Pd-Doped SSZ-13 for Low-T NOx Adsorption: an Operando FT-IR Spectroscopy Study

In this study, mechanistic aspects of NO adsorption/desorption over a home-made Pd/SSZ-13 passive NOx adsorber (PNA) catalyst are investigated. Operando FT-IR spectroscopy and microreactor experiments are performed to envisage the performance of the catalyst and the pathway involved in NO adsorption, with particular emphasis to the impact of species such as C3H6 and CO. In the absence of C3H6 and CO, NO is observed to adsorb as nitrosyls (anhydrous and hydrated) over both Pd2+ and Pd+ species, and as nitrates. 80 mu molNO(x)/g(cat) (NO/Pd molar ratio of 0.8) are adsorbed. The stability of nitrosyls is higher in comparison to the nitrates in that the former initially dehydrate and further decompose at elevated temperatures (> 300 degrees C) leading to the evolution of NO. The presence of CO and C3H6 negatively affects the amounts of NO adsorbed (53 and 45 mu molNO(x)/g(cat), respectively) due to the reduction of Pd sites. CO admission to the catalyst forms a variety of carbonyl species over Pd2+, Pd+ and Pd-0 sites which upon NO admission are readily displaced and NO is adsorbed as hydrated/anhydrous nitrosyls of Pdn+. The nitrosyls so formed exhibit lower thermal stability in comparison to nitrosyls observed in the absence of CO and decompose below 300 degrees C. The addition of C3H6 leads to the apparent formation of oxidized species like acetone, acrolein and acetates, besides propylene adsorption. The NO adsorption in the presence of C3H6 leads to the formation of Pdn+(NO)(X) complexes; upon heating the decomposition of this complexes is observed at low temperatures along with propylene and water desorption. Formation of organic nitro-compounds is also observed that decompose at higher temperatures.