Investigation of Diesel Engine Emissions Oxidation and Sulfation Mechanisms

Noble metal (Pt/Pd) based diesel oxidation catalysts (DOCs) represent the most widely used aftertreatment catalysts to treat emissions from diesel engine exhaust. Due to increasingly stringent regulations to control emissions, the DOCs need to be robust and efficient, which in turn requires a complete understanding of the catalytic reactions taking place inside those units. Despite the excellent activity of currently used DOCs towards engine emissions, they are susceptible to deactivation due to sulfur oxides (SOx) in the diesel engine exhaust. Interactions of SOx with the catalyst metal and support can result in sulfate formation which leads to the release of untreated toxic emissions to the air. In this thesis, aforementioned two issues are investigated. Specifically, our study is targeted to (a) fundamental understanding of the emissions oxidation chemistry on DOC, and (b) Identification of mechanism of sulfation of DOC. In the first strand, a comprehensive microkinetic model for primary emissions oxidation (e.g., CO, NO, NH3, HCN, and CH2O) is developed on Pt DOC. The developed microkinetic model was validated against multiple monolith and fixed bed experiments conducted in practically more relevant operating conditions such as dilute emissions concentrations, atmospheric pressure, and short residence times. This approach is extended to explore the reaction kinetics of SOx on the Pt surface. The second part of the work undertaken in this thesis focuses on understanding the sulfation mechanism of DOC utilizing state-of-the-art first principles computations. A systematic study is conducted for SOx interactions on Pt(111) and Pd(111) surfaces using density functional theory (DFT). To understand the surface reaction mechanism involving various SOx species, we investigate the minimum energy pathways and estimated the oxidation barriers for SOx oxidation on both surfaces. As a stepping stone towards understanding sulfation, we successfully implement our first principles computed parameters as inputs into the SO2 oxidation microkinetic model to predict DOC relevant experimental results. Going forward, we explored the SOx interactions with Pt and Pd surfaces in realistic temperature and pressure conditions, and under oxidizing and sulfating environments using first principles thermodynamics approach. For the first time we are able to explain why Pd behaves so differently towards sulfation compared to Pt. Few critical descriptors are identified which can be useful for the future quest of sulfur resistant catalysts materials.