Magnesium oxide as a catalyst support: The influence of chlorine

Abstract

The properties of MgO when used as a support material for Ru-Au catalysts have been investigated by thermal analysis, chemical analysis, surface area measurements and X-ray diffraction. During impregnation the support undergoes a bulk hydration but heating to 673 K restores the oxide. In a sample impregnated by water only, the dehydration occurs at 651 K and the surface area increases from 15 to> 300 m2 g-1, due to the formation of small pores. Great changes in the DTA peak temperature and in the surface area after dehydration are found in the catalysts and in MgO impregnated with HCl solutions. It is shown that these changes are accurately correlated to the amount of chlorine which remains in the solid (deriving from the metal precursor compounds or HCl). The formation of basic magnesium chlorides, shown by X-ray diffraction, seems to favour an easier release of water (i.e., lower DTA peak temperatures and lower apparent activation energies) and to reduce the formation of small pores.