METHANE COMBUSTION OVER CATALYSTS PREPARED BY OXIDATION OF TERNARY PD(15)X(10)ZR(75) (X=CO, CR, CU, MN AND NI) AMORPHOUS-ALLOYS

Catalysts for the combustion of methane have been prepared by controll ed oxidation of various ternary amorphous alloys with the composition X(10)Pd(15)Zr(75) (X = Co, Cr, Cu, Mn and Ni). As a reference material , a Pd25Zr75 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 occurrin g during activation by oxidation in air were followed by thermal analy sis, 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 marke d differences in the crystallization and oxidation behavior of the all oys. 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 oxidatio n (activation) of the samples. XRD indicated the presence of PdO and p oorly crystalline monoclinic and tetragonal ZrO2 in all activated samp les. Kinetic studies were carried out in a fixed-bed microreactor at t emperatures of 300-1000 K and atmospheric pressure using a reactant mi xture with a ratio CH4:O-2 = 1:4. The alloy precursors showed no signi ficant activity mainly due to their low surface area. The activity of the activated (oxidized) alloys depended strongly on component X and d ecreased in the order Pd > Ni > Cu > Co > Mn > Cr. Catalysts derived f rom Pd25Zr75 and Ni10Pd15Zr75 showed relatively good catalytic stabili ty even after ageing for 15 h at 1000 K under reaction atmosphere, whe reas the other catalysts showed strong deactivation at higher temperat ures. Reduction of the catalysts by hydrogen with subsequent reoxidati on of the reduced components by the reaction atmosphere in most cases led to more active catalysts.