CO2 reactivity with Mg2NiH4 synthesized by in situ monitoring of mechanical milling

CO₂ capture and conversion are a key research field for the transition towards an economy only based on renewable energy sources. In this regard, hydride materials are a potential option for CO₂ methanation since they can provide hydrogen and act as a catalytic species. In this work, Mg₂NiH₄ complex hydride is synthesized by in situ monitoring of mechanical milling under a hydrogen atmosphere from a 2MgH₂:Ni stoichiometric mixture. Temperature and pressure evolution is monitored, and the material is characterized, during milling in situ, thus providing a good insight into the synthesis process. The cubic polymorph of Mg₂NiH₄ (S.G. Fm3[combining macron]m) starts to be formed in the early beginning of the mechanical treatment due to the mechanical stress induced by the milling process. Then, after 25 hours of milling, Mg₂NiH₄ with a monoclinic (S.G. C12/c1) structure appears. The formation of the monoclinic polymorph is most likely related to the stress release that follows the continuous refinement of the material's microstructure. At the end of the milling process, after 60 hours, the as-milled material is composed of 90.8 wt% cubic Mg₂NiH₄, 5.7 wt% monoclinic Mg₂NiH₄, and 3.5 wt% remnant Ni. The as-milled Mg₂NiH₄ shows high reactivity for CO₂ conversion into CH₄. Under static conditions at 400 °C for 5 hours, the interactions between as-milled Mg₂NiH₄ and CO₂ result in total CO₂ consumption and in the formation of the catalytic system Ni-MgNi₂-Mg₂Ni/MgO. Experimental evidence and thermodynamic equilibrium calculations suggest that the global methanation mechanism takes place through the adsorption of C and the direct solid gasification towards CH₄ formation.

Changing the dehydrogenation pathway of LiBH4–MgH2via nanosized lithiated TiO2

Nanosized lithiated titanium oxide (Li_xTiO₂) noticeably improves the kinetic behaviour of 2LiBH₄ + MgH₂.

A new potassium-based intermediate and its role in the desorption properties of the K-Mg-N-H system

New insights into the reaction pathways of different potassium/magnesium amide-hydride based systems are discussed. In situ SR-PXD experiments were for the first time performed in order to reveal the evolution of the phases connected with the hydrogen releasing processes. Evidence of a new K-N-H intermediate is shown and discussed with particular focus on structural modification. Based on these results, a new reaction mechanism of amide-hydride anionic exchange is proposed.

Ca(BH4)2-Mg2NiH4: on the pathway to a Ca(BH4)2 system with a reversible hydrogen cycle

The Ca(BH4)2-Mg2NiH4 system presented here is, to the best of our knowledge, the first described Ca(BH4)2-based hydride composite that reversibly transfers boron from the Ca-based compound(s) to the reaction partner. The ternary boride MgNi2.5B2 is formed upon dehydrogenation and the formation of Ca(BH4)2 upon rehydrogenation is confirmed.

On the chemical state and distribution of Zr- and V-based additives in reactive hydride composites

Reactive hydride composites (RHCs) are very promising hydrogen storage materials for future applications due to their reduced reaction enthalpies and high gravimetric capacities. At present, the materials' functionality is limited by the reaction kinetics. A significant positive influence can be observed with addition of transition-metal-based additives. To understand the effect of these additives, the chemical state and changes during the reaction as well as the microstructural distribution were investigated using x-ray absorption near-edge structure (XANES) spectroscopy and anomalous small-angle x-ray scattering (ASAXS). In this work, zirconium- and vanadium-based additives were added to 2LiBH4-MgH2 composites and 2LiH-MgB2 composites and measured in the vicinity of the corresponding absorption edge. The measurements reveal the formation of finely distributed zirconium diboride and vanadium-based nanoparticles. The potential mechanisms for the observed influence on the reaction kinetics are discussed.