| 1. |
- Feng, Yingxin, 1994, et al.
(författare)
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High-Temperature Reaction Mechanism of NH 3 -SCR over Cu-CHA: One or Two Copper Ions?
- 2024
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Ingår i: Journal of Physical Chemistry C. - 1932-7447 .- 1932-7455. ; 128:16, s. 6689-6701
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Tidskriftsartikel (refereegranskat)abstract
- Cu-exchanged chabazite (Cu-CHA) shows good performance for selective catalytic reduction of nitrogen oxides using NH3 as a reducing agent (NH3-SCR). The temperature dependence of the activity has a characteristic nonmonotonic behavior with a minimum in the range 300-350 °C. The minimum signals that different reaction mechanisms or active sites dominate at low and high temperatures. The low-temperature mechanism is believed to occur over a pair of mobile [Cu(NH3)2]+ complexes, whereas the high-temperature mechanism should proceed over framework-bound Cu ions. To explore the NH3-SCR reaction over framework-bound Cu ions, we use first-principles calculations combined with mean-field microkinetic simulations. We find that the reaction proceeds over a single framework-bound Cu ion and that the first step is NO and O2 coadsorption. The coadsorption competes with NH3 adsorption, and the NH3-SCR rate is largely determined by the adsorption energy of NH3. Combining the high-temperature kinetic model with our previous low-temperature model for NH3-SCR over pairs of mobile [Cu(NH3)2]+ complexes makes it possible to describe the nonmonotonic behavior of the reaction rate. The work provides a detailed mechanistic understanding of the role and transformation of different forms of Cu ions during low- and high-temperature standard SCR in Cu-CHA.
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| 2. |
- Janssens, Ton, et al.
(författare)
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The [(NH 3 ) 4 Cu 2 O 2 ] 2+ -Peroxo Complex as the Key Intermediate for NH 3 -SCR Activity and Deactivation of Cu-CHA Catalysts.
- 2024
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Ingår i: ChemCatChem. - 1867-3899 .- 1867-3880. ; In Press
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Forskningsöversikt (refereegranskat)abstract
- In NH3-SCR over Cu-CHA catalysts in the low-temperature range 150–300 °C, the activation of oxygen occurs via oxidation of a pair of mobile (NH3)2CuI-complexes located in the cages of the zeolite. In this step, a reactive [(NH3)4Cu2O2]2+-peroxo complex (μ-η2,η2-peroxo diamino dicopper(II)-complex) is formed. The chemistry of this complex determines several catalytic properties of the Cu-CHA catalyst. The reaction of NO with the [(NH3)4Cu2O2]2+-peroxo complex governs the NH3-SCR activity. A reaction of the [(NH3)4Cu2O2]2+-peroxo complex with ammonia hinders the reaction of NO with the complex, thus leading to an inhibition of the NH3-SCR reaction. Finally, the deactivation in presence of SO2 is due to a reaction of SO2 with the [(NH3)4Cu2O2]2+-peroxo complex, leading to the formation of Cu−S compounds in the catalyst. In this review, the characterization and the reactions of the [(NH3)4Cu2O2]2+-peroxo complex with NO, NH3, and SO2, and mean-field kinetic models based on first principles calculations for NH3-SCR activity and SO2 poisoning are discussed.
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| 3. |
- Singh, Shivangi, 1996, et al.
(författare)
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Mechanism for Cu-enhanced hydrothermal stability of Cu-CHA for NH 3 -SCR
- 2024
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Ingår i: Catalysis Science and Technology. - 2044-4753 .- 2044-4761. ; 14:12, s. 3407-3415
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Tidskriftsartikel (refereegranskat)abstract
- Exposure of acidic zeolite-based catalysts to water at high temperatures generally leads to deactivation due to dealumination. In Cu-CHA zeolite, which is a preferred catalyst for the selective catalytic reduction of NO by NH3 (NH3-SCR), the acidic protons in the zeolite are partially exchanged by Cu ions. The presence of Cu has been measured to reduce the rate of dealumination, thus stabilizing the catalyst. To understand the stabilizing effect of Cu, density functional theory calculations, ab initio thermodynamics and microkinetic modeling are used to compare the reaction mechanism for the dealumination of H-CHA to Cu-CHA. For H-CHA, we find that dealumination leads to the formation of mobile Al(OH)3H2O (extra-framework aluminum) species, whereas for Cu-CHA, formation of framework bound Cu-Al species is thermodynamically preferred over Al(OH)3H2O, which results in the increased stability of Cu-CHA. The formation of mobile Al(OH)3H2O in Cu-CHA is, moreover, associated with a high energy barrier. The phase diagrams show the formation of Al(OH)3H2O and Al2O3 from H-CHA and that high temperatures favor the formation of Al2O3. For Cu-CHA, high temperatures lead to the formation of CuO and Al2O3, which is favored over Al(OH)3H2O + CuO. The microkinetic model shows that the formation of Al(OH)3H2O in the presence of water starts at 380 K and 800 K in H-CHA and Cu-CHA, respectively. Additionally, the time evolution of the Al(OH)3H2O coverage at 923 K reveals that the process of dealumination is significantly faster for H-CHA as compared to Cu-CHA, which is in accordance with the measured increased stability.
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