稀有金属(英文版) 2019,38(04),292-298

Thermal stability of Si-doped V₂O₅/WO₃-TiO₂ for selective catalytic reduction of NO_x by NH₃

Xun-Zhe Shao Hong-You Wang Meng-Long Yuan Jie Yang Wang-Cheng Zhan Li Wang Yun Guo Guan-Zhong Lu

Laboratory for Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science and Technology

作者简介：*Li Wang,e-mail:wangli@ecust.edu.cn;

收稿日期：3 May 2017

基金：financially supported by the National Key Research and Development Program of China (No. 2016YFC0204300);the National High Technology Research and Development Program of China (No. 2015AA034603);the National Natural Science Foundation of China (Nos. 21333003 and 21571061);the "Shu Guang" Project of the Shanghai Municipal Education Commission (No. 12SG29);the Commission of Science and Technology of Shanghai Municipality (No. 15DZ1205305);

Thermal stability of Si-doped V₂O₅/WO₃-TiO₂ for selective catalytic reduction of NO_x by NH₃

Xun-Zhe Shao Hong-You Wang Meng-Long Yuan Jie Yang Wang-Cheng Zhan Li Wang Yun Guo Guan-Zhong Lu

Laboratory for Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science and Technology

Abstract：

The selective catalytic reduction of NO_x with NH₃(NH₃-SCR) is a very effective technology to control the emission of NO_x, and the thermal stability of NH₃-SCR catalyst is very important for removal of NO_x from diesel engines. In this work, V₂O₅/WO₃-TiO₂(VWT) and SiO₂-doped V₂O₅/WO₃-TiO₂(VWTSi₁₀) catalysts were prepared by impregnation method and characterized by Brunauer-Emmett-Teller(BET), X-ray diffraction(XRD),Raman, temperature programmed reduction by hydrogen(H₂-TPR), X-ray photoelectron spectroscopy(XPS) and temperature programmed desorption by ammonia(NH₃-TPD). The doping of SiO₂ promotes the thermal stability of V₂O₅/WO₃-TiO₂ for NH₃-SCR significantly. After calcination at 650 ℃ for 50 h, the operation window of 10%SiO₂-doped V₂O₅/WO₃-TiO₂ is 220-480℃, while the maximum NO_x conversion on V₂O₅/WO₃-TiO₂ is about 77%. The presence of SiO₂ obviously blocks the transformation of TiO₂ from anatase to rutile and stabilizes the dispersion of VO_x and WO₃ on the surface. It is available for the existence of V⁴⁺ and the amount of surface acid sites increases, which inhabits the NH₃ oxidation at the high temperature range and promotes NH₃-SCR activity.

Keyword：

V₂O₅/WO₃-TiO₂; SiO₂; NH₃-SCR; Thermal stability;

Received： 3 May 2017

1 Introduction

Diesel engines have been widely used due to their high fuel efficiency and power output.However,the removal of NO_x from diesel engine exhaust is a major challenge in environmental catalysis and air pollution control [ 1] .The selective catalytic reduction of NO_x with NH₃ (NH_3-SCR)is one of the best available technologies to control the emission of NO_x [ 2] .

Among the current commercial SCR catalysts,V₂O₅/WO₃-TiO₂ has also been used successfully since 2005 in European on-road mobile applications [ 3] .There are still some problems for the V₂O₅/WO₃-TiO₂ catalyst [ 4, 5, 6] ,such as the thermal deactivation in high temperature exhaust gas [ 7] .When the diesel exhaust temperature exceeds 600℃,it will lead to TiO₂ sintering with crystal phase transforming from anatase to rutile and accompanying loss of surface area,which causes aggregation or even volatilization of vanadium species [ 8, 9] .

Casanova et al. [ 4, 5, 7] found that the presence of rare earths elements (Tb,Er) could lead to the formation of rare earth vanadates which hinder phase structure transformation of TiO₂ and surface area loss.The composite supports were also used to improve high temperature activity and stability,such as TiO_2-Al₂O₃,TiO_2-ZrO₂ and TiO₂-SiO₂ [ 10, 11, 12] .Among them,SiO₂ is widely used due to its large specific surface area and excellent thermal stability.Kobayashi et al. [ 13] reported that V₂O₅/TiO_2-SiO₂behaved better SCR activity and higher N₂ selectivity than V₂O₅/TiO₂ due to the remarkably stronger acidity,higher BET surface area and a lower crystallite sizes of TiO₂,resulting in a good thermal stability and a higher vanadia dispersion.Pan et al. [ 14] reported that SiO₂ doping could keep Lewis acid unchanged and significantly increased Bronsted acid,which had been proven to be beneficial for the low temperature activity.In addition,it could easily generate V⁴⁺.Zhao et al. [ 15] found that co-introduction of SiO₂ and Al₂O₃ into TiO₂ greatly increased BrunauerEmmett-Teller (BET) surface area and the amount of surface acid sites,which promoted SCR activity.Therefore,the Si-doped V₂O₅/WO₃-TiO₂ catalyst may not only improve the high temperature activity but also improve the thermal stability under harsh conditions.

In this work,the SiO₂-doped V₂O₅/WO₃-TiO₂ were prepared and the effects of silica on structure,surface properties and catalytic performance of V₂O₅/WO₃-TiO₂catalysts were investigated.

2 Experimental

2.1 Catalysts preparation

SiO₂-doped WO₃-TiO₂ was prepared by impregnation TiO₂-WO₃ powder (90 wt%TiO₂-10 wt%WO₃,Kailong Lanfeng New Material Technology Co.,Ltd) with tetraethyl orthosilicate [ 16] ,which was denoted as WTSi_x(where x is the content of SiO₂ respect to WO₃-TiO₂).The catalysts were prepared by conventional incipient impregnation method and the oxalic acid solution of NH₄VO₃ was used as precursor.After impregnation,the samples were dried overnight at 100℃and calcination at 550 or 650℃for 6 h to obtain the fresh catalysts,which were assigned to VWT-550℃and VWTSi_x-650℃.After further calcination at 550 or 650℃for 50 h,the VWT-550℃-50 h and VWTSi_x-650℃-50 h were obtained.The V₂O₅ loadings in all catalysts were 3 wt%.

2.2 Activity test

The steady-state NH₃-SCR activities of catalysts were tested in a fixed-bed reactor at atmospheric pressure with0.3 ml catalysts (250-380μm).The feed gas containing500×10^-6 NO,500×10^-6 NH₃,5 vol%O₂,2 vol%H₂O (when used),100×10^-6 SO₂ (when used) and balanced argon passed through the catalytic bed at a flow rate of 300 ml·min^-1.The NO_x concentration of outlet gas was detected from 100 to 550℃by an online Chemiluminescence NO-NO₂-NO_x analyzer (Thermal-Scientific Model42i-HL).The steady NO_x conversion was obtained after keeping at each set temperature for 30 min.The temperature range corresponding to more than 80%NO_x conversion was defined as the operation window.

The NH₃ oxidation activities were measured by a Nicolet 6700 (Thermo Scientific) with gas analysis cell(2 m optical path) with 500 min^-1 NH₃ and 5 vol%O₂ in argon as a feed from 100 to 550℃.The catalysts were kept at each set temperature for 30 min to obtain a stable conversion.

2.3 Catalyst characterization

The powder X-ray diffraction (XRD) patterns were recorded on a BrukerD8 Focus X-ray diffractometer with Cu Kαradiation,the mean crystallite sizes of TiO₂ were calculated based on Scherer equation.The surface areas of samples were measured on a Micromeritics ASAP 2400instrument by N₂ adsorption-desorption at-196℃.The Raman spectra of the catalysts were obtained on a confocal micro-Raman apparatus (IDSpec Aurora,China) using Ar⁺laser with a charged coupled device (CCD) detector,the wavelength of laser was 632.8 nm and the exposure time was 20 s.

Temperature programmed reduction by hydrogen (H₂-TPR) was performed on a commercial temperature-programming system.100 mg catalyst was heated in 5 vol%H₂/N₂ (40 ml·min^-1) from 100 to 800°C at the rate of10℃·min^-1 after pretreated at 500°C for 1 h in a flow of40 ml-min^-1 N₂.Temperature programmed desorption by ammonia (NH₃-TPD) experiment was performed using the same apparatus.100 mg catalyst was pretreated at 500℃

for 1 h in N₂ (50 ml·min^-1) and cooled down to 120℃,then 10 vol%NH₃/N₂ (50 ml·min^-1) passed through catalyst bed for 1 h to obtain saturation.After purging with N₂for 1 h,the temperature was raised up to 700°C at the rate of 10℃·min^-1.

X-ray photoelectron spectroscopy (XPS) spectra were obtained on a Thermo ESCALAB 250 spectrometer with a monochromatized Al KαX-ray source.All binding energies were calibrated by the deposit C 1s at 284.6 eV.

3 Results and discussion

3.1 NH3-SCR activity

The activities of SiO₂-doped V₂O₅/WO₃-TiO₂ catalysts for NH₃-SCR reaction are shown in Fig.la.The operation window was 185-470℃on VWT-550℃catalyst.The doping of SiO₂ slightly decreases the light-off activity,but the activity in the high temperature range is enhanced significantly,which broadens the operation window.Among them,VWTSi₁₀-550℃behaves the optimum performance with the operation window of 205-520℃.When the content of SiO₂ exceeds 10%,the high temperature activity decreases gradually with the increase of SiO₂content.

After calcination at 650℃,the light-off curve of VWT-650℃obviously shifts to higher temperature,and NO_x conversion at the high temperature range also decreases significantly,resulting in a narrow operation window of235-390℃(Fig.1b).For VWT-650℃-50 h,the maximum NO_x conversion is about 77%.However,VWTSi₁₀-650℃shows nearly same SCR activity as VWTSi₁₀-550℃.Furthermore,the light-off property and high temperature activity slightly decrease with the calcination time extending to 50 h,and operation window still is220-480°C,indicating that the deposition of SiO₂ dramatically improves the thermal stability of VWT catalyst.

Fig.1 Catalytic performances of a,b,e NH₃-SCR and c,d NH₃ oxidation on different catalysts

As shown in Fig.1c,d,VWTSi₁₀-550℃behaves lower activity for NH₃ oxidation than VWT-550℃.After calcination at 650℃for 6 and 50 h,the NH₃ oxidation activities of all catalysts increase.However,the increasing extent of VWTSi₁₀ is obviously smaller than that of VWT,which indicates that the doping of SiO₂ suppresses NH₃oxidation and results in higher SCR activity in higher temperature range.

The activities of SiO₂-doped V₂O₅/WO₃-TiO₂ catalysts for NH₃-SCR reaction on the condition of existing H₂O and SO₂ are shown in Fig.1e.The operation window is220-450℃on VWT-650℃catalyst.The doping of SiO₂slightly increases the light-off activity,and the activity in the high temperature range is also enhanced significantly,which broadens the operation window.After extending the calcination time to 50 h,the catalytic activities decrease significantly.However,the catalytic activity of VWTSi₁₀-650℃-50 h is higher than that of VWT-650℃-50 h,and it shows the temperature operation window in the range of240-440℃.Hence,the doping of SiO₂ could improve the high catalytic activity and enhance the stability of resisting H₂O and SO₂.

3.2 BET surface and XRD

XRD patterns are shown in Fig.2,and the a^verage crystallite sizes of TiO₂ are calculated and listed in Table 1.For VWT-550℃,only anatase-TiO₂ phases are detected,and the peaks corresponding to V₂O₅ and WO₃ are not observed,suggesting that V₂O₅ and WO₃ are highly dispersed on TiO_2.After calcination at 650^℃,the diffractionpeaks corresponding to WO₃ are detected on VWT-650℃.When the calcination time increases to 50 h,the peak intensities of WO₃ greatly enhance,and significant rutileTiO₂ could also be detected.Meanwhile,the surface area of VWT-650℃catalyst decreases to 7.1 m²·g^-1,accompanying the significant increase of crystallite sizes of TiO₂from 45.2 to 61.0 nm.

Fig.2 XRD patterns of different catalysts

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Table 1 Textural and structural properties of catalysts

^a Measured by N₂ adsorption-desorption experiment ^b Obtained by XRD data ^c Obtained by XPS analysis

There is no observable difference between the structure of VWT-550℃and VWTSi₁₀-550℃,but the deposition of SiO₂ decreases the crystallite sizes of TiO₂ and increases the surface area significantly.After the calcination at650℃for 6 h,the surface area of VWTSi₁₀-650℃decreases by about 41%but is much higher than that of VWT-650℃,while the crystallite size of TiO₂ also slightly increases from 17 to 19.8 nm.Further extending the calcination time to 50 h,only anatase-TiO₂ is detected on VWTSi₁₀-650℃-50 h,and the surface area of VWTSi_10-650℃-50 h and crystallite sizes of TiO₂ are about 4.6 and 0.41 times that of VWT-650℃-50 h,respectively.Combing BET and XRD results,it could be concluded that the doping of SiO₂ improves the surface area and thermal stability of VWT significantly.

3.3 Raman

The Raman spectra of VWT and VWTSi₁₀ catalysts calcined at different temperature are shown in Fig.3.All catalysts show the characteristic bands of anatase TiO₂ at145,197,396,515 and 635 cm^-1 [ 17] ,and the band intensities increase with the increase of calcination temperature and time.

For VWT-550℃,a weak band assigned to the stretching vibration of W=O could be at 803 cm^-1 [ 18] ,which is enhanced significantly after calcination at 650℃for 6 h.The new bands of crystalline WO₃ are observed at270,324 and 713 cm^-1 on VWT-650℃,corresponding to the W-O-W deformation and the bending vibrations of W=O,respectively.Meanwhile,the bands assigned to the bending vibration of the bridging V-O-V,V=O and the stretching vibration of V=O are detected at 285,303 and994 cm^-1,respectively [ 19, 20, 21] .For VWT-650℃-50 h,the Raman spectrum is similar with that of VWT-650℃,except that the bands of TiO₂,WO₃ and V₂O₅ are strengthened.These results show the agglomeration of W and V species on the VWT.

For VWTSi₁₀-550℃,only the characteristic bands of TiO₂ are detected,and the peak intensities are much weaker than those of VWT-550℃,which coincides with the results of XRD,the doping of SiO₂ induces the structural distortion and decreases the crystallite size of TiO₂.A weak band of V₂O₅ is detected at 994 cm^-1 on VWTSi₁₀-650℃.After further calcination at 650℃for 50 h,the bands corresponding to WO₃ and V₂O₅ are observed at 803and 994 cm^-1,respectively,but their intensities are significantly weaker than those of VWT-650℃and VWT-650℃-50 h,indicating that the modification of SiO₂ could improve the dispersion and thermal stability of V and W species significantly.

3.4 H2-TPR

H₂-TPR profiles of VWT and VWTSi₁₀ catalysts calcined at different temperatures are shown in Fig.4.For VWT-550℃,the overlapping reduction peak at 300-650℃could be assigned to the co-reduction of monomeric/polymeric surface vanadia species (V⁵⁺/V⁴⁺) to V³⁺and W⁶⁺→W⁴⁺ [ 22, 23, 24] .The reduction peak centered at around 800℃could be ascribed to the reduction of W⁴⁺to W⁰ [ 24, 25] .The doping of SiO₂ slightly decreases the reduction ability of VWT,and all reduction peaks shift to higher temperature,which agrees with the little lower SCR catalytic activity in the low temperature range,as shown in Fig.1b.

Compared with those on VWT-550℃and VWTSi₁₀-550℃,the co-reduction peaks of V and W species on VWT-650℃and VWTSi₁₀-650℃distinctly shift to high temperature and the peak intensity decreases,indicating the reducibility decrease of surface V and W species.Meanwhile,a new reduction peak generated at around600-750℃could be assigned to the reduction of W⁶⁺to W⁴⁺of bulk phase WO₃ [ 22, 26] .Combined with XRD and Raman results,the decreased reduction ability of VWT-650℃and VWTSi₁₀-650℃in the low temperature range may be induced by the agglomeration and separation of V and W species.

Fig.3 Raman spectra of different catalysts in the Raman shift range of a 100-700 cm^-1 and b 700-1200 cm^-1

Fig.4 H_2-TPR profiles of different catalysts

By increasing calcination time to 50 h,the reduction peak intensity in the temperature range of 300-600℃further decreases.The presence of SiO₂ suppresses the decrease of reduction peak,and the reduction peak area of VWTSi₁₀-650℃-50 h is still larger than that of VWT-650℃,implying that the V and W species could keep stable on VWTSi₁₀-650℃-50 h and the reduction ability is maintained.Combined with the results of Fig.1b,the doping of SiO₂ could enhance the thermal stability of VWT catalyst,and VWTSi_10-650℃-50 h shows much higher activity than VWT-650℃and VWT-650℃-50 h through the stabilization of V and W species on the surface.

3.5 XPS analysis

The V 2p XPS spectra are shown in Fig.5.Compared with the catalysts calcined at 550℃,the peaks of V 2p shift to higher binding energy after calcination at 650℃for 6 and50 h,which indicates the increase of oxidation state of V species.The ratio of V⁵⁺listed in Table 1 which is obtained by the fitting of V 2p spectra according to the Ref. [ 27] also confirms that the ratio of V⁵⁺increases with the increase of calcination temperature and time.It is suggested to be induced by the increase of crystalline V species on the surface of catalyst [ 28] ,which coincides with results of Raman and XRD.

The results in Table 1 also show that the doping of SiO₂obviously decreases the surface concentration of V,which may be induced by the larger surface area of catalysts containing SiO₂.Meanwhile,the surface ratio of V⁵⁺on VWTSi₁₀ is obviously less than that of VWT calcined at the same condition,indicating that the presence of SiO₂improves the dispersion of V species on the surface and the stability of V⁴⁺.Kobayashi et al. [ 13] reported that the modification of SiO₂ was available for the existence of V⁴⁺on V₂O₅/TiO_2.Lee et al. [ 29] reported that VO_x (x<2.5)could improve the reduction and/or re-oxidation properties of catalysts.H_2-TPR result (Fig.4) confirms that the reduction peak of V species shifts to higher temperature after calcination at 650°C.

Fig.5 XPS spectra of V 2p in different catalysts

3.6 NH3-TPD

NH_3-TPD profiles of different catalysts are shown in Fig.6.A wide NH₃ desorption peak and a relative weak shoulder peak are observed on VWT-550℃at the temperature range of 100-300℃(αpeak) and 300-500℃,corresponding to the Bronsted and Lewis acid sites,respectively [ 15, 20, 30] .The desorption peak area of VWTSi₁₀-550℃is much larger than that of VWT-550℃,which indicates that the presence of SiO₂ improves the amount surface acid sites significantly.After calcination at650℃,αpeak decreases significantly on VWT-650℃,which could be ignored on VWT-650℃-50 h.However,αpeak is maintained on VWTSi₁₀-650℃,and has a slight decrease even on VWTSi₁₀-650℃,indicating that the doping of SiO₂ enhances the stability of surface acid sites significantly.

In addition,two strong desorption peaks are observed on the catalysts calcined at 650℃at the range of250-500℃.Busca et al.reported that VO_x and WO_x could generate both strong Bronsted and Lewis acid sites [ 1] .He et al. [ 31] reported that oligomeric surface V species could produce more acid sites than isolated V species.Combined with Raman results,it could be drawn that isolated or highly dispersed VO_x behaves relatively weak Bronsted acid,and the aggregation of VO_x and/or WO_x would produce new strong acid sites after calcination at 650℃.

Fig.6 NH₃-TPD profiles of different catalysts

As mentioned above,the doping of SiO₂ not only improves surface area of catalyst,but also stabilizes the structure of TiO₂,which could stabilize and improve the dispersion of VO_x and WO₃ species on the surface.The isolated or highly dispersed VO_x species is available for the existence of V⁴⁺and increases the amount of surface acid sites,which could inhabit NH₃ oxidation at the high temperature range and promote NH_3-SCR activity [ 32] even after calcination at 650℃.

4 Conclusion

The SiO_2-doped 3 wt%V₂O₅/WO_3-TiO₂ catalysts were prepared and the catalytic performances for NH_3-SCR were investigated.Among them,VWTSi_10-550℃behaves the maximum operation window of 205-520℃,which still maintains at the temperature range of 220-480℃even after calcination at 650℃for 50 h,while the maximum NO_x conversion is about 77%on VWT-650℃.The characterization results show that the doping of SiO₂obviously increases the surface area of catalysts and blocks the transformation of TiO₂ from anatase to rutile after calcination at 650℃,which could stabilize and improve the dispersion of VO_x and WO₃ species on the surface.The existence of V⁴⁺and stabilized surface acid sites would inhabit the NH₃ oxidation at the high temperature range and promote NH_3-SCR activity.

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