V. Paul-Boncour

Size-Dependent Hydrogen Sorption in Ultrasmall Pd Clusters Embedded in a Mesoporous Carbon Template

Hydrogen sorption properties of ultrasmall Pd nanoparticles (2.5 nm) embedded in a mesoporous carbon template have been determined and compared to those of the bulk system. Downsizing the Pd particle size introduces significant modifications of the hydrogen sorption properties. The total amount of stored hydrogen is decreased compared to bulk Pd. The hydrogenation of Pd nanoparticles induces a phase transformation from fcc to icosahedral structure, as proven by in situ XRD and EXAFS measurements. This phase transition is not encountered in bulk because the 5-fold symmetry is nontranslational. The kinetics of desorption from hydrogenated Pd nanoparticles is faster than that of bulk, as demonstrated by TDS investigations. Moreover, the presence of Pd nanoparticles embedded in CT strongly affects the desorption from physisorbed hydrogen, which occurs at higher temperature in the hybrid material compared to the pristine carbon template.

Influence of the support on the catalytic properties of nickel/ceria in carbon monoxide and benzene hydrogenation

Abstract In this work the catalytic properties of nickel supported on various supports (Al 2 O 3 , SiO 2 , CeO 2 ) in syngas conversion are compared. The influence of the temperature of reduction pretreatment was studied. The characterization of the catalysts was performed by temperature programmed reduction, isothermal reduction, CO and H 2 chemisorption, X-ray diffraction, X-ray absorption spectroscopy, magnetization and X-ray photoelectron spectroscopy. The modification of the catalytic properties of Ni/CeO 2 catalysts with reduction pretreatment is correlated to the transformation of the CeO 2 support and to strong interactions between these species and metal particles.

Pd–Pt alloys: correlation between electronic structure and hydrogenation properties

Abstract Palladium and its alloys have been extensively studied because of their faculty to store reversibly hydrogen isotopes. Here, the substitution of palladium with platinum is investigated. Thermodynamical studies have shown an anomalous behaviour regarding to the classical models. This original behaviour is explained by the study of the electronic structure of the binary solid solutions. The drastic decrease of the hydrogen solubility in the Pd–Pt alloys is accounted for by the filling up of the palladium conduction band by the valence electrons of platinum. The anomalous decrease of the stability of the hydride is explained by the large broadening of the valence band due to the substitution.

Structural and magnetic properties of RFe2H5 hydrides (R=Y, Er)

The structural and magnetic properties of RFe 2 hydrides (R=Y, Er) synthesized under high hydrogen pressures of 10 kbar and 373 K, have been investigated. Both saturated RFe 2 H 5 hydrides absorb about 5 H/f.u. and crystallize in the same type of orthorhombic structure (Imm2 space group), with a=5.437(1) A, b=5.850(1) A and c=8.083(1) A for YFe 2 H 5 and a=5.424(1) A, b=5.793(1) A and c=8.009(1) A for ErFe 2 H 5 . The large hydrogen pressure, necessary to reach 5 H/f.u. can be attributed to a partial filling of all types of available interstitial sites. YFe 2 H 5 displays a weak ferromagnetic behavior, which is almost independent of the temperature. ErFe 2 H 5 shows also a weak magnetization at high temperature whereas below 20 K its magnetization sharply increases due to Er magnetic ordering.

Hydrogen induced phase transitions in YMn2

Abstract X-ray and magnetic investigations of the YMn2Hx hydrides at temperatures ranging from room temperature down to 4.2 K for the concentration range x≤1.2 are presented and analyzed. It is shown, that in the concentration range 0.5 1. The observed phase transitions are interpreted as of martensitic type. In the border ranges for x 0.8 a complex behaviour with several two phase ranges as a function of temperature is observed. For x On the basis of the presented data a phase diagram is proposed. The observed phase transitions are discussed and interpreted in terms of Mn–Mn magnetic interactions in dependence of the lattice expansion caused by hydrogen, taking into account the possible ordering of the hydrogen atoms.

In situ neutron powder diffraction measurements of the absorption and desorption of hydrogen (deuterium) in (La,Ce)Ni2 compounds: Amorphization and recrystallization

Abstract The structural transformation of RNi2 Laves phases (≡ La, Ce) on absorption of hydrogen and deuterium was studied using X-ray powder diffraction at room temperature, 90 and 120 °C and using time-resolved neutron powder diffraction at room temperature. For the intermetallic compounds, superlattice lines indexed in a double cubic cell are observed in addition to the diffraction peaks of the cubic C-15-type structure. In the case of in situ absorption measurements, small increases in the lattice parameters of the crystalline phases LaNi2.18 and CeNi2.16 are observed and the formation of an amorphous phase is detected from the first stage of the absorption. Under equilibrium conditions at room temperature, the lattice parameter of the crystalline phase LaNi2.18 increases as previously, whereas that of CeNi2.16 is found to decrease with increasing deuterium content. A mechanism for the segregation of the cerium atoms from CeNi2.16 leading to a compound rich in cerium vacancies and cerium hydride is proposed on the basis of the results of the high temperature (90 and 120 °C) hydrogen absorption experiments. The thermal desorption of the amorphous hydride CeNi2.16H3.65 (studied by neutron diffraction) leads to a recrystallization of the intermetallic compound CeNi2 + y after the formation of intermediate CeNix phases.

Temperature- and pressure-induced structural transitions in rare-earth-deficient (R = Y, Sm, Gd, Tb) Laves phases

Electrical resistivity, thermal expansion, and temperature-dependent x-ray diffraction measurements on compounds give mutually consistent evidence for structural phase transitions at 740 K, 550 K, 600 K, and 450 K respectively for R = Y, Sm, Gd, Tb; 0 < x < 0.05. Arguments are given as to why most of the rare-earth - nickel compounds with the 1:2 ratio do not crystallize in the simple cubic Laves phase (C15 type) but show a superstructure of the cubic Laves phase at room temperature and at ambient pressure. This superstructure with the space group and a doubled cell parameter is characterized by ordered vacancies on the R sites. It is shown that the observed structural instabilities result in transitions to the cubic Laves phase (space group ), however with disordered vacancies at high temperatures. High-pressure x-ray powder diffraction experiments show that the phase transition in shifts down to room temperature for a pressure of 27 GPa.

The influence of hydrogen on the magnetic properties and electronic structures of intermetallic compounds: YFe2–D2 system as an example

Abstract The magnetic and electronic properties of YFe 2 D x deuterides (1.2≤ x ≤4.2) have been studied by bulk magnetic measurements and X ray absorption spectroscopy (XAS). The magnetic moment increases with deuterium content whereas the Curie temperature increases from 560 K ( x =0) to 720 K ( x =1.2) then decreases almost linearly down to 300 K ( x =4.2). For x =1.2, 1.75 and 1.9, structural transitions from distorted structures to C15 cubic structure occurs below the Curie temperature and are therefore related to deuterium ordering. The analysis of the XAS spectra at the Fe–K edge shows a progressive filling of the 4p-3d band with increasing deuterium content. The evolution of the magnetic and electronic properties of the YFe 2 D x deuterides are discussed in relation with the evolution of the Fe–Fe distances in these compounds and band structure calculations.

Structural and magnetic properties of RFe2Dx deuterides (R = Zr, Y and x ≥ 3.5) studied by means of neutron diffraction and 57Fe Mössbauer spectroscopy

The structure of Y Fe2D3.5 at 290 K has been refined in a monoclinic superstructure using synchrotron radiation and neutron diffraction experiments. The evolution of the 57Fe Mossbauer spectra versus temperature for ZrFe2Dx and Y Fe2Dx deuterides with large deuterium content has been studied. For x = 3.5 both ZrFe2Dx and Y Fe2Dx show an increase of the Fe magnetization compared to the parent intermetallic compound, related to the increase of the cell volume. For Y Fe2Dx the Fe moment decreases to complete disappearance as x increases from 3.5 to 5 due to the stronger influence of the Fe–D bonding compared to the volume increase. The evolution of the isomer shift can be attributed to an almost pure volumetric effect in the case of the Y Fe2 deuterides, whereas for ZrFe2D3.5 additional D–Fe charge transfer has to be taken into account.

New magnetic phenomena in TbNi2

Structural investigations at room temperature revealed that TbNi2 does not crystallize in the Laves phase structure, but shows a superstructure of the Laves phase with the space group F-43m. Susceptibility, specific heat, magnetostriction and magnetoresistance measurements on polycrystalline specimens showed an additional magnetic phase transition at TR = 14 K below the Curie temperature of TC = 36±0.2 K. In order to clarify the nature of this magnetic phase transition at 14 K, elastic neutron diffraction below and above TR and TC was performed. The analysis of these data showed that this transition at TR is due to the rotation of the Tb moments on three of the total of eight non-equivalent Tb sites in the rhombohedrally distorted unit cell in the magnetic ordered state. This rotation of these Tb moments is out of the [111] direction into a plane perpendicular to the space diagonal. The cause for this magnetic instability is due to an interplay of the regularly arranged vacancies in the superstructure and the crystal field level position which has been studied by inelastic neutron scattering.