Mark Kuhn

Electronic properties of gold on Mo(110): d → s,p charge redistribution and valence band shifts

Abstract The interaction between Au atoms and Mo(110) has been investigated using photoelectron spectroscopy and ab initio self-consistent field calculations. The formation of AuMo bonds induces shifts toward higher binding energy (0.3–0.7 eV) in the core levels and valence d band of gold. This is accompanied by an important redistribution of charge, in which Au loses 5d electrons and gains (6s,6p) electrons. The positive binding-energy shifts in the Au 4f levels and 5d band reflect the effects of a Mo-induced reduction in the Au 5d electron population.

Interaction of sulfur with Pt(111) and Sn/Pt(111): Effects of coverage and metal--metal bonding on reactivity toward sulfur

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 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 TD...

Photoemission studies of S/Co/Mo(110) and S/Ni/Mo(110) surfaces: Co- and Ni-promoted sulfidation of Mo(110)

Abstract The properties of a series of X Mo (110) and S/X/Mo(110) surfaces (X = Co or Ni) have been examined using photoemission, thermal desorption spectroscopy and ab initio SCF calculations. The bimetallic bonds in the Co Mo (110) and Ni Mo (110) systems are complex, involving Co(3d,4s)→Mo(5s,5p) and Ni(3d,4s)→Mo(5s,5p) electron transfers and a Mo(4d)→Mo(5s,5p) rehybridization. These redistributions of charge lead to positive core-level shifts for all the metals. The exposure of Mo(110) to large amounts of S 2 gas produces only a chemisorbed layer of sulfur, without forming molybdenum sulfides. The sulfidation of Mo occurs after exposing Co Mo (110) and Ni Mo (110) surfaces to S 2 . Co and Ni promote the formation of molybdenum sulfides by transferring charge to Mo (favoring in this way an electrophilic attack of S on Mo), and by changing the structure of the surface (making it easier for the penetration of S into the bulk of the sample). Co and Ni exhibit a unique ability to enhance the Mo↔S interactions. A comparison of the behavior of several admetals on S Mo (110) surfaces indicates that the “promotional effect” of an admetal on the sulfidation of Mo increases in the following order: Ag ≈ Zn TMS y MoS 2 catalysts (TM = Zn, Cu, Co or Ni) in hydrodesulfurization (HDS) reactions and the trends found for the sulfidation of Mo in S/TM/Mo(110) surfaces. Systems that contain Co and Ni display the largest HDS activity and the strongest Mo↔S interactions.

Electronic properties of Pt in bimetallic systems: photoemission and molecular-orbital studies for PtAl surface alloys

Abstract The adsorption of Pt on polycrystalline Al leads to the formation of surface alloys. The electronic properties of these systems have been examined using XPS and ab initio SCF calculations. The PtAl surface alloys display a Pt(5d) band that appears at much higher binding energy (≈ 1.8 eV) than in metallic Pt. This is accompanied by positive shifts in the Pt 4f (≈ 1.2 eV) and Al 2s (≈ 0.2 eV) levels. The PtAl bond is complex, involving an Al(3s, 3p) → Pt(6s, 6p) charge transfer and a Pt(5d) → Pt(6s, 6p) rehybridization that localize electrons in the region between the two metal centers.

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....

Adsorption of Au on Ru(001): electronic perturbations and the nature of the bimetallic bond

Abstract The electronic properties of Au Ru (001) surfaces have been investigated using high-resolution photoemission spectroscopy with synchrotron radiation, and ab initio self-consistent-field calculations. For submonolayer coverages of Au, the interaction between the Au 5d and Ru 4d bands produces an interface state that appears at ∼ 1.5 eV. A monolayer of Au in contact with Ru(001) has a Au 4 f 7 2 binding energy of ∼ 0.25 eV higher than that of the surface atoms of Au(111). The AuRu bond is complex with Au losing 5d electrons and gaining 6s and 6p electrons. This redistribution of charge affects the stability of the bimetallic bond.

Coadsorption of Zn and S on Mo(110): weakening of the ZnMo bond and Zn-promoted sulfidation of Mo

The coadsorption of Zn and S on Mo(110) has been investigated using TDS, XPS and XAES. Zn atoms supported on clean Mo(110) desorb at 670 (first layer), 510 (second layer) and 480 K (multilayer). The MoZn bond is considerably stronger (∼ 10 kcal/mol) than the ZnZn bond. The strong Mo ↔ Zn interaction leads to shifts of ∼ 0.35 eV toward lower binding energy in the Zn 3d and 2p levels, and in the Zn L3M45M45 Auger transition. At submonolayer coverages (θS + θZn 1 ML) at 300 K produces zinc sulfides and chemisorbed sulfur, without forming molybdenum sulfides. At 600–700 K, zinc promotes the formation of molybdenum sulfides by favoring the migration of sulfur from the surface into the lattice of the Mo substrate. In the ZnxS/MoyS/Mo(110) systems, the zinc sulfides decompose from 750–950 K, whereas the molybdenum sulfides dissociate at much higher temperatures (1200–1350 K).

Repulsive interactions between sulfur and gold on Ru(0001)

Abstract There is no bonding between Au and S atoms coadsorbed on top of Ru(0001). Au compresses S into small islands of high local coverage, causing dramatic changes in the kinetics of sulfur desorption. The compression reduces the adsorption energy of sulfur on Ru (0001), favoring the desorption of S 2 in a sharp peak around 1170 K, and preventing the diffusion of S into the bulk of Ru. The presence of sulfur induces a weakening of ≈ 25 kJ mol in the strength of the Au-Ru(0001) bond. The Au a ⇐ S a interactions are repulsive due to the electron-acceptor nature of both adsorbates.

Electronic and chemical properties of Pd in bimetallic systems: interaction of Pd with Rh(111)

The nature of the metal-metal interactions in a series of PdRh(111) surfaces has been examined using thermal desorption mass spectroscopy, core- and valence-level photoemission, CO chemisorption, and ab initio self-consistent field calculations. A monolayer of Pd supported on Rh(111) exhibits a Pd 3d52 binding energy ∼0.2 eV higher than that of the surface atoms of Pd(100), and desorbs at ∼1390 K. The desorption temperature of CO from this PdRh(111) system is 50–70 K lower than those seen on Pd(111) and Rh(111), indicating a weakening of 3–5 kcal mol−1 in the strength of the CO adsorption bond. These results are compared with data reported in the literature for bimetallic surfaces that contain Pd and Rh, and general trends are explained in terms of a simple model.

A comparison of the reaction of S2 with metallic copper, Cu2O and Cu/ZnO : electronic properties and reactivity of copper

The adsorption of S2 on polycrystalline copper, Cu2O and Cu/ZnO surfaces has been examined by using synchrotron-based photoemission plus INDO/S and ab initio self-consistent-field calculations. S2 mainly interacts with the metal centers of Cu/ZnO, Cu2O and ZnO. The chemical affinity of these systems for sulfur increases following the sequence: ZnO<Cu2O<Cu/ZnO<Cu. The copper atoms in the Cu2O and Cu/ZnO systems display band structures that are substantially different from that of metallic copper, and these electronic perturbations affect their reactivity towards S2. Once copper is bonded to oxygen on ZnO or in Cu2O, its electron density decreases, making more difficult the formation of Cu→S2 dative bonds and the breaking of the S–S bond. The poisoning of Cu/ZnO catalysts by sulfur is discussed in the light of these results.