Preparation Of Co-containing Mixed Oxides With Large Surface Area And Their Catalytic Properties

Catalytic combustion has attracted much attention due to high combustionefficiency as well as low emission of exhaust gas such as CO, NO_x and unburnedhydrocarbons. For long time the supported noble metal catalysts have been generallyused for methane combustion. However, supported noble metal catalysts, althoughwith outstanding activity, are not fully satisfactory based on high price and scarceresource. Therefore there is a strong demand for the development of high activity, andlow-cost catalysts for the combustion of methane. Perovskite LaCoO₃ has beenextensively studied due to high catalytic activity for methane combustion and highthermal ability. However, the high calcination temperature of the preparedLaCoO₃catalysts inevitably leads to large crystallite size and lower specific surfacearea of the catalysts (less than 5 m²/g), so the potential applications of these materialsas catalysts are limited. Therefore, in order to increase the specific surface area of theperovskite-type catalysts, the new synthesis method of the catalysts will be required.In the paper, a series of La_0.8Sr_0.2CoO₃ catalysts were prepared by differentsynthesis methods and used successfully for the methane combustion. The selectedtechniques included the alanine solution combustion method, polyacrylamide-alaninegel method, polyacrylamide-glycine gel method and epoxide gel method. The effectsof organic fuels, coefficient ofφ, rare earths (Nd,Ce), transition metals (Ni,Cu) andcalcination temperature of catatlysts on methane combustion were studied. Theprepared samples were characterized by means of XRD, FI-IR, BET and H₂-TPRmethods.1. The effects of the preparation methods of catatlysts on methane combustionwere studied. The results indicate that La_0.8Sr_0.2CoO₃ catalyst prepared by alaninesolution combustion method has higher catalytic activity for methane combustion,whose T₅₀ and T₁₀₀ were 470℃and 550℃, respectively. It can be explained in termsof the smaller average crystal size, the higher pecific surface are, the lower activationenergy and more mobile chemical-adsorped oxygen in the surface and vacancy. 2. The effects of organic fuels on the structure and the catalytic activities ofLa_0.8Sr_0.2CoO₃ mixed oxides were studied. The results indicate that all La_0.8Sr_0.2CoO₃mixed oxides have large surface area, whose structure and catalytic activities arerelated to the corresponding organic fuels. The catalytic activity of La_0.8Sr_0.2CoO₃catalyst prepared by alanine solution combustion synthesis is the best among allsamples.3. The effects of stoichiometric ratio of organic fuel to oxidizer (φ) on structureand catalytic activities of the catalysts were studied. The results indicate that allLa_0.8Sr_0.2CoO₃ mixed oxides with the different coefficient ofφhave perovskitestructure, whose structure and catalytic activities alter regularly with the changecoefficient ofφ. Whenφis equal to 1.52, the catalytic activity of La_0.8Sr_0.2CoO₃mixed oxides catalyst is the best, whose T₅₀ and T₁₀₀ are respectively 460℃and 540℃, respectively.4. The La_0.8Sr_0.2CoO₃ catalysts of A-site doped different rare earths (Nd,Ce) andB-site doped different transition metals (Ni,Cu) were synthesized by alanine solutioncombustion method. The catalytic activity of La_0.8Sr_0.2CoO₃ mixed oxides for CH₄combustion was studied. The sequences of activities are La_0.8Sr_0.2CoO₃＞Ce_0.8Sr_0.2CoO₃＞Nd_0.8Sr_0.2CoO₃＞La_0.8Sr_0.2Co_0.5Ni_0.5O₃＞La_0.8Sr_0.2Co_0.5Cu_0.5O₃,.5. La_0.8Sr_0.2CoO₃ mixed oxides were prepared by alanine solution combustionmethod and calcined at 600, 700 and 800℃, respectively. The influence of thecalcination temperature of the mixed oxides on the performance of methanecombustion was studied. The catalysts calcined at 700℃has the higher surface area,smaller crystallize, and it has the best activity for methan combustion.