| 1. |
- Creci, Simone, 1992, et al.
(författare)
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Methoxy ad-species in MFI zeotypes during methane exposure and methanol desorption followed by in situ IR spectroscopy
- 2021
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Ingår i: Catalysis Today. - : Elsevier BV. - 0920-5861. ; 369, s. 123-128
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Tidskriftsartikel (refereegranskat)abstract
- The formation and evolution of methoxy ad-species in MFI zeotypes after CH4 exposure, and during temperature programmed desorption of CH3OH have been investigated in situ with diffuse reflectance Fourier transform infrared spectroscopy. Fe and/or Al atoms have been incorporated in framework position prior to crystallization and the influence of the resulting acidity on CH4 activation and CH3OH desorption has been examined. The results show that the presence of Fe promotes CH4 activation and that methanol is more strongly bound to the zeotype in the presence of Al. Because CH4 activation and CH3OH extraction are two of the key steps in the direct conversion of methane to methanol, our results indicate that Al-free zeotypes containing Fe atoms pinpoint important catalyst design parameters needed for this reaction.
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| 2. |
- 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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| 3. |
- Han, Joonsoo, 1990, et al.
(författare)
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N 2 O Formation during NH 3 -SCR over Different Zeolite Frameworks: Effect of Framework Structure, Copper Species, and Water
- 2021
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Ingår i: Industrial & Engineering Chemistry Research. - : American Chemical Society (ACS). - 1520-5045 .- 0888-5885. ; 60:49, s. 17826-17839
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Tidskriftsartikel (refereegranskat)abstract
- The formation characteristics of N2O were investigated with respect to copper-functionalized zeolites, i.e., Cu/SSZ-13 (CHA), Cu/ZSM-5 (MFI), and Cu/BEA (BEA) and compared with the corresponding zeolites in the H form as references to elucidate the effect of the framework structure, copper addition, and water. Temperature-programmed reduction with hydrogen showed that the CHA framework has a higher concentration of Cu2+ (Z2Cu) compared to MFI and BEA. The characterizations and catalyst activity results highlight that CHA has a framework structure that favors high formation of ammonium nitrate (AN) in comparison with MFI and BEA. Moreover, AN formation and decomposition were found to be promoted in the presence of Cu species. On the contrary, lower N2O formation was observed from Cu/CHA during standard and fast SCR reactions, which is proposed to be due to highly stabilized AN inside the zeolite cages. On the other hand, significant amounts of N2O were released during heating due to decomposition of AN, implying pros and cons of AN stability for Cu/CHA with possible uncontrolled N2O formation during transient conditions. Additionally, important effects of water were found, where water hinders AN formation and increases the selectivity for decomposition to NO2 instead of N2O. Thus, less available AN forming N2O was observed in the presence of water. This was also observed in fast SCR conditions where all Cu/zeolites exhibited lower continuous N2O formation in the presence of water.
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| 4. |
- Bergman, Susanna L., et al.
(författare)
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In-situ studies of oxidation/reduction of copper in Cu-CHA SCR catalysts: Comparison of fresh and SO2-poisoned catalysts
- 2020
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Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 269
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Tidskriftsartikel (refereegranskat)abstract
- SO2-poisoning results in deactivation of Cu-CHA SCR under standard SCR conditions; however regeneration at 700 °C completely restores the SCR performance. To understand the nature of these effects, Cu-species in the fresh and poisoned catalysts were characterized by in-situ temperature-dependent time-resolved Cu K-edge X-ray absorption spectroscopy using the multivariate curve resolution alternating least squares (MCR-ALS) approach and continuous Cauchy wavelet transforms. The extracted chemically-meaningful reference spectra of Cu-species were analyzed by DFT-assisted XANES calculations. Cu-bisulfates were found as the most energetically favorable poisoned Cu-species. The response of Cu-species to a reducing environment differs in the fresh and SO2-poisoned catalysts. Differences in reducibility are related to the formation of quasi-linear Cu-complexes in the SO2-poisoned catalyst formed during heating in H2/He. Heating in H2/He leads to partial desulfurization of the poisoned catalyst. Cooling in H2/He after heating results in more facile formation of Cu-metal clusters in fresh catalyst than in SO2-poisoned.
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| 5. |
- Di, Mengqiao, 1994, et al.
(författare)
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Chasing PtO x species in ceria supported platinum during CO oxidation extinction with correlative operando spectroscopic techniques
- 2022
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 409, s. 1-11
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Tidskriftsartikel (refereegranskat)abstract
- Industrially relevant, highly dispersed, Pt/ceria and reference Pt/alumina catalysts with narrow Pt particle size distributions have been prepared, characterised ex situ and studied for CO oxidation by operando infrared and X-ray absorption spectroscopy. At high CO conversions, spectator CO ad-species on ionic platinum are observed while the CO oxidation proceeds on Pt particles in a high oxidation state exhibiting significant Pt[sbnd]O coordination. During the protracted catalytic extinction, the CO coverage builds up gradually while the Pt oxidation state and Pt[sbnd]O coordination remain high because of interactions with ceria. The observed CO oxidation at high CO coverage is suggested to involve sites at the platinum-ceria boundary that cannot be CO self-poisoned. This behaviour is in stark contrast to that of Pt/alumina, which shows removal of platinum oxides formed during CO oxidation and the classical drop in catalytic activity caused by rapid CO self-poisoning when reaching a critical temperature.
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| 6. |
- Ek, Martin, 1984, et al.
(författare)
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Probing surface-sensitive redox properties of VOx/TiO2 catalyst nanoparticles
- 2021
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3372 .- 2040-3364. ; 13:15, s. 7266-7272
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Tidskriftsartikel (refereegranskat)abstract
- Redox processes of oxide materials are fundamental in catalysis. These processes depend on the surface structure and stoichiometry of the oxide and are therefore expected to vary between surface facets. However, there is a lack of direct measurements of redox properties on the nanoscale for analysing the importance of such faceting effects in technical materials. Here, we address the facet-dependent redox properties of vanadium-oxide-covered anatase nanoparticles of relevance to, e.g., selective catalytic reduction of nitrogen oxides. The vanadium oxidation states at individual nanoscale facets are resolved in situ under catalytically relevant conditions by combining transmission electron microscopy imaging and electron energy loss spectroscopy. The measurements reveal that vanadium on {001} facets consistently retain higher oxidation states than on {10l} facets. Insight into such structure-sensitivity of surface redox processes opens prospects of tailoring oxide nanoparticles with enhanced catalytic functionalities.
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| 7. |
- Ek, Martin, 1984, et al.
(författare)
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Redox dynamics of 2D crystalline vanadium oxide phases on high-index anatase facets
- 2023
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Ingår i: Nanoscale. - 2040-3372 .- 2040-3364. ; 15:21, s. 9503-9509
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Tidskriftsartikel (refereegranskat)abstract
- Vanadium oxides exist in a multitude of phases with varying structure and stoichiometry. This abundance of phases can be extended through the use of other oxides as supports, and through redox treatments. However, the combined effects of different supports and redox treatments can be difficult to identify, particularly when present as different terminating facets on nanoparticles. Here, we examine structural dynamics of 2D vanadium oxides supported on anatase TiO2 nanoparticles, correlated with changes in oxidation state, using in situ transmission electron microscopy imaging and electron energy loss spectroscopy. As the average oxidation state is reduced below V(iv), an ordered cubic V(ii) phase is observed exclusively at the high-index {10l} facets of the support. This local accommodation of highly reduced states is necessary for explaining the observed range of average oxidation states. In turn, the findings show that oxidation states extending from V(v)-V(iv) to V(ii) can be simultaneously stabilized by different supporting oxide surfaces during exposure to atmospheres with controlled redox potential.
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| 8. |
- 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. ; In Press
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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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| 9. |
- Härelind, Hanna, 1973, et al.
(författare)
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Influence of the Carbon–Carbon Bond Order and Silver Loading on the Formation of Surface Species and Gas Phase Oxidation Products in Absence and Presence of NOx over Silver-Alumina Catalysts
- 2012
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 2:8, s. 1615-1623
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Tidskriftsartikel (refereegranskat)abstract
- The influence of carbon–carbon bond order, here systematically represented by prototypical C2H6, C2H4, and C2H2, on the formation of oxidation products and surface species in the absence and presence of NO has been studied for silver-alumina catalysts with different silver loadings (2 and 6 wt %). The catalysts were prepared with a sol–gel method including freeze-drying, which results in small silver species uniformly distributed throughout the alumina matrix. The performance of the catalysts was investigated by temperature programmed extinction-ignition experiments using a continuous gas flow reactor. The evolution of surface species during reactant step-response experiments was studied in situ by diffuse reflection Fourier transform infrared spectroscopy. The results show that activation for oxidation generally proceeds more easily with increasing bond order of the hydrocarbon. For example, C2H2 shows the highest conversion at low temperatures. Furthermore, the use of hydrocarbons with high bond order, that is, C2H2, as reductant for lean NOx reduction results not only in the highest peak activity but also in considerable high activity in a wide temperature range mainly thanks to high activity at low temperature. With increasing silver loading, the oxidation reactions are favored such that both the hydrocarbon and the NO activation occur at lower temperatures. Several types of adsorbates, for example, carbonate, acetate, formate, enolic and isocyanate/cyanide surface species, are present on the catalyst during reaction. Generally the nature of the surface species and likely also the surface processes are more influenced by the carbon–carbon bond order than the silver loading. This needs to be considered when designing catalysts for emission control systems. Especially for applications using homogeneous fuels with short hydrocarbons, this may provide opportunities to tailor the catalyst functionality for the needs at hand.
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| 10. |
- Isapour Toutizad, Ghodsieh, 1986, et al.
(författare)
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In situ DRIFT studies on N2O formation over Cu-functionalized zeolites during ammonia-SCR
- 2022
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Ingår i: Catalysis Science and Technology. - : Royal Society of Chemistry (RSC). - 2044-4753 .- 2044-4761. ; 12:12, s. 3921-3936
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Tidskriftsartikel (refereegranskat)abstract
- The influence of the zeolite framework structure on the formation of N2O during ammonia-SCR of NOx was studied for three different copper-functionalized zeolite samples, namely Cu-SSZ-13 (CHA), Cu-ZSM-5 (MFI), and Cu-BEA (BEA). The evolution of surface species during the SCR reaction at different temperatures was monitored with step-response experiments using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) at different reaction conditions. Also, density functional theory (DFT) calculations were performed to assist the interpretation of the experimental results. The DRIFTS results indicate that NO+ and nitrate species are the main products formed during NO oxidation, and NO appears to adsorb on both Cu-Lewis and Al-Lewis acid sites. The DFT calculations for NO adsorption on the SSZ-13 sample reveal adsorption at Bronsted acid sites with similar adsorption energies but with a slight difference in NO+ stretching vibrations in the DRIFT spectra. Within the standard SCR reaction, in the O-H stretching region, the number of NH3 molecules adsorbed on the Bronsted acid sites is higher for the small-pore size sample compared to the medium- and large-pore zeolites. The obtained DRIFTS results for nitrate species are supported by DFT calculations by simulating the IR spectra of mobile and framework bound nitrate species, which both have a signature at 1604 cm(-1) associated with the O-N bond on NO3-. It is revealed that N2O is produced in a higher amount at lower temperatures for all three samples irrespective of the NO/NO2 ratio. Furthermore, the obtained results from both DRIFTS studies and flow reactor experiments show the higher formation of N2O for the large-pore zeolite compared to the medium- and small-pore zeolite.
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