Nb₂O₅ catalysts, we can see from Fig. 6(B) that the concentration of the weak acid sites increases at a higher (Ce, Cr)_xO₂ to Nb₂O₅ ratio, while the concentration of the strong acid sites and the total acidity decrease, along with the R_s/w values (which represent the ratio of strong acid concentration to weak acid concentration). Similar results have been reported elsewhere [4, 21], as the surface strong acid sites are more likely to be covered by the metal hydroxide deposited during the catalyst preparation.

The correlations between the surface acidity and the TOF values, as well as the maximum C₂H₃Cl concentrations, are shown in Fig. 6(C). It can be seen that the maximum C₂H₃Cl concentrations exhibit a positive correlation with increasing surface acidity. Since C₂H₃Cl is recognized as the byproduct obtained during the dehydrochlorination of DCE, such a correlation reveals that the surface acid sites can promote the adsorption and dehydrochlorination of DCE to C₂H₃Cl. Meanwhile, the revealed order of intrinsic acidity is not consistent with the order observed for the intrinsic catalytic activity, as the TOF values tends to increase first and then decrease with increasing acidity. This result indicates that the surface acid property is not the sole factor determining the performances of the y(Ce, Cr)_xO₂/Nb₂O₅ catalysts.

Our previous study on the (Ce, Cr)_xO₂/zeolite system [21] demonstrated that both the acidic and the redox properties play important roles in the deep oxidation of CVOCs. The acid sites of zeolites (especially the strong acid sites) can promote the adsorption and dehydrochlorination of DCE to C₂H₃Cl, whereas the high oxidizing ability of (Ce, Cr)_xO₂ facilitates the quick oxidation of the organic byproducts to form CO₂. The synergistic effect between the surface acid sites and the oxidation sites contributes to improving the catalytic performance. Although either the strength or the concentration of the strong acid sites over Nb₂O₅ is lower than that over HZSM-5 zeolites, it is still reasonable to assume that a similar synergistic effect exists between (Ce, Cr)_xO₂ and Nb₂O₅ in the y(Ce, Cr)_xO₂/Nb₂O₅ catalysts, as the acid sites of Nb₂O₅ can promote the adsorption and dehydrochlorination of DCE to C₂H₃Cl, while the high oxidizing ability of (Ce, Cr)_xO₂ can facilitate further C₂H₃Cl oxidation. Such synergy can explain why the y(Ce, Cr)_xO₂/Nb₂O₅ catalysts show enhanced intrinsic activities during their performance evaluation.

H₂-TPR experiments were also performed to test the reducibilities of the catalysts. Fig. 7 shows the H₂-TPR profiles of all the y(Ce, Cr)_xO₂/Nb₂O₅ catalysts. For (Ce, Cr)_xO₂, three H₂ consumption peaks are observed: peak α (at ~344 ℃), which can be related to the reduction of Cr⁶⁺ to Cr³⁺; peak γ, which is attributed to the reduction of either surface/subsurface O or small Cr₂O₃ particles; whereas, the peak observed at a high temperature (above 700 ℃) can be assigned to the reduction of CeO₂ lattice O [28]. The redox properties of (Ce, Cr)_xO₂ have been detailly discussed in our previous studied [11, 13, 28]. It is revealed that the formation of small Cr₂O₃ particles can increase the amount of oxygen vacancies in CeO₂, which would contribute to the mobility of active oxygen species and is beneficial for the destruction of the byproducts produced in the process of DCE oxidation. The Nb₂O₅ support also shows two H₂ consumption peaks: peak β (at ~410 ℃), attributed to the reduction of surface/subsurface O, and peak δ (at ~580 ℃), related to the reduction of small Nb₂O₅ particles [29, 32-34]. The reduction peaks of Nb₂O₅ appear at higher temperatures and display smaller areas compared to those of (Ce, Cr)_xO₂, indicating that its reducibility is obviously lower than that of (Ce, Cr)_xO₂. For the y(Ce, Cr)_xO₂/Nb₂O₅ catalysts, as the (Ce, Cr)_xO₂ content increases, the peak δ that corresponds to Nb₂O₅ reduction shifts to a lower temperature and eventually overlaps with the peak γ of (Ce, Cr)_xO₂. This type of peak shift and overlap implies the possible existence of a "Nb^m+-O-Ceⁿ⁺" strong interaction at the (Ce, Cr)_xO₂-Nb₂O₅ interface [35, 36] that can promote the reduction of Nb₂O₅. The total peak area of "γ+δ" increases first and then decreases with increasing (Ce, Cr)_xO₂ to Nb₂O₅ mass ratio. 0.25(Ce, Cr)_xO₂/Nb₂O₅ shows the largest "γ+δ" peak area, which suggests the strongest "Nb^m+-O-Ceⁿ⁺" interaction among all the samples, due to the highly dispersed state of (Ce, Cr)_xO₂ over Nb₂O₅ (as seen in the XRD and UV-Raman results). Similar trends can be seen by relating "γ+δ" peak area sequence to the catalytic activities sequence, suggesting that the redox property promoted by strong "Nb^m+-O-Ceⁿ⁺" interaction is favorable for DCE destruction. Similarities can be observed between the trend in the "γ+δ" peak areas and that of the catalytic activities, suggesting that the redox property is favorable for DCE degradation. Besides, more Cr⁶⁺ species would appear with increasing (Ce, Cr)_xO₂ content, as peak α also grows bigger with increasing (Ce, Cr)_xO₂ content. Our previous studies [11, 13, 14] have demonstrated that the highly oxidizing Cr⁶⁺ species is beneficial for the deep oxidation of the C₂H₃Cl byproduct, and thus, the maximum C₂H₃Cl concentration decreases obviously with increasing (Ce, Cr)_xO₂ to Nb₂O₅ mass ratio. However, more Cr⁶⁺ species does not necessarily mean better activities (see TOF values), which is another clear evidence of the fact that both the redox and the acidic properties contribute to the elimination of DCE and that a synergistic effect between (Ce, Cr)_xO₂ and Nb₂O₅ is essential for improving the catalytic performance.

In summary, from the results detailed above, it can be seen that the synergistic effect between the redox and the acid properties plays an important role in promoting the intrinsic performance of the y(Ce, Cr)_xO₂/Nb₂O₅ catalysts for the deep oxidation of DCE. This synergistic effect is a result of the strong acid sites of the Nb₂O₅ support first promoting DCE adsorption and dehydrochlorination, while the strong oxidation sites of (Ce, Cr)_xO₂ favor the deep oxidation of the reactant, intermediates, and byproducts. Proper selection of the (Ce, Cr)_xO₂ to Nb₂O₅ mass ratio can not only enhance the existing strong metal-support interaction in (Ce, Cr)_xO₂/Nb₂O₅, but also facilitate the synergistic effect. In this study, the 0.25(Ce, Cr)_xO₂/Nb₂O₅ catalyst displays the best activity. Besides, compared to our previous study on the (Ce, Cr)_xO₂/HZSM-5 system, Nb₂O₅ exhibits weaker acid and stronger redox properties than HZSM-5, therefore, less of the byproduct C₂H₃Cl is observed for (Ce, Cr)_xO₂/Nb₂O₅ during DCE elimination.

A series of (Ce, Cr)_xO₂/Nb₂O₅ catalysts with different (Ce, Cr)_xO₂ to Nb₂O₅ mass ratios were synthesized by the deposition-precipitation method and evaluated for the deep oxidation of DCE. Analysis of the structure and the texture of the catalysts reveal that the (Ce, Cr)_xO₂ mixed oxide is highly dispersed in the matrix of Nb₂O₅ support. The results of evaluation of the activities show that an appropriate (Ce, Cr)_xO₂ to Nb₂O₅ mass ratio (which, in this study, is 0.25) can noticeably improve the intrinsic catalytic activity for DCE elimination. Further characterizations suggest that the main reason for the catalytic performance obtained is the synergistic effect that exists between (Ce, Cr)_xO₂ and Nb₂O₅. The strong acid sites of Nb₂O₅ promote the adsorption of DCE and its subsequent dehydrochlorination to form C₂H₃Cl, while the strong oxidizing ability of (Ce, Cr)_xO₂ contributes to the deep oxidation of the reactant, intermediates, and byproducts. Therefore, the synergy is a result of the coexistence of abundant strong acid sites and oxidation sites on the (Ce, Cr)_xO₂/Nb₂O₅ catalysts. The higher catalytic activity and selectivity indicate that the (Ce, Cr)_xO₂/Nb₂O₅ catalysts deserve more research attention and may be promising for industrial application.