[Zurück]

Ch. Rameshan, K. Anic, A. Bukhtiyarov, G. Rupprechter:
"Cobalt Oxide Model Catalyst as Alternative to Noble Metal Catalysts";
Poster: 48. Jahrestagung Deutscher Katalytiker, Weimar; 11.03.2015 - 13.03.2015.

Cobalt oxide has recently turned out to be a novel, highly active heterogeneous catalyst for many industrial important reactions. Most important, cobalt oxide-based catalysts hold a unique potential for replacing or reducing the demand for critical materials (noble metals and rare earth oxides) for oxidations. However the origin of the high cobalt oxide activity in catalysis is still not well explained.

We have chosen a thin cobalt oxide film with a well-defined structure as model catalysts for low temperature CO oxidation. The cobalt oxide film was grown on the surface of an Ir(100) single crystal by physical vapor deposition (PVD) in O2 background with subsequent post-oxidation [1-3]. With this routine a 8 ML thick Co3O4(111) or CoO(111) film were prepared. The characterization of the films has been performed by low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS).

Furthermore, we investigated the state of the active component and the interaction of CO, CO2 and gas mixture (CO+O2) with different surfaces of cobalt oxide by XPS, polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) and temperature programmed desorption (TPD) in the temperature range 200 K - 470 K and in the pressure from UHV to 100 mbar.

It has been shown that CO does not adsorb on Co3O4 films at the studied temperatures and pressures. Using XPS and PM-IRAS it has been shown that for both Co3O4(111) and CoO(111), depending on the conditions of CO adsorption, we obtained appearance of two different carbon-containing species which could be identified as carbonates and elementary carbon. Furthermore, in case of Co3O4(111) film CO adsorption leads to partial reduction of cobalt oxide, for higher gas pressures even at room temperature ("pressure gap"). The thermal stability of carbonates and elementary carbon formed during CO adsorption on Co3O4(111) film was investigated by TPD. It has been shown that both desorbed from the surface at 520 K -700 K as CO2, at the same time the Co3O4(111) film has been reduced to CoO(111). It could be shown by IRAS, that CO adsorbs in an on top geometry on CoO and upon subsequent reduction Co(0) is formed.

Furthermore the surface hydroxylation of Co3O4 and CoO and its influence on the adsorption and reaction properties was investigated. It could be shown that hydroxyl groups have an impact on the desorption mechanism on both oxides.

Experiments showed that it is possible to recover the film (initial state of composition and structure) after CO adsorption and desorption experiments by an oxidation treatment at 550 K and subsequent annealing in UHV to 670 K.