Hydrogen generation by aluminum‐water reaction in acidic and alkaline media and its reaction dynamics

The additives AlCl₃, CoCl₂, Al(OH)₃, Ca(OH)₂, and NaAlO₂ are added to water to regulate its pH value (pH = 2‐13) in this study. The effects of media on the aluminum‐water reaction are investigated. Up to an increase in temperature, the hydrogen generation rate in different media increases. H⁺, OH^?, Cl^?, or Co produced from the additive favors the initial removal of the oxide film and aluminum corrosion. Therefore, the initial hydrogen generation rate increases in acidic and alkaline media. The synergistic effect of the formed fresh Co and Cl^? catalyzes aluminum‐water reactions. However, the amount of hydrogen decreases with increasing mass of CoCl₂ because of agglomeration of the catalyst Co. The higher concentration of OH^? ions aids hydrogen generation. However, the reaction rate became slow after the rapid consumption of OH^?, when the concentration of OH^? was relatively small. Hydrogen is quickly generated and Al is completely reacted upon following additions of Al due to the cooperation between H⁺, Cl^?, OH^? ions, and the formed Al(OH)₃.     

Thermal energy storage by the chemical reaction augmentation of heat transfer and thermal decomposition in the CaO/Ca(OH)2 powder

The present paper is concerned with the utilization of a thermal decomposition reaction, Ca(OH)₂CaO + H₂O, for energy storage. One of the important problems in this case is how to heat up and decompose the powder of Ca(OH)₂ effectively, where the thermal conduction is poor.In this study, the effect of copper plates, which are placed in the powder of Ca(OH)₂ as heat-transfer fins, is investigated experimentally and numerically. The results show that the Cu-plates are very effective for heat transfer and the thermal decomposition, and that the optimum configuration of the Cu-plate is 5–10 cm in height and 0.5–1 cm in interval for the condition of this study.     

Thermal energy storage by the chemical reaction augmentation of heat transfer and thermal decomposition in the CaOCa(OH)2 powder

The present paper is concerned with the utilization of a thermal decomposition reaction, Ca(OH)₂?CaO + H₂O, for energy storage. One of the important problems in this case is how to heat up and decompose the powder of Ca(OH)₂ effectively, where the thermal conduction is poor.In this study, the effect of copper plates, which are placed in the powder of Ca(OH)₂ as heat-transfer fins, is investigated experimentally and numerically. The results show that the Cu-plates are very effective for heat transfer and the thermal decomposition, and that the optimum configuration of the Cu-plate is 5–10 cm in height and 0.5–1 cm in interval for the condition of this study.     

The kinetics research of thermochemical energy storage system Ca(OH)2/CaO

As one of the most promising thermochemical energy storage medium, research on the Ca(OH)₂/CaO system provides an important way of understanding energy storage/release rates of the entire energy storage system. In this paper, a high‐precision thermogravimetric analysis is adopted to investigate thermal decomposition processes of the Ca(OH)₂ samples in pure N₂ atmosphere at different heating rates. The results demonstrate that during the thermal decomposition process, two weight loss processes respectively occur during 623.15 ~ 773.15 and 873.15 ~ 973.15 K, and the weight loss rates are close to 21% and 2% severally. Multi‐heating rate methods are applied to the study of thermal decomposition dynamics. Findings show that the obtained kinetic parameters are related to reaction conversion, heating rate, and the chosen model‐methods. To further understand the decomposition mechanism of Ca(OH)₂, differential method, integral method, and multiple scanning method are used to deal with the experimental data. Through the most probable mechanism function analysis, under certain experimental conditions, thermal decomposition kinetics model of Ca(OH)₂ accords well with the shrinking cylinder mechanism. These conclusions provide theoretical bases for applying the Ca(OH)₂/CaO system to the thermochemical energy storage field. Copyright © 2016 John Wiley & Sons, Ltd.     

Ammonia decomposition over nickel catalysts supported on alkaline earth metal aluminate for H2 production

The Ni catalysts supported on alkaline earth metal aluminate compounds, Ni/AM-Al-O (AM = Mg, Ca, Sr, Ba) were synthesized to investigate the influence of their basic property on NH₃ decomposition activity. The basic strength of the catalysts was confirmed to correspond to that of added alkaline earth metal in the support materials (Ni/Mg–Al–O < Ni/Ca–Al–O < Ni/Sr–Al–O < Ni/Ba–Al–O) from CO₂-TPD measurement. This basic strength of the catalysts could influence the catalytic activity for NH₃ decomposition, which increased in order of the Ni/Mg–Al–O < Ni/Ca–Al–O < Ni/Sr–Al–O < Ni/Ba–Al–O catalysts. NH₃-TPSR showed that the strong basic property weakened H₂ adsorption but slightly strengthened N₂ adsorption for the catalysts except for the Ni/Mg–Al–O catalyst. From the kinetic analysis, the absolute value of the H₂ reaction order decreased with increasing basic strength of the catalysts, indicating that the strong basic property of the catalysts could alleviate the H₂ inhibition in ammonia decomposition.     

Effects of additives on the hydrogen generation of Al–H2O reaction at low temperature

Water displacement method is used to study the influence of temperatures (60–80°C), additives (Na₂CO₃, NaCl, Na₂CO₃/NaCl) and concentrations on the reaction characteristics and kinetics of Al–H₂O. Results show that the reaction rate and the hydrogen yield are enhanced with the increase of the temperature or by adding Na₂CO₃. The reaction rate is decreased by adding NaCl, but which has less effect on the hydrogen yield. For the mixture additive, Na₂CO₃ plays a key role in improving the hydrogen yield and the reaction rate. The influence degree of different factors is analyzed by orthogonal method. The most obvious factor is additive, but additive concentration has a minimum influence. The solid products are collected and analyzed by X‐ray diffraction and transmission electron microscopy. Al, Al(OH)₃ and AlO(OH) are detected. The spherical particles are obviously found at the initial reaction stage. However, they change to flocs at the end of reaction. Kinetic analysis shows that the reaction mechanism of Al–H₂O is changed by adding Na₂CO₃ or mixture, but it is not affected by adding NaCl. Moreover, the apparent activation energy of Al–H₂O is 74.49 kJ mol^?1, while it is only 43.03 kJ mol^?1 for Al–H₂O with 5 wt% Na₂CO₃ addition. Copyright © 2017 John Wiley & Sons, Ltd.     

Hydrogen generation from Al/NaBH4 hydrolysis promoted by Li-NiCl2 additives

On-demand hydrogen generation from solid-state Al/NaBH₄ hydrolysis activated by Li-NiCl₂ additives are elaborated in the present paper. Hydrogen generation amount and rate can be regulated by changing Al/NaBH₄ weight ratio, Li and NiCl₂ amount, hydrolytic temperature, etc. The optimized Al−10 wt.% Li−15 wt.% NiCl₂/NaBH₄ mixture (weight ratio of 1:1) yields 1778 ml hydrogen/1 g mixture with 100% efficiency within 50 min at 323 K. The improved hydrolytic performance comes from the effect of Li-NiCl₂ additives, which decrease aluminum particle size in the milling process and produce the catalytic promoter BNi₂/Al(OH)₃ in the hydrolytic process. Compared with the conventional reaction of Al and NaBH₄ in water, there is an interaction of Al/NaBH₄ hydrolysis which improves the hydrolytic kinetics of Al/NaBH₄ via the catalytic effect of hydrolysis by-products Al(OH)₃, BNi₂, and NaBO₂. The Al/NaBH₄ mixture may be applied as a portable hydrogen generation material. Our experimental data lay a foundation for designing practical hydrogen generators.     

Hydrogen generation by the reaction of Al with water promoted by an ultrasonically prepared Al(OH)3 suspension

A high-activity Al(OH)₃ suspension is prepared by the reaction of Al with water using an ultrasonic procedure. The above Al(OH)₃ suspension could considerably promote the Al-water reaction and hydrogen-generation under ambient condition. This Al(OH)₃ suspension has a good stability in air and its activity increases with decreasing the particle sizes of Al powder to prepare it. The mechanism analyses reveal that the Al(OH)₃ particles in ultrasonically prepared Al(OH)₃ suspensions are very fine, which could effectively dissociate water molecules and promote the hydration of the passive oxide film on Al particle surfaces, speeding up the Al-water reaction. As the present Al(OH)₃ suspension is chemically neutral and there is no special treatment for Al powder, the present method provides a viable way to generate hydrogen for portable application.     

Kinetics study on the generation of hydrogen from an aluminum/water system using synthesized aluminum hydroxides

Kinetics study on the generation of hydrogen from an Al/water system is performed. The reaction is affected by three major factors such as the concentration of hydroxyl ions (pH values), catalysts, and temperature. However, these factors are interacted and sometimes difficult to separate. This study demonstrates how these factors affect the generation of hydrogen in an Al/water system. Aluminum hydroxide, Al(OH)₃ (bayerite phase), synthesized using a chemical solution method, is proved to be a very effective catalyst for the reaction of Al and water. Approximately 95% yield (1300 mL) of hydrogen is produced from 1 g Al in 10 mL water using 3 g Al(OH)₃ catalyst at room temperature within 1 minute. The generation rate of hydrogen is accelerated due to the catalyst Al(OH)₃ and the exothermic heat. In this report, a ball‐mixing process, the ratio of Al:Al(OH)₃:H₂O, and the reacting temperatures are investigated to clarify the effect of catalyst Al(OH)₃. The synthesized Al(OH)₃ catalyst is found to reduce the activation energy of Al/water reaction from 158 kJ/mol to 73.3~76.9 kJ/mol. The roles of hydroxyl ions (ie, pH values), temperature, and catalyst on this phenomenal reaction are explained using a kinetics study and the concept of Fick first law. The 3 factors all improve the flux of hydroxyl ions through the passive Al₂O₃ layer; therefore, the generation of hydrogen is enhanced.     

Hydrogen system using novel additives to catalyze hydrogen release from the hydrolysis of alane and activated aluminum

Herein, we present a new system for the generation of hydrogen for use in portable power systems utilizing a two-step process that involves the thermal decomposition of α-AlH₃ (10 wt% H₂) followed by the hydrolysis of the activated aluminum (Al*) byproduct to release additional H₂. This study focuses on the use of promoter additives (PA) to catalyze the hydrolysis of Al*. Our study has shown that the addition of water to a Al*:PA composite results in an instantaneous release of hydrogen at room temperature, without the use of transition metal catalysts. This secondary reaction increases the overall hydrogen content of the material even when the weight of the added water is accounted for. Additionally, a one-step process, in which water is added directly to the α-AlH₃:PA composite, was also examined. Large amounts of H₂ and heat are released immediately following the addition of water and could serve a means to shorten the start-up time of the fuel cell as well as assist in the thermal decomposition of α-AlH₃. Our study compares the use of different PA’s and presents novel composites made of α-AlH₃ and ionic hydride additives in an attempt to obtain the best performance of a hydrogen source based on α-AlH₃. The composites were characterized by TGA-RGA, XRD, and SEM before and after H₂ release.     

Effect of CaO hydration and carbonation on the hydrogen production from sorption enhanced water gas shift reaction

Sorption enhanced water gas shift reaction (SEWGS) based on calcium looping is an emerging technology for hydrogen production and CO₂ capture. SEWGS involves mainly two reactions, the catalytic WGS reaction and the bulk carbonation of CaO with CO₂, and the solid product is CaCO₃, and the Ca(OH)₂ may be formed from the reaction of CaO with H₂O with the presence of steam in gas phase. The effect of Ca(OH)₂ and CaCO₃ on the catalytic WGS reaction and carbonation reaction was studied in a fluidized bed reactor. It was found that the hydrated sorbent and CaCO₃ did not show any catalytic reactivity toward WGS reaction at 400 °C. When the temperature was increased to 500 °C and 600 °C, the catalytic reactivity of hydrated sorbent was recovered partially, but this will depend on the steam fraction in gas phase, the recovery of fresh CaO surface from dehydration of Ca(OH)₂ may be the reason of catalytic reactivity recovery. CaCO₃ can catalyze the WGS reaction at the high-temperature (>600 °C), this may due to the CaCO₃ decomposition and recarbonation processes in which the CaO is transiently formed. The possible mechanism was discussed.     

Enhanced hydrogen generation from graphite-mixed aluminum hydroxides catalyzed Al/water reaction

The high catalytic effect of a graphite-mixed Al(OH)₃ nanoparticles (NPs) for the hydrogen generation from the reaction of Al and water is reported. These graphite-mixed Al(OH)₃ NPs were synthesized through a simple solvothermal procedure. Characterization using powder X-ray diffraction, field emission scanning electron microscopy (FESEM) and transmission electron microscope (TEM) were carried out. The results show that small amounts of graphite added into Al(OH)₃ NPs could significantly enhance the hydrolysis reaction of Al and H₂O reaction, releasing 1360 mL g^-1 hydrogen in 20 min at room temperature. This reaction accomplishes 100% yield of hydrogen using 1 g modified Al(OH)₃ in 1 g Al and 200 g water. The synthesized graphite-mixed Al(OH)₃ exhibits good activity-stability, which can be used for multiple Al/water reactions. This work demonstrates a novel and possible way to generate hydrogen for portable devices.     

Effect of CaO addition on acid properties of Ni–Ca/Al2O3 catalysts applied to ethanol steam reforming

Ni/Al₂O₃ catalysts containing 5 wt% of Ni and modified by addition of CaO (0–5 wt%) were tested in ethanol steam reforming reaction in order to reduce the dehydration ethanol reaction, which produces ethylene that may polymerize and produce coke. The catalysts were prepared by impregnation (I) and co-precipitation (C) methods. All catalysts were investigated for ethanol steam reforming and the catalytic performance was compared in terms of additive addition. The catalysts 5Ni–5Ca/Al (I) and 5Ni–5Ca/Al (C) were less selective to ethylene production and therefore were characterized by the following techniques: energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), temperature programmed reduction (TPR), X-ray absorption near edge structure (XANES), specific surface area by the BET method, scanning electron microcopy (SEM) and isopropanol decomposition reaction. By comparing the catalysts, the 5Ni–5Ca/Al (I) catalyst presented the lowest acidity and carbon deposition, and also presented no deactivation in 24 h of catalytic test.     

Achieving high performance for aluminum stabilized Li7La3Zr2O12 solid electrolytes for all solid‐state Li‐ion batteries: A thermodynamic point of view

For the solid‐state reaction synthesis of Al containing Li₇La₃Zr₂O₁₂, various precursors have been used. Since there is a lack of general agreement for choosing precursors, a quantitative approach to build a consensus is required. In this study, a thermodynamic point of view for selecting the precursors in the field of Li₇La₃Zr₂O₁₂ synthesis was covered according to the Gibbs free energy and enthalpy change of precursors' decomposition reactions. In terms of Gibbs free energy change calculations, LiOH, La(OH)₃, and Al(OH)₃ were favorable whereas, LiOH, La₂O₃, and Al(OH)₃ were the preferred precursors for the enthalpy change calculations. Pellets prepared by using the favored precursors calculated from enthalpy change showed improved densification, higher ionic conductivity (2.11 × 10^?4 S/cm), and lower activation energy (0.23 eV) compared with Gibbs free energy change. As a thermodynamically favored aluminum precursor, Al(OH)₃ was discussed in the present study and hinders the ionic conductivity in comparison to Al₂O₃.     

Hydrogen generation by splitting water with Al–Li alloys

In order to prevent the inert alumina film from forming on the surface of Al metal particles, Li is added into Al to form Al–Li alloy. It can improve the reactivity of Al with water. The prepared Al–Li alloy can rapidly split water to produce hydrogen. With increasing Li content of alloy, the hydrogen generation rate is promoted. The ultimate hydrogen yields of samples can reach 100%. The effect of initial water temperature on the hydrogen generation has been investigated. Even in the water at 0 °C, hydrogen can also be produced rapidly. Composition of solution has some effect on the hydrogen generation. Especially, Mg²⁺ or NO₃^? has negative influence on the hydrogen generation and can reduce the ultimate hydrogen yield of alloy. Longer air exposure time will also decrease the ultimate hydrogen yield. After reaction, Al and Li enter into the residue in the form of LiAl₂(OH)₇·2H₂O and LiAl₂(OH)₇·xH₂O or Al(OH)₃. After calcinations, these reaction by‐products can be easily recycled by existing metallurgical process. Copyright © 2012 John Wiley & Sons, Ltd.     

The synergistic effect of Ca(OH)2 on the process of lignite steam gasification to produce hydrogen-rich gas

The synergistic effect of Ca(OH)₂ prepared by the wet-mixing method on lignite steam gasification process at different temperatures (700–900 °C) was analyzed in a spout-fluid bed reactor. Firstly, to avoid disturbance of volatile and tar, active carbon was used as a model compound. On the one hand, Ca(OH)₂ effectively catalyzed the water-gas shift (WGS) reaction to improve H₂ concentration, but the performance was weaker at higher temperature due to the enhancement of boudouard reaction and the weakening of WGS reaction. On the other hand, it was found that the (CO+2CO₂)/H₂ ratio of syngas produced at 700 °C in the presence of Ca(OH)₂ was 0.82, which was much lower than that of the other cases, owning to the absorption of CO₂. The synergistic effect was observed at this temperature, for the adsorption of CO₂ altered equilibrium of the WGS reaction and further improved H₂ concentration. Then two kinds of Chinese lignite (HLH and XM) were selected to further study the performance of Ca(OH)₂ on optimizing the lignite steam gasification process. In the presence of Ca(OH)₂, tar and char yields greatly reduced at the same reaction temperature, whereas the gas yields significantly increased. As a catalyst, Ca(OH)₂ can not only promote solid–gas reaction to decrease char yield, but also accelerate tar decomposition to reduce its yield in syngas. Based on GC–MS data, it can be deduced that Ca(OH)₂ has different catalytic activity on the steam reforming of tar with different molecular structures. Contrast to Class 4, tars of aliphatic hydrocarbons, Class 2 and Class 5 were clearly catalytic reformed. Hydrogen-rich gas can be produced at 800 °C and 900 °C owning to the catalytic effect of Ca(OH)₂, but the highest H₂ concentration was found at 700 °C due to the additional effect of CO₂ absorption, which was supported by the results of thermogravity experiments.     

HCl Dry Removal with Modified Ca-Based Sorbents at Moderate to High Temperatures

Modified Ca-based sorbents were obtained by adding sodium alkali into Ca(OH)2 and CaCO3. Reactive properties of modified Ca-based sorbents with acidic gases were investigated through reacting with gaseous HC1 at 450-760℃, and SEM and XRD technologies were adopted to get information on the reaction mechanism. Experimental data showed that HC1 dry removal efficiencies increased with temperature before 700℃ for all of the investigated sorbents, and there existed improved sorbents that corresponded to the highest removal efficiencies under the similar conditions. SEM photographs exhibited morphology difference between original and improved sorbents both before and after the reaction; and displayed that improved sorbents formed more porous product layers than original sorbents especially at higher temperature when product sintering became heavier, which is favorable to HC1 dry removal. XRD analysis showed that (1) improved Ca(OH)2 and CaCO3 were less crystalline than original lime and limestone; (2) the re     

Catalytic performance of Ni–Al nanoparticles fabricated by arc plasma evaporation for methanol decomposition

The catalytic properties of Ni-25 at% Al (Ni25Al) nanoparticles fabricated by arc plasma evaporation toward methanol decomposition were studied at temperatures ranging from 513 to 753 K. The Ni25Al nanoparticles showed much higher activity than gas atomized Ni25Al powders. They showed a high degree of selectivity for methanol decomposition into H₂ and CO. Side reactions such as methanation and water-gas shift reaction were suppressed to a high temperature of 673 K, which is hardly achieved for common Ni catalysts. Detailed characterization of the Ni25Al nanoparticles showed that they were composed of Ni, Ni₃Al, and Al₂O₃ phases with Ni and Al oxides on the surface of the Ni and Ni₃Al phases. The Ni oxides were reduced to Ni phase by a hydrogen reduction prior to methanol decomposition, while the Al oxides remained unchanged. It is supposed that the Ni phase provided the active sites for methanol decomposition, and the Ni₃Al and Al₂O₃ phases acted as supports for the Ni phase. Probably the Ni₃Al and Al₂O₃ phases provided good resistance to agglomeration of the Ni phase during the reaction, which might contribute to maintain the high catalytic performance of the nanoparticles for methanol decomposition.     

Novel amorphous aluminum hydroxide catalysts for aluminum–water reactions to produce H2 on demand

Production of aluminum (Al) through electrolysis is an energy intensive process. Al metal with its very high specific energy density of 28.8 MJ kg^?1 can serve as an excellent energy storage vector. Al reacts with water (reaction (1)) at room temperature producing clean H₂ thus providing an alternative to compressed gas storage.

(1)2Al + 6H₂O → 2Al(OH)₃ + 3H₂

Reaction (1) does not proceed easily due to the presence of a 2–4 nm passive alumina surface layer. Aluminum hydroxide (Al(OH)₃) catalyst disrupts this layer, sustaining reaction (1). The present work focuses on the catalysis aspects of reaction (1) by amorphous Al(OH)₃ produced by urea hydrolysis of aluminum nitrate. H₂ yields of up to 100% were obtained at 45 °C. Spent reaction product mixtures are autocatalytic with successive additions of Al micropowder (MP) yielding up to 133 ml H₂/minute and reaction completion within 10 min at pH ≈ 10. There is an induction time initially due to the presence of dissolved ions, followed by production of H₂. Reaction by-products are easily recyclable back to Al metal through subsequent calcining and electrolysis by the Al refining industry, which constitutes the energy storage part of the cycle.     

Hydrogen generation from the reaction of Al pretreated in acidic or alkaline solution and water

Al and Al₂O₃ film react with strong acid or alkaline solution, bring the extensive corrosion. To decrease the corrosion, Al is first pretreated with a small amount of HCl, NaOH, NaAlO₂ and a mixture of NaAlO₂+Al(OH)₃ in this work. Al pretreatment allows for the rapid removal of oxide film, shortens the induction time and ensures the initial Al–H₂O reaction rate. Typically, immersion of the pretreated Al by a mixture of NaAlO₂+Al(OH)₃ into water, generates hydrogen rapidly without an induction time, and the average H₂ generation rate reaches 5.5 mL min⁻¹. As the Al–H₂O reaction proceeds, the potential changes, which is similar to hydrogen evolution of pretreated Al in water. Hydrogen generated rapidly with the consecutive addition of Al, and the initial hydrogen generation rate reaches ~37 mL min⁻¹. Therefore, Al pretreatment by a mixed alkaline solution is an effective method to accelerate hydrogen generation for the first cycle. Rapid and consecutive hydrogen generation by the Al–H₂O reaction could provide on-demand and high-purity hydrogen, meet some equipment requirements and promote the competition in renewable-energy sources.