MICROSTRUCTURAL AND SURFACE-AREA DEVELOPMENT DURING HYDROGEN REDUCTION OF MAGNETITE

Pure, slightly promoted and heavily promoted magnetites were reduced i sothermally in hydrogen at temperatures in the range 598-798 K. The mi crostructural development of the catalyst changed with reduction tempe rature and promoter addition. At lower temperatures there was an incre ase in the surface area and a decrease in the iron crystallite size of the reduced magnetite. In general, a narrower pore size distribution and smaller pore size diameters developed at lower temperatures. Activ ation energy values of 69, 88 and 118 kJ mol-1 were measured for the p ure, slightly and heavily promoted magnetite, respectively. In pure ma gnetite, the heat of carbon monoxide adsorption and carbon deposition per unit surface area decreased with decreasing reduction temperature. This effect can be explained by oxygen remaining at the catalyst surf ace at low temperatures. Increasing promoter addition also caused a de crease in the beat of carbon monoxide adsorption and carbon deposition per unit surface area. However, these results can not be explained by either the presence of oxygen sites at the surface or by the decrease in surface area fraction covered by iron. Our results suggest that th e main effect of alkali addition is manifested in the decrease in the heat of adsorption of carbon monoxide and in the inhibition of the rea ction C(ads) --> C(deposited), keeping the carbon atoms in the adsorbe d state for longer periods of time and reducing coke deposition.