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

The first and repeated isotherms were measured in hydrogen reduced mag netites, using nitrogen, carbon monoxide and carbon dioxide as adsorba tes. Based on a modification of the method developed by Emmett and Bru nauer (P.H. Emmett and S. Brunauer, J. Am. Chem. Soc., 59 (1937) 310-3 15). we were able to estimate the fraction of the total surface area c overed by iron. alkali and oxygen sites. In pure magnetite, as the red uction temperature increases, iron covers a higher percentage of the t otal surface area at the expense of oxide containing sites. By contras t, in promoted magnetites, the fraction of the surface covered by iron remains constant with reduction temperature. There is, however, an in crease in the alkali surface area fraction at the expense of the oxyge n-containing sites. Volumetric measurements suggest that in pure magne tite the adsorption of carbon monoxide is linear and that one of every two adsorbed carbon monoxide molecules remains chemisorbed on the iro n. The latter indicates that a ''bridge'' type of adsorption predomina tes over chemisorption. Alkali promotion seems to inhibit the developm ent of the linearly adsorbed carbon monoxide molecule, and enhances th e amount of carbon monoxide chemisorbed. This effect increases with in creasing reduction temperature and promoter addition.