Hydrogen absorption by intermetallics M2Fe (M = Zr,Hf, Ti) and their effect on the catalytic aromatization of ethane

The absorption properties of the hydride-forming intermetallics M₂Fe(M = Zr, Hf, Ti) and their effect on the aromatization of ethane over Pt,Ga/HCVM catalyst within the temperature range 400–450 °C are studied. The absorption capacity of the intermetallics under aromatization conditions and their efficiency as hydrogen acceptors are shown to decrease in the order Zr₂Fe > Ti₂Fe > Hf₂Fe.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1726–1729, October, 1994.     

Magnetic dilution effect on the charge transfer phase transition and the ferromagnetic transition for an iron mixed-valence complex, (n-C3H7)4N[FeII1−xZnIIxFeIII(dto)3] (dto = C2O2S2)

Iron mixed-valence complex, (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] (dto = C₂O₂S₂) shows a new-type of phase transition coupled with spin and charge around 120 K, where the charge transfer between the Fe^II and Fe^III sites occurs reversibly, and shows the ferromagnetic transition at 7 K. To investigate the magnetic structure and its dimensionality of (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃], we have synthesized a mixed crystal system, (n-C₃H₇)₄N[Fe^II_1?xZn^II_xFe^III(dto)₃], and measured its magnetic properties. In this system, the magnetic moment is reduced with increasing of Zn ratio. Moreover, the ferromagnetic interaction changes to the antiferromagnetic one and the remnant magnetization disappears between x = 0.48 and 0.96, while the charge transfer between the Fe^II and Fe^III sites disappears above x = 0.26. In this paper, we present the magnetic dilution effect on the charge transfer phase transition and the ferromagnetic transition by means of magnetic susceptibility measurement and ⁵⁷Fe Mössbauer spectroscopy.     

Magnetoelectric perovskite (Bi0.5Pb0.5)(Fe0.5Zr0.5)O3: Preparation,structural and magnetic properties

The perovskite (Bi_0.5Pb_0.5)(Fe_0.5Zr_0.5)O₃ was synthesized by solid-state reaction in an attempt to find magnetoelectric materials, in which ferroelectricity and ferromagnetism coexist. This complex perovskite has been studied by X-ray and neutron powder diffraction in combination with magnetic measurements. The compound crystallizes in the orthorhombic space group Pbam with a ～ √2a_p, b ～ 2√2a_p and c ～ 2a_p (with a_p ～ 4.057 Å). The field and temperature dependence of the magnetization combined with neutron diffraction data showed antiferromagnetic behavior with the Neel temperature, T_N ～ 450 K. Rietveld refinements of neutron powder diffraction data collected at different temperatures, between 10 and 700 K, have been carried out in order to extract information about the thermal evolution of the nuclear and magnetic structures. A distorted orthorhombic perovskite structure was found within the whole temperature interval. The Bi/Pb and Fe/Zr ions were found to be partially ordered over the perovskite A-site and disordered over the B-site. The neutron diffraction patterns of the (Bi_0.5Pb_0.5)(Fe_0.5Zr_0.5)O₃ sample showed evidence of a long-range magnetic ordering below T_N with a propagation vector k = (0,0,0) and an antiferromagnetic arrangement of the magnetic moments of the Fe³⁺ cations in the B-site. This is consistent with an A_y-type magnetic structure. The factors governing the structural and magnetic properties of (1 ? x)BiFeO₃–xPbZrO₃ solid solutions are discussed and compared with those of pure BiFeO₃ and PbZrO₃. A solid solution strategy for developing magnetoelectric properties in BiFeO₃-based compounds is described, with the aim of realizing both a spontaneous polarization and magnetization at room temperature.     

Magnetische Messungen in den Systemen (Zr,Hf)—(Fe,Co,Ni)—O

The phases Zr₃Fe, Zr₃CoO _x (x=0,35;1);Zr₃NiO _x (x=0,2; 1); Hf₃NiO are measured magnetically in the range between 1300°K and 90°K. Whereas Zr₃Fe is ferromagnetic, the other isotypical structures are paramagnetic.

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Mit 2 Abbildungen     

Magnetische Eigenschaften von Hf2Fe und Hf2FeH x

The weakly temperature dependent paramagnetic compound Hf₂Fe becomes ferromagnetic by hydrogen absorption, theCurie-temperature of the resulting hydrides depends on the hydrogen content.

     

Magnetic properties of RE2MnGe6 (RE = La,Ce) and YMn0.3Ge2 germanides

The magnetic data of RE₂MnGe₆ (RE = La, Ce) and YMn_0.3Ge₂ are reported. La₂MnGe₆ and Ce₂MnGe₆ crystallize in the orthorhombic Ce₂CuGe₆ structure type, space group Amm2 (No. 38). The non-stoichiometric YMn_0.3Ge₂ compound crystallizes with the orthorhombic CeNiSi₂-type structure (space group Cmcm (No. 36)). The studied RE₂MnGe₆ (RE = La, Ce) intermetallics are characterized by ferromagnetic properties with Curie temperatures 177 (La) and 150 K (Ce), respectively. For YMn_0.3Ge₂ the low-field magnetic measurements indicate the antiferromagnetic property below 395 K with the small ferromagnetic component. The values of the magnetic moments in the ordered state indicate the ferromagnetic ordering in La₂MnGe₆ and complex magnetic order with the ferromagnetic component in YMn_0.3Ge₂ and Ce₂MnGe₆. The hysteresis loop and values of the coercivity field indicate that these compounds are soft magnetic materials.     

Hydrogen in amorphous TM33Zr67 (TM=Fe, Co, Ni) alloys

Hydrogen sorption properties and some corresponding changes in the crystallization of amorphous TM₃₃Zr₆₇ (TM=Fe, Co, Ni) alloys have been investigated. Relatively large amount of hydrogen was found to dissolve into the amorphous alloys during electrochemical hydrogen charging. The microstructural evolution during annealing of H-charged Ni₃₃Zr₆₇ was studied as well. The weaker bonded hydrogen desorbs in a large temperature range (440–625 K) before the crystallization of the amorphous alloys to start. A hydride phase (ZrH₂) was found to form during annealing the H-charged amorphous Ni₃₃Zr₆₇ alloy. During heating at constant heating rate the hydride decomposes at about 715 K and formation of Zr₂Ni immediately takes place. The final microstructure of the Zr₂Ni, crystallized from the H-charged matrix, is noticeably finer compared to the material crystallized from the H-free amorphous alloy, most probably due to the higher temperature of Zr₂Ni formation in the H-charged amorphous alloy than in the H-free sample.     

Low temperature heat capacity of Nd2Zr2O7 pyrochlore

Heat capacity of neodymium zirconate (Nd₂Zr₂O₇) with pyrochlore structure was measured by adiabatic calorimetry and the hybrid adiabatic relaxation method in the temperature range (0.45 to 400) K. Its excess component was obtained by comparison with the heat capacity of the lanthanum zirconate. A thermal anomaly was observed below T=7.2 K. From the heat capacity measurements, the thermodynamic functions of Nd₂Zr₂O₇ were determined.     

Volumetric study of (2-methoxyethanol + tetrahydrofuran + cyclohexane) at T=298.15 K

The excess molar volumes V_m^E at T=298.15 have been determined in the whole composition domain for (2-methoxyethanol + tetrahydrofuran + cyclohexane) and for the parent binary mixtures. Data on V_m^E are also reported for (2-ethoxyethanol + cyclohexane). All binaries showed positive V_m^E values, small for (methoxyethanol + tetrahydrofuran) and large for the other ones. The ternary V_m^E surface is always positive and exhibits a smooth trend with a maximum corresponding to the binary (2-methoxyethanol + cyclohexane). The capabilities of various models of either predicting or reproducing the ternary data have been compared. The behaviour of V_m^E and of the excess apparent molar volume of the components is discussed in both binary and ternary mixtures. The results suggest that hydrogen bonding decreases with alcohol dilution and increases with the tetrahydrofuran content in the ternary solutions.     

Untersuchung von Eisen(II)phosphiten im Hinblick auf die Ausbildung von Wasserstoffbindungen

Ferrous phosphites FeHPO₃·3 H₂O, FeH₂P₂O₅, FeH₄P₂O₆·1/2 H₂O and FeH₁₀P₄O₁₂·4 H₂O were studied using X-ray powder patterns, thermography, electron reflectance spectroscopy, magnetic susceptibility measurements andRaman and infrared molecular spectra. The results obtained indicate that all the phosphites studied contain an approximately octahedral coordination sphere consisting of oxygen atoms, which produces a weak crystal field around the iron atom. Phosphite FeHPO₃·3 H₂O contains an HPO₃ ^2– anion with the symmetry decreased from point groupC _3v toC _s. Ferrous diphosphite FeH₂P₂O₅ contains a non-linear P–O–P bond with a valence angle of 159°. The hydrogen bonds between the water molecule and the HPO₃ ^2– anion indicate a positive hydratation of the phosphite anion. Medium-strong or strong hydrogen bonds, 258–270 pm long, correspond to mutual interactions of the anions in the crystal lattice of hydrogenphosphites. Hydrogen bonding in hydrogenphosphites causes a decrease in theDq values.

     

Ferroelectric and photoelectrochemical studies of lead-free Ba0.925Bi0.05 □0.025(Ti0.65Zr0.30Sn0.05)O3 ceramic and its application to Rhodamine B oxidation under solar light

New materials with general formula Ba_0.925Bi_0.05□_0.025(Ti_0.65Zr_0.30Sn_0.05)O₃ (symbolized BBiTZS) were prepared by high-temperature solid-state reaction. The room X-ray diffraction pattern suggests a cubic perovskite structure. The thermal variation of the permittivity at different frequencies showed a relaxor ferroelectric behavior. The Raman spectroscopy was in accordance of the dielectric measurements. The diffuse phase transition parameters were determined from the modified Curie–Weiss law, while the relaxor behavior was highlighted by a good fit to the Vogel–Filcher relationship. The capacitance-potential (C_sc^?2 ? E) graph at pH ～ 7 is characteristic of n-type behavior with a flat band potential (E_fb) of ?0.52 V_SCE and an electron density (N_A) of 2.7 × 10¹⁸ cm^?3. The Electrochemical Impedance Spectroscopy, measured in the region (1 mHz - 10⁵ Hz), showed a semicircle, assigned with the bulk property (27 kΩ cm²); a constant phase element (CPE) is responsible for the depletion angle (?9°). With an optical of 2.00 eV, the ferroelectric BBiTZS possesses appealing photocatalytic capability and was successfully experimented for the oxidation of Rhodamine B (Rh B) exposed to solar light. According to the band diagram, electrons from the conduction band of BBiTZS are injected into dissolved oxygen, resulting in O₂ radicals, which are employed in the Rh B oxidation. UV–Visible spectrophotometry was used to follow the Rh B discoloration. On a solar irradiation of 97 mW cm^?2, a conversion of 50% is achieved in Rh B solution (10 mg L^?1) within 100 min., and the degradation follows a first order kinetic model with a half photocatalytic-life of 90 min.     

Successful location of tin dopant cations on surface sites of anatase-type TiO2 crystallites evidenced by 119Sn Mössbauer spectroscopic probe and XPS techniques

The present study provides the first experimental evidence for the stabilization of tin dopant cations immediately on the surface of an oxide having a tetragonal structure. ¹¹⁹Sn Mössbauer spectra of the dopant, introduced by air annealing into the bulk of anatase microcrystals, showed that it was located, in the tetravalent state, in somewhat distorted octahedral sites of a unique type. On the contrary, the reduced tin species, formed upon subsequent hydrogen annealing the Sn⁴⁺-doped samples, are found to occupy different sites being characterized by two sets of the isomer shift δ and quadrupole splitting ΔE_Q values (δ_I = 3.25 mm s⁻¹, ΔE_QI = 1.75 mm s⁻¹; and δ_II = 2.85 mm s⁻¹, ΔE_QII = 1.71 mm s⁻¹). Either of them implies both the divalent state of tin atoms and their presence at low-coordination sites that can be assigned to the surface of crystallites. Mössbauer spectra of Sn^4+←2+ daughter ions, formed upon contact with air of Sn²⁺, consist of a symmetrically broadened peak characterized by only slightly different average values of both the isomer shift (<δ> = 0.07 mm s⁻¹) and quadrupole splitting (<ΔE_Q> = 0.50 mm s⁻¹), as compared to the δ and ΔE_Q values for the bulk-located Sn⁴⁺. However, considerable broadening of Sn^4+←2+ doublet components (Γ = 0.97 mm s⁻¹) allows one to suggest that these secondary formed ions remain distributed over the non equivalent sites inherited from their Sn²⁺ precursors. The occurrence of Sn^4+←2+ at surface sites is independently proven by XPS measurements that revealed a greater than 10-fold enrichment with tin of 3–5 nm thick surface layers.     

Kinetics of non-isothermal crystallization in Cu50Zr43Al7 and (Cu50Zr43Al7)95Be5 metallic glasses

The crystallization kinetics of Cu₅₀Zr₄₃Al₇ and (Cu₅₀Zr₄₃Al₇)₉₅Be₅ metallic glasses was studied using differential scanning calorimetry (DSC) at four different heating rates under non-isothermal condition. The glass transition temperature T _g, the onset temperature of crystallization T _x, and the peak temperature of crystallization T _p of the two metallic glasses were determined from DSC curves. The values of various kinetic parameters such as the activation energy of glass transition E _g, activation energy of crystallization E _p, Avrami exponent n and dimensionality of growth m were evaluated from the dependence of T _g and T _p on the heating rate. The values of E _g and E _p, calculated from many different models, are found to be in good agreement with each other. The average values of the Avrami exponent n are (2.8 ± 0.4) for Cu₅₀Zr₄₃Al₇ metallic glass and (4.2 ± 0.3) for (Cu₅₀Zr₄₃Al₇)₉₅Be₅ metallic glass, which are consistent with the mechanism of two-dimensional growth and three-dimensional growth, respectively. Finally, the parameter H _r, S, and crystallization enthalpy ΔH _c are introduced to estimate the glass-forming ability and thermal stability of metallic glasses. The result shows that the addition of Be improves the glass-forming ability and thermal stability of Cu₅₀Zr₄₃Al₇ metallic glass.     

Jahn-Teller distortion,ferromagnetic coupling,and electron delocalization in a high-spin Fe–Fe bonded dimer

Fe₂(N,N’-diphenylformamidinate)₄, 1, first synthesized in 1994, is one of very few non-organometallic compounds with Fe–Fe distances, (2.46 Å) suggestive of an Fe–Fe bond. The electronic structure of 1 has been unclear because of its distorted D₂ geometry, as well as its reported S = 4 ground state. Computational investigations using DFT methods have shown that the D₂ geometry is the result of a Jahn-Teller distortion away from D₄ symmetry, in which the ground state would be orbitally degenerate. Broken symmetry methods have shown that ferromagnetic coupling between the two high-spin Fe(II) ions in 1 is a consequence of spin delocalization caused by a three-electron σ bond and a weaker three-electron δ bond between the Fe atoms. The relationship between ferromagnetic coupling and an Fe–Fe bond is established from results using hybrid functionals having variable amounts of Hartree-Fock exchange, which is found, surprisingly, to mitigate Fe–Fe bonding.     

Neutron powder diffraction study of the magnetic and crystal structures of SrFe2(PO4)2

The crystal and magnetic structures of SrFe²⁺₂(PO₄)₂ have been determined by neutron powder diffraction data at low temperatures (space group P2₁/c (no. 14); Z=4; a=9.35417(13) Å, b=6.83808(10) Å, c=10.51899(15) Å, and β=109.5147(7)° at 15 K). Two magnetic phase transitions were found at T₁=7.4 K (first-order phase transition) and T₂=11.4 K (second-order phase transition). The transition at T₂ was hardly detectable by dc and ac magnetization measurements, and a small anomaly was observed by specific heat measurements. At T₁, strong anomalies were found by dc and ac magnetization and specific heat. The structure of SrFe₂(PO₄)₂ consists of linear four-spin cluster units, Fe2-Fe1-Fe1-Fe2. Below T₁, the propagation vector of the magnetic structure is k=[0,0,0]. The magnetic moments of the inner Fe1-Fe1 atoms of the four-spin cluster unit are ferromagnetically coupled. The magnetic moment of the outer Fe2 atom is also ferromagnetically coupled with that of the Fe1 atom but with spin canting. The four-spin cluster units form ferromagnetic layers parallel to the [−101] plane, while these layers are stacked antiferromagnetically in the [−101] direction. Spin canting of the outer Fe2 atoms provides a weak ferromagnetic moment of about 1 μ_B along the b-axis. The refined magnetic moments at 3.5 K are 4.09 μ_B for Fe1 and 4.07 μ_B for Fe2. Between T₁ and T₂, a few weak magnetic reflections were observed probably due to incommensurate magnetic order.     

Effects of Fe-doping of ceria-based materials on their microstructural and dynamic oxygen storage and release properties

The structural/textual characteristics and dynamic oxygen storage capacity (DOSC) of Fe_0.1Ce_0.9O_x and Fe_0.1Ce_0.6Zr_0.3O_x samples prepared by sol–gel method are investigated by X-ray powder diffraction (XRD), Raman, Hydrogen temperature-programmed reduction (H₂-TPR), X-ray photoelectron spectroscopy (XPS) and mass spectrometry with CO/O₂ transient pulses. The dynamic oxygen storage capacity and rate are largely promoted by Fe doping, and their thermal stability is enhanced by Fe and Zr co-doping. The DOSC (at 673 K) are ordered as: Fresh: Fe_0.1Ce_0.6Zr_0.3O_x (566.6 μmol/g) > Fe_0.1Ce_0.9O_x (551.8 μmol/g) > Ce_0.67Zr_0.33O₂ (287.5 μmol/g) > CeO₂ (140.3 μmol/g); Annealed_1,173K: Fe_0.1Ce_0.6Zr_0.3O_x (101.6 μmol/g) > Ce_0.67Zr_0.33O₂ (45.3 μmol/g) > Fe_0.1Ce_0.9O_x (44.9 μmol/g) > CeO₂ (43.3 μmol/g). The H₂-TPR results showed that Fe-incorporation improve the total oxygen storage capacity (TOSC) of mixed oxide and low temperature activity. The TOSC are ordered as: Fe_0.1Ce_0.9O_x (1.53 mmol/g) > Fe_0.1Ce_0.6Zr_0.3O_x (1.42 mmol/g) > Ce_0.67Zr_0.33O₂ (1.16 mmol/g) > CeO₂ (0.88 mmol/g). XRD and Raman results indicate that Fe_0.1Ce_0.9O_x and Fe_0.1Ce_0.6Zr_0.3O_x are characterized with the fluorite-type cubic structure similar to CeO₂. TPR and XPS analyses reveal that the introduction of Fe into ceria and ceria-zirconia mixed oxides strongly modified the structural and textural properties, which influenced the kinetics of bulk oxygen diffusion.     

In Situ Synchrotron X-ray Diffraction Studies of Hydrogen-Desorption Properties of 2LiBH4–Mg2FeH6 Composite

Adding a secondary complex metal hydride can either kinetically or thermodynamically facilitate dehydrogenation reactions. Adding Mg₂FeH₆ to LiBH₄ is energetically favoured, since FeB and MgB₂ are formed as stable intermediate compounds during dehydrogenation reactions. Such “hydride destabilisation” enhances H₂-release thermodynamics from H₂-storage materials. Samples of the LiBH₄ and Mg₂FeH₆ with a 2:1 molar ratio were mixed and decomposed under three different conditions (dynamic decomposition under vacuum, dynamic decomposition under a hydrogen atmosphere, and isothermal decomposition). In situ synchrotron X-ray diffraction results revealed the influence of decomposition conditions on the selected reaction path. Dynamic decomposition of Mg₂FeH₆–LiBH₄ under vacuum, or isothermal decomposition at low temperatures, was found to induce pure decomposition of LiBH₄, whilst mixed decomposition of LiBH₄ + Mg and formation of MgB₂ were achieved via high-temperature isothermal dehydrogenation.     

Effect of reducing gas on the hydrogen production by thermo-oxidation of water over 1%Rh/Ce0.6Zr0.4O2

We have studied the effect of the reducing gas (H₂, CO and CH₄) on the hydrogen production by thermo-oxidation of water over the 1%Rh/Ce_0.6Zr_0.4O₂ catalyst prepared by impregnation. The catalyst is characterized by hydrogen chemisorption (Hc), before and after catalytic decomposition of water, temperature-programmed desorption, temperature-programmed reduction, X-ray diffraction and scanning electron microscopy. The catalyst is reduced in situ at 500 °C (4 h) under H₂, CO or CH₄ flows and flushed with Ar gas. Then, pulses of water (1 μL/pulse) are injected at 500 °C under Ar flow (30 mL/min). The results show clearly that the reducing gas has a strong effect on the H₂ production which follows the order: H₂?>?CH₄?>?>?CO. H₂ chemisorption measurements at room temperature highlight a strong metal–support interaction over fresh reduced catalysts which decreases after water decomposition (reduced centers?+?H₂O?→?oxidized centers?+?H₂).

     

The excess molar volumes and enthalpies of (N-methyl-2-pyrrolidinone + an alcohol) atT = 298.15 K and the application of the ERAS theory

Excess molar volumes V_m^EatT =  298.15 K and atmospheric pressure are reported for (N -methyl-2-pyrrolidinone  +  propan-2-ol, or butan-1-ol, or butan-2-ol, or 2-methylpropan-1-ol ). TheV_m^E have been calculated from measured values of density using the vibrating tube technique. The results are discussed in terms of the hydrogen bonding and other intermolecular association. Excess molar enthalpiesH_m^E at T =  298.15 K and atmospheric pressure are reported for (N -methyl-2-pyrrolidinone  +  propan-1-ol, or propan-2-ol, or butan-1-ol, or butan-2-ol, or 2-methylpropan-1-ol). The H_m^Ehave been obtained using flow calorimetry. The experimental results have been correlated and compared with the results from the Extended Real Associated Solution (ERAS) theory. The parameters adjusted to the mixtures properties are two cross association parameters and the interaction parameter responsible for the exchange energy of the van der Waals interactions. Self-association parameters of the alcohols and NMP are taken from the literature.     

Structure, bonding, and magnetic response in two complex borides: Zr2Fe1−δRu5+δB2 and Zr2Fe1−δ(Ru1−xRhx)5+δB2

Polycrystalline samples of two complex intermetallic borides Zr₂Fe_1−δRu_5+δB₂ and Zr₂Fe_1−δ(Ru_1−xRh_x)_5+δB₂ (δ=ca. 0.10; x=0.20) were synthesized by high-temperature methods and characterized by single-crystal X-ray diffraction, energy dispersive spectroscopy, and magnetization measurements. Both structures are variants of Sc₂Fe(Ru_1−xRh_x)₅B₂ and crystallize in the space group P4/mbm (no. 127) with the Ti₃Co₅B₂-type structure. These structures contain single-atom, Fe-rich Fe/Ru or Fe/Ru/Rh chains along the c-axis with an interatomic metal-metal distance of 3.078(1) Å, a feature which makes them viable for possible low-dimensional temperature-dependent magnetic behavior. Magnetization measurements indicated weak ferrimagnetic ordering with ordering temperatures ca. 230 K for both specimens. Tight-binding electronic structure calculations on a model “Zr₂FeRu₅B₂” using LDA yielded a narrow peak at the Fermi level assigned to Fe-Fe antibonding interactions along the c-axis, a result that indicates an electronic instability toward ferromagnetic coupling along these chains. Spin-polarized calculations of various magnetic models were examined to identify possible magnetic ordering within and between the single-atom, Fe-rich chains.