Steady-state multiplicity and superadiabatic extinction waves in the oxidation of CO/H2 mixtures over a Pt/Al2O3-coated monolith

Steady-state multiplicity was observed in the catalytic oxidation of mixtures of H-2 and CO over a cordierite monolith wash-coated with Pt/Al2O3. Increasing the CO concentration or decreasing the H-2 concentration across the hysteresis limit results in extinction, whereas decreasing the CO concentration or increasing the H-2 concentration across the other hysteresis limit causes ignition. CO inhibits the ignition of H-2, whereas H-2 assists the ignition of CO. Extinction of the reaction due to a step increase of CO or a step decrease of H-2 occurred in the form of a downstream-moving wave accompanied by a sharply localized superadiabatic temperature excursion. Axial temperatures were recorded by five equally spaced thermocouples. The superadiabatic rise in temperature and the speed of the extinction wave are related in a complex way to the size of the CO jump into the hysteresis zone. The greater the H-2 jump downward in concentration, the lower the maximum temperature and the greater the speed of the reaction front.