Pd-dilution approaches for more sustainable catalysts for passive NOx adsorption

Pranaw  Kunal, Oak Ridge National Laboratory

Recent automobile-emission-control research has been focused on both removal of toxic pollutants as well as increasing overall fuel efficiency. Emission standards over the years have gotten more stringent and shaped this field significantly. Of specific importance is recently issued “150 °C Challenge” which requires a 90% reduction of criteria pollutant gases at this low temperature for useful vehicle lifetimes. While catalytic versatility of platinum group metals (PGMs) is unmatched, their volatile prices are an obstacle for broader adaptability of new after-treatment-technologies.

Considering the above mentioned multi-faceted challenge, PGM-dilution and synthesis of bimetallic systems is a promising avenue for simultaneously addressing all issues. Two metals can be strategically selected to synthesize PGM-diluted compositions which will not only be more economical but also superior to the monometallic counterparts in terms of overall activity and durability.

Scope of this study is to explore Pd-based bimetallic catalysts for passive NOx adsorption (PNA) applications. Firstly, we completed a study using ion-exchanged pure-Pd/SSZ-13. Pd onto SSZ-13 was loaded and evaluated under USDRIVE protocol conditions to obtain a baseline performance. Thereafter, three bimetallic combinations: Pd₅₀Fe₅₀, Pd₅₀Cu₅₀, and Pd₅₀Ag₅₀ supported on SSZ-13 were studied. Comparative evaluations showed that activity varies in order Pd₅₀Cu₅₀>Pd₅₀Ag₅₀>Pd₅₀Fe₅₀. Remarkably, Pd₅₀Cu₅₀ and Pd₅₀Ag₅₀ were ≈1.7x and 1.2x respectively, more active than pure Pd. While gradual but systematic deactivation issues akin to pure Pd are still observed in these bimetallic systems, performance of Pd₅₀Cu₅₀ is superior to the pure-Pd over extended trials. Attempts are currently underway to understand and resolve these issues. Results from various treatments such as high temperature ageing and their advantages in the form of optimal ion-exchanged sites will also be discussed.