Remarkable hydrogen storage properties of MgH2 doped with VNbO5

The present work concerns the catalytic effect of VNbO5, a ternary oxide prepared via a solid-state route, on the sorption performance of MgH2. Three doped systems, namely 5, 10 and 15 wt% VNbO5– MgH2 have been prepared by ball milling and thoroughly characterized. Hydrogen sorption, evaluated by temperature programmed desorption experiments, revealed a significant reduction of the desorption temperature from 330 1C for the un-doped sample (prepared and tested for comparison) to 235 1C for the VNbO5-doped sample. Furthermore, more than 5 wt% of hydrogen can be absorbed in 5 minutes at 160 1C under 20 bar of hydrogen, which is remarkable compared to the 0.7 wt% achieved for the un-doped system. The sample doped with 15 wt% of additive, showed good reversibility: over 5 wt% of hydrogen with negligible degradation even after 70 consecutive cycles at 275 1C and 50 cycles at 300 1C. The kinetics analysis carried out by Kissinger’s method exhibited a considerable reduction of the activation energy for the desorption process. Finally, pressure-composition-isotherm experiments conducted at three different temperatures allowed estimating the thermodynamic stability of the system and shed light on the additive role of VNbO5

Novel processing route for the fabrication of bulk high-entropy metal diborides

A single high-entropy phase material with hexagonal structure is produced by a two-steps processing method. Elemental reactants are first remarkably converted by Self-propagating High-temperature Synthesis (SHS). The completion of the chemical transformation to the desired (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 phase and its concurrent consolidation up to 92.5% relative density is achieved by processing the SHS powders at 1950 °C via Spark Plasma Sintering. It is clearly evidenced that the use of the SHS technique is extremely beneficial to promote the formation of high-entropy ceramics, as compared to the time consuming ball milling treatment alternatively adopted

Mechanically activated metathesis reaction in NaNH2–MgH2 powder mixtures

The present work addresses the kinetics of chemical transformations activated by the mechanical processing of powder by ball milling. In particular, attention focuses on the reaction between NaNH2 and MgH2, specific case studies suitably chosen to throw light on the kinetic features emerging in connection with the exchange of anionic ligands induced by mechanical activation. Experimental findings indicate that the mechanical treatment of NaNH2–MgH2 powder mixtures induces a simple metathetic reaction with formation of NaH and Mg(NH2)2 phases. Chemical conversion data obtained by X-ray diffraction analysis have been interpreted using a kinetic model incorporating the statistical character of the mechanical processing by ball milling. The apparent rate constant measuring the reaction rate is related to the volume of powder effectively processed during individual collisions, and tentatively connected with the transfer of mechanical energy across the network formed by the points of contact between the powder particles trapped during collisions.H2020-MSCA-IF-2015 Grant Number #70795

High-entropy transition metal diborides by reactive and non-reactive spark plasma sintering: A comparative investigation

The direct synthesis and consolidation by SPS (1950 °C, 20 min, 20 MPa) of high-entropy (Hf0.2Mo0.2Zr0.2Nb0.2Ti0.2)B2 from elemental powders resulted in a multiphase product. An increase of the heating rate determined a change of the mechanism governing the synthesis reaction from gradual solid-state diffusion to rapid combustion regime, while the final conversion degree was 67 wt.%. The sintered product displayed a non-uniform microstructure with the presence of 10–15 μm sized pores, due to volatilization phenomena occurring during the combustion synthesis reaction. In contrast, when the SPS process was preceded by powder synthesis via SHS, a homogeneous single-phase ceramic was obtained. Clear benefits are derived by the use of SHS, able to provide very shortly powders with elemental species very well intermixed, so that the obtainment of (Hf0.2Mo0.2Zr0.2Nb0.2Ti0.2)B2 during the subsequent SPS stage is strongly promoted. The resulting 92.5% dense product shows superior oxidation resistance with respect to individual borides prepared with the same method.ARCHIMEDES project sponsored by Regione Autonoma della Sardegna (Italy) - Fondo di Sviluppo e Coesione (FSC) 2014-2020 (Cod. RAS: RASSR88309, Cod. CUP: F76C18000980002). One of the authors (G.T.) performed her activity in the framework of the International PhD in Innovation Sciences and Technologies at the University of Cagliari, Italy. One of us (G.C.) acknowledges the results obtained in this manuscript as quite important for the “Ithermal” and “Generazione E” projects, sponsored by Sardegna Ricerche, Italy (Cod. CUP: F21I18000130006) and by the Italian Ministry of Education, University and Research, Italy (Cod. CUP: B96G18000560005), respectivel

Non-monotonic variation of the grain size in Cu nanopowders subjected to ball milling

Ball milling (BM) a Cu nanopowder resulted in an increase of the average grain size from 8 to 52 nm, followed by a gradual decrease to 19 nm. In contrast, the grain size of coarse-grained Cu decreased monotonically from 290 nm to 19 nm. Fitting a model to the kinetic curves indicates that the two processes have similar activated volumes during collisions. It also reveals that particles over 100 nm are formed when nanoparticles are compressed during a collision for the first time.University of Cagliari and performed within the European Community Horizon 2020 Programme, COST Action CA15102 Solutions for Critical Raw Materials under Extreme Conditions (CRM_EXTREME

Bentonite as a refining agent in waste cooking oils recycling: flash point, density and color evaluation

Clarification of waste recycled cooking oil (WCO) is very important in order to refine the intermediate regenerated base resulting from the previous steps. Bentonite has been historically employed as a filling material for oil refining filters due to its easy availability and its cheap price. In the present communication our early results from the filtration of degummed WCO through a pad of bentonite are presented. In particular, the variation of density, flash point and color have been monitored prior and after the filtration process and compared with samples of non-filtered WCO. An early classification of the bentonite employed has been conducted on the basis of FT-IR and XRD measurements

Chemical effects induced by the mechanical processing of granite powder

Starting from 1970s, the use of mechanical forces to induce chemical transformations has radically changed vast areas of metallurgy and materials science. More recently, mechanochemistry has expanded to core sectors of chemistry, showing the promise to deeply innovate chemical industry while enhancing its sustainability and competitiveness. We are still far, however, from unveiling the full potential of mechanical activation. This study marks a step forward in this direction focusing on the chemical effects induced on the surrounding gaseous phase by the mechanical processing of granite. We show that fracturing granite blocks in oxygen can result in the generation of ozone. The refinement of coarse granite particles and the friction between fine ones are also effective in this regard. Combining experimental evidence related to the crushing of large granite samples by uniaxial compression and the ball milling of coarse and fine granite powders, we develop a model that relates mechanochemical ozone generation to the surface area effectively affected by fracture and frictional events taking place during individual impacts. We also extend the investigation to gaseous phases involving methane, oxygen, benzene and water, revealing that chemical transformations occur as well

Solid state hydrogen storage in alanates and alanate-based compounds: a review

The safest way to store hydrogen is in solid form, physically entrapped in molecular form in highly porous materials, or chemically bound in atomic form in hydrides. Among the different families of these compounds, alkaline and alkaline earth metals alumino-hydrides (alanates) have been regarded as promising storing media and have been extensively studied since 1997, when Bogdanovic and Schwickardi reported that Ti-doped sodium alanate could be reversibly dehydrogenated under moderate conditions. In this review, the preparative methods; the crystal structure; the physico-chemical and hydrogen absorption-desorption properties of the alanates of Li, Na, K, Ca, Mg, Y, Eu, and Sr; and of some of the most interesting multi-cation alanates will be summarized and discussed. The most promising alanate-based reactive hydride composite (RHC) systems developed in the last few years will also be described and commented on concerning their hydrogen absorption and desorption performanc

Theoretical study on molten alkali carbonate interfaces

The properties and structure of relevant interfaces involving molten alkali carbonates are studied using molecular dynamics simulation. Lithium carbonate and the Li/Na/K carbonate eutectic mixture are considered. Gas phases composed of pure CO2 or a model flue gas mixture are analyzed. Similarly, the adsorption of these gas phases on graphene are studied, showing competitive CO2 and N2 adsorption that develops liquid-like layers and damped oscillation behavior for density. The interaction of the studied carbonates with graphene is also characterized by development of adsorption layers through strong graphene–carbonate interactions and the development of hexagonal lattice arrangements, especially for lithium carbonate. The development of molten salts–vacuum interfaces is also considered, analyzing the ionic rearrangement in the interfacial region. The behavior of the selected gas phases on top of molten alkyl carbonate is also studied, showing the preferential adsorption of CO2 molecules when flue gases are considered.European Union’s H2020- MSCA-RISE-2016-CO2MPRISE-73487

Insights into carbon nanotubes and fullerenes in molten alkali carbonates

The properties of single-walled carbon nanotubes and carbon fullerenes in molten alkali carbonates (MACs) were studied as a function of the considered nanomaterial and the ions into the molten salt using classical molecular dynamics simulations. The adsorption and confinement in carbon nanotubes is developed by efficient interaction of carbonate ions in the inner and outer walls of the nanotubes whereas alkali cations do not show a remarkable interaction with the nanomaterial. Analogous solvation mechanisms are inferred for carbon fullerenes with large disruption of the liquid structuring of MAC at high fullerene concentrations. The solvation ability of the studied lithium–sodium–potassium carbonate eutectic mixture for both types of nanomaterials is essential for considering this fluid in the development of composite materials for advanced technological applications.European Union H2020- MSCA-RISE-2016-CO2MPRISE-73487