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  • 1
    Electronic Resource
    Electronic Resource
    Reaction of H2S with MgO(100) and Cu/MgO(100) surfaces: Band-gap size and chemical reactivity (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 8077-8087 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The interaction of H2S, SH, and S with MgO(100) and Cu/MgO(100) surfaces has been investigated using synchrotron-based high resolution photoemission and density functional calculations. Metallic magnesium reacts vigorously with H2S fully decomposing the molecule at temperatures below 200 K. In contrast, the Mg atoms in MgO exhibit a moderate reactivity. At 80 K, most of the H2S molecules (∼80%) chemisorb intact on a MgO(100) surface. Annealing to 200 K induces cleavage of S–H bonds leaving similar amounts of H2S and SH on the surface. The complete disappearance of H2S is observed at 300 K, and the dominant species on the oxide is SH which is coadsorbed with a small amount (∼10%) of atomic S. The adsorbed SH fully decomposes upon heating to 400 K producing S adatoms that are stable on the surface at temperatures well above 500 K. The results of density functional calculations indicate that the bonding interactions of SH and S with pentacoordinated Mg sites of a flat MgO(100) surface are strong, but the bonding of the H2S molecule is relatively weak. Defect sites probably play an important role in the dissociation of H2S. Cu adatoms facilitate the decomposition of H2S on MgO(100) by providing electronic states that are very efficient for interactions with the frontier orbitals of the molecule. The rate of H2S decomposition on MgO is substantially lower than those found on Cr3O4, Cr2O3, ZnO, and Cu2O. For these systems, the smaller the band-gap in the oxide, the bigger its reactivity towards H2S. Theoretical calculations indicate that this trend reflects the effects of band–orbital mixing. The electrostatic interactions between the dipole of H2S and the ionic field generated by the charges in an oxide play only a secondary role in the adsorption process. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480141
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  • 2
    Electronic Resource
    Electronic Resource
    The adsorption of sulfur on Rh(111) and Cu/Rh(111) surfaces (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 3064-3073 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reaction of S2 with Rh(111) and Cu/Rh(111) surfaces has been investigated using synchrotron-based high-resolution photoemission, thermal desorption mass spectroscopy and ab initio self-consistent-field calculations. At 100 K, the adsorption of S2 on Rh(111) produces multilayers of Sn species (n=2–8) that desorb between 300 and 400 K, leaving a film of RhSx on the sample. S2 dissociates upon adsorption on clean Rh(111) at 300 K. An adsorption complex in which S2 is bridge bonded to two adjacent Rh atoms (Rh–S–S–Rh) is probably the precursor state for the dissociation of the molecule. The larger the electron transfer from Rh(111) into the S2(2πg) orbitals, the bigger the adsorption energy of the molecule and the easier the cleavage of the S–S bond. On Rh(111) at 300 K, chemisorbed S is bonded to two dissimilar adsorption sites (hollow and probably bridge) that show well separated S 2p binding energies and different bonding interactions. Adsorption on bridge sites is observed only at S coverages above 0.5 ML, and precedes the formation of RhSx films. The bonding of S to Rh(111) induces a substantial decrease in the density of d states that the metal exhibits near the Fermi level, but the electronic perturbations are not as large as those found for S/Pt(111) and S/Pd(111). Cu adatoms significantly enhance the rate of sulfidation of Rh(111) through indirect Cu↔Rh↔S2 and direct Cu↔S–S↔Rh interactions. In the presence of Cu there is an increase in the thermal stability of sulfur on Rh(111). The adsorption of S2 on Cu/Rh(111) surfaces produces CuSy and RhSx species that exhibit a distinctive band structure and decompose at temperatures between 900 and 1100 K: CuSy/RhSx/Rh(111)→S2(gas) +Cu(gas)+S/Rh(111). © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475697
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  • 3
    Electronic Resource
    Electronic Resource
    Electronic and Chemical Properties of Ag/Pt(111) and Cu/Pt(111) Surfaces: Importance of Changes in the d Electron Populations (1994)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 98 (1994), S. 11251-11255 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100095a004
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  • 4
    Electronic Resource
    Electronic Resource
    Chemistry of NO2 on CeO2 and MgO: Experimental and theoretical studies on the formation of NO3 (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 9929-9939 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In environmental catalysis the destruction or removal of nitrogen oxides (DeNOx process) is receiving a lot of attention. Synchrotron-based x-ray absorption near-edge spectroscopy, high-resolution photoemission, and first-principles density-functional calculations (DFT-GGA) were used to study the interaction of nitrogen dioxide with CeO2 and MgO. The only product of the reaction of NO2 with pure CeO2 at 300 K is adsorbed nitrate. The NO3 is a thermally stable species which mostly decomposes at temperatures between 450 and 600 K. For the adsorption of NO2 on partially reduced ceria (CeO2−x), there is full decomposition of the adsorbate and a mixture of N, NO, and NO3 coexists on the surface of the oxide at room temperature. Ce3+ cations can assist in the transformation of NO and NO2 in DeNOx operations. Adsorbed NO3 (main product) and NO2 are detected after exposing MgO to NO2 gas. A partial NO2,ads→NO3,ads transformation is observed on MgO(100) from 150 to 300 K. DFT-GGA calculations show strong bonding interactions for NO2 on Mg sites of this surface, and dicoordination via O, O is more favorable energetically than monocoordination via N. The NO2,ads species disappears from magnesium oxide at temperatures below 600 K, whereas part of the NO3,ads is stable up to temperatures near 800 K. MgO can be very useful as a sorbent for trapping NO2. A general trend is found after comparing the chemical behavior of NO2 on different types of oxides (CeO2, MgO, TiO2, Fe2O3, CuO, ZnO). On all these systems, the main product after adsorbing NO2 at 300 K is nitrate with minor amounts of chemisorbed NO2 and no signs of full decomposition of the adsorbate. This trend and the results of DFT-GGA calculations indicate that NO2 is very efficient for the nitration (i.e., formation of NO3 as a ligand) of metal centers that are missing O neighbors in oxide surfaces. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481629
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  • 5
    Electronic Resource
    Electronic Resource
    Reaction of S2 and SO2 with Pd/Rh(111) surfaces: Effects of metal–metal bonding on sulfur poisoning (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 3138-3147 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The surface chemistry of S2 and SO2 on Rh(111), Pd/Rh(111) and polycrystalline Pd has been investigated using synchrotron-based high-resolution photoemission and ab initio self-consistent-field calculations. Pd adatoms lead to an increase in the rate of adsorption of S2 on Rh(111), but they are less reactive than atoms of pure metallic palladium: Rh(111)〈Pd/Rh(111)〈Pd. The adsorption of sulfur induces a large reduction in the density of states (DOS) near the Fermi level of Pd/Rh(111) surfaces. The decrease in the DOS is smaller than in S/Pd(111) but bigger than in S/Rh(111). The chemistry of SO2 on Rh(111), Pd/Rh(111), and Pd is rich. At 100 K, SO2 adsorbs molecularly on these systems. Above 200 K, the adsorbed SO2 decomposes (SO2,a→Sa+2Oa) or transforms into SO3/SO4 species. The molecular SOx species disappear upon annealing to 450 K and only atomic S and O remain on the surfaces. A Pd monolayer supported on Rh(111) is not very active for the dissociation of SO2. In this respect, the Pd1.0/Rh(111) system is less chemically active than pure Pd or Rh(111). The electronic perturbations associated with the Pd–Rh bonds reduce the electron donor ability of Pd, weakening the interactions between the Pd 4d orbitals and the lowest unoccupied molecular orbitals of S2 and SO2. The behavior of the S2/Pd/Rh(111) and SO2/Pd/Rh(111) systems shows that bimetallic bonding can reduce the reactivity of Pd towards sulfur-containing molecules. A very large drop in reactivity can be expected when Pd is bonded to s,p or early transition metals. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477910
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  • 6
    Electronic Resource
    Electronic Resource
    Reaction of S2 and H2S with Sn/Pt(111) surface alloys: Effects of metal–metal bonding on reactivity towards sulfur (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 4052-4062 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The surface chemistry of S2 and H2S on polycrystalline Sn, Pt(111), and a ((square root of 3)×(square root of 3))R30°-Sn/Pt(111) surface alloy has been investigated using synchrotron-based high-resolution photoemission and ab initio self-consistent-field calculations. At 100–300 K, S2 chemisorbs and reacts on polycrystalline tin to form metal sulfides. The reactivity of pure tin toward sulfur is large even at a temperature as low as 100 K. In contrast, tin atoms in contact with Pt(111) interact weakly with S2 or H2S. Tin does not prevent the bonding of S to Pt in a ((square root of 3)×(square root of 3))R30°-Sn/Pt(111) surface alloy, but the alloy is less reactive toward H2S than polycrystalline Sn or pure Pt(111). At room temperature, S2 and H2S adsorb dissociatively on Pt sites of ((square root of 3)×(square root of 3))R30°-Sn/Pt(111). Upon the dosing of S2 and H2S to ((square root of 3)×(square root of 3))R30°-Sn/Pt(111), one sees the formation of only a chemisorbed layer of sulfur (i.e., no sulfides of tin or platinum are formed). The Pt–Sn bond is complex, involving a Sn(5s,5p)→Pt(6s,6p) charge transfer and a Pt(5d)→Pt(6s,6p) rehybridization that localize electrons in the region between the metal centers. These phenomena reduce the electron donor ability of Pt and Sn, and the metals are not able to respond in an effective way to the presence of species that are strong electron acceptors like S2, HS, and S. The redistribution of charge produces surfaces that have a remarkable low reactivity toward sulfur. When compared to other admetals (Cu, Zn, Ag, Au), tin is the best choice as a site blocker that can enhance the tolerance of Pt reforming catalysts to sulfur poisoning. The Sn/Pt system illustrates how a redistribution of electrons that occurs in bimetallic bonding can be useful for the design of catalysts that are less sensitive to the presence of S-containing molecules.© 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477005
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  • 7
    Electronic Resource
    Electronic Resource
    Interaction of sulfur with Pt(111) and Sn/Pt(111): Effects of coverage and metal–metal bonding on reactivity toward sulfur (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 11284-11292 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In the chemical and petrochemical industries, Pt-based catalysts are very sensitive to sulfur poisoning. Synchrotron-based high-resolution photoemission, thermal desorption mass spectroscopy (TDS), and first-principles density-functional slab calculations were used to study the adsorption of sulfur on Pt(111) and a p(2×2)-Sn/Pt(111) surface alloy. Our results show important variations in the nature of the bonding of sulfur to Pt(111) depending on the coverage of the adsorbate. For small coverages, θS〈0.3 ML, atomic sulfur is the most stable species. The adsorbate is bonded to hollow sites, has a large adsorption energy (〉75 kcal/mol), and desorbs as S. The Pt–S bonds are mainly covalent but sulfur induces a significant decrease in the density of Pt 5d states near the Fermi level. When the sulfur coverage increases on the surface, θS〉0.4 ML, there is a substantial weakening in the Pt↔S interactions with a change in the adsorption site and a tendency to form S–S bonds. Desorption of S2 is now observed in TDS and the S2p core levels shift to higher binding energy. At coverages near a full monolayer, S2 is the most stable species on the surface and its adsorption energy is ∼45 kcal/mol. Similar trends are observed for the adsorption of sulfur on a p(2×2)-Sn/Pt(111) surface alloy, but the adsorbate↔substrate interactions are weaker than on Pt(111). The formation of Pt–Sn bonds reduces the reactivity of Pt toward sulfur. Electronic effects associated with bimetallic bonding can be useful for controlling or preventing sulfur poisoning. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1327249
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  • 8
    Electronic Resource
    Electronic Resource
    Phase transformations and electronic properties in mixed-metal oxides: Experimental and theoretical studies on the behavior of NiMoO4 and MgMoO4 (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 935-945 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Mixed-metal oxides play a relevant role in many areas of chemistry, physics, and materials science. We have examined the structural and electronic properties of NiMoO4 and MgMoO4 by means of synchrotron-based time-resolved x-ray diffraction (XRD), x-ray absorption near-edge spectroscopy (XANES), and first-principles density functional theory (DFT) calculations. Nickel molybdate can exist in two phases (α and β). Mo is in a near tetrahedral environment in the β-phase, whereas in the α-phase the metal exhibits a pseudo-octahedral coordination with two very long Mo–O distances (2.3–2.4 Å). The results of DFT calculations indicate that the α-phase of NiMoO4 is ∼9 kcal/mol more stable than the β-phase. On the other hand, in the case of magnesium molybdate, an α-NiMoO4-type phase is ∼13 kcal/mol less stable than β-MgMoO4. These trends in stability probably result from variations in the metal–metal repulsion within the α-phases of the compounds. For the α→β transition in NiMoO4, the DFT calculations predict an energy barrier of ∼50 kcal/mol. An apparent activation energy of ∼80 kcal/mol can be derived from the time-resolved XRD experiments. The degree of ionicity in MgMoO4 is larger than that in NiMoO4. The nickel molybdate displays a large density of states near the top of the valence band that is not observed in the magnesium molybdate. This makes NiMoO4 more chemically active than MgMoO4. A similar type of correlation is found between the electronic and chemical properties of NiMoO4, CoMoO4, and FeMoO4. The DFT results and Mo LII-edge XANES spectra show big differences in the splitting of the Mo 4d orbitals in the α- and β-phases of the molybdates. The line shape in the O K-edge essentially reflects the behavior seen for the 4d orbitals in the Mo LII-edge (i.e., mainly O 1s→Mo 4d electronic transitions). The Mo LII- and O K-edges in XANES can be very useful for probing the local symmetry of Mo atoms in mixed-metal oxides. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480619
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  • 9
    Electronic Resource
    Electronic Resource
    Adsorption of phosphorus trifluoride, hydroxyl, mercapto, and methyl on silver: a quantum-chemical study (1991)
    s.l. : American Chemical Society
    Langmuir 7 (1991), S. 1206-1214 
    Details
    ISSN: 1520-5827
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/la00054a031
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  • 10
    Electronic Resource
    Electronic Resource
    Chemical and Electronic Properties of Bimetallic Surfaces (1995)
    s.l. : American Chemical Society
    Accounts of chemical research 28 (1995), S. 477-478 
    Details
    ISSN: 1520-4898
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ar00060a001
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