Reoxidation and Deactivation of Supported Cobalt Fischer−Tropsch Catalysts

Abstract

The role of water in the deactivation of Al2O3-supported cobalt Fischer−Tropsch catalysts has been studied at different water partial pressures. In addition, model studies at nonreacting conditions using H2O/H2 feeds were carried out in order to study the possible reoxidation of cobalt by water. High-pressure gravimetry, temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS) were used as characterization methods in the model studies. The kinetic studies showed significant deactivation when water was added to the feed in fixed-bed reactor experiments. In the model studies, a large extent of bulk cobalt reoxidation was only observed in the absence of H2. Only a small amount of reoxidation could be observed when H2 was present, even at high H2O/H2 ratios. However, XPS studies indicated significant surface oxidation of cobalt at a lower H2O/H2 ratio. Surface oxidation or oxidation of highly dispersed cobalt phases is therefore concluded to be responsible for the observed deactivati...