Abstract
CO hydrogenation has been studied on cobalt foils as model catalysts for Fischer–Tropsch (FT) synthesis. The effect of pretreatment (number of calcinations and different reduction times) for cobalt foil catalysts at 220 °C, 1 bar, and H2/CO = 3 has been studied in a microreactor. The foils were examined by scanning electron microscopy (SEM). It was found that the catalytic activity of the cobalt foil increases with the number of pretreatments. The mechanism is likely an increase in the available cobalt surface area from progressively deeper oxidation of the foil, supported by surface roughness detected by SEM. The highest FT activity was obtained using a reduction time of only 5 min (compared to 1 and 30 min). Prolonged reduction caused the sintering of cobalt crystallites, while too short of a reduction time led to incomplete reduction and small crystallites susceptible to low turn-over frequency from structure sensitivity. Larger crystals from longer reduction times gave increased selectivity to heavier components. The paraffin/olefin ratio increased with the increasing number of pretreatments due to olefin hydrogenation favored by enhanced cobalt site density. From the results, it is suggested that olefin hydrogenation is not structure sensitive, and that mass transfer limitations may occur depending on the pretreatment procedure. Produced water did not influence the results for the low conversions experienced in the present study (<6%).
Highlights
Fischer–Tropsch synthesis (FTS), i.e., CO hydrogenation, is divided into high-temperature and low-temperature synthesis
Regeneration by oxidation at 550 ◦ C after deactivation during the catalytic reaction restored the initial high activity. They concluded that the pretreatment procedure with reduction/oxidation cycles modified the surface of the cobalt foil by developing a CoO layer of a characteristic size and dispersion on the metallic surface of the foil
Series I is based on three separate foils using reduction times of 1, 5, and 30 min, respectively, one foil is used in series with sequential change in reduction times and intermittent while one foil is used in series II with sequential change in reduction times and intermittent FT
Summary
Fischer–Tropsch synthesis (FTS), i.e., CO hydrogenation, is divided into high-temperature and low-temperature synthesis. Regeneration by oxidation at 550 ◦ C after deactivation during the catalytic reaction restored the initial high activity They concluded that the pretreatment procedure with reduction/oxidation cycles modified the surface of the cobalt foil by developing a CoO layer of a characteristic size and dispersion on the metallic surface of the foil. During CO2 hydrogenation, Jnioui et al observed that the pre-oxidation of a cobalt foil increased the rate for CO or CH4 formation by 2 or 3 orders of magnitude [12]. They suggested that the increased rates were due to the creation of surface defects by the alternate oxidations and reductions.
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