Mechanism of NH3 Selective Catalytic Reduction Reaction for NO x

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The Mechanism of NH-SCR Reaction for NO Removal from Diesel Engine Exhaust and Hydrothermal Stability of Cu-Mn/Zeolite Catalysts Min Cheng, Boqiong Jiang, Shuiliang Yao, Jingyi Han, Shuang Zhao, Xiujuan Tang, Jiawei Zhang, and Ting Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09339 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 15, 2017

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The Journal of Physical Chemistry

The Mechanism of NH3-SCR Reaction for NOx Removal from Diesel Engine Exhaust and Hydrothermal Stability of Cu-Mn/Zeolite Catalysts

Min Cheng, Boqiong Jiang*, Shuiliang Yao, Jingyi Han, Shuang Zhao, Xiujuan Tang, Jiawei Zhang, Ting Wang School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou 310018, China Corresponding author: Tel: +86 571 28008235; Fax: +86 571 28008215 E-mail address: [email protected] (B. Jiang)

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ABSTRACT Zeolite-supported catalysts are effective in selective catalytic reduction (SCR) of NOx from the diesel engine exhaust, whereas the reaction mechanism remains unclear. In this work, in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFT) was used to investigate the SCR reaction mechanism on Cu-Mn/ZSM-5 and Cu-Mn/SAPO-34. Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) reaction ways were both involved in the SCR reaction. In the L-H reaction way, NO2 and NH4+ were two important intermediates. The transformation of bidentate nitrates to monodentate nitrates, and further to NO2 played an important role. The amount of bidentate nitrates and monodentate nitrates on Cu-Mn/ZSM-5 was much less than that on Cu-Mn/SAPO-34. It led to less NO removal through L-H reaction way on Cu-Mn/ZSM-5. The formation and consumption of coordinated NH3, as the intermediate in E-R reaction way, was similar on both catalysts. Therefore, the different catalytic activity between the two catalysts should be mainly due to the L-H reaction way, the main reaction way at low temperature. The hydrothermal aging treatment significantly reduced the amount of NH4+ and NOx complexes on Cu-Mn/ZSM-5, and the variation of the SCR reaction through L-H way was also the main reason for different hydrothermal stability of the two catalysts.

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1. INTRODUCTION Selective catalytic reduction (SCR) by ammonia/urea is the most effective technology to remove NOx from diesel engines.1,2 The conventional three-way catalysts (TWC) for NOx removal from gasoline vehicles exhaust cannot efficiently reduce NOx from diesel engine exhaust, due to the high contents of oxygen and moisture in diesel engine exhaust.3 In the past few years, academic researchers were devoted to find effective catalysts to increase SCR efficiency for diesel engines. Several catalysts, such as Ag/Al2O3,4 V2O5-WO3/TiO25 and FeTiOx6 have been investigated. However, these catalysts are not satisfactory to remove NOx from diesel engine exhaust, due to the wide temperature range of the exhaust (150-500 oC)7 and the high temperature (up to 650 oC) during the diesel particulate filter (DPF) regeneration.8 Since Cu/ZSM-5 zeolite was developed for its unique catalytic activity for decomposition of nitrogen oxides,9 researchers have extensively studied the active reaction temperature, copper species and active sites of Cu/ZSM-5, as well as the intermediates in NH3-SCR reactions. Cu/ZSM-5 exhibited over 90% of NO conversion from 250 °C to 350 °C.10 Recently, Cu-exchanged chabazite (CHA) zeolites, especially Cu/SAPO-34, have attracted great attention for eliminating NOx pollution.11-13 Compared to Cu/ZSM-5, Cu/SAPO-34 has higher performance and better potential applications, such as the hydrothermal stability.14 In our previous study,15 Mn was introduced to both Cu/ZSM-5 and Cu/SAPO-34. By doing so, the activity of the catalysts was further improved when the temperature of the exhaust was below 200°C. The catalysts were satisfactory to NOx removal from diesel engine exhaust for light duty, with typical temperature ranging from 150 °C to 250 °C.16 Page 3 of 35



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It is crucial to have clear understanding of the reaction process and inherent mechanism to further enhance catalytic activity, which has not yet been well defined. It was reported that Cu-active sites were formed on both isolated Cu2+ and Cu-oxo clusters,17 and Cu2+ species were reported to be the active sites for NH3 adsorption.18 The adsorbed NH3 could be converted to coordinated NH3 and NH4+, which then reacted with NOx complexes,19 but few studies have defined which NH3 species are more important for NOx removal from diesel engine exhaust at low-temperature. As for NOx complexes, some researchers have defined that the adsorbed NO2 and nitrates species played an important role in the SCR reaction.20 However, most of these studies focused on the NOx removal from stationary source. Although the SCR reaction in diesel engine exhaust is similar to that in the flue gas, the reaction parameters, such as temperature, O2 concentration, and H2O concentration, are different. Some researchers found that nitrates and nitrites are the main NOx adsorption species react with coordinated NH3 and NH4+ for NOx removal from diesel engine exhausts,21 and the nitrates were proposed as the key intermediates,22 but it is still ambiguous the transformation of the nitrates and the reaction way between the NOx complexes and NH3 species. Without the well-defined mechanism, the main parameters influence the catalytic activity and stability is unclear. In order to well define the mechanism of the SCR reaction on Cu-Mn/zeolite catalysts at low temperature, in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) was used to study the reaction process and find out the key intermediates which influence the activity of the catalysts. The transformation of the surface species that adsorbed on the catalysts were analyzed in detail. According to the DRIFT results, the mechanism of the SCR reaction was proposed. The results Page 4 of 35


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were further used to investigate the difference of the hydrothermal stability of the two catalysts, since the hydrothermal stability of catalysts is an important issue for heat produced during DPF regeneration.

2. EXPERIMENTAL 2.1. Catalyst Preparation and Hydrothermal Aging Treatment. Cu-Mn bimetal catalysts with different Cu/Mn ratios were prepared by a two-step liquid ion-exchange method.23 Zeolites (10 g H-ZSM-5 or H-SAPO-34) were added to ammonium nitrate (100 ml, 1 mol/L) and stirred continuously at 90 °C for 2 h. The NH4+/ZSM-5 or NH4+/SAPO-34 powders were obtained by filtration and rinsed thoroughly with deionized water, and then dried at 120 °C for 12 h. NH4+/ZSM-5 or NH4+/SAPO-34 (10

g)

was

added

to

50

ml

of

solution

of

Cu(CH3COO)2·H2O

and

Mn(CH3COO)2·4H2O with Cu/Mn ratio of 3:2, and the total metal concentration was controlled at 0.2 mol/L. The mixture was stirred vigorously at room temperature for 6 h, and then filtered and rinsed thoroughly with deionized water. The solid obtained was dried at 110 °C for 12 h and then calcined at 500 °C for 5 h to obtain the catalysts. To investigate their hydrothermal stability, the catalysts were aged in a quartz tube reactor at 750 °C in 10% H2O/air, with a total flow of 1000 cm3/min for 24 h. The catalysts with hydrothermal aging treatment were noted as Cu-Mn/ZSM-5(HT) and Cu-Mn/SAPO-34(HT). 2.2. In situ DRIFTS Experiments. In situ DRIFTS experiments were performed using a Nicolet is50 FTIR spectrometer equipped with a MCT detector and a high-temperature in situ reaction chamber (Harrick Praying Mantis). The DRIFTS spectra were collected with a resolution of 4 cm−1 in the range of 4000-650 cm−1 with Page 5 of 35



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the Kubelka-Munk (K-M) format. Before the experiments, the powder of catalysts was loaded into the DRIFTS sample cell and gradually heated up to 500 °C with a rate of 10 °C/min in He flow and held for 1 h, and then cooled to 200 °C. The background spectra were recorded before the samples were exposed to the reactants.

3. RESULTS 3.1. The Spectra of Fresh Catalyst 3.1.1.

Adsorption

of

NO+O2

on

the

Catalysts.

Cu-Mn/ZSM-5

and

Cu-Mn/SAPO-34 were exposed to 300 ppm NO+14% O2 for 90 min, and then purged by He for 30 min. Several bands were observed, as shown in Figure 1. The band around 1625 cm−1 was attributed to NO2 adsorbed on catalysts,21,24 and the bands at 1605 and 1575 cm−1 were assigned to monodentate nitrate25,26 and bidentate nitrate,27,28 respectively. The band at 1295 cm−1 was attributed to bridged nitrate,29 which was detected only on Cu-Mn/SAPO-34 with low intensity. The bands at 3546 and 3665 cm−1 due to the interaction between basic OH and NOx were observed.30 The band due to adsorbed NO2 formed on Cu-Mn/ZSM-5 reached the highest intensity at 90 min, and that on Cu-Mn/SAPO-34 reached the highest at 30 min. It indicated that NO2 was adsorbed more easily on Cu-Mn/SAPO-34 than on Cu-Mn/ZSM-5. The amount of bidentate nitrate and monodentate nitrate on Cu-Mn/SAPO-34 was obviously stronger than that on Cu-Mn/ZSM-5.

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Figure 1 DRIFT spectra of (a) Cu-Mn/ZSM-5 and (b) Cu-Mn/SAPO-34 exposed to 300 ppm NO+14% O2 and purged by He for 30 min at 200 °C. 3.1.2.

Adsorption

of

NH3+O2

on

the

Catalysts.

Cu-Mn/ZSM-5

and

Cu-Mn/SAPO-34 was exposed to 300 ppm NH3+14% O2 for 90 min, and then purged by He for 30 min. As shown in Figure 2, in the vibration region of 2000-4000 cm−1, bands at 3615, 3356 (or 3333), 3275 and 3185 cm−1 were detected on both catalysts. The band at 3615 cm−1 could be attributed to OH stretching vibration.31,32 In the N-H stretching region, the bands at 3356 (or 3333) and 3275 cm−1 were due to the NH4+ group,33,34 and the band at 3185 cm−1 was due to coordinated NH3.35 In the bending vibration region of 1000-2000 cm−1, the band at 1467 cm−1 and the broad band around 1738 cm−1 could be attributed to NH4+ on Brønsted acid sites,26,36,37 and the bands at Page 7 of 35



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1628 and 1194 (or 1216) cm−1 could be assigned to NH3 coordinated to the Lewis acid sites.31,38,39 The band at 1339 or 1322 cm−1 could be attributed to the symmetric deformation of NH3 coordinated which was bonded to one new type of Lewis acid sites.40 The bands of NH4+ on Brønsted acid sites was much stronger than those of coordinated NH3 on Lewis acid sites on the two catalysts, indicating that there were a lot of Brønsted acid sites. Between the two catalysts, the amount of NH4+ on Cu-Mn/SAPO-34 was even higher.

Figure 2 DRIFT spectra of (a) Cu-Mn/ZSM-5 and (b) Cu-Mn/SAPO-34 exposed to 300 ppm NH3+14% O2 and after purged by He for 30 min at 200 °C. 3.1.3. The Adsorption of NH3+O2 on the Catalysts Pretreated with NO+O2. As shown in Figure 3, the catalysts were firstly pretreated with 300 ppm NO+14% O2 for Page 8 of 35


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90 min and then purged by He for 30 min. When the 300 ppm NH3+14% O2 were introduced into the cell, the amount of adsorbed NO2 (1625 cm−1) on the two catalysts gradually decreased and then disappeared. It seemed that adsorbed NO2 could react with NH3 species easily. Since the amount of monodentate nitrate was very low on Cu-Mn/ZSM-5, it was difficult to analyze its variation. The band due to bidentate nitrate on Cu-Mn/ZSM-5 did not change significantly with the introduction of NH3+O2. Different from Cu-Mn/ZSM-5, both the bidentate nitrate and monodentate nitrate on Cu-Mn/SAPO-34 were consumed. NH4+ appeared 5 min later than coordinated NH3. It should be noted that when the catalysts were just exposed to 300 ppm NH3+14% O2 without pretreatment, the bands of NH4+ and coordinated NH3 were detected simultaneously (Figure 2). The delayed appearance of NH4+ indicated that NH4+ was consumed by NOx species and could not accumulate on the catalysts. The formation of NH3 species on Cu-Mn/SAPO-34 at 25 min was much earlier and stronger than that of Cu-Mn/ZSM-5 at 50 min, and the band due to NH3 species on Cu-Mn/SAPO-34 was also much stronger with the catalyst purged by NH3+O2 for 90 min (Figure S1). It should be noted that with the pretreatment of NO+O2, the amount of NH4+ on catalysts was greatly enhanced (Figure S2 and S3). It indicated that NO absorption produced new Brønsted acid sites, promoting the formation of NH4+.29

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Figure 3 DRIFT spectra of (a) Cu-Mn/ZSM-5 and (b) Cu-Mn/SAPO-34 exposed to 300 ppm NO+14% O2 followed by exposure to 300 ppm NH3+14% O2 for various times at 200 °C. 3.1.4. The Adsorption of NO+O2 on the Catalysts Pretreated with NH3+O2. The catalysts were firstly pretreated with 300 ppm NH3+14% O2 for 90 min and purged by He for 30 min. When 300 ppm NO+14% O2 was introduced into the cell, the intensity of the band at 1467 and 1738 cm−1 due to NH4+ on Brønsted acid sites decreased firstly, followed by the drop of band intensity of coordinated NH3 (Figure 4). It seemed that both NH4+ and coordinated NH3 participated in the reaction. The bands of nitrates formed on Cu-Mn/SAPO-34 appeared at 30 min, much earlier than that on Cu-Mn/ZSM-5 at 55 min. It can be deduced that in the reaction, NOx could be adsorbed on Cu-Mn/SAPO-34 more easily than on Cu-Mn/ZSM-5. Otherwise, for the NOx complexes formed on the Cu-Mn/SAPO-34, the amount of bidentate nitrate was much higher than that of monodentate nitrate. When NO+O2 was just introduced, the band at 1575 cm−1 due to bidentate nitrate was firstly detected with high intensity. With the reaction proceeding, it decreased with the increases of monodentate nitrate (1605 cm−1). When NH4+ was still visible, the band of adsorbed NO2 (1625 cm−1) could not be detected, i.e., NO2 appeared after NH4+ was absolutely consumed. With Page 10 of 35


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the appearance of the band due to adsorbed NO2, the amount of monodentate nitrate decreased on both catalysts. Compared to the complete disappearance of monodentate nitrate on Cu-Mn/ZSM-5, there was still some monodentate nitrate remained on Cu-Mn/SAPO-34. After the catalysts were purged by NH3+O2 for 90 min, the amounts of adsorbed NO2 on the two catalysts were similar, whereas more nitrates could be detected on Cu-Mn/SAPO-34 than on Cu-Mn/ZSM-5.

Figure 4 DRIFT spectra of (a) Cu-Mn/ZSM-5 and (b) Cu-Mn/SAPO-34 exposed to 300 ppm NH3+14% O2 followed by exposure to 300 ppm NO+14% O2 for various times at 200 °C. 3.2. The Spectra of the Catalysts with Hydrothermal Aging Treatment

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3.2.1. Adsorption of NO+O2 on the Catalysts. In Figure 5, Cu-Mn/ZSM-5 (noted as Cu-Mn/ZSM-5(HT)) and Cu-Mn/SAPO-34 (noted as Cu-Mn/SAPO-34(HT)) with hydrothermal aging treatment, were exposed to 300 ppm NO+14% O2 for 90 min, and purged by He for 30 min. The bands of monodentate nitrate (1605 cm−1), bidentate nitrate (1575 cm−1) and adsorbed NO2 (1625 cm−1) could be detected on the surface of Cu-Mn/ZSM-5(HT), and the bands due to the interaction between basic OH and NOx (3546 and 3665 cm−1) were observed. The intensities of these bands were different with those on fresh Cu-Mn/ZSM-5. Although there was no obvious decrease of nitrates, the amount of adsorbed NO2 decreased significantly, which indicated that it was difficult for the NO2 to be adsorbed on Cu-Mn/ZSM-5(HT). Compared with those on fresh Cu-Mn/SAPO-34, the intensities of the bands due to NOx complexes on Cu-Mn/SAPO-34(HT) had little variation. Band of bidentate nitrate at 1575 cm−1 was first detected, and the bands formed at 1605 and 1625 cm−1 were due to monodentate nitrate and adsorbed NO2, respectively. The intensity of all the bands due to NOx complexes

on

Cu-Mn/SAPO-34(HT)

was

much

higher

than

that

on

Cu-Mn/ZSM-5(HT).

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Figure 5 DRIFT spectra of (a) Cu-Mn/ZSM-5(HT) and (b) Cu-Mn/SAPO-34(HT) exposed to 300 ppm NO+14% O2 for various times and after purging by He for 30 min at 200 °C. 3.2.2. Adsorption of NH3+O2 on the Catalysts. In Figure 6, Cu-Mn/ZSM-5(HT) and Cu-Mn/SAPO-34(HT) were exposed to 300 ppm NH3+14% O2 for 90 min, and then purged by He for 30 min. Compared with fresh Cu-Mn/ZSM-5, the amount of NH4+ on Brønsted acid sites at 1467 and 1738 cm−1 decreased significantly to almost invisible level on Cu-Mn/ZSM-5(HT), whereas there was only slight variation of the amount of coordinated NH3. It indicated that hydrothermal aging treatment had the significant

effect

only

on

NH4+

formed

on

Brønsted

acid

sites.

On

Cu-Mn/SAPO-34(HT), hydrothermal aging treatment had little negative effect on NH3 adsorption. NH4+ and coordinated NH3 formed on Cu-Mn/SAPO-34(HT) at 20 min, and the bands due to NH3 species was also much stronger after the catalyst were purged with NH3+O2 for 90 min. Other bands, especially the bands due to NH4+ at 1467 cm−1 on the Cu-Mn/SAPO-34(HT) were stronger than those on fresh Cu-Mn/SAPO-34. Furthermore, the bands due to coordinated NH3 also increased slightly. The results above indicated that the hydrothermal aging treatment could promote the adsorption of NH3 on Cu-Mn/SAPO-34, which was significantly different Page 13 of 35



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with Cu-Mn/ZSM-5. Although the amounts of coordinated NH3 were similar on both catalysts, the band due to NH4+ (1467 and 1738 cm−1) on Cu-Mn/SAPO-34(HT) was obviously stronger than that on Cu-Mn/ZSM-5(HT).

Figure 6 DRIFT spectra of (a) Cu-Mn/ZSM-5(HT) and (b) Cu-Mn/SAPO-34(HT) exposed to 300 ppm NH3+14% O2 for various times and after purging by He for 30 min at 200 °C. 3.2.3. The Adsorption of NH3+O2 on the Catalysts Pretreated with NO+O2. In Figure 7, the catalysts with hydrothermal aging treatment were firstly pretreated by 300 ppm NO+14% O2 for 90 min and then purged by He for 30 min. When 300 ppm NH3+14% O2 were introduced to Cu-Mn/ZSM-5(HT), the band of adsorbed NO2 (1625 cm−1) decreased, while the bands of nitrates did not change obviously. With the Page 14 of 35


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continuous flow of NH3+O2, the NH4+ and coordinated NH3 at around 3000 cm−1 were formed at 50 min, and the bands were much weaker than those in Figure 3. The bands of coordinated NH3 on Lewis acid sites at 1194 and 1339 cm−1 were formed on Cu-Mn/ZSM-5(HT) as the fresh catalyst, whereas the bands due to NH4+ on Brønsted acid sites at 1467 and 1738 cm−1 were hardly formed. It was totally different with the formation of NH4+ on fresh Cu-Mn/ZSM-5, indicating that the Brønsted acid sites damaged by hydrothermal aging treatment could not be recovered by NOx adsorption on Cu-Mn/ZSM-5(HT). For Cu-Mn/SAPO-34(HT), when 300 ppm NH3+14% O2 were introduced into the cell, the band of adsorbed NO2 also decreased firstly. The band of nitrate species gradually decreased and eventually disappeared. Both NH4+ on Brønsted acid sites (1467 and 1738 cm−1) and coordinated NH3 on Lewis acid sites (1628, 1322 and 1216 cm−1) could be formed. Compared to fresh Cu-Mn/SAPO-34, the intensity of all the NH3 adsorption bands increased slightly on Cu-Mn/SAPO-34(HT). Although hydrothermal aging treatment reduced the amount of NOx complexes on Cu-Mn/ZSM-5(HT), adsorbed NO2 could still participate in the reaction. However， the adsorption of NO+O2 did not increase the amount of Brønsted acid sites on Cu-Mn/ZSM-5(HT), and the bands due to NH4+ on Brønsted acid sites still showed quite low intensity. In contrast, the hydrothermal aging treatment had little influence on Cu-Mn/SAPO-34, as higher amount of NH3 species could be formed even sooner on Cu-Mn/SAPO-34(HT).

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Figure 7 DRIFT spectra of (a) Cu-Mn/ZSM-5(HT) and (b) Cu-Mn/SAPO-34(HT) exposed to 300 ppm NO+14% O2 followed by exposure to 300 ppm NH3+14% O2 for various times at 200 °C. 3.2.4. NO+O2 Adsorption on the Catalysts Pretreated with NH3+O2. In Figure 8, the catalysts with hydrothermal aging treatment were pretreated with 300 ppm NH3+14% O2 for 90 min and then purged by He for 30 min. When 300 ppm NO+14% O2 was introduced into the cell, coordinated NH3 (1339 and 1194 cm−1) was consumed with the continuous flow of NO+O2 on Cu-Mn/ZSM-5(HT). Since the amount of NH4+ was very low, the variation of the band at 1467 cm−1 was unapparent. The bands of adsorbed NO2 and nitrates species on Cu-Mn/ZSM-5(HT) were much

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weaker than that on fresh Cu-Mn/ZSM-5, which meant that it was difficult for the nitrates species and adsorbed NO2 to be formed during the reaction. For Cu-Mn/SAPO-34(HT), with the introduction of NO+O2, the amount of NH4+ groups (1467 cm−1) decreased to undetectable level at 45 min, and coordinated NH3 disappeared later. The bands due to bidentate and monodentate nitrate were detected at the same time. About 5 min later, the band at 1625 cm−1 attributed to adsorbed NO2 was formed. Compared with the fresh Cu-Mn/SAPO-34, the amount of adsorbed NO2 on Cu-Mn/SAPO-34(HT) was higher, and other bands were almost the same with the fresh sample (shown in Figure S5). The hydrothermal aging treatment had little effect on the adsorption of NH3 and NO on Cu-Mn/SAPO-34, and the intermediates could be formed on Cu-Mn/SAPO-34(HT). In contrast, the surface of Cu-Mn/ZSM-5 was significantly different after hydrothermal aging treatment, and the adsorption of both NH3 and NO was restrained.

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Figure 8 DRIFT spectra of (a) Cu-Mn/ZSM-5(HT) and (b) Cu-Mn/SAPO-34(HT) exposed to 300 ppm NH3+14% O2 followed by exposure to 300 ppm NO+14% O2 for various times at 200 °C.

4. DISCUSSION 4.1. The NH3-SCR Reaction Mechanism on the Fresh Catalysts. The DRIFTS results of NH3 adsorption (Figure 2) showed that NH3 adsorption led to two main species, viz. NH4+ adsorbed on the Brønsted acid sites and coordinated NH3 adsorbed on the Lewis acid sites. These two species were formed simultaneously and the amount of NH4+ was higher than that of coordinated NH3. When NO+O2 was introduced into the sample, the intensity of all the bands due to NH4+ and coordinated NH3 decreased gradually. It should be noted that on the surface of these two catalysts, the consumption of NH4+ was faster than that of coordinated NH3 (Figure 4), indicating that NH4+ was more active than coordinated NH3. Therefore, NH4+ should be an important intermediate in NH3-SCR reaction. When NH3+O2 was introduced to the catalyst pretreated by NO+O2 (Figure 3), adsorbed NO2 (the band at 1625 cm−1) was consumed prior to nitrates species (monodentate nitrate and bidentate nitrate). In this process, coordinated NH3 was detected earlier than NH4+. When the catalysts were purged with NH3+O2 for a longer period, NH4+ could be detected. The absence Page 18 of 35


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of NH4+ in the first 30 min might be due to the reaction between NH4+ and NOx species. After NO was adsorbed on fresh catalysts, adsorbed NO2, monodentate nitrate and bidentate nitrate were formed (Figure 1). On Cu-Mn/ZSM-5, the band due to adsorbed NO2 was much stronger than the nitrates, and the amount of bidentate nitrate was higher than that of monodentate nitrate (Figure 1a). In comparison, the amount of nitrates (monodentate nitrate and bidentate nitrate) formed on Cu-Mn/SAPO-34 was much higher (Figure 1b). When NO+O2 was introduced to the catalysts pretreated by NH3+O2 (Figure 4), nitrates were observed firstly on the surface of these two catalysts, and adsorbed NO2 appeared gradually when NH4+ was almost vanished. It should be noted that adsorbed NO2 and nitrates were formed simultaneously during NO adsorption on the surface of fresh samples (Figure 1). It proved that adsorbed NO2 could react with NH3 species more easily than the nitrates, and it might be consumed by NH3 species on the surface of catalysts pretreated by NH3+O2. When NH3+O2 was introduced to the catalyst pretreated by NO+O2 (Figure 3), adsorbed NO2 vanished quickly, and nitrates were gradually consumed, which further proved that adsorbed NO2 was more active than other NOx species. In this process, the bands corresponded to coordinated NH3 was observed earlier than the NH4+, which was detected after adsorbed NO2 totally vanished. Taking into consideration the formation of adsorbed NO2 took place only after the NH4+ was totally consumed (Figure 4), it seemed that these two species could not accumulate on catalysts at the same time, and there might be reaction between adsorbed NO2 and NH4+. In Figure 3, it was found that the bands assigned to nitrates species also decreased, indicating that nitrates could take part in the reaction besides adsorbed NO2. It was reported that nitrates could be transformed Page 19 of 35



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to NO2.41 In order to reveal the transformation process, the variation of the NOx complexes was investigated. Since the amount of nitrates on Cu-Mn/SAPO-34 was much higher than that on Cu-Mn/ZSM-5, the process could be obtained through the investigation of nitrates on Cu-Mn/SAPO-34 (Figure 4b). During the first 30 min of the NO adsorption on Cu-Mn/SAPO-34 pretreated by NH3+O2, the amount of bidentate nitrate was higher than that of monodentate nitrate. It was interesting that with continuous feeding of NO+O2, bidentate nitrate did not further accumulate on Cu-Mn/SAPO-34. In contrast, the intensity of the band due to bidentate nitrate became lower accompanied with the increases of monodentate nitrate. Therefore, it was reasonable to assume that bidentate nitrate was transformed to monodentate nitrate.42 It was reported that when bidentate nitrate was converted into monodentate nitrate, there would be a large number of Brønsted acid sites formed on the surface of the catalysts. In our study, we found that the amount of NH4+ formed on Brønsted acid sites on the catalyst pretreated by NO+O2 was significantly higher than that on fresh catalyst (Figure S2 and S3). It indicated that a large number of Brønsted acid sites were generated after the NO adsorption, accompanied with the conversion of bidentate nitrate to monodentate nitrate. When the catalysts were treated by NO+O2 for 90 min, the band due to adsorbed NO2 became stronger with the decreases of monodentate nitrate. The NO adsorption on the Cu-Mn/SAPO-34 at different temperatures (Figure S6) also showed that with the increases of temperature, the band due to adsorbed NO2 became stronger with decreases of the monodentate nitrate. These two results strongly suggested that adsorbed NO2 could be formed from the decomposition of monodentate nitrate.29

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It was suggested that isolated Cu2+ ions, as the most active sites in SCR reaction,43,44 were coordinated to three framework oxygen atoms and three water molecules as an octahedron,23 and the coordination of water should be weak.45 In our study of Cu/zeolite, Mn/zeolite and Cu-Mn/zeolite (shown in Figure S7 and S8), the NOx species and NH3 species formed on Cu-Mn/zeolite were similar to those on Cu/zeolite, and the introduction of Mn just enhanced the amount of the intermediates. The intermediates formed on Mn/zeolite were quite different. Therefore, the reaction on Cu-Mn/zeolite should be similar to that on Cu/zeolite, and Mn mainly promoted the formation of the intermediates. When NO was adsorbed on the catalysts, bidentate nitrate was coordinated to the metal ions exchanged on the zeolites, accompanied with the desorption of water (procedure 1 in Scheme 1). It was proved the transformation of bidentate nitrates to monodentate nitrates (procedure 2), and then further to NO2 (procedure 4), produced large amount of Brønsted acid sites. With the new forming Brønsted acid sites, NH4+ could be formed on the catalysts (procedure 3 and 5). On the other hand, the gaseous NH3+O2 could also replace the water molecule on the catalyst to be coordinated NH3 (procedure 3). The reaction between NO2 and NH4+ (procedure 6) was defined as the Langmuir-Hinshelwood (L-H) reaction way.46 There was another reaction between coordinated NH3 and gaseous NO (procedure 7) as the Eley-Rideal (E-R) way.47 In the presence of O2, the dehydration reaction would take place between the hydroxyls and the reaction could go on (procedure 8).48

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Scheme 1 NH3-SCR reaction way of NO and NH3 oxidation on the catalysts It was known from Figure 2 that the amount of ammonia species was similar on the Cu-Mn/SAPO-34 and Cu-Mn/ZSM-5, and there was no significant difference in the consumption rate of coordinated NH3 (Figure 4). Therefore, the reaction in the E-R reaction way should be similar on the two catalysts. The difference in the activities between Cu-Mn/SAPO-34 (90% at 200 °C) and Cu-Mn/ZSM-5 (65% at 200 °C) in Figure S9 should be mainly due to the difference in the L-H reaction way. Although the amount of NH4+ (Figure 2) and adsorbed NO2 (Figure 1) on Cu-Mn/ZSM-5 and Cu-Mn/SAPO-34 was also similar, the amount of nitrates (especially the monodentate nitrate) formed on Cu-Mn/ZSM-5 was much lower than that on Cu-Mn/SAPO-34. When NO+O2 was introduced to the Cu-Mn/ZSM-5 catalyst pretreated by NH3 (Figure 4a), the intensity of the bands due to NH4+ and coordinated NH3 decreased, and the nitrates (monodentate nitrate and bidentate nitrate) could be detected at 55 min. When NH4+ was completely consumed, the formation of adsorbed NO2 was Page 22 of 35


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accompanied by the decrease of nitrates, and the band corresponded to monodentate nitrate almost vanished. Therefore, it should be concluded that the transformation of bidentate nitrate to monodentate nitrate (procedure 2) was much slower than that of monodentate nitrate to adsorbed NO2 (procedure 4) on Cu-Mn/ZSM-5. Thus, there was not enough monodentate nitrate converted to adsorbed NO2, and the amount of adsorbed NO2 participating in the reaction was limited. On Cu-Mn/SAPO-34, NO adsorption (Figure 1b) led to formation of not only adsorbed NO2, but also large amount of nitrates, especially monodentate nitrate. When NO was introduced to the catalyst pretreated by NH3 (Figure 4b), the intensity of the band due to NH4+ on Cu-Mn/SAPO-34 decreased during the first 20 min, and nitrates accumulated on the catalyst with higher amount of bidentate nitrate than that of monodentate nitrate. When the NH4+ was completely consumed, the amount of adsorbed NO2 become higher, accompanied with the decreases of nitrates, and the consumption of bidentate nitrate was faster than that of monodentate nitrate. Large amount of bidentate nitrate on Cu-Mn/SAPO-34 was then transformed to monodentate nitrate (procedure 2), and then further to adsorbed NO2 (procedure 4). Therefore, there was enough adsorbed NO2 taking part in the reaction with NH4+ on Cu-Mn/SAPO-34 (procedure 6), and the L-H reaction way could take place, which was much more smoothly than that on Cu-Mn/ZSM-5 due to the adsorption and transformation of nitrates. Considering the difference of the activity at low temperature (Figure S9), it could be concluded that the L-H way influenced the NOx removal significantly at low temperature. Although some researchers reported that the L-H reaction way led to the formation of by-product N2O,49,50 it was seen in Figure S10 that only N2 was observed in gaseous products, and there was almost no signal of N2O during reaction process. Page 23 of 35



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4.2. NH3-SCR reaction mechanism of on the catalysts with hydrothermal aging treatment. As shown in Figure 5a, after hydrothermal aging treatment of Cu-Mn/ZSM-5 at 750 °C, adsorbed NO2 was difficult to be formed on Cu-Mn/ZSM-5(HT), with as much as only one fifth the amount on the fresh Cu-Mn/ZSM-5. Furthermore, little nitrate was detected on Cu-Mn/ZSM-5(HT). From Figure 6a, it could be known that the hydrothermal aging treatment did not influence the amount of coordinated NH3 significantly； However, the NH4+ adsorbed on the Brønsted acid sites was almost undetectable. When NH3+O2 was introduced to Cu-Mn/ZSM-5(HT) pretreated by NO+O2 (Figure 7a), adsorbed NO2 and nitrates were consumed very slowly. It might be caused by the low amount of NH4+, which was an important intermediate reacting with adsorbed NO2. After NH3+O2 was introduced into the system for 45 min, the amount of adsorbed NO2 and nitrates adsorbed on Cu-Mn/ZSM-5(HT) decreased slightly, and coordinated NH3 was observed. However, NH4+ was still undetectable. The absence of NH4+ suggested that the Brønsted acid sites on Cu-Mn/ZSM-5(HT) were destroyed by the hydrothermal aging treatment. Since the amount of nitrates on Cu-Mn/ZSM-5(HT) was very low, the transformation of bidentate nitrate to monodentate nitrate could hardly take place, which inhibited another way to form Brønsted acid sites. When NO+O2 was introduced to the Cu-Mn/ZSM-5(HT) pretreated by NH3+O2 (Figure 8a), the adsorbed NO2 and nitrates were formed simultaneously at 60 min, and the intensity of the bands due to these species was significantly weaker than that on the fresh Cu-Mn/ZSM-5 (Figure 4a). On fresh Cu-Mn/ZSM-5, nitrates were detected firstly, and then adsorbed NO2 appeared. Therefore, it further proved the lack of Brønsted acid sites on Cu-Mn/ZSM-5(HT) leading to the absence of NH4+ reacted with adsorbed NO2. Page 24 of 35


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Considering the sharp decrease of adsorbed NO2 and NH4+ formed on Cu-Mn/ZSM-5(HT), it could also be concluded that the SCR reaction through the L-H reaction way was significantly restrained. On the other hand, the amount and consumption rate of coordinated NH3 on Cu-Mn/ZSM-5(HT) was almost equal to the fresh catalyst (Figure 8a), and the gas phase concentration of NO was constant. Therefore, the reaction between coordinated NH3 and NO through the E-R reaction way should be similar to that of the fresh Cu-Mn/ZSM-5. As shown in Figure S11, the catalytic activity of fresh Cu-Mn/ZSM-5 was 65% at 200 °C, whereas the catalytic activity of Cu-Mn/ZSM-5(HT) was only 30%. This difference was attributed to the inhibition of the L-H reaction way, which should be the dominant reaction way at low temperature. The MS results (Figure S10) showed that although there was no by-product detected, while the amount of N2, as the main product of the SCR reaction, was reduced significantly. It further suggested that inhibition of the L-H reaction way did influence the SCR reaction on Cu-Mn/ZSM-5(HT). Results in Figure 5b and Figure 6b indicated that NOx complexes and NH3 species could be easily formed on Cu-Mn/SAPO-34(HT). The formation of some important intermediates, especially adsorbed NO2, NH4+ and coordinated NH3, was not restrained, and the amount of NH4+ adsorbed on the Brønsted acid sites even increased slightly (Figure S3). When NH3 was introduced to the Cu-Mn/SAPO-34(HT) pretreated by NO+O2 (Figure 7b), the amount of adsorbed NO2 decreased firstly and then nitrates were gradually consumed. Therefore, adsorbed NO2 could take part in the reaction on Cu-Mn/SAPO-34(HT), and the nitrates could also be converted to adsorbed NO2 like that on the fresh Cu-Mn/SAPO-34. With NH3+O2 flow introduced continuously into the cell, coordinated NH3 was observed firstly, and NH4+ was Page 25 of 35



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detected after the adsorbed NO2 vanished. This phenomenon was similar to the fresh Cu-Mn/SAPO-34. Therefore, it could be concluded that hydrothermal aging treatment had little effect on the reaction between NH4+ and adsorbed NO2 on Cu-Mn/SAPO-34(HT). After the pretreatment of NH3+O2, the amount of NH4+ adsorbed on the Brønsted acid sites was even higher than that of the fresh Cu-Mn/SAPO-34. When NO+O2 was introduced, the consumption of NH4+ was much faster than that of coordinated NH3, and bidentate nitrates were transformed to monodentate nitrate at almost the same speed as on the fresh Cu-Mn/SAPO-34. The NH4+ was completely consumed and large amount of adsorbed NO2 was formed on Cu-Mn/SAPO-34(HT). Due to the increases of NH4+, which could react with similar amount of adsorbed NO2 on the fresh Cu-Mn/SAPO-34, the catalytic activity of Cu-Mn/SAPO-34(HT) increased to nearly 100% at 200 °C. Therefore, hydrothermal aging treatment had no negative effect on Cu-Mn/SAPO-34, or even improved the catalytic activity due to the slight increase of Brønsted acid sites. The MS results (Figure S10) showed that SCR reaction Cu-Mn/SAPO-34(HT) could take place with little byproduct. The

amount

of

coordinated

NH3

formed

on

Cu-Mn/ZSM-5(HT)

and

Cu-Mn/SAPO-34(HT) was similar to the fresh catalysts, and the consumption rate of the coordinated NH3 did not vary a lot. It could be understood that the reaction through E-R reaction way was not significantly influenced by the hydrothermal aging treatment. For L-H reaction way, the amount of NH4+ and adsorbed NO2 on Cu-Mn/ZSM-5(HT) was restrained by hydrothermal aging treatment. The transformation from bidentate nitrates to monodentate nitrates could not be observed, and the generation of new Brønsted acid sites from this process was inhibited. Thus, Page 26 of 35


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the variation of the physical and chemical properties of Cu-Mn/ZSM-5 inhibited the reaction

through

L-H

reaction

way.

The

decreases

of

the

activity

of

Cu-Mn/ZSM-5(HT) (from 65% to 30%) might be mainly due to the variation of L-H reaction way, and it further proved that the L-H reaction way is the dominant reaction way at low temperature. On Cu-Mn/SAPO-34(HT), the adsorbed NO2 and NH4+ and their conversion were similar to the fresh Cu-Mn/SAPO-34, and it should be the main reason for difference in the effect of hydrothermal aging treatment on Cu-Mn/ZSM-5 and Cu-Mn/SAPO-34.

5. CONCLUSION In this study, the mechanism of the SCR reaction on Cu-Mn/ZSM-5 and Cu-Mn/SAPO-34 was investigated by in-situ DRIFTS. Different hydrothermal stability between these two catalysts was studied from the perspective of reaction mechanism. The main conclusions were as follows: (1) The L-H reaction way and the E-R reaction way were involved simultaneously in the NH3-SCR reaction on Cu-Mn/ZSM-5 and Cu-Mn/SAPO-34. There was no obvious difference between the reactions through E-R reaction way on the two catalysts, while the activities of the two catalysts were different through the L-H reaction way. (2) Adsorbed NO2 and NH4+ were the main active intermediates of the NOx complexes and NH3 species reacted directly through L-H reaction way. The transformation of nitrate was also defined. On Cu-Mn/SAPO-34, adsorbed NO2 was formed from the decomposition of monodentate nitrate, which was transformed from the bidentate nitrate. On Cu-Mn/ZSM-5, the amount of nitrates was very low, and

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adsorbed NO2 could not be obtained from the transformation of nitrates, leading to lower catalytic activity in comparison with Cu-Mn/SAPO-34. (3) After hydrothermal aging treatment, high amount of NOx species, including adsorbed NO2 and nitrates, were still formed on Cu-Mn/SAPO-34(HT), while the amount of NH4+ increased. On Cu-Mn/ZSM-5(HT), the amount of adsorbed NO2 reduced significantly and the NH4+ was hardly formed. Thus, the reaction through L-H reaction way took place more successfully on Cu-Mn/SAPO-34(HT) than on Cu-Mn/ZSM-5(HT), leading to higher hydrothermal stability of Cu-Mn/SAPO-34. (4) On Cu-Mn/ZSM-5(HT), reaction through E-R reaction way was not influenced significantly, and the decreases of the activity caused by the hydrothermal aging treatment was mainly due to L-H reaction way. The lower stability of Cu-Mn/ZSM-5 than Cu-Mn/SAPO-34 was also due to the difference of the SCR reaction through L-H reaction way. Combined the difference of activity and stability between the two catalysts, it might be concluded that the L-H reaction way should be the dominant way at low temperature. ACKNOWLEDGMENT The project is financially supported by the Natural Science Foundation of Zhejiang Province (LY14E080001), and Key Project of Zhejiang Provincial Science and Technology Program (2012C03003-4). SUPPORTING INFORMATION AVAILABLE The adsorption of NH3+O2 for 90 min on Cu-Mn/SAPO-34 with pretreatment of NO+O2 (Figure S1); The promotional effect of the pretreatment of NO+O2 on the NH3 adsorption on Cu-Mn/ZSM-5 (Figure S2) and Cu-Mn/SAPO-34 (Figure S3); The adsorption of NH3+O2 for 90 min on Cu-Mn/SAPO-34(HT) with pretreatment of Page 28 of 35


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NO+O2 (Figure S4); The comparison of NO adsorption on Cu-Mn/SAPO-34 and Cu-Mn/SAPO-34(HT)

(Figure

S5);

The

variation

of

NO

adsorption

on

Cu-Mn/SAPO-34 at different temperatures (Figure S6); The adsorption of NO and NH3 on Cu/zeolite, Mn/zeolite, Cu-Mn/zeolite (Figure S7 and S8); Catalytic activity of Cu-Mn/ZSM-5 and Cu-Mn/SAPO-34 (Figure S9); The production of in the tail gas of in situ DRIFT (Figure S10); The hydrothermal stability of the catalysts (Figure S11); The variation of H2O in the tail gas during the reaction (Figure S12). REFERENCES (1) Busca, G.; Lietti, L.; Ramis, G.; Berti, F. Chemical and mechanistic aspects of the selective catalytic reduction of NOx by ammonia over oxide catalysts: A review. Appl.

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Mechanism of NH3 Selective Catalytic Reduction Reaction for NO x

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