Investigating the effects of SO2 on Pt-Pd + Mn0.5Fe2.5O2 spinel dual layer catalysts during lean/rich cycling conditions for emission control in natural gas engines

Natalia  Diaz Monrenegro, University of Virginia

Current natural gas engine aftertreatment systems struggle to fully oxidize CH₄ emissions from the engine exhaust, causing the release of unburned CH₄, a powerful greenhouse gas, into the atmosphere. Since aftertreatment systems rely on noble metal-based catalysts to reduce CH₄ emissions, an improved and cost-effective catalytic aftertreatment system is needed for the complete oxidation of CH₄ at low temperatures, as well as simultaneously reduce CO and NO_X emissions and be resistant to common catalyst poisons, such as SO₂. Periodic operation has been shown to improve low-temperature activity in natural gas engines exhaust aftertreatment catalysts especially for catalysts supported on oxygen storage materials (i.e. CeO₂). In this work, the CH₄ oxidation activity during periodic operation was studied over a Pt-Pd+Mn_0.5Fe_2.5O₂ spinel dual layer catalyst in the presence and absence of SO₂, where the spinel layer acts as an oxygen storage material. Under simulated-exhaust conditions, light-off experiments in the absence of SO₂ showed that periodic conditions lowered methane T₅₀ by ~100°C compared with no modulation. CO conversion was unaffected by periodic conditions; however, NOx conversion reached 100% conversion at lower temperatures. The promotional effect of periodic conditions in CH₄, CO and NO_X conversion is affected by the frequency and amplitude of the cycle, which correlate to O availability from the spinel. Focusing on methane oxidation, periodic conditions improved CH₄ oxidation activity at low temperatures; however, the extent of the improvement is dependent on the oxygen concentration during the oxygen-abundant phase, and it brought the formation of partial oxidation products at high temperatures. The extent of SO₂ impact on CH₄ oxidation was a function of modulation conditions and feed gas compositions.

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