Surface Studies of Lanthanum-Sodium Catalysts for the Oxidative Coupling of Methane Modified with Group IVA elements

Abstract

The pioneeristic paper of Keller and Bhasin showed the possibility of direct conversion of methane to higher hydrocarbons—ethylene and ethane—through the so-called Oxidative Coupling of Methane (OCM). Nowadays efficient catalysts have been proposed in the technical literature that allows feasibility studies of this potential technology with respect to the consolidated steam reforming of ethane to produce ethylene. However, fundamental studies are still required to improve the performance of the catalysts. Attention was focused on the characterization of surface basicity intended as negative charge availability. This has been investigated in a family of oxidic catalysts, composed of Group IVA (Ti, Zr) elements, lanthanum and sodium. Previously acquired evidence indicates that the active elements are mainly lanthanum and sodium, so that general considerations can be derived almost independently of the tetravalent component. This chapter describes how catalytic performances were related to surface chemical composition using the partial charge of atoms, obtained according to Sanderson's postulate of the electronegativity equalization, and how the surface was investigated by means of X-ray Photoelectron Spectroscopy (XPS). Detailed knowledge of the surface, particularly of the negative charge distribution, and the possibility of its simulation with more sophisticated approaches—for example, quantum chemistry calculations—can improve the comprehension and the prediction of the performance of a given material.