Showing papers on "Chemical state published in 1996"

Specific Adsorption of a Bisulfate Anion on a Pt(111) Electrode. Ultrahigh Vacuum Spectroscopic and Cyclic Voltammetric Study

Abstract: We addressed in this study the process of specific adsorption of anions at the metal/solution interface. We focused on the nature of the surface chemical bond that accounted for the phenomenon of adsorption specificity in the context of bisulfate coverage and structural information. While we limited our investigations to bisulfate adsorption on the Pt(111) electrode in sulfuric and mixed sulfuric/perchloric acid media, our conclusions have general significance in explaining ionic adsorption events in electrochemistry. We used core-level electron energy loss spectroscopy, auger electron spectroscopy, low energy electron diffraction, and cyclic voltammetry. Our findings show that in the studied range of sulfuric acid concentration (10-4−10-1 M) the maximum anion coverage is 0.34 ± 0.02 monolayer (ML) and that this coverage corresponds to a highly ordered Pt(111)(√3 × √3)R30° surface structure. S2p core-level and LMM Auger electron spectra indicate that the chemical state of bisulfate sulfur is +6, as in the...

The chemical state of sulfur in carbon-supported fuel-cell electrodes

Abstract: Vulcan carbon is the favored support for fuel-cell electrocatalysts, but as-received it contains high levels of sulfur (ca. 5000 ppm or greater) which could potentially poison the fuel-cell electrochemistry. The chemical state of the sulfur in platinum/carbon electrocatalysts has been examined by measuring its oxidation state via x-ray photoelectron spectroscopy at different points during the preparation of a mock fuel-cell electrode. Also monitored were the presence of sulfate in the aqueous wash from the electrocatalysts and the cyclic voltammetry of the electrocatalysts after each preparation step. Our studies indicate that the platinum catalytically oxidizes some of the covalent sulfur in the vulcan carbon to sulfate when water, heat, and strong physical contact between Pt and C are all present. These conditions are attained during the preparation of typical fuel-cell electrodes. Most of the zero-valent sulfur remains in the carbon after treatment, however, and appears not to be initially in contact with the Pt. This remaining unoxidized sulfur may be a source of poisoning to the Pt electrocatalyst with long-term electrochemical use, particularly at the fuel-cell cathode.

X‐ray photoelectron spectroscopy characterization of radio frequency reactively sputtered carbon nitride thin films

Abstract: Carbon nitride thin films prepared by radio frequency reactive sputtering of graphite in pure nitrogen plasma have been characterized by x‐ray photoelectron spectroscopy (XPS) for probing the chemical bonding in the films. The multiple binding energy values obtained for the C 1s and N 1s photoelectrons in the film suggest that both the C and N atoms exhibit at least three types of chemical states, manifestative of different types of the C–N bonding present in the material. The presence of theoretically predicted β‐C3N4 phase in our C–N films has been suggested on the basis of XPS and optical data.

Chemical state of Cd in apatite phosphate ores as determined by EXAFS spectroscopy

Abstract: Abstract Natural apatites used in fertilizer industries often contain trace amounts of Cd, which may reach concentrations of several tens to a few hundred parts per million. Cd is not eliminated during the production of phosphate fertilizers, and its concentration in the final product can exceed environmental norms. Knowledge of the chemical state of Cd in apatite ores is a prerequisite for the design of technical processes of extraction. In the present study, Cd K-edge EXAFS spectroscopy was used to investigate the structural environment of Cd present in sedimentary apatite ores from West Africa. These apatites are fluorinated and contain goethite, quartz, and crandallite as ancillary phases detected by X-ray diffraction or EXAFS spectroscopy. Cd K-edge EXAFS spectra for two natural samples were analyzed and compared with those for Cd-containing reference minerals, including hydroxylapatite, goethite, otavite, and crandallite. A good spectral resemblance was observed between natural products and synthetic apatite containing small amounts of Cd. This spectral likeness indicates that the majority of Cd atoms are diluted in the apatitic framework and do not form Cd10(PO4)6(OH,F)2 clusters. This finding was confirmed by quantitative analysis of the EXAFS spectra, which indicated that Cd atoms are surrounded by nearest 0 atoms at 2.33 Å, next-nearest P atoms at ~3.53 Å, and a third-nearest shell of Ca atoms at ~4.02 Å. A comparison of these data with those obtained for synthetic apatites allowed us to assess that Cd occupies both Ca crystallographic sites with a slight preference for the Ca2 site.

Role of hetero-junctions in oxide semiconductor gas sensors

Abstract: A new design concept of semiconductor gas sensors, which introduces a foreign antenna material to a gas in problem, is described for the detection of dilute odorous gases such as HN 3 and H 2 S. The antenna material, dispersed on the surface of a semiconducting oxide, interacts with the gas sensitively and selectively, and the resulting change in its chemical state induces a change in semiconducting properties of the underlying oxide through hetero-junctions. Typical examples of such sensors include NH 3 sensors using Au and Pt WO 3 and an H 2 S sensor using CuO--SnO 2 .

Potassium promoter in industrial ammonia synthesis catalyst : Studies by surface ionization

Abstract: Industrial ammonia synthesis catalysts, as fresh (pre-reduced and passivated) and used (passivated) flat samples were studied in a molecular beam UHV apparatus, using a surface ionization detector. The desorption of the K promoter from the catalyst at 870–1080 K could be characterized, by measuring angular distributions and simultaneous Arrhenius plots, as a function of the angle around the sample. The two activation energies observed for K desorption from the fresh catalyst were 260 kJ/mol (2.7 eV) and 330 kJ/mol (3.4 eV); the used catalyst had only one, much smaller activation barrier at 164 kJ/mol (1.70 eV). The chemical state of potassium in the catalyst changes from a stronger bound ionic, probably in the form of FeOK, to a weaker, more covalent one during the industrial process. A large difference in work functions between the fresh (3.9 eV) and used (1.7 eV) catalyst was observed. This seems to be due to formation of K oxides on the surface of the used catalyst, caused by enrichment of potassium at the surface during the industrial use.

Correlation between dielectric constant and chemical structure of sodium silicate glasses

Abstract: The chemical structure of sodium aluminosilicate glasses is determined by high resolution x‐ray photoelectron spectroscopy (XPS) as silicon is gradually replaced by aluminum. A well‐defined chemical state is found for silicon, aluminum, and sodium atoms, while three different environments are identified for oxygen atoms corresponding to Si–O–Si, Si–O–Al, and Si–O–Na bonds. The binding energy of Na 1s photoelectrons increases significantly with increasing aluminum substitution while that of Al 2p and components of O 1s photoelectrons remains approximately constant. Thus, the ionicity of sodium increases with aluminum amount, but the over all electron density around silicon, aluminum, and different types of oxygen atoms remains unchanged. The dielectric constant of the glasses increases with increasing aluminum substitution. It is analyzed in terms of the polarizabilities of constituent structural units, viz., silicon tetrahedra, nonbridging oxygen–sodium ion pairs, and aluminum tetrahedron–sodium ion pairs...

Behaviour of a cyanide-derived Fe/Al2O3 catalyst during Fischer-Tropsch synthesis

Abstract: The interaction of carbon monoxide and hydrogen with an alumina-supported iron catalyst has been studied at temperatures ranging from 300 to 523 K and at a pressure of 103 kPa. The species produced and adsorbed on the catalyst surface have been examined by infrared spectroscopy, while the chemical state of the iron within the catalyst was investigated by magnetization measurements and Mossbauer spectroscopy. At 300 K the main type of adsorbed species on the iron particle surface appeared to be molecular carbon monoxide and formate groups. The amount of the latter species increases considerably at elevated reaction temperatures up to 473 K. Changes in the infrared absorption bands of CO, and in the magnetization and Mossbauer data indicate that under the applied conditions the surface properties of the iron particles are affected by the synthesis gas, but that the bulk remains metallic. The increased Fischer-Tropsch activity at more elevated temperatures, viz. 473 and 523 K, is accompanied by significant changes in the in situ recorded infrared spectra. The formation of hydrocarbons is evident from the well developed absorption bands in the 3000-2800 cm−1 range, due to (a)symmetric stretching vibrations of CH2 and CH3 groups. Assignment of the various absorption bands in the 1700-1200 cm−1 wavenumber range is speculative due to (partial) coincidence of the bands and the gradual change in chemical composition of the catalyst particles. At higher temperatures the bulk of the iron particles reacts rapidly to a mixture of carbides. Initially the unstable e-Fe2C is formed which reacts to e′-Fe2.2C upon prolonged periods of time of reaction and/or increased reaction temperatures. These carbides appeared to be thermally unstable: upon heating in helium they are converted into a mixture of χ-Fe5C2 and metallic Fe. Interaction of the reduced iron catalysts with ethylene or methane at 523 K gives rise to absorption bands at 1555, 1345 and 1050 cm−1, which are assigned to CHx adsorbed species. Upon reaction with ethylene also a band at 2160 cm−1 was observed, which was assigned to an ethylidyne species.

Electron spectroscopic investigation of Sn coatings on glasses.

Abstract: Float glasses of different thicknesses and a conducting tin oxide glass have been investigated using Photo and Auger Electron Spectroscopy induced by AlKalpha X-rays. On the basis of measured chemical XPS shifts in the binding energies the chemical state of Sn (+2 or +4) incorporated on the float glasses could not be assigned. The use of the Auger parameter allows to separate relaxation and chemical contributions. The derived true chemical shifts of Sn on float-glasses are larger than those of SnO and/or SnO(2) due to the larger ionic environment of the glass matrix. Ar(+) or HF etching reveals that the concentration of Sn decreases exponentially as a function of depth from the surface.

Relative intensity of the Kβ5 X-ray line

Abstract: The relative intensity of the Kβ5 (K-MIV,V) X-ray line as a function of the atomic number of the emitting elements is very strongly enhanced around Z = 24 (chromium) relative to predictions of the single-particle model for this electric dipole (E1) forbidden transition. The enhancement is attributed to solid state or chemical effects. The Kβ5 transition can be E1-allowed because in chemical compounds the outermost 3d level forms the valence shells, while in metals it becomes a broad band. The intensity of this line can therefore vary with the chemical state. We have determined the K β 5 K β 1 intensity ratio that results from proton impact on Ca, Ti and Cr, and we have collected the experimental data available in the literature. The influence of the sharp increase in the Kβ5 intensity on the K β K α intensity ratio and on X-ray analytical methods (e.g. proton induced X-ray emission, electron probe microanalysis, etc.) is discussed.

Chemical effects in TiO 2 and titanates due to bombardment with Ar + and O 2 + ions of different energies (3.5-10 keV)

Abstract: Compositional and chemical changes in TiO2 and Ph, Ni, Al and Ba titanates induced by bombardment with Ar+ and O2+ ons of different energies have been studied quantitatively by XPS. An increase of preferential loss of oxygen and, in case of PbTiO3, of lead has been observed when increasing the Ar+ ion energy from 3.5 to 10 keV. Because of oxygen loss, Ti4+ species reduce to Ti3+ and Ti2+. In addition, the loss of oxygen from PbTiO3 and NiTiO3 leads to the metallic state of nickel and lead, whereas aluminium and barium in Al2TiO5 and BaTiO3 maintain their chemical state (i.e., Al3+ and Ba2+). Bombardment with OZ ions of PbTiO3 and NiTiO3 leads to a partial reduction of Pb and Ni. This metallization and the preferential loss of lead are more efficient at higher ion energies for both, O2+ and Ar+ bombardment. The results are discussed in terms of chemical stabilities and the possibility of oxygen diffusion in the bombarded oxides.

Surface analysis of doped lanthanide cobalt perovskites by X-ray photoelectron spectroscopy

Abstract: Laboratory for Inorganic Materials Science, Faculty of Chemical Technology, University of Twente, 7500 AE Enschede, The Netherlands High oxygen fluxes through mixed-conducting oxi- des with fluoride and perovskite structure are well known [1-3]. According to [4] the selective feeding of oxygen into high temperature electrochemical reactors generally comprises: (i) providing an electrochemical cell comprising a first zone and a second zone separated from the first by a solid multi- component membrane; (ii) raising the temperature of the electrochemical cell from about 300 °C to about 1400 °C; (iii) passing the oxygen-containing gas in surface contact with the membrance, and (iv) passing the methane in contact with the membrane surface in the second zone. It is evident that under a high temperamre working regime, the permeability of oxygen, as well as all of the other physical and chemical parameters, are very important for the realization of a highly effective chemical process at a large oxygen partial pressure gradient. The catalytic activity of such an oxidation process, e.g. of CO, methane, etc., is a function of the chemical state of the surface elements and decreases monotonically with the decrease of the surface atomic ratio of the 3-rd elements. In the work reported here the chemical state of the surface of an Sr-doped lanthanide cobalt perovskite (La0.3Sr0.7CoO3_~, 3LSC) was the object of an investigation carried out with the help of electron spectroscopy for chemical analysis (ESCA), includ- ing the X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) methods. In our work the method of preparation of perovskite ceramic samples developed by van Doorn and Bouwmeester [5] was accepted. The materials used were La(NO3)3.6H20,

Surface chemical state populations in the molecular beam epitaxy deposition of BaF2 on GaAs by x‐ray photoelectron spectroscopy and heavy‐ion backscattering spectroscopy

Abstract: X‐ray photoelectron spectroscopy (XPS) and heavy‐ion backscattering spectroscopy (HIBS) were used to investigate the chemical states of barium atoms in submonolayer BaF2 depositions on GaAs (100) substrates. XPS measurements showed barium atoms to have two different chemical states at the BaF2/GaAs interface. The relative abundance of these two states was also determined by XPS. The number of barium atoms in each state was found by normalizing integrated XPS peak intensities to HIBS measurements of the total number of barium atoms on the surface. The results were used to formulate a model of a two‐stage deposition mechanism whereby BaF2 first reacts with gallium at the surface and dissociates, releasing gaseous GaF. This reaction is apparently self‐limiting, resulting in a barium ‘‘template’’ layer that enables subsequent BaF2 molecules to form an epitaxial (100)‐oriented film.

Structural and chemical characterization of vapor-deposited polythiophene films

Abstract: The chemical structure, including changes in charge distribution upon doping, for vapor‐deposited polythiophene films has been investigated using x‐ray photoelectron spectroscopy (XPS). Two different methods of doping, coevaporation with FeCl3 and exposure to iodine vapor, are contrasted in this study and several chemical states are observed for the ‘‘dopants’’ associated with these complexes. In general, the FeCl3 codeposited complexes have considerably higher conductivities than the iodine exposed films (10–25 S/cm as compared to 0.01 S/cm). Furthermore, besides being a more effective doping mechanism, the FeCl3 codeposited complexes are much more stable upon exposure to atmosphere and also in the ultrahigh vacuum environment of the XPS system. Atomic force microscopy images of the same films show clear morphological differences between the as‐deposited, the FeCl3 codeposited, and the iodine doped films. The as‐deposited film shows a fibrillar‐type structure while the FeCl3 codeposited film is observed ...

Transition metal nitride and carbide nanoparticles

Abstract: CO2 laser pyrolysis of reactant gases has been used to produce a wide variety of dispersed, single crystal nanoparticles (average size 2 to 20 nm). This chapter discusses the production of nitrides (oxynitrides) and carbides (oxycarbides) of Group 6B elements (Mo and W) and Fe by this technique. The emphasis is on the characterization of the atomic order in the particle and the chemical state of the particle surface. The catalytic properties of these particles for coal liquefaction and heteroatom removal from model compounds is also addressed briefly.

Chromium and Nitrogen Segregation in Thin Oxide Layers Formed on the Surface of 17Cr-Ni-Mo-N Austenitic Steels Studied by Angle Resolved XPS

Abstract: Angle-resolved X-ray photoelectron spectroscopy (AR-XPS) has been used for characterizing thin oxide layers formed on the surface of 17Cr-Ni-Mo-N austenitic steels. It was confimed that chromium and nitrogen segregation takes place on the surface by in-situ heating at 973 K under ultra high vacuum. The effective thickness and concentration of their segregated layer was estimated from the relationship between the concentration and the take-off angle in AR-XPS. AR-XPS study was also made for specimens without and with segregated surface layers which were subsequently exposed to air at room temperature. The surface segregation of chromium and nitrogen was found to considerably inhibit oxidation at room temperature, and such feature was rather distinct compared with the ferritic steel case by chromium segragation alone. These results on the surface layers were consistent with changes in the observed chemical state denoted by Fe 2p and Cr 2p due to the surface segregation.

A search for interstellar gas-phase CO2 gas : Solid state abundance ratios

Abstract: We present searches for gas-phase CO2 features in the ISO-SWS infrared spectra of four deeply embedded massive young stars, which all show strong solid CO2 absorption. The abundance of gas-phase CO2 is at most 2 10 7 with respect to H2, and is less than 5% of that in the solid phase. This is in strong contrast to CO, which is a factor of 10-100 more abundant in the gas than in solid form in these objects. The gas/solid state ratios of CO2 ,C O and H 2O are discussed in terms of the physical and chemical state of the clouds.

Interface properties of thin oxide layers grown on strained SiGe layers at low temperatures

Abstract: The chemical state and the electrical properties of the interfaces of thin oxide films grown on strained layers using plasma and thermal oxidation have been studied in detail. X-ray photoelectron spectroscopy studies show no Ge pile-up at the oxide/substrate interface. In the case of plasma oxidation, Ge at the oxide surface is found to be in a fully oxidized state, while the formation of an intermediate oxidized state is observed in the case of low-temperature thermal oxidation. High-frequency (1 MHz) capacitance - voltage (C - V) and conductance - voltage (G - V) measurements have indicated the growth of good quality gate oxides. The fixed oxide charge and interface state densities are comparable to those of low-temperature-grown metal - oxide - semiconductor capacitors on Si with aluminium gates.