Palladium/Beta zeolite passive NOx adsorbers (PNA): Clarification of PNA chemistry and the effects of CO and zeolite crystallite size on PNA performance

Khivantsev, Konstantin; Jaegers, Nicholas R.; Kovařík, Libor; Prodinger, Sebastian; Derewinski, Miroslaw A.; Wang, Yong; Gao, Feng; Szanyi, János

doi:10.1016/j.apcata.2018.10.021

Cited by 101 publications

(140 citation statements)

References 27 publications

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“…It is very important to note that for both 0. [8][9][10] We also note that upon dehydration either in He or O2, the color of the sample changes from light yellow to light pink (Fig. S3).…”

Section: Resultsmentioning

confidence: 93%

“…We have also shown that Pd/SSZ-13 with atomically dispersed Pd was capable of simultaneously adsorbing NO and CO from vehicle exhaust streams with unrivaled efficiency under practical conditions (i.e., in the presence of water in the stream) at low temperatures. [8][9][10] The importance of their adsorption capacity lies in the fact that the current state-of-the-art selective catalytic reduction (SCR) materials 61 cannot effectively convert NOx to N2 under practical conditions at temperatures lower than 200 ⁰C. To circumvent this problem, Pd/SSZ-13 can be used as a low temperature passive NOx adsorber (PNA).…”

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confidence: 99%

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Stabilization of Super Electrophilic Pd+2 Cations in Small-Pore SSZ-13 Zeolite

Khivantsev

Jaegers²,

Koleva³

et al. 2019

Preprint

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We provide the first observation and characterization of super-electrophilic metal cations on a solid support. For Pd/SSZ-13 the results of our combined experimental (FTIR, XPS, HAADF-STEM) and density functional theory study reveal that Pd ions in zeolites, previously identified as Pd +3 and Pd +4 , are in fact present as super electrophilic Pd +2 species (charge-transfer complex/ion pair with the negatively charged framework oxygens). In this contribution we re-assign the spectroscopic signatures of these species, discuss the unusual coordination environment of "naked" (ligandfree) super-electrophilic Pd +2 in SSZ-13, and their complexes with CO and NO. With CO, non-classical, highly positive [Pd(CO)2] 2+ ions are formed with the zeolite framework acting as a weakly coordinating anion (ion pairs). Non-classical carbonyl complexes also form with Pt +2 and Ag + in SSZ-13. The Pd +2 (CO)2 complex is remarkably stable in zeolite cages even in the presence of water. Dicarbonyl and nitrosyl Pd +2 complexes, in turn, serve as precursors to the synthesis of previously inaccessible Pd +2 -carbonyl-olefin [Pd(CO)(C2H4)] and -nitrosyl-olefin [Pd(NO)(C2H4)] complexes. Overall, we show that zeolite framework can stabilize super electrophilic metal (Pd) cations, and show the new chemistry of Pd/SSZ-13 system with implications for adsorption and catalysis.

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“…It is very important to note that for both 0. [8][9][10] We also note that upon dehydration either in He or O2, the color of the sample changes from light yellow to light pink (Fig. S3).…”

Section: Resultsmentioning

confidence: 93%

mentioning

confidence: 99%

Stabilization of Super Electrophilic Pd+2 Cations in Small-Pore SSZ-13 Zeolite

Khivantsev

Jaegers²,

Koleva³

et al. 2019

Preprint

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“…The NO/Pd storage ratios summarized in Table 1 also show that the storage capacity for these materials seems to follow the same trend, with 1 wt% Pd/SSZ-13 and 1 wt% Pd/BEA storing more NOx than 1 wt% Pd/ZSM-5. As we have previously demonstrated, indeed Pd ions replaced sites created by the presence of Al atoms in the framework (in the H-form of zeolite, Bronsted acid protons) and that is the key requirement of their high dispersion [35,36]. Although, often times higher Si/Al ratios favor higher hydrothermal stability, this leads to agglomeration of Pd into inactive PdOx agglomerates, and the PNA storage capacity is lost.…”

Section: Experimental Methodsmentioning

confidence: 90%

“…In order to understand the changes at the molecular level, we turned to magic angle spinning solid state NMR technique which is known to provide invaluable information on the Al, and thus proton, distribution in zeolites: we performed high-field magic angle spinning (MAS) 27 Al NMR experiments on both fresh and aged Pd/SSZ-13 samples [30][31][32][33][34]36] (Fig. 12).…”

Section: Insight Into the Structural Changes Of Fresh And Hta Pd/ssz-mentioning

confidence: 99%

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

Khivantsev¹,

Jaegers²,

Kovařík³

et al. 2019

Preprint

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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 27Al 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+) ions further corroborate the 27Al 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. 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.

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“…The ability to understand and interpret the signatures of water under reactive environments has become increasingly important as in situ and operando NMR studies become more prominent. The non-destructive insight provided for applications such as geochemistry or catalysis reveals much about the chemical systems, where the structure of water in porous materials and on surfaces play an important role in reactivity and modifying the active centers [4][5][6][7][8] . In protonated MFI zeolite, for instance, water has been shown to interact with Brønsted acid sites to form hydrated hydronium ions which can serve as the catalytically active center 4 .…”

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confidence: 99%

Thermal perturbation of NMR properties in small polar and non-polar molecules

Jaegers

Wang

2020

Sci Rep

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Water is an important constituent in an abundant number of chemical systems; however, its presence complicates the analysis of in situ 1 H MAS NMR investigations due to water's ease of solidification and vaporization, the large changes in mobility, affinity for hydrogen bonding interactions, etc., that are reflected by dramatic changes in temperature-dependent chemical shielding. To understand the evolution of the signatures of water and other small molecules in complex environments, this work explores the thermally-perturbed nMR properties of water in detail by in situ MAS nMR over a wide temperature range. our results substantially extend the previously published temperature-dependent 1 H and 17 O chemical shifts, linewidths, and spin-lattice relaxation times over a much wider range of temperatures and with significantly enhanced thermal resolution. The following major results are obtained: Hydrogen bonding is clearly shown to weaken at elevated temperatures in both 1 H and 17 o spectra, reflected by an increase in chemical shielding. At low temperatures, transient tetrahedral domains of H-bonding networks are evidenced and the observation of the transition between solid ice and liquid is made with quantitative considerations to the phase change. the 1 H chemical shift properties in other small polar and non-polar molecules have also been described over a range of temperatures, showing the dramatic effect hydrogen bonding perturbation on polar species. Gas phase species are observed and chemical exchange between gas and liquid phases is shown to play an important role on the observed nMR shifts. the results disclosed herein lay the foundation for a clear interpretation of complex systems during the increasingly popular in situ nMR characterization at elevated temperatures and pressures for studying chemical systems.

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Palladium/Beta zeolite passive NOx adsorbers (PNA): Clarification of PNA chemistry and the effects of CO and zeolite crystallite size on PNA performance

Cited by 101 publications

References 27 publications

Stabilization of Super Electrophilic Pd+2 Cations in Small-Pore SSZ-13 Zeolite

Stabilization of Super Electrophilic Pd+2 Cations in Small-Pore SSZ-13 Zeolite

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

Thermal perturbation of NMR properties in small polar and non-polar molecules

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