The relationship between catalyst morphology and performance in the oxidative coupling of methane

A detailed study of the oxidative coupling of methane over magnesium oxide and lithium-doped MgO catalysts is presented. The morphology of a number of different catalysts has been examined by detailed transmission electron microscopy and the results have been correlated with catalyst performance, in particular selectivity to C2 hydrocarbons. Three samples of magnesium oxide have been prepared by different methods. Magnesium oxides prepared from the hydroxide, (ex OH), and from burning magnesium ribbon in air show similar morphology, exposing largely {100} planes; they also show very similar catalytic selectivity and specific activity. The ribbon residue material, however, has a cube length which is greater than that of the ex OH material by a factor of 5–10. Steps and corner sites are therefore present in much greater density on the ex OH sample than on the ribbon residue, and, since catalyst performance is unchanged, it is clear that these sites play no significant part in the catalysis over these materials. The active site is therefore located on the planar {100} surfaces. The most selective magnesium oxide catalyst was prepared by thermal decomposition of magnesium hydroxycarbonate and exposed a greater proportion of higher index mean crystal planes, e.g., {l11}, than the less selective forms of magnesium oxide. It is suggested that an additional selective site is present in this form of magnesium oxide, with density related to morphology but not directly to surface area, perhaps a "bottom step" site. The morphology/performance relationship has also been examined for lithium-doped magnesium oxide catalysts. In agreement with previous studies, addition of lithium causes a loss of surface area and of the morphology characteristic of the precursor magnesium oxide; the grain size also increases, grain boundary dislocations become evident, and dislocations are also observed in the bulk of the grains. These are immobile and of the type 12〉110〈, pinned by the presence of lithium ions. The emergence of a dislocation at the surface of the crystallite provides a locus for [Li+O−] centres, thought to be the active site in methyl radical generation in methane coupling. Similar dislocations are observed in Au/MgO catalysts, which are much less selective to C2 hydrocarbons than is pure MgO.