Perovskite-type mixed metal oxide catalysts have been studied for application in catalytic combustion. Catalytic activity for combustion with perovskite catalysts depends on their composition and preparation method. In this study, spray decomposition method was devised for preparation of perovskite catalysts with the combination of spray drying and citrate methods. For the comparison of preparation methods, perovskite catalyst was prepared by two different method (spray decomposition and coprecipitation method). Complete formation of perovskite at lower temperatures can be achieved due to the simultaneous drying and decomposition in the spray decomposition method. The obtained perovskite catalysts exhibit larger surface area and higher catalytic activity for combustion, compared to those obtained from the coprecipitation method. Specifically, the calcination of the perovskite catalyst obtained from the spray decomposition method at 700℃ for 4 h yields the highest catalytic activity for combustion.
Catalytic activities for combustion of CO, methane and propane were measured over $LaMO_3$ (M = Mn, Fe, Co, Ni) prepared from the spray decomposition method. Depending on the type of B metal, the catalytic activities for combustion vary. It has been found that the higher catalytic activities for combustion can be obtained with $LaMnO_3$ and $LaCoO_3$ since activity of $ABO_3$-type perovskites depends on their component oxides $BO_x$.
Partial substitution of metal ion in the perovskite structure affects catalytic activity for combustion. To study the effect of A-site substitution on catalytic activity, La substituted by Sr, Ce and Ag in the $LaMO_3$ (M = Mn, Fe, Co, Ni) perovskites were prepared by spray decomposition method. With the substitution of Ag for La in the $LaMO_3$ (M = Mn, Fe, Co, Ni), the peaks of X-ray diffraction patterns corresponding to Ag-metal appeared. It is indicated that small portion of Ag was present as metallic state in $La_0.7Ag_0.3MO_3$ (M = Mn, Fe, Co, Ni) catalysts.
The binding energy of Mn obtained from XPS measurements did not change with the substitution of Ag and Sr for La in the $LaMnO_3$ perovskite that may indicate no change of the oxidation state.
Base on the result of temperature programmed reduction obtained by TGA, it is note that all the catalysts prepared by spray decomposition method consist of nonstoichiometric metal oxide composites.
$O_2$-TPD indicates that the partial replacement of Ag into the perovskite structure increases the weak adsorption of oxygen at lower temperatures. Especially, the $La_0.7Ag_0.3MnO_3$ catalyst had a largest amount of oxygen desorption at room temperature compared to the other catalysts with different compositions.
The $La_0.7Ag_0.3MnO_3$ catalyst exhibits the highest catalytic activity for combustion among the tested catalysts with different combustible gases that may affect the activity more or less. It is worth to note that the CO oxidation over the $La_0.7Ag_0.3MnO_3$ catalyst occurs at room temperature and the complete conversion is attained at 100℃ for $30,000 h^{-1}$. The catalytic activity for combustion with the perovskite catalysts is believed to be affected by the desorption of oxygen.
Alumina-supported $La_0.7Ag_0.3MnO_3$ catalyst was prepared to deposit most perovskite at the outer rim or periphery of the spherical alumina pellet by adding citric acid to metal nitrate solution. The supported catalyst is the first order dependence for propane and 1/2th or zeroth order for oxygen with increasing the concentration of propane in combustion. The stability of combustion in a fluidized bed combustor with propane is improved significantly using more than 10wt% of 30 wt% $La_0.7Ag_0.3MnO_3/γ-Al_2O_3$.
