Investigations of degradation in LNT catalysts systems: from noble metal to novel perovskite materials

Abstract

The reduction in emissions from mobile and stationary sources has become a global priority. With the impacts of global warming already being felt, all avenues for reduction in greenhouse gas emissions are being investigated. Proposed miles per gallon standards of 54.5 MPG by 2025 have made increased engine efficiency a priority for all auto manufacturers. Lean-burn diesel engines are one potential strategy to reach the MPG targets, however current automotive catalysts struggle to reduce NO¬x emission in the oxidative environments of lean-burn diesel engines. Lean NOx trap (LNT) catalysis is an ideal approach to reducing NO¬x in oxidative environments. LNT catalysts are similar to traditional three-way catalysts in that they are inserted directly into engine exhausts and require no additional reductant systems. These catalysts can be improved by investigation into the specific catalytic components. Platinum serves to oxidize NO, facilitates NOx storage, and reduce NOx. Barium stores NOx during lean phase operation, and common additives such as cerium improve tolerance to chemical thermal and physical degradation. As catalysts formulations are changed it is extremely important to understand how these catalysts will perform over the lifetime of operation. The following work has taken a systematic approach to investigating aging characteristics of LNT catalysts, from simplistic noble metal alumina supported materials, to novel platinum free ceramic catalysts. A global kinetic model has been developed, and through the implementation of adaptive kinetic parameters, degradation of NO oxidation has been accurately modeled with changes in noble metal morphology. Investigations into aging of Pt/Ba/Al2O3 and Pt/Ba/Ce/Al2O3 have shown that NO oxidation profiles change substantially with thermal aging and that both the oxidation state and particle size of Pt affects conversion. Investigation of NOx storage over Pt/Ba/Ce/Al2O3 after thermal aging have shown that thermal aging induces formation of BaAl2O4 at aging temperatures as low as 600ºC and small amounts of BaAl2O4 can promote storage. Finally, a Pt-free LaSrCoO3¬ catalyst has been shown to both oxidize and store NOx but was unable to perform NOx reduction. The work lays the foundation for investigations of non-platinic perovskites catalysts which represent the next generation of LNT catalysts.