Nowadays, finding cheap and non-toxic materials able to reversibly store high amounts of hydrogen is a challenge in the renewable energy field.Metal sulfides seem to be promising candidates to this purpose.Titanium sulfides are reported to be particularly interesting but their ability to store hydrogen remains unclear.In this work, titanium based sulfides TiS 2 and TiS 3 with two-dimensional nanostructures have been synthesized by solid-gas reaction between titanium powder and sulfur at temperatures between 500-600 ºC.The morphology and crystal structure of Ti-sulfides were characterized by SEM (scanning electronic microscopy) equipped with EDX (energy dispersion X-ray) and XRD (X-ray diffraction), respectively.Their thermal stability was examined by TGA (thermal gravimetric analysis).Their hydrogenation properties have been determined by manometric means using a Sieverts system and by DSC-HP (high-pressure differential scanning calorimetric).Ti-sulfides hardly absorb/adsorb hydrogen for hydrogen pressures up to 80 bar and reaction temperatures up to 300 °C.

Mg-Ti nanostructured samples with different Ti contents were prepared via compaction of nanoparticles grown by inert gas condensation with independent Mg and Ti vapour sources. The growth set-up offered the option to perform in situ hydrogen absorption before compaction. Structural and morphological characterisation was carried out by X-ray diffraction, energy dispersive spectroscopy and electron microscopy. The formation of an extended metastable solid solution of Ti in hcp Mg was detected up to 15 at% Ti in the as-grown nanoparticles, while after in situ hydrogen absorption, phase separation between MgH2 and TiH2 was observed. At a Ti content of 22 at%, a metastable Mg-Ti-H fcc phase was observed after in situ hydrogen absorption. The co-evaporation of Mg and Ti inhibited nanoparticle coalescence and crystallite growth in comparison with the evaporation of Mg only. In situ hydrogen absorption was beneficial to subsequent hydrogen behaviour, studied by high pressure differential scanning calorimetry and isothermal kinetics. A transformed fraction of 90% was reached within 100 s at 300 °C during both hydrogen absorption and desorption. The enthalpy of hydride formation was not observed to differ from bulk MgH2.

In this investigation, the H-sorption kinetics of Mg6Pd and Mg6Pd1-xTMx (TM = Ag, Cu, Ni) pseudo-binary compounds at the TM solubility limit have been studied by isothermal hydrogen absorption, thermal desorption spectroscopy and in situ neutron diffraction. Among all studied compounds, the fastest absorption kinetics takes place for the Ni-substituted one. The fit of the absorption curves to established model equations for solid-gas reaction shows that hydrogenation is controlled by diffusion. As for desorption, the peak temperature for the Ni-substituted compound is 90 K below that of MgH2/Mg system and is characterised by a low activation energy of 68 kJ/molH(2). To better understand these results, neutron diffraction experiments during in situ thermal desorption of deuterated Mg6Pd and Mg6Pd0.25Ni0.75 compounds were carried out. These experiments demonstrate a synergetic effect between MgH2 and Mg2NiH4 hydrides as responsible for the remarkable kinetics of the Ni-containing compound. (C) 2015 Elsevier B.V. All rights reserved.

The Bureau Commun de Référence (BCR) sequential extraction scheme and micro-synchrotron-based X-ray fluorescence (μ-SXRF) analysis were used to determine the Cu fractionation in a calcareous vineyard soil and a synthetic soil (mixture of seven constituents: calcite, birnessite, ferrihydrite, goethite, lignocellulosic residue, kaolinite, and quartz) at different Cu contamination rates (190, 1270, and 6350 mg kg−1 of Cu) and aging times (1, 30, 92, and 181 days). The Cu distribution in the spiked vineyard and synthetic soils was different from the original vineyard one and was influenced by the loading level. The newly added Cu was preferentially present in the acid soluble fraction. Aging of the contaminated vineyard and synthetic soils during 6 months led to the redistribution of Cu from the weakly bound acid soluble fraction to the strongly bound reducible one. The evolution with time could satisfactorily be simulated by the Elovich diffusion model for the synthetic soils. It was less significant as less marked in the contaminated vineyard soil than in the synthetic one, even though the trends observed in both were similar. This study supported the hypothesis that “simple” synthetic models could be used to approach the Cu fractionation and its evolution with time in vineyard soils.

Mg6Pd nanoparticles confined inside a carbon host possess better thermodynamics and kinetics of hydrogenation than the analogous bulk alloy. Nanosizing also alters the structural alloy properties.

Mg-6(Pd,TM) (TM = Ag, Cu and Ni) pseudo-binary compounds have been synthesized at the TM solubility limit to determine the influence of TM on the thermodynamics and reaction pathways of the Mg6Pd H system. All compounds exhibit a two-plateau pressure behaviour, being the value of the high plateau pressure well above that of the Mg/MgH2 system. Such destabilization is explained by the formation of different Mg (Pd,TM) intermetallics and/or Mg2NiH4 hydride phases during the hydrogenation reaction. The formation of these phases not only increases the enthalpy of hydrogenation but also enhances disorder leading to a limited destabilization of the hydrogenated state. This compensation effect is characterized by a linear correlation between enthalpy and entropy terms. In addition, this work also provides the assessment at 623 K of the ternary Mg Pd Cu phase diagram in the Mg-rich corner. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

MgH2, MgH2–TiH2 nanocomposites and their deuterated analogues have been obtained by reactive ball milling and their kinetic and cycling hydrogenation properties have been analysed by isotope measurements and high-pressure differential scanning calorimetry (HP-DSC). Kinetics of material synthesis depends on both Ti-content and the isotopic nature of the gas. For pure Mg, the synthesis is controlled by isotope diffusion in Mg and therefore MgH2 forms faster than MgD2. For the MgH2–TiH2 nanocomposites, the synthesis is controlled by the efficiency of milling. Kinetics of reversible hydrogen/deuterium sorption in nanocomposites have been studied at 548 K. The rate limiting step is isotope diffusion for absorption and Mg/MgH2 interface displacement for desorption. HP-DSC measurements demonstrate that the TiH2 phase acts as a gateway for hydrogen sorption even in presence of MgO and provides abundant nucleation sites for Mg and MgH2 phases. The 0.7MgH2–0.3TiH2 nanocomposite exhibits steady hydrogen storage capacity after 100 cycles of absorption–desorption.

Structural properties and reversible deuterium uptake of MgD2-TiD2 nanocomposites have been studied by joint X-ray and neutron diffraction analyses to shed light on the extremely fast hydrogenation kinetics of these materials. (1 - x)MgD2-xTiD(2) nanocomposites with compositions ranging between x = 0 and 0.5 have been prepared by reactive ball milling of Mg and Ti powders under deuterium pressure. They consist of mixtures of MgD2 (beta-and gamma-polymorphs) and epsilon-TiD2 phases homogenously distributed at the nanoscale with crystallite sizes below 15 run. Minor phase miscibility is detected with Mg solubility in the TiD2 phase up to 8 at.% and Ti solubility in the beta-MgD2 up to 7 at.% Ti. At moderate temperatures and pressures (T < 600 K, P-D2 < 1 MPa) reversible deuterium loading in MgD2-TiD2 nanocomposites only occurs through the beta-MgD2 to Mg transformation. Mg/MgD2 thermodynamics is not modified as gamma-MgD2 and Ti solubility in beta-MgD2 are metastable and do not operate during reversible deuterium loading. However, the TiD2 phase allows for outstanding D-sorption kinetics in the Mg/MgD2 system. This paper demonstrates that TiD2 inclusions limit the grain growth of Mg and MgD2 phases allowing for short D-diffusion paths. Furthermore, we provide evidence that the TiD2 phase also favors H-mobility through the existence of coherent coupling between TiD2 and Mg/MgD2 phases and the presence of sub-stoichiometric MgD2-eta and TiD2-eta phases.

En este articulo se analiza y discute, desde el punto de vista de la Ciencia de Materiales, el sistema energetico solar hidrogeno, mas de cuarenta anos despues de su formulacion inicial. El hidrogeno, combustible limpio y de uso diverso, es una de las principales opciones para intentar disminuir la dependencia de nuestro sistema energetico actual de los combustibles fosiles. Para ello, sin embargo, han de superarse aun algunas barreras, tanto en la produccion limpia de hidrogeno como en su uso y en su acumulacion. El punto de vista adoptado en este articulo es el de la ciencia de materiales como motor para resolver los “cuellos de botella”, que actualmente existen, ante una posible implantacion de un sistema energetico basado en el hidrogeno como combustible. En este articulo se describen, resumidamente, las etapas esenciales del esquema energetico que nos ocupa para, despues, centrarnos en la que probablemente sea la etapa mas decisiva: la produccion de H2. En particular y principalmente en la fotogeneracion de H2 que, a partir de agua, se produce al iluminar la interfase de un semiconductor y un electrolito acuoso con radiacion de longitud de onda adecuada. En la fotogeneracion de H2 mencionada, el interes actual se centra en encontrar nuevos materiales semiconductores (o nuevas estructuras) que tengan caracteristicas y respuestas convenientes para que el proceso de fotogeneracion se torne aceptable. Se discutiran las ventajas e inconvenientes de este metodo y las expectativas en el campo derivadas del uso de nuevos materiales, principalmente semiconductores, como el TiS3 o el PdS, y materiales afines.

The reaction kinetics and reversibility for hydrogen sorption were investigated for supported Mg2Cu nanoparticles on carbon. A new preparation method is proposed to synthesize the supported alloy nanoparticles. The motivation of using a support is to separate the nanoparticles to prevent sintering at elevated temperatures. Supported nanocrystallites with an average size of 20 nm were obtained on porous graphite and larger particles (similar to 300 nm) on non-porous graphite by first deposition of metallic Cu species, using solution impregnation, followed by addition of molten Mg and hydrogenation. The temperature for hydrogen release of the 20 nm particles was much lower (similar to 150 degrees C) than the micron-sized material, and the reaction was reversible with the same improved kinetic performance after several hydrogen sorption cycles. The 20 nm Mg2Cu crystallites had a lower activation energy for the hydrogen desorption reaction compared to the bulk material (97 (+/- 9) and 128 (+/- 6) kJ mol(-1) respectively). A desorption enthalpy of 66 (+/- 3) kJ mol(-1) and an entropy value of 126 (+/- 10) J mol K-1 were found for this system. The use of a porous carbon support was beneficial for obtaining Mg2Cu nanoparticles, which improved the hydrogen sorption kinetics.

To improve the hydrogen storage properties of Mg6Pd and to reduce its cost, Pd has been partly substituted by Ni at the solubility limit of the Mg6(Pd,Ni) ρ-phase. The attained composition is Mg6.2Pd0.25Ni0.65 as determined by Energy Dispersive X-Ray (EDX) and X-Ray Diffraction (XRD). Hydrogenation of this compound has been investigated by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM–EDX), Pressure-Composition-Isotherms (PCI) and thermal desorption analysis. On absorption, it decomposes in two steps as evidenced by two distinct plateau pressures. At low pressure, a partial segregation of Mg and Ni out of the pseudo-binary Mg6.2Pd0.25Ni0.65 ρ-phase occurs leading to the formation of MgH2, Mg2Ni and Mg6Pd0.7Ni0.3 phases. At high pressure, the Mg6Pd0.7Ni0.3 phase disproportionates into MgH2, Mg2NiH4, MgPd and Mg5Pd2 phases. The hydrogenation reaction is reversible providing a hydrogen capacity of 5.6wt.% H. The reaction enthalpy of the high pressure plateau is less negative than for pure Mg. Furthermore, the activation energy for H-desorption exhibits a dramatic decrease for hydrogen contents above 4wt.% H, i.e. after the alloy disproportionation.

Zn isotopic fractionation caused by sorption on 2-Lines ferrihydrite (Fh2L) and goethite was investigated to assess the role of reactions at the Fe-oxyhydroxide/water interface in changes of the isotopic distribution of Zn. Since sorption reactions are ubiquitous in Earth’s surface environments, it is important to evaluate their influence on the isotopic distribution of Zn before it can be used to track and quantify contributions of various sources and/or biogeochemical processes involving this element. Our results show that Zn isotopes are fractionated upon sorption on Fe-oxyhydroxides with an enrichment of the heavy isotopes present on the solid’s surface. This fractionation appears to proceed through an equilibrium mechanism and yields different (Δ66/64Zn)sorbed–aqueous values for Zn sorption on goethite [(Δ66/64Zn)sorbed–aqueous around +0.29‰] and Fh2L [(Δ66/64Zn)sorbed–aqueous around +0.53‰]. These different magnitudes of Zn fractionation are related to structural differences between Zn complexes existing on the surface of goethite (octahedrally coordinated Zn by oxygen atoms) and Fh2L (tetrahedrally coordinated Zn by oxygen atoms), as evidenced by Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy and CD-MUSIC modeling. These results show the importance of accounting for reactions at the Fe-oxyhydroxide/water interface when dealing with the isotopic distribution of Zn at the Earth’s surface. Considering the large range of other possible sorbents (Mn or Al oxides, phyllosilicates, carbonates, biologic surfaces, etc.) and the importance of reactions at sorbent/water interfaces for other non-traditional stable isotopes (i.e. Cr, Fe, Ni and Cu) that are increasingly used in environmental studies, these results emphasize the need for further experimental studies that are needed to quantify the isotopic fractionation of these elements possibly accompanying their sorption.

The biogeochemistry of trace elements (TE) is largely dependent upon their interaction with heterogeneous ligands including metal oxides and hydrous oxides of iron. The modeling of TE interactions with iron oxides has been pursued using a variety of chemical models. The objective of this work is to show that it is possible to model the adsorption of protons and TE on a crystallized oxide (i.e., goethite) and on an amorphous oxide (HFO) in an identical way. Here, we use the CD-MUSIC approach in combination with valuable and reliable surface spectroscopy information about the nature of surface complexes of the TE. The other objective of this work is to obtain generic parameters to describe the binding of the following elements (Cd, Co, Cu, Ni, Pb, and Zn) onto both iron oxides for the CD-MUSIC approach. The results show that a consistent description of proton and metal ion binding is possible for goethite and HFO with the same set of model parameters. In general a good prediction of almost all the collected experimental data sets corresponding to metal ion binding to HFO is obtained. Moreover, dominant surface species are in agreement with the recently published surface complexes derived from X-ray absorption spectroscopy (XAS) data. Until more detailed information on the structure of the two iron oxides is available, the present option seems a reasonable approximation and can be used to describe complex geochemical systems. To improve our understanding and modeling of multi-component systems we need more data obtained at much lower metal ion to iron oxide ratios in order to be able to account eventually for sites that are not always characterized in spectroscopic studies.

The bioavailability and the potential toxicity of heavy metals in the environment depend on their speciation in the soil and the soil solution. The analytical task is rendered difficult because the individual chemical species are often present at nano- and picomolar concentration in natural systems.To determine the free metal ion (FMI) concentration, both analytical and computational approaches were used. The recently developed Donnan Membrane Technique (DMT) succeeded in measuring the free metal ion concentration. In a first analytical step, some Zn smelter impacted topsoil (0-5 cm) columns were linked to the DMT cell and metal concentrations were measured at different pHs. The measured free metal concentrations in the soil solution were compared to those obtained from model calculations. In the second part, metal concentrations in the soil solution are predicted with the soil major constituents characteristics.These combined approaches allow a good description of the soil solution chemistry. Cd and Zn are mainly in solution as free aquo-ion that will migrate easily. The speciation of Pb and Cu is regulated by the dissolved organic matter. Their transport will be controlled by organic matter mobility. Cd, Zn and Pb speciation in the soil is controlled by organic matter, metallic oxides (Fe, Mn) and newly formed Zn-bearing minerals.All these calculated speciation and distribution are in reasonable agreement with EXAFS data previously published for the same soil sample. (C) 2002 Elsevier Science B.V All rights reserved.

Suspended solids from several rivers of the Amazon Basin (Brazil) have been studied in both the particulate (> 0,2 mum) and colloidal (0.2 mum-5kD) fractions through a combination of tools including Fourier transform infrared spectroscopy (FT-IR), electron paramagnetic resonance (EPR) and potentiometric measurements. Particulate and colloidal matter are organo-mineral and organic, respectively. Signatures of the particulate fractions distinguish Rio Amazonas/Rio Solimoes rivers and the northern basin rivers (Rio Negro, Rio Branco, Rio Trombetas). In the latter, kaolinites are the dominant clays and exhibit close crystal chemistry characters such as degree of disorder. FT-IR of colloids exhibits similar signatures of humic substances in all rivers. By contrast,, EPR reveals various species which qualitatively and quantitatively differentiate the riverborne colloids: trivalent iron occurring as specific complexes with organic functional groups or as oxides sensu lato, divalent manganese, organic free radicals. The observed differences may be inherited from the weathering processes working in the main pedoclimatic regions drained by the studied rivers. Insights from potentiometric measurements of the recovered colloids also evidence a humic like reactivity consistent with carboxyl and phenol moieties, the relative contributions of which differentiate the Rio Negro and Rio Solimoes/Amazonas.

Colloidal and particulate natural organic matter was fractionated and concentrated from the Amazon and Rio Negro rivers using tangential-flow filtration (TFF). Flow field-flow fractionation, with UV absorbance detection, was used to investigate the molecular weight distributions of the organic colloids. To further characterize the nature of the Rio Negro colloids, the size distributions of the iron and aluminum concentrations were determined by off-line graphite furnace atomic absorption spectrometry analysis of fractions collected during the FFF separation. The size distributions obtained by FFF were considerably smaller than expected from the stated pore size of the TFF membranes. These results demonstrate that care must be taken in using TFF to classify the size distribution of organic colloids and associated elements present in rivers. Total metal complexing capacities (TMCC) of the colloids were determined by voltametry at various pH using lead as a probe. Total binding capacities were measured by automated proton titration of the particulate and the colloidal fractions. To describe Pb binding together with Al competition and the presence of mineral forms of Al in the particulate fraction, the NICA–Donnan approach was used. The fitted model lines agree reasonably well with the experimental results, suggesting that natural ligands behave like purified humic or fulvic acids. This result is surprising due to the extreme treatment humic substances are subjected to during purification. The only major difference between the binding constant values obtained for purified humic substances and the TFF fractions is a higher affinity constant value for the carboxylic groups of the latter. An interesting result is the agreement in the metal binding for the different fractions at pH 4 and 5, which implies that the type of groups present in these fractions are similar