Fischer-Tropsch synthesis (FTS) has been considered a promising way to produce clean liquid fuels from syngas (H2+CO) derived from coal/biomass gasification or methane reforming. Iron-based catalysts are highly promising for the FTS due to their high activity and low cost. Furthermore, iron-based catalysts have particular merits when they are used in the FTS using hydrogen-deficient syngas (H2/CO≤1.0), due to their potential activity for a water-gas shift (WGS) reaction. With low-temperature FTS (200-280 ℃), a precipitation technique is known to be the most practical method for preparation of iron-based catalysts. The precipitated iron-based catalysts are usually composed of hematite (α-Fe2O3) in an as-prepared state, which are inactive for the FTS. Thus, the as-prepared precipitated iron-based catalysts are conventionally reduced and carburized into iron carbides, active phases for the FTS, in a CO-containing atmosphere prior to the reaction. But, it is potentially desirable to use the as-prepared catalysts without an extra pre-activation process from the view point of operation efficiency and facility simplification.
Ferrihydrite is a poorly crystalline iron-oxyhydroxide with the general formular of FeOOH·nH2O, which may form in iron-containing aqueous environments. The ferrihydrite has great potential as a catalyst or a catalyst support due to its high surface area, large pore volume, and small crystallite size. Recently, some studies have reported the possibility of using ferrihydrite as a catalyst for the FTS. However, detailed studies on the ferrihydrite as a FTS catalyst such as reduction/carburization behavior analyses and activation studies have not yet been performed. Therefore, there is a need for further investigation into the ferrihydrite-based FTS catalysts.
In this study, the author developed highly reducible and carburizable ferrihydrite-based FTS catalysts. The ferrihydrite-based catalysts were prepared with chemical promoters, Cu and K, and a structural promoter, SiO2 through a combination of a co-precipitation technique and a wet impregnation method . Firstly, temperature-programmed reduction using H2 (H2-TPR) and CO (CO-TPR) was carried out to investigate the reduction and carburization behavior of ferrihydrite-based Fe/Cu/K/SiO2 catalysts for use in FTS. Unlike pure ferrihydrite, the ferrihydrite-based catalysts did not pass through the intermediate decomposition step of ferrihydrite (Fe9O2(OH)23) into thermal stability induced by SiO2. For the ferrihydrite-based catalysts, the reduction of ferrihydrite into magnetite occurred in two stages because the reduction promoter, Cu, is not homogeneously distributed on the catalyst surfaces. The Cu-rich sites are likely to be reduced in the first stage, and the Cu-lean sites may be reduced in the second stage. After the ferrihydrite is reduced to magnetite, the reduction process of magnetite was similar to that for conventional hematite-based FTS catalysts: ‘magnetite → metallic iron’ and ‘magnetite → wustite (FeO) or fayalite (Fe2SiO4) → metallic iron’ in the H2 atmosphere; ‘magnetite → iron carbides’ in the CO atmosphere.
Secondly, FTS was carried out over ferrihydrite-based catalysts activated by different reducing agents: syngas (H2+CO), CO and H2. The syngas activation successfully changed the ferrihydrite-based catalysts into an active and stable catalytic structure with χ-carbide (Fe2.5C) and ε`-carbide (Fe2.2C). The crystal structure of the catalysts obtained by syngas activation was similar to the structure obtained by CO activation; this similarity was probably due to the peculiar reduction behavior of the ferrihydrite-based catalysts, which exhibit much greater reducibility in CO atmosphere than in H2 atmosphere. The performance of the catalysts activated by syngas was much higher than the performance of the catalysts activated by H2 and was comparable to the performance of the catalysts activated by CO.
Finally, the author prepared ferrihydrite- and hematite-based Fe/Cu/K/SiO2 catalysts, and evaluated the catalytic performance of both catalysts either with or without carrying out pre-activation treatment using syngas. The author found a novel characteristic of ferrihydrite-based catalysts, that is, spontaneous activation of ferrihydrite-based catalysts during the FTS (1.5 MPa, H2/CO=1.0), which is unlike conventional hematite-based catalysts. Even though no activation pre-treatment was carried out, the ferrihydrite-based catalysts showed a high and stable CO conversion about 80% during the entire reaction time (~114 h) after a short induction period of about 20 h. The overall performance of spontaneously activated ferrihydrite-based catalysts was comparable to that of pre-activated catalysts. In contrast, the conventional hematite-based catalysts displayed a low CO conversion below 20% when they are not pre-activated. This strongly demonstrates that the ferrihydrite-based catalysts are highly advantageous for industrial FTS processes because the as-prepared catalysts can be used for the FTS without requiring an extra pre-activation process.