| 1. |
- Chen, Lin, 1990, et al.
(author)
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A comparative test of different density functionals for calculations of NH3-SCR over Cu-Chabazite
- 2019
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In: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 21:21, s. 10923-10930
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Journal article (peer-reviewed)abstract
- A general challenge in density functional theory calculations is to simultaneously account for different types of bonds. One such example is reactions in zeolites where both van der Waals and chemical bonds should be described accurately. Here, we use different exchange-correlation functionals to explore O2 dissociation over pairs of Cu(NH3)2+ complexes in Cu-Chabazite. This is an important part of selective catalytic reduction of NOx using NH3 as a reducing agent. The investigated functionals are PBE, PBE+U, PBE+D, PBE+U+D, PBE-cx, BEEF and HSE06+D. We find that the potential energy landscape for O2 activation and dissociation depends critically on the choice of functional. However, the van der Waals contributions are similarly described by the functionals accounting for this interaction. The discrepancies in the potential energy surface are instead related to different descriptions of the Cu-O chemical bond. By investigating the electronic, structural and energetic properties of reference systems including bulk copper oxides and (Cu2O2)2+ enzymatic crystals, we find that the PBE+U approach together with van der Waals corrections provides a reasonable simultaneous accuracy of the different bonds in the systems.
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| 2. |
- Chen, Lin, 1990, et al.
(author)
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A Complete Multisite Reaction Mechanism for Low-Temperature NH3-SCR over Cu-CHA
- 2020
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In: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 10:10, s. 5646-5656
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Journal article (peer-reviewed)abstract
- The dynamic character of the active centers has made it difficult to unravel the reaction path for NH3-assisted selective catalytic reduction (SCR) of nitrogen oxides over Cu-CHA. Herein, we use density functional theory calculations to suggest a complete reaction mechanism for low-temperature NH3-SCR The reaction is found to proceed in a multisite fashion over ammonia-solvated Cu cations Cu(NH3)(2+) and Bronsted acid sites. The activation of oxygen and the formation of the key intermediates HONO and H2NNO occur on the Cu sites, whereas the Bronsted acid sites facilitate the decomposition of HONO and H2NNO to N-2 and H2O. The activation and reaction of NO is found to proceed via the formation of nitrosonium (NO+) or nitrite (NO2-) intermediates. These low-temperature mechanisms take the dynamic character of Cu sites into account where oxygen activation requires pairs of Cu(NH3)(2+) complexes, whereas HO-NO and H3N-NO coupling may occur on single complexes. The formation and separation of Cu pairs is assisted by NH3 solvation. The complete reaction mechanism is consistent with measured kinetic data and provides a solid basis for future improvements of the low-temperature NH3-SCR reaction.
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| 3. |
- Chen, Lin, 1990, et al.
(author)
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Activation of oxygen on (NH3–Cu–NH3)+ in NH3-SCR over Cu-CHA
- 2018
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In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 358, s. 179-186
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Journal article (peer-reviewed)abstract
- Cu-CHA materials are efficient catalysts for NH 3 –SCR of NO x in oxygen excess. A crucial step in the reaction is oxygen (O 2 ) activation, which still is not well understood. Density functional theory calculations in combination with ab initio thermodynamics and molecular dynamics are here used to study O 2 dissociation on Cu(NH 3 ) 2 + species, which are present under NH 3 –SCR conditions. Direct dissociation of O 2 is found to be facile over a pair of Cu(NH 3 ) 2 + complexes whereas dissociation on a single Cu(NH 3 ) 2 + species is unlikely due to a high activation energy. The presence of NO promotes oxygen dissociation on both single and pairs of Cu(NH 3 ) 2 + complexes. Nitrites and nitrates are easily formed as O 2 dissociates, and NO adsorption over nitrates leads to facile formation of NO 2 . The results stress the importance of ligand-stabilized Cu species in Cu-CHA catalysts for NH 3 –SCR.
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| 4. |
- Chen, Lin, 1990, et al.
(author)
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Effect of Al-distribution on oxygen activation over Cu-CHA
- 2018
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In: Catalysis Science and Technology. - : Royal Society of Chemistry (RSC). - 2044-4753 .- 2044-4761. ; 8:8, s. 2131-2136
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Journal article (peer-reviewed)abstract
- Cu(NH 3 ) 2 + -pairs in chabazite (CHA) have been suggested to activate oxygen during lowerature selective catalytic reduction of nitrogen oxides with ammonia (NH 3 -SCR). As charge neutrality requires that each Cu-complex is associated with a framework Al, the Al-distribution may affect Cu(NH 3 ) 2 + -pair formation and subsequent oxygen activation. Here, density functional theory calculations in combination with ab initio molecular dynamics simulations are used to explore Cu(NH 3 ) 2 + -pair formation and oxygen activation in Cu-CHA. The Al-distribution is found to markedly affect the probability for Cu(NH 3 ) 2 + -pair formation. Moreover, the molecular dynamics simulations reveal a low-energy reaction path for O 2 activation and dissociation. The facile O 2 dissociation suggests that Cu-pair formation rather than O 2 activation governs the lowerature NH 3 -SCR activity. The results indicate that precise synthesis of Cu-exchanged chabazite with respect to Al-distribution may enhance the catalytic activity.
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| 5. |
- Chen, Lin, 1990, et al.
(author)
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Interpretation of NH 3 -TPD Profiles from Cu-CHA Using First-Principles Calculations
- 2019
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In: Topics in Catalysis. - : Springer Science and Business Media LLC. - 1572-9028 .- 1022-5528. ; 62:1-4, s. 93-99
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Journal article (peer-reviewed)abstract
- Temperature-programmed desorption (TPD) with ammonia is widely used for zeolite characterization revealing information on acidity and adsorption sites. The interpretation of TPD measurements is, however, often challenging. One example is the NH 3 -TPD profile from Cu-chabazite (Cu-CHA) which generally is deconvoluted in three peaks with contributions from NH 3 on Lewis acid sites, copper sites and Brønsted acid sites. Here, we use density functional theory calculations combined with kinetic simulations to analyze this case. We find a large number of possible species, giving rise to overlapping features in the NH 3 -TPD. The experimental low-temperature peak (below 200∘C) is assigned to NH 3 desorption from Lewis acid sites together with NH 3 desorption from a [Cu(II)(OH)(NH3)3]+ complex. The intermediate-temperature peak (250-350∘C) is attributed to decomposition of a linear [Cu(I)(NH3)2]+ complex and a residual from [Cu(II)(OH)(NH3)3]+. The high-temperature peak is predicted to have contributions from Brønsted acid sites (NH4+), [Cu(I)NH3]+ and [Cu(II)(NH3)4]2+. The present work shows that NH 3 -TPD from Cu-CHA can be reconciled with copper complexes as NH 3 storage sites.
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| 6. |
- Feng, Yingxin, 1994, et al.
(author)
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A First-Principles Microkinetic Model for Low-Temperature NH3 Assisted Selective Catalytic Reduction of NO over Cu-CHA
- 2021
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In: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 11:23, s. 14395-14407
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Journal article (peer-reviewed)abstract
- A first-principles microkinetic model is developed to investigate low-temperature ammonia assisted selective catalytic reduction (NH3-SCR) of NO over Cu-CHA. The reaction proceeds over NH3-solvated Cu-sites by the formation of H2NNO and HONO, which decompose to N2 and H2O over Brønsted acid sites. Non-selective N2O formation is considered by H2NNO decomposition over the Cu-sites. The adsorption of NH3 at oxidized Cu-sites is found to inhibit the reaction at low temperatures by hindering NO adsorption. For the reactions, we nd positive reaction orders with respect to NO and O2, whereas the reaction order with respect to NH3, is negative. The reaction orders and the obtained apparent activation energy are in good agreement with experimental data. A degree of rate control analysis shows that NH3-SCR over a pair of Cu(NH3)+2 is mainly controlled by NO adsorption below 200 C, whereas the formation of HONO and H2NNO becomes controlling at higher temperatures. The successful formulation of a first-principles microkinetic model for NH3-SCR rationalizes previous phenomenological models and links the kinetic behaviour with materials properties, which results in unprecedented insights in the function of Cu-CHA catalysts for NH3-SCR.
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| 7. |
- Feng, Yingxin, 1994, et al.
(author)
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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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In: Journal of Physical Chemistry C. - 1932-7447 .- 1932-7455. ; 128:16, s. 6689-6701
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Journal article (peer-reviewed)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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| 8. |
- Feng, Yingxin, 1994, et al.
(author)
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The Role of H+- and Cu+-Sites for N2O Formation during NH3-SCR over Cu-CHA
- 2021
-
In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 125:8, s. 4595-4601
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Journal article (peer-reviewed)abstract
- The mechanism for N2O formation over CHA and Cu-CHA zeolite catalysts during NH3-SCR is investigated using density functional theory calculations. Direct NH4NO3 decomposition, which is commonly regarded as the main source of N2O, is found to be associated with high barriers in the absence of Brønsted acid sites. Although Brønsted acid sites promote NH4NO3 decomposition, it is still a highly activated process. Low-temperature N2O formation is instead found to be connected with an NO + NH3 reaction over Cu-sites. In particular, N2O can be formed from H2NNO with a low barrier over Cu-OOH-Cu complexes, which are proposed intermediates in the catalytic cycle for NH3-SCR over Cu-CHA. This finding provides an explanation for the experimentally observed low-temperature N2O formation and the relation between Cu loading and N2O formation. The proposed mechanisms open up strategies to enhance the selectivity to N2 during NH3-SCR.
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| 9. |
- Janssens, Ton, et al.
(author)
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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
-
In: ChemCatChem. - 1867-3899 .- 1867-3880. ; In Press
-
Research review (peer-reviewed)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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| 10. |
- Shwan, Soran, 1984, et al.
(author)
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Solid-State Ion-Exchange of Copper into Zeolites Facilitated by Ammonia at Low Temperature
- 2015
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In: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 5:1, s. 16-19
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Journal article (peer-reviewed)abstract
- The effect of the gas phase during solid-state ion-exchange of copper into zeolites was studied by exposing physical mixtures of copper oxides (CuI2O and CuIIO) and zeolites (MFI, *BEA and CHA) to various combinations of NO, NH3, O2 and H2O. It is shown that heating these mixtures to 250°C results in active catalysts for the selective catalytic reduction of NO with NH3 (NH3-SCR), indicating that the Cu has become mobile at that temperature. Such treatment allows for a fast (
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