OP-II-3

OP-I-4No variation of the selectivity and only the products of the total oxidation weredetected during methane oxidation over Pd catalysts indicating an essentiallydifferent mechanism of oscillations in comparison with Ni and Co catalysts. Undercertain conditions the unusual behaviour, namely the phase shift between oscillationsof CO 2 and H 2 O concentrations was observed.The application of TG analysis in combination with on-line mass-spectrometryallowed to identify the variation of the Pd state during one cycle of the oscillationsand to correlate various states of Pd with a catalytic activity. It was demonstrated thatthe nature of the oscillations was connected not only with periodic oxidation andreduction of a Pd catalyst as suggested in earlier studies [3], but also carbondeposition-removal played an essential role in the mechanism of the oscillations. Itwas shown that exactly the accumulation of carbon in a Pd catalyst was the origin ofthe antiphase oscillations of CO 2 and H 2 O concentrations.The mathematical model was developed, which can simulate the dynamicbehaviour of the surface coverages, the concentrations of dissolved carbon andoxygen, catalyst temperature and partial pressures of reactants. It was demonstratedthat the reduction-oxidation processes of the Pd catalyst could lead to theappearance of the reaction rate self-sustained oscillations. Different modifications ofthe model, considering lateral interactions in the adlayer and/or carbon diffusion inthe deep layers of the catalyst will be discussed. It will be demonstrated how theproperties of the observed experimentally oscillations, namely the phase shiftbetween the oscillations of the reaction products CO 2 and H 2 O can help in thediscrimination of the mathematical models.AknowlegementThis work was supported by the Russian Foundation for Basic Researches (grant N 08-03-00364.References[1]. V.Yu. Bychkov, Yu.P. Tyulenin, M.M. Slinko, V.N. Korchak, Surface Science 603 (2009) 1680.[2]. M.G. Slinko “Catalysis in Industry” 1 (2006), 5.[3]. X.L. Zhang, C.S.-M Lee, D.M.P. Mingos and D.O. Hayward, Appl. Catal. A: Gen., 240 (2003) 183.44