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Article

Improved Sulfur Resistance of COMMERCIAl V2O5-WO3/TiO2 SCR Catalyst Modified by Ce and Cu

1
Center for Excellence in Regional Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China
2
Key Laboratory of Urban Environment and Health, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China
3
University of Chinese Academy of Sciences, Beijing 100049, China
*
Authors to whom correspondence should be addressed.
Submission received: 10 June 2021 / Revised: 16 July 2021 / Accepted: 25 July 2021 / Published: 27 July 2021

Abstract

:
The accumulation of NH4HSO4 leads to the deactivation of commercial V2O5-WO3/TiO2 catalyst (VWTi) in practical application. The commercial catalyst is modified with 0.3 wt. % Ce and 0.05 wt. % Cu (donated as VWCeCuTi), and its sulfur resistance is noticeably improved. After loading 20 wt. % NH4HSO4, the NOx conversion of VWCeCuTi-S remains 40% at 250 °C, higher than that of VWTi-S (25%). Through a series of characterization analyses, it was found that the damaged surface areas and acid sites are the key factors for the deactivation of S-poisoned samples. However, surface-active oxygen and NO adsorption are increased by NH4HSO4 deposition, and the L–H mechanism is promoted over S-poisoned samples. Due to the interaction between V, Ce and Cu, the surface-active oxygen over VWCeCuTi-S is increased, and then NO adsorption is promoted. In addition, VWCeCuTi-S obtains a higher V5+ ratio and a better redox property than VWTi-S, which in turn accelerates the NH3-SCR reaction. More NO adsorption and encouraged reaction contribute to the better sulfur resistance of VWCeCuTi.

Graphical Abstract

1. Introduction

Nitrogen oxides (NOx) emitted from stationary and mobile sources have attracted much attention since they can cause a series of environmental issues, such as greenhouse effects, acid rain, ozone depletion and photochemical smog [1]. Selective catalytic reduction of NOx with ammonia (NH3-SCR) is considered to be the most effective and common technology in controlling NOx emissions [2]. As a typical commercial NH3-SCR deNOx catalyst, the V2O5-WO3/TiO2 catalyst shows excellent medium temperature activity, high N2 selectivity and good thermal stability [3,4]. Despite the low sensitivity to SO2, deactivation resulting from SO2 poisoning does occur over V2O5-WO3/TiO2 catalyst with service time [5,6,7]. The accumulation of NH4HSO4 (ABS) forming at low load has been reported as the primary cause for the catalyst deactivation in practical application [8,9,10].
To prolong the service life of the catalyst, great efforts have been invested to improve the sulfur resistance of V2O5-WO3/TiO2 catalyst [11,12,13,14,15]. Xu et al. [13] reported that Sb-modified monolithic catalyst (V2O5-Sb2O3/TiO2) exhibited a better sulfur resistance than the commercial V2O5-WO3/TiO2 catalyst due to the lower deposition of ABS. Li et al. [14] doped Ce to VWTi catalyst, and the results showed that the addition of Ce could weaken the stability of ABS and promote the reaction between NH4+ species from ABS and NO, leading to a better ABS resistance. Ye et al. [15] found that the introduction of Sb2O5 and Nb2O5 to VWTi SCR catalyst could enhance the decomposition of ABS, and promote the reaction between ABS and NO, contributing to a better sulfur resistance. Kang et al. [16] physically mixed V2O5-WO3/TiO2 catalyst with Fe2O3 samples, and the physically mixed catalysts exhibited higher catalytic stability and SO2 resistance than the V2O5-WO3/TiO2 catalyst. However, the sulfur content in these studies is far less than that in the practical industry after running for a long time.
Recently, because of their unique features, Ce and Cu have been used to improve the SCR performance of commercial VWTi catalyst. Liang et al. [17] added 3% of the CeO2 to V2O5-WO3/TiO2 catalyst and found that its low-temperature NO conversion and sulfur and water resistance were improved. Peng et al. [18] and Liu et al. [19] modified V2O5-WO3/TiO2 catalyst with Ce and reported that its NO conversion and alkali resistance were enhanced. Wang et al. [20] introduced Cu and Fe to V2O5-WO3/TiO2 catalyst and found that Cu or Fe addition, especially Cu, improved the catalytic performance of V2O5-WO3/TiO2. Additionally, Ali et al. [21] suggested that the addition of Cu, even in fairly small amounts, could improve the NH3-SCR performance of CeO2/TiO2 catalyst due to the interaction between Ce and Cu. All of the above studies demonstrate that some interactions exist between V, Ce and Cu, contributing to better SCR performances. However, to our knowledge, there is little research on investigating the cooperative effect of Ce and Cu over the V2O5-WO3/TiO2 catalyst for the selective catalytic reduction of NO by NH3.
Taking the advantage of interactions between Ce, Cu and V, in this work, commercial V2O5-WO3/TiO2 catalyst was first modified with Ce(SO4)2 and Cu(NO3)2, and then the sulfur resistance of modified catalyst was investigated by loading a certain amount of NH4HSO4 on it through a wet impregnation method. Finally, the structural properties as well as chemical properties were systematically studied.

2. Results and Discussion

2.1. NOx Conversion

Considering the decomposition temperature of ABS, the NH3-SCR performance of fresh and ABS-deposited catalysts were evaluated at 250 °C and the results are presented in Figure 1. The NOx conversion of VWTi catalyst stabilizes at about 73% in the whole test process, and that of VWCeCuTi is about 84%. The activities of VWTi and VWCeCuTi in a larger temperature range of 200–400 °C were also tested. As shown in Figure S1, VWCeCuTi shows a general trend of better performance than VWTi. These indicate that the addition of Ce and Cu improves the activity of VWTi catalyst. After being deposited by 20 wt. % ABS, the NOx conversion of VWTi-S declines sharply to 25%, and that of VWCeCuTi-S declines to 40%, implying that ABS deposition severely damages the activity of catalysts. Nevertheless, VWCeCuTi-S possesses a higher activity than VWTi-S. N2O is a harmful byproduct of the NH3-SCR reaction, and its concentration in the de-NOx process is presented in Figure S2. During the whole process, the concentration of N2O over all catalysts is no more than 2 ppm, indicating that ABS deposition has no distinct influence on N2O formation.

2.2. Structure Investigation

XRD patterns of fresh and S-poisoned catalysts are displayed in Figure 2. Only the peaks belonging to anatase TiO2 could be detected on all samples, indicating that neither modification nor ABS deposition makes a difference to the crystalline structure of TiO2 support. Active species and ABS are highly dispersed or amorphous on the surface. N2 physisorption isotherms of fresh and S-poisoned catalysts are presented in Figure S3. Similar isotherms attributed to type IV with H1 hysteresis loops are observed on all samples, suggesting that modification and ABS deposition do not change the pore type of VWTi catalyst. BET surface area, total pore volume and average pore diameter based on N2 physisorption isotherm are summarized in Table 1. No big differences in surface areas exist between VWTi and VWCeCuTi. However, their specific surface areas are seriously damaged by ABS deposition, dropping from 44.1 and 41.3 m2·g−1 to 23.7 and 23.5 m2·g−1, respectively. Meanwhile, the total pore volumes of S-poisoned samples decrease significantly, while their average pore diameters increase to some extent. This may be because some small pores are blocked by ABS. Though ABS deposition blocks pores, which is a main factor for sulfur poisoning, there is no big differences between the two S-poisoned samples.
Sulfur species over these catalysts were analyzed by TGA, and the relative profiles are shown in Figure 3. The profiles of all samples are divided into three stages: Step I (below 200 °C) could be ascribed to the desorption of adsorbed H2O; Step II (200–535 °C) is assigned to the decomposition of NH4HSO4 or (NH4)2SO4; Step III (535–900 °C) is attributed to the decomposition of bulk sulfate species such as vanadium sulfate [22,23,24]. The weight losses at each stage are summarized in Table 2. It was found that there is a small amount of sulfur species on fresh samples, coming from the preparation processes of catalysts. After loading ABS, the weight losses over both samples increase significantly at Stage II, and show a slight increase at Stage III, indicating sulfur species over the two S-poisoned samples are mainly in the form of NH4HSO4 or (NH4)2SO4, and the contents of sulfur species on the two samples are almost the same.

2.3. Surface Analysis by XPS

XPS analysis was carried out to gain an insight into the near surface atomic compositions and chemical states of elements. The obtained XPS spectra of Ti 2p, S 2p, V 2p, Ce 3d and O 1s are displayed in Figure 4, and their relative atomic concentrations are listed in Table 3 and Table S1. As shown in Figure 4a, the binding energies of Ti 2p3/2 (458.8 eV) and Ti 2p1/2 (464.5 eV), which are characteristic peaks of Ti4+ in TiO2, are detected over a VWTi sample, illustrating that Ti atoms are in the +4 oxidation state on the catalyst surface [25,26]. Compared with VWTi, no obvious variations in the binding energies of Ti 2p peaks are observed over VWCeCuTi, indicating that the addition of Ce and Cu nearly has no effect on the TiO2 support, which may be due to the low contents of these additives. Given the deposition of ABS, the peaks shift towards higher binding energies, indicating that the electron cloud density around Ti atoms is decreased. It should be caused by the interaction between TiO2 and ABS, which may decrease the coordination number of Ti and shorten the Ti-O bond through the Ti–O–S bond, leading to an electron-deficit state of Ti atoms [27,28,29].
In Figure 4b, after deconvolution, two peaks centered at ca. 169.7 and ca. 168.6 eV are obtained, which are the characteristic peaks of S6+ in bidentate sulfate, and could be assigned to S 2p1/2 and S 2p3/2, respectively [6,15,23,30]. The S 2p spectra observed on fresh samples are due to the small amount of S contained in commercial catalyst. After loading ABS, the intensities of S 2p spectra increase significantly, suggesting the deposition of sulfate species on S-poisoned samples, which is in good agreement with the data collected from TGA. No characteristic peaks belonging to S4+ are detected over these catalysts, indicating that sulfite species likely do not exist on these catalysts’ surface, which is probably because all samples are calcined in the air during their preparation processes.
V 2p spectra are shown in Figure 4c. After deconvolution, two peaks located at ca. 516.7 and ca. 515.7 eV are observed, which could be attributed to V5+ and V4+, respectively [31,32]. It has been reported that a superior content of V5+ exerts a promotion effect on the dehydrogenation process of ammonia species, which is a key step in the NH3-SCR reaction [33,34]. V5+ ratio is calculated by V5+/(V4+ + V5+), and the results are shown in Table 2. After introducing Ce and Cu, the V5+ ratio of VWTi increases from 42.2 to 54.1%, which is due to the interaction between V, Ce and Cu through the redox cycles of V5+/V4+, Ce4+/Ce3+ and Cu+/Cu2+ [35,36]. Moreover, the V5+ ratio is increased by loading ABS, which are 58.5 and 63.9% over VWTi-S and VWCeCuTi-S respectively. It indicates that ABS deposition increases the valence state of surface V atoms, in accordance with other studies [15,34,37]. These studies demonstrate that electrons from V atoms would be attracted by sulfate anions with stronger electronegativity, which is responsible for the increased valence state of V atoms on the ABS-deposited samples. Nevertheless, the surface concentration of V decreases by more than half after ABS deposition, which could be attributed to the coverage of surface V sites by ABS [23,29]. As a result, the contents of V5+ on S-poisoned samples are still lower than those of fresh ones. In addition, VWCeCuTi-S obtains a higher V5+ content in favor of the catalytic activity when compared with VWTi-S in isolation.
Ce 3d spectra of VWCeCuTi and VWCeCuTi-S are presented in Figure 4d. The bands labeled as “v” and “u” correspond to Ce 3d5/2 and Ce 3d3/2 spin–orbit components, respectively. Furthermore, the bands v and u are related to the 3d104f1 initial electronic state of Ce3+ species, while the rest of the bands are ascribed to the 3d104f0 initial electronic state of Ce4+ species, revealing the co-existence of Ce3+ and Ce4+ on both samples [38,39,40]. Furthermore, Ce3+ is demonstrated to produce oxygen vacancies by generating a charge imbalance, and then promote NO oxidation [14,41]. For the concentration of surface Ce atoms, it also decreases after loading ABS due to the coverage of catalyst surface. No obvious Cu 2p peaks are observed because of the low content of Cu species, as shown in Figure S4.
Figure 4e displays O 1s spectra of the series samples. After deconvolution, O 1s spectra are fitted into three characterization peaks, attributed to the lattice oxygen (Oγ) at ca. 530.1–530.3 eV, surface adsorbed oxygen (Oβ) like hydroxyl species at ca. 531.6–531.8 eV and weakly bonded oxygen species (Oα) at ca. 532.8–533.0 eV, respectively [5,42]. It has been reported that Oβ is more active in oxidation reaction and beneficial to NO adsorption [43,44,45]. The ratio of Oβ is calculated by Oβ/(Oα + Oβ + Oγ) and is summarized in Table 2. After introducing Ce and Cu, the Oβ ratio increases from 12.1 to 13.1%. Compared with fresh samples, the ratios of Oβ over both S-poisoned samples increase significantly, the main reason for which is the formation of abundant hydroxyl species through hydration of SO42− [23,46]. In addition, the O 1s bands shift to higher binding energies after ABS deposition, indicating that the electron cloud density around O atoms is decreased by ABS, owing to the stronger electronegative of SO42−. However, comparing the two poisoned samples, Oβ ratio over VWCeCuTi-S is 27.4%, higher than that over VWTi-S (22.9%). Then, the more surface-active oxygen over VWCeCuTi-S may contribute to more NO adsorption, which will be further clarified below.

2.4. Redox Property

It is generally considered that redox property plays an important role in NH3-SCR reaction, and is usually investigated by H2-TPR. As Figure 5 shows, two reduction peaks gradually appear on the profile of VWTi catalyst with temperature increasing. The first one centered at 603 °C could be assigned to the reduction of V5+ to V3+, and the second one centered at 690 °C could be ascribed to the reduction of W6+ to W4+ [31,47]. After loading Ce and Cu, a new peak at 523 °C appears, which could be ascribed to the reduction of Ce4+ to Ce3+ [47,48,49]. Because of its low content, no peaks belonging to the reduction of Cu2+ are detected. It is generally recognized that a lower reduction peak implies a stronger reducibility [37,50], so VWCeCuTi exhibits a stronger redox property than VWTi.
A strong H2 consumption peak at 597 °C is detected over VWTi-S, which could be attributed to the couple reduction of V5+ and SO42− species [6]. Compared with VWTi, the peak position of V5+ to V3+ remains nearly unchanged over VWTi-S, but the intensity increases significantly due to the reduction of surface SO42− species. Compared with VWCeCuTi, both reduction peaks of VWCeCuTi-S shift to higher temperatures (523–548 °C and 603–624 °C), indicating that its reducibility is slightly inhibited by ABS deposition. The intensity of the first consumption peak also increases sharply due to the overlapped reduction of Ce4+ and SO42− species. With a lower reduction peak at 548 °C, VWCeCuTi-S still exhibits a better redox property than VWTi-S.

2.5. Surface Acidity and NO Adsorption

In situ DRIFT spectra of NH3 adsorption were collected to investigate the acid property of fresh and S-poisoned catalysts. In each test, the sample pre-adsorbs 2000 ppm NH3 for 30 min, followed by purging with N2 for 10 min, and the spectra of four catalysts are summarized in Figure 6. On VWTi catalyst, five NH3 adsorption bands are detected between 1800 and 1000 cm−1. The bands centered at 1604 and 1259 cm−1 could be assigned to asymmetric and symmetric bending vibrations of the N-H bonds in NH3 coordinately linked to Lewis acid sites, the bands at 1668 and 1450 cm−1 could be attributed to asymmetric and symmetric bending vibrations of NH4+ species on the Brønsted acid sites, and the band at 1365 cm−1 could be ascribed to the amide (NH2) species [51,52]. The same bands are observed over the VWCeCuTi sample, but the intensities of bands at 1668 and 1450 cm−1 are much higher than those over VWTi, suggesting that Brønsted acid sites are increased by the modification with Ce and Cu. However, only one weak band that belongs to amide (NH2) species could be detected over S-poisoned samples. This result is different from the findings of some SO2 in situ poisoning studies, in which Brønsted acid sites increase after SO2 and H2O treatment due to the formation of surface OH species through the hydration of SO42− [31,53]. In this work, the seriously damaged surface acid sites may be a result of ABS cover [54,55], which is another important factor for the deactivation of S-poisoned samples.
NO adsorption over fresh and S-poisoned catalysts were investigated by in situ DRIFTS. Each sample was first exposed to 2000 ppm NO + 5% O2 for 30 min, purged by N2 for 10 min, and then spectra were collected. As displayed in Figure 7, only a weak band at 1615 cm−1 attributed to gaseous NO2 is recorded over VWTi and VWCeCuTi, indicating that NO adsorption over fresh samples is weak [56]. However, three obvious bands at 1676, 1484 and 1348 cm−1 are observed over S-poisoned samples, which could be ascribed to N2O4, monodentate nitrate and cis-N2O22−, respectively [57,58,59]. The result suggests that the adsorption ability of NO is efficiently improved after ABS deposition. Furthermore, the intensity of the band at 1484 cm−1 over VWCeCuTi-S is much stronger than VWTi-S, indicating that the formation of monodentate nitrate is largely increased by modification with Ce and Cu. Combined with the result of O 1s spectra, the stronger NO adsorption could be attributed to the more surface-active oxygen over VWCeCuTi-S.

2.6. Reaction between Ammonia and Adsorbed Nitrogen Oxides Species

As discussed above, NO adsorption over S-poisoned catalysts is improved, so further experiments were conducted to investigate if these adsorbed NOx species could participate in the NH3-SCR reaction. Figure 8 presents the DRIFT spectra of VWTi-S and VWCeCuTi-S in a flow of 2000 ppm NH3 after catalysts pre-adsorbed NO + O2 for 30 min and purged with N2 for 10 min. As shown in Figure 8a, after introducing NH3, the intensities of NO adsorption peaks over VWTi-S decrease gradually, and disappear completely within 5 min. Subsequently, a new band (1327 cm−1) ascribed to the NH2 species appears. It indicates that all the adsorbed NOx species could take part in the SCR reaction. Similarly, as shown in Figure 8b, NO adsorption peaks over VWCeCuTi-S decrease and disappear within 3 min after the introduction of NH3, and a new peak (1327 cm−1) assigned to the NH2 species appears. These results demonstrate that the SCR reaction over S-poisoned samples involves adsorbed ammonia species as well as adsorbed NOx species, following the L–H mechanism. Given the stronger intensity and faster disappearance of NO adsorption peaks over VWCeCuTi-S, it is inferred that the NH3-SCR reaction over VWCeCuTi-S is faster than that over VWTi-S.
According to our previous work [35], the NH3-SCR reaction over VWTi and VWCeCuTi mainly follows the Eley–Rideal mechanism, in which adsorbed ammonia species react with gas-phase NO. However, after loading ABS, the Langmuir–Hinshelwood mechanism (adsorbed ammonia species react with adsorbed NOx species) is promoted by enhanced NOx adsorption. Furthermore, the reaction is enhanced by Ce and Cu modification.

2.7. Possible Mechanism of Ce- and Cu-Promoting Effects on S-Poisoned Commercial Catalyst

According to the results of in situ DRIFTS experiments, Brønsted acid sites over VWTi are significantly increased by the modification with Ce and Cu, promoting NH3 adsorption. Meanwhile, NO adsorption over VWTi and VWCeCuTi is weak, indicating that Ce and Cu modification does not change the mechanism of VWTi (following the E–R mechanism), but enhances the activity by providing more Brønsted acid sites. After introducing ABS, on one hand, acid sites over S-poisoned samples decrease sharply; on the other hand, NO adsorption is significantly enhanced, especially that over VWCeCuTi-S, indicating that the L–H mechanism is promoted over S-poisoned samples. Combined with the results of BET, TGA and XPS, the decrease in acid sites is mainly due to the coverage of catalyst surface by ABS, leading to the deactivation of S-poisoned samples. However, based on the XPS result of O 1s, the surface-active oxygen is increased by ABS deposition, which is suggested to be beneficial to NO adsorption [45,60,61]. Additionally, the surface-active oxygen over VWCeCuTi-S is more than that over VWTi-S. In our previous study, it is inferred that electron transfer exists between V, Ce and Cu through the following redox processes [35]:
V5+ + Ce3+→Ce4+ + V4+
V4+ + Cu2+→Cu+ + V5+
Cu+ + Ce4+→Ce3+ + Cu2+
These redox cycles contribute to the co-existence of Ce4+/Ce3+ and Cu+/Cu2+ on VWCeCuTi as well as VWCeCuTi-S. Meanwhile, it has been reported that Ce3+ and Cu+ could create oxygen vacancy, and produce active oxygen [21,23], as shown below:
O 2 + 2 Ce 3 + 2 O ad + 2 Ce 4 +
O 2 + 2 Cu + 2 O ad + 2 Cu 2 +
Therefore, because of the interaction between V, Ce and Cu, VWCeCuTi-S obtains more surface-active oxygen than VWTi-S, and then performs stronger NO adsorption. Additionally, the interaction between V, Ce and Cu makes VWCeCuTi-S obtain a higher V5+ ratio and better redox property than VWTi-S. Ammonia activation is a key step in the NH3-SCR reaction, which mainly happens on V5+ = O for commercial VWTi catalyst, and a superior redox property would be better for the activation [3,33,34]. So, the higher V5+ ratio and better redox property would promote the NH3-SCR reaction over VWCeCuTi-S. Overall, Ce and Cu modification promotes NO adsorption and NH3-SCR reaction on VWCeCuTi-S, leading to a better performance than VWTi-S. The proposed mechanism of sulfur resistance over VWCeCuTi is generalized in Scheme 1.

3. Experimental

3.1. Catalyst Preparation

A commercial V2O5-WO3/TiO2 SCR catalyst (donated as VWTi below) from a coal-fired power plant in China was used in this investigation, of which the contents of V2O5 and WO3 were 2% and 3%, respectively. First, the modified catalyst containing 0.3 wt. % Ce and 0.05 wt. % Cu (donated as VWCeCuTi below) was prepared by the wet impregnation method according to our previous work [35]. Second, for the S-poisoned samples, 20 wt. % ABS, which was determined according to the results in Figure S5, were separately deposited on VWTi and VWCeCuTi by the impregnation method. The detailed steps were as follows: impregnating both samples with a certain amount of NH4HSO4 solution for 12 h at room temperature, drying the mixtures at 105 °C for 12 h, and then calcining them at 300 °C for 5 h in air. The obtained catalysts are crushed and sieved to 40–60 mesh and donated as VWTi-S and VWCeCuTi-S, respectively.

3.2. NH3-SCR Activity Test

NOx conversion was measured in a fixed-bed quartz reactor (ø10 mm × 600 mm). In each test, 0.6 mL of sample was placed in the middle of the reactor, proceeding for 12 h at 250 °C. The reaction gas mixture consisted of 500 ppm NO, 500 ppm NH3, 5 vol. % O2, and N2 as the balance gas. The total flow rate was 600 mL/min, corresponding to the GHSV of 60,000 h−1. The inlet and outlet concentrations of NO, NO2 and N2O were monitored by an Antaris IGS flue gas analyzer (Thermo Scientific, America), and NOx conversion is defined as follows:
NO x   conversion   =   N O x , i n N O x , o u t N O x , i n
where N O x , i n and N O x , o u t represent the inlet and outlet concentrations of NOx (NO + NO2), respectively.

3.3. Catalyst Characterization

The powder X-ray diffraction (XRD) patterns were determined by a X’Pert Pro XRD diffractometer (PANalytical B.V., Holland) using Cu Kα radiation. N2 adsorption-desorption experiments were conducted on a NOVA 2000e surface area and pore size analyzer (Quantachrome, USA) at 77 K. Prior to each test, the sample was degassed under vacuum at 250 °C for 6 h. Thermal Gravimetric Analysis (TGA) was carried out on a Netzch thermoanlyzer (TG-209-F3) with a heating rate of 10 °C/min from 40 to 950 °C in a N2 flow (60 mL/min). X-ray photoelectron spectroscopy (XPS) was performed on an AXIS Supra by Kratos Analytical Inc., using Al Kα radiation (hv = 1486.6 eV, 150 W). Narrow region scans were acquired using a pass energy of 40 and a 0.1 eV step size, and all spectra were calibrated by C 1s (284.8 eV).
A temperature-programmed reduction of H2 (H2-TPR) was recorded on a ChemBET-3000 TPR-TPD chemisorption analyzer (Quantachrome, USA). Prior to each test, about 0.05 g of sample was treated in Ar for 1 h at 300 °C, and then cooled to room temperature. Following this, 5% H2/Ar was switched on, and the temperature was heated to 800 °C linearly with a rate of 10 °C/min. The outlet gas was detected by an online mass spectrometer (MS, DYCOR LC-D100, Ametek Company, USA).
In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) experiments were carried out by a Bruker Vertex 70 infrared spectrometer, equipped with KBr optics and a liquid-nitrogen-cooled MCT detector. Prior to each experiment, the sample was preheated in a N2 flow (80 mL/min) at 300 °C for 1 h and then cooled to 250 °C. Background spectrum was recorded before introducing reactant gases into the cell. DRIFT spectra were recorded through accumulating 64 scans at a resolution of 4 cm−1.

4. Conclusions

The sulfur tolerance of commercial V2O5-WO3/TiO2 catalyst is visibly enhanced by modification with Ce(SO4)2 and Cu(NO3)2. After depositing 20 wt. % NH4HSO4, the NOx conversion of VWCeCuTi-S remains 40% at 250 °C, higher than that of VWTi-S (25%). To explore the contributing factors, various characterization experiments were carried out. It was found that the specific surface areas and surface acid sites are severely damaged by ABS deposition, which are the key factors for the deactivation of S-poisoned samples. However, surface-active oxygen and NO adsorption are increased, and the L–H mechanism is promoted over S-poisoned samples. Based on the results of XPS and H2-TPR, the V5+ ratio, surface-active oxygen and redox property over VWCeCuTi-S are enhanced by the interaction between V, Ce and Cu when compared with VWTi-S. More surface-active oxygen would promote greater NO adsorption. In addition, the higher V5+ ratio and better redox property would accelerate the NH3-SCR reaction over VWCeCuTi-S, in accordance with the results of in situ DRIFTS. The increased NO adsorption and promoted L–H reaction contribute to the better sulfur resistance of VWCeCuTi.

Supplementary Materials

The following are available online at https://0-www-mdpi-com.brum.beds.ac.uk/article/10.3390/catal11080906/s1, Figure S1: NOx conversion of VWTi and VWCeCuTi, Figure S2: N2O concentration of fresh and S-poisoned samples, Figure S3: N2 physisorption isotherms of fresh and S-poisoned catalysts, Figure S4: XPS spectra of Cu 2p, Figure S5: NOx conversion of VWTi catalysts with different NH4HSO4 loading amount at 250 °C: (a) VWTi, (b) VWTi-S-1%, (c) VWTi-S-5%, (d) VWTi-S-10%, (e) VWTi-S-20% and (f) VWTi-S-50%, Table S1: Surface atomic concentration from XPS.

Author Contributions

Conceptualization, H.L., J.C. and J.W.; Formal analysis, H.L. and J.M.; Funding acquisition, J.C. and J.W.; Investigation, H.L. and X.Y.; Project administration, H.L. and J.W.; Resources, Y.C.; Supervision, J.C.; Validation, H.L.; Writing—original draft, H.L. Authorship must be limited to those who have contributed substantially to the work reported. All authors have read and agreed to the published version of the manuscript.

Funding

This work is financially supported by Science and Technology Department of Fujian Province, China [2020Y0085], the Cultivating Project of Strategic Priority Research Program of Chinese Academy of Sciences, Grant No. XDPB1902, and Youth Innovation Promotion Association, Chinese Academy of Sciences [2020309].

Acknowledgments

The authors are grateful to the tests offered by the Analysis and Testing Center, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. NOx conversion of (a) VWTi, (b) VWCeCuTi, (c) VWTi-S and (d) VWCeCuTi-S.
Figure 1. NOx conversion of (a) VWTi, (b) VWCeCuTi, (c) VWTi-S and (d) VWCeCuTi-S.
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Figure 2. XRD patterns of fresh and S-poisoned catalysts.
Figure 2. XRD patterns of fresh and S-poisoned catalysts.
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Figure 3. TGA profiles of fresh and S-poisoned catalysts.
Figure 3. TGA profiles of fresh and S-poisoned catalysts.
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Figure 4. XPS spectra of (a) Ti 2p, (b) S 2p, (c) V 2p, (d) Ce 3d and (e) O 1s.
Figure 4. XPS spectra of (a) Ti 2p, (b) S 2p, (c) V 2p, (d) Ce 3d and (e) O 1s.
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Figure 5. H2-TPR profiles of fresh and S-poisoned samples.
Figure 5. H2-TPR profiles of fresh and S-poisoned samples.
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Figure 6. In situ DRIFT spectra of NH3 adsorption over fresh and S-poisoned samples at 250 °C.
Figure 6. In situ DRIFT spectra of NH3 adsorption over fresh and S-poisoned samples at 250 °C.
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Figure 7. In situ DRIFT spectra of NO adsorption over fresh and S-poisoned samples at 250 °C.
Figure 7. In situ DRIFT spectra of NO adsorption over fresh and S-poisoned samples at 250 °C.
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Figure 8. In situ DRIFT spectra of the reaction between NH3 and adsorbed NO + O2 species over (a) VWTi-S and (b) VWCeCuTi-S at 250 °C.
Figure 8. In situ DRIFT spectra of the reaction between NH3 and adsorbed NO + O2 species over (a) VWTi-S and (b) VWCeCuTi-S at 250 °C.
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Scheme 1. Proposed mechanism of sulfur resistance over VWCeCuTi catalyst.
Scheme 1. Proposed mechanism of sulfur resistance over VWCeCuTi catalyst.
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Table 1. Textural properties of fresh and S-poisoned catalysts.
Table 1. Textural properties of fresh and S-poisoned catalysts.
SampleBET Surface Area (m2·g−1)Total Pore Volume (cm3·g−1)Average Pore Diameter (nm)
VWTi44.10.2320.7
VWCeCuTi41.30.2221.4
VWTi-S23.70.1424.3
VWCeCuTi-S23.50.1526.2
Table 2. Weight losses of catalysts during the TGA processes.
Table 2. Weight losses of catalysts during the TGA processes.
SamplesStep I (wt. %)Step II (wt. %)Step III (wt. %)
VWTi0.70.20.8
VWCeCuTi0.501.6
VWTi-S1.713.62.3
VWCeCuTi-S1.713.92.6
Table 3. Surface atomic ratio and concentration from XPS.
Table 3. Surface atomic ratio and concentration from XPS.
Samples Atomic Ratio (%)Atomic Concentration (at%)
V5+/(V5+ + V4+)Oβ/(Oα + Oβ + Oγ)VCe
VWTi42.212.10.5/
VWCeCuTi54.113.10.60.2
VWTi-S58.522.90.2/
VWCeCuTi-S63.927.40.20.1
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Li, H.; Yi, X.; Miao, J.; Chen, Y.; Chen, J.; Wang, J. Improved Sulfur Resistance of COMMERCIAl V2O5-WO3/TiO2 SCR Catalyst Modified by Ce and Cu. Catalysts 2021, 11, 906. https://0-doi-org.brum.beds.ac.uk/10.3390/catal11080906

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Li H, Yi X, Miao J, Chen Y, Chen J, Wang J. Improved Sulfur Resistance of COMMERCIAl V2O5-WO3/TiO2 SCR Catalyst Modified by Ce and Cu. Catalysts. 2021; 11(8):906. https://0-doi-org.brum.beds.ac.uk/10.3390/catal11080906

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Li, Huirong, Xianfang Yi, Jifa Miao, Yanting Chen, Jinsheng Chen, and Jinxiu Wang. 2021. "Improved Sulfur Resistance of COMMERCIAl V2O5-WO3/TiO2 SCR Catalyst Modified by Ce and Cu" Catalysts 11, no. 8: 906. https://0-doi-org.brum.beds.ac.uk/10.3390/catal11080906

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