Synchrotron-based High-resolution Photoemission Research Articles

Reaction of S2 and SO2 with Pd/Rh(111) surfaces: Effects of metal–metal bonding on sulfur poisoning

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.

Chemistry of SO 2 on Ru(001): formation of SO 3 and SO 4

The interaction of SO 2 with Ru(001) at temperatures of 300 and 100 K has been studied using synchrotron-based high-resolution photoemission, INDO/S self-consistent field calculations, and a thermochemical analysis based on the bond order conservation Morse potential (BOC-MP) formalism. At 300 K and small exposures of SO 2, sulfur dioxide completely decomposes (SO 2→S a+2O a), leaving atomic oxygen and two kinds of sulfur species on the Ru surface (S and S n+O). Several coordination modes for adsorbed SO 2 (η 1-S a-top, η 1-S bridge, η 2-O,O bridge, η 2-S,O bridge) were examined using INDO/S and the BOC-MP formalism. It was found that adsorption geometries in which the molecule is dicoordinated via O,O or S,O are the most probable precursors for dissociation. For large exposures of SO 2 at 300 K, SO 3 and SO 4 species appear on the Ru(001) surface, thus confirming the theoretical predictions derived from the BOC-MP method which indicate that formation of SO 3 and SO 4 can take place only when the number of empty adsorption sites is limited. This prediction was also proved during the adsorption of SO 2 on an O/Ru(001) surface with θ O=0.25 ML, producing similar types of species as at saturation of SO 2 on Ru (S n+O, SO 4 and S). Experiments performed at a surface temperature of 100 K showed a very rich surface chemistry. Five different surface species were observed after SO 2 dosing: atomic S, S n+O, SO 2, SO 3 and SO 4. Annealing to 260 K leaves mainly atomic S and SO 4 on the surface and the latter is completely decomposed by 350 K.

The bonding of sulfur to Pd surfaces: photoemission and molecular–orbital studies

The adsorption and dissociation of S 2 on Pd(111) and polycrystalline Pd were studied using synchrotron-based high-resolution photoemission spectroscopy and ab initio SCF calculations. The photoemission results show several sulfur species with distinct electronic properties. The formation of Pd–S bonds induces large positive binding-energy shifts in the core and valence levels of Pd. After chemisorbing sulfur on the Pd surfaces, there is a substantial decrease in the electron density that the metal exhibits near the Fermi level and a simultaneous drop in the electron population of its 4d band. Pd is more sensitive to sulfur than other metals frequently used in automotive exhaust catalysts (Rh and Pt).

Surface Chemistry of SO2 on Sn and Sn/Pt(111) Alloys: Effects of Metal−Metal Bonding on Reactivity toward Sulfur

The surface chemistry of SO2 on polycrystalline Sn, Pt(111), and a ( x )R30°-Sn/Pt(111) surface alloy has been investigated using synchrotron-based high-resolution photoemission and ab initio self-...

Reaction of S2 and H2S with Sn/Pt(111) surface alloys: Effects of metal–metal bonding on reactivity towards sulfur

The surface chemistry of S2 and H2S on polycrystalline Sn, Pt(111), and a (∛×∛)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 (∛×∛)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 (∛×∛)R30°-Sn/Pt(111). Upon the dosing of S2 and H2S to (∛×∛)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.

Surface Chemistry of SO2 on Zn and ZnO: Photoemission and Molecular Orbital Studies

The reaction of SO2 with polycrystalline Zn and ZnO surfaces has been investigated using synchrotron-based high-resolution photoemission spectroscopy and ab initio self-consistent-field calculations. The chemistry of SO2 on Zn surfaces is quite complex and depends on both the temperature of adsorption and the SO2 exposure. At 300 K, SO2 dissociates on a clean Zn surface to form atomic sulfur and atomic oxygen (SO2,gas → Sa + 2Oa; SO2,gas → SOgas + Oa). The Zn ↔ SO2 bonding interactions induce a significant weakening of the S−O bonds. The theoretical calculations suggest the η2-O,O and η2-S,O bonding conformations of SO2 as the two possible precursors for the dissociation of the molecule. The dissociation reactions are much more exothermic than the formation of SO3 or SO4: SO2,gas + nOa → SOx, where x = 3 or 4. At high SO2 exposures (300 K), when most surface sites are blocked and dissociation of SO2 cannot occur, SO3 and SO4 are formed on the Zn surface. Adsorption at 100 K suppresses the SO2 dissociatio...

Reaction of H2S and S2 with Metal/Oxide Surfaces: Band-Gap Size and Chemical Reactivity

The adsorption and dissociation of H2S and S2 on a series of oxide (Al2O3, Cr2O3, Cr3O4, Cu2O, ZnO) and metal/oxide (Cu/Al2O3, Cu/ZnO) surfaces have been studied using synchrotron-based high-resolution photoemission. H2S and S2 mainly interact with the metal centers of the oxides. At 300 K, H2S undergoes complete decomposition. The rate of decomposition on Al2O3 is much 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 toward S-containing molecules. The results of ab initio SCF calculations for the adsorption of H2S, HS, and S on clusters that resemble the (0001) face of α-Al2O3, α-Cr2O3, and ZnO show that the S-containing species interact stronger with Cr or Zn than with Al centers. These theoretical results and the trends seen in the experimental data indicate that the reactivity of an oxide mainly depends on how well its bands mix with the orbitals of H2S or HS. The electrostatic interactions between the dipole of ...

The adsorption of sulfur on Rh(111) and Cu/Rh(111) surfaces

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).

Reaction of S2 with ZnO and Cu/ZnO Surfaces: Photoemission and Molecular Orbital Studies

The adsorption of S2 on ZnO and Cu/ZnO has been investigated using synchrotron-based high-resolution photoemission spectroscopy. On dosing a clean ZnO surface with S2 at 300 K, the molecule dissociates. The S is associated first with Zn and at medium coverages with Zn−O sites. When the sulfur coverage is increased to θS = 0.5 ML, evidence is found for sulfur bound purely to the O sites of ZnO. The sulfur species associated with O and the Zn−O sites are unstable at temperatures above 500 K. Possible reaction pathways for the dissociation of S2 on ZnO(0001)−Zn and Zn(1010) surfaces were studied using ab initio SCF calculations. At low sulfur coverages, an adsorption complex in which S2 is bridge bonded to two adjacent Zn atoms (Zn−S−S−Zn) is probably the precursor state for the dissociation for the molecule. It is possible to get much higher coverages of sulfur on ZnO (0.7 ML) than on Al2O3 (0.1 ML) at similar S2 exposures. This, in conjunction with results previously reported for H2S adsorption on Cr2O3 a...

H 2 S adsorption on chromium, chromia, and gold/chromia surfaces: Photoemission studies

The reaction of H2S with chromium, chromia, and Au/chromia films grown on a Pt(111) crystal has been investigated using synchrotron-based high-resolution photoemission spectroscopy. At 300 K, H2S completely decomposes on polycrystalline chromium producing a chemisorbed layer of S that attenuates the Cr 3d valence features. No evidence was found for the formation of CrSx species. The dissociation of H2S on Cr3O4 and Cr2O3 films at room temperature produces a decrease of 0.3–0.8 eV in the work function of the surface and significant binding-energy shifts (0.2–0.6 eV) in the Cr 3p core levels and Cr 3d features in the valence region. The rate of dissociation of H2S increases following the sequence: Cr2O3<Cr3O4<Cr. For chromium, the density of states near the Fermi level is large, and these states offer a better match in energy for electron acceptor or donor interactions with the frontier orbitals of H2S than the valence and conduction bands of the chromium oxides. This leads to a large dissociation probability for H2S on the metal, and a low dissociation probability for the molecule on the oxides. In the case of Cr3O4 and Cr2O3, there is a correlation between the size of the band gap in the oxide and its reactivity toward H2S. The uptake of sulfur by the oxides significantly increases when they are “promoted” with gold. The Au/Cr2O3 surfaces exhibit a unique electronic structure in the valence region and a larger ability to dissociate H2S than polycrystalline Au or pure Cr2O3. The results of ab initio SCF calculations for the adsorption of H2S on AuCr4O6 and AuCr10O15 clusters show a shift of electrons from the gold toward the oxide unit that enhances the strength of the Au(6s)↔H2S(5a1,2b1) bonding interactions and facilitates the decomposition of the molecule.