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

Abstract

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 in situ 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. In situ investigation of the catalysts indicated wetting of the TiO2 support by CoO(x) and partial encapsulation of metallic Co by TiO(2-x).

X-ray spectroscopic and scattering methods applied to the characterisation of cobalt-based Fischer–Tropsch synthesis catalysts

“…This remains to be fully understood, but is thought to be due in part to the interfacial CoO/TiO 2 sites present in the materials and to the TiO 2 decorating (and blocking reactant access to) some of the Co surface area in the fully reduced material obtained only at higher temperatures where oxide migration also occurs [98]. ]…”

The CO2 dissociation mechanism on the small copper clusters—the influence of geometry

“…The barrier for C-O dissociation on anionic Cu 8 cluster amounts to approximately 1.5 eV, while it is more favorable for anionic Cu 7 cluster, where it amounts to 1.0 eV. This is caused by the different geometry of the active site-the dissociation occurring at the edge of the cluster results in the raise of the energetic barrier, as it was already observed for the Cu 6 cluster.…”

“…To conclude, the transformation between structures is easy, especially for the anionic Cu 7 and Cu 8 clusters, where the relative energies do not exceed 0.15 eV. For anionic Cu 6 cluster, such transformation needs at least 0.45 eV and can occur easier for neutral cluster, where only 0.24-0.29 eV is required to overcome energetic minimum.…”

Co–Rh Nanoparticles for the Hydrogenation of Carbon Monoxide: Catalytic Performance Towards Alcohol Production and Ambient Pressure X-Ray Photoelectron Spectroscopy Study

“…The F-T synthesis was performed in a home-built fixed bed flow reactor at 20 bar as described elsewhere [17,18]. The catalysts were mixed with SiC and then loaded into the center of a 1/4 00 diameter stainless steel reactor tube.…”