FeOx-VOx-WOx-MnOx-CeOx/TiO 2 as a catalyst for selective catalytic reduction of NOx with NH 3 and the role of iron

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1 Indian Journal of Chemistry Vol. 54A, June 2015, pp FeOx-VOx-WOx-MnOx-CeOx/TiO 2 as a catalyst for selective catalytic reduction of NOx with NH 3 and the role of iron Su Yu a, *, Dong Guojun*,b, Zhao Yuan b, Zhang Yufeng b & Wang Yajie b a School of Chemistry and Materials Science, Heilongjiang University, Harbin, , PR China suyu-0451@163.com b College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin, , PR China dgj1129@163.com Received 3 February 2015; revised and accepted 29 May 2015 The preparation of FeOx-VOx-WOx-MnOx-CeOx/TiO 2 catalyst by the ultrasound-substep-wet impregnation method for removing NOx from diesel engine exhausts has been demonstrated along with the excellent low-temperature selective catalytic reduction activity over a broad temperature window of C. The specific surface area, catalyst structure, composition and SCR reactivity tests of NO with NH 3 at low temperatures have been investigated by nitrogen adsorption, X-ray diffraction, X-ray photoelectron spectroscopy, H 2 temperature programmed reduction, NH 3 temperature programmed desorption and in situ DRIFTS measurements. The results show that the excellent activity is due to the well-dispersed active components, the moderate ratio of V 4+ /V 5+ and (Mn 2+ +Mn 3+ )/Mn 4+, the excellent redox property and the abundant acid sites. Iron plays the important role of inhibiting the agglomeration of metal oxide during the catalyst sintering and converting V and Mn to their higher oxidation states. Keywords: Catalysts, Oxides, Nitrogen oxides, Reduction, Iron, Vanadium, Tungsten, Manganese, Cerium, Titanium Nitrogen oxides (NOx) are the major atmospheric pollutants which can produce several environmental problems due to their physical and chemical effects, such as photochemical smog, acid rain and ozone hole, etc. 1,2 Nowadays, selective catalytic reduction (SCR) of NO with ammonia is the most efficient technique for abating NO emission from stationary sources. Low-temperature SCR of nitrogen oxides with ammonia is a promising technique to remove NOx in mobile sources. The commercial catalyst V 2 O 5 -WO 3 (MoO 3 )/TiO 2 has high activity and stability in SCR of NO with ammonia under o C 3,4. However, SO 2 and the high concentration of ash, e.g., K 2 O, CaO and As 2 O 3, in flue gas reduce the catalysts performance and durability because the catalyst is always placed prior to dust precipitation and flue gas desulphurization systems 5. Therefore, focus is on developing low-temperature catalysts capable of being effective at the end of the exhaust system, without requiring reheating. Recently, Mn-based catalysts have been reported to have excellent activity in low-temperature SCR reactions 6-8. Mn 4+ species and their redox process may be the reason for the high activity in the SCR reaction of NO with NH 3 at low temperature Specifically, it has been demonstrated that MnOx-CeO 2 and MnOx- TiO 2 catalysts have high activity in the SCR of NO by NH 3 at low temperature However, the activity of Mn-based catalysts decreased significantly during SCR reaction in the presence of SO As a promoter, CeO 2 has been studied extensively for the SCR of NO by NH 3 due to the large oxygen storage, the release capacity and the unique redox couple Ce 3+ /Ce 4+ which has the ability to shift between CeO 2 and Ce 2 O 3 under oxidizing or reducing conditions, respectively 15. Addition of Ce can also promote the sulfur resistance of Mn/TiO 2 catalysts. Iron oxide is widely used in catalytic applications either as a promoter or as an active component, and most of the iron-containing SCR catalysts are Fe 2 O 3 - loaded or Fe 3+ -exchanged zeolites types. Brückner et al. 16 proposed that the accessible Fe 3+ species wase the active component in the SCR of NO with NH 3. Qi and Yang 17 observed that the addition of Fe could improve the low-temperature SCR activity of the Mn/TiO 2 catalyst through enhancing the oxidation of NO to NO 2. Wu et al. 18 investigated the effect of transition metal addition on the manganese-based catalyst for SCR of NOx and found that iron had the most favorable effect on the catalytic activity. They proposed that the addition of iron significantly improved the dispersion of Mn and Ti and thus

2 YU et al.: ROLE OF Fe IN NH 3 -SCR OF NOx WITH FeOx-VOx-WOx-MnOx-CeOx/TiO enhanced the low-temperature activity of the Mn/Ti catalyst. Therefore, iron may be a good candidate to improve the low-temperature SCR activity. As V 2 O 5 -WO 3 /TiO 2 and MnOx-CeOx/TiO 2 catalysts exhibit different excellent properities in NH 3 -SCR reaction, we combined both together by ultrasound-substep-wet impregnation method with iron as a promoter. The performance of the FeOx-VOx-WOx-MnOx-CeOx/TiO 2 catalyst in NH 3 - SCR reaction and the role of iron in the studied catalysts systems have been investigated herein. Materials and Methods Preparation of catalysts All the catalysts studied in this work were prepared by an ultrasound-substep-wet impregnation method. For the preparation of FeOx-VOx-WOx-MnOx- CeOx/TiO 2 catalyst, the oxalate was first dissolved in distilled water. Then, ammonium metavanadate (NH 4 VO 3 ) and ammonium tungstate hydrate (H 40 N 10 O 41 W 12 xh 2 O) were added into the solution successively keeping the mass ratio of V 2 O 5 :TiO 2 = 1%, WO 3 :TiO 2 =3%. When NH 4 VO 3 and H 40 N 10 O 41 W 12 xh 2 O were dissolved completely, Mn, Ce, Fe were added separately into the V-W solution step by step keeping the molar ratio of Mn:Ti = 0.4, Fe:Ti = 0.1, Ce:Ti = Throughout the process, the solution needed ultrasound after each component was added into the solution until the solution became transparent. Lastly, a commercial carrier, anatase TiO 2 powder, was added into the solution. Then the solution was desiccated at 100 o C for 12 h in the presence of air and calcinated at 500 C for 4 h in presence of air. The calcined sample was crushed and sieved to mesh. The preparation method of the other four catalysts, viz., V 2 O 5 -WO 3 /TiO 2, FeOx-VOx- WOx/TiO 2, MnOx-CeOx/TiO 2, FeOx-MnOx-CeOx/TiO 2 was similar to that for the FeOx-VOx-WOx-MnOx - CeOx/TiO 2 catalyst; the detailed component ratios are listed in Table 1. Characterization of catalysts BET surface areas of the catalysts were measured by N 2 adsorption-desorption using a Builder SSA instrument (Builder Company, China). The X-ray photoelectron spectroscopy (XPS) measurements were made using a K-Alpha spectrometer (Thermo Fisher SCIENTIFIC, America). Al Kα radiation (1486.6eV) was used as the source, with vacuum degree as kpa, working voltage as 12.5 kv and current as 20 ma. The X-ray powder diffraction (XRD) patterns were examined on a Rigaku D/max-TTR-III diffractometer at a scan rate of 10 /min in the 2θ range from using Cu Kα radiation (λ = nm). H 2 temperature-programmed reduction (TPR) and NH 3 temperature-programmed desorption (TPD) were conducted using a home-made GC-TCD setup (GC-14C, Shimadzu). About 100 mg of the sample was pretreated in argon flow at 300 C for 1 h and cooled to room temperature prior to temperature programmed runs. For the NH 3 -TPD experiment, the NH 3 adsorption on the catalyst was performed with feeding anhydrous NH 3 gas for about 10 min at the room temperature. Then the catalyst was sufficiently purged by argon stream to remove the excessive adsorbate. The TPD was carried out by heating the sample in argon (80 ml/min) from 50 C to 600 C with a heating rate of 10 C/min. The TPR method was used to measure the redox performance of the catalysts and was carried out with a linear heating rate (10 C/min) from 50 C to 800 C in a mixture of 5% H 2 -Ar at a flow rate of 30 ml/min. In situ DRIFTS measurements The in situ DRIFTS experiments were performed on an FTIR spectrometer (Nicolet 6700) equipped with a Harrick DRIFT cell containing KBr windows and MCT detector. Prior to each experiment, the sample was pretreated at 300 o C for 1 h in a flow of argon. The background spectrum was collected in flowing argon and automatically subtracted from the sample spectrum. The reaction conditions were controlled as follows: 120 ml min -1 total flow rate, 0.1% (v/v) NH 3, and argon balance. All spectra were recorded by accumulating 100 scans with a resolution of 4 cm -1. Table 1 Component ratios of the studied catalysts Samples H 2 C 2 O (g) V 2 O 5 /TiO 2 (g) WO 3 /TiO 2 (g) Mn/Ti (mol) Fe/Ti (mol) Ce/Ti (mol) TiO 2 (g) VOx-WOx/TiO FeOx-VOx-WOx/TiO MnOx-CeOx/TiO FeOx-MnOx-CeOx/TiO FeOx-VOx-WOx-MnOx-CeOx/TiO

3 746 INDIAN J CHEM, SEC A, JUNE 2015 Catalytic activity The activity of the catalyst was carried out with a fixed-bed tubular reactor operating at the atmospheric pressure from 150 C to 430 C with the heating rate of 10 C/min. The gas flow was controlled by the mass flow meters. The reaction temperature was measured by a type K thermocouple inserted into the catalyst bed. A mixture of 0.1% (v/v) NO, 0.11% (v/v) NH 3 and 3% (v/v) O 2 in Ar was passed through 0.4 g catalyst at a flow rate of 120 ml min -1. The inlet and outlet concentrations of NO, N 2 O and NO 2 were measured by a gas analysis system, PFEIFFER GSD320 (Thermo Star TM, Germany). NOx conversion (%) was obtained by the following equation: [NO] [NO] [NO ] 2[N O] = in out 2 out 2 out NO x conversion (%) 100% [NO] in Results and Discussion Catalytic activity Figure 1 shows the catalytic activities during the reduction of NO by NH 3 carried out over different samples. Initially, both V 2 O 5 -WO 3 /TiO 2 and FeOx-VOx- WOx/TiO 2 catalysts exhibit a poor low temperature activity which increased quickly when the temperature was above 250 C. However, the NO conversion of V 2 O 5 -WO 3 /TiO 2 catalyst was always under 80% even at high temperatures. Fe-V-W/TiO 2 catalyst exhibited a higher NO conversion than V 2 O 5 -WO 3 /TiO 2 at high temperature regions, and reached nearly 100% when the temperature was above 400 o C. On the other hand, MnOx-CeOx/TiO 2 and Fe-Mn-Ce/TiO 2 catalysts, exhibited a significant low temperature activity, but showed poor activity at high temperatures due to the high oxidation activity of MnOx resulting in the NH 3 oxidation by oxygen 19. The addition of iron not only increased the low temperature activity of MnOx-CeOx/TiO 2 catalyst but also improved its high temperature activity. The NO conversion of FeOx-VOx-WOx-MnOx-CeOx/TiO 2 composite catalyst was always above 80% at low temperature and reached the maximum of 93% at 270 o C. The activity of FeOx- VOx-WOx-MnOx-CeOx/TiO 2 was considerably higher than that of the other four catalysts, and the temperature window was broadened and shifted to lower temperature. Therefore, the promoter iron improved both Mn-Ce/TiO 2 and V-W/TiO 2 catalysts activity at different temperature regions. Phase analysis of the catalysts Figure 2 shows the XRD patterns of the prepared samples. It can be seen that similar diffraction peaks can be observed in all catalysts, and the main diffraction peaks can be directly indexed to the anatase phase TiO 2. Diffraction peaks at 25.2 and 37.8 correspond to the anatase phase TiO 2 (101) and (004). No crystal phase of active components emerged, which indicated that all the active components were highly dispersed on the catalysts surface. This may be attribute to the moderate calcination temperature and the moderate content of the active components. Both the anatase phase TiO 2 and well dispersed active components are beneficial to the NH 3 -SCR DeNOx activity. BET studies The specific surface area and the number of atoms on unit area of the catalysts are summarized in Table 2. Compared to TiO 2 (126 m 2 /g), specific surface area of Fig. 1 Effect of reaction temperature on NO conversion for urea- SCR over different catalysts. [React. cond.: 0.11% NH 3 ; 0.1% NO; 3% O 2 ; GHSV=30000h -1 ]. Fig. 2 XRD patterns of different catalysts.

4 YU et al.: ROLE OF Fe IN NH 3 -SCR OF NOx WITH FeOx-VOx-WOx-MnOx-CeOx/TiO Table 2 Specific surface area and the number of surface atoms per unit area of the catalysts Samples A BET Comp. (wt%) Number of surface atoms per unit area (1/nm 2 ) (m 2 /g) V W Mn Ce V W Mn Ce V+W Mn+Ce TiO VOx-WOx/TiO FeOx-VOx-WOx/TiO MnOx-CeOx/TiO FeOx-MnOx-CeOx/TiO FeOx-VOx-WOx-MnOx-CeOx/TiO all the catalysts decreased significantly because the activity components possess abundant holes on the TiO 2 surface. Hence, the BET surface area generally decreased after loading the activity components on the TiO 2. The BET surface area was not decreased by adding iron, which may reduce the exposure area of TiO 2. Conversely, the addition of iron increased the BET surface area of V 2 O 5 -WO 3 /TiO 2 and MnOx- CeOx/TiO 2 catalysts indicating that iron promoted the active components dispersion. The atomic ratios on catalysts surface from XPS results are listed in Table S1 (Supplementary Data). The number of atoms on unit area was calculated as N number = WN A / MA BET, where N number denotes the atom number of elements on unit area, W represents weight percentage composition of elements (wt%), N A is Avogadro constant, M is relative atomic mass of elements, and A BET is BET surface area. The number of atoms on the catalyst surface (Table 2)increased while N number decreased which proves conclusively that the enhancement of coverage of the components as and good dispersion of the components on the surface were promoted by the addition of iron. This is due to the presence of Fe 2 O 3 around MnOx and V 2 O 5 on the catalyst surface which inhibited the agglomeration of metal oxide during the catalyst sintering, and enhanced transition metal dispersion 20,21 resulting in increase of the BET surface area. On improving the dispersion and the coverage of the active components, more active acid sites would be exposed and shall promote the SCR activity, which is in accordance with NH 3 -TPD results. In addition, the specific surface area of Mn series catalysts were generally higher than V series catalysts because V blocked the aperture more easily than Mn, which is in accordance with the conclusions of Vogt 22 and Motonobu Kobayshi 23 that large loading amount of vanadium can decrease the specific surface area. The catalytic activity and the BET results show that high specific surface area would improve the SCR DeNOx activity. The SCR activity was also influenced by other more important factors, which is why the FeOx-VOx-WOx-MnOx-CeOx/TiO 2 catalyst was indeed more active than FeOx-MnOx-CeOx/TiO 2 even while showing a little lower surface area. XPS analysis Surface information on catalysts about Fe 2p, Mn 2p and V 2p were evaluated by XPS (Fig. 3). (The ratios of V 4+ /V 5+ and (Mn 2+ +Mn 3+ )/Mn 4+ species determined by XPS are listed in Table S2, Supplementary Data). The binding energy of ev and ev are attributed to V and V (Fig. 3a), indicating that vanadium exists on the surface mainly in the form of V 4+ and V 5+ (Ref. 25). The binding energy centered at about ev, ev and ev may be assigned to Mn 4+, Mn 3+ and Mn 2+ according to the literature 26,27 (Fig. 3b). The Fe 2p spectra exhibited in Fig. 3c show the binding energy centered at ~711.4 ev, which may be assigned to Fe 3+ and the binding energy centered at ~712.6 ev ascribed to Fe 3+ bonded with hydroxyl groups ( Fe III -OH). This assignment is supported by the satellite component observed at ~719 ev, which is the fingerprint of the Fe 3+ species 28. The binding energy 29 at 710 ev and ev may be attributed to Fe 2+. High ratio of V 4+ /V 5+ was proposed to facilitate the SCR activity by Abon 30 and Broclawik 31. However, we found V 4+ /V 5+ ratio decreased with the addition of iron (Table S2), indicating that the addition of iron enhanced the oxidation state of V on the catalyst surface. The ratio of V 4+ /V 5+ (2.91) in the V-W/TiO 2 catalyst with poor activity was much higher than that of Fe-V-W/TiO 2 (1.43) and Fe-V-W-Mn-Ce/TiO 2 (0.96) catalysts with better activity. According to this result, it can be inferred that increasing the V 4+ concentration can improve the denox activity when V exists mainly in the form of V 5+ on the catalyst surface. When V exists mainly in the form of V 4+ on

5 748 INDIAN J CHEM, SEC A, JUNE 2015 Fig. 3 XPS spectra of the catalysts. [(a) V2P 3/2 ; (b) Mn2P 3/2 ; (c) Fe2P 3/2 ]. the surface, the denox activity improved with increasing the V 5+ concentration. Hence, we believe that the V 4+ /V 5+ ratio close to 1.0 on the surface is best for the SCR activity. In our previous study, we found that the V-W-Cu-Mn-Ce-Ti-O/CC catalyst with V 4+ /V 5+ ratio of 1.12 had better activity 32. This is also very close to 1.0, and in accordance with our present hypothesis. It can also be seen that on adding iron to Mn-Ce/TiO 2 catalyst, the oxidation state of Mn was enhanced and decreased the (Mn 2+ +Mn 3+ )/Mn 4+ Ratio. In addition, the ratio of (Mn 2+ +Mn 3+ )/Mn 4+ in Fe-V-W-Mn-Ce/TiO 2 catalyst with best activity was 0.99, that is to say the concentration of manganese in low state and in high state were approximately the same. We believe that when the concentrations of the elements in low oxidation state and in high oxidation state were approximately the same, the oxidation reaction rate and the reduction reaction rate would be balanced, which is an advantage for the redox reaction cycle. Since the redox elements existed as V 5+ /V 4+, Mn 4+ /(Mn 3+ +Mn 2+ ) and Fe 3+ /Fe 2+ pairs on the surface, such redox electron pairs exhibit excellent potential redox ability, which is higher than the inserting O species and can also facilitate electron and oxygen transfer. H 2 -TPR analysis H 2 -TPR profiles of various catalyst samples are showed in Fig. 4. The amount of H 2 consumed by the catalyst samples is given in the following sequence: FeOx-VOx-WOx-MnOx-CeOx/TiO 2 (210 µmol g -1 ) > FeOx-MnOx-CeOx/TiO 2 (146 µmolg -1 ) > MnOx- CeOx/TiO 2 (140 µmol g -1 ) > FeOx-VOx-WOx/TiO 2 (68 µmol g -1 ) > V 2 O 3 -WO 3 /TiO 2 (65 µmol g -1 ). The two reduction peaks at 516 C and 800 C characterized the reduction profile of V 2 O 3 - WO 3 /TiO 2. The reduction peak at 516 o C corresponded to the reduction of V 5+, and the peak at 800 o C was attributed to the reduction of WO 3. After adding iron to V 2 O 3 -WO 3 /TiO 2 catalyst, a new

6 YU et al.: ROLE OF Fe IN NH 3 -SCR OF NOx WITH FeOx-VOx-WOx-MnOx-CeOx/TiO Fig. 4 H 2 -TPR profiles of different catalysts. [1, FeOx-VOx- WOx-MnOx-CeOx/TiO 2 ; 2,. FeOx-MnOx-CeOx/TiO 2 ; 3, MnOx- CeOx/TiO 2 ; 4,. FeOx-VOx-WOx/TiO 2 ; 5, V 2 O 5 -WO 3 /TiO 2. Expt. cond.: 5 % H 2 -Ar at a flow rate of 30 ml/min.]. reduction peak at 568 o C emerged which corresponded to the reduction of Fe 3+. The peak at 800 o C disappeared which may due to the enhanced interaction between WO 3 and TiO 2 that made it more difficult to reduce WO 3, while the reduction peak at 516 o C is attributed to V 5+ shifted to the lower temperature. However, in SCR reaction mechanism, V 5+ reacts with NH 3 to form intermediate species, and hence V 5+ transfers to V 4+ is the key process in the SCR reaction, and has little to do with the reduction of WO 3. Therefore, Fe-V-W/TiO 2 catalyst exhibits higher activity than V 2 O 3 -WO 3 /TiO 2 catalyst. The profile of MnOx-CeOx/TiO 2 catalyst showed two peaks at 403 o C and 530 o C. The reduction peak at 403 o C can be attributed to the two reduction processes of MnO 2 to Mn 2 O 3 and Mn 2 O 3 to Mn 3 O The reduction peak at 530 o C corresponds to the reduction of Mn 3 O 4 to MnO 33. The reduction temperature 34, 35 of CeO 2 and TiO 2 are both above 550 o C, and hence the reduction peaks in the profiles do not include the reduction peak of CeO 2 and TiO 2 (Fig. 4 (curves 1, 2, 3)). After adding iron into Mn-Ce/TiO 2 catalyst, all the reduction peaks of MnOx-CeOx/TiO 2 were shifted to low temperature. The peak at 352 o C still belongs to the reduction of MnO 2 to Mn 2 O 3, but the peak at 381 o C probably includes the reduction process of Fe 3 O 4 to FeO instead of that of Mn 3 O 4 to MnO 36. The peak at 530 o C which belongs to Mn 3 O 4 to MnO is shifted to 156 o C, while the small peak above 500 o C is attributed to Fe 3 O 4 to FeO 37,38. As the reduction ability of Fe n+ is much weaker than that of Mn n+39, the addition of iron has little Fig. 5 TPD spectra of NH 3 over the catalysts. [1, FeOx-VOx- WOx-MnOx-CeOx/TiO 2 ; 2, FeOx-MnOx-CeOx/TiO 2 ; 3, MnOx- CeOx/TiO 2 ; 4, FeOx-VOx-WOx/TiO 2 ; 5, V 2 O 5 -WO 3 /TiO 2. Expt. cond.: inlet pure NH 3 10/min -1 ]. influence on the H 2 consumption. Hence the major H 2 consumption is still attributed to the reduction of Mn n+. The introduction of iron promoted the dispersion of Mn species, which made it easier to reduce manganese oxide.,this may be the primary reason why all the reduction peaks of Fe-Mn-Ce/TiO 2 catalyst were shifted to the lower temperature as compared to with MnOx-CeOx/TiO 2 catalyst. The reduction temperature of V 5+ is much higher than Mn n+, hence, we attribute the reduction peak at 361 o C (Fig. 4, curve 1) to the reduction of Mn n+ and the peak at 455 o C to the reduction of V 5+. According to the concentration of various ions from XPS results (Table S2, Supplementary Data), the concentration of V 5+ and Mn 4+ in FeOx-VOx-WOx-MnOx-CeOx/TiO 2 catalyst is much higher than that of the other four catalysts, thus the two peaks at 361 o C and 455 o C are much stronger, showing FeOx-VOx-WOx-MnOx- CeOx/TiO 2 catalyst to have excellent redox ability. H 3 -TPD studies The acidic sites distribution of the catalysts was determined by ammonia TPD measurements. The TPD patterns of the samples are shown in Fig. 5 (The adsorption amount of NH 3 calculated according to TPD results are listed in Table S3, Supplementary Data). It has been established in the literature 40,41 that the desorption temperature of physisorbed NH 3 is usually below 100 o C. The desorption peaks below 100 o C of the samples were all very small indicating that the physisorbed NH 3 has been bellowed off by

7 750 INDIAN J CHEM, SEC A, JUNE 2015 argon before the temperature programmed process. The desorption peak at 140 o C,320 o C,and 520 o C are attributed to weak acid sites, mid-strong acid sites and strong acid sites respectively. As Ti has large specific surface area and better NH 3 adsorption ability than Mn and V compounds, the weak acid sites at 140 o C were probably supplied by Ti 4+ (Ref. 42). In addition, V and W on the catalyst surface can also provide partial weak acid sites, the area of this weak acid peak of V series catalysts being larger than Mn series. After adding iron, the acidic sites distribution became broaden and the area of the peak became larger since iron improved the dispersion of active compounds and exposed more active sites. For the Mn series catalysts, the peak at 350 o C belonged to the mid-strong acid sites created by Mn n+ (Ref. 43). The added iron exists on the catalyst surface in the form of Fe 3+ which may create strong acidity of Lewis acid sites 43. This is probably why the peak at the high temperature (>500 o C) became larger after the addition of iron. It is to support this statement, that the adsorption and desorption characteristics of NH 3 over various catalysts were investigated using the in situ FTIR measurements in Fig. 6. For the FeOx-VOx-WOx-MnOx-CeOx/TiO 2 catalyst, the active compounds were well dispersed on the catalyst surface, and the interaction between the transition metal not only enriched the acidic sites with different intensity but also increased the amount of NH 3 adsorption. This may be a key issue for Fig. 6 DRIFTS spectra of different catalysts. exposed to 1000 ppm NH3 at 70 o C. [1, V 2 O 5 -WO 3 /TiO 2 ; 2, FeOx-VOx- WOx/TiO 2 ; 3, MnOx-CeOx/TiO 2 ; 4, FeOx-MnOx-CeOx/TiO 2 ; 5, FeOx-VOx-WOx-MnOx-CeOx/TiO 2. Expt. cond.: exposed to 1000 ppm NH3 at 70 C]. broadening of the reactive temperature window of FeOx-VOx-WOx-MnOx-CeOx/TiO 2 catalyst. In-situ FTIR studies In situ FTIR experiments were performed to gain a better understanding of the molecular behavior of ammonia and to acquire information about surface species (Fig. 6). The peaks at 1200 cm -1, 1450 cm -1, 1620 cm -1, 1700 cm -1 were observed for all the catalysts. The adsorption bands at 1450 cm -1 and 1700 cm -1 were assigned to the ammonium ions (NH + 4 ) on the Brønsted acid sites 44,45, while 1200 cm -1 and 1620 cm -1 may be assigned to the coordinated NH 3 on the Lewis acid sites 46. As we can see from FTIR spectra of the V series catalysts, Brønsted acid sites on the surface are much stronger than in the other catalysts because V and W provided a large number of Brønsted acid sites. Hence, we attribute the weak acid sites peak at the low temperature in NH 3 - TPD profile to the Brønsted acid sites which play an important role at high SCR reaction temperature. This is probably why the V series catalysts present better high temperature activity than other series catalysts. In the Mn series catalysts despite the Brønsted acid sites being weaker, the Lewis acid sites are much stronger than in V series catalysts. Thus, the mid-strong acid site created by Mn n+ in NH 3 -TPD profiles was assigned as weak Lewis acid. When iron was added to the catalysts, the NH 2 peak became stronger, apparently because Fe 3+ would facilitate NH 3 to form -NH 2 at high temperatures. Hence, the high temperature peak (>500 o C) in NH 3 -TPD profiles was attributed to strong Lewis acid. As Fe 3+ provided stronger Lewis acid sites than Mn n+, the high temperature peak of TPD profiles increased with the addition of iron. The position of the peaks in FTIR profiles are + different due to the different induction effects to NH 4 and NH - 2 caused by different transition metals. The Lewis acid sites and Brønsted acid sites, both abundant on Fe-V-W-Mn-Ce/TiO 2 catalyst, showed in FTIR profiles which is in agreement with the NH 3 -TPD results. Conclusions FeOx-VOx-WOx-MnOx-CeOx/TiO 2 catalyst, prepared by ultrasound-substep-wet impregnation method, has been developed for removing NOx from diesel engine exhausts. The catalyst showed excellent activity over a broad temperature window of o C. The XRD and BET results showed that the active components were well dispersed on the catalyst surface.

8 YU et al.: ROLE OF Fe IN NH 3 -SCR OF NOx WITH FeOx-VOx-WOx-MnOx-CeOx/TiO The elements existed on the catalyst surface in different oxidation states, which contributed to the transfer of the electron and oxygen. The ratio of V 4+ /V 5+ and (Mn 2+ +Mn 3+ )/Mn 4+ were both close to 1.0, when the catalyst showed the best activity. In addition, the FeOx- VOx-WOx-MnOx-CeOx/TiO 2 catalyst exhibited excellent redox property as shown by H 2 -TPR measurements. NH 3 -TPD measurements showed that there were abundant acidic sites on the catalyst surface with different intensities which may be the real reason why it had the broad active temperature window. Iron existed in the form of Fe 2 O 3 on the catalyst surface. The presence of Fe 2 O 3 around the metal oxide on catalyst surface may inhibit the agglomeration of metal oxide during the catalyst sintering process. With the addition of iron, the oxidation state of V and Mn were changed to the higher state. The reduction peak of both V 5+ and Mn n+ are shifted to lower temperature, which indicated that the addition of iron increased the redox property. Supplementary Data Supplementary data associated with this article, i.e., Figs S1 S3, are available in the electronic form at Acknowledgment This work was supported by Fundamental Research Funds for the Central Universities (HEUCF ) and Advanced Technique Project Funds of The Manufacture and Information Ministry, PR China. References 1 Mauzerall D L, Sultan B & Kim N, Atoms Environ, 39 (2005) Twigg M V, Appl Catal B: Environ, 70 (2007) 2. 3 Oliver K & Martin E, Appl Catal. B: Environ, 75 (2008) Maria A L V, Marzia C, Alessandro T & Guido B, Appl Catal B: Environ, 75 (2007) Tang F S, Xu B L & Shi H H. Appl Catal B: Environ, 94 (2010), Huang J H, Tong Z Q & Huang Y, Appl Catal B: Environ, 78(2008), Reddy E P, Neeraja E, Sergey M, Boolchand P & Smirniotis P G, Appl Catal B: Environ, 76 ( 2007) Peña D A, Uphade B S & Smirniotis P G, J Catal, 221 (2004) Boningari T, Panagiotis G & Smirniotis, J Catal, 288 (2012) Mahnaz P, Abdolsamad Z M, Alimorad R, Jafar T & Yadollah M, Appl Surf Sci, 279 (2013) Chizhong W, Shijian Y, Huazhen C, Yue P & Junhua L, J Mol Catal A: Chem, 376 (2013), Sang M L, Kwang H P & Sung C H, Chem Eng J, (2012) Xie W, Yuying Z & Jinxian L, Catal Commun, 37 (2013) Tang N, Liu Y, Wang H & Wu Z, J Phys Chem C, 115 (2011) Li J F, Yan N Q, Qu Z, Qiao S H, Yang S J, Guo Y F, Liu P & Jia J P, Environ Sci Technol, 44 (2010) Kumar M S, Brückner W & Brückner A J Catal, 239 (2006) Gongshin Q, Ralph T & Yang, Appl Catal B: Environ, 44 (2003) Wu Z B & Jiang B Q, Appl Catal B: Environ, 79 (2008) Krishna K, Seijger G B F, van den Bleek C M & Calis H P A, Chem Commun, 10 (2002) Qi G & Yang R T, Appl Catal B: Environ, 44 (2003) Morikawa A, Suzuki T & Kanazawa T, Appl Catal B: Environ, 78 (2008) Vogt E T C, Boot A, Vandille A J, Geus J W, Janssen F J J G & Vonden K F M G, J Catal, 114 (1988) Motonobu K, Appl Catal B: Environ, 60 (2005) Mestl G, Catal Rev, 40 (1998) Najubar M, Brocławik E, Gȯra A, Camra J, Białas A & Wesełucha-Birczyńska A, Chem Phys Lett, 325 (2000) Li X H, Zhang S L, Jia Y, Liu X X & Zhong Q, J Nat Gas Chem, 21 (2012) Gillot B, Buguet S, Kester E, Baubet C & Tailhades P, Thin Solid Films, 357 (1999) Shijian Y, Yongfu G, Naiqiang Y, Daqing W, Hongping H, Jiangkun X & Zan Q, Appl Catal B: Environ, 101 (2011) Descostes M, Mercier F, Thromat N, Beaucaire C & Gautier-Soyer M, Appl Surf Sci, 165 (2000) Abon M, Herrmann J M & Volta J C, Catal Today, 71 (2001) Broclawik E, Gora A & Najbar M, J Mol Catal A: Chem,,166 (2001) Dong G J, Luo X, Yang Z, Liang T & Zhang W O, J Fuel Chem Tech, 37 (05) (2009) Ettireddy P R, Ettireddy N, Mamedov S, Boolchand P & Smirniotis P G, Appl Catal B: Environ, 76 (2007) Sun Y, Hla S S, Duffy G J, Cousins A J, French D, Morpeth L D, Edwards J H & Roberts D G, Int J Hydrogen Energ, 36 (2011) Zhu H, Qin Z, Shan W, Shen W & Wang J, J Catal, 225 (2004) Reddy A S, Chen C Y, Chen C C, Chien S H, Lin C J, Lin K H, Chen C L & Chang S C, J Mol Catal A, Chemical, 2010, 318 (1/2): Jerzy Z, Ilona Z, Leszek Z & Zbigniew K, Appl Catal A: Gen, 2010, 381 (1/2) Hayshi H, Chen, L Z, Tago T, Masahiro K & Katsuhiko W, Appl Catal A: Gen, 231 (1/2)(2002) Yang W, Zhang R, Chen B, Bion N, Duprez D & Royer S, J Catal, 295(2012) Cheng L S, Yang R T & Chen N, J Catal, 164 (1996) Long R Q & Yang R T, J Catal, 207 (2002) Liu F, Li J & Woo S I, Energy Environ Sci, 5 (2012) Zhang R D, Yang W, Luo N, Li P X, Lei Z G & Chen B H, Appl Catal B: Environ, 146 (2014) Peña D A, Uphade B S, Reddy E P & Smirniotis P G, J Phys Chem B, 108 (2004) Centeno M A, Carrizosa I & Odriozola J A, Appl Catal B: Environ, 29 (2001) Liu F D, He H, Ding Y & Zhang C B, Appl Catal B: Environ, 93 (2009) 194.