English

• 0.   Introduction

  Nitrogen oxides (NO_x) emitted from automobile exhaust or chemical manufacturing industrials evoke serious environmental problems, including acid rain, photochemical smog, and greenhouse effects. The selective catalytic reduction (SCR) of NO_x with ammonia (NH₃) is the most effective means to remove NO_x species^[1-2]. Transition metal (Fe, Mn, Co, Cr and Ni), zeolite-based catalysts have been developed to solve this problem^[3-4]. In particular, Mn-based zeolite catalysts exhibit excellent NO removal efficiency because of their variable valence state, strong redox capability and abundant acidic sites. Wang et al.^[5] reported that composite SCR catalysts composed of MNO_x and multi-walled carbon nanotube (MWCNT) demonstrated excellent activity, and the NO_x conversion rate was more than 90% at low temperature of 190 ℃. Lou et al.^[6] reported that Mn/ZSM-5 catalysts exhibited comparable SCR reaction activity in the low-temperature range of 170~350 ℃. Yu et al.^[7] found that the MnSAPO-34 molecular sieve catalysts prepared at 550 ℃ exhibited the best SCR activity with the NO conversion nearly as high as l00%, and the catalytic activity was rapidly improved at 250~300 ℃.

  It is worth noting that different catalyst carriers also have important effects on SCR activity. Al₂O₃ have been widely studied as low-temperature SCR catalyst supports^[8-9] because the surface of the Al₂O₃ carrier is modified with many hydroxyl groups, thus are beneficial for the oxidation of NO into NO₂ and maintaining the reaction between nitrogen oxides and ammonia at low temperatures. For example, Xie et al.^[10] reported that the NO conversion rate reached around 80% at 200 ℃ over CuO/Al₂O₃ catalysts. Activated carbon is another widely used carrier for the SCR catalyst because of the strong adsorption capability for NO molecules at low temperatures. The research results have confirmed that activated carbon combined with CuO exhibited high SCR activity at low temperature of 200 ℃^[11]. In addition, the abundant Lewis acid sites on the surface of TiO₂ are beneficial for the adsorption and activation of ammonia during the SCR reaction. Therefore, TiO₂ can also be employed as carriers for the SCR catalyst.

  Kato et al.^[12] reported that the removal efficiency of NO could reach more than 60% from 250 to 450 ℃ for the Fe₂O₃/TiO₂ composite catalysts. Recent studies have also showed that Ti-SBA-15, Ti-MCM-41, TS-1 and Ti-grafted SiO₂ can provide abundant acid sites due to the incorporation of titanium^[13-15].

  SiO₂ nanotubes with high surface area are considered as ideal supports for the dispersion of the active components of the SCR catalyst and enrichment of target gases. Moreover, it should be indicated that if the titanium can be incorporated into the skeleton of SiO₂ nanotubes, the strong redox capability and large oxygen storage capacity can be achieved within surface concentration. In this study, Mn and Ti co-doped SiO₂ nanotubes (Mn/TiSNTs) were rationally designed and synthesized via assembling of several methods such as co-polycondensation and co-precipitation. The obtained Mn/TiSNTs catalysts exhibited significant SCR activity under low reaction temperatures due to the synergistic effect of different active components and SiO₂ nanotube supports with high surface area.

  1.   Experimental

  1.1   Preparation of Mn/TiSNT catalysts

  The Ti-containing SiO₂ nanotubes (TiSNT) with different Si/Ti molar ratios were synthesized via a sol-gel method. The 1.00 g of Pluronic F127 was dissolved in 60 mL of 2 mol·L^-1 HCl in a glass container with magnetic stirring followed by 2.8 g of tetraethyl orthosilicate. Tetrabutyl titanate was dissolved into 3 mL of toluene, and the resulting solution was slowly added into this solution. The solution was stirred at 250 r·min^-1 and 11 ℃ for 24 h in a covered container. The gel was transferred into Teflon-lined autoclaves and heated to 100 ℃ for 24 h. The product was filtered, washed and dried in a vacuum oven at 55 ℃. The as-synthesized product was calcined at 350 ℃ in air for 5 h. The synthesized samples were hereafter denoted as Ti(x)SNT where x represents the Si/Ti molar ratio.

  According to the previous research, the Mn/TiSNT catalyst with optimized 5.5%(w/w) Mn loading shows the largest specific surface area of 430 m²·g^{-1 [16]}. The Mn/Ti(x)SNT catalysts were similarly prepared by precipitation with NH₃. A specific proportion of the Ti(x)SNT sample was ion exchanged with appropriate amounts of manganese acetate under magnetic stirring at room temperature for 24 h. Ammonia was slowly added to adjust the pH value to 11. The solution was then filtered, washed with deionized water and dried at 100 ℃ overnight followed by calcination at 350 ℃ for 2 h. The synthesized catalyst samples were hereafter denoted as Mn/Ti(x)SNT, where x represents the Si/Ti molar ratio.

  1.2   Characterizations

  XRD patterns of the products were obtained using a Rigaku D/MAX2500 diffractometer with a Cu Kα radiation source (λ=0.154 nm), a tube voltage of 40 kV, and a tube current of 100 mA in the 2θ range of 5°~70° with a scanning rate of 3°·min^-1. TEM images were obtained by using JEM-2100 (with operation voltage of 200 kV). The N₂ adsorption-desorption isotherms were determined using a Quanta-chrome Autosorb-iQ2-MP N₂ adsorption instrument. All of the samples were held in a vacuum at 300 ℃ for 5 h prior to measurement to ensure the elimination of water and other superfluous species. The micropore volume was measured via a t-plot method. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method. UV-Raman spectroscopy was conducted on a Thermo Fisher Scientific DXR Raman spectrometer. A laser line at 325 nm was employed as the excitation source. The UV-Vis DRS spectra were obtained on a Shimadzu UV-2450 UV-Vis spectrophotometer from 200 to 800 nm. The atomic concentrations on the sample surfaces were evaluated using XPS on a Kratos Analytical AXIS Ultra DLD spectrometer. The binding energy of the C1s peak (284.8 eV) was used as an internal standard. The TPD of NH₃ (NH₃-TPD) determined the number of different acid sites and their strengths for the catalysts using a Micromeritics AutoChem 2920 automated catalyst characterization system.

  Prior to test, ~30 mg of the catalyst was pretreated with high-purity N₂ at 40 mL·min^-1 and 500 ℃ for 60 min. Then, physical absorbed ammonium was removed via helium under equivalent conditions. The TPD operation was conducted next a heating rate of 10 ℃·min^-1 from 100 to 800 ℃. The amount of desorbed NH₃ was determined via a thermal conductivity detector (TCD). The TPR runs were carried out with a linear rate (10 ℃·min^-1) in pure N₂ containing 5%(V/V) H₂ at a flow rate of 30 mL·min^-1.

  1.3   NH₃-SCR activity testing

  The catalytic activities of the Mn/Ti(x)SNT samples were investigated using a custom-made fixed bed. For each sample, about 500 mg of the catalyst was placed in a quartz tube reactor with 1 cm in diameter. This was mixed with quartz sand to ensure the smooth passage of the reaction gas through the reactor. The reaction gas was composed of 8%(V/V) O₂, 600 mg·L^-1 NO, 600 mg·L^-1 NH₃ and 5%(V/V) H₂O. The balance was N₂, 300 mL·min^-1 total flow rate and a gas hourly space velocity (GHSV) of 36 000 h^-1 was employed. The concentration of NO in the reactors outlet gas was analyzed via a gas analyzer (FGA-4100, Guangdong Foshan Analytical Instrument Co., Ltd.).

  The NO conversion (Eq.(1)) and N₂ selectivity (Eq.(2)) were respectively calculated as follows:

  $ {\rm{NO}}\;{\rm{conversion = }}\left( {C_{{\rm{NO}}}^{{\rm{in}}} - C_{{\rm{NO}}}^{{\rm{out}}}} \right)/C_{{\rm{NO}}}^{{\rm{in}}} \times 100\% $

  (1)

  $ {{\rm{N}}_{\rm{2}}}\;{\rm{selectivity = }}C_{{{\rm{N}}_{\rm{2}}}}^{{\rm{out}}}/\left( {C_{{\rm{NO}}}^{{\rm{in}}} - C_{{\rm{NO}}}^{{\rm{out}}}} \right) \times 100\% $

  (2)

  Where ${C_{{\rm{NO}}}^{{\rm{in}}}}$ and ${C_{{\rm{NO}}}^{{\rm{out}}}}$ are the respective concentra-tions of NO at the inlet and the outlet of the reactor, $C_{{{\rm{N}}_{\rm{2}}}}^{{\rm{out}}}$ is the concentration of N₂ at the outlet.

  2.   Results and discussion

  2.1   Morphology and composition analysis

  Fig. 1 shows TEM images of Ti-containing SNT samples with different Si/Ti molar ratios. At relatively high Si/Ti molar ratios such as Ti(20)SNT, Ti(15)SNT and Ti(10)SNT, the worm-like tubular morphology was clearly observed, which are illustrated in Fig. 1(A~C). Meanwhile, TEM images clearly confirmed the hollow structure of the worm-like rods. However, when the molar ratio of Si/Ti was reduced to 5(Ti(5)SNT), the worm-like tubular morphology disappeared, in other words, the tubular morphology of Ti(5)SNT was destroyed, as shown in Fig. 1D. This is mainly due to the higher content of titanium precursor and fast hydrolysis rate of titanium precursor, which signifi-cantly affect the assembly of the template and the SiO₂ precursor resulting in the formation of tubular structures.

  Figure 1

  

  Figure 1.  TEM images of Ti-containing SNT with different Si/Ti molar ratios

  (A) Ti(20)SNT; (B) Ti(15)SNT; (C) Ti(10)SNT; (D) Ti(5)SNT

  下载: 全尺寸图片 幻灯片

  Moreover, the elemental composition of Mn/Ti(x)SNT samples was measured by a Varian Vista-AX inductively coupled plasma optical emission spectrometer (ICP-OES). The Si/Ti test ratio (n_Si/n_Ti) of Ti-containing SNT was close to the theoretical value, the results are shown in Table 1.

  Table 1

  

  Table 1.  Element composition of Ti-containing SNT with different Si/Ti molar ratios

  下载: 导出CSV

  Sample	wSi/wTi	Mass fraction of Mn/ %	nSi/nTi
  Ti(20)SNT	10.78	5.5	18
  Ti(15)SNT	7.60	5.7	13
  Ti(10)SNT	5.02	5.5	9
  Ti(5)SNT	2.44	5.4	4

  2.2   XRD patterns

  XRD patterns of Ti-containing SNT samples with different Si/Ti molar ratios are shown in Fig. 2. For Ti-containing SNT, an intense diffraction peak located at 23.4° is clearly observed, and the peak can be attributed to the characteristic peaks of amorphous silica. For Ti(5)SNT, two weak diffraction peaks were located at 25.2° and 27.2°. Both peaks could be attributed to anatase TiO₂ phase. This was possibly because the hydrolysis rate of the titanium precursor was faster and played a leading role resulting in the formation of TiO₂ nanoparticles. The Si source could not form a tubular structure. When Si/Ti molar ratio was more than 5, the anatase phase was not detected in the XRD patterns. It may be speculated that the relatively small amount of titanium could not be detected due to the small particles of titanium. The excessive amount of the titanium precursor will affect the formation of SiO₂ hollow nanotube structures.

  Figure 2

  

  Figure 2.  XRD patterns of Ti-containing SNT with different Si/Ti molar ratios

  下载: 全尺寸图片 幻灯片

  2.3   UV-Vis DRS analysis

  UV-Vis DRS spectroscopy was used to understand the nature and coordination of the Ti species in the Ti-containing SNT. UV-Vis DRS spectroscopy of the Ti-containing SNT samples with different Si/Ti molar ratios are shown in Fig. 3. A strong absorbance band at 220 nm was observed on Ti(20)SNT, Ti(15)SNT and Ti(10)SNT samples. These were attributed to isolated framework titanium in tetrahedral coordination. The Ti atoms likely substitute for Si atoms in the skeleton of SNT structures with the formation of a Ti-O-Si-Ti band^[17-18]. No absorbance band was observed in the SNT sample. A strong absorbnce band at 310~340 nm was observed for the Ti(5)SNT sample, which indicated the presence of polytitanium (Ti-O-Ti)_n clusters^[19], implying the formation of a crystalline TiO₂ phase. No absorbance band was seen at 220 nm in UV-Vis DRS spectroscopy. The Ti atoms do not exist in the skeleton of the SNT structure. In contrast, its peak is too weak to be masked. TEM images do not show a tubular morphology, thus we suspect that the hydrolysis rate of the titanium precursor is faster, which will result in the formation of polytitanium (Ti-O-Ti)_n clusters.

  Figure 3

  

  Figure 3.  UV-Vis DRS spectra of Ti-containing SNT with different Si/Ti molar ratios

  下载: 全尺寸图片 幻灯片

  Based on these characterization results, we synthesized Ti-containing silicon nanotubes with defined hollow tubular structures. The addition of titanium affects the formation of the tubular structures. With Si/Ti molar ratio was fixed over 5 (as Ti(20)SNT, Ti(15)SNT and Ti(10)SNT samples), the Ti species embedded into the framework of SNT and were served as Ti atoms in tetrahedral coordination. When the molar ratio of Si/Ti was 5, the Ti species existed as polytitanium (Ti-O-Ti)_n clusters, which distorted the tetrahedral environment.

  2.4   N₂ adsorption-desorption

  Fig. 4a shows the N₂ adsorption-desorption isotherms of Mn/TiSNT catalyst samples. It is shown that all of the Mn/TiSNT samples exhibited classical Ⅳ-type isotherms with an obvious H4 hysteresis loop as defined by IUPAC. This indicates that a mesopore structure existed in these catalysts. There are two capillary condensation steps in the adsorption isotherms indicating that the catalysts had two types of mesopores. The hysteresis loop at relatively low pressure corresponds to the inner void of the hollow nanospheres, and the hysteresis loop in the relatively high pressure is ascribed to the interparticle void formed from nanosphere packing. Fig. 4b exhibits the narrow mesopore distributions of Mn/TiSNT samples, and the pore diameter increased with the decreasement of Ti. Table 2 shows the BET surface area, pore volume and average pore diameter of Mn/TiSNT catalysts. The BET surface area, pore volume and average pore diameter of catalysts decreased with an increasing amount of doped titanium. When the Si/Ti molar ratios of the Mn/TiSNT catalysts were over 5, the BET surface area, pore volume and average pore diameter of the Mn/TiSNT catalysts decreased slightly with increasing amounts of doped titanium. When the Si/Ti molar ratio of the Mn/TiSNT catalyst was 5, the surface area decreased dramatically from 435 to 286 m²·g^-1 due to the morphology transformation of hollow SiO₂ nanotubes.

  Figure 4

  

  Figure 4.  (a) Nitrogen adsorption-desorption isotherms of Mn/TiSNT catalysts with different Si/Ti molar ratios; (b) Pore distributions of Mn/TiSNT catalysts

  下载: 全尺寸图片 幻灯片

  Table 2

  

  Table 2.  Textural properties of Mn/Ti(x)SNT catalysts

  下载: 导出CSV

  Catalyst	BET surface area/(m2dg-1)	Pore volume/(cm3dg-1)	Average pore diameter/nm
  Mn/Ti(20)SNT	460	0.011 0	11.4
  Mn/Ti(15)SNT	450	0.010 1	10.2
  Mn/Ti(10)SNT	435	0.009 6	9.3
  Mn/Ti(5)SNT	286	0.007 3	8.0

  2.5   XPS results

  The catalysts are characterized by XPS to evaluate the oxidation state of Mn and to estimate the concentrations of Mn on the surface of the Mn/TiSNT catalysts. Fig. 5 presents the Mn2p XPS spectra of the Mn/Ti(20)SNT and Mn/Ti(10)SNT catalysts, which consist of asymmetrical Mn2p_3/2 and Mn2p_1/2 vibrational peaks with binding energies of about 642.3 and 653.8 eV, respectively. Deconvolution fitting of the Mn2p_3/2 and Mn2p_1/2 peaks yields four distinct peaks centered at 642.1, 653.4, 643.8 and 656.1 eV. These were slightly shifted relative to standard literature values.

  Figure 5

  

  Figure 5.  Mn2p XPS spectra of Mn/TiSNT catalyst with different Si/Ti molar ratios

  下载: 全尺寸图片 幻灯片

  The asymmetric Mn2p_3/2 peak indicated the presence of a mixed-valence manganese species. The Mn2p_3/2 peaks near 656.1 eV and the Mn2p_1/2 peaks near 643.8 eV were assigned to Mn^4+[20-21] proving the presence of the MnO₂ species on the catalyst surface. XRD patterns show no MnO₂ crystals on the catalysts. The Mn2p_3/2 and Mn2p_1/2 peaks at approximately 653.4 and 642.1 eV were assigned to Mn^3+[22-23], proving the presence of the Mn₂O₃ species on the catalyst surface.

  The O1s core level peak of the Mn/Ti(20)SNT and Mn/Ti(10)SNT catalysts are shown in Fig. 6. The O1s spectra of all catalysts show two distinct peaks with binding energies of 532.3~534.1 eV and 529.1~530.2 eV, which were assigned to the weakly surface-adsorbed oxygen ions (O_adsorbed) and the lattice oxygen (O_lattice), respectively (The atom ratios of O_lattice to O_adsorbed was shown in Table 3)^[24-25]. The introduction of the Ti species to the silicon nanotubes results in major changes in the content of surface-adsorbed oxygen. It is obvious that the intensity of surface-adsorbed oxygen increased with the decreasing Si/Ti molar ratio. This is because Ti incorporation leads to charge imbalance, and this leads to the formation of vacancies and unsaturated chemical bonds on the catalyst surface.

  Figure 6

  

  Figure 6.  O1s XPS spectra of Mn/TiSNT catalysts with different Si/Ti molar ratios

  下载: 全尺寸图片 幻灯片

  Table 3

  

  Table 3.  Atom ratios of O_lattice to O_adsorbed

  下载: 导出CSV

  Sample	Mn/Ti(5)SNT	Mn/Ti(10)SNT	Mn/Ti(15)SNT	Mn/Ti(20)SNT
  nO(lattice)/nO(adsorbed)	6.11%	45.4%	7.31%	0.94%

  It is well known that surface-adsorbed oxygen plays an important role in NH₃-SCR, and this can promote the oxidation of NO to NO₂. Therefore, an increase in surface-adsorbed oxygen on the catalyst surface has a positive effect on the SCR reaction. When the Si/Ti molar ratio was 5, the amount of surface-adsorbed oxygen decreased sharply. This may be related to its morphology and the formation of polytitanium (Ti-O-Ti)_n clusters that decreased the catalytic activity of Mn/Ti(5)SNT catalysts.

  2.6   SCR activity

  The NO conversion rates for the SCR reaction were evaluated on Mn/TiSNT catalysts with different Si/Ti molar ratios between 50 and 350 ℃ (Fig. 7A). The Si/Ti molar ratio impressed an obvious influence on the catalytic performance of the Mn/TiSNT catalyst. As the reaction temperature increases, the NO conversion rate of all Mn/TiSNT catalysts increased initially, then they reached the highest conversion rate and maintained at this level for a while. The conversion rates subsequently decreased with an increase in the operating temperature. Fig. 7B also shows the N₂ selectivity for these catalysts, all of which exhibited high selectivity through the entire temperature range.

  Figure 7

  

  Figure 7.  (A) Catalytic performance for SCR of NO with ammonia and (B) N₂ selectivity of Mn/TiSNT catalysts with different Si/Ti molar ratios as a function of temperature

  下载: 全尺寸图片 幻灯片

  The Mn/Ti(10)SNT catalysts possess the highest SCR activity at low temperature amongest all catalysts studied. The Mn/Ti(10)SNT catalyst endows NO conversion rates as high as 90% at 135 ℃, and shows excellent catalytic activity from 135~325 ℃ (NO conversion over 90%). When the amount of titanium doping is excessive, the catalytic activity of the Mn/Ti(5)SNT catalyst was drastically decreased. The NO_x removal activity of the Mn/TiSNT catalysts follows the order of Ti(10)SNT > Ti(15)SNT > Ti(20)SNT > Ti(5)SNT. These results clearly suggest that higher titanium doping decreased the deNO_x activity of Mn/TiSNT catalysts.

  2.7   NH₃-TPD and H₂-TPR analysis

  NH₃-TPD was used to determine the catalysts strength and amount of different acid sites-the acidity of the catalyst is beneficial for the adsorption and activation of NH₃. The NH₃-TPD curves for all samples contained three desorption peaks (Fig. 8). The peaks from 100 to 250 ℃ were attributed to ammonium species adsorbed at weak Lewis acid sites or weakly adsorbed NH₃^[26]. The peaks from 250 to 400 ℃ were assigned to ammonia adsorbed on strong Brnsted acid sites^[27]. The peaks from 400 to 600 ℃ were due to strong Brnsted acid sites formed by the interaction of Brnsted acid sites with extra-framework titanium species^[28-29].

  Figure 8

  

  Figure 8.  NH₃-TPD curves of Mn/TiSNT catalysts with different Si/Ti molar ratios

  下载: 全尺寸图片 幻灯片

  For the Mn/Ti(10)SNT catalyst, the intensity of the peak in the temperature range of 400~600 ℃ was significantly higher than other catalysts with different Si/Ti molar ratios. This indicates that when the Si/Ti molar ratio was 10, the amount of acid sites of the catalyst increase-especially the strong acid sites on the catalyst surface. This promotes the adsorption and activation of NH₃ on the surface of the catalyst. Therefore, the SCR activity of the catalyst was improved at low-temperature regions.

  The H₂-TPR profiles for Mn/TiSNT catalysts with different Si/Ti molar ratios are presented in Fig. 9. The redox properties of the catalysts are affected by the amount of doped titanium. The Mn/TiSNT catalyst exhibited a broad reduction peak from 200 to 400 ℃; this corresponded to the following successive reduction process: MnO₂→Mn2O₃→Mn₃O₄→MnO^[30-31]. The reduction peak of the Mn/TiSNT catalysts increased with an increasing amount of doped titanium. The results show that Ti doping could enhance the redox ability and oxygen storage capacity of the Mn/TiSNT catalyst. With an increasing amount of doped titanium, the reduction peak gradually shifted to a higher temperature. This indicated that the redox reaction of the catalyst occurred at a higher temperature. However, the reduction peaks shifted to higher temperatures, which implying the redox activities of the catalysts were reduced by the amount of doped titanium. The H₂-TPR results showed that Mn/Ti(10)SNT had the largest area for the reduction peak of all catalysts, i.e., it had the strongest redox and oxygen storage capacity. This was consistent with the results of the catalytic activity testing.

  Figure 9

  

  Figure 9.  H₂-TPR curves of Mn/TiSNT catalysts with different Si/Ti molar ratios

  下载: 全尺寸图片 幻灯片

  3.   Conclusions

  Ti-containing SNT (TiSNT) with different Si/Ti molar ratios had been synthesized via a sol-gel and co-condensation method. When the Si/Ti molar ratio was more than 5, a hollow tubular morphology was clearly observed. When the Si/Ti molar ratio was located at 5 (Ti(5)SNT), the hollow morphology of Ti(5)SNT is destroyed. The Mn/TiSNT catalysts with different Si/Ti molar ratios were prepared via an impregnation method. Their performances for SCR treatment of NO_x with NH₃ were evaluated. Among the Mn/TiSNT catalysts prepared, the Mn/Ti(10)SNT catalyst was the best for SCR of NO. The results indicate that over 90% of NO conversion was achieved at a low temperature of 135 ℃. Meanwhile, the NO conversion rate remained larger than 90% from 135 to 325 ℃. When the Si/Ti molar ratio was more than 5, the catalyst had a large specific surface area indicating that it could provide a high active surface. XPS results show that the surface-adsorbed oxygen of the Mn/Ti(10)SNT catalyst was the highest, which was favorable for SCR reaction. The TPR studies show that the Mn/Ti(10)SNT catalyst had the strongest redox capability and huge oxygen storage capacity. In addition, the superior activity is ascribed to the abundant acidic sites in Mn/Ti(10)SNT catalysts, which will promote the adsorption and activation of NH₃ on the surface of the catalysts.

• 1. [1]

     Kapar J, Fornasiero P, Hickey N. Catal. Today, 2003, 77:419-449 doi: 10.1016/S0920-5861(02)00384-X

  2. [2]

     Nowak R. Science, 1994, 263:1217-1219 doi: 10.1126/science.8122100

  3. [3]

     Fedeyko J M, Chen B, Chen H Y. Catal. Today, 2010, 151:231-236 doi: 10.1016/j.cattod.2009.12.015

  4. [4]

     Nedyalkova R, Shwan S, Skoglundh M, et al. Appl. Catal. B, 2013, 138:373-380 http://www.sciencedirect.com/science/article/pii/S0926337313001616

  5. [5]

     Wang L S, Huang B C, Su Y X, et al. Chem. Eng. J., 2012, 192:232-241 doi: 10.1016/j.cej.2012.04.012

  6. [6]

     Lou X R, Liu P F, Li J, et al. Appl. Surf. Sci., 2014, 307:382-387 http://www.sciencedirect.com/science/article/pii/S0169433214008058

  7. [7]

     Yu C L, Chen F, Dong L F, et al. Environ. Sci. Pollut. Res., 2017, 24:7499-7510 doi: 10.1007/s11356-017-8375-0

  8. [8]

     Wang X B, Wu S G, Zou W X, et al. Chin. J. Catal., 2016, 37:1314-1323 doi: 10.1016/S1872-2067(15)61115-9

  9. [9]

     Boukha Z, De La Torre U, Pereda-Ayo B, et al. Top. Catal., 2016, 59:1002-1007 doi: 10.1007/s11244-016-0581-3

  10. [10]

      Xie G Y, Liu Z Y, Zhu Z P. J. Catal., 2004, 224:42-49 doi: 10.1016/j.jcat.2004.02.016

  11. [11]

      Jiang X, Lu P, Li C T, et al. Environ. Technol., 2013, 34:591-598 doi: 10.1080/09593330.2012.707232

  12. [12]

      Long R Q, Yan R T. J. Catal., 2002, 207:158-165 doi: 10.1006/jcat.2002.3545

  13. [13]

      Chen L H, Li X Y, Tian G, et al. Angew. Chem. Int. Ed., 2011, 50:11156-11161 doi: 10.1002/anie.v50.47

  14. [14]

      Ko Y, Kim S J, Kim M H, et al. Microporous Mesoporous Mater., 1999, 30:213-218 doi: 10.1016/S1387-1811(99)00010-4

  15. [15]

      Li L D, Wu P, Yu Q, et al. Appl. Catal. B, 2010, 94:254-262 doi: 10.1016/j.apcatb.2009.11.016

  16. [16]

      Ye Y Z, Shen F, Wang H N, et al. J. Chem. Sci., 2017, 129:765-774 doi: 10.1007/s12039-017-1299-x

  17. [17]

      Charbonneau L, Kaliaguine S. Appl. Catal. A, 2017, 533:1-8 doi: 10.1016/j.apcata.2017.01.001

  18. [18]

      Zhang W Z, Frba M, Wang J L, et al. J. Am. Chem. Soc., 1996, 118:9164-9171 doi: 10.1021/ja960594z

  19. [19]

      Marchese L, Maschmeyer T, Gianotti E, et al. J. Phys. Chem. B, 1997, 101:8836-8838 doi: 10.1021/jp971963w

  20. [20]

      Shen B X, Zhang X P, Ma H Q, et al. J. Environ. Sci., 2013, 25:791-800 doi: 10.1016/S1001-0742(12)60109-0

  21. [21]

      Qi G S, Yang R T. J. Phys. Chem. B, 2004, 108:15738-15747 doi: 10.1021/jp048431h

  22. [22]

      Qu Y F, Guo J X, Chu Y H, et al. Appl. Surf. Sci., 2013, 282:425-431 doi: 10.1016/j.apsusc.2013.05.146

  23. [23]

      Hou S C, Cao Y, Xiong W, et al. Ind. Eng. Chem. Res., 2006, 45:7077-7083 doi: 10.1021/ie060269c

  24. [24]

      Wu Z B, Jin R B, Liu Y, et al. Catal. Commun., 2008, 9:2217-2220 doi: 10.1016/j.catcom.2008.05.001

  25. [25]

      Jampaiah D, Tur K M, Venkataswamy P, et al. RSC Adv., 2015, 5:30331-30341 doi: 10.1039/C4RA16787B

  26. [26]

      Rivallan M, Ricchiardi G, Bordiga S, et al. J. Catal., 2009, 264:104-116 doi: 10.1016/j.jcat.2009.03.012

  27. [27]

      Martins G V A, Berlier G, Bisio C, et al. J. Phys. Chem. C, 2008, 112:7193-7200 doi: 10.1021/jp710613q

  28. [28]

      Kuehl G H, Timken H K C. Microporous Mesoporous Mater., 2000, 35:521-532 http://www.sciencedirect.com/science/article/pii/S1387181199002474

  29. [29]

      Dwyer J. Stud. Surf. Sci. Catal., 1988, 37:333-354 doi: 10.1016/S0167-2991(09)60611-X

  30. [30]

      Gac W. Appl. Catal. B, 2007, 75:107-117 doi: 10.1016/j.apcatb.2007.04.002

  31. [31]

      Lv G, Bin F, Song C L, et al. Fuel, 2013, 107:217-224 doi: 10.1016/j.fuel.2013.01.050

• 

  Figure 1  TEM images of Ti-containing SNT with different Si/Ti molar ratios

  (A) Ti(20)SNT; (B) Ti(15)SNT; (C) Ti(10)SNT; (D) Ti(5)SNT

  下载: 全尺寸图片 幻灯片
  

  Figure 2  XRD patterns of Ti-containing SNT with different Si/Ti molar ratios

  下载: 全尺寸图片 幻灯片
  

  Figure 3  UV-Vis DRS spectra of Ti-containing SNT with different Si/Ti molar ratios

  下载: 全尺寸图片 幻灯片
  

  Figure 4  (a) Nitrogen adsorption-desorption isotherms of Mn/TiSNT catalysts with different Si/Ti molar ratios; (b) Pore distributions of Mn/TiSNT catalysts

  下载: 全尺寸图片 幻灯片
  

  Figure 5  Mn2p XPS spectra of Mn/TiSNT catalyst with different Si/Ti molar ratios

  下载: 全尺寸图片 幻灯片
  

  Figure 6  O1s XPS spectra of Mn/TiSNT catalysts with different Si/Ti molar ratios

  下载: 全尺寸图片 幻灯片
  

  Figure 7  (A) Catalytic performance for SCR of NO with ammonia and (B) N₂ selectivity of Mn/TiSNT catalysts with different Si/Ti molar ratios as a function of temperature

  下载: 全尺寸图片 幻灯片
  

  Figure 8  NH₃-TPD curves of Mn/TiSNT catalysts with different Si/Ti molar ratios

  下载: 全尺寸图片 幻灯片
  

  Figure 9  H₂-TPR curves of Mn/TiSNT catalysts with different Si/Ti molar ratios

  下载: 全尺寸图片 幻灯片

  Table 1.  Element composition of Ti-containing SNT with different Si/Ti molar ratios

  Sample	wSi/wTi	Mass fraction of Mn/ %	nSi/nTi
  Ti(20)SNT	10.78	5.5	18
  Ti(15)SNT	7.60	5.7	13
  Ti(10)SNT	5.02	5.5	9
  Ti(5)SNT	2.44	5.4	4

  下载: 导出CSV

  Table 2.  Textural properties of Mn/Ti(x)SNT catalysts

  Catalyst	BET surface area/(m2dg-1)	Pore volume/(cm3dg-1)	Average pore diameter/nm
  Mn/Ti(20)SNT	460	0.011 0	11.4
  Mn/Ti(15)SNT	450	0.010 1	10.2
  Mn/Ti(10)SNT	435	0.009 6	9.3
  Mn/Ti(5)SNT	286	0.007 3	8.0

  下载: 导出CSV

  Table 3.  Atom ratios of O_lattice to O_adsorbed

  Sample	Mn/Ti(5)SNT	Mn/Ti(10)SNT	Mn/Ti(15)SNT	Mn/Ti(20)SNT
  nO(lattice)/nO(adsorbed)	6.11%	45.4%	7.31%	0.94%

  下载: 导出CSV
•