Methane combustion over catalysts prepared by oxidation of ternary Pd 15X 10Zr 75 (X = Co, Cr, Cu, Mn and Ni) amorphous alloys

Abstract

Catalysts for the combustion of methane have been prepared by controlled oxidation of various ternary amorphous alloys with the composition X 10Pd 15Zr 75 (X = Co, Cr, Cu, Mn and Ni). As a reference material, a Pd 25Zr 75 amorphous alloy was used. The oxidation of the amorphous precursors led, in some cases, to highly active catalysts for the deep oxidation of methane. The chemical and morphological changes occurring during activation by oxidation in air were followed by thermal analysis, X-ray diffraction and BET surface area measurements. Component X had a strong influence on these changes and consequently on the final activity of the materials. The thermoanalytical studies revealed marked differences in the crystallization and oxidation behavior of the alloys. The BET surface area of the precursor alloys was <0.1 m 2/g and increased, depending on component X, to 1.1–34.1 m 2/g upon oxidation (activation) of the samples. XRD indicated the presence of PdO and poorly crystalline monoclinic and tetragonal ZrO 2 in all activated samples. Kinetic studies were carried out in a fixed-bed microreactor at temperatures of 400–1000 K and atmospheric pressure using a reactant mixture with a ratio CH 4:O 2= 1:4. The alloy precursors showed no significant activity mainly due to their low surface area. The activity of the activated (oxidized) alloys depended strongly on component X and decreased in the order Pd > Ni > Cu > Co > Mn > Cr. Catalysts derived from Pd 25Zr 75 and Ni 10Pd 15Zr 75 showed relatively good catalytic stability even after ageing for 15 h at 1000 K under reaction atmosphere, whereas the other catalysts showed strong deactivation at higher temperatures. Reduction of the catalysts by hydrogen with subsequent reoxidation of the reduced components by the reaction atmosphere in most cases led to more active catalysts.