Study on dehydrogenation and oxidation kinetics mechanisms of micron α-AlH3 in an oxidative atmosphere

Aluminum hydride (AlH₃) exhibits attractive properties, such as high hydrogen/energy storage, relatively good stability, and low dehydrogenation temperature. Thus, AlH₃ has appreciable prospects as a component in solid propellant for promoting the specific impulse of rocket engines and for effectively reducing the erosion of engine nozzles. The TG-MS, SEM, XRD, XPS, and EDS results show that the thermal reaction of AlH₃ is divided into three stages: (1) Dehydrogenation (below 210 °C, 2AlH₃→2Al+3H₂) starts from the inherent defects on the surface with an incomplete decomposition due to the passivation reaction in which an amorphous Al₂O₃ layer is formed to encapsulate the contained hydrogen. This is accompanied by nucleation and growth of Al nuclei from the outer particles to the inner particles and the formation of H₂O via oxidation of the generated hydrogen. (2) The primary oxidation of Al (210–650 °C, 4Al+3O₂→2γ-Al₂O₃) is attributed to a discontinuous layer of γ-Al₂O₃, which is transformed from amorphous Al₂O₃ that results in the reaction of naked residual Al and O₂. (3) The secondary oxidation of residual Al (above 650 °C, 4Al+3O₂→2α-Al₂O₃) occurs because of the crystal conversion from γ-Al₂O₃ to α-Al₂O₃, which leads to the shrinkage of the oxide shell and to the formation of cracks. Also, melting of residual Al breaks the shell, and this induces further oxidation. The results obtained for the microscopic kinetics mechanisms of dehydrogenation and oxidation of AlH₃, show a clear direction for research regarding modifications of AlH₃ as the theoretical foundation and are beneficial for the wide use of AlH₃ in applications such as solid propellant as an energetic material and as a hydrogen source for fuel cells.     

Stability,reactivity and decomposition kinetics of surface passivated α-AlH3 crystals

As a novel high energy fuel, aluminum hydride (AlH₃) has great potential in the field of solid propellants because of its high hydrogen capacity, which can significantly improve the specific impulse of solid propellants. In order to improve the stability of α-AlH₃, hydrochloric acid has been used to stabilize AlH₃ and the stabilization mechanism has been investigated. Various characterization techniques including scanning electron microscopy, X-ray electron spectrometer, X-ray diffraction, thermal analysis, and vacuum stability test have been employed to investigate the morphology, crystal structure, thermal stability, and decomposition kinetics of raw and passivated α-AlH₃. The results showed that the honeycomb-like structures could be formed on the surface of α-AlH₃ after passivation. First of all, the initial decomposition temperatures of the passivated samples were slightly increased. In particular, for the optimized sample with 105 min passivation time (AlH₃-105min), the initial decomposition temperature (173.4 °C) is increased by 5.6 °C. Moreover, the total decomposition time (1652 min) is improved by about 50% than that of the raw sample (1098 min). Besides, the decomposition activation energies (E_a) of passivated samples are much higher than that of the raw sample (84.8 kJ mol^?1), in which the optimized sample (AlH₃-105min) reaches 107.1 kJ. mol^?1. The decomposition kinetics model may change from 3-D nucleation and nucleus growth model to 2-D nucleation and nucleus growth model. It demonstrates the passivated samples have a lower decomposition rate and higher thermal stability. The stabilization mechanism is as follows: removing the impurities on the surface and accelerating the hydrolysis reaction of AlH₃ to generate complete and dense oxide layers.     

Mechanochemical synthesis of the α-AlH3/LiCl nano-composites by reaction of LiH and AlCl3: Kinetics modeling and reaction mechanism

Mechanochemical activation-assisted synthesis, compared to conventional wet methods, exhibits advantages such as higher chemical homogeneity and better quality of the resulting nano-crystals, in preparing nano-sized aluminum hydride (AlH₃). Solid/liquid state milling is effective and convenient for production of α-AlH₃/LiCl nano-composites through a mechanochemical reaction of LiH and AlCl₃ in [2-Eim] OAc. However, the kinetics of this process has not been thoroughly studied. In this work, we studied the kinetics of mechanochemical synthesis of α-AlH₃/LiCl nano-composite to understand the reaction process and find the optimum milling parameters (including shortest time to achieve high-quality product). We performed isothermal desorption tests for as-milled samples at 80 °C. Structures of the products were analyzed using X-ray powder diffraction. Johnson-Mehl-Avrami (JMA) model was adopted to describe the process, which contains transformation fraction and chemical reaction controlled regime. Kinetics of mechanochemical reaction at the beginning was limited by diffusion rate of the reagent, and later controlled by nucleation of AlH₃. By fitting the experimental data, apparent activation energy for the mechanochemical reaction was calculated as 39.6 kJ/mol.     

Simultaneous dehydrogenation of 1,4- butanediol to γ-butyrolactone and hydrogenation of benzaldehyde to benzyl alcohol mediated over competent CeO2–Al2O3 supported Cu as catalyst

Hydrogen being a dynamically impending energy transporter is widely used in hydrogenation reactions for the synthesis of various value added chemicals. It can be obtained from dehydrogenation reactions and the acquired hydrogen molecule can directly be utilized in hydrogenation reactions. This correspondingly avoids external pumping of hydrogen making it an economical process. We have for the first time tried to carryout 1,4-butanediol dehydrogenation and benzaldehyde hydrogenation simultaneously over ceria-alumina supported copper (Cu/CeO₂–Al₂O₃) catalyst. In this concern, 10 wt% of Cu supported on CeO₂–Al₂O₃ (3:1 ratio) was synthesized using wet impregnation method. The synthesized catalyst was then characterized by various analytical methods such as BET, powder XRD, FE-SEM, H₂-TPR, NH₃ and CO₂-TPD, FT-IR and TGA. The catalytic activity towards simultaneous 1,4-butanediol dehydrogenation and benzaldehyde hydrogenation along with their individual reactions was tested for temperature range of 240 °C–300 °C. The physicochemical properties enhanced the catalytic activity as clearly interpreted from the results obtained from the respective characterization data. The best results were obtained with 10 wt% of Cu supported on CeO₂–Al₂O₃ (3:1 ratio) catalyst with benzaldehyde conversion of 34% and 84% selectivity of benzyl alcohol. The conversion of 1,4-butanediol was seen to be 90% with around 95% selectivity of γ-butyrolactone. The catalyst also featured physicochemical properties namely increased surface area, higher dispersion and its highly basic nature, for the simultaneous reaction towards a positive direction. In terms of permanence, the Cu/CeO₂–Al₂O₃ (10CCA) catalyst was quite steady and showed stable activity up to 24 h in time on stream profile.     

Improvement of the proton exchange membrane fuel cell (PEMFC) performance at low-humidity conditions by adding hygroscopic γ-Al2O3 particles into the catalyst layer

In this study, hygroscopic γ-alumina particles were added into the catalyst layer of membrane electrode assemblies (MEAs) to improve the wettability and performance of PEMFC at low-humidity conditions. Hygroscopic γ-alumina particles with a BET surface area of 442 m² g⁻¹ and an average pore diameter of 9 nm were synthesized by a three-step sol–gel procedure. Uniform Pt/C/γ-alumina catalyst ink was prepared by utilizing an ultrasonic method, and then sprayed on commercial hydrophobic carbon clothes to serve as the catalyst layer. The water contact angles of the catalyst layer with various amounts of γ-alumina additions 0%, 10%, 20% and 40% were measured to be 136°, 109°, 79° and 0°, respectively. Effect of adding γ-alumina particles into the catalyst layer on the single cell performance was investigated under different temperatures of the electrode humidifier. The increased wettability of the cathode catalyst layer with γ-alumina addition reduced the cell performance due to water flooding, which demonstrates the hygroscopic characteristic of γ-alumina particles. On the other hand, when the γ-alumina particles were added into the anode catalyst layer, it was found that the MEA with 10% γ-alumina addition had the highest current density at anode humidifier temperatures ranging from 25 to 55 °C. Nevertheless, the MEA with 40% γ-alumina addition into the anode catalyst layer showed the lowest current density because of the high electrical resistance of the catalyst layer and the water flooding in the anode caused by excess water absorption. The increased wettability of the anode catalyst layer by an appropriate amount of γ-alumina additions also enhances the water adsorption of the anode due to back diffusion.     

Enhanced photoelectrochemical performance of an α-Fe2O3 nanorods photoanode with embedded nanocavities formed by helium ions implantation

Hematite is a prospective semiconductor in photoelectrochemical (PEC) water oxidation field due to its suitable bandgap for the solar spectrum absorption. Nevertheless, the low transfer and separation efficiency of the charge carriers are restricted by its diffusion length of hole which is 2–4 nm and further reduce the PEC performance. Here, we report an innovative method, by introducing nanocavities into the α-Fe₂O₃ nanorod arrays photoanodes through helium ions implantation, to improve the charge carriers' transfer and separation efficiency and further to enhance water oxidation performance. The result indicates that, the photocurrent density of nanocavities embedded α-Fe₂O₃ photoanode (S2-A sample) reaches 1.270 mA/cm² at 1.6 V vs. RHE which is 1-fold higher than that of the pristine α-Fe₂O₃ (0.688 mA/cm²) and the photocurrent density of S2-A sample reaches 0.652 mA/cm² at 1.23 V vs. RHE. In this work, the ion implantation combined with post annealing method is found to be a valid method to improve the photoelectrochemical performance, and it also can be further used to modify the other semiconductor photoelectrodes materials.     

Improvement in the hydrogenation-dehydrogenation performance of a Mg–Al alloy by graphene supported Ni

Mg-based materials are very promising candidates for hydrogen storage. In this paper, the graphene supported Ni was introduced to the Mg₉₀Al₁₀ system by hydrogenation synthesis (HS) and mechanical milling (MM). The 80 wt%Ni@Gn catalyst was synthesized by a facile chemical reduction method. The microstructures of the catalyst and composite show that Ni nanoparticles are well supported on the surface of graphene and they are dispersed uniformly on the surface of MgH₂ particles. After heating to 450 °C and holding at 340 °C for 2 h subsequently under 2.0 MPa hydrogen pressure, all the samples are almost completely hydrogenated. According to the temperature programmed desorption test, the Mg₉₀Al₁₀-8(80 wt%Ni@Gn) composite could desorb 5.85 wt% H₂ which comes up to 96% of the theoretical hydrogen storage capacity. Moreover, it shows the optimal hydriding/dehydriding performance, absorbing 5.11 wt% hydrogen within 400 s at 523 K, and desorbing 5.81 wt% hydrogen within 1800 s at 573 K.     

Hydrogen desorption behaviors of γ-AlH3: Diverse decomposition mechanisms for the outer layer and the inner part of γ-AlH3 particle

Aluminium hydride (AlH₃) is a promising hydrogen storage material because it possesses a high theoretical hydrogen capacity of 10.01 wt%. However, the stability and decomposition mechanism of some AlH₃ polymorphs (e.g., γ-AlH₃) still remain unclear. In this work, the hydrogen desorption behaviours of γ-AlH₃ with or without TiF₃ addition were investigated by hydrogen desorption measurement and thermal analysis. It was revealed that the decompositions of the outer layer and the inner part of γ-AlH₃ particle follow diverse decomposition mechanisms. The outer layer of γ-AlH₃ particle tends to decompose directly, while the inner part of γ-AlH₃ particle prefers to first transform to more stable α-AlH₃ and then decompose. TiF₃ addition significantly lowers the temperature for the direct decomposition of outer layer γ-AlH₃ by about 30 °C but scarcely impacts the decomposition of inner part γ-AlH₃, which further confirms the decomposition mechanism of γ-AlH₃. It was suggested that the particle size plays an important role in the thermal stability of γ-AlH₃. The results of this work will help understanding the decomposition mechanisms of other AlH₃ polymorphs for hydrogen storage.     

Enhanced performance of Ca-doped Pt/γ-Al2O3 catalyst for cyclohexane dehydrogenation

The promotion effects of Ca on catalytic performance of Ca-doped Pt/Al₂O₃ catalyst were investigated by varying Ca content and impregnation orders. The Ca-doped Pt/Al₂O₃ catalyst with atomic rate Ca/Pt = 5 exhibited the highest catalytic activity and stability. Detailed analyses show that low Ca content (Ca/Pt ＜ 5) could not efficiently promote hydrogen spillover from Pt-Al interface to the support and reduce the strong acid sites on the support. However, high Ca content (Ca/Pt > 5) decreases the initial activity due to the coverage of Ca on the support and increases the amount of Pt strongly interacting with Ca, which inhibits Pt reduction. Furthermore, the Ca promotion effect is more pronounced when Ca is added prior to Pt due to surface modification of Ca on Al₂O₃ support. This modified catalyst possesses more dispersed Pt, lower acidity and larger amount of spilled-over hydrogen.     

Mixed reforming of methane over Ni–K/CeO2–Al2O3: Study of catalyst performance and reaction kinetics

Syngas can be effectively produced by mixed reforming of methane (MRM). In this work, the performance of Ni–K/CeO₂–Al₂O₃ catalyst in this process was investigated in a fixed-bed reactor in the 923–1073 K range. Both potassium and ceria are renowned for improving the performance of Ni catalyst in the reforming process. The influence of reaction conditions (viz. temperature, space time, feed composition and time-on-stream) on the conversion of two reactants CH₄ and CO₂, yield of the products H₂ and CO and the H₂/CO ratio in syngas were studied. At T = 1073 K and W/Q₀ = 0.17 g-h/L (here, W and Q₀ denote catalyst mass and volumetric flow rate of feed), conversions of CH₄ and CO₂ were 91.2 and 80.1%. When S/C ratio (or steam-to-carbon ratio) in feed increased from 0.2 to 0.5 mol/mol, H₂/CO ratio at T = 1073 K changed from 1.32 to 2.14 mol/mol. The catalyst performed stably for 50 h of time-on-stream. Reaction kinetics was studied between 973 and 1073 K and power law kinetic model was suggested. The apparent activation energy values for consumption of CH₄ and CO₂ were found to be 33.3 and 45.5 kJ/mol, respectively. This work is expected to aid catalyst development and reactor design for the MRM process.     

Improvement of elevated temperature performance of LiMn1·5Ni0·5O4 cathode material by surface modification with LiCoO2

The surface-modified LiMn_1·5Ni_0·5O₄ materials by LiCoO₂ were prepared by a sol-gel method to improve the electrochemical performance of LiMn_1·5Ni_0·5O₄ and were characterised by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and transmission electron microscopy (TEM)-EDS. X-ray diffraction results indicate that all the samples (modified and pristine samples) have cubic spinel structures, and XRD, FTIR and TEM-EDS data reveal the formation of a solid solution contained Li–Co–Mn–Ni–O on the surface of particles. For the electrochemical properties, the modified material demonstrated dramatically enhanced reversibility and stability at elevated temperature. These improvements are attributed to the formation of the solid solution, and thus-formed solid-solution phase on the surface of LiMn_1·5Ni_0·5O₄ particle reduces the dissolution of Mn ion and stabilises the structure of the cathode material during the charge–discharge process.     

Electrochemical performance of porous α-LiVOPO4/C composite by solution combustion synthesis method

Porous α-LiVOPO₄/C composite was prepared by the solution combustion synthesis method followed by annealing at 500°C for a short annealing duration of just 50?min. The synthesised material was characterised by XRD, SEM, TEM, N₂ adsorption–desorption isotherms and charge–discharge tests. When tested at 0.05C rate for Li ion insertion properties, α-LiVOPO₄/C composite exhibits good cycle capability with an negligible capacity degradation for the first 31 cycles. However, it shows poor rate capability with initial discharge specific capacities of 127.6?mAh?g^??1 (0.05C), 109.1?mAh?g^??1 (0.1C) and 74.9?mAh?g^??1 (0.2C), respectively.     

γ-MgH2 induced by high pressure for low temperature dehydrogenation

The formation of metastable γ-MgH₂ upon application of ultra-high pressure and its dehydrogenation properties were studied. Magnesium-nickel alloy (14 wt.% Ni) was hydrogenated and compressed at ultra-high pressures of 2.5 and 4 GPa. The phase composition and desorption properties of the products were investigated. Powder X-ray diffraction indicated that some α-MgH₂ converted to γ-MgH₂ during compression. This resulted in the onset of hydrogen desorption at 60 °C under vacuum. Our findings thus show that application of ultra-high pressure can facilitate the formation of γ-MgH₂, which has a lower dehydrogenation temperature (≤200 °C) than α-MgH₂, which desorbs at temperatures above 300 °C. The metastable phase possessed a high hydrogen storage capacity of at least 4.5 wt.%. These properties revealed the potential of γ-MgH₂ as a future hydrogen storage material.     

Improvement of high-rate capability of alkaline Zn–MnO2 battery

Electrolytic dendritic-zinc powders of high surface area are prepared from an alkaline solution by a galvanostatic electrodeposition method. The surface morphology and microstructure of these powders are examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Cylindrical AA-size alkaline zinc–manganese dioxide (Zn–MnO₂) batteries made with powders in anode gels are assembled and tested. The electrochemical characteristics of the batteries are evaluated by means of the ac impedance method and the constant-current discharge experiments. It is found that the high-rate performance of cells with dendritic-zinc powders is much better than that of cells with conventional molten-zinc powders.     

Design of efficient Mn-doped α-Fe2O3/Ti-doped α-Fe2O3 homojunction for catalyzing photoelectrochemical water splitting

To produce clean chemical fuel of hydrogen efficiently, applying photocatalysts for conducting photoelectrochemical water splitting is indispensable. Hematite (α-Fe₂O₃) has been considered as one of the most effective photocatalysts for water oxidation due to excellent visible-light responses, high stability and source abundance properties, but low electrical conductivity and slow oxidation evolution kinetics limit its application. In this study, a novel α-Fe₂O₃ homojunction is constructed via doping Ti and Mn in two layers using two-step hydrothermal synthesis followed by one-step annealing process. Co-doping effect of Ti and Mn in α-Fe₂O₃ and growing sequence of Mn doped α-Fe₂O₃ (Mn:Fe₂O₃) and Ti doped α-Fe₂O₃ (Ti:Fe₂O₃) are also investigated to illustrate the efficient design of Mn:Fe₂O₃/Ti:Fe₂O₃ homojunction. The optimized Mn:Fe₂O₃/Ti:Fe₂O₃ electrode shows the highest photocurrent density of 2.10 mA/cm² at 1.60 V_RHE respectively comparing to those of 0.10, 1.20 and 0.22 mA/cm² for Ti:FeOOH, Ti:Fe₂O₃ and α-Fe₂O₃ electrodes. The outstanding performance of Mn:Fe₂O₃/Ti:Fe₂O₃ homojunction is attributed to the smaller charge-transfer resistance, higher carrier density, and less charge recombination. This work gives a rational design for hematite-based photocatalysts and successfully attains greatly improved photocatalytic ability for water oxidation. Development of homojunction using heteroatom doping in thus verified to be highly applicable on synthesizing promising photocatalysts.     

Evaluation of adding flash tank to solar combined ejector–absorption refrigeration system

Performance of the absorption cooling system is still a challenge due to the coefficient of performance (COP) that is generally poor when compared with the conventional vapor compression cycle. High solar radiation in hot climates is usually associated with high ambient temperature and consequently peak cooling demand. Absorption cooling cycles can be powered by solar but the performance is limited by heat source temperature (solar collector) and high ambient temperature that can affect the condensation process. Efficiency enhancement of the system components is essential to increase the COP of the system. A modification in the combined absorption–ejector cooling system is adopted. Adding a removable flash tank between the condenser and evaporator could improve entrainment ratio of the ejector, along with improving the cooling effect inside the evaporator. A computer simulation program is developed to evaluate the performance of the modified combined cycle using aqua-ammonia (NH₃–H₂O) refrigerant. The performance of the proposed combined cooling cycle is compared with basic absorption, and combined absorption–ejector cooling cycles. Results showed a significant improvement in the COP of the modified cycle at different operating conditions. Cooling effect and capacity of the evaporator is enhanced due to the reduction of flash gas delivered to the evaporator. Furthermore, the flash tank optimized the ejector entertainment ratio and consequently increasing the condenser pressure. This optimization will enable the system to perform well in hot climates where the condenser efficiency is limited by ambient temperature.     

Hydrogen production by catalytic decomposition of methane over Fe based bi-metallic catalysts supported on CeO2–ZrO2

Methane decomposition to produce hydrogen was studied over iron based bimetallic catalysts supported on cerium-zirconium oxide in a continuous flow fixed bed reactor at 700 °C. 15 wt% Fe/CeZrO₂ was prepared by wetness impregnation and the promoted Fe catalysts (15 Fe-5 Co/CeZrO₂ and 15 Fe-5 Mo/CeZrO₂) were prepared by co-impregnation technique. Mo promoted Fe catalyst exhibited the maximum surface area of 24.08 m²/g. X-ray diffraction studies revealed that Fe₂O₃, Co₃O₄ and MoO₃ were the phases present in freshly calcined catalysts, while the reduced catalysts consisted of phases including elemental Fe, Mo and Fe–Co alloys. Both X-ray diffraction and temperature programmed reduction studies confirmed the complete reduction of metal oxide species under H₂ at 700 °C. The catalytic activity of Fe/CeZrO₂ was enhanced upon addition of Co and Mo as promoters. The initial hydrogen yield on 15 Fe-5 Mo/CeZrO₂ was ~90% and it decreased with increase in time on stream (TOS), and finally stabilized around ~50% after 125 min of TOS. The Co promoted catalyst exhibited similar activity while the initial hydrogen yield on 15 Fe/CeZrO₂ was ~83% and dropped to ~33% after 125 min of TOS. Graphitic carbon, Fe₃C and Mo₂C phases were observed in the XRD patterns of spent catalysts along with elemental Fe and Fe–Co alloy. It was evident from temperature programmed oxidation results that coke formation which deactivates the catalyst was dominant in 15 Fe/CeZrO₂ when compared to the promoted (Co and Mo) Fe catalysts where carbon nanostructures were dominant. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) confirmed the formation of carbon nanostructures on the surface of spent catalysts. The Fe based catalysts supported both tip and base-growth mechanisms for the growth of carbon nanostructures.     

Enhanced dehydrogenation of LiBH4·NH3–LiAlH4 composites

Constructing reactive composite is a feasible approach to promote the dehydrogenation of high-capacity B-/Al-based complex hydrides, such as LiBH₄ and LiAlH₄. In this work, LiBH₄·NH₃ was firstly synthesized by exposing LiBH₄ to liquefied NH₃. In the heating process, the NH₃ was released from LiBH₄·NH₃ instead of reacting with LiBH₄. The LiBH₄·NH₃ was further manually mixed with LiAlH₄ at different ratio, and the resultant composites started releasing hydrogen in the range of 90–110 °C, which is due to the reaction between LiAlH₄ and NH₃. The residual LiBH₄ decomposed at ～400 °C, which is lower than that of pure LiBH₄. The total hydrogen desorption amount of the composite reaches up to maximum ～6.53 wt.%, without NH₃ release. The present work shows a new compositing way of complex hydrides for high-capacity hydrogen storage.     

Achievement of hydrogen production from autothermal steam reforming of methanol over Cu-loaded mesoporous CeO2 and Cu-loaded mesoporous CeO2–ZrO2 catalysts

Hydrogen production by oxidative steam reforming of methanol (OSRM) or autothermal steam reforming of methanol (ASRM) was investigated over Cu-loaded mesoporous CeO₂ and Cu-loaded mesoporous CeO₂–ZrO₂ catalysts, synthesized via a nanocasting process using MCM-48 as a hard template, followed by a deposition–precipitation technique. Various Cu contents were loaded on the mesoporous CeO₂ and CeO₂–ZrO₂ supports. The fresh and spent catalysts were characterized by N₂ adsorption–desorption, X-ray diffraction, temperature-programmed oxidation, and X-ray photoelectron spectroscopy. The ASRM results showed that 9 wt% Cu loading onto mesoporous CeO₂ and CeO₂–ZrO₂ provided the best catalytic performance with 100% methanol conversion and 60% H₂ yield at 350° and 300 °C, respectively. Furthermore, the time-on-stream stability testing of the 9 wt% Cu loading catalyst was at 168 h, and the CO selectivity of these two catalysts indicated that the addition of ZrO₂ into the catalyst reduced the CO selectivity during the ASRM process.