| 1. |
- Arvidsson, Adam, 1990, et al.
(författare)
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Metal dimer sites in ZSM-5 zeolite for methane-to-methanol conversion from first-principles kinetic modelling: is the [Cu-O-Cu]2+ motif relevant for Ni, Co, Fe, Ag, and Au?
- 2017
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Ingår i: Catalysis Science and Technology. - : Royal Society of Chemistry (RSC). - 2044-4753 .- 2044-4761. ; 7:7, s. 1470-1477
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Tidskriftsartikel (refereegranskat)abstract
- Direct methane-to-methanol conversion is a desired process whereby natural gas is transformed into an energy-rich liquid. It has been realised at ambient pressure and temperature in metal ion-exchanged zeolites, where especially copper-exchanged ZSM-5 has shown promising results. The nature of the active sites in these systems is, however, still under debate. The activity has been assigned to a [Cu-O-Cu]2+ motif. One remaining question is whether this motif is general and also active in other metal-exchanged zeolites. Herein, we use first-principles microkinetic modelling to analyse the methane-to-methanol reaction on the [Cu-O-Cu]2+ motif, for Cu and other metals. First, we identify the cluster model size needed to accurately describe the dimer motif. Starting from the [Cu-O-Cu]2+ site, the metal ions are then systematically substituted with Ni, Co, Fe, Ag and Au. The results show that activation of Ag and Au dimer sites with oxygen is endothermic and therefore unlikely, whereas for Cu, Ni, Co and Fe, the activation is possible under realistic conditions. According to the kinetic simulations, however, the dimer motif is a plausible candidate for the active site for Cu only. For Ni, Co and Fe, close-to-infinite reaction times or unreasonably high temperatures are required for sufficient methane conversion. As Ni-, Co- and Fe-exchanged ZSM-5 are known to convert methane to methanol, these results indicate that the Cu-based dimer motif is not an appropriate model system for these metals.
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| 4. |
- Becker, Elin, 1981, et al.
(författare)
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Methane oxidation over alumina supported platinum investigated by time-resolved in situ XANES spectroscopy
- 2007
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 252:1, s. 11-17
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Tidskriftsartikel (refereegranskat)abstract
- In situ time-resolved X-ray absorption spectroscopy and mass spectrometry were used to correlate changes in catalyst surface composition with catalytic activity for methane oxidation over alumina supported Pt. Different transient experiments (i.e., pulsing of oxygen or hydrogen to an otherwise constant gas composition) were performed to study the methane oxidation kinetics. Changes in the surface O/Pt ratio were monitored by the introduction of a new analysis method of the white line area corresponding to the Pt L-III-edge XANES spectra. The relevance of the method was confirmed by first-principles calculations demonstrating how hydrogen and oxygen adsorbates modify the electronic structure of Pt. The experimental results show that during the gas-phase transients, the surface O/Pt ratio changes, which in turn affects the methane oxidation rate. Activity maxima are observed for an intermediate surface O/Pt ratio. An oxygen-rich surface seems to hinder the dissociative adsorption of methane, leading to low methane oxidation activity at oxygen excess.
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| 6. |
- Bosio, Noemi, 1993, et al.
(författare)
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Interface Reactions Dominate Low-Temperature CO Oxidation Activity over Pt/CeO 2
- 2022
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:38, s. 16164-16171
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Tidskriftsartikel (refereegranskat)abstract
- First-principles-based kinetic Monte Carlo simulations and kinetic experiments are used to explore CO oxidation over Pt/CeO2. The simulations compare CO oxidation over a ceria-supported 1 nm particle with simulations of a free-standing particle and Pt(111). The onset of the CO oxidation over ceria supported Pt is shifted to lower temperatures compared to the unsupported systems thanks to a Mars-van Krevelen mechanism at the Pt/CeO2 interface perimeter, which is not sensitive to CO poisoning. Both the Mars-van Krevelen mechanism and the conventional Langmuir-Hinshelwood mechanism over the Pt nanoparticle are contributing to the conversion after the reaction onset. The reaction orders in CO and O2 are compared experimentally for Pt/CeO2 and Pt/Al2O3. The reaction orders over Pt/CeO2 are close to zero for both CO and O2, whereas the corresponding reaction orders are-0.75 and 0.68 over Pt/Al2O3. The measured zero orders for Pt/CeO2 show the absence of CO/O2 site competition and underline the relevance of interface reactions. The measurements for Pt/Al2O3 indicate that the main reaction path for CO oxidation over Pt is a conventional Langmuir-Hinshelwood reaction. The results elucidate the interplay between condition-dependent reaction mechanisms for CO oxidation over Pt supported on reducible oxides.
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| 7. |
- Florén, Carl-Robert, 1988, et al.
(författare)
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Modelling complete methane oxidation over palladium oxide in a porous catalyst using first-principles surface kinetics
- 2018
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Ingår i: Catalysis Science and Technology. - : Royal Society of Chemistry (RSC). - 2044-4753 .- 2044-4761. ; 8:2, s. 508-520
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Tidskriftsartikel (refereegranskat)abstract
- A comprehensive model is developed for complete methane oxidation over supported palladium. The model is based on first-principles microkinetics and accounts for mass and heat transport in a porous catalytic layer. The turnover frequency (TOF) is simulated for wet exhaust gas compositions, exploring the effects of temperature and total pressure on the TOF. Three different temperature regimes are identified each with different dependency on the total pressure. The regimes originate from temperature and pressure dependent coverages of carbon dioxide and water, which are the most abundant surface species hindering methane dissociation at low temperatures. The TOF is controlled by surface kinetics below 400 °C whereas above 500 °C and up to 8 atm, internal mass transport is controlling. A combination of kinetics, external and internal mass transport controls the TOF at other reaction conditions. The physically meaningful model paves the way for extrapolation and optimization of catalyst design parameters for high catalytic efficiency.
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| 8. |
- Florén, Carl-Robert, 1988, et al.
(författare)
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Multiscale reactor modelling of total pressure effects on complete methane oxidation over Pd/Al2O3
- 2019
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Ingår i: Catalysis Science and Technology. - : Royal Society of Chemistry (RSC). - 2044-4753 .- 2044-4761. ; 9:12, s. 3055-3065
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Tidskriftsartikel (refereegranskat)abstract
- A two-dimensional multiscale model is developed for complete methane oxidation in a continuous flow reactor. The model considers mass and heat transfer for a porous alumina supported palladium catalyst coated on a ceramic monolith substrate and the surface kinetics are described by a first-principles microkinetic model for complete methane oxidation over PdO(101). The temperature dependent conversion for a synthetic exhaust gas composition shows a delayed ignition but a higher conversion at elevated temperatures when the total pressure is increased from 1 to 10 atm. The simulations reveal a temperature and total pressure dependent operating point where the methane conversion is maximized. Analysis of the kinetics shows that the reaction is suppressed by bicarbonates, hydroxyl species and water originating from adsorbed carbon dioxide and water from the gas phase. The reaction order with respect to water and carbon dioxide at 1 atm is -0.94 and -0.99, respectively, and decreases with increasing total pressure. The developed model paves the way for exploring how design parameters and reaction conditions influence the complete methane oxidation reaction.
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| 10. |
- Fouladvand, Sheedeh, 1984, et al.
(författare)
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Methane oxidation over Pd supported on ceria-alumina
- 2012
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Ingår i: Proceedings of the 9th International Congress on Catalysis and Automotive Pollution Control (CAPoC9), Brussels, Belgium, August 29-31.
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Konferensbidrag (refereegranskat)
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