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

Abstract

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 NO_x adsorber applications. Emphasis is placed on understanding the effects of key exhaust components, particularly H₂O and O₂, under conditions relevant to low-temperature NO_x 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 °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 °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 °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.