Phase change stability of CaCl2·6H2O

A CaCl₂·6H₂O composition with a slight excess of water was found very stable on repeated phase change (over 1000 cycles) in a heat cycle test (35 18°C), using vertical glass tubes of 950 mm height. The mole ratio of water/CaCl₂, n, was 6.11, less than that of the peritectic composition (n = 6.14). Dissolved NaCl had superior nucleating ability to that of common barium salts under certain experimental conditions. This result was attributed to a “memory effect”. The dissolved NaCl decreased the heat of vaporization of water about 15 per cent. NaF had a similar effect to that of NaCl, because NaF powder reacted with CaCl₂ in solution yielding NaCl and CaF₂. A brief discussion of CaCl₂·4H₂O formation is presented.     

Heat-transfer properties of a heat-of-fusion store based on CaCl2·6H2O

The heat-transfer processes during crystallization and melting of calcium chloride hexahydrate in a heat-storage unit were studied experimentally. The heat transfer through the solid phase is often a limiting factor for the rate of crystallization. For the melting process, convective heat transfer through the melt dominates. For calculating the performance of heat-of-fusion stores, simplified models are developed based on the experimental observations made. An example is given of how the model can be used when dimensioning a storage unit in a solar space-heating system.     

An incongruent heat-of-fusion system—CaCl2·6H2O—Made congruent through modification of the chemical composition of the system

Incongruent melting with phase separation is a frequent cause of failure of heat-of-fusion systems. In devices based on CaCl₂·6H₂O, formation of a tetrahydrate poses such problems. This paper describes means of making the incongruent system congruent, i.e. ensuring that the tetrahydrate never is the stable species, through chemically modifying the system. Thus, addition of SrCl₂·6H₂O to a CaCl₂---H₂O system causes the solubility of CaCl₂·6H₂O to decrease and CaCl₂·4H₂O to increase. The reasons for the different effects on the hexahydrate and the tetrahydrate are related to decreased activities in the solid state through formation of a solid solution in the former case and to the decreased activities in the liquid state in the latter case (i.e. the phenomenon of freezing point depression). If the addition of SrCl₂·6H₂O is sufficiently large (about 2 wt per cent), the melting point maximum for CaCl₂·6H₂O coincides with the peritectic point for equilibrium between the hexahydrate, the tetrahydrate, and the solution.With technical grade materials, tetrahydrate formation is a more severe problem as they contain impurities (KCl and NaCl) which have an opposite influence on the solubilities of the CaCl₂ hydrates as described above for SrCl₂·6H₂O. Addition of Ca(OH)₂ is shown to be an effective tool in suppressing tetrahydrate formation in these cases. It acts through neutralizing the excess of chloride and through formation of the phase CaCl₂·CaO·2H₂O.     

Direct contact thermal energy storage system using Na2CO3·10H2O solution

The volumetric coefficient of heat transfer and the energy storage capacity in a direct contact thermal energy storage system using Na₂CO₃·10H₂O solution as thermal energy storage medium have been investigated. Hot kerosene was used as a heat transfer fluid. The experiments were carried out by bubbling hot kerosene from the bottom of a column containing Na₂CO₃·10H₂O solution. The column used in this experiment was made from glass of 3 mm in thickness with the inside diameter of 7 cm and 100 cm in height. The effects of kerosene flow rate and kerosene bubble diameter to the volumetric coefficient of heat transfer and the storing rate of energy have been studied. The volumetric coefficient of heat transfer was strongly affected by the flow rate of the kerosene. The effect of the kerosene flow rate on the storing rate of energy was relatively high, while the effect of the bubble diameter was small.     

Heat-transfer characteristics of a latent heat storage system using MgCl2 · 6H2O

Heat-transfer characteristics have been determined for the circular finned and unfinned-tube units during the freezing of magnesium chloride hexahydrate (MgCl₂ · 6H₂O) used as a phase-change material (PCM) with a melting temperature of 116.7 °C. The effects on the heat-transfer characteristics have been determined of the inlet temperature and the flow rate of air used as the heat-transfer fluid (HTF). With the unfinned-tube unit, the heat-transfer coefficients obtained between the PCM and the tube are larger than the calculated values based on the theory of steady-state heat conduction due to the dendritical crystal growth of PCM. The ratio of the heat-transfer coefficient of the finned-to the unfinned-tube systems is about 3.5 within the finned section and decreases gradually far from the finned section with an increase in crystal volume. The total amounts of heat recovered have been correlated in terms of the Fourier, Stefan, and Reynolds numbers to provide basic design data for circular finned- and unfinned-tube heat-storage units.     

Effects of deposition variables on spray-deposited MnO2 thin films prepared from Mn(C2H3O2)2·4H2O

Undoped MnO₂ thin films have been prepared by a modified spray pyrolysis technique under various deposition conditions and the effects of different variables on electrical and optical properties have been studied in detail. It is found that substrate temperature, spray rate, solution concentration, carrier air pressure and post-deposition heat-treatment, spray outlet to substrate distance play important role in obtaining optimum films.Electrical conductivity study shows an anomaly in conductivity at a temperature 323 K and its thickness dependent resistivity follows Fuchs–Sondheimer theory. The Hall effect and thermoelectric studies indicate that the deposited sample is an n-type semiconductor. Optical study in the entire wavelength 0.3–2.5 μm range exhibits a high transmittance in the visible as well as in the near infrared. Calculation from optical data, the sample exhibits a band gap at 0.28 eV, which also supports the value obtained from the Hall effect study. These studies may be of importance for the applications of this material in energy efficient surface coating devices.     

Temperature effect on the electrical properties of pyrolytic MnO2 thin films prepared from Mn(C2H3O2)2·4H2O

Spray deposited MnO₂ thin films onto glass substrate were subjected to a post-deposition heat treatment and the effects of temperature on electrical transport properties were studied in details. The heating and cooling cycles of the samples are reversible after successive heat-treatments in air and vacuum. The films were polycrystalline in structure and the oxygen chemisorption–desorption process was found to play an important role in controlling the electronic properties. Various grain-boundary and energy band parameters were calculated by taking conventional extrinsic semiconductor theory and grain boundary trapping models into account. The samples were non-degenerate n-type semiconductors. The transport properties are interpreted in terms of Seto's model which was proposed for polycrystalline semiconducting films. The inter-crystallite boundaries of the thin films play an important role in the transport properties.     

Influence of water addition on structural and electrochemical behaviour of liquid ammoniates: A study of LiClO4·4NH 3 ammoniate/H2O mixtures

The ammoniates of lithium perchlorate (LiClO₄•xNH₃) can be used as liquid or solid electrolytes in lithium batteries. The compounds possess a very low ammonia pressure and a large electroactivity range at room temperature. The presence of water in the electrolyte is one of the main problems for the lithium anode. From calorimetric measurements on the water-ammoniate system and an X-ray powder diffraction study on crystalline compounds that appear on lithium dipped in the electrolyte, a maximum water content, x = 0.2, can be derived. Above this limit, the thermodynamic properties of the compounds change and the lithium reacts dramatically with the electrolyte. Electrochemical stability ranges behave, over all the water molar fractions studied, in accordance with structural features.     

The preparation of TiO2–nitrogen doped by calcination of TiO2·xH2O under ammonia atmosphere for visible light photocatalysis

The investigation on incorporating nitrogen group into titanium dioxide in order to obtain powdered visible light-active photocatalysts is presented. The industrial hydrated amorphous titanium dioxide (TiO₂·xH₂O) obtained directly from sulphate technology installation was modified by heat treatment at temperatures of 100–800 °C for 4 h in an ammonia atmosphere. The photocatalysts were characterized by UV–VIS–DR and XRD techniques. The UV–VIS–DR spectra of the modified catalysts exhibited an additional maximum in the VIS region (, ) which may be due to the presence of nitrogen in TiO₂ structure. On the basis of XRD analysis it can be supposed that the presence of nitrogen does not have any influence on the transformation temperature of anatase to rutile. The photocatalytic activity of the modified photocatalysts was determined on the basis of decomposition rate of phenol and azo-dye (Reactive Red 198) under visible light irradiation. The highest rate of phenol degradation was obtained for catalysts calcinated at 700 °C (6.55%), and the highest rate of dye decomposition was found for catalysts calcinated at 500 and 600 °C (ca. 40–45%). The nitrogen doping during calcination under ammonia atmosphere is a very promising way of preparation of photocatalysts which could have a practical application in water treatment system under broader solar light spectrum.     

溴化锂吸收式制冷技术研究进展

回顾了近十年来有关溴化锂吸收式制冷技术的发展及主要研究成果。H₂O/LiBr作为一种广泛应用的吸收式制冷工质对,具有优良的热力学性能与环保特性,但存在结晶、腐蚀和循环性能低等问题。文章简述了醇类、盐混合物、离子液体及纳米颗粒等添加剂对H₂O/LiBr溶液传热传质、防结晶及防腐蚀等性能的提升;介绍了关键部件吸收器和发生器的理论及实验研究现状;回顾了吸收式制冷系统循环优化的研究成果。通过归纳分析,总结溴化锂吸收式制冷技术存在的一些问题及未来发展趋势,为后续的研究提供参考。     

Aging of LiFePO4 upon exposure to H2O

The effect of H₂O on carbon-coated LiFePO₄ particles was investigated by chemical analysis, structural analysis (X-ray diffraction, SEM, TEM), optical spectroscopy (FTIR, Raman) and magnetic measurements. Upon immersion in water, part of the product floats while the main part sinks. Both the floating and the sinking part have been analyzed. We find that the floating and sinking part only differ by the amount of carbon that partly detaches from the particles upon immersion in water. Exposure to H₂O results in rapid attack, within minutes, of the surface layer of the particles, because the particles are no longer protected by carbon. The deterioration of the carbon coat is dependent on the synthesis process, either hydrothermal or solid-state reaction. In both cases, however, the carbon coat is permeable to water and fails to protect the surface of the LiFePO₄ particles. The consequence is that this immersion results in the chemical attack of LiFePO₄, but is restricted to the surface layer of the particles (few nanometers-thick). In case the particles are simply exposed to humid air, the carbon coat protects the particles more efficiently. In this case, the exposure to H₂O mainly results in the delithiation of the surface layer, due to the hydrophilic nature of Li, and only the surface layer is affected, at least for a reasonable time of exposure to humid air (weeks). In addition, within this timescale, the surface layer can be chemically lithiated again, and the samples can be dried to remove the moisture, restoring the reversible electrochemical properties.     

Hydrogen permeation through a Pd-based membrane and RWGS conversion in H2/CO2, H2/N2/CO2 and H2/H2O/CO2 mixtures

This study investigated the effect of gases such as CO₂, N₂, H₂O on hydrogen permeation through a Pd-based membrane −0.012 m² – in a bench-scale reactor. Different mixtures were chosen of H₂/CO₂, H₂/N₂/CO₂ and H₂/H₂O/CO₂ at temperatures of 593–723 K and a hydrogen partial pressure of 150 kPa. Operating conditions were determined to minimize H₂ loss due to the reverse water gas shift (RWGS) reaction. It was found that the feed flow rate had an important effect on hydrogen recovery (HR). Furthermore, an identification of the inhibition factors to permeability was determined. Additionally, under the selected conditions, the maximum hydrogen permeation was determined in pure H₂ and the H₂/CO₂ mixtures. The best operating conditions to separate hydrogen from the mixtures were identified.     

缺陷态Bi2MoO6光热催化CO2和H2O还原成CO的性能

在不同温度下采用乙二醇溶剂热合成法合成Bi₂MoO₆催化剂(BMO-x,x=140、160、180),BMO-160在450℃下煅烧的样品为BMO。用XRD、BET、SEM、EDS、UV-visDRS、XPS、in-situ DRIFTS等表征方法研究其理化特性,建立BMO和BMO-x(x=140、160、180)催化剂的反应性能与孔道结构、材料形貌和缺陷空位的构效关系。结果表明,适宜的溶剂合成温度可以形成更高的孔隙率,调控氧空位的占比,有助于产生更好的催化性能。其中,BMO-160的CO产率更高,这是由于在160℃溶剂热合成温度下制备的样品形貌更优,氧空位占比适中。     

Low voltage H2O electrolysis for enhanced hydrogen production

In this study, we report the ability to split H₂O into hydrogen at a reduced voltage by the influence of sulfur dioxide (SO₂) and anode tolerance materials. This will improve the energy consumption for the production of hydrogen. Hydrogen is produced at the cathode while the anode electrode is bathed in sulfur dioxide and water to form sulfuric acid by the application of potential in the form of electrical energy. In the presence of SO₂, the theoretical equilibrium voltage requirement is 0.19 V, thereby reducing the thermochemical free energy to less than one-sixth of its initial value, that is, from 56 to 9.18 kcal/mole. By using SO₂ to scavenge the anode we have in practice reduced the equilibrium voltage to 0.6 V. Based on different electrode configurations, ruthenium oxide (RuO₂) electrocatalyst deposited on silicon (Si) electrode exhibited superior performance for the low voltage H₂O electrolysis.     

Hydrogen production by catalytic decomposition of selected hydrocarbons and H2O dissociation over CeZrO2 and Ni/CeZrO2

Presented paper deals with the catalytic decomposition of hydrocarbons (methane and toluene) in the aspect of H₂ production and types of obtained carbon deposits. The catalyst used in our studies was nickel supported on ceria–zirconia (Ni/CeZrO₂). The aim of this work was to investigate the reactivity of obtained carbon deposits with H₂O. Both issues are of great importance for determining the mechanisms of carbon deposits formation and their suppression during steam reforming reaction.     

Solar hydrogen generation with H2O/ZnO/MnFe2O4 system

The hydrogen generation reaction in the H₂O/ZnO/MnFe₂O₄ system was studied to clarify the possibility of whether this reaction system can be used for the two-step water splitting to convert concentrated solar heat to chemical energy of H₂. At 1273 K, the mixture of ZnO and MnFe₂O₄ reacted with water to generate H₂ gas in 60% yield. X-ray diffractometry and chemical analysis showed that 48 mol% of Mn^II (divalent manganese ion) in the A-site of MnFe₂O₄ was substituted with Zn^II (divalent zinc ion) and that chemical formula of the solid product was estimated to be Zn_0.58Mn^II_0.42Mn^III_0.39Fe_1.61O₄ (Mn^III: trivalent manganese ion). Its lattice constant was smaller than that of the MnFe₂O₄ (one of the two starting materials). From the chemical composition, the reaction mechanism of the H₂ generation with this system was discussed. Since the Mn ions in the product solid after the H₂ generation reaction are oxidized to Mn³⁺, which can readily release the O²⁻ ions as O₂ gas around 1300 K, the two-step of H₂ generation and O₂ releasing seem to be cyclic.     

Photoreduction of carbon dioxide with H2 and H2O over TiO2 and ZrO2 in a circulated photocatalytic reactor

The photocatalytic reduction of carbon dioxide (CO₂) was studied in a self-designed circulated photocatalytic reaction system under titanium dioxide (TiO₂, Degussa P-25) and zirconium oxide (ZrO₂) photocatalysts and reductants at room temperature and constant pressure. The wavelengths of incident ultraviolet (UV) light for the photocatalysis of TiO₂ and ZrO₂ were 365 and 254 nm, respectively. Experimental results indicated that the highest yield of the photoreduction of CO₂ were obtained using TiO₂ with H₂+H₂O and ZrO₂ with H₂. Photoreduction of CO₂ over TiO₂ with H₂+H₂O formed CH₄, CO, and C₂H₆ with the yield of 8.21, 0.28, and 0.20 μmol/g, respectively, while the photoreduction of CO₂ over ZrO₂ with H₂ formed CO at a yield of 1.24 μmol/g. The detected reaction products supported the proposition of two reaction pathways for the photoreduction of CO₂ over TiO₂ and ZrO₂ with H₂ and H₂O, respectively. Additionally, a one-site Langmuir-Hinshewood (L-H) kinetic model was successfully applied to simulate the photoreduction rate of CO₂.     

The thermodynamic analysis of two-step conversions of CO2/H2O for syngas production by ceria

Due to the challenges of demands on alternative fuels and CO₂ emission, the conversion of CO₂ has become a hot spot. Among various methods, two-step conversion of CO₂ with catalyst ceria (cerium oxide, CeO₂) appears to be a promising way. Solar energy is commonly employed to drive the conversion systems. This article proposes a solar-driven system with fluidized bed reactors (FBR) for CO₂/H₂O conversions. N₂ is used as the gas of the heat carrier. The products of CO/H₂ could be further used for syngas. To evaluate the capability of the system for exporting work, the system was analysed on the basis of the Second Law of Thermodynamics and the reaction mechanism of ceria. Heat transfer barriers in practical situations were considered. The lowest solar to chemical efficiency is 4.86% for CO₂ conversion, and can be enhanced to 43.2% by recuperating waste heat, raising the N₂ temperature, and increasing the concentration ratio. The analysis shows that the method is a promising approach for CO₂/H₂O conversion to produce syngas as an alternative fuel.     

On the explosion limit of syngas with CO2 and H2O additions

In this study, the pressure-temperature explosion limits of H₂/CO/O₂/CO₂/H₂O mixtures are analyzed computationally and theoretically. The result indicates that with the increase of H₂O and CO₂ mole fractions, the explosion temperatures are increased around all the three explosion limits. Furthermore, the increase of the explosion temperature is higher with the addition of H₂O compared with the addition of CO₂ near the second explosion limit. In addition, the increase of the explosion temperature near the first and third explosion limit is almost the same for the addition of these two inter gases. The single-limit expression analysis shows that the first explosion limit is mainly controlled by the changing of the oxygen concentration with the addition of inert gas; the changing of the second explosion limit is caused by the changing of the reaction rate of H + O₂ (+M)→HO₂ (+M), which is indirectly through the Chaperon efficiency of the third body recombination reaction, and the effect of inert gas addition on the third explosion limit is mainly caused by the changing of the hydrogen concentration.     

Kinetics of the NH reaction with H2 and reassessment of HNO formation from NH + CO2, H2O

The reaction of ground-state NH with H₂ has been studied in a high-temperature photochemistry (HTP) reactor. The NH(X³Σ) radicals were generated by the 2-photon 193 nm photolysis of NH₃, following the decay of the originally produced NH(A³Π) radicals. Laser-induced fluorescence on the transition at 336 nm was used to monitor the progress of the reaction. We obtained , with ±2σ precision limits varying from 12 to 33% and corresponding accuracy levels from 23 to 39%. This result is in excellent agreement with that of Rohrig and Wagner [Proc. Combust. Inst. 25 (1994) 975] and the data sets can be combined to yield . Starting with this agreement, it is argued that their rate coefficients for NH + CO₂ could not be significantly in error [Proc. Combust. Inst. 25 (1994) 975]. This, combined with models of several combustion systems, indicates that HNO + CO cannot be the products, contrary to their suggestion [Proc. Combust. Inst. 25 (1994) 975]. Ab initio calculations have been performed which confirm this conclusion by showing the barriers leading to these products to be too high compared to the measured activation energies. The calculations indicate the likelihood of formation of adducts, of low stability. These then may undergo further reactions. The NH + H₂O reaction is briefly discussed and it is similarly argued that HNO + H₂ cannot be the products, as had been previously suggested.