A heterogeneous surface model for the “steady-state” kinetics of the boudouard reaction

Abstract

An analysis is presented of the steady-state kinetics of the Boudouard reaction, based upon a heterogeneous surface description of carbon. A specific, simplified case of the resultant general heterogeneous surface model (HSM), involving a linear carbon active site distribution and the simple three-step oxygen exchange mechanism for the underlying kinetics, is shown to exhibit the cardinal steady-state rate forms that have appeared in the literature for this reaction. Thus, an HSM description can be viewed as unifying the various steady-state rate formulations. Analysis of the HSM indicates that the actual behavior exhibited by any particular set of gasification rate data in the chemically controlled regime is dependent on surface heterogeneity, as well as temperature and the relative amounts of CO 2 and CO. Both first order (e.g., at low P CO 2 ) and half order (e.g., at high P CO 2) dependence on P CO 2 seem to be the most likely behavior, with zeroth order dependence also possible at very high P CO 2 . Although the coupling of the rate parameters to the active site distribution of the carbon implies that rigorously there are no truly “intrinsic” rate parameters for the Boudouard reaction, limited numerical results are presented that suggest that at least for some carbons there seems to be a limited range over which some of the parameters vary. It is concluded that energetic heterogeneity of carbon surfaces should be taken into account in kinetic analyses. Interpretation of rate data along these lines promises to yield additional insight into the gasification process.