Development of Catalytic Materials for High Temperature Catalytic Combustion

Abstract

Catalytic combustion has many advantages over conventional combustion. Recently, a great deal of interest has been focussed on development of the catalytic combustor, in which reduction of nitrogen oxides and attainment of high energy efficiency can be achieved simultaneously by stable combustion of lean fuels. Development of thermally stable catalyst is by all means needed for high temperature catalytic combustion. From a systematic study on the effect of additives on thermal stabilization of alumina, it has been revealed that the loss of surface area caused by sintering could be successfully suppressed when additives lead to the formation of hexaaluminate phase. In particular, hexaaluminate fine particles prepared by hydrolysis of corresponding alkoxides are useful as a thermally stable support for the catalyst. Among the series of cationsubstituted barium hexaaluminate (BaMAl₁₁O₁₉: M=Cr, Mn, Fe, Co and Ni), the Mn-system showed the highest activity for methane combustion of the ease of the reduction/oxidation cycle of Mn ions in the hexaaluminate lattice. The surface areas and catalytic activities of Mn-substituted hexaaluminates are significantly enhanced when the mirror plane cations are partially replaced by the aliovalent cation of the same size. In the Sr_1-xLa_xMnAl₁₁O₁₉ system calcined at 1, 300°C, Sr-Substitution resulted in the Inaximum surface area of 23.8m²/g, and the maximum combustion activity (T_10%=500°C) at x=0.2. The honeycomb catalyst composed of Mn-substituted hexaaluminate was successfully demonstrated for promoting high efficiency catalytic combustion. The anisotropic grain growth, which results in planar morphology with a small aspect ratio, contributes to the retention of large surface area by hexaaluminate powders.