Syntheses And Catalytic Oxidation Performances Of High Surface Area Ceria-based Composite Oxides

With the development of world economy and the increase of human activities, the whole globe has been facing more serious environmental problems and the shortage of petroleum sources arising from the extensive usage of energy. Natural gas has many advantages, such as its abundant reserves, low cost, convenient usage, high thermal efficiency, small pollution, etc, and so it is regarded as one of the most alternative energy. Methane, as the main constituent of natural gas, can completely be converted into carbon dioxide and water that are harmless to environment. But the traditional flame combustion of methane is low-efficient, and the uncombusted methane along with the by-products CO, NOx is harmful to environment. To overcome the above drawbacks, the catalytic combustion of methane has been carried out and become a hot research topic in the past several decades.On the other hand, the emissions of chlorinated volatile organic compounds (CVOCs) in chemical industries are very harmful to environment and human health, and catalytic combustion is also an effective method to eliminate these CVOCs.Combustion catalysts mainly include noble metal catalysts, transitional metal oxides catalysts, solid acid catalysts and so forth. Although noble metal catalysts are usually considered to have high activity, their shortcomings are also obvious, such as high cost and facile deactivation. Solid acid catalysts have relatively low activity and usually are used with the incorporation of noble metal. However, transitional metal oxides catalysts have not only good activity, but also high thermal stability. CeO2 as one of the most important rare-earth metal oxides is used widely as the promoters for transitional metal oxides catalysts or even as the main active components of the catalysts. Therefore, in this paper, several novel methods are adopted to prepare high-surface-area ceria and ceria-based composite materials, and the combustion of methane and trichloroethylene that is one of the CVOCs family members are used as the model reactions to investigate the catalytic activities and stabilities of these prepared materials. The main achievements are concluded as follows. Flowerlike mesoporous CeO2 microspheres were prepared hydrothermally via different systems including glucose/acrylic acid, glucose/propanoic acid, glucose/n-butylamine and fructose/acrylic acid systems. CeO2 microspheres prepared in glucose/acrylic acid system have rich mesopores and high surface area up to 211 m2/g. A possible mechanism is proposed for the formation of the flowerlike CeO2 microspheres. In addition, La, Pr and Mn doped flowerlike CeO2 microspheres with high surface areas are also prepared.2. The catalytic activities of La or Pr doped CeO2 microspheres for methane combustion.Flowerlike mesoporous CeO2 microspheres are clearly superior to general CeO2 in the catalytic combustion of methane. But CeO2 is not good catalysts for methane combustion operated at high temperature, due to its poor thermal stability. Doping a proper amount of La or Pr into flowerlike CeO2 microspheres not only improves their thermal stability but increases their activity. It has been found that flowerlike CeO2 microspheres doped with 30mol% La or 20 mol%Pr perform the best activity. The increase of oxygen vacancy concentration caused by doping La or Pr can improve the oxygen mobility, resulting in the enhancement of the catalytic activity of flowerlike CeO2 microspheres for methane combustion.3. The catalytic activities of Mn-doped CeO2 microspheres for trichloroethylene combustion.Flowerlike mesoporous CeO2 microspheres have very good catalytic activity for trichlorothylene combustion, which is better than bulk CeO2. However, they are easily subjected to fast deactivation because of strong adsorption of Cl species coming from the destruction of tricloroethylene on their surface. Flowerlike mesoporous CeO2 microspheres adoped with Mn show much better activity and stability than pure flowerlike mesoporous CeO2 microspheres. It is proposed that introducing Mn can remove Cl species adsorbed on the surface of CeO2 microspheres via the cycle of MnOx→MnOyClx (MnCl2)→MnOx at 200℃in air atmosphere, and then improves the activity and stability of CeO2 microspheres.In addition, the CeO2-MnOx mixed oxides microspheres with 21mol% Mn have better catalytic performance than other CeO2-MnOx microspheres, and also display much better activity than the CeO2-MnOx mixed oxides prepared by the conventional citric acid method and coprecipitation method. High surface area, high oxygen mobility and rich mesopores are regarded as the main reasons of their high catalytic activity.4. The catalytic activities of high-surface-area Ce-Co composite oxides for methane combustion.The Ce-Co composite oxides with relatively high surface area can be prepared by two-step calcination of Ce-Co citrate complex:the calcination at high temperature in nitrogen and then at lower temperature in air. Calcination under nitrogen can produce carbon species deriving from citric acid to hinder the sintering of Ce-Co particles at high temperature, and then these carbon species are combusted at lower temperature in air. Therefore, Ce-Co composite oxides with high surface area are produced. A proper ratio of citric acid/total metal salts (mole ratio) is 2:1 and too less citric acid added cannot produce enough carbon species to hinder sintering of Ce-Co particles. Because of high surface area, Ce-Co composite oxides prepared by this novel method have higher activity than those calcined directly at the same temperature in air.Moreover, Ce-Co composite oxides with the Ce/Co=1/3 (bulk atomic ratio) perform better activity than those with other investigated ratios. The actual ratio of Ce/Co on the surface is 0.9/1, that is, Ce is dispersed richly on the surface compared with Co, which is favorable for close contact of Ce and Co elements and a strong interaction between them, resulting in its high activity and stability for methane combustion. Pure Co3O4 has a high activity but poor thermal stability, and Ce-Co composite oxides with excessive CeO2 have not only low activity but also poor stability.