OBSERVED KINETICS OF AN EXOTHERMIC REACTION ON A TEMPERATURE-CONTROLLED CATALYTIC WIRE

Experiments and simulations of the exothermic oxidation of CO on a temperature-controlled Pt wire are presented which elucidate the impacts of transport processes and temperature nonuniformities on the observed steady-state kinetic and multiplicity features. Rate multiplicity was observed over a wide range of average wire temperatures (T(w) = 100-350-degrees-C), feed compositions (CO/O2 = 0.01-1), and total pressures (5-760 torr). Several of the multiplicity features are atypical of conventional isothermal CO oxidation behavior, such as a (T(w), CO/O2) multiplicity region (bifurcation map) having an extinction branch with a local maximum, which is indicative of a pitchfork singularity. At sufficiently low T(w) and high CO/O2 an unexpected high rate state was sustained in addition to the expected CO-inhibited, low rate state. Visual inspection of the wire while in the high rate state revealed a glowing hot spot near the center of the wire. Simulations using a hot wire model comprised of detailed, isothermal Pt-catalyzed CO oxidation kinetics and the pertinent transport processes help to elucidate the observations. The apparent kinetic anomalies are shown to be a manifestation of temperature nonuniformities along a wire of uniform activity and thermal conductivity. The generality of the observed multiplicity features, the observation of hot spots, and the rational model construct rules out a more complicated kinetic mechanism that would be necessary to explain the data. Close similarities between the CO oxidation data, the simulations, and previously reported data for other Pt-catalyzed oxidations (of propylene and ammonia) suggest that the thermokinetically-induced nonuniformities are a general phenomenon. The results underscore the danger of using ohmic temperature control to study the isothermal kinetics of a metal catalyzed reaction. Model simulations are presented which help to identify conditions for which the impact of nonuniformities can be minimized.