Аңдатпа
A thermally stable manganese oxide catalyst for the deep oxidation of lean CH 4 mixtures with air to CO 2 was developed and characterized. To prepare this catalyst, new approaches to the synthesis of polyoxide catalysts based on Mn modified with La, Ce, Ba, and Sr by supporting them from nitrate solutions onto alumina granules stabilized with 2% Ce were used. The catalyst gave the degree of CH 4 oxidation to CO 2 at a level of 90–98% at a CH 4 concentration of 0.2–4.0% in air (space velocity of 10 × 10 3 h −1 ; temperature of 973 K). The heating of a Mn-containing sample in air to 1373 K had no negative effect on the conversion of CH 4 ; an insignificant decrease in the catalyst activity (by ∼10%) was observed only on heating above 1473 K. The degree of CH 4 oxidation to CO 2 depended only slightly on O 2 and CH 4 concentrations varied over the ranges 2–20 and 0.5–4.0%, respectively. With the use of physicochemical techniques (XRD analysis, BET, electronic diffuse-reflectance spectroscopy, TPD, TPR, and TPO), it was found that the catalyst underwent considerable phase transformations as the temperature was increased to 1473 K: Mn 2 O 3 clusters, which occurred at 873 K, were partially converted into LaMnO 3 perovskites above 1173 K or LaMnAl 11 O 19 hexaaluminates above 1273 K. Oxygen that is the constituent of the latter species participated in the oxidation of CH 4 to CO 2 .