Showing papers on "Chemical state published in 1992"

Bulk and surface analysis of a Fe-P-O oxydehydrogenation catalyst

Abstract: Combination of X-ray photoelectron spectroscopy and X-ray microprobe analysis gives complementary information on the composition and on the chemical state of the elements iron and phosphorus present in a Fe-P-O oxydehydrogenation catalyst. On the fresh catalyst iron is present mainly as Fe3+ species whereas after a typical test, which selectively transforms isobutyric acid to methacrylic acid, both Fe2+ and Fe3+ are present. Phosphorus, in comparison to iron, is not homogeneously distributed in the catalyst particles and there exists a phosphorus enrichment at the surface. Positive and negative charge balance reveal that hydrogen phosphate-like species are present in such a catalyst. This is consistent with the observation that this system is active with large amounts of water in the feed.

Active to passive transition in the oxidation of silicon carbide at high temperature and low pressure in molecular and atomic oxygen

Abstract: The active to passive transition in the oxidation of sintered α-SiC has been determined by a thermodynamic approach and then experimentally under flowing air at high temperatures (1673–1973 K) and low pressures (oxygen partial pressure from 200–2100 Pa). Then, the physico-chemical behaviour of samples was compared for two different environments such as air-plasma atmosphere generated by microwaves, and a molecular atmosphere. The thermodynamic calculation does not predict any variation of the oxidation transition when changing the chemical state of oxygen (O2 or O) but experimentally the domain characterized by the formation of a passive layer of silica is extended to lower pressure under atomic oxygen (for the same temperature range) than in the case of molecular oxygen.

Palladium-alumina catalysts : a diffuse reflectance study

Abstract: Palladium catalysts are prepared from various precursors (chloride, nitrate, acetylacetonate) by using different methods of precursor bonding on 2 types of alumina. The chemical state and environment of Pd is studied by UV-visible diffuse reflectance spectroscopy along the successive preparation stages. The species formed upon interaction of precursors with the carriers are identified and their transformations upon calcination and reduction are studied. The reactivity of oxide «clusters» towards NH 3 is used as a criterion of their dispersion

Description of chemical reactions in terms of the properties of the electron density

Abstract: Chemical reactions can be described by utilizing the topological features of the electron density distribution ϱ( r ). This implies defining atoms in molecules, chemical bonds, molecular graphs, molecular structure, structural stability, and changes in molecular structure from a topological point of view. Looked at in this way, chemical reactions are transitions from one structural region to another passing through one or several unstable structures, which can be associated with the transition state (s) of chemical reactions. An alternative way of describing chemical reactions is given by the analysis of the Laplace concentration − V 2 ϱ( r ). This leads to a more qualitative account of changes in molecular structure owing to chemical reactions. It also has the advantage of bridging the gap between molecular-orbital-based and ϱ( r ) -based descriptions of chemical reactions and molecular activity.

Correction of peak shift and classification of change of X-ray photoelectron spectra of oxides as a result of ion sputtering

Abstract: Round-robin experiments have been performed to correct binding energy shifts of XPS spectra for oxides by charge-up which results from irradiation of x-rays or ion sputtering and to determine criteria for the change of chemical state resulting from Ar ion sputtering. Al 2 O 3 , SiO 2 , MgO, TiO 2 and NiO, in plate and powder form, were used for the experiments.

Reactive N+2 ion bombardment of GaAs{110}: A method for GaN thin film growth

Abstract: We have measured using x‐ray photoelectron spectroscopy the surface composition, chemical state, and band bending of a GaAs{110} surface exposed to medium energy (0.5 and 3.0 keV) reactive N+2 ion bombardment at room temperature as a function of ion dose. Thermally stable, thin GaN films are formed at a rate is limited by ion‐induced physical desorption of the surface layer. After annealing the sample, a small band bending shift toward higher kinetic energy (0.15 eV) is observed which results from the ion‐induced formation of midgap charge acceptor states. The implication of these results on the use of GaN thin films for surface passivation of GaAs in device applications is described.

Particle beam-induced reactions versus thermal degradation in PMDA-ODA polyimide

Abstract: Chemical reactions induced by heat and particle bombardment at low energy in PMDA-ODA were studied by XPS and REELS. Different chemical reactions involving the chemical state of oxygen and nitrogen are shown to occur with different products and rates depending on the peculiar way of energy deposition. In particular, thermal treatments proceed mainly by decarbonylation of imidic rings, leaving the ether linkage in the PMDA-ODA unit essentially unaffected. Evidence is found that thermal treatment results in the formation of large condensed aromatic systems, possible precursors for true graphitization at higher termperature

Electronic states of BaBiO3- delta and K-doping effects studied by photoelectron spectroscopy

Abstract: Changes in the electronic states for Ba1-xKxBiO3- delta ( delta =0 and 0.5) due to various surface treatments (fracturing, scraping and ion sputtering) have been studied by X-ray photoelectron spectroscopy. It is found that marked spectral changes occur as a result of scraping and sputtering; the core levels are broadened and the intensity at the top of the valence bands is reduced. These results are discussed in terms of the reduction of elements and damage in the surface region. The above results cause the authors to conclude that fracturing these samples is suitable as a cleaning procedure for this system. The Bi 4f, Ba 4d and O 1s core levels have very low binding energies and are shifted by 0.2-0.4 eV to lower binding energies on 50% substitution of K for Ba. The Bi 4f line seems to contain a single chemical state for BaBiO3- delta , and more than one chemical state for Ba0.5K0.5BiO3- delta . The top of the valence band is found to be 0.5 eV below the Fermi level for BaBiO3- delta , while a clear Fermi edge appears for the K-doped material in the valence band spectra from both X-ray and ultraviolet photoelectron spectroscopy. The electronic states of BaBiO3- delta and K doping effects are discussed and compared with band calculations.

The surface state and catalytic properties of Pt black after O2-H2 cycles

Abstract: XPS and UPS of a Pt black catalyst after customary H2-O2 regeneration shows considerable amounts of residual C as well as surface OH/H2O species Surface C could not be removed even by O2 at 800 K Oxygenates are stable even after H2 treatment up to 750 K Their chemical state has been tentatively identified by comparing XPS and UPS results Catalytic transformations of n-hexane on Pt black treated analogously is reported and the effect of surface species on catalytic properties discussed Possible consequences of the presence of stable surface OH/H2O species on H2-O2 titrations are mentioned

Catalytic activity of layered α-(tin or zirconium) phosphates and chromia-pillared derivatives for isopropyl alcohol decomposition

Abstract: Layered α-zirconium and α-tin phosphates (α-ZrP and α-SnP) and their chromia-pillared homologues have been evaluated from the point of view of surface acidity. The FT-IR of pyridine adsorption and the dehydration of 2-propanol at 180 °C were used as chemical probes to measure acid sites. The studied materials were characterized by X-ray diffraction, surface area measurements and X-ray photoelectron spectroscopy. The latter technique was also used to reveal the chemical state of both fresh and used on-stream catalysts. Under the experimental conditions used, α-SnP presents a moderate Bronsted and Lewis acidity, which is absent in α-ZrP. In contrast, all the thermally stable porous materials, obtained by pillaring α-SnP and α-ZrP with chromium(III) oxide, show a marked enhancement of the acidity with respect to the original phosphates. These acid sites are strong enough to still retain a small proportion of pyridine upon outgassing at 500 °C, but insufficient to perform methanol decomposition into hydrocarbons. Interestingly, the 2-propanol conversion in the presence of air led to both dehydration and dehydrogenation, this latter functionality being activated in an alcohol-helium stream.

Study on polished and etched surfaces of polar (111) CdTe by x-ray photoelectron spectroscopy and grazing-incidence x-ray diffraction

Abstract: Several etchants have been applied to polar (111)Cd and (111)Te surfaces of CdTe. Induced surface layers were analyzed by x‐ray photoelectron spectroscopy and grazing‐incidence x‐ray diffraction for studying compositions, chemical states, and crystalline phases of respective layers. A bromine/methanol etch led to a layer composed of mainly tellurium oxides on both (111) surfaces. Oxidation depth, however, was larger at the (111)Te side. Etching with HCl subsequently removed the oxides. A film made of crystalline tellurium was, however, produced with the thickness being larger on the (111)Te surface than on the other surface. A fairly thick layer consisting of TeO2 and crystalline Te for etching with N solution (H2O2:H2O:HF=2:2:3 v/v), formed on both surfaces with the cadmium having been severely depleted, especially on the (111)Te surface.

Characterization study of polycrystalline tin oxide surfaces before and after reduction in CO

Abstract: Polycrystalline tin oxide surfaces have been examined before and after reduction in 40 Torr of CO at 100 and 175 C using Auger electron spectroscopy (AES), electron spectroscopy for chemical analysis (ESCA), ion scattering spectroscopy (ISS) and electron stimulated desorption (ESD) The changes in the surface composition and chemical states of the surface species generally are subtle for the reductive conditions used However, significant changes do occur with regard to the amounts and the chemical forms of the hydrogen-containing species remaining after both the 100 and 175 C reductions

Influences of copper on physico-chemical and catalytic properties of ZSM-5 zeolites in the reaction of ethene aromatization

Abstract: Copper-containing ZSM-5 zeolites were used as catalysts for ethene aromatization. Physico-chemical properties of the catalysts were studied by means of electron paramagnetic resonance (EPR), electron spectroscopy for chemical analysis (ESCA), and electron microscopy. ESCA spectra show that the surface layer of fresh catalysts is enriched in aluminium whereas copper distribution is nearly uniform. The addition of copper enhances activity and yield in the aromatization reaction. This fact and the changes in the chemical state and distribution of copper after catalytic experiments, regeneration and reduction lead to the conclusion that copper plays an important role in the studied reaction.

Thermal and surface characterizations of 25.5 (wt.%) CeO2–2.5 Y2O3–72 ZrO2 fine powder

Abstract: By means of X-ray photoelectron spectroscopy (XPS), simultaneous differential thermal analysis (DTA) and thermogravimetric analysis (TGA), changes of valence state and surface chemical composition of a 25.5 (wt.%) CeO2–2.5 Y2O3–72 ZrO2 fine powder have been studied as a function of temperature (up to 1550°C) and atmosphere (flowing 90% N2-10% H2 and static air). The XPS results lead to the evaluation of the valence state of cerium, zirconium and yttrium, both in the starting powder and in thermal-treated powders. The analysis of the characteristic satellite structures, binding energies and peak shapes of the Ce 3d spectra show that cerium in the starting powder is confirmed to be Ce(IV) with a small amount of Ce(III), and that after thermal treatment the cerium is either unaffected or partially or fully reduced to Ce(III). The DTA and TGA thermograms confirm this redox behaviour and disclose, for air treatment, the presence of a strong, sharp exothermic peak at ∼410°C, which is interpreted as being due to the oxidation of a small amount of Ce2O3. On the contrary, under reducing conditions in the temperature range 630–730°C, the powder is affected by a remarkable weight loss owing to the Ce(IV) reduction to Ce(III) oxide. The change of valence state of zirconium, due to the reduction of ZrO2 to ZrO2−x under reducing conditions at high temperature, has also been monitored by XPS. Furthermore, on high temperature (≥900°C)-treated powders, XPS results show the occurrence of segregation phenomena of both bulk dissolved impurities, such as silicon, sodium, iron and aluminium, as well as yttrium stabilizing oxide. The Wagner two-dimensional chemical state plots for silicon, sodium and aluminium indicate that the chemical nature of the segregated phase can be ascribed to a silicate of sodium and yttrium with the presence of aluminium at higher temperatures. The effect of the above-mentioned chemical aspects on the final properties of sintered materials is also discussed.

Analysis of organic contaminations on Si(100) by thermal desorption spectroscopy

Abstract: Temperature and chemical state selective analysis of organic contaminants on Si(100) present after storage in air can be performed using thermal desorption spectroscopy (TDS). Samples stored for different times in air were characterized by XPS and then subjected to TDS. Hydrocarbon fragments were observed as well as a significant partial pressure of formaldehyde occurring as a decomposition product from higher molecular contaminants. Effects of treatment with HF and heating under reduced pressures of molecular oxygen were studied. The paper describes also some general analytical features of the technique.

Influence of Incorporated Copper on the Formation, Solid State Structure and Electrochemical Properties of g-FeOOH

Abstract: It is well known, that the rate of the atmospheric corrosion of iron is slowed down considerably, if the metal is alloyed by small amounts of Cu. This effect is specifically pronounced during those times, when the metal surface is covered by thin electrolyte layers. As then the oxygen reduction is rate determining for the corrosion rate, the kinetics of the oxygen reduction is slowed down on Cu-containing steels with respect to pure iron. On pure iron oxygen is reduced within the oxide scale at electron conducting oxide crystals. Such oxides contain high amounts of Fe2+ states and result from an electrochemical reduction of γ-FeOOH. Therefore it is quite likely, that on Cu-containing steel the solid state properties of the oxide scale are changed such, that the electronic conductivity decreases. – In this study model electrodes of Cu-doped γ-FeOOH are investigated. The morphology of the scale is analysed by SEM, the chemical state of Cu and Fe by XPS and the structure of the scale using FT-IR spectroscopy. The results clearly show, that the morphology of the Cu-doped oxide is changed significantly and XPS data suggest, that the reducibility of the doped oxide is much less than the one of undoped γ-FeOOH.

Improving the interpretation of x‐ray photoelectron and Auger electron spectra using numerical methods

Abstract: X‐ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) have long been used for sputter depth profiling for the determination of sample chemical composition as a function of depth. Often in XPS and AES, peak overlap or a subtle change in chemistry makes the chemical analysis of the elements in a material difficult. By using a variety of numerical methods, such as linear least squares fitting and target factor analysis, it is possible to improve the analysis and interpretation of depth profiles. The use of numerical methods can aid in the interpretation of chemical state information and in most cases significantly improves the detection limits of specific elements. An XPS sputter depth profile of tantalum silicide on silicon and an AES profile of a multilayer structure of gold, tantalum, and silicon carbide are shown as examples of the improvements offered by applying numerical methods.

Joint use of XPS and Auger techniques for the identification of chemical state of copper in spent catalysts

Abstract: X-ray photoelectron and x-ray-induced Auger spectroscopic analyses of various copper-containing catalysts have been used to reveal the chemical state of copper during on-stream in the dehydrogenation of methanol to methyl formate. Three catalyst systems have been selected for this purpose: (1) supported preparations (Cu/S, where S=ZrO 2 , ZnO; MgO or graphite; (2) mixed oxides of the type LaMn 1-x CuO 3 (x=0.4-0.6); (3) copper-containing bentonite and sepiolite

Analysis of passivating oxide and surface contaminants on GaAs (100) by temperature‐dependent and angle resolved x‐ray photoelectron spectroscopy, and time‐of‐flight secondary ion mass spectrometry

Abstract: Surface contaminations on GaAs (100) wafers have been analyzed by means of temperature‐dependent and angle resolved x‐ray photoelectron spectroscopy (TOXPS, ARXPS), and time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS). First, the dependence of the composition, chemical state, and diffusion behavior of the passivating thermal oxide on different annealing temperatures was investigated. The phases of this oxide can be described by a multilayer model and are consistent with the Ga–As–O equilibrium phase diagram. Second, the nature and amount of impurities at ambient and elevated temperatures was studied. No metal contaminants were found within the sensitivity of TOF‐SIMS. The residual amount of carbon contamination at the desorption temperature of the oxide depends on the amount of photon irradiation at ambient temperature.

X-ray photoelectron spectroscopy, optical absorption and thermoluminescence studies in CaSO4: Tm

Abstract: XPS studies of the core and molecular orbital electronic levels of CaSO4: Tm have shown that the material is in its proper chemical state (sulfate) and that it is quite resistant to radiation damage. However, at very high doses of X-ray irradiation, in the MGy range, the phosphor shows brown discoloration associated with partial decomposition of the sulfate into sulfite, thiosulfate and smaller amounts, of sulfide. This process of chemical reduction involving the breakup of oxygen atoms from the sulfate ion causes a band tailing at the valence band towards lower binding energy. Similar effects, but to a smaller extent are caused if the phosphor is sintered at about 1000°C. The sintered phosphor shows increased optical absorption in the UV at about 225 nm and it responds to UV light by yielding thermoluminescence. A thermoluminescence peak at approximately 300°C can be generated by UV light alone in the sintered material.