First-principles study of the H2 splitting processes on pure and transition-metal-doped Al (111) surfaces

By performing first-principles calculations, H₂ splitting processes on pure and transition metal (TM) atom substituted Al (111) surfaces were examined. Corrected reaction pathways with splitting energy barriers (0.99 eV) lower than those in previous studies (1.28 eV) were obtained. By further analyzing the H₂ splitting process on the 3d-TM-atom-doped Al (111) surface, the relationship of the catalysis effect and the electron donation-back donation process on TM 3d orbitals were examined in detail. Finally, to confirm the possibility of reducing the partially oxidized Al (111) surface with an H₂ molecule, the surface reduction process was studied by using the climb-image nudged elastic band (CI-NEB) method systematically.     

Augmented hydrogen adsorption on metal (Mg,Mn) doped α-phase TeO2: A DFT investigation

TeO₂, as a promising gas sensor material, has been extensively studied for its capacity to detect hydrogen with high sensitivity. First-principles calculations were applied to explore the adsorption properties of hydrogen (H₂), carbon dioxide (CO₂), methane (CH₄), and hydrogen sulfide (H₂S) on TeO₂ doped with either Mg or Mn to explore this compound's potential as hydrogen sensors. Hydrogen is more readily adsorbed on pure-TeO₂, Mg–TeO₂ and Mn–TeO₂ than CO₂, CH₄ and H₂S molecules by calculating their adsorption energy and charge transfer; the sequence of adsorption strength is H₂>H₂S > CO₂>CH₄. The hydrogen molecules and pure-TeO₂, Mg–TeO₂ and Mn–TeO₂ form H–O bonds with lengths of 0.98, 0.98 and 0.99 Å, respectively, indicating that chemical adsorption is dominant between them. The adsorption of hydrogen leads to significant changes in the density of states (DOSs) of pure-TeO₂, Mg–TeO₂ and Mn–TeO₂, which may lead to changes in their electrical conductivity. Moreover, the larger diffusion coefficients for hydrogen on the surfaces of pure-TeO₂, Mg–TeO₂ and Mn–TeO₂ relative to other gases indicates that hydrogen diffuses readily in TeO₂-based sensing materials, and the higher gas concentration contributes to improvements in response performance. This finding offers a theoretical basis for experimental explorations of the influence of metal dopants on TeO₂ hydrogen sensing performance.     

Cooperatively enhanced catalytic properties of Ti@Al(100) near-surface alloy for aluminum hydrogenation

We have carried out detailed first-principles studies of the catalytic properties of Ti@Al(100) near-surface alloy. The single Ti atom, (0,2) Ti–Ti pair, and [0,2] Ti doping domain have better catalytic performances. These species doped in the top surface can develop back-bonding interaction with H₂ to catalyze the splitting, which however on the other hand hinder the dissociated H atoms to diffuse. Doped in the subsurface, they can also enhance hydrogen interaction on aluminum to catalyze H₂ splitting. The activation energies are 0.80, 0.68, and 0.48 eV for Ti atom, (0,2) pair, and [0,2] doping domain, respectively. Without Ti–H bond, the dissociated H atom could diffuse away with small energy cost. The structural expansion induced by titanium doping, the lower electronegativity of Ti, and the more valence electrons of Ti may cooperatively facilitate the charge transfer from the above Al atoms to H₂ molecule, accounting for the enhanced splitting properties.     

Interaction of hydrogen and platinum over a B2 FeTi (110) Slab

In this work, we present a density functional theory (DFT) study of hydrogen interaction with Pt on a B2 FeTi (110) metallic surface. DFT is used to trace relevant orbital interactions and to discuss the electronic consequences of incorporating H on Fe-Ti bonding. We determined the optimal location for Pt and, then, for adsorbed hydrogen. In addition, we followed the density of states and changes in chemical bonding both in the surface and the adsorbates. The overlap population analysis reveals metal-metal bond breaking after hydrogen adsorption, thus being the inter-metallic bond the most affected one.     

Hydrogen storage in lithium-decorated benzene complexes

Based on first-principles calculations, we find Li-decorated benzene complexes are promising materials for high-capacity hydrogen storage. Lithium atoms in the complexes are always positively charged, and each one can bind at most four H₂ molecules by a polarization mechanism. Therefore, a hydrogen uptake of 8.6 wt% and 14.8 wt% can be achieved in isolated C₆H₆–Li and Li–C₆H₆–Li complexes, respectively. The binding energy in the two cases is 0.22 eV/H₂ and 0.29 eV/H₂, respectively, suitable for reversible hydrogen storage. Various dimers may form, but the hydrogen storage capacity remains high or uninfluenced. This study provides not only a promising hydrogen storage medium but also comprehensions to other hydrogen storage materials containing six-carbon rings.     

Understanding hydrogen diffusion mechanisms in doped α-Fe through DFT calculations

Hydrogen embrittlement is detrimental to structural metals during applications. Herein, we explore the hydrogen diffusion mechanisms in doped α-Fe using first-principles calculations. We prove that the hydrogen trap is a thermodynamically spontaneous process, and doping will decrease the hydrogen adsorption energy due to the change of adsorption sites. Furthermore, hydrogen diffusion from surface to subsurface will determine the diffusion rate. Mo, Mn and C are beneficial to the increase of the energy barrier of hydrogen diffusion from the surface to subsurface and in the bulk. The current work provides a promising path towards enhancing the hydrogen diffusion barrier in α-Fe.     

Highly hydrogen permeable Nb–Ti–Co hypereutectic alloys containing much primary bcc-(Nb,Ti) phase

In order to develop alloys combing high hydrogen permeability with large resistance to the hydrogen embrittlement, microstructures and hydrogen permeability (Φ) have been investigated for the as-cast alloys on the straight line connecting the eutectic {TiCo + (Nb, Ti)} phase and the Nb-rich primary (Nb, Ti) phase in the Nb–Ti–Co system. The alloys on the above-mentioned line consist of the TiCo compound and the (Nb, Ti) solid solution. The value of Φ increases with increasing temperature and volume fraction of the primary (Nb, Ti) phase. The most Nb-rich Nb₆₀Ti₂₁Co₁₉ alloy shows the highest Φ value of 3.99 × 10⁻⁸ (mol H₂m⁻¹ s⁻¹ Pa^−0.5) at 673 K, which is 2.6 times higher than that of pure Pd. The present work demonstrates that highly hydrogen permeable alloys are obtainable in the Nb rich Nb–Ti–Co ones on the straight line connecting the eutectic {TiCo + (Nb, Ti)} phase and the Nb-rich primary (Nb, Ti) phase.     

Hydrogen release from a mixture of NaBH4 and Mg(OH)2

Hydrogen generating reaction between sodium borohydride, NaBH₄, and magnesium hydroxide, Mg(OH)₂ (brucite), was studied. Reaction rate was found to depend on the degree of reactants homogenization and/or their particle size. Kinetic of the reaction was studied in isothermal approach in the temperature range of 240–360 °C. It is shown that the reaction obeys 2D diffusion mechanism and its activation energy is 155.9 kJ/mol. Powder XRD analysis and Raman spectroscopy reveal that mechanically activated mixture of NaBH₄ and Mg(OH)₂ reacts yielding MgO as the only crystalline phase in the temperature range of 240–318 °C. At higher temperatures a new crystalline tetragonal phase of as yet undetermined composition is developed.     

First-principles studies of K1−xMxMgH3 (M = Li,Na, Rb,or Cs) perovskite hydrides for hydrogen storage

The structural stability and hydrogen release properties of M-doped KMgH₃ (M = Li, Na, Rb, or Cs) were examined using density functional theory (DFT) calculations. The reaction enthalpies (ΔH) of the four possible dehydrogenation reaction pathways were calculated using the doped structures with different phases (, Pnma, and R3c). The most favorable reaction pathway among these four pathways was found. Among the dopants investigated, the most promising dopant for this reaction was Li. In addition, the application of pressure was found to be useful for tuning the reaction enthalpies of the dehydrogenation reactions. Overall, the results present an efficient means of designing new promising perovskite-type hydrides for hydrogen storage.     

First-principles study of the hydrogen absorption at Σ5 symmetrical tilt grain boundary in B2-TiFe alloy

We present a density functional theory (DFT) study of the hydrogen-metal interaction in the B2-TiFe alloy with Σ5(310) symmetrical tilt grain boundary (GB) and (310) free surface (FS). The influence of hydrogen on the electronic properties of alloy with GB and FS is analyzed for different hydrogen sorption sites. The hydrogen absorption/adsorption, binding and segregation energies are calculated at GB and FS. Our calculations reveal that H segregates more strongly to the surface than to the GB that results in decrease in the Griffith work, i.e., H makes the fracture of the GB easier.     

Atom doping in α-Fe2O3 thin films to prevent hydrogen permeation

Prevention of hydrogen (H) penetration into passive films and steels plays a vital role in lowering hydrogen damage. This work reports effects of atom (Al, Cr, or Ni) doping on hydrogen adsorption on the α-Fe₂O₃ (001) thin films and permeation into the films based on density functional theory. We found that the H₂ molecule prefers to dissociate on the surface of pure α-Fe₂O₃ thin film with adsorption energy of −1.18 eV. Doping Al or Cr atoms in the subsurface of α-Fe₂O₃ (001) films can reduce the adsorption energy by 0.03 eV (Al) or 0.09 eV (Cr) for H surface adsorption. In contrast, Ni doping substantially enhances the H adsorption energy by 1.08 eV. As H permeates into the subsurface of the film, H occupies the octahedral interstitial site and forms chemical bond with an O atom. Comparing with H subsurface absorption in the pure film, the absorption energy decreases by 0.01–0.22 eV for the Al- and Cr-doped films, whereas increases by 0.82–0.96 eV for the Ni-doped film. These results suggest that doping Al or Cr prevents H adsorption on the surface or permeation into the passive film, which effectively reduces the possibility of hydrogen embrittlement of the underlying steel.     

Hydrogen sorption kinetics of MgH2 catalyzed with titanium compounds

Identification of effective catalyst is a subject of great interest in developing MgH₂ system as a potential hydrogen storage medium. In this work, the effects of typical titanium compounds (TiF₃, TiCl₃, TiO₂, TiN and TiH₂) on MgH₂ were systematically investigated with regard to hydrogen sorption kinetics. Among them, adding TiF₃ leads to the most pronounced improvement on both absorption and desorption rates. Comparative studies indicate that the TiH₂ and MgF₂ phases in situ introduced by TiF₃ fail to explain the superior catalytic activity. However, a positive interaction between TiH₂ and MgF₂ is observed. Detailed comparison between the effect of TiF₃ and TiCl₃ additive suggests the catalytic role of F anion. XPS examination reveals that new bonding state(s) of F anion is formed in the MgH₂ + TiF₃ system. On the basis of these results, we propose that the substantial participation of F anion in the catalytic function contributes to the superior activity of TiF₃.