Hydrolytic reaction of waste aluminum foils for high efficiency of hydrogen generation

Hydrolyzed waste aluminum foil in low alkaline aqueous solution and concomitant additives are evaluated to generate hydrogen gas. The result of hydrogen generation using wasted Al foil is efficient in comparison with traditional Al powder. A polythene film coated on waste Al foil was removed by immersing into nitric acid for 5 h prior to hydrolyzed reaction. Low alkaline solution (0.75 M NaOH) combined with Bi additives at elevating temperature (70 °C) in waste Al foil-water hydrolysis system enable to increase hydrogen generation rate to 30 ml s^?1 g^?1 and total volume 1300 ml g^?1. The optimized result is attributed to the micro-galvanic cell formation between Al/Bi and removing hydroxide on Al foil surface by alkaline solution. In this report we develop low cost and waste recovery by hydrolyzing waste Al foil. High efficiency of hydrogen generation is achieved by low alkaline concentration and reducing activation energy. Al oxidation mechanism is explained by the linear-parabolic growth model and polarization curves indicate that corrosion potential of Al foils did not abruptly degrade and the corrosion capability with reliability were verified.     

Effect of crystalline phases of aluminum hydroxide catalysts on Al-water reaction

Previous works indicated that aluminum hydroxide could be acted as catalyst to speed up the hydrolysis reaction of Al, and the catalytic activity of aluminum hydroxide is related to its microscopic properties. In this work, the effect of crystalline phases of aluminum hydroxide catalysts on Al hydrolysis is researched systematically. The results show that the hydrogen production performance of Al hydrolysis catalyzed by aluminum hydroxide with different crystalline phases is obviously different, and the catalytic activity of different aluminum hydroxides is in the order of boehmite > bayerite > gibbsite. A possible mechanism is given, which reveals that the surface hydroxyl groups of aluminum hydroxide act as anchoring sites for the adsorption of H₂O molecules and decrease the dissociative adsorption energy of H₂O molecules on aluminum hydroxides surface, promoting the hydrolysis reaction of Al. Consequently, the difference in catalytic activity of aluminum hydroxide with different crystalline phases is attributed to the difference in surface hydroxyl group density. This study provides a new idea for the synthesization of aluminum hydroxide with excellent catalytic activity.     

Silicic acid ionization and calculation of silica solubility at elevated temperature and pH application to geothermal fluid processing and reinjection

Utilization of geothermal liquids is often complicated by the deposition in process equipment of dissolved salts and minerals, particularly amorphous silica. Herein we compute the solubility of silica as a function of temperature and pH in vapor saturated liquid water to 300°C and pH 12. Ionization of dissolved silica (silicic acid) at alkaline pH enhances the solubility. The ionization of silicic acid is commonly expressed: (A) H₄SiO₄ = H₃SiO₄^? + H⁺, for which reaction the enthalpy of ionization L_A is highly temperature dependent and changes sign along the vapor saturated curve of liquid water. We advocate the alternative neutralization reaction (B) H₄SiO₄ + OH^? = H₃SiO₄^? + H₂O, which results in a linear relationship between log K_B and T^?1. The equilibrium constant for this reaction in vapor saturated liquid water from 0 to 300°C is given by log k_B = 1479/T ? 0·6496 based on selected literature data.An iterative algorithm is developed to compute solubility of silica polymorphs as a function of temperature and pH in circumstances where literature data are lacking, using the neutralization reaction and neutral-pH solubility data. Conditions are specified under which certain approximations which greatly simplify the computational problem are valid. The mean ionic activity coefficient for H₃SiO₄^? in saturated silica solutions is estimated and shows excellent agreement with the Debye - Huckel theory for conditions where the latter is valid. Determination of this activity coefficient allows prediction of molal solubility from true thermodynamic equilibrium constants for the several participating reactions by combining our equation (7) and Table 1. This bypasses use of the more cumbersome molal concentration products. The results of computation compare favorably with the few data available in the literature. Application of these results to the problems of scaling control in process equipment and to reinjection of processed geothermal fluids suggests that alkalization receive consideration as a chemical control on scaling. This procedure would simultaneously facilitate fluid reinjection, and may be a superior alternative to currently proposed acidification procedures for some cases.     

Catalytic steam reforming of methanol over highly active catalysts derived from PdZnAl-type hydrotalcite on mesoporous silica MCM-48

A kind of composite material PdZnAl(HT)/MCM-48 was synthesized by dispersing PdZnAl-type hydrotalcite (denoted as PdZnAl(HT)) on mesoporous silica MCM-48. PdZnAl(HT) was confirmed to be formed on the MCM-48 in small particles, and the small PdZnAl(HT) particles easily collapsed during increasing the temperature. A kind of novel PdZn(Al)O/MCM-48 catalyst was obtained after calcining and reducing the PdZnAl(HT)/MCM-48 precursor. PdZn alloy species were formed on the PdZnAl(HT)/MCM-48 after reducing in H₂ at 673 K. PdZn(Al)O/MCM-48-2 (with a mass ratio of PdZn(Al)O to MCM-48 = 1) had a large BET surface area (431 cm² g^?1) and small size of PdZn particles (4.1 nm) at the same time. In the steam reforming of methanol, the catalytic stability of PdZn(Al)O/MCM-48-2 was much higher than that of the Cu-based catalyst CuZn(Al)O at 503 K. The methanol conversion over PdZn(Al)O/MCM-48-2 greatly increased with increasing reaction temperature and reached 99% at 513 K. PdZn(Al)O/MCM-48-2 showed higher catalytic activity than PdZnAl(HT) and PdZn/MCM-48 (imp) at the same reaction temperature. The initial CO₂ selectivity and H₂ selectivity over PdZn(Al)O/MCM-48-2 at 503 K were 99.6 and 99.4%, respectively. Moreover, PdZn(Al)O/MCM-48-2 showed the highest rate of H₂ production among various catalysts in the steam reforming of methanol. In a long-time operation, the methanol conversion over PdZn(Al)O/MCM-48-2 decreased from 75.8 to 68.5% after 50 h on stream at 503 K. The size of PdZn particles did not increase after 50 h on stream but the carbonaceous deposits on the catalyst surface caused the deactivation. The deactivated catalyst could be regenerated by calcining in the air at 723 K followed by reducing in the H₂ at 673 K. The carbonaceous deposits were eliminated by calcining in the air and the PdZn active species were formed again by reducing in H₂.     

Chemical equilibrium analysis for hydrolysis of magnesium hydride to generate hydrogen

Magnesium hydride is a promising hydrogen source because of its high mass density of hydrogen, 15.2%, when it is hydrolyzed; MgH₂ + 2H₂O = Mg(OH)₂ + 2H₂ + 277 kJ. However, a magnesium hydroxide, Mg(OH)₂, layer forms rapidly on the surface of the unreacted MgH₂ as the pH increases, hindering further reaction. The purpose of this study is to find acids that could effectively accelerate the reaction by using a chemical equilibrium analysis where the relationships of pH to concentration of ionized Mg were calculated. For the best performing acid, the calculated and measured relationships were compared, and the effects of acid concentration on hydrogen release were measured. The analysis revealed that citric acid and ethylenediamine-tetraacetic acid were good buffering agents. The calculated and measured relationships between pH and concentration of ionized Mg were in good accord. Hydrogen release improved considerably in a relatively dilute citric acid solution instead of pure distilled water. The maximum amount of hydrogen generated was 1.7 × 10³ cm³ g⁻¹ at STP after 30 min. We estimated the exact concentration of citric acid solution for complete MgH₂ hydrolysis by a chemical equilibrium analysis method.     

Hydrogen inhibition in wet dust removal systems by using calcium lignosulfonate (CLS)

Wet dust removal systems pose hydrogen fire and explosion risks because accumulated aluminium dust can react with water to produce hydrogen gas. Traditionally, hydrogen sensors, alarm devices, explosion-proof electrical components and pressure relief devices are installed in wet dust removal systems to mitigate such risks. However, these safety strategies cannot fundamentally prevent the occurrence of hydrogen fires and explosions. In this work, calcium lignosulfonate (CLS), which is an abundant, inexpensive and renewable chemical, is used to inhibit hydrogen production. Through a series of hydrogen inhibition experiments using CLS solution, a hydrogen inhibition method is proposed. The hydrogen evolution curves of aluminium particles after reaction with CLS solutions at different concentrations reveal that when the concentration of the CLS solution reaches 0.5 g/L, essentially no hydrogen gas is produced. Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) are used to characterize the aluminium particles before and after reaction. The film formation behaviour of CLS on the surface of aluminium particles is characterized. The results show that CLS is a good inhibitor and that the adsorption of CLS on the aluminium particle surface obeys the Langmuir adsorption isotherm. Additionally, Fourier transform infrared (FT-IR) analysis is conducted to reveal the physicochemical mechanism of hydrogen inhibition. The application of CLS solution in wet aluminium dust removal systems results in the maximum reduction in hydrogen explosion risk.     

Effect of surface tension on the temperature of burning metal droplets

An effect of Laplace pressure resulting in elevated boiling temperatures for fine burning metal droplets is discussed. The effect is evaluated for both Al and Mg. An increase in the boiling temperature suggests that the temperature of fine Al particles can become equal to or even higher than the temperature of the vapor phase Al flame. The high particle surface temperature may be associated with accelerated surface reaction rates. For Al, such surface reactions may generate vapor-phase aluminum suboxides, resulting in a reduced heat effect and extended burn times for fine particles and nanoparticles. For Mg, the particle temperature remains below the vapor phase flame temperature despite an elevated boiling point. For fine particles, surface reaction producing solid MgO growing directly on the particle surface is possible.     

Preparation and application of nanoporous carbon templated by silica particle for use as a catalyst support for direct methanol fuel cell

Polymer-silica composites were synthesized by resorcinol–formaldehyde polymerization in the presence of uniform size silica particles. After carbonization and subsequent removal of the silica template, these polymer-silica composites turned into nanoporous carbon xerogel with high surface area and large pore size. By controlling the initial pH of the carbon precursor solution, nanoporous carbons with different textural properties could be fabricated. As the pH of the reaction mixture was decreased, the pore size of nanoporous carbon increased because the aggregated silica particles, i.e., larger templates, were generated in low pH condition. Although the pore size of nanoporous carbon was increased with decreasing pH, the templating effect was reduced with decreasing pH, leading to the formation of significant microporous carbon framework in carbon xerogels. For DMFC application, the PtRu (1:1) alloy was supported on nanoporous carbons and activated carbon by a formaldehyde reduction method. It was revealed that the textural properties of carbon supports played important roles in the metal dispersion and DMFC performance of the supported PtRu catalysts. The support with large pore size and high surface area (especially, meso-macropore area) was favorable for high dispersion of PtRu catalyst and easy formation of triple-phase boundary. Microporous framework, resulted from the destruction of structural integrity, was insufficient for high dispersion of PtRu species. The catalysts with higher metal dispersion and structural integrity showed higher catalytic activities in the methanol electro-oxidation and the DMFC performance test.     

Catalytic reduction of CO2 by Al–Sn-CNTs composite for synthesizing formic acid

Electrochemical reduction and photocatalytic reduction of CO₂ have attracted more and more attention, but they also face the problem of low utilization efficiency of electricity and solar energy. In this study, a new strategy of applying a novel Al–Sn-CNTs composite in electrochemical corrosion process was proposed to reduce CO₂ without additional electricity and light. In the Al–Sn-CNTs/CO₂ system, micro galvanic cell with Al as anode and Sn or CNTs as cathode was formed, and CO₂ was reduced to formic acid on the cathode surface. The cumulative formic acid concentration of the Al–Sn-CNTs/CO₂ system achieved 21.18 mg/L within 60 min under the conditions of initial pH 9.0, Cl⁻ concentration 10 mmol/L, and Al–Sn-CNTs composite dosage 2 g/L. Based on the morphology, crystal structure and electrochemical test results of the Al–Sn-CNTs composite, a possible mechanism of CO₂ reduction to formic acid in the Al–Sn-CNTs/CO₂ system was proposed.     

Preparation of Ni–Cu/Mg/Al catalysts from hydrotalcite-like compounds for hydrogen production by steam reforming of biomass tar

Ni–Cu/Mg/Al bimetallic catalysts were prepared by the calcination and reduction of hydrotalcite-like compounds containing Ni²⁺, Cu²⁺, Mg²⁺, and Al³⁺, and tested for the steam reforming of tar derived from the pyrolysis of biomass at low temperature. The characterizations with XRD, STEM-EDX, and H₂ chemisorption confirmed the formation of Ni–Cu alloy particles. The Ni–Cu/Mg/Al bimetallic catalyst with the optimum composition of Cu/Ni = 0.25 exhibited much higher catalytic performance than the corresponding monometallic Ni/Mg/Al and Cu/Mg/Al catalysts in the steam reforming of tar in terms of activity and coke resistance. The catalyst gave almost total conversion of tar even at temperature as low as 823 K. This high performance was related to the higher metal dispersion, larger amount of surface active sites, higher oxygen affinity, and surface modification caused by the formation of small Ni–Cu alloy particles. In addition, the Ni–Cu/Mg/Al catalyst showed better long-term stability than the Ni/Mg/Al catalyst. No obvious aggregation and structural change of the Ni–Cu alloy particles were observed. The coke deposition on the Ni–Cu/Mg/Al catalyst was approximately ten times smaller than that on the Ni/Mg/Al catalyst, indicating good coke-resistance of the Ni–Cu alloy particles.     

Hydrogen generation through massive corrosion of deformed aluminum in water

Aluminum, one of most reactive metals, rapidly corrodes in strong acidic or alkaline solutions but passivates at pH of about 5–9. We have determined that the passivation of aluminum in this range of pH, and in particular in regular tap water, can be substantially prevented after milling of aluminum with water-soluble inorganic salts (referred to as “WIS”), such as KCl or NaCl. Ensuing corrosion of Al in tap water, with accompanying release of hydrogen and precipitation of aluminum hydroxide, at normal pressure and moderate temperatures (∼55 °C) is rapid and substantial. For example, ∼92% of the Al in the Al–WIS system when milled for 1 h and ∼81% when milled for 15 min, corrodes in 1 h, with the release of 1.5 mol of hydrogen per each mole of Al consumed in the reaction. Besides gaseous hydrogen, only solid aluminum hydroxides were formed as the reaction byproducts, opening up the possibility of straightforward recycling of the system. The effects of WIS concentration, chemistry of other additives, powder particle size, temperature, and milling conditions on the reaction kinetics are reported.     

Further studies of the anodic dissolution in sodium chloride electrolyte of aluminium alloys containing tin and gallium

As part of a programme to develop a high power density, Al/air battery with a NaCl brine electrolyte, the high rate dissolution of an aluminium alloy containing tin and gallium was investigated in a small volume cell. The objective was to define the factors that limit aluminium dissolution in condition that mimic a high power density battery. In a cell with a large ratio of aluminium alloy to electrolyte, over a range of current densities the extent of dissolution was limited to ∼1000 C cm⁻² of anode surface by a thick layer of loosely bound, crystalline deposit on the Al alloy anode formed by precipitation from solution. This leads to a large increase in impedance and acts as a barrier to transport of ions.     

Effect of synthesis conditions on characteristics of the precursor material used in NiO·OH/Ni(OH)2 electrodes of alkaline batteries

The synthesis of nickel hydroxide occurs by many stages. When the precipitating reagent is NH₄OH solution, the precipitation of nickel hydroxide occurs between pH 8.0 and 8.6. For pH between 8.6 and 10.0, a soluble complex such as [Ni(NH₃)₆]²⁺ is formed. The precipitation of nickel hydroxide happens again after the pH equals 10.0. Finally, there occurs the ageing of α-Ni(OH)₂. A mixture of α-Ni(OH)₂ and β-Ni(OH)₂ phases is formed when the solid state reaction is not totally completed. One adsorbed layer becomes very hard with the exit of the water intercalated in the α-Ni(OH)₂. In presence of KOH solution occurs the formation and the ageing of α-Ni(OH)₂. Synthesis was characterized by the following techniques: X-ray diffraction, Fourier transform infrared spectroscopy (FT-IR), differential thermal analysis (DTA) and gravimetric thermal analysis (GTA), and specific surface area and UV–vis spectroscopy.     

Synergistic effect on hydrolytic sodium borohydride adding waste Al for hydrogen generation

Synergistic effect of sodium borohydride (NaBH₄) mixed with waste aluminum in hydrolytic system were investigated to improve the hydrogen generation (HG) efficiency. The alkaline aqueous originated from NaBH₄ hydrolysis impels the passive aluminum hydroxide film of Al surface to be dissolved, thus, mutual effect of HG amount increasing by 88% and 50% for Al powder and waste Al can, respectively. To add acid catalyst in the mutual mixture system prompts to significantly accelerate HG rate from 100 ml/min˙g to 400 ml/min˙g but raising temperature is not obviously found. It is speculated that exothermic effect dominates the NaBH₄ hydrolytic reaction and eliminates the temperature effect. In this study, the mutual hydrolyzed reaction of NaBH₄ and waste Al provides ecofriendly environment, low cost and enables to achieve higher HG rate and further uses for portable/mobile fields.     

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.     

Electrochemical preparation of a novel,effective and low cast catalytic surface for hydrogen evolution reaction

The formation of platinum nucleus on the freshly polished aluminum (Al) and anodized aluminum electrodes (Al₂O₃/Al) was studied by cyclic voltammetry. Results showed that the deposition of platinum on freshly polished aluminum from an aqueous 0.5 M phosphate buffer solution containing H₂PtCl₆ takes place rapidly through the electroreduction of dissolved Pt (IV) ions. At shorter deposition times, small particles of platinum crystals were formed on the aluminum and the surface coverage was imperfect. At longer deposition times, the size of the platinum crystals increases while their number decreases due to the coalescence and agglomeration processes. The electrodeposition of Pt on the Al electrode was conveniently carried out over the Al₂O₃/Al electrode. The electrochemical and catalytic activities of the Pt/Al and Pt/Al₂O₃/Al electrodes were studied in 0.1 M H₂SO₄ solution. In cyclic voltammetry, the two pair symmetric peaks appeared in 0.1 M H₂SO₄ solution which was attributed to the formation of strongly (H_s) and weakly bounded hydrogen (H_w). The occurrence of the third anodic hydrogen peak (H_3rd) was revealed at low scan rate and in high concentration of H₂SO₄. At potentials more negative than −0.3 V vs. SCE, the current is mainly due to hydrogen evolution reaction. The influence of the various parameters such as deposition method and amount of platinum, sulfuric acid concentration and medium temperature on the hydrogen evolution reaction is described. Finally the kinetic of the hydrogen evolution reaction is also discussed on the Pt/Al electrode.     

Hydrogen generation by aluminum corrosion in seawater promoted by suspensions of aluminum hydroxide

Nowadays, new processes of H₂ generation from water via Al corrosion are mainly limited by Al passivation. Here we report on the systematic assessment of H₂ production by corrosion of Al in seawater suspensions prepared with NaAlO₂. The reported results are encouraging, since it was observed that seawater suspensions tested can prevent Al passivation during H₂ evolution, reaching 100% yields at ca. 700 cm³ H₂ min⁻¹. XRD analysis revealed the formation of solid Al(OH)₃ (bayerite) in initial seawater suspensions. So, model suspensions were prepared using NaAlO₂ + Al(OH)₃ in distilled water, which even improved the results obtained in seawater. Suspended particles of Al(OH)₃ act as nuclei in a mechanism of seeded crystallization, which prevents Al surface passivation. Moreover, a synergistic effect of Al(OH)₃ suspensions in combination with NaAlO₂ solutions was key in promoting Al corrosion. The effect of NaCl in aqueous suspensions was also studied, but it was insignificant compared to this synergistic effect. The composition of suspensions was optimized and a 0.01 M NaAlO₂ solution with 20 g dm⁻³ Al(OH)₃ was selected as candidate to generate H₂ at pH ca. 12 with high efficiency. Consecutive runs of the selected composition were performed obtaining ca. 90% yields in all of them.     

Hydrogen generation by the reaction of Al with water using oxides as catalysts

Hydrogen generation by the reaction of pure Al powder in water with the addition of Al(OH)₃, γ‐Al₂O₃, α‐Al₂O₃, or TiO₂ at mild temperatures was investigated. It was found that the reaction of Al with water is promoted and the reaction induction time decreases greatly by the above hydroxide and oxides. X‐ray diffraction analyses revealed that the hydroxide and oxide phases have no any change during the Al–water reaction, indicating that they are just as catalysts to assist the reaction of Al with water. A possible mechanism was proposed, which shows that hydroxide and oxides could dissociate water molecules and promote the hydration of the passive oxide film on Al particle surfaces. Copyright © 2013 John Wiley & Sons, Ltd.     

Cobalt oxide preparation from waste LiCoO2 by electrochemical–hydrothermal method

Cobalt ions, extracted from waste LiCoO₂ by using a nitric acid leaching solution, are potentiostatically transformed into cobalt hydroxide on a titanium electrode and cobalt oxide is then obtained via a dehydration procedure. In linear sweep voltammetry, distinct cathodic current peak is observed and indicates that hydroxide ions are formed near the electrode via the electroreduction of dissolved oxygen and nitrate ions give rise to an increase in the local surface pH of the titanium. Under appropriate pH conditions, island-shaped cobalt hydroxide is precipitated on the titanium substrate and heat treatment of the cobalt hydroxide results in the formation of cobalt oxide.     

Hydrogen generation from hydrolysis of aluminum/graphite composites with a core–shell structure

Hydrogen generated by hydrolysis of metal aluminum with water is promising for portable fuel cell applications. However aluminum would not react with water to yield hydrogen at ordinary conditions due to the passive oxide film formed on its surface. In the present investigation, the aluminum/graphite composite were prepared by a ball milling process in an attempt to improve the reactivity of aluminum, using sphere-shape aluminum particles and laminate graphite as the initial materials and 2 wt% NaCl as the milling-assisted agent. The TEM observation showed that the Al particles are covered by graphite to form a core–shell structure. Such a Al/graphite composite material exhibited a pronounced hydrolysis reactivity with tap water to generate hydrogen while Al alone did not react with water. The presence of graphite could lower the hydrogen generation reaction temperature below 45 °C. Increasing the reaction temperature could obtain an increased hydrogen generation rate and the maximum hydrogen generation rate of 40 cm³ min⁻¹ g⁻¹ Al was obtained when the reaction temperature was increased to 75 °C. Prolonging milling time could also improve the Al hydrolysis reactivity in the composite particularly at a relatively low temperature. The XRD results identified that the hydrolysis byproducts are bayerite (Al(OH)₃) and boehmite (AlOOH). The microstructure-related hydrolysis reaction mechanism was finally proposed.