| 1. |
- Broqvist, Peter, 1975, et al.
(författare)
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A DFT Study on CO Oxidation over Co3O4
- 2002
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 210:1, s. 198-206
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Tidskriftsartikel (refereegranskat)abstract
- A pilot investigation regarding the CO oxidation mechanism at the Co3O4(110) surface is performed by means of first-principles calculations based on density functional theory. Preferred adsorption of CO is found to occur at a surface Co3+ site. A possible mechanism for CO2(g) formation of the Mars–van Krevelen type is also looked into via the monitoring of two inequivalent oxygen abstraction routes. Experimental observations regarding partial surface deactivation are discussed based on surface intermediates. The oxidative property of the Co3O4 substrate, i.e., Co3++e−→Co2+, is found to be crucial for the O abstraction step. This implies that CoAl2O4 is inactive in CO oxidation, in agreement with experiment.
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| 2. |
- Broqvist, Peter, 1975, et al.
(författare)
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Characterization of NOx species adsorbed on BaO: Experiment and theory
- 2004
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Ingår i: Journal of Physical Chemistry B. - 1520-5207 .- 1520-6106. ; 108:11, s. 3523-3530
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Tidskriftsartikel (refereegranskat)abstract
- Charged NO, species (x = 1,2,3) formed upon adsorption of NO2 on BaO are characterized by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and theoretical vibrational analysis using the density functional theory (DFT). Experiments at 30degreesC reveal nitrite formation. At T > 250degreesC, shifts associated with formation of nitrates are observed, indicating an interconversion of oxygen atoms among adsorbates. The theoretical study includes single and pairwise NO2 adsorption on (BaO)(9) clusters. As has been reported previously [Broqvist, P.; Panas, I.; Fridell, E.; Persson, H. J. Phys. Chem. B 2002, 106, 137], an additional energy gain is calculated for the second adsorbed NO2. A vibrational analysis of the investigated adsorption configurations supports the interpretation of nitrite to nitrate interconversion on the BaO surface. Moreover, the calculations demonstrate the sensitivity in the NO2 vibrational band splitting with respect to adsorption configuration.
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| 3. |
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| 4. |
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| 5. |
- Broqvist, Peter, 1975, et al.
(författare)
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NOx storage on BaO(100) surface from first principles: a two channel scenario
- 2002
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 106:1, s. 137-145
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Tidskriftsartikel (refereegranskat)abstract
- NO2 adsorption at a BaO(100) surface is investigated by means of spin polarized GGA density functional theory. A periodic supercell procedure is employed, and two redox reaction channels are mapped out, involving two chemisorbed NO2 molecules per supercell. The chemisorption is studied in two subsequent steps. The reaction paths are initiated by NO2 adsorption in the form of a nitrite over a Ba2+ site. This generates an electron hole among the surrounding surface oxygen atoms. A reaction path branching occurs as the second NO2 either (a) acts as surface oxidant, forming a surface nitrite−peroxide pair by releasing NO(g), or (b) binds to an O-surf site to form a formal surface nitrate. A redox reaction involving surface nitrite−nitrate interconversion is also addressed. The computed results are employed to interpret experimental observations of surface nitrites, peroxides, NO(g) desorption, and surface Ba(NO3)2 formation. The understandings are discussed in the context of the NOx storage concept of lean-burn catalysis.
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| 6. |
- Broqvist, Peter, 1975, et al.
(författare)
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NOx storage on BaO: theory and experiment
- 2004
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Ingår i: Catalysis Today. - : Elsevier BV. - 0920-5861. ; 96:1-2, s. 71-78
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Tidskriftsartikel (refereegranskat)abstract
- We review our understanding of the NO2 interaction with BaO. The presented picture has evolved from calculations using the density functional theory, reactor measurements and vibrational spectroscopy of surface species during NO2 storage on a BaO/Al2O3 catalyst and BaO powder. The DFT calculations predict nitrite formation on the BaO(1 0 0) surface upon NO2 adsorption. A particular stable adsorption configuration is a nitrate-nitrite pair geometry, with the nitrate bonded to a barium cation and the nitrite involving a surface oxygen anion. This configuration is supported by vibrational spectroscopy. Only nitrite formation is observed for NO exposure to BaO powder at low temperatures, whereas signatures at similar to250degreesC reveal the formation of surface nitrates. Continuous NO2 adsorption at this temperature results in surface Ba(NO3)(2), as well as bulk Ba(NO3)(2), formation.
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| 7. |
- Broqvist, Peter, 1975
(författare)
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On the role of Alkaline Earth Metal Oxides in Environmental Catalysis
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In heterogeneous catalysis, metal oxides have various roles: as catalysts, as support material for catalytically active metal particles, as promoters, as stabilizers, or as storage materials. Although their main application is as support material, their role as promoter or storage material becomes increasingly important as emission legislations are becoming more stringent. One such case is the catalytic cleaning of emissions from lean burn gasoline and diesel engines. Running the engine at lean conditions implies that the presently employed catalytic concepts are incapable of continuous NOx reduction. In this thesis, density functional theory (DFT) has been used to study the role of alkaline earth metal oxides, both as a storage material and as a support material for catalytically active metals. The main focus has been NOx storage in barium containing NOx storage and reduction (NSR) catalysts. As the storage material is experimentally ill-defined, different models for the storage material have been investigated (BaO(100), (BaO)9 cluster and BaCO3(110)). An atomistic understanding of species formed upon NO2 exposure to the storage material is presented. The mechanistic understanding comprises a pair-wise adsorption of NO2 with enhanced stability compared with single NO2 adsorption. Moreover, for NO2 storage in BaCO3, the pair-wise mechanism is shown to be crucial for effective surface decarbonation. Ab initio molecular dynamics calculations for NO2 adsorption on BaO(100) have been performed to study the dynamics of adsorbed NO2 molecules. Nitrites and nitrates are found to be mobile on the short time scale of the simulations (~4 ps). The role of oxides as model support material has been studied considering Pt atom, dimer and film adsorption on MgO(100) and BaO(100). For MgO(100), Pd, Ag and Au adsorption have been studied also. Both the metal/metal-oxide bond and support effects on the CO-metal bond upon CO adsorption have been explored. The support is found to prepare Pt for CO adsorption, resulting in both an enhanced Pt-CO bond and support-PtCO bond. However, this effect is only observed for mono-layer metal adsorption. Finally, the CO oxidation reaction is studied over a MgO supported Au nanoparticle. Special focus is given to adsorption and reaction at low-coordinated Au sites at the Au/MgO perimeter and to the effects of dopants. Keywords DFT, adsorption, catalysis, NOx storage, CO oxidation, supported catalysts, metal oxide, BaO, MgO, BaCO3, Au, slab, cluster
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| 8. |
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| 9. |
- Broqvist, Peter, 1975, et al.
(författare)
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Surface properties of alkaline earth metal oxides
- 2004
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 554:2-3, s. 262-271
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Tidskriftsartikel (refereegranskat)abstract
- Structural and energetic surface properties of the alkaline earth metal oxides MgO, CaO, SrO, and BaO are investigated within the density functional theory. In particular, structural distortions (relaxation and rumpling) and surface energies are studied for the (100) and (110) surfaces. A different sign of the rumpling is calculated for MgO compared to the other oxides. This difference is carefully examined and discussed in terms of differences in electronic properties, as well as nearest and next-nearest neighbor interactions. The stability of the results is investigated with respect to model approximations, such as the finite number of atomic layers used to describe semi-infinite surfaces, and choice of exchange-correlation energy functional.
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| 10. |
- Broqvist, Peter, 1975, et al.
(författare)
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The nature of NOx species on BaO(100): An ab initio molecular dynamics study
- 2005
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Ingår i: Journal of Physical Chemistry B. - 1520-5207 .- 1520-6106. ; B109, s. 15410-15416
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics of NO., species adsorbed on BaO(100) have been investigated with ab initio molecular dynamics simulations at a temperature of 300 degrees C. Nitrites are found to continuously interconvert between different adsorption configurations. For both nitrites and nitrates, diffusion events between anion sites are observed. These findings support the use of spillover mechanisms often postulated in mechanistic models of catalysts based on the NO, storage and reduction concept. The large number of possible adsorption configurations are reflected in broad calculated vibrational signatures. These results explain the corresponding property observed in experimental infrared measurements of NO, species on BaO. The dynamic response of the BaO(100) surface is found to strongly depend on the nature of the surface- adsorbate interaction. The largest distortions are predicted for nitrite adsorption
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| 11. |
- Broqvist, Peter, 1975, et al.
(författare)
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Toward a Realistic Description of NOxStorage in BaO: The Aspect of BaCO3
- 2005
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Ingår i: Journal of Physical Chemistry B. ; 109, s. 9613-9621
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Tidskriftsartikel (refereegranskat)abstract
- NOx storage over hexagonal BaCO3(110) is investigated using first-principles calculations. Special focus is put on the importance of surface decarbonation. Upon decarbonation, supported BaO quasi-molecules are formed and a small drive toward (BaO)(n) cluster formation is predicted. Introduction of NO2 makes the decarbonation energetically relevant, while forming NO2-BaO-NO2 units, on the decarbonated surface. With this configuration, it is possible to replace all surface carbonates with nitrites and nitrates, forming a BaCO3 supported BaNO3NO2 overlayer. Thermodynamic considerations are employed to elaborate on the thermal stability of the formed NOx overlayers.
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| 12. |
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| 15. |
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| 16. |
- Hammadi, Souzan, et al.
(författare)
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Impact of temperature on short-range charge ordering in LiFePO4/FePO4
- 2024
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 109:14
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Tidskriftsartikel (refereegranskat)abstract
- Energy is stored in a LiFePO4 battery electrode through the intercalation of Li. As Li incorporate into the crystal lattice of Fe(III)PO4, electrons reduce Fe(III) into Fe(II). The interactions of Li and its vacant site (Va) with these localized electrons (holes), so-called polarons, cause phase separation during battery operation. These fundamental interactions are however difficult to quantify using standard electronic structure calculations. In this paper, we utilize DFT+? with occupation matrix control to compute interaction energies at varying Li-Fe(II) and Va-Fe(III) pair separations. The increased energy with separation warrants the use of an electrostatic description. The DFT+? data are fitted to a Coulombic potential with two-body corrections and used in a Monte Carlo scheme. The coordination of the species determines their short-range ordering, showing that the Li and Va create chains bridged by their associated polarons which dissociate into dimers at higher temperatures. This dissociation happens at higher temperatures for Va than for Li, indicating a more pronounced clustering behavior during the formation of FePO4. Notably, a significant amount of uncoordinated Li exists at elevated temperatures, challenging the simplified picture of complete Li-Fe(II) pairing.
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| 17. |
- Mirsakiyeva, Amina, et al.
(författare)
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Initial Steps in PEO Decomposition on a Li Metal Electrode
- 2019
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 123:37, s. 22851-22857
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Tidskriftsartikel (refereegranskat)abstract
- Poly(ethylene oxide) (PEO) is the most widely used compound as a solid-state (solvent-free) polymer electrolyte for Li batteries, mainly due to its low glass transition temperature (T-g) and ability to dissolve Li salts. It is also frequently suggested that its cathodic stability renders it possible to operate with Li metal anodes in the design of high energy density storage devices. However, little is still known about the true interfacial chemistry between Li metal and PEO and how these two materials interact with each other. We are here exploring this relationship by the means of density functional theory (DFT)-based modeling. Using bulk structures and isolated PEO chains, we have found that there is a strong thermodynamic driving force to oxidize Li metal into lithium oxide (Li2O) when PEO is decomposed into C2H4 and H-2, irrespectively of the PEO oligomer length. Explicit modeling of PEO on a Li(100) surface reveals that all steps in the decomposition are exothermic and that the PEO/Li metal system should have a layer of Li2O between the polymer electrolyte and the metal surface. These insights and the computational strategy adopted here could be highly useful to better tailor polymer electrolytes with favorable interfacial properties.
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