Sorption of hydrogen onto titanate nanotubes decorated with a nanostructured Cd3[Fe(CN)6]2 Prussian Blue analogue

Nanostructured films of cadmium hexacyanoferrate (III), Cd₃[Fe(CN)₆]₂ have been deposited on the surface of titanate nanotubes (TiNT) by ion exchange with CdSO₄, followed by reaction with K₃[Fe(CN)₆] in an aqueous suspension. The composite demonstrates a significantly higher hydrogen storage uptake than pure Cd₃[Fe(CN)₆]₂ and TiNT. At a temperature of 77 K and a pressure 100 bar, the hydrogen uptake for the composite is approximately 12.5 wt %, whereas only 4.5 wt % and 4 wt % are achieved for the TiNT and Cd₃[Fe(CN)₆]₂ respectively. Electron microscopy and infrared spectroscopy show that Cd₃[Fe(CN)₆]₂ is uniformly distributed on the surface of the nanotubes forming a discontinuous nanostructured film with a well developed interface, which allows efficient interaction with the support. The possible reasons for the high uptake of hydrogen in the composite are discussed.     

Surface-induced thermal decomposition of [Ru(dcbpyH)2-(CN)2] on nanocrystalline TiO2 surfaces: Temperature-dependent infrared spectroscopy and two-dimensional correlation analysis

Thermal degradation mechanism of the self-assembled thin films of [Ru(dcbpyH)₂-(CN)₂] (Ruthenium 505, R505) anchoring on TiO₂ surfaces via its carboxylate group has been examined by temperature-dependent diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The CN stretching bands of R505 at 2000-2100 cm⁻¹ appeared to change drastically at ≈140 °C on TiO₂ surfaces, whereas a major CN peak at ∼2090 cm⁻¹ disappeared at a much higher temperature above ≈250 °C in their solid states. Two-dimensional (2D) correlation analysis was introduced to explain the thermal desorption behaviors of the Ruthenium dye. Multiple peaks of the CN stretching vibrations are more clearly resolved in the 2D correlation analysis. More complicated features in the CN stretching vibrational spectra on TiO₂ than those of the solid states suggest a substantial interaction of the CN groups with the TiO₂ surfaces.     

Photogalvanic behaviour of [Cr2O2S2(1-Pipdtc)2(H2O)2] in aqueous DMF

The photogalvanic behaviour of [Cr₂O₂S₂(1-Pipdtc)₂(H₂O)₂] was performed in a Honda Cell using 100% DMF and different percentages of water-DMF systems. The maximum potential of 200 mV with 18 μA of current was generated in DMF. The experiments were performed at different pH conditions and added dyes. The system was found to generate a maximum potential of 86, 67 and 36 mV at pH∼4, 7 and 8, respectively, at 65 °C. Malachite green and methylene blue (10⁻⁴ M) produced maximum potential of 100 and 82 mV, respectively. The system was reversible when the irradiated solution was aerated immediately. When the solution was kept in dark for a long time (12 h) and then aerated, the solution was found to be irreversible. The Cr(V) is photoreduced to Cr(IV) with the light irradiation and the unstable Cr(IV) reverts to Cr(V) by aerial oxidation.     

Behavior of the monophosphate tungsten bronzes (PO2)4(WO3)2m (m = 4 and 6) in electrochemical lithium insertion

The electrochemical lithium insertion process has been studied in the family of monophosphate tungsten bronzes (PO₂)₄(WO₃)_2m, where m = 4 and 6. Structural changes in the pristine oxides were followed as lithium insertion proceeded. Through potentiostatic intermittent technique, the different processes which take place in the cathode during the discharge of the cell were analysed. The nature of the bronzes Li_x(PO₂)₄(WO₃)_2m formed was determined by in situ X-ray diffraction experiments. These results have allowed establishment of a correlation with the reversible/irreversible processes detected during the electrochemical lithium insertion. Measurements of resistivity showed that upon lithium insertion, the metallic pristine oxides become insulating.     

Surface adsorption and encapsulated storage of H2 molecules in a cagelike (MgO)12 cluster

Cluster-based materials are candidate materials for solid-state hydrogen storage owing to their special geometric and electronic structures. The surface adsorption and the encapsulated storage of H₂ molecules in a cagelike (MgO)₁₂ cluster have been studied using density functional theory (DFT) calculations including a dispersion interaction. The results revealed that the cagelike (MgO)₁₂ cluster surface can adsorb 24 H₂ molecules with an average adsorption energy of 0.116 eV/H₂, which brings about a gravimetric density of 9.1 wt%. Compared with dispersion-corrected DFT calculations, the traditional DFT method substantially underestimates the surface adsorption strength. According to symmetric configurations, a maximum capacity of six H₂ molecules can be stored in the interior space of the cagelike (MgO)₁₂ cluster. The encapsulated H₂ molecules are trapped by stepwise energy barriers of 0.433–2.550 eV, although the storage is an endothermic process. The present study will be beneficial for hydrogen storage in cagelike clusters and assembled porous materials.     

Biomass-derived hierarchical porous CdS/M/TiO2 (M = Au,Ag, pt,pd) ternary heterojunctions for photocatalytic hydrogen evolution

We demonstrate a general method for the synthesis of biomass-derived hierarchical porous CdS/M/TiO₂ (M = Au, Ag, Pt, Pd) ternary heterojunctions for efficient photocatalytic hydrogen evolution. A typical biomass—wood are used as the raw sources while five species of wood (Fir, Ash, White Pine, Lauan and Shiraki) are chosen as templates for the synthesis of hierarchical porous TiO₂. The as-obtained products inherited the hierarchical porous features with pores ranging from micrometers to nanometers, with improved photocatalytic hydrogen evolution activity than non-templated counterparts. Noble metals M (M = Pt, Au, Ag, Pd) and CdS are loaded via a two-step photodeposition method to form core (metal)/shell (CdS) structures. The photocatalytic modules—CdS(shell)/metal (core)/TiO₂ heterostructures, have demonstrated to increase visible light harvesting significantly and to increase the photocatalytic hydrogen evolution activity. The H₂ evolution rates of CdS/Pd/TiO₂ ternary heterostructures are about 6.7 times of CdS/TiO₂ binary heterojunctions and 4 times higher than Pd/CdS/TiO₂ due to the vertical electron transfer process. The design of such system is beneficial for enhanced activity from morphology control and composition adjustment, which would provide some new pathways for the design of promising photocatalytic systems for enhanced performance.     

Synthesis and dehydrogenation of LiCa(NH2)3(BH3)2

We report the synthesis of a new hydrogen storage material with a composition of LiCa(NH₂)₃(BH₃)₂. The theoretical hydrogen capacity of LiCa(NH₂)₃(BH₃)₂ is 9.85 wt.%. It can be prepared by ball milling the mixture of calcium amidoborane (Ca(NH₂BH₃)₂) and lithium amide (LiNH₂) in a molar ratio of 1:1. The experimental results show that this material starts to release hydrogen at a temperature as low as ca. 50 °C, which is ca. 70 °C lower than that of pure Ca(NH₂BH₃)₂ possibly resulting from the active interaction of NH₂⁻ in LiNH₂ with BH₃ in Ca(NH₂BH₃)₂. ca. 4.1 equiv. or 6.8 wt.% hydrogen can be released at 300 °C. The dehydrogenation is a mildly exothermic process forming stable nitride products.     

C2H2M (M = Ti,Li) complex: A possible hydrogen storage material

The hydrogen storage capacity of Ti-acetylene (C₂H₂Ti) and Li-acetylene (C₂H₂Li) complex has been tested using second order Møller Plesset method with different basis sets. Single Ti(Li) decorated acetylene complex can adsorb maximum of five(four) hydrogen molecules, which corresponds to the gravimetric hydrogen storage capacity of 12(19.65) wt % and it meets the target of 9 wt % by 2015 specified by US Department of Energy. The hydrogen adsorption energies with zero point energy and Gibbs free energy correction show that hydrogen adsorption on C₂H₂Ti is energetically favourable for a wide range of temperature and that is unfavourable on C₂H₂Li complex even at a very low temperature. Atom centered density matrix propagation molecular dynamics simulations reveal that four H₂ molecules remain adsorbed on C₂H₂Ti complex at 300 K. Though H₂ uptake capacity of C₂H₂Li complex is higher than that of C₂H₂Ti complex, the thermochemistry results favour to C₂H₂Ti complex over C₂H₂Li complex as a possible hydrogen storage media.     

Hydrogen storage properties and microstructures of Ti0.7Zr0.3(Mn1−xVx)2 (x = 0.1, 0.2, 0.3, 0.4, 0.5) alloys

Hydrogen storage properties, activation performance and thermodynamics of Ti_0.7Zr_0.3(Mn_1−xV_x)₂ (x = 0.1, 0.2, 0.3, 0.4, 0.5) alloys and associated microstructures and surface chemical states were investigated by hydrogenation measurements and relevant structure and surface characterization methods. The results showed that the phase composition of the alloy changed from single C14 Laves phase (x ≤ 0.2) to coexistent Laves phase and V-based BCC solid solution phase with increasing V content (x ≥ 0.3). The V in the alloys catalyzed hydrogen dissociation and improved resistivity to oxygen poisoning, so that the alloys could be easily and quickly activated at 293 K even after being exposed in air for a long time. The hydrogen storage capacity of the alloy increased and the plateau pressure decreased with increasing V content. The x   = 0.2 and 0.3 alloys exhibited the best reversible hydrogen storage capacities of above 1.8 wt% at 1 kPa–4 MPa and 293 K. The relative partial molar enthalpy |ΔH|$\left|\Delta H\right|$ increased but the relative partial molar entropy |ΔS|$\left|\Delta S\right|$ decreased with increasing V content, and deviated from the linear relationship for x = 0.4 and 0.5 alloys due to coexisted BCC phase in the alloys.     

Destabilization of LiBH4 by MH2 (M = Ce, La) for hydrogen storage: Nanostructural effects on the hydrogen sorption kinetics

Destabilization of LiBH₄ by addition of metal hydrides or borohydrides is a powerful strategy to develop new promising hydrogen storage systems. In this study, we compare the destabilization behavior of the LiBH₄ by addition of MH₂ (M = La, Ce). A notable improvement in the hydrogen desorption temperature, the rate and the weight percentage of hydrogen released is observed for LiBH₄-MH₂ with respect to LiBH₄. Formation of LaB₆ and CeB₆ after dehydriding of the composites is proved by PXRD. Remarkable hydrogen storage reversibility of LiBH₄-MH₂ composites is confirmed under moderate conditions: 400 °C and 6.0 MPa of hydrogen pressure for 4 h without catalyst. The LiBH₄-LaH₂ composite exhibits improved hydrogen desorption performance compared with LiBH₄-CeH₂ composite, but the hydrogen storage reversibility is inferior. Notably, the LiBH₄-CeH₂ nanocomposite produced by in situ formation of CeH₂ from Ce(BH₄)₃-LiH displays excellent hydrogen storage properties. The addition of ZrCl₄ as a catalyst improves dehydriding kinetics. The mechanism underlying the enhancement in the LiBH₄-MH₂ composites is also discussed. Our study is the first work about reversible hydrogen storage in LiBH₄-LaH₂.     

Ab initio calculations on energetics and electronic structures of cubic Mg3MNi2 (M = Al, Ti, Mn) hydrogen storage alloys

Mg₃MNi₂ (M = Al, Ti, Mn) ternary intermetallic compounds with cubic structure are a new type of potential hydrogen storage alloys. Using ab initio density functional theory (DFT) calculations, the energetics and electronic structures of Mg₃MNi₂ (M = Al, Ti, Mn) compounds are systematically investigated. The optimized structural parameters including lattice constants and internal atomic positions are close to experimental data determined from X-ray powder diffraction. The calculated results of formation enthalpy ΔH_form show that the stabilities of cubic Mg₃MNi₂ (M = Al, Ti, Mn) compound, compared with hexagonal Mg₂Ni, increase in the order of Mg₃MnNi₂, Mg₂Ni, Mg₃TiNi₂ and Mg₃AlNi₂, whereas the stabilities of their saturated Mg₃MNi₂H₃ (M = Al, Ti, Mn) hydrides, compared with monoclinic Mg₂NiH₄, decrease in the order of Mg₂NiH₄, Mg₃AlNi₂H₃, Mg₃TiNi₂H₃ and Mg₃MnNi₂H₃. Further calculations of hydrogen desorption enthalpy ΔH_des indicate that these cubic Mg₃MNi₂ (M = Al, Ti, Mn) compounds possess promising dehydrogenation properties for their relatively lower ΔH_des values. Among of them, the dehydrogenation ability of Mg₃TiNi₂ is the most pronounced. Analysis of electronic structures suggests that the strong covalent bonding interactions between Ni and M within cubic Mg₃MNi₂ (M = Al, Ti, Mn) are dominant and directly control the structural stabilities of these compounds.     

Co-doping schemes to enhance H2 evolution under visible light irradiation over SrTiO3:Ni/M (M = La or Ta) prepared by spray pyrolysis

Two photocatalysts, SrTiO₃:Ni/La and SrTiO₃:Ni/Ta, were prepared by continuous spray pyrolysis. The effects of the co-dopants on hydrogen evolution over the uncalcined photocatalysts were evaluated under visible light irradiation. The co-doping of La³⁺ into SrTiO₃:Ni transformed the charge structure and increased the presence of Ni²⁺ at the expense of Ni³⁺ in the host lattice structure. The co-doping of Ta⁵⁺ into SrTiO₃:Ni also increased the Ni²⁺/Ni³⁺ ratio around the Ti⁴⁺ ions. Compared with SrTiO₃:Ni, SrTiO₃:Ni/La showed a 3 times greater rate of hydrogen evolution under visible light irradiation and SrTiO₃:Ni/Ta, a 4 times greater rate. The co-doping levels required for optimized hydrogen evolution over SrTiO₃:Ni/La and SrTiO₃:Ni/Ta prepared by spray pyrolysis were smaller than those prepared by other methods. Spray pyrolysis also produced particles with large surface areas and high roughnesses.     

Photocatalytic water splitting on protonated form of layered perovskites K0.5La0.5Bi2M2O9 (M = Ta; Nb) by ion-exchange

Protonated layered perovskite oxides H_1.9K_0.3La_0.5Bi_0.1Ta₂O₇ (HKLBT) and H_1.6K_0.2La_0.3Bi_0.1Nb₂O_6.5 (HKLBN), which were prepared from K_0.5La_0.5Bi₂M₂O₉(M = Ta; Nb)(KLBT(N)) by H ion-exchange in 3M HCl solution, were found as good photocatalysts for water splitting under UV light irradiation. The characterization by XRD, ICP and TG revealed that HKLBT(N) retained layered structure of their parent materials KLBT(N) after HCl treatment. An amount of exfoliation of Bi during the protonated process caused the decrease of contribution of Bi 6p in conduction band (CB) and thus resulted in more negative CB potential. HKLBT(N) showed considerable higher photocatalytic activity for H₂ and/or O₂ evolution than KLBT(N) in the absence of sacrificial reagents, which was attributed to the higher position of conduction band and the layered structure after acid treatment. It was concluded that the interlayer modification via ion-exchange for layered K_0.5La_0.5Bi₂M₂O₉ (M = Ta; Nb) is a potential way to construct novel photocatalysts with high activity for water splitting.     

Facile reduction of nitrophenols: Comparative catalytic efficiency of MFe2O4 (M = Ni,Cu, Zn) nano ferrites

Nano ferrites of the formula MFe₂O₄ (M = Ni, Cu, Zn), synthesized using sol–gel technique, were employed to catalyze the reductive transformation of nitrophenols to aminophenols. The catalytic reduction was carried out in the excess of NaBH₄ as reducing agent in aqueous medium at room temperature. CuFe₂O₄ and NiFe₂O₄ were found to be active for the reduction of nitrophenols with significant difference in their activities whereas ZnFe₂O₄ was found to be inactive. The kinetics of the reduction of nitrophenols to aminophenols was also investigated. The reaction followed pseudo first order kinetics. The first order rate constant values for 30 mol% of CuFe₂O₄ and NiFe₂O₄ for the reduction of 2-nitrophenol were observed to be 3.68 min⁻¹ and 0.33 min⁻¹ respectively. The rates of reduction for the three isomers of nitrophenol were also studied and were observed to follow the order – 2-nitrophenol > 4-nitrophenol > 3-nitrophenol. The selective formation of aminophenol was confirmed using LC-MS, ¹H NMR and FT-IR spectroscopic techniques.     

Mechanisms of H2 generation for metal doped Al16M (M = Mg and Bi) clusters in water

We have systematically investigated the hydrolysis mechanism of metal doped Al₁₆M (M = Al, Mg and Bi) clusters with H₂O molecules and proposed a reasonable elucidation for the experimentally observed fast H₂ generation rate and high H₂ yield in the Al–Bi based composite. Mg and Bi showed negative effect on the dissociation process of the first H₂O molecule, but accelerated further H₂ generation process. The investigation of persistent hydrolysis reactions demonstrated that the proton-transfer way makes the aluminum–water reaction a lasting process in the long-term H₂ generation in existence of Bi atom, which explains not only the previously observed fast H₂ generation rate but also high H₂ yields in the Bi added Al powder. Our experimental results of hydrogen generation form Al–Bi (Mg) mixture and water are in good agreement with the theory prediction. The facilitated hydrolysis reaction in Al₁₆Bi cluster is attributed to the weakened hydroxide adsorption with the presence of Bi in the aluminum cluster, which is the key factor to accelerate the proton-transfer process.     

Reversible hydrogen storage from 6LiBH4-MCl3 (M = Ce, Gd) composites by in-situ formation of MH2

Different destabilized LiBH₄ systems with several interacting components are being explored for hydrogen storage applications. In this study, hydrogen sorption properties of as-milled 6LiBH₄-MCl₃ composites (M = Ce, Gd) are investigated by X-ray diffraction, differential scanning calorimetry and thermovolumetric measurements. The chemical interaction between metal halides and LiBH₄ decreases the dehydrogenation temperature in comparison with as-milled LiBH₄. Hydrogen release starts at 220 °C from the decomposition of M(BH₄)₃ formed during milling and proceeds through destabilization of LiBH₄ by in-situ formed MH₂. The dehydrogenation products CeB₆-LiH and GdB₄-LiH can be rehydrided under moderate conditions, i.e 400 °C and 6.0 MPa of hydrogen pressure for 2 h without catalyst. A new 6LiBH₄-CeCl₃-3LiH composite shows promissory hydrogen storage properties via the formation by milling of CeH_2+x. Our study is the first work about reversible hydrogen storage in LiBH₄-MCl₃ composites destabilized by in-situ formed MH₂.     

Synthesis and improved electrochemical performances of porous Li3V2(PO4)3/C spheres as cathode material for lithium-ion batteries

Spherical Li₃V₂(PO₄)₃/C composites are synthesized by a soft chemistry route using hydrazine hydrate as the spheroidizing medium. The electrochemical properties of the materials are investigated by galvanostatic charge-discharge tests, cyclic voltammograms and electrochemical impedance spectrum. The porous Li₃V₂(PO₄)₃/C spheres exhibit better electrochemical performances than the solid ones. The spherical porous Li₃V₂(PO₄)₃/C electrode shows a high discharge capacity of 129.1 and 125.6 mAh g⁻¹ between 3.0 and 4.3 V, and 183.8 and 160.9 mAh g⁻¹ between 3.0 and 4.8 V at 0.2 and 1 C, respectively. Even at a charge-discharge rate of 15 C, this material can still deliver a discharge capacity of 100.5 and 121.5 mAh g⁻¹ in the potential regions of 3.0-4.3 V and 3.0-4.8 V, respectively. The excellent electrochemical performance can be attributed to the porous structure, which can make the lithium ion diffusion and electron transfer more easily across the Li₃V₂(PO₄)₃/electrolyte interfaces, thus resulting in enhanced electrode reaction kinetics and improved electrochemical performance.     

MS2 (M = W, Mo) photosensitive thin films for solar cells

Photosensitive textured WS₂ and MoS₂ films can be obtained by the techniques of reactive sputtering and solid state reaction, as long as the substrates used are each coated with a 10–20 nm Ni layer. When MS₂ (M = W, Mo) layers are deposited onto these substrates and then annealed for half an hour at 1073k in an argon atmosphere, textured films crystallized in the 2H-MS₂ structure are obtained, with their c crystallite axes perpendicular to the plane of the substrate. The films are nearly stoichiometric. The crystallinity enhancement of the films can be attributed to an improvement in the crystallization process related to liquid NiS phases present at the grain boundaries during annealing. Residual phases (Ni_xS_y; Ni;…) are distributed at the grain boundaries and do not strongly disturb the properties of the WS₂ and MoS₂ films. The optical absorption spectra are similar to those of single crystals, and the high photosensitivity of the films is attributed to a grain size enhancement by the NiS phase.     

Kinetics and modeling study of a Mg(BH4)2/Ca(BH4)2 destabilized system

Borohydrides such as Mg(BH₄)₂ and Ca(BH₄)₂ continue to attract attention as potential hydrogen storage materials because of their high hydrogen content. In this study the desorption kinetics of Mg(BH₄)₂, Ca(BH₄)₂ and a 5Mg(BH₄)₂/Ca(BH₄)₂ mixture of the two were compared in the two-phase region at the same temperature and at a constant pressure thermodynamic driving force. The rate of hydrogen desorption from the two-phase region in the 5Mg(BH₄)₂/Ca(BH₄)₂ mixture was faster than that from either of the constituents. This indicated that Ca(BH₄)₂ was able to serve as a destabilizing agent for Mg(BH₄)₂. The results also showed that the desorption rates from the two-phase region were much faster than those from the single phase region. Modeling studies showed that the rate of hydrogen release from Mg(BH₄)₂, during the first 80% of the reaction, is diffusion controlled while in Ca(BH₄)₂ the reaction rate is phase boundary controlled. In the mixture the rate appears to be under the mixed control of both processes.     

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.