Catalytic effects of MgFe2O4 addition on the dehydrogenation properties of LiAlH4

Study on the catalytic roles of MgFe₂O₄ on the dehydrogenation performance of LiAlH₄ was carried out for the first time. Notable improvement on the dehydrogenation of LiAlH₄–MgFe₂O₄ compound was observed. The initial decomposition temperatures for the catalyzed LiAlH₄ were decreased to 95 °C and 145 °C for the first and second stage reactions, which were 48 °C and 28 °C lower than the milled LiAlH₄. As for the desorption kinetics performance, the MgFe₂O₄ doped-LiAlH₄ sample was able to desorb faster with a value of 3.5 wt% of hydrogen in 30 min (90 °C) while the undoped LiAlH₄ was only able to desorb 0.1 wt% of hydrogen. The activation energy determined from the Kissinger analysis for the first two desorption reactions were 73 kJ/mol and 97 kJ/mol; which were 31 and 17 kJ/mol lower as compared to the milled LiAlH₄. The X-ray diffraction result suggested that the MgFe₂O₄ had promoted significant improvements by reducing the LiAlH₄ decomposition temperature and faster desorption kinetics through the formation of active species of Fe, LiFeO₂ and MgO that were formed during the heating process.     

Intensive investigation on hydrogen storage properties and reaction mechanism of the NaBH4-Li3AlH6 destabilized system

Notable effects of Li₃AlH₆ on the hydrogen storage properties of the NaBH₄ are studied intensively. Li₃AlH₆ is synthesized by milling 2LiH-LiAlH₄ mixture for 12 h. The best molar ratio of the NaBH₄- Li₃AlH₆ destabilized system is 1:1 which has decomposed at two stages; Li₃AlH₆ decomposition stage at 170 °C and NaBH₄decomposition stage at 400 °C. As no significant effect on the decomposition temperature between 1 h and 24 h of milling time can be observed, the 1-hour milling preparation method is selected for the characterization. Isothermal absorption has shown that the system is able to absorb 4.2 wt% and 6.1 wt% of hydrogen in 60 min at 330 °C and 420 °C under 30 atm of hydrogen pressure. In contrast, only about 3.4 wt% and 3.7 wt% of hydrogen can be absorbed by the milled NaBH₄ under a similar condition. Meanwhile, the system is able to desorb 2.0 wt% and 4.1 wt% of hydrogen in 60 min at 330 °C and 420 °C in isothermal desorption while only 0.3 wt% and 2.1 wt% can be released by the milled NaBH₄ under the similar condition. The decomposition activation energy and enthalpy of the NaBH₄ stage are calculated to be 162.1 kJ/mol and 68.1 kJ/mol H₂. Based on the X-ray diffraction analysis, Na, Al and AlB₂ are formed during the dehydrogenation process. The formation of Al and AlB2 are the keys to the improvement of hydrogenation properties. It is concluded that Li₃AlH₆ is a good destabilizing agent for the NaBH₄ system.     

CoFe2O4 synthesized via a solvothermal method for improved dehydrogenation of NaAlH4

The effect of cobalt ferrite (CoFe₂O₄) nanopowder synthesised through a solvothermal method on the dehydrogenation properties of sodium alanate (NaAlH₄) was studied for the first time. The onset decomposition temperature of NaAlH₄ is significantly reduced after milling with CoFe₂O₄, in which the 10 wt% CoFe₂O₄-doped NaAlH₄ sample starts to decompose at ～100 °C. In contrast, the as-milled NaAlH₄ begins to decompose at ～200 °C, ～100 °C higher than the doped sample. With respect to desorption kinetic at constant temperature of 150 °C, the 10 wt% CoFe₂O₄-doped NaAlH₄ sample desorbed ～2.2 wt% hydrogen within 30 min, whereas the as-milled NaAlH₄ only desorbed below 0.2 wt% hydrogen. The Kissinger plot exhibited that the apparent activation energy (E_A) for hydrogen release from NaAlH₄ is significantly reduced after adding with 10 wt% CoFe₂O₄-doped NaAlH₄. The E_A values for the first and second stage dehydrogenation of the 10 wt% CoFe₂O₄-doped NaAlH₄ composite are calculated as 80.3 and 88.2 kJ/mol, respectively, and these values are reduced at approximately 34.3 and 30.5 kJ/mol compared with the as-milled NaAlH₄ (114.6 and 118.7 kJ/mol, respectively). Based on the X-ray diffraction result, the enhancement of desorption properties of NaAlH₄ with the presence of CoFe₂O₄ is presumably due to the synergistic catalytic effect played by new active species (Co₃O₄ and Fe) that in situ formed during the desorption process.     

Enhancing the dehydrogenation properties of LiAlH4 using K2NiF6 as additive

Lithium alanate (LiAlH₄) is a material that can be potentially used for solid-state hydrogen storage due to its high hydrogen content (10.5 wt%). Nevertheless, a high desorption temperature, slow desorption kinetic, and irreversibility have restricted the application of LiAlH₄ as a solid-state hydrogen storage material. Hence, to lower the decomposition temperature and to boost the dehydrogenation kinetic, in this study, we applied K₂NiF₆ as an additive to LiAlH₄. The addition of K₂NiF₆ showed an excellent improvement of the LiAlH₄ dehydrogenation properties. After adding 10 wt% K₂NiF₆, the initial decomposition temperature of LiAlH₄ within the first two dehydrogenation steps was lowered to 90 °C and 156 °C, respectively, that is 50 °C and 27 °C lower than that of the аs-milled LiAlH₄. In terms of dehydrogenation kinetics, the dehydrogenation rate of K₂NiF₆-doped LiAlH₄ sample was significantly higher as compared to аs-milled LiAlH₄. The K₂NiF₆-doped LiAlH₄ sample can release 3.07 wt% hydrogen within 90 min, while the milled LiAlH₄ merely release 0.19 wt% hydrogen during the same period. According to the Arrhenius plot, the apparent activation energies for the desorption process of K₂NiF₆-doped LiAlH₄ are 75.0 kJ/mol for the first stage and 88.0 kJ/mol for the second stage. These activation energies are lower compared to the undoped LiAlH₄. The morphology study showed that the LiAlH₄ particles become smaller and less agglomerated when K₂NiF₆ is added. The in situ formation of new phases of AlNi and LiF during the dehydrogenation process, as well as a reduction in particle size, is believed to be essential contributors in improving the LiAlH₄ dehydrogenation characteristics.     

Reversible hydrogen storage in a 3NaBH4/YF3 composite

A 3NaBH₄/YF₃ hydrogen storage composite was prepared through ball milling and its hydrogen sorption properties were investigated. It is shown that NaBH₄ does not react with YF₃ during ball milling. The dehydrogenation of the composite starts at 423 °C, which is about 100 °C lower than the dehydrogenation temperature of pure NaBH₄, with a mass loss of 4.12 wt%. Pressure–Composition–Temperature tests reveal that the composite has reversible hydrogen sorption performance in the temperature range from 350 °C to 413 °C and under quite low hydrogenation plateau pressures (<1 MPa). Its maximum hydrogen storage capacity can reach up to 3.52 wt%. The dehydrogenated composite can absorb 3.2 wt% of hydrogen within 5 min at 400 °C. Based on the Pressure–Composition–Temperature analyses, the hydrogenation enthalpy of the composite is determined to be −46.05 kJ/mol H₂, while the dehydrogenation enthalpy is 176.76 kJ/mol H₂. The mechanism of reversible hydrogen sorption in the composite involves the decomposition and regeneration of NaBH₄ through the reaction with YF₃. Therefore, the addition of the YF₃ to NaBH₄ as a reagent forms a reversible hydrogen storage composite.     

Integrated Ni/Nb2O5 nanocatalytic agent dose for improving the hydrogenation/dehydrogenation kinetics of reacted ball milled MgH2 powders

Reactive ball milling (RBM) technique was employed to synthesize ultrafine powders of MgH₂, using high energy ball mill operated at room temperature under 50 bar of a hydrogen gas atmosphere. The MgH₂ powders obtained after 200 h of continuous RBM time composed of β and γ phases. The powders possessed nanocrystalline characteristics with an average grain of about 10 nm in diameter. The time required for complete hydrogen absorption and desorption measured at 250 °C was 500 s and 2500 s, respectively. In order to improve the hydrogenation/dehydrogenation kinetics of as synthesized MgH₂ powders, three different types of nanocatalysts (metallic Ni, Nb₂O₅ and (Ni)_x/(Nb₂O₅)_y) were utilized with different weight percentages and independently ball milled with the MgH₂ powders for 50 h under 50 bar of a hydrogen gas atmosphere. The results showed that the prepared nanocomposite MgH₂/5Ni/5Nb₂O₅ powders possessed superior hydrogenation/dehydrogenation characteristics, indexed by low values of activation energy for β-phase (68 kJ/mol) and γ-phase (74 kJ/mol). This nanocomposite system showed excellent hydrogenation/dehydrogenization behavior, indexed by the short time required to uptake (41 s) and release (121 s) of 5 wt% H₂ at 250 °C. At this temperature the synthesized nanocomposite powders possessed excellent absorption/desorption cyclability of 180 complete cycles. No serious degradation on the hydrogen storage capacity could be detected and the system exhibited nearly constant absorption and desorption values of +5.46 and −5.46 wt% H₂, respectively.     

Desorption properties of LiAlH4 doped with LaFeO3 catalyst

The addition of a catalyst and ball milling process was found to be one of the efficient method to reduce the decomposition temperature and improve the desorption kinetics of lithium aluminium hydride (LiAlH₄). In this paper, a transition metal oxide, LaFeO₃ was used as a catalyst. Decomposition temperature of the 10 wt% of LaFeO₃-doped LiAlH₄ system was found to be lowered from 143 °C to 103 °C (first step) and from 175 °C to 153 °C (second step), respectively. In isothermal desorption kinetics, the amount of hydrogen released of the doped sample was improved to 3.9 wt% in 2.5 h at 90 °C. Meanwhile, the undoped sample had released less than 1.0 wt% of hydrogen under the same condition. The activation energy of the LaFeO₃-doped LiAlH₄ sample was measured to be 73 kJ/mol and 90 kJ/mol for the first two dehydrogenation reactions compared to 107 kJ/mol and 119 kJ/mol for the undoped sample. The improvements of desorption properties were the results from the formation of LiFeO₂, Fe and La or La-containing phase during the heating process.     

Enabling easy and efficient hydrogen release below 80 °C from NaBH4 with multi-hydroxyl xylitol

NaBH₄ has been widely considered as a low-cost hydrogen storage material with high gravimetric hydrogen capacity of about 10 wt%. However, its strong thermodynamic stability severely hinders the application of NaBH₄ to obtain hydrogen. In this study, multi-hydroxyl xylitol, for the first time, was chosen as a protic hydrogen carrier to destabilize and react with NaBH₄. The NaBH₄-xylitol composites were easily prepared through hand-milling, which leads to the fast liberation of hydrogen from moderate temperatures lower than 80 °C with more than 3.6 wt% hydrogen desorption capacity. Besides, over 80% of hydrogen could be efficiently released in NaBH₄-xylitol composite. The dehydrogenation process is proved to be completely impurity-free and controllable by altering the NaBH₄/xylitol ratios and operating temperatures. The reaction between NaBH₄ and xylitol is investigated to be similar to an alcoholysis procedure, however no observable liquid phase takes place before and after the dehydrogenation of NaBH₄-xylitol composite, showing a great potential to produce hydrogen under low temperatures through solid-state carriers.     

Effect of milling duration on hydrogen storage thermodynamics and kinetics of Mg-based alloy

Mechanical milling is widely recognized as the best method to prepare nano-structured magnesium based hydrogen storage materials. The composites La₇Sm₃Mg₈₀Ni₁₀ + 5 wt% TiO₂ (named La₇Sm₃Mg₈₀Ni₁₀–5TiO₂) whose structures are nano-crystal and amorphous accompanied by great hydrogen absorption and desorption properties were fabricated by mechanical milling. The research focuses on the effect of milling duration on the thermodynamics and dynamics. The instruments of researching the gaseous hydrogen storing performances include Sievert apparatus, DSC and TGA. The calculation of dehydrogenation activation energy was realized by applying Arrhenius and Kissinger formulas. The calculation results show the specimen milled for 10 h exhibits the optimal activation performance and hydrogenation and dehydrogenation kinetics. Extending or shrinking the milling duration will lead to the degradation of hydrogen storage performances. The as-milled (10 h) alloy at the full activated state can absorb 4 wt% hydrogen in 87 s at 473 K and 3 MPa and release 3 wt% H₂ in 288 s at 573 K and 1 × 10⁻⁴ MPa. The changed milling durations have little impact on the thermodynamic properties of experimental samples and the enthalpy change (ΔH) of the alloy milled for 10 h is 74.23 kJ/mol. Moreover, it is found that the as-milled (10 h) alloy displays the minimum apparent activation energy of dehydrogenation (59.1 kJ/mol), suggesting the optimal hydrogen storing property of the as-milled (10 h) alloy.     

Synthesis of low-cost biomass charcoal-based Ni nanocatalyst and evaluation of their kinetic enhancement of MgH2

In this study, a low-cost biomass charcoal (BC)-based nickel catalyst (Ni/BC) was introduced into the MgH₂ system by ball-milling. The study demonstrated that the Ni/BC catalyst significantly improved the hydrogen desorption and absorption kinetics of MgH₂. The MgH₂ + 10 wt% Ni/BC-3 composite starts to release hydrogen at 187.8 °C, which is 162.2 °C lower than the initial dehydrogenation temperature of pure MgH₂. Besides, 6.04 wt% dehydrogenation can be achieved within 3.5 min at 300 °C. After the dehydrogenation is completed, MgH₂ + 10 wt% Ni/BC-3 can start to absorb hydrogen even at 30 °C, which achieved the absorption of 5 wt% H₂ in 60 min under the condition of 3 MPa hydrogen pressure and 125 °C. The apparent activation energies of dehydrogenation and hydrogen absorption of MgH₂ + 10 wt% Ni/BC-3 composites were 82.49 kJ/mol and 23.87 kJ/mol lower than those of pure MgH₂, respectively, which indicated that the carbon layer wrapped around MgH₂ effectively improved the cycle stability of hydrogen storage materials. Moreover, MgH₂ + 10 wt% Ni/BC-3 can still maintain 99% hydrogen storage capacity after 20 cycles. XRD, EDS, SEM and TEM revealed that the Ni/BC catalyst evenly distributed around MgH₂ formed Mg₂Ni/Mg₂NiH₄ in situ, which act as a “hydrogen pump” to boost the diffusion of hydrogen along with the Mg/MgH₂ interface. Meanwhile, the carbon layer with fantastic conductivity enormously accelerated the electron transfer. Consequently, there is no denying that the synergistic effect extremely facilitated the hydrogen absorption and desorption kinetic performance of MgH₂.     

Characterization of microstructure,hydrogen storage kinetics and thermodynamics of a melt-spun Mg86Y10Ni4 alloy

Ternary Mg₈₆Y₁₀Ni₄ alloy was successfully prepared by vacuum induction melting and subsequent melt-spinning technique. The phase composition and microstructure of the melt-spun and hydrogenated samples were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy measurements. The melt-spun alloy had an amorphous structure, and it transformed into nanocrystalline during the first hydrogenation process. The hydrogenated sample was composed of MgH₂, Mg₂NiH₄, YH₂, and a small amount of YH₃. The hydrogen absorption/desorption kinetics and thermodynamics were measured by Sievert's apparatus at various temperatures. It was found that the melt-spun Mg₈₆Y₁₀Ni₄ alloy could be fully activated after five hydrogenation and dehydrogenation cycles at 380 °C, and it exhibited a reversible gravimetric hydrogen storage capacity of about 5.3 wt%. The enhanced hydrogen sorption kinetics during the first few cycles can be attributed to the increased specific surface caused by the pulverization and cracking of the alloy particles. The activation energy for dehydrogenation reaction was determined to be 67 kJ/mol and 71 kJ/mol by using Arrhenius equation and Kissinger equation respectively. The thermodynamics of the sample was also evaluated by pressure–composition–isotherms, and the results shown that the enthalpy and entropy changes of Mg/MgH₂ transformation in the Mg₈₆Y₁₀Ni₄ alloy were slightly higher than that of pure Mg/MgH₂.     

Study on reversible hydrogen uptake and release of 1,2-dimethylindole as a new liquid organic hydrogen carrier

Indole derivatives have been considered as promising liquid organic hydrogen carriers (LOHCs) for onboard hydrogen storage applications. Here a new member of indole family, 1,2-dimethylindole (1,2-DMID), was reported as a potential liquid organic hydrogen carrier with a hydrogen storage content of 5.23 wt%, a meting point of 55 °C and a boiling point of 260 °C. Full hydrogenation and dehydrogenation of 1,2-DMID can be achieved with fast kinetics under mild conditions. The hydrogenation of 1,2-DMID followed the first order kinetics with an apparent activation energy of 85.1 kJ/mol. Dehydrogenation of fully hydrogenated product, octahydro-1,2-DMID was conducted over 5 wt% Pd/Al₂O₃ at 170–200 °C. The stored hydrogen can be completely released at 180 °C in 3 h and at 200 °C in 1 h. The energy barrier of dehydrogenation of octahydro-1,2-DMID was calculated to be 111.9 kJ/mol 3 times cycles of hydrogenation and dehydrogenation were employed to test the recycle ability of 1,2-DMID. The structures of intermediates were also discussed by means of Material Studio calculations.     

Synergetic effect of reactive ball milling and cold pressing on enhancing the hydrogen storage behavior of nanocomposite MgH2/10 wt% TiMn2 binary system

Intermetallic TiMn₂ compound was employed for improving the de/rehydrogenation kinetics behaviors of MgH₂ powders. The metal hydride powders, obtained after 200 h of reactive ball milling was doped with 10 wt% TiMn₂ powders and high-energy ball milled under pressurized hydrogen of 70 bar for 50 h. The cold-pressing technique was used to consolidate them into 36-green buttons with 12 mm in diameter. During consolidation, the hard TiMn₂ spherical powders deeply embedded into MgH₂ matrix to form homogeneous nanocomposite bulk material. The apparent activation energies of hydrogenation and dehydrogenation for the fabricated buttons were 19.3 kJ/mol and 82.9 kJ/mol, respectively. The present MgH₂/10 wt% TiMn₂ nanocomposite binary system possessed superior hydrogenation/dehydrogenation kinetics at 225 °C to absorb/desorb 5.1 wt% hydrogen at 10 bar/200 mbar H₂ within 100 s and 400 s, respectively. This new system revealed good cyclability of achieving 414 cycles within 600 h continuously without degradations. For the present study, the consolidated buttons were used as solid-state hydrogen storage for feeding proton-exchange membrane fuel cell through a house made Ti-reactor at 250 °C. This nanocomposite system possessed good capability for providing the fuel cell with hydrogen flow at an average rate of 150 ml/min. The average current and voltage outputs were 3 A and 5.5 V, respectively.     

Effect of Ti-based nanosized additives on the hydrogen storage properties of MgH2

MgH₂-based nanocomposites were synthesized by high-energy reactive ball milling (RBM) of Mg powder with 0.5–5 mol% of various catalytic additives (nano-Ti, nano-TiO₂, and Ti₄Fe₂O_x suboxide powders) in hydrogen. The additives were shown to facilitate hydrogenation of magnesium during RBM and substantially improve its hydrogen absorption-desorption kinetics. X-ray diffraction analysis showed the formation of nanocrystalline MgH₂ and hydrogenation of nano-Ti and Ti₄Fe₂O_x. The possible reduction of TiO₂ during RBM in hydrogen was not observed, which is in agreement with lower hydrogenation capacity of the corresponding composite, 5.7 wt% for Mg + 5 mol% nano-TiO₂ compared to 6.5 wt% for Mg + 5 mol% nano-Ti. Hydrogen desorption from the as-prepared composites was studied by Thermal Desorption Spectroscopy (TDS) in vacuum. A significant lowering of the hydrogen desorption temperature of MgH₂ by 30–90 °C in the presence of the additives is associated with lowering activation energy from 146 kJ/mol for nanosized MgH₂ down to 74 and 67 kJ/mol for MgH₂ modified with nano-TiO₂ and Ti₄Fe₂O_0.3 additives, respectively. After hydrogen desorption at 300–350 °C, these materials are able to absorb hydrogen even at room temperature. It is shown that nano-structuring and addition of Ti-based catalysts do not decrease thermodynamic stability of MgH₂. The thermodynamic parameters, obtained from hydrogen desorption isotherms for the Mg–Ti₄Fe₂O_0.3 nanocomposite, ΔH_des = 76 kJ/mol H₂ and ΔS_des = 138 J/K·mol H₂, correspond to the reported literature values for pure polycrystalline MgH₂. Hydrogen absorption-desorption characteristics of the composites with nano-Ti remain stable during at least 25 cycles, while a gradual decay of the reversible hydrogen capacity occurred in the case of TiO₂ and Ti₄Fe₂O_x additives. Cycling stability of Mg/Ti₄Fe₂O_x was substantially improved by introduction of 3 wt% graphite into the composite.     

The consequence of silicon additive in isothermal decomposition of hydrides LiH,NaH, CaH2 and TiH2

The study focuses on hydrogen desorption characteristics of lithium hydride (LiH), sodium hydride (NaH), calcium hydride (CaH₂), and titanium hydride (TiH₂) which permits a possible path regarding challenging goals of US DOE standards. This research reported the consequences of enrichment in hydrogen uptake 7.02 wt% in lithium hydride, 7.71 wt% in sodium hydride, 5.91 wt% in calcium hydride, and 5.00 wt% in titanium hydride using silicon as an additive when evaluated with the help of volumetric technique. The ball milling process with silicon additive for LiH, NaH, CaH₂, and TiH₂ shows depletion in dehydrogenation temperatures 523 K, 453 K, 488 K, and 463 K respectively. Similarly, the reduction in the activation energies reported due to ball milling process with silicon additive are 37 kJ/mol for lithium hydride, 42 kJ/mol for sodium hydride, 56 kJ/mol for calcium hydride and 45 kJ/mol for titanium hydride compared with crystalline powder samples of the respective materials. The outcome of Fourier-transform infrared spectroscopy of milled hydride samples after decomposition intimate rapid decrease in transmittance intensities due to hydrogen release because of the destabilization effect caused by silicon additive. The porosity and sponginess in high-resolution Transmission electron microscopy images after dehydrogenation reveals the hydrogen desorption from the sample materials.     

The enhanced de/re‐hydrogenation performance of MgH2 with TiH2 additive

Catalytic effects of TiH₂ on hydrogenation/dehydrogenation kinetics of MgH₂ were investigated in this study. The TG analysis showed that the addition of the x wt% TiH₂ exhibited lower onset temperature of 160°C which is 100°C and 190°C lower than as‐milled and as‐received MgH₂. The dehydrogenation and hydrogenation kinetics were significantly improved compared with the pure MgH₂. The activation energy for the hydrogen desorption of MgH₂ was reduced from ?137.13 to ?77.58 kJ/mol by the addition of TiH₂. XRD and XPS results showed that the phase of TiH₂ remained same during the dehydrogenation without any intermediate formation confirming its role as catalyst.     

Mechanochemical modification of LiAlH4 with Fe2O3 - A combined DFT and experimental study

LiAlH₄ is a promising material for hydrogen storage, having the theoretical gravimetric density of 10.6 wt% H₂. In order to decrease the temperature where hydrogen is released, we investigated the catalytic influence of Fe₂O₃ on LiAlH₄ dehydrogenation, as a model case for understanding the effects transition oxide additives have in the catalysis process. Quick mechanochemical synthesis of LiAlH₄ + 5 wt% Fe₂O₃ led to the significant decrease of the hydrogen desorption temperature, and desorption of over 7 wt%H₂ in the temperature range 143–154 °C. Density functional theory (DFT)-based calculations with Tran-Blaha modified Becke-Johnson functional (TBmBJ) address the electronic structure of LiAlH₄ and Li₃AlH₆. ⁵⁷Fe Mössbauer study shows the change in the oxidational state of iron during hydrogen desorption, while the ¹H NMR study reveals the presence of paramagnetic species that affect relaxation. The electron transfer from hydrides is discussed as the proposed mechanism of destabilization of LiAlH₄ + 5 wt% Fe₂O₃.     

Two-dimensional Mo2C: An efficient promoter for hydrogen storage and release from a liquid organic hydrogen carrier

Two-dimensional Mo₂C (2D-Mo₂C) is reported for the first time as an effective promoter of a Pt/Al₂O₃ catalyst for both the hydrogenation and dehydrogenation of the liquid organic hydrogen carrier (LOHC) pair, dibenzyltoluene (DBT) and perhydro-dibenzyltoluene (H18-DBT), respectively. Addition of 6.2 wt% 2D-Mo₂C to a Pt/Al₂O₃ catalyst resulted in a significant increase in both the degree of hydrogenation and dehydrogenation compared to the unpromoted catalyst. An analysis of the initial (120 min) perhydro-DBT dehydrogenation kinetics in the temperature range of 270–330 °C, resulted in a reduction in apparent activation energy from 119.5 ± 3.8 kJ/mol for the Pt/Al₂O₃ catalyst to 110.4 ± 5.6 kJ/mol for the 6.2 wt% 2D-Mo₂C/Pt/Al₂O₃ catalyst. The 6.2 wt% 2D-Mo₂C/Pt/Al₂O₃ catalyst was also more stable than the unpromoted catalyst over several consecutive cycles of hydrogenation and dehydrogenation. Catalyst characterization showed that addition of 2D-Mo₂C resulted in an increase in particle size and electron density of the Pt, which enhanced both the hydrogenation and dehydrogenation reactions, despite the fact that the 2D-Mo₂C alone was inactive for both reactions.     

Enhancing hydrogen storage performance of MgH2 through the addition of BaMnO3 synthesized via solid-state method

Currently, magnesium hydride (MgH₂) as a solid-state hydrogen storage material has become the subject of major research owing to its good reversibility, large hydrogen storage capacity (7.6 wt%) and affordability. However, MgH₂ has a high decomposition temperature (>400 °C) and slow desorption and absorption kinetics. In this work, BaMnO₃ was synthesized using the solid-state method and was used as an additive to overcome the drawbacks of MgH₂. Interestingly, after adding 10 wt% of BaMnO₃, the initial desorption temperature of MgH₂ decreased to 282 °C, which was 138 °C lower than that of pure MgH₂ and 61 °C lower than that of milled MgH₂. For absorption kinetics, at 250 °C in 2 min, 10 wt% of BaMnO₃-doped MgH₂ absorbed 5.22 wt% of H₂ compared to milled MgH₂ (3.48 wt%). Conversely, the desorption kinetics also demonstrated that 10 wt% of BaMnO₃-doped MgH₂ samples desorbed 5.36 wt% of H₂ at 300 °C within 1 h whereas milled MgH₂ only released less than 0.32 wt% of H₂. The activation energy was lowered by 45 kJ/mol compared to that of MgH₂ after the addition of 10 wt% of BaMnO₃. Further analyzed by using XRD revealed that the formation of Mg_0·9Mn_0·1O, Mn₃O₄ and Ba or Ba-containing enhanced the performance of MgH₂.     

Hydrogen generation from the ball milled composites of sodium and lithium borohydride (NaBH4/LiBH4) and magnesium hydroxide (Mg(OH)2) without and with the nanometric nickel (Ni) additive

The composites of (NaBH₄+2Mg(OH)₂) and (LiBH₄+2Mg(OH)₂) without and with nanometric Ni (n-Ni) added as a potential catalyst were synthesized by high energy ball milling. The ball milled NaBH₄-based composite desorbs hydrogen in one exothermic reaction in contrast to its LiBH₄-based counterpart which dehydrogenates in two reactions: an exothermic and endothermic. The NaBH₄-based composite starts desorbing hydrogen at 240 °C. Its ball milled LiBH₄-based counterpart starts desorbing at 200 °C. The latter initially desorbs hydrogen rapidly but then the rate of desorption suddenly decelerates. The estimated apparent activation energy for the NaBH₄-based composite without and with n-Ni is equal to 152 ± 2.2 and 157 ± 0.9 kJ/mol, respectively. In contrast, the apparent activation energy for the initial rapid dehydrogenation for the LiBH₄-based composite is very low being equal to 47 ± 2 and 38 ± 9 kJ/mol for the composite without and with the n-Ni additive, respectively. XRD phase studies after volumetric isothermal dehydrogenation tests show the presence of NaBO₂ and MgO for the NaBH₄-based composite. For the LiBH₄-based composite phases such as MgO, Li₃BO₃, MgB₂, MgB₆ are the products of the first exothermic reaction which has a theoretical H₂ capacity of 8.1 wt.%. However, for reasons which are not quite clear, the first reaction never goes to full completion even at 300 °C desorbing ∼4.5 wt.% H₂ at this temperature. The products of the second endothermic reaction for the LiBH₄-based composite are MgO, MgB₆, B and LiMgBO₃ and the reaction has a theoretical H₂ capacity of 2.26 wt.%. The effect of the addition of 5 wt.% nanometric Ni on the dehydrogenation behavior of both the NaBH₄-and LiBH₄-based composites is rather negligible. The n-Ni additive may not be the optimal catalyst for these hydride composite systems although more tests are required since only one n-Ni content was examined.