Preparation Of Manganese-based Oxides And Its Properties On The Selective Catalytic Oxidation Of Ammonia

Ammonia (NH3) is a kind of typical toxic industrial waste gas and does serious harm to human health and ecological environment. Therefore, in recent years, the control of ammonia has attracted widespread attention.At present, there are several technologies to eliminate ammonia, and selective catalytic oxidation is a promising method for its efficiency, convenience and no secondary pollution. Usually, noble metal catalysts show superior low-temperature catalytic performance but with poor N2 selectivity. On the contrary, transition metal catalysts exhibit good N2 selectivity, but poor catalytic activity at low temperature. In this thesis, manganese based oxides were prepared by different synthetic method and their ammonia catalytic activities were measured. Furthermore, the relationship between the structure and the activities was built through investigating the properties of catalysts. The main contents of the work are as follows:1. The Mn-Ce oxides catalysts were prepared by precipitation method and their ammonia catalytic activities were measured. The results showed that, when urea acted as precipitant with the ratio n(urea)/n(Mn+Ce)=10:1, the 400℃ calcined Mn-Ce oxides catalyst with Mn/Ce ratio of 4:1 showed an excellent catalytic activity for ammonia oxidation with complete conversion temperature of 200℃.2. The Mn-Ce oxides catalysts were prepared by hydrothermal precipitation method and their ammonia catalytic activity were measured. The results showed that, the adjustment of the manganese source, as well as the reaction conditions, could obviously affect the ammonia oxidation performance of the catalyst. The Mn-Ce oxides catalyst with both Mn2+ and MnO4-as manganese source and then synthesized at 140℃ showed a better ammonia oxidation activity. The complete conversion temperature of the catalyst was 180℃. Combined with the characterization results, it can be concluded that the addition of KMnO4 could improve the specific surface area of the Mn-Ce oxides catalyst, and facilitate the diffusion of Mn4+ into the cubic fluorite structure of CeO2 which resulted in the formation of Mn-Ce-O solid solution. Moreover, the addition of KMnO4 alsopromoted the flow of oxygen and improved the redox ability. All these effects contributed to the improvement of the ammonia oxidation activity.3. The manganese oxides catalysts were prepared by hydrothermal precipitation method and their ammonia catalytic activity were measured. The results showed that, the MnO2(UH) catalyst prepared by the hydrothermal precipitation method achieved the complete ammonia oxidation at 170℃ with N2 selectivity of 49%. Compared with the MnO2(H) catalyst and commercial MnO2, the results of XRD, BET, H2-TPR, SEM, XPS, NH3-TPD showed that adding a certain amount of urea during the synthesis process could modulate the structure of catalysts.Particles dispersed more homogeneously with smaller size and the morphology transformed from nanorods to nanoplate while the crystal phase retained as α-MnO2. Furthermore, a large number of nanosheets clumped into flower ball, significantly increased the specific surface area of the catalyst, leading to a large number of adsorption sites and oxygen vacancies on the catalyst surface. This could promote the adsorption and activation of NH3 as well as dioxygen, and improve the ammonia oxidation activity of the catalyst ultimately.