Bifurcation features of mixtures containing CO and hydrocarbons in diesel oxidation catalyst

A model of a monolith reactor with radial gradients in the washcoat is used to study the ignition-extinction of pure CO, C2H6 and C3H6 and of their mixtures on a Pt/Al2O3 diesel oxidation catalyst. The ignition temperature of the individual species oxidation increases upon an increase in feed concentration of either CO or C3H6 and decreases upon an increase in feed concentration of C2H6. This is because CO, C3H6 have negative order kinetics and C2H6 has positive order kinetics. Here, we determine the steady-state hysteresis behavior of conversion versus feed temperature during co-oxidation of CO and hydrocarbons (C3H6, C2H6). A single S-shaped bifurcation diagram of conversion versus inlet temperature is obtained during the co-oxidation of all feed mixtures of CO and C3H6 because the activation energies of CO and C3H6 are close to each other. The presence of CO in the feed inhibits the oxidation of propylene during the co-oxidation of CO and C3H6. In contrast, the ignition temperatures of pure CO and pure C2H6 are much different and CO ignites at a much lower temperature than C2H6. A double-S shaped bifurcation diagram is obtained when hysteresis exists for both CO and C2H6 during the co-oxidation. The oxidation of CO present in the feed increases the catalyst temperature and this reduces the C2H6 ignition temperature. The separated ignition is transformed to a simultaneous ignition when the CO concentration in the feed is sufficiently high. We determine the boundary in the parameter space of feed compositions and other operating conditions that separate the simultaneous and separate ignition. The application of this result for the design of DOCs for treating exhaust gases at lower inlet temperatures is discussed.