Effect of chromium on magnetite formation

Abstract

Chromium-doped magnetite is the traditional catalyst used in the high temperature shift reaction in industrial processes. This work reports a method for preparing this solid, by heating chromium-doped iron(III)hydroxoacetates (IHA), prepared both by coprecipitation and by impregnation. This method provides the production of the catalyst in its active form avoiding the step of activation in industrial processes. It was noted that the presence of chromium affects magnetite formation as well as its characteristics. IHA containing chromium produces magnetite at higher temperature than does the pure IHA. Chromium-doped magnetites have higher surface areas, are less crystalline, have lower Fe(II)/Fe(III) ratios and are catalysts more active than those obtained from plain IHA. These effects depend on the preparation method of the precursor. It was also found that the impregnated sample is made of aggregates of a few crystals whereas the coprecipitate is made of smaller polycrystalline particles. In addition, the coprecipitation method leads to a more even distribution of chromium in solids. These observations are consistent with the best performance shown by the catalyst prepared by coprecipitation.