585 divacancy-defective germanene as a hydrogen separation membrane: A DFT study

As sustainable and clean energy, hydrogen is the most attractive and promising energy source in the future. Membrane separation is attractive due to its high hydrogen separation performance and low energy consumption. Van-der-Waals-corrected density functional theory (DFT) calculations are performed to investigate the hydrogen separation performance of 585 divacancy-defective germanene (585 germanene). It is found that the 585 germanene presents a surmountable energy barrier (0.34 eV) for hydrogen molecule passing through the membrane, and that membrane exhibits extremely high selectivity for H₂ molecules over CO, CO₂, N₂, CH₄ and H₂S molecules in a wide range of temperatures. Meanwhile, the hydrogen permeance of 585 germanene can reach 1.94 × 10^?7 mol s^?1 m^?2 Pa^?1 at the low limit temperature of methane reforming (at 450 K), which is higher than the industrially acceptable gas permeance. With high selectivity and permeance, the 585 germanene is a promising candidate for hydrogen separation.     

Highly selective 3D porous graphene membrane for organic gas separation derived from polyphenylene

In this paper, a 3D nanoporous carbon molecular sieve (CMS) membrane is proposed to investigate the diffusion and separation properties of ethylene/methane and ethylene/acetylene binary mixtures permeating through the structural deformated carbon nanotube (CNT) channels. Combining the results obtained from density functional theory (DFT) calculations and molecular dynamics (MD) simulations, we find that the organic gas permeability and selectivity can be effectively ameliorated by fine-tuning the geometric structure of CNTs gas separation channels. By virtue of the intrinsic structural characteristics, this hybrid CMS configuration established elliptical cylinder channels to separate the organic gas molecules with similar molecular size. Compared with channels with a circular cross section, the gas selectivity for channels with an elliptical cross section is larger, and it increases with an increasing pressure. The selectivity of ethylene over acetylene (methane) increased to ~13.8 (5.5) in deformed CNTs channels, which is more than doubled over the original CNT channels. This distinguished hybridization configuration may pave a promising avenue to utilize gas separation materials.     

Fullerene-impregnated IRMOFs for balanced gravimetric and volumetric H2 densities: A combined DFT and GCMC simulations study

This paper aimed to study the effects of fullerene (C₆₀) impregnation on the isoreticular metal-organic framework (IRMOF) materials MOF-650 (ZnO₄ nodes were connected to azulenedicarboxylate linkers), MOF-5(ZnO₄ nodes were connected to benzenedicarboxylate linkers), and IRMOF-10 (ZnO₄ nodes were connected to biphenyldicarboxylate linkers) for H₂ storage, these IRMOFs had similar structures but different pore volumes and organic linkers. Density functional theory (DFT) and grand canonical monte Carlo (GCMC) calculations indicated that C₆₀ plays an important role in balancing the gravimetric and volumetric H₂ densities of the IRMOFs. The C₆₀@IRMOFs revealed improved volumetric density when H₂ was undersaturated but reduced gravimetric density under H₂ saturation. The saturated gravimetric H₂ density of the IRMOFs was decided by the free volume. At 77 K, C₆₀@MOF-650 had a gravimetric H₂ density of 5.3 wt% and volumetric H₂ density of 42 g/L under 10 bar, and C₆₀@IRMOF-10 had a gravimetric H₂ density of 7.4 wt% and volumetric H₂ density of 43 g/L under 18 bar. These values nearly meet the United States Department of Energy (DOE) gravimetric and volumetric H₂ density ultimate targets (gravimetric H₂ density, 6.5 wt%; volumetric H₂ density, 50  g/L) under ambient pressures. Among the studied IRMOFs, C₆₀@MOF-650 and C₆₀@IRMOF-10 demonstrated the best H₂ storage properties at 233 and 298 K.     

Computational study of CO2 methanation over two-dimensional molybdenum carbide catalysts

Two-dimensional molybdenum carbide (2D-Mo₂C) is thought to be promising for catalytic hydrogenation of CO₂ to CH₄, but little is known about its catalytic reaction mechanism. In this work, we investigate the hydrogenation of CO₂ to CH₄ on 2D-Mo₂C using density functional theory. Our calculations show that Mo on the surface can efficiently decompose CO₂ to CO and O, and also H₂ to H. The hydrogenation of CO produces CHO that is readily deoxygenated to CH, and CH is selectively hydrogenated to produce CH₄. Interestingly, the embedded Ir₁ on 2D-Mo₂C can act as a single-atom promoter to improve the performance of CO₂ methanation, while on the other hand maintaining its high selectivity for CH₄. This work provides insight into the mechanism of 2D-Mo₂C-catalyzed CO₂ methanation reactions and suggests a strategy to improve the performance of such catalysts through single-atom promoters.     

Catalytic mechanisms of TiH2 thin layer on dehydrogenation behavior of fluorite-type MgH2: A first principles study

DFT calculations were carried out to investigate hydrogen release and diffusion behaviors. Results demonstrated that MgH₂/TiH₂ interface is thermodynamically stable with negative adhesion energy of −1.33 J/m² with respect to the individual MgH₂ and TiH₂ slabs. The formation of MgH₂/TiH₂ interface alters the interstice structure and space of the interstitial sites where H atoms located and then significantly lowers the dehydrogenation energy of hydrogen releasing from both the MgH₂ and TiH₂ slabs nearby the interface comparing the bulk MgH₂ and TiH_2. The smallest dehydrogenation energy of 0.06 eV/H could be reached when H releases from MgH₂ side. The study also illustrates that the existence of the MgH₂/TiH₂ interface promotes the diffusion of hydrogen vacancy. The lowest diffusion barrier of hydrogen vacancy in the MgH₂ slab (from the sublayer to the frontier layer to the interface) is estimated as 0.21 eV. Based on the present study, one can deduce that the dehydrogenation of the MgH₂/TiH₂ system will start by H releasing from MgH₂ slab, which generates H vacancies near the interface, then the interior H of MgH₂ migrates to the H vacancies (diffuse of H vacancies in the opposite direction) and releases. The TiH₂ acts as a catalyst promoting the generation and diffusion of H vacancies in MgH₂. Therefore synthesizing of MgH₂/TiH₂/MgH₂ sandwich structure could be an effective approach to promote the dehydrogenation process of MgH₂, and an ideal structure owning geometric hydrogen capacity of 6.45 wt%.     

Synthesis,structural characterization and thermal decomposition studies of (N2H5)2B12H12 and its solvates

A series of salts of the B₁₂H₁₂²⁻ anion has been prepared: a solvent-free (N₂H₅)₂B₁₂H₁₂, its solvates – (N₂H₅)₂B₁₂H₁₂·H₂O, (N₂H₅)₂B₁₂H₁₂·2(CH₃CN), (N₂H₅)₂B₁₂H₁₂·(CH₃OH), and the salt of a protonated azine – [(CH₃)₂CNNHC(CH₃)₂]₂B₁₂H₁₂. These compounds have been synthesized from the commercially available precursors via one- or two-step procedures and fully identified on the basis of single-crystal and powder X-ray diffraction. At room temperature (N₂H₅)₂B₁₂H₁₂ crystallizes in C2/c space group, with a = 18.480(5) Å, b = 6.5344(19) Å, c = 13.106(4) Å and β = 131.911(16)^o, V = 1177.8(7) ų, Z = 4. While this compound nominally contains ca. 10.7 wt% of hydrogen, it thermally decomposes above 200 °C releasing mainly N₂ and NH₃, with H₂ being only the minor gaseous product. Contrary to the recently reported case of hydrazinates of borohydrides, doping with 5 mol% of FeCl₃ does not increase the relative amount of hydrogen significantly, however, it alters the ratio of N₂ and NH₃.     

Influence of intermediate layers in tubular carbon membrane for gas separation performance

The main objective of this study is to determine the best intermediate layer for tubular carbon membranes for H₂ and He separation. Intermediate layer was applied to strengthen interfacial adhesion between selective carbon layers and tubular support. Three different intermediate layers (alumina powder, carbon molecular sieve (CMS), and carbon pencil) had been evaluated to compare their influence towards the performance of gas separation of the carbon membrane. Tubular carbon membrane was fabricated from PI/NCC-based polymer blends which had been carbonized under Argon atmosphere at 800 °C with a heating rate of 3 °C/min. Based on the scanning electron microscopic (SEM) observations, carbon membrane with alumina powder as an intermediate layer had formed a smoother surface compared to other types of intermediate layers. A high performance of tubular carbon membrane was obtained by employing alumina powder as an intermediate layer, which exhibited the best selectivity of H₂/N₂ and He/N₂ of 447.31 ± 1.45 and 471.72 ± 2.19, respectively.     

Metal (oxide) modified (M= Pd,Ag, Au and Cu) H2SrTa2O7 for photocatalytic CO2 reduction with H2O: The effect of cocatalysts on promoting activity toward CO and H2 evolution

Ag, Pd, Au, Cu₂O as cocatalysts were loaded on the layered H₂SrTa₂O₇ (HST) for photocatalytic CO₂ reduction with H₂O. The characterization revealed that cocatalysts loaded on the surface of HST can effectively promote the separation of photogenerated electrons and holes due to the formation of Schottky barrier or p-n junction, thus enhancing photocatalytic activity. Of note, Ag, Pd, Au, Cu₂O loading exhibited obviously different performance on promoting photocatalytic activity of HST toward CO evolution and H₂ evolution because of the different overpotentials of CO evolution and H₂ evolution on loaded photocatalysts. Cocatalysts with low overpotentials of CO or H₂ evolution act as active sites for CO or H₂ evolution, thus controlling the selectivity toward CO or H₂. The Au/HST exhibited high activity for only H₂ evolution (17.5 μmol g⁻¹ h⁻¹) due to relative low overpotential for H₂ evolution (0.67 V) while the Cu₂O/HST exhibited high activity only for CO evolution (0.23 μmol g⁻¹ h⁻¹) due to relative low overpotential for CO evolution (0.40 V). The Pd/HST sample exhibits high photocatalytic activity for both CO and H₂ evolution rates due to the low overpotential for CO and H₂ evolution, reaching 4.0 and 4.7 times of bare HST, respectively. This work here gives an in-depth understanding of the effect of cocatalysts on promoting photocatalytic activity and selectivity and can also give guidance to design photocatalysts with high activity and selectivity for photocatalytic CO₂ reduction with H₂O.     

First-principles study of LiBH4 nanoclusters interaction with models of porous carbon and silica scaffolds

Density functional theory calculations of an interaction of LiBH₄ represented by n = 2−6 and 12 formula units nanoclusters with models of activated carbon and porous silica show that on both non-defective substrates only physisorption is observed for all cluster sizes. The binding energies are low, reaching up to −43 kJ/mol for smallest clusters. The charge transfer between LiBH₄ and the support is not observed. On defective graphene (LiBH₄)₂ may adsorbed dissociatively. Hydrogens detached from BH₄ groups saturates under-coordinated C atoms while the binding between BH₃ moiety and underlying C atoms restores sp³-hybridization in the BH₄ group. The dissociative adsorption of LiBH₄ clusters leads to the retrieval of the three-fold coordination of the C atoms, the subsequent (LiBH₄)₂ physisorps with the differential heat of adsorption not exceeding −46 kJ/mol. The present calculations indicate that chemical interaction between matrix and lithium borohydride, leading to a destabilization of LiBH₄, takes place until substrate's defects remain unsaturated.     

Promotion of H2 adsorption performance on InN monolayer by embedding Cu atom: A first-principles study

Hydrogen energy as a clean energy has great application potential, and finding efficient hydrogen storage materials has become the current research hotspot. This work studied the structure, electronic properties, thermodynamic properties and H₂ adsorption performance of InN, N-defect (V_N–InN), In-defect (V_In–InN), Cu atom substitutes N atom embedded InN (Cu/V_N–InN) and Cu atom substitutes In atom embedded InN (Cu/V_In–InN) by density functional theory (DFT). The results show that all of the five InN materials have good thermal stability at room temperature (300 K), and the structural stability of the defective InN increases after embedding of Cu atom. Meanwhile, the hydrogen interaction on the five InN materials was investigated. Cu/V_In–InN has the best performance for H₂ adsorption among the five InN materials. The adsorption energy for Cu/V_In–InN can reach ?0.769 eV, which is 4.5 times better than original InN nanosheet. After adsorbing 5H₂ molecules, the average adsorption energy is ?0.399 eV that indicates Cu/V_In–InN structure still has possibility of adsorbing more hydrogen molecules and it has the potential to become a new hydrogen storage material.     

A DFT study on production of hydrogen from biomass-derived formic acid catalyzed by Pt–TiO2

Production and storage of hydrogen from biomass component by using efficient catalysts, it can finely maintain the future energy of the world and reduce human dependence on fossil fuels. Hydrogen production mechanism via formic acid decomposition on the TiO₂ anatase (101) and Pt–TiO₂ surfaces in the solvent (water) and gaseous conditions performed by density functional theory (DFT) calculation. Regarding to the proposed routes, decomposition reaction of formic acid on TiO₂ surface incline to be followed by second route in the water which is acceptable in terms of energy. Decomposition reaction of formic acid on Pt–TiO₂ surface prefers to do it via first route (rotation around CO bond of formic acid) in solvent conditions. Furthermore, adsorption energy and geometric changes of formic acid on TiO₂ anatase (101) and Pt–TiO₂ surface in gaseous and solvent conditions were clearly studied.     

Strong organic acid-assistant synthesis of holey graphitic carbon nitride for efficient visible light photocatalytic H2 generation

A facile and cost-effective method was developed for the synthesis of holey N-deficient graphitic carbon nitride nanosheets (FCN) using trifluoroacetic-acid-treated urea as a precursor. The role of trifluoroacetic acid on the composition, structure and photocatalytic performance of the prepared catalysts was carefully investigated. The obtained samples displayed laminated porous morphology with nitrogen defects, larger specific surface areas, extended range of spectral response and enhanced electron mobility of charge carriers. Consequently, the optimized catalyst FCN-400 exhibited superb photocatalytic performance and excellent cycling stability for hydrogen evolution. The hydrogen evolution rate over FCN-400 reached 309.3 μmol/h under visible light irradiation, which is 11.3-fold of that of urea-derived graphitic carbon nitride (27.3 μmol/h).     

Influence of Pt decoration on the hydrogen storage performance of cup-stacked carbon nanotubes: A DFT study

The hydrogen adsorption behaviour of cup-stacked carbon nanotubes (CSCNTs) decorated with the platinum atom at four positions of the conical graphene layer (CGL) is investigated using density functional theory. The optimization shows that the inside lower edge position (IL) results have the best hydrogen adsorption parameters among the four positions. The Pt–H₂ distance is 1.54 Å, the H–H bond length (l_H-H) is 1.942 Å, and the hydrogen adsorption energy (E_ads) is 1.51 eV. The hydrogen adsorption of CSCNTs decorated by Pt at the IL position also has larger E_ads and l_H-H than the Pt-doped planar graphene, Pt-doped single-wall carbon nanotubes and Pt-doped carbon nanocones. The Pt atom at the IL position has a more significant polarization effect on the adsorbed H₂, it has trends to convert H₂ into two separate H atoms. While the hydrogen adsorption behaviour at other positions belongs to the Kubas coordination, the l_H-H and the E_ads increased not significantly.     

Selective and tunable H2 adsorption/sensing performance of W-doped graphene under external electric fields: A DFT study

W-doped graphene and its selective gas adsorption/sensing performance are studied through first-principles density functional theory (DFT) calculations. A single W atom is stably anchored into the graphene plane with a high binding energy of ?9.325 eV. The W-doped graphene interacts more strongly with H₂ compared to NH₃, CH₄, CO, SO₂ or H₂S. The H₂ adsorption system also has a higher adsorption energy of ?1.035 eV. Furthermore, the W-doped graphene exhibits the highest sensor response to H₂ with the largest number of transferred charges and the biggest change in the band gap. A negative electric field improves the interaction between the H₂ and the W-doped graphene by increasing the adsorption energy and promoting charge transfer. However, the adsorption of the H₂ is significantly weakened upon the application of a positive electric field; the adsorbed H₂ is easily desorbed from the W-doped graphene with a modulated recovery time as short as ～4.099 s at room temperature (300 K) upon a +0.4 V Å^?1 increase in the electric field. These results reveal that the W-doped graphene has promising selective and tunable H₂ adsorption/sensing performance upon the application of external electric fields.     

Sustainable H2 generation via steam reforming of biogas in membrane reactors: H2S effects on membrane performance and catalytic activity

This study proposes the steam reforming of a synthetic biogas stream containing 200 ppm of H₂S, carried out in a non-commercial supported Pd–Au/Al₂O₃ membrane reactor (7–8 μm selective layer thickness) at 823 K and 150 kPa over a non-commercial Rh(1%)/MgAl₂O₄/Al₂O₃ catalyst. This system is able to recover almost 80% of the total hydrogen produced during the reaction and shows good resistance to the H₂S contamination, as confirmed by stable methane conversions for more than 400 h under operation. For comparison, the same reaction was carried out in a commercial self-supported Pd–Ag membrane (150 μm wall thickness), yielding a hydrogen recovery equal to 40% at 623 K and 200 kPa, and presenting stable methane conversions for less than 200 h under operation due to the effect of the H₂S contamination.     

Experimental evaluation of graphene oxide/TiO2-alumina nanocomposite membranes performance for hydrogen separation

In recent years, graphene oxide membranes showed interesting performances in terms of high permeating flux and perm-selectivity in several applications of gas separation because of their inherent properties combined to a low energy consumption. In this paper, a graphene oxide layer is coated on modified TiO₂-alumina tubular substrate in order to prepare graphene oxide nanocomposite membranes useful for hydrogen separation. Nanocomposite graphene oxide membrane samples were obtained by using vacuum deep coating method, depositing the graphene oxide solution as single layers on TiO₂-alumina substrate. Temperature and pressure variations were evaluated to achieve high H₂ permeance, high H₂/CO₂ selectivity and membrane performance stability during the experimental tests. Furthermore, it was found that the temperature increase causes a perm-selectivity (H₂/N₂ and H₂/CO₂) decrease, while the transmembrane pressure increase involves a general improvement of the perm-selectivity.     

Experimental investigation of natural polysaccharide-based mixed matrix membrane modified with graphene oxide and Pd-nanoparticles for enhanced gas separation performance

In this work, we proposed a mixed matrix membrane prepared by using a glycerol modified guar gum (GGP) polymer matrix incorporated with graphene oxide (GO). The influence of varying GO concentration on the gas separation performance was investigated and 2 wt% was found to be the optimum concentration for high performance. The 2 wt% GO mixed matrix membranes were further modified with Pd nanoparticles. When GO, and Pd nanoparticles were mixed, CO₂ permeability increased by 49.94%, while the permeability of H₂ gas molecules decreased by 98.11%, respectively, compared to the pristine GGP membrane. The selectivity of CO₂/H₂ was obtained as 18.27. The glass transition temperature of the membrane increased from 85 to 95.2 °C, tensile strength and elongation of the break were significantly improved by 29.09% and 84.37% through the addition of Pd and GO into the membrane. The scanning electron microscopy revealed a dense top surface after GO nanosheets incorporation. Further, the thermogravimetric analysis proposes that the modified membrane is thermally stable than GGP. Henceforth, the study suggests GO incorporation and Pd nanoparticles modification of guar gum membrane is a promising gas separation membrane with potentially high selectivity for CO₂ gas.     

Preparation,characterization and gas permeation properties of PDMS/PEI composite asymmetric membrane for effective separation of hydrogen from H2/CH4 mixed gas

In this work, PDMS/PEI membranes were synthesized and sorption and permeation of H₂/CH₄ mixture were studied. The influence of pressure, temperature and feed composition were investigated. It was shown that permeances increased and selectivity decreased with an increment in the feed temperature. Increasing feed pressure caused a decline in gas permeance and increased selectivity. Higher concentrations of hydrogen in the feed declined the selectivity. The effect of different non-solvents was explained by their effect on precipitation time and it was concluded that water made the membrane denser while isopropanol forms a sponge-like structure. Coagulation bath temperature made the membrane denser. Film casting and dip-coating techniques were used to prepare selective membranes. Obtained selectivity results introduced dip-coating as a better method than film casting. Sequential coating improved selectivity of the prepared membrane. Finally, sequential coating with different concentrations was applied and enhanced selectivity significantly from about 22 to more than 70.     

Development of high performance amine functionalized zeolitic imidazolate framework (ZIF-8)/cellulose triacetate mixed matrix membranes for CO2/CH4 separation

High cost and complex fabrication process of inorganic membranes and lower position of pristine polymeric membranes in the Robeson upper bound curve urged the researchers to develop mixed matrix membranes (MMMs). Cellulose acetate being most commercially used polymer, dominates the market of CO₂ separation mainly because of low cost and environmental friendly resource. In the present study, MMMs consists of amine functionalized zeolitic imidazolate framework (NH₂-ZIF-8) and cellulose triacetate were fabricated for the first time. NH₂-ZIF-8 was used as a filler because the pore size of ZIF-8 is between the kinetic diameter of separating gases (CO₂ and CH₄). Moreover,  NH₂ group attached on the surface of ZIF-8 has affinity with condensable gases like CO₂. Morphology, crystallinity, tensile strength and functional groups of fabricated membranes were investigated using different analytical techniques. Results revealed that the increase of feed pressure has increased CO₂ permeability and decreased permselectivity. However, improvements in gas separation performance were observed with the addition of nanofiller. Best position in Robeson's upper bound curve at 4 bar was obtained with 10 wt% loading with CO₂ permeability and CO₂/CH₄ permselectivity of 218 barrer and 13.84, respectively. The improvement in the gas separation performance with loading is attributed to the increased diffusion coefficients as well as solubility coefficients, which was increased to 33% and 3.8%, respectively.