oa Study of the redox properties of Co3O4/SiO2 catalysts

Background: Cobalt catalysts are employed in catalyzing several important and selective reactions, one of which is the Fischer-Tropsch (F-T) reaction that is becoming of particular importance because of its application in converting natural gas and coal into liquid fuels. Supported cobalt catalysts are used most commonly for F-T reactions because of their high activity, high selectivity for forming linear hydrocarbons, low activity for the water gas shift reaction and lower cost compared to noble metal catalysts. In these reactions, cobalt is supported on an silica oxide material. Objectives: To investigate the oxidation isotherms of 15 wt% Co3O4/SiO2 using the coulometric titration (CT) technique and to study the effect of the reduction gas type on the Co3O4/SiO2 isotherm. Methods: The sample to be analysed was placed in an alumina crucible that was then inserted into the centre of the YSZ tube (Fig.1). The sample was exposed to a gas mixture of H2-H2O for full reduction. Reduction also was carried out using a mixture of CO-CO2. 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 CT for reduced 15 wt% Co3O4/SiO2 (Fig. 2) exhibit two well-defined transitions. The first transition corresponds to the oxidation of Co to CoO and the second oxidation corresponds to CoO to Co3O4. Figure 3 represents the comparison of the Co/SiO2 oxidation isotherms at 873 K due to reduction using a mixture of H2/H2O and CO/CO2. The results indicate that the isotherms exhibit the same pattern but they are different in their P(O2) values. The transitions due to CO/CO2 reduction appear at lower P(O2) values. Conclusions: Coulometric titration results show that 15 wt% Co3O4/SiO2 exhibit two well-defined steps due to Co3O4/SiO2 full reduction. This indicates that there is no strong interaction between Co and silica.