Theoretical investigation of adsorption and dissociation of H2 on (ZrO2)n (n=1-6) clusters

The structure, vibration, and electronic structure of H₂ molecule adsorbed on (ZrO₂)_n (n = 1-6) clusters were investigated with density functional theory. We found that H₂ is easily absorbed on the top Zr atoms of (ZrO₂)_n (n = 1-6) clusters. The Zr₅O₁₀H₂ cluster has the lowest binding energies in the Zr_nO_2nH₂ (n = 1-6) clusters. By analyzing vibrational frequency and Mulliken charge, the H-O and Zr-H bonds were found to be formed in different sized Zr_nO_2nH₂ clusters. The dissociation mechanism of H₂ shows that the charge transfers from (ZrO₂)_n cluster to H₂ due to the important role of the orbital hybridization between the cluster and H₂ molecule. With increasing the number of H₂ molecule adsorbed on (ZrO₂)_n clusters, the adsorption favors to the sites with low coordinate number, and these adsorption modes present a symmetrical tendency.     

Photocatalytic hydrogen production over CuGa2−xFexO4 spinel

Recovery of hydrogen from industrial H₂S waste using spinel photocatalyst was studied. Spinel metal oxide photocatalysts (CuGa_2−xFe_xO₄ for x = 0.8, 0.6 and 0.4) were synthesized by ceramic route. They were loaded with 0.5 and 1 wt% noble metal oxide, RuO_2. Their XRD pattern revealed a single phase cubic spinel crystalline structure for all the catalysts. SEM displayed small size cubic particles with the particle size decreasing with the decrease in iron content. 1 wt% RuO₂ loaded CuGa_1.6Fe_0.4O₄ decomposed H₂S in aqueous 0.5 M KOH solution under visible light (λ ≥ 420 nm) irradiation and generated H₂ to the tune of 10,045 μmol/h, giving rise to a high quantum efficiency of 21% at 510 nm.     

A novel lithium decorated N-doped 4,6,8-biphenylene for reversible hydrogen storage: Insights from density functional theory

Two-dimensional (2D) carbon-based (C-based) and carbon-nitrogen (C–N) materials have great potential in the energy harvest and storage fields. We investigate a novel carbon biphenylene (C468) consisting of four-, six- and eight-membered rings of sp² carbon atoms (Fan et al., Science, 372:852-6 (2021)) for hydrogen storage. Using first-principles based Density functional theory calculations, we study the geometrical and electronic properties of C468 and N-doped C468. Lithium (Li) atoms were symmetrically adsorbed on both sides of the substrate, and their adsorption positions were determined. The maximum gravimetric density of hydrogen (H₂) adsorbed symmetrically on both sides of Li atom was studied within the scope of physical adsorption process (−0.2 eV/H₂ ∼ −0.6 eV/H₂). Li-decorated C468 can adsorb 8 upper hydrogen molecules and 8 lower hydrogen molecules, and Li-decorated N-doped C468 can adsorb 9 upper hydrogen molecules and 9 lower hydrogen molecules. The gravimetric densities of Li-decorated C468 and Li-decorated N-doped C468 can reach 9.581 wt% and 10.588 wt%, respectively. Our findings suggest significant insights for using Li-decorated C468 and Li-decorated N-doped C468 as hydrogen storage candidates and effectively expand the application scope of C-based materials and C–N materials.     

Effect of Mg content on structure,hydrogen storage properties and thermal stability of melt-spun Mgx(LaNi3)100−x alloys

Amorphous Mg_x(LaNi₃)_100−x (x = 40, 50, 60, 70) alloys with ribbon shape (5 mm wide, 0.2 mm thick) have been prepared by rapid solidification, using a melt-spinning technique. Their microstructure, hydrogen storage properties and thermal stability were studied by means of XRD, SEM, PCTPro2000 and DSC analysis, respectively. The results indicated that when Mg_x(LaNi₃)_100−x alloys have been hydrogenated at 573 K under 2 MPa hydrogen pressure, LaH₃ phase is formed in the case of x (x = 40, 50, 60, 70), Mg₂NiH₄ phase formed in the case of x (x = 40, 50, 60, 70), Mg₂NiH_0.3 phase formed in the case of x (x = 40, 50), and MgH₂ phase formed in the case of x = 70. Experimental data of hydrogen desorption kinetics, tested at 523 K, 573 K and 623 K, are in good agreement with Avrami–Erofeev equation. The maximum hydrogen absorption capacity is 2.71 wt.% for Mg₇₀(LaNi₃)₃₀ and 2.35 wt.% for Mg₇₀(LaNi₃)₃₀, the increase of hydrogen desorption capacity is in the order of x = 70 > x = 60 > x = 50 > x = 40. Based on DSC analysis, the activation energies for dehydrogenation of these samples are calculated to be 122 ± 2 kJ/mol (x = 40) > 101 ± 3 kJ/mol (x = 50) > 84 ± 5 kJ/mol (x = 60) > 64 ± 3 kJ/mol (x = 70), which are in agreement with the results of hydrogen desorption kinetics.     

Enhancement of hydrogen-storage performance of MgH2 by Mg2Ni formation and hydride-forming Ti addition

MgH₂, rather than Mg, was used as a starting material in this work. A sample with a composition of MgH₂–10Ni–4Ti was prepared by reactive mechanical grinding. Activation of the sample was completed after the first hydriding cycle. At n = 1, the sample desorbed 2.53 wt% H for 10 min, 3.99 wt% H for 20 min, 4.58 wt% H for 30 min, and 4.68 wt% H for 60 min at 593 K under 1.0 bar H₂. At n = 2, the sample absorbed 3.59 wt% H for 5 min, 4.55 wt% H for 25 min, and 4.60 wt% H for 45 min at 593 K under 12 bar H₂. The inverse dependence of the hydriding rate on the temperature in the initial stage and the normal dependence of the hydriding rate on the temperature in the later stage were discussed. The rate-controlling step for the dehydriding reaction of activated MgH₂–10Ni–4Ti was analyzed as the chemical reaction at the hydride/α-solid solution interface.     

Stabilized Li-decoration and enhanced hydrogen storage on reduced graphene oxides

Hydrogen storage properties of Li-decorated graphene oxides containing epoxy and hydroxyl groups are studied by using density functional theory. The Li atoms form Li₄O/Li₃OH clusters and are anchored strongly on the graphene surface with binding energies of −3.20 and −2.84 eV. The clusters transfer electrons to the graphene substrate, and the Li atoms exist as Li⁺ cations with strong adsorption ability for H₂ molecules. Each Li atom can adsorb at least 2H₂ molecules with adsorption energies greater than −0.20 eV/H₂. The hydrogen storage properties of Li-decorated graphene at different oxidation degrees are studied. The computations show that the adsorption energy of H₂ is −0.22 eV/H₂ and the hydrogen storage capacity is 6.04 wt% at the oxidation ratio O/C = 1/16. When the O/C ratio is 1:8, the storage capacity reaches 10.26 wt% and the adsorption energy is −0.15 eV/H₂. These results suggest that reversible hydrogen storage with high recycling capacities at ambient temperature can be realized through light-metal decoration on reduced graphene oxides.     

Photo-catalytic hydrogen production over Fe2O3 based catalysts

The hydrogen photo-evolution was successfully achieved in aqueous (Fe_1−xCr_x)₂O₃ suspensions (0 ≤ x ≤ 1). The solid solution has been prepared by incipient wetness impregnation and characterized by X-ray diffraction, BET, transport properties and photo-electrochemistry. The oxides crystallize in the corundum structure, they exhibit n-type conductivity with activation energy of ∼0.1 eV and the conduction occurs via adiabatic polaron hops. The characterization of the band edges has been studied by the Mott Schottky plots. The onset potential of the photo-current is ∼0.2 V cathodic with respect to the flat band potential, implying a small existence of surface states within the gap region. The absorption of visible light promotes electrons into (Fe_1−xCr_x)₂O₃-CB with a potential (∼−0.5 V_SCE) sufficient to reduce water into hydrogen. As expected, the quantum yield increases with decreasing the electro affinity through the substitution of iron by the more electropositive chromium which increases the band bending at the interface and favours the charge separation. The generated photo-voltage was sufficient to promote simultaneously H₂O reduction and SO₃²⁻ oxidation in the energetically downhill reaction (H₂O + SO₃²⁻ → H₂ + SO₄²⁻, ΔG = −17.68 kJ mol⁻¹). The best activity occurs over Fe_1.2Cr_0.8O₃ in SO₃²⁻ (0.1 M) solution with H₂ liberation rate of 21.7 μmol g⁻¹ min⁻¹ and a quantum yield 0.06% under polychromatic light. Over time, a pronounced deceleration occurs, due to the competitive reduction of the end product S₂O₆²⁻.     

Theoretical investigation of molecular hydrogen adsorption and dissociation on AlnV(n = 1–13) clusters

We present density functional calculations of H₂ adsorption and dissociation on small-sized Al_nV clusters for n = 1–13. The growth pattern for Al_nV (n = 2–4, 8, 10–12) clusters is V atom occupying a peripheral position of Al_n clusters. And the growth pattern for Al_nV (n = 6, 9, and 13) clusters is V-substituted Al_n+1 clusters. It is found that the V atom substituted the surface atom of Al_n+1 cluster and occupies a peripheral position. H₂ is easily physically absorbed on the top V atom of Al_nV (n = 1–13) clusters with a side-on orientation rather than an end-on orientation because of the more effective orbital overlap in the side-on orientation. The reaction of Al_nV with H₂ would produce Al_nVH₂ because of large exothermic energy changes and relatively small activation energies especially for AlV and Al₇V, which might serve as highly efficient and low-cost catalysts for hydrogen dissociation.     

Doping the transition metal atom Fe,Co, Ni into C48B12 fullerene for enhancing H2 capture: A theoretical study

The feasibility of transition metal coated fullerene cages M₁₂C₄₈B₁₂(M = Fe, Co, and Ni) for hydrogen storage is investigated by the pseudopotential density functional theory. Fe₁₂C₄₈B₁₂(Co₁₂C₄₈B₁₂ and Ni₁₂C₄₈B₁₂) adsorbs 60(48 and 48) H₂ with moderate average adsorption energy of 0.50(0.45 and 0.32) eV/H₂. The gravimetric hydrogen density of Fe₁₂C₄₈B₁₂(Co₁₂C₄₈B₁₂ and Ni₁₂C₄₈B₁₂) can reach 8.7(6.8 and 6.8) wt%. The Dewar–Kubas interaction dominates the adsorption of H₂ on the outer surface of Fe₁₂C₄₈B₁₂(Co₁₂C₄₈B₁₂ and Ni₁₂C₄₈B₁₂). Therefore, the stable M₁₂C₄₈B₁₂(M = Fe, Co, and Ni) cages can be applied as candidates for hydrogen storage under near-ambient conditions.     

Improvement of hydrogen-storage properties of MgH2 by Ni,LiBH4, and Ti addition

In this work, differently from our previous work, MgH₂ instead of Mg was used as a starting material. Ni, Ti, and LiBH₄ with a high hydrogen-storage capacity of 18.4 wt% were added. A sample with a composition of MgH₂–10Ni–2LiBH₄–2Ti was prepared by reactive mechanical grinding. MgH₂–10Ni–2LiBH₄–2Ti after reactive mechanical grinding contained MgH₂, Mg, Ni, TiH_1.924, and MgO phases. The activation of MgH₂–10Ni–2LiBH₄–2Ti for hydriding and dehydriding reactions was not required. At the number of cycles, n = 2, MgH₂–10Ni–2LiBH₄–2Ti absorbed 4.09 wt% H for 5 min, 4.25 wt% H for 10 min, and 4.44 wt% H for 60 min at 573 K under 12 bar H₂. At n = 1, MgH₂–10Ni–2LiBH₄–2Ti desorbed 0.13 wt% H for 10 min, 0.54 wt% H for 20 min, 1.07 wt% H for 30 min, and 1.97 wt% H for 60 min at 573 K under 1.0 bar H₂. The PCT (Pressure–Composition–Temperature) curve at 593 K for MgH₂–10Ni–2LiBH₄–2Ti showed that its hydrogen-storage capacity was 5.10 wt%. The inverse dependence of the hydriding rate on temperature is partly due to a decrease in the pressure differential between the applied hydrogen pressure and the equilibrium plateau pressure with the increase in temperature. The rate-controlling step for the dehydriding reaction of the MgH₂–10Ni–2LiBH₄–2Ti at n = 1 was analyzed.     

Li-decorated graphyne as high-capacity hydrogen storage media: First-principles plane wave calculations

Taking into account the van der Waals correction, the characteristics of the Li-decorated graphyne as the hydrogen storage medium have been explored using first-principles plane wave calculations. We find that Li atom can be adsorbed not only over the center of large hexagon (H_L site) but also over the center of small hexagon (H_S site). For double-side Li decorations, there are 14H₂ molecules can be adsorbed on Li-decorated graphyne primitive cell with the adsorption energy of 0.19 eV/H₂. As a result, the hydrogen storage capacity of 13.0 wt% can be obtained. This suggests that the Li-decorated graphyne system can serve as a high-capacity hydrogen storage medium.     

TiO2 nanotubes coupled with nano-Cu(OH)2 for highly efficient photocatalytic hydrogen production

Noble-metal-free Cu(OH)₂/TNTs (TNTs: TiO₂ nanotubes) nanocomposite photocatalysts were successfully prepared by loading nano-Cu(OH)₂ on TNTs via a hydrothermal-precipitation process. These were then characterized in terms of morphology and physicochemical properties by employing TEM, XRD, XPS, BET, UV–Vis DRS and PL. The effects of Cu(OH)₂ loading, amount of catalyst on the photocatalytic hydrogen production performance of Cu(OH)₂/TNTs were investigated in detail in aqueous methanol solution under UV irradiation. The results show that, compared with pure TNTs, the TNTs loaded with highly dispersed 8 wt% Cu(OH)₂ exhibited remarkably improved activity for hydrogen production (the largest quantity of evolved hydrogen was ca. 14.94 mmol h⁻¹ g⁻¹ catalyst) with good photostability. This high activity is attributed to the strong synergistic function of Cu(OH)₂/TNTs, including suitable potential of Cu(OH)₂/Cu (E⁰ = −0.222 V) between conduction band (−0.260 V) of TNTs and the reduction potential of H⁺/H₂ (E⁰ = 0.000 V), a unique tubular microstructure of TNTs coated with nano-Cu(OH)₂, large BET specific surface area and high dispersion of Cu(OH)₂. Furthermore, a process mechanism for methanol/water decomposition over Cu(OH)₂/TNTs is proposed to understand its high activity.     

Hydrocracking of Jatropha oil over Ni-H3PW12O40/nano-hydroxyapatite catalyst

The Ni-H₃PW₁₂O₄₀/nano-hydroxyapatite catalyst with H₃PW₁₂O₄₀ (HPW) loading was prepared by impregnation method and performed through hydrocracking of Jatropha oil in a fixed-bed reactor. The catalyst was characterized by N₂ adsorption–desorption, powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption of ammonia (NH₃-TPD), thermogravimetric analysis (TGA). The conversion of Jatropha oil over Ni-HPW (30 wt%)/nHA was 100%, the liquid yield of liquid product was 83.4%, the ratio of i/n-paraffins was 1.64 at 360 °C, 3 MPa, H₂/oil (v/v) = 600 and LHSV = 2 h⁻¹. The pour point of final product oil was −28 °C and the catalyst was used without sulfurization.     

Preparation and characterization of NbF5-added Mg hydrogen storage alloy

A sample with a composition of 95 wt% Mg-5 wt% NbF₅ (named Mg-5NbF₅) was prepared by reactive mechanical grinding using Mg instead of MgH₂ as a starting material. Its hydriding and dehydriding rates were then measured under nearly constant hydrogen pressures. The activation of Mg-5NbF₅ was not required, and Mg-5NbF₅ had an effective hydrogen storage capacity, which was defined as the quantity of hydrogen absorbed for 60 min, of 5.50 wt%. At the first cycle (n = 1) at 593 K, the sample absorbed 4.37 wt% H for 5 min and 5.50 wt% H for 30 min under 12 bar H₂, and desorbed 1.03 wt% H for 5 min, 4.66 wt% H for 30 min, and 5.43 wt% H for 60 min under 1.0 bar H₂. Reactive mechanical grinding of Mg with NbF₅, which formed MgH₂, MgF₂, NbH₂, and NbF₃ by the reaction of 11 Mg + 7NbF₅ + 3H₂ → MgH₂ + 10MgF₂ + 2NbH₂ + 5NbF₃, is considered to create defects, to produce reactive clean surfaces, and to reduce the particle size of Mg. The XRD pattern of Mg-5NbF₅ dehydrided at n = 3 revealed Mg, small amounts of β-MgH₂ and MgO, and very small amounts of MgF₂ and NbH₂. An increase in the dehydriding rate of Mg-5NbF₅ was attempted by adding Ni to Mg-5NbF₅. Mg-5NbF₅ had higher initial hydriding and dehydriding (after the incubation period) rates and a larger effective hydrogen storage capacity than Mg-10NbF₅, Mg-10MnO, and Mg-10Fe₂O₃, which were reported to have quite high hydriding rate and/or dehydriding rate.     

Theoretical studies on hydrogen adsorption properties of lithium decorated diborene (B2H4Li2) and diboryne (B2H2Li2)

Using ab initio based quantum chemical calculations, we have studied the structure, stability and hydrogen adsorption properties of different boron hydrides decorated with lithium, examples of the corresponding anions being dihydrodiborate dianion, B₂H₂²⁻ and tetrahydrodiborate dianion, B₂H₄²⁻ which can be considered to be analogues and isoelectronic to acetylene (C₂H₂) and ethelene (C₂H₄) respectively. It is shown that there exists a B-B double bond in B₂H₄Li₂ and a B-B triple bond in B₂H₂Li₂. In both the complexes, the lithium sites are found to be cationic in nature and the calculated lithium ion binding energies are found to be very high. The cationic sites in these complexes are found to interact with molecular hydrogen through ion-quadrupole and ion-induced dipole interactions. In both the complexes, each lithium site is found to bind a maximum of three hydrogen molecules which corresponds to a gravimetric density of ∼23 wt% in B₂H₄Li₂ and ∼24 wt% in B₂H₂Li₂. We have also studied the hydrogen adsorption in a model one-dimensional nanowire with C₆H₄B₂Li₂ as the repeating unit and found that it can adsorb hydrogen to the extent 9.68 wt% and the adsorption energy is found to be −2.34 kcal/mol per molecular hydrogen.     

A density functional theory and grand canonical Monte Carlo simulations study the hydrogen storage on the Li-decorated net-τ

To find ideal hydrogen storage media, hydrogen storage performance of Li decorated net-τ has been investigated by first-principles calculations. Maximum 6 Li atoms are adsorbed on net-τ, with the average binding energy of 2.15 eV for per Li atom. Based on 6Li-decorated net-τ, up to twenty H₂ molecules are adsorbed, with a high H₂ storage capacity of 12.52 wt% and an appropriate adsorption energy of 0.21 eV/H₂. Finally, H₂ uptake performance is measured by GCMC simulations. Our results suggest that Li-decorated net-τ may be a promising hydrogen storage medium under realistic conditions.     

Photophysical and photocatalytic water splitting performance of stibiotantalite type-structure compounds,SbMO4 (M = Nb,Ta)

Metal oxides with ferroelectric properties are considered to be a new family of efficient photocatalysts. Here, we investigate stibiotantalite type-structure compounds, SbMO₄ (M = Nb, Ta), with layered crystal structures, and ferroelectric properties as photocatalysts for hydrogen generation from the splitting of pure water. Both compounds were prepared by a conventional solid-state reaction method, and their optical properties, electronic band structure, and photocatalytic water splitting performance were characterized and evaluated. Diffuse reflectance analysis showed that both compounds have moderate band gaps of 3.7 eV for SbTaO₄ and 3.1 eV for SbNbO₄ (cf. 3.0 eV for TiO₂). Mott–Schottky analysis reveals that their conduction-band edge potentials are higher than the water reduction (hydrogen evolution) potential (0 V vs. RHE), indicating both compounds can generate hydrogen from water splitting. The photocatalytic water splitting performance was conducted by using pure water and UV-light irradiation, and photocatalytic H₂ production was confirmed for both compounds. After loading RuO₂ cocatalyst, the rates of hydrogen evolution of SbNbO₄ and SbTaO₄ were 24 μmol/g h and 58 μmol/g h, respectively. It was concluded that both compounds can be used as photocatalysts for water splitting under UV irradiation. The photocatalytic activity difference in both compounds was discussed with regard to electronic band structure and dipole moment difference, resulting from their crystal structures.     

A reversible hydrogen storage material of Li-decorated two-dimensional (2D) C4N monolayer: First principles calculations

Two-dimensional (2D) materials can be regarded as potential hydrogen storage candidates because of their splendid chemical stability and high specific surface area. Recently, a new dumbbell-like carbon nitride (C₄N) monolayer with orbital hybridization of sp³ is reported. Motivated from the above exploration, the hydrogen adsorption properties of Li-decorated C₄N monolayer are comprehensively investigated via first principles calculations based on the density functional theory (DFT). It is found that the Dirac points and Dirac cones exists in the Brillouin zone (BZ) from the calculated electronic structure and indicates the C₄N can be used as an excellent topological material. Also, the calculated phonon spectra demonstrate that the C₄N monolayer owns a strong stability. Moreover, the calculated binding energy of decorated Li atom is bigger than its cohesive energy and results in Li atoms disperse over the surface of C₄N monolayer uniformly without clustering. In addition, the Li₈C₄N complex can capture up to 24H₂ molecules with an optimal hydrogen adsorption energy of −0.281 eV/H₂ and achieves the hydrogen storage density of 8.0 wt%. The ab initio molecular dynamics (AIMD) simulations suggest that the H₂ molecules can be desorbed quickly at 300 K. This study reveals that Li-decorated C₄N monolayer can be served as a promising hydrogen storage material.     

Enhanced ethanol steam reforming by CO2 absorption using CaO,CaO*MgO or Na2ZrO3

This paper presents results of thermodynamic analysis and experimental evaluation of hydrogen production by steam reforming of ethanol (SRE) combined with CO₂ absorption using a mixture of a solid absorbent (CaO, CaO*MgO and Na₂ZrO₃) and a Ni/Al₂O₃ catalyst. Thermodynamic analysis results indicate that a maximum of 69.5% H₂ (dry basis) is feasible at 1 atm, H₂O/C₂H₅OH = 6 (molar ratio) and T = 600 °C. whereas, the addition of a CO₂ absorbent at 1 atm, T = 600 °C and H₂O/C₂H₅OH/Absorbent = 6:1:2.5, produced a H₂ concentration of 96.6, 94.1, and 92.2% using CaO, CaO*MgO, and Na₂ZrO₃, respectively. SRE experimental evaluation achieved a maximum of 60% H₂. While combining SRE and a CO₂ absorbent exhibited a concentration of 96, 94, and 90% employing CaO, CaO*MgO, and Na₂ZrO₃, respectively at 1 atm, T = 600 °C, SV = 414 h⁻¹ and H₂O/C₂H₅OH/absorbent = 6:1:2.5 (molar ratio).     

New fan blade-like core-shell Sb2TixSy photocatalytic nanorod for hydrogen production from methanol/water photolysis

New photocatalysts of Sb₂Ti_xS_y (x = 0, 0.5, 1.0, 1.5 mol and y = 3, 4, 5, 6 mol) fan blade-like core-shell nanorods have been designed ultimately to enhance hydrogen production. The nanorods of 500 nm long and 60–100 nm wide are Sb₂S₃ nanorod surrounded by an amorphous TiS₂ membrane, showing absorption band edges of above 600 nm. The evolution of H₂ from methanol/water (1:1) photo-splitting over Sb₂Ti_xS_y nanorods in the liquid system is doubled, compared to that over pure Sb₂S₃. Particularly, 52 μmol of H₂ gas is produced after 10 h when 0.5 g of Sb₂Ti_1.0S₅ nanorods is used at pH = 7, and the performance is increased by more than 50% at higher pH. Based on cyclic voltammetry (CV) and UV-Visible absorption spectra, the high photocatalytic activity can be attributed to the existence of an appropriate band-gap state, which includes the scope of the redox potential of water in Sb₂Ti_1.0S₅ nanorods, resulting in the promotion of the redox reaction of water.