Ceria modified MnOx/TiO2 as a superior catalyst for NO reduction with NH3 at low-temperature
Introduction
Selective catalytic reduction (SCR) of NOx with NH3 is an effective process for cleaning NOx from stationary sources [1]. In order to avoid the deactivation by SO2 and dust, SCR reactor is suggested to be located downstream of the particle control and the desulfurizer devices where the flue gas temperature is usually below 150 °C. Especially after wet desulfurization process, the flue gas will be cooled down even below 100 °C. Therefore, it is needed to develop superior SCR catalysts with high activities at low-temperature.
Manganese oxides (MnOx) have been studied extensively as low-temperature SCR catalysts since they contain various types of labile oxygen, which are important to complete the catalytic cycle. Supported manganese oxides, such as MnOx/Al2O3 [2], MnOx/TiO2 [3], [4], and Mn deposited over carbon–ceramic cellular monolith [5] have been shown to be active for low-temperature SCR of NO with NH3 in the presence of O2.
As an inexpensive and a relatively nontoxic material, ceria (CeO2) is potentially advantageous for catalytic applications. Since it has two stable oxidation states, Ce4+ and Ce3+, oxygen could be stored and released by ceria via the redox shift between Ce4+ and Ce3+. Ceria could promote the oxidization of NO to NO2, thereby increases the catalyst activity for SCR of NO with NH3 [6]. Therefore, Mn–Ce mixed-oxide is believed to be a potential catalyst with high activity in low-temperature SCR reaction. Recently, Qi and Yang [7] have dealt with Mn–Ce mixed-oxide and found that it yielded nearly 100% NO conversion at 150 °C. Carja et al. [8] developed Mn–Ce/ZSM-5 catalyst in an aqueous phase which exhibited 75–100% NO conversion at a broad temperature window (240–500 °C). Mn–Ce/AC/C catalyst has also been studied by Hao et al. [9] which yields 78% NO conversion at 100 °C.
Based on our previous study on MnOx/TiO2, Ce doped MnOx/TiO2 was prepared by sol–gel method in this work. The main object was to seek catalysts with higher activities at low-temperature even below 100 °C.
Section snippets
Catalyst preparation
Tetrabutyl titanate (0.1 mol), manganese nitrate (0.04 mol), ethanol (0.8 mol), water (0.6 mol) and acetic acid (0.3 mol) were mixed under vigorous stirring at room temperature to form transparent sol. The molar ratio of precursor used above is the optimal value obtained in our previous study for MnOx/TiO2 [4]. Certain amount (0–0.02 mol) of cerium nitrate was added during this process for different catalysts. The sol transformed to gel after stabilized at room temperature for two weeks. The gel was
Catalytic activity tests
Fig. 1 shows NO conversion for the prepared catalysts. All the prepared catalysts did not provide NO conversion below 40 °C. It is found from Fig. 1 that the activity was greatly improved with the addition of Ce within the low-temperature range (60–150 °C). Especially for Ce(0.07)MnTi, 84% NO conversion was obtained at 80 °C and no NO could be detected in outlet at the temperature above 120 °C. Fig. 1 also shows the SCR activity decreased in the following sequence: Ce(0.07)MnTi > Ce(0.05)MnTi >
Conclusion
Cerium modified MnOx/TiO2 catalysts showed high activity for the low-temperature SCR of NO with NH3. About 84% NO conversion was obtained on the Ce(0.07)MnTi catalyst at 80 °C with a GHSV of 40,000 h−1. In the case of Ce(0.07)MnTi, BET surface area and pore volume were 50% greater than Ce(0)MnTi. XPS results indicated that the chemisorbed oxygen concentration on catalyst surface could doubly increase with the introduction of Ce. TPR investigation verified that the addition of Ce could enhance the
Acknowledgments
The project is financially supported by the National Natural Science Foundation of China (NSFC-20577040), New Century Excellent Scholar Program of Ministry of Education of China (NCET-04-0549) and Excellent Young Scholar Program of Zhejiang University.
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