Stoichiometric oxidation of methane on structured Pt/Pd + mixed metal oxide spinels: Enhanced conversion with feed modulation

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 structured, single- or 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 and NO conversions compared to a steady-state feed having the same overall 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). In the absence of spinel, the modulation enhancement is negatively impacted at high methane conversions. Flow reactor and kinetics studies are conducted to elucidate the underlying mechanism responsible for enhanced CH₄ and NOx conversion. 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. Feed modulation at an intermediate frequency between the two regimes leads to enhanced oxidation rate, enabling a balance between metallic and oxidic PGM crystallites that are favorable for methane activation. Fig. 1 compares the transient CH₄ conversion for PGM-only (2(a)) and PGM + MFO catalyst (2(b)). During the transition from net lean (low CH₄ conversion) to net rich (high CH₄ conversion), a pronounced enhancement in the CH₄ conversion is encountered for the PGM + MFO catalyst, suggesting participation by the spinel. Implications of the findings in terms of precious metal loading reduction will be discussed.

Figure 1. Dependence of CH₄ conversion during lean-rich cyclcing for Pt/Pd catalyst without (a) and with Mn_0.5Fe_2.5 O₄ spinel (b).