MICROSTRUCTURAL AND SURFACE-AREA DEVELOPMENT DURING HYDROGEN REDUCTION OF MAGNETITE

Pure, slightly promoted and heavily promoted magnetites were reduced isothermally in hydrogen at temperatures in the range 598-798 K. The microstructural development of the catalyst changed with reduction temperature and promoter addition. At lower temperatures there was an increase 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. Activation energy values of 69, 88 and 118 kJ mol-1 were measured for the pure, slightly and heavily promoted magnetite, respectively. In pure magnetite, 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 surface at low temperatures. Increasing promoter addition also caused a decrease 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 the main effect of alkali addition is manifested in the decrease in the heat of adsorption of carbon monoxide and in the inhibition of the reaction C(ads) --> C(deposited), keeping the carbon atoms in the adsorbed state for longer periods of time and reducing coke deposition.