Effects of temperature on oxidation characteristics of NO catalyzed by Mn-Ce/γ-Al2O3 from diesel engine exhaust

With the aim of studying the effect of Mn-Ce catalysts on the NO oxidation activity, a series of xMnyCe/γ-Al2O3(x:y is mole ratio, x=4, 6, 8, 10; y=10) catalysts were synthesized by a sol-gel method. The samples were dried at 110℃ for 24 h, calcined in air for 1 h at 300℃ and then for 5 h at 500℃ to obtain the required 40-60 mesh powder.The effect of metallic Mn and Ce on their microstructure and catalytic properties were investigated by X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) analysis. According to the results of analysis, the diffraction peaks of Mn2O3 became stronger and then shifted to weaker with the value of x increasing from 4 to 10, while Mn2O3 reached up to its peak value when x was 6. The grain size of cerium in the form of CeO2 was 26 nm as indicated by Scherrer equation. CeO2 diffraction peak shifted to a higher angle, due to the cell shrinkage caused by the fact that a part of Ce4+ ions were replaced by Mn4+ and Mn3+, which improved the oxygen vacancy concentration and increased the activity of catalyst. The dissymmetric peak of Mn 2p3/2 observed in XPS spectra proved that Mn3+ and Mn4+ were both present in the xMn10Ce/γ-Al2O3 catalyst. MnO2 could be reducted to MnO while MnO would be oxidated to MnO2 by the lattice oxygen generated by CeO2. And the peak of O 1s indicated that the content of lattice oxygen of 6Mn10Ce/γ-Al2O3 and 8Mn10Ce/γ-Al2O3 was different, which was mainly because of the different Mn/Ce ratio. The higher level of O was more favorable to the oxidative of NO. Furthermore, the effects of temperature on the catalytic oxidation activity of NO were investigated based upon a tubular fixed bed reactor in the range of 150-450℃ with an inside diameter of 10 mm and plugged between two silica wool layers to prevent the sample being blew away. The gases used in test were 500 ppm NO, 10% O2, with N2 in balance and a space velocity of 55 000/h. Results show that NO2 concentration over 6Mn10Ce/γ-Al2O3 catalysts reached stable after 900 s under 250℃, while the stable time reduced to 570 s at 300℃ and slowed down with the rising of temperature. NO conversion rate under different Mn/Ce ratios first increased and then decreased with the rise of temperature and reached up to the peak value at 300℃. It should be noticed that NO conversion rate would decrease as the further increase of temperature because of NO generated by the thermodynamics of NO2. In addition, NO conversions of all the catalysts kept almost the same in the temperature range from 400 to 450℃, due to the accelerated thermal decomposition of NO2 under the influence of high temperature. The xMn10Ce/γ-Al2O3 (x≥6) catalysts showed better NO catalytic oxidation activity, over the temperature range from 250℃ to 350℃. Among all the catalysts, 6Mn10Ce/γ-Al2O3 catalyst showed the highest catalytic activity at low temperature, and NO conversion rate reached up to 44.8% at 200℃ and 83.6% at 300℃,respectively. The reason was that the properties of the catalysts depended mainly on the active components, especially the Mn/Ce ratio. The results also indicated that MnOx was the main contributor for NO oxidation, and the catalysts showed better oxidation capacity with the increase of MnOx. The NO oxidation activity followed the trend 6Mn10Ce/γ-Al2O3> 8Mn10Ce/γ-Al2O3>10Mn10Ce/γ-Al2O3>4Mn10Ce/γ-Al2O3. © 2016, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.