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

• DOI: 10.1021/acs.jpcc.8b01007
• Publication Date: 2018-05-01T18:13:36Z
• Authors: Konstantin Khivan...; Feng Gao; Libor Kovarik; Yong Wang; János Szanyi
• Keywords: 02 engineering and technology; 0210 nano-technology

Model Pd/SSZ-13 with high dispersion of Pd ions (0.1 and 1 wt % Pd) was synthesized. The material characterized Fourier transform infrared (FTIR) cryo-scanning transmission electron microscopy. Adsorption NO leads to the formation Pd(II)–NO Pd(I)–NO complexes as well NO+ species that replace residual H+ (extra-framework) sites. These nitrosyl have notable thermal stability, resistance decomposition under vacuum at 200 °C. Addition molecular oxygen NO-containing stream improves NOx storage Pd/H-SSZ-13. In particular, addition O2 slightly increases amount complex νNO ∼1865 cm–1, whereas low frequency band 1805 assigned Pd(I)–NO, decreases in intensity. Simultaneously, polydentate nitrate appear small amounts, contributing temperature release stage during a passive adsorber (PNA) cycle. concentration (characterized by broad centered 2170 cm–1), presence O2, intensity profoundly contributes increased capacity store it temperatures >140 H2O/O2, does not perform satisfactorily PNA, but CO PNA shifts peak >320 °C, where selective catalytic reduction catalysts are most effective. With aid FTIR spectroscopy, we reveal mixed carbonyl–nitrosyl Pd(II)(NO)(CO) CO. Because shielding Pd(II) ion from excess water such stable coordinatively saturated complexes, performance is improved Therefore, demonstrate that, besides adsorbed on Pd, (NO+) extra-framework positions chabazite important for storage. Furthermore, role promoting elucidated, thus highlighting utility combined spectroscopic approach (in materials testing) derive structure/PNA relationships identify new avenues improve performance.