Oxidation of zeolite acid sites in NO/O2 mixtures and the catalytic properties of the new site in NO oxidation

Loiland, Jason A.; Lobo, Raúl F.

doi:10.1016/j.jcat.2015.02.009

Cited by 45 publications

(33 citation statements)

References 76 publications

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“…12,13 Previous mechanistic studies of the NH3-SCR reaction indicated that the NO + species formed in zeolite pores play a significant role. [14][15][16] Szanyi et al…”

Section: Introductionmentioning

confidence: 99%

Lean NO_x Capture and Reduction by NH₃ via NO⁺ Intermediates over H-CHA at Room Temperature

et al. 2021

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The oxidation of NO to NO2 and the subsequent reduction by NH3 via a NO + intermediate over a proton-type chabazite zeolite (H-CHA) were investigated by the combination of in situ infrared (IR) spectroscopy and density functional theory (DFT) calculations. The in situ IR spectral results indicate that the NO + species formed under a flow of NO + O2 at 27-250 °C are more stable at lower temperatures over both H-CHA and copper-cation-exchanged CHA zeolite (Cu-CHA). The Arrhenius plot (T = 27-120 °C) shows a negative apparent activation barrier energy (−11.5 kJ mol⁻ 1 ) for the formation of NO + species under the NO + O2 flow over H-CHA. The time course of the IR spectra at 27 °C shows that NO is oxidized by O2 to NO2 and then further converted via N2O4 to NO + and NO3⁻. The subsequent exposure to NH3 at 27 °C reduces the NO + species to N2. DFT calculations revealed that Brønsted acid sites in zeolite pores promote the dissociation of N2O4 intermediates into NO + and NO3⁻ species with a low activation barrier (15 kJ mol⁻ 1 ). Moreover, the computed activation barrier for the reduction of NO + species by NH3 was considerably low (6 kJ mol⁻ 1 ). The experimental and theoretical results of this study demonstrate the high potential of Cu-free H-CHA zeolites for promoting the lean NOx capture to form NO + species and the subsequent reduction by NH3 at room temperature.

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“…12,13 Previous mechanistic studies of the NH3-SCR reaction indicated that the NO + species formed in zeolite pores play a significant role. [14][15][16] Szanyi et al…”

Section: Introductionmentioning

confidence: 99%

Lean NO_x Capture and Reduction by NH₃ via NO⁺ Intermediates over H-CHA at Room Temperature

et al. 2021

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“…NO oxidation is also observed on zeolites due to confinement effects, which are studied separately with Reactive Forcefield (ReaxFF) molecular dynamics (MD) simulations at a realistic operating temperature of PNAs. 10,11 Experimental studies conducted with water as the co-feed suggest a negative impact of water on the activity of PNAs. 1−3 Moreover, theoretical calculations carried out by Mandal et al in the presence of water suggest a decrease in the NO binding energies.…”

Section: Introductionmentioning

confidence: 99%

The Operating Cycle of NO Adsorption and Desorption in Pd–Chabazite for Passive NO_x Adsorbers

et al. 2021

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Pd-doped chabazite (Pd/CHA) offers unique opportunities to adsorb and desorb NO x in the target temperature range for application as a passive NO x adsorber (PNA). The ability of Pd/CHA to trap NO x emissions at low temperatures (<200 °C) is facilitated by the binding of NO x species at various Pd sites available in the CHA framework. Density functional theory (DFT) simulations are performed to understand Pd speciation in CHA and the interaction of NO with Pd/CHA to explain the mechanisms of NO adsorption, oxidation, and desorption processes. The calculations are used to elucidate the important role of Pd1+ cationic species, anchored at 6MR-3NN, in providing a strong (E b = −272 kJ/mol) NO adsorption site in Pd/CHA. For NO release, the redox transformation of Pd species comes into play and Pd1+ species are suggested to transform into cationic Pd2+, [PdOH]+, or [Pd–O–Pd]2+ species, all of which show significantly reduced NO binding (−116, −153, and −117 kJ/mol, respectively) as compared to Pd1+. This enables NO desorption at the operating temperature of a downstream catalyst for subsequent catalytic reduction.

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“…The peak at around 2165 cm −1 is assigned to NO + adsorbed on acid Brønsted sites in the zeolite framework. This surface species has previously been reported to absorb IR in the region of 2220-2128 cm −1 for different types of zeolites [7,[26][27][28]30,31]. The intensity of IR absorbance in this region significantly decreased for PdLTA after 900 • C hydrothermal aging.…”

Section: Characterizationmentioning

confidence: 52%

Hydrothermal Aging of Pd/LTA Monolithic Catalyst for Complete CH4 Oxidation

2020

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Palladium-based catalysts are known to provide high CH4 oxidation activity. One drawback for these materials is that they often lose activity in the presence of water vapor due to the formation of surface hydroxyls. It is however possible to improve the water vapor tolerance by using zeolites as support material. In this study, we have investigated Pd supported on thermally stable LTA zeolite with high framework Si/Al ratio (Si/Al = ~44) for CH4 oxidation and the effect of hydrothermal aging at temperatures up to 900 °C. High and stable CH4 oxidation activity in the presence of water vapor was observed for Pd/LTA after hydrothermal aging at temperatures ≤ 700 °C. However, aging at temperatures of 800–900 °C resulted in catalyst deactivation. This deactivation was not a result of structural collapse of the LTA zeolite as the LTA zeolite only showed minor changes in surface area, pore volume, and X-ray diffraction pattern after 900 °C aging. We suggest that the deactivation was caused by extensive formation of ion-exchanged Pd2+ together with Pd sintering. These two types of Pd species appear to have lower CH4 oxidation activity and to be more sensitive to water deactivation compared to the well dispersed Pd particles observed on the LTA support prior to the hydrothermal aging. By contrast, Pd/Al2O3 was generally sensitive to water vapor no matter of the aging temperature. Although the aging caused extensive Pd sintering in Pd/Al2O3, only minor deterioration of the CH4 oxidation activity was seen. The results herein presented show that Pd/LTA is a promising CH4 oxidation catalyst, however Pd rearrangement at high temperatures (≥800 °C) is one remaining challenge.

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Oxidation of zeolite acid sites in NO/O2 mixtures and the catalytic properties of the new site in NO oxidation

Cited by 45 publications

References 76 publications

Lean NO_x Capture and Reduction by NH₃ via NO⁺ Intermediates over H-CHA at Room Temperature

Lean NO_x Capture and Reduction by NH₃ via NO⁺ Intermediates over H-CHA at Room Temperature

The Operating Cycle of NO Adsorption and Desorption in Pd–Chabazite for Passive NO_x Adsorbers

Hydrothermal Aging of Pd/LTA Monolithic Catalyst for Complete CH4 Oxidation

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Oxidation of zeolite acid sites in NO/O2 mixtures and the catalytic properties of the new site in NO oxidation

Cited by 45 publications

References 76 publications

Lean NOx Capture and Reduction by NH3 via NO+ Intermediates over H-CHA at Room Temperature

Lean NOx Capture and Reduction by NH3 via NO+ Intermediates over H-CHA at Room Temperature

The Operating Cycle of NO Adsorption and Desorption in Pd–Chabazite for Passive NOx Adsorbers

Hydrothermal Aging of Pd/LTA Monolithic Catalyst for Complete CH4 Oxidation

Contact Info

Product

Resources

About

Lean NO_x Capture and Reduction by NH₃ via NO⁺ Intermediates over H-CHA at Room Temperature

Lean NO_x Capture and Reduction by NH₃ via NO⁺ Intermediates over H-CHA at Room Temperature

The Operating Cycle of NO Adsorption and Desorption in Pd–Chabazite for Passive NO_x Adsorbers