Superior Performance of Fe1–xWxOδ for the Selective Catalytic

Article pubs.acs.org/est

Superior Performance of Fe1−xWxOδ for the Selective Catalytic Reduction of NOx with NH3: Interaction between Fe and W Hui Wang, Zhenping Qu,* Shicheng Dong, Hongbin Xie, and Chen Tang Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Sciences and Technology, Dalian University of Technology, Linggong Road 2, Dalian, 116024, China S Supporting Information *

ABSTRACT: Novel iron−tungsten catalysts were first developed for the selective catalytic reduction of NOx by NH3 in diesel exhaust, achieving an excellent performance with a wide operating temperature window above 90% NOx conversion from 225 or 250 to 450 °C (GHSVs of 30 000 or 50 000 h−1). It also exhibited a pronounced stability and relatively high NOx conversion in the presence of H2O, SO2 and CO2. The introduction of W resulted in the formation of α-Fe2O3 and FeWO4 species obtained by HRTEM directly. The synergic effect of two species contributed to the high SCR activity, because of the increased surface acidity and electronic property. The FeWO4 with octahedral [FeO6]/[WO6] structure acted as the Brønsted acid sites to form highly active NH4+ species. Combining DFT calculations with XPS and UV−vis results, it was found that the fine electron interaction between α-Fe2O3 and FeWO4 made the electron more easily transfer from W6+ sites to Fe3+ sites, which promoted the formation of NO2. Judging by the kinetics and SCR activity studies, the Fe0.75W0.25Oδ with an appropriate W amount showed the strongest interaction, and thereby the lowest activation energy of 39 kJ•mol−1 and optimal catalytic activity. These findings would be conducive to the reasonable design of NH3−SCR catalysts by adjusting the fabrication. and displayed a relatively excellent low-temperatureNH3−SCR activity, but its reaction operating temperature window is still narrow (200−325 °C) with above 90% NOx conversion. It is commonly accepted that the catalytic behavior of iron oxides is sensitive to the intrinsic nature of the iron species, including oxidation state, coordination circumstance, dispersion and stability.6,7 Therefore, researchers have been focusing on the structure modification of Fe2O3 lattice by addition of foreign ions (e.g., V, Mn, Ti, or Ce) to improve the textural, structural, acid−base, electronic and redox properties and thus catalytic activity. He et al.6 have reported the electron cloud of Fe3+ species in FeTiOx crystallite catalyst could be induced by titanium through the formation of Fe3+-(O)2-Ti4+structure and thus improved the redox properties of iron species. Tungsten is widely used as a stabilizer and promoter in the catalysts, which provides the high thermal stability, acidity and gives a wider temperature window for NH3−SCR reaction, the inhibition of NH3 oxidation.8 And meanwhile tungsten itself possesses the unique electronic properties, high melting point complementary to those of Fe2O3 in many ways. Luo et al. have highlighted the effect of interface modification which enhanced the

1. INTRODUCTION The preeminent fuel efficiency of diesel engines compared with their petrol counterparts comes at a cost of elevated pollutant emissions, such as nitrogen oxides (NOx) which have significant adverse effects on the environment as well as on humans, and therefore the removal of NOx is critical.1,2 Among the various technologies developed to reduce NOx from diesel engines, NH3−SCR could be accepted as a high potential technology to reduce NOx emissions from heavy-duty diesel engines in an excess of O2.1 Although WO3(MoO3)-V2O5/ TiO2 catalyst has been commercialized, some problems still exist for the extensive industry application, such as a narrow operation temperature window (300−400 °C), the toxicity of V2O5 to human health and ecological environment, and the high conversion of SO2 to SO3.3 Hence, it is highly desirable to develop an eco-friendly SCR catalyst with excellent activity and N2 selectivity in a broad temperature range, together with the prevention of the ammonia slip. Among the active components tested so far, iron-based catalysts particularly stand out due to its relatively high activity and N2 selectivity, the lower cost and higher abundance.4,5 However, the use of iron oxides catalysts for this application is especially challenging because of a relatively narrow temperature operating window, relatively poor thermal stability and the higher performance of ammonia oxidation.5,6 In our previous research,5 the Fe2O3/CNTs has been investigated © XXXX American Chemical Society

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July 24, 2016 November 2, 2016 November 18, 2016 November 18, 2016 DOI: 10.1021/acs.est.6b03589 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology interfacial charge transfer and thus promoted the photocatalytic activity of the [email protected] Herein, there is great potential for building iron−tungsten composite oxides to utilize the often complementary physical/chemical properties of these two dissimilar metal oxides. In the present study, we reported a facile “stepwise urea-assisted” method to design a novel Fe−W mixed oxides for the first time. Meanwhile, we systemically investigated its NH3−SCR performance under different working conditions. The iron−tungsten oxides catalysts presented the excellent NH3−SCR activity, high N2 selectivity and strong resistance against H2O CO2 and SO2 in a wide temperature range. The two kinds of species in the catalyst were distinguished and their contributions to the catalytic activity were also evaluated. These iron−tungsten catalysts could be a very competitive catalyst for the practical application in controlling the NOx emission from diesel engines.

Figure 1. NOx conversion as a function of temperature over various Fe1−xWxOδ catalysts (inset: N2 selectivity).

2. EXPERIMENTAL SECTION 2.1. Synthesis of Catalysts. Fe1−xWxOδ samples (x = 0.15, 0.25, 0.35, 0.50) were synthesized by “stepwise urea-assisted” method. Calculated amounts of Fe(NO 3 ) 3·9H 2O were dissolved in the deionized water at room temperature. Then amount of ammonium metatungstate was added to the solution to form suspension. Next, calculated amounts of urea were dissolved in the solution. The aqueous solution was aged at 90 °C for 24 h. Finally, the precipitate, after filtering, washing and drying, was calcined at 500 °C for 5 h. The details of the preparation process of Fe2O3 and WOx were described in the Supporting Information (SI). 2.2. Activity and Kinetic Measurement. Catalytic performance was evaluated by using a fixed-bed quartz tubular reactor. Reaction conditions were as follows: 500 ppm of NO, 500 ppm of NH3, 3 vol % O2, 10 vol % H2O (when used), 5 vol % CO2 (when used), 200 ppm of SO2 (when used), He balance and 300 mL/min total flow rate. The gas including NO, NH3, and NO2 were analyzed by a NO−NO2−NOx Analyzer (42iHL, Thermo Scientific), and the product N2 was detected by an online gas chromatograph (GC 7890II) equipped with a 5A molecular sieves column. The details of kinetic experiments were exhibited in the SI. 2.3. Catalyst Characterization and Density Function theory (DFT). The morphologies of the catalysts were studied using transmission electron microscopy (TEM, Tecnai G2 20STwin). In situ diffuse reflectance infrared Fourier transform spectra (in situ DRIFT) were collected from 800 to 4000 cm−1 at a spectral resolution of 4 cm−1 on a Bruker Vector FTIR spectrometer equipped with a high-sensitive MCT detector cooled by liquid nitrogen. The mixed gas steam rate was fixed at 100 mL/min. Electronic structure calculations based on DFT were carried out using CASTEP code in Materials Studio.10 A tight convergence of the plane-wave expansion was obtained with a kinetic energy cutoff of 300 eV. The atomic charges were calculated using the Hirshfeld approach. The modeled supercell contain (2*2*2) crystallographic unit cells in optimization runs to demonstrate the distribution of electron charge density (ECD) for FeWO4.11,12

h−1. The pure Fe2O3 exhibited the low activity and its highest NOx conversion was only 75%. The addition of W afforded a remarkable increment of NOx conversion. With the increase in W to 0.25, the Fe0.75W0.25Oδ catalyst showed a considerably broad operation temperature window with above 90% NOx conversion from 250 to 450 °C. Meanwhile, an increased N2 selectivity was obtained and maintained at above 90% over the whole temperature range (inset in Figure 1). The slowdown of N2 selectivity was observed when the temperature increased, owing to the N2O formation as the main out-product (SI Figure S1). However, the further addition of W amount led to the decrease in NOx conversion. In the past years, a number of iron containing materials have been used as NH3−SCR catalysts (SI Table S1). He et al.6 have reported that the iron-titanate catalyst presented a narrow temperature range 225−350 °C above 90% NOx conversion. Similarly, the activity of rod-Fe2O3 catalyst dropped to about 80% NOx conversion when the temperature was increased to 400 °C.4 The activity of V2O3−WO3/TiO2 catalyst as a typical commercial catalyst under the same conditions of this work in our laboratory had also been tested, and its reaction temperature window was only from 300 to 400 °C (Table S1). Apparently, the as-synthesized iron−tungsten catalyst with an optimal W amount (x = 0.25, Fe0.75W0.25Oδ catalyst) outperformed the above-mentioned catalyst, achieving an excellent activity with a broad operation temperature window of 250−450 °C (NOx conversion >90%). The behavior of catalytic activity via the reaction rates and activation energies (Ea) was further studied on the serial Fe−W oxides, and the results are shown in SI Figure S2 and Table 1. The addition of tungsten (x ≤ 0.25) led to a monotonic Table 1. Structural, Chemical and Kinetics Parameters of Fe1‑xWxOδ Serial Catalysts

sample Fe2O3 Fe0.85W0.15Oδ Fe0.75W0.25Oδ Fe0.65W0.35Oδ Fe0.50W0.50Oδ WO3

3. RESULTS AND DISCUSSION 3.1. Catalytic Activity and Kinetics. 3.1.1. NH3−SCR Performance and Kinetics study. Figure 1 shows the SCR activity of Fe1−xWxOδ serial catalysts under GHSV of 50 000 B

XRD

XPS

UV−vis

Ea (kJ· mol−1)

average size Fe2O3 (nm)

ratio of Oα (%)

bandgap (eV)

60 51 39 45 59

22 12 8 4

29 36 41 31 24 20

2.04 1.92 1.87 1.90 1.95 2.67

DOI: 10.1021/acs.est.6b03589 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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obvious decrease was obtained and the reaction rate of NOx conversion still remained at a stable level after the CO2 supply was stopped. The effect of inhibition was negligible. Similarly, no deactivation took place during the switching of 200 ppm of SO2 into the NH3−SCR feed. That manifests the deactivation caused by CO2 and SO2 could be totally suppressed at 300 °C. The presence of 10 vol % H2O made the reaction rate decrease and level off in 10 h. After H2O was cut off, the rate of NOx conversion was rapidly restored to nearly the original value and was kept at this level for the rest of the operation, indicating the inhibition of water is reversible. This reaction pattern demonstrates that the Fe0.75W0.25Oδ catalyst displays the relatively good CO2/SO2/H2O resistance, thus differing from the traditional iron-base oxides that are relatively severely or/ and irreversibly deactivated by H2O, SO2 and CO2.5,13,14 Moreover, the Fe0.75W0.25Oδ catalyst also showed the excellent long-term stability for SCR reaction (SI Figure S4). A high catalytic activity with a constant reaction rate of NO x conversion during a 12 h successive running duration was observed. Herein, the selected Fe0.75W0.25Oδ catalyst is a potential candidate as an NH3−SCR catalyst in the practical application for the NOx abatement from diesel exhaust. 3.2. Confirmation of Composition and Structure (XRD and HRTEM). The pure Fe2O3 displayed well-defined diffraction peaks attributed to typical hematite (SI Figure S5).4 As for the WOx sample, only a well crystallized monoclinic WO3 phase with sharp and intense diffraction peaks was observed.15 The W addition led to the considerable attenuation of diffraction peaks along with the broadening of fwhm (full width at half-maximum), which relates with a decrease in crystallite size of α-Fe2O3. The particle size of the αFe2O3 calculated by using Scherrer’s formula is 22 and 4−12 nm for pristine Fe2O3 and iron−tungsten samples, respectively (listed in Table 1). No characteristic peaks attributed to WO3 was found on all composite oxide samples, but a broad bump around 2θ = 20−35° was gradually intensified with the increase in W amount, suggesting that a new phase may be formed as highly dispersed state or/and amorphous state which are beyond the detection limit of XRD. These results demonstrate that the introduction of W species has a strong inhibition effect on the crystallization of the α-Fe2O3 phase. To further accurately confirm the metal species and morphology in detail, HRTEM was performed on Fe1−xWxOδ catalysts (x = 0.15, 0.25, 0.35). The TEM images in Figure 3a− c showed that the as-selected Fe−W catalysts were assembled from small particles ( 0.25), the Fe−W catalysts showed an increase in Ea value from 45 to 59 kJ•mol−1. The Fe0.75W0.25Oδ catalyst displayed the optimal NH3−SCR activity and the lowest activation energy. These NH3−SCR performance results indicate that a synergistic effect for the NH3−SCR reaction may exist between Fe and W species and their contributions to the catalytic activity will be deeply evaluated in the subsequent sections. 3.1.2. SCR Activity Under Different Conditions. In the practical utilization of DeNOx process for diesel engines, the SCR reaction temperature may have sudden increase in some working conditions, resulting in the activity variation of catalysts. The NOx conversion as a function of reaction temperature over Fe0.75W0.25Oδ catalyst under different calcination temperatures were reported in Figure 2A. When

Figure 2. NOx conversion on Fe0.75W0.25Oδ catalyst under different calcined temperature (A) and GHSVs (B).

the calcination temperature was increased from 500 to 700 °C, the NOx conversion witnessed a downward trend. But the Fe0.75W0.25Oδ catalyst calcined at an extremely high temperature of 700 °C still displayed a rather high NOx conversion exceeding 80% from 275 to 450 °C. The conversions of NOx over Fe0.75W0.25Oδ catalyst under different GHSVs are shown in Figure 2B. With the increase of GHSV from 30000 h−1 to 100 000 h−1, the NOx conversion gently declined at low temperature below 300 °C while the high temperature SCR activity was not obviously affected. Remarkably, the NOx conversion could still be sustained above 80% from 250 to 450 °C under the high GHSV of 100 000 h−1. It indicates that the selected Fe0.75W0.25Oδ catalyst possesses the high durability to the large space velocity and high calcination temperature, which is advantageous to the practical use. It has been known that the composition of diesel exhaust is complicated, such as CO2, SO2 or H2O. Durability experiments were performed under the condition where conversions were significantly lower ( 0.25), the energy

severest deviation driven by the inductive effect of Fe2O3. Meanwhile, it has also been found that the Fe 2p peaks shifted to the lower binding energy (Figure 5B). It further convincingly evidence that the strong electron interaction between iron and tungsten occurs, leading to a high concentration of holes of W species and a high electron density of iron species on Fe2O3. Further addition of W (0.25 < x ≤ 0.50), the W 4f peaks shifted to relatively lower binding energy instead of continuing to shift toward the higher binding energy. Correspondingly, the Fe 2p peaks showed a diminishing shift to a higher binding energy with the further addition of W. That might result from the weakened interfacial inductive effect between iron and tungsten when the W amount is high. Among the Fe−W oxides catalysts, Fe0.75W0.25Oδ sample possessed the highest binding energy of W 4f peaks (34.0 eV) and lowest binding energy of Fe 2p peaks (709.2 eV), which displayed the strongest electron interaction between FeWO4 and α-Fe2O3. The high electron density of iron species in these sample are beneficial to facilitate the adsorption and activation of oxygen species and thereby the catalytic activity. The sub-bands of O 1s (Figure 5C) at higher binding energy (529.2−531.5 eV) was the result of surface adsorbed oxygen, while that at lower binding energy (526.7−528.9 eV) was attributed to the lattice oxygen O2− of the metal oxides (denoted as Oβ).21 The values of the XPS O 1s core-level binding energies and the calculated ratio of Oα are listed in SI Table S2 and Table 1, respectively. It was found the ratio of Oα had an order of Fe2O3 < Fe0.85W0.15Oδ < Fe0.75W0.25Oδ > Fe0.65W0.35Oδ > Fe0.50W0.50OδWO3 > WO3. Usually, Oα was E

DOI: 10.1021/acs.est.6b03589 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 6. Time-resolved difference in situ DRIFTS of NH3 adsorption over Fe2O3 (A) and Fe0.75W0.25Oδ (B) catalysts at 200 °C.

cm−1 and a week band at 1615 cm−1 detected on Fe2O3 catalyst can be assigned to the bending vibrations of the N−H bonds in the NH3 coordinated to the Lewis acid sites.23 During the following purge with helium, a certain amount of NH3 adsorbed on the weak acid sites was removed, resulting in a slight decrease in the band 1208 cm−1 and the disappearance of the band at 1615 cm−1. It is worth pointing out that no obvious peaks related to NH4+ species were observed on the surface of Fe2O3. In contrast, after the addition of W, several bands at 1200, 1262, 1427, 1606, and 1680 cm−1 appeared simultaneously and slightly reduced during the following exposure to He only. Besides of 1200 and 1606 cm−1, the new band observed at 1262 cm−1 was also attributed to the symmetric deformation of ammonia coordinatively which bonded to one new type of Lewis acid sites. Compared to Fe2O3, the bands intensity of adsorbed ammonia species bound to Lewis (1200, 1262, and 1606 cm−1) was greatly increased over Fe0.75W0.25Oδ catalyst. While the bands at 1680 and 1427 cm−1 could arise from the asymmetric and symmetric deformation of NH4+ (δs (NH4+) and δas (NH4+)) bound to Brønsted acid sites.24 Furthermore, a negative band at 2006 cm−1 was evident in this spectrum due to species that disappeared or were perturbed upon ammonia adsorption. At this position, the band could be

band gap gradually increased but still lower than that of pure oxides. The interaction between FeWO4 and α-Fe2O3 is weakened with the further addition of W, which is in accord with the XPS analysis. The correlation between the band gap and the activation energies (Ea) of Fe1−xWxOδ catalysts (0 < x ≤ 0.50) could be established in Figure 5E. An analogous linear relationship between the band gap and activation energies was found and the Ea value diminished with a decrease in band gap energy. Fe0.75W0.25Oδ catalyst possessed the strongest electron effect as evidenced by the lowest band gap (1.87 eV) and highest (lowest) binding energies of W 4f (Fe 2p), and thus showed the lowest activation energies of 39 kJ·mol−1. In light of the above observations, it firmly deduces that the coexistence of α-Fe2O3 and FeWO4 species with an optimal ratio is very important to enhance the NH3−SCR activity because of the strong electron effect. 3.3.2. Surface Acidity (in Situ DRIFTS). The adsorption and activation of NH3 play a significant role in the DeNOx processes, which could be obtained by studying DRIFTS spectra of the adsorbed NH3 over these catalysts. The spectra of the adsorbed ammonia on the Fe2O3 and Fe0.75W0.25Oδ samples are given in Figure 6. The samples were exposed to 500 ppm of NH3 for 30 min at 200 °C. A major band at 1208 F

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Figure 7. Time-resolved difference in situ DRIFTS of NOx adsorption over Fe2O3 (A) and Fe0.75W0.25Oδ (B) catalysts at 200 °C.

assigned to the first overtone of 2v (WO) stretching mode of surface octahedral [WO 6 ] clusters consisted in FeWO4 species.25 Herein, this emerging inverse band indicates that some reactions occurred between WO group and ammonia, which may be one of the incentives for the generation of Brønsted acid sites. Schwidder et al.26 have proposed a promoting effect of Brønsted acidity on the low temperature NH3−SCR activity over Fe-MFI catalysts. It can be thus inferred that the addition of tungsten can provide more acid sites to adsorb and active NH3 species, especially for NH4+ species, which could contribute to their remarkable NH3−SCR activity. 3.3.3. Verification of the Adsorbed NOx Species (in Situ DRIFTS). To acquire information regarding different NOx adsorbed species, the NOx adsorption properties were also tested using in situ DRIFTS (Figure 7). After NO+O2 adsorption for 30 min at 200 °C, two obvious bands at 1535 and 1570 with a weak shoulder at 1630 cm−1 were observed on Fe2O3 catalyst. The bands at 1570 cm−1 could be attributed to the νs(NO) mode of bidentate nitrate, whereas the bands at 1535 and 1630 cm−1 was assigned to the νas(NO2) mode of

monodentate nitrate and adsorbed NO2.26,27 After purge, the band at 1630 cm−1 disappeared along with the increasing of other bands, indicating that the NO2 formed on Fe2O3 could be further oxidized to nitrate species. For the tungsten added catalyst, the DRIFTS spectra of NOx adsorption were quite different from that of Fe2O3. Compared to the Fe2O3, only small amount of nitrate species was formed on the surface of Fe−W oxides catalyst. Two new prominent bands at 1229, 1606 cm−1 and a weak band at 1431 cm−1 developed. The former two bands were ascribed to νa(NO2) of nitrite and adsorbed NO2, while the later was assigned to monodentate nitrate.28,29 Noticeably, the NO2 species is relatively weakly bounded to the adsorption sites, since a majority of NO2 species was removed during the following purge with helium. These observations suggest that the addition of W can facilitate the oxidation of NO to NO2 but inhibit the further oxidation to NO3− species, which would make the reaction process to be a “fast-SCR”. The different adsorption behavior of NOx on Fe0.75W0.25Oδ catalyst would be also due to the electron interaction between α-Fe2O3 and FeWO4 species in accordance with XPS, UV−vis results and DFT calculation. This electronic G

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(5) Qu, Z.; Miao, L.; Wang, H.; Fu, Q. Highly dispersed Fe2O3 on carbon nanotubes for low-temperature selective catalytic reduction of NO with NH3. Chem. Commun. 2015, 51 (5), 956−958. (6) Liu, F.; He, H.; Zhang, C. Novel iron titanate catalyst for the selective catalytic reduction of NO with NH3 in the medium temperature range. Chem. Commun. 2008, 17, 2043−2045. (7) Wang, H.; Qu, Z.; Xie, H.; Maeda, N.; Miao, L.; Wang, Z. Insight into the mesoporous FexCe1‑xO2‑δ catalysts for selective catalytic reduction of NO with NH3: Regulable structure and activity. J. Catal. 2016, 338, 56−67. (8) Can, F.; Berland, S.; Royer, S.; Courtois, X.; Duprez, D. Composition-dependent performance of CexZr1−xO2 mixed-oxidesupported WO3 Catalysts for the NOx storage reduction-selective catalytic reduction coupled process. ACS Catal. 2013, 3 (6), 1120− 1132. (9) Bai, S.; Zhang, K.; Sun, J.; Luo, R.; Li, D.; Chen, A. Surface decoration of WO3 architectures with Fe2O3 nanoparticles for visiblelight-driven photocatalysis. CrystEngComm 2014, 16 (16), 3289−3295. (10) Segall, M. D.; Philip, J. D. L.; Probert, M. J.; Pickard, C. J.; Hasnip, P. J.; Clark, S. J.; Payne, M. C. First-principles simulation: ideas, illustrations and the CASTEP code. J. Phys.: Condens. Matter 2002, 14 (11), 2717. (11) Wang, W.; Hu, L.; Ge, J.; Hu, Z.; Sun, H.; Sun, H.; Zhang, H.; Zhu, H.; Jiao, S. In situ self-assembled FeWO4/graphene mesoporous composites for Li-Ion and Na-Ion batteries. Chem. Mater. 2014, 26 (12), 3721−3730. (12) Cid-Dresdner, H.; Escobar, C. The crystal structure of ferberite, FeWO4. Zeitschrift für Kristallographie-Crystalline Materials 1968, 127, 61. (13) Liu, Z.; Millington, P. J.; Bailie, J. E.; Rajaram, R. R.; Anderson, J. A. A comparative study of the role of the support on the behaviour of iron based ammonia SCR catalysts. Microporous Mesoporous Mater. 2007, 104 (1−3), 159−170. (14) Apostolescu, N.; Geiger, B.; Hizbullah, K.; Jan, M. T.; Kureti, S.; Reichert, D.; Schott, F.; Weisweiler, W. Selective catalytic reduction of nitrogen oxides by ammonia on iron oxide catalysts. Appl. Catal., B 2006, 62 (1−2), 104−114. (15) Szilágyi, I. M.; Fórizs, B.; Rosseler, O.; Szegedi, Á .; Németh, P.; Király, P.; Tárkányi, G.; Vajna, B.; Varga-Josepovits, K.; László, K.; Tóth, A. L.; Baranyai, P.; Leskelä, M. WO3 photocatalysts: Influence of structure and composition. J. Catal. 2012, 294, 119−127. (16) Zhang, J.; Zhang, Y.; Yan, J.-Y.; Li, S.-K.; Wang, H.-S.; Huang, F.-Z.; Shen, Y.-H.; Xie, A.-J. A novel synthesis of star-like FeWO4 nanocrystals via a biomolecule-assisted route. J. Nanopart. Res. 2012, 14 (4), 1−10. (17) Rajagopal, S.; Bekenev, V. L.; Nataraj, D.; Mangalaraj, D.; Khyzhun, O. Y. Electronic structure of FeWO4 and CoWO 4 tungstates: First-principles FP-LAPW calculations and X-ray spectroscopy studies. J. Alloys Compd. 2010, 496 (1−2), 61−68. (18) Almeida, M. A. P.; Cavalcante, L. S.; Morilla-Santos, C.; Dalmaschio, C. J.; Rajagopal, S.; Li, M. S.; Longo, E. Effect of partial preferential orientation and distortions in octahedral clusters on the photoluminescence properties of FeWO4 nanocrystals. CrystEngComm 2012, 14 (21), 7127−7132. (19) Almeida, M. A. P.; Cavalcante, L. S.; Morilla-Santos, C.; Filho, P. N. L.; Beltrán, A.; Andrés, J.; Gracia, L.; Longo, E. Electronic structure and magnetic properties of FeWO4 nanocrystals synthesized by the microwave-hydrothermal method. Mater. Charact. 2012, 73, 124−129. (20) Shpak, A. P.; Korduban, A. M.; Medvedskij, M. M.; Kandyba, V. O. XPS studies of active elements surface of gas sensors based on WO3−x nanoparticles. J. Electron Spectrosc. Relat. Phenom. 2007, 156− 158, 172−175. (21) Wu, S.; Yao, X.; Zhang, L.; Cao, Y.; Zou, W.; Li, L.; Ma, K.; Tang, C.; Gao, F.; Dong, L. Improved low temperature NH3-SCR performance of FeMnTiOx mixed oxide with CTAB-assisted synthesis. Chem. Commun. 2015, 51 (16), 3470−3473. (22) Yu, F.; Cao, L.; Huang, J.; Wu, J. Effects of pH on the microstructures and optical property of FeWO4 nanocrystallites

effect resulted in the high electron density of Fe2O3, leading to the feasibility of oxygen activation and thereby promoting the NO2 formation at these sites. Simultaneously, the further oxidation of NO2 to nitrate was restrained because this reaction was accompanied by a release of electrons. In summary, the introduction of W caused the formation of finely dispersed α-Fe2O3 and FeWO4 as evidenced by XRD (Figure S5 in SI) and HRTEM (Figure 3), appearance of structural defects obtained by the DFT calculation (Figure 4) and modification of electronic structure (Figure 5 and Figure S8 in SI), and thereby the lower Ea value and enhanced activity for NH3−SCR (Figure 1 and Figure S2 in SI). The synergistic effect of α-Fe2O3 and FeWO4 species were responsible for the superior catalytic behavior to NH3−SCR. On the one hand, the presence of FeWO4 provided Brønsted acid sites for the formation NH4+ as highly active species. On the other hand, the electron interaction between α-Fe2O3 and FeWO4 resulted in a high electron density of iron species, which was beneficial to form NO2 species. Therefore, tuning on the adsorption and activation of NH3 and the formation of NO2 through introducing appropriate tungsten species could greatly promote the NH3−SCR catalytic activity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b03589. Related table and figures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-411-84708083; e-mail: [email protected]. ORCID

Zhenping Qu: 0000-0003-0948-2029 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Nature Science Foundation of China (21377016, 21577014), the Natural Science Foundation of Liaoning Province (2014020011) and Program for Changjiang Scholars and Innovative Research Team in University (IRT_13R05). We also appreciate Dr. Nobutaka Maeda for his assistance in the studies of in situ Diffuse Reflectance Infrared Fourier Transform Spectra (in situ DRIFT).

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REFERENCES

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DOI: 10.1021/acs.est.6b03589 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.est.6b03589 Environ. Sci. Technol. XXXX, XXX, XXX−XXX