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 NOX 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+Mn0.5Fe2.5O2 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 NOX 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.