N2O Formation Pathways over Zeolite-Supported Cu and Fe Catalysts

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N2O formation pathways over zeolitesupported Cu- and Fe- catalysts in NH3-SCR Dong Zhang, and Ralph T. Yang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03405 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 9, 2018

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N2O formation pathways over zeolite-supported Cu- and Fecatalysts in NH3-SCR Dong Zhang, Ralph T. Yang* Department of Chemical Engineering, University of Michigan, Ann Arbor, MI, USA Address: 3074 H.H. Dow, 2300 Hayward Street, Ann Arbor, MI 48109-2136, USA.

Abstract: N2O is a common by-product in the NH3-SCR reaction. We analyzed the N2O formation pathways in NH3-SCR over various catalysts (Cu-ZSM-5, Fe-ZSM-5, Cu-SAPO-34,

Fe-SAPO-34,

Cu-SSZ-13

and

Fe-SSZ-13),

aided

by

catalyst

characterization using XRD, XPS, EDS Mapping and NH3-TPD. The results showed that the NH3 non-selective catalytic reduction was the major N2O formation pathway for most of the Cu-catalysts. The N2O formation at lower temperatures (< 300oC) originated mainly from decomposition of NH4NO3. In addition, NH3 non-selective oxidation was another reaction that formed N2O especially at higher temperatures. The N2O resulting from the Eley-Rideal mechanism was also favored at higher temperatures. The decomposition of NO to N2O and O2 also led to N2O formation, although its contribution was minimal. The absence of N2O yield over most Fe-catalysts could be attributed to active N2O decomposition and N2O-SCR reactions. Moreover, varying O2 and H2O concentrations in the feed exerted strong influence on both N2O formation and SCR activity. Decrease in O2 level from 14% to 3% led to continual decline in N2O formation 1

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but had no effect on SCR activity until reaching a threshold concentration of 2%. H2O in lower concentrations (2-3%) facilitated N2O formation and NO conversion due to increase in Brønsted acidity, while H2O in higher concentrations (> 5%) led to suppression of these reactions due to the coverage of active sites.

Keywords: N2O formation; NH3-SCR; Cu-catalysts for SCR; Fe-catalysts for SCR; zeolites for SCR; 1. INTRODUCTION Selective catalytic reduction (SCR) with ammonia via urea injection in the presence of excessive oxygen is the most prevailing method for abating nitrogen oxides (NOx) in the exhaust gases from diesel engine combustion. 1 Nitrous oxide (N2O) is generated as an undesirable byproduct in the reaction. It is a greenhouse gas which has a global warming potential (GWP) 265–298 times that of CO2 and is now being regulated by US Environmental Protection Agency.2 Catalyst is the key in SCR technology and the activity exerts tremendous effects on the abatement of NOx and N2O emissions. A great variety of SCR catalysts have been reported in the literature including noble metals (e.g., Pt3, Pd4, Rh5, Au, Ag6) and transition metals (e.g., V7, W8, M9, Cu10, Fe11, Mn, Ce12). The noble-metal catalysts are highly active but costly, while the transition metal catalysts are less costly and more resistant to poisoning. In particular, the Cu-based catalysts exhibit high activities at low 2


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temperatures (300°C).13 As a result, these catalysts have been extensively studied in recent years.14-18 Apart from the active metal species, the support also plays an important role. Al2O36, TiO29, honeycomb monolith19 and zeolites13,

18, 20

are widely used as

supports for catalysts. Zeolites offer large specific surface area, high pore volume, uniform pore structure with shape selectivity and acid sites, which are desirable for catalytic reactions. Among zeolites, ZSM-5, SAPO-34 and SSZ-13 supported Cu and Fe catalysts have shown high activities and durabilities for ammonia-SCR.16,

21-29

In

particular, the Cu-SSZ-13 have been already used in diesel trucks. Compared with Cu-SSZ-13, Cu-SAPO-34 is also a promising candidate for NH3-SCR because it is more resistant to relatively harsh hydrothermal treatment. It has also drawn increasing attentions recently.13 Many studies on N2O formation in NH3-SCR over Cu- and Fe-based catalysts have been undertaken. Aylor et al. proposed that N2O could be formed by decomposition of Cu+(NO)2, an intermediate in NH3-SCR, over Cu-ZSM-5 catalysts.30 Rahkamaa-Tolonen et al. suggested that Cu-zeolite catalysts generated more N2O than Fe-zeolite catalysts under the same conditions.31 Lee et al. investigated the effects of hydrothermal aging on N2O yield over Cu-ZSM-5 and Cu-SSZ-13. It was found that small amounts of N2O were generated over these catalysts before and even after hydrothermal aging. They also proposed that NH4NO3, the intermediate formed in SCR, decomposed into N2O.32 3


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De-La-Torre et al. developed a kinetic model for N2O formation over Cu-CHA zeolite catalysts for NH3-SCR.33 Konduru et al. discussed the transformation of NO to N2O during SCR and the pathway of N2O decomposition over Cu-ZSM-5.34 Leistner et al. investigated N2O formation patterns over Cu/SSZ-13, Cu/SAPO-34 and Cu/BEA and proposed that N2O at either lower or higher temperature occurred on different copper sites or followed different mechanisms.34 Cho et al. studied the N2O generation over Fe-zeolite catalysts for SCR. A significant increase in the amount of N2O was generated for high NOx conversion due to side reactions with ammonia, NO2 and O2.35 Yang et al. proposed two mechanisms of N2O formation during low-temperature NH3-SCR over Mn-Fe spinel. They demonstrated that the Eley-Rideal mechanism (reaction of adsorbed ammonia species with gaseous NO) and Langmuir-Hinshelwood mechanism (reaction of adsorbed ammonia species with adsorbed NOx species) both led to N2O formation.36 Devadas et al. examined the influence of NO2 on NH3-SCR over Fe-ZSM-5 and found that the addition of NO2 contributed to the production of N2O at lower and intermediate temperatures. However, the N2O is absent at higher temperature which could be a result of N2O decomposition and N2O-SCR reactions.37 Kim et al. also noted the suppression of N2O formation over the

Fe-zeolite-promoted

V2O5-WO3/TiO2-based

catalysts.

However,

the

SCR

performance was compromised which was likely due to the enhanced NH3 oxidation to NO.38 Rivallan et al. investigated the catalytic conversion mechanism of N2O to N2 and 4


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O2 over Fe-ZSM-5 catalyst.39 Although much understanding has been obtained on N2O formation in NH3-SCR, there are still some questions needing further understanding. 1. There are many mechanisms (or pathways) that contribute to the N2O formation such as Eley-Rideal, Langmuir-Hinshelwood mechanism, ammonia oxidation, etc. Is there any difference in the mechanisms or pathways leading to the N2O generation over different catalysts? Besides, does the reaction temperature have effects on the pathways? 2. It has been reported that N2O can decompose over Cu and Fe catalysts.40-41 How effective are the Cu and Fe catalysts in N2O decomposition (at different temperatures)? 3. Based on past research37, 42-43, the introduction of 5% H2O in the feed, which is the typically presented in the exhaust of diesel trucks, leads to the suppression in SCR performance and N2O production. Does H2O of different concentration have the similar effects on the SCR activity and N2O production? 4. It is known that there exists a threshold O2 concentration beyond which the NO conversion levels off. Will there be a similar threshold for N2O formation? In this work, we performed the N2O formation pathway analysis over the commercially important Cu- and Fe-based ZSM-5, SAPO-34 and SSZ-13. These catalysts were evaluated under 4 conditions with different reactant gas compositions: Standard SCR condition (NO, NH3, O2, H2O), (NH3, O2, H2O), (NH3, NO, H2O) and (NO, H2O) in the temperature range of 150-550°C. The effects of H2O of different 5


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concentrations on N2O yield were investigated. N2O decomposition and N2O-SCR over Cu and Fe catalysts were also studied. The catalysts were further characterized by XRD, XPS, EDS Mapping and NH3-TPD.

2. EXPERIMENTAL 2.1 Preparation of catalysts Seven different catalysts, Cu-ZSM-5 IE, Fe-ZSM-5 IE, Fe-ZSM-5 IWI, Cu-SAPO-34, Fe-SAPO-34, Cu-SSZ-13 and Fe-SSZ-13 were prepared according to the methods described below. Cu-ZSM-5 I.E. was prepared by ion-exchange (I.E.) procedure. 2g of NH4-ZSM-5 (Si/Al=10) was calcined at 550°C for 6h to produce H-ZSM-5. The resulting H-ZSM-5 was added to 250 mL of 0.1M CuCl2 solution with constant stirring. After ion-exchange for 24h at room temperature, the product was filtered and washed with deionized water. The ion-exchange procedure was performed twice. The final product was dried at 100°C overnight and calcined in air at 550°C for 4h. Fe-ZSM-5 I.E. was prepared by the same procedure. 2g of NH4-ZSM-5 (Si/Al=10) was added to 250 mL of 0.1M FeCl2 solution with constant stirring. The following procedure was the same as that used for preparing Cu-ZSM-5 I.E. Fe-ZSM-5 I.W.I. was prepared by impregnating FeCl2 onto the NH4-ZSM-5 (Si/Al=10) via incipient wetness impregnation (I.W.I.). The Fe loading was 1.6 wt%. The 6


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samples were dried at 110°C for 12h and calcined in air at 550°C for 4h. Cu-SAPO-34 and Cu-SSZ-13 were prepared by the one-pot synthesis procedure which can be found elsewhere. In brief, to synthesize Cu-SAPO-34, the starting gel with the molar ratio of 1 Al2O3, 1.14 P2O5: 0.57 SiO2: 76.42 H2O: 0.080 Cu-TEPA: 3.38 PA (propylamine) was prepared by adding H3PO4, AlOOH into deionized water under constant stirring. The SiO2 was then added into it. After stirring for 1h, CuSO4·5H2O was introduced into this gel, followed by adding TEPA (tetraethylenepentamine). After 1h, 1.4 g of PA was added into the above gel under vigorous stirring to achieve a pH value in the range of 7-9. After being stirred for 12h at room temperature, the resulting gel was transferred to a stainless-steel reactor with Teflon liner and heated at 180°C for 48h. The product was washed with deionized water and dried in air at 80°C overnight. It was further washed in 1 mol·L-1 NH4NO3 solution to remove the unattached copper cations. After being washed with deionized water and dried at 100°C, it was calcined at 550°C for 8h in the air.14 The Cu-SSZ-13 was synthesized from the starting gel with the molar ratio of 3.5 Na2O: 1.0 Al2O3: 15 SiO2: 200: H2O: 3.0 Cu-TEPA. It was prepared by dissolving 0.514g of NaAlO2 and 0.38g of NaOH in 5.136g of deionized water. 1.07 g of TEPA was added to the above solution with vigorous stirring for 5 min. 0.735g of CuSO4·5H2O was then added into it. After stirring for 1h, 3.487g of silica sol (LUDOX AS-40) was added into the above gel with vigorous stirring for 3h. After that, the gel was transferred to a 7


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stainless-steel reactor with Teflon liner at 140°C for 4-6 days. The product was filtered, washed with deionized water and dried at 100°C overnight. The Cu-SSZ-13 was ion exchanged with 1 mol·L-1 NH4NO3 solution at 80°C for 12h. After washing with deionized water and drying at 100°C for 12h, the products were calcined at 550°C for 8h in the air.44 Fe-SAPO-34 I.E. or Fe-SSZ-13 I.E. was prepared via the ion-exchange procedure. The NH4-SAPO-34 or NH4-SSZ-13 was obtained first by dispersing 2g Cu-SAPO-34 or Cu-SSZ-13 into 100 mL 0.1 M NH4NO3 solution with constant stirring at 80°C for 8h followed by filtering and rinsing. This step was carried out twice. The resulting NH4-SAPO-34 or NH4-SSZ-13 was then added into 0.1M FeCl2 solution with constant stirring at room temperature for 24h followed by filtering and rinsing. This ion-exchange procedure was also repeated once. The final product was obtained after calcination in air at 500°C for 4h. It is worth noting that the residual copper species may exist in the as-synthesized Fe-SAPO-34 I.E. and Fe-SSZ-13 I.E. As long as Fe species were vast majority, the samples should be considered as Fe-catalysts rather than Fe-Cu-catalysts.

2.2 Characterization The powder X-ray diffraction (XRD) measurements were carried out using Rigaku Rotaflex D/Max-C system with Cu Kα radiation. The samples were loaded on a sample holder with 1-mm depth. The diffractometer data were obtained in the range of 5-50° 8


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The X-ray photoelectron spectroscopy (XPS) spectra were measured on Kratos Axis Ultra spectrometer at a pass energy of 20 eV, with Al Kα as the exciting X-ray source. The binding energies of Cu 2p and Fe 2p were calibrated against the C 1s signal (284.5 eV) of contaminant carbon. Ammonia temperature-programmed desorption (NH3-TPD) was conducted using a thermogravimetric analyzer (TGA-50, Shimadzu). 10 mg of sample was loaded onto the reactor. The pretreatment involved heating samples in helium at the flow rate of 20 mL/min from room temperature to 300°C at the heating rate of 10°C/min. It was further kept at 300°C for 30 min, followed by cooling down to room temperature. The gas mixture of 500 ppm NH3/helium at the flow rate of 20 mL/min passed through the sample for 3h. Then, the adsorption gas was purged with helium for 30 minutes to eliminate NH3 in the effluent. The measurements were performed from room temperature to 700°C at the heating rate of 10°C/min. The weight loss of sample was monitored to determine the desorption of NH3. The derivative curve of the weight loss versus temperature yielded the NH3-TPD profile. The surface chemical composition of samples was determined using a Philips XL30FEG scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDX). 2.3 Catalytic activities The activity measurement of SCR was carried out in a quartz U-tube reactor at the 9


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atmospheric pressure. The temperature, controlled by an Omega temperature controller, was varied from 150 to 550°C as measured by a K-type thermocouple. Data at each temperature took at least 30 minutes to assure steady state. In each run, 30 mg of catalysts (20-40 meshes) was used. The reactant gas was obtained by blending different gases. The typical gas compositions are listed in Table 1. The feed gas flow rate was 200 mL·min-1. The outlet concentration of NOx was continuously monitored by a chemiluminescence NO/NOx analyzer (42C, Thermo Environmental Instruments Inc.). The N2O outlet concentration was analyzed using a gas chromatograph (Shimadzu 8A) equipped with a Porapak Q column and a TCD detector. Table 1. Inlet gas compositions for activity and reaction analyses Condition

Inlet gas composition

C1

500 ppm NH3, 500 ppm NO, 14% O2 and 2% H2O (optional)

C2

500 ppm NH3, 14% O2 and 2% H2O (optional)

C3

500 ppm NH3, 500 ppm NO and 2% H2O (optional)

C4

500 ppm NO and 2% H2O (optional)

C5

500 ppm NH3, 500 ppm NO, 0-14% O2 and 2% H2O

C6

500 ppm NH3, 500 ppm NO, 14% O2 and 0-5% H2O

C7

50 ppm N2O

C8

50 ppm N2O and 500 ppm NH3

10


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3. RESULTS AND DISCUSSION 3.1 Characterization of catalysts 3.1.1

EDAX elemental analysis

Table 2 shows the metal contents of catalysts determined or estimated by EDAX. The specific loadings of Cu and Fe on the surface of samples are shown. It should be noted that the surface elemental distribution may be different from the overall content. For example, the overall Fe loading in Fe-ZSM-5 I.W.I. was indeed 1.6 wt% according to our preparation procedure. The Table 2 gave the metal loading of the sample surface, which was 4.20 wt%. It implied that some ex-framework iron species existed on the zeolite surface. Despite the problem, it still provided reasonable estimates of the metal contents of the catalysts. For example, the Cu and Fe content of Fe-SAPO-34 I.E. was 1.50 and 9.42 wt%, respectively. Also, by comparing the Cu/Al ratio (0.03) with Fe/Al ratio (0.25) of Fe-SAPO-34 I.E., we can see the ratio of Fe was far greater than that of Cu. It suggested most Cu species had been exchanged by Fe. Although residual Cu did exist in Fe-SAPO-34 I.E. it can still be regarded as Fe-SAPO-34 rather than Fe-Cu-SAPO-34. It also applied to Fe-SSZ-13 I.E. Furthermore, the Si/Al ratio of SAPO-34 and SSZ-13 based catalysts was given in Table 2. The Si/Al for SAPO-34 catalysts was 0.36 while that for SSZ-13 catalysts was 3.63. As we know, the higher Si/Al ratio will lead to the decrease in acid sites and increase in acid strength. The differences in Si/Al might have effects on the N2O formation. 11


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Table 2. Metal contents of catalysts Sample

Cu (wt%)

Fe (wt%)

Cu/Al *

Fe/Al *

Si/Al *

Cu-ZSM-5 I.E.

3.85

-

0.37

-

-

Fe-ZSM-5 I.E.

-

9.3

-

0.98

-

Fe-ZSM-5 I.W.I.

-

4.2

-

-

-

Cu-SAPO-34

10.43

-

0.21

0.36

Fe-SAPO-34 I.E.

1.5

9.42

0.03

0.25

Cu-SSZ-13

12.6

-

0.55

3.63

Fe-SSZ-13 I.E.

2.6

10.53

0.11

0.54

The data noted with (*) is calculated based on the atomic ratio.

3.1.2

XRD

The XRD patterns of catalysts are shown in Figure 1. It can be seen that the diffraction peaks belonging to ZSM-5 can be seen in Cu-ZSM-5 I.E., Fe-ZSM-5 I.E. and Fe-ZSM-5 I.W.I., indicating the zeolitic framework in these catalysts were preserved.18 Meanwhile, the diffraction peaks of CHA zeolite in Fe-SAPO-34 I.E. diminished significantly in comparison with Cu-SAPO-34. It implied that the zeolitic structure collapsed during ion-exchange. In contrast, it is interesting to note that the diffraction peaks of CHA zeolite were quite obvious in Fe-SSZ-13 as well as in Cu-SSZ-13, 12


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suggesting the CHA zeolitic structure was well preserved after ion-exchange. A number of reports can be found on Fe-SSZ-13 prepared via ion-exchange procedure.

44-47

.

However, the fewer reports on Fe-SAPO-34 catalysts in the literature were prepared by “one-pot” synthesis—incorporating iron species during the preparation of SAPO-34.23 In addition, the patent published by Li et al. pointed out that it is difficult to prepare Fe-SAPO-34 using conventional ion-exchange method due to the relatively small pore opening in the CHA zeolite framework.48 He also proposed an approach similar to the “one-pot” synthesis. Therefore, it may be concluded that Fe-SAPO-34 was not able to maintain the zeolitic framework structure during ion-exchange. Furthermore, it should be noted that the diffraction peaks of copper or iron oxide were not observable in all samples, which indicated that the metal content was either too low to be detected or they were in highly dispersed states.14

13


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Figure 1. XRD patterns of catalysts. 3.1.3

NH3-TPD

Before the discussion, we need to address one problem: each catalyst sample can give small amounts of side N-containing products, such as N2, N2O, and NO via a reaction between adsorbed NH3 and the lattice oxygen atoms at high temperatures, in addition to the majority of NH3 desorbed from the surface. In our experiments, it was impossible to differentiate small oxygen loss from weight loss caused by NH3 desorption using TGA, meaning it might cause some errors in the NH3-TPD data. However, according to Zheng et al.49, the reasonable NH3-TPD results can be obtained using TGA.

14


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Therefore, our NH3-TPD results were believed to be reasonable too. Figure 2 depicts the NH3-TPD results of all catalysts. For Cu-ZSM-5 I.E., Fe-ZSM-5 I.E., Fe-ZSM-5 I.W.I., Cu-SAPO-34 and Fe-SAPO-34, the spectra showed similar patterns, with two desorption peaks centered at around 90 and 150-200°C. The peak at 90°C can be ascribed to the physically adsorbed NH3 on the sample while that at 150-200°C was related to the ammonia that was adsorbed on the weak Lewis acid sites.20, 50

Compared with Cu-ZSM-5 I.E., the peak associated with Lewis acid sites on

Fe-ZSM-5 I.E. was more intense, which might be a result of the higher loading of iron in Fe-ZSM-5 I.E. than that of copper in Cu-ZSM-5 I.E. as evidenced by the EDAX results. Similarly, the reduced peak intensity of Lewis acid in Fe-ZSM-5 I.W.I. compared with Fe-ZSM-5 I.E. may be explained by the lower iron loading as well. Meanwhile, those peaks related to Lewis acids in Cu-SAPO-34 and Cu-SSZ-13 were much broader compared to those for the ZSM-5-based catalysts. Also, the temperatures of those peaks in Cu-SAPO-34 and Cu-SSZ-13 have shifted to higher temperatures, indicating their Lewis acid site strengths were stronger. As for Fe-SAPO-34 I.E. and Fe-SSZ-13 I.E., TPD peaks at 300-600°C can be observed in addition to the ones at lower temperatures of corresponding Cu-SAPO-34 and Cu-SSZ-13. It implied the peaks at higher temperatures were the characteristics of Fe species. On Fe-SAPO-34 I.E., three peaks can be identified in this range. A comparison with previous work 45, 51 showed that the one at around 340°C might be due to ammonia desorption from Fe active centers while the other peak at 15


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450°C was due to Brønsted acid sites formed by Al & Fe in zeolite framework. The peak at 550°C was attributed to the Brønsted acid sites formed by Al & Fe in the zeolite framework and extra-framework positions. In contrast, there was only one peak at 450°C related to the Brønsted acid sites that can be identified in Fe-SSZ-13. It is interesting to note the desorption peaks of NH3 at around 400°C in some samples are missing. It does not necessarily mean the Brønsted acid sites were absent in these samples. The absence of peaks at higher temperatures was associated with the equipment that we used—TGA. According to Zheng, who also used TGA to examine the NH3-TPD profiles. It was found that some Brønsted acid sites were too weak to be detected by TGA. As discussed by Parvulescu et al.

52

, Lewis acid sites play an important role in

NH3-SCR mechanism because the key intermediates in the reaction, NH and NH2 species, can be formed as NH3 is adsorbed on Lewis acid sites. Brønsted acid sites were also indispensable due to the fact that NH3 was activated to NH4+ on Brønsted acid sites in the zeolite framework. Then the gaseous NO and NO2 and NO3- species can react with the NH4+ to form N2 and H2O.53 Furthermore, compared with Cu-SAPO-34, Cu-SSZ-13 had more acid sites, as evidenced by the intense peak at around 150-200°C. Meanwhile, compared with Fe-SSZ-13 I.E., the Fe-SAPO-34 I.E. had stronger Brønsted acid sites, as evidenced by the peaks presented at the higher temperatures. This is in accordance with the data in Table 2 that the SSZ-13 based catalysts had a much higher Si/Al ratio than SAPO-34 16


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based ones. Finally, although the zeolitic structure of Fe-SAPO-34 I.E. had collapsed (Figure 1), it is still necessary to use it for further investigation. This is because a large quantity of Lewis and Brønsted acid sites existed in it, which were beneficial for SCR. Besides, Schwidder and Lee54-55 proposed that isolated Fe3+ cations in the catalysts served as the active sites for SCR. And the isolated Fe3+ species were independent of the zeolitic structure. Therefore, it is still interesting to evaluate the SCR activity and N2O formations over it.

Figure 2. NH3-TPD profile of catalysts.

17


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3.1.4

XPS

XPS measurements were performed to characterize the distribution of Cu species on Cu-ZSM-5 I.E., Cu-SAPO-34 and Cu-SSZ-13 and that of Fe species on Fe-ZSM-5 I.E., Fe-ZSM-5 I.W.I., Fe-SAPO-34 I.E. and Fe-SSZ-13 I.E. Figure 3 presented the XPS spectra of the specific elements in the catalysts. For all the Cu-catalysts, the peaks at around 932.8 and 951.0 eV represented the Cu+ species. Additionally, the ones at around 933.6 and 953.8 eV pertained to Cu2+ species in tetrahedral and octahedral coordination, respectively The shake-up peaks at 940-945 eV also represented the existence of Cu2+ species on the surface.56 The data of surficial Cu2+/Cu+ in the copper-containing samples were given in Table S1. It can be seen that the relative intensity of Cu+-related peaks is much weaker than that of Cu2+-related peaks in both Cu-ZSM-5 I.E. and Cu-SSZ-13. In contrast, peaks of Cu+ species are more intense than ones of Cu2+ species in Cu-SAPO-34, indicating more Cu+ species existed on the surface. From the deconvolution of XPS spectra of Fe 2p in the Fe-catalysts, two distinct bands centered at 711-712 and 713-715 eV can be observed. These peaks were attributed to the binding energies of iron species in FeO and Fe2O3, respectively.18 This result implied that the iron in the samples was presented in two valences of +2 and +3. The catalysts with Fe2+ possessed enhanced redox ability and hence better SCR activities.57

18


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` Figure 3. XPS profile of catalysts.

3.2 NH3-SCR activity tests The SCR activities of ZSM-5 based catalysts were measured. As shown in Figure 4, all catalysts exhibited high activities from 150-550°C. Compared with Cu-ZSM-5 I.E., Fe-ZSM-5 I.E. showed superior catalytic activity in the tested temperature range (150-550°C), without an observable decrease at the higher temperature. Meanwhile, the NO conversion over Cu-ZSM-5 I.E. dropped from 90% to 70% as the reaction temperature increased from 400 to 550°C. As shown by previous work, e.g., Sultana et al. 17

, the Cu-catalysts are supposed to have better activity at a lower temperature while

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Fe-catalysts are likely to show better performance at a higher temperature. The lower NO conversion of Cu-catalysts over higher temperature range can be explained by the fact that highly active copper species for the undesired NH3 oxidation by O2 leads to the decrease in NH3 concentration needed for SCR reactions. Meanwhile, at lower temperatures, NH3 is likely to be adsorbed on the iron species in Fe-catalysts, which inhibits the SCR activity and leads to the inferior NO conversion compared to Cu-catalysts. However, in our experiments, the Fe-ZSM-5 I.E. showed higher activity than Cu-ZSM-5 I.E. in the whole temperature range, which can be attributed to the higher iron loading on the surface of Fe-ZSM-5 I.E.—9.30 wt%, which was much higher than the copper loading (3.85%) on the surface of Cu-ZSM-5 I.E. The higher iron loading compensated for the iron sites occupied by NH3. Compared with Fe-ZSM-5 I.E., Fe-ZSM-5 I.W.I. showed lower SCR activity, especially in the lower temperature range. The NO conversion of Fe-ZSM-5 I.E. and Fe-ZSM-5 I.W.I. was 58% and 28% respectively. The inferior performance of Fe-ZSM-5 I.W.I. was mainly due to the lower iron content (4.20 wt%) than that of the Fe-ZSM-5 I.E. sample (9.30 wt%).

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Figure 4. SCR activities of ZSM-5 zeolite-supported catalysts. Conditions: 500 ppm NH3, 500 ppm NO, 14% O2 and 2% H2O, Balance = He, total flowrate = 200 mL/min, 30 mg sample.

The SCR performance of Cu-SAPO-34, Fe-SAPO-34 I.E., Cu-SSZ-13 and Fe-SSZ-13 I.E. are shown in Figure 5.

More direct comparisons can be made among

these four catalysts because they had similar metal contents. It can be seen that the NO conversion in the lower temperature range (150-300°C) was higher for Cu-SAPO-34 and Cu-SSZ-13 than that for the corresponding Fe-SAPO-34 I.E. and Fe-SSZ-13 I.E. This was because NH3 was likely to be adsorbed on the iron species in Fe-catalysts, as explained earlier. However, unlike Cu-SAPO-34, there was no drastic drop of NO conversion over Cu-SSZ-13 at higher temperatures (400-550°C). As suggested by Wang

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et al.

58

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, NH3 oxidation over Cu-SAPO-34 was stronger than that over Cu-SSZ-13 at

higher temperatures, which led to the decreased NO conversion over Cu-SAPO-34. It is also interesting to note that the SCR activity of Fe-SAPO-34 I.E. was still competitive although the zeolitic structure collapsed based on XRD analysis. Schwidder and Lee54-55 proposed that isolated Fe3+ cations in the catalysts served as the active sites for SCR. A possible explanation for the good activity of Fe-SAPO-34 I.E. was that the isolated Fe3+ species that were not affected by the collapse of zeolitic structure acted as the active centers on the surface of Fe-SAPO-34. Another possible explanation could be obtained from the XPS analysis of the catalysts (i.e., Figure 3). As discussed in the literature, the Fe2p3/2 binding energies of iron depends on its oxidation state; lower oxidation states lead to lower binding energies (and it is near 708 eV for Fe0)

18, 44

. It was observed that high

SCR activities were related to iron oxides of lower oxidation states. The results shown in Figure 3 (with strong XPS bands at 710-712 eV) and Figures 4-5 (high activities) are in agreement with this observation.

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Figure 5. SCR activities of SAPO-34 and SSZ-13 zeolite-supported catalysts. Conditions: same as in Figure 4.

3.3 N2O formation over various catalysts N2O product analyses were made for each catalyst under 4 feed gas conditions (C1-C4, Table 1). The results are shown in Figure 6 - Figure 12. For each figure, the sub-figure on the left-hand side is the condition without H2O while the one on the right is the condition with 2% H2O in the feed. Under each condition, the possible reactions or reaction pathways that generate N2O are summarized, as shown below. Under Condition C1 (500 ppm NH3, 500 ppm NO, 14% O2 and 2% H2O (optional)), also referred to as the standard SCR condition, the possible reactions are:

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4NO+4NH3+3O2= 4N2O + 6H2O

(1)

2NH3 + 2O2 =N2O + 3H2O

(2)

2NH3+4NO = 2N2 +N2O＋3H2O

(3)

3NO = N2O + NO2

(4)

4NO = 2N2O + O2

(5)

Under Condition C2 (500 ppm NH3, 14% O2 and 2% H2O (optional)): Reaction (2) is possible. Under Condition C3 (500 ppm NH3, 500 ppm NO and 2% H2O (optional)): Reactions (3), (4) and (5) are possible. Under Condition C4: (500 ppm NO and 2% H2O (optional)): Reactions (4) & (5) are possible. To investigate the effects of O2 on the NO conversion and N2O formation over the specific catalyst, experiments under condition C5 were performed. Under Condition C5: (500 ppm NH3, 500 ppm NO, 0-14% O2 and 2% H2O): Reactions (1-5) are possible. To investigate the effects of H2O on the NO conversion and N2O formation over the specific catalyst, experiments under condition C6 were performed. Under Condition C6: (500 ppm NH3, 500 ppm NO, 14% O2 and 0-5% H2O): Reactions (1-5) are possible. 24


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To study the decomposition of N2O and N2O-SCR reaction over the specific catalyst, experiments under C7 and C8 were carried out separately. 2N2O = 2N2 + O2

(6)

3N2O + 2NH3 = 4N2 + 3H2O

(7)

Under Condition C7 (50 ppm N2O): Reaction (6) is possible. Under Condition C8 (50 ppm N2O and 500 ppm NH3): Reactions (6) and (7) are possible.

3.3.1

N2O yield analyses

Figure 6 shows the N2O yields over Cu-ZSM-5 I.E. under various conditions. When 2% H2O was introduced, the N2O emission under C1 was the highest among all, reaching 45 ppm at 350°C. Under C2, C3 and C4, the N2O yields were not prominent—less than 5 ppm in the tested temperature range. However, there was an increasing trend of N2O yields under C2 and C3 as the temperature was raised. Meanwhile, the N2O decomposition from NO barely occurred under C4. Furthermore, when H2O was absent in the feed, N2O emission under C1 dropped significantly, only reaching around 15 ppm at 350°C. In comparison, the increase of N2O yields under C2 and C3 became more obvious under C2 and C3. Based on the isotopic labeling experiments carried out by Kumthekar and Ozkan 59, the main reaction for N2O emission over vanadia catalyst was Reaction (1). This reaction 25


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is known as NH3 non-selective catalytic reduction (NSCR).60 There are many mechanistic explanations for N2O formation under condition C1. One of the most prevalent explanations is a Langmuir−Hinshelwood type mechanism, in which the reaction of adsorbed NH3 species with adsorbed NOx species leads to ammonium nitrate (NH4NO3) formation.36 It was proposed that the NH4NO3 formed at the lower temperatures (

N2O Formation Pathways over Zeolite-Supported Cu and Fe Catalysts

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