Heats of argon adsorption on microporous magnesium oxide powders

Abstract

Isosteric heats of adsorption of argon on MgO smoke and on five samples produced from Mg(OH)₂ have been determined from adsorption isotherms. Heats of adsorption of the isolated atom at various positions on different surface planes, in very small intercrystallite pores and at position of “contact” of adjacent crystals, have been calculated theoretically. The theoretical curve of isosteric heat as a function of adsorbate concentration on a plane (100) surface is close to the experimental curve for the MgO smoke above ∼20 % coverage. The microcrystalline oxides prepared from Mg(OH)₂ yield higher heats of adsorption. From the theoretical calculations this is not explicable as adsorption on surfaces other than (100) nor in terms of “surface heterogeneities”, but is consistent with adsorption in very small pores or at points of crystal-crystal contact, where the adsorbate atom is close to more lattice ion neighbours than on the plane surface. The variations in heats of adsorption between the different samples are consistent with the changes in microstructure brought about by different calcination treatments.