| 1. |
- Andersen, Casper Welzel, et al.
(författare)
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Redox-Driven Migration of Copper Ions in the Cu-CHA Zeolite as Shown by the InSitu PXRD/XANES Technique
- 2017
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Ingår i: Angewandte Chemie - International Edition. - : Wiley. - 1433-7851. ; 56:35, s. 10367-10372
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Tidskriftsartikel (refereegranskat)abstract
- Using quasi-simultaneous insitu PXRD and XANES, the direct correlation between the oxidation state of Cu ions in the commercially relevant deNOx NH3-SCR zeolite catalyst Cu-CHA and the Cu ion migration in the zeolitic pores was revealed during catalytic activation experiments. A comparison with recent reports further reveals the high sensitivity of the redox-active centers concerning heating rates, temperature, and gas environment during catalytic activation. Previously, Cu+ was confirmed present only in the 6R. Results verify a novel 8R monovalent Cu site, an eventually large Cu+ presence upon heating to high temperatures in oxidative conditions, and demonstrate the unique potential in combining insitu PXRD and XANES techniques, with which both oxidation state and structural location of the redox-active centers in the zeolite framework could be tracked.
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| 2. |
- Bello, Estefania, et al.
(författare)
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NH3-SCR catalysts for heavy-duty diesel vehicles : Preparation of CHA-type zeolites with low-cost templates
- 2022
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Ingår i: Applied Catalysis B. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 303
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Tidskriftsartikel (refereegranskat)abstract
- Computer-assistance allows selecting the most adequate low-cost organic structure directing agents (OSDAs) for the crystallization of Al-rich CHA-type zeolites. The host-guest stabilization energies of tetraethylammonium (TEA), methyltriethylammonium (MTEA) and dimethyldiethylammonium (DMDEA), in combination with Na, were first theoretically evaluated. This ab-initio analysis reveals that two TEA show a serious steric hindrance in a cha cavity, whereas two MTEA would present excellent host-guest confinements. The synthesis of Al-rich CHA-type zeolites has been accomplished using TEA and MTEA. Electron diffraction and high-resolution transmission electron microscopy reveal large CHA-domains with narrow faulted GME-domains in the CHAtype material synthesized with TEA, confirming the better OSDA-directing roles of MTEA cations towards the cha cavity, in good agreement with DFT calculations. Cu-exchanged Al-rich CHA-type samples achieved with MTEA and TEA show excellent catalytic activity and hydrothermal stability for the selective catalytic reduction (SCR) of NOx with ammonia under conditions relevant for future heavy duty diesel conditions.
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| 3. |
- Feng, Yingxin, 1994, et al.
(författare)
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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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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 11:23, s. 14395-14407
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Tidskriftsartikel (refereegranskat)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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| 4. |
- 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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| 5. |
- Shwan, Soran, 1984, et al.
(författare)
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Solid-State Ion-Exchange of Copper into Zeolites Facilitated by Ammonia at Low Temperature
- 2015
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 5:1, s. 16-19
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Tidskriftsartikel (refereegranskat)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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| 6. |
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| 7. |
- Skoglundh, Magnus, 1965, et al.
(författare)
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Copper mobility in zeolite-based SCR catalysts
- 2017
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Ingår i: Presented at the 254th American Chemical Society National Meeting & Exposition, Washington D.C., USA, August 20-24, 2017.
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Konferensbidrag (refereegranskat)abstract
- Selective catalytic reduction with ammonia (NH3-SCR) is an effective, well-established method to eliminate nitrogen oxides (NOx) in oxygen excess for stationary and mobile applications. Titania-supported vanadia catalysts are traditionally used for NH3-SCR. This type of catalyst is effective in the range 300-450°C, but the NOx reduction efficiency decreases at both lower and higher temperatures. The efficiency of the NH3-SCR process can be improved significantly by using catalysts based on copper-exchanged zeolites and zeotypes, due to their high activity around 200°C. Solid-state ion-exchange in a mixture of copper oxide and zeolite is an efficient way to prepare such catalysts, but this process usually requires high (>700°C) temperatures. The ion-exchange can be considerably affected by appropriate choice of atmosphere during the process. It is shown that the copper-exchange is possible at unprecedented low temperatures, as low as 250°C, in presence of ammonia. The influence of the treatment conditions on the copper-exchange and the mechanism of the reaction-driven ion-exchange process will be presented and discussed. Such copper-exchanged zeolite structures with high copper loading are potentially interesting catalysts for a number of technical applications.Powder mixtures of Cu2O or CuO and zeolite with either CHA, MFI or *BEA framework structure were exposed to well-defined gas atmospheres at constant temperature. After the treatment, the SCR activity of the samples was determined by steady state and transient flow reactor experiments, and the physicochemical properties of the samples were characterized with bulk and surface sensitive characterization techniques. Furthermore, first-principles calculations were used to investigate the energetic conditions for the ion-exchange process.We show that in the presence of ammonia, copper becomes mobile at considerably lower temperatures,
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| 8. |
- Skoglundh, Magnus, 1965, et al.
(författare)
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Mobility of copper in zeolite-based SCR catalysts
- 2017
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Ingår i: Presented at the 25th North American Catalysis Society Meeting, Denver, Colorado, USA, June 4-9 2017.
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Konferensbidrag (refereegranskat)abstract
- Selective catalytic reduction with ammonia (NH3-SCR) is a well-established and effective method to eliminate nitrogen oxides (NOx) in oxygen excess for stationary and mobile applications. Vanadia supported on titania was the first NH3-SCR catalyst that was commercialized. This type of catalyst is effective around 300-450°C, however at lower or higher temperatures, the efficiency of the catalyst to reduce NOx decreases. To increase the overall NOx reduction, high SCR activity around 200°C is required and copper-exchanged zeolites are interesting candidates in this respect. Solid-state ion-exchange in a mixture of copper oxide and zeolite is an efficient method to prepare such catalysts, but the process usually requires high (>700°C) temperatures. The ion-exchange process with copper oxides and zeolites can be considerably affected inpresence of reactive atmospheres. It is shown that the copper-exchange is possible at unprecedented low temperatures, as low as 250°C, when facilitated by ammonia. The influence of the treatment conditions on the copper-exchange and the mechanism of the ion-exchange process will be presented and discussed. Such copper-exchanged zeolite structures with high copper loading are potentially interesting catalysts for a number of technical applications.Powder mixtures of CuO or Cu2O and zeolite with either the MFI, *BEA or CHA framework structure were exposed to well-defined gas atmospheres at constant temperature. After the treatment the SCR activity was determined by steady state and transient flow reactor experiments, and the physico-chemical properties of the samples were characterized with bulk and surface sensitive characterization techniques. Furthermore, density functional theory calculations were used to investigate the energetic conditions for the ion-exchange process. We show that copper in the presence of ammonia becomes mobile at considerably lower temperatures,
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| 9. |
- Wang, Xueting, 1991, et al.
(författare)
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Direct measurement of enthalpy and entropy changes in NH3 promoted O2 activation over Cu−CHA at low temperature
- 2021
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Ingår i: ChemCatChem. - : Wiley. - 1867-3899 .- 1867-3880. ; 13:11, s. 2577-2582
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Tidskriftsartikel (refereegranskat)abstract
- Oxygen activation is a key step in the selective catalytic reduction of nitrogen oxides with ammonia (NH -SCR) over Cu-chabazite. We present direct measurements of oxygen adsorption at low temperatures over [NH −Cu−NH ] complexes and framework-bound Cu species in Cu-chabazite with Si/Al=14 using isothermal microcalorimetry combined with mass spectrometry. The enthalpy change for O adsorption over [NH −Cu−NH ] complexes at 200 °C is determined to be −79 kJ/mol. By fitting a Langmuir isotherm, the corresponding entropy change is determined to be −142 J/(mol*K) at 10 % O . The results show that O adsorption at low temperatures over [NH −Cu−NH ] complexes is more facile than on framework-bound Cu species. The experimental results are in agreement with density functional theory calculations showing a lower barrier for O activation over the [NH −Cu−NH ] complexes as compared to the framework-bound Cu species.
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