Reactivity Retention of Modern Converters for Stringent Emission Control

The growing emphasis on environmental protection and sustainability has resulted in increasingly stringent emission regulations for automotive manufacturers, as demonstrated by the upcoming EURO 7 and 2027 EPA standards. Significant advancements in cleaner combustion and effective aftertreatment strategies have been made in recent decades to increase the engine efficiency while abiding by the emission limits. Among the exhaust aftertreatment strategies, three-way catalyst has remained the primary solution for stoichiometric burn engines due to its high conversion efficiency and ability to simultaneously allow both oxidative and reductive reactions in a single stage with spatial separation due to the oxygen storage capabilities of ceria. However, fuel and lubricant-borne sulfur and phosphorus compounds have been shown to have a significant long-term effect on the activity of three-way catalysts, particularly during the lean-rich transitions and oxygen storage processes. In the present study, the impact of sulfur contamination on the conversion efficiency and oxygen storage capacity of the three-way catalyst has been investigated on a heated flow reactor bench platform. The influence of sulfur accumulation on the water-gas shift reaction and activity of ceria has been studied. Additionally, the process of sulfur removal at high temperature (~700-750°C) has also been explored. Relevant engine-out exhaust conditions from the SI engine platform, including flow, temperature, and exhaust species (individually), were replicated on a heated aftertreatment flow bench during contamination and regeneration cycles. A comprehensive analysis of species before and after the catalyst sections was performed using Fourier-transformed infrared (FTIR) and mass spectrometers to study and quantify the conversion and formation of species including sulfur species (sulfur dioxide and hydrogen sulfide) and hydrogen, under different catalyst conditions. The conversion selectivity of sulfur species during regeneration is also investigated. The results show that sulfur contamination causes a substantial reduction in oxygen storage capacity. Effective sulfur removal required a combination of high temperature (~700°C or higher) and lean-rich cycling; absence of either condition resulted in incomplete desulfation and the selectivity towards sulfur dioxide and hydrogen sulfide was largely dependent on the reductant species used during high temperature desulfation.