Research on the deactivation mechanism of a denitration catalyst WO₃–V₂O₅/TiO₂ at a coal-fired power plant

Abstract

The spent and fresh V₂O₅–WO₃/TiO₂ monolith catalysts were collected from a coal-fired power plant. The de-NO_x efficiency dropped by 20% after the fresh catalyst was used for 30 000 h. Then, the catalysts and the fly ash attached to spent catalysts were collected and analyzed. It was found that the relative amount of Si and Al increased by 80.84% and 2.26 times, respectively, which indicated that a lot of sediments deposited on the surface of the catalyst. Moreover, the content of Na, K, Ca and Fe increased in different degrees. A few new elements, such as Cl, Zn and Pb, appeared on the surface of the deactivated catalyst, and all of these elements had bad effects on the activity. Some kinds of ammonium salts and sulfates emerged on the fly ash, which showed that the catalysts were poisoned by SO₂. The special area decreased only by 4.39 m² g⁻¹. The V³⁺/(V⁴⁺ + V⁵⁺) ratio in the catalyst increased from 0.09 to 0.45 after deactivation, and V⁴⁺ decreased by about 39.54%, which caused the deactivation of the catalyst. The surface acidity of the deactivated catalyst decreased a lot, which might be the immediate cause of deactivation. The particle size of TiO₂ increased due to sintering. The main causes for the deactivation may be described as active sites decreased, poisoned and covered.

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1. Introduction

Nitrogen oxides (NO_x) lead to a number of environmental problems, such as photochemical smog, acid rain, haze and greenhouse effect.^1,2 Most of NO_x come from coal-fired power plants in China. To eliminate the NO_x pollution, the selective catalytic reduction (SCR) of NO_x with NH₃ is used for its high activity.^3,4 Numerous types of catalytic systems have been extensively investigated.⁵ Moreover, two dimensional materials became an important platform to design single site catalysts for various reactions, such as CO₂ reduction and CO oxidation.^5–14 However, among these catalysts, V₂O₅–WO₃/TiO₂ is the most effective and popular catalyst used in most of coal-fired power plants for its high de-NO_x efficiency and its tolerance towards SO₂.^15,16

However, the V₂O₅–WO₃/TiO₂ catalyst, similar to most of catalysts, faces the problem of deactivation after being used for a long term.¹⁷ The lifespan of the V₂O₅–WO₃/TiO₂ catalyst is closely related to the type of coal, the combustion method, and working conditions in coal-fired power plants.¹⁸ Meanwhile, the cost of catalyst occupies most of SCR system. In general, the V₂O₅–WO₃/TiO₂ catalyst cannot play the role of de-NO_x after being used for about 3 years. To extend the life-span of the catalyst by modification, it is necessary to identify the possible causes of its deactivation.

Otherwise, the spent catalyst faces the disposal problem.¹⁹ A few valuable metals still remain in the spent catalyst, such as W, V and Ti. On one hand, the deactivated catalyst may be used again after being regenerated by a proper way. On the other hand, recovering the valuable metals from the spent catalyst by suitable methods is also an effective way to reuse the resource. Different regeneration methods are based on the specific causes of deactivation.^20,21 The recovery products are affected by the substances deposited on the surface of the catalyst. Therefore, it is essential to study the deactivation mechanism of SCR catalysts at coal-fired power plant.

The spent honeycomb monolith SCR catalyst was obtained from a coal fired power plant, which was used continuously for about 30 000 h. The fresh catalyst was also collected for a comparison in the experiment. In this study, the catalytic activity of fresh and spent catalysts was studied in a laboratory, and the deactivation mechanism of the spent catalyst was discussed.

2. Material and methods

2.1 Commercial SCR catalysts

The specimens of fresh and deactivated catalysts were deposed in the middle part of the catalyst in the plant. The ash was collected from the superficies of the deactivated catalysts by an air blower. The pore diameter of the fresh and deactivated catalysts were 8.08 mm and 8.04 mm, respectively, while the wall thickness were 1.00 mm and 1.10 mm, respectively. There were a few gray and red-gray sediments on the surface of the pore. The color of the fly ash was gray, and a few granulated substances were found in the ash. The fly ash on the surface of the deactivated catalyst was also collected for the research. For further studying, we defined the fresh catalyst as FC, the deactivated catalyst as DC and the fly ash on the catalyst as FA.

2.2 Catalytic activity tests

FC and AC were smashed to below 149 μm for the test. The SCR catalytic activity tests of FC and AC were carried out in a fixed-bed stainless steel reactor tube with 0.6 g of the samples. The test temperature was in the range from 240 °C to 420 °C. The test system consisted of four parts: simulated gases and mass flow meters, a gas mixing section, a reactor and an analyzer section. The experimental layout of the catalytic activity test is shown in Fig. 1. The components of the simulated gas stream were: c(NO) = 500 mg m⁻³, c(NH₃) = 500 mg m⁻³, ϕ(O₂) = 5% with N₂ as the balance gas in the experiment. The de-NO_x efficiency of FC and DC was calculated by eqn (1).where NO_x(in) is the concentration of the gas stream in the inlet of the reactor and NO_x(out) is the NO concentration of the gas stream in the outlet.

Fig. 1 The experimental layout of the catalytic activity test equipment ((1) gas tanks and cylinders (2) mixing reactor (3) reactor (4) flue gas analyzer (5) air outlet (6) pump).

2.3 Chemical characterization

The elemental content of DC, FC, and FA was evaluated on an X-ray fluorescence spectrometer (XRF, PFX-235, Thermo, USA). The valence of W, V, Ti and O was detected by X-ray photoelectron spectroscopy (XPS, QUANTUM 2000X, Physical Electronics Company, USA). The BET surface area of FC and DC was measured using a Micromeritics ASAP 2020 apparatus. The crystalline phase of FC, DC and FA was detected via X-ray diffraction (XRD, X'Pert Pro, PANalytical B.V., Holand). The particle morphology of the catalyst was examined by a scanning electron microscope (SEM, S-4800, Hitachi, Japan). The main chemical functional groups of the catalyst were identified by their Fourier transform infrared spectra (FT-IR, Nicolet iS10, Thermo, USA). NH₃-TPD was detected by an AutoChem II 2920 (NH₃-TPD, AutoChem II 2920).

3. Results and discussion

3.1 De-NO_x efficiency

The efficiency of FC and DC in the experiment is presented in Fig. 2. As shown in Fig. 2, the denitrification efficiency of FC was higher than 85% in a temperature range of 300–420 °C. However, the de-NO_x efficiency of DC decreased significantly in the test temperature window. DC showed only 71.43% in the experiment at 420 °C, which cannot satisfy the requirement of the power plant. The value of the de-NO_x efficiency of DC was below 80% in the test temperature window. The trend of the de-NO_x efficiency of DC and FC was similar at the test temperature, which indicated that DC and FC showed a relatively high de-NO_x efficiency at a high temperature. Moreover, the catalyst was a medium and high temperature denitration catalyst, which caused the de-NO_x efficiency to increase with the test temperature in an almost linear relationship.

Fig. 2 The de-NO_x efficiency of FC and DC as a function of the test temperature.

3.2 XRF

It was found that about 7 g of FA was collected when 100 g of DC gathered. Therefore, it can be inferred that about 7 g of FA deposited on the surface of 100 g of DC after being used for 30 000 h. The composition of FC and DC was identified by XRF, and FA was also analyzed by XRF. The substances on the catalyst, appeared or vanished, may have a great influence on the NH₃-SCR reaction. The content change of TiO₂ could be ignored as compared with other ingredients. TiO₂ was selected as the reference for its high content in the catalyst. So, the changing substances were considered in the research. The metal oxide ratio of MO_x/TiO₂ between FC and 100 g DC + 7 g FA was considered and presented in Fig. 3.

Fig. 3 MO_x/TiO₂ between FC and DC + FA.

As shown in Fig. 3, the oxide ratio of WO₃/TiO₂ remained constant, which indicated that WO₃ was stable. The relative content of SiO₂ increased by 80.84% in DC + FA. Additional SiO₂ was from the fly ash in the flue gas. SiO₂ may deposit on the surface or block the pore of the catalyst. The relative content of SO₃ increased by 18.11%, which may have existed as CaSO₄, (NH₄)₂SO₄ or NH₄HSO₄. The sulfates had a negative effect on the NH₃-SCR reaction,²² which could destroy V=O.

The relative content of Fe₂O₃ increased by 2.26 times, and the large amount of Fe was from the coal. The Fe might take place the V active sites.²³ The relative content of MgO increased by 1.18 times. Although the poisoning effect of Mg on the catalyst was less than that of K, Na and Ca, it could also lead to a decrease in activity.²⁴ The relative content of Al₂O₃ increased by 2.26 times, and the large amount of Al was from the coal. Al may deposit on the surface of the catalyst. A few researchers found Al₂(SO₄)₃, which could decrease the specific surface area.²⁵ The relative content of CaO increased by 36.71%, which could decrease the adsorbed ammonia amount.²⁶ Moreover, Ca could work together with a few inorganic matters, such as NO, C, SO₂ or P₂O₅.^24,27

The oxide ratio of K₂O/TiO₂ and Na₂O/TiO₂ increased by 102.76% and 15.79%, respectively. Many researchers have carried out experiments on the effect of alkali metals on catalysts.²⁸ The alkali metals can cause lots of changes in the catalyst, such as blocked catalyst channels, decreased reducibility and reduced surface acidity sites. The poisoning effect of a few alkali salts on the V₂O₅–WO₃/TiO₂ catalyst follows the sequence Na₂CO₃ < K₂CO₃ < Na₂SO₄ < K₂SO₄ < NaCl < KCl.²⁹ K can coordinate to Brønsted acidic sites.^30,31 Also, substances containing K, such as KCl can induce the catalysts sintered, decrease V atoms on the surface, and change the valence state of V.³²

Besides, a few elements such as Pb, Cl, As and Zn appeared in DC and FAwere also proved to be poisonous and harmful to the catalyst. Pb could poison the active polytungstate species, inhibit NH₃ adsorption, and decrease the Brønsted acidic sites.^33,34 In addition, the collective effect of Pb and SO₂ could not only decrease the surface acidic sites but also decrease the redox sites.³⁵ This can promote to form N₂O above 300 °C, and destroy the active V–OH sites.³⁶ Both the Lewis and Brønsted acidic sites decreased when H₂AsO₄ or HAsO₄⁻ formed. The surface active oxygen increased and the oxidizability of catalysts enhanced, when the catalyst was poisoned by As, which resulted in increased NH₃ oxidation and reduced SCR selectivity at high temperatures.³⁷

Moreover, the effect of Zn poisoning for V₂O₅–WO₃/TiO₂ was rarely reported. The amount of Mn⁴⁺ and chemisorbed oxygen species in the Mn/TiO₂ catalyst decreased after being poisoned by Zn. Zn also affected the reducibility, surface acidity and NO adsorption ability of the catalyst.^38,39

In addition, a certain amount of V was found in FA, which indicated an outflow of some active components during the operational process.

3.3 XRD

Fig. 4a shows the XRD patterns of FC, DC and FA. Only the peaks of anatase TiO₂ are displayed for FC and DC. There were no obvious peaks of SiO₂, V₂O₅ and WO₃ in FC and DC due to their low concentrations and high dispersion. It could be clearly seen that the crystal form of TiO₂ did not change between FC and DC.

Fig. 4 The XRD patterns of FC, DC (a) and FA (b).

The XRD patterns of FA are also shown in Fig. 4b. The diffraction peak of SiO₂ appeared in FA according to Fig. 4b. Some SiO₂ were added as a skeleton material with other ingredients in the catalyst preparation stage. Most of SiO₂ was from the coal of combustion, which covered the active sites on the catalyst surface. Too much SiO₂ and other ingredients on the surface of the catalyst may lead to a decrease in its efficiency. Meanwhile, CaSO₄ was found in FA. CaSO₄ deposited on the surface of the catalyst could restrict the NH₃-SCR reaction. Furthermore, (NH₄)₂SO₄ and NH₄HSO₄ were also observed. (NH₄)₂SO₄ and NH₄HSO₄ were formed from NH₃ and SO₂ during the operational stage, which were harmful for the active site V₂O₅ in the catalyst.

Former researches had pointed out that the main sediment sulfates of (NH₄)₂SO₄ and NH₄HSO₄ were formed under the temperature of 300 °C, while CaSO₄ was generated as the main sulfate at higher temperatures.^40,41 In addition, Al₂SiO₅ emerged in FA, which may form from SiO₂ and Al₂O₃ in the catalyst or in the flue gas, which may also cover the active sites.

On the basis of the Scherrer equation, the mean dimensions of TiO₂ particles for FC and DC were 18.9 and 20.3 nm, respectively. Hence, it could be observed that DC was sintered, which led to an increase in the mean particle size of TiO₂.

3.4 Surface morphology

The SEM images of FC and DC are shown in Fig. 5. As shown in Fig. 5a, FC showed the relatively regular smooth morphology without an impurity particle distribution. In contrast, DC illustrates a coarse surface and a particle agglomeration or hardness to some extent. As shown in Fig. 5b, c, e and f, a few substances with a trod-like structure, a block structure or a globular structure appeared on the surface of DC. There were a certain amount of SiO₂, Al₂O₃ and CaO as skeletal structures in FC. The skeletal structures did not appear on the surface of FC for low activity. After eroding for 30 000 h continuously, a few Si–Ca–Al skeletal structures bared as trod-like structures on the surface of DC. The block structure matters were sintered with the catalyst and other ingredients in the flue gas. A lot of substances generated in the flue gas might deposit on the surface of the catalyst, and a few of them reacted with the flue gas and ingredients of the catalyst. These matters exhibited all kinds of morphologies after deposited or reacted for a certain period of time. A certain amount of S emerged on the surface of DC might be sulfate. SO₂ existed in the flue gas, and V₂O₅ could be the catalyst for SO₂ to be converted to SO₃. SO₃ was transformed to ammonium sulfate and calcium sulfate via reacting with other ingredients, such as NH₃ or CaO. The substances deposited on the catalyst surface could be considered as contaminants. They could not only cover the active sites and destroy the pores, but also change the structure of the active sites.

Fig. 5 SEM photos of samples (FC (a–d); DC (b–e), (c–f)).

3.5 Specific surface area

The specific surface area, pore structure and pore volume were also under the attention for their influence on the catalytic reaction. In the research, the value of specific surface area of FC was 64.30 m² g⁻¹, while the corresponding value for DC was 59.91 m² g⁻¹. A decrease in the specific surface area was limited, which indicated that the specific surface area did not play a main role in the deactivation.

The curves about adsorption–desorption isotherms and pore distribution of FC and DC are also shown in Fig. 6a and b, respectively. Both FC and DC exhibited type IV adsorption–desorption hysteresis loops, which indicated that FC and DC both were porous materials. The pore size distribution of FC and DC was calculated by the DFT approach according to adsorption–desorption hysteresis loops. The major pores of the FC and DC were mesoporous. As shown in Fig. 6a, the meso-pore peak centers of FC were at 9, 10 and 13 nm, while the peaks of DC were at 13, 15 and 20 nm. The major pores did not show obvious changes between FC and DC. Above all, the specific surface area, pore structure and pore volume did not exhibit an important influence on the deactivation.

Fig. 6 Pore size distribution (a) and adsorption–desorption curve of FC and DC (b).

3.6 XPS

The XPS analysis was also conducted to understand the chemical states on the surfaces of FC and DC. A special attention was paid to elements of Ti, W, V and O, which are displayed in Fig. 7.

Fig. 7 The XPS spectrum of FC and DC ((a) W; (b) V; (c) Ti; (d) O).

Fig. 7a shows the W 4f XPS spectrum of FC and DC. The binding energy of 37.1 eV was attributed to Ti 3p,⁴² and the spectrum around 35.6 and 37.7 eV corresponded to W⁶⁺ 2p_7/2 and W⁶⁺ 2p_5/2.⁴³ It can be seen from Fig. 7a that only spectra of Ti 3p and W⁶⁺ were detected over FC, revealing that tungsten oxide existed in a hexavalent state when FC was not used in the SCR system. The peaks located at 34.6 and 36.7 eV were attributed to W⁵⁺ 2p_7/2 and W⁵⁺ 2p_5/2, respectively.⁴⁴ WO₃ can store electrons^45,46 and WO₃–TiO₂ can provide Lewis acidic sites for ammonia adsorbtion.^47,48 As shown in Fig. 7a, W⁵⁺ 2p appeared in DC, which indicated that a certain amount pentavalent tungsten was also generated. The content of W⁵⁺ occupied 5.14% in DC, and W⁵⁺ might be poisoned by heavy metals, such as Pb, which could restrain the active polytungstate species to be formed again.³⁴ There were a few clear evidences about the valence change of W in V₂O₅–WO₃/TiO₂, the decrease in W⁶⁺ might have some influence on the deactivation.

It is proposed that the peaks around 516.4–517.4 eV and 515.6–516.1 eV were attributed to V⁵⁺ and V⁴⁺, respectively, while the peak at 513.1–514.7 was assigned to V³⁺.^49–51 As compared with FC, the valence states of V in DC showed a few changes. V³⁺/(V⁴⁺ + V⁵⁺) in the catalyst increased from to 0.09 to 0.45. As known, V³⁺ had no catalytic activity in the NH₃-SCR reaction, while the higher concentration of V⁴⁺ could contribute to a high SCR activity.^52–54 V⁴⁺ decreased from 80.14% in FC to 40.60% in DC, which contributed to the deactivation.

It can be seen from Fig. 7c that only Ti⁴⁺ was detected over FC and DC. TiO₂ was stable when used for about 30 000 h. As shown, a slight decrease about the binding energies of DC was observed with Ti⁴⁺ 2p_3/2 shifted from 458.7 to 458.6 eV, and Ti⁴⁺ 2p_1/2 shifted from 464.4 to 464.3 eV. The titanium species in DC had a higher density electron cloud than titanium species in FC according to the binding energies of Ti⁴⁺ 2p_3/2 between FC and DC owing to the electrons around other species, such as tungsten, transferred to the titanium species. Moreover, the inductive effect of V was also related to the electron density of Ti⁴⁺.⁵⁵

The binding energies of 529.0–530 eV and 531.3–531.9 eV were assigned to the lattice oxygen (O_β) and chemisorbed oxygen (O_α), respectively.^56–58 As shown in Fig. 7d, both O_α and O_β were observed over FC and DC, and O 1s of DC shifted to the lower binding energy as compared to FC. Some higher electronegativity substances might reduce the binding energy of O. Surface chemisorbed oxygen was proven to be the most active oxygen during the SCR process. The O_α amount in DC decreased from 67.22% to 60.75%, which had a significant effect in the oxidation reaction.⁵⁹

3.7 FT-IR

FT-IR was used to characterize the catalyst functional groups of FC and DC, and the results are shown in Fig. 8. The FT-IR spectra had bending vibrations of OH (3440) and HOH stretching (around 1636). Otherwise, the FT-IR spectra of FC and DC showed different shapes. The weak peak around 1020 cm⁻¹ of FC may be assigned to V=O.⁶⁰ The bands at 1102 and 1190 cm⁻¹ for DC could be attributed to the characteristic peaks of SO₄²⁻.⁶¹ It could be implied that a certain amount of sulfate was generated on the surface of DC, which was similar to the results of the elemental study. Due to SO₂ in the flue gas, different sulfates, such as (NH₄)₂SO₄, NH₄HSO₄ and CaSO₄, formed on the used catalyst.⁶¹

Fig. 8 FT-IR spectra of FC and DC.

3.8 NH₃-TPD measurements

The surface acidity of the NH₃-SCR catalyst plays a crucial role in the reaction process. The NH₃ adsorption step on the surface is important to covert NO_x to N₂. To explore the changes of the surface acidity between FC and DC, NH₃-TPD was conducted in the research (Fig. 9). The TPD curve for FC showed three peaks around 213 °C, 385 °C and 530 °C. As shown in Fig. 9, the intensities of the desorption peaks of DC decreased a lot, which indicated that the acidic sites of the catalyst were seriously poisoned.

Fig. 9 The NH₃ adsorption–desorption spectrum of FC and DC.

4. Conclusions

It is not easy to conclude the specific factors about the impacts on the SCR catalyst activity due to the complexity of the substances and the variable conditions of the flue gas. According to the experimental results of this research, the relative amount of Si and Al increased by 80.84% and by 2.26 times, respectively, which severely blocked and covered the active sites. The amount of alkali metals, such as K and Na increased a lot, which were harmful to the acidic sites. Other elements (Fe, Mg, Ca, etc.) could cover the active sites, which could lead to the deactivation of the catalyst. A few substances, particularly As, could terribly destroy the acidic sites and V–OH. The new substances played a great role on the catalyst deactivation, such as the sulfates formed on the catalyst. The special area decreased by 4.39 m² g⁻¹, which was not a main factor for deactivation. V³⁺/(V⁴⁺ + V⁵⁺) in the catalyst increased from to 0.09 to 0.45 after deactivation, and the V⁴⁺ amount decreased by about 39.54%. The surface acidity decreased a lot after being used for a long time. The valence change of V atoms might affect the reducibility of the catalyst surface, and lead to the deactivation of catalyst. The substances deposited on the surface could lead to a change in the special surface area and cover the active sites, causing the catalyst deactivated. Sulfates, such as CaSO₄, (NH₄)₂SO₄, and NH₄HSO₄, could cover the active sites and reduce the NH₃ adsorption amount. In short, the deactivation pathways can be explained as three ways (Fig. 10): (1) the active sites were reduced, and some V₂O₅ was washed away. (2) The active sites were poisoned by various elements (K, Na, S, As, Pb, Zn, Ca, etc.), which caused V³⁺/(V⁴⁺ + V⁵⁺) to increase and NH₃ adsorption amount to decrease. (3) The active sites were covered by substances, such as SiO₂, CaSO₄, (NH₄)₂SO₄ and NH₄HSO₄, which restricted the catalytic reaction.

Fig. 10 The deactivation pathways.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

The authors gratefully acknowledge the financial support from the Fundamental Research Funds for the Central Universities (2010YH14).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra06812h

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