Hydrogen dissociation and diffusion on Ni- and Ti-doped Mg(0001) surfaces

Facilitating Hydrogen Dissociation over Dilute Nanoporous Ti-Cu Catalysts

The dissociation of H₂ is an essential elementary step in many industrial chemical transformations, typically requiring precious metals. Here, we report a hierarchical nanoporous Cu catalyst doped with small amounts of Ti (npTiCu) that increases the rate of H₂-D₂ exchange by approximately one order of magnitude compared to the undoped nanoporous Cu (npCu) catalyst. The promotional effect of Ti was measured via steady-state H₂-D₂ exchange reaction experiments under atmospheric pressure flow conditions in the temperature range of 300-573 K. Pretreatment with flowing H₂ is required for stable catalytic performance, and two temperatures, 523 and 673 K, were investigated. The experimentally determined H₂-D₂ exchange rate is 5-7 times greater for npTiCu vs the undoped Cu material under optimized pretreatment and reaction temperatures. The H₂ pretreatment leads to full reduction of Cu oxide and partial reduction of surface Ti oxide species present in the as-prepared catalyst as demonstrated using in situ ambient pressure X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. The apparent activation energies and pre-exponential factors measured for H₂-D₂ exchange are substantially different for Ti-doped vs undoped npCu catalysts. Density functional theory calculations suggest that isolated, metallic Ti atoms on the surface of the Cu host can act as the active surface sites for hydrogen recombination. The increase in the rate of exchange above that of pure Cu is caused primarily by a shift in the rate-determining step from dissociative adsorption on Cu to H/D atom recombination on Ti-doped Cu, with the corresponding decrease in activation entropy that it produces.

Breaking the Passivation: Sodium Borohydride Synthesis by Reacting Hydrated Borax with Aluminum

A significant obstacle in the large-scale applications of sodium borohydride (NaBH₄ ) for hydrogen storage is its high cost. Herein, we report a new method to synthesize NaBH₄ by ball milling hydrated sodium tetraborate (Na₂ B₄ O₇  ⋅ 10H₂ O) with low-cost Al or Al₈₈ Si₁₂ , instead of Na, Mg or Ca. An effective strategy is developed to facilitate mass transfer during the reaction by introducing NaH to enable the formation of NaAlO₂ instead of dense Al₂ O₃ on Al surface, and by using Si as a milling additive to prevent agglomeration and also break up passivation layers. Another advantage of this process is that hydrogen in Na₂ B₄ O₇  ⋅ 10H₂ O serves as a hydrogen source for NaBH₄ generation. Considering the low cost of the starting materials and simplicity in operation, our studies demonstrate the potential of producing NaBH₄ in a more economical way than the commercial process.

Roles of Ti-Based Catalysts on Magnesium Hydride and Its Hydrogen Storage Properties

Magnesium-based hydrides are considered as promising candidates for solid-state hydrogen storage and thermal energy storage, due to their high hydrogen capacity, reversibility, and elemental abundance of Mg. To improve the sluggish kinetics of MgH2, catalytic doping using Ti-based catalysts is regarded as an effective approach to enhance Mg-based materials. In the past decades, Ti-based additives, as one of the important groups of catalysts, have received intensive endeavors towards the understanding of the fundamental principle of catalysis for the Mg-H2 reaction. In this review, we start with the introduction of fundamental features of magnesium hydride and then summarize the recent advances of Ti-based additive doped MgH2 materials. The roles of Ti-based catalysts in various categories of elemental metals, hydrides, oxides, halides, and intermetallic compounds were overviewed. Particularly, the kinetic mechanisms are discussed in detail. Moreover, the remaining challenges and future perspectives of Mg-based hydrides are discussed.

The performance of adsorption, dissociation and diffusion mechanism of hydrogen on the Ti-doped ZrCo(110) surface

Compared with pristine ZrCo(110), the adsorption, dissociation, and successive diffusion of hydrogen on the Ti-decorated ZrCo(110) have been investigated based on first-principles calculation. For the purpose of having fast absorption kinetics, both activation processes need to overcome small energy barriers. The adsorption energies of molecular as well as atomic hydrogen on the Ti-decorated ZrCo(110) surface were calculated using first-principles calculations with the periodic density functional theory (DFT). The H₂ molecule on ZrCo(110) and Ti-doped ZrCo(110) surfaces could be spontaneously partially dissociated due to the interaction with the substrate surfaces, producing H atoms strongly chemisorbed to the hollow sites. The H₂ dissociation energy barrier and the H diffusion barrier were also determined. Our results show that the activation energy for H₂ dissociation on the decorated surface (0.052 eV) is much smaller than that of the pure surface (0.524 eV), elucidating that the activation condition of H₂ on the pure ZrCo(110) is more severe than that on the Ti-doped surface. Particularly, Ti-decoration facilitates the H₂ dissociation. Moreover, the re-desorption performance of the two dissociated H atoms is improved by lowering the energetic barrier from 1.798 eV (on the pure surface) to 1.315 eV (on the decorated surface). The calculations also reveal that decorating the surface with Ti eliminates the barrier for the into-bulk penetration of a hydrogen atom. Based on the local density of states, the Bader charge and differential charge density, as well as the influence of the Ti atom on topological properties were analyzed. Theoretical results presented in this study are generally in well accordance with the available theoretical and experimental data.

First principles study on stability and hydrogen adsorption properties of Mg/Ti interface

The hydrogenation and stability properties of the Mg/Ti interface are studied by first-principles calculations. The strain of lattice and movement of ions were imposed to search for a stable Mg/Ti interface. The anti-symmetrical configuration was found to be the most stable. The easiest transition pathway from anti-symmetrical to symmetrical configuration may be through the diagonal direction with no energy barrier. The hydrogen adsorption at distinguished positions in the Mg/Ti interface is investigated. The negative hydrogen adsorption energy reaches -0.991 eV at the top site in the interface, which will highly favor the thermodynamic stability of the Mg/Ti interface. The electronic structure is studied and it was found that the Ti acts as a hydrogen atom 'capturer' and strong interactions between H and its surrounding Ti and Mg atoms are expected. Thus, inserting Ti layers could create an interfacial zone where the adsorptions of hydrogen atoms may get stabilized and therefore improve the hydrogen storage properties of Mg.

Chemical evolution via beta decay: a case study in strontium-90

Using (90)Sr as a representative isotope, we present a framework for understanding beta decay within the solid state. We quantify three key physical and chemical principles, namely momentum-induced recoil during the decay event, defect creation due to physical displacement, and chemical evolution over time. A fourth effect, that of electronic excitation, is also discussed, but this is difficult to quantify and is strongly material dependent. The analysis is presented for the specific cases of SrTiO(3) and SrH(2). By comparing the recoil energy with available threshold displacement data we show that in many beta-decay situations defects such as Frenkel pairs will not be created during decay as the energy transfer is too low. This observation leads to the concept of chemical evolution over time, which we quantify using density functional theory. Using a combination of Bader analysis, phonon calculations and cohesive energy calculations, we show that beta decay leads to counter-intuitive behavior that has implications for nuclear waste storage and novel materials design.

The role of steps in the dissociation of H(2) on Mg(0001)

The role of steps in the dissociation of molecules on metal surfaces has been extensively investigated in the past. In particular, both theoretical calculations and experimental results for H(2) dissociation on transition metal (TM) surfaces show that steps can significantly increase the reactivity, leading to higher metal-H binding energies and lower activation energies. Here we have used density functional theory (DFT) with the generalized gradient approximation (GGA) to investigate the role of steps on the Mg(0001) surface in the dissociation of H(2) and the binding of H to the metal surface. Our results follow those found for TM surfaces as far as H adsorption energies are concerned, namely that adsorption energies are higher near the steps. However, we find that the activation energy for the dissociation of hydrogen is hardly affected by the presence of steps, with a DFT-GGA value of 0.85 eV, only marginally lower than the value 0.87 eV found on the flat Mg(0001) surface.