David E. Ramaker

Correlation of Water Activation, Surface Properties, and Oxygen Reduction Reactivity of Supported Pt–M/C Bimetallic Electrocatalysts Using XAS

An analysis of X-ray absorption spectroscopy XAS data X-ray absorption near-edge structure XANES and extended X-ray absorption fine structure EXAFS at the Pt L3 edge for Pt‐M bimetallic materials M = Co, Cr, Ni, Fe and at the Co K edge for Pt‐Co is reported for Pt‐M/C electrodes in HClO4 at different potentials. The XANES data are analyzed using the method, which utilizes the spectrum at some potential V minus that at 0.54 V reversible hydrogen electrode RHE representing a reference spectrum. These data provide direct spectroscopic evidence for the inhibition of OH chemisorption on the cluster surface in the Pt‐M. This OH chemisorption, decreasing in the direction Pt Pt‐Ni Pt‐Co Pt‐Fe Pt‐Cr, is directly correlated with the previously reported fuel cell performance electrocatalytic activities of these bimetallics, confirming the role of OH poisoning of Pt sites in fuel cells. EXAFS analysis shows that the prepared clusters studied have different morphologies, the Pt‐Ni and Pt‐Co clusters were more homogeneous with M atoms at the surface, while the Pt‐Fe and Pt‐Cr clusters had a “Pt skin.” The cluster morphology determines which previously proposed OH inhibition mechanism dominates, the electronic mechanism in the pres

Chloride Ingress into Aluminum Prior to Pitting Corrosion An Investigation by XANES and XPS

Two distinct chloride (Cl - ) species were detected on and/or in the passive oxides of polycrystalline Al samples, which were anodically polarized below the stable pitting potential in Cl - -containing solutions. Chloride was found to be present as an adsorbed specie at the surface of the Al oxide, as well as an incorporated specie within the passive oxide. The two species of Cl - were recorded by X-ray absorption near edge structure (XANES), using both an electron yield detector and an X-ray fluorescence detector, and by X-ray photoelectron spectroscopy (XPS). Electron yield XANES and XPS results indicate that adsorbed Cl - migrates from the solution/Al oxide interface into the passive Al oxide film, prior to stable pit initiation. Cl - migration occurs once a critical anodic potential or critical adsorbed Cl - concentration is reached. The migration of Cl - is followed by a loss of oxidized Al from the passivating film, as determined by XPS, and can be attributed to (i) metastable pitting events or (ii) oxide dissolution. The ingress of Cl - into the oxide appears to be a key factor for the onset of metastable pitting or passive film dissolution.

The past, present, and future of auger line shape analysis

Abstract This review critically evaluates the suitability of Auger spectral line shape analysis as a source of electronic structure information. Methods for extracting the true Auger line shape from the raw data and a theoretical framework for semiquantitative interpretation of that line shape are presented. A wide range of recent applications concentrating on the line shapes of the low Z metals (Be, Li, Na. Mg, and Al). the line shapes of C and Si, the transition metals, and, finally, those of the metal oxides and halides are considered. Spectra for gas-phase molecules, adsorbed molecules. and solids are examined. Finally, new methods for controlling the background, the initial state, the spin polarization, and the angle of escape are discussed, along with requirements for improving the theory. Over 350 references are included.

Comparison of photon-stimulated dissociation of gas-phase, solid and chemisorbed water

Abstract Recent electron- and photon-stimulated desorption (ESD/PSD) data for H2O in the condensed phase and chemisorbed on GaAs(110) and Ti(001) are interpreted utilizing previously published photoemission, electron coincidence and Auger data along with theoretical calculations. Comparison with fragmentation data from the gas phase indicates that only two hole-one electron type states are effective for desorption in condensed or molecularly chemisorbed hydrogen bonded water. The 1b2−1 excitation, which effectively dissociates H2O gas via predissociation, is ineffective in the condensed phase because of the presence of intermolecular decay mechanisms which compete with the predissociation process. Hydrogen bonding reduces the effectiveness of the “2a2−” excitation for H+ desorption. The 1b1−24a1 and 1b1−13a1−14a1 two hole-one electron states are sufficiently long lived: occupation of the strongly antibonding 4a1 orbital also makes them repulsive. These properties make the two hole-one electron states the most persistent for H+ desorption from the H2O phases studied. The core level PSD spectrum from solid D2O is also interpreted. All of the results are found to be comparable to previously reported results for CO.

In situ X-ray absorption spectroscopy as a unique tool for obtaining information on hydrogen binding sites and electronic structure of supported Pt catalysts: towards an understanding of the compensation relation in alkane hydrogenolysis

L2 and L3 X-ray absorption near edge spectra (XANES) on supported Pt particles, with and without chemisorbed hydrogen, are shown to reflect the type of hydrogen-binding site on the Pt surface. FEFF8 ab initio multiple scattering calculations are used to determine XANES spectral fingerprints for the atop vs threefold H binding sites on Pt. Comparison of the experimental XANES data with the theoretical fingerprints, and further theoretical results, show that the acid/base properties of the support have a profound influence on the hydrogen coverage, and therefore on the mode of hydrogen adsorption on the Pt surface. As the electron richness of the support oxygen atoms increases (i.e., with increasing alkalinity of the support), the H coverage increases and the hydrogen-binding site of the strongly adsorbed hydrogen changes from atop to threefold. This site change is primarily responsible for the observed changes in previously reported kinetic data, which show an increase in negative order (roughly from −1. 5t o−2.5) in hydrogen partial pressure for neopentane hydrogenolysis with increasing support alkalinity. This change in negative order directly reflects the greater number of vacant Pt sites that must be available to allow adsorption of the neopentane. A compensation relation is found in the kinetic data of Pt on different supports resulting directly from this change in hydrogen coverage. This implies that the experimentally determined kinetic parameters are apparent values. These apparent values are correlated to the intrinsic kinetic parameters via the thermodynamic properties of the sorption of the reactants, described by the Temkin equation. The TOF of neopentane hydrogenolysis over several catalysts, measured in previous work, decreases with the increasing alkalinity of the support. This can now be directly explained as the result of the change in hydrogen coverage using a Frumkin isotherm, implying that the neopentane adsorption becomes weaker with increased hydrogen coverage. These conclusions, that hydrogen drives the catalysis, are further supported by density functional calculations on small Pt 4 clusters, which show that the acid/base properties of the support have a much larger direct influence on Pt–H bonding than on Pt–CH n bonding.  2003 Elsevier Science (USA). All rights reserved.

Enhanced Oxygen Reduction Activity in Acid by Tin-Oxide Supported Au Nanoparticle Catalysts

Gold nanoparticles supported on hydrous tin-oxide (Au-SnO{sub x}) are active for the four-electron oxygen reduction reaction in an acid electrolyte. The unique electrocatalytic of the Au-SnO is confirmed by the low amount of peroxide detected with rotating ring-disk electrode voltammetry and Koutecky-Levich analysis. In comparison, 10 wt % Au supported on Vulcan carbon and SnO{sub x} catalysts both produce significant peroxide in the acid electrolyte, indicating only a two-electron reduction reaction. Characterization of the Au-SnO{sub x} catalyst reveals a high-surface area, amorphous support with 1.7 nm gold metal particles. The high catalytic activity of the Au-SnO is attributed to metal support interactions. The results demonstrate a possible path to non-Pt catalysts for proton exchange membrane fuel cell cathodes.

Bonding information from Auger spectroscopy

Abstract The use of Auger Spectroscopy to obtain bonding and electronic structure information is reviewed. The methods for extracting the Auger lineshape from the experimental spectrum are described; in particular the background substraction and loss deconvolution techniques are presented. A prescription is given for quantitatively interpreting the lineshape utilizating an empirically determined one-electron DOS and atomic Auger matrix elements. Final state hole-hole correlation or localization effects on the lineshape are emphasized. The carbon KVV lineshapes of graphite, diamond, and the carbides are examined. Localization effects in the lineshapes are correlated with ionic bonding character.

Interpretation of the Al K- and LII/III-edges of aluminium oxides: differences between tetrahedral and octahedral Al explained by different local symmetries

The Al K- and LII/III-edge XANES of aluminium oxide are interpreted using empirical molecular orbital theory (EHMO) and ab initio self-consistent field real space multiple scattering calculations (FEFF8). Most features in the XANES at the K- and LII/III-edges are interpreted as shape resonances; although some fine structure, visible at both edges, arises from multiple scattering over the medium range (~15 A). The change in local symmetry between octahedral and tetrahedral Al explains the observed differences in the electronic structure. First, Al p–d hybridization is allowed only in tetrahedral symmetry, resulting in a lower absorption edge in tetrahedral Al than in the octahedral. Second, only in octahedral Al do the oxygen orbitals near the valence band maximum (the HOMOs) have the right symmetry to mix with the Al p orbitals just above the band gap (the LUMOs). This gives a more screened core hole in the octahedral case. Calculations on distorted octahedral Al sites reveal both p–d and s–d hybridizations; however, the latter is less prominent. The diffuse d orbitals, which hybridize with the p or s orbitals in tetrahedral or distorted octahedral symmetry, are primarily responsible for the fine structure in the near-edge region (0–15 eV) that is determined by medium-range scattering (up to ~15 A). The observed difference in the magnitude of this fine structure at the K- and LII/III-edges is caused by the different degrees of d orbital hybridization with the s and p orbitals.

Extracting Auger lineshapes from experimental data

Abstract A method is presented for extracting Auger lineshapes from an experimental, derivative Auger electron spectrum. The method involves removing the background after numerically integrating the derivative spectrum. This procedure is contrasted with the “dynamic background subtraction” method and similar methods which remove the background before integration. A treatment of electron energy loss is included. A functional form for the total electron emission for the Li salts of SO 4 3− and PO 4 3− is presented and compared to electron-loss data. Electron escape depth and spectrometer response corrections to the observed Auger signal are also considered. The S and P LVV Auger lineshapes are presented for the third row oxyanions, SO 4 2− PO 4 3− .

Dramatically enhanced cleavage of the C-C bond using an electrocatalytically coupled reaction.

This paper describes a generalized approach for the selective electrocatalytic C-C bond splitting in aliphatic alcohols at low temperature in aqueous state, with ethanol as an example. We show that selective C-C bond cleavage, leading to carbon dioxide, is possible in high pH aqueous media at low overpotentials. This improved selectivity and activity is achieved using a solution-born co-catalyst based on Pb(IV) acetate, which controls the mode of the ethanol adsorption so as to facilitate direct activation of the C-C bond. The simultaneously formed under-potentially deposited (UPD) Pb and surface lead hydroxide change the functionality of the catalyst surface for efficient promotion of CO oxidation. The resulting catalyst retains an unprecedented ability to sustain the full oxidation reaction pathway on an extended time scale of hours as opposed to minutes without addition of Pb(IV) acetate.