NATURAL MANGANESE OXIDES AS CATALYSTS FOR OXIDATIVE COUPLING OF METHANE - A STRUCTURAL AND DEGRADATION STUDY

Natural manganese oxides were evaluated as oxidative coupling catalyst s to convert methane into C-2-hydrocarbons. Reactions were done in a c yclic redox mode in which the oxidized catalyst was reacted with metha ne at 850 degrees C in the absence of oxygen. The bulk and surface ana lyses (XRD, XPS, FT-IR) indicate that the catalyst deactivation in the methane atmosphere is related to reduction of manganese ions, oxygen depletion of the catalyst surface and formation of manganese carbide s pecies. It is proposed that the XPS line of O1s electrons, Eb=528.6 eV should be assigned to the surface oxygen bound to the reduced Mn2+ ca tion with close oxygen vacancies, and lines of Mn2p(3/2) electrons, Eb =641 eV and C1s at 282.5 eV may be assigned to the surface manganese c arbide. (C) 1997 Elsevier Science S.A.