(692a) Forced Dynamic Operation for Enhanced Conversion during Methane Oxidation on Dual-Layer Pt/Pd + Mn0.5Fe2.5O4 Spinel: Kinetic and Reactor Modeling

We investigate the use of structured catalytic monoliths coated with Al₂O₃-supported Pt-Pd and mixed metal oxide spinel for coupled CH₄ oxidation and NOx reduction. A series of dual-layer monoliths containing Pt-Pd/Al₂O₃ (PGM) and Mn_0.5Fe_2.5O₄/Al₂O₃ spinel (MFO) show that a combination of a lean/rich modulation and spinel addition gives enhanced methane conversion compared to a steady state feed with the same composition. Up to a 100 °C decrease in the methane conversion light-off temperature is obtained for an application-relevant feed (CH₄ + NO + H₂ + CO + O₂ + H₂O + CO₂).

Flow reactor and kinetics studies were conducted to elucidate the underlying mechanism responsible for enhanced CH₄ and NOx conversion. The data show shows enhanced CH₄ conversion on Pt-Pd/Al₂O₃ + Mn_0.5Fe_2.5O₄/Al₂O₃ dual layer catalyst compared to Pt-Pd/Al₂O₃ single layer catalyst, during feed modulation over a range of amplitudes and frequencies and temperatures. The mechanism is linked to suppression of O₂ inhibition on the methane oxidation rate near the stoichiometric neutral point. The methane oxidation rate dependence on O₂ partial pressure reveals a rate maximum separating O₂ adsorption limited and O₂ inhibition regimes, along with multiplicity for a finite range of O₂ concentration. Feed modulation between the two regimes leads to an enhanced oxidation rate. The addition of the spinel to Pt-Pd significantly enhances CH₄ conversion, with the existence of an optimal modulation frequency that depends on temperature.

A monolith reactor model is developed that contains independently measured kinetics of methane oxidation, methane steam reforming, and oxygen uptake and release. The model confirms our working hypothesis that the role of spinel is critical in providing efficient oxidation of partial oxidation products CO and H₂ which inhibit anaerobic CH₄ reaction with H₂O.