Structure/function relationships in Nd2O3-doped MgO catalysts for the methane coupling reaction

MgO doped with Nd2O3 has been evaluated as a catalyst for the oxidative coupling of methane (OCM) reaction, It is demonstrated that significant improvements in methane conversion efficiency and selectivity to C-2 products (as compared to pure MgO) can be achieved. The level of catalytic improvement is shown to be dependent on the Nd2O3 concentration and the precise catalyst preparation method. Detailed transmission electron microscopy studies have been performed on the Nd2O3/MgO catalysts prepared by an impregnation route and by a number of coprecipitation methods. Six distinct neodymia morphologies on the MgO support have been identified. These are (i) crystalline thin films, (ii) Nd2O3 crystallites attached to the MgO support, (iii) isolated Nd2O3 needles, (iv) epitaxial microclusters, (v) disordered thin films, and (vi) isolated clusters. The relative proportions of these Nd2O3 morphologies in the catalysts are also shown to depend upon the neodymia loading and the preparation route. Cross-correlation between microstructural observations and catalytic performance data for these materials enable us to deduce that the disordered neodymia films, which probably contain some dissolved Mg2+ ions, are largely responsible for the improvement in OCM performance. (C) 1997 Academic Press.