PNNL

Abstract

Addition of molecular oxygen to NO-containing stream improves NOx storage of the Pd/H-SSZ-13. Model Pd/SSZ-13 with high dispersion of Pd ions (0.1 and 1 wt% Pd) was synthesized. The material was characterized with FTIR and cryo-STEM. 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 are characterized by notable thermal stability, with resistance to decomposition under high vacuum at 200 ºC. Addition of O2 to NO slightly increases the amount of Pd(II)-NO complex with ?NO at ~1865 cm-1 whereas the low frequency ?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 PNA cycle. The concentration of NO+ (characterized by the broad IR band centered at 2170 cm-1), in the presence of O2, increases in intensity profoundly and contributes to the increased capacity of Pd/SSZ-13 to store NOx and release it at temperatures >140 ºC. In the presence of H2O/O2, 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 ºC where SCR 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. Due to shielding of the Pd(II) ion from excess water and selective formation of stable coordinately saturated Pd(NO)(CO) complexes, the PNA performance is improved by CO. Therefore, we demonstrate that beside 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.