High Resolution Pdf Measurements On Ag Nanoparticles

The quantitative analysis of structural defects in Ag nanoparticles was addressed in this work. We performed atomic scale structural characterization by a combination of x-ray diffraction (XRD) using the Pair Distribution Function analysis (PDF) and High Resolution Transmission Electron Microscopy (HRTEM). The XRD measurements were performed using an innovative instrumentation setup to provide high resolution PDF patterns. © 2009 American Institute of Physics.1092102106Rocha, T.C.R., Winnischofer, H., Westphal, E., Zanchet, D., Formation Kinetics of Silver Triangular Nanoplates (2007) J. Phys. Chem. C, 111, p. 2885Petkov, V., Jeong, I.-K., Chung, J.S., Thorpe, M.F., Kycia, S., Billinge, S.J.L., High Real-Space Resolution Measurement of the Local Structure of Ga 1-xIn xAs Using X-Ray Diffraction (1997) Phys. Rev. Lett, 83, pp. 4089-409

A near ambient pressure XPS study of Au oxidation

The surface of a gold foil under ozone oxidation was examined by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and scanning electron microscopy (SEM). Our in-situ observations show that a surface oxide phase is formed during the exposure to ozone; however this phase decomposes in vacuum and even in the presence of ozone at temperatures higher than 300°C. Assuming an oxide overlayer completely covering the Au surface, the thickness of the oxide phase was estimated to be between 2.9 and 5.8 Å by energy-dependent XPS depth profiling. The surface oxidation led to structural modifications of the gold surface. These morphological changes do not disappear even under vacuum. In the Au 4f spectra, an additional component at low binding energy (83.3 eV), which appears during/after O3 treatment, is assigned to the presence of low-coordinated atoms which appear on the Au surface as a result of surface restructuring under oxidation. Ex-situ SEM images demonstrate that only the region of the sample that was exposed to O3 shows the presence of ridges on the Au surfac

The Oxidation of Rhenium and Identification of Rhenium Oxides During Catalytic Partial Oxidation of Ethylene An In Situ XPS Study

Rhenium is catalytically active for many valuable chemical reactions, and consequently has been the subject of scientific investigation for several decades. However, little is known about the chemical identity of the species present on rhenium surfaces during catalytic reactions because techniques for investigating catalyst surfaces in-situ – such as near-ambient-pressure X-ray photoemission spectroscopy (NAP-XPS) – have only recently become available. In the current work, we present an in-situ XPS study of rhenium catalysts. We examine the oxidized rhenium species that form on a metallic rhenium foil in an oxidizing atmosphere, a reducing atmosphere, and during a model catalytic reaction (i.e. the partial-oxidation of ethylene). We find that, in an oxidizing environment, a Re₂O₇ film forms on the metal surface, with buried layers of sub-oxides that contain Re⁴⁺, Re²⁺ and Re^δ+ (δ ∼ 1) species at the Re₂O₇/Re interface. The Re²⁺ containing sub-oxide is not a known bulk oxide, and is only known to exist on rhenium-metal surfaces. The Re₂O₇ film sublimes at a very low temperature (ca. 150 ℃), while the Re⁴⁺, Re²⁺ and Re^δ+ species remain stable in oxidizing conditions up to at least 450 ℃. In a reducing atmosphere of H₂, the Re²⁺ species remain on the surface up to a temperature of 330 ℃, while Reδ+ species can be detected even at 550 ℃. Under conditions for partial-oxidation of ethylene, we find that the active rhenium catalyst surface contains no bulk-stable oxides, but consists of mainly Re²⁺ species and small amounts of Re⁴⁺ species. When the catalyst is cooled and inactive, Re₂O₇ is found to form on the surface. These results suggest that Re²⁺ and Re⁴⁺ species may be active species in heterogeneous rhenium catalysts

Insights into the Electronic Structure of the Oxygen Species Active in Alkene Epoxidation on Silver

Extensive density functional theory calculations of the O1s binding energies, adsorption energies, and the experimentally measured in situ X-ray photoelectron spectra of oxygen on silver are reported in an effort to clarify which species are present during ethylene epoxidation. We find that the O1s binding energy of an oxygen adatom increases near linearly with its adsorption energy due to the ionic nature of the Ag/O interaction. Thus, contrary to widespread assignments, a weakly bound oxygen adatom does not account for the electrophilic species with an O1s binding energy of 530 eV that is thought to be active in ethylene epoxidation. Instead, we show that the only species with O_1s binding energies near 530 eV are covalently bound, which we find in our calculations, for example, when hydrogen or carbon are present

Promotion Mechanisms of Iron Oxide Based High Temperature Water Gas Shift Catalysts by Chromium and Copper

The Cr and Cu promotion mechanisms of high temperature water–gas shift (HT-WGS) iron oxide catalysts, synthesized by coprecipitation, were investigated as a function of reaction conditions. XRD and in situ Raman characterization showed that the initial calcined catalysts consisted of the Fe₂O₃ (hematite) bulk phase and transformed to the Fe₃O₄ (magnetite) phase during the HT-WGS reactions. In situ NAP-XPS and HS-LEIS surface analysis revealed that Cr was surface enriched as Cr⁶⁺ for the initial catalyst and reduced to Cr³⁺ during the HT-WGS reactions, with the Cr³⁺ dissolving into the bulk iron oxide lattice forming a solid solution. In situ NAP-XPS, XANES, and HS-LEIS characterization indicated that Cu was initially present as Cu²⁺ cations dissolved in the Fe₂O₃ bulk lattice and reduced to metallic Cu⁰ nanoparticles (∼3 nm) on the external surface of the iron oxide support during the HT-WGS reactions. In situ HS-LEIS surface analysis also suggests that ∼1/3 of the surface of the Cu nanoparticles was covered by a FeO_x overlayer. The CO-TPR probe demonstrated that Cr does not chemically promote the iron oxide catalyst and that only Cu is a chemical promoter for the iron oxide HT-WGS catalysts. The Cu promoter introduces new catalytic active sites that enhance the reaction rates of the WGS reactions

Chemical Synthesis And Structural Characterization Of Highly Disordered Ni Colloidal Nanoparticles

This work focuses on synthetic methods to produce monodisperse Ni colloidal nanoparticles (NPs), in the 4-16 nm size range, and their structural characterization. Narrow size distribution nanoparticles were obtained by high-temperature reduction of a nickel salt and the production of tunable sizes of the Ni NPs was improved compared to other methods previously described. The as-synthesized nanoparticles exhibited spherical shape and highly disordered structure, as it could be assigned by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Annealing at high temperature in organic solvent resulted in an increase of nanoparticle atomic ordering; in this case, the XRD pattern showed an fcc-like structure. Complementary data obtained by X-ray absorption spectroscopy confirmed the complex structure of these nanoparticles. 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Thermodynamic and spectroscopic properties of oxygen on silver under an oxygen atmosphere

We report on a combined density functional theory and experimental study of the O1s binding energies and X-ray Absorption Near Edge Structure (XANES) of a variety of oxygen species on Ag(111) and Ag(110) surfaces. Our theoretical spectra agree with our measured results for known structures, including the p(Nx1) reconstruction of the Ag(110) surface and the p(4x4) reconstruction of the Ag(111) surface. Combing the O1s binding energy and XANES spectra yields unique spectroscopic fingerprints, allowing us to show that unreconstructed atomic oxygen is likely not present on either surface at equilibrium conditions under oxygen chemical potentials typical for ethylene epoxidation. Furthermore, we find no adsorbed or dissolved atomic species whose calculated spectroscopic features agree with those measured for the oxygen species believed to catalyze the partial oxidation of ethylene