Repeated Redox Cycles Research Articles

Montmorillonite‐Supported Iron Oxide for Hydrogen Storage by Chemical Looping

AbstractA technology for hydrogen storage is proposed based on the repeated reduction of magnetite (Fe3O4+4 H2→3 Fe+4 H2O) and the oxidization of metallic iron (3 Fe+4 H2O→Fe3O4+4 H2). Herein, we prepared three montmorillonite (MMT)‐supported iron oxide samples by using a sol–gel method and demonstrated their application in hydrogen storage through a chemical looping process. The redox performance of the Fe‐MMT samples was studied at 600 °C in a fixed‐bed reactor. The results showed that 70 Fe‐MMT (with 70 wt % Fe2O3) exhibited relatively good redox performance owing to its excellent anti‐agglomeration ability and sintering resistance. After the redox performance tests, Fe2SiO4 was observed in all Fe‐MMT samples. It was worth noting that the diffraction intensity of Fe2SiO4 in 70 Fe‐MMT was much weaker than in 30 Fe‐MMT (30 wt % Fe2O3) and 50 Fe‐MMT (50 wt % Fe2O3), which was owed to the high MMT content in the 30 Fe‐MMT and 50 Fe‐MMT samples. The formation of Fe2SiO4 in repeated redox cycles could reduce the stability and recyclability of Fe‐MMT samples. This might be the major reason for the better hydrogen storage performance of 70 Fe‐MMT compared with 30 Fe‐MMT and 50 Fe‐MMT.

CO production from CO2 via reverse water–gas shift reaction performed in a chemical looping mode: Kinetics on modified iron oxide

Carbon monoxide production from carbon dioxide via isothermal reverse water–gas shift chemical looping (RWGS-CL) is studied with a modified iron oxide oxygen carrier material (80wt% Fe2O3–Ce0.5Zr0.5O2). The material is characterized by TEM, XRD and thermogravimetry at temperatures from 750°C to 850°C and gas mole fractions of H2 and CO2 from 0.05 to 0.75, respectively. High temperature and high reactant concentrations favor the oxidation and reduction of the material during repeated redox cycles. The reaction rate of reduction is always faster than that of oxidation applying the same gas concentration of H2 and CO2, respectively. The long term stability of the material is investigated with 500 redox cycles in a plug flow reactor. The material shows gradual deactivation lowering the CO yield during the first 100 redox cycles. After that, a steady state CO yield is reached for the next 400 redox cycles. Deactivation is attributed to surface sintering which results in slower reaction kinetics. TG data was used for a kinetic analysis applying the master plot method. The experimental data for oxidation and reduction indicated reaction mechanisms, which are well described by a reaction order and a geometrical contraction model. After parameter estimation, a good agreement between the model and the TG data was achieved with the reaction order and geometrical contraction model for oxidation and reduction, respectively. The RWGS-CL process can be used for sustainable CO production from CO2 if the energy for the process and for H2 production is supplied by renewable sources.

Kinetics of oxidative cracking of n‐hexane to olefins over VOx/Ce‐Al2O3 under gas phase oxygen‐free environment

The kinetics of oxidative cracking of n‐hexane to olefins using lattice oxygen of VOx/Ce‐Al2O3 is investigated. The TPR/TPO analysis shows a consistent reducibility (79%) of VOx/Ce‐Al2O3 in repeated redox cycles. The total acidity of the sample is found to be 0.54 mmol/g with 22% are strong acid sites that favors olefin selectivity. The oxidative cracking of n‐hexane in a fluidized CREC Riser simulator gives approximately 60% olefin selectivity at 30% n‐hexane conversion. A kinetic model is developed considering (1) cracking, (2) oxidative dehydrogenation (ODH), and (3) catalyst deactivations. The proposed cracking mechanism considers adsorption, C–H and C–C bond fission and desorption as elementary steps and implemented by pseudo steady state hypothesis. A Langmuir‐Hinshelwood mechanism is found to represent the ODH reactions. The developed model fits the experimental data with favorable statistical indicators. The estimated specific reaction rate constants are also found to be consistent with the product selectivity data. © 2016 American Institute of Chemical Engineers AIChE J, 63: 130–138, 2017

Enhanced performance of manganese ore as oxygen carrier for chemical-looping with oxygen uncoupling (CLOU) by combination with Ca(OH)2 through spray-drying

Chemical-looping with oxygen uncoupling (CLOU) is an innovative combustion technology with inherent capture of the CO2 and potential to drastically reduce the cost of capture. The process requires two interconnected reactors, between which the oxygen carrier particles are circulated while carrying oxygen from the combustion air to the fuel. In this work, the reactivity and mechanical stability of five materials were studied; three natural ores and two materials which were combinations of an ore and Ca(OH)2. The overall aim was to investigate the feasibility of making a reactive and mechanically stable material using cheap raw materials and an industrially relevant particle manufacturing process.The two combined materials behaved differently from their natural counterparts. The Brazilian ore+Ca(OH)2 showed a decrease in reactivity towards methane, but higher reactivity towards syngas in comparison to the pure ore. The South African ore+Ca(OH)2 showed a major improvement in reactivity towards both syngas and methane, which could be attributed to formation of a perovskite-structure material with significant CLOU properties. A comparison between the pure ores and the combined materials show that the addition of Ca(OH)2 had generally a beneficial impact on the mechanical stability of the oxygen carriers. As a significant change in the particle size distribution was seen for all oxygen carrier materials after repeated redox cycles with long reduction periods, the particle disintegration was likely caused by the chemical phase transformations occurring inside the particles, rather than by mechanical forces.

Transformation of clay-sized minerals in soils exposed to prolonged regular alternation of redox conditions

The direction of the transformation of Fe-bearing minerals under harshly changing redox conditions is still under debate. Some studies suggest preferential accumulation of weakly crystalline Fe oxides while other studies showed that repeated redox cycles favour the presence of crystalline phases. Since characterized by distinct redox cycles, paddy soils are ideal for studying redox-related transformations of Fe oxides and Fe-bearing clay minerals. We analysed changes in the Fe mineral assemblage upon long-term reduction–oxidation cycles along a chronosequence of 100, 700, and 2000-year-old paddy soils developed on comparable parent material relative to two non-irrigated counterparts.The clay mineral assemblage, as indicated by X-ray diffraction, showed no significant differences between paddy and non-paddy soils as well as no changes with time under paddy management.Speciation of Fe mineral phases in top- and subsoil was accomplished by Mössbauer spectroscopy at room temperature and 4.2K, in combination with X-ray diffraction and selective chemical extractions (dithionite–citrate–bicarbonate, NH4 oxalate). We found ferrous and ferric Fe in silicates, nano- to micro-crystalline goethite and haematite, as well as a ferrihydrite-type phase. In topsoils, prolonged paddy cultivation resulted in general loss of silicate- and oxide-bound Fe in the topsoils and the remaining Fe oxide assemblage became more and more dominated by organic-rich weakly crystalline phases. Total contents of Fe oxides in subsoils increased due precipitation of Fe leached from the topsoils. Micro-crystalline goethite was the predominating Fe oxide phase in the upper subsoil, likely due to Fe2+-catalysed solid-state transformation of freshly precipitated hydrous Fe oxide phases. Lepidocrocite formed in the deeper subsoil, probably promoted by less CO2 in the soil water. Thus, paddy cultivation causes different Fe mineral weathering, forming, and transforming environments, with the depth differentiation of these environments becoming increasingly distinct with time under paddy management, i.e., the number of redox cycles the soils underwent.

Hydrogen Oxidation Mechanisms on Perovskite Solid Oxide Fuel Cell Anodes

The most widely used Solid Oxide Fuel Cell (SOFC) anode materials, Ni-cermets, are susceptible to coking in hydrocarbon fuels as well as many coal- and bio-derived syngas compositions. Ni-based anodes are also poisoned by fuel impurities such as hydrogen sulfide, and can be structural vulnerable during repeated redox cycling. These problems can be largely avoided by implementing fuel processing measures, but not without sacrificing the overall power plant cost effectiveness and efficiency. A number of different conducting oxide materials have been proposed as anodes that can avoid the above-mentioned problems with Ni-based anodes. However, these oxide anodes exhibit much higher polarization resistance than anodes containing Ni or other catalytic metals. The mechanisms that limit hydrogen oxidation rates in these oxide anodes have not been determined. Several studies of chromite, titanate-manganite, chromite-manganite, and doped ceria anodes have suggested that hydrogen adsorption was a possible rate-limiting step, along with hydrogen electrochemical oxidation. Recent electrochemical impedance spectroscopy (EIS) measurements on SOFCs with SrTi0.3Fe0.7O3-δ (STF)-Gd0.1Ce0.9O2 (GDC) anode as a function of hydrogen partial pressure provided direct evidence that a H2adsorption mechanism was an important rate-limiting step in the anode reaction. Here we present new results on (La,Sr)(Ga,Mg)O3 (LSGM) electrolyte-supported SOFCs with STF and (La,Sr)(Cr,Fe)O3-δ (LSCrFe) based oxide anodes. The results show clear evidence that adsorption becomes the rate-limiting step at low temperature and low fuel hydrogen partial pressure. Figure 1 shows an example of the current-voltage results for an STF anode cell, where there is a limiting current that decreases with decreasing hydrogen partial pressure. Figure 2 shows that the limiting current decreases with decreasing temperature, indicating that it is an activated process and hence not a gas diffusion limitation. A model accounting for hydrogen adsorption and electrochemical oxidation as possible rate-limiting steps is developed and used to fit current-voltage and EIS results from the oxide anode cells. The fits, shown in Figures 1 and 2, are good for cell voltages from open circuit down to ~ 0.5 V. Possible reasons for the deviation at lower cell potentials will be discussed in the talk. It is also shown that the model is consistent with results for other oxide anodes, suggesting that the dissociative adsorption of H2can be an important rate-limiting step for various oxide anodes under SOFC operating conditions, especially lower temperature and hydrogen partial pressure. Moreover, both the experimental data and model fitting results suggest that the precipitation of metal particles at the anode surface, like Ru in LSCrFe based anodes and Ni in STF based anodes, effectively promote hydrogen adsorption. Figuer captions: Fig. 1 Current-voltage data and fitting results for LSGM electrolyte supported cell with STF anode at 800 oC versus the hydrogen partial pressure Fig. 2 Current-voltage data and fitting results for LSGM electrolyte supported cell with STF anode at various temperatures under wet H2 Figure 1

Titanium-substituted lanthanum strontium ferrite as a novel electrode material for symmetrical solid oxide fuel cell

In this study, La0.3Sr0.7Ti0.3Fe0.7O3-δ (LSTF) is evaluated as a novel symmetrical electrode on SDC(Sm0.2Ce0.8O1.9)-YSZ(yttria-stabilized zirconia)-SDC three-layer electrolyte. Impedance spectroscopy results indicate the polarization resistances of anode and cathode are 0.18 and 0.04 Ω cm2 at 900 °C, respectively, suggesting that the micron-scale LSTF possesses high electrocatalytical activity for both oxygen reduction and hydrogen fuel oxidation reactions. A single cell consisting of LSTF symmetrical electrode and YSZ electrolyte support (∼400 μm) shows a maximum power density (MPD) of 374 mW cm−2 at 900 °C using wet H2 as fuel. Furthermore, the LSTF electrode shows good short-term stability in wet H2 at 800 °C and reliable redox stability during the repeated redox cycles. The present results indicate that the La0.3Sr0.7Ti0.3Fe0.7O3-δ is a promising electrode material for symmetrical SOFCs.

Kinetics of oxygen uncoupling of a copper based oxygen carrier

Here, an oxygen carrier consisting of 60wt% CuO supported on a mixture of Al2O3 and CaO (23wt% and 17wt% respectively) was synthesised by wet-mixing powdered CuO, Al(OH)3 and Ca(OH)2, followed by calcination at 1000°C. Its suitability for chemical looping with oxygen uncoupling (CLOU) was investigated. After 25 repeated redox cycles in either a thermogravimetric analyser (TGA) or a laboratory-scale fluidised bed, (with 5vol% H2 in N2 as the fuel, and air as the oxidant) no significant change in either the oxygen uncoupling capacity or the overall oxygen availability of the carrier was found. In the TGA, it was found that the rate of oxygen release from the material was controlled by intrinsic chemical kinetics and external transfer of mass from the surface of the particles to the bulk gas. By modelling the various resistances, values of the rate constant for the decomposition were obtained. The activation energy of the reaction was found to be 59.7kJ/mol (with a standard error of 5.6kJ/mol) and the corresponding pre-exponential factor was 632m3/mol/s. The local rate of conversion within a particle was assumed to occur either (i) by homogeneous chemical reaction, or (ii) in uniform, non-porous grains, each reacting as a kinetically-controlled shrinking core. Upon cross validation against a batch fluidised bed experiment, the homogeneous reaction model was found to be more plausible. By accurately accounting for the various artefacts (e.g. mass transfer resistances) present in both TGA and fluidised bed experiments, it was possible to extract a consistent set of kinetic parameters which reproduced the rates of oxygen release in both experiments.

Activation and Reactivity of Iron Oxides as Oxygen Carriers for Hydrogen Production by Chemical Looping

We report the activation principle, the different forms of activated oxygen carriers, and the optimal activation method for iron-based oxygen carriers. The activation of an oxygen carrier is commonly referred to as the enhancement of the oxygen transfer capacity of an oxygen carrier by repeated reduction and oxidation reactions, which is mainly attributed to enhanced intraparticle ionic/gaseous diffusivity over the support material. Accurate knowledge of the activation process for an oxygen carrier is a key issue in the development of particles and optimization of a chemical looping system. In this study, we found that two key factors, the interaction between iron oxide and the support as well as the localized hematite-lower oxides stress, are fundamental reasons for the activation of iron-based oxygen carriers. The dispersion of the iron oxide in the activated particles was determined by the iron oxide–support interaction. The larger the localized stress applied at the interface between the hematite and lower oxides, the more cracks/pores were created, facilitating gas diffusion. We observed the different forms of the activated particles by altering the reduction and oxidation conditions, and the various formations could be explained by the aforementioned key factors. One of the activated particles covered with sintered iron oxide resulting from the outward diffusion of Fe showed enhanced reactivity. However, this particle revealed lower reactivity than the highly dispersed oxygen carrier and the gradual formation of inactive iron due to sintering in repeated redox cycles. In regard to the optimal activation method, the particles subjected to conventional redox cycles (reduction–steam oxidation–air oxidation) were slowly activated as the cycles repeated, while particles subjected to redox cycles without steam oxidation were rapidly activated during the redox cycles due to larger localized stresses at the interface between the hematite and Fe than between the hematite and magnetite. We expect these findings will be useful in developing high-performance oxygen carriers and activating them efficiently.

Kinetic characterization on reductive reactivity of iron(III) oxides in surface sediments of the East China Sea and the influence of repeated redox cycles: Implications for microbial iron reduction

Reductive reactivity of iron(III) [Fe(III)] oxides in surface sediments of the East China Sea (ECS) and its variations while the sediments were subjected to repeated redox cycles were characterized by three rate parameters: m0 (the initial amount of ascorbate-reducible Fe(III) oxides), k′ (the rate constant, s−1) and γ (a parameter describing heterogeneity of reactivity), obtained by applying a generalized dissolution rate law to time-dependent dissolution data in a buffered ascorbate solution (pH 7.5). The spatial pattern of the m0 values is generally controlled by that of the clay fraction, but the values cannot be quantitatively corresponding to any specific reactive Fe(III) pools operationally defined by the conventional chemical extractions. Unlike the m0, the spatial patterns of k′ and γ are more complex and may be controlled by more intricate factors relevant to Fe redox chemistry. At most sites studied, Fe(III) oxides have lower reactivity (k′) and higher heterogeneity (γ) than those of fresh ferrihydrite. Repeated redox cycles rendered m0 and γ apparently decreased mainly during the initial 2–3 cycles due to mineral ripening and neoformation of “purified” Fe(III) oxides, respectively. Although the initial 1–2 cycles resulted in a “temporary” increase in Fe(III)-oxide reactivity, prolonged cycles eventually rendered the reactivity decreasing relative to the initial value. An a priori assumption that long-term redox cycles in natural sediments could enhance reductive reactivity of Fe(III) oxides as well as heterogeneity of the reactivity is unwarranted.

Selenium redox cycling during weathering of Se-rich shales: A selenium isotope study

Selenium isotopes are becoming an important paleoenvironmental proxy. However, few studies have focused on the behavior of Se isotopes during oxidative weathering. In this paper, a comprehensive set of Se isotopic composition and concentration data were collected from the weathering profiles of Se-rich shales of the Permian Maokou Formation in Yutangba and Shadi, China to investigate Se isotopic fractionation and Se enrichment during weathering processes. The δ82/76Se in fresh shales (148±118mg/kg Se, 1SD, n=40) from Shadi and Yutangba drill cores varies from −1.69‰ to 1.74‰ with an average of 0.40±0.71‰ (1SD, n=40), consistent with the range in other Phanerozoic shales, suggesting that Se isotopes are not strongly fractionated during Se sequestration in the primary sedimentary environment. However, the strongly weathered Se-rich shales from Shadi and Yutangba profiles are isotopically lighter with average δ82/76Se values of −1.96±1.08‰ (1SD, n=5) and −1.08±1.83‰ (1SD, n=23), respectively. These data suggest that Se isotopes can be fractionated during oxidation and reduction processes associated with weathering, with heavier isotopes removed preferentially during oxidative weathering of shales. Such a shift, if found to be a global phenomenon, would have implications for models of the global Se cycle and interpretation of Se isotope data from past biogeochemical regimes.Locally altered shales exposed in a quarry at Yutangba are extremely enriched in Se with 1642±1505mg/kg (1SD, n=45), approximately 10 times greater than that in unaltered drill core samples. These rocks display very strong variation in δ82/76Se over short distances, with a single 60cm transect showing the most strongly negative and positive δ82/76Se values (−14.20‰ to +11.37‰) observed to date in natural samples. This suggests that Se has undergone multiple cycles of oxidation, mobilization, and re-reduction, resulting in a Se-rich redox front that has migrated downward through the organic-rich shales over time. δ82/76Se values vary sharply over distances as small as 10cm, indicating that Se redox conditions change strongly with position and are controlled by fractures and rock layering. Our data and a simple conceptual model suggest that zones of increased permeability that are accessed first by infiltrating waters are isotopically light, whereas less accessible zones are heavy. Repeated redox cycling accentuates this pattern. Furthermore, the average δ82/76Se in Se-rich shales at the Yutangba weathering system is 0.45±5.77‰ (1SD, n=39), identical with that (0.40±0.71‰) in fresh shales from same locality, implying that Se released by weathering accumulates in the redox front with little loss. The strong Se isotopic fractionation occurring in the redox front implies that Se isotopes can be extensively used in tracing geochemical processes of Se in groundwater system, especially related to fractures.

Development of Sinter-Resistant Core–Shell LaMnxFe1–xO3@mSiO2 Oxygen Carriers for Chemical Looping Combustion

This work investigates the possibility of using LaMn0.7Fe0.3O3.15@mSiO2 as oxygen carriers for chemical looping combustion (CLC). CLC is a new combustion technique with inherent separation of CO2 from atmospheric N2. LaMn0.7Fe0.3O3.15@mSiO2 core–shell materials were prepared by coating a layer of mesostructured silica around the agglomerated perovskite particles. The oxygen carriers were characterized using different methods, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 sorption, hydrogen temperature-programmed reduction (H2-TPR), and temperature-programmed desorption of oxygen (TPD-O2). The reactivity and stability of the carrier materials were tested in a special reactor, allowing for short contact time between the fluidized carrier and the reactive gas [Chemical Reactor Engineering Centre (CREC) fluidized riser simulator]. Multiple reduction–oxidation cycles were performed. TEM images of the carriers showed that a perfect mesoporous silica layer was formed around samples with 4, 32, and 55 nm in thickness. The oxygen carriers having a core–shell structure showed higher reactivity and stability during 10 repeated redox cycles compared to the LaMn0.7Fe0.3O3.15 core. This could be due to a protective role of the silica shell against sintering of the particles during repeated cycles under CLC conditions. The agglomeration of the particles, which occurred at high temperatures during CLC cycles, is more controllable in the core–shell-structured carriers, as confirmed by SEM images. XRD patterns confirmed that the crystal structure of all perovskites remained unchanged after multiple redox cycles. Methane conversion and partial conversion to CO2 were observed to increase with the contact time between methane and the carrier. Indeed, more oxygen from the carrier surface, grain boundaries, and even from the bulk lattice was released to react with methane. Upon rising the contact time, less CO was formed, which is desirable for CLC application. Increasing the reaction temperature and methane partial pressure lead to enhanced conversions of CH4 under CLC conditions.

Hydrogen Storage and Release by Redox Reaction of Iron Oxide Medium with Mo and Zr Additives

Chemical hydrogen storage and release of iron-based oxide mediums were investigated by hydrogen reduction and water splitting oxidation (Fe3O4 + 4H2 ⇌ 3Fe + 4H2O). In this study, all metal oxide mediums were prepared by coprecipitation method using urea solution as precipitant. The redox reactions of the mediums were conducted using a fixed bed quartz reactor under atmospheric pressure. The theoretical amount of hydrogen storage that can be obtained from the redox reaction of iron oxide is calculated to be 4.8wt% on the basis of 1g-Fe. However, in case of using the iron oxide medium without additives, the medium was rapidly deactivated due to the agglomeration of Fe metals in the hydrogen reduction step of repeated redox cycles. In this study, therefore, Mo and Zr additives were added to iron oxide to improve the reactivity of the medium and to prevent the agglomeration of that. As a result, the reactivity for oxidation of the mediums was largely improved with the addition of Mo additive. It was concluded that change in the valence of Mo cations affected the redox behavior of the mediums.

Pd2+/Pd0 Redox Cycling in Hexagonal YMn0.5Fe0.5O3: Implications for Catalysis by PGM-Substituted Complex Oxides

Complex oxides--containing at least two different cations on crystallographically distinct sites--have recently been shown to display redox cycling of platinum group metals (PGMs), such as Pd; for example, Pd-substituted complex oxides can reversibly extrude metallic Pd under reducing conditions and then reincorporate Pd(2+) ions into the lattice under oxidizing conditions. The title compounds, YMn(0.5)Fe(0.5-x)Pd(x)O(3-δ) (0 ≤ x ≤ 0.07) crystallizing in the noncentrosymmetric YMnO(3) structure, were prepared using a sol-gel process at 800 °C, and the structures were refined from high-resolution synchrotron X-ray powder diffraction data. Their redox cycling behavior was monitored using synchrotron X-ray diffraction and EXAFS studies. In contrast to the previously studied complex oxide host compounds, YMn(0.5)Fe(0.5-x)Pd(x)O(3-δ) is only modestly tolerant to cycling: repeated redox cycling leads to the formation of PdO, which, on the time-scale of the oxidation cycles, does not reincorporate in the complex oxide lattice. Both oxidized and reduced samples were tested for the oxidation of CO to CO(2) under CO-lean conditions. YMn(0.5)Fe(0.5-x)Pd(x)O(3-δ) performs essentially as well as previously studied YFe(1-x)Pd(x)O(3-δ). The CO oxidation light-off characteristics of the hexagonal hosts are very similar to finely dispersed PdO. Despite evidence that Pd is almost fully dispersed as divalent ions in the host lattice, which is presumably accompanied by the concurrent creation of oxygen vacancies (2 Pd(2+):1 V(O(2-))), the as-prepared hexagonal materials do not display any significant improvement in catalytic activity as a function of Pd substitution level. This suggests that the corner-connected trigonal bipyramids that characterize this structural family do not enable the transport of oxygen through the bulk of the lattice. The study casts light on factors in the solid-state chemistry of precious metal-substituted complex oxides that influence the efficacy of redox cycling of the precious metal, and catalytic performance.

In-situ patterned intra-anode triple phase boundary in SOFC electroless anode: An enhancement of electrochemical performance

Co-existence of intra-anode and conventional triple phase boundary (TPB) is reported for the first time in core (YSZ)–shell (Ni) electroless anode cermet. A mathematical model is proposed for determination of intra-anode TPB length and has been validated experimentally. Retention of longer intra-anode TPB even after repeated redox cycling is responsible for relatively lower conductivity degradation of such cermets. Existence of intra-anode TPB in electroless anode results in significant improvement of electrochemical performance. Single cells with electroless anode exhibit a current density of 2.5 A cm −2 compared to cells with conventional anode (1.7A cm −2) at 800 °C, 0.7 V.

Structures and Redox Characteristics of Electron-Deficient Vanadyl Phthalocyanines

The first single-crystal X-ray structures of substituted vanadyl phthalocyanine materials reveal the high-valence vanadium ions (denoted as V(IV)), whose coordination by a highly electron-deficient ligand is facilitated by an axial oxo group. The metal center of the hydrophilic V═O core, encapsulated in F-rich hydrophobic pockets, reaches a coordination number of 6 by binding an additional H(2)O that, in turn, hydrogen-bonds with ketones, resulting in solvent-induced variable solid-state architectures. Fluoroalkyl (R(f)) ligand substituents hinder π-π stacking interactions and favor ordered long-range packing, as well as the facile formation of film materials that exhibit high thermal stability and oxidation resistance. Reversible redox chemistry and spectroscopic studies in both solution and the solid-state indicate single-site isolation in both phases and an R(f)-induced propensity for electron uptake and inhibition of electron loss. Repeated redox cycles reorganize the thin films to accommodate Li(+) ions and facilitate their migration. The facile reduction, combined with high stability and ease of sublimation imparted by the R(f) scaffold that suppresses oxidations, recommends the new materials for sensors, color displays, electronic materials, and redox catalysts, as well as other applications.

Chemical hydrogen storage and release properties using redox reaction over the Cu-added Fe/Ce/Zr mixed oxide medium

The chemical hydrogen storage (hydrogen reduction) and release (water-splitting oxidation) properties of the Cu-added Fe/Ce/Zr mixed oxide medium were investigated. The media with Cu content ranging from 0 to 5 wt% were prepared by a co-precipitation method using urea as a precipitant. The hydrogen reduction and the water-splitting oxidation on the medium were tested by temperature programmed reduction/oxidation (TPR/TPO) and repeated isothermal redox cycles at 550 °C for reduction and 350 °C for oxidation. The initial reduction rates and oxidation rates of the media increased with increasing the amount of the Cu additive. In addition, the reactivity of the medium for water-splitting oxidation was enhanced as the CeO 2/ZrO 2 ratio increased. Especially, the Fe-based mixed oxide medium with Cu/CeO 2/ZrO 2 contents of 3/30/10 wt% (Cu(3%)-Fe-CeO 2/ZrO 2(3/1)) showed superior performance in chemical hydrogen storage and release. As the results of isothermal redox cycles using the medium, the total amount of hydrogen evolved in water-splitting oxidation was maintained at ca. 8.5 mmol g −1-medium (ca. 1.8 wt% hydrogen storage amounts on the basis of the total medium) over 15 repeated redox cycles.

Effects of CeO 2 additive on redox characteristics of Fe-based mixed oxide mediums for storage and production of hydrogen

Effects of CeO 2 additive to Fe-based mixed oxide mediums with Rh and ZrO 2 for chemical hydrogen storage were investigated in terms of stability and reactivity of the mediums in water splitting oxidation with repeated redox cycles. The mediums with CeO 2 content ranging from 0 to 30 wt% were prepared by co-precipitation method using urea solution as a precipitant. The hydrogen storage and release properties were investigated during repeated isothermal redox cycles at 823 K for reduction with hydrogen and 623 K for oxidation with water vapor under atmospheric pressure. The amount of hydrogen produced by the mediums, both with and without CeO 2, was maintained at an almost constant level over ten repeated redox cycles. However, the oxidation rates of the mediums without CeO 2 were decreased during repeated redox cycles while that increased with increasing CeO 2 contents. Especially, the mediums added with 30 wt% of CeO 2 (FRZC-30) showed high activity and stability for ten redox cycles, the degree of hydrogen storage was almost maintained ca. 1.9 wt% on the basis of total amount of the medium.

Hydrogen Production by Redox of Cation-Modified Iron Oxide

A series of modified iron oxides, which can store and supply pure hydrogen for polymer electrolyte fuel cells through repeated redox reactions (Fe3O4 + 4H2 ↔ 3Fe + 4H2O), were prepared by impregnating Fe or Fe2O3 powder with an aqueous solution containing foreign metal additives. Effects of various metal additives in the modified samples on the hydrogen production from water decomposition were investigated. Among all the metal cation additives examined, Mo was the most favorable one for enhancing the reoxidation performance of the reduced modified samples at lower temperatures. The addition of Mo to Fe or Fe2O3 powder not only decreased the temperature of water decomposition significantly (i.e., increased the production of hydrogen at lower temperatures) but improved the stability of the modified samples during repeated redox cycling. The temperature of H2 production decreased from 489 to 289 °C for Mo-modified Fe and from 527 to 283 °C for Mo-modified Fe2O3 in the fourth cycle. No obvious decrease in the...

Copper N 2S 2 Schiff base macrocycles: The effect of structure on redox potential

A series of bis-salicylidene based N 2S 2 copper macrocycles were prepared, structurally characterised and subjected to electrochemical analysis. The aim was to investigate the effects of length of polymethylene chains between either the imine donors or the sulfur donors on redox state and potential of the metal. The complexes structurally characterised had either distorted square planar or tetrahedral geometries depending on their oxidation state (Cu 2+ or Cu +, respectively), and the N–(CH 2) n –N bridge was found to be most critical moiety in determining the redox potential and oxidation state of the copper macrocycles, with relatively little change in these properties caused by lengthening the S–(CH 2) n –S bridge from two to three carbons. In fact, a weakness was observed in the complexes at the sulfur donor, as further lengthening of the S–(CH 2) n –S methylene bridge to four carbons caused fission of the carbon–sulfur bond to give dimeric rings and supramolecular assemblies. Cu + complexes could be oxidised to Cu 2+ by tert-butylhydroperoxide, with a corresponding change in the spectrophotometric properties, and likewise Cu 2+ complexes could be reduced to Cu + by treatment with β-mercaptoethylamine. However, repeated redox cycles appeared to compromise the stability of the macrocycles, most probably by a competing oxidation of the ligand. Thus the copper N 2S 2 macrocycles show potential as redox sensors, but further development is required to improve their performance in a biochemical environment.