Preparation And Characterization Of Substitued Hexaaluminate Catalysts For Methane Combustion

Catalytic combustion is an effective method to suppress the emission of nitrogen oxide from combustors. This potential application in energy generation industry has attracted many research interests in recent decade. The effect of catalytic combustion technology on industry application is to find suitable catalysts, with excellent activity and high heat-resistant ability. Generally, noble metal catalysts have high catalytic activity. However, their high cost and poor high temperature stabilities are obvious disadvantages. Perovskite catalysts are also not ideal for catalytic combustion at high temperature. Hexaaluminates are considered to be one of the most suitable materials for high-temperature catalytic combustion of methane because of their unique crystal structure. The aim of this paper is to find advanced hexaaluminate type catalysts, with high catalytic activity and high thermal stability. The co-precipitation, sol-gel and reverse microemulsion methods were employed to prepare catalysts. Based on LaAl₁₂O_19-δ. catalytic combustion catalyst was synthesized by substitution of different metal ions. The effects of preparation methods and various ions composite substitution on the samples crystal structure and performance of the catalysts were investigated by means of BET, XPS, XRD, SEM, TG-DTA , TPR and miniature fixed bed with methane combustion as model reaction.The co-precipitation method was employed to prepare LaMnAl₁₁O_19-δ catalyst sample for methane combustion. Calcination temperature and duration time was considered in the synthesis experiment. The XRD results showed that the crystal phase of hexaaluminates was formed when sample catalysts were calcined at 1200℃for 4 hours. The precipitator and precipitation temperature were also factors affected on the sample performances. Investigation showed salvolatile was better than the others in the process of co-precipitation. Under the precipitation temperature of 90℃and molar ratio (salvolatile/ M²⁺)of 1.5, the catalyst sample had higher activity , which light-off T₁₀% and total conversion T₉₀% were 474℃and 662℃, respectively.The metal chelated-gel and hydrolysis of metal alkoxide methods were employed to synthesize catalyst samples. During the process of the metal chelated-gel method, the catalyst, which the sucrose was used as chelating agent, exhibited higher catalytic activity in methane combustion. In the process of the hydrolysis of metal alkoxide method, the catalyst had the highest activity when the molar ratio of H₂O/ A1(OC₃H₇)₃ is equal to 1.5.How to obtain steady system of reverse microemulsion is a key factor to prepare catalyst sample with reverse microemulsion method. Experiment results indicated that the optimum ratio of Triton X-100 / n-hexanol / cyclohexane / water was 1:1:2:1. when the temperature was 25℃and the concentration of precursors was 0.75mol/L, a steady reverse microemulsion was formed under above ratio. This system can be used as the reaction media to prepare hexaaluminate catalysts. K ions were used as mirror plane cations to prepare substituted hexxaluminate catalysts. Experimental results showed that the catalyst samples had good crystal structure and lower temperatures of initial and complete conversion for methane combustion. A series of K₂Mn_xAl_12-xO_19-δ samples were prepared by reverse microemulsion-hydrolysis of metal alkoxide method. The K₂MnAl₁₁O_1-δ catalyst had higher activity with T₁₀%as 458℃and T₉₀% as 676℃. Increased of Mn ions in precursor would be result in the perovskite phase in the samples.The effects of various ions substitution on performance of the catalysts were investigated in above three methods. The results showed that the radius of Mn²⁺ and Fe³⁺ were similar to Al³⁺, so Mn²⁺ and Fe³⁺ easily accessed hexaaluminate skeleton and promoted the formation of pure single heaaluminate phase. While the other ions( Ni, Co, Cu, Zn, Zr, et al.), which had larger ionic radius , were difficult to enter hexaaluminate skeleton. The introduction of single Mn could improve the catalytic activity at low temperature and make methane light-off temperature shift to lower value. Single Fe could make methane completely convert at lower temperature. The LaFeMnAl₁₀O_19-δ catalyst which were prepared by any methods possessed the lowest temperatures of initial and complete conversion of methane, probably due to the more redox capacity of active sites. After 100h on-stream, no obvious decrease of catalytic activity was observed for LaFeMnAl₁₀O_19-δ sample. It can be concluded that LaFeMnAl₁₀O_19-δ catalyst had excellent catalytic performance and thermal stability for methane combustion.The mixture produced in reverse microemulsion can be easily converted to hexaaluminate catalysts with higher surface area and activity compared to that produced by co-precipitation or sol-gel methods. The results of TPR and XPS showed that Mn ion was existed in Mn²⁺ and Mn³⁺ in the catalyst, while the Fe ion was existed in Fe³⁺.