Dynamics of NO and N2O Decomposition over Cu–ZSM-5 under Transient Reducing and Oxidizing Conditions

doi:10.1006/jcat.2000.3046

Journal of Catalysis

Volume 196, Issue 2, 10 December 2000, Pages 271-286

https://doi.org/10.1006/jcat.2000.3046 Get rights and content

Abstract

N₂O and NO decomposition pathways on Cu–ZSM-5 have been investigated by monitoring the adsorbate dynamics and changes in reactant and product concentrations using infrared spectroscopy (IR) and mass spectrometry (MS) under transient reducing and oxidizing conditions. Transient reducing and oxidizing conditions were produced by the H₂, CO, and O₂ pulses into either N₂O or NO streams. IR and MS studies under the transient conditions revealed that adsorbed O produced from N₂O exhibited different reactivity and dynamics from the adsorbed O produced during NO decomposition. The differences in reactivity and dynamics of the adsorbed O were evidenced by the fact that (i) adsorbed O from N₂O interacted with CO/H₂ to produce two humps in the H₂O and O₂ concentration profiles; adsorbed O from NO reacted with CO/H₂ that led to only one hump in the H₂O and O₂ concentration profile, and (ii) addition of the O₂ pulse led to reaction of O₂ with adsorbed O from NO, resulting in oxidation of Cu⁺ in Cu⁺(NO) to Cu²⁺ in Cu²⁺(NO₃⁻); addition of O₂ did not lead to any reaction with adsorbed O from N₂O decomposition. N₂O decomposition is proposed to proceed via Cu⁺–ON₂, Cu²⁺O⁻, and Cu²⁺O⁻–ON₂ with Cu⁺–ON₂ serving as a precursor for N₂ formation and Cu²⁺O⁻ as a precursor for O₂ formation. NO decomposition proceeds via Cu⁺(NO), Cu²⁺O⁻, and Cu²⁺(NO₃⁻) with Cu⁺(NO) serving as a precursor for NO dissociation. Cu⁺ in Cu⁺(NO) is different from that of Cu⁺ in Cu⁺–ON₂. The former may be associated with Al(OH)₄⁻ of the zeolite, the latter with Si(OH)₄⁻.

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The mechanism of NO and N<inf>2</inf>O decomposition catalyzed by short-distance Cu(I) pairs in Cu-ZSM-5: A DFT study on the possible role of NO and NO<inf>2</inf> in the [Cu–O–Cu]<sup>2+</sup> active site reduction
2018, Journal of Catalysis
The reactivity between NO and the oxidized form of a short-distance dinuclear Cu-ZSM-5 catalyst (ZCu₂O) was investigated. ZCu₂O, which contains the [CuOCu]²⁺ bridge coordinated at the opposite T11 positions of the M6 ring of ZSM-5, is obtained by the spin-forbidden decomposition of N₂O on the reduced form of the catalyst, ZCu₂, with an activation energy of about 18 kcal mol⁻¹. The further addition of NO to the [CuOCu]²⁺ unit of ZCu₂O occurs in the doublet state without activation energy and gives NO₂. After desorption, which requires 39.9 kcal mol⁻¹, NO₂ decomposes on a second ZCu₂O site, giving NO again and O₂. Three reaction paths were defined for the latter reaction, with activation energies ranging from about 30 to 42–43 kcal mol⁻¹. Final O₂ desorption is endothermic. The effect of enthalpy and Gibbs free energy contributions at 298.15 and at 773 K was also shown and discussed. According to the present calculations, the [CuOCu]²⁺ bridge can easily be broken by reaction with NO but the desorption and further decomposition of NO₂ are characterized by energetics which make the above mechanism slower than the spin-allowed decomposition of N₂O on similar sites, already reported in the literature. The above conclusions were based on a kinetic analysis according to the Energetic Span Model.
Effect of copper precursors on the catalytic performance of Cu-ZSM-5 catalysts in N<inf>2</inf>O decomposition
2018, Chinese Journal of Chemical Engineering
Five Cu-ZSM-5 catalysts were obtained by treating Na-ZSM-5 (Si/Al ratio = 15) with aqueous solutions of different Cu precursors (CuCl₂, Cu(NO₃)₂, CuSO₄, Cu(CH₃COO)₂, and ammoniacal copper (II) complex ion). After being pretreated in flowing He at 500 °C to form active Cu⁺, these catalysts exhibited quite different activities in catalytic decomposition of N₂O. CZM-AC(II) (prepared by ammoniacal copper (II) complex ion) with 9.4 wt% Cu content was the most active among these Cu-ZSM-5 catalysts, achieving almost complete N₂O conversion at 400 °C. CZM-CA (prepared using Cu(CH₃COO)₂ as the Cu precursor) with 2.8 wt% Cu content was the second most active catalyst among these Cu-ZSM-5 catalysts, achieving almost complete N₂O conversion at 425 °C. CZM-CC, CZM-CN, and CZM-CS prepared by using CuCl₂, Cu(NO₃)₂, or CuSO₄ as the Cu precursor with similar Cu contents (≈ 1.7 wt%) were the least active among these Cu-ZSM-5 catalysts, achieving ca. 90% N₂O conversion at 500 °C. XRD, ICP, SEM, TEM, EDX-mapping, and CO-IR experiments were conducted to characterize relevant samples. The superior activity of CZM-AC(II) can be attributed to the high contents of total Cu⁺ and dimeric Cu⁺ among these samples. The influence of co-fed O₂ or H₂O on the catalytic performance of typical samples was also studied.
Direct decomposition of NO on metal-loaded zeolites with coexistence of oxygen and water vapor under unsteady-state conditions by NO concentration and microwave rapid heating
2017, Catalysis Today
The direct decomposition of nitric oxide (NO) into nitrogen and oxygen was successfully catalyzed on copper- and iron-exchanged ZSM-5 zeolites through microwave (MW) rapid heating. Under the developed methodology, NO is first concentrated in the microcage of the zeolite structure and the adsorbed NO is subsequently decomposed by efficient energy irradiation of MWs without desorption of NO as intact molecules. Lower temperatures favor NO adsorption/concentration inside the zeolite cavities. Under the investigated conditions, the coexistence of oxygen and water vapor exert little adverse influence on the adsorption properties. Rapid and short-term heating of zeolite catalysts under MW irradiation allow the reaction to be repeated at higher temperatures without considerable deterioration of catalytic performance. With the optimization of the adsorption sequences at lower temperatures, heating, and the catalytic reaction for adequate duration, we expect the developed two-step NO decomposition process to find suitable practical applications.
Effect of the crystal size of Cu-ZSM-5 on the catalytic performance in N<inf>2</inf>O decomposition
2015, Materials Chemistry and Physics
Citation Excerpt :
This is explained by the fact that the exchange capacity of each ZSM-5 sample is associated to the Si/Al ratio, whereas the Si/Al ratio is identical for them (Table 2). Cu+ species has been widely perceived as active sites for N2O decomposition [25,26,32,35,36,45,51]. In situ CO-IR was used to monitor the oxidation state of Cu species.
Four Cu-ZSM-5 samples with different average crystal sizes (from about 20 nm to 10 μm) were synthesized and tested in the catalytic decomposition of N₂O (N₂O = N₂ + 1/2O₂). Against our intuition, we found that Cu-ZSM-5 with the smallest crystal size (about 20 nm) showed the lowest catalytic activity whereas Cu-ZSM-5 with the biggest crystal size (about 10 μm) showed the highest catalytic activity, although their Cu contents were similar. Relevant characterization was conducted employing XRD, SEM, TEM, ICP, XRF, N₂ adsorption–desorption, CO-IR, H₂-TPR, and O₂-TPD. The influence of co-fed O₂ or H₂O on catalytic activity was also studied.
Silicalite-1@Cu-ZSM-5 core-shell catalyst for N<inf>2</inf>O decomposition
2015, Journal of Molecular Catalysis A: Chemical
Silicalite-1 particles with an average size of 260 nm were used as seeds to prepare Silicalite-1@ZSM-5 core-shell crystals with sizes on the order of 500 nm. A Silicalite-1@Cu-ZSM-5 catalyst was then prepared by ion exchange with copper acetate. Upon appropriate pretreatment in He at 500 °C, the core-shell catalyst showed much higher activity in N₂O decomposition than Cu-ZSM-5 prepared using commercial ZSM-5 as the precursor. Characterization of relevant samples was conducted employing N₂ adsorption-desorption, XRD, ICP, XRF, SEM, TEM, XPS, CO-IR, H₂-TPR, and O₂-TPR. It was found that Silicalite-1@Cu-ZSM-5 has more dimetric Cu⁺ upon thermal pretreatment in He. In addition, less O₂ can be adsorbed on Silicalite-1@Cu-ZSM-5. Both factors contribute to the much higher activity of Silicalite-1@Cu-ZSM-5. The effect of co-fed O₂ and H₂O vapor on the catalytic performance was also studied.
Adsorption and co-adsorption of NO and water on LaCu-ZSM5
2014, Microporous and Mesoporous Materials
Citation Excerpt :
The formation of nitrate species upon the interaction of NO with Cu-ZSM5 has been widely studied by FTIR [19–25] showing the formation of mononytrosil and of both bidentate and chelating nitrates over copper sites. Formation of nitrates was observed also in the absence of O2 in the feed as the result of the reaction between copper mono- and di-nitrosyl species [23,24]. At temperature as high as 180 °C nitrates prevail on less stable nitrosyl species [25].
The effect of water on NO adsorption on LaCu-ZSM5 has been studied by in-situ FTIR at 50 °C identifying for which copper sites NO adsorption competes with water. The relative thermal stability of NO and H₂O ad-species have been also determined by increasing the sample temperature from 50 to 400 °C. Water was removed from zeolite at temperature lower than that necessary to desorb NO, as both bridged and chelating nitrates, from copper sites.
A quantitative analysis of the adsorption of NO on the zeolite previously contacted with water was made by carrying out adsorption tests at temperatures far from those activating any DeNOx reaction in a lab-scale PFR reactor. The variation of the amount of adsorbed NO with the temperature of H₂O desorption has been evaluated increasing the desorption temperature from 125 to 500 °C after zeolite saturation with water at 125 °C. The total recovery of the amount of NO adsorbed on the dry zeolite demonstrated the reversibility of water deactivation.

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¹: Present address: Oak Ridge National Laboratory, Oak Ridge, Tenneśsee.

²: To whom correspondence should be addressed. E-mail: [email protected].

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Regular ArticleDynamics of NO and N2O Decomposition over Cu–ZSM-5 under Transient Reducing and Oxidizing Conditions

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Dynamics of NO and N₂O Decomposition over Cu–ZSM-5 under Transient Reducing and Oxidizing Conditions