The influence of the reaction medium of oxidative methane coupling on the nature of bulk defects of the Li/CaO system

Abstract

The relationship of the catalytic activity of Li/CaO (0 ⩽ Li ⩽ 1.7 at%) in CH4 oxidative coupling with the defectness of the matrix has been studied. It was shown that the activity of undoped CaO increases with the rise of concentration of anion and cation vacancies. The concentration and nature of defects in the pure CaO are independent of the composition of the gas (O2, N2O, CH4 or reaction mixture). The incorporation of Li ions into the CaO lattice and also the treatment of Li/CaO with the reaction mixture lead to a change of the character of bulk defects and catalytic properties of the oxide. The signals from O3−, CO2− and LiCO3− centers are detected in the EPR spectra of γ-irradiated Li/CaO catalysts after the contact with the CH4-O2 mixture. Such defects are not observed when a CH4-N2O mixture is used. Simultaneously, the replacement of O2 by N2O in the reaction mixture is accompanied by the decrease of the CO formation rate and the increase of activity in the C2-product formation.