Effect of Nb promoter on the structure and performance of iron titanate

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Kinetics, Catalysis, and Reaction Engineering

Effect of Nb promoter on the structure and performance of iron titanate catalysts for the selective catalytic reduction of NO with NH

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Kai Cheng, Bing Liu, Weiyu Song, Jian Liu, Yongsheng Chen, Zhen Zhao, and Yuechang Wei Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01441 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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Industrial & Engineering Chemistry Research

Effect of Nb promoter on the structure and performance of iron titanate catalysts for the selective catalytic reduction of NO with NH3

Kai Cheng, †, ‡ Bing Liu, †, § Weiyu Song, †, § Jian Liu, *, † Yongsheng Chen, ‡ Zhen Zhao, † Yuechang Wei †

（† State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China ‡

Department of Mechanical and Automation Engineering, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China）

* Corresponding author: Email address: [email protected]; §: All authors have equal contribution as the first author Postal Address: 18# Fuxue Road, Chang Ping District, Beijing, 102249, China, Tel: 86-10-89732278, Fax: 86-10-69724721

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Iron titanate catalysts with variable Nb content was synthesized by the homogeneous precipitation method, and they were applied to the NH3-SCR. The widest performance temperature window is obtained on the 0.05Nb-0.95Fe-Ti-O catalyst between 200 to 400 oC with a NO conversion higher than 90% at a gas hourly space velocity (GHSV) of 50,000 h-1. The addition of Nb improves the acidity of Fe-Ti-O catalyst, while decreases its reducibility. The excellent performance of 0.05Nb-0.95Fe-Ti-O is attributed to its suitable acidity and reducibility. Both Brønsted and Lewis acid sites over 0.05Nb-0.95Fe-Ti-O catalyst are active in the reaction, and the formation of reactive monodentate nitrate species and cis- N2O22− also promote the performances. Keywords: NH3-SCR; Nb promoter; Acidity and reducibility; DFT calculations; NOx adsorbed species.

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1. Introduction Nitrogen oxides (NOx), mainly emitted from fossil fuels combustion and mobile exhaust gas, have been major atmospheric pollutants. These species could result in acid rain, photochemical smog, haze weather and ozone depletion.1-3 Selective catalytic reduction (SCR) of NOx with NH3 is the current technology for the NOx abatement, and V-based catalysts are the most widely used. However, there are still several problems with this catalyst system, including the relatively narrow working temperature window (300~400 oC), the toxicity of active vanadium species, and low N2 selectivity at high temperatures.4,5 Thus, it is important to develop an environmentally friendly SCR catalyst to replace V-based catalysts. Fe-based catalysts exhibit excellent SCR performance and N2 selectivity in a wide temperature window, such as Fe/ZSM-5,6 Fe-Cu-Ox/CNTs-TiO2,7 and Fe2O3/TiO2.8 Novel iron titanate catalysts are reported to be efficient SCR catalysts in the medium temperature range (250~ 400 oC).9-13 The SCR activities of the iron titanate catalysts are low at the low temperature (below 250 oC). Thus, it is very important to further enhance the performance of Fe-Ti catalysts at low temperatures for the practical application. Recently, there were a few reports that the amorphous state catalysts were used in the SCR reaction. Zhang et al.14 reported that Ce-Ti amorphous oxide showed higher performance than its crystalline counterpart at low temperatures. The Ce-O-Ti short-range order species with the interaction between Ce and Ti in atomic scale was confirmed to be the active sites using various technologies. He et al.15 also reported that the active crystallites in the iron titanate catalyst were mainly in the form of a specific edge-shared Fe3+-(O)2-Ti4+ structure, and had large surface area, pore volume and abundant surface defects supplying rich catalytically active sites for the NH3-SCR reaction. Niobium is often used as a promoter to enhance the performance of the SCR catalysts due to its strong acidity. Nb-promoted CeZrOx catalyst shows excellent NH3-SCR performance, N2 selectivity and SO2/H2O resistance. The addition of Nb to CeZrOx catalysts not only results in strong redox property and high surface area, but also promotes the adsorption and activation of NH3 and NOx species.16 It is reported that the introduction of Nb

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to V2O5/TiO2 catalyst enhanced the NH3-SCR performance and N2 selectivity,17 which is mainly due to strong acidity. However, most studies are focused on the effect of Nb addition on the acid property, the relationship between acid and redox property has not been understood well. In this work, a series of Nb-Fe-Ti-O catalysts were prepared by a homogeneous precipitation method and was applied in the NH3-SCR process. A unique amorphous 0.05Nb-0.95Fe-Ti-O catalyst exhibited excellent SCR performance. The effect of the acidity and reducibility on SCR performance of Nb promoted iron titanate catalysts is studied by a combination of experimental and theoretical computation. Furthermore, NH3-SCR mechanism and the role of Nb species in iron titanate catalyst are also proposed.

2. Experimental 2.1. Catalyst preparation Iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O, Sinopharm Chemical Reagent Co., Ltd, > 98%), Titanium(Ⅳ) sulfate (Ti(SO4)2, Sinopharm Chemical Reagent Co., Ltd, > 98%), Niobium(V) oxalate hydrate (C10H5NbO20, Sinopharm Chemical Reagent Co., Ltd, > 99%) or relevant metal nitrates were used as precursors. Ammonium hydroxide (NH3·H2O, Sinopharm Chemical Reagent Co., Ltd, 25 wt%) was added dropwise into a precursor solution under continuous stirring. The precipitate was filtrated and washed using distilled water, dried at 100 oC for 12 h, and calcined at 400 oC for 6 h at ambient conditions. For the sake of brevity, the catalysts were denoted as (1-x)Nb-xFe-Ti-O (x = 1, 0.99, 0.98, 0.95, 0.92, 0.9), where (1-x)Nb-xFe-Ti-O represents the Nb/Fe/Ti atomic ratios and equals to (1-x):x:1. 2.2. Catalyst characterization The crystalline phases of the samples were detected via powder X-ray diffractometer (XRD) technique. XRD patterns were obtained on a Shimadzu XRD 6000 advance diffractometer equipped with Cu Kα (λ = 0.1542 nm) radiation in the 2θ range of 10–80o at a scanning rate of 4◦/min. N2 adsorption-desorption measurement was used to determine the surface area, pore distribution and pore volume of the catalysts. N2 sorption isotherms were recorded with a Micromeritics TriStar II 3020 analyzer at 77 K. The samples were degassed at 350 oC for 12 h -4-


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prior to measurements. Temperature programmed reduction with hydrogen (H2-TPR) was performed on a Chemisorb 2720 TPx chemisorption analyzer to study the redox property of the catalysts. 100 mg sample was pretreated by a flowing Ar (25 ml/min) at 350 oC for 30 min to remove pre-adsorbed H2O and cooled to room temperature in a N2 flow. Afterwards, H2-TPR started from room temperature to 500 oC at a rate of 10 oC/min in a flowing 10% H2/Ar (50 ml/min). Pyridine was used as a probe to count the surface acid sites on the catalysts. FTIR spectroscopy of pyridine adsorbed samples was performed in a MAGNAIR 560 FT-IR instrument with a resolution of 4 cm-1. Before adsorbing pyridine, all the samples were treated in situ under vacuum at 400 °C for 1 h, followed by the adsorption of purified pyridine vapor at room temperature for 20 min. Then, the system was degassed and evacuated at 200 oC, IR spectra were recorded. in-situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) was used to study NH3-SCR reaction mechanism. In situ DRIFTS were carried out on a Thermo Nicolet IS50 spectrometer, which was equipped with an MCT/A detector. The catalyst was loaded in a Harrick IR cell and heated to 400 oC under a flowing N2 (50 ml/min) for 60 min. A background spectrum was collected under a nitrogen atmosphere. IR spectra were recorded by averaging 32 scans with a resolution of 4 cm−1. 2.3. Catalytic Performance Catalytic activities of NH3-SCR of NOx were evaluated in a fixed bed quartz reactor (i.d. 6 mm). The feed gas consisted of 1000 ppm NO, 1000 ppm NH3, 3vol.% O2, and N2 as the balance gas. The gas flow rate was 500 mL·min−1. Each sample was 0.4 g for evaluation and GHSV is 50,000 h-1. The concentrations of NO and NO2 at the inlet and the outlet were measured by a SIGNAL4000 VM NOx analyzer. In addition, the concentrations of NH3, NO, NO2 and N2O in the outlet were determined by a Thermo Nicolet iS-50 spectrometer. For NH3-SCR reaction, the main reaction is shown as follows, 4NH3+4NO+O2→4N2+6H2O, While the side reactions are 4NH3+5O2→4NO+6H2O, 4NH3+3O2→2N2+6H2O, and -5-



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2NH3+2O2→N2O+3H2O. As the total volume remained the constant in the reaction, the NOx conversion and N2 selectivity of NH3-SCR were calculated as follows. The errors of the determination of NOx conversion and N2 selectivity are ±0.5% and ±1.5%, respectively. NO x conversion ( % ) =

N 2Selectivity(%) =

NO x (in) − NO x (out)

2[N 2 ]out

NO x (in)

*100%

2[N 2 ]out *100% + 2[N 2 O]out + [NO2 ]out

(1)

(2)

2.4. Computational details All the density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP) to illustrate the role of Nb species in the catalysts.18,19 The spin-polarized DFT+U approach20 was used, with Ueff = 4.0 eV applied to Ti 3d electrons. This Ueff value is consistent with previous theoretical studies.21, 22

The projector-augmented wave method was used to represent core-valence interactions.23

Valence electrons were described by a plane wave basis with an energy cutoff of 400 eV. The generalized gradient approximation with the Perdew-Burke-Ernzerh of function was used to model electronic exchange and correlation.24 Gaussian smearing with a width of 0.05 eV was used to improve the convergence of states near the Fermi level. The Brillouin zone was sampled at the Gamma point. Optimized structures were obtained by minimizing the forces on each ion until they were less than 0.05 eV/Å. The adsorption energy was defined as: Eads = E(adsorbate+surface) - E(adsorbate) - E(surface) where E(adsorbate+surface) is the total energy of the adsorbate interacting with the surface. E(adsorbate) and E(surface) are the energies of the free adsorbate in gas phase and the bare surface, respectively. A negative value corresponds to exothermic adsorption, with more negative value corresponding to stronger adsorption. According to Liu's study,15 we modeled Fe-Ti catalyst as shown in Figure 1a, which consists of 12 Ti atoms, 12 Fe atoms and 42 O atoms. A vacuum gap of 12 Å was used. During geometry optimization, all atoms were allowed to relax.

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3. Results and discussion 3.1 NH3-SCR catalytic performance Figure 2 exhibits the results of NOx conversion (a) and N2 selectivity (b) over (1-x)Nb-xFe-Ti-O (x = 1, 0.99, 0.98, 0.95, 0.92, 0.9) catalysts in the temperature range of 150 – 400 oC with a GHSV of 50,000 h-1. To confirm the thermodynamic equilibrium of the used gas mixture, a blank test was carried out without catalyst placed in the fixed-bed quartz reactor, the results are also shown in Figure 2. It shows almost no conversion, so the reactant gases seldom react with each other at 150 – 400 oC. The Fe-Ti-O catalyst shows the NOx conversion above 90% among 275 – 350 oC. When appropriate amounts of niobium are introduced into the Fe-Ti-O catalyst, a significant influence on catalytic activity is observed. The temperature range for optimum NO conversion (>99%) extents towards lower temperature, corresponding to active window broadening. NOx conversion markedly increases at low temperature range with the increase of Nb content. The 0.05Nb-0.95Fe-Ti-O catalyst shows the best performance, over which the NOx conversion reaches above 90% at about 200 – 400 oC. However, further increasing Nb content leads to NOx conversion lowering. For the 0.08Nb-0.92Fe-Ti-O and 0.1Nb-0.9Nb-Ti-O catalysts, the activity temperature windows are much narrower than that of the 0.05Nb-0.95Fe-Ti-O catalyst. It is also noted that the NOx conversion decreases above 350 oC due to the occurrence of unselective oxidation of NH3. Considering the selectivity of N2 over (1-x)Nb-xFe-Ti-O catalysts, N2 selectivity decreases seriously at higher temperatures for the Fe-Ti-O catalyst. The addition of Nb to the Fe-Ti-O catalyst results in the enhancement of N2 selectivity.25 To further illustrate the differences between catalysts, the NH3-SCR performance of (1-x)Nb-xFe-Ti-O catalysts is also evaluated with a high GHSV of 800,000 h-1. As shown in Figure 3a, all the catalysts show maximum NOx conversion below 80% between 150 and 400 ºC. 0.05Nb-0.95Fe-Ti-O catalyst shows the highest NOx conversion above 200 ºC and reaches the maximum at 76% at 350 oC. Further increase the temperature leads to a decline in the NOx conversion. H2O and SO2 are inevitable in the exhaust stream and they always cause a certain extent of decrease in NH3-SCR activity. The effects of H2O and SO2 on the NH3-SCR performance of the 0.05Nb-0.95Fe-Ti-O sample are studied at 300 oC with a GHSV of

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800,000 h-1 and the results are shown in the Figure 3b. When 5% H2O is introduced into the stream, the NOx conversion decreases from 45% to 37% and then remains constant for 12 h during the activity test. After removing H2O, the NOx conversion is recovered to the initial value. These results show that the presence of water vapor may inhibit the NH3-SCR performance at 300 oC, which may be due to the reversible competitive adsorption between H2O and NH3 molecules on the acid sites.26-28 When 100 ppm SO2 is added into the gas flow, the NOx conversion decreases from 45% to 31%, then no obvious change in NOx conversion is observed in the following hours. After SO2 is cut off, NOx conversion is not recovered, indicating that the deactivation was irreversible. 3.2. Physical properties of (1-x)Nb-xFe-Ti-O catalysts Powder XRD patterns of (1-x)Nb-xFe-Ti-O catalysts are shown in Figure 4. No obvious sharp diffraction peaks are observed besides some broad bumps on the Fe-Ti-O catalyst. The results show that the Fe-Ti-O catalyst mainly consists of amorphous iron titanate, which is believed to be the real active phase.15 XRD patterns of (1-x)Nb-xFe-Ti-O catalysts exhibit no significant variation. Table 1 shows the BET surface areas of the catalysts. The samples exhibit surface area and pore volume in the range of 194.5 – 244.5 m2/g and 0.3 – 0.4 cm2/g, respectively. The 0.05Nb-0.95Fe-Ti-O catalyst exhibits the largest surface area at 244.5 m2/g. 3.3. Redox properties of (1-x)Nb-xFe-Ti-O catalysts H2-TPR of (1-x)Nb-xFe-Ti-O catalysts is performed to study the redox properties of the catalysts. Figure 5 shows H2-TPR results of all the Nb-Fe-Ti-O catalysts. As shown in Figure 5, a reduction peak is observed at 316 oC for the Fe-Ti-O catalyst due to the reduction of iron species and the Nb addition modifies the redox properties of (1-x)Nb-xFe-Ti-O catalysts.29 With the increasing of Nb content from 0 to 0.1, the temperature of the reduction peak shows a monotonic increase from 316 to 384 oC. The amount of H2 consumption below 500 oC is shown in Table. 2 used CuO as a calibration reference. The H2 consumption decreases with the increase of Nb content. These results indicate that the reducibility of the catalysts become difficult with the addition of Nb content. DFT is also used to calculate the oxygen vacancy formation energy and to predict the reducibility of catalysts. A lower oxygen vacancy formation energy corresponds to a higher reducibility. The oxygen vacancies on the Fe-Ti-O and Nb-Fe-Ti-O catalysts are shown in -8-


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Figure 6. The oxygen vacancy formation energy of the Fe-Ti-O catalyst is calculated to be 1.78 eV, which is lower than that of the Nb-Fe-Ti-O catalyst (3.84 eV). The result suggests that the formation of oxygen vacancy becomes more difficult with the introduction of Nb and thus decreases the reducibility of the Fe-Ti-O catalyst, which is in line with H2-TPR results. It has been recognized in previous studies that NO oxidation to NO2 can facilitate the NH3-SCR reaction.30–32 Therefore, due to the decrease of reducibility, the addition of Nb prevents NO oxidation to NO2 and thus affects the NH3-SCR performance when Nb substitution amount is above 0.05. 3.4. Surface Properties of (1-x)Nb-xFe-Ti-O catalysts NH3 adsorption is one of the most critical steps in the NH3-SCR reaction.33,34 FTIR spectroscopy of pyridine adsorbed the Fe-Ti-O and 0.05Nb-0.95Fe-Ti-O catalysts after outgassing at 40, 200 and 350 oC are shown in Figure 7. The peaks appearing at around 1540 cm-1 are attributed to pyridinium ions coordinated to Brønsted sites. The peaks appearing at around 1440, 1455 and 1630 cm−1 are assigned to pyridinium coordinated to Lewis sites. Compared with the spectrum evacuated at 200 oC, the intensities of the bands at 1440 cm-1 attributed to L acid are much stronger after evacuation at 40 oC. It is also noted that there appears a new peak at 1574 cm-1 after degassing at 40 oC, which is attributed to the vibration mode of Py H-bonded with surface. Accordingly, it shows that there are considerable L acid sites in the samples. However, the quantity of B acid sites is limited, and most of Py molecules are adsorbed on the surface by hydrogen bond after degassing at 40 oC. Both Brønsted and Lewis acid sites are observed on the Fe-Ti-O and 0.05Nb-0.95Fe-Ti-O catalysts at 200 oC. The amount of Lewis and Brönsted sites is quantified by Py-IR spectroscopy and the results are shown in the Table. 3. After the introduction of 5% Nb content, the amount of acid sites measured by IR spectroscopy of adsorbed pyridine is correspondingly much greater. After outgassing at 200 oC, the amount of Lewis acid sites for the 0.05Nb-0.95Fe-Ti-O and Fe-Ti-O catalysts is 0.159 and 0.091 mmol/g, which is much higher than that of Brönsted acid sites at 0.011 and 0.006 mmol/g, respectively. The result indicates that Lewis acid sites prevail over Brønsted acid sites. The more Lewis acid sites over the 0.05Nb-0.95Fe-Ti-O catalyst would be assigned to the addition of Nb element with sufficient Lewis acid sites. In order to elucidate the effect of Nb on the acidity, the optimized structures of NH3 -9-



adsorption at L-acid and B-acid sites of the Fe-Ti-O and Nb-Fe-Ti-O catalysts from DFT calculations are presented in Figure 8. The adsorption energies of NH3 at the L-acid and B-acid sites are -0.88 and -0.89 eV for the Fe-Ti-O catalyst from the DFT calculation, respectively. For the Nb-Fe-Ti-O catalyst, the adsorption energies of NH3 at the L-acid and B-acid sites are -1.37 and -2.07 eV, respectively. Thus, NH3 adsorption on the Nb-Fe-Ti-O catalyst is stronger than that on the Fe-Ti-O catalyst, indicating that the introduction of Nb facilitates NH3 adsorption and improves the acidity. These results agree with the experimental results. The experimental performance testing results in Figure 2 show that the catalytic performance varies with the Nb content and reaches a maximum when Nb:Fe molar ratio is 0.05:0.95. The theoretical results account for this catalytic performance. The increase of the NH3-SCR performance may be attributed to the fact that the introduction of Nb improves the acidity of the Fe-Ti-O catalyst. However, at the same time, the addition of Nb decreases the reducibility of the Fe-Ti-O catalyst. The decrease of the NH3-SCR performance is attributed to the decrease of reducibility when Nb:Fe molar ratio is more than 0.05:0.95. Therefore, the acidity and reducibility have a synergistic effect on the catalytic performance. 3.5. in-situ DRIFTS Studies 3.5.1 Adsorption of NH3 Figure 9 shows in-situ DRIFT spectra of NH3 adsorption on the 0.05Nb-0.95Fe-Ti-O catalyst at 30–500 oC. Several vibration bands are observed in the range of 1000–4000 cm−1 after NH3 adsorption at room temperature. The peaks at 1598 and 1199 cm−1 are attributed to the asymmetric and symmetric bending vibrations of NH3 coordinately linked to Lewis acid site, respectively. All these peaks related to Lewis acid site also disappear completely at 450 o

C due to thermal desorption. The peaks at 3358, 3241 and 3146 cm−1 are ascribed to the N-H

stretching vibration modes of the coordinated NH3, while the peak at 1449 cm−1 is attributed to the asymmetric bending vibration of NH4+ species on Brønsted acid site.35–37 These adsorbed NH3 species desorb when the temperature increases, and disappear completely at 200 °C. There are two negative bands observed at around 3673 and 1626 cm-1, which may be due to the hydroxyl consumption through interaction with NH3 to form NH4+. It is noted that a new peak appears at 1359 cm-1 at 200 °C, which is attributed to wagging and scissoring - 10 -


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vibrations of -NH2 species. It is formed by hydrogen abstraction from NH3 coordinated to Lewis acid site.38 The results show that the decomposition and desorption of the adsorbed NH3 species on the surface of (1-x)Nb-xFe-Ti-O catalyst are easier on account of the strong interaction among Fe, Ti and Nb at the Fe3+-O-Ti4+ interface. 3.5.2 Co-adsorption of NO and O2 in-situ DRIFTS of NO+O2 desorption on the 0.05Nb-0.95Fe-Ti-O catalyst at 30–400 oC is shown in Figure 10. The peaks are detected at 1613, 1584, 1552, 1484, 1287 and 1238 cm-1 at 30 oC, which are attributed to bridging (1613 and 1238 cm-1) nitrate species, monodentate (1552 and 1287 cm-1), and adsorbed bidentate (1584 cm-1).39,40 When the temperature increases, the peak intensities of the bridging, monodentate, and bidentate nitrate species gradually decrease and their peaks completely disappear at 400 oC. Interestingly, a new peak at 1364 cm-1 appears at 150 oC. The intensity of this peak increases with further elevation of the temperature. It is reported that this monotonously growing species is attributed to cisN2O22− species.41 These species are believed to be active species in the NH3-SCR reactions. Thus, the introduction of Nb results in more active cis- N2O22− species on the 0.05Nb-0.95Fe-Ti-O catalyst and improves catalytic performance. The DFT calculations show that NH2 intermediate is formed from N-H bond breaking of the adsorbed NH3 species (Figure 11a). The in-situ DRIFT results have confirmed the existence of N2O2 species in the NH3-SCR reaction on the Nb-Fe-Ti-O catalysts. Zhang et al. proposed that N2O2 species react with NH3 more favorably than other nitrates species using DRIFTS method.41 To illustrate how N2O2 species is formed, we investigated the adsorption of two NO molecules on the surface oxygen vacancy of the Nb-Fe-Ti-O catalyst by DFT calculations. The results show that the adsorbed N2O2 species (Figure 11b) is spontaneously formed when the two NO molecules are adsorbed on the vacancy, with a high adsorption energy of -4.34 eV. 3.5.3 Reactions between nitrogen oxides and ammonia adspecies To investigate whether absorbed NH3 could react with NO + O2, the catalyst was first pretreated with NH3 for 60 min, followed by N2 purging to remove weakly absorbed NH3 at 200 oC, NO + O2 was then introduced into the cell for 30 min. The resulting spectra are shown in Figure 12. The coordinated NH3 related to Lewis (1208 and 1605 cm-1) and Brønsted acid - 11 -



sites (1437 and 1665 cm-1) are observed on the 0.05Nb-0.95Fe-Ti-O catalyst with feeding NH3. The negative band at ~1370 cm-1 may be attributed to the coverage of residual sulfate species (νas S=O) from Ti(SO4)2 precursor by adsorbed NH3. 11 When switching the gas to NO + O2, the intensities of all peaks assigned to ammonia species decrease and the peaks disappear in 5 min. Meanwhile, some peaks attributing to NOx species appear (1617, 1578, 1555 and 1372 cm-1). The results show that the NH3 species bonded to both Brønsted and Lewis acid sites on the 0.05Nb-0.95Fe-Ti-O catalyst should be involved in the reaction. 3.5.4 Reactions between ammonia and adsorbed nitrogen oxides species To confirm whether absorbed NO + O2 could react with NH3, the catalyst was first pretreated with NO + O2 for 60 min, followed by purging with N2 to remove weakly bonded NOx species at 200 oC, NH3 was then introduced into the cell for 60 min, and the corresponding results are displayed in Figure 13. The intensities of monodentate nitrate species (1549 cm-1) decreases and the peak at 1370 cm-1 assigned to cis- N2O22− (Figure 11) disappears after 2 min. The results indicate that these absorbed species react with NH3 species in the reaction. Meanwhile, the adsorbed NH3 species is observed. Therefore, monodentate nitrate and cis- N2O22− species react more favorably with NH3 than other nitrates adspecies. 3.6. Reaction mechanism The observation of -NH2 and cis-N2O22− intermediates found in in-situ DRIFTS suggest the following reaction mechanism on the Nb-Fe-Ti-O catalyst. As shown in Scheme 1, the reaction mechanism consists of four steps. In the step I, NH3 adsorbs at the L-acid site of the Nb-Fe-Ti-O catalyst, and then reacts with NO to form N2 and H2O. B-acid site is generated on the surface of the Nb-Fe-Ti-O catalyst at the end of step I. In the step II, NH3 binds to B-acid site to form adsorbed NH4 species which then reacts with NO, leading to the formation of N2 and H2O. Oxygen vacancy is formed on the surface of the Nb-Fe-Ti-O catalyst at the end of the step II. Next, in the step III, N2O2 species is formed on the oxygen vacancy and then reacts with NH3, leading to the formation of N2 and H2O. Two B-acid sites is generated on the surface at the end of the step III. Finally, in the step IV, the Nb-Fe-Ti-O surface is regenerated to the original state by O2 molecule, completing the whole catalytic cycle. The catalytic cycle in Scheme 1 indicates that both acidity and reducibility in the Nb-Fe-Ti-O catalyst play important roles in the reaction. After these four steps, a catalytic cycle is completed, and the - 12 -


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catalyst is regenerated to the original state. Our proposed reaction scheme is consistent with the stoichiometry of the reaction, which is 4NH3 + 4NO + O2 = 4N2 + 6H2O.42–44

4. Conclusions The catalytic performance of the Fe-Ti-O catalyst for NH3-SCR is improved by Nb doping. The acidity and reducibility have a synergistic effect on the catalytic performance. The addition of Nb improves the acidity of the Fe-Ti-O catalyst, while decreases its reducibility. The in-situ DRIFTS result and DFT calculations demonstrate that both the Brønsted and Lewis acid sites over the 0.05Nb-0.95Fe-Ti-O catalyst are active in the reaction, and the formation of reactive monodentate nitrate species and cis- N2O22− also promote the performances. The widest performance temperature window is obtained on the 0.05Nb-0.95Fe-Ti-O catalyst between 200–400 oC with an NO conversion higher than 90%. The excellent performance of the 0.05Nb-0.95Fe-Ti-O catalyst is attributed to its suitable acidity and reducibility.

Acknowledgements This work was financially supported by National Natural Science Foundation of China (U1662103, 21673290), Beijing Natural Science Foundation (2182060) and HKSAR Innovation and Technology Commission (ITC) with the ITF Project No. ITS/422/16.

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- 13 -



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- 17 -



Table captions Table 1 The textural and structural properties of all the catalysts Table 2 The reduction peaks and H2 consumption amount of all the catalysts Table 3 The acid amount of the 0.05Nb-0.95Fe-Ti-O and Fe-Ti-O catalysts

- 18 -


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Page 19 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1 The textural and structural properties of all the catalysts Samples

SBETa (m2·g-1)

Vmicb (cm2·g-1)

Average pore diameter (nm)

Fe-Ti-O

194.5

0.3

4.5

0.01Nb-0.99Fe-Ti-O

215.5

0.4

5.9

0.02Nb-0.98Fe-Ti-O

236.9

0.3

4.2

0.05Nb-0.95Fe-Ti-O

244.5

0.3

5.1

0.08Nb-0.92Fe-Ti-O

219.8

0.3

4.1

0.1Nb-0.9Fe-Ti-O

210.2

0.3

5.1

a

Calculated by BET method

b

Calculated by t-plot method

- 19 -



Table 2 The reduction peaks and H2 consumption amount of all the catalysts The Temperature of Maximum

H2 Consumption Amount

Reduction Peak (oC)

per Gram (mmol/g) a

Fe-Ti-O

316.4

3.57

0.01Nb-0.99Fe-Ti-O

348.8

3.34

0.02Nb-0.98Fe-Ti-O

350.0

2.30

0.05Nb-0.95Fe-Ti-O

369.9

2.27

0.08Nb-0.92Fe-Ti-O

375.4

2.16

0.1Nb-0.9Fe-Ti-O

384.8

1.17

Sample

a

The amount of H2 consumption below 500 oC was quantified by Ag2O used as a calibration reference.

- 20 -


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Page 21 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3 The acid amount of the 0.05Nb-0.95Fe-Ti-O and Fe-Ti-O catalysts Sample 0.05Nb-0.95Fe-Ti-O

Fe-Ti-O

Temperature

L-Acid Amount

B-Acid Amount

(oC)

(mmol/g)

(mmol/g)

40

0.214

0.012

200

0.159

0.011

350

0.121

0.009

40

0.144

0.006

200

0.091

0.006

350

0.058

0.004

- 21 -



Figure captions Figure 1. Calculated structures of (a) FeTiOx and (b) FeNbTiOx. Figure 2. NOx conversion (a) and N2 selectivity (b) as a function of reaction temperature over (1-x)Nb-xFe-Ti-O catalysts. Reaction conditions: [NO] = [NH3] = 1000 ppm, [O2] = 3%, balance N2, total gas flow rate 500 ml/min, and GHSV = 50,000 h-1. Figure 3. NOx conversion as a function of reaction temperature over (1-x)Nb-xFe-Ti-O catalysts (a) and time at 300 oC in the presence of 5% H2O or 100 ppm SO2 over 0.05Nb-0.95Fe-Ti-O catalyst (b). Reaction conditions: [NO] = [NH3] = 1000 ppm, [O2] = 3%, balance N2, total gas flow rate 500 ml/min, and GHSV = 800,000 h-1 Figure 4. X-ray diffraction patterns of (1-x)Nb-xFe-Ti-O . Figure 5. H2-TPR profiles of (1-x)Nb-xFe-Ti-O catalysts Figure 6. Oxygen vacancy formation on (a) FeTiOx and (b) FeNbTiOx. Figure 7. FT-IR spectra of pyridine adsorbed on FeTiOx and 0.05Nb-0.95Fe-Ti-O after degassing at 200 oC. Figure 8. NH3 adsorption at (a) L-acid site and (b) B-acid site of FeTiOx; NH3 adsorption at (c) L-acid site and (d) B-acid site of FeNbTiOx. Figure 9. In-situ DRIFTS of NH3 desorption on 0.05Nb-0.95Fe-Ti-O measured at 30-500 oC. Figure 10. In-situ DRIFTS of NO+O2 desorption on 0.05Nb-0.95Fe-Ti-O measured at 30-400 o

C.

Figure 11. Formation of (a) NH2 species and (b) N2O2 species on FeNbTiOx. Figure 12. In-situ DRIFTS over 0.05Nb-0.95Fe-Ti-O as a function of time in a flow of NH3 after the catalysts was pre-exposed to a flow of NO + O2 for 60 min followed by N2 purging for 30 min at 200 oC. Figure 13. In-situ DRIFTS over 0.05Nb-0.95Fe-Ti-O as a function of time in a flow of NO + O2 after the catalysts was pre-exposed to a flow of NH3 for 60 min followed by N2 purging for 30 min at 200 oC. Scheme 1. Our proposed catalytic cycle of NH3-SCR reaction on FeNbTiOx catalysts. (The active sites in these four steps are marked with red color. Ov represents the oxygen vacancy on the surface.)

- 22 -


Page 22 of 38

Page 23 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1.

- 23 -



100

(a)

90 80

NOx Conversion/%

70 60

Fe-Ti-O 0.01Nb-0.99Fe-Ti-O 0.02Nb-0.98Fe-Ti-O 0.05Nb-0.95Fe-Ti-O 0.08Nb-0.92Fe-Ti-O 0.1Nb-0.9Fe-Ti-O blank test

50 40 30 20 10 0 150

200

250

300

350

400

o

Temperature/ C

100

N2 Selectivity/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 38

98

Fe-Ti-O 0.01Nb-0.99Fe-Ti-O 0.02Nb-0.98Fe-Ti-O 0.05Nb-0.95Fe-Ti-O 0.08Nb-0.92Fe-Ti-O 0.1Nb-0.9Fe-Ti-O

96

94

(b)

150

200

250

300

350

o

Temperature/ C

Figure 2.

- 24 -


400

Page 25 of 38

100

(a)

90

Fe-Ti-O 0.01Nb-0.99Fe-Ti-O 0.02Nb-0.98Fe-Ti-O 0.05Nb-0.95Fe-Ti-O 0.08Nb-0.92Fe-Ti-O 0.1Nb-0.9Fe-Ti-O

80

NOx Conversion/%

70 60 50 40 30 20 10 0 150

200

250

300

350

400

o

Temperature/ C

60

Added in

50

NOx Conversion / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cut off

40

30

20

With H2O With SO2

10

(b) 0 0

2

4

6

8

10

Time / h

Figure 3.

- 25 -


12

14

16


0.1Nb-0.9Fe-Ti-O

0.08Nb-0.92Fe-Ti-O 0.05Nb-0.95Fe-Ti-O

intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 38

0.02Nb-0.98Fe-Ti-O

0.01Nb-0.99Fe-Ti-O

Fe-Ti-O

FeTiO3 in JCPDS

10

20

30

40

50

60

2 theta(degree)

Figure 4.

- 26 -


70

80

Page 27 of 38

0.1Nb-0.9Fe-Ti-O 0.08Nb-0.92Fe-Ti-O Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.05Nb-0.95Fe-Ti-O 0.02Nb-0.98Fe-Ti-O 0.01Nb-0.99Fe-Ti-O

Fe-Ti-O

100

150

200

250

300

350

400

o

T/ C

Figure 5.

- 27 -


450

500


Figure 6.

- 28 -


Page 28 of 38

Page 29 of 38

o

0.6

L

40 C L

Absorbance/a.u.

0.5

L+B

0.4 L 0.3

B

0.05Nb-0.95Fe-Ti-O

0.2 0.1 Fe-Ti-O

0.0 1700

1650

1600

1550

1500

1450

1400

-1

Wavenumber/cm

L

o

200 C

Absorbance/a.u.

L+B

L

0.3

0.2

B 0.05Nb-0.95Fe-Ti-O

0.1

Fe-Ti-O

0.0 1700

1650

1600

1550

1500

1450

1400

-1

Wavenumber/cm

L

0.25 o

L+B

350 C L

0.20

Absorbance/a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.15

B 0.10 0.05Nb-0.95Fe-Ti-O

0.05 Fe-Ti-O

0.00 1700

1650

1600

1550

1500

1450

-1

Wavenumber/cm

Figure 7.

- 29 -


1400


Figure 8.

- 30 -


Page 30 of 38

0.6

o

500oC 450 oC 400oC 350oC 300oC 250oC 200 oC 150oC 100oC

3673

0.4 0.2

30 C

0.0 -0.2

4000

3500

2000

1598

-0.8

1359

-0.6

1800

1600

Wavenumbers/cm

1400 -1

Figure 9.

- 31 -


1199

1449

-0.4

3358 3241 3146

Absorbance/a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1626

Page 31 of 38

(b)

1200

1000


0.8

o

400 C

0.6

o

350 C

0.4

o

300 C o

250 C

0.2

o

200 C

0.0

o

150 C

-0.4

30 C

o

-0.6 4000

3500

2000

1800

1600

1400 -1

Wavenumbers/cm

Figure 10.

- 32 -


1287 1238

100 C

1364

o

-0.2

1613 1584 1552 1484

Absorbance/a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 38

1200

1000

Page 33 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 11.

- 33 -


1617 1578 1555

0.4 0.3

30 min 20 min 10 min 5 min

0.2 0.1

3 min 0.0

-0.3 4000

3500

2000

1800

1600

1400 -1

Wavenumbers/cm

Figure 12.

- 34 -


1208

1437

-0.2

0 min 1665 1605

-0.1

1 min 3367 3267 3167

Absorbance/a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 38

1372


1200

1000

Page 35 of 38

0.3

60 min

0.2

0.0

30 min 20 min 10 min 5 min

-0.1

3 min 1 min

0.1

2000

1800

1549

1600

1203

3500

1422 1370

4000

1689

-0.3

1608

0 min -0.2 3362 3267 3161

Absorbance/a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1400

1200

-1

Wavenumbers/cm

Figure 13.

- 35 -


1000


Scheme 1.

- 36 -


Page 36 of 38

Page 37 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphical Abstract

- 37 -



Research Highlights ● The NH3-SCR activity and selectivity of the Fe-Ti-O catalyst is enhanced by small Nb doping. ● The widest activity temperature window is obtained on the 0.05Nb-0.95Fe-Ti-O catalyst between 200–400 oC with GHSV = 50, 000 h-1. ● There is a trade-off effect between acidity and reducibility. 0.05Nb-0.95Fe-Ti-O catalyst shows the high activity due to suitable acidity and reducibility.

- 38 -


Page 38 of 38

Effect of Nb promoter on the structure and performance of iron titanate

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