Contributions of phase composition and defect structure to the long term stability of Li/MgO catalysts

The phase composition and defect structure of the system Li2O-MgO was investigated in terms of the long term stability of Li/MgO catalysts. The Li content was varied from 0 to 7 mol.%. Pure Li center dot MgO solid solutions were prepared via a special washing procedure. Li contents below 0.04 wt.% were stabilized within the MgO host lattice, whereas higher Li contents were found to segregate as Li2O and Li2CO3 phases. The catalytic activity in the oxidative coupling of methane was found to decay for all catalysts over a period of 19 h on stream, accompanied by a loss Of Li as LiOH. Li in the Li center dot MgO solid solution was found to be more stable in the lattice than in the surface region of the solid. However, impedance measurements on transition metal stabilized Li/MgO catalysts indicated that even the Li ions within the Li center dot MgO solid solution are not sufficiently stabilized. Thus, neither the Li compounds nor the dissolved Li ions within the Li/MgO solution seem to be truly stable at 750 degrees C under catalytic conditions.