Rich methane oxidation on Pt/Pd/Al2O3: Steady state performance, multiplicity features, and spatial patterns

Abstract

Performance features are reported for the catalytic sub-stoichiometric (rich) oxidation of methane on a Pt + Pd/Al2O3 washcoated monolith over a range of feed temperatures and O2/CH4 for both dry and wet feeds. Near isothermal steady state multiplicity and co-existing states with different methane conversions are presented. The methane conversion vs. O2 feed concentration consists of a higher conversion regime at lower concentrations and a lower conversion, inhibited regime at higher concentration. The conversion maximum is accompanied by an overlap of the two regimes in the form of a clockwise hysteresis loop. Spatial concentration profiles for the dry feed reveal an upstream oxidation zone and downstream stream reforming zone. Measurements extending beyond the catalyst imply the co-existence of active and inhibited states. Addition of water promotes oxidation and steam reforming while the multiplicity and nonuniform states persist. Underlying mechanistic aspects responsible for the inhibition, rate multiplicity and co-existing states are discussed.