urther investigated. When SO2 concentration is 0.01%, activities of Mn-Ce-M were performed at 250˚C, as shown in Figure 6. For all the catalysts, after importing SO2 into the reactant gas, the NO conversion is kept or slightly increases within 50 min and then begins to decrease. The performance improvement for the time being can be attributed to the enhancement of NH3 adsorption on the acid site formed from the SO2 oxidation. After a period of time, a lot of sulfate were formed and adhered to the catalyst surface which leads to catalytic active decrease. Use of Mn-Ce-Pr and Mn-Ce-Y catalysts, the NO conversion are lower than that of Mn-Ce catalyst at initial stage, but decreased slowly and begin to ex-

Figure 6. SCR activities of Mn-Ce-M0.025 catalysts in the presence of SO2 (Reaction condi- tions: φ(NO) = 1000 ppm; φ(NH3) = 1200 ppm; φ(O2) = 5%; N2 as balance; GHSV = 10,000 h1; SO2 = 0.01%; reaetion temperature = 250˚C).

ceed that of Mn-Ce catalyst after 150 min in SO2 condition. After 275 min in SO2 condition, the NOx conversion decrease from 88.8% to 73.5% over Mn-Ce catalyst, 87.1% to 77.1% over Mn-Ce-Pr catalyst and 88.7% to 75% over Mn-Ce-Y. The results demonstrated that Pr presents better tolerance to the SO2 than Y element. Doping W and Zr into the Mn-Ce catalyst not only improve the catalytic performance but also enhance the catalyst’s sulfur tolerance. In all catalysts, Mn-Ce-W catalyst presented the best performance with 88.1% NO conversion in SO2 condition after 275 min.

3.5. FT-IR Analysis

In order to study the catalyst sulfur tolerance, the fresh catalyst and deactivated Mn-Ce-M catalysts with SO2 was carried out by FT-IR spectroscopy to detect the adsorbed species on the Mn-Ce-M catalyst. The FTIR spectra for the fresh Mn-Ce catalyst, deactivated Mn-Ce and Mn-Ce-W catalysts with SO2 deactivated catalysts with SO2 are shown in Figure 7. The adsorption peak at 1634 cm−1 band was commonly considered as O-H stretching vibration peak [8] . Comparing to fresh Mn-Ce catalyst, four new bands appeared when the catalysts were exposed to flue gas with SO2. NH4+ was adsorbed on catalyst acid site at 1400 cm−1 and three bands between 1048 - 1129 cm−1 were identified as adsorption peaks [9] [10] . The results from the FTIR spectra indicated sulfate formation on the catalysts.

There into, the adsorption peak of NH4+ was weakest on Mn-Ce catalyst, but the strongest peaks were formed. That is to say, over Mn-Ce catalyst, a little ammonium sulfate or ammonium bisulfate was formed, but a lot of metal sulphates deposited. So, the maximum sulfate formation was observed on Mn-Ce catalyst, which may be the main reason of the catalyst inactivation. Metal doping into the Mn-Ce catalyst could inhibit the sulfate formation, especially the suppression to the deposition of metal sulfate, which could lead to the block of catalyst pores or channels. The weakest peaks presented over Mn-Ce-W catalyst, which proved that W addition could greatly inhibit the sulfate formation. Then Mn-Ce-W catalyst presented the optimal performance in the aspect of sulfur tolerance.

4. Conclusion

Mn-based catalysts were prepared via ultrasonic immersing method for the selective catalytic reduction (SCR) of NOx from fuel gas. Transition metals effect on the Catalysts’ DeNOx efficiency and tolerance to sulfur were investigated. SEM and XRD results demonstrate metal elements high dispersion on TiO2 and uniform particle size about 500 nm to 800 nm. Sulfur tolerance analysis indicated that transition metals M can improve the catalysts’ performance when SO2 exists in the fuel gas. Mn-Ce-W catalyst performs the best NOx conversion of 93.2% at 200˚C and 98.4% at 250˚C, respectively. From the FTIR results, the weakest peak over the Mn-Ce-W catalyst which proved W element inhibit sulfate formation, especially metal sulfate, and this is the reason of good sulfur tolerance for the catalysts.

Figure 7. FT-IR spectra of fresh and deactivated catalysts with SO2. (a) Fresh Mn-Ce; (b) de- activated Mn-Ce catalysts; (c) deactivated Mn-Ce-W0.025 catalysts.

Acknowledgements

We acknowledge the support of the National Support Plan (2011BAA04B07) and Beijing Institute of Technology Basic Research Fund (20111042014).

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NOTES

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