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Kinetics, Catalysis, and Reaction Engineering

Selective catalytic reduction of NOx with NH3 over novel Fe-Ni-Ti catalyst Zhiming Liu, Xu Feng, Zizheng Zhou, and Qi Xu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01387 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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Industrial & Engineering Chemistry Research

Selective catalytic reduction of NOx with NH3 over novel Fe-Ni-Ti catalyst Zhiming Liu a, c*, Xu Fenga, c, Zizheng Zhoua, c, Qi Xub∗ a

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China

b

Jiangsu Collaborative Innovation Center for Ecological Building Materials and Environmental Protection Equipments, Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, Yancheng Institute of Technology, Yancheng 224051, China

c

Beijing Key Laboratory of Energy Environmental Catalysis, Beijing University of Chemical Technology, Beijing 100029, China

* Corresponding author. Tel: +86-10-64427356; E-mail: [email protected] (Z. Liu)； [email protected] (Q. Xu)

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Abstract: A novel Fe-Ni-Ti composite oxide prepared via the hydrothermal method has been developed for the selective catalytic reduction of NOx with NH3. This environmentally benign catalyst showed high activity and excellent selectivity to N2, which is superior to that of Fe-Ti and Ni-Ti catalysts. Catalyst characterization results revealed that over Fe-Ni-Ti catalyst the dual redox cycles (Fe3+ + Ni2+ ↔ Fe2+ + Ni3+, Ti4+ + Ni2+ ↔ Ti3+ + Ni3+) are crucial for the enhanced activity. The synergetic effect among Fe, Ni and Ti leads to not only the increased redox property, but also improved surface acidity. DRIFT experiments demonstrated that more reactive NH3/NH4+ and M-NO2 nitro species formed over Fe-Ni-Ti catalyst, thus resulting in the efficiently catalytic removal of NOx.

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1. Introduction Selective catalytic reduction of NOx by NH3 (NH3-SCR) has been demonstrated to be an effective method for the control of NOx emitted from stationary source and mobile source.1,

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V-based catalysts are widely used as a traditional commercial

de-NOx catalyst. But the arrow temperature window of 300-400℃，formation of N2O at high temperatures, and vanadium poisoning problem have limited its application.3, 4 Therefore, vanadium-free metal oxides for the catalytic reduction of NOx have been intensively studied recently. Among the metal oxides for the reduction of NOx by NH3,5-7 Fe-based catalyst has attracted more and more attention due to its environmentally benign character as well as the low price.8, 9 Fe-Ti spinel oxide exhibited high NOx reduction activity between 300-400 ℃.10 Liu et al.11 reported that FexTiOy was active for the NOx reduction at medium temperatures. The partial substitution of Fe by Mn in FexTiOy catalyst leads to higher activity, however, the selectivity to N2 is decreased due to the formation of N2O, especially above 200 ℃.12 Karami et al.13 found that the N2 selectivity is also decreased as the introduction of Cr to Fe-Ti catalyst, although the activity can be enhanced. The addition of Fe to Ce/TiO2 resulted in improved low-temperature activity and its resistance against SO2.14 The added Fe can improve the Brønsted acid sites, which is beneficial for the NH3-SCR.15 In the NOx reduction by NH3 pristine Ni oxide is inactive, however, the selectivity to N2 is very high and nearly 100% selectivity can be obtained.16 The introduction of Ni to Mn/TiO2 catalyst contributes to forming active MnO2 species, thus resulting in 3



higher SCR activity.17 The synergetic effect between Ni and Mn can lead to enhanced acid sites,18 which facilitating the adsorption of NH3. The promoting effect of Ni on Ce-based catalyst was also reported. The introduction of NiO to CeO2 nanorods induces more Ce3+ and Oα formed, both of which contributes to improving the SCR activity. As a result, NiO/CeO2 nanorods exhibited much higher activity than the pure CeO2 catalyst.19 The synergetic effect between Ni and Ce also existed over Ni-Ce-Ti mixed oxide catalyst.20 To date, seldom research on the Fe-Ni-Ti composite oxide for the NOx reduction by NH3 has been reported. Considering the redox behavior of Fe oxides, the excellent N2 selectivity of Ni oxide and the high SO2 durability of TiO2, herein a novel Fe-Ni-Ti mixed oxide catalyst has been fabricated and it is highly active and selective for the NOx reduction. Based on the catalyst characterization, the synergetic effect among Fe, Ni and Ti over Fe-Ni-Ti catalyst has been revealed.

2. Experimental 2.1. Catalyst preparation Fe-Ni-Ti catalysts were fabricated by the hydrothermal method, which is a useful method for the synthesis of composite oxide catalyst over which the strong interaction among different metal species exists.20, 21 Herein Fe(NO3)2·3H2O, Ni(NO3)3·6H2O and Ti(SO4)2 were the precursors and the preparation procedures are the same as those reported previously.20 The precipitate obtained after the hydrothermal reaction were dried and calcined at 500 ℃ for 6 h in air. In addition, Fe-Ti and Ni-Ti catalysts were 4


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also synthesized by the same method. These catalysts were designated by FexNiyTi1-x-y(x, y represents the molar ratios of Fe/(Fe+Ni+Ti) and Ni/(Fe+Ni+Ti), respectively). 2.2. Catalytic test Catalytic performance evaluation was performed in a fixed-bed reactor using 0.12 g sample of 40-60 meshes. The reaction mixture in a N2 stream consisted of 500 ppm NO, 500 ppm NH3, 5% O2, 0 or 50 ppm SO2, 0 or 5% H2O, and GHSV is 140,000 h-1. In order to calculate the NOx conversion and N2 selectivity,20 the concentrations of NOx, NH3 and N2O were monitored continuously by a chemiluminescence NO/NO2 analyzer (Thermo Scientific, model 42i-HL) and a FTIR spectrometer (Gasmet FTIR DX4000). 2.3. Catalyst characterization BET surface area and pore structure of the catalysts were determined by N2 adsorption-desorption isotherms20 obtained on a Micromeritics ASAP 2020 HD88. The catalysts were degassed at 300 ℃ for 4 h before measurement. XRD was performed on a Bruker D8 ADVANCE X-ray diffractometer with Cu Kα radiation. XPS spectra were recorded on the electron spectrometer (VG Scientific, ESCALab220i-XL) with Mg Kα as the excitation source. The binding energy was calibrated by that of C 1s peak (284.8 eV). NH3-TPD and H2-TPR were obtained on a chemisorption analyzer (Micromeritics AutoChem II 2920). In NH3-TPD experiment, the sample was pretreated under N2 atmosphere at 350 ℃ for 1h, then cooled down to 100 ℃.Subsequently the sample 5



was fed with 10%NH3/N2 until it was saturated, then it was purged with N2 to blow away the physisorption NH3. Finally, the NH3-saturated samples was heated at 10 ℃ min−1 to 550 ℃. For H2-TPR, the sample was pretreated at 300 ℃ for 1 h under N2 atmosphere, then cooled down to room temperature, subsequently it was heated in 10% H2/N2 gas flow to 800 ℃ at 10 ℃ min-1. 2.4. In-situ DRIFTs study In-situ DRIFT spectra were collected on Nicolet NEXUS 6700 FTIR spectrometer, which is equipped with smart collector and MCT detector. All the samples were pretreated in N2 flow at 400 ℃ for 1 h prior to each experiment, then the background spectrum was collected in N2 flow at each temperature. The sample spectrum can be obtained by subtracting the background spectrum at the same temperature. For transient studies, the sample was pre-adsorbed NH3 (or NO+O2) flow for 60 min at 350 ℃, then switching the gas to N2 to remove the weakly adsorbed species, followed switching to NH3 (or NO+O2) to record the dynamic spectra. The resolution for all spectra collected is 4 cm−1 by accumulating 100 scans.

3. Results and discussion 3.1. Catalytic performance NOx reduction activities and N2 selectivities of Fe0.3Ti0.7, Ni0.2Ti0.8 and Fe0.3Ni0.2Ti0.5 catalysts are exhibited in Figure 1. Ni0.2Ti0.8 is inactive for NOx reduction by NH3. Fe0.3Ti0.7 catalyst is more active than Ni0.2Ti0.8 catalyst, with the maximum NOx conversion of 84% obtained at 350 ℃ . Above 350 ℃ , NOx 6


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conversion is declined significantly. In contrast, Fe0.3Ni0.2Ti0.5 catalyst is much more active than Fe0.3Ti0.7 and Ni0.2Ti0.8 catalysts between 300-450 ℃，in which over 90% NOx conversion is obtained. Regarding on the N2 selectivity, nearly 100% N2 selectivity is achieved on Fe0.3Ni0.2Ti0.5 catalyst. In contrast, for Fe0.3Ti0.7 catalyst the N2 selectivity is decreased with increasing the reaction temperature. Therefore, the synergistic effect existed over Fe0.3Ni0.2Ti0.5 due to the co-existence of Fe and Ni, which not only resulted in high NH3-SCR activity but also superior N2 selectivity. Among different Fe-Ni-Ti catalysts evaluated Fe0.3Ni0.2Ti0.5 is the most active one (see Figure S1). Under practical working conditions H2O and SO2 are usually present, and can exert inhibiting effect on the activity of NH3-SCR catalyst.22 Therefore, the effects H2O and SO2 on the activity of Fe0.3Ni0.2Ti0.5 catalyst at 350 ℃ were investigated and the results were illustrated in Figure 2. A slight inhibiting effect of H2O is observed. It can be seen that the inhibiting effect of SO2 is more noticeable than the effect of H2O, more decrease of NOx conversion (94% →80%) appeared due to the introduction of SO2. In the co-existence of H2O and SO2 the inhibiting effect becomes a little bit more noticeable. The suppression is due to the formed ammonium sulfate on the surface of the catalyst.23 After cutting off H2O (or SO2/H2O+SO2), the NOx conversion were recovered to the initial level. So the inhibiting effect of H2O (or SO2/H2O+SO2) is reversible. 3.2. Characterization of catalyst 3.2.1. Structure properties 7



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Table 1 presents the specific surface area and total pore volume of Fe0.3Ti0.7, Ni0.2Ti0.8 and Fe0.3Ni0.2Ti0.5 catalysts. Fe0.3Ni0.2Ti0.5 catalyst is of much larger surface area than those of Fe0.3Ti0.7 and Ni0.2Ti0.8 catalysts. Larger BET surface area could benefit for the NH3-SCR activity.15 From the XRD patterns shown in Figure 3, the diffraction peaks related to anatase TiO2 and rutile TiO224, 25 are detected in both Fe0.3Ti0.7 and Ni0.2Ti0.8 catalysts. In addition, NiTiO3 phase20 is also observed over Ni0.2Ti0.8 catalyst. Compared with Ni0.2Ti0.8 catalyst, the diffraction peaks over Fe-Ni-Ti catalysts becomes much weaker due to the introduction of Fe species, and even no obvious peak appeared on Fe0.3Ni0.2Ti0.5, suggesting this catalyst is of amorphous structure. The change of the structure reflects a strong interaction among Fe, Ni and Ti existed, which inhibits the crystallization of the FeOx, NiOx and TiO2 phase. The amorphous structure of Fe0.3Ni0.2Ti0.5 leads to improved surface area as shown in Table 1. 3.2.2. XPS analysis The chemical state of the surface species were measured by XPS and the results were illustrated in Figure 4. Fe 2p XPS spectra for Fe0.3Ti0.7 and Fe0.3Ni0.2Ti0.5 catalysts are shown in Figure 4(a). On Fe0.2Ti0.8 the peak at 711.5 eV is ascribed to Fe3+.24 Compared with Fe0.2Ti0.8, the Fe 2p peak over Fe0.3Ni0.2Ti0.5 catalyst was moved to lower BE by about 1.0 eV, suggesting that some Fe2+ was formed due to the introduction of Ni. For Fe0.3Ni0.2Ti0.5 catalyst, the peaks assigned to Fe3+ (711.9 eV) and Fe2+ (710.2 eV) can be observed after peak deconvolution.24, Fe2+/(Fe2++Fe3+) ratio is about 43%. 8


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The

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Figure 4(b) exhibited the Ni 2p XPS spectra for Ni0.2Ti0.8 and Fe0.3Ni0.2Ti0.5 catalysts. The Ni 2p3/2 spectra can be divided into two sub-bands: the band at 854.6 eV ascribed to Ni2+ and that at 856 eV corresponded to Ni3+.20, 28, 29 In addition, the band at high binding energy (861.1-867.7 eV) is assigned to the satellite peak.20 The Ni3+/(Ni2++Ni3+) ratio over Fe0.3Ni0.2Ti0.5 is about 62.4%, which is significantly higher than that of Ni0.2Ti0.8 catalyst (37.1%). Over Fe0.3Ni0.2Ti0.5 catalyst, the electron can be transferred from Ni to Fe species by the redox cycle (Fe3++ Ni2+ ↔ Fe2++ Ni3+), thus leading to the high ratio of Ni3+ and Fe2+ species. Previous research proposed that the reduction of Fe3+ to Fe2+ is crucial for high-temperature activity of Fe-based catalyst.27 The formed Fe2+ over Fe0.3Ni0.2Ti0.5 catalyst accounts for its higher activity than that of Fe0.3Ti0.7 catalyst above 300 ℃ as shown in Figure 1(a). Figure 4(c) illustrates the Ti 2p spectra. The band at 458.8 eV over both Fe0.3Ti0.7 and Ni0.2Ti0.8 is related to Ti4+,30, 31 and the band position shifts toward to lower BE by 1.0 eV over Fe0.3Ni0.2Ti0.5 catalyst. It implies some Ti3+ formed,30, 32 which can be ascribed Ti4+ accepts electron from Ni2+. This electron transfer between Ni2+/Ni3+ and Ti4+/Ti3+ was also reported by Chen et al..29 In addition, the O 1 s XPS information of Fe0.3Ti0.7, Ni0.2Ti0.8 and Fe0.3Ni0.2Ti0.5 catalysts is shown in Figure 4 (d). The peak located at 531-532 eV is assigned to Oα (surface chemisorbed oxygen), while that at 529.0-530.0 eV is ascribed to Oβ (lattice oxygen).33-35 It is evident that Oα/(Oα+Oβ) over Fe0.3Ni0.2Ti0.5 is much higher than those of Fe0.3Ti0.7 and Ni0.2Ti0.8 catalysts, suggesting that more surface adsorbed oxygen existed over Fe0.3Ni0.2Ti0.5 catalyst. Previous research demonstrated that the 9



surface adsorbed oxygen is beneficial for the NOx reduction.23, 24 3.2.3. Redox property (H2-TPR) The redox property of catalyst is closely related to its NH3-SCR performance. Therefore, H2-TPR was conducted to investigate the redox ability of catalyst. As shown in Figure 5, Fe0.3Ti0.7 and Ni0.2Ti0.8 catalysts showed a single reduction peak at 375 ℃ and 493 ℃, respectively. For Fe0.3Ti0.7Ox catalyst, the reduction peak corresponds to the reduction of Fe3+ to Fe2+.24, 27 In the case of Ni0.2Ti0.8 catalyst, the reduction peak can be interpreted as the reduction of Ni2+ to Ni0.18 For Fe0.3Ni0.2Ti0.5 catalyst, there are two reduction peaks: the peak at 312 ℃ is due to the reduction of Fe3+ and the one at 352 ℃ is ascribed to the reduction of Ni2+. It is evident that the reduction temperatures of Fe and Ni species are lower than that of Fe0.3Ti0.7 and Ni0.2Ti0.8 catalysts. The two reduction peaks overlapped, which indicates the co-reduction of Fe and Ni occurred. The synergetic effect makes the reduction of Fe3+ and Ni2+ become easier to proceed. Therefore, the redox property of Fe0.3Ni0.2Ti0.5 catalyst is enhanced, which promotes the NOx reduction. 3.2.4. Surface acidity property (NH3-TPD) The surface acidity property was a key factor for the activity of the NH3-SCR catalysts. NH3-TPD profiles of Fe0.3Ti0.7, Ni0.2Ti0.8 and Fe0.3Ni0.2Ti0.5 catalysts were illustrated in Figure 6. For Fe0.3Ti0.7 and Ni0.2Ti0.8 catalysts, only one peak related to weakly absorbed NH3 or NH4+

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appears at about 160 ℃ . In contrast, over

Fe0.3Ni0.2Ti0.5 catalyst there are two desorption peaks at 173 and 430 ℃. The former peak is assigned to the desorption of weakly absorbed NH3 or NH4+, and the latter one 10


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can be assigned to the strongly absorbed NH3 or NH4+ species.7, 22 It is evident that the area of the NH3 desorption peaks of Fe0.3Ni0.2Ti0.5 is higher than those of Fe0.3Ti0.7 and Ni0.2Ti0.8 catalysts. Therefore, over Fe0.3Ni0.2Ti0.5 catalyst the co-existence of Fe and Ni resulted in increased surface acidity, which would promote the adsorption and activation of NH3.36 3.3. In-situ DRIFTS study 3.3.1. NH3 adsorption NH3 adsorption on the surface of the catalyst is crucial for the NOx reduction by NH3.4, 7, 15 Therefore, the adsorption of NH3 on Fe0.3Ti0.7, Ni0.2Ti0.8 and Fe0.3Ni0.2Ti0.5 catalysts was investigated by DRIFTS and the spectra were recorded in Figure 7. As shown in Figure 7(a), over Fe0.3Ti0.7 catalyst several bands at 3352, 3262, 3160, 1604, 1469, 1426, 1321, 1162 cm-1 appeared. The bands at 3352, 3262, 3160 cm-1 are assigned to N-H stretching vibration of the coordinated NH3.33, 37 And the peaks at 1469 and 1426 cm-1 are assigned to NH4+ species on Brønsted acid sites,4, 15, 23 while those at 1604, 1321, 1162 cm-1 are attributed to NH3 linked to Lewis acid sites.20,33 For Ni0.2Ti0.8 catalyst (see Figure 7(b)), besides the N-H stretching vibration modes of the coordinated NH3 (3361, 3262 and 3153 cm-1), NH4+ on Brønsted acid sites (1469 and 1421 cm-1), NH3 on Lewis acid sites (1595 and 1165 cm-1), the intermediates of ammonia oxidation (1369 cm-1)38 and NH2 species (1554 cm-1)21, 39 are also observed. As for Fe0.3Ni0.2Ti0.5 catalyst (see Figure 7(c)), the intensities of the peaks assigned to NH3 on Lewis acid sites (1321 cm-1) and NH4+ on Brønsted acid sites (1469 and 1426 cm-1) are obviously higher than those on Fe0.3Ti0.7 and Ni0.2Ti0.8 catalysts. In addition, 11



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new peaks assigned to NH3 on Lewis acid sites (1232 and 1136 cm-1)15, 21 and NH4+ on Brønsted acid sites (1671 cm-1)33 are also observed. Even at 350 ℃, the adsorption peaks of NH3 are still relatively strong. This fact indicates both the Lewis acid and Brønsted acid sites over Fe0.3Ni0.2Ti0.5 catalyst are enhanced, which is in accordance with the NH3-TPD results. More acid sites generated over Fe0.3Ni0.2Ti0.5 catalyst will facilitate the absorption and activation of NH3, which is crucial for the NOx reduction. 3.3.2. NO+O2 adsorption The DRIFT spectra of NO+O2 adsorption at different temperatures over Fe0.3Ti0.7, Ni0.2Ti0.8 and Fe0.3Ni0.2Ti0.5 catalysts are presented in Figure 8. For Fe0.3Ti0.7 catalyst (see Figure 8(a)), several bands at 1614, 1584, 1540, 1285 and 1246 cm-1 are observed, which can be ascribed to the adsorbed NO2,4, 37 bidentate nitrate,2,15 monodentate nitrate,15 chelating nitrite15 and bridging nitrate,22, 40 respectively. Above 150 ℃ the peak located at 1285 cm-1 disappear, while the others are still observed. For Ni0.2Ti0.8 catalyst (see Figure 8(b)), besides the peaks ascribed to NO2, bidentate nitrate, monodentate nitrate, trans-N2O22− species and bridging nitrate, a new peak related to trans-N2O22− species (1488 cm-1) appeared. And the new peaks ascribed to M-NO2 nitro (1365 cm-1)41 and trans-N2O22− species (1465 cm-1)27,

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appear when

temperature increased to 350 ℃. As shown in Figure 8(c), over Fe0.3Ni0.2Ti0.5 catalyst all the adsorbed NOx species are weaker than other two catalysts except the M-NO2 nitro species, whose intensity is evidently higher than that of Ni0.2Ti0.8 catalyst. Therefore, the co-presence of Fe and Ni suppressed the adsorption of most NOx species but promotes the formation of M-NO2. 12


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3.3.3. Reactivity of adsorbed species In order to clarify the reactivity of the adsorbed species, transient experiment is carried out. The dynamic changes of in-situ DRIFT spectra at 350 ℃ in Figure 9 can reflect the reactivity of adsorbed NH3. After adsorbing NH3 for 60 min, a series of peaks assigned to absorbed NH3 species are observed. When the catalyst was purged with N2 and then the gas was switched to NO+O2, the peaks attributed to absorbed NH3 species diminished and disappeared thoroughly within 10 min. Meanwhile, some new peaks appeared at 1540, 1465 and 1370 cm-1, which are assigned to monodentate nitrate, trans-N2O22− and M-NO2 nitro species, respectively. This fact indicates that both the adsorbed NH3 on Lewis acid sites and NH4+ on Brønsted acid sites are reactive and they participate in the reduction of NOx. The reactivity of NOx species was also studied at 350 ℃, the obtained dynamic spectra is shown in Figure 10. After exposing the catalyst to NO+O2 flow for 60 min, monodentate nitrate, trans-N2O22− species and M-NO2 nitro species are observed. After switching to NH3, no obvious decrease of the peaks ascribed to monodentate nitrate and trans-N2O22− species was observed. However, the intensity of the band assigned to M-NO2 nitro species decreased noticeably as the reaction going on. Moreover, some peaks assigned to NH3 species appear after switching to NH3 for 10 min. This result suggests M-NO2 nitro species is reactive, while monodentate nitrate and trans-N2O22− species are inactive. The strong interaction between different metal oxides can leads to the change of the physical structure.22, 30 XRD analysis showed that the co-presence of Fe, Ni and Ti 13



suppresses the crystallization of the FeOx, NiOx and TiO2 phase, thus forming an amorphous structure, which contributes to the NOx reduction.15 More importantly, the strong interaction among Fe, Ni and Ti induces the chemical state changes of the active sites. Ni2+ loses electron and transfers them to Fe3+ and Ti4+, resulting in the formation of Fe2+ and Ti3+. Therefore, three redox couples (Fe2+/Fe3+, Ni2+/Ni3+ and Ti3+/Ti4+) existed over Fe0.3Ni0.2Ti0.5 catalyst, and they can form two redox cycles (Fe3+ + Ni2+ ↔ Fe2+ + Ni3+, Ti4+ + Ni2+ ↔ Ti3+ + Ni3+). The redox cycles can promote the electron transfer among Fe, Ni and Ti species, which leads to improved redox property as illustrated in Figure 5. The improved redox property is beneficial for the NOx reduction.15 The existence of Fe2+ and Ti3+ might induce the formation of defects on the surface of the catalyst. The defects would facilitate the formation of more activated oxygen atom, which then promotes the formation of M-NO2 nitro species. As shown in Figure 10, the M-NO2 nitro species is reactive for the NOx reduction. Besides the redox property, the acidity of the catalyst is very important for its NOx reduction activity.7, 22, 23 The promoted electron transfer due to the dual redox cycles would facilitate the adsorption and activation of NH3.20 Compared with Fe0.3Ti0.7 and Ni0.2Ti0.8 catalysts, more Lewis acid sites and Brønsted acid sites were generated over Fe0.3Ni0.2Ti0.5 catalyst. Even the temperature is increased up to 350 ℃, the intensities of the peaks related to the adsorption NH3 are still relatively strong. This promotion of acid sites was also reported in the previous study by doping Ni into Mn-Ti catalyst.29 As shown in Figure 9, both monodentate nitrate and trans-N2O22− are inactive to react with NH3. Similar phenomenon was also reported in the case of CeWTi 14


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catalyst.37 The inactive nitrate would cover the active sites, thus inhibiting the adsorption of the reactants (NH3 or/and NO), then causing the deactivation of the catalyst.42 For Fe-Ni-Ti catalyst, the promoted acidity would promoted the adsorption of NH3, moreover, the adsorption of inactive monodentate nitrate and trans-N2O22− species is suppressed (see Figure 8(c)). Therefore, more active sites are available for the adsorption and activation of NH3 and M-NO2 nitro species. In-situ DRIFTs studies showed that over Fe-Ni-Ti catalyst the absorbed NH3 or NH4+ can react with gas phase NO to form N2 by the Eley-Rideal (E-R) mechanism. On the other hand, the reaction between activated NH3 (or NH4+) and M-NO2 also occurred via the Langmuir-Hinshelwood (L-H) mechanism. The synergetic effect among Fe, Ni and Ti not only leads to improved redox property but also increased acidity of the catalyst, both of which promotes the NH3-SCR of NOx.

4. Conclusion Environmentally benign Fe-Ni-Ti composite oxide has been developed for the NOx reduction by NH3, and it showed high activity and N2 selectivity. The strong interaction among Fe, Ni and Ti leads to the formation of amorphous structure, the dual redox cycles (Fe3+ + Ni2+ ↔ Fe2+ + Ni3+, Ti4+ + Ni2+ ↔ Ti3+ + Ni3+) and enhanced Lewis acid sites as well as Brønsted acid sites. Mechanism investigation via in situ DRIFTs demonstrated the NOx reduction proceeds by both E-R mechanism and L-H mechanism. Over Fe-Ni-Ti catalyst the synergetic effect contributes to forming reactive NH3/NH4+ and M-NO2 nitro species, and consequently improving the 15



NH3-SCR performance.

Acknowledgments The authors acknowledge financial support from the National Key R&D Program of China (2017YFC0210700), the National Natural Science Foundation of China (21677008, 21611130170), the Beijing Municipal Natural Science Foundation (8162030), and Jiangsu Collaborative Innovation Center for Ecological Building Materials and Environmental Protection Equipments (YCXT201604).

Supporting Information Available：The activities of different Fe-Ni-Ti catalysts are shown in Figure S1. The information is available free of charge on the ACS website.

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Catalysts for Low-Temperature Selective Catalytic Reduction of NO with NH3: I. Evaluation and Characterization of First Row Transition Metals. J. Catal. 2004, 221, 421. (17) Thirupathi, B.; Smirniotis, P. G. Nickel-doped Mn/TiO2 as an Efficient Catalyst for the Low-Temperature SCR of NO with NH3: Catalytic Evaluation and Characterizations. J. Catal. 2012, 288, 74. (18) Cai, S.; Zhang, D.; Shi, L.; Xu, J.; Zhang, L.; Huang, L.; Li, H.; Zhang, J. Porous Ni-Mn Oxide Nanosheets in Situ Formed on Nickel Foam as 3D Hierarchical Monolith De-NOx Catalysts. Nanoscale 2014, 6, 7346. (19) Maitarad, P.; Han, J.; Zhang, D.; Shi, L.; Namuangruk, S.; Rungrotmongkol, T. Structure-Activity Relationships of NiO on CeO2 Nanorods for the Selective Catalytic Reduction of NO with NH3: Experimental and DFT Studies. J. Phys. Chem. C 2014, 118, 9612. (20) Liu, Z.; Liu, H.; Feng, X.; Ma, L.; Cao, X.; Wang, B. Ni-Ce-Ti as a Superior Catalyst for the Selective Catalytic Reduction of NOx with NH3. Mol. Catal. 2018, 445, 179. (21) Liu, Z.; Yi, Y.; Zhang, S.; Zhu, T.; Zhu, J.; Wang, J. Selective Catalytic Reduction of NOx with NH3 over Mn-Ce Mixed Oxide Catalyst at Low Temperatures. Catal. Today 2013, 216, 76. (22) Liu, Z.; Feng, X.; Zhou, Z.; Feng, Y.; Li, J. Ce-Sn Binary Oxide Catalyst for the Selective Catalytic Reduction of NOx by NH3. Appl. Surf. Sci. 2018, 428, 526. (23) Shan, W.; Liu, F.; He, H.; Shi, X.; Zhang, C. A Superior Ce-W-Ti Mixed Oxide

19



Catalyst for the Selective Catalytic Reduction of NOx with NH3, Appl. Catal. B 2012, 115, 100. (24) Liu, Z.; Liu, Y.; Chen, B.; Zhu, T.; Ma, L. Novel Fe-Ce-Ti Catalyst with Remarkable Performance for the Selective Catalytic Reduction of NOx by NH3, Catal. Sci. Technol. 2016, 6, 6688. (25) Wu, Z.; Jin, R.; Wang, H.; Liu, Y. Effect of Ceria Doping on SO2 Resistance of Mn/TiO2 for Selective Catalytic Reduction of NO with NH3 at Low Temperature, Catal. Commun. 2009,10, 935. (26) Han, J.; Meeprasert, J.; Maitarad, P.; Nammuangruk, S.; Shi, L.; Zhang, D. Investigation of the Facet-Dependent Catalytic Performance of Fe2O3/CeO2 for the Selective Catalytic Reduction of NO with NH3. J. Phys. Chem. C 2016, 120, 1523. (27) Chen, Z.; Wang, F.; Li, H.; Yang, Q.; Wang, L.; Li, X. Low-Temperature Selective Catalytic Reduction of NOx with NH3 over Fe-Mn Mixed-Oxide Catalysts Containing Fe3Mn3O8 Phase. Ind. Eng. Chem. Res. 2012, 51, 202. (28) Wan, Y.; Zhao, W.; Tang, Y.; Li, L.; Wang, H.; Cui, Y.; Gu, J.; Li, Y.; Shi, J. Ni-Mn Bi-Metal Oxide Catalysts for the Low Temperature SCR Removal of NO with NH3. Appl. Catal. B 2014, 148, 114. (29) Chen, L.; Li, R.; Li, Z.; Yuan, F.; Niu, X.; Zhu, Y. Effect of Ni Doping in NixMn1-xTi10 (x=0.1-0.5) on Activity and SO2 Resistance for NH3-SCR of NO Studied with in Situ DRIFTS. Catal. Sci. Technol. 2017, 7, 3243. (30) Liu, Z.; Yi, Y.; Li, J.; Woo, S. I.; Wang, B.; Cao, X.; Li, Z. A 20


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Superior Catalyst with Dual Redox Cycles for the Selective Reduction of NOx by Ammonia. Chem. Commun. 2013, 49, 7726. (31) Oku, M.; Wagatsuma, K.; Kohikib, S. Ti 2p and Ti 3p X-ray Photoelectron Spectra for TiO2, SrTiO3 and BaTiO3. Phys. Chem. Chem. Phys. 1999, 1, 5327. (32) Seo, P. W.; Cho, S. P.; Hong, S. H.; Hong, S. C. The Influence of Lattice Oxygen in Titania on Selective Catalytic Reduction in the Low Temperature Region. Appl. Catal. A 2010, 380, 21. (33) Liu, Z.; Liu, Y.; Li, Y.; Su, H.; Ma, L. WO3 Promoted Mn-Zr Mixed Oxide Catalyst for the Selective Catalytic Reduction of NOx with NH3. Chem. Eng. J. 2016, 283, 1044. (34) Lee, K. J.; Kumar, P. A.; Maqbool, M. S.; Rao, K. N.; Song, K. H.; Ha, H. P. Ceria Added Sb-V2O5/TiO2 Catalysts for Low Temperature NH3 SCR: Physico-Chemical Properties and Catalytic Activity. Appl. Catal. B 2013, 142, 705. (35) Kang, M.; Park, E. D.; Kim, J. M.; Yie, J. E. Manganese Oxide Catalysts for NOx Reduction with NH3 at Low Temperatures. Appl. Catal. A 2007, 327, 261. (36) Liu, Z.; Su, H.; Chen, B.; Li, J.; Woo, S. I. Activity Enhancement of WO3 Modified Fe2O3 Catalyst for the Selective Catalytic Reduction of NOx by NH3. Chem. Eng. J. 2016, 299, 255. (37) Chen, L.; Li, J.; Ge, M. DRIFT Study on Cerium-Tungsten/Titania Catalyst for Selective Catalytic Reduction of NOx with NH3. Environ. Sci. Technol. 2010, 44, 9590. 21



(38) Qi, G.; Yang, R. Characterization and FTIR Studies of MnOx-CeO2 Catalyst for Low-Temperature Selective Catalytic Reduction of NO with NH3. J. Phys. Chem. B 2004, 108, 15738. (39) Qi, G.; Yang, R.; Chang, R. MnOx-CeO2 Mixed Oxides Prepared by Coprecipitation for Selective Catalytic Reduction of NO with NH3 at Low Temperatures. Appl. Catal. B 2004, 51, 93. (40) Peng, Y.; Wang, C.; Li, J. Structure-Activity Relationship of VOx/CeO2 Nanorod for NO Removal with Ammonia. Appl. Catal. B 2014, 144, 538. (41) Wu, Z.; Jiang, B.; Liu, Y.; Wang, H.; Jin, R. DRIFT Study of Manganese/ Titania-Based Catalysts for Low-Temperature Selective Catalytic Reduction of NO with NH3. Environ. Sci. Technol. 2007, 41, 5812. (42) Kijlstra, W. S.; Brands, D. S.; Smit, H. I.; Poels, E. K.; Bliek, A. Mechanism of the Selective Catalytic Reduction of NO with NH3 over MnOx/Al2O3, J. Catal. 1997, 171, 219.

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Table 1. Textual properties of Fe0.3Ti0.7, Ni0.2Ti0.8 and Fe0.3Ni0.2Ti0.5 catalysts BET surface area Catalyst

Pore volume

(m2 g−1)

(cm3 g−1)

Pore diameter (nm)

Fe0.3Ti0.7

85.0

0.30

14.1

Ni0.2Ti0.8

76.0

0.23

12.0

Fe0.3Ni0.2Ti0.5

166

0.28

6.81

23



Figure 1. NOx conversion (a) and N2 selectivity (b) as a function of reaction temperature over Fe0.3Ti0.7, Ni0.2Ti0.8 and Fe0.3Ni0.2Ti0.5 catalysts (Conditions: 500 ppm NO, 500 ppm NH3, 5% O2, GHSV= 140,000 h-1). Figure 2. Response of NOx conversion to the intermittent feed of H2O (or SO2/ H2O +SO2) over Fe0.3Ni0.2Ti0.5 catalyst at 350 ℃(Conditions: 500 ppm NO, 500 ppm NH3, 5% O2, 5%H2O, 50 ppm SO2, GHSV= 140,000 h-1). Figure 3. XRD patterns of Fe0.3Ti0.7, Ni0.2Ti0.8 and Fe0.3Ni0.2Ti0.5 catalysts. Figure 4. XPS spectra of Fe 2p (a), Ni 2p(b), Ti2p(c) and O1s(d) for different catalysts. Figure 5. H2-TPR profiles of Fe0.3Ti0.7, Ni0.2Ti0.8 and Fe0.3Ni0.2Ti0.5 catalysts. Figure 6. NH3-TPD profiles of Fe0.3Ti0.7, Ni0.2Ti0.8 and Fe0.3Ni0.2Ti0.5 catalysts. Figure 7. In-situ DRIFT spectra of NH3 adsorption on Fe0.3Ti0.7(a), Ni0.2Ti0.8(b) and Fe0.3Ni0.2Ti0.5(c) catalysts at different temperatures. Figure 8. In-situ DRIFT spectra of NO+O2 adsorption on Fe0.3Ti0.7(a), Ni0.2Ti0.8(b) and Fe0.3Ni0.2Ti0.5(c) catalysts at different temperatures. Figure 9. The dynamic change of the in situ DRIFT spectra in a flow of NO + O2 over Fe0.3Ni0.2Ti0.5 catalyst pre-exposed NH3 at 350 ℃. Figure 10. The dynamic change of the in situ DRIFT spectra in a flow of NH3 over Fe0.3Ni0.2Ti0.5 catalyst pre-exposed NO + O2 at 350 ℃.

24


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Page 25 of 36

Figure 1

100 (a)

Conversion of NOx(%)

80 60

Fe0.3Ni0.2Ti0.5 Fe0.3Ti0.7 Ni0.2Ti0.8

40 20 0 200

250

300

350

400

450

Temperature(°C) 100 (b)

80

Selectivity to N2(%)

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


60 Fe0.3Ni0.2Ti0.5 Fe0.3Ti0.7

40

Ni0.2Ti0.8

20 0 200

250

300

350

Temperature(°C)

25


400

450


Figure 2

100

80

Conversion of NOx (%)

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

on

60

off

40 5% H2O 50 ppm SO2 5% H2O + 50 ppm SO2

20

0 0

1

2

3

4

5

6

7

8

9

10 11 12

Reaction time (h)

26


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Figure 3

∆ : Anatase TiO2

∆

• : Rutile TiO2 ♦ : NiTiO3 ♦ •∆

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


•

∆ ∆ ∆ ♦

∆ ♦

•

Ni0.2Ti0.8 •∆

∆

∆

Fe0.1Ni0.1Ti0.8 Fe0.1Ni0.2Ti0.7 Fe0.2Ni0.2Ti0.6 Fe0.2Ni0.3Ti0.5 Fe0.3Ni0.2Ti0.5 Fe0.3Ti0.7

10

20

30

40

50

60

70

2θ(degree)

27


80

90


Figure 4

(a) Fe Fe

2+

3+

Intensity(a.u.)

Fe0.3Ni0.2Ti0.5

Fe0.3Ti0.7

730

725

720

715

710

705

Binding energy(eV)

(b)

3+

Ni ratio Ni Ni

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 28 of 36

2+

3+

Fe0.3Ni0.2Ti0.5

62.4%

Ni0.2Ti0.8

37.1%

870

865

860

Binding energy(eV)

28


855

850

Page 29 of 36

(c)

Intensity(a.u.)

Ti 2p

Fe0.3Ni0.2Ti0.5 Ni0.2Ti0.8 Fe0.3Ti0.7

468

464

460

456

Binding energy(eV)

(d) Oβ

Oα ratio

Oα

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


Fe0.3Ni0.2Ti0.5

65.1%

Ni0.2Ti0.8

42.6%

Fe0.3Ti0.7

27.3%

536

534

532

530

Binding energy(eV)

29


528

526


Figure 5

352 312 Fe0.3Ni0.2Ti0.5

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 30 of 36

375

Fe0.3Ti0.7 493

200

400

Ni0.2Ti0.8

600

Temperature(°C)

30


800

Page 31 of 36

Figure 6

Fe0.3Ni0.2Ti0.5 Fe0.3Ti0.7

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


Ni0.2Ti0.8

100 150 200 250 300 350 400 450 500 550

Temperature(°C)

31



Figure 7 1162

1604

3262

1321

(a)

0.2

Absorbance(a.u.)

350°C 300°C 250°C 200°C 150°C

3352 3160

1469 1426

100°C

40003500 2000

1800

1600

1400

1200

1000

-1

Wavenumber(cm ) 1369

3262

350°C

Absorbance(a.u.)

1165

(b)

0.2

300°C 250°C 200°C 150°C

40003500 2000

1800

1600

1469 1421

1595 1554

3361 3153

100°C

1400

1200

1000

-1

Wavenumber(cm ) 1232

1604

3262

1390 1321

(c)

0.2

350°C

Absorbance(a.u.)

300°C 250°C 200°C 150°C

40003500 2000

1800

1600

1400

1136

1469 1426

1544

1671

100°C 3352 3160

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

1200

-1

Wavenumber(cm ) 32


1000

Page 32 of 36

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Figure 8 1614

1540

1246

(a)

0.2

Absorbance(a.u.)

350°C 300°C 250°C 200°C

2000

1800

1285

100°C

1584

150°C

1600

1400

1200

1000

-1

Wavenumber(cm )

1465

1614 1540

Absorbance(a.u.)

350°C

1365

(b)

0.2

300°C 250°C 200°C

2000

1800

1285 1246

1488

100°C

1584

150°C

1600

1400

1200

1000

-1

Wavenumber(cm )

1370

1540

350°C

1465

(c)

0.2

Absorbance(a.u.)

300°C 250°C 200°C 150°C 100°C

2000

1800

1600

1285 1246

1614 1584

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

Wavenumber(cm ) 33


1000


NO+O2 60min

1465

1540

0.2

1370

Figure 9

Absorbance(a.u.)

NO+O2 30min NO+O2 20min NO+O2 10min NO+O2 5min NO+O2 2min

40003500 2000

1800

1600

1400 -1

Wavenumber(cm )

34


1230

1321

1390

1469

1604 1544

NH3 60min 3252 3262 3160

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

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1200

1000

Page 35 of 36

Figure 10

1232

1604

3352 3262 3160

0.2

NH3 60min NH3 30min NH3 20min NH3 10min NH3 5min NH3 2min

40003500 2000

1800

1600

1465

1370

NO+O2 60min 1540

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 -1

Wavenumber(cm )

35


1200

1000


Table of contents 100

80

Conversion of NOx (%)

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

60

Fe3+ + Ni2+↔ Fe2+ + Ni3+ Ti4+ + Ni2+↔ Ti3+ + Ni3+

40

Dual redox cycles Fe0.3Ti0.7 Ni0.2Ti0.8 Fe0.3Ni0.2Ti0.5

20

0 200

250

300

350

400

Reaction temperature (℃)

36


450

Page 36 of 36

Selective Catalytic Reduction of NOx with NH3 ... - ACS Publications

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