Effect of surface treatment investigated by gas adsorption on metal-oxide particles

Abstract

Studies of surface gas adsorption are useful because knowledge of the physical/chemical phenomena occurring on various adsorbents, including MgO, is expected to provide important information to study the fine mechanisms of gas molecules on surface. Quite often, however, surface protecting gases, such as water and hydroxylates, inhibit the interaction of gas molecules on metal oxide surfaces. However, no systematic investigations have been made on the methods of removing the protecting atomic layers. To address these questions, cubic shapes of MgO particles were synthesized by using a combustion method with RF power in air, and the functions and the species of protecting layers were investigated by using methane-adsorption isotherms. The methane adsorption studies on MgO(100) powders were carried out under different heat-treatment conditions: a set of pure powders that had been exposed for weeks in air after preparation, and another set of heat-treated powders that had been kept in a vacuum lower than 10(-6) Torr at 750 degrees C for 48 hours and never in contact with air after that. Methane isotherms measured below 77 K coherently revealed that the heat treated samples showed additional isotherm steps, indicating an existence of a clean surface. The dominant species of protecting molecules on the surface identified by using a residual gas analyzer (RGA) is water, which evaporates near 100 degrees C.