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A comparative study of different doped metal cations on the reduction, acidity and activity of Fe9M1Ox (M = Ti4+, Ce4+/3+, Al3+) catalysts for NH3-SCR reaction Jingfang Sun, Yiyang Lu, Lei Zhang, Chengyan Ge, Changjin Tang, Haiqin Wan, and Lin Dong Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03080 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 18, 2017

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Industrial & Engineering Chemistry Research

A comparative study of different doped metal cations on the reduction, acidity and activity of Fe9M1Ox (M = Ti4+, Ce4+/3+, Al3+) catalysts for NH3-SCR reaction Jingfang Suna, Yiyang Lua, Lei Zhangd, Chengyan Gec, Changjin Tanga, Haiqin Wanb, #, Lin Donga, *

a

, Jiangsu Key Laboratory of Vehicle Emissions Control, School of Chemistry and Chemical

Engineering, Nanjing University, Nanjing 210093, PR China b

State Key Labrotary of Pollution Control and Resource Reuse, School of the Environment,

Nanjing University, Nanjing 210093, PR China c

School of Chemistry and Chemical Engineering, Yancheng Institute of Technology, Yancheng,

224051, PR China d

Chongqing Three Gorges University, School of Environmental and Chemical Engineering,

chongqing 404001, PR China

#

Corresponding author. Email addresses: [email protected] * Corresponding author. Email addresses: [email protected]

1

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ABSTRACT Selective catalytic reduction of NOx with NH3 reaction was investigated over a series of Fe-containing samples prepared by co-precipitation method. The doping effect of introducing representative cations (M=Ti4+, Ce3+/4+, Al3+) with apparent different acidity and redox properties was systematically studied. The results indicated that suitable acidity and redox property as well as the balance between them were extremely important for NH3-SCR reaction. Ti4+-doped catalysts showed the most prominent effect, obtaining 4-fold increase in NOx conversion compared to Fe2O3 at 100 ℃, due to their strong acidity and slight reducibility. Samples with Ce3+/4+-doped, which colud be facilely reduced but lacked sufficient acidity, got lower catalytic activity than Al3+-doped samples with relatively stronger acidity. Thus, the activity followed: Fe9Ti1Ox > Fe9Al1Ox > Fe9Ce1Ox. In addition, Fe9Ti1Ox catalyst exhibited a ~80% NOx conversion and remained stable in the presence of SO2 and H2O at 250 °C for 80 h, making it a suitable candidate for NH3-SCR reaction.

Keywords: Fe-based catalysts; Co-precipitation method; Doping effect; NH3-SCR reaction 2


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1. INTRODUCTION Rapid urbanization and industrial development have caused serious air pollution. Among various air pollutants, nitrogen oxides (NOx) from flue gases of stationary (coal-fired power plants) and mobile (lean-burn diesel engines) sources lead to serious environmental problems, such as haze, ozone depletion and acid rain et al..1,2 As well, more and more stringent regulations are issued with ever-increased environmental awareness. Hence, great efforts are being made to minimize NOx emissions, and selective catalytic reduction of NOx with NH3 (NH3-SCR) is widely used as an effective and low cost NOx removal method. 3 The currently used industrial catalysts for NH3-SCR process are based on TiO2-supported V2O5-WO3 or V2O5-MoO3 oxides because of their high NOx removal efficiency at 300-400 °C and excellent resistance to SO2 poisoning. However, these vanadium-based catalysts also have some inevitable shortcomings, including their high cost, narrow temperature window, low N2 selectivity at high temperature and the toxicity of vanadium pentoxide .4,5 As a result, great attentions have been paid to reduce the vanadium loading or thoroughly replace vanadium with other active metal components, such as Fe, Mn, Ce, or Cu.6-10 Iron oxide is well known as an active ingredient or promoter in NH3-SCR catalysts. Till now, many kinds of iron-based catalysts have been reported, such as Fe-V/AC,11 Fe2O3/CeO2,12 MnFe/Al2O313, Fe/CNT14, Fe-Mn Spinel,15,16 Co-Fe mixed oxide,10 FeMnTiOx mixed oxide,17 FexTiyOz,18,19 Fe3+-exchanged Fe-SSZ-13,20 Fe-Beta21 and Fe-BEA.22,23 Yang et al. have incorporated Ti4+ species into γ-Fe2O3 to decrease the oxidization ability of Fe3+, thus inhibiting the catalytic oxidization of NH3 to NO over γ-Fe2O3 at high temperatures.24 Li et al. have reported that iron-tungsten catalysts showed excellent activity in a wide temperature range and outstanding H2O/SO2 durability above 300 °C. They explained that after introducing WOx species, an enhanced interaction between iron and tungsten promoted the formation of surface adsorbed oxygen species and increased the surface acidity, which lead to an improvement in the catalytic performance.25 Previous works on this topic are mainly concentrated on increasing the catalytic activity by introducing different 3



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foreign cations into Fe2O3. However, there are few reports which systematically tune the reducibility or acidity of the doped FexM1-xOz catalysts and perform a comparative study. Take into account that TiO2, CeO2 and Al2O3 are widely used components in catalytic materials, with their attractive merit of low prices and wide availbility. In addition, their acid-base properties are markably different: TiO2 is a typical acidic oxide, Al2O3 is known as an amphoteric oxide and CeO2 mainly shows basic property. Their redox properties are also completely different. Besides, TiO2 is also well known for its favourable resistance to SO2 poisoning in NH3-SCR reaction.26,27 Al2O3 is widely employed as a rigid support material with high surface area, excellent adsorption capacity and good heat stability.28 As for CeO2, its excellent reducibility and remarkable oxygen storage capacity (OSC) made it capable to be applied as NH3-SCR catalytic materials.29 Therefore, these materials are appropriate candidates for investigatng dopant effects with varied acidity-redox properties. Herein, we study the doping effect in NH3-SCR reaction over iron-based catalysts using Ti4+, Ce3+/4+, Al3+ as representative doping cations. The obtained samples were characterized by means of N2-physisorption, XRD, HRTEM, in situ DRIFTS, NH3-TPD and H2-TPR technologies, then assessed for NH3-SCR model reaction. With the purpose of investigating the effects of dopants on the physical and textural properties, redox behavior, acidity and catalytic performances of the catalysts in NH3-SCR reaction.

2. EXPERIMENTAL SECTION 2.1 Catalyst preparation The Fe9M1Ox (M=Ti, Ce, Al) catalysts were synthesized by co-precipitation method using Fe(NO3)3·9H2O as iron precursor and Ti(SO4)2, Ce(NO3)3·6H2O and Al(NO3)3·9H2O as doped ion sources with an Fe-M molar ratio of 9:1. The precipitator was NH3·H2O. After washing by deionized water for several times, the obtained samples were dried in air at 110 ℃ for 12 h and then calcined at 450 ℃ for 5 4


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h. In addition, pure Fe2O3 was prepared via the same method.

2.2 Catalyst characterization X-ray diffraction (XRD) was performed on a Philips X'pert Pro diffractometer with Ni-filtered Cu Kα1 radiation (λ= 0.15408 nm) at 40 kV and 40 mA. The specific surface areas of the catalysts were determined using a Micrometrics ASAP-2020 analyzer through Brunauer-Emmett-Teller (BET) method at liquid N2 temperature. Approximately 0.1 g of sample was initially degassed in a N2/He mixture at 300 °C for 4 h before the adsorption measurement. The pore size distributions were calculated from the desorption branch of N2 adsorption isotherm using the Barrett-Joyner-Halenda (BJH) algorithm. X-ray photoelectron spectroscopy (XPS) data were obtained on a PHI 5000 Versa Probe high performance electron spectrometer, with a monochromatic Al Kα radiation (1486.6 eV, 15 kW). To compensate for surface charge effects, adventitious C 1s (284.6 eV) were used to calibrate the binding energies. H2-temperature programmed reduction (H2-TPR) was conducted on a quartz U-tube reactor connected to a TCD detector. The sample (25 mg) was frist pretreated in a N2 stream at 200 °C for 1 h, and then switched to a H2-Ar stream (7 % H2 by volume) at a temperature rate of 10 °C min-1. NH3-temperature programmed desorption (NH3-TPD) experiments were carried out on a multifunction chemisorption analyzer with a TCD detector. The sample (0.1 g) was frist pretreated with N2 (40 ml min-1, 600 °C, 1 h). After cooling to room temperature, a flow of NH3 (1 vol.%, 10 ml min-1) was switched on for 1 h until the adsorption was saturated. Subsequently, the sample was flushed again with highly purified N2 gased at 100 °C for 1 h to remove the weakly adsorbed NH3. Finally, the temperature was raised up to 600 °C (10 °C min-1) and corresponding data were collected. In situ DRIFT spectra were obtained on a Nicolet Nexus 5700 FTIR spectrometer at a spectral resolution of 4 cm-1 (number of scans, 32) using a high-sensitivity MCT detector cooled by liquid N2. Prior to adsorption experiments, the sample was 5



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pretreated in N2 (400 °C, 1 h) to eliminate physisorbed water and other impurities. Background spectra were collected at each target temperature during cooling process and automatically subtracted from the sample spectra. Flows of NH3-N2 (500 ppm NH3) or NO-N2+O2-N2 (500 ppm NO and 5% O2 by volume) were switched on for about 45 min till adsorption saturation at the required temperature. After this, weakly adsorbed molecules were purged by N2, and desorption/reaction performances were studied by recording the spectra at each desired temperature from 30 °C to 400 °C or collecting the samples under the opposite gases atmosphere (NO-N2+O2-N2/ NH3-N2) as a function of time. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) photos were performed on a JEM-2100 microscope (200 kV). The sample was ultrasonically suspended in ethanol, and then deposited on a carbon film supported on copper grids.

2.3. Catalytic performance test Catalytic performance of these samples for NH3-SCR reaction was determined in a fixed-bed quartz reactor tube. The composition of feed stream was fixed with 500 ppm NO, 500 ppm NH3, 100 ppm SO2 (when used), 5 % H2O (when used), 5 % O2, and N2 in balance. The catalytic reactions were carried out at a gas flow rate of 100 ml min-1, and a GHSV of 60 000 h-1. An online Nicolet IS10 spectrometer equipped with a gas cell was used to determine the concentrations of the gases. The NH3-SCR activities of the catalysts were expressed by the following equations: NO conversion (%) = N2 selectivity (%) =

[NO]in – [NO]out [NO]in

×100

[NO]in – [NO]out +[NH 3 ]in – [NH 3 ]out – [NO 2 ]out – 2[N 2 O]out [NO]in – [NO]out  [NH 3 ]in – [NH 3 ]out

×100

3. RESULTS AND DISCUSSION 3.1 Catalytic test result 3.1.1 Reaction performance In order to obtain information about how the doped cations affected the reaction 6


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performance of Fe2O3, NH3-SCR catalytic activity of these synthesized catalysts were tested and the results are shown in Figure 1(a) and (b). The highest NOx conversion for pure Fe2O3 sample was only 50 % in a temperature range of 100-400 °C. After introducing doped cations (M=Ti4+, Ce3+/4+, Al3+), the activity was significantly enhanced. The NOx conversion was by the order of Fe9Ti1Ox > Fe9Al1Ox > Fe9Ce1Ox. Moreover, Fe9Ti1Ox sample not only showed the best catalytic activity but also possessed the widest operating temperature window, with over 80% NOx conversion in the range of 150- 350 °C. Acidity and redox properties are two important aspects for NH3 adsorption and activation.30 Presumably, these two factors may also be regulated by the doping effect to different degrees. Ti4+ has both redox and acidity properties, which can partly explain the optimal performance of Fe9Ti1Ox catalysts. The higher NOx conversion of Fe9Al1Ox compared to Fe9Ce1Ox indicates that acidity is perhaps more necessary for Fe-based catalysts. Furthermore, N2 selectivity were also increased at temperatures above 250 ℃ after doping the cations, and followed the same order as NOx conversion: Fe9Ti1Ox > Fe9Al1Ox > Fe9Ce1Ox > Fe2O3. According to the literature, N2 selectivity declination at high temperature might be attributed to the unselective catalytic oxidation of ammonia,24, 28 so we can suggest that the redox properties of Fe2O3 at high temperature may be suppressed to a certain extent due to the doping operation.

3.1.2 Effect of H2O and SO2 on the catalytic performance In practical applications, the effluent gases of coal-fired power plants always contain some amount of SO2. Therefore, 100 ppm SO2 was injected into the reaction system to explore its influence on the catalytic activity of Fe9M1Ox samples and the corresponding results were displayed in Figure 2(a). A similar trend was exhibited by all of these catalysts during the test process. After SO2 was introduced, NOx conversion of Fe9M1Ox catalyst first declined and then gradually stabilized. Fe9Ti1Ox exhibited the best SO2 resistance with a stabilized NOx conversion of approximately 80%, which might be owe to the favorable SO2 resistance of TiO2.31 While for Fe9Ce1Ox and Fe9Al1Ox catalysts, the activity showed an obvious decrease and the 7



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final NOx conversion declined to ~ 60 %. It is generally known that water is also an inevitable containment in practical reactions, so it is also necessary to investigate the influence of water and SO2 coexisted in flue gases. Thus, H2O and SO2 durability of Fe9Ti1Ox at 225 °C and 250 °C were evaluated through a long-time tests and the results are illustrated Figure 2(b) and Figure 2(c), respectively. As shown in Figure 2(b), after introducing H2O, the NOx conversion of Fe9Ti1Ox catalyst remained unchanged for several hours, which represented 5 vol. % H2O in the feed gases at 225 °C has negligible influence on the catalytic performance of Fe9Ti1Ox. When SO2 was also injected, the catalytic performance declined continuously and dropped to 60 % after 85 hours. While for the H2O and SO2 durability test at 250 °C, as shown in Figure 2(c), it can be seen that for the first 3 h, without H2O, NOx conversion approached 95 %. When H2O was introduced, NOx conversion had a slightly decline and then remained stable for several hours. Afterwards, 100 ppm SO2 was also injected into the reaction system, we can find that the activity showed very little change, remaining stable for the rest of the time (10-82 h). Thus, Fe9Ti1Ox catalyst showed a ~80 % NOx conversion in the presence of H2O and SO2 at 250 °C. This shows that doping in Ti4+ is an effective solution for the enhancement of H2O and SO2 durability at 250 °C

32-35

. To gain

further insight into the catalytic performance of Fe9Ti1Ox catalyst and to understand the different performances arising from different doping cations (M=Ti4+, Ce3+/4+, Al3+), we characterized their structural and chemical properties.

3.2 Physical and textural properties (XRD, BET and TEM) XRD was adopted to identify the crystal structures of Fe9M1Ox series catalysts. It can be seen in Figure 3 that pure Fe2O3 displayed well-defined diffraction peaks, which corresponded to hematite Fe2O3 (PDF-ID 84-0309). While for Fe9M1Ox catalysts, the emergence of broad bump diffraction patterns indicated the formation of amorphous phase or microcrystalline phase,36 and thereby the maximized interaction between Fe and Mn+ cations (Fe-O-M species, M= Ti4+, Ce3+/4+, Al3+).37 This is 8


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conductive to the improvement of pore volumes, specific surface areas and catalytic activity.37-39 Similar phenomenon was also observed by Li et al., they found that Ce-O-Ti short-range order species with an atomic scale was on the amorphous Ce-Ti oxide sample that showed better activity than its crystalline counterpart at lower temperatures.40 Furthermore, the diffraction peaks of Fe9Ti1Ox and Fe9Ce1Ox were relatively smoother than those of Fe9Al1Ox, which could be explained by the stronger Fe-O-Ti (Ce) interactions between Fe3+/2+ and the reducible cations Ti4+ and Ce4+/3+.

Figure S1(a) shows the N2 adsorption-desorption isotherms plots of Fe9M1Ox catalysts. These isotherms were classified to type IV as defined by IUPAC,37, 38 which belong to mesoporous materials with pore diameters between 2-50 nm. For pristine Fe2O3, the hysteresis loop exhibited a typical H3 type due to the slitlike pore structure. After doping other cations, hysteresis loop type changed to H2 type due to the ink-bottle-shaped pore structure, suggesting the injection operation altered the pore structure.42,43 The pore size distribution displayed in Figure S1(b) shows that Fe9M1Ox mainly possessed pore diameters around 4 nm, which are obviously smaller than those of Fe2O3.

BET surface areas, pore volumes and average pore diameters of the samples calculated from the isotherms are shown in Table S1. Fe9M1Ox catalysts had larger surface areas, pore volumes and smaller pore diameters compared to pristine Fe2O3, which can be attributed to the inhibition of individual crystallization during coprecipitation,44 as exhibited by XRD.

The effects of different doping cations (M=Al3+, Ce3+/4+, Ti4+) on the surface morphologies and degree of crystallinity were characterized by TEM, together with insert SAED (selected area of electronic diffraction). As shown in Figure 4(a) and 4(e), pristine Fe2O3 were mainly present in irregular elliptical forms with particle sizes of approximately 30-50 nm. Preferentially exposed crystal planes (110) and (012) with interplanar spacing of 0.25 nm and 0.35 nm were detected.44 SAED result inserted in 9



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Figure 4(a) indicated Fe2O3 particles were polycrystalline. For Fe9Al1Ox sample in Figure 4(b), the particle size decreased significantly compared to Fe2O3, with an irregular particle size around 15 nm. SAED pattern (insert) expressed that Fe9Al1Ox sample was also well crystallized and the finger distance of 0.25 nm, 0.35 nm and 0.22 nm were in line with the lattice distance of (110), (012) and (113) planes of hematite Fe2O3. Fe9Ce1Ox and Fe9Ti1Ox samples showed wormhole-like floccules aggregated from small round nanoparticles below 10 nm. Considering the absence of legible diffraction spots, but that some sporadic diffraction spots were scattered on the amorphous diffraction rings in the SAED results, we concluded that Fe9Ti1Ox and Fe9Ce1Ox samples mainly consisted of amorphous phases and included little remaining crystalline Fe2O3. Crystal lines could only be found in the bulk Fe2O3 particles, which was consistent with the XRD and BET results.

3.3 Reduction properties (H2-TPR) Reduction behavior is an important aspect for NH3-SCR reaction. Therefore, H2-TPR was employed to investigate the redox properties of these samples, and the corresponding results are presented in Figure 5. As has been previously reported, 25,41 the reduction of pure Fe2O3 happened stepwise, labeled as  (200-400 °C) and  (400-800 °C). Peak  could be ascribed to the reduction of Fe2O3 to Fe3O4, and peak  originated from the multiple reduction of Fe3O4 to FeO and Fe0. From the onset reduction temperatures listed in Table 1, we can find that Fe9Ti1Ox and Fe9Ce1Ox had an initial reduction temperature of ~ 184 °C, which were much lower than that of Fe2O3, indicating the reducibility of Fe9Ti1Ox and Fe9Ce1Ox were improved by the doping process. Furthermore, the integrated area ratio / of Fe9M1Ox samples were bigger than that of bulk Fe2O3. A larger ratio is known to be due to a greater extent of reduction, which may be explained by the size-effect and the Fe-O-M (M=Al3+, Ce3+/4+, Ti4+) interaction. Thus we can suppose that the extent of reduction of Fe9M1Ox catalysts were regulated, helping to both improve the activity in low temperatures and the inhibition of NH3 oxidation in high temperatures.25 10


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3.4 Surface chemical states analysis (XPS) XPS characterization was further employed to investigate the surface oxygen species of catalysts, and two fitted peaks are presented in Figure 6(a). The sub-bands at lower binding energies were ascribed to lattice oxygen species, denoted as O. The O peaks correspond to surface adsorbed oxygen species, such as O22- or O-.45 Because of their high mobility, surface oxygen species are more reactive than lattice oxygen species, thus are more beneficial to the oxidation of NO to NO2, facilitating the ‘‘fast NH3-SCR” process.42,

46

From Table 2, it can be clear seen that the O ratio in

Fe9M1Ox samples were significantly increased, indicating the formation of more surface oxygen species. Moreover, Fe9Ti1Ox had a comparable surface adsorbed oxygen to Fe9Ce1Ox, both higher than Fe9Al1Ox. This suggests that the doped Ti4+ and Ce3+/4+ cations were more beneficial for oxygen mobility.

Similarly, the interactions between Fe2+/3+ species and doped cations (Al3+, Ce3+/4+, Ti4+) were investigated by analyzing the chemical environments of Fe3+/2+ species. As shown in Figure 6(b), well-defined peaks assigned to Fe 2p3/2 at 710.5 eV and Fe 2p1/2 at 724.5 eV were detected for all the samples. At the same time, a satellite peak at ~719 eV, a characteristic feature of Fe3+ species, also appeared.10, 38 The peak positions of Fe9M1Ox clearly shifted to higher binding energy compared to the bulk Fe2O3. This may be due to an influence on the electron cloud of iron species in Fe9M1Ox samples by the doped cations.38, 42 In Figure 6(c), the Fe 2p3/2 spectra were fitted with two peaks for each sample, the peaks located at 710 eV and 711 eV were attributed to Fe2+ and Fe3+, respectively.45,

47

As shown in Table 2, the relative

concentration of Fe2+ increased after doping the caions, and could be ranked in the order: Fe9Ti1Ox ≈ Fe9Ce1Ox > Fe9Al1Ox > Fe2O3. Generally speaking, the appearance or change in content of an intermediate valence state is often caused by enhanced interactions, imbalanced electric charge or the generation of surface oxygen vacancies, and these factors usually lead to improvements in the catalytic performance.48, 49 Thus, we can speculate that increasing the Fe2+ content should be favorable to promote the NH3-SCR reaction. To summarize the XPS results, we can conclude that the 11



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introduction of foreign cations (Al3+, Ce3+/4+, Ti4+) exhibited some positive effects relative to the pure Fe2O3. Moreover, stronger Fe-O-Ti (Ce) interactions resulted in a higher amount of active Fe2+ species, surface oxygen species and better redox properties.

3.5 Surface acidity (NH3-TPD) Surface acidity of catalysts can efficiently affect the adsorption and desorption behaviors of NH3, which further leads to different catalytic performances in NH3-SCR reaction. In the present work, NH3-TPD was chosen to investigate the surface acidity of the catalysts, the results were shown in Figure 7. It is well known that the temperature of desorption peak is related to surface acid strength. Thus, peaks below 300 oC can be attributed to the desorption of NH3 from weak acid sites, corresponding to the desorption of physisorbed NH3, while the peaks at the temperatures above 300 o

C can be ascribed to the desorption of NH3 from strong acid sites, due to the

desorption of chemisorbed NH3 species.45 The area under the desorption peak is proportional to the number of acid sites. Herein, we can deduce that doping foreign cations greatly enhanced NH3 adsorption ability, thus promoted the NH3-SCR performance. Moreover, the peak areas for Fe9Ti1Ox and Fe9Al1Ox were larger than Fe9Ce1Ox, which meant Fe9Ti1Ox and Fe9Al1Ox had more surface acid sites than Fe9Ce1Ox.

3.6 In situ DRIFT analysis 3.6.1 NH3 adsorption The influence of doping cations on surface acidity was further characterized by in situ NH3-DRIFT. Figure 8 showed the spectra of NH3 desorption recorded as a function of temperature. The bands in the region of 1140-1240 cm-1 and 1580-1610 cm-1 could be assigned to Lewis acid sites, whereas the bands located in 1415-1465 cm-1 and 1670-1695 cm-1 were ascribed to coordinated NH4+ chemisorbed on Brønsted acid sites.33, 2, 49,50 All of these infrared vibration signals weaken gradually with increasing temperature. However, the bands ascribed to Lewis acid sites 12


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exhibited larger peak areas and higher intensity stability compared to Brønsted acid sites at the same temperature, indicating that there were more Lewis acid sites and they were stronger than the Brønsted acid sites. Furthermore, consistent with NH3-TPD results, the peaks of Fe9Ti1Ox showed the highest intensity, meaning Fe9Ti1Ox can adsorb and activate the most NH3 species, resulting in its excellent catalytic performance compared to the other catalysts.

3.6.2 The reaction of NO+O2 with pre-adsorbed NH3 To explore the reaction pathways of adsorbed NH3 species and NO on the Fe9M1Ox catalysts, the interaction between NO+O2 with pre-adsorbed NH3 species as a function of time were studied using in situ DRIFT spectra. As shown in Figure 9, the bands at 1660-1550 cm-1 and 1250-1220 cm-1 corresponding to Lewis acid sites were observed on all of the samples. Moreover, the bands intensity of Lewis acid sites for Fe9Ti1Ox were stronger than those of Fe9Al1Ox and Fe9Ce1Ox, which represented more abundant thermal stable surface acid sites were presented in Fe9Ti1Ox sample at 250 oC. When NO+O2 were injected, the bands attributed to absorbed NH3 species diminished rapidly, accompanying with the generation of nitrate species (1628 cm-1 for bridging bidentate nitrates, 1601 and 1204 cm-1 for bridging monodentate nitrates, 1265 cm-1 for bidentate nitrates).25, 49, 51 The phenoemnon indicates that Eley-Rideal (E-R) reaction pathway proceeded via pre-adsorbed NH3 and the NOx gas over Fe9M1Ox samples.10 Based on the above results, it seems that doping Ti4+ cations promoted the adsorption and activation of NH3 molecules, and thus further improved the performance of NH3-SCR reaction.

3.6.3 Reaction of NH3 with pre-adsorbed NO+O2 In addition, in situ DRIFT spectra were obtained through exposing NH3 molecular upon samples pre-adsorbed by NO+O2 species at 250 oC as a function of time, and the reslults were shown in Figure 10. It can be clearly seen that prior to the injection of NH3 gas, the surfaces of catalysts were dominated by various nitrates species, such as bridging nitrates (1611-1615 cm-1), bidentate nitrates (1006-1009 cm-1, 1520-1540 13



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cm-1 and 1580-1583 cm-1) and monodentate nitrates (1213-1230 cm-1 and 1489 cm-1).52, 53 Similar reslults were detected after the introduction of NH3 even for the catalysts doped with different cations. Unlike in the case of pre-adsorbed NH3 in Figure 9, most of the pre-adsorbed NOx species were stable and did not change even after the injection of NH3 for 30 min. Bands corresponding to adsorbed NH3 species were also not observed. These results indicated that NH3 molecules not only can not displace the pre-adsorbed NOx from the catalyst surfaces, but also do not react with the pre-adsorbed NOx species. This suggestes the NH3-SCR reaction did not proceed via Langmuir-Hinshelwood (L-H) mechanism.54 These results demonstrated that the choice of doping cations for Fe-based catalysts was essentially determined by suitable balancing of acidity and redox properties. Take into account the intrinsic redox properties of pure Fe2O3, the selected Ti4+ cation, which showed the best acidity and some redox properties, exhibited better catalytic performance compared to the weakly acidic Al3+ or basic Ce3+/4+ dopants. Additionally, the reducible cations (Ti4+, Ce3+/4+) facilitated the formation of Fe-O-M species, which in turn optimized the redox behavior of Fe2O3 at different temperatures, thus resulting in better low temperature NOx conversion and high temperature N2 selectivity. Moreover, doping with acidic cations (Ti4+, Al3+) provided abundant acid sites for the activation of NH3 in NH3-SCR reaction. A possible mechanism for the reaction of Fe9M1Ox catalysts at 250 °C was proposed, suggesting that the reaction between adsorbed NH3 species and gaseous NO and O2 molecules (E-R process) was the crucial NH3-SCR mechanism (Scheme 1).

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4. CONCLUSIONS The present work compared the influence of different doped cations on the structure, morphology, reduction, acidity and NH3-SCR catalytic performance of Fe9M1Ox (M=Ti4+, Ce3+/4+, Al3+) catalysts. Combining with the characterizations of the series catalysts, following conclusions can be obtained: (1) Ti4+ doped Fe9Ti1Ox catalyst exhibited better catalytic performance than Fe9Ce1Ox and Fe9Al1Ox, which can be attributed to the appropriate regulation of redox properties and acidity of Fe2O3 by Ti4+. (2) The interactions between doping reducible cations and Fe3+ lead to a higher ratio of surface oxygen species and Fe2+ species, and also enhanced the reducibility at low temperatures. (3) Fe9Ti1Ox showed a pronounced ~ 80% conversion of NOx and remained stable in the presence of SO2 and H2O at 250 °C for 80 h. Thus it suitable, as a candidate of NH3-SCR catalyst.

ASSOCIATED CONTENT: Supporting information: N2 adsorption-desorption isotherms plots, BET surface areas, pore volumes and average pore diameters of the Fe9M1Ox samples calculated from the isotherms.

ACKNOWLEDGMENTS The financial supports of National High-tech Research and Development (863) Program of China (2015AA03A401), the National Natural Science Foundation of China (21303082, 21607019, 21607122) are gratefully acknowledged.

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Table 1. Peak information of H2-TPR results of Fe9M1Ox catalysts Table 2 XPS results of Fe9M1Ox catalysts Figure 1. Catalytic performance of Fe9M1Ox catalysts for NH3-SCR reaction: (a) NOx conversion; (b) N2 selectivity. Reaction conditions: 500 ppm NO, 500 ppm NH3, 5% O2, GHSV = 60,000 h-1. Figure 2. The influence of SO2 and H2O on NOx conversion in the NH3-SCR reaction: (a) SO2 durability over Fe9M1Ox catalysts at 250 °C, (b) H2O/H2O+SO2 durability over Fe9TiOx catalysts at 225 °C, (c) H2O/H2O+SO2 durability over Fe9TiOx catalysts at 250 °C. Reaction conditions: 500 ppm NO, 500 ppm NH3, 5 % O2, 100 ppm SO2 (when used), 5 % H2O (when used), GHSV = 60,000 h-1. Figure 3. XRD patterns of Fe9M1Ox catalysts. Figure 4. TEM (a-d), HRTEM (e-h) images and SAED (insert) pattens of Fe2O3 (a, e), Fe9Al1Ox (b, f), Fe9Ce1Ox (c, g) and Fe9Ti1Ox (d, h) catalysts. Figure 5. H2-TPR profiles for Fe9M1Ox catalysts. Figure 6. XPS spactra of O 1s (a), Fe 2p (b) and Fe 2p3/2 (c) of the catalysts. Figure 7. NH3-TPD results of the Fe9M1Ox catalysts. Figure 8. In situ DRIFT spectra of NH3 desorption over (a) Fe9Al1Ox, (b) Fe9Ce1Ox and (c) Fe9Ti1Ox in the temperature range of 100-400 oC Figure 9. In situ DRIFT spectra of NO+O2 adsorption over (a) Fe9Al1Ox, (b) Fe9Ce1Ox and (c) Fe9Ti1Ox samples pre-adsorped NH3 at 250 oC Figure 10. In situ DRIFT spectra of NH3 adsorption over (a) Fe9Al1Ox, (b) Fe9Ce1Ox and (c) Fe9Ti1Ox samples pre-adsorped NO+O2 at 250 oC Scheme 1. Possible reaction mechanism of Fe9M1Ox catalysts doping with different cations at 250 °C

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Table 1. Peak information of H2-TPR results of Fe9M1Ox catalysts. Initial reduction temperatures (°C) 253 184 184 270

Samples Fe2O3 Fe9Ti1Ox Fe9Ce1Ox Fe9Al1Ox

Peak Area  (a.u.) 51 53 34 51

Peak Area  (a.u.) 489 338 236 253

A/A 0.10 0.16 0.14 0.20

Table 2 XPS results of Fe9M1Ox catalysts. Atomic concentration (%)

AFe2+/(AFe3++AFe2+) （%）

O

Fe

M

C

A O/(AO+AO) (%)

Fe2O3

47.60

21.73

---

30.67

19.4

32.2

Fe9Al1Ox

50.32

19.54

3.27

26.87

25.7

41.3

Fe9Ce1Ox

48.10

18.39

3.60

29.91

31.5

58.2

Fe9Ti1Ox

51.02

21.01

2.56

25.41

31.2

57.6

Samples

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Figure 1. Catalytic performance of Fe9M1Ox catalysts for NH3-SCR reaction: (a) NOx conversion; (b) N2 selectivity. Reaction conditions: 500 ppm NO, 500 ppm NH3, 5% O2, GHSV = 60,000 h-1.

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Figure 2. The influence of SO2 and H2O on NOx conversion in the NH3-SCR reaction: (a) SO2 durability over Fe9M1Ox catalysts at 250 °C, (b) H2O/H2O+SO2 durability over Fe9TiOx catalysts at 225 °C, (c) H2O/H2O+SO2 durability over Fe9TiOx catalysts at 250 °C. Reaction conditions: 500 ppm NO, 500 ppm NH3, 5 % O2, 100 ppm SO2 (when used), 5 % H2O (when used), GHSV = 60,000 h-1.

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Figure 3. XRD patterns of Fe9M1Ox catalysts.

Figure 4. TEM (a-d), HRTEM (e-h) images and SAED (insert) pattens of Fe2O3 (a, e), Fe9Al1Ox (b, f), Fe9Ce1Ox (c, g) and Fe9Ti1Ox (d, h) catalysts.

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Figure 5. H2-TPR profiles for Fe9M1Ox catalysts.

Figure 6. XPS spactra of O 1s (a), Fe 2p (b) and Fe 2p3/2 (c) of the catalysts.

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Figure 7. NH3-TPD results of the Fe9M1Ox catalysts.

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Figure 8. In situ DRIFT spectra of NH3 desorption over (a) Fe9Al1Ox, (b) Fe9Ce1Ox and (c) Fe9Ti1Ox in the temperature range of 100-400 oC

Figure 9. In situ DRIFT spectra of NO+O2 adsorption over (a) Fe9Al1Ox, (b) Fe9Ce1Ox and (c) Fe9Ti1Ox samples pre-adsorped NH3 at 250 oC

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Figure 10. In situ DRIFT spectra of NH3 adsorption over (a) Fe9Al1Ox, (b) Fe9Ce1Ox and (c) Fe9Ti1Ox samples pre-adsorped NO+O2 at 250 oC

Scheme 1. Possible reaction mechanism of Fe9M1Ox catalysts doping with different cations at 250 °C.

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