O(-) identified at high temperatures in CaO-based catalysts for oxidative methane dimerization

MgO and CaO are catalytically active for the oxidative dimerization of methane. The basic step involves a transfer of charge between adsorbed CH4 molecules and the catalyst surface. The catalytic activity has been ascribed to O - states. A new technique, Charge Distribution Analysis has been used to study undoped, NaOH- and La 2 O 3 - promoted CaO catalysts and CaO single crystals. CDA allows to obtain information about mobile charge carriers. CDA utilizes the fact that a dielectric, when placed in an electric field gradient, experiences a force. This force depends on the dielectric polarization which contains contributions from the ideal crystal, from local defects that rotate, from charges that diffuse, and from surface charges. In combination with magnetic susceptibility data CDA allows to unambiguously determine that the mobile charge carriers that appear in the CaO-based catalysts above 550-600 °C are O' states. Their precursors are preoxy anions, O2 2- .