NO_x emissions have caused a series of environmental problems in the recent years, including serious harm to our lives and belongings. Stationary source emissions (such as coal-fired power plants) account for ~60% of the total NO_x emissions. Ammonia-selective catalytic reduction (NH₃-SCR) is an effective method to eliminate NO_x from stationary sources, and the denitration catalyst is the key unit for this technique [1]. V₂O₅-WO₃/TiO₂ is the commercial catalyst for the NH₃-SCR process; it exhibits excellent denitration performance between 300 and 400 ℃ [2]. However, this type of catalyst typically suffers from serious deactivation caused by alkali (alkaline-earth) metals, heavy metals, water vapor, and sulfur dioxide during operation [3-5]. For alkali (alkaline-earth) metal poisoning, it has been widely reported that the degree of poisoning was positively related to the alkalinity of the metal, i.e., K > Na > Ca > Mg [6, 7]. The proportion of medium-alkali coal (alkali metal content 0.3%–0.5%) and high-alkali coal (alkali metal content > 0.5%) in China is ~28.94%; however, the traditional V₂O₅-WO₃/TiO₂ catalyst exhibits poor performance resistance to alkali metal poisoning, especially to K poisoning [8]. For example, Kamata et al. [9] reported that the activity of the V₂O₅-WO₃/TiO₂ catalyst was lost almost completely at 350 ℃, when the mass fraction of K₂O is 1%. Therefore, it is paramount to improve the anti-alkali metal poisoning of the V₂O₅-WO₃/TiO₂ catalyst to satisfy applications under a high-alkali metal condition.

The reduction in surface acidity [10, 11] and redox ability [7, 12] are two major factors in the deactivation of NH₃-SCR catalysts. Li et al. [13] reported that the mechanism of alkali (alkali-earth) metal poisoning of the V₂O₅-WO₃/TiO₂ catalyst involved the passivation of surface acid sites and bulk tungsten species, and the suppression and reducibility of active surface oxygen species. Peng et al. [14] conducted a theoretical study regarding the effects of alkali poisoning on vanadium-based catalysts based on DFT calculations. The results indicated that doping the alkali metal could lower the reducibility of the active component V₂O₅, and could decrease surface acidity.

Ceria has received considerable attention recently in NH₃-SCR reactions because of its excellent redox properties. Some researchers have reported Ce⁴⁺-modified vanadium-based catalysts for NH₃-SCR reactions, such as V₂O₅-Ce(SO₄)₂/TiO₂, VO_x/Ce_xZr_1-xO₂, and V₂O₅-CeO₂/TiO₂ catalysts [1, 15, 16]. These researchers suggested that Ce⁴⁺ addition could not only promote the surface acidity and redox ability of the catalysts, but also enhance their alkali metal resistance.

Zirconia is an acid-based amphoteric oxide used widely as a support and co-catalyst, e.g. WO₃-ZrO₂, ZrO₂-SO₄^2-, MnO_x-ZrO₂, and CeO₂-ZrO₂ support [17-21]. Additionally, CeO₂-ZrO₂ is a good support for the NH₃-SCR reaction. Vuong et al. [16] reported V₂O₅ supported on CeO₂-ZrO₂ is better than that on pure CeO₂ or ZrO₂ support. This is likely related to the -O-Ce-O-V(=O)-O-Zr-O- moiety, which may benefit oxygen transport. Shen et al. [22] explored the influence of Ce-Zr ratios on Mn/Ce-Zr catalysts, and found that the presence of ZrO₂ in the catalyst enhanced the BET specific surface area and surface acidity efficiently. Furthermore, researchers have demonstrated that Zr⁴⁺ addition can improve the thermal stability of the catalyst [23, 24]. For example, Shi et al. [24] reported that the introduction of Zr⁴⁺ efficiently increased the high-temperature activity and N₂ selectivity of the V₂O₅-WO₃/TiO₂ catalyst, because Zr⁴⁺ inhibited the shrinking of catalyst surface area and growth of the TiO₂ crystallite size at high temperatures.

To the best of our knowledge, modifying with Ce⁴⁺ can enhance the tolerance of alkali metals, and adding Zr⁴⁺ can improve the thermal stability, specific surface area, and oxygen transport performance. However, the V₂O₅-WO₃/TiO₂ catalyst modified by co-doping of Ce⁴⁺ and Zr⁴⁺ to improve the denitration performance and K-poisoning resistance has not yet been reported. Based on the V₂O₅-WO₃/TiO₂ catalyst, herein we successfully synthesized a novel V₂O₅-WO₃/TiO₂-CeO₂-ZrO₂ catalyst for the selective catalytic reduction of NO_x with NH_3. It demonstrates good denitration performance and excellent anti-K poisoning performance. Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption of NH₃ (NH₃-TPD), and temperature-programmed reduction of H₂ (H₂-TPR) were used to characterize both fresh and poisoned catalysts. Moreover, the NH₃-SCR mechanism over these catalysts was explored through in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technology.

The TiO₂-CeO₂-ZrO₂ support with a molar ratio of Ti:Ce:Zr = 9:0.5:0.5 was prepared by a co-precipitation method. First, the required amounts of TiCl₄, Ce(NO₃)₃‧6H₂O and Zr(NO₃)₄‧5H₂O were dissolved with ultrapure water to form a solution at room temperature. Next, excess ammonia was added to the solution with vigorous stirring until pH = 10, and the resulting solution was again stirred for 3 h, followed by aging for 24 h. The precipitate was subsequently filtered and washed several times with ultrapure water until a pH change was not observed (pH = 7, measured by pH test paper) and no Cl^– appeared (detected by the solution of AgNO₃); the obtained cakes were dried overnight at 110 ℃, and were finally calcined at 450 ℃ for 5 h.

The V₂O₅-WO₃/TiO₂-CeO₂-ZrO₂ catalyst was prepared by the impregnation method. The V₂O₅ was 1 wt% and the WO₃ was 5 wt%. Ammonium metavanadate and ammonium tungstate were mixed in the oxalic acid solution in the desired proportion. The appropriate amount of TiO₂-CeO₂-ZrO₂ support was impregnated by stirring for 1 h, dried overnight at 110 ℃, and subsequently calcined at 450 ℃ for 5 h. For comparison, the V₂O₅-WO₃/TiO₂, V₂O₅-WO₃/TiO₂-ZrO₂, and V₂O₅-WO₃/TiO₂-CeO₂ catalysts with a molar ratio of Ti:Ce (or Zr) = 9:1 were synthesized using the same method.

The K-poisoned catalysts were prepared using the wet impregnation method with a KNO₃ solution (K₂O was 1 wt%), subsequently evaporated at 110 ℃ using an oil bath and calcined at 450 ℃ for 5 h. Herein, we denoted the V₂O₅-WO₃/TiO₂, V₂O₅-WO₃/TiO₂-ZrO₂, V₂O₅-WO₃/TiO₂-CeO₂, and V₂O₅-WO₃/TiO₂-CeO₂-ZrO₂ catalysts as VWT, VWTZ, VWTC, and VWTCZ, respectively, and the K-poisoning V₂O₅-WO₃/TiO₂, V₂O₅-WO₃/TiO₂-ZrO₂, V₂O₅-WO₃/TiO₂-CeO₂, and V₂O₅-WO₃/TiO₂-CeO₂-ZrO₂ catalysts as KVWT, KVWTZ, KVWTC, and KVWTCZ, respectively, for simplicity.

Activity and H₂O + SO₂ tolerance were measured in a quartz reactor using a 200-mg catalyst (40–60 mesh). The feed gas mixture contained 500 ppm NO, 500 ppm NH₃, 5% O₂, 5% H₂O (when used), 100 ppm SO₂ (when used), and N₂ as the balance gas. The gas hourly space velocity was 60000 mL‧g^–1‧h^–1. The concentrations of NO_x, NH₃, and N₂O were detected by a flue gas analyzer, NH₃ analyzer, and N₂O analyzer, respectively. The NO_x conversion of the catalysts and N₂ selectivity were determined from the following equations:

(1)

(2)

The XRD patterns of the samples were collected on the Philips X'Pert3 Powder diffractometer between 10° and 90° using a Ni-filtered Cu K_α radiation. The scan rate is 10°/min, while the step size is 0.02°. The operating voltage and current of the X-ray tube are 40 kV and 40 mA, respectively.

N₂ adsorption-desorption isotherms were measured with a Belsorp-max analyzer. Prior to each measurement, samples were degassed under vacuum at 300 ℃ for 4 h. The specific surface areas of the samples were calculated by the BET method.

The temperature-programmed desorption of NH₃ (NH₃-TPD) and the temperature-programmed reduction of H₂ (H₂-TPR) were performed using a chemisorption analyzer (TP-5076). For NH₃-TPD, the sample first was exposed to 1% NH₃-He gas flow for 1 h at 100 ℃, and was flushed under He for 1 h; subsequently, it was desorbed under He gas flow at a rate of 10 ℃/min up to 700 ℃. For H₂-TPR, the sample was subjected to a 5% H₂-N₂ gas flow at a rate of 10 ℃/min, up to 900 ℃. The gas flow rate was set as 30 mL/min; each sample was pretreated at 300 ℃ in He for 1 h before testing.

X-ray photoelectron spectroscopy (XPS) was performed using a PHI 5000 VersaProbe system with monochromatic Al K_α radiation (1486.6 eV), and an accelerating power of 15 KW. All the binding energies were calibrated by the C 1s peak at 284.6 eV.

The in situ DRIFTS experiments were performed on the Nicolet 5700 FT-IR spectrometer. The sample was pretreated with a high-purified N₂ stream at 450 ℃ for 1 h to remove the absorbed impurities. The sample background of each target temperature was collected during the cooling process. The sample was exposed to a controlled stream-of-reaction atmosphere at a rate of 50 mL/min for 1 h to be saturated at the ambient temperature. Further, the spectra of the adsorbed species, by subtracting the corresponding background reference, were subsequently collected as the sample was heated from room temperature to 450 ℃ at a rate of 10 ℃/min.

The NH₃-SCR performance of various catalysts measured at temperatures ranging from 100 to 500 ℃ is shown in Fig. 1. The activity of all the catalysts displays a volcano curve with increasing reaction temperature. The activity can be enhanced significantly with the addition of Ce⁴⁺ and Zr⁴⁺, and the reaction temperature window has been expanded. In the temperature range of 300–400 ℃, which is the operating temperature window of vanadium-based catalysts, more than 97% of NO_x conversion can be achieved. The VWT and VWTZ catalysts are completely inactivated when K is introduced, but the VWTCZ catalyst still exhibits the best activity of approximately 75%, which is also higher than that of the VWTC catalyst. Peng et al. [14] synthesized the V-CeW/Ti catalyst to improve the anti-alkali performance of VW/Ti catalysts. They found that when 1 wt% K was introduced, the V-CeW/Ti catalyst exhibited better catalytic activity than VW/Ti catalyst; however, only approximately 40% NO conversion can be achieved. Shen et al. [25] reported that the addition of SiO₂ can improve the anti-K poisoning performance of the VWT catalyst. When K with a K/V molar ratio of 0.5 was introduced, the anti-K poisoning performance of the VWT catalyst was enhanced after the introduction of SiO₂; however, only 68% of NO can be removed. Compared with these catalysts, the VWTCZ catalyst prepared in the present work exhibits better anti-K poisoning performance, thereby indicating its potential to be used for the denitration of flue gas containing K. One-hundred percent N₂ selectivity is achieved for all the modified catalysts (because KVWT and KVWTZ catalysts are completely inactivated, we no longer discuss their N₂ selectivity and tolerance of H₂O and SO₂ herein). The results above indicate that the co-existence of Ce⁴⁺ and Zr⁴⁺ is crucial for improving the anti-K poisoning performance of the VWT catalyst, and that some synergetic effects occur between them.

The flue gas contains H₂O and SO₂, which are known to suppress NO_x conversion. We explored the H₂O + SO₂ tolerance of those catalysts at 350 ℃, and the results are as shown in Fig. 2. The NO_x conversion of these catalysts is highly stable within 120 min before the introduction of H₂O and SO₂, revealing that the NH₃-SCR reaction reaches a steady state. The activity decreases by approximately 20% from 120 to 600 min with the introduction of H₂O and SO₂; this result is caused by the competitive adsorption between the reactants and H₂O + SO₂, the generation of surface hydroxyl groups, and the deposition of sulfates [26-28]. Finally, the NO_x conversion recovers partially after eliminating H₂O and SO₂ in the last 120 min, thus suggesting that the deactivation of these catalysts suffers from reversible and irreversible deactivations. Interestingly, the activity of the VWTCZ catalyst is still as high as 75% (15% higher than the VWT catalyst), providing evidence that the catalyst co-doped by Ce⁴⁺ and Zr⁴⁺ demonstrates outstanding resistance toward water and sulfur.

The XRD results of the obtained samples are shown in Figs. 3 and 4. As shown in Fig. 3, only the diffraction peaks assigned to anatase TiO₂ can be observed in the XRD patterns of all the supports [PDF-ICDD 21-1272]. We observed that the peaks of all supports slightly shifted to a low-angle direction when compared with pure TiO₂ (as shown in the partial enlarged view in Fig. 3), thus providing evidence that Ce⁴⁺ and Zr⁴⁺ are incorporated into the lattice of TiO₂. This further causes lattice expansion owing to the radius of Ce⁴⁺ (0.92 Å ) and Zr⁴⁺ (0.86 Å ), which is larger than that of Ti⁴⁺ (0.68 Å ) [29, 30]. It is noteworthy that an obvious decrease in the intensity of the diffraction peaks occurred in the modified supports when compared with pure TiO₂. Moreover, the full-widths at half maximum height of the TiO₂-ZrO₂, TiO₂-CeO₂, and TiO₂-CeO₂-ZrO₂ supports are 0.458, 0.604, and 0.563, respectively (calculated from the strongest diffraction peak around 2θ = 25.3°), and are larger than that of pure TiO₂ (0.399). These results demonstrate that the doping of Ce⁴⁺ and Zr⁴⁺ can inhibit the growth of TiO₂ grains effectively. The decrease in the grain size leads to an increase in the specific surface area, thus promoting the dispersion of the active species and enhancing the catalyst activity. No new crystal phases are detected when V₂O₅, WO₃, and K₂O are introduced (Fig. 4), thereby indicating that V₂O₅, WO₃, and K₂O are distributed in an amorphous state and/or a highly dispersed state on the surface of these supports.

The textural properties of these supports were measured by BET, and the corresponding results are displayed in Table 1. The specific surface area and the total pore volume of the supports are increased after Ce⁴⁺ and Zr⁴⁺ doping, and the catalyst co-doped by Ce⁴⁺ and Zr⁴⁺ has the largest specific surface area and total pore volume, which is consistent with the XRD results. The larger specific surface area is conducive to the dispersion of the active species, and the increase in the total pore volume assists with the full contact of the reactant molecules with the catalyst, as well as the diffusion of reactant molecules and product molecules, thus resulting in excellent catalytic performance.

The surface acidity of the catalysts was tested by NH₃-TPD, which is recognized as a key factor for the NH₃-SCR reaction [28]. Fig. 5 exhibits the NH₃-TPD curves of the fresh and K-poisoning catalysts. Three desorption peaks (i.e., I, II, and III) for the VWT catalyst appeared, resulting from the desorption of physisorbed NH₃ and from the desorption of chemisorbed NH₃ on weak and medium-strong acid sites, respectively [30]. A new peak is detected when Ce⁴⁺ and Zr⁴⁺ are co-doped, and is related to the strong acid (labeled as IV). Moreover, the amount of chemisorbed NH₃ is increased significantly. There is an obvious decrease after K addition, especially for the VWT and VWTZ catalysts, which is primarily owing to the combination of K with vanadium to form a V-O-K species that reduces surface acidity by hindering the adsorption of NH₃ on Lewis or Brønsted acid sites. Notably, the acid amount of the VWTCZ catalyst is best retained after K poisoning, and the retention is 59%, which is much higher than that of other catalysts (Table 2). Peng et al. [14] reported that the contact between the K species and the CeO₂ surface is bonded easily to form a Ce-O-K structure, thereby reducing the adsorption of K on the active V species, based on the DFT study. It is well known that Zr⁴⁺ can increase the thermal stability of Ce⁴⁺, which is beneficial in providing more Ce⁴⁺ to combine with the K species. Therefore, the active V species can be protected better by the co-doping of Ce⁴⁺ and Zr⁴⁺. Consequently, the results of the NH₃-TPD indicate that the addition of Ce⁴⁺ and Zr⁴⁺ could not only increase the surface acidity of the VWT catalyst, but also effectively reduce the impact of K poisoning.

The redox properties of various catalysts were detected by H₂-TPR, which is another important factor for the NH₃-SCR reaction [26]. As shown in Fig. 6, two primary reduction peaks are observed for all the catalysts at 400–650 ℃. For the VWT catalyst, the peak centered at 480 ℃ can be ascribed to the reduction of V⁵⁺→V³⁺; the peak centered at 548 ℃ can be assigned to the reduction of W⁶⁺→W⁴⁺ [31-33]. The stronger reduction peaks of the VWTCZ catalyst is caused by the good dispersion of V₂O₅ and WO₃ species on the surface of TCZ support, as well as the overlapped reduction of surface Ce⁴⁺ to Ce³⁺ [34]. According to literature [35-37], we found that the improvement in the redox properties for catalysts is beneficial to oxygen migration, such as the transfer of lattice oxygen to form surface oxygen in CeO₂, and that the surface oxygen can be easily reduced to generate oxygen vacancy, thereby promoting the formation of surface-chemisorbed oxygen species. As is known, the surface chemisorbed oxygen species can oxidize NO to NO₂ effectively, thereby improving the catalytic performance through a "fast NH₃-SCR" route. With the addition of K, we found that the reduction peaks shift to higher temperatures, thereby demonstrating that K poisoning lessens the reducibility of the primary active species (V and W oxides). However, regarding the Ce⁴⁺ and Zr⁴⁺ co-doped catalysts, the effect of K poisoning on the peak temperature is the smallest (29 and 13 ℃, respectively). Furthermore, we calculated the H₂ consumption of these catalysts before and after K poisoning. It is found that the H₂ consumption of VWTCZ is greater than that of the VWT catalyst, and the retention of the KVWTCZ is the largest (Table 3). These results suggest that the redox ability can be protected more effectively by co-doping with Ce⁴⁺ and Zr⁴⁺. In summary, Ce⁴⁺ and Zr⁴⁺ co-doping can enhance the redox property of the VWT catalyst, and inhibit the decrease caused by K poisoning effectively.

XPS measurements were performed to better understand the chemical state of vanadium on the surface of these V-based catalysts, and the results are presented in Fig. 7. According to literature, the binding energies of V⁵⁺, V⁴⁺, and V³⁺ are in the ranges of 516.4–517.2, 515.7–516.2, and 515.2–515.7 eV, respectively [38, 39]. The ratio of V⁵⁺ is 38.0% (Table 4) for the VWTCZ catalyst, which is higher than that of the VWT catalyst. When K is introduced, the V⁵⁺ ratio of the catalysts decreases, especially in the VWT catalyst (decrease from 36.8% to 20.0% in Table 4). Topsoe et al. [40] reported that the decrease in V⁵⁺ caused the deterioration in the redox properties of the catalyst. Therefore, the addition of K leads to decreased activity. However, the reduction in the VWTCZ catalyst is slight (decrease from 38.0% to 34.4%, as shown in Table 4), indicating that the co-doping of Ce⁴⁺ and Zr⁴⁺ could effectively reduce the effect of K on the redox properties of the catalyst. This result is consistent with the results of H₂-TPR.

We performed in situ DRIFTS experiments to explore the further interaction between the catalyst and the reaction molecules, as well as the influence of Ce⁴⁺ and Zr⁴⁺ co-doping and K poisoning. Finally, an NH₃-SCR reaction mechanism on the VWTCZ catalyst was proposed.

The in situ DRIFT spectra of NH₃ adsorption over the fresh and K-poisoning catalysts as a function of temperature are given in Fig. 8. For the VWT catalyst, the bands at 1683, 1605, and 1193 cm^–1 at room temperature (25 ℃) can be attributed to the asymmetric bending vibration of N-H in coordinated NH₃ that is adsorbed on Lewis acid sites (labeled as L acid) [41-43]. The band at 1459 cm^–1 can be assigned to a symmetric bending vibration of the NH₄⁺ species on Brønsted acid sites (labeled as B acid) [44]. With the increased temperature, the bands from the B acid disappear completely at 200 ℃; however, the bands of L acid remain, even at 450 ℃. This result demonstrates that the L acid is more stable than the B acid. A new band is detected at 1428 cm^–1 at 200 ℃, which is related to the adsorbed -NH₂ species [42].

The band at 1622 cm^–1 can be related to the surface O-H stretching vibration modes when Ce⁴⁺ and Zr⁴⁺ are introduced [45]. The band at 1156 cm^–1 can be attributed to the L acid, and a new band for the adsorption of NH₃ on the L acid sites appears at 1560 cm^–1 [42]. The intensity of the B acid and L acid over the VWTCZ catalyst are stronger than that of the VWT catalyst at all temperatures. After the K addition, the intensity of the L acid and B acid over the KVWT catalyst are weakened; however, the KVWTCZ catalyst is maintained effectively. These observations demonstrate that the co-doping of Ce⁴⁺ and Zr⁴⁺, as well as the amounts of acid sites on the catalyst surface, can increase the strengths of the L acid and B acid; K poisoning will reduce the surface acidity of the catalyst and affect the adsorption of NH₃, while Ce⁴⁺ and Zr⁴⁺ co-doping can effectively reduce the effect of K on the surface acidity of the catalyst. The results are in good agreement with those of NH₃-TPD.

Fig. 9 displays the in situ DRIFT spectra of surface-adsorbed species resulting from the co-adsorption of NO + O₂ at different temperatures over the fresh and K-poisoning catalysts. For the VWT catalyst, the bands at 1632 and 1292 cm^–1 can be attributed to the adsorbed NO₂ molecules and linear nitrites [41, 42]. When Ce⁴⁺ and Zr⁴⁺ are doped, the intensity of the adsorbed NO₂ molecules and linear nitrites are strengthened. At the same time, two new bands appeared at 1581 and 1233 cm^{– 1}, which are related to the bidentate nitrates and the chelating bidentate nitrates [42, 46]; this result is caused by the oxidation of NO to NO₂. Several new bands are observed at 1598, 1511 and 1380 cm^–1 when K is introduced, and they are assigned to the monodentate nitrates, bidentate nitrates and cis-N₂O₂^2– species, respectively [42, 47, 48]. However, the bands at 1632 and 1233 cm^–1 are weakened. This result shows that the addition of K reduces the ability of the catalysts to oxidize NO to NO₂, while NO₂ is the key factor for the "fast NH₃-SCR" route. These phenomena indicate that the doping of Ce⁴⁺ and Zr⁴⁺ can enhance the adsorption and oxidation of NO to form different types of nitrates; K poisoning increases the adsorption of NO but reduces the redox properties of the catalysts and attenuates the oxidation of NO to NO₂. However, the addition of Ce⁴⁺ and Zr⁴⁺ can reduce this effect effectively, and the results are consistent with those of H₂-TPR.

The VWTCZ catalyst demonstrates excellent physicochemical properties, good H₂O + SO₂ tolerance, outstanding catalytic performance, and optimal anti-K-poisoning performance. NO + O₂ + NH₃ co-adsorption was measured by in situ DRIFTS to reveal the reaction mechanism, and the results are presented in Fig. 10. The bands of bridging nitrates (1638 and 1216 cm^–1), bidentate nitrates (1600 cm^–1), L acid (1683 cm^–1), and B acid (1442 cm^–1) are observed when the VWTCZ catalyst is exposed to the mixed gases [41, 42, 44, 45]. These results demonstrate that both NO and NH₃ are absorbed simultaneously on the surface of the VWTCZ catalyst, thereby revealing that the selective catalytic reduction of NO by NH₃ over the catalyst follows the Langmuir-Hinshelwood (L-H) mechanism. Furthermore, compared with the in situ DRIFTS results of NH₃ adsorption, the bands of the L acid and B acid are stronger in the mixture gases because the presence of NO + O₂ increases the intensity of acid sites on the surface of the catalyst. The bands of the adsorbed NO_x and adsorbed NH₃ species still appeared at the corresponding positions for the KVWTCZ catalyst, thus suggesting that K poisoning does not change the reaction mechanism. However, the intensity of these bands is weakened slightly owing to K poisoning.

In this work, a novel V₂O₅-WO₃/TiO₂-CeO₂-ZrO₂ catalyst was synthesized, and exhibited outstanding catalytic performance and excellent anti-K poisoning performance at 300–400 ℃. By exploring the effects of Ce⁴⁺ and Zr⁴⁺ co-doping as well as K-poisoning on the physicochemical properties of V₂O₅-WO₃/TiO₂ catalyst, several conclusions were obtained, as follows: (1) Ce⁴⁺ and Zr⁴⁺ species could be doped into the lattice of TiO₂ to inhibit the grain growth. This led to the increase in the BET specific surface area and total pore volume. (2) For Ce⁴⁺ and Zr⁴⁺ co-doping, the surface acidity and redox properties were enhanced, and the effect of K-poisoning on the surface acidity and the redox properties of the catalyst were reduced effectively. This may be due to the combination of Ce⁴⁺ with K to form Ce-O-K, while Zr⁴⁺ could increase its thermal stability, allowing more Ce⁴⁺ to bind with K, thus protecting the active V⁵⁺ species. (3) NO, O₂, and NH₃ could interact with the VWTCZ catalyst simultaneously, indicating that the NH₃-SCR reaction over VWTCZ catalyst followed an L-H mechanism. Furthermore, K-poisoning did not change the reaction mechanism.