Rate multiplicity and oscillations in single species surface reactions

The steady states for surface reaction of a single species in an isothermal stirred reactor are examined for various functional forms of adsorption, desorption, and reaction rates. Adsorbate-adsorbate interactions, vacant site reaction requirements, precursor intermediates, and other deviations from Langmuir kinetics are shown to introduce factors in rates which can be approximated as e^-αθ or (1-θ)^m. It is shown that these factors in the reaction term can lead to multiple steady states, although with three steady states at least one is stable, producing hysteresis but no oscillatory states. A factor e^-αθ in the desorption rate can produce multiple steady states and oscillations. However, this occurs only when the adsorption-desorption equilibrium isotherm predicts multiple surface phases. The number and stability of steady states then depends on whether the reactor load line intersects in stable, metastable, or unstable branches of the adsorption isotherm. Introduction of several simultaneous rate anomalies and relaxation of adsorption-desorption equilibrium assumptions can lead to multiple steady states, but rate oscillations can only be obtained when multiple surface phases are predicted. Oscillations will then only be observed when the metastable branches are sufficiently stable that a single uniform phase exists on the surface because coexistence of multiple surface phases tends to eliminate both oscillations and hysteresis.