Regular ArticleImprovement of catalytic activity over Mn-modified CeZrOx catalysts for the selective catalytic reduction of NO with NH3
Graphical abstract
Introduction
NOx emissions from the combustion of fossil fuels have long been recognized as one of the most threatening atmospheric pollutants globally [1], [2], [3]. A variety of technologies have been developed to reduce NOx emissions from both stationary and mobile combustion facilities and devices, among which selective catalytic reduction (SCR) with NH3 as a reducing agent has attracted great attention for its high NOx conversion, low cost, and being environmentally friendly [4], [5], [6]. The main reactions in a typical SCR process include:4NO + 4NH3 + O2 → 4 N2 + 6H2O4NO + 2NH3 + 2O2 → 3 N2 + 6H2O2NO2 + 4NH3 + O2 → 3 N2 + 6H2ONO + NO2 + 2NH3 + 2O2 → 2 N2 + 3H2O
The commonly adopted commercial denitration catalyst is the vanadium based catalyst (V2O5-MoO3 (WO3)/TiO2), which has many shortcomings, such as high reaction temperature (300–400 °C), narrow operating temperature window and biological toxicity [7], [8], [9]. In recent years, V-free composite metal oxide catalysts have attracted increased interests by both academia and industry researchers for their high rates of NOx conversion, and broadened operating temperature ranges [10], [11]. In particular, Ce-Zr mixed metal oxide catalysts have been extensively studied for their excellent SCR performance and low cost. Li et al. [12] synthesized a WO3-CeO2-ZrO2 catalyst for selective catalytic reduction (SCR) of NO with NH3, reaching complete conversion at 250–500 °C and the superb catalytic performance was attributed to the superior redox properties of CeO2-ZrO2, and the presence of highly dispersed or amorphous WOx species. Masaaki et al. [13] loaded rhodium into a ceria-zirconia solid solution by incipient wetness impregnation for SCR by unburned hydrocarbons in the presence of O2. However, there are still some deficiencies in the Ce-Zr catalysts, such as poor SCR performance at low temperatures and still narrow operating temperature window. Manganese oxides (MnOx) involve several kinds of labile oxygen which is favorable to the redox cycle. Therefore, Mn-based catalysts are well-known for its distinguished SCR effect at low temperatures and may be an effective and economical choice as SCR catalysts [14].
Based on the hypothesis of the combined advantages of Mn-based and Ce-Zr mixed metal oxides catalysts, we synthesized a series of MnCeZrOx catalysts and tested for NH3-SCR in the present work. Specific attention was paid to the effect of Mn/Ce molar ratio on the SCR performance. The physical and chemical properties of the Mn-doping catalysts were systematically characterized using Brunauer-Emmertt-Teller (BET), X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), H2-Temperature Program (H2-TPR) and in situ Fourier transform infrared spectroscopy (FTIR). The reaction pathway over the Mn0.25Ce0.5Zr0.25Ox catalyst was also proposed based on the intermediate species observed in the SCR process.
Section snippets
Catalyst preparation
MnxCe0.5Zr0.5−xOy (x = 0, 0.1, 0.2, 0.25, 0.3, 0.4, 0.5) catalysts with different Mn/Ce molar ratios were synthesized via the impregnation-NH3·H2O coprecipitation method. The precursors manganese acetate ((CH3COO)2Mn·4H2O), cerium nitrate (Ce(NO3)3·6H2O), zirconium nitrate (Zr(NO3)4·5H2O) were dissolved in 200 mL deionized water with different Mn/Zr molar ratios. Then ammonia (25 wt%) was added into the solution slowly until the pH reached 9.5. The mixture was then stirred for 3 h at room
NH3-SCR activity
It can be seen from Fig. 1a, NO conversions for the Ce0.5Zr0.5Ox, Mn0.1Ce0.5Zr0.4Ox, Mn0.2Ce0.5Zr0.3Ox, Mn0.25Ce0.5Zr0.25Ox, Mn0.3Ce0.5Zr0.2Ox and Mn0.4Ce0.5Zr0.1Ox catalysts calcined at 550 °C first increased as temperature increased, remained stable and then decreased as the temperature increased further in the range of 50–350 °C. The NOx removal efficiency at low temperatures followed the order: Mn0.25Ce0.5Zr0.25Ox > Mn0.4Ce0.5Zr0.1Ox > Mn0.3Ce0.5Zr0.2Ox > Mn0.2Ce0.5Zr0.3Ox > Mn0.1Ce0.5Zr0.4O
Conclusions
A series of MnCeZr mixed oxide catalysts were prepared using the impregnation-NH3·H2O coprecipitation method and tested for NH3-SCR. The best SCR performing catalyst was found to be Mn0.25Ce0.5Zr0.25Ox calcined at 500 °C which achieved NO conversion > 80% in the temperature range of 60–350 °C. The doping of manganese into the mixed oxides played a key role in improving the SCR activity and the optimum ratio of Mn/Ce was found to be 1/2. The characterization of the catalysts using XRD, BET,
Acknowledgements
This work was supported financially by the Key Project of the National Ministry of Science and Technology (No. 2016YFC0204204), the Major Program of the National Natural Science Foundation of China (No. 21590813), the National Natural Science Foundation of China (Nos. 21377015 and 21577012), the Program of Introducing Talents of Discipline to Universities (B13012), and the Key Laboratory of Industrial Ecology and Environmental Engineering, China Ministry of Education.
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