| 1. |
- Paul, Jan, et al.
(författare)
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The vibrational bands of carbon monoxide bound to hemes or metal surfaces
- 1985
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Ingår i: Biochimica et Biophysica Acta - Protein Structure and Molecular Enzymology. - : Elsevier BV. - 0167-4838 .- 1879-2588. ; 832:3, s. 257-264
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Tidskriftsartikel (refereegranskat)abstract
- Infrared spectra of imidazole carbonyl complexes of 2,4-substituted hemes are presented. An increased CO stretch frequency is accompanied by a lowered FeC vibrational energy. Hartree-Fock-Slater electron structure calculations discern π and σ contributions to the observed shifts of vibrational energies. We conclude that an enhanced electron availability manifests itself as (i) a lowered ferric/ferrous reduction potential, (ii) increased filling of the 2π orbital of liganded CO which in turn reduces νCO and increases νFec, and (iii) increased basicity of the liganded CO. Analogies between CO liganded to heme and CO adsorbed onto metal surfaces are discussed.
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| 2. |
- Hoffmann, F.M., et al.
(författare)
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A FT-IRAS study of the vibrational properties of CO adsorbed on Cu/Ru(001) : II: The dispersion of copper
- 1987
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 87:3, s. 1857-1865
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
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| 4. |
- Paul, Jan, et al.
(författare)
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A working procedure for the evaluation of exafs phase shifts : application to Cu(111)/Cs
- 1985
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Ingår i: Chemical Physics Letters. - 0009-2614 .- 1873-4448. ; 117:5, s. 511-513
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Tidskriftsartikel (refereegranskat)abstract
- A theoretical procedure is presented to evaluate EXAFS phase shifts. The procedure is based on a self-consistently determined ground-state electron density and an energy-dependent complex scattering potential. Most EXAFS or NEXAFS/XANES schemes used today apply a constant imaginary part of the scattering potential. We illustrate our procedure by calculations on cesium atoms adsorbed onto Cu(111).
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| 5. |
- Paul, Jan, et al.
(författare)
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Alkali hydride formation on aluminum
- 1988
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Ingår i: Catalysis: theory to practice. - : Chemical Inst. of Canada. ; , s. 1890-1897
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Konferensbidrag (refereegranskat)
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| 6. |
- Paul, Jan, et al.
(författare)
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Alkali promoted CO bond weakening on aluminum : a comparison with transition metal surfaces
- 1987
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 86:9, s. 5188-5195
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
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| 8. |
- Paul, Jan, et al.
(författare)
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CO adsorption on clean and oxidized Al(110)
- 1986
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 130:3, s. 160-163
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 9. |
- Paul, Jan, et al.
(författare)
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Co2 conversion and oxalate stability on alkali promoted metal surfaces : sodium modified Al(100)
- 1988
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Ingår i: Catalysis Letters. - 1011-372X .- 1572-879X. ; 1:12, s. 445-455
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
- Paul, Jan, et al.
(författare)
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Decomposition of water on clean and oxidized aluminum(100)
- 1986
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 90:21, s. 5321-5324
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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