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Abstract

Background: Cobalt catalyzed Fischer-Tropsch synthesis (FTS) is a process for converting natural gas to liquid fuels. Supported cobalt catalysts are common for F-T reactions because of their high activity and selectivity for forming linear hydrocarbons. Oxide supporters affect the catalytic properties of cobalt significantly. Co and oxide support interact and lead to irreversible deactivation of the catalyst. Promoters, e.g., Pd, Ru and Pt are added to the cobalt oxide supported catalysts for various reasons, such as to increase the reducibility of Co and to act as a source for hydrogen spillover. Objectives: To investigate the oxidation isotherms of cobalt and cobalt supported oxide catalysts using coulometric titration (CT) and to examine the effect of the rhenium (Re) promoter on the reducibility of cobalt on an alumina catalyst. Methods: The sample was exposed to a gas mixture containing 10% H2, 3% H2O and 87% N2 for full reduction. The P(O2) was then established by equilibrium between H2 and H2O: P(O2)½ =K-1 * P(H2O)/P(H2) Oxygen was then pumped into the cell electrochemically by passing a current through the Pt electrodes (Fig. 1). The open-circuit potential across the electrodes (V) was measured in order to calculate P(O2) from the Nernst equation: V= RT/4F Ln PO2/0.21 Results: The oxidation isotherms obtained from the CT for reduced bulk cobalt oxide (Co3O4) (Fig. 2) and promoted catalyst (Fig. 3) exhibit two well-defined transitions. The first transition is corresponding to the oxidation of Co to CoO and the second oxidation is corresponding to CoO to Co3O4. On the other hand, the oxidation isotherms of reduced 15 wt% Co3O4/Al2O3 exhibit one transition, which is corresponding to CoO to Co3O4 (Fig. 4). It is clear that the Co/alumina compound has a limited degree of reduction, which may be due to the strong interaction between Co and alumina resulting in a non-reducible form of a Co-alumina compound (Figs. 5-7). This explains that rhenium enhanced the reduction of the cobalt/alumina catalyst to a metallic cobalt but it did not have an effect on the extent of reduction of the cobalt oxide Co3O4 to CoO. Conclusions: Coulometric titration and thermal gravimetric analysis results show that a rhenium promoter has facilitated the reduction of Co3O4 to Co (Fig. 8).

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/content/papers/10.5339/qfarf.2012.EEP1
2012-10-01
2019-10-20
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