Autonomous and forced oscillations during methane oxidation over cobalt catalysts

Oscillatory behaviour during methane oxidation over a cobalt foil has been studied using on-line mass-spectrometry and video recording of the colour of the catalyst surface. It was demonstrated that during oscillatory behaviour, periodic transitions of the catalyst surface from an oxidised state (dark colour) to a reduced state (light colour) occurred together with the variation of the catalyst temperature. Oscillations over the cobalt foil appeared at higher temperatures (860-950 degrees C) and had longer periods, in comparison with oscillatory behaviour which was observed earlier over a nickel foil. The application of TGA in combination with TPR experiments revealed the differences in nickel and cobalt redox properties which are responsible for the variation in the properties of the oscillations. Forced oscillations could be obtained in a low temperature region (700-860 degrees C) if bare chromel. and alumel wires were spot-welded separately to the cobalt foil. It was shown that in this case the chromel wire induced oscillatory behaviour of the whole cobalt foil. Complicated mixed mode oscillations detected at higher temperatures were shown to be the result of the coupling of high frequency oscillations produced by the unshielded chromel-alumel thermocouple and low frequency oscillations appearing over the cobalt foil. (c) 2007 Elsevier B.V. All rights reserved.