Effect of Sn Doping on Alkali Metal Resistance of V/UiO-66 Catalyst

Abstract

V/UiO-66 catalyst and modified Sn-V/UiO-66 catalyst were prepared by impregnation method with UiO-66 as carrier, and their potassium poisoning was simulated. The reaction and deactivation mechanisms of vanadium-titanium catalyst were analyzed. Results demonstrate that the crystallographic form of catalysts barely changes after K-loading, and the specific surface area of the catalysts has an irregular fluctuation. The catalyst activity is decreased rapidly after the alkali metal was loaded due to the degraded metal redox performance and the rapidly decreased surface acid content of the catalysts. The addition of Sn can enhance the interaction between VO_x and other components, which thereby increases the V⁵⁺ content and reducibility of VO_x. Thus, VO_x on the surface of Sn-V/UiO-66 catalysts can provide more acid sites. The poisoned K-Sn-V/UiO-66 catalyst still has high total acid content and strong redox property, presenting the high resistance of Sn-V/UiO-66 catalyst against alkali metal poisoning.

Keywords

UiO-66 catalyst; catalytic denitration; NH₃-SCR; modification

Nitrogen oxides are one of the primary sources of air pollution caused by the combustion of fossil materials^[1], which causes environmental problems, such as photochemical smog, acid rain, greenhouse effect, and serious health problems^[2‒5]. Therefore, reducing nitrogen oxides becomes the primary goal of environmental protection. Currently, the ammonia selective catalytic reduction (NH₃-SCR) is a mature and the most widely used method for stationary source denitrification^[6‒8]. Through catalyst, the reducing agent converts the nitrogen oxides in gas into harmless nitrogen and water. The cost of catalysts usually accounts for 30%‒40% of the total cost.

In the selective catalytic reduction (SCR) reactor, there are two main types of arrangement: high-dust and low-dust. In the high-dust environment, SCR reactor is placed at the downstream of the economizer and the upstream of the air preheater with the flue gas temperature of approximately 350 °C. In the low-dust environment, SCR reactor is located behind the desulfurization and dust removal devices. The most widely used high-dust SCR catalyst is V₂O₅-MoO₃(WO₃)/TiO₂ because its active temperature is 300‒400 °C^[9‒10]. Neverthe-less, the high-dust environment contains many toxic substances, including Na, K, Ca, and As, which can poison the catalysts and degrade their performance.

The poisoning effects of alkali metals on catalysts have been extensively researched. Generally, it is believed that Na and K poisoning is a chemical poisoning: Na and K occupy the acid sites on the catalyst surface and reduce the NH₃ adsorption capacity, thereby reducing SCR activity^[11‒13]. CaO poisoning, as a physical poisoning, is caused by the generation of CaSO₄ and the blocking of catalyst micropores^[14‒15]. Among the alkali metals in the flue gas, K has the most severe effect on the catalyst. Therefore, it is necessary to improve the anti-alkali metal poisoning ability of catalysts and to prolong the service life of catalysts. The provision of more sacrificial acid sites and the establishment of capture sites to protect active sites are the main solutions^[16‒18]. It is demonstrated that the Sn oxides exhibit unique electrochemical and chemical properties, which can improve the redox performance of catalysts at low temperatures and control the acidity of catalyst substrates^[19].

The active substance Sn was added to the V/UiO-66 catalyst in this research, and the anti-alkali metal poisoning ability of catalysts was improved by increasing the acid sites and enhancing the surface oxidation performance.

1 Experiment

An appropriate amount of ammonium metavanadate (NH₄VO₃) was dissolved in hot water at 90 °C. Then the monoethanolamine was added dropwise until a transparent solution was obtained. When the mixture was cooled down to room temperature, UiO-66 without and with stannous sulfate was separately added into the solution and mixed thoroughly. After drying at 105 °C for 12 h and calcining at 350 °C for 3 h, the catalysts with 3wt% V₂O₅/UiO-66 and those with 1wt% SnO₂/UiO-66 were obtained, which were named as V/UiO-66 and V-Sn/UiO-66 catalysts, respectively.

The impregnation method was used to simulate the K-poisoning of catalysts. The K₂SO₄ solution with molar ratio of K:V=2:1 was prepared. The prepared catalysts were immersed in K₂SO₄ solution for 5 h, then dried at 105 °C for 8 h, and calcined at 350 °C for 4 h. The poisoned catalysts were named as K-V/UiO-66 and K-V-Sn/UiO-66 catalysts.

The X-ray diffractometer (XRD, D8 advanced XRD by Bruker, Germany) was used for catalyst characterization. The current was 40 mA, the voltage was 40 kV, Cu Kα was used as the radiation source, and the scanning range was 2θ=10°~80° with the scanning step of 0.02°.

The N₂ physical adsorption test of the catalyst was performed by the Autosorb-iQ physical adsorption instrument (Quantachrome Company, United States). The catalyst specimens of 0.25‒0.3 g were pretreated by vacuuming at 240 °C for 2 h and then placed in liquid nitrogen (-196 °C) for tests. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method.

The H₂-temperature programmed reduction (TPR) of the catalysts was conducted by Auto Chem II 2920 Chemisorber (Mack Co., Ltd). The specimen (50 mg, particle size of 0.30‒0.45 mm) was pretreated in pure Ar atmosphere at 300 °C for 0.5 h. The temperature rose from 50 °C to 800 °C at a heating rate of 10 °C/min in the atmosphere of 10vol% H₂/Ar with flow rate of 20 mL/min. The thermal conductivity detector (TCD) was used to measure the consumption of H₂ in this process.

The NH₃-temperature programmed desorption (TPD) of the catalysts was conducted by Auto Chem II 2920 Chemisorber (Mack Co., Ltd). The specimen (100 mg, particle size of 0.30‒0.45 mm) was pretreated in a pure He atmosphere (flow rate of 20 mL/min) at 400 °C for 0.5 h. After cooling to 50 °C, 5vol% NH₃/N₂ with flow rate of 20 mL/min was introduced to adsorb NH₃ for 0.5 h. Then, the pure He was purged for 1 h. Finally, the temperature rose to 500 °C with heating rate of 10 °C/min. TCD was used to measure the desorbed NH₃ content during this process.

The catalysts were characterized by X-ray photoelectron spectroscope (XPS, Thermo Scientific ESCALAB 250Xi). The vacuum degree of the analysis chamber was 8×10^-10 Pa, the excitation source was Al Kα (hv=1253.6 eV), the working voltage was 12.5 kV, the filament current was 16 mA, and the signal accumulation was conducted for 10 cycles.

The activity evaluation of the catalyst was conducted in a stainless-steel SCR fixed reactor with external heating in a tube furnace. The schematic diagram of experiment setup is shown in Fig.1. The simulated gas composition was 0.05vol% NO, 0.05vol% NH₃, 6.00vol% O₂, and N₂ (equilibrium carrier gas). The catalyst amount was 5 mL, the total gas flow was 1000 mL/min, and the space velocity was 12 000 h^-1. The catalyst activity was evaluated at 100‒340 °C, and the NO content before and after the reaction was detected by German Testo 350 flue gas analyzer. The NO removal rate η is calculated by Eq.(1), as follows:

$$\eta =\frac{V_{\mathrm{N}\mathrm{O}-\mathrm{i}\mathrm{n}}-V_{\mathrm{N}\mathrm{O}-\mathrm{o}\mathrm{u}\mathrm{t}}}{V_{\mathrm{N}\mathrm{O}-\mathrm{i}\mathrm{n}}}\times \mathrm{100}\mathrm{\%}$$

(1)

Fig.1  Schematic diagram of experiment device for catalyst activity evaluation

where V_NO-in is the volume of ingoing NO gas and V_NO-out is the volume of outgoing NO gas.

2 Results and Discussion

2.1 XRD analysis of catalysts

Fig.2 illustrates XRD patterns of different catalysts. Typical diffraction peaks of tetragonal ZrO₂ can be observed at 2θ=29.7°, 34.6°, 49.6°, and 59.2° of all catalysts, and the corresponding crystal plane are (003), (012), (104), and (113), respectively^[20]. The diffraction peaks of vanadium oxide and tin oxide cannot be observed. There are three possible reasons for this phenomenon: (1) the contents of vanadium oxide and tin oxide are low; (2) the diffraction peaks of vanadium oxide and tin oxide are covered by ZrO₂ diffraction peaks; (3) the vanadium oxide and tin oxide are uniformly dispersed on the carrier surface in the amorphous. Comparing the V/UiO-66 and Sn-V/UiO-66 catalysts, it can be seen that the peak intensity of the crystal phase is weakened after Sn addition into the V/UiO-66 catalyst, indicating a strong interaction between the active component and the substrate. Thus, Sn⁴⁺ is distributed randomly in the ZrO₂ lattice, reducing the crystallinity and size of ZrO₂ carrier particles. Furthermore, the number of electrons and oxygen vacancies increases, which also improves the reactions^[21].

Fig.2  XRD patterns of different catalysts

The diffraction peak of K₂SO₄ cannot be observed in the K-V/UiO-66 and K-Sn-V/UiO-66 catalysts, indicating that the K content is low. In addition, K element is mainly distributed in a monolayer state and highly dispersed on the catalyst carrier surface. Besides, the diffraction angle and peak intensity barely change after K₂SO₄ is loaded, which suggests that the alkali metal loading does not significantly impact the crystal structure of catalyst. Therefore, the reduction in catalyst denitration activity after K-poisoning is not directly related to the crystal phases of catalysts.

2.2 BET analysis of catalysts

Table 1 shows the BET specific surface area of V/UiO-66, Sn-V/UiO-66, K-V/UiO-66, and K-Sn-V/UiO-66 catalysts. The Sn-V/UiO-66 catalyst has the specific surface area of 220.87 m²·g^-1, which is approximately 2.4 times greater than that of the V/UiO-66 catalyst (93.16 m²·g^-1), indicating that the Sn addition significantly increases the specific surface area of catalyst. It is also observed that the Sn⁴⁺ gathers at the grain boundary of UiO-66. It is worth noting that due to the similar ionic radii of Sn⁴⁺ (0.069 nm) and Zr⁴⁺ (0.072 nm), a large number of Sn ions are inserted into the UiO-66 lattice, replacing some Zr⁴⁺ ions and thereby forming a novel skeleton structure. The smaller radius of Sn⁴⁺ ions results in the inhabitation of particle growth by reducing the grain boundary energy. Consequently, the specific surface area of specimen increases after Sn doping^[22]. The NH₃-SCR reaction occurs at the catalyst surface. It is believed that the high specific surface area of the catalyst can provide more active adsorption centers for the reaction molecules and intermediate products in the gas-solid reaction, which effectively promotes the phase transfer, therefore enhancing the denitrification activity of the catalysts^[23].

Table 1  BET specific surface area of different catalysts (m²·g^-1)

V/UiO-66	Sn-V/UiO-66	K-V/UiO-66	K-Sn-V/UiO-66
93.16	220.87	95.99	165.80

It is found that after K-poisoning, the specific surface area of catalyst becomes smaller, because the alkali metal salt particles are distributed on the catalyst surface and some pores are blocked. The specific surface area of the V/UiO-66 catalyst is 93.16 m²·g^-1, whereas that of the K-V/UiO-66 catalyst is 95.99 m²·g^-1, indicating that the specific surface area is slightly increased by the K-poisoning. However, for the Sn-V/UiO-66 catalyst, the specific surface area is decreased from 220.87 m²·g^-1 to 165.80 m²·g^-1 after K-poisoning. This result suggests that the K-poisoning leads to the reduction in specific surface area of catalysts. It can be observed that the specific surface area of the catalyst fluctuates after K loading^[24]. However, the catalyst activity does not change significantly.

2.3 H₂-TPR analysis of catalysts

H₂-TPR tests were used to characterize the effect of alkali metals on the redox ability of catalysts. Fig.3 shows the H₂-TPR curves of different catalysts. It is known that the dispersed and multimeric vanadium oxides undergo a multi-step reduction process of V⁵⁺→V⁴⁺→V³⁺→V²⁺ at 400‒600 °C. The superimposed reduction peaks of vanadium oxides occur at 400 and 550 °C^[25]. Compared with the V/UiO-66 catalyst, the K-Sn-V/UiO-66 catalyst exhibits a greater reduction peak area and its peak shifts towards the lower temperature area. With Sn addition, the interaction between Sn and Zr increases the redox ability of the catalyst and facilitates the reduction of V-O component. The redox properties of catalysts play a significant role. In addition, the lower the reduction initiation temperature of the catalyst, the stronger the redox ability of the catalyst^[26]. Combined with the NH₃-SCR results of catalyst activity, it can be seen that the Sn-V/UiO-66 catalyst with strong redox performance exhibits better catalytic activity than the V/UiO-66 catalyst with poor redox performance does. This result suggests that the redox property determines the catalyst activity in a significant manner.

Fig.3  H₂-TPR curves of different catalysts

Compared with the V/UiO-66 catalyst, the center temperature of the reduction peak shifts to the higher temperature area for the K-V/UiO-66 catalyst. According to Ref.[27], the catalyst reducibility depends on the grain arrangement and grain structure of catalyst. The K-poisoning causes the rearrangement of crystals due to the interactions between K and complex oxides. The poor alignment state results in the poor reducibility of catalyst, as indicated by the shifting of the peak temperature towards the high temperature area. This phenomenon also infers that the activity of surface oxides is significantly reduced after K-poisoning. Thus, the VO_x can hardly be reduced and the redox performance of catalyst is reduced, which affects the SCR process, such as NH₃ activation and NO oxidation. Therefore, the Sn doping can improve the redox performance of V/UiO-66 catalyst and effectively protect the catalyst from K-poisoning.

2.4 NH₃-TPD analysis of catalysts

The surface acidity of catalyst evaluated by NH₃-TPD process can significantly influence the NH₃-SCR reaction^[25]. Fig.4 shows the NH₃-TPD curves of different catalysts. It can be seen that the NH₃-TPD curve of V/UiO-66 catalyst has a prominent NH₃ desorption peak at 75‒150 °C. After adding Sn, the Sn-V/UiO-66 catalyst shows two NH₃ desorption peaks at 75‒150 and 375‒425 °C, which are attributed to the desorption of NH₄⁺ at the Brønsted acid site and the desorption of NH₃ at the Lewis acid site^[28], respectively. Compared with the NH₃ at Lewis acid sites, NH₄⁺ at Brønsted acid sites is more unstable and more prone to desorption. Fig.4 also shows that the Sn-V/UiO-66 catalyst has a larger desorption peak area than the V/UiO-66 catalyst does. This phenomenon indicates that the Sn addition can increase the number of Brønsted acid sites and Lewis acid sites on the catalyst surface to a certain extent, which is beneficial to the adsorption and activation of NH₃^[29].

Fig.4  NH₃-TPD curves of different catalysts

When alkali metals are loaded on the catalyst, the acid content decreases rapidly, especially for the V/UiO-66 catalyst. This result is consistent with the results in Ref.[30]. It is reported that the potassium can affect both the Lewis acid and Brønsted acid on the catalyst surface, i.e., the number and strength of acid sites are reduced by K-poisoning. This is mainly because in SCR catalyst, V-OH exists as the Brønsted acid site, providing the active acid sites for SCR reactions; the reducing agent NH₃ is adsorbed on these active sites, thereby promoting the activation and removal of NO. After the alkali metal is loaded, K preferentially adsorbs on the acid sites on catalyst surface and combines with the V element to form V-O-K components, which hinders the adsorption of NH₃ on the Brønsted acid site, thereby reducing the surface acidity. The decreased Lewis acid may be related to the reduction in the specific surface area of the catalyst, and it is speculated that the alkali metal compounds occupy or cover some active sites. The surface acidity of catalyst significantly influences the NH₃ adsorption amount. K-poisoning reduces the surface acidity of the catalyst, therefore decreasing the SCR activity of the catalyst. It is worth noting that the total acid content of the poisoned K-Sn-V/UiO-66 catalyst is higher than that of the V/UiO-66 catalyst, indicating that the Sn addition can effectively reduce the effect of K-poisoning on the surface acidity of catalyst. In brief, the Sn addition can provide abundant acid sites for the catalyst and improve the anti-alkali metal poisoning property of catalyst.

2.5 XPS analysis of catalysts

The surface properties of the catalyst are crucial to the catalytic reaction. Fig.5a shows the overall XPS spectra of the surfaces of different catalysts. As for the V/UiO-66 and Sn-V/UiO-66 catalysts, C 1s, V 2p, Zr 3d, O 1s, and Sn 3d peaks can be observed according to their XPS spectra. As for the K-V/UiO-66 and K-Sn-V/UiO-66 catalysts, K 2p peak can be observed. The low peak intensity of K 2p is related to the small content of K₂SO₄ on the catalyst surface.

Fig.5  XPS spectra of different catalysts: (a) overall spectra; (b) Sn 3d; (c) O 1s; (d) V 2p; (e) Zr 3d

Fig.5b shows XPS spectra of Sn 3d of Sn-V/UiO-66 and K-Sn-V/UiO-66 catalysts. It can be seen that the Sn 3d spectra contain two peaks: Sn 3d_3/2 and Sn 3d_5/2. For the Sn-V/UiO-66 catalyst, the binding energies of Sn 3d_3/2 and Sn 3d_5/2 peaks are 486.78 and 495.08 eV, respectively, which indicates that Sn exists as Sn⁴⁺ in the Sn-V/UiO-66 catalyst. After K₂SO₄ is deposited on Sn-V/UiO-66 catalyst, the binding energy of Sn 3d shifts to the lower binding energy area, and the decrease range is within 0.4 eV. The deposition of K₂SO₄ on Sn-V/UiO-66 does not change the oxidation state of Sn, and Sn still exists as SnO₂.

Fig.5c shows XPS spectra of O 1s of different catalysts. The binding energy of lattice oxygen (O^2-, denoted as O_β), is 530.1‒530.3 eV, and that of the adsorbed oxygen (O^-/O₂^2-, denoted as O_α) on catalyst surface is 531.5‒531.8 eV. According to Ref.[31], the adsorbed oxygen on catalyst surface is considered as the main surface-active oxygen components due to its high mobility, therefore playing an essential role in oxidation reactions. High O_α content leads to the strong oxidization ability, thereby promoting the NO oxidation into NO₂ and the rapid SCR reaction. It can be seen from Table 2 that the adsorbed oxygen content on the surface of V/UiO-66 catalyst is 29.83%, and that of the Sn-V/UiO-66 catalyst is 30.44%. After adding Sn, the adsorbed oxygen content of the catalyst is slightly increased.

Table 2  Oxygen contents on different catalyst surfaces (%)

Catalyst	Oα	Oβ
V/UiO-66	29.83	70.17
K-V/UiO-66	31.08	68.92
Sn-V/UiO-66	30.44	69.56
K-Sn-V/UiO-66	39.54	60.46

After K-poisoning, the adsorbed oxygen content on the surface of V/UiO-66 catalyst changes slightly from 29.83% to 31.08%, while that of the Sn-V/UiO-66 catalyst increases from 30.44% to 39.54%. The surface-active oxygen significantly increases, resulting in the specific reactivity of K-Sn-V/UiO-66 catalyst. In addition, the binding energy of O 1s in catalysts after K-poisoning shifts to the lower energy area, which indicates that K coordinates with V in the catalyst and occupies the V-OH and V=O active sites on the catalyst surface to form V-O-K components, thereby increasing the electron cloud density in the outer valence electron layer of the oxygen nucleus.

Fig.5d illustrates XPS spectra of the V 2p of different catalysts. The binding energy of V⁴⁺ is 516.5‒516.7 eV, and that of V⁵⁺ is 517.5‒517.7 eV. For the vanadium-based catalysts, the redox cycle between V⁴⁺ and V⁵⁺ is crucial for the catalytic activity. In SCR reaction, V⁴⁺ and V⁵⁺ on the catalyst surface mainly exist in the form of V-OH and V=O, respectively. NO in the flue gas is oxidized into NO₂, and V⁵⁺=O is reduced to V⁴⁺-OH, which promotes the fast SCR reaction and improves the low-temperature activity of the catalyst. Metastable V⁴⁺ can promote the occurrence of this cyclic transition^[27]. The proportion of V⁴⁺ in the Sn-V/UiO-66 catalyst is 29.51at%, which is lower than that in the V/UiO-66 catalyst (35.47at%), as shown in Table 3. It is speculated that after adding Sn, Sn⁴⁺ oxidizes partial low-valence V⁴⁺ into high-valence V⁵⁺. After K-poisoning, for the V/UiO-66 catalyst, the proportion of V⁴⁺ decreases from 35.47at% to 25.04at%. The decrease in V⁴⁺ content inevitably leads to the significant reduction in the activity of SCR reaction.

Table 3  V contents in different catalysts (at%)

Catalyst	V4+	V5+
V/UiO-66	35.47	64.53
K-V/UiO-66	25.04	74.96
Sn-V/UiO-66	29.51	70.49
K-Sn-V/UiO-66	27.05	72.95

Fig.5e illustrates XPS spectra of Zr 3d in different catalysts. The binding energy of Zr 3d_5/2 is 184.6‒184.9 eV, while that of Zr 3d_3/2 is 182.3‒182.6 eV. Zr element mainly exists as Zr⁴⁺ on the ZrO₂ surface^[32]. As shown in Fig.5, the binding energies of Zr 3d_5/2 and Zr 3d_3/2 in V/UiO-66 catalyst are 184.73 and 182.42 eV, respectively. After Sn addition, the binding energies of Zr 3d_5/2 and Zr 3d_3/2 in Sn-V/UiO-66 catalyst are increased to 184.88 and 182.57 eV, respectively. This result indicates that the binding energy of Zr surface increases, the interaction between the catalyst and the active components is enhanced, and therefore the denitration activity is improved. After K-poisoning, the binding energy of Zr 3d_5/2 in V/UiO-66 catalyst decreases from 184.73 eV to 184.65 eV, and that of Zr 3d_3/2 decreases from 182.42 eV to 182.30 eV. Therefore, the decrease in denitrification activity is caused by the reduced interactions between catalyst and active components due to the K-poisoning.

2.6 Denitration performance test of catalyst

Fig.6 shows the denitration activity curves of different catalysts. It can be seen that the denitrification activity of V/UiO-66 and Sn-V/UiO-66 catalysts is increased with increasing the temperature, and the highest efficiency approaches 100% within a wide temperature range. The temperature window of V/UiO-66 catalyst is 180‒340 °C, while that of Sn-V/UiO-66 catalyst is 160‒340 °C. Therefore, the temperature window extends slightly to the low-temperature area after the Sn modification. After K-poisoning, the SCR activity of the V/UiO-66 catalyst decreases significantly, and the highest NO conversion ratio decreases from 100% to 60%. Although the activity of Sn-V/UiO-66 catalyst also reduces to a certain extent, the NO conversion ratio, namely NO removal ratio, is still more than 80% at 200‒340 °C. Therefore, Sn doping significantly enhances the resis-tance of catalyst against alkali metal K-poisoning.

Fig.6  Denitration activity curves of different catalysts

2.7 Discussion

According to the Lewis acid mechanism, SCR reaction process is as follows:

NH3+V5++O2-→V4+…NH2+OH-

(2)

V4+…NH2+NO→V4+…NH2NO

(3)

V4+…NH2NO→V4++N2+H2O

(4)

2V4++1/2O2→2V5++O2-

(5)

2OH-→H2O+O2-

(6)

When K is loaded onto the catalyst, it reacts with the active sites, resulting in a significant decrease in NH₃ adsorption capacity. In addition, the redox performance of catalyst is reduced after K-poisoning, which affects the SCR reaction processes, such as NH₃ activation and NO oxidation, and ultimately leads to the catalyst deactivation^[33].

The VO_x components on the surface of Sn-V/UiO-66 catalyst interact with other components, such as V-O-Sn and V-O-Zr, therefore improving the reducibility of VO_x components on the catalyst and increasing the proportion of V⁵⁺ components in the catalyst, which results in more Brønsted acid sites on the catalyst surface.

In addition, Sn⁴⁺ is highly acidic, and SnO₂ can quickly chemisorb NH₃ and form a coordination state with NH₂ on its surface. Therefore, SnO₂ can enhance the NH₃ adsorption capacity of catalyst, more NH₂ in the coordination state is generated, and the denitration efficiency of catalyst is im-proved. The total acid content of the poisoned K-Sn-V/UiO-66 catalyst is still higher than that of the V/UiO-66 catalyst, indicating that adding Sn can reduce the impact of K-poisoning on the surface acidity of the catalyst, lead to high acid content, and enhance the anti-alkali metal poisoning effects.

3 Conclusions

1) Compared with those of the V/UiO-66 catalyst, Sn-V/UiO-66 catalyst has smaller grain size, larger specific surface area, higher surface acid content, stronger redox capacity, and a large number of surface-active oxygen components. Sn-V/UiO-66 catalyst has excellent denitration efficiency, which is stable of nearly 100% at 180‒340 °C.

2) The K-Sn-V/UiO-66 catalyst converts 80% NO_x, whereas the V/UiO-66 catalyst only converts 60% NO_x.

3) Potassium salts can cause catalyst poisoning, therefore decreasing the catalyst activity. The potassium salt weakens the interactions between the active components and the catalyst carrier and reduces the surface acidity and redox property of catalyst. The poisoned K-Sn-V/UiO-66 catalyst still maintains the excellent redox property and high acid content, presenting superb resistance against alkali metal poisoning.