The Superior Hydrothermal Stability of Pd/SSZ-39 in Low Temperature Passive NOx Adsorption (PNA) and Methane Combustion

Abstract

We successfully synthesized uniform SSZ-39 with an average crystal size of about a micron. Pd (0.7 - 3 wt%) was supported on SSZ-39 with Si/Al ratio ~12. The as-synthesized materials were characterized by FTIR, XRD, Helium Ion Microscopy, HAADF-STEM imaging, 27Al, 29Si and H solid state NMR spectroscopic techniques. <br>FTIR studies with CO and NO probe molecules reveal that the 0.7 wt% Pd/SSZ-39 material with Si/Al ~12 has the majority of Pd dispersed atomically as isolated Pd(II) and Pd(II)-OH centers, and thus can be used as a low-temperature passive NOx adsorber. Pd(II)-NO, Pd(II)(OH)(NO) and Pd(II)(CO)(NO) complexes form during PNA in this material. We compare this PNA material directly with the Pd/SSZ-13 system (with Si/Al ratio ~12) and show its superior hydrothermal stability. Remarkably, Pd/SSZ-39 with Si/Al ratio ~12 survives hydrothermal aging up to 815 ºC in 10% H2O/Air vapor for 16 hours without significant loss in activity. The SSZ-39 crystal structure remains intact during hydrothermal aging up to 1,000 ºC as we elucidate it with XRD and HAADF-STEM imaging/EDS mapping. However, changes to the framework during such harsh hydrothermal treatment significantly change the NOx release profiles during PNA as evidenced by high-field 27Al NMR on fresh and aged Pd/SSZ-39 samples as well as PNA performance measurements. <br>Besides PNA application, these hydrothermally very stable materials (3 wt% Pd on SSZ-39 with Si/Al ratio ~12) can be used as a robust methane combustion catalyst under industrially relevant conditions (GHSV~600,000hr-1). This catalyst shows minimal deactivation after both harsh hydrothermal aging at 750 and 800 ºC, and prolonged time on stream (105 hrs) at 425 ⁰C. In contrast, both 3wt% Pd/alumina and 3wt% SSZ-13 supported samples lose a significant portion of their activity.<br>

Palladium/Ferrierite versus Palladium/SSZ‐13 Passive NOx Adsorbers: Adsorbate‐Controlled Location of Atomically Dispersed Palladium(II) in Ferrierite Determines High Activity and Stability**

“…We show the excellent performance of these materials under practically relevant conditions. Moreover, due to the presence of a specific Pd site in the FER micropores resistant to water hydrolysis during hydrothermal aging at 800 °C (and even 850 °C) in 10 % H 2 O/O 2 /N 2 mix for 16 hours, no changes in the adsorption of NOx occur, a feature that has not been previously achieved with any PNA material [6–19] . Although previously Pd/SSZ‐39 [14] showed stability of NOx uptake after aging at 800 °C, the NOx release profile broadened and shifted to higher temperatures which was not desirable (at CO levels ≈250 ppm in ref.…”

“…Conceptually, NOx is adsorbed at low temperature and released at temperatures when Cu/SSZ‐13 is active for SCR (200 °C and higher). As it was recently shown, the active sites in these Pd‐containing materials are isolated Pd ions located in the micropores of various zeolites [6–20] . Structure‐adsorption property relationships have been revealed for a range of Pd/zeolite materials (SSZ‐13, BEA, MFI, MWW, SSZ‐39) and guidelines for the preparation of high‐loadings of atomically dispersed materials have been disclosed [6–27] .…”

“…Furthermore, ammonia cannot be delivered successfully to the catalyst at temperatures <180 °C when urea is used as the source of ammonia. In order to address the low‐temperature cold start problem, Pd/zeolite materials were introduced as passive NOx adsorbers (PNA) [6–22] . Conceptually, NOx is adsorbed at low temperature and released at temperatures when Cu/SSZ‐13 is active for SCR (200 °C and higher).…”

“…As it was recently shown, the active sites in these Pd‐containing materials are isolated Pd ions located in the micropores of various zeolites [6–20] . Structure‐adsorption property relationships have been revealed for a range of Pd/zeolite materials (SSZ‐13, BEA, MFI, MWW, SSZ‐39) and guidelines for the preparation of high‐loadings of atomically dispersed materials have been disclosed [6–27] . More recently, FER materials were used to support Pd and their PNA performance has been evaluated [21, 22] .…”

Correction to: Passive NO Adsorption on Hydrothermally Aged Pd-Based Small-Pore Zeolites

“…We previously found that Cu-SSZ-39 with the AEI structure showed superior HTS to Cu-SSZ-13 with the CHA structure, due to the more tortuous threedimensional 8 member-ring channels [15]. As PNA material as well as methane combustion catalyst, Khivantsev found that Pd/SSZ-39 showed excellent HTS and survived hydrothermal aging up to 815 °C without a significant loss of activity of PNA performance [16,17].…”