Interaction of water with 1% Li/MgO: dc conductivity of Li/MgO catalyst for methane selective activation

Abstract

The dc conductivity of pure magnesium oxide and lithium-doped magnesium oxide has been investigated using the four-electrodes method in He flow between 673 and 1173 K. The influence of oxygen and water vapour on the surface conductivity was also studied. For the lithium-doped magnesium oxide, the activation energy of the conduction was 25 kcal mol^–1 at low temperature (673–873 K) and 49 kcal mol^–1 at high temperature (973–1173 K). The effect of replacing water with heavy water on the conduction activation energy of lithium-doped magnesium oxide was observed. The activation energy when using D₂O was 8 kcal mol^–1 higher than when using H₂O in the high-temperature range, while the isotopic effect was not significant in the low-temperature region. Temperature-programmed desorption (TPD) of water was performed on magnesium oxide and lithium-doped magnesium oxide. Remarkable hydrogen release was observed at temperatures above 873 K on lithium-doped magnesium oxide.

A vehicle mechanism for proton conductivity by way of surface OH^– was suggested at low temperature, while holes (O^–) generated from hydrogen evolution in which O—H dissociation becomes the most important step were proposed as charge carriers at high temperature. A mechanism where multi O—H bonds rupture to produce a hole was proposed to explain the extensive isotope effect. A probable mechanism for the formation of the active sites for methane activation at high temperatures is discussed.