Nitrogen oxides (NOx) are very harmful to the environment and contribute to acid rain, photochemical smog, ozone depletion and global warming . The selective catalytic reduction (SCR) of NOx with NH3 is a well-established and efficient process for the elimination of NOx emissions . However, the catalyst plays an important role in the SCR system. The activity of the catalyst determines the denitrification efficiency of the SCR system. The commercial catalyst for de-NOx process is carried out at 350˚C - 400˚C . With the aim of avoiding the inactivation of the catalyst caused by sulfur dioxide and dust, the SCR equipment is usually installed at the downstream of desulfurization for reducing sulfur dioxide and dust pollution to a minimum . However, the temperature of flue gas from desulfurization is usually low. Therefore, the development of superior low temperature SCR catalysts is getting more and more attention.
For SCR catalysts, V2O5/TiO2 catalysts have attracted much attention due to their low temperature activity and environmental friendliness  - . Therefore, the study of modified V2O5/TiO2 catalytic system has great practical significance . In order to improve the activity of the catalyst, many efforts have been paid to modify the material of catalysts. However, the effect of ion doping on the activity of catalyst strongly depends on many factors such as the dopant concentration, the distribution of the dopant, the configuration of doping ions and so on . Zhu et al. prepared Nb-doped V2O5-WO3/TiO2 catalysts for the NH3-SCR reaction and found the addition of Nb2O5 could improve the SCR activity at low temperature. As a result, the thermal treatment at 400˚C could regenerate the deactivated catalyst and get SCR activity recovered. The particle and monolith catalysts both kept stable NOx conversion at 225˚C with high concentration of H2O and SO2 during the long time tests . Hu et al. reported that the dopant of Ce could enhance the surface chemisorbed oxygen on the Na poisoning V2O5-WO3/TiO2 catalysts, facilitate the redox cycle, and increase the intensity of acid sites due to the newly formed Brønsted acid sites stemmed from , thereby promoting catalytic activity and Na poisoning resistance . Zhao et al. found that the S-doped vanadium-titanium catalyst can prevent the phase transition from anatase to rutile, producing crystal defects and reducing the band gap. And the addition of sulfur can bring more NH3 adsorbent, while increasing the catalytic activity of NO on NH3-SCR . In order to increase the activity of the catalyst, it is also very necessary to investigate the preparation conditions and reaction conditions. Wang et al. experimentally studied Fe2O3 particle catalysts in the low temperature selective catalytic reduction (SCR) of NO with NH3 . The results show that oxygen concentration, [NH3]/[NO] molar ratio, GHSV and other factors have a great influence on catalytic activity.
In this study, Ti3+ self-doped V2O5/TiO2 catalysts were prepared by sol-gel method and impregnation method. The effects of oxygen concentration, [NH3]/[NO] molar ratio and the GHSV on its performance were investigated. At the same time, the preparation conditions of the support and different doping amount of the Ti3+ were investigated.
2.1. Catalyst Preparation
The titanium dioxide support was prepared by the sol-gel method. Briefly, 12 mL tetrabutyl orthotitanate (98%, TBOT), 48 mL absolute ethanol (AR), 3.8 mL acetylacetone (AR, Hacac) and a certain amount of aluminium acetylacetonate (98%, Al(acac)3) were mixed and stirred for 1 h to obtain solution A while solution B consisted of 60 mL absolute ethanol and 7 mL H2O. Solution B was dropwise added into solution A by titration funnel under magnetic stirring. After stirring for 1 h at room temperature, the sol was heated at 50˚C for 4 h in water bath and then subsequently dried at 80˚C for 6 h. After that the gel was calcined at 350˚C for 2 h in air. Finally, the titanium dioxide support was obtained.
The loading of V2O5 was prepared by impregnation with the requisite amount of ammonium metavanadate (the V2O5 content in all catalysts is 1% by weight). The NH4VO3 was dissolved in 20 mL deionized water at 60˚C, and impregnated by contacting the Al-Ti support. The mixture was heated for 4 h at 60˚C in a water bath. Then obtained mixture was dried at 120˚C for 6 h and calcined at 350˚C for 4 h. Samples with different ratios of Ti3+ were sighed as x-VTi which x represented the molar percentage of Al(acac)3 to TBOT.
2.2. Characterization of Catalysts
Powder X-ray diffraction (XRD) patterns of all samples were obtained on a XD-3 diffractometer with Cu Ka radiation (k = 0.15418 nm) (Beijing Purkinje General Instrument Co., Ltd., China) operated at 36 kV and 30 mA. Intensity data were reported in the 2θ range from 10˚ to 80˚, with a step size of 0.04˚.
Scanning electron microscopy (SEM) produced by Hitachi was used to observe the morphologies of catalysts under the accelerating voltage of 15 kV.
2.3. Catalytic Activity Measurement
Experiments to investigate the catalytic activity of the catalyst were carried out in a fixed-bed flow reactor at 120˚C - 330˚C containing 0.30 g catalyst under atmospheric pressure. The reactor was heated by a temperature-controlled furnace. They were used to control the gas flow of mass flowmeters. The total gas flow rate was 100 mL/min. The reaction gas components were as follows: 500 ppm NO, 500 ppm NH3, 5% O2 and balanced N2. The gas hourly space velocity was 23,885 h−1. Different space velocities were obtained by changing the gas total flow or the volume of catalyst used. The mixed gas went into the reactor and the NO and NO2 (NOX = NO + NO2) concentrations were monitored by a Testo350 flue gas analyzer (Testo, Germany). The reaction system was kept for 1 h at each reaction temperature to reach a steady state before the analysis of the outlet gas was performed. The NOX conversion could be defined as Equation (1):
There and represent the concentrations of NOX in the inlet and outlet gas stream respectively.
3. Results and Discussion
3.1. Activity of Catalyst
The activity of different Ti3+ doping amount is tested, and the results are shown in Figure 1. The denitration activity of all Ti3+ self-doped V2O5/TiO2 catalysts increased with the increase of temperature. Compared with the 0-VTi catalyst, the activity efficiency is significantly improved. The catalytic activity of 0.2-VTi was the highest, and reached 88% at 210˚C. Therefore, when the Al(acac)3/TBOT ratio is 0.2%, Ti3+ self-doping has the best effect on the catalyst.
The XRD patterns of the prepared catalysts are shown in Figure 2. It can be seen from Figure 2 that the sample after Ti3+ self-doping (the 0.2-VTi catalyst) shows the major crystalline phase of anatase TiO2. However, the 0-VTi catalyst shows trace amount of rutile phase. This indicates that the Ti3+ doping can inhibit the formation of rutile phase. In addition, there is no characteristic peak of V2O5 was detected, indicating that V element may be well dispersed on the catalyst surface  .
Figure 3 shows the SEM images of the synthesized 0.2-VTi catalyst sample. SEM image shows that the composite consists of irregular shaped aggregates. Figure 3 shows there is no obvious change of structure between fresh catalyst (Figure 3(a)) and used catalyst (Figure 3(b)). Moreover, no single V2O5 particles or clusters were found on the surface of TiO2. This indicates that V2O5 species may cover the surface of TiO2 particles in the form of a single thick layer. This is consistent with the results of XRD .
Figure 1. Effect of Ti3+ self-doped V2O5/TiO2 catalyst on NOX conversation.
Figure 2. XRD patterns of the catalysts with various Ti3+ doping.
Figure 3. SEM micrographs of the 0.2-VTi catalyst, (a) fresh, (b) used.
3.2. Effect of the Calcined Temperature of the Support on SCR Activity
Figure 4 shows the NOx conversion as a function of temperature (from 120˚C to 330˚C) in the NH3-SCR reaction over Ti3+ self-doped V2O5/TiO2 catalysts of the different calcined temperature of the support. From the result, it was clear to be found that the NOx conversion is the highest when the support calcinations temperature is 350˚C. It presented an over 80% NOx conversion within the temperature range of 200˚C - 330˚C, especially at 240˚C - 330˚C where the NOx conversion was increased to a maximum of exceed 99%. However, the catalytic activity at low temperature calcinations is relatively low. This may be because the low temperature calcined catalyst has a low degree of crystallinity . And in appearance, the support prepared at a low temperature appeared black, because of incomplete calcinations and residual carbon, resulting in low denitrification efficiency. It can also be seen from Figure 4 that the catalytic activity declines when the support was calcined at high temperatures.
3.3. Effect of O2 Concentration on SCR Activity
It was found through previous studies that oxygen is very important for SCR of
Figure 4. Effect of Ti3+ catalysts of various calcined temperature of the support. Reaction conditions: [NO] = [NH3] = 500 ppm, O2 = 5%, balance N2, GHSV = 23,885 h−1, total flow rate 100 mL/min.
NO with NH3 at low temperature . Figure 5 shows the effect of oxygen concentration on SCR activity at 210˚C. It was clear that the NOx conversion over the catalyst increased with the increasing of oxygen concentration, especially when the oxygen concentration was less than 3%. The 36% NOx conversion was obtained in the presence of without O2 at 210˚C. This indicates that the catalyst has the ability to catalyze and reduce in the absence of O2. However, the NOx conversion increases significantly as the O2 concentration increased to 3%. This shows that O2 has a great influence on the catalytic activity. The NOx conversion increased to 88.1% when O2 reached 5%. However, the catalytic activity remained basically stable with the continued increase of O2 concentration. Therefore, the optimal O2 concentration is 5%.
3.4. Effect of the [NH3/NO] Molar Ratio on SCR Activity
It is well known that the [NH3]/[NO] molar ratio is a key parameter in SCR of NO with NH3 . Figure 6 shows the effect of different [NH3/NO] molar ratio on catalytic activity at 210˚C. It can be clearly seen that the catalytic activity changes significantly when the molar ratio of [NH3]/[NO] is less than one, and the activity rose rapidly as the [NH3]/[NO] molar ratio increases. When the [NH3]/[NO] molar ratio is 1, the catalytic activity reached over 88%. However, the NOx conversion varied more gradually when a higher molar ratio of [NH3]/[NO] was used. The continued increase of the NH3 concentration will not increase the NOx conversion. Therefore, the [NH3]/[NO] molar ratio of 1 is advisable for this experiment.
Figure 5. Effect of the O2 concentration on catalytic activity at 210˚C. Reaction conditions: [NO] = [NH3] = 500 ppm, balance N2, GHSV = 23,885 h−1, total flow rate 100 mL/min.
Figure 6. Effect of [NH3]/[NO] molar ratio on catalytic activity at 210˚C. Reaction conditions: [NO] = 500 ppm, O2 = 5%, GHSV = 23,885 h−1, balance N2, total flow rate 100 mL/min.
3.5. Effect of GHSV on SCR Activity
The GHSV is an important parameter that has a great influence on the denitrification efficiency of SCR catalysts. This work also measured the effect of GHSV on SCR activity, as shown in Figure 7. It can be clearly seen that the catalytic activity decreases with the increasing of GHSV. And the catalytic activity was the best at 23,885 h−1, in particular, the catalytic activity reached 88.1% at 210˚C and
Figure 7. Effect of GHSV on catalytic activity. Reaction conditions: balance N2, total flow rate are 23,885 h−1, 31,050 h−1 and 35,828 h−1 respectively.
the activity exceeded 99% between 240˚C - 330˚C. This is because when the GHSV is too high, the contact time between the reactants and the catalyst is short and the reaction is not sufficient, the conversion rate is low.
3.6. Effect of the Reaction Time on SCR Activity
In order to evaluate the stability of the catalytic activity, a comprehensive experiment was carried out at a standard reaction of catalyst activity test at 210˚C. The results are shown in Figure 8. It was found that the NO conversion was always kept over 80% for the catalyst continued to react for over 1750 min·s, indicating that the catalyst had better activity and stability.
3.7. Effect of H2O and SO2 on SCR Activity
Sulfur dioxide as another important pollution was in the flue gas inevitably . The effect of H2O and SO2 on the activity of the catalyst was investigated experimentally at 210˚C, and the result is exhibited in Figure 9. When the 500 ppm SO2 was added in firstly, the obvious decrease of NO conversion was not observed. The NOx conversion decreased from 88.1% to about 78% after a 250 min·s 'SCR reaction process. When the supply of SO2 was cut off at 250 min, NOx concentration in the exhaust recovered gradually to the original level. Then 5% H2O was introduced at 450 min, and the experimental results showed that the NOx conversion decreased rapidly. When the H2O is turned off at 600 min, the NOx conversion is quickly restored. Therefore, Ti3+ self-doped V2O5/TiO2 catalyst has superior SO2 and H2O resistance.
On the basis of the above results, it can be concluded that the catalytic activity of
Figure 8. Effect of reaction time on catalytic activity at 210˚C. Reaction conditions: [NO] = 500 ppm, O2 = 5%, GHSV = 23,885 h−1, balance N2, total flow rate 100 mL/min.
Figure 9. Effect of H2O and SO2 on catalytic activity at 210˚C. Reaction conditions: [NO] = 500 ppm, O2 = 5%, SO2 = 500 ppm, H2O = 5%, GHSV = 23,885 h−1, balance N2, total flow rate 100 mL/min.
V2O5/TiO2 catalyst at low temperature is significantly affected by the self-doping of Ti3+, which can reach up to 80% at 200˚C. The investigation showed that the activity of the catalyst is the best when the oxygen concentration is 5% and the [NH3]/[NO] molar ratio is 1. At the same time, the effect of GHSV on the catalyst is investigated. It is found that the catalyst also has a wide active range at a large GHSV. In addition, the experimental results showed that the Ti3+ self-doped V2O5/TiO2 catalyst has good stability and better SO2 and H2O resistance during a long time.
This work was supported by the Postgraduate Research & Practice Innovation Program of Jiangsu Province (No. SJCX17_0572). This research was supported financially 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 (15KJB610003) and Research Foundation of Jiangsu University (15JDG156).
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