Catalytic Combustion Over Me-hexaaluminate (Me=Mn, Fe, Co, Ni) Catalysts

The main advantages of catalytic combustor are ultra-low emissions of nitrogen oxides (NO_X), carbon monoxide (CO) and unburned hydrocarbons (UHC), and to stabilise combustion of low heating value gas (such as coal/biomass gasified gas, coal-bed gas) at relatively low temperatures. Catalytic combustion of gas is one of the most promising processes to produce environmentally clean energy. However, the utilization is prevented by development of catalyst life. Nowadys, the research of catalytic combustion focuses on how to prepare the catalysts own low ignition temperature and high-temperature stable.In this work,hexaaluminate catalysts LaMeAl₁₁O₁₉(Me=Mn,Fe,Co,Ni,NiCe)were prepared by co-precipitation method. The catalysts were characterized by means of XRD, BET, XPS, in-siut DRIFT and the catalyst activity in the catalytic combustion of simulating gasified biomass (3%CO,2%H₂,1%CH₄,500ppmNH₃,balance N₂) was test in a tubular quartz reactor. The effect of diffrent transition-metal ions substiuttion on catalytic structure and combustion activity was investgiated.It is found that the percursosr were aclcined at 1200℃for 4 hours leads to the formation of a final monophasic materials with a MP structure and maintain high surface areas. In these structures, the valence state of La, Al and O is respectively La³⁺, Al³⁺, O^2-. As far as trnasition metalions is concerned, the introduecd Fe ions is trivalent, Co and Ni ions is both dvialent, whereas the Mn ions is either dvialent or tvriaient.The prepared substituted hexaaluminate catalysts are active in the catalytic combustion. In particular the Fe- and Mn-substituted samples show the highest activity in the combustion of methane, which is the least active biogas fuel. Ni-substituted samples have the maximum burning rate, and doping of Ce can obviously improve catalytic activity. The oxidation of NH₃ follows closely that of CO for all catalysts. Generally the selectivity for NO_X is proportional to the oxidation activity of the catalyst and ammonia contained in the biogas is oxidized to N₂ but large amounts of NO are also formed, and little NO₂ is formed under high tempreture. In whole process of combustion, the concertration of N₂O is digit ppm. The results of in-situ DRIFT and combustion test show that ammonia contained in the gas is oxidized by an imide (NH) mechanism, in which the first step yields NH, and then the NH reacts with atomic oxygen (O) to form nitroxyl (HNO) and further conversion to N₂ or nitrous oxide (N₂O), or NH could even react with molecular O₂ to produce nitric oxide (NO).