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Article

Tungsten Oxide Modified V2O5-Sb2O3/TiO2 Monolithic Catalyst: NH3-SCR Activity and Sulfur Resistance

by
Liping Liu
1,2,
Xiaodong Wu
2,*,
Yue Ma
2,
Jinyi Wang
1,
Rui Ran
2,
Zhichun Si
3 and
Duan Weng
2
1
Huaneng Clean Energy Research Institute, Beijing 102209, China
2
School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
3
Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China
*
Author to whom correspondence should be addressed.
Processes 2022, 10(7), 1333; https://doi.org/10.3390/pr10071333
Submission received: 21 May 2022 / Revised: 25 June 2022 / Accepted: 6 July 2022 / Published: 8 July 2022
(This article belongs to the Section Environmental and Green Processes)
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Abstract

:
In this study, a V2O5-Sb2O3/TiO2 monolithic catalyst was modified by introducing WO3. The WO3-modified catalyst exhibited enhanced catalytic activity in the measuring temperature range of 175–320 °C. The changes in dispersion of vanadia species were investigated by ultraviolet-visible (UV-Vis) spectroscopy and H2 temperature-programmed reduction (H2-TPR). A durability test was conducted in a wet SO2-containing atmosphere at 220 °C for 25 h. The sulfate deposition was estimated by temperature-programmed decomposition (TPDC) of sulfates, thermo-gravimetric (TG) analysis, and temperature-programmed desorption (TPD) of NH3. Isothermal SO2 oxidation and temperature-programmed surface reaction (TPSR) of NH4HSO4 with NO were performed. Based on these characterizations, effects of WO3 modification on the sulfate tolerance of the catalyst were explored.

1. Introduction

Nitrogen oxides (NOx) emitted from stationary and mobile sources cause many environmental pollution problems, including photochemical smog, ozone depletion, acid rain, and greenhouse effects [1,2,3]. Selective catalytic reduction (SCR) of NOx with ammonia has been widely recognized as one of the most effective technologies for NOx abatement [4,5,6,7,8]. V-based SCR catalysts have been widely commercially applied due to their high catalytic performance and good stability in the temperature region of 300–400 °C [9,10,11]. In stationary sources, to gain appropriate reaction temperature for catalysts, the SCR unit is commonly located upstream of the desulfurization and dust-removal facilities. However, high concentrations of SO2 and ash in the exhaust gas may result in severe catalyst poisoning and shorten the service life of the catalyst in this case [12,13]. Alternatively, an SCR unit is located downstream of desulfurization and dust-removal units to prevent the catalyst from being exposed to high concentrations of SO2 and ash, and to increase the recovery efficiency of waste heat [13,14]. It should be noted that uncaptured SO2 still exists in the flue gas in this case, and the flue temperature is lowered to a temperature below 300 °C [15,16]. Thus, it is of significance to exploit the SCR catalyst with high NOx conversion regarding low-temperature exhaust gas containing SO2.
Metal oxide catalysts (e.g., Mn-, V-, Ce-based catalysts) have been widely investigated for their low-temperature NH3-SCR activities, owing to their advantages of low cost and easy preparation [17,18,19,20,21,22,23,24,25]. Among these catalysts, V-based catalysts are considered a preferable choice for SO2-containing flue gas purification at low temperatures, due to their superior sulfur resistance [26,27]. However, the NOx conversions of V-based catalysts at low temperatures still need to be further improved to meet the increasingly stringent emission regulations. Over the past decades, several attempts to modify V-based catalysts have been reported. For instance, Zhang et al. [28] prepared F-doped V2O5/TiO2 catalysts with improved NH3-SCR activity at low temperatures, and they ascribed the enhanced activity to the fact that F-doping could improve the interaction of V2O5 with TiO2 and facilitate the formation of reduced vanadia. Ma et al. [29] studied the effect of CeO2 modification on the low-temperature NH3-SCR activity of the V2O5-WO3/TiO2 catalyst. They demonstrated that V-O-Ce bridging bonds were formed in the catalyst, and the reducibility of the catalyst was enhanced due to the synergistic effect between Ce and V. The introduction of WO3 can improve the NH3-SCR activity of the catalyst by increasing the redox property and Brönsted acidity [30]. Indeed, Lietti et al. [31] observed that the V2O5-WO3/TiO2 catalyst has both higher transient and steady-state reactivity in SCR than the binary V2O5/TiO2 counterpart. They reported the presence of a specific synergistic effect between V and W, which promoted the reoxidation of reduced vanadia [31]. Paganini et al. [32] also connected the superior redox properties of V2O5-WO3/TiO2 to the V–W electronic interaction. Chen et al. [27] reported that the introduction of tungsten benefits the generation of more low-valence vanadium species due to its promotional effect of capturing and transferring electrons, and thus the WO3 modified catalyst exhibits superior low-temperature SCR activity to that of V2O5/TiO2 catalyst after hydrothermal aging treatment. In contrast, Jaegers et al. attributed [33] the promotion effect of WO3 to structural effects, including inducing the formation of oligomeric vanadia sites and generating adjacent surface sites rather than the electronic effect through in-situ spectroscopic measurements (i.e., MAS NMR, Raman and EPR). Conversely, the role of WO3 as an acidity promoter is widely accepted. By combining experimental characterizations with DFT calculations, Sun et al. [34] studied the promotion effect of tungsten oxide on SCR activity of V2O5-WO3/Ti0.5Sn0.5O2 catalyst. They found that the number of the Brønsted acid sites increases with the loading amount of WO3, resulting in higher SCR activity. Nuguid et al. [35] developed a characterization technique of modulated excitation Raman spectroscopy to access the mechanistic information that was currently unachievable with steady-state Raman experiments. They clarified that only a defined portion of VOx species acted as catalytic active centers, which were coordinately unsaturated. Additionally, TiO2 acted as an NH3 reservoir and was indirectly involved in the SCR reaction.
The durability of low-temperature SCR catalysts in the presence of SO2 also receives great attention. Kang et al. [36] constructed low-temperature SCR catalysts by mechanically mixing commercial V2O5-WO3/TiO2 with Fe2O3, and the obtained catalysts exhibited higher catalytic stability in the presence of SO2 than V2O5-WO3/TiO2. They discovered that the formation of ammonium sulfates was hindered due to the production of iron sulfates from adjacent Fe2O3. Furthermore, iron sulfate could assist the NH3-SCR reaction by supplying additional Brønsted acid sites. In recent years, antimony (Sb) has attracted increasing attention as an additive to promote the SCR performance of V-based catalysts [37]. Phil et al. [38,39] found that the Sb-doped V2O5/TiO2 catalyst demonstrated not only higher NOx conversions, but also better SO2 resistance at low temperatures than catalysts containing other promoters (Pb, B, Cu, and P). It has been demonstrated in our previous work that the Sb-modified V2O5/TiO2 catalyst indicated excellent deNOx performance in the presence of SO2 because the introduction of Sb2O3 can not only weaken the SO2 oxidation activity, but also enhance the reactivity of NH4HSO4 with NO [40]. However, the low-temperature SCR activity of VSbTi monolithic catalyst must be further improved to meet increasingly stringent emission regulations [2].
This study aimed to introduce WO3 to V2O5-Sb2O3/TiO2 (VSbTi) catalyst to further improve catalytic performance at low temperatures. The VSbTi and W-modified monolithic catalysts were prepared by an extrusion-molding method. The effects of WO3 addition on the surface properties, catalytic activity, and durability were clarified. Furthermore, the sulfur-poisoning mechanism of the modified catalyst was illustrated.

2. Experimental

2.1. Catalyst Preparation

VSbTi and VSbWTi cuboid monolithic catalysts were fabricated by an extrusion-molding method, using a vacuum mixing extruder (Zibo Shenyun Machinery, China, QLJ-150L). Firstly, the VSbTi catalyst (the mass ratio of V2O5:Sb2O3:TiO2 = 2.7:2.0:95.3) was prepared by mixing mesoporous TiO2 powders (Millennium Chemicals DT51, Baltimore, Maryland, USA, SBET = 81 m2/g) in an aqueous precursor solution containing 0.5 M antimony acetate (Sinopharm, Beijing, China, 97%) and 1.1 M ammonium metavanadate (Beijing Chemical Reagent, Beijing, China, 99.9%). After stirring for 2 h, the suspension was titrated with 20 wt.% ammonia hydroxide (Sinopharm Chemical Reagent, Beijing, China, GR) until the pH reached 10, and then stirred for another 6 h. Subsequently, the resulting precipitates were separated by filtration and desiccation. The additives of stearic acid, polyethylene oxide, and carboxymethyl cellulose were used to facilitate the extrusion process. The VSbTi monolithic catalyst was obtained through extrusion molding and calcination at 550 °C for 6 h. The size of the monolithic catalysts was 36 mm × 36 mm × 200 mm (length), and the size of the cubic channel was 6 mm × 6 mm. The VSbWTi monolithic catalyst (the mass ratio of V2O5:Sb2O3:WO3:TiO2 = 2.7:2.0:1.5:93.8) was prepared by the same method but with the addition of 0.1 M ammonium paratungstate (Sinopharm, Beijing, China, 99.5%) as a W precursor. The SO2 resistance test was performed in a feed stream of 1000 ppm NO/1000 ppm NH3/350 ppm SO2/10% H2O/N2 at 220 °C for 25 h, and the corresponding sulfated catalysts after the durability test were denoted as VSbTi-S and VSbWTi-S, respectively.

2.2. Activity Measurement

Isothermal NH3-SCR activity evaluation was conducted between 175 and 320 °C in a fixed-bed stainless-steel reactor. Both the stainless-steel reactor and monolithic catalysts were cuboid in shape. The monolithic catalyst was exposed to a feed stream of 1000 ppm NO/1000 ppm NH3/350 ppm SO2/5% O2/10% H2O/N2 with a gas hourly space velocity of (GHSV) of 5000 h−1. The effluent gases were monitored by a flue gas analyzer (ecom EN-2F, RBR, Germany) when the reaction reached equilibrium. The NOx conversion was calculated according to the following equation:
NO x   conversion   ( % ) = [ NO ] in − [ NO ] out − [ NO 2 ] out [ NO ] in   ×   100
To evaluate the activity of the catalysts for SO2 oxidation, an isothermal reaction was performed in the same apparatus to that of SCR activity measurements with a feed stream consisting of 500 ppm SO2, 5% O2 and N2 in balance (GHSV 50,000 h−1). The produced SO3 was absorbed by an aqueous solution containing 80 wt.% isopropyl alcohol. After reaction for 6 h, the obtained solution was titrated with 0.01 M Ba(ClO4)2·3H2O using alizarin red S as an indicator. The SO2 conversion was estimated according to the following equation:
SO 2   conversion   ( % ) = [ SO 3 ] out [ SO 2 ] in × 100

2.3. Characterizations

Considering the homogeneous property of monolithic catalysts prepared by extrusion molding, the catalyst powders were prepared for the following characterizations by crushing a middle fraction of monolithic catalysts and grinding the fraction in a mortar for 2 min. The crystallization structure of the catalysts was obtained by using an X-ray diffractometer (D8 Advance, Bruker, Germany) with a Cu Kα radiation (λ = 1.5418 Å), operating at an angle range of 20–80° and a scanning rate of 6 °/min.
The specific surface areas of the samples were measured using N2 adsorption at −196 °C by the four-point Brunauer–Emmett–Teller (BET) method on an automatic surface analyzer (F-Sorb 3400, Gold APP Instrument). The catalysts were degassed at 200 °C for 2 h prior to the measurements.
Ultraviolet-visible (UV-vis) spectroscopy was conducted on a UV-2600 spectrophotometer (Shimadzu, Japan). BaSO4 was used as a reference. The spectra were collected at room temperature (RT) in a wavelength region of 200–800 nm.
The temperature-programmed decomposition (TPDC) of sulfates deposited on the catalysts was performed in N2 (50 mL/min) from RT to 750 °C at a heating rate of 10 °C/min. The released gases were monitored by a mass spectrometer (Omnistar 200, Germany).
The thermogravimetric (TG) measurement of the catalyst was performed on a Mettler Toledo TGA/DSC instrument from RT to 700 °C at a rate of 10 °C /min.
H2 temperature-programmed reduction (H2-TPR) was performed on a chemisorption analyzer (AutoChem II 2920, Micromeritics, USA) using a thermal conductivity detector (TCD). Prior to the test, 50 mg of samples were treated in O2/N2 (50 mL/min) at 200 °C for 30 min, then cooled down to RT. Only a low flow velocity was allowed for the chemisorption analyzer. To ensure sufficient contact between the samples and gaseous oxygen, a flow containing a high content of O2/N2 (20%) was employed for pretreatment before a H2-TPR test. After purging in Ar (50 mL/min) for 20 min, the feed gas was changed into 10% H2/Ar (50mL/min). Subsequently, the temperature was heated to 550 °C at a rate of 10 °C/min.
Temperature-programmed desorption of ammonia (NH3-TPD) was performed on a Nicolet 380 FTIR gas analyzer (Thermo Fisher Scientific, Waltham, MA, USA). Prior to each test, the sample was pretreated in 5% O2/N2 (500 mL/min) at 200 °C for 30 min, then cooled down to 100 °C. Afterward, the sample was exposed to 1000 ppm NH3/N2 until saturation and then purged in N2 to remove physically adsorbed NH3. The adsorption saturation was recognized by the variation of NH3 concentration less than 10 ppm around the inlet concentration within 10 min. The sample was heated to 500 °C at 10 °C /min in N2 for NH3 desorption.

3. Results

3.1. NH3-SCR Activity

Figure 1a depicts the NH3-SCR activities of VSbTi and VSbWTi monolithic catalysts. With the introduction of WO3, the VSbWTi catalyst exhibits higher NOx conversions at temperatures below 300 °C. The NOx conversion reaches 90% at ca. 210 °C and 230 °C over VSbWTi and VSbTi, respectively. Because V-based catalysts produce little N2O at low temperatures [41,42], the product from NOx conversion is nearly 100% N2 in the reaction. To measure the durability of the monolithic catalysts, a SO2 resistance test was performed at 220 °C after the NH3-SCR activity test, and the results are portrayed in Figure 1b. It is readily anticipated that the initial NOx conversions in Figure 1b would be slightly lower than those in Figure 1a at the same temperature since the catalysts have been aged during the NH3-SCR activity test. Compared with VSbTi, the VSbWTi catalyst deactivates more rapidly upon exposure to SO2 and H2O. After poisoning for 25 h, a drop of 12% in NOx conversion occurs over VSbTi catalyst while VSbWTi exhibits a more evident decrease of 20%, so the poisoned catalysts indicate similar NOx conversion (71–72%) at the end of the test. In real applications, the poisoned catalysts would undergo vapor washing and thermal regeneration when the NOx conversion fell beneath a threshold value (e.g., 70%, depending on operation conditions). These results indicate that the introduction of WO3 to VSbTi catalyst leads to higher deNOx activity in the fresh state but lower sulfate tolerance.

3.2. Structural Properties

To understand the differences between VSbTi and VSbWTi in SO2 resistance, crystalline structures of the fresh and sulfated catalysts were investigated. Characteristic diffraction peaks ascribed to anatase TiO2 were observed in the XRD patterns of all the catalysts (Figure S1). No diffraction peaks assigned to vanadium oxide, tungsten oxide or antimony oxide were detected over all the catalysts, implying that these metal oxides exist in the forms of low crystallinity or well dispersion on the titania support. Diffraction features of crystalline sulfates were not observed on the sulfated catalyst.
The BET surface areas (SBET) of VSbTi, VSbWTi, VSbTi-S, and VSbWTi-S are 37, 33, 30, and 27 m2/g (Table S1), respectively. Compared with pure TiO2 (DT51), a significant decrease in SBET was found in all the catalysts, which is mainly associated with the introduction of impregnated metal oxides, including V2O5, Sb2O3, and WO3. A similar phenomenon was also reported by Albonetti et al. [43], in which the SBET of TiO2 (DT51) decreases to 36 m2/g after impregnation with 3 wt.% V2O5. As for the fresh catalysts, the introduction of WO3 led to a further decrease in SBET, which may be due to both the reduced content of TiO2, and the pore blockage of the support by the added WO3. As expected, the surface areas of sulfated catalysts decline, in comparison with the fresh counterparts, which can be explained by the pore-blocking effect of deposited sulfates [44]. In detail, the sulfur oxides react with ammonia and transform into ammonium sulfates, blocking the pores of the TiO2 support [45,46].
UV-vis spectroscopy was adopted to identify the surface vanadium species in the fresh and sulfated catalysts. As indicated in Figure 2, the spectra are dominated by the absorption edge assigned to the O2−→Ti4+ charge transfer (CT) transition of anatase TiO2 at around 410 nm [21,47]. The broad absorption edge in the visible region of 450–550 nm is associated with the O-V CT transition of vanadium oxide, which is sensitive to the coordination structure of vanadia species [48,49], namely, the longer the adsorption edge wavelength representing the higher the coordination of polymeric vanadia species [49,50]. For the fresh catalysts, the adsorption edge position shifts from 534 to 542 nm after W modification. According to the literature [51], there is little contribution of WO3 toward the absorption in the region of 400–550 nm with WO3 addition lower than 7 wt.% in the TiO2/SiO2 catalyst. In our study, the loading of WO3 is 1.5 wt.%, so the contribution of WO3 to spectroscopic features could be ignored. Thus, it can be deduced that the introduction of WO3 facilitates the redispersion of vanadia and results in the polymerization of vanadia species. Interestingly, the adsorption edge of the W-modified catalyst shifts significantly to a lower wavelength of 516 nm after sulfation, while the adsorption edge decreases slightly to 532 nm for VSbTi-S. This suggests that sulfation treatment leads to a more significant loss of polymeric vanadia species for the WO3-modified catalyst. there is weak absorption at wavelengths higher than 600 nm for the fresh catalysts. This should be attributed to the presence of the highly polymerized vanadia species, which results in the band broadening and red-shift to a wavelength higher than 600 nm [52]. After sulfation, these highly polymerized vanadates disappear over VSbWTi-S and VSbTi-S since no absorption at the wavelength above 600 nm could be observed anymore. A similar phenomenon has also been discovered by Youn et al. [48].
To compare the sulfate deposition on the sulfated catalysts, the outlet gases (i.e., SO2, NH3, H2O, and O2) during the temperature-programmed decomposition process were analyzed by MS, and the results are provided in Figure 3. The releases of H2O, O2, and NH3 at temperatures below 230 °C are attributed to the water, NH3, and other surface-adsorbed impurities (e.g., hydroxyl and carbonate groups originated from the decomposition of additives during the calcination process) adsorbed on the catalysts [53]. In the temperature range of 230–370 °C, NH3 release is observed, which is attributed to the decomposition of (NH4)2SO4 to NH4HSO4 and NH3 [20,54,55]. Subsequently, NH4HSO4 decomposes into NH3, SO2, O2, and H2O at higher temperatures (370–530 °C). It has been reported that the decomposition of metal sulfates occurs at higher temperatures than ammonium sulfates [56,57]. Clearly, a much larger amount of NH3, SO2, O2, and H2O are released from VSbWTi-S in the range of 230–710 °C (Figure 3b). This indicates that larger amounts of ammonium sulfates and metal sulfates deposit on the W-modified catalyst during sulfation. The bimodal feature of the SO2 signal in the range of 330–460 °C is likely related to the decomposition of ammonium bisulfates bonded to different metal sites [58,59]. It is noted that the O and SO2 signals do not follow the same profile. In Figure 3b, the SO2 signal indicates a high contribution at around 400 °C and a smaller one close to 600 °C. When observing the O signal, the case is the opposite, with a low contribution at around 400 °C and a large contribution above 600 °C. The disappeared O signal at around 400 °C can be explained by NH3 oxidation. It may be observed that the release window of NH3 overlaps with those of SO2 and O2 at temperatures of about 400 °C due to the decomposition of ammonium sulfates. Part of the released O2 is consumed with the catalytic oxidation of NH3, resulting in a reduced O signal. At temperatures above 600 °C, the decomposition of metal sulfates does not produce NH3, and the released O2 is well maintained. Based on previous studies [36,54,60,61] and our work, the scheme of the formation and decomposition of AS and ABS is summarized in Figure S2.
The sulfates deposit on the catalysts were quantified by TG and the profiles are portrayed in Figure 4. The weight loss data are summarized in Table 1. Fresh catalysts only indicate one significant weight loss stage (Stage I), which is associated with the desorption of surface-adsorbed H2O. Aside from this, sulfated catalysts display two other typical weight loss features (Stages II and III), and their weight losses in Stage I are larger than those of fresh counterparts due to H2O and NH3 adsorption during the sulfation treatment. Combining the MS results with the previous studies [40,62], the weight loss in Stage II should be assigned to the decomposition of ammonium sulfates, and that in Stage III can be ascribed to the decomposition of metal (i.e., V, Sb, Ti and W) sulfates. As listed in Table 1, the amount of ammonium sulfates and metal sulfates deposited on VSbWTi-S is about 9 and 3 times of that on VSbTi-S, respectively. This implies that WO3 modification significantly accelerates the sulfate deposition on the catalyst, in accordance with the MS results.

3.3. Surface Properties

To evaluate the redox properties of the fresh and sulfated catalysts, H2-TPR tests were performed and the results are illustrated in Figure 5. Both fresh catalysts indicate one major reduction peak between 350 to 500 °C, assigned to the reduction from V5+ to V3+ [48,63]. This peak shifts towards higher temperatures with nearly unchanged H2 consumption after WO3 modification, which is attributed to the transformation of monomeric VOx to polymeric VOx species [48,64], in line with the UV-vis results (Figure 2). As for VSbWTi-S, a typical bimodal reduction characteristic is observed. The sharp peak centered at 448 °C arises from the reduction of sulfates, and the shoulder at 418 °C is assigned to the reduction of vanadia. Additionally, its total H2 consumption (634 μmol/g) is much larger than the fresh counterpart due to the contribution of sulfate reduction. VSbTi-S indicates a similar bimodal reduction curve but with much smaller H2 consumption (317 μmol/g). These results verify that much more sulfates deposit on the W-modified catalyst during sulfation. The low-temperature reduction peak shifts toward lower temperature after sulfation, and the temperature drop for VSbWTi-S (21 °C) is much larger than that of VSbTi-S (4 °C), demonstrating a more severe loss of polymeric vanadia species for the former catalyst. It should be noted that some gases other than H2 can also contribute to the TCD response and interfere with the TPR profiles, which would cause some disturbance to the above H2 consumption analysis.
Figure 6 portrays the NH3-TPD curves of the catalysts. Both fresh catalysts exhibit similar desorption curves, illustrating that the introduced WO3 has little effect on the surface acidity of the catalyst. After sulfation, the amount of total NH3 desorption increases by 36% for VSbTi-S and 223% for VSbWTi-S. This can be explained primarily by the deposited sulfates that can also serve as acid sites [65,66]. Additionally, the decomposition of ammonium sulfates deposited on the catalyst surface contributes to NH3 production during NH3-TPD measurement, as clarified by MS analysis (Figure 3). The significantly increased NH3 desorption of VSbWTi-S consolidates that W modification has a detrimental impact on the SO2 resistance of VSbWTi catalyst that much more sulfates deposit on the W-modified catalyst upon sulfation.

3.4. Reactivity of Ammonium Bisulfate with NO

The sulfate generation on the catalyst is closely associated with the catalytic oxidation of SO2. Thus, the SO2 oxidation activity of fresh catalysts was measured, and the results are illustrated in Figure S3. Compared with VSbTi, VSbWTi exhibits a stronger ability to oxidize SO2 to SO3. As the first step of sulfate deposition, it is consistent with the result that abundant sulfates deposit on VSbWTi-S. NH4HSO4 (ABS) is generally the primary type of ammonium sulfate at low temperatures. Its accumulation on the catalyst depends on not only the generation of this salt, but also its decomposition via reaction with NO and O2 to produce N2, H2O, and SO2 [20].
Figure 7 indicates the TPSR profiles of the fresh catalysts impregnated with ABS in a NO + O2 atmosphere without NH3. In this case, ABS becomes the only source of ammonia for SCR reactions, and the variations of NOx conversion reflect the reactivity of deposited NH4HSO4 with NO over different catalysts. Two NOx conversion peaks occur at 195–200 °C and 370–400 °C, corresponding to the reaction of NO with NH4+ in NH4HSO4 and that with NH3 coming from NH4HSO4 decomposition, respectively [60]. The low-temperature peak appears similar for the two catalysts, while the high-temperature peak is significantly retarded to a higher temperature (400 °C) over ABS/VSbWTi. These results indicate that the WO3 modification not only promotes the generation of sulfates by enhanced catalytic oxidation of SO2 to SO3, but also weakens the decomposition of ammonium bisulfate with lower reactivity of ABS with NO. As a result, more severe deposition of ammonium sulfate species takes place on VSbWTi-S. Similarly, phenomena of more metal sulfate deposition may exist over this catalyst.

4. Discussion

It has been demonstrated in our previous work that Sb2O3-modified V2O5/TiO2 catalyst (denoted as VSbTi) exhibited excellent deNOx performance in the presence of SO2 because the introduction of Sb2O3 can not only weaken the SO2 oxidation, but also enhance the reactivity of NH4HSO4 with NO [40]. In this work, WO3 is further introduced into VSbTi monolithic catalyst for improving the low-temperature SCR activity to meet increasingly stringent emission regulations (Figure 1a). The UV-vis and H2-TPR results indicate that some monovanadates are transformed into polymeric vanadates in the WO3 modified catalyst. It is well known that polymeric vanadate is more beneficial for NH3-SCR reactions than monovanadate. These results suggest that the introduction of WO3 in the VSbTi catalyst could change the dispersion of vanadia species and form more polymeric vanadates, which account for the enhanced NH3-SCR activity of the VSbWTi catalyst. However, the durability of low-temperature SCR catalysts in the presence of SO2 is always another important concern, and the WO3 modification results in more severe deactivation in durability tests with SO2 (Figure 1b). It has been discovered that more ammonium sulfates and metal sulfates deposit on the W-modified catalyst as confirmed by the UV-vis (Figure 2), MS-TPD (Figure 3), TG (Figure 4), H2-TPR (Figure 5), and NH3-TPD results (Figure 6). Moreover, the UV-vis and H2-TPR results demonstrate more losses of polymeric vanadates over VSbWTi-S compared with VSbTi-S. The serious physical coverage of active sites by sulfates and the transformation of polymeric vanadates to metal sulfates are responsible for the more serious deactivation of the WO3-modified catalyst upon sulfur attacking from the reaction atmosphere.
To further elucidate the different sulfur tolerances of the catalysts, SO2 oxidation test and TPSR test with ABS were employed. Compared with VSbTi, higher catalytic oxidation of SO2 to SO3 is achieved over the W-modified catalyst (Figure S3). As indicated in Figure S2, this reaction is the initial step of the formation of sulfates on the catalyst during NH3-SCR reactions. The generated SO3 reacts with both NH3 and catalyst components to deposit AS/ABS and metallic sulfates, respectively. These sulfates not only block the pore of catalysts (Table S1), but also reduce the availability of vanadate active sites [67,68,69]. The deposition of sulfates depends on the competition between the formation of sulfates and their consumption. The latter factor can be evaluated by the catalytic reaction of ABS (as a representative of deposited sulfates) with NO. The TPSR test (Figure 7) illustrates that the introduction of WO3 hinders the consumption of impregnated ABS with NO, which is in line with the result that WO3 modified catalyst displayed low sulfur resistance [70]. In this way, much more ammonium sulfates and metal sulfates are deposited over the VSbWTi catalyst upon sulfur exposure. The modification of WO3 promotes the NH3-SCR activity at low temperatures (<300 °C) but lowers sulfur resistance. These results are expected to give inspiration for designing strategies to gain satisfying low-temperature SCR catalysts under different application scenarios.

5. Conclusions

In this work, VSbTi and VSbWTi monolithic catalysts were prepared by an extrusion-molding method. WO3 modification is found to promote the low-temperature (<320 °C) NH3-SCR activity of V2O5-Sb2O3/TiO2 catalyst, but weaken its sulfur resistance at 220 °C. On the one hand, WO3 modification changes the dispersion of vanadia species to form more polymeric vanadates, which are more active sites than monomeric vanadates. On the other hand, the introduction of WO3 leads to enhanced SO2 oxidation ability and retarded reactivity of deposited NH4HSO4 with NO. As a result, more ammonium sulfates and metal sulfates deposit on the WO3-modified catalyst during the durability test in the presence of SO2 and H2O, and lower sulfur resistance was discovered for this catalyst.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr10071333/s1, Figure S1: XRD patterns of the catalysts; Figure S2: Scheme of formation and decomposition of AS and ABS; Figure S3: SO2 oxidation curves of the catalysts. Reaction conditions: SO2 = 500 ppm, O2 = 5%, N2 in balance, and GHSV = 50,000 h−1; Table S1: BET surface areas of catalysts.

Author Contributions

Experimental preparation and operation, writing—original draft, and employment of software, L.L. and Y.M.; writing—review and editing, X.W. and J.W.; supervision, R.R. and Z.S.; funding acquisition, D.W. All authors have read and agreed to the published version of the manuscript.

Funding

The authors gratefully acknowledge financial support from the National Key Research and Development Program of China (No. 2017YFC0211202).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. NOx conversions of the monolithic catalysts: (a) NH3-SCR activity; (b) durability test at 220 °C. Reaction conditions: NO = NH3 = 1000 ppm, O2 = 5%, H2O = 10%, SO2 = 350 ppm, N2 in balance, and GHSV = 5000 h−1.
Figure 1. NOx conversions of the monolithic catalysts: (a) NH3-SCR activity; (b) durability test at 220 °C. Reaction conditions: NO = NH3 = 1000 ppm, O2 = 5%, H2O = 10%, SO2 = 350 ppm, N2 in balance, and GHSV = 5000 h−1.
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Figure 2. UV-vis spectra of the VSbTi, VsbWTi, VsbTi-S and VsbWTi-S catalysts.
Figure 2. UV-vis spectra of the VSbTi, VsbWTi, VsbTi-S and VsbWTi-S catalysts.
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Figure 3. Gaseous products during the temperature-programmed decomposition of (a) VSbTi-S and (b) VsbWTi-S catalysts.
Figure 3. Gaseous products during the temperature-programmed decomposition of (a) VSbTi-S and (b) VsbWTi-S catalysts.
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Figure 4. TG profiles of the VSbTi, VSbWTi, VSbTi-S, and VSbWTi-S catalysts.
Figure 4. TG profiles of the VSbTi, VSbWTi, VSbTi-S, and VSbWTi-S catalysts.
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Figure 5. H2-TPR curves of the VSbTi, VSbWTi, VSbTi-S, and VSbWTi-S catalysts.
Figure 5. H2-TPR curves of the VSbTi, VSbWTi, VSbTi-S, and VSbWTi-S catalysts.
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Figure 6. NH3-TPD profiles of the VSbTi, VSbWTi, VSbTi-S, and VSbWTi-S catalysts.
Figure 6. NH3-TPD profiles of the VSbTi, VSbWTi, VSbTi-S, and VSbWTi-S catalysts.
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Figure 7. TPSR profiles of NH4HSO4 with NO over the catalysts impregnated with ABS. Reaction conditions: NO = 1000 ppm, O2 = 5%, N2 in balance and GHSV = 100,000 h−1.
Figure 7. TPSR profiles of NH4HSO4 with NO over the catalysts impregnated with ABS. Reaction conditions: NO = 1000 ppm, O2 = 5%, N2 in balance and GHSV = 100,000 h−1.
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Table
Table 1. Weight loss of sulfated catalysts derived from the TG results.
Table 1. Weight loss of sulfated catalysts derived from the TG results.
CatalystWeight Loss (%)
Stage II (230–460 °C)Stage III (460–670 °C)
VSbTi-S0.280.38
VSbWTi-S2.541.27
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Liu, L.; Wu, X.; Ma, Y.; Wang, J.; Ran, R.; Si, Z.; Weng, D. Tungsten Oxide Modified V2O5-Sb2O3/TiO2 Monolithic Catalyst: NH3-SCR Activity and Sulfur Resistance. Processes 2022, 10, 1333. https://doi.org/10.3390/pr10071333

AMA Style

Liu L, Wu X, Ma Y, Wang J, Ran R, Si Z, Weng D. Tungsten Oxide Modified V2O5-Sb2O3/TiO2 Monolithic Catalyst: NH3-SCR Activity and Sulfur Resistance. Processes. 2022; 10(7):1333. https://doi.org/10.3390/pr10071333

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Liu, Liping, Xiaodong Wu, Yue Ma, Jinyi Wang, Rui Ran, Zhichun Si, and Duan Weng. 2022. "Tungsten Oxide Modified V2O5-Sb2O3/TiO2 Monolithic Catalyst: NH3-SCR Activity and Sulfur Resistance" Processes 10, no. 7: 1333. https://doi.org/10.3390/pr10071333

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