Dehydrogenation Properties Research Articles

Improved Dehydrogenation Properties of LiBH4 Using Catalytic Nickel- and Cobalt-based Mesoporous Oxide Nanorods.

Lithium borohydride (LiBH4 ) with a theoretical hydrogen storage capacity of 18.5 wt % has attracted intense interest as a high-density hydrogen storage material. However, high dehydrogenation temperatures and limited kinetics restrict its practical applications. In this study, mesoporous nickel- and cobalt-based oxide nanorods (NiCo2 O4 , Co3 O4 and NiO) were synthesized in a controlled manner by using a hydrothermal method and then mixed with LiBH4 by ball milling. It is found that the dehydrogenation properties of LiBH4 are remarkably enhanced by doping the as-synthesized metal oxide nanorods. When the mass ratio of LiBH4 and oxides is 1:1, the NiCo2 O4 nanorods display the best catalytic performance owing to the mesoporous rod-like structure and synergistic effect of nickel and cobalt active species. The initial hydrogen desorption temperature of the LiBH4 -NiCo2 O4 composite decreases to 80 °C, which is 220 °C lower than that of pure LiBH4 , and 16.1 wt % H2 is released at 500 °C for the LiBH4 -NiCo2 O4 composite. Meanwhile, the composite also exhibits superior dehydrogenation kinetics, which liberates 5.7 wt % H2 within 60 s and a total of 12 wt % H2 after 5 h at 400 °C. In comparison, pure LiBH4 releases only 5.3 wt % H2 under the same conditions.

Enhanced thermal diffusivity and dehydrogenation of 2LiNH2[sbnd]MgH2 by doping with super activated carbon

Doping with the additives in metal-N–H system has been regarded as one of the most effective approaches to improve its hydrogen storage properties. Herein, we prepared super activated carbon (SuperC) through the activation of commercial activated carbon by KOH and evaluated its effect on dehydrogenation properties of 2LiNH2MgH2. Our studies show that doping with SuperC could effectively lower its dehydrogenation temperatures. For instance, 2LiNH2MgH2–10 wt% SuperC can release 4.86 wt% of hydrogen upon heating up to 300 °C with the onset and peak dehydrogenation temperatures of 71 °C and 168 °C, respectively. Moreover, the release of byproduct NH3 was successfully suppressed. Measurement of thermal diffusivity suggests that the enhanced dehydrogenation properties may be ascribed to the improved heat transfer for solid-solid reaction resulting from doping with SuperC.

Thermodynamic Properties and Reversible Hydrogenation of LiBH4–Mg2FeH6 Composite Materials

In previous studies, complex hydrides LiBH4 and Mg2FeH6 have been reported to undergo simultaneous dehydrogenation when ball-milled as composite materials (1 − x)LiBH4 + xMg2FeH6. The simultaneous hydrogen release led to a decrease of the dehydrogenation temperature by as much as 150 K when compared to that of LiBH4. It also led to the modified dehydrogenation properties of Mg2FeH6. The simultaneous dehydrogenation behavior between stoichiometric ratios of LiBH4 and Mg2FeH6 is not yet understood. Therefore, in the present work, we used the molar ratio x = 0.25, 0.5, and 0.75, and studied the isothermal dehydrogenation processes via pressure–composition–isothermal (PCT) measurements. The results indicated that the same stoichiometric reaction occurred in all of these composite materials, and x = 0.5 was the molar ratio between LiBH4 and Mg2FeH6 in the reaction. Due to the optimal composition ratio, the composite material exhibited enhanced rehydrogenation and reversibility properties: the temperature and pressure of 673 K and 20 MPa of H2, respectively, for the full rehydrogenation of x = 0.5 composite, were much lower than those required for the partial rehydrogenation of LiBH4. Moreover, the x = 0.5 composite could be reversibly hydrogenated for more than four cycles without degradation of its H2 capacity.

Dehydrogenation properties of LiBH4 modified by Mg from first-principles calculations

First-principles calculations were performed to investigate the dehydrogenation characteristics of LiBH4 modified by Mg. Our results show that Mg modification can destabilize LiBH4, by reducing the energy cost required for H-desorption. The decreased bonding strength between Li-B/H, the presence of semiconducting/metallic character as well as the formation of MgB bond are responsible for the destabilization of LiBH4. Mg-for-Li-substituted compound with rather low dehydrogenation energy exhibits good dehydrogenation performance.

Catalytic effect of nickel phthalocyanine on hydrogen storage properties of magnesium hydride: Experimental and first-principles studies

The high dehydrogenation temperature of magnesium hydride MgH2 is still the main obstacle to its practical application as a solid-state hydrogen storage medium. Using experimental and first-principles calculations approaches, we, for the first time, investigate the catalytic effect and mechanism of nickel phthalocyanine on the dehydrogenation properties of MgH2. The results display that a small amount of nickel phthalocyanine can promote MgH2 dehydrogenation at significantly decreased temperatures by more than 90 °C relative to milled pristine or graphene-added MgH2 system. However, the agglomeration of MgH2 particles is not evidently alleviated through nickel phthalocyanine addition. When MgH2 is milled with graphene firstly and then the obtained mixture is further milled with nickel phthalocyanine, the dehydrogenation properties and agglomeration of MgH2 particles can be synergistically improved to some extent. The first-principles calculations of dehydrogenation enthalpy and binding energy account for the experimental differences in catalysis and aggregation-resistance abilities of nickel phthalocyanine and graphene on MgH2 particles. Notably, the NiN4-inserted graphene is predicted to be an ideal additive for MgH2, which combines the synergetic catalysis-confinement effect of nickel phthalocyanine and graphene on MgH2 particles. Analysis of electronic structures reveals that the excellent catalytic effect of nickel phthalocyanine on MgH2 can be ascribed to the more electron transfer between nickel phthalocyanine and MgH2, which induces the significantly weakened bond strength of MgH and decreased dehydrogenation enthalpy of MgH2.

First principles study of dehydrogenation properties of alkali/alkali-earth metal doped Mg7TiH16

First principles calculations were performed to investigate the influence of vacancy and alkali/alkali earth metals (Li, Na, K and Ca) doping on the dehydrogenation properties of Mg7TiH16. The occupation energy of dopants and dehydrogenation energy of defected Mg7TiH16 were calculated. It was found that dopants prefer to substitute for Mg atom leading to a distortion of the structure of the matrix, and therefore destabilize the system. All doped systems were found to possess a lower hydrogen release energy than undoped Mg7TiH16. The obtained results indicate that alkali/alkali-earth metals represent excellent dopants for Mg7TiH16 resulting in an improvement of its dehydrogenation characteristics by lowering its equilibrium temperature/pressure.

Improved hydrogen storage properties of MgH2 with Ni-based compounds

The nanoscaled Ni-based compounds (Ni3C, Ni3N, NiO and Ni2P) are synthesized by chemical methods. The MgH2-X (X = Ni3C, Ni3N, NiO and Ni2P) composites are prepared by mechanical ball-milling. The dehydrogenation properties of Mg-based composites are systematically studied using isothermal dehydrogenation apparatus, temperature-programmed desorption system and differential scanning calorimetry. It is experimentally confirmed that the dehydrogenation performance of the Mg-based materials ranks as following: MgH2Ni3C, MgH2Ni3N, MgH2NiO and MgH2Ni2P. The onset dehydrogenation temperatures of MgH2Ni3C, MgH2Ni3N, MgH2NiO and MgH2Ni2P are 160 °C, 180 °C, 205 °C and 248 °C, respectively. The four Mg-based composites respectively release 6.2, 4.9, 4.1 and 3.5 wt% H2 within 20 min at 300 °C. The activation energies of MgH2Ni3C, MgH2Ni3N, MgH2NiO and MgH2Ni2P are 97.8, 100.0, 119.7 and 132.5 kJ mol−1, respectively. It' found that the MgH2Ni3C composites exhibit the best hydrogen storage properties. Moreover, the catalytic mechanism of the Ni-based compounds is also discussed. It is found that Ni binding with low electron-negativity element is favorable for the dehydrogenation of the Mg-based composites.

Hierarchical Ga-MFI Catalysts for Propane Dehydrogenation

We report the synthesis, characterization, and enhanced propane dehydrogenation properties of hierarchical Ga-MFI zeolite catalysts synthesized by two different methods: (i) repetitive branching and (ii) utilization of a long chain alkyl SDA. Structural, compositional, and morphological characterizations confirm that the Ga-MFI catalyst materials have hierarchical structures including micropores and mesopores in a single particle and show that Si/Ga ratios comparable to bulk Ga-MFI catalysts can be obtained. Acid site analysis using NH3-TPD and pyridine adsorption followed by in situ IR spectroscopy allowed quantification of the number and types of acid sites present and show that the hierarchical catalysts have considerably higher Lewis acid site concentrations than the bulk catalysts. The hierarchical Ga-MFI catalysts show superior PDH performance (at 600 °C) compared to bulk Ga-MFI catalysts. Propane conversion rates are increased 2–6-fold and propylene selectivities are 10–100% higher. We also examine...

Dehydrogenation properties and mechanisms of MgH2-NiCl2 and MgH2-NiCl2-graphene hydrogen storage composites

The four hydrogen storage systems including pure MgH2, MgH2-5 wt%NiCl2, MgH2-10 wt%NiCl2 and MgH2-10 wt%NiCl2-10 wt%graphene were prepared by ball-milling in this work. Using experimental X-ray diffraction, scanning electron microscopy and differential scanning calorimetry testing methods in combination with first-principle calculations, the dehydrogenation properties and mechanisms of NiCl2 single-doped and NiCl2-graphene co-doped MgH2 composites were systematically investigated. Experimental results show that the NiCl2 single-doping is conductive to decreasing the size of MgH2 grains and particles. The co-doping of NiCl2 and graphene not only reduces the size of MgH2 grains and particles, but also contributes to the uniformity of MgH2 particles. As compared with milled pure MgH2, the dehydrogenation peak temperatures are decreased by 24 °C and 47 °C for the 10 wt%NiCl2 single-doped and 10 wt%NiCl2-10 wt%graphene co-doped MgH2 systems, respectively. It is demonstrated that the co-doping of NiCl2 and graphene exhibits the synergistic effects of confinement and catalysis on improving the dehydrogenation properties of MgH2. The first-principle calculations indicate that the co-doping of NiCl2 and graphene leads to the distortion of MgH2 atomic-configuration and results in the charge transfer between the dopants and MgH2, which induce the weakened structural stability and decreased dehydrogenation enthalpy of MgH2.

First principle study of strain effect on structural and dehydrogenation properties of complex hydride LiBH4

Using the first-principles calculations based on density functional theory (DFT), the structure, stability, thermodynamic and kinetic properties of complex hydride LiBH4 under different biaxial strains have been investigated. The results show that the free strain LiBH4 involves a high stability and the biaxial tensile or compressive strain lowers the hydrogen desorption enthalpy of this system. Further, the diffusion activation energy of hydrogen atom in LiBH4 is also decreased which can accelerate the hydrogenation kinetic of LiBH4.

Local Structures and Heterogeneity of Silica-Supported M(III) Sites Evidenced by EPR, IR, NMR, and Luminescence Spectroscopies.

Grafting molecular precursors on partially dehydroxylated silica followed by a thermal treatment yields silica-supported M(III) sites for a broad range of metals. They display unique properties such as high activity in olefin polymerization and alkane dehydrogenation (M = Cr) or efficient luminescence properties (M = Yb and Eu) essential for bioimaging. Here, we interrogate the local structure of the M(III) surface sites obtained from two molecular precursors, amides M(N(SiMe3)2)3 vs siloxides (M(OSi(OtBu)3)3·L with L = (THF)2 or HOSi(OtBu)3 for M = Cr, Yb, Eu, and Y, by a combination of advanced spectroscopic techniques (EPR, IR, XAS, UV-vis, NMR, luminescence spectroscopies). For paramagnetic Cr(III), EPR (HYSCORE) spectroscopy shows hyperfine coupling to nitrogen only when the amide precursor is used, consistent with the presence of nitrogen neighbors. This changes their specific reactivity compared to Cr(III) sites in oxygen environments obtained from siloxide precursors: no coordination of CO and oligomer formation during the polymerization of ethylene due to the presence of a N-donor ligand. The presence of the N-ligand also affects the photophysical properties of Yb and Eu by decreasing their lifetime, probably due to nonradiative deactivation of excited states by N-H bonds. Both types of precursors lead to a distribution of surface sites according to reactivity for Cr, luminescence spectroscopy for Yb and Eu, and dynamic nuclear polarization surface-enhanced 89Y NMR spectroscopy (DNP SENS). In particular, DNP SENS provides molecular-level information about the structure of surface sites by evidencing the presence of tri-, tetra-, and pentacoordinated Y-surface sites. This study provides unprecedented evidence and tools to assess the local structure of metal surface sites in relation to their chemical and physical properties.

Synergetic effects of K, Ti and F on the hydrogen storage properties of the Li[sbnd]N[sbnd]H system

In this paper, the hydrogen storage properties of the LiNH2LiH system doped with K2TiF6 were investigated and discussed. Interestingly, the hydrogen storage properties are significantly enhanced by introducing K2TiF6 into the LiNH2LiH system. By doping 5 mol% K2TiF6 in the LiNH2LiH system, we obtain the hydrogen desorption peak temperature (233 °C) at a heating rate of 10 °C min−1, which is approximately 66 °C lower than that of the pristine LiNH2LiH system. Moreover, the system begins to desorb H2 at 75 °C, which is approximately 124 °C lower than in the pristine LiNH2LiH system. The isothermal desorption kinetics at 250 °C and 300 °C clearly reflects the dramatically improved kinetic properties. Additionally, the reversibility of the LiNH2LiH system can be drastically enhanced by adding K2TiF6. We propose that the dehydrogenation property of the K2TiF6-doped LiNH2LiH sample is improved by the synergetic effects of K, Ti and F.

Hydrogen storage behavior of LiBH4 improved by the confinement of hierarchical porous ZnO/ZnCo2O4 nanoparticles

Abstract LiBH4@xZZCO composites are successfully synthesized by confining LiBH4 in hierarchical porous ZnO/ZnCo2O4 (ZZCO) nanoparticles via a chemical impregnation method, and the effect of ZZCO on the dehydrogenation performance is systematically investigated. It is found that dehydrogenation properties of LiBH4 are significantly improved, especially for the LiBH4@2ZZCO composite. The onset desorption temperature of LiBH4@2ZZCO is decreased to 169 °C, and the majority of hydrogen release occurs at 275 °C, much lower than raw LiBH4. In addition, 8.7 wt% H2 could be released below 500 °C. Moreover, the apparent activation energy (Ea) has been reduced from 146 kJ/mol (pure LiBH4) to 120.22 kJ/mol. The improved dehydrogenation properties are attributed to the synergistic effect of nanoconfinement and destabilization of ZZCO nanoparticles.

Improving dehydrogenation properties of Mg/Nb composite films via tuning Nb distributions

To investigate the effect of Nb on the dehydrogenation properties of Mg–Nb composite films, Mg/Nb eight-layer film and Mg-10 at% Nb alloy film with the similar Mg-to-Nb atomic ratio were prepared by magnetron sputtering. Results show that both Mg/Nb eight-layer film and Mg-10 at% Nb alloy film exhibit excellent de/hydrogenation properties. Mg-10 at% Nb alloy film starts to release hydrogen at 108 °C, and its desorption peak temperature is lower to 146 °C, which is much better than that of pure MgH2 under the same condition. Scanning electron microscopy (SEM) results demonstrate that the dispersive Nb nanoparticles in Mg/Nb eight-layer film may serve as nucleation sites for Mg ↔ MgH2 reactions, which can provide channels for H diffusion. For Mg-10 at% Nb alloy film, the uniform distributions of Nb can accelerate the hydrogen diffusion and effectively improve the dehydrogenation kinetics for MgH2. This study provides an enlightening way for designing and preparing Mg-based composite films with excellent dehydrogenation properties.

Structure and hydrogen storage properties of mechanically alloyed Ti-V alloys

Ti-V alloys are potential candidates for hydrogen storage materials. In this study, mechanical alloying under an argon atmosphere was used to produce Ti2−xVx nanocrystalline alloys (x = 0.5, 0.75, 1, 1.25, 1.5). Shaker type ball mill was used. An objective of the present study was to investigate an influence of chemical composition and method of production on hydrogenation and dehydrogenation properties of Ti-V alloys. X-ray diffraction analyses revealed formation of BCC solid solution after 14 h of milling. It is the first time of obtaining this phase directly from mechanical alloying method. HRTEM images confirmed formation of nanocrystalline materials. Synthesized materials were studied by a conventional Sievert's type apparatus at 303 K. It was observed that the maximum hydrogen storage capacity is increased with increased Ti content in the alloy. Ti1.5V0.5 alloy showed high hydrogen storage capacity at room temperature, which reached about 3.67 wt.%. Simultaneously, it was noticed that Ti-rich alloys form more stable hydride phases than V-rich alloys. Observed properties resulted mainly from structure of studied materials.

Synergic effects of VLi and Ti doping on hydrogen desorption on LiBH4 (010) surface: A first-principles investigation

Ti-doping and Li-vacancy (VLi) crucially affect the dehydrogenation properties of LiBH4 surface. However, theoretical investigations on individual Ti or VLi could not completely explain experimental observations. In this article, we investigated the synergistic effects of co-existing Ti and VLi on the dehydrogenation properties of LiBH4 (010) surface. Our result shows mutual stabilization between Ti-dopant and Li-vacancy, implying expectable co-existence of Ti and VLi. Thermodynamic destabilization from composite Ti + VLi defect agrees with experiments better than that from single Ti or VLi. The kinetic barrier on Ti + VLi decorated surface also becomes closer to experimental result. Therefore, the co-existing Ti and VLi synergistically and crucially affect the dehydrogenation thermodynamics and kinetics on LiBH4 surface. The electronic structure further reveals strong HH, BB, and TiB bonds as well as weakened BH bond in transition states on Ti + VLi co-existed surface, which is the main factor of low kinetic barrier.

Alkali metal silanides α-MSiH3: A family of complex hydrides for solid-state hydrogen storage

The alkali metal silanides α-MSiH3 appear to be a promising family of complex hydrides for solid-state hydrogen storage. Herein the structural, energetic and electronic properties of α-MSiH3 silanides (M = Li, Na, K, Rb, Cs) and MSi Zintl phases are systematically investigated for the first time by using first-principles calculations method based on density functional theory. The structural parameters of α-MSiH3 and MSi including lattice constants and atomic positions are determined through geometry optimization. The obtained results are close to the experimental data analysed from X-ray and neutron powder diffraction. The calculations of formation enthalpy show that α-KSiH3, α-RbSiH3 and α-CsSiH3 silanides are easier to be synthetized relative to α-LiSiH3 and α-NaSiH3, which interprets well the lower thermostabilities of experimental α-LiSiH3 and α-NaSiH3. Nevertheless, LiSi, KSi and CsSi phases are easier to be formed relative to NaSi and RbSi. The calculations of hydrogen desorption enthalpy reveal that the dehydrogenation abilities of α-MSiH3 silanides along the decomposition path of α-MSiH3→MSi + H2 are gradually enhanced in the order of α-CsSiH3, α-RbSiH3, α-KSiH3, α-NaSiH3 and α-LiSiH3, which may be originated from their decreasing thermostabilities. From a comprehensive point of view including hydrogen storage capacity, thermostability and dehydrogenation ability, α-KSiH3 (∼4.29 wt%) is identified as the most promising alkali metal silanide for reversible hydrogen storage. Analysis of electronic structures indicates that a significant charge transfer leads to positively charged M ions and negatively charged SiH3 complex, which constitutes the ionic bonding between them. The bonding within SiH3 complex not only involves the covalent hybridization between Si (3s) (3p) and H (1s) orbitals, but also exhibits some ionic bond characteristics due to the partial charge transfer from Si to H. The covalent bonding interactions between H and Si atoms within SiH3 mainly dominate the thermostabilities and dehydrogenation properties of α-MSiH3 silanides.

Improved dehydrogenation performance of NaAlH 4 using NiFe 2 O 4 nanoparticles

Abstract The dehydrogenation properties of commercial NaAlH 4 ball milled with a NiFe 2 O 4 nanoparticle catalyst have rarely been studied. In this work, temperature programmed desorption analysis indicates that the onset temperature of three dehydrogenation steps of NaAlH 4 + 3 mol% NiFe 2 O 4 is 72, 132 and 82 °C lower than those of as-milled NaAlH 4 , respectively. The dehydrogenation capacity of NaAlH 4 could be significantly improved by adding NiFe 2 O 4 . Furthermore, NaAlH 4 + 3 mol% NiFe 2 O 4 displays good cycle stability at 150 °C and only a slight capacity loss after five cycles. The apparent activation energies of NaAlH 4 + 3 mol% NiFe 2 O 4 for the three dehydrogenation steps are 54.3, 73.1 and 113.1 kJ mol −1 , respectively, as determined by differential scanning calorimetry and Kissinger analysis. From the analysis of X-ray diffraction, the new phases after dehydrogenation are NaFeO 2 and Al 3 Ni. The reaction between NiFe 2 O 4 and NaAlH 4 during desorption is revealed.

Effect of Cu promoter and alumina phases on Pt/Al2O3 for propane dehydrogenation

We investigated the effects of different Cu weight ratio on θ or γ-Al2O3 which were impregnated with platinum in terms of catalytic activity for propane dehydrogenation and physicochemical properties. 1.5 wt% Pt, 0-10 wt% Cu catalyst supported on θ-Al2O3 or γ-Al2O3 was prepared by incipient wetness co-impregnation. Enhanced Pt dispersion by increasing Cu contents in γ-Al2O3 supported catalyst was confirmed via XRD and XPS. Pt and CuO was separated in Pt-Cu/θ-Al2O3, but Pt-Cu alloy was identified after reduction treatment. Also, adding Cu in Pt/Al2O3 makes catalyst’s acidity lower and this property led to increased propylene yield in propane dehydrogenation. However, Pt3Cu was not good for yield of PDH, which was confirmed in Pt-10Cu/θ-Al2O3 through XRD.

Role of milling time and Ni content on dehydrogenation behavior of MgH2/Ni composite

The effects of ball milling time and Ni content on the dehydrogenation performance of MgH2/Ni composite were systematically investigated. The structural evolution of ball milled MgH2+x%Ni (x=0, 2, 4, 8, 20, 30, mass fraction) samples during mechanical milling process and dehydrogenation properties were investigated by a series of experimental techniques. The results show that the desorption kinetics is independent of particle size, grain size and defects as the temperature is above 380 oC. The desorption kinetics is improved by prolonged milling time due to refined and uniformly distributed Ni. The formation of Mg2Ni after dehydrogenation is proposed to explain the degradation of hydrogen storage properties of MgH2 during de-/hydrogenation cycling process. The desorption activation energy of MgH2 decreases with the increase of Ni content due to the catalytic effect of Ni. It is found Ni favors the nucleation of magnesium phase and accelerates the recombination of hydrogen atoms.