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  • 51. Probst, Michael
    et al.
    Injan, Natcha
    Megyes, Tuende
    Bako, Imre
    Balint, Szabolcz
    Limtrakul, Jumras
    Nazmutdinov, Renat
    Mitev, Pavlin D.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    A gold cyano complex in nitromethane: MD simulation and X-ray diffraction2012Inngår i: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 539, s. 24-29Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The solvation structure around the dicyanoaurate(I) anion (Au(CN)(2)) in a dilute nitromethane (CH3NO2) solution is presented from X-ray diffraction measurements and molecular dynamics simulation (NVT ensemble, 460 nitromethane molecules at room temperature). The simulations are based on a new solute-solvent force-field fitted to a training set of quantum-chemically derived interaction energies. Radial distribution functions from experiment and simulation are in good agreement. The solvation structure has been further elucidated from MD data. Several shells can be identified. We obtain a solvation number of 13-17 nitromethane molecules with a strong preference to be oriented with their methyl groups towards the solute. (C) 2012 Elsevier B.V. All rights reserved.

  • 52. Raymand, David
    et al.
    Jacobsson, T. Jesper
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Edvinsson, Tomas
    Investigation of Vibrational Modes and Phonon Density of States in ZnO Quantum Dots2012Inngår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, nr 12, s. 6893-6901Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The ability to understand the phonon behavior in small metal oxide nanostructures and their surfaces is of great importance for thermal and microelectronic applications in successively smaller devices. Here the development of phonons in successively larger ZnO wurtzite quantum dots (QDs) is investigated. Raman spectroscopic measurements for particles from 3 to 11 nm reveal that the E-2 Raman active optical phonon at 436 cm(-1) is the first mode to be developed with a systematic increase with particle size. We also find a broad phonon band at 260-340 attributed to surface vibrations. The E-1-LO mode at 585 cm(-1) is the next to be developed while still being strongly suppressed in the confined particles. Other modes found in bulk ZnO are not developed for particles below 11 nm. Results from density functional theory showed an excellent agreement with the experimental molecular vibrations in the zinc acetate precursor and phonon modes in bulk ZnO. To elucidate the vibration behavior and phonon development in the ZnO QDs under nonzero temperature conditions and incorporating surface reconstruction, we performed reactive force field calculations. We show that the experimentally developed phonon modes in the QDs are the ones expected from dynamic theory. In particular, we show that the surface phonon modes in the very outermost surface (5 angstrom) can explain the observed broad phonon band and give the precise relation between the intensity of the surface and bulk phonons as the particle size increases. Calculations with temperatures between 50K and 1000K also show distinction of temperature effects in the material and that the phonon peaks are not generally shifted when the system is heated and quantum confined but instead reveal a dependence on the symmetry of the phonon mode.

  • 53. Sandstrom, M.
    et al.
    Persson, I.
    Jalilehvand, F.
    Lindquist-Reis, P.
    Spangberg, D.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Hydration of some large and highly charged metal ions2001Inngår i: Journal of Synchrotron Radiation, ISSN 0909-0495, E-ISSN 1600-5775, Vol. 8, s. 657-659Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    EXAFS studies of metal ions with hydration numbers higher than six in aqueous solution, often show asymmetric distribution of the metal-oxygen bond distances. The hydration number can be determined from a correlation with the bond distance. The mean Ca-O distance 2.46(1) Angstrom shows the calcium( II) ion to be eight-hydrated in a wide asymmetric distribution. Theoretically calculated EXAFS oscillations for individual snapshots from an MD simulation show large variations. The scandium( III) ion is surrounded by two groups of about eight water molecules, with the mean Sc-O distance 2.185(6) Angstrom. The yttrium( III) ion coordinates eight waters in an asymmetric distribution at 2.368(5) Angstrom, and the lanthanum( III) ion 6 + 3 water molecules at 2.52(2) and 2.65(3) Angstrom, respectively. For the the uranium( IV) and thorium(IV) ions, the M-O distances 2.42(1) and 2.45(1) Angstrom, respectively, indicate hydration numbers close to 10.

  • 54. Shpakov, V.
    et al.
    Gotte, A.
    Baudin, M.
    Woo, T.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    MgO(001) surface phonons from ab initio calculations2005Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, nr 19Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    An ab initio study of the surface phonons in high-symmetry points of the Brillouin zone has been performed for the clean MgO(001) surface in the framework of the density functional theory (DFT) using plane waves and a lattice-dynamical treatment with a supercell approach. It is found that a calculation of the surface phonon modes using the DFT is a competitive approach compared to earlier shell-model calculations in the literature. The strongly localized phonon modes can be calculated with a reasonable accuracy using thin slab systems and a modest plane-wave cutoff.

  • 55.
    Sjödahl, Johan
    et al.
    KTH, Skolan för kemivetenskap (CHE), Kemi, Analytisk kemi.
    Kempka, Martin
    KTH, Skolan för kemivetenskap (CHE), Kemi, Analytisk kemi.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Thorsén, Anders
    Roeraade, Johan
    KTH, Skolan för kemivetenskap (CHE), Kemi, Analytisk kemi.
    Chip with twin anchors for reduced ion suppression and improved mass accuracy in MALDI-TOF mass spectrometry2005Inngår i: Analytical Chemistry, ISSN 0003-2700, E-ISSN 1520-6882, Vol. 77, nr 3, s. 827-832Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A new sample target for matrix-assisted laser desorption/ionization mass spectrometry is described. The target consists of pairs of elevated hydrophilic anchor surfaces, positioned in proximity onto a microchip. The anchors are used to obtain separate preparations of sample and external standard, while both anchor surfaces are irradiated simultaneously by the laser pulse. Using a standard, based on six peptides, a 2-fold improvement in mass accuracy is observed. Also, ion suppression is significantly reduced. With a one peptide calibration standard, 22 tryptic fragments from a BSA digest are detected using the twin-anchor concept, whereas only 14 fragments are detected when the sample and standard are laser-ablated as a mixture from a conventional anchor target. A volume of similar to30 pL of sample solution of angiotensin I is transferred to the anchor surface, under a thin layer of a perfluorocarbon, to prevent a concentration bias due to evaporation. With this arrangement, a detection limit of 1.5 amol was achieved with a signal-to-noise ratio of 22:1.

  • 56. Skorodumova, N. V.
    et al.
    Baudin, M.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Surface properties of CeO2 from first principles2004Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 69, nr 7Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The surface energies and electronic structure of (111), (110), and (100) surfaces of CeO2 have been studied using the first-principles projector augmented-wave method within the local density approximation (LDA) and generalized gradient approximation (GGA) and are compared with molecular dynamics (MD) simulations at 10 K. Slabs of the same thicknesses were used for the two methods (lamellar three-dimensional boxes in the case of density functional theory and two-dimensional isolated slabs in the MD simulations). The polar (100) surface of CeO2 was modeled as an oxygen-terminated surface with half of the oxygen atoms moved from one surface to the other to fulfill the stoichiometric formula. Among a number of different possible terminations, the checkerboard termination was found to be the most stable. Out of all three low-index surfaces, the (111) surface was found to be the most stable, in agreement with experiment and previous calculations. Our calculated surface energy is 1.0 J/m(2) (LDA) and 0.7 J/m(2) (GGA). Both values are lower than most calculated surface energies for this surface in the literature, including our present MD results (1.4 J/m(2)). For (110) our surface energies (LDA, GGA, and MD) are 40%-60% higher than for (111). The structural relaxation is considerably larger for (110) than for (111) and shows excellent agreement between LDA, GGA, and MD. Also the electronic density of states shows larger differences between surface and bulk for the (110) surface than for (111), with, for example, a decrease of the band gap between the valence band (essentially O 2p) and the empty Ce 4f band in the (110) surface region compared to the bulk.

  • 57. Skorodumova, N. V.
    et al.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Johansson, Börje
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Structural and electronic properties of the (100) surface and bulk of alkaline-earth metal oxides2005Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, nr 12Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Electronic and structural properties of the (100) surfaces and bulk of MgO, CaO, SrO, and BaO have been studied using the projector augmented wave (PAW) method within the local density approximation and generalized gradient approximation. Relaxed (100) MgO shows the positive rumpling of 2.2% whereas (100) CaO, (100) SrO, and (100) BaO exhibit increasingly negative rumpling and surface contraction. (100) CaO is a border case between the two trends and the sign of the rumpling of this surface can be changed at distortion. We find that the surface structures obtained as a result of relaxation are largely determined by an increasing interaction between the cation semicore states and oxygen p states observed in the oxide series.

  • 58. Spangberg, D.
    et al.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Effective three-body potentials for Li+(aq) and Mg2+(aq)2003Inngår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 119, nr 14, s. 7263-7281Artikkel, forskningsoversikt (Fagfellevurdert)
    Abstract [en]

    A method for the extraction of effective three-body potential parameters from high-level ab initio cluster calculations is presented and compared to effective pair potentials extracted at the same level. Dilute Li+(aq) and Mg2+(aq) solutions are used as test cases and long molecular-dynamics simulations using these newly developed potentials were performed. Resulting thermodynamical, structural, and dynamical properties are compared to experiment as well as to the empirical effective pair potentials of Aqvist. Moreover, a new time-saving method for the correction of cluster energies computed with a relatively cheap ab initio method, to yield expensive, high-level ab initio energies, is presented. The effective pair approach is shown to give inconsistent results when compared to the effective three-body potentials. The performance of three different charge compensation methods (uniform charge plasm, Bogusz net charge correction, and counter ions) is compared for a large number of different system sizes. For most properties studied here, the system-size dependence is found to be small for system sizes with 256 water molecules or more. However, for the self-diffusion coefficients, a 1/L dependence is found, i.e., a very large system-size dependence. A very simple method for correcting for this deficiency is proposed. The results for most properties are found to compare reasonably well to experiment when using the effective three-body potentials.

  • 59. Spangberg, D.
    et al.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Many-body potentials for aqueous Li+, Na+, Mg2+, and Al3+: Comparison of effective three-body potentials and polarizable models2004Inngår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 120, nr 10, s. 4829-4843Artikkel, forskningsoversikt (Fagfellevurdert)
    Abstract [en]

    Many-body potentials for the aqueous Li+, Na+, Mg2+, and Al3+ ions have been constructed from ab initio cluster calculations. Pure pair, effective pair, effective three-body, and effective polarizable models were created and used in subsequent molecular dynamics simulations. The structures of the first and second solvation shells were studied using radial distribution functions and angular-radial distribution functions. The effective three-body and polarizable potentials yield similar first-shell structures, while the contraction of the O-O distances between the first and second solvation shells is more pronounced with the polarizable potentials. The definition of the tilt angle of the water molecules around the ions is discussed. When a proper definition is used, it is found that for Li+, Mg2+, and Al3+ the water molecules prefer a trigonal orientation, but for Na+ a tetrahedral orientation (ion in lone-pair direction) is preferred. The self-diffusion coefficients for the water molecules and the ions were calculated; the ionic values follow the order obtained from experiment, although the simulated absolute values are smaller than experiment for Mg2+ and Al3+.

  • 60. Spangberg, D.
    et al.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    The solvation of Li+ and Na+ in acetonitrile from ab initio-derived many-body ion-solvent potentials2004Inngår i: Chemical Physics, ISSN 0301-0104, E-ISSN 1873-4421, Vol. 300, nr 03-jan, s. 165-176Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Several Li--(+) and Na+-acetonitrile models were derived from ab initio calculations at the counterpoise-corrected MP2/ TZV++(d,p) level for distorted ion-(MeCN)(n) clusters with n = 1, 4 and 6. Two different many-body ion-acetonitrile models were constructed: an effective three-body potential for use with the six-site effective pair model of Bohm et al., and an effective polarizable many-body model. The polarizable acetonitrile model used in the latter model is a new empirical model which was also derived in the present paper. Mainly for comparative purposes, two ion-acetonitrile pair potentials were also constructed from the ab initio cluster calculations: one pure pair potential and one effective pair potential. Using all these potential models, MD simulations in the NPT ensemble were performed for the pure acetonitrile liquid and for Li+(MeCN) and Na+(MeCN) solutions with 1 ion in 512 solvent molecules and with a simulation time of at least 120 ps per system. Thermodynamic properties, solvation-shell structure and the self-diffusion coefficient of the ions and of the solvent molecules were calculated and compared between the different models and with experimental data, where available. The Li+ ion is found to be four-coordinated when the new many-body potentials are used, in contrast to the six-coordinated structure obtained for the pure pair and effective pair potentials. The coordination number of Na+ close to six for all the models derived here, although the coordination number becomes slightly smaller with the many-body potentials. For both ions, the solvent molecules in the first shell point their nitrogen ends towards the cation, while in the second shell the opposite orientation is the most common.

  • 61. Spangberg, D.
    et al.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Lindqvist-Reis, P.
    Jalilehvand, F.
    Sandstrom, M.
    Persson, I.
    Model extended X-ray absorption fine structure (EXAFS) spectra from molecular dynamics data for Ca2+ and Al3+ aqueous solutions2000Inngår i: Journal of Physical Chemistry B, ISSN 1089-5647, Vol. 104, nr 45, s. 10467-10472Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Theoretical extended X-ray absorption fine structure (EXAFS) spectra have been computed from molecular dynamics (MD) simulation data. Based on MD-generated molecular geometries, EXAFS spectra were generated with the FEFF6 program, commonly used in the analysis of experimental EXAFS spectra. The effects of multiple scattering and second-shell neighbors on the theoretical spectra were evaluated for CaCl2(aq) and AlCl3(aq), i.e., for two solutions with very different cation hydration structures. The effects are significantly larger for the more structured Al3+ solution, The theoretical Ca2+(aq) spectra are compared with new experimental EXAFS data.

  • 62. Spangberg, D.
    et al.
    Rey, R.
    Hynes, J. T.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Rate and mechanisms for water exchange around Li+(aq) from MD simulations2003Inngår i: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 107, nr 18, s. 4470-4477Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The solvent exchange rate of aqueous Li+ has been determined by employing the reactive flux method in combination with very long molecular dynamics simulations based on effective three-body ion-water potentials. The mechanisms for the resulting exchange events have been studied in considerable detail and analyzed in terms of the five commonly discussed exchange classes: associative (A), dissociative (D), and interchange (1, 1a, 1d). Furthermore, the stereochemistry (cis, trans) of each exchange has been monitored and correlated with the exchange classes. Most of the exchanges are associative or associative interchanges and of a trans type, although the trans exchanges are less probable on a purely statistical basis.

  • 63. Wojcik, M. C.
    et al.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Siegbahn, H. O. G.
    Structure and predicted near edge x-ray absorption fine structure spectra for the surface of liquid formamide from molecular-dynamics simulation2000Inngår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 113, nr 8, s. 3374-3380Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A modification of an existing model for formamide interactions in the liquid phase makes it possible to study the bulk liquid and its free surface at conditions near the triple point, i.e., 300 K and essentially zero pressure, by (MD) molecular-dynamics simulation. Density profiles and orientation profiles in the surface region show the depth and gradual loss of the molecular ordering in the transition from bulk vapor to the bulk liquid. Near edge x-ray absorption fine structure (NEXAFS) resonance intensity spectra are computed from the orientation profiles, so that comparison should be possible with experimental depth resolved NEXAFS spectra.

  • 64. Yang, Z. X.
    et al.
    Woo, T. K.
    Baudin, M.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Atomic and electronic structure of unreduced and reduced CeO2 surfaces: A first-principles study2004Inngår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 120, nr 16, s. 7741-7749Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The atomic and electronic structure of (111), (110), and (100) surfaces of ceria (CeO2) were studied using density-functional theory within the generalized gradient approximation. Both stoichiometric surfaces and surfaces with oxygen vacancies (unreduced and reduced surfaces, respectively) have been examined. It is found that the (111) surface is the most stable among the considered surfaces, followed by (110) and (100) surfaces, in agreement with experimental observations and previous theoretical results. Different features of relaxation are found for the three surfaces. While the (111) surface undergoes very small relaxation, considerably larger relaxations are found for the (110) and (100) surfaces. The formation of an oxygen vacancy is closely related to the surface structure and occurs more easily for the (110) surface than for (111). The preferred vacancy location is in the surface layer for CeO2(110) and in the subsurface layer (the second O-atomic layer) for CeO2(111). For both surfaces, the O vacancy forms more readily than in the bulk. An interesting oscillatory behavior is found for the vacancy formation energy in the upper three layers of CeO2(111). Analysis of the reduced surfaces suggests that the additional charge resulting from the formation of the oxygen vacancies is localized in the first three layers of the surface. Furthermore, they are not only trapped in the 4f states of cerium.

  • 65. Yang, Z. X.
    et al.
    Woo, T. K.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Strong and weak adsorption of CO on CeO2 surfaces from first principles calculations2004Inngår i: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 396, nr 06-apr, s. 384-392Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The adsorption of CO on (1 1 1), (1 1 0) surfaces of ceria (CeO2) was studied using projector-augmented wave (PAW) method based density-functional theory within generalized gradient approximation (GGA). It is found that there exist different adsorption features for CO on (1 1 1) and (1 1 0) surfaces. While only weak adsorptions are found on (1 1 1) surface, both weak and strong adsorptions exist on (1 1 0) surface. Electrostatic interactions were involved in the weak interactions, while covalent bonding was developed in the strong adsorptions. The strongly adsorbed CO on the (1 1 0) surface of CeO2 forms carbonate species as found in the IR experiments.

  • 66. Yang, Zongxian
    et al.
    Lu, Zhansheng
    Luo, Gaixia
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Oxygen vacancy formation energy at the Pd/CeO2(111) interface2007Inngår i: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 369, nr 02-jan, s. 132-139Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The atomic and electronic structure of Pd adsorption on the unreduced and reduced ceria (111) surfaces is studied using first principles projector-augmented-wave (PAW) method based density functional theory (DFT) within generalized gradient approximation (GGA) and with the inclusion of on-site Coulomb interaction (GGA + U). It is found that there exist different adsorption features for Pd on different surfaces (unreduced and reduced): (1) on the unreduced CeO2(111) surface, Pd prefers to be adsorbed on the O-bridge site (O-2-site); (2) on the reduced CeO2(111) surface, the adsorption is much stronger than that on the unreduced CeO2(111) surface; (3) the formation of an oxygen vacancy is easier at the Pd/CeO2(111) interface than that on the clean CeO2(111) surface, indicating that the Pd adatom enhances the oxygen storage capacity (OSC) of ceria, and the interaction between the palladium catalyst and the ceria support enhances the lower temperature activity of the catalyst. The mechanisms for this promotion in the OSC of ceria at the Pd/ceria interface are discussed.

  • 67. Yang, Zongxian
    et al.
    Luo, Gaixia
    Lu, Zhansheng
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Oxygen vacancy formation energy in Pd-doped ceria: A DFT+U study2007Inngår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 127, nr 7Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Using the DFT+U method, i.e., first principles density functional theory calculations with the inclusion of on-site Coulomb interaction, the effects of Pd doping on the O vacancy formation energy (E-vac) in CeO2 has been studied. We find that E-vac is lowered from 3.0 eV in undoped ceria to 0.6 eV in the Pd-doped compound. Much of this decrease can be attributed to emerging Pd-induced gap states above the valence band and below the empty Ce 4f states. These localized defect states involve the Pd ion and its nearest neighbors, which are also the main acceptors of the extra electrons left on reduction. The effect of the Pd dopant on the geometric structure is very modest for CeO2 but considerable for CeO2-x.

  • 68. Yang, Zongxian
    et al.
    Woo, Tom K.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Adsorption of NO on unreduced and reduced CeO2 surfaces: A plane-wave DFT study2006Inngår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 600, nr 22, s. 4953-4960Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The adsorption of NO on the (111) and (110) surfaces of ceria (CeC2) was studied using projector-augmented wave (PAW) method based density-functional theory within the generalized gradient approximation (GGA). Several adsorption sites for NO on the stoichiometric surfaces are found, all with weak molecule-surface interaction. The adsorption on the reduced surfaces is much stronger. The O-ends of the adsorbed NO molecules fill the oxygen vacancies and the N-O bonds are elongated. If two such adsorbed NO molecules, residing at neighbouring sites, meet, their N-ends will form a strong N-N bond with little or no barrier. This is an intermediate step towards dissociation of free N-2 which is calculated to be strongly thermodynamically driven.

  • 69. Yang, Zongxian
    et al.
    Woo, Tom K.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Effects of Zr doping on stoichiometric and reduced ceria: A first-principles study2006Inngår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 124, nr 22Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The Zr doping in CeO2 may change the reduction properties and therefore the redox properties of CeO2. Using first-principles density functional theory with the inclusion of on-site Coulomb interaction for a 96-atom supercell, these effects are studied by comparing the differences in atomic structures, electronic structures, and reduction energies of the doped CeO2 and those of the nondoped CeO2. It is found that (1) Zr doping of the ceria structure results in important modifications involving nonequivalent O atoms; (2) the oxygen anions (still four-coordinated) next to the doping center show considerably lower reduction energies (by 0.6 eV) and larger displacements ('' higher mobilities ''); (3) an O vacancy is most easily created close to the Zr centers, therefore the Zr-doping centers might serve as nucleation centers for vacancy clustering; and (4) the electrons left by the released oxygen localize on two Ce cations neighboring the vacancy, which results in the reduction of two Ce4+ ions.

  • 70. Zhukovskii, Y. F.
    et al.
    Kotomin, E. A.
    Herschend, B.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Jacobs, P. W. M.
    The adhesion properties of the Ag/alpha-Al2O3(0001) interface: an ab initio study2002Inngår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 513, nr 2, s. 343-358Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Ab initio computer simulations of the atomic and electronic structure of the Ag/alpha-Al2O3(0 0 0 1) (corundum) interface have been performed for a periodic two-dimensional slab model using the Hartree-Fock method and a posteriori electron correlation corrections. We have considered both Al- and O-terminated corundum substrate surfaces. The dependence of the adhesion energy on the interfacial distance has been analyzed for the two most favorable Ag adsorption positions over corundum and for two different metal coverages (a 1/3 monolayer (NIL) of the Ag(l 1 1) crystallographic plane and a full Ag(I 1 1) monolayer). The two different terminations (Al- and O-) give rise to qualitatively different results. The former case corresponds to the most stable termination of the pure corundum (0 0 0 1) substrate where small adhesion energies per Ag atom (0.15-0.25 eV for I ML and 0.40-0.55 eV for 1/3 NIL) are accompanied by minor interfacial charge transfer, indicating physisorption, which may be explained by a weak atomic polarization. In contrast, for O-terminated corundum, substantial adhesion energies (3-5 eV per Ag atom at I ML coverage and 6-11 eV for 1/3 ML) combined with noticeable charge transfer from silver atoms towards the substrate (0.5e to 0.9e) are clear indications of a strong interfacial ion bonding. For both terminations, the observed difference in Ag adhesion energies for 1/3 NIL and I NIL coverages arises from a transition from directed Ag-O bonding towards a more delocalized electron density distribution in the complete monolayer. The results of our calculations are compared with available experimental studies and theoretical simulations for various Me/Al2O3 interfaces.

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