III International Conference

PP-I-1more time in the oxidised state than in the reduced state. Simultaneous measurements ofconcentrations in the gas phase revealed that the oxidised state was characterised by loweractivity of methane oxidation, while the extent of methane conversion was much greater onthe reduced surface. CO and H 2 concentrations reached their maxima over the completelyreduced surface. These thermokinetic oscillations have much in common with oscillatorybehaviour which was observed earlier over a nickel foil [1]. It can be supposed that similar toNi catalysts thermokinetic oscillations over Co catalysts could arise from periodic oxidationand reduction of the catalyst surface, the variation of the selectivity of the process and thecompetition of reactants for free active sites [2]. However there are also some differencesbetween oscillatory behaviour over Ni and Co catalysts. Oscillations over the cobalt foilappeared at higher temperatures (860-950 o C), and had longer periods, in comparison withoscillatory behaviour which was observed earlier over a nickel foil (650-860 o C) [1]. Theapplication of TGA in combination with TPR experiments revealed the differences in nickeland cobalt redox properties which are responsible for the variation in the properties of theoscillations. It was demonstrated that metallic cobalt oxidised much more extensively thannickel, and pre-oxidised Co needed much more time for reduction by H 2 or CH 4 .Forced oscillations could be obtained in a low temperature region (700-860 o C), if barechromel and alumel wires were spot-welded separately to the cobalt foil. It was shown that inthis case the chromel wire induced oscillatory behaviour of the whole cobalt foil. Complicatedmixed mode oscillations detected at higher temperatures were shown to be the result of thecoupling of high frequency oscillations produced by the unshielded chromel–alumelthermocouple and low frequency oscillations originating over the cobalt foil.The coupling of oscillations from two various foils has been studied in a quartz crossshapedreactor. The antiphase temperature oscillations were the result of the coupling of theNi and the Co foils, while in the case of two Ni foils completely synchronous oscillationshave been observed indicating that the strength of coupling via the gas phase was very strong.AcknowledgementThis work was supported by the Russian Foundation for Basic Researches(grant N 05-03-33128).References1. V. Yu. Bychkov, Yu. P. Tyulenin, V. N. Korchak, E. L. Aptekar, Appl.catal. A: Gen., 3042(2006) 21.2. M.M. Slinko, V.N. Korchak, N.V. Peskov, Appl. Catal. A: Gen., 303 (2006) 2588