INTERACTION OF WATER WITH 1-PERCENT LI/MGO - DC CONDUCTIVITY OF LI/MGO CATALYST FOR METHANE SELECTIVE ACTIVATION

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.