Boosting low-temperature selective catalytic reduction of NOx with NH3 of V2O5/TiO2 catalyst via B-doping

Hanghang Li,Wei Zhao,,Licheng Wu,Qian Wang,,Danhong Shang,Qin Zhong

1 School of Energy &Power Engineering,Jiangsu University,Zhenjiang 212013,China

2 School of Environmental and Chemical Engineering,Jiangsu University of Science and Technology,Zhenjiang 212003,China

3 School of Chemical Engineering,Nanjing University of Science and Technology,Nanjing 210094,China

Keywords:B-doping Catalyst SCR NOx Support

ABSTRACT A series of B-doped V2O5/TiO2 catalysts has been prepared the by sol-gel and impregnation methods to investigate the influence of B-doping on the selective catalytic reduction (SCR) of NOx with NH3.X-ray diffraction,Brunauer-Emmett-Teller specific surface area,scanning electron microscope,X-ray photoelectron spectroscopy,temperature-programmed reduction of H2 and temperature-programmed desorption of NH3 technology were used to study the effect of the B-doping on the structure and NH3-SCR activity of V2O5/TiO2 catalysts.The experimental results demonstrated that the introduction of B not only improved the low-temperature SCR activity of the catalysts,but also broadened the activity temperature window.The best SCR activity in the entire test temperature range is obtained for VTiB2.0 with 2.0%doping amount of B and the NOx conversion rate is up to 94.3% at 210°C.The crystal phase,specific surface area,valence state reducibility and surface acidity of the active components for the as-prepared catalysts are significantly affected by the B-doping,resulting in an improved NH3-SCR performance.These results suggest that the V2O5/TiO2 catalysts with an appropriate B content afford good candidates for SCR in the low temperature window.

1.Introduction

With the development of economy,ever-increasing atmospheric pollution problem is one of the most serious challenges facing us recently.The major air pollutant is nitrogen oxides(NOx),which are originating from the stationary and mobile sources,including coal-fired power plants and vehicle engines,et al.NOxemissions can not only cause acid rain,fog and haze,and photochemical smog,but also cause serious harm to human health [1–5].In response to the current environmental problems,the country puts forward the slogan of ‘‘fighting the blue sky”and has formulated more and more stringent emission regulations to reduce environmental pollution caused by exhaust emissions[6–8].More recently,selective catalytic reduction(SCR)technology is considered to be the most effective approach to reduce NOxemissions[9–11].This technology converts the NOxin the exhaust gas into non-toxic N2and H2O under the action of a catalyst,thereby reducing air pollution.

The catalyst is the core component of the SCR system.The currently used catalyst is mainly V2O5/TiO2and V2O5-WO3/TiO2based-systems [12].However,the low temperature activity of these catalysts are usually not good due to their narrow active temperature window (300–400 °C) [13].The working conditions of the automobile are changeable during the driving process,and the exhaust gas temperature is usually lower than 300°C,which is not conducive to the catalytic effect of the catalyst.Therefore,this is also a major disadvantage of the catalyst.In view of the shortcomings of poor low-temperature activity and the narrow active temperature window of the V2O5/TiO2catalysts,further improvement of the catalytic performance is required.

Studies have shown that adding some metal and/or non-metal elements in the catalyst preparation process can improve the low temperature SCR activity of the catalysts [14].For example,Maet al.studied the effect of CeO2doping on the activity of V2O5-WO3/TiO2catalysts [15].They found that the Ce doping can form V-O-Ce bonds,and the interactions between Ce and V species could enhance the reduction ability of the catalyst.Thereby,the NOxconversion rate of the catalyst can reach more than 80% in a wide temperature range of 190–450°C.Zhaoet al.prepared Fdoped V2O5/TiO2catalysts by the sol-gel and impregnation methods [16].They found that F-doping could promote the conversion of NO to NO2by generating more oxygen vacancies.Yanget al.reported that Cr doping in V2O5/TiO2catalyst can increase the number of weak acid and medium acid sites on the catalyst surface and increase the low-temperature reduction ability of the catalyst,resulting in above 90%NOxconversion rate from 160 to 300°C[17].Niuet al.used Tm as a dopant to modify Mn/TiO2and discovered that the Tm0.1Mn/TiO2as NH3-SCR catalyst had excellent lowtemperature catalytic performance [18].

Based on above considerations,the aim of this paper is to investigate the influence of B-doping on the NH3-SCR activity of V2O5/TiO2catalysts,and thus a series of B-doped V2O5/TiO2catalysts is prepared by the sol-gel and impregnation methods.The formation of the as-prepared samples is confirmed by X-ray diffraction,SEM images and X-ray photoelectron spectroscopy.Among all samples,the VTiB2.0with 2.0%doping amount of B exhibits the best activity in the entire test temperature range,and the NOxconversion rate is up to 94.3%at 210°C.The enhanced NH3-SCR activity is ascribed to a combination of mechanisms.

2.Experimental

2.1.Preparation of B-doped V2O5/TiO2 catalysts

B-doped TiO2support was prepared by the sol-gel method.First,35 ml of absolute ethanol and 10 ml of tetrabutyl titanate(C16H36O4Ti) were mixed and stirred with a magnetic stirrer for 1 h to obtain a solution A.Then,35 ml of absolute ethanol,4 ml of glacial acetic acid,10 ml of deionized water and a certain amount of boric acid (H3BO3) were mixed and stirred with a magnetic stirrer for 1 h to obtain a solution B.The solution A was slowly added to the solution B and continue stirred for 1 h.The resultant solution was heated at 40°C water bath to gel for 4 h,at which time the sol turned into a milky white gel.After dried at 80 °C for 12 h,and the obtained materials were calcinated at 600 °C for 3 h,affording the desired product TiBx(x=0.5,1.0,1.5,2.0,2.5 and 3.0,respectively,which represents the mass ratio of B and TiO2,namelymB/mTiO2×100).The loading of vanadium oxide onto the TiBxsupport,containing 1% (mass) active component V2O5,was performed by the impregnation method.Required quantity of ammonia vanadate (NH4VO3) was added to the distilled water with the calculated amount of TiBxsupport.After heated for 4 h at 60 °C in a water bath,the obtained mixture was dried in air at 120 °C for 6 h and calcined at 350 °C for 4 h.The corresponding sample is labeled as VTiBx(x=0.5,1.0,1.5,2.0,2.5 and 3.0,respectively).For comparison,the sample defined as VTiB0was also prepared using the similar procedures in the absence of H3BO3.

2.2.Catalyst activity measurements

The catalyst activity measurements were carried out on a programmable temperature evaluation device equipped with a fixed-bed quartz tubular flow reactor(6 mm of internal diameter).The amount of catalyst required is 0.3 g.The reaction temperature is 120–390°C.The total gas flow rate is 100 ml?min-1.The reaction space velocity(GHSV)is 27,549 h-1.The mixed gas composition is NO(5.00×10-4),NH3(5.00×10-4),O2(5%),and N2as the balance gas.The concentrations of NO and NO2are measured by the Testo350 flue gas analyzer.Before each measurement of the NOxcontent,it needs to be maintained for 2 h to reach a steady state.The conversion rate of NOxis calculated by the following formula:

where NOx=NO+NO2,[NOx]inand[NOx]outrepresent the NOxconcentration in the inlet and outlet gas streams,respectively.

2.3.Catalyst characterization

The powder X-ray diffraction (XRD) pattern was performed on the Bruker D8 advanced diffractometer (Bruker,Germany) with Cu Kα radiation (λ=0.15406 nm).And the step size is 0.02°.

The surface area and pore structure of the catalyst were determined by N2physical adsorption at -196°C using Micromeritics ASAP2020.Before each analysis,the samples were degassed under vacuum at 150°C for 4 h.The specific surface area is calculated by the Brunauer-Emmett-Teller (BET) formula.The Barrett-Joyner-Halenda (BJH) formula is used to calculate the pore volume and pore diameter.

The surface morphology and roughness of the catalyst were observed by scanning electron microscope (SEM) (JSM-7800F,Japan JEOL Company).The SEM image was taken at a magnification of 14,000 at an acceleration voltage of 15 kV.

X-ray photoelectron spectroscopy(XPS)was analyzed by ESCALAB 250Xi photoelectron spectrometer (Thermo Fisher Scientific Co.).Al KaX-ray is the excitation source and the voltage is 14 kV.The electronic binding energy of the elements is calibrated using C1s (284.8 eV).

The temperature-programmed reduction of H2(H2-TPR) experiment was carried out on the AutoChem II 2920 analyzer(Micromeritics,USA).About 0.1 g of catalyst sample was used.Before the H2-TPR experiment,the sample was placed in a quartz tube reactor and pretreated at 300°C for 30 min under N2(30 ml?min-1) to remove the impurities adsorbed on the surface.Then,the temperature was increased to 780°C at a heating rate of 10°C?min-1under 5%H2/N2(30 ml?min-1).The H2consumption was measured by a thermal conductivity detector (TCD).

The temperature-programmed desorption of NH3(NH3-TPD)experiment was performed on the AutoChem II 2920 analyzer(Micromeritics,USA).0.2 g catalyst was heated from room temperature to 300°C for 30 min at a heating rate of 10°C?min-1under N2gas flow,and cooled down to 50°C.After that,N2-NH3mixed gas(2% NH3by volume) was switched.The catalyst sample was saturated at 50°C for 60 min.Then the weakly adsorbed NH3gas was removed by purging with N2to stabilize the baseline.Finally,the temperature was increased to 700°C at a rate of 10 °C?min-1to collect NH3-TPD desorption data.

3.Results and Discussion

3.1.Catalyst activity

In order to study the effect of different B doping amount on the NOxconversion rate of the catalyst,a series of catalysts(VTiBx)was prepared in the present work.The catalyst activity test experiment was carried out in the temperature range of 120–390 °C,and the results are shown in Fig.1.It can be seen from Fig.1 that there is a significant difference in the corresponding activities of the VTiBxcatalysts with different B doping amounts.But compared with the VTiB0catalyst prepared without B doping,the activities of VTiBxare significantly improved,especially in the low temperature section.The activity of all catalysts is increased with the increasing of temperature,but exhibits a downward trend above 300°C.Among all samples,the VTiB2.0catalyst has the best activity in the entire test temperature range and the NOxconversion rate can reach 94.3% at 210°C.This shows that B-doping can improve the low temperature activity of the catalyst.Further increasing the doping amount of B,the activity of the catalyst in the low temperature section does not change obviously.But the SCR activity is significantly decreased above 360 °C,demonstrating that more B doping is not better and there is an optimal value.More importantly,compared with the VTiB0catalyst,the active temperature window of the B-doped catalysts widens toward the low temperature range,confirming that B-doping can effectively broaden the active temperature window of the catalyst for efficient lowtemperature SCR activity.

Fig.1.The influence of different B doping amount on the NOx conversion efficiency.Reaction conditions:[NO]=[NH3]=500 μl?L-1,[O2]=5% (volume) and N2 as balance gas,GHSV=27,549 h-1.

3.2.Characterization of catalysts

3.2.1.X-ray diffraction (XRD) analysis

In order to study the effect of B-doping on the crystal phase structure of the V2O5/TiO2catalysts,the prepared samples were characterized by XRD,the results being shown in Fig.2.It can be seen that the two catalysts show characteristic diffraction peaks of anatase TiO2at 2θ=25.2°,37.7°,48.0°,54.2°,55.0°,62.6°,68.8°,70.1° and 75.0° [19–22].However,for the VTiB0catalyst,the characteristic peaks of rutile TiO2are also observed at 2θ=27.3°,36.0°,41.1°,56.5° and 63.9° [23–25].The XRD results indicate that B-doping can promote the transformation of TiO2from rutile to anatase phase,and consistent with previous reported results[26].It is known that anatase TiO2can provide more active sites,which is conducive to the SCR reaction [27].In addition,no diffraction peaks of vanadium oxide (VOx) are detected in both samples,indicating that VOxis highly dispersed and/or exists in the amorphous form.

Fig.2.XRD spectra of VTiB0 and VTiB2.0 catalysts.A:anatase,R:rutile.

3.2.2.BET specific surface area and pore structure

The measurement results of the BET specific surface area and total pore volume of the catalyst samples are shown in Table 1.It can be seen from the Table 1 that the specific surface area of the VTiB0catalyst is 3.77 m2?g-1,while the specific surface area of the VTiB2.0catalyst is 15.61 m2?g-1,which is about 4 times as large as that of the VTiB0catalyst.This indicates that the B-doping can greatly increase the specific surface area of the VTiB0catalyst.The larger specific surface area of the catalyst is beneficial for the improvement of the catalytic activity[28,29].Thus,the VTiB2.0catalyst has a higher SCR activity than the VTiB0catalyst.Meanwhile,the total pore volume of the catalyst (VTiB2.0) obtained by the Bdoping is significantly increased compared to VTiB0,which is also conducive to the adsorption of reactants on the catalyst surface,and can accelerate the SCR reaction [30].

3.2.3.Scanning electron microscope (SEM)

Fig.3 shows the SEM and corresponding elemental images of the catalyst samples.Comparing Fig.3(a)and 3(b),it can be found that the morphology of the catalyst VTiB2.0doped by B is more rough in comparison with that of VTiB0,which is beneficial to increase the specific surface area,as observed in Table 1.The catalyst with high surface area could expose more active sites to improve the SCR activity [31].As shown in Fig.3(c)–(e),the Ti,O and V elements have a relatively homogeneous distribution,and consistent with the composition of VTiB0.In contrast,the signal of B element is also observed for VTiB2.0in addition to Ti,O and V (Fig.3(f)–(i)),demonstrating the B-doped V2O5/TiO2was successfully prepared.

Table 1 The specific surface area and pore structure of the catalysts

3.2.4.X-ray photoelectron spectroscopy (XPS) analysis

The XPS technology can study the surface element composition and chemical state of the catalyst.In order to study the effect of Bdoping on the composition of the catalyst,XPS analysis was performed on the VTiB0and VTiB2.0catalysts (Fig.4).The XPS spectra of Ti 2p for catalysts are illustrated in Fig.4(a).Two characteristic peaks near 458.88 and 464.63 eV are attributed to Ti 2p3/2and Ti 2p1/2,respectively [32,33],which indicates that the Ti element in the catalyst exists mainly in the form of Ti4+[34].Compared with VTiB0catalyst,the Ti 2p photoelectron bands of VTiB2.0catalyst shift to the high binding energy in a small degree,which indicates that the electronegativity and the ability to bind electrons of Ti are increased after B-doping[35].Fig.4(b)shows the V 2p3/2XPS spectra of the VTiB0and VTiB2.0samples.The peaks with binding energies of 517.3 and 516.6 eV correspond to V5+and V4+,respectively[36–38].According to the calculation results,the ratio of V4+/(V4++V5+) in VTiB0and VTiB2.0catalysts is 0.33 and 0.44,respectively.This indicates that B-doping can increase the content of V4+in the catalysts.Zhanget al.pointed out that both V4+and V3+in the reduced state are active sites for the formation of super oxygen atom O2-,which is beneficial to improve the low temper-ature denitration activity of the catalyst[39].In addition,the activity of SCR reaction is closely related to the conversion between V5+and V4+[40].All these results indicate that the B-doping can improve the low temperature activity of the catalysts.

Fig.3.SEM and corresponding elemental images of VTiB0 catalyst (a,c,d and e) and VTiB2.0 catalyst (b,f,g,h and i).

Fig.4(c)presents the O 1s XPS spectra of the catalysts.By comparison,it can be found that the binding energy of O 1s after Bdoping shifts to the high field direction.This may be due to the content and electronegativity of adsorbed oxygen has been increased by the introduction of B,resulting in increased oxygen vacancy rate [41].Jinget al.pointed out that adsorbed oxygen is the most active oxygen,and its existence is conducive to the SCR reaction [42].This may be an important reason that the B-doping improves SCR activity of the catalysts.The XPS spectra of B 1s of VTiB2.0samples are displayed in Fig.4(d).The characteristic peak at 192.23 eV can be attributed to the Ti-O-B bond,suggesting that B ions have been entered into the TiO2lattice[43,44].The presence of interstitial boron could promote the transformation between Ti4+and Ti5+,which is consistent with the results of Ti 2p analysis.The characteristic peak near 192.88 eV can be attributed to the B-O bond in B2O3[45,46].

3.2.5.Redox property

In the SCR reaction,the oxidation-reduction ability of the catalyst has an important influence on the performance of the catalyst[47,48].Therefore,the reducibility of VTiB0and VTiB2.0catalysts were measured by H2-TPD,and the results are presented in Fig.5.A reduction peak is found in the range of 300–600 °C for the VTiB2.0catalyst,which may be attributed to the transition from V5+to V3+[49].However,five reduction peaks are found in the range of 50–750°C for the VTiB0catalyst.The four reduction peaks in the range of 350–700 °C may be assigned to the reduction of vanadium oxide (VOx),while the reduction peak in the range of 250–300°C may be ascribed to the physical adsorption of H2on the catalyst surface.It has been reported that there are four reduction peaks in the unloaded vanadium oxide,located at 675,714,788 and 869°C,corresponding to the following chemical reduction process:V2O5→V6O13→V2O4→V6O11→V2O3,respectively [50].Compared with previous reported unsupported vanadium oxide,the reduction peak of VTiB0shifts to the low temperature direction.This may be caused by the vanadium oxide supported on TiO2.More importantly,the reduction temperature of the B-doped catalyst shifts toward the low temperature as a whole,and the intensity of the reduction peak is enhanced.Previous studies have found that the lower temperature reduction peak of the catalyst is accordance with the stronger reduction ability of the catalyst[51].This also indicates that the B-doping is beneficial to the dispersion of the active component V2O5on the surface of TiO2.

3.2.6.Acid property

It has been reported that the acidity of the catalyst surface plays an important role in the SCR reaction [52,53].With the aim to study the effect of B-doping on the acidity of the catalyst surface,NH3-TPD analysis was performed on the VTiB0and VTiB2.0catalysts.Fig.6 demonstrates the NH3-TPD profiles of the catalysts.It seems that there exist two major NH3desorption peaks for both samples in the range of 50–700°C.The total amount of NH3adsorbed is determined by the peak area of the TPD curve.According to the NH3-TPD curve,the peaks at low and high temperatures correspond to the weak acid and strong acid sites,respectively[54,55].For the VTiB2.0catalyst,two desorption peaks appear in the range of 50–500 °C.The first desorption peak centered in the range of 100–200°C could be attributed to the physical adsorption of NH3molecules on the catalyst surface and someions bound to the weak Br?nsted acid (B acid).The desorption peak between 300 and 400°C could be attributed to the adsorption of NH3on the strong acid position,corresponding to the Lewis acid (L acid)[56,57].Compared with VTiB0catalyst,the desorption peaks of VTiB2.0catalyst are shifted to low temperature region,and the peak area and intensity of VTiB2.0catalyst are significantly increased.These results are consistent with the amount and acidity strength of the catalysts were increased.Obviously,the introduction of B can increase the number of Br?nsted acid on the catalyst surface and promote the adsorption and activation of NH3,which should be an important factor for the improved SCR activity[58].Previous reports have shown that NH4+adsorbed at the B acid site plays an important role in the SCR reaction [59].

Fig.4.The XPS spectra of VTiB0 and VTiB2.0 catalysts:(a) Ti 2p,(b) V 2p,(c) O 1s and (d) B 1s.

Fig.5.H2-TPR spectra of VTiB0 and VTiB2.0 catalysts.

Fig.6.NH3-TPD profiles of VTiB0 and VTiB2.0 catalysts.

On the basis of above experimental results,the improved SCR activity of VTiB2.0is attributed to the formation of abundant anatase phase TiO2,which provides more active sites for SCR.The introduction of B can greatly increase the specific surface area and total pore volume of the VTiB0catalyst,which is beneficial for the adsorption of reactants on the catalyst surface and accelerating the SCR reaction rate.The increased V4+content and redox ability for VTiB2.0also have a positive influence on the SCR denitration efficiency.Moreover,the amount and acidity strength of the catalysts are increased by the B-doping,promoting the adsorption and activation of NH3,which also contributes to the SCR catalytic performance.

4.Conclusions

B-doped V2O5/TiO2catalysts have been prepared by the sol-gel and impregnation methods,and were investigated for SCR of NOxwith NH3.The experimental results indicated that the B-doping not only improved the low-temperature activity of the catalysts,but also broadened the activity temperature window.When the doping amount of B is 2.0%,the best activity in the entire test temperature range is obtained and the NOxconversion rate is up to 94.3% at 210°C.According to the characterization results,it is found that the B-doping can inhibit the transformation of anatase phase TiO2to rutile phase,promote the dispersion of active components,increase the specific surface area and pore volume,and enhance the acidity and activity of the catalyst surface sites;all these factors are conducive to improving the low temperature denitration activity of the catalysts.These results demonstrate that the V2O5/TiO2systems with an appropriate content of B elements afford good candidates for SCR in the low temperature window.Our future work will focus on the design and preparation of novel V2O5/TiO2-based materials with efficient NH3-SCR performance in the real exhaust gas.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was funded by the National Natural Science Foundation of China (51506077),the Natural Science Foundation of Jiangsu Province (BK20150488),the Natural Science Foundation of Jiangsu High School (15KJB430007) and the Research Foundation of Jiangsu University (15JDG156).