Pd-dilution approaches for highly efficient passive NOx adsorbers

Pranaw  Kunal, Oak Ridge National Laboratory

Dynamic, and high prices of platinum group metals (PGMs) are major driving force necessitating alternatives for catalytic processes including emission control reactions. While catalytic versatility of PGMs is unapparelled, associated price volatility presents a major challenge – deactivation issues upon prolonged usage is yet another bottleneck. One of the approaches for simultaneously addressing these two main issues is via PGM-dilution. Two metals can be strategically selected to attain bimetallic compositions which may exhibit catalytic behaviors superior to the monometallic counterparts as well as better long-term stability while also being economically more sustainable.

In light of the above, scope of this study is to explore applicability of Pd-based bimetallic compositions for passive NOx adsorption (PNA). Field of PNA is relatively new and provides an effective way to control overall vehicular emissions. These materials store NOx in the initial/passive stages of vehicle operation and release them only when the engine warms up (>180 °C) therefore, operate in conjunction with downstream selective catalytic reduction counterparts to enable desired NOx-mitigation. Historically, Pd-based zeolites have been the workhorse for this reaction, recent surges in Pd-prices however, warrant development of more-efficient and economic alternatives.

In this case, we have studied PNA performance using zeolite based PdX bimetallic compositions. Following, our initial control study using ion-exchanged Pd/SSZ-13 for PNA under directly applicable industrial conditions, here we have synthesized and evaluated three novel bimetallic compositions: Pd53Cu47, Pd62Ag38, and Pd62Co38/SSZ-13. Interestingly, initial NOx:Pd uptake values for Pd53Cu47, Pd62Ag38, and Pd62Co38 were respectively ~2.1x, ~1.4x and ~1.3x higher than similarly ion-exchanged Pd/SSZ-13. While performance decline as previously observed for pure-Pd were still seen in these bimetallic systems, Pd53Cu47 performed better than pure-Pd in both uptake and durability. Pd62Ag38 and Pd62Co38 on the other hand showed a higher extent of degradation during performance repeats up to 10 trials. Synergistic and distinct interaction between the two comprising metals in such catalysts are clearly evident by not only better NOx-uptake but also differences in extent of uptake decline after 10 trials. Characterization results using analytic techniques such as DRIFTS, TEM, XRD etc. will also be discussed to explain the origins of such bimetallic-favorable interactions attainable using solution-phase ion-exchange synthesis. Results from treatments such as high temperature ageing and their effects if any, on the PNA performance of these materials will also be covered.