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Kinetics and multiple rate states of platinum -catalyzed carbon monoxide oxidation

Martha Ellen Garske, University of Massachusetts - Amherst

Abstract

Surface-catalyzed oxidation reactions commonly exhibit multiple steady states. While multiplicity often arises from a coupling of transport and chemical rate processes, it can also result from intrinsic interactions in the catalytic sequence. Carbon monoxide oxidation catalyzed by noble metals notoriously exhibits multiplicity behavior. In addition, reported multiplicity features of CO oxidation at different pressure conditions indicate apparent discrepancies. Observed multiplicity behavior can provide insight into reaction kinetics.^ An experimental study of CO oxidation on an electrically-heated Pt wire was carried out over a wide range of average wire temperature, gas composition, and total pressure conditions to better understand the kinetics and multiplicity behavior. The wire temperature-CO pressure bifurcation map had the shape of an upward opening cusp at atmospheric pressure for sufficiently low CO/O$\sb2$. If the total pressure was decreased or the CO concentration was increased, the extinction branch exhibited a local maximum, indicative of a pitchfork singularity.^ The kinetics and isothermal multiplicity features of CO oxidation were analyzed using data from this research and others, encompassing a wide range of total pressure. It was demonstrated that the commonly-used three-step Langmuir-Hinshelwood mechanism, although capable of predicting the observed multiplicity behavior, is incapable of predicting the correct kinetic behavior in the limits of low and high CO surface coverage. Four models which are more descriptive of the catalytic phenomena were developed and were shown to correctly predict the kinetic and multiplicity features. Model simulations over a wide total pressure range help link ultrahigh vacuum and atmospheric pressure kinetic behavior.^ Two of these kinetic models were incorporated into a nonisothermal model. Simulations demonstrated that use of hot-wire anemometry can lead to temperature nonuniformities along a wire with a constant length-average temperature. These nonuniformities can significantly alter the observed kinetics and isothermal rate multiplicity features. Both models adequately predicted the observed behavior when the wire was nonisothermal, i.e., bifurcation diagrams exhibiting isolated states and bifurcation maps indicative of a pitchfork singularity. In addition, these models correctly predicted the observed experimental trends with regard to changes in total pressure and changes in mass transfer resistances. ^

Subject Area

Engineering, Chemical

Recommended Citation

Martha Ellen Garske, "Kinetics and multiple rate states of platinum -catalyzed carbon monoxide oxidation" (January 1, 1991). Doctoral Dissertations Available from Proquest. Paper AAI9132853.
http://scholarworks.umass.edu/dissertations/AAI9132853

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