Molecular Level Understanding of How Oxygen and Carbon Monoxide Improve NOx Storage in Palladium/SSZ-13 Passive NOx Adsorbers: The Role of NO+ and Pd(II)(CO)(NO) Species

Model Pd/SSZ-13 with high dispersion of Pd ions (0.1 and 1 wt % Pd) was synthesized. The material was characterized with Fourier transform infrared (FTIR) and cryo-scanning transmission electron microscopy. Adsorption of NO leads to the formation of Pd(II)-NO and Pd(I)-NO complexes as well as NO+ species that replace residual H+ (extra-framework) sites. These nitrosyl species have notable thermal stability, with resistance to decomposition under high vacuum at 200 C-degrees. Addition of molecular oxygen to NO- containing stream improves NOx storage of the Pd/H-SSZ-13. In particular, addition of O-2 to NO slightly increases the amount of Pd(II)- NO complex with nu(NO) at similar to 1865 cm(-1), whereas the low frequency nu(NO) band at 1805 cm(-1), assigned to Pd(I)-NO, decreases in intensity. Simultaneously, polydentate nitrate species appear in small amounts, contributing to the high temperature NOx release stage during a passive NOx adsorber (PNA) cycle. The concentration of NO+ (characterized by the broad infrared band centered at 2170 cm(-1)), in the presence of O-2, increases in intensity profoundly and contributes to the increased capacity of Pd/SSZ-13 to store NOx and release it at temperatures >140 C-degrees. In the presence of H2O/O-2, Pd/SSZ-13 does not perform satisfactorily as PNA, but the addition of CO to the stream improves the PNA storage capacity and shifts the NOx release peak temperature to >320 C-degrees, where selective catalytic reduction catalysts are the most effective. With the aid of FTIR spectroscopy, we reveal the selective formation of a mixed carbonyl-nitrosyl complex Pd(II)(NO)(CO) in the presence of CO. Because of shielding of the Pd(II) ion from excess water and selective formation of such stable coordinatively saturated Pd(II)(NO)(CO) complexes, the PNA performance is improved by CO. Therefore, we demonstrate that, besides NO species adsorbed on Pd, nitrosyl ions (NO+) in extra-framework positions of chabazite are important for PNA storage. Furthermore, the important role of CO in promoting PNA performance is elucidated, thus highlighting the utility of the combined spectroscopic approach (in addition to materials performance testing) to derive structure/PNA performance relationships and identify new avenues to improve the PNA performance.