Subsequent Reoxidation Research Articles

Synthesis of YFeO<sub>3-δ</sub> and LaFeO<sub>3-δ</sub> Perovskites with High Catalytic Activity in Carbon Oxidation Reactions

YFeO3-δ (δ = 0.26) and LaFeO3-δ (δ = 0.5) perovskites with a high specific surface and oxygen non-stoichiometry was firstly synthesized by pyrolysis of polymer-salt compositions. It was shown that the catalytic oxidation of carbon in the presence of these complex oxide systems proceeds in the range of 400 - 700 °С, with a maximum temperature at 556 °С for YFeO3-δ; and 380 - 620 °С ,with a maximum temperature at 501 °С for LaFeO3-δ, in one-stage mode for both. By means of thermal analysis and diffractometry, it was shown that there is no contribution to the soot oxidation mechanism by cyclic perovskite surface transformations, due to the reduction of metal oxides by the soot and their subsequent reoxidation. It has been established that the basis of the catalytic reaction mechanism for both perovskites is the presence of oxygen vacancies on the surface of complex oxides.

Single-Source Alkoxide Precursor Approach to Titanium Molybdate, TiMoO5, and Its Structure, Electrochemical Properties, and Potential as an Anode Material for Alkali Metal Ion Batteries.

Transition-metal oxide nanostructured materials are potentially attractive alternatives as anodes for Li ion batteries and as photocatalysts. Combining the structural and thermal stability of titanium oxides with the relatively high oxidation potential and charge capacity of molybdenum(VI) oxides was the motivation for a search for approaches to mixed oxides of these two metals. Challenges in traditional synthetic methods for such materials made development of a soft chemistry single-source precursor pathway our priority. A series of bimetallic Ti-Mo alkoxides were produced by reactions of homometallic species in a 1:1 ratio. Thermal solution reduction with subsequent reoxidation by dry air offered in minor yields Ti2Mo2O4(OMe)6(OiPr)6 (1) by the interaction of Ti(OiPr)4 with MoO(OMe)4 and Ti6Mo6O22(OiPr)16(iPrOH)2 (2) by the reaction of Ti(OiPr)4 with MoO(OiPr)4. An attempt to improve the yield of 2 by microhydrolysis, using the addition of stoichiometric amounts of water, resulted in the formation with high yield of a different complex, Mo7Ti7+xO31+x(OiPr)8+2x (3). Controlled thermal decomposition of 1–3 in air resulted in their transformation into the phase TiMoO5 (4) with an orthorhombic structure in space group Pnma, as determined by a Rietveld refinement. The electrochemical characteristics of 4 and its chemical transformation on Li insertion were investigated, showing its potential as a promising anode material for Li ion batteries for the first time. A lower charge capacity and lower stability were observed for its application as an anode for a Na ion battery.

Redox-driven changes in water-dispersible colloids and their role in carbon cycling in hydromorphic soils

Redox fluctuations in hydromorphic soils can influence ecosystem functions by altering the cycling of organic carbon (OC) and other elements in both the aqueous and solid phases throughout the soil profile. Most studies focusing on the mobility of dissolved OC in rice paddy soils have often disregarded the contribution of colloidal particles. We provide a detailed chemical and size-related characterization of water-dispersible soil colloids and their depth distribution in two soil profiles under long-term temperate paddy and non-paddy management, by asymmetric flow field-flow fractionation. Anoxic conditions enhanced colloid dispersion with a preferential release of the finer colloid fractions (5 and 4-fold increase in the < 30 nm and 30–240 nm fractions, respectively). The mobility of OC-rich, iron (Fe) and aluminium (hydr)oxide and aluminosilicate colloids along the soil profile was probably responsible for their depletion in the topsoil and a corresponding accumulation in the deeper illuvial horizons compared to the non-paddy soil. However, the release, percolation and subsequent reoxidation of Fe2+ was also shown to be a plausible mechanism leading to the formation of fine colloids in the subsoil. Redox-driven changes in colloid distribution were also linked to the differences in OC and pedogenetic Fe stocks in these two agro-ecosystems.

Additive Effect of Iron and Copper Sulfates in Heteropolyacid Solution As a Cathode Reaction Mediator on the Performance of Redox Flow PEFCs

Since the Industrial Revolution, we have relied heavily on fossil resources. Even today, they are indispensable for our cultural activities, but environmental problems such as global warming and soil pollution caused by their combustion have been concerned. These issues have grown an interest in alternative fuels. In this situation, hydrogen has been regarded as promising next-generation energy sources in terms of energy content and environmental load. Moreover, fuel cells, which directly convert the chemical energy of hydrogen into electrical energy, are cleaner and more efficient than conventional power generation devices.Polymer electrolyte fuel cells (PEFCs) have been already commercialized as power sources for automobiles due to low operating temperature and high current density. However, the carbon-supported platinum or platinum-based alloy is widely applied as the cathode. Therefore, these fuel cells are very costly and required to reduce the platinum usage for more widespread application of PEFCs.We proposed a redox flow PEFC as one of the solutions to this problem [1]. The redox flow PEFC employing the heteropolyacid solution of H6PV3Mo9O40 as a redox mediator for the cathode reaction was constructed. In this system, the cathode is free from noble metals for electrocatalyst. The electrochemical reduction of heteropolyanion over the carbon cathode and the subsequent re-oxidation of the partially reduced heteropolyanion by exposure to the dissolved oxygen in the regenerator are important processes for the continuous power generation. Especially, the improvement of re-oxidation rate contributes to a stable operation with higher current density. Therefore, the development of catalyst to promote this process will lead to the improved performance.For the heteropolytungstates of H3PW12O40 and H5SiVW11O40, it has been reported that the re-oxidation of their reduced anion species are efficiently catalyzed by a combination of copper ion and iron ion [2].The catalytic pathway consists of a reversible reaction between Cu(II) and Fe(II), followed by fast oxidation of reduced heteropolyanion and Cu(I) by Fe(III) and O2, respectively, to regenerate Fe(II) and Cu(II). In this study, therefore, the influence of the addition of iron and copper sulfates to the H6PV3Mo9O40 solution on the cell performance was evaluated. As a result, the obtained current density and the re-oxidation rate were improved by the addition. Also, in the discharge operation at a constant current the terminal voltage remained higher in using the solution with these sulfates than without additives. Accordingly, the addition of iron and copper sulfates to H6PV3Mo9O40 solution contributed to the improvement of the performance of the redox flow PEFCs.-------------------------------------------------------------------------------------[1] T. Matsui, E. Morikawa, S. Nakada, T. Okanishi, H. Muroyama, Y. Hirao, T. Takahashi, K. Eguchi. ACS Appl. Mater. Inter.18119-18125 (2016).[2] M. Kim, M. Chamack, Y. V. Geletii, C. L. Hill, Inorg. Chem. 311-318 (2018)

Lattice Expansion in Optimally Doped Manganese Oxide: An Effective Structural Parameter for Enhanced Thermochemical Water Splitting

Earth abundant transition-metal oxides have attracted broad interest for thermochemical production of renewable fuels and other gaseous commodities. Despite progress, a major challenge remains in achieving fast and reversible redox kinetics as well as large oxygen exchange capacities. Here, we present insights on the optimal doping of manganese oxide nanocrystals for their efficient and stable utilization as redox material for thermochemical water splitting. The detailed investigation of the evolution of the material properties over a broad range of possible Ce-Mn compounds reveals a single key structural parameter affecting the thermochemical performance. We observe that the expansion of the MnO lattice is essential for activating its reduction from oxide to carbide and thus for H2O splitting during its subsequent reoxidation. This is optimally achieved for a very narrow window of dopant concentration peaking at 3% Ce content, which provides the largest distortion of the manganese oxide crystal lattice. In contrast, smaller or higher Ce amounts of 1 and 5%, respectively, result in significantly smaller lattice expansions either due to an insufficient dopant amount or to the segregation of Ce in large CeO2 domains. We use these findings to propose a mechanism for the enhancement of the redox kinetics of this metal oxide, which may provide guidance for the design of a family of future materials for thermochemistry.

Growth of Ultrathin Single-Crystalline IrO2(110) Films on a TiO2(110) Single Crystal.

The growth of a flat, covering, and single-crystalline IrO2(110) film with controlled film thickness on a single-crystalline TiO2(110) substrate is reported. The preparation starts with a deposition of metallic Ir at room temperature followed by a post-oxidation step performed in an oxygen atmosphere of 10-4 mbar at 700 K. On this surface, additional Ir can be deposited at 700 K in an oxygen atmosphere of 10-6 mbar to produce a IrO2(110) layer with variable thicknesses. To improve the crystallinity of the resulting IrO2(110) layer, the final film was post-oxidized in 10-4 mbar of O2 at 700 K for 5 min. The surface-sensitive techniques of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED) are employed to characterize the morphology, crystallinity, and electronic structure of the prepared ultrathin IrO2(110) films and how these films decompose upon annealing under ultrahigh vacuum (UHV) conditions. STM provides evidence that the IrO2(110) films start already to reduce at 465 K under UHV conditions. Upon annealing to 605 K under UHV the reduction of IrO2 intensifies (XPS), but the oxide film can readily be restored by re-oxidation in 10-4 mbar of O2 at 700 K. Thermal decomposition at 725 K leads, however, to severe reduction of the IrO2(110) layer (XPS, STM) that cannot be restored by a subsequent re-oxidation step. The utility of the IrO2(110)-TiO2(110) system as model electrodes is exemplified with the electrochemical oxygen evolution reaction in an acidic environment.

Effects of reaction atmosphere on physico-chemical properties of V-doped FeNb11O29

FeNb11O29, pure or doped with metal transition ions, is a very promising material with advanced multifunctionalities and interesting applicative perspectives. It is isostructural with Nb12O29, one of the rare compounds in which Nb displays a local magnetic moment and shows both antiferromagnetic ordering and metallic conductivity at low temperatures. In this work we have synthesized and studied Fe0.8V0.2Nb11O29 monoclinic powders. In particular we monitored the effects on structural, electronic and magnetic properties in samples produced in different atmospheres to stabilize cations with different oxidation states.We have demonstrated that the reaction atmosphere influences the phase homogeneity, the crystallite size and the amount of paramagnetic centres, with a transformation of Fe3+ in Fe2+ when an inert atmosphere is used, as proved by the absence, in this case, of any electron paramagnetic resonance signal. Also the Raman spectra result to be affected due to the change of coordination polyhedra. Subsequent re-oxidation of reduced powders brings to the monophasic iron niobate again containing Fe3+ demonstrating the reversibility of redox process. This reversibility is accompanied by a complete restoring of monoclinic structure evidenced by X-ray diffraction data and by Raman measurements, which allowed also to follow in situ the spectral changes induced by laser heating.

Effect of Thermal Aging on Local Structure and Three-Way Catalysis of Cu/Al2O3

Cu supported on Al2O3, prepared by impregnation, was thermally aged at different temperatures, and the influence of thermal aging on the local structure, redox behavior of Cu, and catalytic activity for a stoichiometric NO–CO–C3H6–O2 reaction was investigated. Crystalline CuO was mainly formed on Al2O3 after thermal aging at ≤700 °C, whereas aging at higher temperatures induced Cu2+ incorporation into tetrahedral (Td-Cu2+) rather than octahedral (Oh-Cu2+) sites of γ-Al2O3. Despite its lower surface area, thermally aged Cu/Al2O3 with Td-Cu2+ sites showed higher catalytic performance for the stoichiometric NO–CO–C3H6–O2 reaction compared with the as-prepared catalyst, especially for NO reduction. Td-Cu2+ was reduced to Td-Cu+ during reaction with CO and/or C3H6, and NO could be reduced in subsequent reoxidation of Td-Cu+ to Td-Cu2+ by NO. This redox behavior is more probable on Td-Cu rather than crystalline CuO, resulting in enhancement of NO reduction during the three-way catalyst reaction.

Maximum fluorescence and electron transport kinetics determined by light-induced fluorescence transients (LIFT) for photosynthesis phenotyping

Photosynthetic phenotyping requires quick characterization of dynamic traits when measuring large plant numbers in a fluctuating environment. Here, we evaluated the light-induced fluorescence transient (LIFT) method for its capacity to yield rapidly fluorometric parameters from 0.6 m distance. The close approximation of LIFT to conventional chlorophyll fluorescence (ChlF) parameters is shown under controlled conditions in spinach leaves and isolated thylakoids when electron transport was impaired by anoxic conditions or chemical inhibitors. The ChlF rise from minimum fluorescence (Fo) to maximum fluorescence induced by fast repetition rate (Fm−FRR) flashes was dominated by reduction of the primary electron acceptor in photosystem II (QA). The subsequent reoxidation of QA− was quantified using the relaxation of ChlF in 0.65 ms (Fr1) and 120 ms (Fr2) phases. Reoxidation efficiency of QA− (Fr1/Fv, where Fv = Fm−FRR − Fo) decreased when electron transport was impaired, while quantum efficiency of photosystem II (Fv/Fm) showed often no significant effect. ChlF relaxations of the LIFT were similar to an independent other method. Under increasing light intensities, Fr2′/Fq′ (where Fr2′ and Fq′ represent Fr2 and Fv in the light-adapted state, respectively) was hardly affected, whereas the operating efficiency of photosystem II (Fq′/Fm′) decreased due to non-photochemical quenching. Fm−FRR was significantly lower than the ChlF maximum induced by multiple turnover (Fm−MT) flashes. However, the resulting Fv/Fm and Fq′/Fm′ from both flashes were highly correlated. The LIFT method complements Fv/Fm with information about efficiency of electron transport. Measurements in situ and from a distance facilitate application in high-throughput and automated phenotyping.

Redox cycles with doped calcium manganites for thermochemical energy storage to 1000 °C

Redox cycles of doped calcium manganite perovskites (CaMnO3−δ) are studied for cost-effective thermochemical energy storage at temperatures up to 1000 °C for concentrating solar power and other applications. If the thermodynamics and kinetics for heat-driven reduction can be tailored for high temperatures and industrially accessible low O2 partial pressures (PO2⩾10-4 bar), perovskite redox cycles can offer high specific energy storage at temperatures much higher than state-of-the-art molten-salt subsystems. To this end, a range of A-site and B-site doped CaMnO3−δ were screened for their reducibility at 900 °C and PO2≈10-4 bar via thermogravimetric analysis. For compositions with high reducibility, notably A-site doped Ca1−xSrxMnO3−δ (x=0.05 and 0.10) and B-site doped CaCryMn1−yO3−δ (y=0.05 and 0.10), oxygen non-stoichiometry δ with respect to temperature and PO2 were measured and used to fit thermodynamic parameters of a two-reaction, point-defect model of the redox process for the two prominent crystalline phases (orthorhombic and cubic) that the perovskites occupy during the cycle. The fits compare favorably to differential scanning calorimetry measurements with the magnitude of the overall reduction enthalpies decreasing as the degree of reduction increases and the perovskites shift from orthorhombic to cubic crystalline phases. Based on thermodynamic limits, redox cycles of both Ca1−xSrxMnO3−δ compositions between air at 500 °C and PO2≈10-4 bar at 900 °C can store and release up to ≈700 kJ kg−1 with over 50% of the total energy stored as chemical energy. This is approximately 140 kJ kg−1 more chemical energy than the thermodynamic limits for CaCryMn1−yO3−δ compositions under the same cycle conditions. Approaching these thermodynamic limits for the specific energy storage of these redox cycles in a concentrating solar plant requires fast kinetics for perovskite reduction in the solar receiver and for reoxidation in the heat recovery reactor. Isothermal packed-bed redox cycling experiments of Ca1−xSrxMnO3−δ and CaCryMn1−yO3−δ compositions at temperatures up to 1000 °C show that reoxidation is fast compared to reduction. Thus, specific thermochemical energy storage is limited by residence times available for high-temperature reduction. The Sr-doped compositions approach higher fractions (≈90% or more) of the specific energy storage equilibrium limit after 300 s of reduction in the packed bed configuration above 800 °C and completely reoxidize in ⩽20 s in air. Non-isothermal cycling with heating from 500 °C to 900 °C in low PO2≈10-4 bar and subsequent reoxidation during cooling in air back to 500 °C demonstrate excellent chemical stability over 1000 cycles for all doped CaMnO3−δ compositions tested. The results suggest that these redox cycles may offer a viable energy storage subsystem with long-term stability for future concentrating solar plants and other high-temperature energy storage applications.

Uranium adsorption and subsequent re-oxidation under aerobic conditions by Leifsonia sp. - Coated biochar as green trapping agent

It has generally been assumed that the immobilization of U(VI) via polyphosphate accumulating microorganisms may present a sink for uranium, but the potential mechanisms of the process and the stability of precipitated uranium under aerobic conditions remain elusive. This study seeks to explore the mechanism, capacity, and stability of uranium precipitation under aerobic conditions by a purified indigenous bacteria isolated from acidic tailings (pH 6.5) in China. The results show that over the treatment ranges investigated, maximum removal of U(VI) from aqueous solution was 99.82% when the initial concentration of U(VI) was 42 μM, pH was 3.5, and the temperature was with 30 °C much higher than that of other reported microorganisms. The adsorption mechanism was elucidated via the use of SEM-EDS, XPS and FTIR. SEM-EDS showed two peaks of uranium on the surface. A plausible explanation for this, supported by FTIR, is that uranium precipitated on the biosorbent surfaces. XPS measurements indicated that the uranium product is most likely a mixture of 13% U(VI) and 87% U(IV). Notably, the reoxidation experiment found that the uranium precipitates were stable in the presence of Ca2+ and Mg2+, however, U(IV) is oxidized to U(VI) in the presence of NO3− and Na+ ions, resulting in rapid dissolution. It implies that the synthesized Leifsonia sp. coated biochar could be utilized as a green and effective biosorbent. However, it may not a good choice for in-situ remediation due to the subsequent re-oxidation under aerobic conditions. These observations can be of some guiding significance to the application of the bioremediation technology in surface environments.

Sensing Selectivity Enhancement of Palladium Oxide Toward VOCs Using Characteristic Response Features below 250oc

Breath analysis has received much attention for its non-invasive monitoring of human pathological conditions. The exhaled volatile organic compounds (VOCs) from patients can be utilized as potential biomarkers of specific diseases and disorders. For instance, acetone can be the indicator of diabetes and ethanol is an important interfering gas during the breath analysis. This work focuses on the improvement of the gas sensing selectivity for ethanol and acetone, by taking advantage of the intriguing redox behavior of palladium oxide (PdO) in the temperature range between 100 and 250oC. Metal oxide semiconductor (MOS) gas sensors have been extensively studied because of their rapid response and high sensitivity. Furthermore, fabrication of MOS gas sensors is simple and compatible to existing semiconductor fabrication processes. In this study, sputter-deposited PdO nanoflake thin films were used as the sensing material. PdO is a p-type semiconductor with a band gap of 2.2eV and thermally stable up to 800oC in atmosphere. In addition, PdO has a high catalytic reactivity toward reducing gases, such as CO and CH4. For the application of gas sensing, PdO is a good sensitizer in gas sensors for sensitivity and selectivity improvement. According to our previous works, PdO demonstrates characteristic sensing responses toward reducing gases due to its distinctive reduction and reoxidation behaviors. The characteristic sensing response may be ascribed to the reduction of PdO and subsequent reoxidation. In general, the gas sensing principle based on MOS gas sensors is described by two sensing mechanism models: ionosorption and oxygen vacancy models. The characteristic redox property of PdO can modify the gas sensing behavior predicted by the two models. Moreover, adsorption of intermediate adspecies on the PdO sensor in operation also affects the sensing response. Conventional MOS sensors use the sensor signal, which is typically determined under a steady sensing condition, as the input variable for the data processing of gas classification. In this case, simple mathematical models, such as principle component analysis (PCA) and linear discriminant analysis (LDA), can be used to classify the input signals. However, these simple models have poor performance in selectivity improvement and thus have limited applications for MOS gas sensors. Since the PdO gas sensor demonstrates dynamic characteristic responses toward VOC gases, we extracted characteristic response features as the input variables for the artificial neural network (ANN) method to improve the selectivity of VOC gas sensing. Figure (a) shows the electrical sensing response of PdO exposed to a dry air gas mixture of VOCs at 250oC. Ammonia is also included in the study because it is a strong reducing agent and an important indicator of renal diseases and asthma. At the early stage of sensing, the sensing current promptly drops upon the exposure to the gases. The sensing current then gradually rises and reaches the equilibrium. The first prompt current drop can be ascribed to reduction of preadsorbed oxygen anions and PdO by VOCs on the basis of ionosorption and oxygen vacancy models. Reducing gases can react with oxygen anions and lattice oxygen atoms, releasing negative charges to the PdO sensor and thus decreasing its conductance. The latter current rise is a result of the formation of metal Pd nano-domain, which is a surface product of the PdO reduction. Chemical characterizations indicate that ethanol, acetone and ammonia undergo dehydrogenation on the PdO surface. These reducing gases can provide a plentiful amount of hydrogen to reduce the PdO substrate, depending on their bond strength and adsorption structure on the sensor. The reduction rate of the PdO substrate greatly depends on the surface coverage of hydrogen adatoms and the sensing temperature. As a result, the PdO sensor yields distinct response profiles when it is exposed to the studied gases at different temperatures. Two characteristic features extracted from the response profiles for the ANN method are depicted in figure (b). The two features are the slope of the conductance rise trace and the ratio of conductance change at two time points. The classification result is listed in table 1, demonstrating a high classification rate. In summary, the gas sensing behavior of PdO toward VOCs was studied. Characteristic features from the electrical response profiles were used as variables for the ANN pattern recognition algorithm to enhance the sensing selectivity of PdO toward VOCs. Figure 1

Rapid Redox Cycle Stability of Doped CaMnO3 Particles for High-Temperature Thermochemical Energy Storage

A-site doped Ca1-xSrxMnO3-δ and B-site doped CaCryMn1-yO3-δ have been studied for high specific thermochemical energy storage (TCES) in concentrating solar power and other high-temperature applications. With low levels of doping (5 and 10%) for both compositions, these perovskites have demonstrated significant reduction in high temperatures at O2 partial pressures P O2 ≈10-4 bar with as much as 450 kJ/kg of chemical energy can be stored (for Ca0.95Sr0.05MnO3-δ) at 900 ºC on top of sensible energy in a TCES system [1]. The viability of such high specific TCES in these perovskites depends on their rates of energy accommodation during heating cycles and their phase and morphological stability during rapid reduction and reoxidation for energy capture and release respectively. In this study, Ca1-xSrxMnO3-δ and CaCryMn1-yO3-δ were studied for reduction at temperatures up to 1000 ºC and tested in extended redox cycling to explore their stability for energy storage and release at such high-temperatures where the materials undergo expansion/compression and phase transitions during these cycles. The results as highlighted below suggest that these materials have significant promise for high-temperature large-scale TCES. Ca1-xSrxMnO3-δ (x = 0.05 and 0.10) and CaCryMn1-yO3-δ (y = 0.05 and 0.10) were synthesized as particles with solid-state-reactive sintering [1] and cycled as particles (diameter range of 250-425 μm) between air P O2 (≈ 0.17 bar) and P O2 ≈10-4 bar both in isothermal experiments up to 1000 ºC and in long-term cycling experiments between 500 and 900 ºC to simulate TCES system operation. Over these conditions, the doped CaMnO3-δ perovskites undergo significant reduction (δ > 0.2 at 1000 ºC) at phase transitions (orthorhombic to cubic to tetragonal perovskite phase), reduction, and associated lattice expansion as observed in hot-stage XRD for similar composition [1]. The isothermal redox cycling tests in conjunction with thermodynamics derived from TGA/DSC measurements as reported elsewhere [2,3] provided a basis for fitting thermodynamically consistent surface reaction rates [4,5] and bulk ionic transport properties to measured transient evolution in oxygen non-stoichiometry δ which went as high as 0.3 for Ca0.95Sr0.05MnO3-δ at 1000 ºC. These experiments are illustrated with ten-cycle averaged plots in Figure 1 with data for Ca0.9Sr0.1MnO3-δ compared to packed bed models [4] used to fit surface kinetic and bulk transport rate expressions. Figure 1 also shows the chemical energy incorporated into the perovskite with increasing δ during reduction based on thermodynamics derived from fitting equilibrium δ over a range of T and P O2. The packed bed results and model fits show fast rates of reoxidation relative to reduction. The rapid reoxidation i.e., oxygen incorporation rates are limited by the inlet flow supply for a substantial fraction of the experiments. The high-degree of reduction and the rapid rates of reoxidation and associated phase transitions raise questions about the stability of these materials in non-isothermal redox cycling for high-temperature TCES system applications. The selected perovskite materials were thus cycled for over 1000 times under non-isothermal conditions to simulate TCES system operation. The long-term cycling for all four doped CaMnO3-δ perovskites occurred with heating from a fully oxidized state at 500 ºC to a reduced state at 900 ºC in P O2 ≈10-4 bar followed by subsequent reoxidation and cooling in air back to 500 ºC. Figure 2 shows the transient evolution of δ during reduction and heating, averaged over successive 100 cycles for all four materials. An initial induction period of increasing maximum δ is followed by a stable period of redox cycling in spite of the rapid reoxidation. Only CaCr0.05Mn0.95O3-δ shows evidence of a significant loss in maximum δ, and this composition showed the slowest to undergo phase transitions in DTA experiments. Ex situ X-ray diffraction shown in Figure 3 of the particles before and after the long-term non-isothermal redox testing show that the particles maintain their dominant perovskite phase with evidence of small fractions of a secondary spinel phase. The stability of these cyclic tests particular for the Ca1-xSrxMnO3-δ reveal the promise of these materials as a media for effective TCES in a range of high-temperature energy storage applications. Reference [1] E.I. Leonidova, I.A. Leonidov, M.V. Patrakeev, V.L. Kozhevnikov, Journal of Solid State Electrochemistry, 15 (2011) 1071-1075. [2] L. Imponenti, K.J. Albrecht, R.J. Braun, G.S. Jackson, ECS Transactions, 72 (2016) 11-22. [3] L. Imponenti, K.J. Albrecht, J.W. Wands, M.D. Sanders, G.S. Jackson, Solar Energy, 151 (2017) 1-13. [4] K.J. Albrecht, "Multiscale Modeling and Experimental Interpretation of Perovskite Oxide Materials in Thermochemical Energy Storage and Conversion in Applications for Concentrating Solar Power", Ph.D. Dissertation, Colorado School of Mines, 2016. [5] R. Merkle, J. Maier, Angewandte Chemie-International Edition, 47 (2008) 3874-3894. Figure 1

Enhanced visible-light photocatalytic activity of Bi2MoO6 nanoplates with heterogeneous Bi2MoO6-x@Bi2MoO6 core-shell structure

A series of Bi2MoO6 (BMO) nanoplates with a heterogeneous Bi2MoO6-x@Bi2MoO6 core/shell structure were successfully obtained by reduction with CaH2 at 140°C and subsequent reoxidation in air under low temperatures (≤250°C). It is found that the oxygen vacancies mainly exist in the core, while the shell is formed by surface disorder layer with few oxygen vacancies, which is almost stoichiometric. The controllable growth of the surface disorder layer can be realized by the reduction and/or reoxidation temperatures. The reoxidized BMO with the surface disorder layer exhibits about five times higher visible-light photocatalytic activity toward phenol degradation and an order of magnitude higher activity toward methylene blue (MB) degradation than those of the unmodified sample. The visible-light absorption is mainly dominated by the oxygen vacancies through the modulation of the band gap structure, which promote the solar light utilization and photogenerated charge carriers. Density functional theory calculation shows that BMO is a direct band gap semiconductor. It is suggested that the surface disorder layer could suppress the direct recombination of photogenerated carriers and improve surface reactivity, and results in the enhancement of the photocatalytic activity. In addition, the existence of the stoichiometric surface disorder can promote the catalyst stability. It is demonstrated that the reduction- reoxidation method applied here is simple, easily control, and beneficial for the design and realization of heterogeneous core-shell nanostructured photocatalysts to improve visible and infrared optical absorption by engineering the surface disorder and the oxygen vacancies inside core of Bi2MoO6 nanoplates and the related nanostructured photocatalysts.

A model of the FAD redox cycle describes the dynamics of the effect of the geomagnetic field on the human visual system

In experimental studies, we could show that the visual threshold of man is influenced by the geomagnetic field. One of the results was that the threshold shows periodic fluctuations when the vertical component of the field is reversed periodically. The maximum of these oscillations occurred at a period duration of 110s. To explain this phenomenon, we chose the process that likely underlies the navigation of birds in the geomagnetic field: the light reaction of the FAD component of cryptochrome in the retina. The human retina contains cryptpochrome like the bird retina. Based on the investigations of Müller and Ahmad (J Biol Chem 286:21033-21040, 2011) and Solov'yov and Schulten (J Phys Chem B 116:1089-1099, 2012), we designed a model of the light-induced reduction and subsequent reoxidation of FAD. This model contains a radical pair, whose interconversion dynamics are affected by the geomagnetic field. The parameters of the model were partly calculated from the data of our experimental investigation and partly taken from the results of other authors. These parameters were then optimized by adjusting the model behaviour to the experimental results. The simulation of the finished model shows that the concentrations of all substances included show really oscillations with the frequency of the modelled magnetic field. After optimization of the parameters, the oscillations of FAD and FADH* show maximal amplitude at a period duration of 110s, as was observed in the experiment. This makes it most likely that the signal, which influences the visual system, originates from FADH* (signalling state).

Full reoxidation of CuMn2O4 spinel catalyst triggered by epitaxial Mn3O4 surface nanocrystals

AbstractTransmission electron microscopy was performed on a model system of a novel CuMn2O4 spinel catalyst in order to study the effects of reduction by carbon monoxide and subsequent reoxidation by oxygen both processed at 600°C. A phase transition from CuMn2O4 spinel into separated MnO and Cu‐rich phases, which are CuO or Cu depending on the reduction period, was observed in the reduced samples. Furthermore, a surface coverage of the MnO grains by epitaxially grown, nanocrystalline Mn3O4 was found in all reduced samples. These Mn3O4 nanocrystals are assumed to act as seed crystals in the subsequent reoxidation step completely reversing the phase transitions and reconstituting single‐phase CuMn2O4 spinel. In addition, superlattice reflections due to cation ordering occurred as a typical feature in the as‐prepared spinel. In the reoxidized spinel, the superlattice reflections were absent, hence indicating a disordered cation distribution.

A novel CeO2–xSnO2/Ce2Sn2O7 pyrochlore cycle for enhanced solar thermochemical water splitting

A novel CeO2–xSnO2/Ce2Sn2O7 pyrochlore stoichiometric redox cycle with superior H2 production capacities is identified and corroborated for two‐step solar thermochemical water splitting (STWS). During the first thermal reduction step (1400°C), a reaction between CeO2 and SnO2 occurred for all the CeO2–xSnO2 (x = 0.05–0.20) solid compounds, forming thermodynamically stable Ce2Sn2O7 pyrochlore rather than metastable CeO2‐δ. Consequently, substantially higher reduction extents were achieved owing to the reduction of CeIV to CeIII. Moreover, in the subsequent reoxidation with H2O (800°C), H2 production capacities increased by a factor of 3.8 as compared to the current benchmark material ceria when x = 0.15, with the regeneration of CeO2 and SnO2 and the concomitant reoxidation of CeIII to CeIV. The H2O‐splitting performance for CeO2–0.15SnO2 was reproducible over seven consecutive redox cycles, indicating the material was also robust. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3450–3462, 2017

Substrate Grain-Dependent Chemistry of Carburized Planar Anodic TiO2 on Polycrystalline Ti.

Mixtures or composites of titania and carbon have gained considerable research interest as innovative catalyst supports for low- and intermediate-temperature proton-exchange membrane fuel cells. For applications in electrocatalysis, variations in the local physicochemical properties of the employed materials can have significant effects on their behavior as catalyst supports. To assess microscopic heterogeneities in composition, structure, and morphology, a microscopic multitechnique approach is required. In this work, compact anodic TiO2 films on planar polycrystalline Ti substrates are converted into carbon/titania composites or multiphase titanium oxycarbides through carbothermal treatment in an acetylene/argon atmosphere in a flow reactor. The local chemical composition, structure, and morphology of the converted films are studied with scanning photoelectron microscopy, micro-Raman spectroscopy, and scanning electron microscopy and are related with the crystallographic orientations of the Ti substrate grains by means of electron backscatter diffraction. Different annealing temperatures, ranging from 550 to 850 °C, are found to yield different substrate grain-dependent chemical compositions, structures, and morphologies. The present study reveals individual time scales for the carbothermal conversion and subsequent surface re-oxidation on substrate grains of a given orientation. Furthermore, it demonstrates the power of a microscopic multitechnique approach for studying polycrystalline heterogeneous materials for electrocatalytic applications.

Polymer Electrolyte Fuel Cells Employing Heteropolyacids as Redox Mediators for Oxygen Reduction Reactions: Pt-Free Cathode Systems.

In this study, the heteropolyacids of H3+xPVxMO12-xO40 (x = 0, 2, and 3) were applied as redox mediators for the oxygen reduction reaction in polymer electrolyte fuel cells, of which the cathode is free from the usage of noble metals such as Pt/C. In this system, the electrochemical reduction of heteropolyacid over the carbon cathode and the subsequent reoxidation of the partially reduced heteropolyacid by exposure to the dissolved oxygen in the regenerator are important processes for continuous power generation. Thus, the redox properties of catholytes containing these heteropolyacids were investigated in detail. The substitution quantity of V in the heteropolyacid affected the onset reduction potential as well as the reduction current density, resulting in a difference in cell performance. The chemical composition of heteropolyacid also had a significant impact on the reoxidation property. Among the three compounds, H6PV3Mo9O40 was the most suitable redox mediator. Furthermore, the pH of the catholyte was found to be the crucial factor in determining the reoxidation rate of partially reduced heteropolyacid as well as cell performance.

Kinetics of O2 Entry and Exit in Monomeric Sarcosine Oxidase via Markovian Milestoning Molecular Dynamics.

The flavoenzyme monomeric sarcosine oxidase (MSOX) catalyzes a complex set of reactions currently lacking a consensus mechanism. A key question that arises in weighing competing mechanistic models of MSOX function is to what extent ingress of O2 from the solvent (and its egress after an unsuccessful oxidation attempt) limits the overall catalytic rate. To address this question, we have applied to the MSOX/O2 system the relatively new simulation method of Markovian milestoning molecular dynamics simulations, which, as we recently showed [ Yu et al. J. Am. Chem. Soc. 2015 , 137 , 3041 ], accurately predicted the entry and exit kinetics of CO in myoglobin. We show that the mechanism of O2 entry and exit, in terms of which possible solvent-to-active-site channels contribute to the flow of O2, is sensitive to the presence of the substrate-mimicking competitive inhibitor 2-furoate in the substrate site. The second-order O2 entry rate constants were computed to be 8.1 × 10(6) and 3.1 × 10(6) M(-1) s(-1) for bound and apo MSOX, respectively, both of which moderately exceed the experimentally determined second-order rate constant of (2.83 ± 0.07) × 10(5) M(-1) s(-1) for flavin oxidation by O2 in MSOX. This suggests that the rate of flavin oxidation by O2 is likely not strongly limited by diffusion from the solvent to the active site. The first-order exit rate constants were computed to be 10(7) s(-1) and 7.2 × 10(6) s(-1) for the apo and bound states, respectively. The predicted faster entry and slower exit of O2 for the bound state indicate a longer residence time within MSOX, increasing the likelihood of collisions with the flavin isoalloxazine ring, a step required for reduction of molecular O2 and subsequent reoxidation of the flavin. This is also indirectly supported by previous experimental evidence favoring the so-called modified ping-pong mechanism, the distinguishing feature of which is an intermediate complex involving O2, the flavin, and the oxidized substrate simultaneously in the cavity. These findings demonstrate the utility of the Markovian milestoning approach in contributing new understanding of complicated enyzmatic function.