Investigation of NO adsorption and desorption phenomena on a Pd/ZSM-5 passive NOx adsorber

Gu, Yuntao; Majumdar, Sreshtha Sinha; Pihl, Josh A.; Epling, William S.

doi:10.1016/j.apcatb.2021.120561

Cited by 29 publications

(15 citation statements)

References 51 publications

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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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Section: Introductionmentioning

confidence: 99%

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

et al. 2022

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“…Experimentally, however, NO adsorption is inhibited by water, and it has been suggested that fully hydrated sites at low temperature do not bind NO. 28 Only upon water desorption with increasing temperature NO starts to bind to partially hydrated or dry active sites. This discrepancy can be rectified by performing single point (SP) calculations with the HSE06 hybrid functional.…”

Section: Resultsmentioning

confidence: 99%

“…This oxidation is commonly attributed to the reduction of palladium which in turn acts as an active site for high temperature NO release. 11,27,28,37 To accommodate the 2|e − | oxidation of NO, reported mechanisms for NO oxidation over Pd/zeolites involve the formation of dimeric active sites analogous to dimeric Cu sites in Cu/zeolites during SCR of NO. 11,12,26,27,38 The assumption of dimeric sites allows formulating mechanisms in which Pd takes formal oxidation states of +2 and +1 and avoids the complete reduction to metallic Pd.…”

Section: Introductionmentioning

confidence: 99%

“…A similar effect of oxygen has been reported by Epling and co-workers for NO release on Pd/ZSM-5. 28 Subsequent to NO release measurements in the absence of oxidants (run 2), we performed NO uptake experiments with a full feed but without performing an oxidative pretreatment (run 3). We found that while the NO uptakes remained similar (Figure S8), Figure 4b clearly shows less NO 2 compared to the initial uptake and release experiment (run 1).…”

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Complete Mechanism for NO_x Adsorption and Release on Atomically Dispersed Palladium in Pd-CHA

Solanki,

Ambast,

Gupta

et al. 2023

ACS EST Eng.

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Palladium exchanged zeolites are an established class of passive NO x adsorber (PNA) that effectively stores and catalytically oxidizes NO at low temperature and releases NO x (x = 1, 2) at higher temperature, enabling its downstream selective catalytic reduction to N 2 . Notably, the presence of water leads to solvated and mobile Pd complexes that behave drastically different from the cationic species anchored to the zeolite framework under dry conditions. Herein, we use theoretical and experimental techniques to develop a complete reaction mechanism for NO adsorption and release on dynamically hydrated, isolated Pd ions within chabazite zeolites (Pd-CHA). van der Waals corrected density functional theory (DFT) calculations, subsequently verified with HSE06 single-point calculations, manifest that NO preferentially binds to the hydrated Pd II site and facilitates the activation of water leading to the transient formation of a Brønsted acid site and hydrated palladium hydroxide. Adsorbed NO reacts with hydroxide to form HONO and Pd changes its formal oxidation state from +2 to +1 upon HONO desorption. HONO subsequently disproportionates to form NO, NO 2 , and H 2 O. The strong binding of NO to hydrated Pd I leads to NO release at temperatures in excess of 200 °C with concomitant reoxidation by NO 2 or O 2 . The complete NO x storage, oxidation, and release mechanisms identified by DFT are consistent with NO x uptake experiments on Pd-CHA. A consistent catalytic cycle for NO oxidation by water on isolated palladium active sites is described, and a mechanistic framework for further improvement of PNA performance is established.

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“…Pd ions including Pd II and [Pd II ĲOH)] are the active sites on Pd/zeolites for NO x adsorption, which form adsorbed species: Pd II ĲNO)ĲOH), Pd II ĲNO) and Pd II ĲCO)ĲNO) in realistic exhaust flow. [10][11][12][13][14][15][16][17][18] Moreover, it is generally accepted that water will hinder NO x adsorption because of the competitive adsorption between water and NO x , 11,14,[19][20][21] and the existence of CO or C 3 H 6 improves PNA performance by shielding the Pd II ions from excess water and preventing the formation of coordinatively saturated complexes. 11,21 However, in the past two years, the negative effect of CO was found on Pd/zeolites for NO x adsorption performance.…”

Section: Introductionmentioning

confidence: 99%

The upgrading role of Al at T1/T2 sites in stabilizing Pd ions over Pd-beta passive NO_x adsorbers under a reducing atmosphere

Zhu

Wang

Zhai

et al. 2022

Catal. Sci. Technol.

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Investigation of NO adsorption and desorption phenomena on a Pd/ZSM-5 passive NOx adsorber

Cited by 29 publications

References 51 publications

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

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

Complete Mechanism for NO_x Adsorption and Release on Atomically Dispersed Palladium in Pd-CHA

The upgrading role of Al at T1/T2 sites in stabilizing Pd ions over Pd-beta passive NO_x adsorbers under a reducing atmosphere

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Investigation of NO adsorption and desorption phenomena on a Pd/ZSM-5 passive NOx adsorber

Cited by 29 publications

References 51 publications

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

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

Complete Mechanism for NOx Adsorption and Release on Atomically Dispersed Palladium in Pd-CHA

The upgrading role of Al at T1/T2 sites in stabilizing Pd ions over Pd-beta passive NOx adsorbers under a reducing atmosphere

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Complete Mechanism for NO_x Adsorption and Release on Atomically Dispersed Palladium in Pd-CHA

The upgrading role of Al at T1/T2 sites in stabilizing Pd ions over Pd-beta passive NO_x adsorbers under a reducing atmosphere