Influence of preparation methods on iron-tungsten composite catalyst for NH3-SCR of NO: The active sites and reaction mechanism

Journal Pre-proofs Full Length Article Influence of preparation methods on iron-tungsten composite catalyst for NH3-SCR of NO: the active sites and reaction mechanism Qiulin Zhang, Yaqing Zhang, Tengxiang Zhang, Huimin Wang, YanPing Ma, JifengWang, Ping Ning PII: DOI: Reference:

S0169-4332(19)33006-5 https://doi.org/10.1016/j.apsusc.2019.144190 APSUSC 144190

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

23 July 2019 16 September 2019 26 September 2019

Please cite this article as: Q. Zhang, Y. Zhang, T. Zhang, H. Wang, Y. Ma, JifengWang, P. Ning, Influence of preparation methods on iron-tungsten composite catalyst for NH3-SCR of NO: the active sites and reaction mechanism, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144190

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Influence of preparation methods on iron-tungsten composite catalyst for NH3-SCR of NO: the active sites and reaction mechanism Qiulin Zhang*,Yaqing Zhang, Tengxiang Zhang, Huimin Wang, YanPing Ma, Jifeng Wang, Ping Ning Faculty of Environmental Science and Engineering, Kunming University of Science and Technology. Kunming, 650500, P.R. China

* Corresponding author. Tel:136-6878-8376 E-mail address: [email protected]

Abstract: A series of iron-tungsten composite oxides catalysts prepared by grinding methods (FW-GR), incipient impregnation (FW-IM), sol-gel (FW-SG) and microemulsion (FW-ME) methods were used for selective catalytic reduction of NO by NH3 (NH3-SCR). The SCR activity was significantly depended on the preparation method, the NO conversions over different catalysts were following the order of FW-GR > FW-SG > FW-ME > FW-IM. The results indicated that the NO conversion was strongly related to the FeWO4 species. It also discovered that the catalyst with higher specific surface area, surface Fe2+ content, surface adsorbed oxygen species, redox potential, surface acidic sites and acidic strength showed better NO reduction activity among all catalysts. In situ DRIFTS results revealed that the main reaction mechanisms of iron-tungsten catalysts were “L-H” at low temperature and “E-R” at high temperature. The SCR reaction over FW-GR sample involved the formation of NH2 intermediate and subsequent reduction by NO, while the formation of NH4NO3 species over FW-IM was easily transformed into N2O species. Keywords: NH3-SCR; active sites; reaction mechanism 1. Introduction Nitrogen oxides (NOx) as one of the major air pollutants have caused adverse effects on ecological environment and human health. The selective catalytic reduction of NO by NH3 (NH3-SCR) is confirmed to be the most efficient technology for the removal of NOx, and more than 80% of the actual industrial de-NOx process is operated via this technique [1]. Although commercial V2O5-WO3(or MoO3)/TiO2 as a widely applied SCR catalyst presents high efficiency for removing of NOx, there are still some disadvantages, such as narrow activity window (300-400°C), high biological toxicity of vanadium and formation of N2O at high temperature [2]. In recent

years,

many

efforts

are

focused

on

the

development

of

an

environmental-friendly SCR catalyst with excellent activity in a wide temperature range for NO removing. Some transition metal exchanged zeolites catalysts (Fe/ZSM-5, Cu/ZSM-5 and Cu/SAPO-34) and non-vanadium transition metal oxide catalysts have been extensively investigated for NH3-SCR reaction [3-8]. The zeolite-based catalysts show excellent NH3-SCR performance and stability in a wide

operating temperature range, but the poor hydrothermal stability and susceptibility to sulfur poisoning of zeolite-based catalysts impede their industrial application [9]. The non-vanadium transition metal oxide catalysts attract much attention for NO removal, however, developing the catalyst with a broad temperature window and excellent resistant to SO2 remain a great challenge [10]. Various transition metal composite oxide catalysts have been shown to be active for NH3-SCR reaction, especially for the catalysts containing Ce, Fe, Mn, Cu, Co oxides, due to their excellent oxygen storage and redox properties [5-20]. Iron oxides are environmental-benign materials and which widely distribute in earth crust. The previous studies have shown that iron oxide containing catalysts were widely investigated for SCR of NO due to its excellent activity and resistant to SO2 [7,21-24]. The outstanding redox cycling between Fe3+ and Fe2+ of iron oxide containing catalysts can promote the activation of NO, and then the SCR performance is enhanced [25]. WO3 is the key component in the commercial SCR catalysts. The promotion in SCR activity of WO3 has also been confirmed by many researchers [8]. It has been reported by several studies that the addition of WO3 into Fe-Ce mixed oxides causes an activity enhancement by inhibiting NH3 oxidation [26]. Recently, Wang et al and Shan et al have discovered that the Iron-tungsten complex oxide catalyst as a candidate for NH3-SCR reaction present super application potential [27,28]. The Iron-tungsten catalysts showed high NO conversions and N2 selectivity in a wide temperature range and undergoing significantly less deactivation even reacts in the presence of H2O + SO2. On the other hand, the previous reports have also confirmed that the surface WOx and FeOx species not only influenced the NH3-SCR activities, but also affected the N2 selectivity of Iron-tungsten catalyst [29,30]. The specific catalytic activity datum of iron-tungsten catalysts of other reports were shown in the following Table S1. Therefore, the tungsten oxide and iron oxide species in Iron-tungsten catalyst play a crucial role in NH3-SCR reaction. It is widely reported that the structural properties and surface species of catalysts are strongly related to preparation methods, and the catalysts prepared by different methods often contribute to different physicochemical properties and catalytic

performance [31,32]. Herein we present the Iron-tungsten catalysts by using four preparation methods: grinding methods, incipient impregnation, sol-gel and microemulsion methods. The relationship between preparation methods and NH3-SCR reaction are investigated. The physicochemical properties are analyzed by BET, XRD, TEM XPS, H2-TPR, NH3-TPD, and Raman techniques, and the in situ DRIFTS is used to understand the adsorbed species and reaction intermediates of NH3-SCR reaction on different Iron-tungsten catalysts. 2. Experimental 2.1. Catalyst preparation 2.1.1. Grinding method (FW-GR) The FW-GR catalyst was prepared by the grinding method. The appropriate amounts of Fe (NO3)3·9H2O and (NH4)6H2W12O40·6H2O (4:1) were milled with urea for 40 minutes in a mortar. And then the resulting gelatinoid sample was dried at 100 °C for 24 h, subsequently calcined at 500 °C in air for 3 h. 2.1.2. Impregnation method (FW-IM) (NH4)6H2W12O40·6H2O was dissolved in deionized water. And then the Fe2O3 was immersed in the solution for 6 h. The mixture was heated at 60 °C for 5 h and dried overnight at 110 °C. The resulting solid was calcined at 500 °C for 3 h in air to obtain the final catalyst. The catalyst was signed as FW-IM. 2.1.3. Sol-gel method (FW-SG) The FW-SG catalyst was prepared by the sol-gel method. The desired amounts of Fe (NO3)3·9H2O and (NH4)6H2W12O40·6H2O were dissolved in deionized water and stirred together. Citric acid was then introduced into the mixed solution (the molar ratio of metal ions and citric acid was 1:1). The mixture was heated at 80 °C under stirring until it became a transparent sol and then dried at 75 °C until the sol transformed to gel. The sample was calcined at 500 °C in air for 3 h. 2.1.4. Micro emulsion method (FW-ME) The FW-ME catalyst was prepared by water-in-oil microemulsion method. Triton

X-100，n-Hexyl alcohol and cyclohexane were homogeneously mixed by continuous magnetically stirring at room temperature. Aqueous (NH4)6H2W12O40·6H2O was introduced to form the transparent solution, followed by adding hydrazine to generate tungsten-hydrazine complex suspension. After reacting for 30 min, Fe (NO3)3·9H2O was put into the suspension and hydrolyzed for several hours. The obtained slurry was washed by ethanol and hot water for several times, dried at 110 °C overnight and calcined at 500 °C in air for 3 h. For all catalysts, the mass ratio of FeOx to WO3 was fixed at 4:1. The samples, denoted as FW. The resulting material was pressed into pellets, crushed and sieved to 40-60 mesh for using. The chemical reagents used in the present work were analytical grade. 2.2. Catalytic performance measurements The activity tests were carried out in a fixed-bed quartz reactor at atmospheric pressure. The simulated gas mixture contained 600 ppm NO, 0 or 600 ppm NH3, 5% O2, 0 or 5% H2O, 0 or 100 ppm SO2 and N2 balanced. The gas hourly space velocity (GHSV) was 60,000 h-1. The concentration of NO and NO2 in the inlet and outlet gas was monitored by the flue gas analyzer (ECOM·J2KN), and the N2O was monitored by gas chromatography. The NO conversion was calculated as follows: NO conversion (%) =

[NO]inlet − [NO]outlet × 100% [NO]inlet

The NO oxidation NO2 ratio was calculated as follows: NO oxidation NO2 ratio (%) =

[NO2 ]outlet × 100% [NO]inlet

2.3. Catalyst characterization The

BET

surface

area

of

the

catalysts

was

obtained

from

N2

adsorption/desorption analysis at -196 °C on the Micromeritics TriStar II 3020 apparatus. Powder XRD pattern was recorded through an X-ray diffraction meter (Bruker D8 ADVANCE) between 20◦ and 80◦ at a step rate of 6 °/min operated at 40 kV and 30 mA with Cu Kα radiation (λ=1.5418 Å). Raman spectra of the synthesized

samples were collected by a Renishaw-2000 Raman spectrometer using an Ar+ laser beam. The intensity data was collected from 100 to 1200 cm-1. The X-ray photoelectron spectroscopy (XPS) studies were carried out on UL-VAC PHI 5000 Versa Probe-II equipment with Al Kα X-ray radiation under UHV. The H2-TPR experiment was conducted through a reactor. The sample (50 mg) was pre-treated in N2 (30 mL min-1) at 400 °C and the temperature was raised from 50 to 850 °C in 5% H2/Ar (30 mL min-1). The H2 consumption signal was continuously monitored by a thermal conductivity detector (TCD). For the NH3-TPD experiments, the catalyst was pre-treated at 400 °C for 40 min in N2 (30 mL min-1). Then the sample was saturated with 4% NH3/He at a flow rate of 30 mL min-1 for about 40 min at 25°C. Desorption was performed by heating the sample in pure N2 from 50 to 600 °C. In situ DRIFTS experiments were performed on a Nicolet iS50 FTIR spectrometer. Prior to experiments, the catalyst was pretreated at 400 °C in 5% O2+N2 for 30 min. The background spectrum was collected in a flowing of N2 atmosphere and subtracted from each sample spectrum. At room temperature, the sample was exposed to the controlled stream of NH3 (600 ppm NH3) or/and NO + O2 - N2 (600 ppm and 5% O2) at a rate of 100 mL min-1 for 30 min to achieve saturation, followed by N2 purging from 50-450 °C. In the transient studies, the sample was exposed to 600 ppm NO + 5% O2 (or 600 ppm NH3) for 30 min, before which it was pre-adsorbed to a flow of 600 ppm NH3 (or 600 ppm NO + 5% O2) for 30 min at 200 °C, subsequently purged with N2 for 15 min. For the co-adsorption of reactant gases around 50-400 °C, the sample was exposed to the flow of 600 ppm NH3 + 600 ppm NO + 5% O2, after that the spectra was recorded. 3. Results analysis 3.1. Catalytic performance

Fig.1. Catalytic activity of the different FW the catalysts for NH3-SCR. Reaction conditions: 600 ppm NO, 0 or 600 ppm NH3, 5% O2, 0 or 5% H2O, 0 or 100 ppm SO2 and balance N2, GHSV = 60,000 h-1. (a) NOx conversion; (b) N2O yield; (c) Effects of H2O and SO2 on the activity of FW-GR catalyst at 300 °C; (d) NO-to-NO2 oxidation activity at 200 °C.

The results of NH3-SCR catalytic activity and the concentration of N2O in the NH3-SCR reaction over FW catalysts were shown in Fig. 1. As shown in Fig. 1(a), the NOx conversion was over 90% on FW-GR, FW-IM, FW-SG and FW-SG at 222, 286, 226 and 249 °C, respectively. Markedly, the FW-GR and FW-SG catalysts could obtain more than 90% NOx conversions at 225-500 °C. Moreover, it was rather obvious that the FW-IM showed the relatively low NH3-SCR efficiency in a narrow operation temperature. Thus, it suggested that the SCR activities of FW catalysts were significantly affected by the preparation method. In addition, the N2 selectivity of FW catalysts was evaluated by monitoring the concentration of N2O in the NH3-SCR process. Fig. 1(b) revealed the concentration of N2O over FW catalysts prepared by diverse ways with the increase of temperature from 150 to 500 °C. As anticipated, the

low N2O concentration (below 38 ppm) with small differences was detected over the FW catalysts. It can be concluded that the FW catalyst prepared by the grinding method exhibited the best NH3-SCR performance. For practical use on diesel engines, high SO2 and H2O durability is usually required for the potential NH3-SCR catalysts. Further studies on the effects of H2O and SO2 on the activity of FW-GR catalyst at 300 °C were carried out. As shown in Fig. 1(c), the introduction of 100 ppm SO2 + 5% H2O into the SCR atmosphere did puny influence the deNOx efficiency over the FW-GR catalyst, and the NO conversion could always maintain above 90% even after 60 h reaction. It was worthy to note that the NO conversion was full recovered after closing H2O and SO2 feed, indicating that FW-GR catalyst was highly resistant to the H2O and SO2 poisoning which was very beneficial to practical use. The NO-to-NO2 oxidation activity of FW catalysts were evaluated by NO oxidation experiment as presented in Fig. 1(d). The NO oxidation ratio was in the order of: FW-GR (25.3%) > FW-SG (24.0%) > FW-ME (18.3%) > FW-IM (10.7%). This fact indicated that the NO was more easily oxidized to NO2 on FW-SG and FW-GR than that of FW-IM and FW-ME. The high NO oxidation activity further facilitated the NH3-SCR reaction via “fast SCR” reaction (4NH3 + 2NO + 2NO2 → 4N2 + 6H2O) [53]. 3.2. XRD and BET results Table 1 BET measurements and crystallite size for the various catalysts. Sample

Surface area

Total pore

Average pore

crystallite

NOx conversion

(m2/g)

volume(cm3/g)

diameter (nm)

size(nm)

at 250°C (%)

FW-GR

97.1

0.23

7.0

26.9

98

FW-IM

16.5

0.11

16.9

25.1

50

FW-SG

72.5

0.21

8.58

25.1

100

FW-ME

64.7

0.21

9.81

25.0

92

Physicochemical characteristics of FW catalysts prepared with different methods were summarized in Table 1. The specific surface areas were 97.1 m2/g for FW-GR, 72.5 m2/g for FW-SG, 64.7 m2/g for FW-ME and 16.5 m2/g for FW-IM, respectively.

It could be seen that the total pore volume of the FW-IM catalyst was 0.11 cm3/g, while the pore volume of other catalysts were almost as twice as that of the FW-IM catalyst. It was generally accepted that the larger surface areas and total pore volume not only lead to the better dispersion of active species but also facilitate the transportations of reactants, thus contributing to the high catalytic performance.

Fig. 2. (a)XRD patterns and (b)XRD partial enlarged view of the FW catalysts.

The XRD results are shown in Fig. 2(a). It can be seen that the peaks for pure Fe2O3 were attributed to hematite (PDF-#79-1741). The slight diffraction patterns of FW catalysts ascribed to Fe2O3, indicating that the weak crystallinity. Moreover, sharp peaks shifted distinctly to smaller angles after introduction of W species. The crystallite dimension of the α-Fe2O3 calculated by using Scherrer’s formula was 21.9 and 25.1-26.9 nm for pristine Fe2O3 and FW catalysts, respectively (listed in Table 1). The crystallite size of FW samples increased with the addition of W species. Considering the approximate ionic radii of Fe3+ (0.645 Å) and W6+ (0.620 Å), the left shift of 2θ angles in Fig. 2(b) could be interpreted that WO3 doped into the unit cell of Fe2O3 [33]. In the case of FW catalysts, no peaks corresponding to WO3 were observed in the XRD patterns despite with 20 wt% WO3, which suggested that tungsten oxides were highly dispersed on the surface of catalyst. Fig. 2(a) also presented that the diffraction intensity of FW-SG and FW-GR were weaker than that of FW-IM and FW-ME, implying the low crystallization of FW-SG and FW-GR catalysts. 3.3. TEM analysis

Fig. 3. TEM images of the FW-GR (a, e), FW-IM (b, f), FW-SG (c, g) and FW-ME (d, h).

In order to explore microstructure of FW catalysts, the typical TEM tests were conducted and the results were shown in Fig. 3. From Fig. 3a-d, The FW catalysts prepared by different ways had the similar iron content, but the morphology of samples was quite different. It could be seen that the catalyst particles of FW-GR and FW-SG were smaller spherule than FW-IM. The FW-ME presented the shape of quadrangular bulk and uneven particles. Further examination by HRTEM showed the interplanar distance of FW samples (Fig. 3e-h). The marked lattice pitch of 0.251 nm and 0.184 nm were corresponded to the (110) and (024) planes of Fe2O3 [34], and the marked lattice pitch of 0.375 nm and 0.461 nm were matched with the (120) and (011) planes of WO3 [35, 36]. The lattice fringe (d = 0.291 nm) belonged to the (111) crystallographic planes of FewO4 [27]. The FeWO4 species apparently emerged on FW-GR, FW-SG and FW-ME catalysts. Combined with the activity test results, it was speculated that the presence of FeWO4 species improved the deNOx performance of FW catalysts. 3.4. Raman spectra study

Fig. 4. Raman spectra of the FW catalysts.

Raman spectroscopy was employed to accurately characterize the phase structure of the FW catalysts (Fig. 4). It could be seen that the bands at 215 and 281 cm-1 attributing to the single-phase of FeWO4 species were observed on FW-GR [37]. For the FW-SG and FW-ME, except for the visible Raman peaks centered at 215, and 281 cm-1, the peaks at 388 and 598 cm-1 were also discovered. It was reported that the band at 388 cm-1 was ascribed to the stretching mode of Fe-O-W bonds [38,39], while the peak at 598 cm-1 was the characteristic peak of the Fe2O3 phase. Interestingly, the Raman signal at 220, 285, 410 and 598 cm-1 contributing to the Fe2O3 phase were shown on FW-IM [40,41], then a distinct peak at 806 cm-1 assigning to W-O stretching modes (A1g) of the crystalline WO3 phase was also presented [35,37,42]. It was reported that the FeWO4 species played a crucial role in promoting the NH3-SCR activity [27,29]. The apparently emerged FeWO4 species on FW-GR, FW-SG and FW-ME was beneficial to the NO conversion. Furthermore, the crystalline WO3 were observed on FW-IM with low catalytic performance. It suggested that the surface FeWO4 species was more conducive to the NO conversion than WO3 [28]. 3.5. XPS and EDS analysis

Fig. 5. XPS of FW catalysts, (a) Fe 2p, (b) W4f, (c) O 1s. Table 2 Surface atomic concentration of the FW catalysts as determined by the XPS results (atomic %) and atomic concentration EDS results (atomic %). XPS Sample

Fe2+/(Fe2++

Fe3+)

EDS Surface atomic concentration

Atomic concentration

(atomic %)

(atomic %)

Oα/(Oα+Oβ) W

Fe

O

W

Fe

O

FW-GR

65.9%

28.1%

4.3

30.2

65.5

1.96

38.03

60.01

FW-IM

46.3%

22.5%

4.7

28.3

67.0

0.89

58.82

40.29

FW-SG

57.5%

27.8%

4.2

30.1

65.7

6.11

77.52

16.37

FW-ME

53.0%

25.4%

4.9

29.1

66.0

7.20

77.59

15.21

The surface chemical composition and oxidation state of these synthetic catalysts were measured by XPS. Fig. 5 illustrated the XPS spectra of Fe 2p, W 4f, and O 1s, respectively. The corresponding surface atomic concentrations and the relative concentration ratio of different oxidation states were summarized in Table 2. The Fe 2p and O 1s peaks were fitted into sub-bands by searching for the optimum

combination of Gaussian bands. Fig. 5(a) showed the deconvoluted Fe 2p XPS of FW catalysts. The peaks detected at about 710.7 and 713.0 eV were assigned to Fe2+ and Fe3+, respectively [43,44]. The visible shake up satellite peaks were presented in all samples, implying the presence of both Fe2+ and crystallographic Fe2O3 species [45]. Since coexistence of Fe2+ in Fe3+ species could make unbalanced and unsaturated chemical bonds, then the quantity of surface chemisorbed oxygen were increased at some extent [45,46]. It was also reported that the intermediate valence of iron species could contribute to the enhancement of the catalytic performance for NH3-SCR. The value of Fe2+ / (Fe 2+ + Fe 3+) calculated by area of the XPS spectra was in the order of: FW-GR (65.9%) > FW-SG (57.5%) > FW-ME (53.0%) > FW-IM (46.3%). This was related to the formation of solid solution (containing Fe-O-W structures) and electron interactions between WOx and FeOx [27, 28, 29]. W 4f7/2 spectra of FW catalysts were presented in Fig. 5(b). According to previous reports [8, 47], the binding energy of W 4f7/2 (35.47-35.68 eV) and W 4f5/2 (37.5-37.78 eV) could be attributed to the W6+ state. It also could be seen from Fig. 4(b) that the binding energy of FW-IM and FW-ME shifted to higher value compared with FW-GR and FW-SG, implying the interaction between iron and tungsten atom was enhanced on the catalysts prepared by grinding and sol-gel methods [45,46]. As depicted in Fig. 5(c), the XPS of O 1s were fitted into two peaks. The O 1s binding energy located at 531-531.5 eV and 530.1-530.3 eV were ascribed to the chemisorbed surface oxygen species (Oα) and the lattice oxygen species (Oβ), respectively [47]. As we known, the surface oxygen Oα possessing excellent mobility and oxidative ability were key factor for oxidation of NO to NO2. During this procedure, NO could react with reactive Oα with enhancing oxidation ability on the active sites and subsequently transformed to NO2 or nitrate species, then the SCR reaction was improved [43,48]. The Oα / (Oα + Oβ) ratio was 28.1%, 22.5%, 27.8% and 25.4% for FW-GR, FW-IM, FW-SG and FW-ME, respectively. It was clear that the FW-GR showed the highest surface chemisorbed oxygen species among the catalysts prepared by different methods. Combined with the NO-to-NO2 oxidation results, it demonstrated that the FW-GR and FW-SG possessed a large amount of surface chemisorbed oxygen species,

which was favor of NO oxidation to NO2. Thus, the abundant surface chemisorbed oxygen species of FW-GR and FW-SG could lead to the excellent NH3-SCR reaction activity at low temperature. In addition, X-ray energy dispersive spectroscopy (EDS) was performed to investigate the chemical composition of mesoporous FW catalysts and the atomic concentration of FW catalysts were shown in Table 2. From Table 2, these results demonstrated that the FW-IM catalyst possessed a relative large amount of the surface W atoms indicating that more crystallization of WO3 species was existed on the surface of FW-IM [47]. As we can see, the grinding method greatly increased the ratio of Oα / (Oα + Oβ) and Fe2+ / (Fe 2+ + Fe 3+) over FW catalysts. More surface adsorbed oxygen and FeWO4 species were observed on FW-GR and FW-SG than those on FW-IM and FW-ME, indicating that the strong interaction between Fe and W species on FW-GR and FW-SG catalysts could produce extra surface activate oxygen participating into the SCR reaction. 3.6. Redox performance study

Fig. 6. H2-TPR results of serial FW catalysts

The redox properties of FeOx-WO3 catalysts were explored by H2-TPR. As shown in Fig. 6, FW-GR and FW-SG exhibited four reduction peaks. The peak centered at 384 °C was attributed to reduction process from part of the Fe2O3 to Fe3O4 [43]. The peak in the range of 506-577 °C could be attributed to the step reduction of Fe2O3→Fe3O4→FeO, whereas the peak from 750 to 850 °C was associated with the reduction of Fe2+ to Fe0 [49,50]. The FW-ME and FW-IM showed two evident reduction peaks at 543-550 and 802-810 °C, which could be ascribed to the reduction

of Fe2O3 to Fe3O4 (543-550°C), Fe3O4→FeO→Fe (802-810 °C), respectively [49,50]. This figure provided an obvious message that the reduction temperatures of peaks shifted to lower temperatures on FW-GR and FW-SG. The onset temperature of the first peak was below 400 °C, which meant that the redox property of FW catalysts was enhanced by the synthesis of grinding and sol-gel methods. Therefore, the excellent reduction property of FW-GR and FW-SG was beneficial to the NH3-SCR reaction [27,28,51]. 3.7. NH3-TPD analysis

Fig. 7. NH3-TPD curves of the four catalysts Table 3. The quantitative analysis data of NH3-TPD for the synthesized catalysts. Sample

Acid amount* (a.u.) / 1E-8 Peak a

Peak b

Peak c

FW-GR

9.1

15

38

FW-IM

1.3

2.2

5.2

FW-SG

14

19

33

FW-ME

5.4

7.2

14

* NH3 signal is detected by Quadrupole mass spectrometer. The acid amounts by the deconvolution of the NH3-TPD curves.

The NH3 adsorption was an important process in NH3-SCR reaction. In order to probe the effect of different preparation method on the acidity of FW catalysts, NH3-TPD experiments were performed on the FW catalysts. As shown in Fig. 7. The NH3-TPD profiles of all samples could be fitted to three NH3 desorption peaks: the low-temperature peak (Peak a) located at about 120 °C attributed to the weakly

adsorbed NH3 on the weak Lewis acid sites, the peak (Peak b) located at about 160 °C due to the NH3 adsorbed on the strong Lewis acid sites, and the high-temperature desorption peak (Peak c) above 300-500 °C assigned to the NH3 desorption from the Brønsted acid sites [14, 49]. Table 3 exhibited the acid amounts of the FW catalysts by the deconvolution of the NH3-TPD curves. The FW catalysts prepared by different methods presented distinctly different acidity, and the acid amount of sequences were FW-SG (14) > FW-GR (9.1) > FW-ME (5.4) > FW-IM (1.3) for weak acidity and FW-GR (38) > FW-SG (33) > FW-ME (14) > FW-IM (5.2) for strong acidity. Moreover, three NH3 desorption peaks of FW-GR shifted distinctly to lower temperature than other FW catalysts. The results indicated a significant enhance in NH3 adsorption over the FW-GR and FW-SG catalysts. For FW-ME or FW-IM, nonetheless, the surface acidities were relatively weak. To inspect the types, intensities and amounts of the surface acid sites in the four catalysts, in situ DRIFTS of NH3 adsorption/desorption was employed [53,54]. 3.8. In situ DRIFTS study It was widely accepted that the adsorption and activation of reactants were the key steps in the catalytic reaction. In the NH3-SCR reaction, the activated intermediates of NH3 and NO could be revealed by the in situ DRIFTS technology. As stated above, the NH3-TPD results confirmed that the surface acid sites presented vary characters of the FW catalysts. It had been reported that the activated intermediates mightily related to the surface acidity of the SCR catalysts [52,54]. Here the in situ DRIFTS experiments were employed to investigate the formation of intermediate products and reveal the reaction mechanism of NH3-SCR reaction over FW catalysts. 3.8.1. NH3 adsorption ability

Fig. 8. In situ DRIFTS of four catalysts pre-treated in flowing 600 ppm NH3 at 50 °C for 30 min and then purged by N2 at 50, 150, 250, 350 and 450 °C.

In situ DRIFTS experiments were conducted to explore the NH3 desorption behavior for further investigating the acid property of different catalysts. The NH3 adsorption/desorption on the FW series catalysts at different temperatures were given in Fig. 8 and Fig. S1. When NH3 was introduced for 30 min and then purged by N2, several vibration bands were detected on all samples. The bands centered at 1606 and 1238-1206 cm-1 could be assigned to the asymmetric (σas) and symmetric (σs) vibration and of the coordinated NH3 linked to Lewis acid sites, respectively [54,55], whereas the bands at 1669 and 1436-1429 cm-1 could be attributed to the NH4+ bound to the Brønsted acid sites [56]. Three peaks on the broad band in the range of 3398-3162 cm-1 were observed in the spectrum belong to contributions from N-H vibration of ammonia adsorbed on the Lewis acid sites [11]. At the beginning of the temperature ramp, the intensity of the bands at 3398-3162, 1669, 1436-1429 and 1238-1206 cm-1 decreased due to NH3 desorption from weak acid sites. Interestingly, the intensity of the peak at 1606 cm-1 increased between 50 and 350 °C, and then decreased with further increasing of temperature (350-450 °C). This revealed that some of the NH3 desorbed from weak Brønsted acid sites (1669 and 1436-1429 cm-1) associated with the -W-OH group might have re-adsorbed onto the Lewis acid sites

(1606 cm-1) connected with -W=O group in this low temperature region [53,57]. As illustrated in Fig. 8, it was obvious that Brønsted acid sites and Lewis acid sites coexisted on these four samples surface, but the distribution of acid sites was significantly different. For FW-GR sample, the intensities of adsorbed ammonia species on Brønsted acid sites were much higher than that of other FW catalysts at different temperatures. For this phenomenon, it could relate to the first overtone of 2v (W=O) stretching mode of surface octahedral [WO6] clusters consisted in FeWO4 species [27]. Herein, this prominent intensities indicated that some reactions occurred between W=O group and ammonia, which may be one of the incentives for the generation of Brønsted acid sites. Nevertheless, the FW-IM sample presented the less acid sites and weaker acid strength, which may be related to no presence of FeWO4 species in FW-IM catalyst. 3.8.2. NO + O2 adsorption ability

Fig. 9. In situ DRIFT spectra of four catalyst samples treated in flowing 600 ppm NO + 5%O2 at 50 °C for 30 min and then purged by N2 at 50, 150, 250, 350 and 450 °C.

To identify the NOx adsorption species on the FW catalysts under reaction temperature, in situ DRIFT spectra were collected at 50-450 °C in a flow of N2 after NO + O2 adsorption. The spectra were recorded and shown in Fig. 9 and Fig. S2. It could be observed that the NO + O2 adsorption on FW catalysts resulted in four common bands at 1611, 1578, 1283 and 1241 cm-1. One additional band at 1197 cm-1

was presented on FW catalysts except FW-IM. These bands assigned to bridging nitrates (1241 cm-1) [28], nitrite (1197 cm-1) [58], bidentate nitrates (1578 cm-1) [58], monodentate nitrates (1283 cm-1) [28,58] and absorbed NO2 species (1611 cm-1) [59,60]. It could be found that the nitrate species were unstable, and the intensities of nitrate species reduced during subsequent exposure to N2 and disappeared on FW-GR, FW-SG, FW-ME at 400 °C and on FW-IM at 300 °C. It also could be clearly seen from Fig. 8 that the intensities of the NOx adsorption species were as follows: FW-GR > FW-SG > FW-ME > FW-IM, indicating that the higher level of nitrate and nitrite species formed upon FW-GR. Which implied the enhanced adsorption and activation ability of NO, facilitating the NO conversion in NH3-SCR reaction [61,62] . 3.8.3. Reaction between adsorbed NH3 and NO + O2 species

Fig. 10. DRIFTS of NO + O2 and pre-adsorbed NH3 species at 200 °C on FW catalysts (a: FW-GR, b: FW-IM, c: FW-SG, d: FW-ME)( conditions: 600 ppm NH3 pre-absorbed followed by N2 purging for 30 min, and then switching to 600 ppm NO + 5% O2.)

The reactivity of pre-adsorbed NH3 with NO + O2 species was also studied on

FW catalysts at 200 °C, the reactions were measured as a function of time. As presented in Fig. 10, When the FW catalyst was exposed to NH3 after 30 min, the coordinated NH3 species (1214 and 1606 cm-1), NH4+ species (1421 cm-1) and N-H stretching region bands (3351-3156 cm-1) were all observed on four FW catalysts [54,62]. In addition, two major bands at 1669 and 1573 cm-1 on FW-GR and a weak band at 1669 cm-1 on FW-IM were detected. The formers were assigned to NH4+ and coordinated NH3 species. After introducing NO + O2, the bands at 1214, 1421, 1669, 3351-3156 cm-1 gradually declined further disappeared at 20 min due to the surface NH3 reacted with NOx. However, it was interesting that the intensity of the peak at 1606 cm-1 gradually decreased and disappeared at 10 min, and then increased with the introduction NO + O2, It interpreted that the adsorption of NH3 species was consumed due to participation in the reaction, and then the new nitrate species gradually accumulated [55,56]. Apart from that, significantly stronger intensity of the DRIFT spectra was detected on FW-GR. In other words, larger amount of surface NH3 adsorption species that can participate in the reaction was formed on FW-GR. 3.8.4. Reaction between adsorbed NO + O2 and NH3 species

Fig. 11. In situ DRIFT spectra of passing NH3 over NO + O2 pre-adsorbed on FW catalysts (a: FW-GR, b: FW-IM, c: FW-SG, d: FW-ME) at 200 ºC.

The reactivity of pre-adsorbed NO + O2 and NH3 species was performed over the FW series catalysts at 200 °C, and the spectra were recorded as a function of time. The corresponding results were shown in Fig. 11. Upon exposure to 600 ppm NH3, The bands of absorbed NO2 species (1610 cm-1) and monodentate nitrates (1542 cm-1) disappeared at 5 min over the FW catalysts [30,52,59,60]. In addition, the band (1578 cm-1) assigned to bridging nitrate also decreased and further vanished on FW-GR. After 10 min, all bands related to NOx were greatly reduced. Simultaneously, the bands assigned to coordinated NH3 (1606 and 1213 cm-1), N-H stretching (3156-3351 cm-1) [30,63,64], NH4+ (1427 cm-1) [59,56,64] all were detected on four FW catalysts. Meanwhile, NH2 (1558 cm-1) [30] and NH4+ (1669 cm-1) were also observed on FW-GR, FW-SG and FW-ME. The NH2 species was confirmed as an important intermediate due to its reaction with gaseous or weakly absorbed NO to form NH2NO, which was decomposed to N2 and H2O, favoring the reduction of NOx [53]. With the time increasing, the surface of FW catalysts was mainly covered by the various adsorbed NH3 species. This further suggested that the higher activity of nitrate species formed upon NO + O2 adsorption could be reacted with NH3 species on four FW catalysts that enhanced the SCR reaction. Here, a significantly larger amount of surface nitrates was formed on FW-GR compared to FW-IM, FW-SG and FW-ME, suggesting the FW-GR possessed more plentiful nitrates that could participate in the reaction.

3.8.5. Co-adsorption of NO + O2 + NH3

Fig. 12. DRIFTS of 600 ppm NH3 + 600 ppm NO + 5% O2 at various temperatures over FW catalysts (a: FW-GR, b: FW-IM, c: FW-SG, d: FW-ME).

In this experiment, the reactants were introduced over FW series catalysts at 50-400 °C in a flow of NO + O2 + NH3. The DRIFT spectra were recorded (As shown in Fig. 12). During the temperature ramp, several bands at 3390-3164, 1669, 1428-1416 cm-1 and 1228-1203 cm-1 corresponding to the surface adsorption of NH3 and NH4+ decreased in spectra intensity starting from 50 °C on FW series catalysts [30,63,64]. Concurrently, the intensities of the bands belonging to bridge NO2- species (1329 cm-1) and nitrite species (1197 cm-1) decreased noticeably only on FW-IM [63,65]. The NO2- species (1329 cm-1) was easy to react with adsorbed NH3 species to form NH4NO2 species, which acted as an active intermediate in SCR process and was easy to decompose to N2 and H2O [63,65]. And absorbed NO2 species (1611 cm-1) were detected on FW-SG and FW-GR. If NO and NO2 were in equivalence concentrations subsistent, the overall SCR reaction rate was increased and the “fast

SCR” reaction occurs [53,59,60]. Furthermore, the intermediate of different FW catalysts were not exactly the same in the warming process. The intensity of the band at 1558 cm-1 increased between 50 and 150°C, and then decreased with increasing temperature on FW-GR and FW-ME. This indicated that the adsorbed NH4+ ions were oxidized or deprotonated to form NH2 or hydrazine, and then it reacted with the NOx species further decomposed to N2 and H2O [5,66]. Meanwhile, the intensities of the band belonging to the NH4NO3 species (1573 cm-1) decreased significantly, and disappeared at 300 °C on FW-IM. According to literature [53], N2O originated from NH4NO3 at low temperature and NH3 oxidation at high temperature. Since 200 °C was the onset temperature of the NH4NO3 decomposition reaction. This might be the reason that the amount of N2O produced by FW-IM catalyst was higher than that of other FW catalysts. 3.9. Discussion In FeOx-WO3 bimetallic composite oxide catalysts, the addition of WO3 was found to restrain crystallization of Iron oxide [27,28]. The results of XRD and Raman analyses showed that iron and tungsten elements existed in the form of low crystalline Fe2O3, highly dispersed WO3 and FeWO4 species. In the FW-GR and FW-SG catalysts, low crystallinity of Fe2O3 played a successive role in increasing the specific surface area of material, and then influenced the dispersion of active species. Therefore, the surface adsorption and activation properties of FW catalyst were affected. According to NH3-TPD and in situ DRIFT adsorption tests, the capacity of adsorbed ammonia and NOx species of FW-GR sample was much higher than that of other FW catalysts. The results of XPS showed that the binding energy of W4f7/2 for FW-IM (35.56 eV) and FW-ME (35.68 eV) presented a slower blue shift compared with FW-GR (35.52 eV) and FW-SG (35.47 eV), implying the strong interaction between iron and tungsten atom on the catalysts prepared by grinding and sol-gel methods. The ratio of Fe2+ / Fe2+ + Fe3+ was in the following sequence FW-GR (65.9%) > FW-SG (57.5%) > FW-ME (53%) > FW-IM (46.3%). The increase of Fe2+ contents contributed to the interaction between iron and the surrounding W atoms, forming the solid solution

(containing Fe-O-W structures). Moreover, W species partially doped into the FeOx matrix and modified the electronic properties of the FeOx structure [27], which caused the defect structure of FeOx, and promoted the formation of oxygen vacancies [47]. On the other hand, the Oα / (Oα + Oβ) ratio was 28.1%, 22.5%, 27.8% and 25.4% for FW-GR, FW-IM, FW-SG and FW-ME, respectively. Since higher oxygen mobility of Oα compared with Oβ, it was expected that the redox ability of FW-GR and FW-SG was enhanced, which was further proved by H2-TPR results. The reduction temperatures of peaks that attributed to the reduction of FeOx shifted to lower temperatures on FW-GR and FW-SG, indicating the reduction property of FW-GR and FW-SG was optimal. Our study indicates that the surface structure and physicochemical property significantly affects NH3-SCR reaction path and activity of the FW catalysts. Activity test results showed that the FW-GR and FW-SG catalysts could obtain more than 90% NO conversions at 225-500 °C. The low N2O concentration (below 38 ppm) with small differences was detected over the FW catalysts. Based on the in situ DRIFT studies of the SCR process over the FW catalysts, specific reaction pathways could be inferred. For FW catalysts, combined with in situ DRIFTS results of the reactivity, it was concluded that the in situ formed adsorbed NOx specie, coordinated NH3 and ionic NH4+ were regarded to be active species might be participated in the SCR reaction on FW catalysts [63,64]. In the low temperature range, the NH2 from the reaction between surface ammonia and nitrates were the key intermediates and further reduced to form N2 and H2O by the NO gas on FW catalysts except for FW-IM. For FW-IM catalyst, the NH4NO3 species reacted with the NOx species further decomposed to form N2O. The NO2- species (1329 cm-1) combined with adsorbed NH3 species to form NH4NO2 and then decomposed to N2 and H2O. So summarizing that in the high temperature range, the adsorbed NH3 species gradually became more important in the NH3-SCR reaction. Below (< 300 °C), the Langmuir-Hinshelwood (L-H) at low temperature and Eley-Rideal (E-R) at high temperature (> 300 °C, adsorbed NOx species drastically consumed at 300 °C) for the NH3-SCR process over FW catalysts. Similar phenomena have also been reported [30,63-65]. In addition,

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Graphical Abstract Iron-tungsten composite oxides catalyst prepared by grinding methods (FW-GR) exhibits excellent catalytic activity for SCR NO by NH3. The FW-GR catalyst possessed higher surface acidic sites and acidic strength, the main reaction mechanisms follows the “L-H” at low temperature and “E-R” at high temperature.

Highlights 1. The iron-tungsten catalysts prepared by grinding and sol-gel methods exhibited high activity for the NH3-SCR of NOx. 2. The NO conversion was strongly related to the FeWO4 species. 3. The iron-tungsten catalysts prepared by grinding leads to increased Lewis and Brønsted acid sites and proper redox property. 4. Grinding method was simple and environmentally friendly.