Deuterium insertion was used to tune the magnetic properties of Y 0.9 Tb 0.1 Fe 2 Laves phase towards an itinerant electron metamagnetic (IEM) behavior. The latter is highly sensitive to chemical changes and external parameters. The structural and magnetic properties of Y 0.9 Tb 0.1 Fe 2 D 4.3 were investigated using various neutron powder diffraction experiments in addition to magnetic measurements under steady and pulsed high magnetic fields up to 60 T. The deuteride crystallizes in a monoclinic structure (Pc Pc space group) with 4.3 D atoms located in 18 tetrahedral interstitial sites. At zero field, it undergoes a ferrimagnetic-antiferromagnetic (FiM-AFM) transition at T M0 = 90 K, accompanied by an anisotropic magnetostriction and a negative cell volume expansion of 0.6 %. A second AFM-PM transition is observed at 146 K. Under pulsed magnetic field at 4.2 K, the deuteride displays a multistep magnetic behavior from ferrimagnetic to a ferromagnetic state, which can be attributed to a stepwise rotation of the Tb moments. The ZFC-FC magnetization curves at low fields exhibit an irreversibility below 90 K, followed by a sharp decrease in magnetization at the FM-AFM transition. Between 90 K and 130 K, the magnetization curves display an IEM behavior, with the transition field increasing linearly with temperature.

Various applications benefit from the use of compounds combining lanthanides and 3d metals because of their fundamental physicochemical properties. To tune compositions for dedicated applications and to overcome the criticality of raw materials or geopolitical tension, a better understanding of the binary, ternary and even quaternary systems is needed. The present work focuses on the ternary Mn-Ni-Sm system, which is particularly interesting for energy storage applications as it forms hydrogen absorbing intermetallic compounds. The fruitful combination of standard X-ray diffraction, anomalous X-ray diffraction close to the Mn 3d edge, X-ray absorption near edge structure, electron probe micro-analysis and first-principles calculations provides a better understanding of the physicochemical properties and phase equilibrium of this ternary system. In particular, the atomic substitution of Mn atoms in the Ni sites has been investigated for the stacking AB2/AB5 Sm2MnyNi7-y system. The Mn atoms substitute preferentially in the AB5 sub-units, whereas Sm vacancies are observed in the AB2 sub-units inducing an antagonist variation in cell volume units. In the SmMnyNi12-y system, the preferential localization of Mn atoms has been highlighted in site 8i for the SmMn4Ni8 compound.

Single-atom catalysts represent an intense topic of research due to their interesting catalytic properties for a wide range of reactions. Clarifying the nature of the active sites of single-atom catalysts under realistic working conditions is of paramount importance for the design of performant materials. We have prepared an Ir single-atom catalyst supported on a nitrogen-rich carbon substrate that has proven to exhibit substantial activity toward the hydrogenation of butadiene with nearly 100% selectivity to butenes even at full conversion. We evidence here, by quantitative operando X-ray absorption spectroscopy, that the initial Ir single atoms are coordinated with four light atoms i.e., Ir-X4 (X = C/N/O) with an oxidation state of +3.2. During pre-treatment under hydrogen flow at 250 °C, the Ir atom loses one neighbour (possibly oxygen) and partially reduces to an oxidation state of around +2.0. We clearly demonstrate that Ir-X3 (X = C/N/O) is an active species with very good stability under reactive conditions. Moreover, Ir single atoms remain isolated under a reducing atmosphere at a temperature as high as 400 °C.

EXAFS analysis of pure elements, binary and ternary equiatomic refractory alloys within the NbZr-Ti-Hf-Ta system is performed at the Nb and Zr K-edges to analyze the evolution of the chemical local environment and the lattice distortion. A good mixing of the elements is found at the atomic scale. For some compounds, a distribution of distances between the central atom and its neighbors suggests a distortion of the structure. Finally, analysis of the Debye-Waller parameters show some correlation with the lattice distortion parameter δ2, and allows to quantify experimentally the static disorder in medium entropy alloys.

At 300 K, Y0.9Gd0.1Fe2Hx hydrides crystallize sequentially with increasing H concentration in various structures related to a lowering of the cubic MgCu2 type structure of the parent alloy: cubic C1, monoclinic M1, cubic C2, monoclinic M2, cubic C3, orthorhombic O. Above 300 K, they undergo a first-order transition at a TO-D temperature driven by order-disorder of hydrogen atoms into interstitial sites. Their magnetic, structural and magnetocaloric properties have been investigated through magnetic measurements, and high-resolution synchrotron diffraction experiments. The magnetization at 5 K decreases slightly from 4 to 3.8 mu(B) for x = 3-3.9 H/f.u., then with a larger slope for higher H content. A discontinuous decrease of the magnetic transition temperature is observed: M1 and C2 hydrides are ferrimagnetic with T-C near 300 K, M2 hydride displays a sharp ferromagnetic-antiferromagnetic transition at TFM-AFM = 144 K, whereas C3 and O hydrides present only a sharp increase of the magnetization below 15 K and a weak magnetization up to room temperature. Negative magnetic entropy variations (Delta S-M) are measured near T-C for the M1 and C2 phases, near TFM-AFM for the M2 phase, whereas positive Delta S-M peaks due to inverse MCE effect are found near TO-D. A structural and magnetic phase diagram is proposed. (c) 2022 Elsevier B.V. All rights reserved.

Y0.9Gd0.1Fe2, which crystallize in a C15 cubic structure, can absorb up to 5 H/f.u. and its pressure-composition isotherm displays a multiplateau behavior related to the existence of several hydrides with different crystal structures. At room temperature Y0.9Gd0.1Fe2Hx hydrides (2.9 <= x <= 5) crystallize in three phases with cubic structure (C1, C2 and C3), two phases with monoclinic structures (M1 and M2), and one phase with orthorhombic structure (O), with the following sequence for increasing H concentration: C1, M1, C2, M2, C3, O. Each phase exists as single phase within a H homogeneity range, and they are separated from each other by two-phase domains. The reductions of crystal symmetry are related to various hydrogen orders into interstitial sites. Weak superstructure peaks were indexed by doubling the cubic cell parameter of the cubic C2 phase. Upon heating, the monoclinic M1 and M2 and the cubic C2 phases undergo order-disorder (O-D) transitions toward a disordered cubic structure CDis. These O-D transitions are reversible with thermal hysteresis effects. The cubic C3 and orthorhombic O phases transform into a disordered cubic phase ac-companied by H desorption. (C) 2021 Published by Elsevier B.V.

The structural and magnetic properties of Y0.9Pr0.1Fe2D3.5 deuteride have been investigated by synchrotron and neutron diffraction, magnetic measurements, and differential scanning calorimetry. Deuterium insertion induces a 23.5 % cell volume increase and a lowering of crystal symmetry compared to the cubic C15 parent compound (Fd-3m SG). The deuteride is monoclinic (P21/c SG) below 330 K and undergoes a first order transition between 330 and 350 K towards a pseudo-cubic structure (R-3m SG) with TO-D = 342(2) K. The compound is ferromagnetic, accompanied by a magnetostrictive effect below TC = 274 K. The analysis of the critical exponents indicates a second order type transition with a deviation from the isotropic 3D Heisenberg model towards the 3D XY model. This implies an easy plane of magnetization in agreement with cell parameter variation showing a planar magnetic orientation. A weak magnetic peak is even observed at the order-disorder transition with a maximum at 343 K. Magnetic entropy variations are characteristic of direct and reverse magnetocaloric effects at TC and TO-D respectively.

Sm2Fe17 compounds are high-performance permanent magnets. Cobalt substitution allows us to further improve their magnetic properties. Depending on the thermal treatment, cobalt-substituted compounds can be synthesized either in the TbCu7 (disordered) or in the Th2Zn17 (ordered) structure type. Rietveld refinement of the number of transition metal dumbbells replacing rare-earth atoms from synchrotron powder diffraction data shows that the TbCu7 disordered structure has the same composition as the ordered one (a transition metal-to-rare earth ratio of 8.5). Then, cobalt site occupancies have been determined in both structures using synchrotron resonant (anomalous) diffraction. Cobalt is found to be absent from the dumbbell sites. The diffraction results are confirmed by Mössbauer spectroscopy.

By carefully controlling the ytterbium concentration, the power factor has been optimized for YbyCo4Sb12, leading to ZT = 1.4 at 750 K in Yb0.15Co4Sb12.

The nature of the active phase (metallic vs. oxidic, metal phase vs. concentrated hydride/ dilute solid solution with hydrogen) in heterogeneous catalysis by supported metals is still a matter of high debate. Here, we have monitored for the first time oxide-supported Pd nanocatalysts (average particle size 4.5 mn) during both CO oxidation (in H-2-free atmosphere) and preferential oxidation of CO in H-2 excess (PROX) by operand X-ray absorption spectroscopy. Under our conditions, the CO conversion in the absence of H-2 is around 30% at 150 degrees C and reaches 100% at 200 degrees C, whereas in the presence of H-2 the conversion reaches a maximum of 15% (at 250 degrees C), in agreement with our previous works using a conventional flow-fixed bed reactor. The active phase for CO oxidation below 200 degrees C is metallic Pd, whereas it is a solid solution of Pd with hydrogen during PROX below 300 degrees C. This work provides a direct evidence of the presence of subsurface/bulk hydrogen as a probable cause of the low PROX performance of supported Pd catalysts.

The local structure and the thermal stability of small and well-dispersed RhHx nanoparticles (average size of 1.4 nm) were studied by in situ X-ray Absorption Spectroscopy. The RhHx nanoparticles are stable at room temperature and undergo a structural transition from hydride (fcc) to metal phase (fcc) with a shrinking of the lattice volume due to the desorption of hydrogen. This phase transition occurs in the temperature range of 150-180 degrees C, in good agreement with the results from thermo-desorption spectroscopy. Above 180 degrees C, the desorbed nanoparticles undertake important coalescence. In situ transmission electron microscopy performed up to 300 degrees C proves that this process cannot be only thermal, thus it may be ascribed to a X-ray beam effect. (C) 2018 Elsevier Inc. All rights reserved.

Carbon supported RhHx particles with an average size of 1.0 nm have been synthesized by a simple liquid impregnation method followed by reduction under hydrogen at 175 °C.1 Two Rh-based nanocatalysts were tested for the hydrogenation of butadiene : hydride RhHx (as-synthesized nanoparticles) or metal Rh (as-synthesized nanoparticles pre-treated at 300 °C under He). Laboratory catalytic tests were performed for the partial hydrogenation of butadiene for different gas feed compositions and temperatures. They proved that the Rh hydride nanocatalyst is more active than its metal counterpart, irrespective of the reaction conditions. However, the apparent activation energies and the selectivities to butenes are almost identical for both Rh and RhHx catalysts, suggesting similar reaction mechanisms. In order to probe in situ the local structure, XAS experiments were carried out in operando conditions on the ROCK beam line. The EXAFS analyses demonstrate the stability of the Rh or RhHx structure throughout the reaction at room temperature: neither hydrogen depletion from the hydride phase nor hydride formation from the metal phase were observed over a period of two hours. As previously suggested theoretically,2 hydrogen adsorption at the nanoparticle surface may be stabilized in the presence of subsurface hydrogen, which would explain the higher activity of the hydride phase relative to the metal catalyst.1. C. Zlotea, Y. Oumellal, M. Msakni, J. Bourgon, S. Bastide, C. Cachet-Vivier and M. Latroche, Nano Lett.15, 4752–4757 (2015). 2. H. A. Aleksandrov, S. M. Kozlov, S. Schauermann, G. N. Vayssilov and K. M. Neyman, Angew. Chem. Int. Ed., 53, 13371–13375 (2014).

A Rh hydride (RhHx) nanocatalyst shows enhanced catalytic hydrogenation activity as compared to its metal counterpart (Rh).

YMn2−xFexH(D)y compounds with high hydrogen or deuterium content (5 ≤ y ≤ 6) were synthesized under high H or D gaseous pressure (P ≈ 0.8 GPa) at 373 K, in order to determine their structural and magnetic phase diagram. For high Mn content (x < 1) they form complex hydrides (deuterides) with y = 6 and crystallize in a cubic K2PtCl6 structure (Fm-3m space group). For 0.2 ≤ x < 1 the complex hydrides coexist with metal hydrides (y = 5) crystallizing in a cubic MgCu2 type structure (Fd-3m space group). Single phase hydrides with MgCu2 type structure are obtained for 1 ≤ x ≤ 1.5 with interstitial H(D) atoms localized in tetrahedral Y2(Mn,Fe)2 sites (y = 5). The cubic cell volume decreases versus Fe content and is 1.4% larger for the hydride compared to the corresponding deuteride. For x > 1.5 the compounds display an orthorhombic distortion (Pmn21 space group) which is due to H or D ordering in 8 interstitial sites. The magnetic measurements of hydrides and deuterides show a decrease of the magnetization compared to their parent compounds whereas a large irreversibility is observed below the magnetic ordering temperature which depends on the Fe content. Neutron diffraction measurements at 2 and 300 K reveal the existence of only short range order magnetic interactions.

Comptes Rendus Chimie - In Press.Proof corrected by the author Available online since lundi 25 avril 2016

Recent advances on synthesis, characterisation and hydrogen absorption properties of ultra-small metal nanoparticles (defined here as objects with average size ≤ 3 nm) are briefly reviewed in the first part of this work. The experimental challenges encountered in performing accurate measurements of hydrogen absorption in Mg- and noble metal-based ultra-small nanoparticles are addressed. The second part of this work reports original results obtained for ultra-small bulk immiscible Pd-Rh nanoparticles. Carbon supported Pd-Rh nanoalloys in the whole binary chemical composition range have been successfully prepared by liquid impregnation method followed by reduction at 300 °C. EXAFS investigations suggested that the local structure of these nanoalloys is partially segregated into Rh-rich core and Pd-rich surface coexisting within the same nanoparticles. Downsizing to ultra-small dimensions completely suppresses the hydride formation in Pd-rich nanoalloys at ambient conditions, contrary to bulk and larger nanosized (5-6 nm) counterparts. The ultra-small Pd90Rh10 nanoalloy can absorb hydrogen forming solid solutions under these conditions, as suggested by in situ XRD. Apart from this composition, common laboratory techniques such as, in situ XRD, DSC and PCI failed to clarify the hydrogen interaction mechanism : either adsorption on developed surfaces or both adsorption and absorption with formation of solid solutions. Concluding insights were brought by in situ EXAFS experiments at synchrotron: ultra-small Pd75Rh25 and Pd50Rh50 nanoalloys absorb hydrogen forming solid solutions at ambient conditions. Moreover, the hydrogen solubility in these solid solutions is higher with increasing Pd content and this trend can be understood in terms of hydrogen preferential occupation in the Pd-rich regions, as suggested by in situ EXAFS. The Rh-rich nanoalloys (Pd25Rh75 and Pd10Rh90) only adsorb hydrogen on the developed surface of ultra-small nanoparticles. In summary, in situ characterization techniques carried out at large scale facilities are unique and powerful tools for in-depth investigation of hydrogen interaction with ultra-small nanoparticles at local level.

In-depth clarification of hydrogen interaction with noble metal nanoparticles and nanoalloys is essential for further development and design of efficient catalysts and hydrogen storage nanomaterials. This issue becomes even more challenging for nanoalloys of bulk-immiscible metals. The hydrogen interaction with bulk-immiscible Pd–Rh nanoalloys (3–6 nm) supported on mesoporous carbon is studied by both laboratory and large scale facility techniques. X-ray diffraction (XRD) reveals a single phase fcc structure for all nanoparticles confirming the formation of nanoalloys in the whole composition range. In situ extended x-ray absorption fine structure (EXAFS) experiments suggest segregated local structures into Pd-rich surface and Rh-rich core coexisting within the nanoparticles. Hydrogen sorption can be tuned by chemical composition: Pd-rich nanoparticles form a hydride phase, whereas Rh-rich phases do not absorb hydrogen under ambient temperature and pressure conditions. The thermodynamics of hydride formation can be tailored by the composition without affecting hydrogen capacity at full hydrogenation. Furthermore, for hydrogen absorbing nanoalloys, in situ EXAFS reveals a preferential occupation of hydrogen for the interstitial sites around Pd atoms. To our knowledge, this is the first study providing insights into the hydrogen interaction mechanism with Pd–Rh nanoalloys that can guide the design of catalysts for hydrogenation reactions and the development of nanomaterials for hydrogen storage.