V.D. Dobrovolsky

Electronic structure of hexagonal tungsten trioxide: XPS, XES, and XAS studies

Abstract X-Ray photoelectron (XPS), emission (XES) and absorption (XAS) spectroscopy methods were used to study the electronic structure of hexagonal tungsten trioxide, h-WO3. Its precursor, hexagonal hydrogen tungsten bronze, HxWO3, and the monoclinic form of tungsten trioxide, m-WO3, were also studied. For the mentioned compounds, both the XPS valence-band and core-level spectra, as well as the O Kα emission bands and the W LIII absorption edges were derived. It was established that, binding energies of both the W 4f and O 1s core-level electrons do not change when going from the tungsten trioxides to HxWO3. A high-energy shift of the inflection point of the XAS W LIII spectrum of the HxWO3 bronze, with respect to its position on the spectrum of pure metallic tungsten, was found to be close to those of the spectra of the two WO3 forms studied. Half-widths of both the XPS valence-band spectra and the O Kα bands increase somewhat in the sequence m-WO3→h-WO3→HxWO3. The formation of a near-Fermi sub-band, which is absent for both the modifications of WO3, was observed on the XPS valence-band spectrum of hexagonal HxWO3. The energy positions of the centers of gravity of the O Kα band remain constant for all the compounds studied.

Peculiarities of interatomic interaction in titaniumhydrides with different content of hydrogen

Both the Ti Kβ 5 and Ti Lα X-ray emission spectra of metallic titanium and its hydrides TiH 1.88 , TiH 2.27 , and TiH 2.52 have been investigated. In the above spectra the differences of energy distributions of Ti p-like and Ti d-like valence states of the hydrides have been observed. The observed peculiarities of interatomic interactions have been explained taking into account that, in the hydrides TiH x>2 , hydrogen atoms begin to occupy octahedral positions in addition to tetrahedral ones. The X-ray Ti K absorption spectra of Ti met , TiO 2 and the hydrides mentioned above have also been studied. Results of investigations of the Ti K absorption spectra indicate that, in the titanium hydrides studied, a charge transfer occurs from Ti to H atoms.

XPS and transmission electron microscopy of themulticomponent hydride-forming alloys

Abstract The photoelectron spectrograms of the Ni2p, Ni3p, Co3p, Mn2p, Al2p,Ols and Cls in LaNi4.6Mn0.4, LaNi4.5Al0.5, MmNi3.5Co0.7Al0.8, and Zr3d, Ti2p, V2p, Ni2p, Co2p, Ols, and Cls inZr0.9VCo0.55Ni0.55 and Zr0.59Ti0.41V0.53Cr0.22 Fe0.20Co0.27Ni0.78 have been obtained.In a surface layer about 2–2.5 nm of both alloys, the main part of nickel atoms are in metallic state(Ni0) and only a third of them in oxidized state (Ni+2, Ni+3). At the same time the concentration of nickel in surface layer of Zr-containing alloy is muchlower as compared with Mm-containing alloy. The chemical state of Co and V atoms is similar toNi atoms state, but their concentrations, especially of V, in surface layer is lower than that ofnickel. The results of XPS have been compared with those of transmission electron microscopy ofthe thin foils of La- and Zr- based alloys. Dark field microphotographs of the oxidized alloysindicate presence of two different phases, which, according to electron microdiffraction, arenickel-based alloy and oxide. On the first stage of oxidation on the surface of foil the fine nickelparticles (about 10 nm) and amorphous oxide phase appear. Complete oxidation of the foil leadsto formation of eutectic-like structure of mixture of metallic and oxide phase. Some difference ofthe oxidized layer structure of investigated alloys have been observed.

XPS and transmission electron microscopy of the multicomponent hydride-forming alloys for electrochemical applications

Abstract Photoelectron spectra of the Ce4d, Ni2p, Ni3p, Co3p, Al2p, O1s, C1s in Mm(Ni,Co,Al) 5 and Zr3d, Ti2p, V2p, Ni2p, O1s, C1s in Zr 1− X Ti X (Ni,V,Co,Cr) 2 alloys have been obtained. In the surface layer, about 2–2.5 nm of both alloys, the main part of the nickel atoms are in the metallic state (Ni 0 ) and only a third of them in oxidized states (Ni +2 , Ni +3 ). At the same time the concentration of nickel in surface layer of Zr-content alloy is much smaller when compared with Mm-content alloy. The chemical state of the Co and V atoms is similar to the Ni atoms state, but their concentrations, especially of V, in the surface layer is smaller than that of nickel. The results of XPS have been compared with the data of transmission electron microscopy of the thin foils of La- and Zr- based alloys. Dark field microphotographs of the oxidized alloys indicate the presence of two different phases, which, according to electron microdiffraction, are nickel-based alloy and oxide. In the first stage of oxidation on the surface of the foil, fine nickel particles (about 10 nm) and an amorphous oxide phase appear. Complete oxidation of the foil leads to the formation of a eutectic like structure consisting of a mixture of metallic and oxide phases. Some difference in the oxidized layer structure of the investigated alloys has been observed.

Influence of Titanium and Iron Additives to Magnesium on Hydrogen-Sorption Properties, Thermal Stability, and Kinetics of Hydrogen Desorption from MgH2 Phase of Mechanical Alloy

We report on studies of the process of cyclic hydriding–dehydriding (eight cycles) of a mechanical alloy synthesized in a ball mill by grinding powder mixture Mg + 10 wt.% TiH2 + 10 wt.% Fe in argon medium. Hydrogen-sorption properties and thermal stability of the mechanical alloy are studied employing thermodesorption spectroscopy at hydrogen pressure of 0.1 MPa. Hydrogen capacity of the mechanical alloy is determined to be 4.9 wt.%, and onset temperature of hydrogen desorption from the alloy under this study equals 220°C. Kinetics of the process of hydrogen desorption from the hydride phases of the mechanical alloy is examined. The present studies have revealed that mechanical treatment of the magnesium powder in Ar medium with both Fe and Ti additives causes a greater effect on improving the kinetics of hydrogen desorption from hydride phase MgH2 of the alloy and decreasing its thermal stability in comparison with that caused by mechanical treatment of the same powder with either Fe or Ti additives in hydrogen medium.

Thermal Stability And H-Desorption Properties Of Mg3Mnni2HX Prepared By Reactive Mechanical Alloying Of Powders Mg, Mn, Ni In H2-Atmosphere

In the present paper, Mg3MnNi2Hx hydride has been synthesized using the reactive mechanical alloying (RMA) process, i.e. employing a simple one-stage technology instead of the complicated four-stage route used in [1] for synthesis of the same hydride (the milling powder of pure Mg, Ni and Mn metals and of Mg2Ni alloy in argon atmosphere followed by their pressing, sintering and hydriding from the gaseous phase). The Mg3MnNi2Hx hydride derived by the RMA method was studied by thermodesorption spectroscopy. Similarly to Mg3MnNi2Hx hydride derived in [1], the hydride obtained in the present paper reveals decreased thermal stability and close hydrogenation capacity (1.35 wt%).

Correlation Between Surface Chemical States and Electrochemical Activities of Alloys AB5 and AB2

The X-ray photoelectron La 3d, Ni 2p, Ni 3p, Al 2p, O Is and C 1s core-level spectra in LaNi4.5Al0.5 and the Zr 3d, Ti 2p, Ni 2p, Co 2p, V 2p, O 1s and C 1s spectra in Zr0.59Ti0.41Ni0.78V0.53Cr0.22Fe0.2Co0.27 and Zr0.9VCo0.55Ni0.55 have been obtained. Results of the XPS studies have been compared with data of polarization measurements as well as of the electrochemical behaviours of the alloys in the potential range -0.95 V ≤ E ≤ -0.2 V. A good correlation of data of the XPS and polarization measurements is established. The present study indicate that the alloy which contained on its surface the greatest amount of catalytic active centres observed by the XPS technique (e.q., segregation of the nickel atoms in the metalic state Ni°) possesses the more flat polarization curve in the range of small currents as well as the smaller potential at some cathodic currents. In the present work the such alloy was LaNi4.5Al0.5, which displayed higher electrochemical activity as compared with those of the Zr-containing alloys.

Analysis of the Interrelation of the Thermal Stability of Hydrides of the Intermetallic Compounds of Composition AB2 with the Nature of Their Chemical Bonds Character Me–H

Charge state of Zr atoms in ZrV2 intermetallic compound and its hydride ZrV2H4 has been studied using the X-ray absorption spectroscopy (XAS) method. Thermal stability of the hydride has been investigated employing the thermodesorption spectroscopy (TDS) method. It has been established the positive charge on Zr ions in ZrV2H4 hydride (i.e., the “transfer” of electronic charge from Zr atoms during formation of the hydride), and this fact indicates the presence of ionic component of metal-hydrogen bonds in the hydride. The conclusion about the existence of correlation between observed increasing thermal stability of ZrV2H4 hydride and ionic component of its metal-hydrogen bonds has been made.

Thermal Stability And Hydrogen Sorption Properties Of The Mgh2 Hydride Derived By The Reactive Milling Of The Mg + 10 Wt% Ti Mixture

Thermal stability and hydrogen sorption properties of the MgH2 phase derived by the reactive milling of the Mg + 10 wt% Ti mixture was studied by the thermodesorption spectroscopy method during the first five cycles of hydrogen sorption-desorption. The addition of Ti decreases decomposition temperature of MgH2by 115 K, if we conclude about decomposition temperature taking into account peaks of speed of a hydrogen release on the desorption spectra derived during the first cycle of heating-cooling in hydrogen atmosphere of the Mg + 10 wt% Ti mechanical alloy. During the second cycle of dehydriding-hydriding, the decomposition temperature of the MgH2 phase of the Mg + 10 wt% Ti composite decreases additionally by 35 K, and the temperature does not change for three following sorption-desorption cycles. It has been established the influence of the method of synthesis of the hydride phase MgH2upon location sites of hydrogen in the phase, and, consequently, upon its decomposition temperature. The addition of titanium to magnesium increases the effect of the above influence and promotes increasing the process of dispersion of magnesium and the MgH2 hydride synthesized due to the reactive milling. Additionally, the addition of titanium promotes decreasing the quantity of oxygen-containing structures, catalytic poisons preventing dissociative hydrogen chemosorption, adsorbed on the surface of MgH2 particles.

Electronic Structure of HyWO3 and WOx Studied by the XPS, XES, and XAS Methods

X-Ray photoelectron (XPS), emission (XES), and absorption (XAS) spectroscopy methods have been used to study the electronic structure of the compounds HyW03 (y=0.24) and WOx (2.0 ≤ x ≤ 3.0). The XPS core-level binding energies (BE’s) and valence-band structures for the above compounds have been determined. The XES O Kα and XAS W LIII spectra for the compounds H0.24WO3 and WOx have been derived. In all the tungsten oxides studied the energy positions of the centers of gravity of the O 2p-like emission bands remain constant. Half widths of both the XPS W 4f and O 1s core-level spectra increase in the sequence WO3 → WO2. The creation of a subband of M-M-interactions with a two-peak structure in the near-Fermi region of the XPS valence-band spectra has been observed for WOx, where 2.0 ≤ x ≤ 2.77. The above subband intensity significantly increased in the sequence WO2.77 → WO2.3 → WO2. The XPS O 1s core-level BE’s remain constant (within the experimental error) for the hydrogen tungsten bronze H0.24WO3 and all the tungsten oxides studied. The O Kα band halfwidth somewhat increases going from the tungsten oxides WOx to the compound H0.24WO3. It was established that, charge states of the tungsten and oxygen atoms in the H0.24WO3 compound are close to those in the tungsten trioxide WO3. The inflection point of the W LIII absorption spectra shifted monotonously towards the higher energies going from WO2 to WO3.