Palladium/ Zeolite Low Temperature Passive NOx Adsorbers (PNA): Structure-Adsorption Property Relationships for Hydrothermally Aged PNA Materials

Abstract

<p>Zeolites with different framework structures (SSZ-13, ZSM-5, BEA) but similar Si/Al ratios and Pd loading (~1 wt%) were synthesized and evaluated as low temperature passive NOx adsorbers (PNA). These materials exhibit high NOx adsorption efficiency with atomically dispersed Pd the active adsorption site. Hydrothermal aging at 750 ºC for 16 hours in the presence of 10% water vapor in air resulted in the formation of PdO nanoparticles in all three samples as evidenced by high energy XRD. Hydrothermal aging of the small-pore Pd/SSZ-13 (Si/Al = 6), which contain ~100-90% atomically dispersed palladium ions, decreases its PNA performance only by ~10-20%, indicating agglomeration of only ~10-20% of all atomically dispersed Pd into PdO. High-field solid state <sup>27</sup>Al NMR studies on the fresh and aged samples substantiate dealumination and significant changes in the distribution of Al (and thus, Brönsted acid) sites after hydrothermal aging. FTIR measurements with NO probe molecule and titration of Brönsted acid sites with nitrosyl (NO<sup>+</sup>)<sup> </sup>ions further corroborate the <sup>27</sup>Al NMR data. Because framework aluminum atoms are the anchoring sites for atomically dispersed Pd ions, their elution from the framework causes the loss of active atomically dispersed Pd species. With the aid of HAADF-STEM imaging and synchrotron XRD studies, we further confirm and visualize the fate of these Pd species – they agglomerate into PdO nanoparticles on the external surface of zeolite. Consequently, these changes lead to the decrease in PNA performance of these materials after hydrothermal aging. The thus formed agglomerates cannot be re-dispersed back to their ionic state due to the loss of framework Al T-sites and/or inherent stability of such large PdO particles.</p> <p>Our study demonstrates that, unlike in previous studies that found increased PNA performance upon HTA, high temperatures hydrothermal aging of PNA materials, that contain atomically dispersed Pd initially, results in a decrease in NOx storage efficiency due to the formation of PdO agglomerates. However, we also highlight the high hydrothermal stability of predominantly atomically dispersed 1-3 wt% Pd/SSZ-13 (Si/Al = 6), whose performance decreases only marginally after prolonged hydrothermal aging at 750 ºC. This study shows that hydrothermally stable passive NOx materials can be prepared using small-pore SSZ-13 zeolite. </p>

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

“…Ryou et al found that hydrothermal aging can activate the passive NO adsorption performance of Pd/SSZ-13 due to the redistribution of Pd from PdO to Pd 2+ ions [14]. Subsequently, Khivantsev et al prepared well-dispersed materials with high Pd dispersion, and agglomeration into PdO nanoparticles from Pd 2+ ions was found to occur during hydrothermal aging [9]. Here, we found that the two processes both took place and that they were closely related to the different zeolite structures and hydrothermal aging conditions.…”

“…They further used the same method to prepare Pd/zeolites with different framework structures (CHA, MFI, BEA) and investigated the hydrothermal stability of these materials. PdO x nanoparticles were formed for all the hydrothermally aged samples, but Pd/SSZ-13 zeolite showed the highest tolerance to hydrothermal aging [9]. So far, the transformation from PdO clusters to atomically dispersed Pd and its reverse process during HTA have both been observed.…”

“…PNA materials consist of noble metals (such as Pd, Pt) and oxides (such as Al 2 O 3 , CeO 2 , WZrO x ) or zeolite supports (such as MFI, BEA, CHA) [5][6][7][8][9][10]. Compared with oxide-supported metals used as PNA materials, metals supported on zeolites are more attractive due to their improved NO adsorption capacity and high SO 2 tolerance, as well as optimal hydrothermal stability (HTS) [9,[11][12][13].…”

“…PNA materials consist of noble metals (such as Pd, Pt) and oxides (such as Al 2 O 3 , CeO 2 , WZrO x ) or zeolite supports (such as MFI, BEA, CHA) [5][6][7][8][9][10]. Compared with oxide-supported metals used as PNA materials, metals supported on zeolites are more attractive due to their improved NO adsorption capacity and high SO 2 tolerance, as well as optimal hydrothermal stability (HTS) [9,[11][12][13]. Recently, researchers found that Pd-SSZ-13, as a representative smallpore zeolite with excellent HTS, showed large NO adsorption capacity and a suitable NO x release temperature due to its easily dispersible Pd species and appropriate pore structure [9,[11][12][13][14].…”

“…Compared with oxide-supported metals used as PNA materials, metals supported on zeolites are more attractive due to their improved NO adsorption capacity and high SO 2 tolerance, as well as optimal hydrothermal stability (HTS) [9,[11][12][13]. Recently, researchers found that Pd-SSZ-13, as a representative smallpore zeolite with excellent HTS, showed large NO adsorption capacity and a suitable NO x release temperature due to its easily dispersible Pd species and appropriate pore structure [9,[11][12][13][14]. Kim et al found that hydrothermal aging (HTA) treatment can activate Pd/SSZ-13 to enhance its NO x adsorption capacity, by promoting the atomic dispersion of Pd from PdO x species [14].…”