Catalytic Performance of Ti3+ Self-Doped V2O5-TiO2 Catalysts for Selective Catalytic Reduction with NH3

Ti3+ self-doping modified V2O5/TiO2 catalyst was prepared by sol-gel and impregnation methods and used for selective catalytic reduction (SCR) of NOx with NH3. Results showed that Ti3+ self-doped V2O5/TiO2 catalyst performed the better catalytic activity. And X-ray diffraction and scanning electron microscopy were used to evaluate the phase composition and morphology of the prepared catalyst. The effects of calcinations temperature of the support, oxygen concentration, [NH3]/[NO] molar ratio and the GHSV on the denitration performance were investigated. It was found that more than 80% NOx conversion was obtained at 210°C when the O2 volume fraction was 5%, the NO concentration was 500 ppm, the [NH3]/[NO] molar ratio = 1 and the GHSV was 23,885 h−1. The results showed that the catalytic activity increased first with the increasing of O2 concentration and [NH3]/[NO] molar ratio, then remained stable. At the same time, the stability of the catalyst was also studied at the temperature of 210°C. The reaction continued for 750 minutes, and the catalytic activity remained above 80%, indicating that the catalyst has a good stability. Moreover, the Ti3+ self-doped V2O5/TiO2 catalyst also showed good SO2 and H2O resistance. Therefore, these findings provide important information to better understand the application of the prepared catalyst.


Introduction
Nitrogen oxides (NO x ) are very harmful to the environment and contribute to How to cite this paper: Zhang, K., Zhao, W., Dou, S.P. and Wang, Q. (2020) Catalytic Performance of Ti 3+ [1]. The selective catalytic reduction (SCR) of NO x with NH 3 is a well-established and efficient process for the elimination of NO x emissions [2]. 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-NO x process is carried out at 350˚C -400˚C [3]. 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 [4]. 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, V 2 O 5 /TiO 2 catalysts have attracted much attention due to their low temperature activity and environmental friendliness [5]- [10]. Therefore, the study of modified V 2 O 5 /TiO 2 catalytic system has great practical significance [11]. 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 [4]. Zhu

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 NH 3 , 5% O 2 and balanced N 2 . 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 NO 2 (NO X = NO + NO 2 ) 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 NO X conversion could be defined as Equation (1)

Activity of Catalyst
The activity of different Ti 3+ doping amount is tested, and the results are shown in Figure 1. The denitration activity of all Ti 3+ self-doped V 2 O 5 /TiO 2 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%, Ti 3+ 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 Figure 4 shows the NO x conversion as a function of temperature (from 120˚C to 330˚C) in the NH 3  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 [18]. 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.

Effect of O2 Concentration on SCR Activity
It was found through previous studies that oxygen is very important for SCR of K. Zhang et al. NO with NH 3 at low temperature [19]. Figure 5 shows the effect of oxygen concentration on SCR activity at 210˚C. It was clear that the NO x conversion over the catalyst increased with the increasing of oxygen concentration, especially when the oxygen concentration was less than 3%. The 36% NO x conversion was obtained in the presence of without O 2 at 210˚C. This indicates that the catalyst has the ability to catalyze and reduce in the absence of O 2 . However, the NO x conversion increases significantly as the O 2 concentration increased to 3%. This shows that O 2 has a great influence on the catalytic activity. The NO x conversion increased to 88.1% when O 2 reached 5%. However, the catalytic activity remained basically stable with the continued increase of O 2 concentration. Therefore, the optimal O 2 concentration is 5%.

Effect of the [NH3/NO] Molar Ratio on SCR Activity
It is well known that the [NH 3 ]/[NO] molar ratio is a key parameter in SCR of NO with NH 3 [14]. Figure 6 shows the effect of different [NH 3

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 Journal of Materials Science and Chemical Engineering 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.

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.

Effect of H2O and SO2 on SCR Activity
Sulfur dioxide as another important pollution was in the flue gas inevitably [12]. The effect of H 2 O and SO 2 on the activity of the catalyst was investigated experimentally at 210˚C, and the result is exhibited in Figure 9. When the 500 ppm SO 2 was added in firstly, the obvious decrease of NO conversion was not observed. The NO x conversion decreased from 88.1% to about 78% after a 250 min·s 'SCR reaction process. When the supply of SO 2 was cut off at 250 min, NO x concentration in the exhaust recovered gradually to the original level. Then 5% H 2 O was introduced at 450 min, and the experimental results showed that the NO x conversion decreased rapidly. When the H 2 O is turned off at 600 min, the NO x conversion is quickly restored. Therefore, Ti 3+ self-doped V 2 O 5 /TiO 2 catalyst has superior SO 2 and H 2 O resistance.

Conclusion
On the basis of the above results, it can be concluded that the catalytic activity of