REOXIDATION AND DEACTIVATION OF SUPPORTED COBALT FISCHER-TROPSCH CATALYSTS

The role of water in the deactivation of Al2O3-supported cobalt Fische r-Tropsch catalysts has been studied at different water partial pressu res. In addition, model studies at nonreacting conditions using H2O/H- 2 feeds were carried out in order to study the possible reoxidation of cobalt by water. High-pressure gravimetry, temperature-programmed red uction (TPR), and X-ray photoelectron spectroscopy (XPS) were used as characterization methods in the model studies. The kinetic studies sho wed 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 H-2. Only a sm all amount of reoxidation could be observed when H-2 was present, even at high H2O/H-2 ratios. However, XPS studies indicated significant su rface oxidation of cobalt at a lower H2O/H-2 ratio. Surface oxidation or oxidation of highly dispersed cobalt phases is therefore concluded to be responsible for the observed deactivation. The possibility of ch anges in the phase distribution of the catalysts caused by the presenc e of high water partial pressures is discussed on the basis of TPR stu dies of H2O/H-2- and H2O/He-treated catalysts.