Single atom Ru on CeO2: effective and stable PNA/RNA material

Janos  Szanyi, Pacific Northwest National Laboratory

Single atom Ru on CeO₂: effective and stable PNA/RNA material

Konstantin Khivantsev, Jinshu Tian, Xavier I.P. Hernandez, Inhak Song, Yong Wang and János Szanyi

Institute for Integrated Catalysis, Pacific Northwest National Laboratory

RICHLAND, WA 99352, USA

Passive NOx adsorbers (PNAs) have been considered and studied extensively in recent years for low temperature (<150 °C) NOx abatement in diesel engine exhaust aftertreatment systems.  The idea behind this technology is to adsorb NOx at engine start up and low temperature operations when the catalyst bed is at temperatures too low for effective NOx reduction.  When the eftertreatment system reaches high enough temperature the thus adsorbed NOx is released and effectively reduced on the SCR catalyst.  The bulk of the studies reported in the literature has been focused on zeolite-based sorbent materials, in particular, Pd ion exchanged small pore zeolites.  These materials have shown high NOx storage capacity, proper NOx release temperature, but not sufficient long-term stability.  During repeated cycling these Pd-based NOx adsorbers lost a significant portion (30-50%) of their uptake capacity (although, most of the performance could be recovered by a high temperature calcination treatment).   The very high price of Pd also added a concern to the high-volume application of these materials in diesel aftertreatmenmt systems.  Ceria-based NOx adsorbers have also been considered for this application, although the concerns about their long-term stability prevented their potential practical application.  The use of Ru in NOx reduction technologies has been mostly avoided due to the concern about potential loss of Ru due to the volatility of RuO₂.  The recently developed atom trapping technique, however, allows us to prepare ceria-based NOx adsorber materials containing single Ru atoms.  These materials (containing Ru at low loadings; 0.1-0.5 wt%) show very promising NOx adsorption properties, and have the potential to operate in aftertreatment systems as storage materials.   Both Ru and the CeO₂ support participate in the NOx storage process: Ru adsorbs NO as nitrosyls, while CeO₂ binds NOx as nitrites and nitrates (NO₂ is effectively produced on these materials and can react with the CeO₂ support).  In this presentation we will discuss the PNA/RNA performance of Ru/CeO₂ materials, their thermal (hydrothermal) stability, performance under long-term cycling and their spectroscopy/microscopy characterization.