Cycle behaviour of iron ores in the steam-iron process

The use of several commercial iron ores usually employed as pigments, to store and supply pure hydrogen by means of the steam-iron process has been proposed and analyzed. The process roughly consists in repeated series of alternate reduction and oxidation steps in which a reducing stream (H₂ + CO, or in general H₂ enriched fuels) reacts with the iron oxide rendering the metal or a partially reduced oxide. Pure hydrogen is released during the re-oxidation with steam. The studied iron ores contain some impurities that accounting minor percentages (<10 wt%) enhance the behaviour of the solid. This improvement regards not only to the reduction and oxidation rate, but especially to the ability of the solid to maintain a given redox capacity along cycles. Also concerning this topic, the effect of the presence of these natural additives has been investigated in order to determine the inert behaviour of methane as a potential reducing agent. This study allowed the determination of the maximum temperature at which carbon formation is inhibited so that the subsequent released hydrogen will not be contaminated by carbon compounds.     

Separation and storage of hydrogen by steam-iron process: Effect of added metals upon hydrogen release and solid stability

During the last decade, the steam-iron process has re-emerged as a possible way to separate and/or storage pure hydrogen through the use of metallic oxides subjected to redox cycles. The most renamed candidate to achieve this goal has traditionally been iron oxide. Nevertheless, the study of its behaviour along repetitive reduction/oxidation stages has shown that the hydrogen storage density diminishes abruptly from the first cycle on.To cope with this problem, the inclusion of a second metal oxide in the solid structure has been tried. Isothermal experiments of reduction with hydrogen rich flows and oxidation with steam have been carried out with Al, Cr and Ce as second metals, in nominal amounts from 1% to 10 mol% added to the hematite structure, which has been synthesized in laboratory by coprecipitation. Series of up to seven cycles (reductions followed by oxidations in a thermogravimetric system acting as differential reactor for the gas) have shown that to that point, an almost repetitive behaviour can be obtained, recovering the magnetite (Fe₃O₄) structure after each oxidation step.Since the second metal oxide does not intervene in the reduction/oxidation process, the optimum content of second metal for each species has been determined with the aim to keep the highest hydrogen storage density along cycles.     

Influence of intrinsic hydrogenation/dehydrogenation kinetics on the dynamic behaviour of metal hydrides: A semi-empirical model and its verification

Metal hydrides can store hydrogen at low pressures and with high volumetric capacity. For the possible application as storage medium in hydrogen stand-alone power systems, large metal hydride hydrogen storage units are usually required. A reliable and verified kinetic correlation is an important tool in the designing process of a larger storage unit. This paper describes kinetic investigation of a AB₅-type alloy and its corresponding hydride, with the purpose of finding a semi-empirical correlation suitable for use in heat and mass transfer modelling and engineering design of metal hydride storage units.     

Low temperature CO oxidation and water gas shift reaction over Pt/Pd substituted in Fe/TiO2 catalysts

We demonstrate the activity of Ti_0.84Pt_0.01Fe_0.15O_2−δ and Ti_0.73Pd_0.02Fe_0.25O_2−δ catalysts towards the CO oxidation and water gas shift (WGS) reaction. Both the catalysts were synthesized in the nano crystalline form by a low temperature sonochemical method and characterized by different techniques such as XRD, FT-Raman, TEM, FT-IR, XPS and BET surface analyzer. H₂-TPR results corroborate the intimate contact between noble metal and Fe ions in the both catalysts that facilitates the reducibility of the support. In the absence of feed CO₂ and H₂, nearly 100% conversion of CO to CO₂ with 100% H₂ selectivity was observed at 300 °C and 260 °C respectively, for Ti_0.84Pt_0.01Fe_0.15O_2−δ and Ti_0.73Pd_0.02Fe_0.25O_2−δ catalyst. However, the catalytic performance of Ti_0.73Pd_0.02Fe_0.25O_2−δ deteriorates in the presence of feed CO₂ and H₂. The change in the support reducibility is the primary reason for the significant increase in the activity for CO oxidation and WGS reaction. The effect of Fe addition was more significant in Ti_0.73Pd_0.02Fe_0.25O_2−δ than Ti_0.84Pt_0.01Fe_0.15O_2−δ. Based on the spectroscopic evidences and surface phenomena, a hybrid reaction scheme utilizing both surface hydroxyl groups and the lattice oxygen was hypothesized over these catalysts for WGS reaction. The mechanisms based on the formate and redox pathway were used to fit the kinetic data. The analysis of experimental data shows the redox mechanism is the dominant pathway over these catalysts.     

Hydrogen-rich gas from catalytic steam gasification of municipal solid waste (MSW): Influence of steam to MSW ratios and weight hourly space velocity on gas production and composition

The present work deals with a study coupling experiments and modeling of catalytic steam gasification of municipal solid waste (MSW) for producing hydrogen-rich gas or syngas (H₂ + CO) with calcined dolomite as a catalyst in a bench-scale downstream fixed bed reactor. The influence of steam to MSW ratios (S/M) on gas production and composition was studied at 900 °C over the S/M range of 0.39–1.04, for weight hourly space velocity (WHSV) in the range of 1.22–1.51 h⁻¹. Over the ranges of experimental conditions examined, calcined dolomite revealed better catalytic performance at the presence of steam. H₂ and CO₂ contents increased with S/M increasing, while CO and CH₄ contents decreased sharply, the contents of CH₄, C₂H₄ and C₂H₆ were relatively small, and the influence of S/M was insignificant. The highest H₂ content of 53.22 mol %, the highest H₂ yield of 42.98 mol H₂/kg MSW, and the highest H₂ potential yield of 59.83 mol H₂/kg MSW were achieved at the highest S/M level of 1.04. Furthermore, there was a good agreement between the experimental gas composition and that corresponding to thermodynamic equilibrium data calculated using GasEq model. Consequently, a kinetic model was proposed for describing the variation of H₂ yield and carbon conversion efficiency with S/M during the catalytic steam gasification of MSW. The kinetic model revealed a good performance between experimental results and the kinetic model.     

Self-ignition combustion synthesis of LaNi5 utilizing hydrogenation heat of metallic calcium

This paper describes self-ignition combustion synthesis (SICS) of LaNi₅ in a pressurized hydrogen atmosphere using metallic calcium as both the reducing agent and the heat source. In this study, the effects of hydrogen on the ignition temperature and the hydrogenation properties of the products were mainly examined. In the experiments, La₂O₃, Ni, and Ca were dry-mixed in the molar ratio of 1:10:6 and then heated up at a hydrogen pressure of 1.0 MPa until the ignition due to the hydrogenation of calcium. For the sake of comparison, the same experiments were performed in a normal argon atmosphere. The results showed that the ignition temperature was drastically lowered by hydrogen; it was only 600 K in the case of hydrogen as compared to 1100 K in the case of argon. The products also exhibited high initial activity and hydrogen storage capacity of 1.54 mass%. The proposed method offers many benefits for using cost-effective rare-earth oxide, saving productive time and energy, improving initial activity of the product and applying to any AB₅-type hydrogen storage alloy.     

Influence of hydrogen and carbon monoxide on reduction behavior of iron oxide at high temperature: Effect on reduction gas concentrations

The purposes of this study are to reduce Fe₂O₃ using hydrogen (H₂) and carbon monoxide (CO) gases at a high temperature zone (500 °C–900 °C) by focusing on the influence of reduction gas concentrations. Reduction behavior of hematite (Fe₂O₃) at high temperature was examined using temperature programmed reduction (TPR) under non-isothermal conditions with the presence of 10% H₂/N₂, 20% H₂/N₂, 10% CO/N₂, 20% CO/N₂ and 40% CO/N₂. The TPR_CO results indicated that the first and second reduction peaks of Fe₂O₃ at a temperature below 660 °C appeared rapidly when compared to TPR_H2. However, TPR_H2 exhibited a better reduction in which Fe₂O₃ entirely reduced to Fe at temperature 800 °C (20% H₂) without any remaining of wustite (FeO) whereas a temperature above 900 °C is needed for a complete reduction in 10% H₂/N₂, 10% and 20% CO/N₂. Furthermore, the reduction of hematite could be improved when increasing CO and H₂ concentrations since reduction profiles were shifted to a lower temperature. Thermodynamic calculation has shown that enthalpy change of reaction (ΔHr) for all phases transformation in CO atmosphere is significantly lower than in H₂. This disclosed that CO is the best reductant as it is a more exothermic, more spontaneous reaction and able to initiate the reduction at a much lower temperature than H₂ atmosphere. Nevertheless, the reduction of hematite using CO completed at a temperature slightly higher compared to H₂. It is due to the presence of an additional carburization reaction which is a phase transformation of wustite to iron carbide (FeO → Fe₃C). Carburization started at the end of the second stage reduction at 600 °C and 630 °C under 20% and 40% CO, respectively. Therefore, reduction by CO encouraged the formation of carbide, slower the reduction and completed at high temperature. XRD analysis disclosed the formation of austenite during the final stage of a reduction under further exposure with high CO concentration. Overall, less energy consumption needed during the first and second stages of reduction by CO, the formation of iron carbide and austenite were enhanced with the presence of higher CO concentration. Meanwhile, H₂ has stimulated the formation of pure metallic iron (Fe), completed the reduction faster, considered as the strongest reducing agent than CO and these are effective at a higher temperature. Proposed iron phase transformation under different reducing agent concentrations are as followed: (a) 10% H₂, 20% H₂ and 10% C; Fe₂O₃ → Fe₃O₄ → FeO → Fe, (b) 20% CO; Fe₂O₃ → Fe₃O₄ → FeO → Fe₃C → Fe and (c) 40% CO; Fe₂O₃ → Fe₃O₄ → FeO → Fe₃C → Fe → F,C (austenite).     

Hydrogen absorption in Ni-catalyzed Mg: A model for measurements in the low temperature range

Magnesium has been deeply studied as a possible hydrogen storage material for both, mobile and static applications. In this article we continued the work presented in our previous paper by modeling the hydrogen absorption in Ni-catalyzed magnesium in the range of pressures of 500 kPa–5000 kPa and temperatures from 423 K to 468 K. A new model based in the Ginstling–Brounshtein diffusion equation was proposed for the hydrogen absorption kinetics. It adds the contribution of the pressure of the gaseous phase and the enthalpy of reaction to the previously mentioned diffusive model. An activation energy for the process was estimated and the value obtained (112 kJ/mol) was concordant with previous values reported in the literature.     

Hydrogen desorption kinetics of the destabilized LiBH4AlH3 composites

LiBH₄ can be destabilized by AlH₃ addition. In this work, the hydrogen desorption kinetics of the destabilized LiBH₄AlH₃ composites were investigated. Isothermal hydrogen desorption studies show that the LiBH₄ + 0.5AlH₃ composite releases about 11.0 wt% of hydrogen at 450 °C for 6 h and behaves better kinetic properties than either the pure LiBH₄ or the LiBH₄ + 0.5Al composite. The apparent activation energy for the LiBH₄ decomposition in the LiBH₄ + 0.5AlH₃ composite estimated by Kissinger's method is remarkably lowered to 122.0 kJ mol^?1 compared with the pure LiBH₄ (169.8 kJ mol^?1). Besides, AlH₃ also improves the reversibility of LiBH₄ in the LiBH₄ + 0.5AlH₃ composite. For the LiBH₄ + xAlH₃ (x = 0.5, 1.0, 2.0) composites, the decomposition kinetics of LiBH₄ are enhanced as the AlH₃ content increases. The sample LiBH₄ + 2.0AlH₃ can release 82% of the hydrogen capacity of LiBH₄ in 29 min at 450 °C, while only 67% is obtained for the LiBH₄ + 0.5AlH₃ composite in 110 min. Johnson?Mehl?Avrami (JMA) kinetic studies indicate that the reaction LiBH₄ + Al → ‘LiAlB’ + AlB₂ + H₂ is controlled by the precipitation and subsequently growth of AlB₂ and LiAlB compounds with an increasing nucleation rate.     

Kinetic study of the hydrogen oxidation reaction on sub-stoichiometric titanium oxide-supported platinum electrocatalyst in acid solution

The kinetics and mechanism of the hydrogen oxidation reaction were studied in 0.5 mol dm⁻³ HClO₄ solution on an electrode based on titanium oxide with Magneli phase structure-supported platinum electrocatalyst applied on rotation Au disk electrode. Pt catalyst was prepared by impregnation method from 2-propanol solution of Pt(NH₃)₂(NO₂)₂ and sub-stoichiometric titanium oxide powder. Sub-stiochiometric titanium oxide support was characterized by X-ray diffraction and BET techniques. The synthesized catalyst was analyzed by TEM technique. Based on Tafel-Heyrovsky-Volmer mechanism the corresponding kinetic equations were derived to describe the hydrogen oxidation current-potential behavior on RDE over the entire potential region. The polarization RDE curves were fitted with derived polarization equations according to proposed model. The fitting shows that the HOR on Pt proceeds most likely via the Tafel-Volmer (TV) pathway in the lower potential region, while the Heyrovsky-Volmer (HV) pathway is operative in the higher potential region. It is pointed out that Tafel equation that has been frequently used for the kinetics analysis in the HOR, can not reproduce the polarization curves measured with high mass-transport rates. Polarization measurements on RDE revealed that the Pt catalyst deposited on titanium suboxide support showed equal specific activity for the HOR compared to conventional carbon-supported Pt fuel cell catalyst.     

Study of the hydrogenation mechanism of LaCuMg8 ternary phase: The decomposition induces kinetics improvement

The hydrogen sorption properties of LaCuMg₈ are investigated. LaCuMg₈ crystallizes in the La₂Mg₁₇ structure type with the lattice parameters a = 10.1254(2) Å and c = 10.0751(2) Å.     

Improvement of hydrogen storage properties of the AB2 Laves phase alloys for automotive application

Hydrogen storage properties of the Ti_1.1CrMn AB₂-type Laves phase alloys, for both low (−30 °C) and high (80 °C) temperature applications, are improved by substituting Zr at Ti site. In agreement with the larger radius of Zr than Ti, the lattice volume of (Ti_1−xZr_x)_1.1CrMn (x=0, 0.05, 0.06 and 0.1) alloys, prepared by arc melting, increases with x. The increase in the Zr content leads to a decrease in the equilibrium hydrogen sorption pressure plateau and faster absorption kinetics, associated with an increase in the hydrogen storage capacity from 1.9 to 2.2 wt% for Ti_1.1CrMn and (Ti_0.9Zr_0.1)_1.1CrMn alloys, respectively. At −5 °C, (Ti_0.9Zr_0.1)_1.1CrMn alloy reversibly absorbs and desorbs 2.2 wt% at 160 bar within 250 s. Based on thermodynamic calculated values, the optimized Zr substituted alloy (Ti_0.9Zr_0.1)_1.1CrMn desorbs hydrogen at 3.2 bar at −30 °C and 135 bar at 80 °C. This is a significant reduction of the sorption pressure plateau as compared with the current technology for mobile applications based on Ti_1.1CrMn alloy with hydrogen desorption plateau above 400 bar at 80 °C. Finally, the mechanism of improved hydrogen storage properties is discussed based on the radius and the hydrogen affinity of the substituting element.     

Single-particle investigation on the activation process of a hydrogen storage alloy

Using an improved apparatus to investigate the activation process of single particles of hydrogen storage alloy LaNi_3.55Co_0.75Mn_0.4Al_0.3, we directly showed the particle's morphology changes during the activation process. Electrochemical properties were systematically monitored during the activation process of the single particles. We finally proposed a new parameter – normalized output rate (NOR) – to evaluate the output performance of the electrode material. In addition to revealing what occurred during the activation process, we provided a program for the quick testing of electrode materials.     

Influence of preparation methods and Zr and Y promoters on Cu/ZnO catalysts used for methanol steam reforming

Binary Cu/ZnO catalysts were prepared using three different methods (coprecipitation, sequential precipitation and homogeneous precipitation) and tested in a methanol steam reforming reaction. Zirconium and yttrium were tested as promoters, and their effects were evaluated in the same reaction. The studied preparation methods influenced the surface area of the Cu-based catalysts and consequently their catalytic activity; however, we verified that surface area was not the only factor influencing activity. Different structural changes in the aurichalcite precursor resulted from the different preparation methods used, and these differences were also observed in the reduced catalysts. An expansion of the Cu lattice with an increase in microstrain were identified and attributed to the formation of a Cu–Zn alloy. Based on the correlation found between these structural changes and the catalytic activity, the Cu–Zn alloy was proposed as active site. We concluded that the preparation methods used influenced Cu dispersion and overall catalyst structure, and Cu–Zn alloy formation resulted from the incorporation of Zn atoms into the Cu lattice. This influence was more pronounced in the catalysts prepared by homogeneous precipitation and coprecipitation. The yttrium promoter did not provide textural or structural advantages. In contrast, the incorporation of Zr promoted both greater Cu dispersion and structural changes in the Cu lattice.     

A semi-global reaction rate model based on experimental data for the self-hydrolysis kinetics of aqueous sodium borohydride

This work studied the self-hydrolysis kinetics of aqueous sodium borohydride (NaBH₄) for hydrogen generation and storage purposes. Two semi-global rate expressions of sodium borohydride and hydrogen ion consumption were derived from an extensive series of batch process experiments where the following parameters were systematically varied: solution temperature (298 K–348 K), NaBH₄ concentration (0.5 wt% to 25.0 wt%), and sodium hydroxide (NaOH) concentration (0.0 wt% to 4.0 wt%). Transient hydrogen generation rates and transient solution pH were measured during the hydrolysis experiments. Given initial conditions (temperature, NaBH₄ concentration, and H⁺ concentration), the two coupled semi-global rate equations can be integrated to obtain the transient time history of H₂ generation (or NaBH₄ consumption) and solution pH (or H⁺ concentration). Comparing analytical results of transient hydrogen generation rate and transient solution pH with experimental data, good agreement was reached for many conditions, especially for elevated solution pH values, levels at which NaBH₄ solutions are used practically.     

Effect of hydrogen absorption/desorption cycling on hydrogen storage properties of a LaNi3.8Al1.0Mn0.2 alloy

The effect of long-term hydrogen absorption/desorption cycling up to 3500 cycles on the hydrogen storage properties of LaNi_3.8Al_1.0Mn_0.2 alloy was investigated. The pressure-composition (PC) isotherms for absorption/desorption and the absorption kinetics were measured at 433 K, 453 K and 473 K. X-ray diffraction analysis revealed that the alloy had a homogeneous hexagonal CaCu₅ type structure and kept this structure even after 3500 cycles, but the diffraction peaks were broadened. The degree of peak broadening was increased with increase of the cycle number, but exhibiting a maximum after initial activation. The shapes of PCT curves after 300, 2000 and 3500 cycles were similar to that after initial activation. It was found that the alloy subjected to 300 cycles did not exhibit significant changes in hydrogen storage capacity, but the long-term cycling up to 2000 and 3500 cycles resulted in obvious decrease in hydrogen storage capacity. The degradation of the hydrogen capacity might be resulted from the formation of the irreversible sites and more stable hydride phase, though no new phase was found after absorption/desorption cycling from XRD pattern as shown in Fig. 6 because of the limitation of XRD analysis sensibility. The hydrogen absorption kinetics after 300 cycles was deteriorated but improved again after 2000 and 3500 cycles compared with that of after initial activation. The changes in hydrogenation properties of the alloy induced by cycling were discussed by considering the crystal structure, lattice strain and pulverization of the sample.     

Influence of the reduction of graphene oxide (rGO) on the structure and photoactivity of CdS-rGO hybrid systems

Solvothermal and chemical reduction of graphene oxide with N₂H₄ or HI affect the surface composition, rupture and delamination degree of reduced graphene oxide (rGO). Higher reduction and stacking of rGO was achieved by chemical reduction with HI, while solvothermal reduction and, especially, the chemical reduction with N₂H₄ lead to higher delamination of rGO. The incorporation of the different rGO to CdS implies changes in the characteristics and photoactivity of the CdS-rGO hybrids. A promoter effect was observed in all CdS-rGO hybrids respect to the photoactivity of bare CdS, observing the better photoactivity on the hybrid in which the graphene oxide was reduced with HI (CdS-rGO/HI). The variations in the photoactivity of CdS-rGO hybrids are analyzed in terms of changes in the structure, surface and light absorption ability of CdS and also analyzing the contact of CdS with rGO. The greater concentration of small CdS nanostructures with strong quantum confinement is in the origin of the enhancement in photoactivity observed in the CdS-rGO/HI hybrid.     

A multifactor study of catalyzed hydrolysis of solid NaBH4 on cobalt nanoparticles: Thermodynamics and kinetics

In the present work, hydrogen generation through hydrolysis of a NaBH_4(s)/catalyst_(s) solid mixture was realized for the first time as a solid/liquid compact hydrogen storage system using Co nanoparticles as a model catalyst. The performance of the system was analysed from both the thermodynamic and kinetic points of view and compared with the classical catalyzed hydrolysis of a NaBH₄ solution. The kinetic analysis of the NaBH_4(s)/catalyst_(s)/H₂O_(l) system shows that the reaction is first order with respect to the catalyst concentration, and the activation energy equal to 35 kJ mol_NaBH4⁻¹. Additionally, calorimetric measurements of the heat evolved during the hydrolysis of NaBH₄ solutions evidence the global process energy (−217 kJ mol_NaBH4⁻¹). Characterization of the cobalt nanoparticles before and after the hydrolysis associated with the calorimetric measurements suggests the “in situ” formation of a catalytically active Co_xB phase through “reduction” of an outer protective oxide layer that is regenerated at the end of reaction.     

Influence of transition metal dopants and temperature on the dehydrogenation and rehydrogenation kinetics of NaAlH4

NaAlH₄ has been doped with ScCl₃, TiCl₃, CeCl₃, and combinations of these additives by high-energy ball milling. The phase composition of the samples after milling was analyzed by X-ray diffraction. Microstructure and catalyst distribution were investigated by scanning electron microscopy. The effect of the additives and their combination on the two dehydrogenation and rehydrogenation steps of suchlike doped NaAlH₄ has been studied under isothermal and isobaric conditions by thermogravimetry under a H₂ back pressure of 1 bar. From these studies it turned out that ScCl₃-doped NaAlH₄ was superior to all other dopants and combinations investigated, both for dehydrogenation and rehydrogenation. For this dopant, the influence of the temperature on the kinetics of each single dehydrogenation and rehydrogenation step and the dehydrogenation kinetics in dependence on the H₂ back pressure were studied in detail.