Study On Preparation Of Mn-Ce Compound Oxides Catalyst And Its Catalytic Wet Air Oxidation Of Butyric Acid

Landfill leachates are a kind of harmful organic wastewater produced in the process of municipal solid waste landfill, with complex composition and high concentration. It has been a demanding problem for municipal environment. Catalytic wet air oxidation is an effective method to degrade organic pollutant into harmless inorganic matters or smaller organic matters that are easy to degrade. It uses air or oxygen as oxidant and works on the condition of high temperature and pressure. In the previous study of our team, catalytic wet air oxidation had been used to degrade landfill leachates taking Co/Bi composite oxides as catalyst. The results gained were quite good. However, to make the reaction undergoing in mild conditions, it is vital to screen and find more effective catalysts. Over the past years, people have done a lot research to find solid catalysts that have potential industrial value. Manganese-Cerium composite oxides, which have been found to be remarkable active in the wet oxidation of several refractory pollutants, were promising candidate. In this paper, the activity of Mn-Ce composite oxides catalyst was assessed by catalytic wet air oxidation of butyric acid existed in landfill leachates, the factors that may affect catalyst structure and activity in co-precipitation was measured by orthogonal design, this can provide support for catalysts selection; the effect of catalyst composition on activity of Mn-Ce composite oxides catalyst was also studied, and this helps people to better understand the mechanisms of reaction. Co-precipitation has been widely used to prepare composite oxides catalysts as in the process of co-precipitation each component can be even mixed and this is very helpful. Mn-Ce composite oxides catalyst was prepared through co-precipitation and applied to catalytic wet air oxidation of butyric acid, the result showed Mn-Ce composite oxides catalyst was very active, 2h-TOC-removal rate achieve over 90%. Compared with 25% removal rate gained in the same reaction conditions using Co/Bi catalyst, Mn-Ce is more active than Co/Bi in the process of butyric acid degradation.In the process of co-precipitation, there are many factors that can affect the activity of catalyst. The factors such as the way materials added, precipitator concentration, reaction time, drying temperature, drying time, calcination temperature and calcination time, etc, were measured by orthogonal design. Orthogonal experiments results analysis revealed that reaction time and calcination temperature had significant influence on catalyst activity; drying temperature had some impact with little influence attributed by other factors.According to the results gained from orthogonal experiments, controlling the main factors that affected the catalytic activity of catalysts in the experiment, eleven manganese and cerium composite oxides with molar ratios ranging from 0 to 100 %, which were expressed as Mn/Ce 0/10, 1/9, 2/8, 3/7, 4/6, 5/5, 6/4, 7/3, 8/2, 9/1, 10/0, were prepared and used in the catalytic wet air oxidation of butyric acid. The results showed Mn/Ce 3/7, 4/6 and 5/5 were the most active catalysts, followed were 7/3 and 6/4, while the activity of 2/8 and 1/9 were the lowest among composite oxides. Pure manganese and cerium oxides had some activity in the reaction; however, their activities were much lower compared to composite oxides catalyst.The kinetic of catalytic wet air oxidation of butyric acid using Mn-Ce catalysts with different ratio was studied. Allometric equation Y=aXb had been used to non-linear curve fit the kinetic data of the degradation of the total organic carbon, the results indicated that Allometric equation was very suitable to describe the process of TOC removal.Specific surface area measurer has been used to characterize the physical properties of catalysts, the results showed both specific surface area and core structure could affect catalyst activity. So catalyst with high activity must have suitable specific surface area and core structure. 11 Mn-Ce composite oxides catalysts were characterized by X-ray diffraction. According to the main features of the patterns, pure cerium oxides were existed mainly in the form of CeO₂, while pure manganese oxides were existed in the form of MnO₂, Mn₂O₃ and MnO. The dominant diffraction peak detected from the Ce relatively rich catalysts Mn/Ce 1/9⁴/6 was CeO₂, with the adding of Mn, the effect between Mn and Ce made the intensity of cerium oxides related peaks decreased. As the increasing of Mn content in the composite oxides, i.e. Mn/Ce 5/5⁹/1, the phase of cerium oxides had been changed dramatically. Not only CeO₂ was detected, but also Ce₂O₃ and manganese oxides. And with the increasing of Mn, the intensity of Mn oxide-related peaks increased and manganese oxides phases became more complicated. This means the interaction of Mn and Ce has significant influence on the activity of Mn-Ce composite oxides catalysts. The interaction of Mn and Ce is beneficial for cerium oxide to promote oxygen storage and release, to enhance oxygen mobility, to form surface and bulk vacancies, and to improve the catalyst redox properties of the composite oxides. It can be inferred that Mn-Ce composite oxides catalysts with Mn/Ce 3/7⁷/3 probably are more active.