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 Mn0.5Fe2.5O4/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.

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

ACS Spring Meeting 2021

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