In-situ and operando FT-IR investigation of Pd speciation in Pd/SSZ-13: The pivotal role of CO and NO adsorption with and without H₂O

In-situ and operando FT-IR spectroscopy was employed to give fundamental insights on CO and NO adsorption and effects on Pd/SSZ-13 investigated for potential passive NOx adsorber applications. Emphasis is placed on understanding the effects of key exhaust components, particularly H2O and O2, under conditions relevant to low-temperature NOx emission control. In-situ CO adsorption experiments at room temperature revealed the coexistence of Pd species with varying local environments and oxidation states. Notably, nearly all Pd2* ions are reduced to Pd*/Pd0 through the combined action of CO and adsorbed water. Operando studies at 50 degrees C confirmed that Pd2* reduction occurs even in the presence of trace amounts of water, while oxygen has minimal influence on Pd speciation in terms of oxidation state or coordination geometry. In-situ NO adsorption at room temperature further demonstrated that the nitrosyl bands near 1860 and 1810 cm-1, well documented in literature, are both constituted by contributions from both Pd2* and Pd* species. Additionally, NO and water together significantly promote Pd2* reduction, as evidenced by post-NO CO adsorption at room temperature with the formation of mixed Pdn*(CO)(NO) complexes, mainly related to Pd+. Under operando conditions performed at 120 and 150 degrees C, hydrated Pd*(NO)(H2O)x species form readily, but full Pd2* reduction is unlikely due to potential re-oxidation by oxygen. Importantly, Pd*(CO)(NO) species is not significantly formed under realistic PNA conditions (80-150 degrees C, presence of H2O and O2). These findings offer critical new insights into Pd redox behavior and speciation in Pd/ SSZ-13, with emphasis on IR band assignments.