Ändra sökning
Avgränsa sökresultatet
12 1 - 50 av 96
RefereraExporteraLänk till träfflistan
Permanent länk
Referera
Referensformat
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annat format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annat språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf
Träffar per sida
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sortering
  • Standard (Relevans)
  • Författare A-Ö
  • Författare Ö-A
  • Titel A-Ö
  • Titel Ö-A
  • Publikationstyp A-Ö
  • Publikationstyp Ö-A
  • Äldst först
  • Nyast först
  • Skapad (Äldst först)
  • Skapad (Nyast först)
  • Senast uppdaterad (Äldst först)
  • Senast uppdaterad (Nyast först)
  • Disputationsdatum (tidigaste först)
  • Disputationsdatum (senaste först)
  • Standard (Relevans)
  • Författare A-Ö
  • Författare Ö-A
  • Titel A-Ö
  • Titel Ö-A
  • Publikationstyp A-Ö
  • Publikationstyp Ö-A
  • Äldst först
  • Nyast först
  • Skapad (Äldst först)
  • Skapad (Nyast först)
  • Senast uppdaterad (Äldst först)
  • Senast uppdaterad (Nyast först)
  • Disputationsdatum (tidigaste först)
  • Disputationsdatum (senaste först)
Markera
Maxantalet träffar du kan exportera från sökgränssnittet är 250. Vid större uttag använd dig av utsökningar.
  • 1. Axelsson, O.
    et al.
    Paul, Jan
    Weisel, M.D.
    Hoffmann, F,M.
    Reactive evaporation of potassium in CO2 and the formation of bulk intermediates1994Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 12, nr 1, s. 158-160Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Potassium vapor condensed in CO2 has been studied by transmission infrared spectroscopy and by total pressure thermal desorption spectroscopy. The results are compared with surface spectroscopic data for CO2 adsorbed on alkali metal overlayers. Both systems show the formation of bulk coordinated oxalate species at cryogenic temperatures and the conversion to carbonate intermediates upon annealing.

  • 2. Axelsson, O.
    et al.
    Shao, Y.
    Paul, Jan
    Hoffmann, F.M.
    A theoretical and experimental study of reaction pathways for the interaction of CO2 with alkali-modified surfaces1995Ingår i: Journal of Physical Chemistry, ISSN 0022-3654, Vol. 99, nr 18, s. 7028-7035Artikel i tidskrift (Refereegranskat)
  • 3.
    Aycik, G.A.
    et al.
    Turkish Atomic Energy Authority, Ankara Nuclear Research and Training Center.
    Paul, M.
    Turkish Atomic Energy Authority, Ankara Nuclear Research and Training Center.
    Sandström, Åke
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Industriell miljö- och processteknik.
    Paul, Jan
    Leaching of radioactive isotopes from ash2003Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

    The aim of the study is to reduce the environmental impact of ash deposits. Ash from coal and biomass combustion, containing uranium and thorium from Yatagan-Silopi and Tuncbilek coal; cesium-137 from forests in northeastern Turkey and central Sweden. Turkey is dependent on coal for power generation and huge volumes of ash (>15 Mton/yr) are produced every year. Because of that certain coals, in particular Yatagan, with known problems from Mo and U leaching to the ground water, and Silopi oil shales/asphaltites were studied. Biomass ash comes from branches, bark and other unused parts of the trees and plants. This ash has low concentration of environmentally hazardous metals, but {sup 137}Cs is a problem in certain regions and it is of interest to investigate the possibilities to leach this metal from the ash. Washing ash through rapid chemical leaching at low pH reduces the slow release of metals from ash due to precipitation and besides it may lead to metal recovery from the ash. Initial experiments were done in batch form, in which the neutralizing capacity at pH 1-1.5 was measured by adding sulfuric acid to maintain pH for mixtures of ash and water. Subsequent experiments were done in bench scale. The process also reduced the metal content of the ash, due to chemical leaching of metal oxides and ion exchange at the surfaces of stable oxides. This means that treated ash will not release further metals and, eventually, relaxes the requirements on depositories and allows the ash from coal to be used as an admixture in cement, and to be used as a fertilizer following after treatment of ash from biomass

  • 4. Chen, W.
    et al.
    Cameron, S.
    Göthelid, M.
    Hammar, M.
    Paul, Jan
    Redox properties of titanium oxides on Pt3Ti1995Ingår i: Journal of Physical Chemistry, ISSN 0022-3654, Vol. 99, nr 34, s. 12892-12895Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The morphology and electronic structure of surface-segregated titanium oxides on Pt3Ti(111) are presented. Core level photoemission spectra at grazing emission reveal two states of oxidation: a dominant and reducible four-valent oxide together with a small amount of a three-valent oxide is produced by oxidation in 0 2 at and below 400°C; an irreducible three-valent oxide by oxidation in 02 at and above 450 °C. The ratio between the active four-valent and the inactive three-valent oxides decreases with increasing oxidation temperature. The probability for reduction by CO is almost unity for the Ti 4+ oxide, and the conclusion must be that the four-valent oxide plays an active role for catalytic reactions. Scanning tunneling measurements relate these observations to changes in the dispersion and nucleation of the oxide overlayer. The four-valent oxide grows as islands with remaining areas open for CO adsorption while the three-valent oxide spreads on and blocks the crystal surface. Photoemission spectra relate these dispersion effects to an electronic interaction between the Ti 3+ oxide and adjacent Pt atoms. The above observations are in accordance with the common picture of dispersion effects in titania-supported SMSI catalysts and prove that interfacial energies play a crucial role whether the dominant phase is metallic or an oxide.

  • 5. Chen, W.
    et al.
    Chulkov, E.
    Paul, Jan
    Band structure calculations of Pt and Pt3Ti1996Ingår i: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 54, nr 4, s. 392-396Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Linearized Augmented Plane Wave (LAPW) calculations of the Pt and Pt3Ti bulk electron structures are presented. We emphasis on hybridization, charge transfer effects and the impact on chemisorption properties Densities of States (DOS) and novel band-structures are also provided. The delocalized character of free electron states documents itself by sp-orbitals largely unaffected by alloy formation i.e. the Pt and Ti projected sp-densities are similar and also akin to the distribution of sp-states for pure platinum. The strong Pt-Ti interaction comes from d and f orbitals with a significantly lowered Pt d center-of-mass for the alloy and an intense Ti d structure above the Fermi level. The f-projected density of states gives two narrow peaks: a Ti peak below the Fermi level and an unoccupied Pt peak near Ef. Together the d and f interactions result in a much lowered DOS(Ef). Our results are of direct relevance for adsorption systems modelling titania supported platinum, a well known catalyst for CO hydrogenation. The ordered alloy, Pt3Ti, forms in the industrial catalyst under reducing conditions at elevated temperatures, a phenomenon related to the Strong Metal Support Interaction (SMSI) effect in catalysis, and much work has addressed special adsorption sites for enhanced methanation rates.

  • 6. Chen, W.
    et al.
    Paul, Jan
    Barbieri, A.
    Hove, M.A. van
    Cameron, S.
    Structure determination of Pt3Ti(111) by automated tensor LEED1993Ingår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 5, s. 4585-4594Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An analysis of low-energy electron diffraction (LEED) I-V spectra from the clean Pt3Ti(111) surface was performed by comparing measured intensities with data calculated using an automated tensor LEED program, which employs a directed search optimization procedure. It was found that the topmost layer is pure Pt and that the other layers have the bulk composition. The first and second interlayer spacings are 2.23+or-0.03 AA and 2.21+or-0.03 AA respectively, corresponding to a contraction of 0.9% and 1.8% of the bulk value. The perpendicular buckling is 0.04 AA +or-0.05 AA in the top layer and 0.15 AA +or-0.04 AA in the second layer. The results are in full accordance with previous investigations of the physical and chemical properties of this surface.

  • 7. Chen, Wenhua
    et al.
    Lu, Hua
    Pradier, Claire-Marie
    Paul, Jan
    Flodström, Anders
    Reduction of NO by C4 hydrocarbons on platinum in the presence of oxygen: influence of sulfur dioxide1997Ingår i: Journal of Catalysis, ISSN 0021-9517, E-ISSN 1090-2694, Vol. 172, nr 1, s. 3-12Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The reduction of NO, in the presence of i-C4H10or i-C4H8and O2, catalyzed by a platinum foil, was studied in order to understand better the catalytic activity of platinum metal, free of any support or dispersion effects. The reaction products were analyzed by mass spectrometry, and the surface was characterized by X-ray photoelectron spectroscopy at different stages of the reaction. A correlation between the catalytic activity for NO conversion and the presence of adsorbed intermediates is demonstrated. The role of oxygen is interpreted as twofold: formation of active intermediates and deactivation of the surface. The effect of traces of sulfur dioxide in the reacting phase on the reduction of NO by isobutene was investigated. The influence of SO2is very much dependent upon its initial concentration in the gas phase. Low concentrations (<15 ppm) promote the reduction of NO, whereas higher levels poison the reaction by a surface site blocking effect.

  • 8. Chen, Wenhua
    et al.
    Paul, Jan
    Intermolecular forces in porphyrin crystals1995Ingår i: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 34, nr 1, s. 199-201Artikel i tidskrift (Refereegranskat)
  • 9. Chen, Wenhua
    et al.
    Severin, Lukas
    Göthelid, Mats
    Hammar, Mattias
    Cameron, Steve
    Paul, Jan
    Electronic and geometric structure of clean Pt3Ti(111)1994Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 50, nr 8, s. 5620-5627Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Photoemission spectra and scanning tunneling microscopy (STM) images of the clean Pt3Ti(111) surface are presented. Grazing-emission core-level spectra show that the topmost layer is pure platinum, modified compared with the Pt(111) surface. The Pt 4f levels at the surface are shifted 0.4 eV toward the Fermi level relative to bulk Pt3Ti while the Pt 4f and Ti 2p levels in the bulk are shifted 0.4 and 1.3 eV to higher binding energy relative to pure bulk platinum and titanium, respectively, Tunneling measurements show a surface with only metallic atoms and a small p(2×2) buckling. Our observations of molecularly adsorbed CO are not compatible with metallic titanium atoms at the surface and the STM data thus indirectly confirm that only platinum atoms are present in the topmost layer. Linear muffin-tin-orbital calculations of the bulk band structure and valence-band photoemission spectra reveal highly hybridized electron states between the Pt d and Ti d levels. The calculations give a minor charge transfer from Ti to Pt, 0.37 electrons per Ti atom, but the large core-level shifts reflect the stability of the alloy and the response to the excitation rather than the amount of charge transfer. The observed segregation of platinum to the surface and the altered electronic structure of the topmost layer, due to interaction with the underlying alloy, are in full agreement with earlier conclusions based on low-energy electron-diffraction measurements and on the chemical properties of the surface.

  • 10. Demirel, B.
    et al.
    Paul, Jan
    Analyses of products from autoclave reactions: derivations of reaction parameters1996Ingår i: Vol. 41, nr 4, s. 1157-1160Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This work presents energetics for high pressure hydroprocessing reactions derived from post analyses of liquid and gaseous products. Specifically, GC and GC/MS were used to follow the product distribution fram the hydrocracking of methyldecalins over zeohte supported palladium and platinum catalysts as a function of temperature. Plain Arrhenius plots summarize key results and reveal possible connections in terms of`activation energies between hydrogen consumption and the amounts of different products. The total cycloalkane production and the consumption ofhydrogen both show a simple temperature dependence with the same{open_quote}activation{close_quote} energies. Methane production varies more rapidly with temperature but can still be described by a single exponential term. The final example, conversion to aromatics, displays a more complicated dependence with an accelerated yield at high temperature. This form of data analyses connects to a new routine for mass balance evaluations and it is now applied to model catalyst performance and to understand optimum reaction conditions. Other branches of this project include surface spectroscopic measurements of fresh, sulfided and used catalysts, characterization of partially hydrogenated naphthalenes and modeling of hydrogen activity at metal sulfides.

  • 11.
    Edelbro, R.
    et al.
    Luleå tekniska universitet.
    Sandström, Åke
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Industriell miljö- och processteknik.
    Paul, Jan
    Full potential calculations on the electron band structures of Sphalerite, Pyrite and Chalcopyrite2003Ingår i: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 206, nr 1-4, s. 300-313Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The metal contents of ore can be as low as 0.4%m This means sophisticated methods of enrichment have to be applied. Better understanding of the processes of flotation and leaching may lead to higher yields and less damage to the environment. The bulk electronic structures of Sphalerite, Pyrite and Chalcopyrite have been calculated within an ab initio, full potential, density functional approach. The exchange term was approximated with the Dirac exchange functional, the Vosko-Wilk-Nusair parameterization of the Cepler-Alder free electron gas was used for correlation and linear combinations of Gaussian type orbitals were used as basis functions. The Sphalerite (zinc blend) band gap was calculated to be direct with a width of 2.23 eV. The Sphalerite valence band was 5.2 eV wide and composed of a mixture of sulfur and zinc orbitals. The band below the valence band located around -6.2 eV was mainly composed of Zn 3d orbitals. The S 3s orbitals gave rise to a band located around -12.3 eV. Pyrite was calculated to be a semiconductor with an indirect band gap of 0.51 eV, and a direct gap of 0.55 eV. The valence band was 1.25 eV wide and mainly composed of non-bonding Fe 3d orbitals. The band below the valence band was 4.9 eV wide and composed of a mixture of sulfur and iron orbitals. Due to the short inter-atomic distance between the sulfur dumbbells, the S 3s orbitals in Pyrite were split into a bonding and an anti-bonding range. Chalcopyrite was predicted to be a conductor, with no band-crossings at the Fermi level. The bands at -13.2 eV originate from the sulfur 3s orbitals and were quite similar to the sulfur 3s bands in Sphalerite, though somewhat shifted to lower energy. The top of the valence band consisted of a mixture of orbitals from all the atoms. The lower part of the same band showed metal character. Computational modeling as a tool for illuminating the flotation and leaching processes of Pyrite and Chalcopyrite, in connection with surface science experiments, is discussed.

  • 12.
    Edholm, O.
    et al.
    Kungliga tekniska högskolan, KTH.
    Nordlander, P.
    Rice University, Houston.
    Chen, W.
    University of Hongkong.
    Ohlsson, I.
    Umeå universitet.
    Smith, M.L.
    Chuparosa Research, Mesa, Arizona.
    Paul, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    The effect of water on the Fe3+/Fe2+ reduction potential of heme2000Ingår i: Biochemical and Biophysical Research Communications - BBRC, ISSN 0006-291X, E-ISSN 1090-2104, Vol. 268, nr 3, s. 683-687Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Hemeproteins can act as catalysts, oxygen carriers or electron conductors. The ferric/ferrous reduction potential Em7 of iron in the center of the prosthetic group ranges from negative values for peroxidases to an extreme positive value for cytochrome a3 with Hb and Mb in the middle [1]. Proteins exercise their influence on Em7 in several ways: via substituents at the periphery of the chelate structure, via the proximal ligand, and via interaction with the surrounding medium, amino acid side chains, or polar solvents. Work on recombined proteins and 2,4-substituted free hemes documented that the first two effects are additive [2]. For the third effect, models of the dielectric media on a molecular level have been successfully applied [3-5]. Em7 has also been empirically correlated to the degree of heme exposure to water [6-8]. The apoprotein/porphyrin and water/porphyrin interfaces are complementary since water molecules fill any empty space in the crevice and surround any pertinent part of heme outside the protein boundary. The present work links to this idea by a combination of statistical mechanics simulations and quantum mechanical calculations comparing heme in water with heme in an apolar environment. Our results show that polarization of the porphyrin π-electron cloud by the field from water dipoles influences Em7. The dominant effect of this and other determinates of iron electron availability is perturbations of delocalized electron density in the porphyrin chelate, reproduced by a model where the prosthetic group is treated as a disc of uniform electron density. The present work is also of interest since the interfacial energy constitutes the main barrier for heme-protein separation [9-11].

  • 13. Edholm, O.
    et al.
    Ohlsson, P.I.
    Smith, M.L.
    Paul, Jan
    The barrier for heme-protein separation estimated by non-equilibrium molecular dynamics simulations1998Ingår i: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 291, nr 5-6, s. 501-508Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In heme-containing proteins the heme group is usually non-covalently bound in a pocket. Molecular dynamics (MD) simulations have been performed to estimate the barrier height for heme-protein separation. In simulations of myoglobin dissolved in water, a force has been applied to pull the heme out of the binding pocket. With forces above 0.5 nN, the heme group is easily pulled out of the pocket in times of the order of tens of picoseconds. With weaker forces, heme release becomes too slow to be monitored in an MD simulation covering a couple of hundred picoseconds. These results are consistent with a free energy barrier to heme release of about 100 kJ/mol. The results show that the main energetic change that occurs during the release is a conversion of heme/protein Lennard-Jones energy into heme/water Lennard-Jones energy. The release is essentially barrierless in energy indicating that the main part of the barrier is entropic.

  • 14.
    Ganemi, B.
    et al.
    Kungliga tekniska högskolan, KTH.
    Björnbom, E.
    Kungliga tekniska högskolan, KTH.
    Demirel, B.
    Center for Applied Energy Research, Lexington, KY.
    Paul, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Zeolite Cu-ZSM-5: material characteristics and NO decomposition2000Ingår i: Microporous and Mesoporous Materials, ISSN 1387-1811, E-ISSN 1873-3093, Vol. 38, nr 2-3, s. 287-300Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Zeolite ZSM-5 (SiO2/Al2O3 ratio 53:1) ion exchanged with Cu2+ to 0%, 74% and 160% was characterized by X-ray diffraction (XRD), Thermogravimetric analysis (TGA), Infrared (IR) spectroscopy, Electron spectroscopy for chemical analysis (ESCA) and ammonia desorption. A more limited set of data was obtained for Cu-ZSM-5-33, ion exchanged with 0%, 104% and 210% Cu2+ ions. All catalysts lose water below 100°C. More strongly bound water, approximately two molecules per Cu2+ ion, emerge at a higher temperature. This corresponds either to an incomplete hydration shell for zeolite-bound Cu2+ ions or to the decomposition of Cu(OH)2 and simultaneous reactive adsorption of copper ions on the inner surface of the zeolite. The process occurs in the same temperature range, 150-350°C, where XRD reveals rearrangements in the H-form of the catalyst. Reactions between the exchangeable cations and the zeolite appear critical for lattice changes and possibly the formation and dispersion of catalytically active centers at these temperatures. Dehydroxylation and water desorption are observed between 350°C and 450°C for H-ZSM-5. This temperature range overlaps with the light-off temperature for direct NO decomposition over Cu-ZSM-5. This coincidence can be rationalized in terms of two effects of enhanced ionic mobility and dynamics of the zeolitic framework. ESCA shows that partial reduction, cupric to cuprous, occurs as a result of annealing in the same temperature range. It has been suggested that NO-derived surface intermediates act as site blockers for the direct decomposition below the light-off temperature until destabilized by lattice movements. The lower stability and thus higher mobility of low SiO2/Al2O3 ratio ZSM-5 zeolites would then rationalize an advantage of these materials as supports in catalysts for direct NO decomposition.

  • 15.
    Ganemi, B.
    et al.
    Kungliga tekniska högskolan, KTH.
    Björnbom, E.
    Kungliga tekniska högskolan, KTH.
    Paul, Jan
    Conversion and in situ FTIR studies of direct NO decomposition over Cu-ZSM51998Ingår i: Applied Catalysis B: Environmental, ISSN 0926-3373, E-ISSN 1873-3883, Vol. 17, nr 4, s. 293-311Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Copper ion-exchanged zeolites ZSMS with SiO2:Al2O3 molar ratios 33 and 53 have been subjected to activity tests for direct decomposition of NO (2000 ppm, GHSV 560-5400 h(-1)). In situ infrared measurements were used to follow the reaction and surface and gas phase compositions. IR studies were also done in excess oxygen with rapid NO2 formation in the gas phase. A high level of overexchange of copper in the zeolite in combination with a low concentration of acid sites, concurrent with a high SiO2:Al2O3 ratio, enhances the conversion of NO. A vibrational band at 1631 cm(-1) is observed below the light-off temperature and interpreted as a bridged nitrate group bound to Cu2+-O-Cu2+ dimers. This band disappears above the light-off temperature but the intensity below this temperature correlates with the catalytic activity. We interpret that these bridge bound nitrate groups act as siteblockers on the active sites for NO conversion and that a tentative reaction intermediate, N2O3, also binds in a bridge configuration to the same Cu2+-O-Cu2+ dimers.A second nitrate group with unidentate coordination and vibrational bands at 1598/1575 cm(-1) probes isolated copper ions.A third infrared band at 2130 cm(-1) confirms previous observations of NO2+-ions bound to the zeolite. We conclude that these species are coordinated to deprotonated and negatively charged sites on the zeolite and that these sites for NO2+ adsorption are blocked by Cu2+ ion-exchange. The 2130 cm(-1) species appear to have no role in direct NO decomposition but the adsorption sites are crucial for the stability of the zeolite and intimately related to ion mobility in the lattice.Prolonged immersion of the zeolite in dilute solutions of copper ions improves the catalyst performance by copper hydroxylation leading to enhanced formation of the above dimers.A high SiO2:Al2O3 ratio leads to more stable catalysts, particularly in combination with a modest overexchange of copper ions. Excessive amounts of copper escalates the deactivation of the Cu-ZSM5 catalyst through the migration and sintering of cupric oxide crystallites.

  • 16. Hirschauer, B.
    et al.
    Söderholm, S.
    Paul, Jan
    Flodström, A.S.
    Large area synthesis of thin alumina films by laser ablation1996Ingår i: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 99, nr 4, s. 285-291Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Al2O3 has been ablated on commercially available 3″ silicon wafers at different distances between the target and the substrate and laser fluencies. ‘Amorphous' Al2O3 (γ-alumina with grain size <20 nm) was grown by pulsed laser deposition at room temperature. The structure, the morphology, the profile and the composition of the produced films have been investigated. Fully oxidised thin films (thickness ≤5 μm) with high uniformity and smoothness were synthesised without additional oxygen gas during the ablation. The quality of the films was independent of the ablation fluency and of the distance between target and substrate.

  • 17. Hoffmann, F. M.
    et al.
    Weisel, M. D.
    Paul, Jan
    Characterization of CO2 adsorption and reaction on single crystal metal surfaces1994Ingår i: Carbon dioxide chemistry: environmental issues / [ed] Jan Paul; Claire-Marie Pradier, Cambridge: Royal Society of Chemistry, 1994, s. 57-65Konferensbidrag (Refereegranskat)
  • 18. Hoffmann, F.M.
    et al.
    Paul, Jan
    A FT-IRAS study of the vibrational properties of CO adsorbed on Cu/Ru(001): I. The structural and electronic properties of Cu1987Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 86, nr 5, s. 2990-2996Artikel i tidskrift (Refereegranskat)
  • 19. Hoffmann, F.M.
    et al.
    Paul, Jan
    A FT-IRAS study of the vibrational properties of CO adsorbed on Cu/Ru(001): II: The dispersion of copper1987Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 87, nr 3, s. 1857-1865Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The dispersion of copper adsorbed on a Ru(001) substrate has been investigated by using Fourier transform-infrared reflection absorption spectroscopy (FT-IRAS) and carbon monoxide as a molecular probe. Copper films evaporated at 85 K show a drastically different CO adsorption behavior compared to annealed films and exhibit a variety of adsorption sites. Characteristic C-O stretching frequencies allow us to identify small copper clusters of 1-4 atoms (2138-2123 cm-1), two-dimensional (2120-2110 cm-1) and three-dimensional (2098 cm-1) copper aggregates. After annealing to 250 K copper films at sub- and monolayer coverages form well-ordered small two- and three-dimensional copper aggregates. Formation of the epitaxial monolayer or islands of copper (2082 cm-1) requires a surprizingly mild annealing temperature of 350 K. Further annealing to 540 K results in increasing domain size of the copper islands or annealing of defect sites of the epitaxial monolayer. Multilayer coverages of copper evaporated at 85 K exhibit C-O stretching frequencies found for high-index copper single crystal surfaces, e.g., (211) and (755). This indicates a large number of surface steps and protruding copper atoms associated with rough films. Annealing to 540 K results in a smooth copper layer with preferential (111) orientation (2075 cm-1). The vibrational data presented here for Cu-Ru(001) agree well with previous reports of CO adsorption on copper single crystals, supported or evaporated films, and matrix-isolated clusters. They further allow us to determine the dispersion of supported Cu-Pt and Cu-Ni catalysts from data in the literature.

  • 20. Hoffmann, F.M.
    et al.
    Weisel, M. D.
    Paul, Jan
    The activation of CO2 by potassium-promoted Ru(001) I. FT-IRAS and TDMS study of oxalate and carbonate intermediates1994Ingår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 316, nr 3, s. 277-293Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The interaction of CO2 with the clean and alkali promoted Ru(001) surface has been studied with time-evolved Fourier transform-infrared reflection absorption spectroscopy (FT-IRAS) and thermal desorption mass spectrometry (TDMS). CO2 adsorbs on the clean Ru(001) surface at 85 K in a physisorbed CO2 monolayer, which desorbs undissociated at 100 K. The interaction of CO2 with a √3 × √3-R30° monolayer of potassium on Ru(001) results in the facile formation of oxalate at 85 K. Oxalate decomposes to carbonate after heating above 150 K, i.e. C2O4 → CO3 + CO. Vibrational spectra suggest, in agreement with theoretical calculations, an ionic carbonate species with D3h symmetry and the molecular plane oriented perpendicular to the surface, although alternate coordinations cannot be completely ruled out. Decomposition of the carbonate starts at 700 K and results in the simultaneous desorption of K and CO2 as major decompositions products, suggesting a reaction pathway of CO3 → CO2 + O and a K:CO3 stoichiometry of ≈1:1. The interaction of CO2 with a multilayer of potassium adsorbed on Ru(001), exhibits similar intermediates as observed for the monolayer, i.e. oxalate and carbonate. However, the overall reaction behavior is more complex and controlled by the penetration of CO2 into the potassium layer, which limits the reaction to only a few (4-5) of the topmost potassium layers. Reaction at 85 K reveals the formation of oxalate, CO2-2 and possibly CO-2 species. Annealing of the multilayer to 425 K results in the formation of carbonate, and the desorption of unreacted potassium. Vibrational spectra characterize an essentially ionic carbonate species with a preferential orientation of the molecular plane perpendicular to the surface, although the vibrational linewidths suggest imperfect ordering of the layer. Further annealing to 550 K results in a well-ordered "bilayer", containing two carbonate species with the molecular plane perpendicular (C2v, first layer) and parallel (C3v, second layer) to the surface, respectively. Decomposition of this layer at 700 K leaves a carbonate monolayer which subsequently decomposes at 750 K. The overall decomposition behavior of the multilayer is complex and is sensitive to the preparation and thickness of the potassium multilayer.

  • 21.
    Hoffmann, Friedrich M.
    et al.
    Department of Science, BMCC−CUNY, New York City.
    Yang, Yixiong
    Chemistry Department, Brookhaven National Laboratory, Upton.
    Paul, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    White, Michael G.
    Chemistry Department, Brookhaven National Laboratory, Upton.
    Hrbek, Jan
    Chemistry Department, Brookhaven National Laboratory, Upton.
    Hydrogenation of carbon dioxide by water: alkali-promoted synthesis of formate2010Ingår i: Journal of Physical Chemistry Letters, Vol. 1, nr 14, s. 2130-2134Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Conversion of carbon,dioxide utilizing protons from water decomposition is likely to provide a sustainable source of fuels and chemicals in the future. We present here a time-evolved infrared reflection absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD) study of the reaction of CO2 + H2O in thin potassium layers. Reaction at temperatures below 200 K results in the hydrogenation of carbon dioxide to potassium formate. Thermal stability of the formate, together with its sequential transformation to oxalate and to carbonate, is monitored and discussed. The data of this model study suggest a dual promoter mechanism of the potassium: the activation of CO2 and the dissociation of water. Reaction at temperatures above 200 K, in contrast, is characterized by the absence of formate and the direct reaction of CO2 to oxalate, due to a drastic reduction of the sticking coefficient of water at higher temperatures

  • 22. Larsson, C.G.
    et al.
    Paul, Jan
    Walldén, L.
    Diffraction of UV excited photoelectrons by an ordered overlayer1983Ingår i: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T4, s. 44-46Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Angle resolved UV photoemission spectra obtained from Cu(111)-(√3 × √3)R30° Xe and Cu(111)-(√3 × √3)R30° CO samples are compared with spectra obtained from the clean substrate. Interest is focussed on the adsorbate induced changes in the Cu3d band range of initial energies. From the similarity between the spectra observed for the ordered Xe and CO overlayers we conclude that the changes are primarily due to diffraction of the photoexcited Cu3d electrons by the adsorbate. This is supported also by calculated photoelectron energy distributions.

  • 23.
    Lindgren, S.Å.
    et al.
    Chalmers University of Technology.
    Paul, Jan
    Walldén, L.
    Chalmers University of Technology.
    Photoemission of backbonding electrons from CO chemisorbed on Cu(III)1981Ingår i: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 84, nr 3, s. 487-490Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Angle-resolved photoclectron energy spectra recorded in the normal direction for a CO-exposed Cu(111) sample show a peak at the Fermi edge for high monolayer coverages. We associate this peak with a filled part of a resonance-broadened CO 2π level.

  • 24. Lindgren, S.Å.
    et al.
    Paul, Jan
    Walldén, L.
    Sodium induced structure in UPS spectra of Cu(111)/Na1985Ingår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 155, nr 1, s. 165-172Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    UPS Spectra of sodium monolayers evaporated onto Cu(111) are presented. We interpret sodium induced structure below the Cu d-band as due to photoemitted 3d electrons which have experienced an energy loss by exciting a sodium monolayer plasmon. An experimental difficulty is that even small amounts (<1/10 monolayers) of water contamination produces structure in the same energy range. This emission is due to the OH 1π orbital. Hydroxide groups may also origin from the alkali source if this is operated at too elevated temperatures.

  • 25. Lindgren, S.Å.
    et al.
    Paul, Jan
    Walldén, L.
    Surface state energy shifts by molecular adsorption: CO on clean and Na covered Cu(111)1982Ingår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 117, nr 1-3, s. 426-433Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Angle resolved photoelectron energy spectra recorded in the near UV show that the surface state 0.4 eV below EF of Cu(111) shifts to higher energy upon CO adsorption. The surface band related emission intensity is reduced by the adsorption at a rate suggesting that each adsorbed molecule wipes out the surface state over an area corresponding to seven surface layer Cu atoms. The surface state energy shift is not as closely related to the adsorbate induced workfunction change as found theoretically and experimentally for alkali adsorbates. The influence on the shift of the workfunction and initial energy of the surface state is studied by preadsorbing Na on the Cu(111) surface.

  • 26.
    Lindgren, S.Å.
    et al.
    Chalmers University of Technology.
    Paul, Jan
    Walldén, L.
    Chalmers University of Technology.
    The Cu conduction band gap at the L point determined by photoemission and electron reflectivity measurements1982Ingår i: Solid State Communications, ISSN 0038-1098, E-ISSN 1879-2766, Vol. 44, nr 5, s. 639-642Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The energies of the Cu conduction band gap edges around the Fermi level at the L point of the Brillouin zone are determined by angle resolved photoemission and electron reflectivity measurements on Na covered Cu(111).

  • 27. Lindgren, S.Å.
    et al.
    Paul, Jan
    Walldén, L.
    Westrin, P.
    Low-energy electron diffraction from close-packed Na1982Ingår i: Journal of Physics. C, Solid State Physics, ISSN 0022-3719, Vol. 15, nr 30, s. 6285-6291Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The authors obtain a homogeneously thick film of close-packed Na with close-packed planes parallel to the surface by evaporating Na onto Cu(111) held at LN2 temperature. Comparison between measured LEED intensity spectra, recorded at LN2 temperature, and theoretical ones, calculated under the assumption that the crystal structure of Na is FCC or HCP, favours the HCP structure. The authors UPS results indicate that if Na is evaporated onto Cu(111) held at RT, it will form three-dimensional clusters on top of a Na film only few atomic layers thick.

  • 28.
    Mehtap, Paul
    et al.
    Luleå tekniska universitet.
    Sandström, Åke
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Industriell miljö- och processteknik.
    Paul, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Bioleaching of sulphidic concentrates ans pH-control with ashes2007Ingår i: World of coal ash: 2007 proceedings ; science, applications and sustainability ; Covington, Kentucky, May 7 - 10, 2007 ; 2007 World of Coal Ash (WOCA) Conference, Lexington, Ky, 2007Konferensbidrag (Refereegranskat)
  • 29. Paul, Jan
    Alkali overlayers on aluminum, alumina, and aluminum carbide1987Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 5, nr 4, s. 664-670Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Adsorption data for potassium and sodium atoms on the surfaces of aluminum metal, aluminum oxide, and aluminum carbide are presented. Low coverage adsorption of K(Na) on Al(100) is characterized by a 4.6(3.2) D surface dipole and desorption around 620(590) K. Increasing the alkali coverage lowers the desorption temperature as well as the effective surface dipole per alkali atom. Clustering on aluminum metal is activated by annealing to around room temperature. The adsorption of K and Na monomers on dehydrogenated Al2O3 is characterized by desorption around 350 K and, for sodium, by cluster formation even at submonolayer coverage. Finally, potassium but not sodium atoms bind strongly to Al4C3 (Id>700 K). This explains a previous observation of potassium at a promoted aluminum surface after exposure to carbon monoxide and annealing to 700 K.

  • 30.
    Paul, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    An unified view on nitrogen oxide abatement2000Ingår i: 12th International Congress on Catalysis: Proceedings of the 12th Icc, Granada, Spain, July 9-14, 2000 / [ed] Avelino Corma, Amsterdam: Elsevier, 2000, s. 1247-1251Konferensbidrag (Refereegranskat)
    Abstract [en]

    A unified model for nitrogen oxide abatement is suggested. The model observes that nitrates are the most common surface species on oxide supported catalysts under literally all conditions below the light-off temperature. The three routes for nitrogen oxide reduction; (i) direct decomposition, (ii) selective catalytic reduction (SCR) with ammonia and (iii) hydrocarbon driven NOx reduction, merely become different ways to destabilize surface coordinated nitrates. Nitrates are decomposed by thermal excitations in direct NO decomposition and by reactions between NO3 and NH4+ in ammonia SCR. Reactions between NO3 and hydrocarbon derived ligands rationalize the importance of partial oxidation products for the third route. The model stresses the significance of Cu2+-O-TM+ dimers for dual ion exchanged catalysts and lattice flexibility for direct decomposition over aluminum rich ZSM5 [TM = Transition Metal].

  • 31. Paul, Jan
    CO dissociation on potassium promoted aluminum1986Ingår i: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 323, s. 701-703Artikel i tidskrift (Refereegranskat)
  • 32.
    Paul, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Environmental monitoring station in Turkey: proposal from the Turkish State Meteorological Service2001Rapport (Övrigt vetenskapligt)
  • 33. Paul, Jan
    Hydrogen adsorption on Al(100)1988Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 37, nr 11, s. 6164-6174Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Adsorption studies of hydrogen and deuterium on the clean (100) surface of aluminum are reported. The probability for H2 (D2) dissociation and subsequent metal-hydrogen bond formation is less than 1:104. This prompted the use of a beam of atomic hydrogen (deuterium). We have adsorbed between 0.01 and 1.0 monolayer and studied annealing sequences between 80 and 750 K with electron-energy-loss spectroscopy (EELS) and thermal-desorption spectroscopy (TDS). At <90 K we observe surface hydrogen coordinated in bridge positions, characterized by an Al-H (-D) vibrational band at 1125 (800) cm-1. Annealing to 150 K causes dominant features to appear at 1750 (1250) and 750 (550) cm-1. These losses are interpreted as the stretching and bending and/or scissoring modes of a terminally coordinated species. Further annealing to 280 K sees the growth of an intense band at 1825 (1325) cm-1, with no correlated low-frequency band. This last annealing signals the formation of another terminally coordinated species, tentatively assigned as ‘‘p(1×1) islands.'' Hydrogen recombination and molecular desorption is observed between 340 and 360 K. We have no indication of any scattering resonance in the elastic energy range 1-9 eV, nor do the angular profiles of the above losses deviate from the profile of the elastic peak. All vibrational modes are thus concluded to be dominated by dipole excitations.

  • 34. Paul, Jan
    Improved air quality on Turkish roads: fuels and exhaust gas treatment1997Ingår i: Air quality management at urban, regional and global scales: proceedings of the 10th regional IUAPPA conference, Gümüssuyu, Istanbul, Turkey, September 23 - 26, 1997 / [ed] S. Incecik, Trans Tech Publications Inc., 1997, s. 145-152Konferensbidrag (Refereegranskat)
  • 35.
    Paul, Jan
    Department of Physics, Chalmers University of Technology, Göteborg, Sweden. Exxon Research and Engineering Company Clinton Township, Annandale, USA.
    Non-local electronic perturbations of metal-carbonyl bonds1985Ingår i: Surfactants, Adsorption, Surface Spectroscopy and Disperse Systems, Berlin: Encyclopedia of Global Archaeology/Springer Verlag, 1985, s. 57-61Kapitel i bok, del av antologi (Övrigt vetenskapligt)
    Abstract [en]

    Metal-carbonyl bonds, as found for carbon monoxide either liganded to metalloporphyrins or chemisorbed onto metal surfaces, are discussed. These two classes of systems are compared with emphasis on short range vs. long range perturbations of the vibrational bands, v MeC and v CO. It is concluded that metalloporphyrins serve as models for terminal bonding on metal surfaces and that data obtained for carbonyl-hemes enable the isolation of non-local electronic effects, which are much discussed in connection with alkali promotors coadsorbed with CO on metal catalysts.

  • 36. Paul, Jan
    Perturbations of metal-ligand bonds: porphyrins and metal surfaces1984Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
  • 37. Paul, Jan
    UPS spectra of H2O, CH3OH and C5H9OH adsorbed onto Cu(111)/Na and Na(cp)1985Ingår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 160, nr 2, s. 599-617Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The present communication presents ultraviolet photoemission spectra (UPS) of three different "alcohols"; water (H2O), methanol (CH3OH), and cyclopentanol (C5H9OH), chemisorbed onto a Cu(111) surface partially covered by sodium atoms as well as onto closely packed sodium films, a free electron adsorbent. Whereas all three alcohols ROH bind reversibly and associatively to Cu(111) they react with adsorbed sodium atoms to metal bound alcoxides RO. The chemisorption bond, characterized by the interaction between O 2pπ orbitals and metal atoms as an electron donor, the alcoxide being the acceptor, is similar for all groups R. The O 2pπ orbitals shift to higher UPS binding energies with increasing electron density, i.e. decreasing rs/ao of the sodium overlayer. Only for HONa, the sterically smallest group R, does the alcoxide growth continue in three dimensions. Although, possibly failing to reproduce the electron density profile of a free electron surface, Hartree-Fock-Slater cluster calculations of small models ROH and RONa3 enable correlations to be made between UPS intensity peaks and one electron orbitals.

  • 38. Paul, Jan
    Vibrational spectra of CO and CO2 adsorbed on potassium modified Fe(100)1989Ingår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 224, nr 1-3, s. 348-358Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    High resolution electron energy loss (EEL) spectra for CO adsorbed on potassium modified Fe(100) are presented. Five CO vibrational bands are clearly distinguishable. Two bands at or above 2000 cm-1 correspond to the *1- and *2-states on the clean surface. Two more bands, residing between 1450 and 1850 cm-1, reveal adsorption at potassium modified sites. The response of these bands to changes in potassium coverage and annealing temperature is highly predictable and similar to the situation on Fe(111) [1]. This suggests a local K: CO coordination. A fifth band is observed around 950 cm-1 and interpreted as CO adsorbed at potassium modified *3-sites, redshifted from 1200 cm-1 at unmofidied sites. The present results are complementary to a previous study of the same adsorption system utilizing photoemission spectroscopy and thermal desorption spectroscopy [2]. Finally, we present novel vibrational and thermal desorption (TDS) data for Fe(100)/K exposed to CO2 and point at similarities between intermediate states following CO and CO2 adsorption on different potassium promoted iron surfaces.

  • 39. Paul, Jan
    et al.
    Cameron, S.D.
    Dwyer, D. J.
    Hoffmann, F.M.
    The interaction of CO and O2 with the (111) surface of Pt3Ti1986Ingår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 177, nr 1, s. 121-138Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The electronic properties of clean and partly oxidized Pt3Ti(111) surfaces have been studied utilizing carbon monoxide both as a probe and as a reducing agent. Vibrational frequencies and desorption profiles of chemisorbed CO as well as ion scattering and angular resolved X-ray photoelectron spectroscopy (XPS) suggest that the first atomic layer of annealed Pt3Ti(111) is quasi-pure platinum. Scarcely any (θ ≈ 0.01) dissociation of CO was observed. Minor shifts of vibrational frequencies and desorption temperatures compared to Pt(111) and a p(2 × 2) "reconstruction" of the clean surface reveal some influence of the bulk. Auger spectroscopy, XPS, and ion scattering all show an increased titanium signal as a result of oxidation. Surface bound atomic oxygen gives a vibrational band around 650 cm-1 which coincides with infrared absorption spectra of TiO2. Flashing with CO shifts the band to 500 cm-1. Correlated with this shift we observe (i) CO2 desorption at a temperature well above that observed for Pt(111)/O, (ii) an altered Ti XPS signal, and (iii) a reduced oxygen concentration. Subsequently adsorbed CO molecules vibrate at the same frequencies as on the bare surface, give the same c(4 × 2) LEED pattern, and desorb at the same temperatures but with reduced intensity, in all proving that the surface oxide only acts as a site-blocker with respect to the metal surface. Our current understanding of these observations is that oxygen creates "islands of TiO2", segregated to the surface but with no electronic influence on remaining areas of the platinum enriched metal surface. The hexacoordinated Ti4+ ions on the surface of these islands are reduced by CO to pentacoordinated Ti3+ species. The vibrational shift, 650 to 500 cm-1, can be understood by the dipole active bands of a triatomic O-Ti4+ -O vibrator compared to a diatomic Ti3+-O vibrator.

  • 40. Paul, Jan
    et al.
    Chen, Wenhua
    Ohlsson, Per-Ingvar
    Smith, Michael
    Heme transfer reactions: an important prerequisite for synthetic oxygen carriers1998Ingår i: Turkish journal of chemistry, ISSN 1300-0527, E-ISSN 1303-6130, Vol. 22, nr 2, s. 103-108Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Electronic changes of iron- and metal free porphyrins are reviewed in light of their importance to modify the ferri/ferro reduction potential and energy dissipation between ligated carbon monoxide and the porphyrin chelate. Substitutions at positions 2 and 4 are effective ways to exercise this influence. Other measures include exposure to water and modifications of axial ligation, including the \lq three-ligand' case. These measures will also influence the stability of recombined hemin-protein entities, as apparent from the kinetics of heme-transfer from a modified donor protein to apomyoglobin, but the stability appears to be related to the bulkiness of 2,4-substitutional groups rather than to electron availability. It is suggested that the altered stability is a secondary effect from enhanced exposure to water in the crevice.

  • 41. Paul, Jan
    et al.
    dePaola, R.A.
    Hoffmann, F.M.
    Vibrational spectroscopy of alkali-molecule interactions and compound formation on metal surfaces1989Ingår i: Physics and chemistry of alkali metal adsorption: [proceedings of the WE-Heraeus-Seminar on alkali metal adsorption held in February 1989 at Bad Honnef, Allemagne] / [ed] H.P. Bonzel; Alexander M. Bradshaw; Gerhard Ertl, Amsterdam: Elsevier, 1989, s. 213-230Konferensbidrag (Refereegranskat)
    Abstract [en]

    Discussion relative aux interactions à courte distance conduisant à la formation de liaisons entre les atomes de métaux alcalins et les atomes et molécules coadsorbées. Les résultats de spectroscopie vibrationnelle et de thermodésorption révèlent la chimie de formation des composés de métaux alcalins sur des surfaces métalliques, dépendant du substrat et de l'ionicité de l'atome alcalin. Présentation d'exemples pour la formation d'oxydes, d'hydrures, d'hydroxydes, de méthylates et de carbonates de métaux alcalins sur du ruthénium et de l'aluminium

  • 42. Paul, Jan
    et al.
    Hoffman, F.M.
    CO adsorption on clean and oxidized Al(110)1986Ingår i: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 130, nr 3, s. 160-163Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Electron-energy loss (EELS) spectra and thermal desorption (TDS) traces of carbon monoxide bound to the (100) surface of aluminum are presented. CO chemisorption on clean Al(100) is characterized by vibrational bands at 440 and 2060 cm-1 and by desorption at 125 K. Oxide "islands", formed by oxidation in O2 at 575 K, have no observed electronic influence on open metallic areas of the adsorbent but merely block CO adsorption sites.

  • 43. Paul, Jan
    et al.
    Hoffmann, F.M.
    Alkali hydride formation on aluminum1988Ingår i: Catalysis: theory to practice: Proceedings / [ed] M. J. Phillips; M. Ternan, Chemical Inst. of Canada , 1988, Vol. 4: Oxide catalysts and catalyst development, s. 1890-1897Konferensbidrag (Refereegranskat)
  • 44. Paul, Jan
    et al.
    Hoffmann, F.M.
    Alkali promoted CO bond weakening on aluminum: a comparison with transition metal surfaces1987Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 86, nr 9, s. 5188-5195Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Data on the adsorption and decomposition of carbon monoxide on alkali promoted Al(100) are presented. CO dissociates on the potassium or sodium promoted surface and aluminum oxide and aluminum carbide form after annealing to 700 K. At intermediate temperatures EELS show alkali-CO complexes with vibrational frequencies ranging from 1060 to 2060 cm-1. A band at 1750 cm-1 was assigned to CO molecules coordinated to bulk potassium. CO vibrational spectra as well as work function measurements reveal an altered alkali dispersion as a function of preannealing temperature. Comparisons are made between the surfaces of aluminum and transition metals with respect to (i) alkali adsorption, (ii) hybridization between metal d states and CO π orbitals, (iii) the magnitude of unscreened (long-range) perturbations, and finally (iv) the energetics of carbide and oxide formation. Potassium but not sodium atoms bind strongly to aluminum carbide (Td>700 K). We suggest that potassium is rare among alkali metals not in its ability to promote CO dissociation but in preventing a downshift of the C 2pz orbital and thus carbide to graphite transformation.

  • 45. Paul, Jan
    et al.
    Hoffmann, F.M.
    Co2 conversion and oxalate stability on alkali promoted metal surfaces: sodium modified Al(100)1988Ingår i: Catalysis Letters, ISSN 1011-372X, E-ISSN 1572-879X, Vol. 1, nr 12, s. 445-455Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    CO2 conversion on alkali promoted metals in aprotic systems has been followed with surface sensitive spectroscopies. New results on sodium modified aluminum(100) are presented and compared with previous studies on magnesium [1], aluminum [2], and bulk alkali metals [3]. Electron energy loss spectra reveal two different states of CO2 adsorption at 100 K and monolayer sodium coverage. Vibrational bands at 650 cm-1 and 2325 cm-1 correspond to weakly bound molecular CO2 and a multitude of bands between 2300 cm-1 and 460 cm-1 to oxalate ions with low, possibly unidentate, coordination. Gentle annealing increases the coordination as apparent by vibrational shifts. This corresponds to oxalate to carbonate conversion, a process which is completed around room temperature. CO desorption was detected at 285 K and Auger measurements reveal a 1:3 C/O stoichiometry after high temperature annealing. We observe no release of CO2 above 110 K but an additional weak state of CO desorption around 470 K. High temperature annealing causes decomposition of all intermediates and leaves the aluminum surface covered with an irreducible carbide and oxide overlayer. We suggest that CO2 reduction and dimerization to C2O4 -2 is a common path to yield carbon deposition on all alkali promoted surfaces in hydrogen deficient systems. In contrast, oxalate decomposition is related to the specific chemistry of each substrate.

  • 46. Paul, Jan
    et al.
    Hoffmann, F.M.
    Decomposition of water on clean and oxidized aluminum(100)1986Ingår i: Journal of Physical Chemistry, ISSN 0022-3654, Vol. 90, nr 21, s. 5321-5324Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper presents electron energy loss spectra (EELS) and thermal desorption (TDS) traces following the adsorption and subsequent annealing of water on A1(100), with and without the presence of a surface oxide. A water molecule will in both cases either (1) reversibly bind at a hydrogen-bonded site or (2) decompose, preferentially to a surface-bound hydroxyl species. The H20 dissociation occurs via H2 evolution only on the bare surface and also via hydrogen abstraction in the presence of a surface oxide. Neither dihydrogen nor water desorption are observed as the hydroxyl species diminish during annealing. Instead, the aluminum hydroxide transforms into aluminum oxide and "trapped" hydrogen atoms, depicted as an Al/A10, hydride.

  • 47. Paul, Jan
    et al.
    Hoffmann, F.M.
    Hydrogen adsorption on alkali modified aluminum1988Ingår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 194, nr 3, s. 419-437Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Coadsorption experiments of alkali metals and hydrogen on Al(100) are reported. The probability for H-H (D-D) dissociation and subsequent atomic adsorption is less than 1:104 at all alkali coverages. As a consequence, each overlayer had to be prepared by exposing the sodium or potassium modified surface to a beam of hydrogen (deuterium) atoms. We observed an attractive alkali-hydrogen interaction at all coverages. This interaction is best described as the formation of an alkali hydride in the presence of excess alkali atoms. The formation of the hydride shifts the recombination and desorption temperature of hydrogen adatoms on Al(100) from around 350 to around 500 K. While no isotope effect was detected on the clean aluminum surface (diffusion limited recombination), a significant 10-15 K shift was observed for both alkali hydrides (bond breaking). Furthermore, both alkali hydrides observed intense vibrational losses, thus revealing the ionic character of the metal-hydrogen bond. Electron energy loss spectra of an annealed monolayer of sodium hydride (deuteride) showed metal-hydrogen stretching bands at 1850 (1350) cm-1 and 1715 (1250) cm-1, a deformation band at 800 (600) cm-1, and a metal-metal band at 200 (200) cm-1. Corresponding peak positions for potassium hydride (deutride) were 1650 (1200) cm-1, 1500 (1100) cm-1, and 775 (585) cm-1. The metal-metal vibration could not be separated from the elastic peak because of the larger mass of potassium compared to sodium.

  • 48. Paul, Jan
    et al.
    Hoffmann, F.M.
    TDS and EELS observations for CO, O2, and CH3OH bound to Ru(0001)/Cu1986Ingår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 172, nr 1, s. 151-173Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The electronic properties of monolayers of copper atoms adsorbed onto a Ru(0001) single crystal surface have been studied with thermal desorption spectroscopy (TDS) and high resolution electron energy loss spectroscopy (EELS) utilizing carbon monoxide (CO), dioxygen (O2), methanol (CH3OH), and to some extent water (H2O) as chemical probes. Whereas a three-monolayer-thick film exhibits most properties of a Cu(111) crystal distinct deviations are found at lower Cu coverages. TDS as well as EELS show a weakened Ru---CO bond and a strengthened Cu---CO bond as a result of metal-metal interaction. The stronger Cu---CO bond is accompanied by a higher probability for O2 dissociation. The mobilities of copper and oxygen atoms are such that annealing to 650 K produces an overlayer structure which is independent of adsorption sequence: Cu/O2 or O2/Cu, but where Ru---O as well as Cu---O vibrations can be identified. Methanol adsorbs reversibly on a monolayer of copper atoms. Metal bound methoxy species are formed in the presence of oxygen atoms. The decomposition paths of such methoxy intermediates alter towards more formaldehyde (CH2O) relative to CO with increasing copper and methoxy coverages.

  • 49. Paul, Jan
    et al.
    Hoffmann, F.M.
    Robbins, J.L.
    Carbon monoxide and carbon dioxide decomposition on bulk polycrystalline alkali metals1988Ingår i: Journal of Physical Chemistry, ISSN 0022-3654, Vol. 92, nr 24, s. 6967-6969Artikel i tidskrift (Refereegranskat)
  • 50. Paul, Jan
    et al.
    Lindgren, S-Å
    Göteborgs universitet.
    Walldén, L.
    Göteborgs universitet.
    Surface state energy shifts by molecular adsorption: CO ON Cu(111)1981Ingår i: Solid State Communications, ISSN 0038-1098, E-ISSN 1879-2766, Vol. 40, nr 4, s. 395-397Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Molecular adsorption induced energy shifts of a surface state is observed for the Cu(111)/CO system. Angle resolved photoelectron energy spectra show that a zone center surface state shifts to higher energy relative to the Cu bulk bands. The effects is discussed in terms of charge transfer, molecular interaction range and absorption site.

12 1 - 50 av 96
RefereraExporteraLänk till träfflistan
Permanent länk
Referera
Referensformat
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annat format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annat språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf