Pore Size Expansion Accelerates Ammonium Bisulfate

Research Article

Cite This: ACS Appl. Mater. Interfaces 2019, 11, 4900−4907 www.acsami.org

Pore Size Expansion Accelerates Ammonium Bisulfate Decomposition for Improved Sulfur Resistance in Low-Temperature ‑ NH3 SCR † ‡ † † † † † † Kai Guo, Gaofeng Fan, Di Gu, Shuohan Yu, Kaili Ma, Annai Liu, Wei Tan, Jiaming Wang, ∥ † † § † ‡ § Xiangze Du, Weixin Zou, Changjin Tang,*, , and Lin Dong*, , , † ‡ School of Chemistry and Chemical Engineering and School of Environment, Nanjing University, Nanjing 210023, China § Jiangsu Key Laboratory of Vehicle Emissions Control, Nanjing 210023, China ∥ School of Chemistry, Sichuan University, Chengdu 610000, China

*S Supporting Information

ABSTRACT: Sulfur poisoning has long been recognized as a bottleneck for the development of long-lived NH3-selective catalytic reduction (SCR) catalysts. Ammonium bisulfate (ABS) deposition on active sites is the major cause of sulfur poisoning at low temperatures, and activating ABS decomposition is regarded as the ultimate way to alleviate sulfur poisoning. In the present study, we reported an interesting finding that ABS decomposition can be simply tailored via adjusting the pore size of the material it deposited. We initiated this study from the preparation of mesoporous silica SBA-15 with uniform one-dimensional pore structure but different pore sizes, followed by ABS loading to investigate the effect. The results showed that ABS decomposition proceeded more easily on SBA-15 with larger pores, and the decomposition temperature declined as large as 40 °C with increasing pore size of SBA-15 from 4.8 to 11.8 nm. To further ascertain the ff real e ect in NH3-SCR reaction, the Fe2O3/SBA-15 probe catalyst was prepared. It was found that the catalyst with larger mesopores exhibited much improved sulfur resistance, and quantitative analysis results obtained from Fourier transform infrared and ion chromatograph further proved that the deposited sulfates were greatly alleviated. The result of the present study demonstrates for the first time the vital role of pore size engineering in ABS decomposition and may open up new opportunities for designing NH3-SCR catalysts with excellent sulfur resistance. ff KEYWORDS: NH3-SCR, low temperature, sulfur poisoning, ammonium bisulfate decomposition, pore size e ect

1. INTRODUCTION Conventionally, deposited ABS on the NH3-SCR catalyst Downloaded via NANJING UNIV on May 22, 2019 at 06:50:17 (UTC). Selective catalytic reduction (SCR) by ammonia has been surface can be removed through water washing or high 11−13 proved to be the most eﬀective technology for removing NO temperature calcination. Chang et al. recently showed that x − from diesel engines and power plants, where the NH3-SCR the V2O5 WO3/TiO2 catalyst poisoned by ABS can be 1,2 14

See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. catalyst is the core of the SCR system. It is well accepted that regenerated under elevated temperature treatment. However, the key to developing practically used NH3-SCR catalysts not both water washing and thermal regeneration display some only relies on excellent denitration efficiency but also high negative effects. For example, washing treatment will inevitably resistance to sulfur oxides, due to the fact that most exhausts generate plenty of wastewater and treating the catalyst at high from mobile sources and stationary sources contain not only temperatures is an energy-required process, which also can nitrogen oxides but also quite a good amount of sulfur induce significant reduction of the surface area and severe 3,4 − − dioxide. For commercial V2O5 WO3(MoO3)/TiO2 cata- sulfation of active components.15 19 From a practical point of lysts, excellent sulfur resistance is shown at 300−420 °C, which view,itishighlyexpectedthatthethermaldriven coincides with the temperature window of coal-fired power plants.5,6 However, when the reaction temperature declines to decomposition of ABS can be accelerated so that it can rival lower values, e.g., 200−250 °C, catalyst deactivation often with ABS deposition. In an ideal case, negligible ABS can be takes place. This is mainly attributed to the cover of sticky deposited on the catalyst surface when the formation and sulfates like ammonium bisulfate (ABS) on the catalyst decomposition rates of ABS reach a dynamic balance. surface.7,8 It was reported that these ammonium salts could not decompose completely until the temperature exceeds 400 Received: September 10, 2018 °C.9 Consequently, they deposit gradually on the catalyst Accepted: January 17, 2019 surface, causing pore plugging and active site blocking.10 Published: January 17, 2019

There are some of the recent studies focused on the thermal water was evaporated at 100 °C and the obtained sample was stability of ABS on a catalyst surface. For example, Zhu et al. subsequently dried at 100 °C overnight. The sample was labeled mS/ SBA-x,wherem represents ABS loading in terms of weight carried out a study on ABS decomposition over the V2O5 catalyst supported on activated carbon (AC) and found that in percentage. The introduction of the active Fe2O3 component on SBA-15 was comparison with TiO2 support, ABS decomposition was 20 performed by a simple wet impregnation method. The calculated promoted due to the reactivity between ABS and AC. Shen · amount (10 wt %) of Fe(NO3)3 9H2O was dissolved in 30 mL of et al. investigated the mechanism of ammonium bisulfate deionized water and 0.5 g of support (SBA-40 and SBA-150) was decomposition over V/WTi catalysts and revealed that added. The suspension was vigorously stirred for 2 h and dried electronic interactions between ABS and metal oxides (titania overnight at 100 °C. Finally, the catalyst was obtained by calcination and tungsta) could weaken the thermal stability of ABS.7,21 at 450 °C for 4 h in a muffle furnace. For simplicity, the catalysts were Recently, Tang et al. reported a novel cation−anion separation denoted as Fe2O3/SBA-40 and Fe2O3/SBA-150. strategy against ABS deposition. They found that by 2.2. Sample Characterization. The structure of synthesized + SBA-15 was determined by small-angle X-ray diffraction (XRD), intercalating NH4 into the layer structure MoO3, the strong + − transmission electron microscopy (TEM), and N2-sorption. A Philips electrostatic interactions between NH4 and HSO4 were ’ ff fi α X pert Pro di ractometer with Ni- ltered Cu K 1 radiation (0.15408 broken, which was advantageous to promote ABS decom- ff 22 nm) was used to collect X-ray powder di raction (XRD) patterns position. Notably, in addition to the impacts from chemical with a step width of 0.06°/s from 0.6 to 5°. The X-ray tube was aspects, Xu et al. and co-workers proposed that the mesopore operated at 40 kV and 40 mA. The specific surface area and pore size − − − channels in the MnO2 Fe2O3 CeO2 TiO2 catalyst played an distribution (PSD) of samples were measured by N2-physisorption at important role in ABS decomposition, based on the −196 °C on a Micromeritics ASAP-3020 analyzer. Before measure- experimental result that a balance between ABS formation ment, the sample was vacuum-pretreated at 300 °C (120 °C for ABS and decomposition in SCR reaction was achieved on the deposited samples) for 3 h and pore size distribution results were − − catalyst with abundant mesopores.23 Nevertheless, in their obtained by the Barrett Joyner Halenda method. Transmission study, due to the multiple components and coupled changes of electron microscopy (TEM) images were taken on a JEM-1011 instrument at an acceleration voltage of 200 kV. The sample was chemical properties (e.g., redox, acid−base) by constructing fl dispersed in A.R. grade ethanol with ultrasonic treatment and the the mesoporous catalyst, the complex in uence from chemical resulting suspension was allowed to dry on a carbon film supported on modification cannot be discriminated, and therefore the copper grids. Fourier transform infrared (FT-IR) spectra of samples detailed role of the pore structure in ABS decomposition is were collected on a Nicolet Is10 FT-IR spectrometer from 400 to still not clear. In consideration that most of the prepared 4000 cm−1. The background was collected first and then subtracted for every sample. The number of scans was 32 at a resolution of 4 catalysts are porous, it is imperative to discover the unique role − of pore parameters in heterogenous catalysis, particularly for cm 1. the development of long-lived SCR catalysts. In the present The decomposition behaviors and products of NH4HSO4 on study, we report a novel finding that irrespective of chemical various mesoporous silica were measured by simultaneous thermal fi analysis and mass spectroscopy (STA-MS). The measurement was modi cations, ABS decomposition can be simply tailored via carried out in a flowing nitrogen atmosphere (60 mL/min) on a adjusting the textual properties of the material it deposited. fi NETZSCH STA-449-F5 combined with a QMS-403D instrument Speci cally, ordered mesoporous silica SBA-15 with uniform and the heating rate was set at 5 °C/min. The reference curve for one-dimensional pore structure and tunable pore size is DSC was operated on an empty crucible. employed as a supporting material, and ABS decomposition is To obtain the quantitative data of surface species (sulfates and found to be promoted in SBA-15 with a larger pore size. A soluble metal ions) after sulfur resistance test, 50 mg of the spent plausible explanation for the accelerated decomposition is catalyst was dispersed in 20 mL of deionized water and ultrasonicated fi proposed based on the Kelvin equation. Finally, to explore the for 2 h, then the solid was ltered out and the liquid phase was kept real effect on sulfur resistance during NH -SCR reaction, a for the following tests. Inductively coupled plasma atomic emission 3 spectroscopy (ICP-AES) was used for analyzing soluble cations in the model Fe2O3/SBA-15 catalyst was constructed and it was sample after the sulfur resistance test on a PerkinElmer Optima 5300 observed that the catalyst with a larger pore size showed much DV instrument with a radio frequency power of 1300 W. The sulfate better sulfur tolerance. We expect that the information anions in the liquid phase were analyzed by ion chromatography obtained in the present research could provide useful guidance (DIONEX ICS-1100AR, Thermofisher scientific, India). The system for the design of NH3-SCR catalysts with superior antisulfur consisted of one IC pump, an injection valve, column heater, an ion- poisoning performance. exchange analytical column (AS19), and a guard column (AG19). Samples were loaded onto a 100 μL sample loop with a syringe. All columns, tubing, and fittings were made of poly-ether-ether-ketone. 2. EXPERIMENTAL SECTION During the IC measurement, 25 mM KOH eluent, 0.38 mL/min flow 2.1. Sample Preparation. SBA-15 with different pore sizes were rate, and 24 mA applied current were maintained. synthesized by a hydrothermal method at different temperatures.24 In 2.3. Activity and Sulfur Resistance Test on Model Catalysts. a typical preparation, 4.0 g of P123 was dissolved in 30 mL of The activity performance of the model catalyst was evaluated in a deionized water and 120 mL of 2.0 M HCl. Tetraethoxysilane (9.0 g) fixed-bed quartz reactor. The feed stream was fixed with 500 ppm ° was added into the solution dropwise under stirring at 40 C for 24 h. NO, 500 ppm NH3,5%O2, 800 ppm SO2 (when used), and Ar in The milky slurry was transferred into a Teflon-lined autoclave for balance, and total gas flow was controlled at 200 mL/min. The hydrothermal treatment at different temperatures (40, 100, 130, and catalyst (200 mg) was sieved to 40−60 mesh. Before the test, it was 150 °C). After that, the obtained white product was collected by pretreated in purified Ar stream at 150 °C for 1 h. Then, the mixed filtration, followed by washing with deionized water and ethanol gases were switched on and the activity data were collected at every several times. The cake was dried at 110 °C overnight and air-calcined target temperature after stabilizing for 1 h in the range of 50−400 °C. at 550 °C for 5 h. The obtained powder was labeled SBA-x, where x The sulfur resistance experiment was carried out in a similar way. represents the operated hydrothermal temperature. After Ar pretreatment, the SCR reaction was performed by holding ° To realize ABS loading on SBA-15, the calculated amount of the reaction temperature at 250 C. SO2 was introduced after the ffl NH4HSO4 (15, 30 wt %) was dissolved in 30 mL of deionized water reaction steady state was reached. The concentration of e uent gases with 0.3 g of SBA-x support added. After vigorous stirring for 2 h, was continuously analyzed on an IS10 FT-IR spectrometer equipped

4901 DOI: 10.1021/acsami.8b15688 ACS Appl. Mater. Interfaces 2019, 11, 4900−4907 ACS Applied Materials & Interfaces Research Article with a 2 m path-length gas cell (2 L volume). The NO conversion and ABS impregnation. It can be seen that ABS deposition results N2 selectivity were calculated based on the following equations in an obvious decrease of the surface area and pore volume of []−[]NO NO the carrier, indicating that the deposited ABS is filled in the NO conversion(%) = in out × 100% []NO (1) pore channels of SBA-15. Previously, it was reported that the in chemical state of ABS on the catalyst surface can affect their 7 fl N2 selectivity(%) decomposition behaviors. To explore any in uence of pore

[NO ]in −[ NO ] out +[ NH 3 ] in −[ NH 3 ] out −[ NO 2 ] out − 2 [ N 2 O ] out size on the chemical state of impregnated ABS in the = mesopores, FT-IR analysis is conducted on 15S/SBA-40 and [NO ]in −[ NO ] out +[ NH 3 ] in −[ NH 3 ] out (2) 15S/SBA-150 samples. In general, for sulfur-contained species, × 100% their bands are centered at the wavenumber range of 800− 1500 cm−1.7,18,277,18,27 However, this range is entangled with 3. RESULTS AND DISCUSSION the Si−O vibrations from mesoporous silica.28,29 Thus, to 3.1. Structure Properties of SBA-15 Synthesized from discriminate the signal of sulfate from that of silica material, Different Hydrothermal Temperatures. Mesoporous silica differential FT-IR spectra obtained by deducting the spectra of SBA-15 exhibits chemical inertness, uniform one-dimensional pure SBA-40 and SBA-150 supports are plotted and the results pore structure, and tunable pore size, and thus is an ideal are displayed in Figure 2. In the differential spectra of 15S/ supporting material to investigate the pore size effect on ABS decomposition. SBA-15 synthesized at different hydrothermal temperatures was first characterized by TEM, and the obtained images are displayed in Figure S1. Ordered channel arrays were observed along the direction of pore arrangement, indicating the formation of uniform one-dimensional pore structures. The large-scale periodicity of mesopores was identified by the appearance of a family of diffraction peaks in small-angle XRD patterns (Figure S2).25 As well, the structural details were revealed by N2-physisorption isotherms and pore size distribution (PSD) curves (Figures S3 and 1). It can be seen

Figure 2. Diﬀerential spectra obtained by subtracting the spectra of pure supports from 15S/SBA-40 and 15S/SBA-150.

SBA-40 and 15S/SBA-150, well-resolved vibration bands due to ABS deposition are present. The broad band at 3500−3000 cm−1 and peaks at 1641 and 1446 cm−1 represent the − + 30,31 vibrations of N HinNH4 , whereas the bands at 1172, 1033, 882, and 588 cm−1 are characteristic of different vibration modes of sulfate and bisulfate species.32 By comparing the two differential spectra, no evident differences Figure 1. Pore size distribution curves of SBA-15 synthesized at in band intensity and peak position are observed, suggesting a different hydrothermal temperatures. negligible influence on the deposition state of ABS by different pore sizes. that all samples display type IV isotherms according to IUPAC 3.3. Decomposition of ABS Supported on SBA-15. A fi classi cation with H1 hysteresis loops, indicative of the typical 15S/SBA-40 sample was employed to approach the formation of cylindrical mesoporous materials.26 As expected, decomposition details of ABS on SBA-15. Thermogravimetry− PSD curves revealed clear differences in pore size. At the differential scanning calorimetry (TG−DSC) and MS hydrothermal temperatures of 40, 100, 130, and 150 °C, SBA- techniques were jointly used to collect the signals during the 15 with pore sizes of 4.84, 6.24, 8.43, and 11.8 nm was decomposition process and the results are displayed in Figure obtained. Taken together, we can safely conclude that uniform 3. Two weight losses are visible from TG curves in the tested one-dimensional mesopores with varying pore sizes are range, and no further weight loss is detected at higher established and the influenceofporesizeonABS temperatures (data were not shown). The weight loss below decomposition can thus be readily investigated. 150 °C can be attributed to the evaporation of absorbed water, 3.2. Location and Chemical State of ABS. The whereas the weight loss between 300 and 400 °C is due to the investigation system of ABS deposited on mesoporous silica decomposition of ABS. Correspondingly, two endothermic was constructed via the wetness impregnation method. Table peaks are shown in the DSC curves. The attribution is further S1 lists the textual properties of the samples before and after confirmed by qualitative analysis from mass spectroscopy. As

4902 DOI: 10.1021/acsami.8b15688 ACS Appl. Mater. Interfaces 2019, 11, 4900−4907 ACS Applied Materials & Interfaces Research Article

Figure 3. STA-MS patterns of 15S/SBA-40. can be seen from MS profiles, the signal of water starts to appear at very low temperature and obtains a peak at 69.2 °C, and then declines to baseline at a temperature below 150 °C. For the SO2 signal, a much broad temperature window is ° displayed. SO2 begins to release at a temperature of 212 C, and the maximal decomposition rate is reached at a temperature of ca. 384 °C. To explore the possible loading amount effect, the decomposition experiment was also carried out on the 30S/ SBA-40 sample and the result is presented in Figure S4.Itis observed that the TG−DSC and MS signals of 30S/SBA-40 display similar trends to 15S/SBA-40, revealing that the decomposition behavior of ABS is not much affected by its loading. Nevertheless, the detailed decomposition temperature changes with ABS loading. The onset and peak decomposition temperatures of 30S/SBA-40 are observed at 255.7 and 392.5 °C, which are much higher than those of 15S/SBA-40 (212.0 and 384.1 °C), suggesting that higher loading of ABS leads to Figure 4. Decomposition temperature of 15 wt % ABS supported on difficult decomposition. SBA-15 with different pore sizes. The influence of pore size on ABS decomposition was − investigated and detailed TG DSC curves are shown in Figure that the enhanced performance was related to reduced S5. For convenience, the peak temperature of the second blockage of the catalyst surface from the condensation of the endothermic DSC peak is taken as an indication of the byproduct, sulfuric acid.33 According to the Kelvin equation, decomposition temperature of ABS on SBA-15. It is found that the condensation of sulfuric acid occurs in smaller pores first for the set of 15S/SBA-x samples, both TG and DSC curves and that larger pores are less vulnerable to condensation. In the show a similar trend with the curves of 15S/SBA-40. All present study, we suppose the similar rule may be held for ABS samples reveal two main weight losses and two endothermic decomposition. peaks. Moreover, it is observable that the decomposition Regarding the status of ABS during thermal treating, it was temperature displays a monotonic reduction with increasing reported that ABS would become sticky and melt at around support pore size (Figure 4), suggesting accelerated decom- 150 °C during temperature programming.9 This is evidenced position of ABS on SBA-15 with a larger pore size. by the STA-MS characterization of pure ABS. As can be seen Remarkably, an almost 40 °C decrease in ABS decomposition from Figure 5, two endothermic peaks centered at 159 and 513 temperature is reached with the increase of the support pore °C are shown in the DSC curve. In contrast, no obvious weight size from 4.84 to 11.8 nm. loss is detected around 160 °C from the TG curve. Therefore, With the employment of inert mesoporous silica as a we can speculate that the phase transition of ABS from solid to supporting material, the influence of chemical factors (redox liquid takes place at this temperature range and ABS actually and/or acid−base properties) on ABS decomposition is experiences a liquid phase before subsequent decomposition at insignificant. As such, we can focus on the impact of physical higher temperatures. factors, specifically, the pore width of SBA-15 in the present It is known that for a liquid phase with a curved surface, such study. Previously, Taira et al. reported that the Pt/TiO2 as the surface of a droplet, the equilibrium vapor pressure can catalyst with a high ratio of larger pores exhibited higher CO be different from vapor pressure at a flat surface. The change of fl − oxidation activity under the in uence of SO2. They proposed vapor pressure due to curved liquid vapor interface can be

4903 DOI: 10.1021/acsami.8b15688 ACS Appl. Mater. Interfaces 2019, 11, 4900−4907 ACS Applied Materials & Interfaces Research Article

Fe2O3/SBA-40 and Fe2O3/SBA-150, respectively. Figure S6 shows the pore size distributions of the two catalysts. As expected, after Fe doping, both catalysts display a decrease in pore size. Nevertheless, uniform pore distribution is still present and distinct pore sizes are apparently displayed, which is helpful for us to investigate the pore size effect. The chemical state of mesoporous silica after iron oxide deposition was studied by FT-IR spectroscopy. Figure S7 presents the IR spectra of SBA-15 and Fe2O3/SBA-15 samples. The vibrations at 1054 and 803 cm−1 are attributed to asymmetric and symmetric stretching of the Si−O−Si bond, and the band at 964 cm−1 is ascribed to the stretching vibration of Si−O− 35,36 − Figure 5. STA-MS patterns of pure ABS. H. After introducing Fe oxide, the vibrations of Si O related bands remain almost the same as that of pure SBA-15. It proves that SBA-15 maintained its chemical properties after well described by the Kelvin equation (eq 3), where p is the impregnating with iron oxides. The significant disparity in the actual vapor pressure, p0 is the vapor pressure when the surface pore size comparable chemical state of two model catalysts was fl γ θ is at, VL is the molar liquid volume, is the surface tension, also much convenient for us to investigate the pore size effect is the contact angle, rp is the curvature radius, R is the gas on ABS decomposition in the practical catalytic process. constant, and T is the temperature. The NO conversion and N2 selectivity results of Fe2O3/ SBA-40 and Fe O /SBA-150 are shown in Figure S8.Itis p 2cosγθVL 2 3 ln = obvious that they have almost identical activity in the range of p rRT 0 p (3) 50−400 °C, suggesting that the chemical properties of active fl ff For liquid phase ABS in SBA-15, it displays a concave surface components (Fe2O3) were also barely in uenced by di erent silica supports. Figure 6 illustrates the NH -SCR performance and the vapor pressure (p) is lower than p0. Thus, the higher 3 vapor pressure of the melted ABS is attained in larger pore of the two catalysts operated in the presence of 800 ppm SO2. ° channels. With the increase of temperature, liquid phase ABS The reaction temperature is controlled at 250 C to avoid fi in larger pores of SBA-15 is more liable to vaporize and the signi cant decomposition of ABS. The initial NO conversions decomposition is easier to take place by taking into are comparable for both samples, which is in line with the consideration the negligible intermolecular interaction forces above activity results. However, a notable difference in reaction of gaseous ABS.34 As a result, the gas−liquid equilibrium will efficiency can be observed from the longtime stability test. For be prompted to the gas side, which further alleviates the Fe2O3/SBA-150, NO conversion reveals no evidence of decline condensation and deposition of ABS on larger pores. Scheme 1 in the entire test range, demonstrating that the catalyst is illustrates the state of ABS on SBA-15 with different pore sizes. barely deactivated after SO2 injection. In contrast, for Fe2O3/ This can well explain the accelerated decomposition of ABS on SBA-40, NO conversion decreases instantly when SO2 is SBA-15 with larger pores. We anticipate that this unique introduced into the feeding gas. After little recovery of activity, character can be used to promote the sulfur resistance ability of NO conversion shows a continuous recession to 36% after the the SCR catalyst operated at low temperatures. 16 h test. Additionally, for the universal concern of the pore ff ff 3.4. Pore Size E ect on Sulfur Resistance during NH3- size e ect, Fe2O3/MCM-41 was also prepared. MCM-41 is a SCR Reaction. A model catalyst with iron oxide supported on kind of mesoporous silica with a similar cylindrical pore SBA-15 is constructed to explore the pore size effect on ABS channel to SBA-15, but it shows a much smaller pore size (2−3 decompositioninpracticalNH3-SCR reaction. For the nm). The result in Figure 6 shows that when MCM-41 was comparison purpose, two SBA-15 supports (SBA-40 and employed, a more rapid decrease of activity was obtained. After SBA-150) are used and the obtained catalysts are denoted as the 16 h test, the NO conversion of Fe2O3/MCM-41 was

Scheme 1. Illustration of ABS Decomposition Behaviors on SBA-15 with Diﬀerent Pore Sizes

4904 DOI: 10.1021/acsami.8b15688 ACS Appl. Mater. Interfaces 2019, 11, 4900−4907 ACS Applied Materials & Interfaces Research Article

resistance test. For sulfate signals, it can be seen from Figure 7b that Fe2O3/SBA-40-spent reveals more intense bands than Fe2O3/SBA-150-spent. The bands at about 1094, 1011, 941, −1 and 787 cm are clear on Fe2O3/SBA-40 after the sulfur ff resistance test. However, the di erential spectrum of Fe2O3/ SBA-150-spent does not show these significant peaks, implying that sulfate species was slightly deposited. Based on FT-IR results, the cover of ammonium sulfate species on the catalyst surface is established. Moreover, by comparing the differential spectra, it can be deduced that the catalyst with larger pore size can greatly reduce the deposition of surface sulfate salts. This is in line with the experimental result operated on bare SBA-15 and suggests that the rule is also applicable to the practical catalytic process. Finally, to acquire quantitative information of the formed sulfate salts (Fe2(SO4)3,NH4HSO4) on the spent catalyst, ICP-AES and ion chromatography experiments were performed. Table 1 shows the concentration of various ions on the

Figure 6. Sulfur resistance tests on Fe O /SBA-40, Fe O /SBA-150, a 2 3 2 3 Table 1. Soluble Ions on the Spent Catalyst Surface and Fe2O3/MCM-41 catalysts. Reaction conditions: 500 ppm NO, −1 500 ppm NH , 5 vol % O , 800 ppm SO , GHSV of ca.15 000 h . 2− 3+ 3 2 2 sample SO4 (mg/L) Fe (mg/L)

Fe2O3/SBA-40-spent 33.814 0.14 fi Fe2O3/SBA-150-spent 11.288 0.21 nally declined to ca. 25%. Apparently, the SO2 tolerance test aThe concentrations of SO 2− and Fe3+ were measured by ion tells us that the catalyst with a larger pore size can restrain SO2 4 poisoning more effectively, and the pore size effect on ABS chromatography and ICP-AES, respectively. decomposition could also be verified on various supports. It is well acknowledged that deactivation of NH3-SCR two spent samples. It can be seen that quite a good amount of catalysts in the sulfur-contained stream can be attributed to the sulfate ions is generated on the spent samples but only a trace − formation of sulfate-related species.8,37 39 To know more amount of Fe3+, indicating that the negligible metal component details about the deactivation, FT-IR spectra were employed to is sulfated during the sulfur resistance test. Thus, under current identify the change of surface species, and the results are reaction conditions, the contribution of deactivation from presented in Figure 7a. It is obvious that the spectra of the chemical poisoning of the active component should be minor. ff spent catalysts display some di erences compared to those of On the other hand, the deposition of sulfate salts on Fe2O3/ the fresh ones. Several new peaks appear after the sulfur SBA-150 is greatly alleviated, as can be evidenced by the fact 2− resistance test and could be reasonably ascribed to the reaction that the amount of SO4 on Fe2O3/SBA-40-spent is almost − byproducts. The broad band emerged in the range of 3000 three times that on Fe2O3/SBA-150-spent. This can well 3500 cm−1 and an intense peak at 1450 cm−1 on the spent account for the distinct sulfur resistance performance between − catalysts were characteristic of the N H stretching mode and Fe2O3/SBA-40 and Fe2O3/SBA-150. Based on the above + 40 NH4 on Brønsted acid sites, implying that ammonium results, we could safely conclude that the textual properties of species are formed on the catalyst surface after the sulfur the catalyst have a significant impact on ABS decomposition,

ﬀ Figure 7. (a) FT-IR spectra of fresh and spent Fe2O3/SBA-15 catalysts and (b) the di erential spectra obtained by subtracting the spectra before reaction from those after reaction.

4905 DOI: 10.1021/acsami.8b15688 ACS Appl. Mater. Interfaces 2019, 11, 4900−4907 ACS Applied Materials & Interfaces Research Article and the catalyst with larger pores can effectively prevent the (4) Liu, Z.; Li, J.; Junaid, A. S. M. Knowledge and Know-how in deposition of ammonium bisulfate, which is in favor of good Improving the Sulfur Tolerance of DeNOx Catalysts. Catal. Today sulfur resistance performance in practical SCR reaction. 2010, 153,95−102. (5) Lietti, L.; Nova, I.; Ramis, G.; Dall’Acqua, L.; Busca, G.; Giamello, E.; Forzatti, P.; Bregani, F. Characterization and Reactivity 4. CONCLUSIONS − of V2O5 MoO3/TiO2 De-NOx SCR Catalysts. J. Catal. 1999, 187, In summary, we reported in the present study the pore-size- 419−435. dependent property of ABS decomposition for low-temper- (6) Gao, R.; Zhang, D.; Liu, X.; Shi, L.; Maitarad, P.; Li, H.; Zhang, ature NH3-SCR reaction. It was found that ABS deposition can J.; Cao, W. 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4907 DOI: 10.1021/acsami.8b15688 ACS Appl. Mater. Interfaces 2019, 11, 4900−4907