EFFECT OF HYDROGEN REDUCTION TEMPERATURE ON THE SURFACE-CHEMISTRY OF PURE AND PROMOTED MAGNETITE

The first and repeated isotherms were measured in hydrogen reduced magnetites, using nitrogen, carbon monoxide and carbon dioxide as adsorbates. Based on a modification of the method developed by Emmett and Brunauer (P.H. Emmett and S. Brunauer, J. Am. Chem. Soc., 59 (1937) 310-315). we were able to estimate the fraction of the total surface area covered by iron. alkali and oxygen sites. In pure magnetite, as the reduction temperature increases, iron covers a higher percentage of the total surface area at the expense of oxide containing sites. By contrast, in promoted magnetites, the fraction of the surface covered by iron remains constant with reduction temperature. There is, however, an increase in the alkali surface area fraction at the expense of the oxygen-containing sites. Volumetric measurements suggest that in pure magnetite the adsorption of carbon monoxide is linear and that one of every two adsorbed carbon monoxide molecules remains chemisorbed on the iron. The latter indicates that a ''bridge'' type of adsorption predominates over chemisorption. Alkali promotion seems to inhibit the development of the linearly adsorbed carbon monoxide molecule, and enhances the amount of carbon monoxide chemisorbed. This effect increases with increasing reduction temperature and promoter addition.