Evidence of Highly Active Cobalt Oxide Catalyst for the Fischer–Tropsch Synthesis and CO₂ Hydrogenation

Hydrogenations of CO or CO₂ are important catalytic reactions as they are interesting alternatives to produce fine chemical feedstock hence avoiding the use of fossil sources. Using monodisperse nanoparticle (NP) catalysts, we have studied the CO/H₂ (i.e., Fischer–Tropsch synthesis) and CO₂/H₂ reactions. Exploiting synchrotron based <i>in situ</i> characterization techniques such as XANES and XPS, we were able to demonstrate that 10 nm Co NPs cannot be reduced at 250 °C while supported on TiO₂ or SiO₂ and that the complete reduction of cobalt can only be achieved at 450 °C. Interestingly, cobalt oxide performs better than fully reduced cobalt when supported on TiO₂. In fact, the catalytic results indicate an enhancement of 10-fold for the CO₂/H₂ reaction rate and 2-fold for the CO/H₂ reaction rate for the Co/TiO₂ treated at 250 °C in H₂ versus Co/TiO₂ treated at 450 °C. Inversely, the activity of cobalt supported on SiO₂ has a higher turnover frequency when cobalt is metallic. The product distributions could be tuned depending on the support and the oxidation state of cobalt. For oxidized cobalt on TiO₂, we observed an increase of methane production for the CO₂/H₂ reaction whereas it is more selective to unsaturated products for the CO/H₂ reaction. <i>In situ</i> investigation of the catalysts indicated wetting of the TiO₂ support by CoO_<i>x</i> and partial encapsulation of metallic Co by TiO<sub>2–<i>x</i>