Promotional Effect of Zr on Thermal Stability of CeTiOxMonolith Catalyst for Selective Catalytic Reduction of NOxwith Ammonia

XU Bao-QiangXU Hai-DiCAO YiLAN LiYANG YiZHANG Yan-HuaLI Yuan-ShanGONG Mao-ChuCHEN Yao-Qiang＊,,(Key Laboratory of Green Chemistry & Technology of the Ministry of Education,College of Chemistry, Sichuan University, Chengdu 6006, China)(Institute of New Energy and Low-Carbon Technology, Sichuan University, Chengdu 6006, China)(Sichuan Provincial Environmental Protection Environmental Catalytic MaterialsEngineering Technology Center, Chengdu 6006, China)(College of Chemical Engineering, Sichuan University, Chengdu 6006, China)

Promotional Effect of Zr on Thermal Stability of CeTiOxMonolith Catalyst for Selective Catalytic Reduction of NOxwith Ammonia

XU Bao-Qiang1XU Hai-Di＊,2, 3CAO Yi1LAN Li1YANG Yi1ZHANG Yan-Hua1LI Yuan-Shan4GONG Mao-Chu1CHEN Yao-Qiang＊,1,3
(1Key Laboratory of Green Chemistry & Technology of the Ministry of Education,
College of Chemistry, Sichuan University, Chengdu 610064, China)
(2Institute of New Energy and Low-Carbon Technology, Sichuan University, Chengdu 610064, China)
(3Sichuan Provincial Environmental Protection Environmental Catalytic Materials
Engineering Technology Center, Chengdu 610064, China)
(4College of Chemical Engineering, Sichuan University, Chengdu 610064, China)

CeTiOxand CeZrTiOxcatalysts were prepared by co-precipitation method and used for selective catalytic reduction of NOxby NH3(NH3-SCR). Thermal aging of the two catalysts was conducted at various temperatures to investigate the effect of Zr on thermal stability of CeTiOxcatalyst. The NH3-SCR performance ofthe catalysts showed that the Zr-modified catalyst exhibited better activity and N2selectivity than CeTiOxcatalyst after high temperature aging at 650, 750 and 850℃. The XRD, Raman and H2-TPR results illustrated that the addition of Zr could prevent Ce species from sintering and inhibit the formation of rutile TiO2. The SEM images of the catalysts revealed that the aggregation of the particles with increasing calcination temperature could be inhibited by addition of Zr. From the XPS spectra of Ce3d, the ratio of Ce3+/Ce4 +of CeTiOxdecreased more sharply than that of CeZrTiOxwith the aging temperature, meaning that more crystal defects and oxygen vacancies were preserved after Zr addition, which led to increased catalytic performance. In addition, NH3-TPD results implied that modified catalyst possessed larger amounts of Br?nsted acid sites after calcining at a same temperature, improving the activity and inhibiting the NH3oxidation. Thus, a conclusion could be drawn that Zr addition improved the thermal stability of the CeTiOxcatalyst.

cerium titanium mixed oxides; zirconium; thermal stability; selective catalytic reduction; NOx

NOxis 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 NOxexhausted from automobile at present[3]. The widely used NH3-SCR catalyst is the commercial V2O5-WO3(MoO3)/TiO2, however, the toxicity of V2O5and 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 NOx.

Recently, CeTiOxhas 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/TiO2catalysts with more 5% Ce showed excellent NH3-SCR performance in the temperature range of 275～400℃. In general, catalytic activity, N2selectivity 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 CeTiOxcatalyst 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 TiO2was easy to transfer to rutile TiO2after high-temperature treatment[13], which results in decreasing the NH3-SCR performance of CeTiOxcatalysts after the above high temperature, so the thermal stability of CeTiOxcatalyst 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 CuOx/WOx-ZrO2[17], Cu-Ce-Mn/ZrO2[18], CeZrTiSiW[16,19]and MnOx-CeO2/WO3-ZrO2[20]also showed good stability.

In this work, CeTiOxand CeZrTiOxcatalysts 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 CeTiOxcatalyst.

1　Experimental

1.1Catalyst preparation

CeTiOxand CeZrTiOxcatalysts were prepared by the conventional co-precipitation method. Cerium nitrate (AR grade, Jinshan, Chengdu, China), zirconylnitrate 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 pHreached approximately 9. The obtained precipitates were dried at 90℃for 24 h and then calcined at 550℃for 3 h in air. The CeTiOxcatalyst, with Ce/Ti mole ratio at 0.2, was denoted as CT. The CeZrTiOxcatalyst, 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.

1.2Catalyst characterization

The surface area, pore size, and pore volume of the catalysts were measured with the Brunauer-Emmett-Teller (BET) method by N2adsorption 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-1intervals 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 1×10-9Pa.

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-1were used. The experiment included four stages: (1) degasification of the sample in Ar at 450℃for 1 h, (2) adsorption of 2% NH3at 80℃for 1 h, (3) isothermal desorption in Ar at 80℃until no NH3was detected, and(4) temperature programmed desorption in Ar (TPD stage) at 8℃· min-1up to 600℃. The detector was a thermal conductivity detector.

Temperature-programmed reduction with H2(H2-TPR) experiments were carried out on a selfassembled experimental equipment with a thermal conductivity detector. All samples (100 mg) were pretreated in a quartz tubular micro-reactor in a flow of pure N2at 450℃for 1 h, and then cooled to room temperature. The reduction was carried out in a flow of 5% H2-95% N2from room temperature to 900℃with a heating rate of 8℃·min-1.

1.3Catalytic 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 NOxconversions and N2selectivity (SN2) were calculated as follows:

φ denoted the volume concentration of the gases

2　Results and discussion

2.1SCR 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 NOxconversion 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 NOxconversion 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.

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

Moreover, N2O, as one of the greenhouse gases, can absorbed infrared radiation with 270-time higher intensity than carbon dioxide (CO2)[1]. So N2selectivity was an important factor to evaluate NH3-SCR performance of CT and Z-CT catalysts. N2selectivity of the two catalysts calcined at different temperatures was shown in Fig.2. All the catalysts had nearly 100% N2selectivity and the Z-CT throughout performed slightly better than CT catalyst, especially N2selectivity 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 N2selectivity of CT catalysts.

2.2XRD and Raman

To clarify the process of phase transition of CT and Z-CTcatalysts 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 TiO2phase 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 TiO2and CeO2crystallites.

It should be noted that rutile TiO2was observed in the profiles of CT750; in contrast, no rutile TiO2could be observed over Z-CT, suggesting that formation of rutile TiO2was inhibited by introduction of Zr. It is well known that the transition from anatase to rutile TiO2would lead to the decrease of specificsurface 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 ZrTiO4solid solution was detected from Z-TC catalyst after calcining at 750℃, which could be the reason that no rutile TiO2was detected in the catalyst with Zr addition.

Fig.2 N2selectivity of the catalysts

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

The phases of TiO2and CeO2was absolutely crystallized after calcination at 850℃, thus the SCR activity of CT was rapidly declined. ZrTiO4was still the main diffraction phase from XRD curve of ZCT850 catalyst, and only small amount of rutile TiO2was observed compared with CT catalyst. Thus, it could be concluded that addition of Zr into CT catalyst inhibited the TiO2phase transition fromanatase to rutile.

Furthermore, there were no XRDpeaks attributed to CeO2species for Z-CT at any calcined temperature; while cubic CeO2could be obviously detected over CT catalysts beside CT550, moreover, the grain size of CeO2was 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 TiO2to rutile TiO2and the sintering of active amorphous CeO2in the process of hightemperature 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) showedthecorrespondingresultsof Ramanspectroscope from 100 to 750 cm-1over CT catalyst. The peaks located at 142, 194 and 642 cm-1were attributed to anatase TiO2, while peaks located at 233, 443 and 610 cm-1were the typical Raman bands due to rutile TiO2. It was obvious that the anatase TiO2was totally transformed into rutile TiO2when the aging temperature rose from 750 to 850℃. The F2gmode of cubic fluorite CeO2phase located at 465 cm-1appeared and became sharper with temperature rising, which was in line with the XRD results of CT.

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-1were attributed to tetragonal ZrO2and the peaks located at 147 and 400 cm-1were attributed to the anatase TiO2. No Raman bands appeared over Z-CT550 and ZCT650, meaning that the catalysts were in the amorphous state. After Z-CT was calcined at 750 and 850℃, peaks belonging to ZrO2and anatase TiO2were observed. But the wave numbers were slightly different compared with the pure ZrO2and anatase TiO2. This might be caused by the formation of ZrTiO4solid 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.

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

2.3SEM

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 ofCT750 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.

Fig.5 SEM images of the catalysts

2.4H2-TPR

Fig.6(A) showed the H2-TPR profiles of the CT catalysts with different calcination temperatures. Two H2reduction 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 CeO2at over 800℃was gently emerged and shifted to higher temperature, which was account of the grain growth of CeO2with 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 H2consumption happened at temperature higher than 800℃, meaning that Ce species were in the crystalline state, which was in accordance with the XRD results.

Compared with CT catalyst after thermal aging, no H2consumption belonging to bulk CeO2was 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 H2consumption of Z-CT was higher than CT, meaning that Zr modified sample showed better reducibility and thus improved the NH3-SCR activity.

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

2.5XPS

The Ce3d XPS spectra for CT and Z-CT catalysts were presented in Fig.7. The XPS peaks denoted as u1and v1represented Ce3+, while those denoted as u, u2, u3, v, v2, and v3could be assigned to Ce4+. It is known that oxidation-reduction cycle occurs between Ceand Ce, while generally most Ce in the catalyst is in the state of Ce[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 u1and v1attributed 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.

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.

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

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

2.6NH3-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 NH3experiments were performed. As shown in Fig.8, a similar broad NH3desorption peak was observedboth 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 NH3desorption 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 NH3adsorption on Z-CT catalysts than CT catalysts make contribution to maintain higher SCR activity and N2selectivity[4,29]of Z-CT catalysts.

Fig.8 NH3-TPD profiles for the catalysts

Table 2 Peak areas of Baand Tbsites and ratios of B/T for the catalysts

As was known that NH3adsorbed on the surface of catalyst in two states: (a) ammonia adsorbed as ammonium ions, over Br?nsted 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 Br?nsted acid sites desorbed at lower temperatures than that on Lewis acid sites[30-31]. For CeTiOxcatalyst, NH3desorption peaks located at 80～200℃were attributed to Br?nsted 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 Br?nsted 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 Br?nsted acid sites. Peak areas of Br?nsted acid sites and total acid sites and ratios of B/T for the catalysts were shown in Table 2. It was obvious that the Br?nsted acidity on CT surface decreased more sharply than that of Z-CT after thermal aging. There was an interesting fact that pure TiO2only showed Lewis acidity and poor catalytic activity for SCR. However, when sulfated, strong Br?nsted acidity was formed[33]and SCR activity was strongly enhanced[34]. This meant that Br?nsted acidity played an important role in the SCR reaction. Moreover, it had been reported that the introduction of Br?nsted acid sites weakened the strong oxidation of ammonia but enhanced the ammonia adsorptioncapacity of catalyst. Therefore, the N2selectivity 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 Br?nsted acidity and therefore improved the activity and N2selectivity.

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 TiO2from anatase to rutile in the process of hightemperature 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 Br?nsted 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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Zr的添加對提高NH3選擇性催化還原NOx整體式催化劑熱穩定性的影響

徐寶強1徐海迪＊,2,3曹毅1蘭麗1楊怡1張艷華1李元山4龔茂初1陳耀強＊,1,3
(1四川大學綠色化學與技術教育部重點實驗室，成都610064)
(2四川大學新能源與低碳技術研究院，成都610064)
(3四川省環境保護環境催化材料工程技術中心，成都610064)
(4四川大學化學工程學院，成都610064)

采用共沉淀的方法制備了CeTiOx和CeZrTiOx兩種用于NH3選擇性催化還原NOx的催化劑。兩種催化劑在不同的高溫條件下進行熱老化以考察Zr的添加對CeTiOx催化劑熱穩定性的影響。NH3-SCR活性數據顯示，經過650、750和850℃的高溫老化處理，Zr改性后的催化劑較CeTiOx催化劑具有更好的催化活性和N2選擇性。XRD、Raman和H2-TPR結果表明添加Zr可以阻止Ce物種的燒結并且抑制金紅石相TiO2的生成。催化劑的SEM圖像顯示Zr的添加可以抑制顆粒隨著焙燒溫度升高而產生的聚集長大。XPS的Ce3d光譜表明，隨著老化溫度的提升，CeTiOx催化劑表面的Ce3+/Ce4+比相對于CeZrTiOx催化劑下降更加明顯。這意味著隨著Zr的添加，更多的晶格缺陷和氧空缺出現在Zr改性催化劑的表面，這有利于催化性能的提升。另外，NH3-TPD結果表明經過相同的高溫老化，改性的催化劑保持了更多的Br?nsted酸性位，提高了催化活性并抑制了氨的氧化。因此，Zr的添加提高了催化劑的熱穩定性能。

鈰鈦復合氧化物；鋯；熱穩定性；選擇性催化還原；NOx

A

1001-4861(2016)03-0517-10

10.11862/CJIC.2016.039

2015-08-11。收修改稿日期：2015-11-27。

四川省科技廳科技支撐項目(No.2012FZ0008)，國家自然科學基金(No.21173153)，國家高技術研究發展計劃項目(863)(No.2013AA065304),四川大學項目(No.2015SCU11056)資助。

＊通信聯系人。E-mail：xuhaidi@scu.edu.cn，nic7501@scu.edu.cn