A combined experimental and theoretical investigation of the effect of forced feed composition cycling for CO oxidation on platinum has been performed. A novel approach to forced composition cycling was examined, in which the phase angle between the two input streams was varied. Reaction rate enhancement is shown to occur, and by varying the phasing of the feed streams it is possible to achieve a global maximum in the time-average reaction rate. This phenomenon can be explained quantitatively by a model based on an adsorbate-induced phase change of the Pt surface combined with CO adsorption selfexclusion. This mathematical model can also quantitatively describe the complex steady-state behavior (uniqueness-multiplicity transitions) observed for this reaction. The predictions of the model have been validated further through a detailed experimental study of the effects of feed flow rate, temperature, size of catalyst charge, and cycling frequency on the instantaneous and time-average conversions during forced cycling of the feed composition.
IntroductionDespite the apparent simplicity of the reaction, a complete mechanistic description of the C O oxidation on supported platinum has yet to be developed. This lack of knowledge is illustrated vividly by the variety of hypotheses proposed to explain the experimentally observed phenomena. For example, for the phenomenon of self-sustained oscillatory behavior it has been speculated that the oscillations could be due to: competitive adsorption of different types of surface CO (Hugo and Jakubith, 1972); coverage-dependent activation energies (Belyaev et al., 1974;Pikios and Luss, 1977); adsorption of inert (Eigenberger, 1978) or coreacting (Mukesh et al., 1982) species; variation of the catalyst surface temperature (Dagonnier et al., 1980;Jensen and Ray, 1982); kinetic nonlinearities in the reaction mechanism (Morton and Goodman, 198 1); catalyst oxidation/ reduction (Sales et al., 1982); variation of the oxygen sticking probability as a function of CO coverage (Ertl et al., 1982;Lynch et al., 1986); interaction of silicon impurities with the catalyst surface (Yeates et al., 1985); and diffusion of carbon to the catalyst surface (Burrows et al., 1987).Several alternative hypotheses have been postulated even for
1796December 1990
William R. C. Graham David T. LynchDepartment of Chemical Engineering University of Alberta Edmonton, Alberta, Canada T6G 2G6 the much less complex phenomenon of steady-state multiplicity. Hegedus et al. (1977) demonstrated that intrapellet diffusional resistances are important, while Chakrabarty et al. (1984) concluded that the interaction of the surface reaction with the adsorption/desorption processes, in the absence of diffusional limitations, is responsible for the multiplicities. Herskowitz and Kenney (1983) found that each of two quite different LangmuirHinshelwood-Hougen-Watson models could adequately predict the values of CO concentration at which transitions from low to high conversions occurred. Graham and Lynch (1 987) showed th...
