English

• NO_x is a kind of the main air pollutants which can cause some environmental issues, such as acid rain and photochemical smog^[1-2]. NH₃-SCR technology is one of the most advanced methods to remove NO_x exhausted from automobile at present^[3]. The widely used NH₃-SCR catalyst is the commercial V₂O₅-WO₃(MoO₃)/TiO₂, however, the toxicity of V₂O5 and the generated high concentration of N₂O above 450 ℃ limited the practical applications of the V-based catalysts for diesel vehicles^[4]. So it is necessary to develop an environmentally friendly NH₃-SCR catalyst to remove NO_x.

  Recently, CeTiO_x has been considered to be a promising catalyst for NH₃-SCR with high activity and excellent selectivity to N₂^[5-6]. Xu et al.^[5] revealed that Ce/TiO₂ catalysts with more 5% Ce showed excellent NH₃-SCR performance in the temperature range of 275~400 ℃. In general, catalytic activity, N₂ selectivity and stability of SCR catalyst are important parameters to evaluate its performance. Therefore, a lot of works including doping Cu^[7-8], Mn^[9-10], W^[4] and Co^[11] into CeTiO_x catalyst was done aiming at further improving its activity and selectivity to N₂. On the other side, Diesel Particulate Filters (DPF) used for eliminating particulate matter (PM) is required by the future regulations. The periodic regeneration of the filter may lead to the exhaust temperature up to 850 ℃ and then reduce the thermal stability of the catalysts^[12]. However, anatase TiO₂ was easy to transfer to rutile TiO₂ after high-temperature treatment^[13], which results in decreasing the NH₃-SCR performance of CeTiO_x catalysts after the above high temperature, so the thermal stability of CeTiO_x catalyst has become a critical issue to be solved. Zr addition could improve the stability and redox property of Ce-based oxygen storage materials which is widely used in TWC (Three-Way-Catalyst)^[14]. It was also reported that Zr based catalysts used for NH₃-SCR usually possess good thermal stability^[15]. Verdier et al.^[12] prepared a series of acidic zirconia materials for NH₃-SCR catalysts which showed good thermal stability after harsh aging. Xu et al.^[16] compared the thermal stability of Ce/ZrTiSiW with commercial V₂O5-based catalyst and Fe-ZSM-5, the results suggested the high tolerance to thermal shock of the zirconia-based catalyst. Other Zr based catalysts, such as CuO_x /WO_x-ZrO₂^[17], Cu-Ce-Mn/ZrO₂^[18], CeZrTiSiW^{[16, 19]} and MnO_x-CeO₂/WO₃-ZrO₂^[20] also showed good stability.

  In this work, CeTiO_x and CeZrTiO_x catalysts were prepared and their NH₃-SCR performances were compared after different calcination temperature. XRD, XPS, H₂-TPR, BET and NH₃-TPD were used to characterize the effect of Zr addition on the thermal stability of CeTiO_x catalyst.

  1   Experimental

  1.1   Catalyst preparation

  CeTiO_x and CeZrTiO_x catalysts were prepared by the conventional co-precipitation method. Cerium nitrate (AR grade, Jinshan, Chengdu, China), zirconyl nitrate hydrate (AR grade, 99%, Yutai, Shangdong, China) and titanylsulfate (AR grade, Dandong, Liaoning, China) were used as precursors. The precursors mixed in an aqueous solution were then precipitated with an alkaline buffer solution until the pH reached approximately 9. The obtained precipitates were dried at 90 ℃ for 24 h and then calcined at 550 ℃ for 3 h in air. The CeTiO_x catalyst, with Ce/Ti mole ratio at 0.2, was denoted as CT. The CeZrTiO_x catalyst, with Zr/Ti mole ratio at 0.3 and Ce/Ti mole ratio at 0.2, was denoted as Z-CT.

  The well-proportioned slurry obtained from mixing appropriate amount of water and the as-prepared powders were coated on honeycomb cordierites (cylinder, diameter: 11 mm, length: 26 mm, bulk: 2.5 cm3, 62 cell·cm^-2, Corning Ltd., USA). Then the monolith catalysts were dried at 90 ℃ for 12 h, followed by calcining at 550 ℃ for 3 h in air. Finally, the monolith catalysts were obtained with the catalyst loading of about 160 g·L^-1.

  CT and Z-CT catalysts were calcined at 650, 750 and 850 ℃ for 3 h as the thermal aged catalysts to investigate the effect of Zr addition on the thermal stability of CT catalyst. The two series of catalysts treated at different temperatures were noted as CT550, CT650, CT750, CT850, Z-CT550, Z-CT650, Z-CT750 and Z-CT850.

  1.2   Catalyst characterization

  The surface area, pore size, and pore volume of the catalysts were measured with the Brunauer-Emmett-Teller (BET) method by N₂ adsorption at-196 ℃ on a Quantachrome automated surface area and pore size analyzer (Autosorb SI). The samples were pretreated at 300 ℃ for 3 h prior to the measurement.

  Powder X-ray diffraction (XRD) experiments were performed on a Rigaku D/max-RA diffractometer using Cu Kα (λ= 0.154 06 nm) radiation. The tube voltage and current were 40 kV and 100 mA, respectively. The XRD powder diffractogram was recorded at 0.03°·s^-1 intervals in the range of 20°~80°. The crystalline phases were identified using reference data from the International Center Diffraction Data (ICDD).

  X-ray photoelectron spectroscopy (XPS) data were obtained by a British Kratos XSAM-800 electron spectrometer using 13 kV high voltage, 20 mA electron current and Mg Kα radiation. The C1s peak (284.8 eV) was used for the calibration of the binding energy values. The pressure in the analytical chamber was about 1×10^-9 Pa.

  Temperature-programmed desorption experiments of NH₃ (NH₃-TPD) were carried out on a fixed bed quartz reactor. A sample mass of 100 mg and a gas flow rate of 30 mL·min^-1 were used. The experiment included four stages: (1) degasification of the sample in Ar at 450 ℃ for 1 h, (2) adsorption of 2% NH₃ at 80 ℃ for 1 h, (3) isothermal desorption in Ar at 80 ℃ until no NH₃ was detected, and (4) temperature programmed desorption in Ar (TPD stage) at 8 ℃·min^-1 up to 600 ℃. The detector was a thermal conductivity detector.

  Temperature-programmed reduction with H₂ (H₂-TPR) experiments were carried out on a self-assembled experimental equipment with a thermal conductivity detector. All samples (100 mg) were pretreated in a quartz tubular micro-reactor in a flow of pure N₂ at 450 ℃ for 1 h, and then cooled to room temperature. The reduction was carried out in a flow of 5% H₂-95% N₂ from room temperature to 900 ℃ with a heating rate of 8 ℃·min^-1.

  1.3   Catalytic activity tests

  The catalytic activity measurement of the prepared monolithic catalysts was carried out in a fixed-bed quartz flow reactor. The concentrations of simulated gases were as follows: 0.1% NO, 0.1% NH₃, 5% O₂, balanced with N₂. The volume of the catalyst used for activity test was 2.5 mL and the total flow rate was 1 250 mL·min^-1, yielding the GHSV of 30 000 h^-1. The original and effluent NO concentrations at different temperatures were continually detected by an IR detector (ANTARIS IGS Analyzer, Thermo scientific). The data were collected after the reaction was stabilized for 30 min at each temperature.

  The NO_x conversions and N₂ selectivity (S_N2) were calculated as follows:

  φ denoted the volume concentration of the gases

  2   Results and discussion

  2.1   SCR performance

  Activities of CT and Z-CT catalysts calcined at different temperatures were carried out to compare their SCR performance. It was shown in Fig. 1(A) that the fresh catalysts calcined at 550 ℃ performed excellent NO_x conversion in a wide temperature range. The activity of the two catalysts declined to different extents after higher temperature calcination. To get a clear knowledge of the decrease of the activity with the change of calcination temperature, NO conversion tested at 360 ℃ as a function of calcination temperature was drawn in Fig. 1(B). It showed that there was hardly any difference between the activities of CT and Z-CT catalysts after calcination at 550 ℃. While Z-CT performed better NO_x conversion than the catalyst without Zr addition after aging at 650, 750 and 850 ℃. Moreover, it was distinct that the catalytic activity of CT dropped more sharply than that of Z-CT catalyst with increasing calcined temperature. In a short word, Z-CT catalysts could keep higher NH₃-SCR activity compared with CT catalysts after thermal aging at different temperature, indicating the addition of Zr improve the thermal stability of the CT catalyst.

  Figure 1.  (A) NO conversion as a function of temperature; (B) NO conversion tested at 360 ℃ as a function of calcination temperature

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  Moreover, N₂O, as one of the greenhouse gases, can absorbed infrared radiation with 270-time higher intensity than carbon dioxide (CO₂)^[1]. So N₂ selectivity was an important factor to evaluate NH₃-SCR performance of CT and Z-CT catalysts. N₂ selectivity of the two catalysts calcined at different temperatures was shown in Fig. 2. All the catalysts had nearly 100% N₂ selectivity and the Z-CT throughout performed slightly better than CT catalyst, especially N₂ selectivity of Z-CT550 and Z-CT650 catalysts was obviously higher than that of CT550 and CT650 catalysts above 350 ℃. The results implied that Zr addition promoted not only the NH₃-SCR activity but also the N₂ selectivity of CT catalysts.

  Figure 2.  N₂ selectivity of the catalysts

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  2.2   XRD and Raman

  To clarify the process of phase transition of CT and Z-CT catalysts with increasing calcination temperature, XRD measurement was performed and the results were shown in Fig. 3. No distinct diffraction peaks could be seen over CT550 and Z-CT550 catalysts from the patterns, suggesting that the amorphous metal oxides may be obtained after calcining at 550 ℃. After thermal aging at 650, cerium oxide phase and anatase TiO₂ phase were observed in the XRD patterns of CT, while no obvious crystallization happened over Z-CT catalyst. It indicated that adding Zr to CT could inhibit the formation of TiO₂ and CeO₂ crystallites.

  Figure 3.  XRD patterns for CT (A) and Z-CT (B) catalysts

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  It should be noted that rutile TiO₂ was observed in the profiles of CT750; in contrast, no rutile TiO₂ could be observed over Z-CT, suggesting that formation of rutile TiO₂ was inhibited by introduction of Zr. It is well known that the transition from anatase to rutile TiO₂ would lead to the decrease of specific surface area and oxygen vacancies^[22], which would result in the decrease of the catalytic activity. Thus, the NH₃-SCR activity of Z-CT catalyst was better than that of CT catalyst after calcined at above 650 ℃. Moreover, a new phase of ZrTiO₄ solid solution was detected from Z-TC catalyst after calcining at 750 ℃, which could be the reason that no rutile TiO₂ was detected in the catalyst with Zr addition.

  The phases of TiO₂ and CeO₂ was absolutely crystallized after calcination at 850 ℃, thus the SCR activity of CT was rapidly declined. ZrTiO₄ was still the main diffraction phase from XRD curve of Z-CT850 catalyst, and only small amount of rutile TiO₂ was observed compared with CT catalyst. Thus, it could be concluded that addition of Zr into CT catalyst inhibited the TiO₂ phase transition from anatase to rutile.

  Furthermore, there were no XRD peaks attributed to CeO₂ species for Z-CT at any calcined temperature; while cubic CeO₂ could be obviously detected over CT catalysts beside CT550, moreover, the grain size of CeO₂ was gradually increased with rising the thermal aging temperature of CT. The result showed that the Zr addition could prevent the Ce species from sintering and hold the lattice defects over Z-CT catalysts under high-temperature thermal aged. It is known that the amorphous state of the active component is more active than its crystalline form, because the former could create much larger amount of crystal defects and oxygen vacancies on the surface of catalyst and then improve the catalytic activity^[2]. It summarized briefly that the introduction of Zr into CT catalyst can inhibit the phase change from anatase TiO₂ to rutile TiO₂ and the sintering of active amorphous CeO₂ in the process of high-temperature thermal aging, which result in the Z-CT catalyst had better activity than CT catalysts after high temperature calcination.

  As a potential complementary characterization of XRD, Raman spectroscope was employed to detect the surface information of both CT and Z-CT catalysts thermally aged at different temperatures. Fig. 4(A) showed the corresponding results of Raman spectroscope from 100 to 750 cm^-1 over CT catalyst. The peaks located at 142, 194 and 642 cm^-1 were attributed to anatase TiO₂, while peaks located at 233, 443 and 610 cm^-1 were the typical Raman bands due to rutile TiO₂. It was obvious that the anatase TiO₂ was totally transformed into rutile TiO₂ when the aging temperature rose from 750 to 850 ℃. The F_2g mode of cubic fluorite CeO₂ phase located at 465 cm^-1 appeared and became sharper with temperature rising, which was in line with the XRD results of CT.

  Figure 4.  Raman spectra for CT (A) and Z-CT (B) catalysts

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  Fig. 4(B) showed the Raman profiles of Z-CT catalyst caicined at different temperatures. The peaks located at 147, 268, 315, 460 and 604 cm^-1 were attributed to tetragonal ZrO₂ and the peaks located at 147 and 400 cm^-1 were attributed to the anatase TiO₂. No Raman bands appeared over Z-CT550 and Z-CT650, meaning that the catalysts were in the amorphous state. After Z-CT was calcined at 750 and 850 ℃, peaks belonging to ZrO₂ and anatase TiO₂ were observed. But the wave numbers were slightly different compared with the pure ZrO₂ and anatase TiO₂. This might be caused by the formation of ZrTiO₄ solid solution, leading to the change of the electronic environment.

  Combined with the results of XRD and Raman, a conclusion could be drawn that the addition of Zr stabilized the structure of CT catalyst.

  2.3   SEM

  The SEM images of the catalysts were shown in Fig. 5 at 100 K magnification. Catalysts calcined at 550 and 650 ℃ seemed to be flocculent and the average sizes of the particles were too fuzziness to statistics. It was obvious that the particle sizes grew with increasing calcination temperature, so it was available to calculate the particle sizes after calcining at 750 and 850 ℃. The average particle sizes of CT750 and CT850 catalysts were about 23 and 45 nm respectively, while the data of Z-CT calcined at the same temperatures were about 19 and 37 nm severally. It was distinctly presented that Zr addition inhibited the aggregation of the particles, and then enhanced the structure stability of CT catalyst.

  Figure 5.  SEM images of the catalysts

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  2.4   H₂-TPR

  Fig. 6(A) showed the H₂-TPR profiles of the CT catalysts with different calcination temperatures. Two H₂ reduction peaks at 579 and 642 ℃ attributed to the surface and bulk Ce species respectively, were observed over CT550 catalyst^[23]. With increasing calcination temperature, a reduction peak of bulk CeO₂ at over 800 ℃ was gently emerged and shifted to higher temperature, which was account of the grain growth of CeO₂ with aging temperature rising confirmed by XRD and Raman. The peak at about 600 ℃ was attributed to the reduction of oxygen linked to surface ceria. After the catalyst was calcined at 850 ℃, the two peaks at 542 and 674 ℃ were very weak. In fact, the actual catalytic activity was controlled by the rates of redox cycles. The rate of cerium oxidation was fast, while the reduction of ceria was generally sluggish^[24]. The fact that Z-CT was less reducible at low temperature was in line with its poor NH₃-SCR performance. Nearly all the H₂ consumption happened at temperature higher than 800 ℃, meaning that Ce species were in the crystalline state, which was in accordance with the XRD results.

  Figure 6.  H₂-TPR profiles of CT (A) and Z-CT (B) catalysts calcined at different temperatures

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  Compared with CT catalyst after thermal aging, no H₂ consumption belonging to bulk CeO₂ was detected after Zr addition. The peak at about 717 ℃ on profile of Z-CT850 could be attributed to the reduction of zirconia. The results meant that Zr prevented the Ce species from sintering, which was in line with the result of XRD. Moreover, the H₂ consumption of Z-CT was higher than CT, meaning that Zr modified sample showed better reducibility and thus improved the NH₃-SCR activity.

  2.5   XPS

  The Ce3d XPS spectra for CT and Z-CT catalysts were presented in Fig. 7. The XPS peaks denoted as u1 and v1 represented Ce³⁺, while those denoted as u, u2, u3, v, v2, and v3 could be assigned to Ce⁴⁺. It is known that oxidation-reduction cycle occurs between Ce (Ⅲ) and Ce (Ⅳ), while generally most Ce in the catalyst is in the state of Ce (Ⅳ)^[25]. But Ce³⁺ could create a charge imbalance, the vacancies and unsaturated chemical bonds on the surface of the catalyst, which will lead to an increase of chemisorbed oxygen on the surface of the catalyst^[26-27]. Therefore, the value of Ce³⁺/Ce⁴⁺, can reflect the redox properties or even the catalytic activity of the catalyst in some extent. The ratio of Ce³⁺/Ce⁴⁺ was a calculation of total area of u1 and v1 attributed to Ce³⁺ divided by the total area of the six peaks assigned to Ce⁴⁺. Values of Ce³⁺/Ce⁴⁺ of different catalysts were listed in Table 1. The values of Ce³⁺/Ce⁴⁺ were always lower than 1 from Table 1, indicating Ce⁴⁺ was the main valence state of Ce in the two series of catalysts.

  Figure 7.  Ce3d XPS for (A) CT and (B) Z-CT catalysts

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  Table 1.  Ratio of Ce³⁺/Ce⁴⁺ of the catalysts calcined at different temperatures

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  Sample	Ratio of Ce3+/Ce4+
  	550 ℃	650 ℃	750 ℃	850 ℃
  CT	62.3%	34.1%	31.1%	24.6%
  Z-CT	62.4%	42.0%	38.2%	35.6%

  Ce³⁺/Ce⁴⁺ of CT550 catalyst were as same as that of Z-CT550 catalyst; however, the values of two catalyst were both gradually decreased with rising the calcination temperature, while Ce³⁺/Ce⁴⁺ of Z-CT catalysts was always evidently higher than that of CT catalysts. More amount of Ce³⁺ represented higher oxygen storage capacity and redox properties of the catalyst^[25], which was in accordance with the result of H₂-TPR and crucial to the SCR reaction. Furthermore, it was reported that the presence of Ce³⁺ could create the surface defects and the number of surface defects would be decreased with the growth of the ceria particles^[28]. From XRD and XPS results, it could be seen that rising aging temperature could be beneficial to the crystallization of ceria nanoparticles and harmful to the maintenance of ceria defects and Ce³⁺.

  Combined with the above analysis, Z-CT catalyst possessed larger amount of Ce³⁺/Ce⁴⁺ than those of CT catalysts, except the catalysts calcined 550 ℃, so higher oxygen storage capacity and redox properties, as well as more surface defects co-contributed to higher NH₃-SCR activities of Z-CT catalysts.

  2.6   NH₃-TPD

  To investigate the effect of calcination temperature on the changes of the surface acid amount and the distribution of acidic sites of CT and Z-CT catalysts, temperature-programmed desorption of NH₃ experiments were performed. As shown in Fig. 8, a similar broad NH₃ desorption peak was observed both in CT550 and Z-CT550 catalysts, indicating that there was little difference of the acid amount and acidic sites between the two catalysts. The intensities of NH₃ desorption peaks were obviously dropped when the calcination temperature rose, but Z-CT catalysts kept higher peak intensity than CT catalysts at the same calcined temperature. The above phenomena illustrated that Z-CT catalysts had higher surface acid and more acidic sites than CT catalysts even if suffered high-temperature thermal aging. Accordingly, more NH₃ adsorption on Z-CT catalysts than CT catalysts make contribution to maintain higher SCR activity and N₂ selectivity^{[4, 29]} of Z-CT catalysts.

  Figure 8.  NH₃-TPD profiles for the catalysts

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  As was known that NH₃ adsorbed on the surface of catalyst in two states: (a) ammonia adsorbed as ammonium ions, over Brönsted acidic-OH surface hydroxyl groups; and (b) molecularly adsorbed ammonia, through a Lewis-type interaction on coordinatively unsaturated cations^[30]. It was well known that ammonia adsorbed on Brönsted acid sites desorbed at lower temperatures than that on Lewis acid sites^[30-31]. For CeTiO_x catalyst, NH₃ desorption peaks located at 80~200 ℃ were attributed to Brönsted acid sites and the Lewis acid sites were at 230~350 ℃^[32]. As was shown in Fig. 8(B), peaks located at about 140 and 190 ℃ were belonged to Brönsted acid sites and peaks located at 245 and 306 ℃ were belonged to Lewis acid sites. For CT catalyst, peak located at about 152 ℃ was belonged to Brönsted acid sites. Peak areas of Brönsted acid sites and total acid sites and ratios of B/T for the catalysts were shown in Table 2. It was obvious that the Brönsted acidity on CT surface decreased more sharply than that of Z-CT after thermal aging. There was an interesting fact that pure TiO₂ only showed Lewis acidity and poor catalytic activity for SCR. However, when sulfated, strong Brönsted acidity was formed^[33] and SCR activity was strongly enhanced^[34]. This meant that Brönsted acidity played an important role in the SCR reaction. Moreover, it had been reported that the introduction of Brönsted acid sites weakened the strong oxidation of ammonia but enhanced the ammonia adsorption capacity of catalyst. Therefore, the N₂ selectivity and activity of catalysts were improved^[35]. The enhancement of NH₃-SCR performance after Zr addition was attributed to the fact that Zr improved the thermal stability of Brönsted acidity and therefore improved the activity and N₂ selectivity.

  Table 2.  Peak areas of B^a and T^b sites and ratios of B/T for the catalysts

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  	Area of B sites			Area of T sites			B/T
  	CT	Z-CT		CT	Z-CT		CT	Z-CT
  550	255.4	414.79		1 101.9	1 059.9		23.1%	39.2%
  650	100.4	284.9		526.5	809.2		19.1%	35.2%
  750	86.38	197.5		440.3	455.8		19.6%	43.3%
  850	38.1	154.8		166.3	225.2		22.9%	68.7%
  a B denoted the Brönsted acid sites; b T denoted total acid sites

  3   Conclusions

  Effect of Zr on the thermal stability of Ce-Ti monolith NH₃-SCR catalyst was investigated in this study. Catalytic activity results showed that Z-CT performed better than CT after high temperature treatment at 650, 750 and 850 ℃. The introduction of Zr into CT catalysts could prevent the Ce species from sintering and inhibit the phase transformation TiO₂ from anatase to rutile in the process of high-temperature thermal aging, suggesting the structural properties of CT catalysts were stabilized by the addition of Zr. More amount of Ce³⁺/Ce⁴⁺ indicated higher redox properties and more surface defects over Z-CT catalysts than CT catalysts, suggesting the redox properties of CT catalysts were stabilized by the addition of Zr. In addition, Zr modified catalysts possessed more Brönsted acidic sites than CT catalysts, suggesting the surface acidic properties of CT catalysts were stabilized by the addition of Zr. Therefore, it could be concluded that the addition of Zr promoted the thermal stability of CT catalysts.

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  Figure 1  (A) NO conversion as a function of temperature; (B) NO conversion tested at 360 ℃ as a function of calcination temperature

  Reaction conditions: φ(NO)=φ(NH₃)=0.1%, φ(O₂)=5%, balance N₂, GHSV=30 000 h^-1

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  Figure 2  N₂ selectivity of the catalysts

  Reaction conditions: φ(NO)=φ(NH₃)=0.1%, φ(O₂)=5%, balance N₂, GHSV=30 000 h^-1

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  Figure 3  XRD patterns for CT (A) and Z-CT (B) catalysts

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  Figure 4  Raman spectra for CT (A) and Z-CT (B) catalysts

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  Figure 5  SEM images of the catalysts

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  Figure 6  H₂-TPR profiles of CT (A) and Z-CT (B) catalysts calcined at different temperatures

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  Figure 7  Ce3d XPS for (A) CT and (B) Z-CT catalysts

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  Figure 8  NH₃-TPD profiles for the catalysts

  B denoted the Brönsted acid sites and T denoted total acid sites

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  Table 1.  Ratio of Ce³⁺/Ce⁴⁺ of the catalysts calcined at different temperatures

  Sample	Ratio of Ce3+/Ce4+
  	550 ℃	650 ℃	750 ℃	850 ℃
  CT	62.3%	34.1%	31.1%	24.6%
  Z-CT	62.4%	42.0%	38.2%	35.6%

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  Table 2.  Peak areas of B^a and T^b sites and ratios of B/T for the catalysts

  	Area of B sites			Area of T sites			B/T
  	CT	Z-CT		CT	Z-CT		CT	Z-CT
  550	255.4	414.79		1 101.9	1 059.9		23.1%	39.2%
  650	100.4	284.9		526.5	809.2		19.1%	35.2%
  750	86.38	197.5		440.3	455.8		19.6%	43.3%
  850	38.1	154.8		166.3	225.2		22.9%	68.7%
  a B denoted the Brönsted acid sites; b T denoted total acid sites

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