Inhibitory features of the thermal oxidation of carbon monoxide. A kinetic foundation to dynamic instabilities in closed vessels

Abstract

Unusual experimental features of the oxidation of carbon monoxide in closed vessels, in which successive ‘ignitions’(non-periodic oscillations) result from a continuous increase of the vessel temperature, are revisited. An isothermal kinetic basis for the interpretation of these events, deduced by numerical methods, is presented. It provides a foundation from which (i) the incomplete spontaneous combustion of CO, discovered more than fifty years ago but not explained hitherto, and (ii) the isothermal oscillatory glows that accompany CO oxidation, may also be interpreted.

The isothermal kinetic scheme, related specifically to CO + O₂ to which some H₂ has been added, comprises 21 elementary reactions. It contains the generally accepted minimum description of H₂ oxidation in vessels in which surface termination of radicals is relatively inefficient. Three additional elementary reactions are the interactions of CO with O, OH and HO₂. The strong inhibitory features of the ignition are traced to the intervention of the termination step: CO + O + M → CO₂+ M coupled to a quadratic interaction that generates oxygen atoms: HO₂+ H → H₂O + O. The successive ignitions are explained very satisfactorily, but a quantitative discrepancy between the predicted and measured relative proportions of CO and H₂ that are consumed in each of the successive ignitions cannot be rationalized solely on the basis of the homogeneous 21-step mechanism. It is suggested that additional surface interactions may play a part. The water-gas equilibrium is used to illustrate this.