SURFACE CHARACTERIZATION OF CARBON-SUPPORTED IRON CATALYSTS FOR CARBON-MONOXIDE HYDROGENATION - INFLUENCE OF CATALYST DISTRIBUTION AND COAL MINERALS

The iron catalysed carbon monoxide hydrogenation (Fischer-Tropsch synthesis) has been studied by on-line combination of a chemical microreactor working under technical conditions with surface analytical methods (photoelectron spectroscopy, secondary ion and thermal desorption mass spectrometry, ion scattering spectroscopy, electron and x-ray microscopy). In contrast to other sample preparation methods, deposition of Fe(CO)5 onto carbon substrates leads to a surface shell with highly dispersed iron particles. Metallic iron, which is the most important species in the reaction, is formed by reduction of the samples in H-2 atmosphere at elevated temperature. A reduction time of 16 h is required to achieve complete reduction of residual FeO at the interface between iron and the carbon support. The activity and selectivity of the catalyst is mainly determined by the existence of a mobile potassium phase which segregates to the surface. A high potassium concentration shifts the selectivity towards long chains, a low concentration lowers the total activity. Degradation of the catalyst during the reaction mainly occurs by two processes: the gradual oxidation of metallic iron to FeO and sintering of iron to larger particles. Iron oxidation can be reversed by reactivation in H-2. Sintering cannot be prevented completely, but seems to be delayed by a high Al content in the carbon substrate.