Phase transformations in iron Fischer-Tropsch catalysts during temperature-programmed reduction

Temperature-programmed reduction using both carbon monoxide (CO-TPR) and hy drogen (H-2-TPR) was used to study the phase transformations in iron cataly sts. High-resolution transmission electron microscopy (HRTEM) and X-ray dif fraction (XRD) allowed us to follow the phase transformations in these iron catalysts during TPR. Two catalysts were used, a model supported catalyst with a nonporous silica support and a precipitated catalyst with no support . Under identical CO-TPR conditions, the supported and unsupported iron cat alysts behaved very differently. For the supported catalyst, three stages o f phase transformation could be identified: hematite to magnetite, magnetit e to iron carbide, and in the third stage significant carbon deposition acc ompanying further carburization. No carbide formation or significant carbon deposition was observed in the unsupported Fe catalyst, due to the presenc e of S impurities in the unsupported catalyst. Most importantly, the result s show that carbon deposition occurs in parallel with carbide formation, no carbon being seen in the catalyst that was not carbided. In these catalyst s, copper facilitates the reduction of iron oxide, especially the transform ation of hematite to magnetite. Segregation of copper from iron oxide cause s the hematite to magnetite transformation to shift to higher temperatures. No interfacial Fe-SiO2 phases could be detected during both CO-TPR and H-2 -TPR. The major role of the silica support is to prevent the sintering of t he Fe phases. (C) 2000 Academic Press.