English

• 0.   Introduction

  Nitrogen oxides (NO_x) and particulates, emitted from the industrial combustion of fossil fuels, are main atmospheric pollutants, causing a variety of environ-mentally harmful effects such as acid rain, smog and PM_2.5^[1-2]. In traditional cleaning systems, the NO_x abate-ment and dust removal mainly employ the selective cat-alytic reduction (SCR) and filtration technology, respec-tively, and the dust removal usually performs after the SCR process because the reaction temperature of the catalyst is between 250 and 350 ℃^[3-5]. As a result, plugging and poisoning the SCR catalyst are easily occurred under high dust system, decreasing the ser-vice life of the catalyst. To address the problems men-tioned above, highly effective catalysts loaded on the porous ceramic filter was proposed for simultaneous removal of particulates and NO_x^[6-8]. Generally, the cata-lytic membrane is mainly comprised of separation layer and support layer. The dust removal was attributed to the separation layer and the catalyst was integrated into the support layer for the NO_x abatement. To guar-antee the performance of the catalytic membrane, developing highly effective catalyst is of critical impor-tance.

  Up to now, the widely used commercial catalyst in SCR is V ₂ O₅-WO₃/TiO₂. Nevertheless, there are same inevitable drawbacks, such as the toxicity of vanadium and inferior performance at low temperature, hindering their wide application^[9-10]. In this regard, environment-friendly catalysts with wide temperature window were designed for the NO_x removal, such as MnO _x, CeO₂, Fe₂O₃ and CuO^[11-14]. Among that, CeO₂ based catalyst attracts increasing attention because of their excellent reducibility and oxygen storage capacity^[15-17]. Neverthe-less, pure CeO₂ catalyst can facilitate the side reaction of NH₃ oxidation and thereby decrease the SCR catalyt-ic activity. Recent studies have indicated that the Ce-Ti oxides catalyst presented excellent NO_x conversion efficiency in a wide temperature range, especially the amorphous Ce-Ti oxides with the formation of new Ce-O-Ti active site^[18-19]. Apart from the composition, the per-formance of the catalyst is also highly related to the microstructure. For instance, the Ce-Mn oxide nano-sphere catalyst demonstrated higher SCR performance than that of Ce-Mn oxide catalyst with a particle struc-ture^[20]. The Mn-Fe catalyst with a nanocage structure delivered a superior SCR performance^[21]. Moreover, ordered mesoporous catalysts have also attracted great attentions owing to the open pore framework and high surface area. It has demonstrated that the mesoporous catalyst can enhance the catalytic activity and broaden the temperature range for the SCR catalyst^[22]. Herein, it is a great potential for cerium based SCR catalyst by combining the titanium oxides with ordered mesopo-rous structure.

  In this work, complete amorphous Ce-Ti oxide series catalyst with highly ordered mesoporous struc-ture for the SCR of NO_x with ammonia was synthesized via a template assisted sol-gel route. The catalyst microstructure and morphology was investigated in de-tail. The obtained catalyst exhibited approximately 100% NO conversion efficiency in a wide temperature range and excellent resistance to SO₂. Subsequently, the catalyst was integrated into the support layer of a rigid ceramic membrane by a wet impregnation proce-dure for simultaneous removal of particulates and NO_x.

  1.   Experimental

  1.1   Preparation of the catalyst

  A series of Ti-Ce mixed catalysts was obtained via a template assisted sol-gel process. Hydrochloric acid (HCl) was added drop wise to tetraethyl titanate (Ti(OC₂H₅)₄) under vigorous stirring at 30 ℃. A solution of n-butyl alcohol (C₄H₉OH) with tri-block copolymer F127 and certain concentrations of Ce(NO₃)₃·6H₂O was then added into it and the stirring was maintained for 2 h to obtain a transparent sol. The molar ratio of n_Ti(OC2H5)4: n_C4H9OH:n_HCl:n_F127:n_{Ce(NO3)3·6H2O} was 1:6:9:0.005:(0~0.2) (denoted as Ce_aTiO_x, where a was the Ce molar fraction). Subsequently, the sol was poured into the Petri-dishes and dried at 20 ℃ and 80%RH for 24 h to form a thin gel layer. The gel was then calcined in air for 450 ℃ for 4 h with the heating and cooling rate of 0.5 ℃·min^-1 to obtain the catalyst. For comparison, the Ce_aTiO_x catalyst without adding template F127 was also prepared and denoted as Bla-Ce_aTiO_x.

  1.2   Preparation of catalytic ceramic filter

  An unsymmetrical SiC-SiO₂ based porous ceramic with a dimension 60/40 nm×100 mm (outer/inner diam-eter×length) and an average pore size of 20 μm was provided by Jiulang High-technology company (Nanjing China). The Ce_0.2TiO_x sol was employed as cat-alyst precursor solution. The catalytic ceramic filter was prepared by a vacuum suction dip-coated method. To guarantee the catalytic activity, the coating procedure was repeated to increase the catalyst loading amount, denoted as Ce_aTiO_x-M-N (N was coating times), and the loading catalyst was detected by a weighing method.

  1.3   Catalyst characterization

  The crystal phase of the catalysts were determined by X-ray diffraction (XRD) test using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ= 0.154 nm) at 40 kV and 30 mA between 20° to 80°. Small-angle X-ray diffraction (SAXRD) patterns of the catalysts were obtained from X-ray diffractometer at 40 kV and 200 mA between 0.5° to 5° (Ultima IV/PSK with Cu Kα radiation, λ=0.154 nm). The pore structure of the samples was measured by the transmission elec-tron microscopy (TEM) on a JEOL JEM-2100 electron microscope at an accelerating voltage of 200 kV. The surface area, pore volume and pore size of the catalysts were obtained using a BELSORPII N₂ adsorption/ desorption apparatus at 77 K. The catalyst chemical composition was analyzed by X-ray photoelectron spec-troscopy (XPS, AXIS-Ultra, Al Kα 1 486.6 eV photons, 15 mA, 15 kV). The surface morphology of the catalytic membrane was obtained by a Hitachi S4800 field emis-sion scanning electron microscope (FE-SEM) with an accelerating voltage of 3 kV.

  1.4   Measurement of the catalytic membrane performance

  The gas permeation of the catalytic membrane with a length of 100 mm was investigated using a home-made equipment. The SCR activity of the catalytic membrane with a length of 20 mm was examined on a laboratory test device (Fig. 1). The model gas was used for the examinations containing 500 mg·L^-1 NH₃, 500 mg·L^-1 NO, 3% (volume fraction) O₂, 5% (volume frac-tion) H ₂O and N₂ as carrier gas, with a temperature ranged from 25 to 400 ℃ at a gas filtration velocity range of 4~10.6 cm·s^-1. In addition, the NO concentra-tions was analyzed by a FTIR analyzer, and the NO conversion (X_NO) was calculated according to the Eq. 1 Where c_NO-in and c_NO-out were the concentrations of NO in the inlet and outlet, respectively.

  Figure 1

  

  Figure 1.  Scheme of the test device for the measurement the performance of the catalytic ceramic filter

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  $ {X_{{\rm{NO}}}} = \frac{{{c_{{\rm{NO - in}}}} - {c_{{\rm{NO - out}}}}}}{{{c_{{\rm{NO - in}}}}}} \times 100\% $

  (1)

  2.   Results and discussion

  2.1   Microstructure and micromorphology analysis of the catalysts

  Fig. 2 shows the XRD patterns of Ce-Ti mixed oxides after calcination at 450 ℃. As observed, the dif-fraction peaks at 25.3°, 38.3°, 48.2°, 54.5°, 62.5° were a typical anatase for the pure TiO₂ material (PDF No.21^-1272). In the presence of Ce, no diffraction peak related to CeO₂ crystalline phase was found, indicating that Ce species were well dispersed on the surface of anatase TiO₂ existed as amorphous phase^[22]. Further-more, the peaks of anatase TiO₂ became weaker with the increase of Ce/Ti molar ratio. No obvious diffrac-tion peak was observed for Ce_0.2TiO_x and Ce_0.3TiO_x, indi-cating the phase transformation of TiO₂ was suppressed and a complete amorphous structure was formed. The possible reason was attributed to the larger ion radii of Ce³⁺/Ce⁴⁺ (R_{Ce^{3 +}} =0.103 nm, R_{Ce ^{4 +}}=0.092 nm, R_{Ti ^{4 +}}=0.065 nm), when the Ce ions replaced the Ti ions to form the Ti-O-Ce species, causing the distortion of TiO₂ lattice and thereby hindering the anatase phase transforma-tion^[23-26]. Furthermore, the electronegativity of Ti (1.54) was larger than that of Ce (1.12), the strength of Ti-O bond and thermal stability of the mesostructured were enhanced by the partial electron transfer, and thus in-hibited the anatase phase transformation of TiO₂ materi-al^[27]. Notably, the short-order Ce-O-Ti species has dem-onstrated to be active sites for the SCR, the occurrence of Ce-O-Ti species could be conducive to improve the SCR performance^[18].

  Figure 2

  

  Figure 2.  XRD patterns of the Ce-Ti mixed oxides (450 ℃)

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  In this study, the ordered mesoporous structure was fabricated via a template assisted sol-gel process. Nevertheless, the assembled ordered pore was prone to collapse with the removal of template and grain growth during the sintering process. To insight into the pore structure, the SAXRD was conducted (Fig. 3a). An obvi-ously diffraction peak for Ce_aTiO_x catalyst was observed and enhanced with the increase in the Ce/Ti molar ratio, except the Ce_0.3TiO_x, indicating the ordered pore structure well retained. In contrast, pure TiO₂ cat-alyst exhibited a weak diffraction peak, manifesting the ordered pore structure collapsed seriously. The possi-ble reason was that the crystal grain growth was suppressed by the presence of the Ce species. Further-more, the large Ce species entered into and consolidated the pore framework, so the thermal stability of ordered pore structure was improved (Fig. 3b). Never-theless, too high a level of Ce doping, Ce_0.3TiO_x, the continuity of the TiO₂ framework can be disturbed and harmed the alignment of mesopores^[28]. Further evi-dence for the pore structure of the catalyst is provided in the TEM images (Fig. 3c~h)). As expected, a disor-dered wormlike structure and a high degree of crystalli-zation was observed for pure TiO₂ catalyst. In contrast, the Ce_0.2TiO_x presented highly ordered hexagonal pore structure with a pore size of ~6.3 nm and no nanocrys-tal structure was observed, manifesting the ordered pore structure was stabilized by the inhibition of the grain growth with the presence of Ce.

  Figure 3

  

  Figure 3.  (a) SAXRD patterns of the Ce-Ti mixed oxides; (b) Mechanism graph of the Ce stabilizing the ordered mesoporous structure; (c~e) TEM images of TiO₂; (f~h) TEM images of Ce_0.2TiO_x (sintering temperature: 450 ℃)

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  To further explore the microstructure, the porosity and surface area of the catalyst was investigated by the N₂ adsorption-desorption isotherms (Fig. 4 and Table 1). All the catalysts showed a type Ⅵ gas adsorption isotherm, which was a typical mesoporous structure. In the presence of Ce species, the assembled ordered pore was well retained, a dramatic increase of the BET (Brunauer-Emmett-Teller) surface area of the Ce_aTiO_x catalysts was observed. With the increase of the Ce amount, the surface area was increased from 103 to 195 m²·g^-1, especially the Ce_0.2TiO_x catalysts with high-ly ordered pore structure delivered the largest surface area. The surface area of Bla-Ce_0.2TiO_x catalysts with-out template hugely decreased to 40 m²·g^-1. Further-more, the pore size of the catalyst was slightly decreased with the increase of the Ce amount because the larger Ce ion entered into the TiO₂ pore framework.

  Figure 4

  

  Figure 4.  N₂ adsorption-desorption isotherms for the catalysts (450 ℃)

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  Table 1

  

  Table 1.  Microstructure of the catalysts (450 ℃)

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  Catalyst	Pore size / nm	BET surface area / (m2·g-1)	Pore volume / (cm3·g-1)	Porosity / %
  TiO2	4.19	103	0.116	30.87
  Ce0.05TiOx	4.55	123	0.128	32.65
  Ce0.1TiOx	3.28	179	0.124	32.45
  Ce0.2TiOx	3.28	195	0.134	33.97
  Ce0.3TiOx	3.34	190	0.125	31.71
  Bla-Ce0.2TiOx	3.71	40	0.046	26.62

  Subsequently, the catalyst was analyzed by XPS to determine its main elements and chemical states (Fig. 5), the detailed atomic fraction of the catalyst is summarized in Table 2. As observed, four elements, namely, Ti, Ce, O and C, were present together on the Ce_a TiO_x surface. With the increase of the Ce content, the content of oxygen slightly was decreased and the content of carbon was increased. For the Ce3d XPS spectrum, it was complicated as a result of the hybrid-ization of the Ce4f with ligand orbitals and partial occu-pation of the 4f orbitals. It was reported that the Ce3d spectra was consisted of two sets of spin-orbital multi-plets, Ce3d_5/2 and Ce3d_3/2, denoted as V and U, respec-tively. The U‴ (916.7 eV), U″ (907.5 eV), U (901.0 eV), V‴ (898.4 eV), V″ (888.2 eV) and V (882.5 eV), were corresponding to the Ce⁴⁺, while the U' (903.5 eV), U₀ (898.8 eV), V' (884.9 eV) and V₀ (880.3 eV) were cor-responding to the Ce^{3+ [29]}. For the SCR catalyst, the Ce³⁺ was conducive to the adsorption of NH₃ to form NH₄⁺, contributing to the low-temperature SCR activi-ty^[30-31]. Based on the fitting calculation of peak area, the Ce_0.2 TiO_x catalyst delivered the highest Ce³⁺/Ce⁴⁺ ratio, and may delivered desirable low-temperature SCR activity.

  Figure 5

  

  Figure 5.  (a) XPS spectrum of Ce_0.2TiO₂; (b) High-resolution XPS spectrum of Ce3d (sintering temperature: 450 ℃)

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  Table 2

  

  Table 2.  Atomic fraction of Ti, O, C and Ce

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  Catalyst	Atomic fraction / %				Atomic ratio of Ce3+ to Ce4+
  	Ti	O	C	Ce
  TiO2	21.34	74.68	3.98
  Ce0.1TiOx	21.02	72.71	4.56	1.71	2.31
  Ce0.2TiOx	20.76	69.05	6.27	3.98	2.40
  Ce0.3TiOx	20.15	65.86	8.34	5.55	1.92

  2.2   Performance of the catalyst membrane

  Fig. 6 shows the surface morphology of the catalytic membrane. As observed, the catalytic membrane was consisted of a support layer and a homogeneous and integrated separation layer with a thickness of ~320 μm. The support layer was constructed by a large SiC particles packed, thus the catalyst nanoparticles with small size can be permeated into the pore of the sup-port membrane. It is a prerequisite for the catalytic membrane with a relatively low differential pressure. However, it is inevitable to enlarge the pressure drop with the catalytic particle permeating in the support layer. Fig. 7a shows the pressure drop of the membrane versus the filtration velocity. The pressure drop of the reactor linearly increased with the increasing of the fil-tration velocity. When the filtration velocity increased from 2.6 to 10.6 cm·s^-1, the differential pressure was increased from 780 to 3 290 Pa. After loading the cata-lyst, the differential pressure increased with the num-ber of coating times due to the permeation of the cata-lytic particle into the pore of the support membrane. However, the increase degree of the differential pres-sure was not significant, indicating that the prepared catalytic filter could fulfil the criteria for its economic use in the hot gas purification^[8].

  Figure 6

  

  Figure 6.  (a) Support layer surface, (b) cross section and (c) seperation layer surface of catalytic membrane

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  Figure 7

  

  Figure 7.  (a) Relationship between differential pressure and filtration velocity; (b) NO conversion of the unsupported catalysts at various temperatures (v=4 cm·s^-1); (c) NO conversion of the Ce_0.2TiO_x catalyst at various velocities; (d) SO₂ tolerance of the Ce_0.2TiO_x catalyst (v=4 cm·s^-1, T=250 ℃); (e) NO conversion of the prepared catalytic membrane (v=4 cm·s^-1)

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  Before the SCR performance of the catalytic mem-brane test, the activity of the unsupported catalysts were evaluated (Fig. 7b~d). Among all of the catalysts, the pure TiO₂ presented the lowest SCR activity and the Ce_aTiO_x catalyst exhibited significant enhancement of the SCR activity. Notably, the ordered mesoporous Ce_0.2 TiO_x exhibited approximately 100% NO conver-sion in a wide temperature range of 175~350 ℃ and demonstrated the highest SCR activity. In contrast, Bla-Ce_0.2TiO_x catalyst showed an obvious decrease in the SCR activity, suggesting the ordered pore structure with the large surface area and open pore framework was contributing to the SCR performance. Meanwhile, the completed amorphous Ce_0.2TiO_x could induce a Ce-O-Ti species because of the strong interactions between Ce and Ti, thus the ordered mesoporous Ce_0.2TiO_x catalyst exhibited a superior SCR catalytic activity^{[19, 32-33]}. Furthermore, the Ce_0.3TiO_x catalyst pres-ent relative low SCR activity in the low-temperature range was mainly attributed to the collapse of ordered pore structure and the formation of Ce-O-Ce.

  Fig. 7c shows the NO conversions over Ce_0.2TiO_x catalyst under different filtration velocity. The increase of the filtration velocity made the 100% NO conversion in a narrow temperature range. For practical applica-tion, SO₂ tolerance was an important factor and the SO₂ tolerance over Ce_0.2 TiO_x catalyst is showed in Fig. 7d. The NO conversion decreased slightly, then reached a steady state, and the NO conversion decreased with the increase of the SO₂ concentration. After switching SO₂ off, the NO conversion recovered quickly and remained unchanged, manifesting the high sulfur resistance of the Ce_0.2TiO_x catalyst. It has been demonstrated that the Ce-Ti catalyst, in the presence of SO₂, did not re-duce the amount of coordinated NH ₃, even promoted the formation of NH₄+, and thereby slightly hindered the NO adsorption^{[19, 34-36]}. It can concluded that the Ce _0.2TiO_x catalyst with high SCR activity and satisfacto-ry SO ₂ tolerance can be a promising catalyst for devel-oping the catalytic membrane.

  Herein, the Ce_0.2TiO_x catalyst was employed to pre-pare the catalytic membrane and the NO conversion of the catalytic membrane was investigated (Fig. 7e). As observed, the Ce_0.2TiO_x-M^-1 presented relative low NO conversion in a temperature range of 150~400 ℃. The Ce _0.2TiO_x-M-2 and Ce_0.2TiO_x-M-3 presented almost simi-lar SCR activity, indicating the Ce_0.2TiO_x-M-2 obtained the optimum catalyst loading amount, corresponding to a mass fraction of 2.17%. Comparing with the powder catalyst, the catalytic membrane exhibited a slightly low NO conversion due to the inhomogeneous gas velocity in the pore channel of the membrane.

  3.   Conclusions

  In this study, a template assisted sol-gel method was employed to prepare highly ordered Ce-Ti catalyst with high SCR activity. In the presence of Ce species, the phase transformation of the catalysts were sup-pressed to form an amorphous phase and the assem-bled ordered pore structure were well retained. Mean-while, the BET result showed that the surface area was significantly improved by the additive of Ce species. After the SCR test, the amorphous and ordered pore structure of the Ce _0.2TiO_x catalyst presented high SCR activity in a wide temperature range of 175~350 ℃. Subsequently, the Ce_0.2TiO_x catalyst was loaded on the filter and delivered an almost similar SCR activity with the catalyst powder.

  
  
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• 

  Figure 1  Scheme of the test device for the measurement the performance of the catalytic ceramic filter

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  Figure 2  XRD patterns of the Ce-Ti mixed oxides (450 ℃)

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  Figure 3  (a) SAXRD patterns of the Ce-Ti mixed oxides; (b) Mechanism graph of the Ce stabilizing the ordered mesoporous structure; (c~e) TEM images of TiO₂; (f~h) TEM images of Ce_0.2TiO_x (sintering temperature: 450 ℃)

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  Figure 4  N₂ adsorption-desorption isotherms for the catalysts (450 ℃)

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  Figure 5  (a) XPS spectrum of Ce_0.2TiO₂; (b) High-resolution XPS spectrum of Ce3d (sintering temperature: 450 ℃)

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  Figure 6  (a) Support layer surface, (b) cross section and (c) seperation layer surface of catalytic membrane

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  Figure 7  (a) Relationship between differential pressure and filtration velocity; (b) NO conversion of the unsupported catalysts at various temperatures (v=4 cm·s^-1); (c) NO conversion of the Ce_0.2TiO_x catalyst at various velocities; (d) SO₂ tolerance of the Ce_0.2TiO_x catalyst (v=4 cm·s^-1, T=250 ℃); (e) NO conversion of the prepared catalytic membrane (v=4 cm·s^-1)

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  Table 1.  Microstructure of the catalysts (450 ℃)

  Catalyst	Pore size / nm	BET surface area / (m2·g-1)	Pore volume / (cm3·g-1)	Porosity / %
  TiO2	4.19	103	0.116	30.87
  Ce0.05TiOx	4.55	123	0.128	32.65
  Ce0.1TiOx	3.28	179	0.124	32.45
  Ce0.2TiOx	3.28	195	0.134	33.97
  Ce0.3TiOx	3.34	190	0.125	31.71
  Bla-Ce0.2TiOx	3.71	40	0.046	26.62

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  Table 2.  Atomic fraction of Ti, O, C and Ce

  Catalyst	Atomic fraction / %				Atomic ratio of Ce3+ to Ce4+
  	Ti	O	C	Ce
  TiO2	21.34	74.68	3.98
  Ce0.1TiOx	21.02	72.71	4.56	1.71	2.31
  Ce0.2TiOx	20.76	69.05	6.27	3.98	2.40
  Ce0.3TiOx	20.15	65.86	8.34	5.55	1.92

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