The feedback mechanism in self-oscillations for CO oxidation over EUROPT-3

Single-crystal Pt is well known to demonstrate oscillatory and complex dyna mic behaviour, for instance, during the catalytic oxidation of CO. This typ e of behaviour is also observed for presently investigated supported Pt cat alyst, EUROPT-3. In the present case self-oscillations have been examined d uring CO oxidation using in situ FTIR, applied to both steady-state experim ents and step response, transient isotopic labelling and concentration prog ramming experiments. The goal of the present investigation was to establish the feedback mechanism that is essential in explaining complex dynamic beh aviour and to apply periodic operation to suppress the self-oscillations. F or low CO/O-2 ratios the reaction demonstrates so-called regime I kinetic b ehaviour with reaction orders in CO and O-2 being, respectively, one and ze ro. Platinum is slowly oxidised, thereby blocking sites for adsorption and subsequent reaction. At a critical point, corresponding to a degree of oxid ation of the Pt of approximately 61%, regime II type behaviour is observed: the system exhibits multiplicity and self-oscillations. With progressing o xidation of Pt both the period and the amplitude of the oscillations increa se. Although the existence of oxidised Pt seems to be correlated to the eme rgence of self-oscillations, quantitative analysis of the oxidation and red uction kinetics of Pt reveals that the dynamics of these reaction steps are at least two orders of magnitude too slow to act as feedback mechanism. A qualitative comparison of other known feedback mechanisms indicates that a phase transition mechanism can describe the observed self-oscillations. It is shown that forced concentration oscillations suppress the self-oscillati ons. Periodic reduction of the catalyst in CO is effective in keeping the a mount of oxidised Pt low, and thereby prevents the system from entering reg ime II. (C) 1999 Elsevier Science Ltd. All rights reserved.