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.     

Mechanistic insights into hydrogen evolution reaction on Ni2B(001) facet using first-principle calculations

In this paper, first-principle calculations based on density functional theory (DFT) were used to investigate the performance and mechanism of the hydrogen evolution reaction (HER) on the typical active (001) facet of the novel electrocatalyst Ni₂B. There were two types of atomic distribution on the Ni₂B (001) surface, namely the B-rich surface and the Ni-rich surface. The investigation of the reaction mechanism revealed that the Volmer-Heyrovsky mechanism was easier to be realized on this Ni₂B (001) facet, and the Heyrovsky reaction was the reaction rate-determining step. The Gibbs free energy(ΔG_H) on the B-rich surface was - 0.02 eV, which was closer to 0 eV than that on the Ni-rich surface of Ni₂B (001). The HER reactivity on the Ni-rich surface was increased by Cr-doping (ΔG_H = - 0.01 eV), which indicated that the introduction of other transition metal atoms might effectively increase the HER electrocatalysis activity of Ni₂B (001) surface. This work paves a new avenue for exploring efficient and durable non-precious metal electrocatalysts for HER in acidic medium.     

Insights into the mechanisms of H2S adsorption and dissociation on CdS surfaces by DFT-D3 calculations

The adsorption and dissociation of H₂S on CdS surfaces is investigated using dispersion-corrected density functional theory (DFT-D3) to provide quantum-level insights into their (photo)catalytic performance for H₂S splitting. Calculations of structural parameters, electronic properties and energies of intermediates adsorption on perfect CdS surfaces indicate that the (110) facet is the most stable surface, while the most active surface (100) is quickly covered by sulfur formed during the reaction, unfavorable for catalyst stability and reuse. Calculations of CdS (110) surfaces with an S vacancy demonstrate that the vacancy serves as an electron donor center and atomic S1 capture center, favoring the adsorption of dissociative species, and significantly reducing the energy barriers and reaction energies for the hydrogen evolution process, hence increasing the CdS surface catalytic performance. These theoretical results complement and reinforce available experimental studies, guiding the rational design of efficient photocatalysts for hydrogen production from H₂S splitting.     

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.     

Interaction of H2S with perfect and S-covered Ni(110) surface: A first-principles study

First-Principles study based on Density functional theory (DFT) calculations are employed to investigate the dissociative mechanism of H₂S adsorption and its dissociation on perfect, and sulfur covered Ni(110) surface. On both surfaces, we probe the site preference for H₂S, HS, H, and S adsorption mechanisms. The results indicate that H₂S is energetically adsorbed on their high symmetry adsorption sites with the preferred short-bridge (SB) site on both surfaces. Furthermore, we found that chemisorption of HS is stronger in contrast to H₂S at favorable short-bridge (SB) with a binding energy of −3.59 eV on perfect Ni(110) surface, and on S-covered Ni(110) surface at the favorable hollow site having a binding energy of −3.57 eV. In the first H₂S dehydrogenation, energy barriers for S–H bond breaking over the clean surface are 0.08–0.46 eV and a little bit higher on the S-covered surface are 0.1–0.78 eV, while in second dehydrogenation the energy barrier on a clean surface is 0.19 eV. For further detail, electronic densities of states and d-band center model are used to characterize the interaction of adsorbed H₂S with both surfaces. Hence, our results show that decomposition of H₂S over perfect and S-covered Ni(110) surface is exothermic and also an easy process. However, kinetically and thermodynamically, the subsistence of surface sulfur avoids the H–S bond breaking process.     

Vibrational and thermodynamic properties of LiBH4 polymorphs from first-principles calculations

The study of phonons describes the thermodynamic properties behavior of compounds with small atoms because phonons have an important influence on its properties. Lithium borohydride, LiBH₄, is one of the suitable materials for hydrogen storage solid state. Although the transformations of Lithium borohydride LiBH₄ were repeatedly studied by experiments and fundamental side, these transformations are still under discussion. In the present work, the mode vibrational analysis of orthorhombic and hexagonal LiBH₄ structures were considered with ab initio lattice-dynamics based on the quasi-harmonic approximation approach as implemented in Phonopy code. The results show that the orthorhombic structure is thermodynamically stable, while the hexagonal structure is unstable owing to the presence of negative mode frequency. The thermal expansion behavior and various thermodynamic properties stability like heat capacity, entropy and Helmholtz energy were also studied and the obtained results are in good agreement with experiments. This shows a deep connection between stability and strength and helps researchers to estimate accurately the thermodynamic performance of LiBH₄ materials.     

Spontaneous dissociation of H2 and high energy barrier of H invasion on W doped Al2O3(0001) surface

In view of the wide use of tungsten in fusion experimental devices and the importance of hydrogen isotopes permeation, here we studied the adsorption, dissociation, diffusion and invasion behavior of hydrogen on W doped α-Al₂O₃ (0001) surface. Based on the first-principle approaches, we found the W substitution for a top surface Al atom is the most energetically favorable. H₂ molecule prefers to be adsorbed on the surface W and spontaneously dissociates into two H anions. Near the W defects, H atoms favor to be adsorbed at the W and Al sites rather than O sites on the surface, and within the subsurface layer H can only bond to W stably. As a result, H migration to subsurface should occur around W with an energy barrier as large as 4.22 eV which is much larger than the 1.91 eV around the O atom on undoped α-Al₂O₃ (0001) surface. These findings suggest that W surface doping is beneficial to α-Al₂O₃ as tritium permeation barrier.     

Hydrogen adsorption and diffusion on doped Zr(0001) surfaces: A first-principles study

The hydrogen adsorption and diffusion behaviors on the clean and a series of element doped Zr(0001) surfaces are studied through first-principles calculations. Among the studied doping elements, Cu, Co, Y, and Mg prefer to substitute Zr on the topmost surface layer, Al, Pd, Ir, and Si are favored from topmost layer to several surface layers down, while Mo is not favored. Independent of the substitution energies, Mo, Co, and Ir induce a symmetry-breaking local distortion surface structure. Based on the obtained geometries, it is found that most dopants promote the hydrogen adsorption on their next nearest neighbor sites but hinder it on the nearest neighbor sites. Most of the dopants also promote both the hydrogen diffusion on the surface plane and the hydrogen penetration into the subsurface layers. The results indicate that element doping may facilitate the hydride nucleation in Zr alloys.     

The role of magnesium on properties of La3-xMgxNi9 (x=0, 0.5, 1.0, 1.5, 2.0) hydrogen storage alloys from first-principles calculations

The present work gives the electronic structures of La_3-xMg_xNi₉ (x = 0.0–2.0) alloys by first-principles calculations using the generalized gradient approximation of Perdew-Wang 91 (GGA-PW91) method within Cambridge Serial Total Energy Package (CASTEP), aiming at gaining insight into the hydrogen storage mechanism of La_3-xMg_xNi₉ alloys modified by Mg. The results show that the La_3-xMg_xNi₉ alloys consist predominantly of interactions between La-Ni, Ni-Ni or/and Mg-Ni. Among them, La-Ni interaction is the major factor controlling the structural stability of the alloys. Mg substitution increases the La-Ni bonding interactions to achieve stable Mg-containing metal matrices for reversible hydrogen absorption-desorption. This is particularly obvious at high Mg composition, as the La-Ni interactions gradually increase with Mg content. The increase of La-Ni interactions coupled with the decrease of Mg-Ni and Ni-Ni interactions will relieve the hydrogen-induced amorphization and disproportionation, and subsequently enhance the cyclic stability of La_3-xMg_xNi₉ alloys at high Mg content. However, Mg substitution for La leads to a subsequent contraction in cell volume, dramatically reducing the reversible H capacity at high Mg composition such as LaMg₂Ni₉. Suitable Mg content in La-Mg-Ni systems, such as an approximately range x = 1.0–1.4 in La_3-xMg_xNi₉ alloys, is required in trade-off between hydrogen storage capacity and cycle life.     

Dissolution,diffusion, and penetration of H in the group VB metals investigated by first-principles method

The group VB metals (V, Nb, Ta) are referred to as one of the most valuable hydrogen separation membrane materials because of their advantages over Pd. To evaluate the hydrogen-permeation performance of the three metals, their structure stability, H-solubility, H-diffusivity, H-permeability, and elastic stiffness tensors have been investigated using the first-principles method. Our results reveal that the tetrahedral interstitial site (TIS) is favorable position for H-occupying, but mainly because H-solution enthalpy is the lowest. H-diffusion coefficient follows the order of V > Nb > Ta, H-permeability and H-permeation flux follow the order of Nb > V > Ta. The attractive interactions between H atoms are weak, so it is impossible to form H₂ molecules inside the metals, and all metal hydride phases present good ductility. The calculated Debye temperature is basically consistent with the experimental value. These results provide fundamental data for the further design of alloy membranes based on VB metals.     

A theoretical study of the ability of 2D monolayer Au (111) to activate gas molecules

The adsorption and activation of gas molecules are investigated substantially in solid-gas heterogeneous catalysis. Here we investigated the interaction between gas molecules and unique two-dimensional monolayer Au (111) structure using density functional theory. It is found that CO₂, H₂O, N₂ and CH₄ molecules are weakly adsorbed on the surface with the adsorption energies between ?0.150 and ?0.250 eV due to van der Waals interaction. While CO, NO, NO₂, and NH₃ molecules are adsorbed more stably with the adsorption energies between ?0.300 and ?0.470 eV. Especially, the bond length of CO is stretched by 0.038 Å and the bond angle of NO₂ is obviously enlarged by 10.460°. The activation originates from the rearrangement of molecule orbitals and the orbitals hybridization between the partial orbitals of gas molecules and Au-5d orbitals. The fundamental analyses of adsorption mechanism and electronic properties may provide guidance for the applications of two-dimensional monolayer metal catalysis.PACSnumbers 73.22.-f, 73.61.-r     

Comparison between hydrogen production via H2S and H2O splitting on transition metal-doped TiO2 (101) surfaces as potential photoelectrodes

Doping is an effective way to engineer the electronic band structure of semiconductor materials and consequently their photocatalytic activity for hydrogen generation. In this work, periodic Density Functional Theory (DFT) was employed to compare the adsorption of H₂S and H₂O molecules on TiO₂(101) anatase surfaces compared to four transition metal-doped TiO₂(101) anatase surfaces; Cr⁴⁺-TiO₂, V⁴⁺-TiO₂, Mn⁴⁺-TiO₂, and Nb⁴⁺-TiO₂. The defect formation energy, molecular adsorption energy, hydrogen splitting energies, geometrical changes, electronic structure and charge transfer characteristics were investigated to determine and compare the changes in adsorption of H₂S and H₂O on the pristine vs. doped surfaces. The defect formation energy calculations revealed the Nb⁴⁺-TiO₂ surface resulted in the highest stability, smallest change in neighboring bond lengths and the highest dopant to surface charge transfer. However, upon H₂S and H₂O adsorption, the calculations concluded that the V⁴⁺-TiO₂ surface resulted in the most stable structure for adsorbed H₂S and lowest hydrogen splitting energy requiment compared to the other dopant metals and the lowest for H₂S vs H₂O, indicating its potential catalytic activity for facile dehydrogenation for industrial applications.     

Micro-mechanism study on the effect of O2 poisoned ZrCo surface on hydrogen absorption performance

Hydrogen storage alloys are usually susceptible to poisoning by O₂, CO, CO₂, etc., which decreases the hydrogen storage property sharply. In this paper, the adsorption characteristics of oxygen on the ZrCo(110) surface were investigated, and the effect of oxygen occupying an active site on the surface on the hydrogen adsorption behavior was discussed. The results show that the dissociation barrier of H₂ is increased by more than 26% after O occupies the active sites on the ZrCo(110) surface, and the probability of H₂ adsorption and dissociation decreases significantly. The adsorption energy of H atoms on the O–ZrCo(110) surface decreased by 18–56%, and the adsorption stability of H decreased. In addition, H atom diffusion on the surface and into bulk are prevented with higher reaction energetic barriers by O occupying active sites. Eventually, the ability of the ZrCo surface to adsorb hydrogen is seriously reduced.     

O-assisted and pristine Au-Pt(100) surfaces: A platform for adsorption and decomposition of H2O

Adsorption and decomposition of water (H₂O) over pristine and oxygen (O) assisted Au-Pt(100) surfaces were systematically explored using ab initio calculations based on density functional theory (DFT). To consider the long-range interaction, semi-empirical dispersion correction (D2) was included in all calculations. The most preferable adsorption sites for H₂O and its fragments such as OH, O, and H, over clean and O-assisted Au-Pt(100) surfaces were determined by examining adsorption energy with different configurations. Our calculations showed that the H₂O prefers top site over Au atom while OH, O, and H prefer to be adsorbed over bridge position. In present study, we determine the best possible co-adsorption sets for considered adsorbates. We further investigated the transition states, dehydrogenation process, and activation energies for extracting H from adsorbed H₂O over both pristine and O-aided Au–Pt(100) surfaces. It was found that the O promotes H₂O dissociation significantly by diminishing the barrier energy. The decisive role played by O in decomposing H₂O molecule is revealed in this work. Our study will further assist experimentalists in designing and synthesizing novel catalysts for dehydrogenation of H₂O in hydrogen production.     

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.     

Experimental study on the interactions of supercritical CO2 and H2O with anthracite

To better understand the mechanism of interactions of supercritical CO₂ (SCCO₂) and H₂O, untreated, SCCO₂ treated and SCCO₂-H₂O treated anthracites were adopted to analyze changes in pore structure, adsorption capacity. Observations from experimental data reveal that mineral and other substances in the fractures and matrix of coal body are dissolved and mobilized by the carbonic acid formed from the mixture of SCCO₂ and H₂O, which may contribute to smaller pore development and enhance the adsorption capacity. These findings may provide new insights into effective and safe storage of CO₂ in coal reservoir.     

Strain effect on structural and dehydrogenation properties of MgH2 hydride from first-principles calculations

The structure, stability, dehydrogenation thermodynamic and kinetic properties of MgH₂ hydride under different biaxial strain conditions were investigated by using first-principles calculations based on the density functional theory (DFT). The results show that either biaxial tensile or compressive strain is likely to cause the structural deformation of MgH₂ crystal, and its lattice distortion becomes severe with increasing magnitude of strain. Due to the contribution of strain energy, the biaxial strain not only weakens the structural stability of MgH₂, but also lowers its hydrogen desorption enthalpy and dehydrogenation temperature. Furthermore, the diffusion activation energy of hydrogen atom in MgH₂ host is also decreased, which results in a remarkable improvement of dehydrogenation properties. Noticeably, the effect of tensile strain in improving dehydrogenation thermodynamics is relatively superior to that of compressive one, while the role of the latter in enhancing dehydrogenation kinetics is relatively stronger than that of the former. Further analysis of electronic structures suggests the strain-induced changes in structural and dehydrogenation properties of MgH₂ are closely associated with the value of total densities of states at the Fermi level as well as the bonding electrons number below Fermi level. These results provide an insight for developing better MgH₂-based nanocomposite hydrogen storage materials by introducing suitable interface misfit strain.     

Density functional and bonding study of hydrogen and platinum adsorption on B2-FeTi (111) slab

We performed a density functional theory study (DFT) of hydrogen interaction with Pt on the B2-FeTi (111) surface. The DFT is employed to trace the relevant orbital interactions and to discuss the electronic changes of incorporating H on the Fe–Ti bonding. We determined the optimal location for Pt and then for adsorbed hydrogen. We also followed the density of states and the changes in the chemical bonding for both adsorbed atoms on the surface. The overlap population analysis reveals metal–metal bond breaking after hydrogen adsorption, being this bond the more affected.     

A safe and clean way to produce H2O2 from H2 and O2 within the explosion limit range

The direct synthesis of hydrogen peroxide (DSH) from hydrogen and oxygen is an attractive production route due to its green nature. However, it faces multiple technical challenges, the biggest being the explosion risk of the flammable gas mixture. Herein we have used microreactors to perform the reaction in an inherently safer way which allows the hydrogen concentration to fall within the explosion limit range. For the first time, we have studied the flame propagation phenomena inside a microreactor to determine the optimum channel dimension for DSH. A mechanism of “fast synthesis and slow destruction” has been proposed via investigation on the influence of channel length and liquid flow rate. Besides, a variety of reaction parameters including gas flow rate, oxygen: hydrogen ratio, catalyst composition and gas pressure have been studied carefully. The successful employment of a microreactor in this case has indicated the potential of using microreactors to inhibit the explosion risks of hazardous processes.