Impact of Hydrothermal Aging on SO2 Poisoning over Cu-SSZ-13

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Applied Chemistry

Impact of hydrothermal aging on SO2 poisoning over Cu-SSZ-13 diesel exhaust SCR catalysts Lai Wei, Dongwei Yao, Feng Wu, Biao Liu, Xiaohan Hu, Xingwen Li, and Xinlei Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04543 • Publication Date (Web): 18 Feb 2019 Downloaded from http://pubs.acs.org on February 20, 2019

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Industrial & Engineering Chemistry Research

Impact of hydrothermal aging on SO2 poisoning over Cu-SSZ-13 diesel exhaust SCR catalysts Lai Wei, † Dongwei Yao,†* Feng Wu,† Biao Liu,† Xiaohan Hu,† Xingwen Li,† Xinlei Wang ‡ †

‡

College of Energy Engineering, Zhejiang University, Hangzhou 310027, China

Department of Agricultural and Biological Engineering, University of Illinois at UrbanaChampaign, Urbana, IL 61801, USA

KEYWORDS: SCR, Cu-SSZ-13, Hydrothermal aging, Sulfur poisoning, TPD

ABSTRACT: The impact of hydrothermal aging on SO2 poisoning over Cu-SSZ-13 selective catalytic reduction (SCR) catalysts were investigated in this study. The catalyst samples were hydrothermally aged at various temperatures for SO2 poisoning and characterized by SO2 temperature-programmed desorption (SO2-TPD), X-ray diffraction (XRD), H2 Temperature programmed reduction (H2-TPR) and NH3-TPD. The performance evaluations were also performed to illustrate the impact on SCR activity and oxidation. The results showed that the CuSO4-like species decreased in the 750°C aged sample via copper migration, while they increased in the 850°C aged sample due to CuOX/CuAlOX formation. The low-temperature SCR activity and oxidation decreased due to sulfur poisoning. For the sulfated sample, however, it was determined that NOX conversion was slightly higher on the mildly hydrothermally aged sample.

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Furthermore, desulfation is easier for the mildly hydrothermally aged sample but more difficult for the severely hydrothermally aged sample.

1. Introduction As one of the major sources of nitrogen oxides (NOX) in the U.S and China, heavy-duty diesel (HDD) has made a considerable contribution to NOX emissions

1-3.

To meet the increasingly

restricted NOX limits in the current emission standards, selective catalytic reduction (SCR) has been widely used as the current leading technology for NOX abatement of heavy-duty diesel

4-7.

The NOX emissions should be held under the limits during the entire lifetime of the SCR converter, which requires better durability of the catalysts 8. Therefore, understanding the mechanism of the SCR catalyst deactivation is critical. There are two vital deactivation factors for an SCR catalyst: hydrothermal aging and sulfur poisoning

7, 9-11.

Although ultralow sulfur

diesel fuel has been used in many countries, the accumulation of sulfur in the SCR catalyst can occur during a long lifetime, not to mention the use of a high sulfur content fuel among developing countries or specific applications 8, 12, 13. Cu-SSZ-13 is one of the most effective copper-based chabazite (Cu-CHA) SCR catalysts for meeting regulations, and much attention has been paid to it because of its excellent thermal stability, wide activity window, outstanding hydrocarbon (HC) resistance and high N2 selectivity 14, 15.

However, its sensitivity to sulfur is one of the primary weakness of Cu-zeolite SCR

catalysts 4. This is due to the poisoning of the Cu active sites by sulfur species 9, 16. The reaction between sulfur species and Cu active sites may eventually form stable sulfate species 4, 13, which has been proven to lead to a decrease in the Cu sites redox ability and the SCR activity at low temperatures ( 750°C and ≤ 850°C), which still has a limited impact on the catalyst SCR performance, some of the isolated Cu2+ will transfer to oxide species in the large cage, leading to an improvement of the unselective NH3 oxidation and a loss of low-temperature activity. Thus, more Cu-SO4-like species can be generated on the copper oxide species after SO2 poisoning 16. In addition, Copper oxides species (CuOX) and Cu aluminate species (CuAlOX) may both form 25. The dealumination also occurs on the Cu-SSZ-13 catalyst during the hydrothermal aging treatment, resulting in not only the loss of crystallinity and Brønsted acid sites on the zeolite framework

22, 26, 27

but also a

change in the quantity of Al-SO4-like species after SO2 exposure. In fact, the small pores of the CHA framework prevent Al migration, thus the dealumination and the following Al-Cu clustering are controlled to a limited degree, but still exist 25.


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Therefore, due to the copper migration, oxides formation, and dealumination, the SO2 poisoning process over the Cu-SSZ-13 catalyst can be affected by hydrothermal deactivation. As a result, the catalyst performance and desulfation are also influenced. However, to the best of our knowledge the change in metal sulfate species over the Cu-SSZ-13 catalyst with different temperatures of hydrothermal aging has been rarely investigated. Luo et al.9 evaluated the sulfur poisoned Cu-SSZ-13 samples which had been hydrothermally aged. Wijayanti et al. 16 performed the SO2-TPD of fresh and 800°C hydrothermally aged samples. Nevertheless, the effect of different levels of hydrothermal aging on the sulfur poisoning process and the catalytic performance over the Cu-SSZ-13 catalyst is not clear. In this research, using SO2-TPD technology, we studied the effect of the change in the sulfate species on the Cu-SSZ-13 catalyst, which had been hydrothermally aged with different temperatures. We also observed how the sulfate species changed at different levels of hydrothermal aging treatment. XRD, H2-TPR, and NH3-TPD were used to characterize the catalyst zeolite framework and active sites after hydrothermal aging. Standard-SCR, NO oxidation, and NH3 oxidation were performed over the catalyst samples to show the influence on the catalytic performance. The SO2 desorption during the performance evaluation was also recorded to illustrate the change of desulfation difficulty. The results of this research are critical for improving the diesel SCR catalyst deactivation mechanism, and provide guidance for future automobile after-treatment system design to minimize SCR deterioration. 2. EXPERIMENTAL SECTION 2.1 Catalyst information and experimental scheme The commercial Cu-SSZ-13 and H-SSZ-13 diesel SCR catalyst powder samples (40-60 mesh) were investigated in this study. The mole Si/Al ratios of both of the catalyst samples are 10.5.


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The Cu-SSZ-13 catalyst sample has a Cu content of 2.9% wt. (~456.7 μmol/g). Since the Cu loading is high enough (Cu/Al≈0.3) according to references 7, 9, 11, two kinds of Cu2+ active sites exist on the catalyst. The H-SSZ-13 catalyst samples were used as a reference for the NH3-TPD and SO2-TPD experiments. A flow reactor system (Figure S1) was used for catalyst deactivation, performance evaluation, SO2-TPD and NH3-TPD in this study. A Multi-Gas 6030 FTIR (MKS, USA) was equipped to analyze the composition of the effluent gas. For all the experiments mentioned above, the amount of catalyst powder sample was 100 mg and the gas hourly space velocity (GHSV) was set to 300,000 h-1. The volume of the catalyst bed, total flow rate, and some other detailed information are listed in the supporting information document (Table S1). 2.2 Deactivation method 2.2.1 Hydrothermal aging To acquire the hydrothermally aged catalyst sample, the fresh Cu-SSZ-13 and H-SSZ-13 catalyst samples were first degreened with a gas consisting of 300 ppm NH3, 300 ppm NO, 10% O2, and 5% H2O balanced with N2 at 550°C for 1 hour. Afterwards, the samples underwent a hydrothermal aging process for four hours with 10% O2, and 5% H2O at 550/750/850°C. The hydrothermally aged samples were labeled as “HTA” with the hydrothermal aging temperature, e.g., HTA-750. In addition, the H-SSZ-13 samples were labeled with “SSZ”, e.g., HTA-SSZ-750. Considering 550°C is within the typical condition temperature range of the diesel exhaust gas, HTA-550 can be regarded as a fresh sample for reference. 2.2.2 Sulfur poisoning To acquire the SO2 poisoned catalyst samples, the fresh catalyst powder was first hydrothermally aged with different temperatures, and then exposed to a gas feed of 100 ppm SO2,


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10% O2, and 5% H2O balanced with N2 at 250°C for 2.5 hours followed by a purge process with pure N2 for 1 hour. The sulfur poisoned samples were named as “SO2” with the hydrothermal aging temperature, e.g., SO2-750. The H-SSZ-13 samples were labeled with “SSZ”, e.g., SO2SSZ-750. To minimize the loosely bound sulfate species, while maximizing the amount of sulfate species since the decomposition of sulfate species on the Cu-SSZ-13 catalyst starts at 300°C, the sulfation temperature was set to 250°C. The concentration of SO2 was set to 100 ppm, as it is the lowest SO2 concentration that can be stably controlled on our system. H2O is needed in the poisoning environment in order to be close to the actual conditions and enhance the formation of sulfate species. The reason why NH3 and NOX were not included is that competitive adsorption exists between SO2 and NOX on the active sites

19.

Additionally, the NH3+SO2 or

NH3+NO+SO2 condition would most likely reduce the accumulation of the stable metal sulfate species and make the poisoned catalyst samples easier to recover, so SO2 poisoning without NH3 causes a more pronounced hard-reversible degradation 16, 20. 2.3 SO2-TPD As a very practical way to quantify the sulfur species, an SO2-temperature-programmed desorption (SO2-TPD) experiment was conducted on the flow reactor system. The catalyst sample was hydrothermally aged at different temperatures and then exposed to a gas feed of 100 ppm SO2, 10% O2, and 5% H2O balanced with N2 at 250°C for 2.5 hours, which is the same as the SO2 poisoning condition, followed by a purge process with pure N2 for 1 hour. During the SO2-TPD process, the temperature was linearly increased up to 850°C at a ramp speed of 10°C/min in N2. 2.4 Performance evaluation of the catalyst


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To understand the effect of deactivation on the catalyst performance, performance evaluations over the fresh and deactivated samples were conducted on the flow reactor system. Details of the inlet gases are listed in Table 1. The evaluation temperature was between 150°C to 550°C, which can be easily reached in a diesel exhaust system 17. After temperature rise and stabilization, the entire performance evaluation at every temperature step consisted of three parts, in order: NOoxidation, NH3-oxidation, and Standard-SCR. Table 1. The performance evaluation experiment over Cu-SSZ-13 catalyst samples Temp. rise

NO-Oxidation

Time /min

~20

20

40

40

NO /ppm

300

300

0

300

0

0

300

300

NH3 /ppm O2 /% H2O /% GHSV /h-1

NH3-Oxidation Standard-SCR

10 5 300,000

2.5 XRD To characterize the CHA framework of the catalyst samples, XPert-3 Powder analyzer (PANalytical B.V., Netherlands) was used for X-ray diffraction (XRD). The transmitting power was 40 kv*40 mA, the scanning step was 0.026°, the scanning time was 30 s and the 2θ range was 5~40°. 2.6 H2-TPR To characterize the Cu active sites of the catalyst samples, an AutoChem II analyzer (Micromeritics, USA) was used for H2 temperature programmed reduction (H2-TPR). The mass of the catalyst sample was 40 mg. The flow rate of the analyzer was set to 30 ml/min (STP). The


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catalyst sample was first pretreated at 500°C for 30 min in air. After cooling down to 80°C, the sample was purged with 1% H2/Ar for 30 min. Then, during the TPR process, the temperature was heated up to 900°C with a heating rate of 10°C/min in the same gas feed. 2.7 NH3-TPD To evaluate the NH3 storage capacity of the hydrothermally aged catalyst samples, an NH3TPD test was conducted on the same flow reactor system. For the NH3 adsorption step, the catalyst sample was exposed to a gas feed of 300 ppm NH3, and 5% H2O balanced with N2 at 200°C for two hours followed by a purge process in 5% H2O/N2 for one hour. Then, during the NH3 desorption process, the temperature was linearly increased up to 600°C at a ramp speed of 10°C/min in 5% H2O/N2. 3. RESULTS AND DISCUSSIONS 3.1 XRD, H2-TPR and NH3-TPD: Impact of hydrothermal aging on the zeolite framework and active sites 3.1.1 XRD and H2-TPR The XRD patterns of the Cu-SSZ-13 catalyst samples hydrothermally aged at different temperatures are displayed in Figure 1(a). The labeled CHA structure peaks of the samples prove that the CHA zeolite structure of the catalyst remained after the hydrothermal aging process. However, after aging at 850°C, the CHA characteristic peaks become weaker, indicating a loss of the zeolite structure’s crystallinity and dealumination. The characteristic peaks of CuOX (CuO: 2θ=35.6° and 38.8°, Cu2O: 2θ=36.44°) can hardly be found in the XRD patterns. However, the formation of the CuOX species cannot be excluded on the hydrothermally aged samples, since the Cu ions are well-dispersed on the catalyst and difficult to massively agglomerate, resulting in the CuOX species barely being detected by XRD 25.


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As displayed in Figure 1(b), the H2-TPR results after smoothing (Adjacent-Averaging, Points of Window: 10) shows that the H2 reduction peak of HTA-550 (200~250°C) moves to a higher temperature range (300-350°C) by hydrothermal aging. According to reference11, there are two major H2-TPR peaks at approximately 250°C and 350°C, which correspond to the reduction of two kinds of Cu2+ species to Cu+: [Cu(OH)]+-Z and Cu2+-2Z, respectively. Thus, the shift of the H2-TPR peak to a higher temperature can be attributed to the migration of Cu species from [Cu(OH)]+-Z to Cu2+-2Z on the hydrothermally aged samples. In addition, another small H2-TPR peak at approximately 200°C can be found in the figure, corresponding to the copper oxide species, which is more easily reducible than Cu2+ ions 26, 27. For the HTA-550 sample, this peak is distinct, although it overlaps with part of the peak centered at approximately 250°C, then it becomes broader in the HT750 sample. This phenomenon proves that some CuOX/CuAlOX species exist in the fresh sample have transformed to other kind of copper species. According to Wang et al.23, the mild hydrothermal aging can result in the transformation of the copper oxide species to isolated Cu2+ in the 6mr. After severe hydrothermal aging, the peak related to the CuOX/CuAlOX species improves in the HTA-850 sample, indicating the growth of the CuOX/CuAlOX generated from the isolated Cu2+.

Figure 1 XRD patterns and H2-TPR profiles of hydrothermally aged Cu-SSZ-13 catalyst samples. (a) XRD results, (b) H2-TPR results.


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3.1.2 NH3-TPD The NH3-TPD results are shown in Figure 2(a). Two distinct NH3 desorption peaks can be found in the profile of the HTA-550 sample: one is centered at approximately 300°C (Peak 1) and the other at 400°C (Peak 2). For the HTA-SSZ-550 sample, only Peak 2 exists (other results of the H-SSZ-13 samples are shown in Figure S3). According to Leistner et al.

27,

the high-

temperature peak most likely represents the ammonia adsorbed onto the Brønsted acid sites. Luo et al.

7, 11

indicated that the high-temperature peak represents the Brønsted acid sites on the

framework, such as Al-OH. Thus, the Peak 2 of the H-SSZ-13 sample is higher than that of the Cu-SSZ-13 sample because the acid sites on the zeolite are not coordinated with the Cu species. The low-temperature peak is mainly related to the two kinds of Cu active sites, [Cu(OH)]+-Z and Cu2+-2Z, which is why Peak 1 cannot be found in the H-SSZ-13 sample. The adsorption of NH3 on the zeolite Lewis acid sites, except for the Cu ions, is very weak and cannot be used as lowtemperature adsorption sites above 200°C. With the quantification of the NH3-TPD profile peak area (Figure S2), the total NH3 storage capacity of the hydrothermally aged samples is presented in Figure 2(b). The NH3-TPD profile of the HTA-750 sample shows that Peak 1 intensifies, Peak 2 decreases, and the total adsorbed NH3 amount remains the same. According to Luo et al. 7, 11, this change in the two NH3-TPD desorption peaks upon mild hydrothermal aging can be attributed to Cu migrated from [Cu(OH)]+-Z to Cu2+-2Z, which is also confirmed by the H2-TPR experiments, resulting in a growth of Cu2+-2Z with more adsorbed NH3 (~2 NH3 molecules on Cu2+-2Z, ~1 NH3 molecule on [Cu(OH)]+-Z), as well as the consumption of the Brønsted active sites on the framework (Cu2+-2Z is doubly coordinated with zeolite, [Cu(OH)]+-Z is singly coordinated with


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zeolite). Dealumination and transformation of Cu2+ to CuOX/CuAlOX did not occur by mild hydrothermal aging, thus the total NH3 storage capacity was almost unchanged. For the HTA850 sample, a decrease of both the low-temperature peak and the high-temperature peak can be observed. The total NH3 storage capacity drops from 517 μmol/g (HTA-550) to 316 μmol/g (HTA-850). The generation of CuOX/CuAlOX and dealumination might both occur after more severe hydrothermal aging treatment (above 750°C)

11, 28, 29,

resulting in the loss of Cu2+ active

sites as well as the Brønsted acid sites on the framework, which weakened the low-temperature and high-temperature NH3 storage capacity.

Figure 2. NH3-TPD results of hydrothermally aged catalyst samples. (a) NH3-TPD profiles, (b) Quantification of total NH3 storage capacity (μmol/g)

3.2 SO2-TPD: Change of sulfate species


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Figure 3. SO2-TPD results of the catalyst samples. (a) SO2-TPD profile Cu-SSZ-13/H-SSZ-13 catalyst samples, (b) Quantification of the total sulfate species (μmol S/g).

The SO2 desorption profiles after smoothing (Adjacent-Averaging, Points of Window: 10) of SO2 poisoned samples with different hydrothermal aging temperatures are displayed in Figure 3(a). Each Cu-SSZ-13 sample has two distinct desorption peaks: a low-temperature peak centered at approximately 450°C (Peak 1), and a high one centered at approximately 700°C (Peak 2). Based on the study of Su et al.

20,

three types of sulfate species exist on the SO2

poisoned Cu-SSZ-13 catalyst in a humid environment: H2-SO4, Cu-SO4 and Al-SO4-like species. The very limited amount of sulfate species decomposes at approximately 450°C (Peak 1), which probably represents the H2-SO4-like form species coordinated with copper at 6mr 9. The location and area of Peak 1 over three samples are almost the same. According to Jangjou et al. 21 and Su et al. 20, the desorption peak (Peak 2) at ~700°C suggests the formation of stable copper sulfate species. The Cu-sulfate usually decomposes at 600~700°C 17. Most of Peak 2 is likely related to the Cu-sulfate formation, which is mostly the sulfate interacting with [Cu(OH)]+-Z in the large cage of the CHA zeolite. It is reported that the Al-SO4 on Cu-SSZ-13 decomposes at approximately 720~800°C

20.

The metal sulfate species with this decomposition temperature


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likely represents the aluminum sulfate species, which is also confirmed by the result of the HSSZ-13 sample (SO2-SSZ-550) shown in Figure 3(a), which has only one SO2 desorption peak (Peak 3) centered at approximately 800°C, since no Cu exists on this catalyst sample. However, the sulfur poisoning of Al is not as serious as that of Cu, since the Al-SO4-like species also provides acid sites and Al is not the main active sites for the SCR reaction. The SO2-TPD results of other H-SSZ-13 catalyst samples are presented in Figure S5(a)-(b). To distinguish the amount of each kind of metal sulfate species by peak-differentiating and imitating is quite difficult, because the decomposition temperatures differ from the reference samples. However, quantifying the total metal sulfate is practicable. Figure 3(b) illustrates the quantities of the total sulfate species by integrating the TPD peaks area in Figure S4(a)~(c). Compared with SO2-550, Peak 2 is obviously weakened by 750°C hydrothermal aging. This illustrates that the amount of copper sulfate species was reduced for the mildly hydrothermally aged sample, since copper ions had migrated from sulfur-sensitive [Cu(OH)]+-Z to the less sulfur-sensitive Cu2+-2Z. Peak 3 almost remains over the SO2-750 sample, since the dealumination is very limited at such a temperature. However, Peak 3 drops after the 850°C hydrothermal aging, indicating that the aluminum sulfate species of SO2-850 decreased. This could be due to the dealumination and Al-Cu clustering caused by 850°C hydrothermal treatment. Interestingly, although the quantity of Al-SO4 continuously declined, Peak 2 intensifies and the total amount of metal sulfate species rises to a great extent. This growth can be attributed to the increase in the Cu-SO4 species. According to Wijayanti et al. 16, the formation of copper sulfate is increased by the presence of copper oxide species over the hydrothermally aged Cu-SSZ-13 catalyst. Thus, the growth of sulfate species over SO2-850 possibly represents the sulfate interacting with CuOX/CuAlOX.


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Another interesting phenomenon is that the maximum location of the SO2-TPD peak shifts to a higher temperature over the SO2-750 sample, and goes back to a lower temperature over the SO2-850 sample. One possible reason for this is the simultaneously changing of the two kinds of metal sulfate species, Cu-SO4 and Al-SO4, which are related to Peak 2 and Peak 3, respectively. In addition, maybe the decomposition temperatures of the Cu-SO4-like species on different Cu sites were different. The change in the forms of the Cu-SO4-like species probably cause a shift in the SO2-TPD peaks.

3.3 Standard SCR and NH3/NO oxidation: Effect on the catalytic performance 3.3.1 Standard-SCR The steady-state standard-SCR performance of the samples are shown in Figure 4 (a)-(b). The change in the NOX conversion and the NH3 conversion over the HTA samples is not distinct. These results support the idea that hydrothermal aging has a limited influence on the steady-state NOX conversion because of the great hydrothermal stability of Cu-SSZ-13 9. The NOX conversion of the HTA-750 sample shows a slight rise at 500-550°C, which was also observed by Luo et al. 9 This can be explained by the weakening of the competitive NH3 oxidation ability at a high temperature over the 750°C hydrothermally aged sample

19, 22,

as will be shown in the

NH3 oxidation part in section 3.3.2. The NH3 oxidation, which starts at approximately 300°C, consumes NH3 and causes a lack of reductant of the SCR 27. The HTA-850 sample has a poorer performance mainly in the low-temperature range and high-temperature range. After 850°C of hydrothermally aging treatment, some of the copper active sites were deactivated by CuOX/CuAlOX formation, and the quantity of Brønsted acid sites were decreased by dealumination. In the low-temperature range, the loss of SCR activity was caused by


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CuOX/CuAlOX formation and reduced the amount of copper active sites 27. The CuOX/CuAlOX species contributes more to high-temperature oxidation. The next section will focus on the change of oxidation. Moreover, the Brønsted acid sites are critical for the SCR process at a hightemperature

30.

Therefore, the activity abatement at a high temperature could possibly be

attributed to the deactivation of copper active sites, the growth of the aggregated CuOX/CuAlOX species and the loss of Bronsted acid sites 23. The results of the SO2 poisoned samples are also shown in Figure 4 (a). It is obvious that the standard SCR activity declines significantly and the SO2-550 sample shows the worst NOX conversion below 350°C, suggesting a significant loss of the active sites9, which were poisoned by the sulfate species. This was also reported by other observations 17. Interestingly, Figure 4(a) also shows that the sulfur-poisoned samples, which had been hydrothermally aged before (SO2750 and SO2-850), reveal better SCR activity compared with SO2-550. Considering that the SO2 tended to interact with [Cu(OH)]+-Z rather than Cu2+-2Z, the migration from [Cu(OH)]+-Z to Cu2+-2Z caused by hydrothermal aging indeed has a contribution to weakening the influence of the SO2 poisoning upon the SCR performance at a low-temperature range, while at the middletemperature range, the conversions of the sulfur-poisoned samples have recovered and all the conversions are almost the same, suggesting the recovery of active sites, which were previously poisoned by sulfates.


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Figure 4. Standard SCR performance over Cu-SSZ-13 catalyst samples. 300 ppm NH3, 300 ppm NO, 10% O2, and 5% H2O balanced with N2, GHSV=300,000 h-1. (a) NOX conversion of standard-SCR, (b) NH3 conversion of standard-SCR.

To more clearly compare the difference of the standard SCR NOX conversion over the hydrothermally aged samples before and after SO2 poisoning, Figure 4 was converted to Figure S6. The difference is computed by the NOX conversion of SO2 poisoned samples minus that of the HTA samples with the same HTA temperature. The declines are quite distinct at a lowtemperature range, especially for SO2-550. It is strange that when the temperature increased above 500°C, the NOX conversion difference of the SO2-550 sample showed a slight growth, but SO2-750 remained almost the same, and the SO2-850 sample decreased slightly. This phenomenon could not be due only to the desulfation of the active sites. As an additional explanation, an oxidation change and desulfation process also might have made some contribution. In fact, both NO and NH3 participated in the desulfation process 31. The change in the oxidation and desulfation ability needs further confirmation by NH3/NO oxidation tests and SO2 desorption results. 3.3.2 NH3 / NO Oxidation


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Figure 5. NH3/NO oxidation performance over Cu-SSZ-13 catalyst samples. 300 ppm NH3 (if needed), 300 ppm NO (if needed), 10% O2, and 5% H2O balanced with N2, GHSV=300,000 h-1. (a) NO2 generation of NO oxidation, (b) NH3 conversion of NH3 oxidation, (c) NOX generation of NH3 oxidation.

The NH3/NO oxidation test is very important for catalyst activity measurements since the change of the Cu active sites can be analyzed through NH3/NO oxidation performance. It is commonly accepted that H-SSZ-13 without Cu species is inert in NO oxidation, and the NO oxidation to NO2 could probe the copper active sites

9, 32.

Meanwhile, both types of Cu active

sites are active for the SCR reaction, and only [Cu(OH)]+-Z is mainly active for oxidation 9. NH3 oxidation has an adverse effect on the SCR reaction. As the reductant of the SCR reaction, the oxidation of NH3 will reduce the NOX conversion of the SCR reaction. The results of the NO2 generation of NO oxidation, NH3 conversion of NH3 oxidation and NO2 generation of NH3 oxidation are shown in Figure 5 (a)-(c). As shown in Figure 5 (a) and (b), the NO/NH3 oxidation activity of HTA-750 decreases to a great extent. The ability of NOX generation for NH3 oxidation, which is shown in Figure 5 (c), exhibits a slight drop at a high temperature (500-550°C) over the HTA-750 sample. The oxidation activity loss of HTA-750 could be due to the migration of copper from [Cu(OH)]+-Z to Cu2+-2Z. Additionally, the decreased selectivity of NOX in the NH3 oxidation proved the transformation of CuOX/CuAlOX to Cu2+-2Z 11. In contrast, the oxidation rate increased over the HTA-850 sample compared with the HTA-750 sample, and the NOX generation of NH3 oxidation rose considerably, even higher than the HTA-550 sample above 350°C. This phenomenon confirms the growth of the CuOX/CuAlOX species, generating more NOX during


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the NH3 oxidation process. Therefore, the oxidation enhancement of HTA-850 above the lowtemperature section could be due to the formation of the oxides species, which is in line with the results of the H2-TPR experiment. After SO2 exposure, the NO/NH3 oxidation activity of the poisoned samples (SO2-550, SO2750 and SO2-850) dropped dramatically below 400°C. The NO2 generation of NO oxidation and NH3 conversion of the NH3 conversion were nearly zero up to 250°C. This suggests that the [Cu(OH)]+-Z active sites, which were in charge of the NO/NH3 oxidation, were almost all poisoned by SO2 exposure at a low-temperature range 9. For the NOX generation of NH3 conversion, with a limited amount of CuOX/CuAlOX, the changes in the SO2-550/HTA-550 and SO2-750/HTA-750 samples were not obvious. However, the decrease in the NOX generation is distinct below 500°C, suggesting that the CuOX sites were also poisoned by sulfate, which agrees with the SO2-TPD results. As the temperature increased above 300°C, the oxidation rate started to recover. Interestingly, the NO2 generation of NO oxidation and NH3 conversion/NOX generation of the NH3 conversion of SO2-550/SO2-750/SO2-850 (except the NO oxidation rate over SO2-850) were all finally beyond that of HTA-550/HTA-750/HTA-850. This phenomenon was also discovered by other research

9, 31,

but unfortunately has not been well explained. Some explanation and the effect of

the oxidation change on the standard SCR will also be discussed in the discussion section. 3.3.3 N2O generation during standard SCR N2O has an even stronger greenhouse effect than CO2. Even though the N2O formed on CuCHA is very limited compared with other kinds of Copper zeolite SCR catalysts

12, 14,

it is still

necessary to evaluate the N2 selectivity by measuring the N2O formation during the SCR reaction. The N2O generation of standard-SCR over hydrothermally aged and SO2 poisoned Cu-SSZ-13


19


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catalyst samples is shown in Figure 6. For each of the hydrothermally aged sample profiles, there is one distinct peak centered approximately 200-250°C, which decreases as the hydrothermal aging temperature rises. The peak centered approximately 200-250°C represents the N2O formed mainly on the [Cu(OH)]+-Z active sites by decomposition of ammonium nitrate

8, 12.

Therefore,

the decrease of this peak can be due to Cu2+ migration for HTA-750 and the transformation of Cu2+ to the CuOX/CuAlOX species for HTA-850. For the SO2 poisoned samples, the first peak almost disappeared. The N2O formation over these three samples essentially increases with the temperature. Among them, SO2-550 had the lowest N2O formation and was far below the other samples. SO2 prefers to react with [Cu(OH)]+Z

21.

Therefore, the formation of N2O at a lower temperature decreased with the hydrothermal

aging temperature for the HTA samples and tended to zero over the SO2-550 samples. The Cu2+2Z active sites migrated from [Cu(OH)]+-Z might have some contribution to the N2O formation at low temperatures. As a result, the N2O formation of sulfur poisoned samples hydrothermally aged before were enhanced in the low-temperature range. At a high temperature, the N2O is mainly generated by unselective ammonia oxidation8. It can be clearly seen that the N2O formation in SO2-750 and SO2-850 is much higher than that of SO2-550. However, we are currently not able to explain the reason for this.


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Figure 6. N2O generation of standard SCR performance over Cu-SSZ-13 catalyst samples. 300 ppm NH3, 300 ppm NO, 10% O2, and 5% H2O balanced with N2, GHSV=300,000 h-1.

3.3.4 SO2 desorption during the performance evaluation To investigate the impact of sulfate decomposition on catalyst activity recovery, the smoothed SO2 desorption data (Adjacent-Averaging, Points of Window: 30) during the catalyst performance evaluations for all of the SO2 poisoned samples at every temperature step are shown in Figure 7. The original results and full temperature results are presented in Figure S7. With the temperature increasing, the SO2 decomposed from the sulfate species on the catalyst desorbed to the exhaust gas intermittently. The decomposition temperature of the sulfate species in the performance evaluation gas environment (with the presence of NH3/NO) is lower than that in the SO2-TPD gas environment (without NH3/NO). The amount of sulfate decomposition of these samples was different at various temperature steps. The peak heights comparison are listed in Table 2. Almost no SO2 was desorbed at 150-250°C, so they are omitted in Figure 7. Table 2. SO2 desorption during the performance evaluation over the Cu-SSZ-13 catalyst samples Temp.

Main Desorption Step

Peak heights

300°C

Temp. rise to 350°C

SO2-750 >SO2-550 >SO2-850

350°C

SCR period

SO2-750 >SO2-550 >SO2-850

SCR period

SO2-550 >SO2-750 ≈SO2-850

NH3 oxidation period

SO2-750 ≈SO2-850 >SO2-550

450°C


SO2-550 >SO2-850 >SO2-750

500°C


SO2-850 >SO2-750 ≈SO2-550

550°C


SO2-850 >SO2-750 >SO2-550

400°C


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For all the poisoned samples, the desulfation can be seen to clearly begin at the temperature rise period from 300-350°C, which was also observed by other researchers

17, 18, 20.

During this

period, sharp SO2 desorption peaks were detected. This first sharp peak could be possibly due to the decomposition of NH4HSO4-like species by the interaction between NH3 and pre-adsorbed sulfur during the previous evaluation step. This proves that the NH3 can reduce the sulfates via the formation of ammonia. When the temperature increased to 350°C and the standard SCR period began, another peak appeared. This phenomenon suggests that the decomposition of NH4SO4-like species generated by NH3 exposure or desulfation by SCR condition start to occur over these sulfur-poisoned samples. Compared with SO2-550, the amount of desorbed SO2 over SO2750 is much lower below 400°C, indicating that mild hydrothermal aging not only decrease the amount of sulfate species but also reduce the desulfation difficulty on the Cu-SSZ-13 catalyst. When the temperature reached 400°C, the decomposition of sulfate began to mainly occur in the NH3 oxidation period for the SO2-550 sample. However, the SO2-750 and SO2-850 samples continually desulfated in the standard SCR period. During the performance evaluation above 400°C, all samples only desorbed SO2 in the NH3 oxidation period, but the quantities were different. At the 550°C performance evaluation step, the sulfate of SO2-850 still decomposed obviously during this period, revealing the increasing difficulty of the desulfation. The SO2 desorption profile of the catalyst samples immediately dropped when the NH3 oxidation period switched to the standard SCR period, suggesting that some stable sulfate species was still left on the catalyst, which requires a higher temperature to fully regenerate. The immediate drop of the SO2 desorption peak at 550°C also suggests that the NH3 oxidation gas environment has a much higher efficiency than the SCR condition at that temperature. All these results agreed with the view of Kumar et al.

31

that the SCR condition, containing NH3 and NO in a locally reducing


22

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environment, is better than the environment only containing NH3 for the chemical desulfation at a lower temperature. When the temperature is increased above 400°C, NH3 can be oxidized to NOX and forms an SCR-like desulfation environment. In contrast, the SCR reaction rate of NO+NH3 is extremely fast in a short reaction zone at such a temperature, leaving no reductant to react with the sulfate backward on the catalyst 31.

Figure 7. SO2 desorption of SO2 poisoned Cu-SSZ-13 catalyst samples during the performance evaluation. (250~550°C)

3.4 DISCUSSION In this study, the SO2-TPD results indicated that different temperatures of hydrothermal aging varied the quantity of the sulfate species formed on the SO2-poisoned Cu-SSZ-13 catalyst. The Al-SO4-like species declined by severe hydrothermal deactivation (850°C) via dealumination. The Cu-SO4-like species first decreased over the 750°C hydrothermally aged sample (SO2-750) via copper migration from [Cu(OH)]+-Z, which SO2 prefers to react with, to a more stable position Cu2+-2Z, which SO2 would hardly be adsorbed on. After undergoing 850°C hydrothermal aging, some isolated Cu2+ transferred to CuOX or CuAlSO4 species, resulting in the


23


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growth of Cu-SO4-like species. Figure 8 illustrates the changes in metal sulfate species over SO2exposed Cu-SSZ-13 catalyst samples, which were hydrothermally aged at different temperatures.

Figure 8. Schematic diagram of the change in metal sulfate species over SO2-poisoned Cu-SSZ13 catalyst samples with different hydrothermal aging temperatures.

For the effect of such a change in sulfate species on the catalyst performance, without the hydrothermal aging treatment, the sulfur poisoned sample (SO2-550) showed a much lower SCR activity/oxidation rate/N2O formation at temperatures below 350°C. The reduced NO oxidation and N2O formation were caused by the poisoning of [Cu(OH)]+-Z. After the hydrothermal aging treatment, the SCR activities of the poisoned samples of SO2-750 and SO2-850 were similar but were higher than SO2-550 at a low temperature due to the migration of Cu species. In general, at a low temperature SO2 poisoning has a greater impact on the Cu-SSZ-13 SCR activity than hydrothermal aging due to the loss of active sites by sulfate covering. When the performance evaluation temperature increased beyond 350°C, the SCR activity of all the poisoned samples (SO2-550/SO2-750/SO2-850) began to be restored. However, the difference is that the oxidation rate of the poisoned samples, which were hydrothermally aged,


24

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generally increased obviously at a high temperature. Some researchers

9, 31

have found the

promotion of the oxidation rate over a sulfated sample at the high-temperature range, but they rarely explained the reason why. This phenomenon could not only be explained by the active site recovery during the performance evaluation but also possibly due to some kind of additional consumption of SO2 on the sulfur poisoned active sites. In fact, the NO and NH3 was involved in the desulfation process. For the SO2-550 sample, the NOX generation of NH3 oxidation remained the same and all the NH3 was oxidized, so the NOX conversion of the standard SCR increased at 550°C, because more NO participated in the desulfation process. For the hydrothermally aged samples, more NO and NH3 participated in the desulfation process, making the oxidation performance seem better than before and resulting in the reduction of standard SCR NOX conversion in a high-temperature range for a lack of reductant. More investigation is required to confirm the effect of hydrothermal aging on the sulfur-poisoned catalyst performance at a high temperature. In summary, compared to the SO2-550 sample, the Cu-SSZ-13 catalyst samples hydrothermally aged at 750°C generally reduced the amount of sulfate species, while increasing the quantity of easy-recoverable sulfate species. Thus, the catalyst desulfation difficulty is reduced. In contrast, the easy-recoverable sulfate species significantly decreased over catalyst samples hydrothermally aged at 850°C. Additionally, the amount of more stable sulfate species, which were decomposed at a higher temperature, were increased over the SO2-850 samples. Thus, the difficulty of desulfation was enhanced. 4. CONCLUSIONS This study was performed to investigate the effect of different levels of hydrothermal aging on the SO2 poisoning process over the Cu-SSZ-13 SCR catalyst and to determine the impact of the


25


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change in the sulfate species on the catalytic performance. The identification of sulfate species over SO2 poisoned samples was conducted using SO2-TPD, and the amount of different kinds of sulfate species was determined for different degrees of hydrothermal aging. The standard SCR, oxidation, N2 selectivity and desulfation performance of the SO2 poisoned catalyst were all influenced by the temperature of hydrothermal aging. The change of the copper active sites after SO2 exposure was highlighted, and the Cu-SO4-like species decreased on the 750°C aged sample via copper migration, but they increased on the 850°C aged sample by CuOX/CuAlOX formation. The low-temperature SCR activity and oxidation were weakened by active sites poisoning. Furthermore, it can be concluded that mild hydrothermal aging reduced the desulfation difficulty. However, more severe hydrothermal aging not only irreversibly deactivated the catalyst but also increased the difficulty of desulfation. This study also raised important questions about the mechanism of SCR activity and oxidation promotion over sulfur-poisoned catalyst samples at high temperatures. Further research is required to explore the nature of the sulfate species’ impact on the SCR reaction chemistry at low temperatures and the detailed desulfation mechanism over the Cu-SSZ-13 catalyst. This work is the comprehensive investigation of the change of sulfate species and the performance evaluation over SO2-poisoned Cu-SSZ-13 catalysts that have been hydrothermally aged at different temperatures. The findings of this study provide important implications for the future design and improvement of SCR catalysts. This work suggests that an appropriate degree of hydrothermal aging before use not only weakens unfavorable NH3 oxidation but also strengthens sulfur resistance, which is a critical part of catalyst durability. ASSOCIATED CONTENT


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The Supporting Information is available free of charge on the ACS Publications website at DOI: Supporting Information. Detailed information of the catalyst samples; Detailed description of the flow reactor system; Additional results of NH3-TPD tests; Additional results of SO2-TPD tests; Difference of standard-SCR performance between SO2 poisoned and hydrothermally aged samples; The calculation of performance evaluation for the catalyst samples; Additional SO2 desorption results of SO2 poisoned samples during performance evaluation. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Dongwei Yao: 0000-0001-7698-514X Funding National Key Research and Development Program (2017YFC0211103) National High-tech Research and Development Program (2013AA065301). ZJU-UIUC Institute Research Program (ZJU R2017.1.43) Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS


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We would like to acknowledge Prof. Xiaoyan Shi and Dr. Jinpeng Du from the EcoEnvironmental Sciences Research center of Chinese Academy of Sciences (Beijing, China) for their help with the H2-TPR test, Dr. Jinyong Luo from Cummins Inc. (Columbus, IN 47202-3005, USA) for guidance on NH3-TPD, and Dr. Lei Ma and Prof. Junhua Li from Tsinghua University (Beijing, China) for their guidance on the characterization over hydrothermally aged catalysts. We also acknowledge Dr. Cong Liu from Argonne National Laboratory (Argonne, IL 60439, USA) for the comments for this paper. This study was financially supported by the National Key R&D Program (2017YFC0211103) and the National High-tech R&D Program (2013AA065301). This study was also supported by the ZJU-UIUC Institute Research Program (ZJU R2017.1.43). REFERENCES (1) Bishop, G. A.; Hottor-Raguindin, R.; Stedman, D. H.; McClintock, P.; Theobald, E.; Johnson, J. D.; Lee, D.; Zietsman, J.; Misra, C., On-road Heavy-duty Vehicle Emissions Monitoring System. Environ. Sci. Technol. 2015, 49, 1639-1645. (2) Abdelmegeed, M. A. E.; Rakha, H., Heavy-Duty Diesel Truck Emissions Modeling. Transport. Res. Rec. 2017, 2627, 26-35. (3) He, L.; Hu, J., Investigating Real-World Emissions of China's Heavy-Duty Diesel Trucks: Can SCR Effectively Mitigate NOx Emissions for Highway Trucks? Aerosol Air Qual. Res. 2017, 17, 2585-2594. (4) Nova, I.; Tronconi, E., Urea-SCR technology for deNOx after treatment of diesel exhausts; Springer: Germany, 2014; Vol. 5. (5) Shen, M.; Li, X.; Wang, J.; Wang, C.; Wang, J., Nature Identification of Cu Active Sites in Sulfur-Fouled Cu/SAPO-34 Regeneration. Ind. Eng. Chem. Res. 2018, 57, 3501-3509. (6) Wang, J.; Peng, Z.; Qiao, H.; Yu, H.; Hu, Y.; Chang, L.; Bao, W., Cerium-Stabilized CuSSZ-13 Catalyst for the Catalytic Removal of NOx by NH3. Ind. Eng. Chem. Res. 2016, 55, 1174-1182. (7) Luo, J.; Kamasamudram, K.; Currier, N.; Yezerets, A., NH3-TPD methodology for quantifying hydrothermal aging of Cu/SSZ-13 SCR catalysts. Chem. Eng. Sci. 2018, 190, 60-67.


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(31) Kumar, A.; Smith, M. A.; Kamasamudram, K.; Currier, N. W.; Yezerets, A., Chemical deSOx: An effective way to recover Cu-zeolite SCR catalysts from sulfur poisoning. Catal. Today 2016, 267, 10-16. (32) Kumar, A.; Smith, M. A.; Kamasamudram, K.; Currier, N. W.; An, H.; Yezerets, A., Impact of different forms of feed sulfur on small-pore Cu-zeolite SCR catalyst. Catal. Today 2014, 231, 75-82.


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For Table of Contents Only


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Table of Contents (TOC)/Abstract graphic (Original size) 339x211mm (96 x 96 DPI)


Impact of Hydrothermal Aging on SO2 Poisoning over Cu-SSZ-13

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