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Article Cite This: ACS Omega 2019, 4, 4927−4935

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New Insights into the Decomposition Behavior of NH4HSO4 on the SiO2‑Decorated SCR Catalyst and Its Enhanced SO2‑Resistant Ability Dong Ye, Ruiyang Qu, Shaojun Liu, Chenghang Zheng, and Xiang Gao* State Key Laboratory of Clean Energy Utilization, State Environmental Protection Center for Coal-Fired Air Pollution Control, Zhejiang University, 38 Zheda Road, 310027 Hangzhou, China

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S Supporting Information *

ABSTRACT: This article illustrates the detailed decomposition behavior of NH4HSO4 on the TiO2 and TiO2−SiO2 supports, along with the effect of SiO2 addition on the sulfur resistance of the corresponding V2O5-based catalysts. For TiO2 support, sulfate species selectively occupied its surface basic hydroxyl groups, while Si−OH groups functioned as the main sites for the accommodation of NH4HSO4 over the TiO2− SiO2 mixed support, enabling its surface sulfate species with higher thermal stability. Compared with NH4+ on the TiO2 surface, NH4+ on the TiO2−SiO2 mixed support was much easier to be consumed during the heating process, hence causing some variations in the decomposition behavior of NH4HSO4. Finally, adding SiO2 enhanced the SO2 tolerance properties of the catalysts to a certain extent. When exposed to the SO2-containing flue gas, the deposition of NH4HSO4 mainly caused serious deactivation of SiO2-free catalyst, while the as-accumulated SO42− also contributed to the declined activity of SiO2-added catalyst. These results ensured the potential commercialization of TiO2−SiO2-based catalysts in the typical lowtemperature selective catalytic reduction systems in the short run and pointed out a strategy to design new catalysts with superior activity and enhanced SO2-tolerant ability.

1. INTRODUCTION Selective catalytic reduction (SCR) of NO with NH3 is the most widespread technology for the abatement of NOx in both mobile and stationary applications.1 For industrial heaters and furnaces, the flue gas temperature is always below 300 °C, which makes commercial SCR catalysts exhibit a disappointing activity. Therefore, low-temperature SCR technology is gaining attention, and recently, several types of catalysts have been reported to possess a satisfactory activity in the temperature region of 100−300 °C.2 However, the formation of NH4HSO4 would inevitably take place when SO2 and H2O are simultaneously added to the reactant gas. The as-produced NH4HSO4 then covers the active sites of the catalysts and negatively affects the SCR reactions, which is the main barrier to the industrialization of low-temperature SCR systems.3 Thus, designing a catalyst with an enhanced sulfur-tolerant ability becomes an important step in solving this problem. The decomposition behavior of NH4HSO4 has been proved to partially determine the sulfur resistance of catalysts in our previous studies.4 It has been confirmed that the deposited NH4HSO4 selectively occupies certain functional groups on the catalyst surfaces.5 Once the deposition amount of NH4HSO4 increases, a deficit in surface sites for the deposition of NH4HSO4 would result in the presence of crystalline ammonium sulfate salts on the catalysts. Compared with amorphous ammonium sulfate salts, crystalline salts have © 2019 American Chemical Society

higher thermal stability. Thus, an effective strategy to improve the catalyst sulfur resistance is to enlarge its specific surface area and increase sites for amorphous NH4HSO4 deposition, which could impede the formation of crystalline ammonium sulfate salts on the catalysts to a certain extent. As a result, an improved NH4HSO4 decomposition behavior would be observed and catalyst sulfur resistance might be in turn promoted. TiO2−SiO2 mixed oxide has been widely utilized as an effective support for SCR catalysts due to its large surface area.6 In terms of the detailed mechanism of NH4HSO4 decomposition over the TiO2−SiO2 mixed oxide, scanty information was available. But some aspects of paramount importance must be taken into consideration. (a) Provided that parts of SO42− are bonded to SiO2 sites, some changes in the decomposition behavior of NH4HSO4 might be caused on the SiO2-containing samples, since there exist some variations in the physicochemical properties between TiO2 and SiO2. (b) Provided that SiO2 modification leads to an enhancement in the decomposition behavior of NH4HSO4, it is important to clarify whether the enlarged surface area is the main reason. Received: November 9, 2018 Accepted: December 24, 2018 Published: March 6, 2019 4927

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For the reasons mentioned above, TiO2 support and TiO2− SiO2 mixed support, along with V2O5−WO3/TiO2 and V2O5− WO3/TiO2−SiO2 catalysts, were synthesized. First, the decomposition behavior of NH4HSO4 on these samples was investigated to demonstrate that NH4HSO4 deposited on the pure supports behaves similarly to that on the corresponding V2O5-based catalysts so that it is reasonable to use the asprepared supports instead of the catalysts to study the detailed decomposition mechanism of NH4HSO4 for simplification. And then, X-ray diffraction (XRD), N2 adsorption, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) were used to study the existing forms of NH4HSO4 on the samples. Finally, sulfur resistance tests were performed to explore the effect of SiO2 addition on the SO2-tolerant ability of the catalysts.

2. RESULTS AND DISCUSSION 2.1. Decomposition Behavior of NH4HSO4 on the Serial Samples. According to previous studies, it has been proved that a vital step in the decomposition of NH4HSO4 would be the transformation of S-containing species from SO42− to SO2 because SO2, instead of SO3, is the unique Scontaining product in this reaction.7 Thus, temperatureprogrammed decomposition (TPDC) method was adopted to study the stability of SO42− on the serial samples, which would be evaluated using the outlet SO2 signal. In the case of Ti support, the outlet SO2 concentration starts to increase at ca. 200 °C, indicating the occurrence of the consumption of SO42− (Figure 1). As the temperature exceeds

Figure 2. DTG profiles of the serial NH4HSO4-deposited samples.

seems that it is appropriate to study the detailed decomposition behavior of NH4HSO4 on the pure support surfaces for simplification because of the unnoticeable change in the decomposition behavior of NH4HSO4 after adding V2O5 and WO3. As the temperature increases, two weight loss peaks are seen for the ABS−Ti sample (centered at 363 and 453 °C), while the ABS−TiSi sample exhibits three weight loss peaks (at 149, 283, and 523 °C). The appearance of the weight loss peaks below 300 °C confirms that adding SiO2 has a promotional effect on the decomposition behavior of NH4HSO4 at low-temperature regions, which is in contrast to the assumptions mentioned above. It seems that doping SiO2 might result in some changes in the reaction routes of NH4HSO4 decomposition. Combined with the SO2 profiles in Figure 1, it seems that the consumption behavior of SO42− can in part explain the presence of the weight loss peaks above 450 °C, while the weight loss peaks belonging to the ABS−TiSi sample below 300 °C may be related to the NH 4 + decomposition behavior, which would be evidenced using the in situ diffuse reflectance infrared Fourier transforms (DRIFTS) experiments. The detailed decomposition behavior of NH4HSO4 on the serial supports was investigated using the in situ DRIFTS method. As shown in Figure 3, characteristic peaks related to NH4+ at ca. 1432 cm−1, bidentate SO42− at ca. 1250 cm−1, and H2O at 1632 cm−1 are present at 100 °C.4a,8 With the increasing temperature, a blue shift of the bands occurs, which is assigned to SO42−, and the peak intensity for NH4+ continuously decreases. After calculating these samples’ relative concentrations of NH4+ and SO42−, it could be found that the consumption of NH4+ occurs prior to that of SO42− during the heating process and NH4+ on the TiSi support exhibits lower thermal stability. Combined with the FTIR spectra in Figure S2, it could be concluded that the appearance of the weight loss peaks below 300 °C can be mainly explained by the decomposition of NH4+ for ABS−TiSi sample, and the simultaneous consumption of NH4+ and SO42− constitutes the weight loss peak centering at ca. 360 °C for NH4HSO4 deposited on the Ti support; the weight loss peaks above 450 °C are largely ascribed to the consumption of SO42−. As mentioned above, during the heating process, NH4+ bonding to sulfate species would be consumed at lowtemperature regions, leaving sulfate species stabilized on the catalyst supports. On further increasing the temperature, the

Figure 1. SO2 profiles of the serial NH4HSO4-deposited samples.

350 °C, a fast upward trend of the outlet SO2 signal is presented, of which the highest point (ca. 72 ppm) centers at ca. 445 °C. For TiSi mixed support, the onset temperature of SO2 release increases to ca. 350 °C; on further increasing the temperature, the outlet signal of SO2 shows a trend of slow increase. And it should be noted that during the whole reaction process, the outlet concentration of SO2 belonging to TiSi mixed support is much lower than that of Ti support. It seems that SO42− on the TiSi mixed support has higher thermal stability, and the decomposition behavior of NH4HSO4 might be inhibited through adding SiO2. Derivative thermogravimetry (DTG) profiles of the serial NH4HSO4-introduced samples are given in Figures 2 and S1. It 4928

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Figure 3. In situ DRIFTS experiments of NH4HSO4 on the (a) Ti support and (b) TiSi support, and (c) relative concentrations of NH4+ and SO42− of the serial samples.

Figure 4. (a) XRD patterns and (b) Raman spectra of the serial samples.

(Figure 4b). For Ti support, five Raman peaks centering at 145, 198, 396, 516, and 639 cm−1 are seen, which could be assigned to anatase TiO2 phase.10 Given that SiO2 is introduced, the peaks for anatase TiO2 phase shift to higher wavenumbers, demonstrating that the TiO bond order increases and the bond distance between Ti and O atoms becomes longer.2a That is to say, doping SiO2 causes lattice distortion of TiO2 because of the different radii between Ti and Si atoms (Ti: 0.2 nm; Si: 0.117 nm), which might in turn produce new sites for the accommodation of NH4HSO4. Additionally, the intensity of the peaks attributed to anatase TiO2 phase decreases for TiSi mixed support, which is consistent with the XRD patterns in Figure 4a that the crystal growth of TiO2 is hindered to some extent upon the addition of SiO2. After the deposition of NH4HSO4, little change has taken place in the serial Raman peaks, demonstrating that catalyst support crystal phases remain unchanged with the introduction of NH4HSO4. Table 1 demonstrates the data of surface areas and pore volumes of the studied samples. Compared with Ti support,

stabilized SO42− begins to decompose. Compared with Ti support, NH4+ on the TiSi sample is much easier to be consumed, on which SO42−, however, is hard to decompose. The reasons for this are still unknown but would be revealed in the next sections through investigating the interactions between NH4HSO4 and these catalyst supports. 2.2. Interactions between NH4HSO4 and Catalyst Supports. 2.2.1. Texture Properties of the Serial Samples. The XRD patterns of the serial samples are presented in Figure 4a. The diffraction peaks belonging to Ti and TiSi samples exhibit typical anatase TiO2 phase (PDF-ICDD 21-1272). It should be noted that the peak intensity of TiSi mixed support is much weaker than that of Ti support, suggesting that introducing SiO2 exerts an inhibitory effect on the formation of TiO2 crystal phase.9 Given the deposition of NH4HSO4, no diffraction peaks indexed to any crystalline ammonium sulfate salts are generated, indicating that NH4HSO4 on the support surfaces is amorphous in structure. In addition to XRD patterns, Raman spectra could be used to further study the physical properties of the serial samples 4929

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species for Si-doped sample, which would be uncovered by the characteristics of catalyst surface hydroxyl groups. As given in Figure 5b, a negative peak centering at 3698 cm−1 is observed with the deposition of NH4HSO4 for TiO2 sample, suggesting that TiO2 surface basic hydroxyl groups are the main sites for the selective occupation of SO42−.15 In the case of Si-decorated sample, a negative peak attributed to Si−OH species appears at 3692 cm−1 after introducing NH4HSO4, indicating that Si− OH groups, rather than TiO2 surface basic hydroxyl groups, might act as the main sites for the accommodation of SO42−.16 These findings would be further evidenced by the XPS method, through which the detailed element information of the serial samples would be revealed. 2.2.3. Element Information of the Serial Samples. Surface element information of the serial samples was investigated using the XPS method. As is illustrated in Figure 6a, the binding energies of the Ti 2p photoelectron peaks are proved to be the characteristics of Ti4+ ions as in TiO2.17 Given that NH4HSO4 is deposited on the Ti support surface, the binding energies of the Ti 2p peaks increase, indicating that electrons around the Ti atoms are attracted by SO42− because of this species’ stronger electronegativity. To some extent, this result is consistent with the conclusions drawn from the FTIR spectra in Figure 5 that Ti−OH species are the unique sites for the accommodation of NH4HSO4 on the Ti surface, while for TiSi support, the shift of the Ti 2p photoelectron peaks is negligible, demonstrating that Ti atom environment remains almost unchanged regardless of the deposition of NH4HSO4, and Ti−OH sites might be free from the occupation of sulfate species. The XPS results of Si atoms are presented in Figure 6b. At ca. 102 eV, one Si 2p photoelectron peak is seen, revealing that Si atoms in the SiO2-containing samples exist in the +4 oxidation state.18 In the meantime, adding NH4HSO4 increases the binding energies of the Si 2p peaks from 101.90 to 102.20 eV, illustrating that NH4HSO4 sometimes interacts with SiO2 and that Si−OH groups might function as the main sites for the occupation of sulfate species. Combined with the FTIR spectra in Figure 5, it could be concluded that different occupation sites of sulfate species mainly explain the occurrence of the variations in the decomposition behavior of NH4HSO4 on the TiSi mixed support. And considering the existence of chemical interactions between NH4HSO4 and catalyst supports, it seems that some changes might take place in N and S atom environments after the doping of SiO2. According to Figure 6c, N species in the ABS−Ti sample exhibit in the form of NH4+, while in the ABS−TiSi sample, some subpeaks are obtained after deconvolution. One peak centering at ca. 401.70 eV is speculated to be the characteristic of NH4+, which is the same with that of the ABS−Ti sample.19 The other peak at ca. 400 eV reveals the existence of adsorbed NH3.20 This result indicates that NH4+ on the TiSi mixed support surface might have lower thermal stability; a vacuum environment could promote the consumption behavior of the bonded NH4+ and then the as-released NH3 would in turn adsorb on the TiSi support, which might be the main reason for the appearance of the subpeak at ca. 400 eV and is consistent with the FTIR spectra in Figure 3 that doping SiO2 causes a larger decline scope of the relative concentration value of NH4+ during the heating process. As shown in Figure 6d, two evident bands corresponding to the S 2p1/2 and S 2p3/2 spin−orbit states of sulfate species are obtained after deconvolution.21 Given the modification of

Table 1. N2 Adsorption Results of the Serial Samples samples

BET surface areas (m2 g−1)

total pore volumes (cm3 g−1)

Ti ABS−Ti TiSi ABS−TiSi Ti(NW) ABS−Ti(NW)

62 23 152 66 258 181

0.14 0.07 0.29 0.22 2.0 1.5

adding SiO2 significantly increases the catalyst support surface area from 62 to 152 m2 g−1. To eliminate the possibility that an enlarged surface area mainly explains the enhanced decomposition behavior of NH4HSO4 on the TiSi sample, TiO2 support with a high surface area of ca. 250 m2 g−1 was synthesized (Table 1), and the thermogravimetry (TG)−DTG results are shown in Figure S3. It can be confirmed that catalyst chemical properties, rather than its textural structures, mainly determine the stability of the deposited amorphous NH4HSO4. 2.2.2. Surface Species of the Serial Samples. FTIR analysis was used to study the nature of N and S species on the supports. In the wavenumber range of 2000−1000 cm−1, no obvious characteristic IR peaks can be detected except the peak ascribed to adsorbed H2O at 1624 cm−1 for Ti sample (Figure 5).11 By contrast, some peaks indexed to residual sulfate

Figure 5. (a) FTIR spectra of the serial samples and (b) characteristics of hydroxyl groups of the serial samples after the deposition of NH4HSO4.

species can still be detected for TiSi mixed support.12 This result in part indicates that SO42− is strongly bonded to TiSi support surface and cannot be completely removed through the washing process, which is consistent with the TPDC profiles in Figure 1 to some extent. Given that NH4HSO4 is deposited on the TiO2 support, characteristic peaks for bidentate sulfate anions centering at 1219, 1134, and 1039 cm−1, along with the NH4+ band at 1400 cm−1, are seen.12,13 The appearance of the peaks at 1219 and 1134 cm−1 is mainly explained by the asymmetric and symmetric stretching vibrations of SO species, whereas the asymmetric stretching of the S−O vibrations constitutes the IR peak at 1039 cm−1.14 As for the ABS−TiSi sample, the bands related to bidentate SO42− begin to shift toward higher and lower wavenumbers. It seems that certain Si-related sites instead of Ti-related ones might act as a reservoir for the accommodation of sulfate 4930

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Figure 6. XPS images: (a) Ti 2p, (b) Si 2p, (c) N 1s, and (d) S 2p.

Figure 7. Proposed decomposition routes of NH4HSO4 on the serial supports.

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SiO2, an increase in the binding energies of the S 2p bands could be detected, which is indicative of the lower electron cloud density around the S atoms on the TiSi mixed support than that on the Ti support. This would exert an inhibitory effect on the transformation of S species from SO42− to SO2 to a certain extent.22 As a result, the release amount of SO2 decreases during heating, and the weight loss peaks mainly attributed to the consumption of SO42− shift toward higher temperatures via adding SiO2. As previously mentioned, once NH4HSO4 is deposited on the catalyst supports, their surface hydroxyl groups would be selectively occupied by sulfate species (Figure 7). During the heating process, NH4+ decomposes first, as the consumption of SO42− follows. Compared with TiO2 sample, SO42− on the TiSi support surface tends to be strongly bonded to Si−OH sites, which would in turn weaken the interactions between sulfate species and NH4+. Consequently, NH4+ on the SiO2-modified sample is easy to be consumed, whereas its surface SO42− exhibits higher stability. 2.3. Sulfur-Tolerant Ability of the Serial Catalysts. Based on the TG−DTG profiles in Figures 2 and S1, the decomposition behavior of NH4HSO4 is enhanced to some extent through adding SiO2. Considering the deactivation mechanism of V2O5/TiO2-based SCR catalysts during the operation in the SO2- and H2O-containing flue gas below 300 °C, it seems that catalyst SO2-tolerant ability might be improved after the doping of SiO2. The time dependency of the relative activity during proceeding SO2 and H2O reaction is depicted in Figure 8. In

Table 2. Elemental Analysis of the Serial Catalysts after the Reaction in the SO2- and H2O-Containing Flue Gas at 250 °C samples

N/S content

VW/Ti (after reaction) VW/TiSi (after reaction) pure NH4HSO4

0.46 0.23 0.44

contrast, this ratio drops to ca. 0.23 for VW/TiSi catalyst, further confirming that ammonia species on the SiO2containing sample possesses lower thermal stability. Therefore, apart from NH4HSO4, the accumulation of sulfate species also contributes to the declined activity of VW/TiSi catalyst when exposed to the SO2- and H2O-containing flue gas. 2.4. Evaluation of the Activity of the Serial Samples. As shown in Figure 9a, these two fresh catalysts exhibit over 90% NOx conversion between 300 and 450 °C, and little change has occurred in the SCR activity with the addition of SiO2. After the deposition of NH4HSO4, a dramatic decrease in the catalytic activity could be observed. Compared with VW/ Ti catalyst, the deactivation of VW/TiSi sample by the deposition of NH4HSO4 is inhibited to a certain extent, which constitutes an important reason for the promoted SO2-tolerant ability of SiO2-decorated catalyst. Apart from NH4HSO4, the accumulated SO 4 2− also explains the decreased NO x conversion of the catalysts in the SO2 deactivation process. Therefore, it is essential to study the effect of SO42− addition on the performance of the serial samples. Similar to the phenomena in Figure 9a, adding SO42− also negatively affects the activity of the serial samples in the whole temperature range, and SiO2-doped catalyst still exhibits higher activity compared to SiO2-free catalyst (Figure 9b). The N and S contents of the serial samples in Table 3 eliminate the possibility that lower deposition amount of NH4HSO4 and SO42− is the main reason for the improved activity of SiO2containing catalyst. As mentioned above, NH4HSO4 would be inevitably produced under the condition that SO2 and H2O are simultaneously added to the reactant gas, which blocks the catalyst pore structures and adversely affects the SCR reactions. That is to say, sulfur resistance of the catalysts is closely related to the performance of their NH4HSO4deposited counterparts. Compared with SiO2-free catalysts, the introduction of SiO2 obviously improves the activity of the NH4HSO4-deposited samples, which partially contributes to the enhancement in the sulfur resistance of SiO2-modified catalyst. It should be noted that once NH4HSO4 is deposited on the catalyst surfaces, NH4+ tends to decompose with SO42− strongly bonded to −OH sites. In other words, the remaining SO42− would also cause decrease in the catalyst activity. And to some extent, the activity of the SO42−-accumulated catalysts also determines the sulfur resistance of their fresh counterparts. Similar to the phenomena of NH4HSO4-deposited ones, the deactivation of the catalysts by SO42− is also impeded via the introduction of SiO2, which further explains the promoted SO2-resistant ability of VW/TiSi catalyst. For these reasons, it seems that TiSi-based SCR catalysts might be commercialized in the short run, and this finding would pave the way for the design of new catalyst systems with superior activity and enhanced SO2-tolerant ability.

Figure 8. Sulfur-tolerant ability of the serial catalysts. Conditions: 667 ppm NO + 667 ppm NH3 + 250 ppm SO2 + 5% H2O + 5% O2 + N2 as balanced gas, gas hourly space velocity (GHSV) = 150 000 mL g−1 h−1.

terms of VW/Ti catalyst, the relative activity falls below ca. 0.85 after 12 h. As for VW/TiSi catalyst, this value remains above ca. 0.94 in the whole deactivation period, suggesting that SiO2-promoted catalyst has an enhanced SO2-resistant ability in the lower-temperature region. The N and S contents of the serial catalysts after the sulfur deactivation period are presented in Table 2. For VW/Ti catalyst, the ratio of N/S content is ca. 0.46, which is almost equal to that of NH4HSO4 (ca. 0.44). It seems that the deposition of NH4HSO4 might mainly explain the deactivation of SiO2-free catalyst by SO2 and H2O. By 4932

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Figure 9. SCR activity of the serial catalysts: (a) fresh catalysts and NH4HSO4-deposited catalysts; (b) fresh catalysts and SO42−-added catalysts. Conditions: 667 ppm NO + 667 ppm NH3 + 5% O2 + N2 as balanced gas, GHSV = 150 000 mL g−1 h−1.

and calcined at 500 °C for 5 h. The molar ratio of Ti/Si was 4:1. The single metal oxide, TiO2 (Ti for short), was prepared using the similar precipitation method. In addition, TiO2 support (labeled as Ti(NW)) with high surface area was prepared using the hydrothermal method.23 Briefly, 2.5 g of P25 TiO2 powder (85%: anatase, 15%: rutile; SBET = ca. 55 m2 g−1) obtained from Degussa was immersed into 70 mL of 10 mol L−1 KOH solution. Then, the mixture was transferred to a Teflon-lined autoclave and heated at 130 °C for 24 h. After cooling down, the obtained white powder was washed with 1 L of 0.1 mol L−1 HCl solution three times and then with deionized water several times to remove the residual K+ and Cl−. Finally, the powder was dried at 105 °C overnight and calcined at 500 °C for 5 h. V2O5−WO3/TiO2 (VW/Ti) and V2O5−WO3/TiO2−SiO2 (VW/TiSi) catalysts, whose contents of V2O5 and WO3 were 3 and 5 wt %, respectively, were prepared using the incipient wetness impregnation method. Calculated amounts of NH4VO3 and H40N10O41W12·xH2O were dissolved in 1 mL of 0.1 mol L−1 oxalic solution, which was mixed with Ti or TiSi support. Afterward, the mixtures were stirred for 1 h and exposed to ultrasonic energy for another 2 h at 50 °C, during which process excess water was removed. All of the prepared samples were dried at 105 °C overnight and finally calcined at 500 °C for 5 h. The deposition of NH4HSO4 was carried out using the solution impregnation method. The as-prepared powders were immersed in NH4HSO4 solution. Then, the mixtures were exposed to ultrasonic energy for ca. 2 h and dried at 105 °C overnight. The samples were designated ABS−Ti, ABS−TiSi, ABS−VW/Ti, and ABS−VW/TiSi. The introduction of SO42− was conducted using the similar impregnation method except that NH4HSO4 solution was replaced by dilute sulfuric acid solution. The samples were labeled as VW/Ti−SO42− and VW/TiSi−SO42−. 4.2. Reaction Systems. In a typical temperatureprogrammed decomposition (TPDC) process, a NH4HSO4loaded sample of 0.3 g was pretreated at 100 °C for 1 h in flowing N2 to remove the adsorbed impurities. And then, the sample was cooled down to room temperature and heated to 450 °C at a heating rate of 10 °C min−1 in N2. The flow rate was 500 mL min−1. A flue gas analyzer (Testo 350) was used

Table 3. Elemental Analysis of the Serial Samples sample

N (wt %)

S (wt %)

ABS−VW/Ti ABS−VW/TiSi VW/Ti−SO42− VW/TiSi−SO42−

1.17 1.04

2.54 2.56 2.81 2.77

3. CONCLUSIONS This article illustrates the detailed decomposition behavior of NH4HSO4 on the Ti and TiSi supports, along with the corresponding V2O5-based catalysts’ low-temperature sulfurtolerant ability. The conclusions are listed below: (1) In the case of Ti support, sulfate species selectively occupies its surface basic hydroxyl groups, while for TiSi mixed support, Si−OH groups function as the main sites for the accommodation of NH4HSO4, which enables its surface SO42− with higher thermal stability. (2) During the heating process, the consumption of NH4+ occurs prior to that of SO42−; NH4+ on the TiSi mixed support possesses lower thermal stability than that on the Ti support. (3) SO2 tolerance properties of the catalysts are enhanced to a certain extent after the addition of SiO2. In the SO2purging period, the declined activity of SiO2-containing catalyst is largely ascribed to the accumulation of SO42− and NH4HSO4, while more serious deactivation of SiO2free catalyst is mainly due to the deposition of NH4HSO4. 4. EXPERIMENTAL SECTION 4.1. Preparation of the Samples. TiO2−SiO2 mixed support (denoted as TiSi) was prepared using the coprecipitation method. Certain amounts of Ti(SO4)2 and tetraethyl orthosilicate were dissolved in dilute sulfuric acid solution and ethanol, respectively. The two obtained solutions were mixed together and slowly added to the excess ammonia solution. After that, the precipitate was obtained through filtration and washed with deionized water several times until no residual SO42− was detected (using Ba(NO3)2 solution). Finally, the obtained powder was dried at 105 °C overnight 4933

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temperature for 1 h to remove the physically adsorbed impurities. Finally, the IR spectra were collected at the serial temperatures (100, 150, 200, and 250 °C) in N2 at a resolution of 4 cm−1 typically averaging 64 scans.4b,c,5 The relative concentrations of NH4+ and SO42− were calculated using the method proposed by our group in the previous studies.4c,24 Based on the obtained IR spectra, the areas of the NH4+ and SO42− peaks at the serial temperatures were calculated. And then, the peak area for NH4+ at different temperatures was divided by that at 100 °C. The obtained values could be labeled as the relative concentrations of NH4+. The relative concentrations of SO42− were calculated using the same method.

to record the outlet signal of SO2, which could reflect the stability of SO42− on the serial samples. Activity tests of the serial catalysts were carried out in the reaction system. The feeding gases contained 667 ppm NO, 667 ppm NH3, and 5% O2 and N2 as the carrier gas. The gas hourly space velocity (GHSV) was 150 000 mL g−1 h−1. The concentrations of NO and NO2 for the gases before and after passing through the catalyst bed were analyzed using a flue gas analyzer (Testo 350). The NOx conversion was calculated using the following equation NOx conversion(%)

■

= 100% × ([NOin ] + [NO2in ] − [NOout ] − [NO2out ]) /([NOin ] + [NO2in ])

(1)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b03128. TG−DTG profiles of (a) ABS−VWTi and (b) ABS− VWTiSi samples; FTIR spectra of (a) ABS−Ti and (b) ABS−TiSi samples after calcination at serial temperatures; and TG−DTG profiles of ABS−Ti(NW) sample (PDF)

Sulfur tolerance tests of the serial catalysts were performed in the reaction system at 250 °C. The reaction condition was similar to that mentioned above except that 250 ppm SO2 and 5% H2O were also purged into the reaction system. Catalyst deactivation status after exposure to the H2O- and SO2containing flue gas was reflected by the relative activity value of X/X0, in which X is the NOx conversion at certain time in the SO2 deactivation process and X0 is the NOx conversion of fresh catalysts. 4.3. Characterizations of the Samples. Thermogravimetric analysis was conducted on the serial NH4HSO4deposited samples using TA Instruments SDT Q500. Prior to each experiment, a sample of 6 mg was pretreated in N2 at 100 °C for 5 min to remove the adsorbed H2O. After that, the tested sample was cooled down to 50 °C and heated to 650 °C at a heating rate of 10 °C min−1 in N2. XRD measurements of the serial samples were conducted on an X-ray diffractometer (RIGAKU D/MAX 2550, Japan). Patterns in the range between 10 and 80° were recorded with a step size of 0.02° (2θ). S and N elemental analysis experiments were performed using an elemental analyzer (Vario Micro). The textural structures of the serial samples were measured by N2 adsorption on a Quantachrome Autosorb-1C instrument. The specific surface areas were determined by the Brunauer−Emmett−Teller (BET) method. The pore size distributions were calculated from the N2 desorption isotherms using the cylindrical pore model (Barrett−Joyner−Halenda model). The nature of bonding of sulfate and ammonia species with catalyst surface was studied using an FTIR spectrophotometer (Nicolet 6700). The IR absorbance spectra were recorded in the range of 4000−400 cm−1. Raman spectra were recorded using a Renishaw inVia spectrometer equipped with an Ar ion laser (514.4 nm line). XPS experiments were performed on a PANalytical X’prt Pro diffractometer. The binding energies of the Ti 2p, Si 2p, N 1s, and S 2p photoelectron peaks were calibrated with C 1s peak (BE = 284.5 eV) as standard. In situ diffuse reflectance infrared Fourier transform (DRIFT) spectra were recorded using a Thermo Nicolet 6700 spectrophotometer. First, the NH4HSO4-free sample loaded in the Harrick IR cell was pretreated at 300 °C under a flowing N2 atmosphere for 1 h to remove the adsorbed impurities. The background spectra at 250, 200, 150, and 100 °C were collected in N2. After cooling down to room temperature, the NH4HSO4-free sample was replaced by the NH4HSO4-deposited one, which was flushed by N2 at room

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 571 87951335. ORCID

Xiang Gao: 0000-0002-1732-2132 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51836006, U1609212, and 51621005). The authors greatly appreciate Dr Duluo Nie’s contribution to this work.

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REFERENCES

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DOI: 10.1021/acsomega.8b03128 ACS Omega 2019, 4, 4927−4935