A. I. Boronin

Effect of dispersion of supported palladium on its electronic and catalytic properties in the hydrogenation of vinylacetylene

Abstract It is found for the gas-phase hydrogenation of vinylacetylene that supported palladium shows an approximately constant specific catalytic activity (turnover number TON) over a wide range of particle sizes, namely, from 40 to 180A, regardless of the nature of the support (silica or γ-alumina). With a decreasing in the size of the palladium particles from 40 to 10Athe specific activity (TON) falls by one order of magnitude, a positive shift of the Pd 3d 5/2 level by 0.6 eV being observed. The low specific activity of highly dispersed palladium is caused by a stronger chemisorption of vinylacetylene on small electron-deficient clusters compared with large particles.

XPS and UPS studies of oxygen adsorption over clean and carbon-modified silver surfaces

Abstract Oxygen adsorption over clean and carbon-modified silver surfaces has been studied by XPS and UPS in the temperature range 300–700 K under O 2 pressures from 10 −3 to 10 Pa. Adsorption over a clean silver surface at T >420 K leads to the appearance of atomically adsorbed oxygen on the surface, which is characterized by E B (O 1s) = 528.5 eV, high reactivity towards CO and a heat of adsorption of 100 kJ/mol. Subsurface oxygen is formed in small amounts. Increasing the O 2 pressure to 10 Pa does not lead to the formation of new adsorption forms. At T E B (C 1s)=287.8 eV and E B (O 1s)=530.6 eV are formed on the surface. During interaction of O 2 with the silver surface containing dissolved carbon in the bulk, the surface is modified by O and C due to CO bond formation in the subsurface region. The O 2 adsorption at high pressures (10 Pa) on the modified surface is characterized by the formation of a new oxygen state with E B =530.3 eV. Based on UPS spectra characteristics (the peak at E B θ 9.5 eV), this state is supposed to be of molecular form.

An XPS investigation of the chemisorption of oxygen on the iridium (111) surface

Using X-ray photoelectron spectroscopy (XPS) to monitor the concentration of adsorbed oxygen on Ir(111), it was found that at temperatures well below the onset of thermal desorption, the adsorbed oxygen disappears from the surface. A pressure of approximately 10−7 Torrof O2 over the crystal surface ensures that the adsorbed oxygen is not removed via reaction with background CO. The implication is that oxygen is diffusing into the bulk of the crystal. Support for this idea comes from the observation of a small, broad peak in the X-ray photoelectron spectrum of the “clean” surface at the same binding energy as the oxygen signal and the difficulty in removing the oxygen which contributes to this peak. Evidently, this peak derives from oxygen dissolved in the Ir lattice. The saturation coverage of oxygen is estimated to be 7.9 × 1014atoms/cm2 by comparison with XPS data for a CO saturated surface. This coverage is consistent with the interpretation that the (2×2) oxygen LEED pattern arises from three independent domains of (2×1) oxygen superstructures rotated 120° with respect to each other.

Effect of Pd/C dispersion on its catalytic properties in acetylene and vinylacetylene hydrogenation

In reactions of the selective hydrogenation of acetylene and vinylacetylene, the specific catalytic activity (turnover number, TON) and selectivity of Pd obtained via reduction of C3H5PdC5H5 anchored on active carbon, within a particle-size range of 30–110A, is roughly constant and close to the TON of Pd over oxide supports (SiO2, Al2O3). The binding energy of the Pd 3d5/2 level of a low-dispersed Pd does not depend significantly on its particle size and the nature of the support. The TON of Pd/C containing particles <30Ais several times lower than that of low-dispersed Pd.

On the nature of the interaction of H2PdCl4 with the surface of graphite-like carbon materials

Abstract Adsorption of H2PdCl4 from aqueous solutions on the surface of graphite-like carbon materials proceeds through two competitive pathways: reduction, which gives rise to Pd ° particles of 6–100nm in size, and formation of π-complexes of PdCl2 with >CC PdCl 2 C +2 HCl ← (2) H 2 PdCl 4 + C → (1) Pd C + 2 Cl C +2 HCl Process (1) is localized near the exterior surface of the porous carbon particles. Process (2) takes place on the whole surface of these particles. The quantitative ratio of the adsorbed palladium species (Pd2+ and Pd °) and also the dispersion and morphology of the metallic particles depend on the mode of contact of the H2PdCl4 solution with the carbon, the composition of the gas phase, the size of the carbon particles, the chemical state of the carbon surface and the conditions of drying. The results obtained are explained in terms of the electrochemical theory of carbon reactivity.

The Thermal Stability of Nanodiamond Surface Groups and Onset of Nanodiamond Graphitization

Abstract The results of study of the surface chemistry of nanodiamonds treated by a mixture of HClO4 and H2SO2 acids are presented. Changes of the composition of surface diamond groups brought up by thermal annealing were monitored by Fourier transformed infrared spectroscopy and temperature‐programmed desorption. The decomposition of oxygen‐containing groups is observed at 300–900°C; CHx groups decompose at 700–1150°C. The process of the annealing of ND was investigated by the X‐ray photoelectron spectroscopy. The clear onset of ND graphitization was observed at temperature of 950°C that is supported by the appearance of sp 2 component in the C1s spectrum. The presence of the N1s peak in spectra of ND is observed in initial and partially graphitized ND annealed up to 1100°C.

An XPS and UPS study of the kinetics of carbon monoxide oxidation over Ag(111)

Abstract The oxidation of carbon monoxide over a Ag(111) catalyst has been studied by XPS and UPS. The kinetics have been determined over the temperature range of 180 to 400 K and found to be of the Langmuir-Hinshelwood type, although the Eley-Rideal mechanism is mimicked. A negative activation energy, −1.7 kcal/mole, and a preexponential, 6 × 10 −18 cm 2 , are found. The former corresponds to the difference in the activation energies for carbon monoxide desorption and for carbon monoxide oxidation (leading to CO 2 desorption). At 90 K, upon carbon monoxide exposure to the active oxygen precovered surface, the O ls and C ls spectral regions show the formation of CO 2 -like and carbonate species; the latter is stable to at least room temperature. That is, at 90 K, the residence time and mobility of CO 2 formed at the surface permits a new surface reaction — the formation of stable surface carbonate. The identifications are based on C and O coverages and on line positions from the literature for Cu/CO 2 and several bulk carbonates. With UPS, the 1π g , the unresolved doublet 1π u and 3σ g , and the 4σ g molecular orbitals of adsorbed CO 2 -like species are identified, as well as the unresolved triplet 1α′ 2 , 1e″ and 4e′ and the unresolved triplet 3e′, 1α″ 2 and 4a′ molecular orbitals of the carbonate species. Surface CO 2 -like species formed by surface oxidation of CO seem to be more strongly bound than reversibly adsorbed CO 2 .

Associative oxygen species on the oxidized silver surface formed under O2 microwave excitation

The experimental methods of X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS, respectively) and the quantum mechanical calculations are applied for analysis of oxygen states on the silver oxide surface. At low temperatures (T<470 K), the silver surface is intensively oxidized by a microwave oxygen discharge to form cuprite Ag2O. Two adsorbed oxygen species of the atomic (dissociative) and molecular (associative) nature can be adsorbed on the cuprite Ag2O surface. A comparison of the UPS data and the DFT calculations of molecular models Ag2–O2 and Ag2–O3 shows that the formation of ozonide-like structures is preferable to that of peroxide species. Thermal stability and the reaction probability of the adsorbed states are investigated.

Decomposition of ethylene and a mechanism of graphite formation on the Pt(110) surface

Abstract Adsorption and decomposition of ethylene as well as deposition of graphite on the (110) plane of platinum in the temperature range from 300 to 1450 K are studied by XPS, LEED, and TPR. It is shown that the process of ethylene decomposition proceeds through a number of intermediate species up to elementary carbon. At a temperature higher than 470 K there is penetration of the carbon into subsurface layers of platinum. This process is accompanied by the formation of a carbide-like phase of sandwich structure (CLPSS) stable at T ⩽700 K responsible for modifying the surface properties. It is found that the mechanism of low-temperature graphite formation on Pt(110) is determined mainly by the decomposition of the CLPSS at T >700 K. The role of different parameters influencing the efficiency of the graphitization is discussed.

Oxidation of the polycrystalline gold foil surface and XPS study of oxygen states in oxide layers

Gold oxide films obtained on the surface of polycrystalline gold foil upon oxidation by oxygen activated by a high-frequency discharge have been studied by X-ray photoelectron spectroscopy. High-frequency O2 activation affords oxide films more than 3–5 nm thick. As follows from Au4f spectra, the surface gold atoms are oxidized to the oxidation state +3. The O1s spectra have a composite shape and are decomposed into four components that characterize nonequivalent states of oxygen in the resulting oxide films. It is assumed that the two major oxygen states (Eb(O1s) = 529.0 and 530.0 eV) correspond to the oxygen atoms in two-and three-dimensional gold oxide Au2O3, respectively. The oxygen states characterized by the higher binding energies (Eb(O1s) = 531.8 and 535.2 eV) likely correspond to molecular oxygen in peroxide and superoxide groups, respectively.