Observed kinetics of an exothermic reaction on a temperature-controlled catalytic wire

Abstract

Abstract 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°C), feed compositions (CO/O 2 = 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/O 2 ) 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/O 2 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-calyzed 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.