The Influence of Si/Al Ratios on Adsorption and Desorption Characterizations of Pd/Beta Served as Cold-Start Catalysts

Jiang, Ming; Wang, Jun; Wang, Jianqiang; Shen, Meiqing

doi:10.3390/ma12071045

Cited by 19 publications

(11 citation statements)

References 47 publications

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“…Density functional theory (DFT) calculations by Mandal et al indicate that a divalent Pd 2+ cation charge-compensated by a 6-MR paired Al site in CHA (denoted as Z 2 Pd, where Z represents an anionic framework charge introduced by Al substitution) is the most thermodynamically stable mononuclear Pd 2+ species, consistent with their experimental Pd-CHA samples that did not show the OH IR stretches near 3660 cm –1 expected for a monovalent [PdOH] + complex (denoted as ZPdOH) . These findings appear consistent with the larger amounts of mononuclear Pd ions (higher Pd 2+ /Al ratios) reported to form on zeolites of higher bulk Al density (lower Si/Al ratios), ,− which on average should contain larger amounts of proximal Al sites. Additionally, the Rietveld refinement of X-ray diffraction (XRD) patterns of dehydrated Pd-exchanged Y zeolites showed mononuclear Pd cations predominantly located in the plane of the 6-MR, likely as Z 2 Pd sites.…”

Section: Introductionsupporting

confidence: 71%

“…NH 4 + back-exchange procedures were adapted from Lee et al and are similar to other Na + back-exchange procedures. ,, Pd-exchanged samples (0.05–0.08 g) were mixed with 25–50 g of 1 M NH 4 NO 3 (99.9 wt % NH 4 NO 3 , Sigma-Aldrich) in PFA containers with a magnetic Teflon stir bar for 24 h at 338 K, unless stated elsewise. The solution was removed from heated oil baths to cool to an ambient temperature before recovering solids via centrifugation, and the Pd content in the decanted and filtered (0.2 μm) supernatant was quantified with AAS.…”

Section: Methodsmentioning

confidence: 99%

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Structural Interconversion between Agglomerated Palladium Domains and Mononuclear Pd(II) Cations in Chabazite Zeolites

et al. 2021

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Palladium-exchanged zeolites are candidate materials for passive NO x adsorption in automotive exhaust aftertreatment, where mononuclear Pd cations behave as precursors to the purported NO x adsorption sites. Yet, the structures of zeolite lattice binding sites capable of stabilizing mononuclear Pd 2+ ions, and the mechanisms that interconvert agglomerated PdO and Pd domains into mononuclear Pd 2+ ions during Pd redispersion treatments, remain incompletely understood. Here, we use a suite of spectroscopic methods and quantitative site titration techniques to characterize mononuclear and agglomerated Pd species on zeolites with varying material properties and treatment history. Aqueous-phase methods to introduce Pd onto NH 4 -form zeolites initially form mononuclear [Pd(NH 3 ) 4 ] 2+ complexes, but subsequent thermal treatments (573−723 K; air) lead to in situ formation of H 2 that first reduces Pd 2+ to metallic Pd domains, which are then oxidized by air to PdO domains. Progressive treatment of Pd-zeolites in air to higher temperatures (723−1023 K) converts larger fractions of agglomerated PdO to mononuclear Pd 2+ , as quantified by H 2 temperature programmed reduction, because higher temperature treatments facilitate Pd redispersion toward deeper locations within chabazite (CHA) crystallites, which is corroborated by complementary titrimetric and spectroscopic data. Pd-CHA zeolites synthesized with similar bulk Pd and framework Al content, but varying framework Al arrangement, provide evidence that six-membered rings (6-MR) hosting paired Al sites (Al−O−(Si−O) x −Al, x = 1, 2) stabilize Pd 2+ ions and that otherwise isolated Al sites can stabilize [PdOH]+ species, identifiable by an IR OH stretch at 3660 cm −1 . These findings clarify the underlying chemical processes and gas environments that cause Pd agglomeration in zeolites and their subsequent redispersion to mononuclear Pd 2+ ions, which prefer binding at 6-MR paired Al sites in CHA, and indicate that higher temperature air treatments lead to more uniform Pd spatial distributions throughout zeolite crystallites.

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Section: Introductionsupporting

confidence: 71%

Section: Methodsmentioning

confidence: 99%

Structural Interconversion between Agglomerated Palladium Domains and Mononuclear Pd(II) Cations in Chabazite Zeolites

et al. 2021

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“…Recently, Pd-exchanged zeolites were proposed as promising materials for this technology. , Highly dispersed Pd ions at exchange sites of a zeolite have been suggested as active sites for NO x storage at low temperatures . Subsequently, many research groups have studied NO x adsorption/desorption properties over Pd zeolites. − Small-pore Pd-SSZ-13 with chabazite (CHA) topology showed advantageous NO x release temperatures and higher hydrothermal stability compared to medium- and large-pore Pd zeolites. , …”

Section: Introductionmentioning

confidence: 99%

Computational Study of Noble Metal CHA Zeolites: NO Adsorption and Sulfur Resistance

et al. 2022

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Noble metal-exchanged small-pore molecular sieves with chabazite topology are promising materials for automotive cold-start NO x emission control applications. A combination of first-principles thermodynamics and density functional theory was applied for the prediction of monomeric palladium, platinum, and ruthenium species formed in 1Al or 2Al sites of six-/eight-membered rings of the SSZ-13 framework in the presence of SO 2 , NO, O 2 , and H 2 O at temperatures between 0 and 1100 K. Calculations using gradient-corrected Perdew−Burke−Ernzerhof (PBE) functional and hybrid Heyd−Scuseria−Ernzerhof (HSE06) functional showed that the binding energy of NO adsorbed on Pd, Pt, or Ru ions is a strong function of exchange−correlation functional. Use of the PBE functional overestimated the binding strengths of NO to Pd, Pt, or Ru ions compared to the HSE06 functional. While PBE led to the adsorption of two NO per Pd, Pt, or Ru ion, HSE06 predicted the adsorption of a single NO. Isolated Pd, Pt, or Ru ions in 1Al sites tended to bind NO stronger than their counterparts in 2Al sites. Both functionals revealed that Pd and Pt ions have more similarities in terms of both NO adsorption and sulfur resistance compared to Ru ions. The results of this study are beneficial for further modeling of passive NO x adsorbers with improved properties to deliver cleaner tailpipe emissions during engine cold start.

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“…Zeolites are widely employed in industrial applications and have many advantages in their use as supports for a DOC catalyst, such as low production cost, high thermal stability, large specific surface area, and water insensitivity. , Zeolites have tunable hydrophilicity and acidity by changing the ratio of silica and alumina of the zeolites. , Zeolites with a higher siliceous content have higher hydrophobicity and better hydrothermal stability than those with a lower siliceous content, and therefore, could be beneficial for a DOC. A few studies have recently investigated the use of zeolites with different SAR in an exhaust gas after-treatment system, and they have mainly focused on the combustion of CH 4 and the passive adsorption of NO x . ,− For example, a Pd/H-beta zeolite with high SAR has been found to have PdO particles with high activity for CH 4 conversion and also high hydrophobicity, which resulted in good stability for the conversion of CH 4 in the presence of water vapor. , The conversion of CH 4 has also been found to be regenerated more easily on a Pd/H-beta zeolite with a high siliceous content than on a Pd/Al 2 O 3 zeolite after the catalysts had been exposed to a gas stream containing SO 2 …”

Section: Introductionmentioning

confidence: 99%

The Effect of Si/Al Ratio on the Oxidation and Sulfur Resistance of Beta Zeolite-Supported Pt and Pd as Diesel Oxidation Catalysts

Woo

Ilmasani

et al. 2021

ACS Eng. Au

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Pt and Pd supported on beta zeolites, with different silica to alumina ratios (SARs), are examined in the form of diesel oxidation catalysts (DOCs) for the oxidation of CO, NO, C3H8, and C3H6. The effect of SARs on the physicochemical properties of the catalysts, their subsequent oxidation activity, and sulfur tolerance was investigated. A beta zeolite with a high SAR has low acidity and high hydrophobicity, which induces the agglomeration of either Pt or Pd in the catalysts during preparation and thermal treatment. The effect of sintering is more severe on Pt than on Pd catalysts. High-SAR zeolites retained Pt in a more metallic fraction in the obtained catalysts, whereas this effect was not significant for the Pd-based catalyst, wherein Pd exists mostly in the oxide form. High-SAR Pt/BEA and Pd/BEA catalysts exhibit better conversion of CO, NO, C3H8, and C3H6 than low-SAR catalysts. A linear relationship for turnover frequency (TOF) with SAR (and with average particle size) was found for all four oxidation reactions, namely, the higher is the SAR, the higher is the TOF. This suggests that the catalysts are tailorable by adjusting the SAR of the zeolite supports. A five-cycle test indicated that zeolite-based catalysts have greater stability than alumina-based catalysts. The activity for NO oxidation was very stable for the high-silicious Pt/BEA catalyst with time-on-stream, which was not the case for the Pt/Al2O3 catalyst. The Pd/BEA catalysts were more susceptible to sulfur poisoning than their Pt counterparts; however, they were easily regenerated. Also, the Pt/BEA catalysts were effectively regenerated, especially for the highest SAR.

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The Influence of Si/Al Ratios on Adsorption and Desorption Characterizations of Pd/Beta Served as Cold-Start Catalysts

Cited by 19 publications

References 47 publications

Structural Interconversion between Agglomerated Palladium Domains and Mononuclear Pd(II) Cations in Chabazite Zeolites

Structural Interconversion between Agglomerated Palladium Domains and Mononuclear Pd(II) Cations in Chabazite Zeolites

Computational Study of Noble Metal CHA Zeolites: NO Adsorption and Sulfur Resistance

The Effect of Si/Al Ratio on the Oxidation and Sulfur Resistance of Beta Zeolite-Supported Pt and Pd as Diesel Oxidation Catalysts

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