Unstable and Oscillatory Behaviour in Heterogeneous Catalysis

With two examples, the oxidation of H2 and CO on Pt carrier catalysts, it is shown that special states of chemisorption and kinetics of surface processes can create periodicity and bistable behaviour of catalytic reactions. The (nonisothermal) oscillations of the H2 oxidation can be traced back to the superposition of a slow oxidation-reduction process under participation of an active and an inactive (blocking) chemisorption species of oxygen, in agreement with TDS measurements. By means of a stability analysis, based on the linearized balance equations, the main features of the oscillatory behaviour can be explained. — With the CO oxidation oscillations under isothermal conditions have been studied that appear as precursors to a quenching of the reaction. A periodic change of the CO coverage (blocking) of the catalyst surface was demonstrated. For the formation of the CO coverage via CO clusters on the surface, in the sense of dissipative structures, a mechanism with co-operation of the chemisorption process and the LH-reaction is shown up. The periodic reactivation of the surface is attributed to the increase of desorption probability (decrease of desorption energy) with increasing packing density of the CO. A quantitative treatment, however, is not yet possible at present, because relationships for the kinetics of non-random distributions on surface sites are missing.

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