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Pore Size Expansion Accelerates Ammonium Bisulfate Decomposition for Improved Sulfur Resistance in Low Temperature NH-SCR 3

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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15688 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 20, 2019

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ACS Applied Materials & Interfaces

Pore Size Expansion Accelerates Ammonium Bisulfate Decomposition for Improved Sulfur Resistance in Low-Temperature NH3-SCR

Kai Guo a, Gaofeng Fan b, Di Gu a, Shuohan Yu a, Kaili Ma a, Annai Liu a, Wei Tan a, Jiaming Wang a, Xiangze Du d, Weixin Zou a, Changjin Tang a, c,*, Lin Dong a,b,c,*

a

School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210023,

China

b

School of Environment, Nanjing University, Nanjing, 210023, China

c

Jiangsu Key Laboratory of Vehicle Emissions Control, Nanjing, 210023, China

d

School of Chemistry, Sichuan University, Chengdu, 610000, China

*Corresponding authors: [email protected], [email protected].

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Abstract:

Sulfur poisoning has long been recognized as a bottleneck for the development of longlived NH3-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 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. Results showed that ABS decomposition proceeded more easily on SBA-15 with larger pores, and the decomposition temperature declined as large as 40 oC with increasing pore size of SBA15 from 4.8 nm to 11.8 nm. To further ascertain the real effect in NH3-SCR reaction, Fe2O3/SBA-15 probe catalyst was prepared. It was found the catalyst with larger mesopores exhibited much improved sulfur resistance, and quantitative analysis from FTIR and ion chromatograph further proved the deposited sulfates were greatly alleviated.


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The result of 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.

Keywords: NH3-SCR; low temperature; sulfur poisoning; ammonium bisulfate decomposition; pore size effect


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1.INTRODUCTION Selective catalytic reduction (SCR) by ammonia has been proved to be the most effective technology for removing NOx from diesel engine and power plant, NH3-SCR catalyst is the core of SCR system1, 2. It is well accepted that the key to developing practically used NH3-SCR catalyst not only relies on excellent denitration efficiency but also high resistance to sulfur oxides, due to the fact that most exhausts from mobile source and stationary source contain not only nitrogen oxides but also quite amount of sulfur dioxide3, 4. For commercial V2O5-WO3(MoO3)/TiO2 catalysts, excellent sulfur resistance is shown at 300-420 oC, which coincides with the temperature window of coal-fired power plant5, 6. However, when the reaction temperature declines to lower values, e.g., 200-250 oC, catalyst deactivation often takes place. This is mainly attributed to the cover of sticky sulfates like ammonium bisulfate (ABS) on catalyst surface7, 8. It was reported that these ammonium salts could not decompose completely until the temperature exceeds 400 oC 9. Consequently, they deposit gradually on catalyst surface, causing pore plugging and active site blocking10. Conventionally, deposited ABS on NH3-SCR catalyst surface can be removed through water washing or high temperature calcination11-13. Chang et al. recently showed that the V2O5WO3/TiO2 catalyst poisoned by ABS can be regenerated under elevated temperature treatment14. However, both water washing and thermal regeneration display some negative effects. For example, washing treatment will inevitably generate plenty of waste water and treating catalyst at high temperatures is energy required process which also can induce a significant reduction of surface area and severe sulfation of active components15-19. From a practical point of view, it is highly expected that the thermal driven decomposition of ABS can be accelerated so that it can


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rival with ABS deposition. In an ideal case, negligible ABS can be deposited on catalyst surface when the formation and decomposition rates of ABS reach a dynamic balance. There are some of the recent studies focused on the thermal stability of ABS on catalyst surface. For example, Zhu et al. carried out a study on ABS decomposition over V2O5 catalyst supported on activated carbon (AC), and found that in comparison with TiO2 support, ABS decomposition was promoted due to reactivity between ABS and AC20. Shen et al. investigated the mechanism of ammonium bisulfate decomposition over V/WTi catalysts and revealed that electronic interactions between ABS and metal oxides (titania and tungsta) could weaken the thermal stability of ABS7, 21.

Recently, Tang et al. reported a novel cation-anion separation strategy against ABS deposition.

They found that by intercalating NH4+ into a layer structure MoO3, the strong electrostatic interactions between NH4+ and HSO4- were broken, which was advantageous to promote ABS decomposition22. Notably, in addition to the impacts from chemical aspects, Xu et al. and coworkers proposed that the mesopore channels in MnO2-Fe2O3-CeO2-TiO2 catalyst played an important role in ABS decomposition, based on the experiment result that a balance between ABS formation and decomposition in SCR reaction was achieved on the catalyst with abundant mesopores23. Nevertheless, in their study, due to the multiple components and coupled changes of chemical properties (e.g. redox, acid-base) by constructing the mesoporous catalyst, the complex influence from chemical modification cannot be discriminated, and therefore the detailed role of pore structure on ABS decomposition is still not clear. In consideration that most of the prepared catalysts are porous, it is imperative to discover the unique role of pore parameters in heterogenous catalysis, particularly for the development of long-lived SCR catalysts. In the present study, we report a novel finding that irrespective of chemical modifications, ABS decomposition can be simply tailored via adjusting textual property of the material it deposited. Specifically, ordered


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mesoporous silica SBA-15 with uniform one-dimensional pore structure and tunable pore size is employed as a supporting material, and ABS decomposition is found to be promoted on SBA-15 with larger pore size. A plausible explanation for the accelerated decomposition is proposed based on the Kelvin equation. Lastly, to explore the real effect on sulfur resistance during NH3-SCR reaction, a model Fe2O3/SBA-15 catalyst was constructed and it was observed that the catalyst with larger pore size showed much better sulfur tolerance. We expect the information obtained in the present research could provide a useful guidance for the design of NH3-SCR catalyst with superior anti-sulfur poisoning performance.


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2.EXPERIMENTAL SECTION 2.1 Sample preparation SBA-15 with different pore sizes were synthesized by a hydrothermal method at different temperatures24. In a typical preparation, 4.0 g P123 was dissolved in 30 ml deionized water and 120 ml 2.0 M HCl. Tetraethoxysilane (TEOS, 9.0 g) was added into the solution dropwise and kept stirring at 40 oC for 24 h. The milky slurry was transferred into Teflon-lined autoclave for hydrothermal treatment at different temperatures (40, 100, 130 and 150 oC). After that, the white product was collected by filtration, followed by washing with deionized water and ethanol for several times. The cake was dried at 110 oC overnight and air-calcined at 550 oC for 5 h. The obtained powder was labeled as SBA-x, where x represents the operated hydrothermal temperature. To realize ABS loading on SBA-15, the calculated amount of NH4HSO4 (15, 30 wt. %) was dissolved in 30 ml deionized water with 0.3 g SBA-x support added. After vigorous stirring for 2 h, water was evaporated at 100 oC and the obtained sample was subsequently dried at 100 oC overnight. The sample was labeled as mS/SBA-x where m represents ABS loading in terms of weight percentage. The introduction of the active Fe2O3 component on SBA-15 was performed by a simple wet impregnation method. Calculated amount (10 wt.%) of Fe(NO3)3·9H2O was dissolved in 30 ml deionized water and 0.5 g support (SBA-40 and SBA-150) was added. The suspension was vigorously stirred for 2 h and dried overnight at 100 oC. Finally, the catalyst was obtained by calcination at 450 oC for 4 h in muffle furnace. For simplicity, the catalysts were denoted as Fe2O3/SBA-40 and Fe2O3/SBA-150.


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2.2 Sample characterization The structure of synthesized SBA-15 was determined by small angle-XRD, TEM and N2sorption. A Philips X’pert Pro diffractometer with Ni-filtered Cu Kα1 radiation (0.15408 nm) was used to collected X-ray powder diffraction (XRD) patterns with a step width of 0.06° per second from 0.6° to 5°. The X-ray tube was operated at 40 kV and 40 mA. Specific surface area and pore size distribution of samples were measured by N2-physisorption at -196 oC on a Micromeritics ASAP-3020 analyzer. Before measurement, the sample was vacuum-pretreated at 300 oC (120 oC for ABS deposited samples) for 3 h and pore size distribution results were obtained by BJH (Barrett-Joyner-Halenda) method. Transmission electron microscopy (TEM) images were taken on a JEM-1011 instrument at an acceleration voltage of 200 kV. The sample was dispersed in A.R. grade ethanol with ultrasonic treatment and the resulting suspension was allowed to dry on carbon film supported on copper grids. Fourier translation infrared spectra (FT-IR) of samples were collected on a Nicolet Is10 FT-IR spectrometer from 400-4000 cm-1. The background was collected firstly and subtracted for every sample. The number of scans was 32 at a resolution of 4 cm-1. The decomposition behaviors and products of NH4HSO4 on various mesoporous silica were measured by STA-MS (simultaneous thermal analysis and mass spectroscopy). The measurement was carried out in flowing nitrogen atmosphere (60 ml/min) on a NETZSCH STA-449-F5 combined with QMS-403D instrument and the heating rate was set at 5 oC/min. The reference curve for DSC was operated on an empty crucible. To obtain the quantitative data of surface species (sulfates and soluble metal ions) after sulfur resistance test, 50 mg spent catalyst was dispersed in 20 ml deionized water and treated by ultrasonic for 2 h, then the solid was filtered out and the liquid phase was kept for the following


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tests. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used for analyzing soluble cations on the sample after sulfur resistance test on a Perkin Elmer Optima 5300DV instrument with a radio frequency power of 1300W. The sulfate anions in the liquid phase were analyzed by ion chromatography (DIONEX ICS-1100AR, Thermofisher scientific, India). The system consisted of one IC pump, an injection valve, column heater, an ion-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 (PEEK). During the IC measurement, 25 mM KOH eluent, 0.38 ml/min flow rate, 24mA applied current was maintained.

2.3 Activity and sulfur resistance test on model catalysts Activity performance of model catalyst was evaluated in a fixed-bed quartz reactor. The feed stream was fixed with 500 ppm NO, 500 ppm NH3, 5% O2, 800 ppm SO2 (when used) and Ar in balance, and total gas flow was controlled at 200 ml/min. The catalyst (200 mg) was sieved to 4060 mesh. Before the test, it was pretreated in purified Ar stream at 150 oC for 1 h. Then the mixed gases were switched on and the activity data were collected at every target temperature after stabilizing for 1h in the range of 50-400 oC. Sulfur resistance experiment was carried out in a similar way. After Ar pretreatment, SCR reaction was performed by holding the reaction temperature at 250 oC. SO2 was introduced after reaction steady state was reached. The concentration of effluent gases was continuously analyzed on an IS10 FTIR spectrometer equipped with a 2 m path-length gas cell (2 L volume). The NO conversion and N2 selectivity were calculated based on the following equations: 𝑁𝑂 conversion(%) =

[𝑁𝑂]𝑖𝑛 ― [𝑁𝑂]𝑜𝑢𝑡 [𝑁𝑂]𝑖𝑛

× 100%

Equation 1


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𝑁2 selecyivity(%) =

[𝑁𝑂]𝑖𝑛 ― [𝑁𝑂]𝑜𝑢𝑡 + [𝑁𝐻3]𝑖𝑛 ― [𝑁𝐻3]𝑜𝑢𝑡 ― [𝑁𝑂2]𝑜𝑢𝑡 ― 2[𝑁2𝑂]𝑜𝑢𝑡 [𝑁𝑂]𝑖𝑛 ― [𝑁𝑂]𝑜𝑢𝑡 + [𝑁𝐻3]𝑖𝑛 ― [𝑁𝐻3]𝑜𝑢𝑡

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× 100%

Equation 2

3.RESULTS AND DISCUSSION 3.1 Structure properties of SBA-15 synthesized from different hydrothermal temperatures Mesoporous silica SBA-15 exhibits chemical inertness, uniform one-dimensional pore structure and tunable pore size, and thus is an ideal supporting material to investigate the pore size effect on ABS decomposition. The SBA-15 synthesized at different hydrothermal temperatures were firstly 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 (Figure S3 and Figure 1). It can be seen that all samples display type IV isotherms according to IUPAC classification with H1 hysteresis loops, indicative of the formation of cylindrical mesoporous materials26. As expected, PSD curves revealed clear differences in pore size. Under the hydrothermal temperatures of 40, 100, 130, 150 oC, SBA-15 with pore sizes of 4.84, 6.24, 8.43, 11.8 nm were obtained. Taken together, we can safely conclude that uniform 1D mesopores with varied pore sizes are established and the influence of pore size on ABS decomposition can thus be readily investigated.


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3.2 Location and chemical state of ABS The investigation system of ABS deposited on mesoporous silica was constructed via a wetness impregnation method. Table S1 lists the textual properties of the samples before and after ABS impregnation. It can be seen that ABS deposition results in an obvious decrease of surface area and pore volume of the carrier, indicating the deposited ABS is filled in the pore channels of SBA15. Previously, it was reported that the chemical state of ABS on catalyst surface can affect their decomposition behaviors7. To explore any influence of pore size on the chemical state of impregnated ABS in the mesopores, FT-IR analysis is conducted on 15S/SBA-40 and 15S/SBA150 samples. In general, for sulfur-contained species, their bands are centered at the wavenumbers range of 800-1500 cm-1

7, 18, 27.

However, this range is entangled with the Si-O vibrations from

mesoporous silica28, 29. Thus, to discriminate the signal of sulfate from that of silica material, differential FT-IR spectra obtained by deducting the spectra of pure SBA-40 and SBA-150 supports are plotted and the results are displayed in Figure 2. In the differential spectra of 15S/SBA-40 and 15S/SBA-150, well-resolved vibration bands due to ABS deposition are present. The broad band at 3500-3000cm-1 and peaks at 1641cm-1 and 1446 cm-1 represent the vibrations of N-H in NH4+

30, 31,

while the bands at 1172 cm-1, 1033 cm-1, 882 cm-1 and 588 cm-1 are

characteristic of different vibration modes of sulfate and bisulfate species32. By comparing the two differential spectra, no evident differences in band intensity and peak position are observed, suggesting negligible influence on deposition state of ABS by different pore sizes.

3.3 Decomposition of ABS supported on SBA-15 A typical 15S/SBA-40 sample was employed to approach the decomposition details of ABS on SBA-15. TG-DSC and MS techniques were jointly used to collect the signals during


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decomposition process and the results are displayed in Figure 3. Two weight losses are visible from TG curves in the tested range, and no further weight loss is detected at higher temperatures (data was not shown). The weight loss below 150 oC can be attributed to evaporation of absorbed water, while the weight loss between 300 oC to 400 oC is due to decomposition of ABS. Correspondingly, two endothermic peaks are shown in the DSC curves. The attribution is further confirmed by a qualitative analysis from mass spectroscopy. As can be seen from MS profiles, the signal of water starts to appear at very low temperature and obtains a peak at 69.2 oC, and then declines to baseline at temperature below 150 oC. For SO2 signal, a much broad temperature window is displayed. SO2 begins to release at temperature of 212 oC, and the maximal decomposition rate is reached at temperature of ca. 384 oC. To explore the possible loading amount effect, decomposition experiment was also carried out on the 30S/SBA-40 sample and the result is presented in Figure S4. It is observed that the TGDSC and MS signals of the 30S/SBA-40 display similar trends with 15S/SBA-40, revealing 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 for the 30S/SBA-40 are observed at 255.7 oC and 392.5 oC, which are much higher than that of 15S/SBA-40 (212.0 oC and 384.1 oC), suggesting that higher loading of ABS leads to difficult decomposition. The influence of pore size on ABS decomposition was investigated and detailed TG-DSC curves are shown in Figure S5. For convenient, the peak temperature of the second endothermic DSC peak is taken as an indication of the decomposition temperature of ABS on SBA-15. It is found that for the set of 15S/SBA-x samples, both TG and DSC curves show a similar trend with the curves of 15S/SBA-40. All samples reveal two main weight losses and two endothermic peaks.


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Moreover, it is observable that the decomposition temperature displays a monotonic reduction with increasing support pore size (Figure 4), suggesting accelerated decomposition of ABS on SBA15 with larger pore size. Remarkably, almost 40 oC decrease in ABS decomposition temperature is reached with rising of support pore size from 4.84 nm to 11.8 nm. With employment of inert mesoporous silica as a supporting material, the influence of chemical factors (redox and/or acid-base property) on ABS decomposition is insignificant. As such, we can focus on the impact from physical factors, specifically, the pore width of SBA-15 in the present study. Previously, Taira et al. reported that Pt/TiO2 catalyst with a high ratio of larger pores exhibited higher CO oxidation activity under the influence of SO2. They proposed that the enhanced performance was related to reduced blockage of catalyst surface from the condensation of byproduct sulfuric acid33. According to the Kelvin equation, the condensation of sulfuric acid occurs in smaller pores first and that larger pores are less vulnerable to condensation. In the present study, we suppose the similar rule may be held for ABS decomposition. Regarding the status of ABS during thermal treating, it was reported that ABS would become sticky and melt at around 150 oC during temperature programming9. This is evidenced by the STAMS characterization of pure ABS. As can be seen from Figure 5, two endothermic peaks centered at 159 oC and 513 oC are shown in the DSC curve. In contrast, no obvious weigh loss is detected around 160 oC from TG curve. Therefore, we can speculate that the phase transition of ABS from solid to liquid takes place at this temperature range and ABS actually experiences a liquid phase before the subsequent decomposition at higher temperatures. It is known that for a liquid phase with a curved surface, such as the surface of a droplet, the equilibrium vapor pressure can be different from vapor pressure at a flat surface. The change of vapor pressure due to curved liquid-vapor interface can be well described by Kelvin equation


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(Equation 3), where p is the actual vapor pressure, p0 is the vapor pressure when the surface is flat, VL is the molar liquid volume, γ is the surface tension, θ is the contact angle, rp is the curvature radius, R is the gas constant, and T is the temperature. 𝑝

𝑙𝑛 𝑝0 =

2𝛾𝑉𝐿cos 𝜃 𝑟𝑝𝑅𝑇

Equation 3

For liquid phase ABS in SBA-15, it displays a concave surface and the vapor pressure (p) is lower than p0. Thus, the higher vapor pressure of the melted ABS is attained in larger pore channels. With increasing of temperature, liquid phase ABS in larger pores of SBA-15 is more liable to vaporize and the decomposition is easier to take place by taking into consideration of the negligible intermolecular interaction forces of gaseous ABS34. As a result, the gas-liquid equilibrium will be prompted to the gas side, which further alleviates the condensation and deposition of ABS on larger pores. Scheme 1 illustrates the state of ABS on SBA-15 with different pore sizes. This can well explain the accelerated decomposition of ABS on SBA-15 with larger pores. We anticipate this unique character can be used to promote sulfur resistance ability of SCR catalyst operated at low temperatures.

3.4 Pore size effect on sulfur resistance during NH3-SCR reaction A model catalyst with iron oxide supported on SBA-15 is constructed to explore the pore size effect on ABS decomposition in practical NH3-SCR reaction. For comparison purpose, two SBA15 supports (SBA-40 and SBA-150) are used and the obtained catalysts are denoted as Fe2O3/SBA40 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


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oxide deposition was studied by FT-IR spectroscopy. Figure S7 presents the IR spectra of SBA15 and Fe2O3/SBA-15 samples. The vibrations at 1054 and 803 cm-1 are attributed to asymmetric and symmetric stretching of Si-O-Si bond, and the band at 964 cm-1 is ascribed to the stretching vibration of Si-O-H35, 36. After introducing Fe oxide, the vibrations of Si-O related bands remain almost the same with that of pure SBA-15. It proves that the SBA-15 maintained its chemical property after impregnating with iron oxides. The significant disparity in pore size comparable chemical state of two model catalysts were also much convenient for us to investigate the pore size effect on ABS decomposition in practical catalytic process. The NO conversion and N2 selectivity results of Fe2O3/SBA-40 and Fe2O3/SBA-150 are shown in Figure S8. It is obvious that they have almost identical activity in the range of 50 oC to 400 oC, suggesting the chemical property of active components (Fe2O3) was also barely influenced by the different silica supports. Figure 6 illustrates the NH3-SCR performance of the two catalysts operated in the presence of 800 ppm SO2. The reaction temperature is controlled at 250 oC to avoid significant decomposition of ABS. The initial NO conversions are comparable for both samples, which is in line with the above activity results. However, notable difference in reaction efficiency can be observed from the longtime stability test. For Fe2O3/SBA-150, NO conversion reveals no evidence of decline in the entire test range, demonstrating the catalyst is barely deactivated after SO2 injection. In contrast, for Fe2O3/SBA-40, NO conversion decreases instantly when SO2 is introduced into the feeding gas. After a little recovery of activity, NO conversion shows a continuous recession to 36% after 16 h test. Additionally, for the universal concern of the pore size effect, Fe2O3/MCM-41 was also prepared. MCM-41 is a kind of mesoporous silica with similar cylindrical pore channel to SBA-15, but it shows much smaller pore size (2-3nm). The result in Figure 6 shows that, when MCM-41 was employed, more rapid decrease of activity was obtained.


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After 16 h test, the NO conversion of Fe2O3/MCM-41 was finally declined to ca. 25%. Apparently, the SO2 tolerance test tells us that the catalyst with larger pore size can restrain SO2 poisoning more effectively, and the pore size effect on ABS decomposition could also be verified on various supports. It is well acknowledged that deactivation of NH3-SCR catalysts in the sulfur-contained stream can be attributed to the formation of sulfate-related species.8, 37-39 In order to know more details about the deactivation, FT-IR spectra was employed to identify the change of surface species, and the results are present in Figure 7a. It is obvious that the spectra of spent catalysts display some differences compared to those of the fresh ones. Several new peaks appear after sulfur resistance test and could be reasonably ascribed to the reaction byproducts. The broad band emerged at range of 3000-3500cm-1 and intense peak at 1450 cm-1 on the spent catalysts were characteristic of N-H stretching mode and NH4+ on Brønsted acid sites40, implying ammonium species are formed on catalyst surface after sulfur resistance test. For sulfate signals, it can be seen from Figure 7b that Fe2O3/SBA-40-spent reveals more intense bands than that of Fe2O3/SBA-150-spent. Bands at about 1094, 1011, 941 and 787cm-1 are clear on Fe2O3/SBA-40 after sulfur resistance test. However, the differential spectrum of Fe2O3/SBA-150-spent does not show these significant peaks, implying sulfate species was slightly deposited. Based on FT-IR results, the cover of ammonia sulfate species on 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 experiment result operated on bare SBA-15 and suggests the rule is also applicable to practical catalytic process. Lastly, to acquire quantitative information of the formed sulfate salts (Fe2(SO4)3, NH4HSO4) on the spent catalyst, ICP-AES and ion chromatography experiments were operated. Table 1


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shows the concentration of various ions on the two spent samples. It can be seen that quite amount of sulfate ions is generated on the spent samples but a trace amount of Fe3+ presents, indicating the negligible metal component is sulfated during sulfur resistance test. Thus, under current reaction conditions, the contribution of deactivation from chemical poisoning of the active component should be minor. On the other hand, the deposition of sulfate salts on Fe2O3/SBA-150 is greatly alleviated, as can be evidenced by the fact that the amount of SO42- on Fe2O3/SBA-40-spent is almost three times that on Fe2O3/SBA-150-spent. This can well account for the distinct sulfur resistance performance between Fe2O3/SBA-40 and Fe2O3/SBA-150. Based on the results above, we could safely conclude that the textual properties of catalyst have a significant impact on ABS decomposition, and the catalyst with larger pores can effectively prevent deposition of ammonium bisulfate, which is in favor of good sulfur resistance performance in practical SCR reaction.

4.CONCLUSIONS In summary, we reported in the present study the pore size dependent property of ABS decomposition for low temperature NH3-SCR reaction. It was found that ABS deposition can be greatly alleviated by adjusting pore size of the materials it deposited, with no need of chemical modification. By using mesoporous silica SBA-15 as a supporting material, obvious acceleration effect of ABS decomposition is observed with enlargement of pore size. This decomposition promoting effect was mainly related to the fact that larger pore size generates higher vapor pressure, which is helpful for ABS vaporization and decomposition. Further sulfur resistance test on model Fe2O3/SBA-15 catalyst showed the catalyst with larger pore sizes exhibited much better durability than the smaller one, validating the promotion effect was also applicable for real catalytic process. The present study brings us a brief cognition on how the catalyst pore structure


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affects ammonium bisulfate decomposition and is expected to be helpful for us to design novel NH3-SCR catalyst with excellent sulfur resistance ability.

Supporting information available:

Small angle XRD, TEM, N2-sorption, TG-DSC patterns of 15S/SBA-15 series, TG-MS of 30S/SBA-40, pore parameter of ABS or Fe2O3 deposited samples and SCR activity with N2 selectivity are included.

Author information

*Corresponding authors

Email address:

[email protected] (Changjin Tang),

[email protected] (Lin Dong)


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Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT

Financial support by the National Natural Science Foundation of China (21773106, 21707066, 21573105) and Environmental Protection Department of Jiangsu Province (2016048) are gratefully acknowledged.


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Figure captions

Figure 1. The pore size distribution curves of SBA-15 synthesized at different hydrothermal temperatures

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

Figure 3. STA-MS patterns of 15S/SBA-40

Figure 4. Decomposition temperature of 15 wt.% ABS supported on SBA-15 with different pore sizes

Figure 5. STA-MS patterns of pure ABS

Figure 6. Sulfur resistance tests on Fe2O3/SBA-40 and Fe2O3/SBA-150 catalysts. Reaction condition: 500ppm NO, 500ppm NH3, 5 vol.% O2, 800ppm SO2, GHSV of ca.15000h-1.


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


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Figure 1. The pore size distribution curves of SBA-15 synthesized at different hydrothermal temperatures


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


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Figure 3. STA-MS patterns of 15S/SBA-40


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Figure 4. Decomposition temperature of 15 wt.% ABS supported on SBA-15 with different pore sizes


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Figure 5. STA-MS patterns of pure ABS


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Figure 6. Sulfur resistance tests on Fe2O3/SBA-40, Fe2O3/SBA-150 and Fe2O3/MCM-41 catalysts. Reaction condition: 500ppm NO, 500ppm NH3, 5 vol.% O2, 800ppm SO2, GHSV of ca.15000 h-1


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


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Scheme 1. Illustration of ABS decomposition behaviors on SBA-15 with different pore sizes


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Table caption

Table 1 Soluble ions on spent catalysts surface


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Table 1 Soluble ions on spent catalysts surface*

Sample

SO42- (mg/L)

Fe3+ (mg/L)

Fe2O3/SBA-40-spent

33.814

0.14

Fe2O3/SBA-150-spent

11.288

0.21

*The concentrations of SO42- and Fe3+ were measured by Ion Chromatography and ICP-AES, respectively.


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*TOC Graphic


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Pore Size Expansion Accelerates Ammonium ... - ACS Publications

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