Coupled Methane and NOx Conversion on Pt + Pd/Al2O3 Monolith: Conversion Enhancement Through Feed Modulation and Mn0.5Fe2.5O4 Spinel Addition

Abstract The performance of a dual-layer Pt-Pd/Al2O3 washcoated monolith promoted by an Al2O3-supported Mn0.5Fe2.5O4 spinel base layer was evaluated with the objective to develop a catalyst with a lower loading of Platinum group metal (PGM) for stoichiometric natural gas vehicle (NGV) emission control. Flow reactor results show that a combination of a lean/rich modulated feed and spinel addition gives enhanced methane and NO conversions compared to a steady-state feed having the same overall composition. Washcoated catalysts – PGM-only, spinel-only, PGM + spinel – were compared over a wide range of operating conditions. The results show a decrease in the methane conversion light-off temperature of up 85 °C for an application-relevant feed (CH4 + NO + H2 + CO + O2 + H2O + CO2) while a moderate 40 °C decrease was observed for a simple feed (CH4 + O2 + H2O + CO2). In the absence of spinel the modulation enhancement is negatively impacted at high methane conversion (> 80%). The data show that the O2 storage and release property of the spinel is needed to fully exploit the modulation enhancement. The mechanism responsible for the enhancement is linked to suppression of O2 inhibition on the methane oxidation rate near the stoichiometric neutral point (λ  = 1). Sufficiently fast cycling achieves a balance between a metallic and oxidic precious metal crystallites favorable for methane activation. The addition of spinel moves this balance closer to the λ  = 1 feed, enabling a high oxidation efficiency with lower byproduct NH3 selectivity. Enhancement in the NO conversion follows the methane conversion due to an easier NO reduction under the slightly rich of stoichiometric feed. Finally, comparison of the PGM + spinel catalyst to PGM-only catalysts shows that the addition of spinel can achieve the same or higher activity up to a certain PGM loading.