Study On The Modification Of MnO_x/?-Al₂O₃ Catalysts For Low Temperature Selective Catalytic Reduction Of NO_x

This paper focues on the development of a novel low temperature catalyst to replace the conventional V-based selective catalytic reduction?SCR? catalysts. MnO_x/?-Al₂O₃ catalysts together with those modified by doping with Mo were prepared and tested to show their performance in the selective catalytic reduction?SCR? at low temperature. In addition, the reaction mechanism of modified MnO_x/?-Al₂O₃ catalysts also was investigated. Summary of the major findings and general conclusions of this research are presented as following:?1? The SCR activities of MnO_x / ?-Al₂O₃ catalyst in different Mn loadings were investigated. The effects of oxygen and reducing gas on SCR reaction were also studied. MnO_x / ?-Al₂O₃ catalysts were characterized by BET, XRD, H₂-TPR, TPD in order to further study the reaction mechanism. The results showed that a removal efficiency of NO was 92% was achieved when the Mn loading was 2 wt%, the operating temperature was 250°C, space velocity was 10,000 m L/?g·h?. It was found that oxygen had negligible effect on SCR process when its concentration exceeded 3%. In terms of the concentration of NH₃, there were no noticeable change in the catalytic performance when the NH₃/NO ratio reached 1.5. The H₂-TPR and XPS results illustrated thatMn₂O₃ and Mn₃O₄ are as the main phases existing on the surface of the catalysts. There were three types of acid sites on the surface of MnO_x/?-Al₂O₃ catalyst. And two kinds of species formed on the surface of Mn O_x/?-Al₂O₃ catalyst when NO was introduced into catalyst.?2? The Mo modified catalysts were characterized by using different techniques, such as BET, XRD, XPS, H₂-TPR, TPD. The results showed that Mo had a prominent modification on MnO_x/?-Al₂O₃ catalysts when Mo loading was 1.25 wt%, and the Mn2Mo1.25 catalyst showed an excellent NO conversion and a wide temperature window?NO conversion 93%, temperature window 150-300°C?, when Mn loading was raised to 3 wt%, SCR reaction activity and reaction temperature window achieved the best?NO conversion 96%, temperature window 100-300°C?. Meanwhile, according to results of BET?XRD?H₂-TPR and XPS analyses, the addition of Mo to the MnO_x/?-Al₂O₃ catalysts inhibited the growth of MnO_x particle size and contributed to maintain the well-dispersion of MnO_x on the surface of the support. Meanwhile, Mo addition also impeded Mn2+ and Mn³⁺ to form MnO_x with higher oxidation states, such as MnO₂. More improtantly, the reducibility of catalysts was enhanced by Mo additive, which was the key to SCR reaction.?3? The surface reaction of catalysts before and after Mo modification was studied by using NH₃-TPD and NO-TPD, the main purpose of which was to reveal the mechanism of low temperature SCR. It was found that Mo promoted the total amount of acid sites on the surface, but inhibited NO adsorption on the surface of the catalysts. However it lowered the desorption temperature of nitrite adsorbed on the surface of catalysts, which was significant to enhance low temperature SCR activity. The SCR reaction started from the transformation of coordinated NH₃ on the Mn³⁺ ions to-NH₂. The-NH₂ then reacted with nitrite adsorbed on the surface to form an intermediate product?-NH₂NO?, which then decomposes into N₂ and H₂ O. Therefore, the number of Mn³⁺ sites and amount of adsorbed nitrite were the two key factors dictating the performance of the low temperature performance of the SCR catalysts.