Efficient reverse water gas shift reaction at low temperatures over an iron supported catalyst under an electric field.

The development of high-performance Fe-based catalysts is attractive because Fe is a cost-effective and earth-abundant element. Application of an external electric field and an appropriate catalytic support to an Fe-based catalyst enabled the reverse water-gas shift reaction to proceed with high activity, selectivity, and durability even at the low temperature of 423 K. The Fe-supported catalyst showed superior CO selectivity (≈ 100%) compared to the Co- or Ni-supported catalyst. The apparent activation energy (5.9 kJ mol-1) over the Fe/Ce0.4Al0.1Zr0.5O2 catalyst under an electric field was much lower than that without an electric field (61.4 kJ mol-1).

Factors governing the protonation of Keggin-type polyoxometalates: influence of the core structure in clusters.

Atomic substitution is a promising approach for controlling structures and properties for developing clusters with desired responses. Although many possible coordination candidates could be deduced for substitution, not all can be prepared. Therefore, predicting the correlation between structures and physical properties is important prior to synthesis. In this study, regarding Keggin-type polyoxometalates (POMs) as a model cluster, the dominant factors affecting the protonation were investigated by atomic substitutions and geometry changes. The valence of Keggin-type POMs and the constituent elements of the cluster shell structure affect the charge and potential distribution, which change the protonation sites. Furthermore, the valence of Keggin-type POMs and the bond length between the core and shell structure determine the protonation energy. These factors also affect the HOMO-LUMO gap, which governs photochemical and redox reactions. These governing factors derived from actual parameters of the α-isomer of Keggin-type POMs enabled us to deduce the protonation energy of the ß-isomer, which is more difficult to prepare and isolate than the α-isomer.

Vanadium-Containing Keggin-Type Polyoxometalates, [VM12O40]3- and [VVM11O40]4- (M = Mo, W): Structural Characterization and Voltammetric, NMR, and EPR Studies Related to Electrochemical Reduction at Framework and Central Vanadium Sites.

Vanadium is accommodated in both the framework (VoutV) and central positions (VinV) in the Keggin-type polyoxometalates (POMs) [VinVVoutVM11O40]4- (M = Mo, W; VinVVoutVM11) and in the central position in [VinVM12O40]3- (VinVM12). The structures of the VinVVoutVM11 class have been determined by X-ray crystallography and compared to those of VinVM12 reported previously. A major feature of interest with POMs is their capacity for very extensive reduction, particularly when protonation accompanies the electron transfer step. With VinVVoutVM11 and VinVM12 POMs, knowledge as to whether reduction occurs at V or M sites and the concomitant dependence on acidity has been obtained. Frozen solution EPR spectra obtained following bulk electrolysis showed that the one-electron reduction of VinVMo12 occurs at the molybdenum framework site to give VinVMoVMo11. In contrast, EPR spectra of one-electron reduced VinVW12 at <30 K are consistent with the electron being accommodated on the central V atom in a tetrahedral environment to give VinIVW12. In the case of VinVVoutVM11, the initial reduction occurs at the framework VoutV site to give VinVVoutIVM11. The second electron is delocalized over the Mo framework in two-electron reduced VinVVoutIVMoVMo10, whereas it is accommodated on the central V site in VinIVVoutIVW11. The distance between VinIV and VoutIV in VinIVVoutIVW11 estimated as 3.5 ± 0.2 Å from analysis of the EPR spectrum is consistent with that obtained in VinVVoutVW11 from crystallographic data. Simulations of the cyclic voltammograms as a function of CF3SO3H acid concentration for the initial reduction processes provide excellent agreement with experimental data obtained in acetonitrile (0.10 M [n-Bu4N][PF6]) and allowed acid association constants to be estimated and compared with the literature values available for [XVoutVM11O40]n- (X = S (n = 3), P and As (n= 4); M = Mo, W). The interpretation of the voltammetric data is supported by 51V NMR measurements on the oxidized VV forms of the POMs.

Selective Electrochemical Hydrogenation of Phenol with Earth-abundant Ni-MoO2 Heterostructured Catalysts: Effect of Oxygen Vacancy on Product Selectivity.

Herein, we report highly efficient carbon supported Ni-MoO2 heterostructured catalysts for the electrochemical hydrogenation (ECH) of phenol in 0.10 M aqueous sulfuric acid (pH 0.7) at 60 °C. Highest yields for cyclohexanol and cyclohexanone of 95 % and 86 % with faradaic efficiencies of ∼50 % are obtained with catalysts bearing high and low densities of oxygen vacancy (Ov ) sites, respectively. In situ diffuse reflectance infrared spectroscopy and density functional theory calculations reveal that the enhanced phenol adsorption strength is responsible for the superior catalytic efficiency. Furthermore, 1-cyclohexene-1-ol is an important intermediate. Its hydrogenation route and hence the final product are affected by the Ov density. This work opens a promising avenue to the rational design of advanced electrocatalysts for the upgrading of phenolic compounds.

Cerium(III) Niobate Layered Perovskites: Abnormal Optical Absorption Modulations by Tuning of B-Site Composition and Perovskite Layer Charge Control.

The cerium-introduced layered perovskite of RbCeTa2O7 has exhibited a specific optical absorption due to metal-to-metal charge transfer transitions between Ce 4f and transition metal d-orbitals to show the unique pale-green coloration, which is different from conventional coloration mechanisms. To further extend the coloring state based on the same mechanism, in this work, a series of the [Ce(Ta,Nb)2O7]- layered perovskites, Rb1-xCsx[Ce(Ta1-xNbx)2O7] (x = 0∼1), with Nb substitutions in the perovskite units have been prepared and investigated in terms of those crystal structures and optical absorption mechanism. The Rietveld analysis using the XRD profile and EXAFS analyses well refined those structures as the Dion-Jacobson-type layered perovskite. The color of solid solutions gradually changed from pale-green to dark reddish-brown with increasing amount of substituted niobium. The unique coloring state change behavior of solid solutions from pale-green to dark reddish-brown depending on the amount of the substituted niobium is not observed in the other layered perovskite analogues (e.g., La and Pr analogues). The first-principles calculation based on the density functional theory method indicated that the band structural change should be a key factor for the coloration modulation. Furthermore, the redox ability through the charge modulation of the perovskite layer, which is a specific function of the cerium-based layered perovskite, was also investigated for the niobate [CeNb2O7]- perovskite layer, resulting in the anisotropic lattice changes similar to those of a Ta analogue with different structural changes in the stacking and in-plane directions. The accompanying change in electronic structure led to a clear modulation in optical absorption, yielding a drastic change in the coloring state from dark brown to yellow.

Electrochemical antioxidant capacity measurement: a downsized system and its application to agricultural crops.

An on-site electrochemical antioxidant capacity measurement system was developed using a screen print electrode (SPE) and circuit tester. The antioxidant capacities of eight antioxidants were evaluated with the handheld electrochemical antioxidant capacity measurement system to compare with those measured with spectroscopic methods, namely, oxygen radical absorbance capacity (ORAC) and 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging assays, as well as the reported electrochemical method with three conventional electrodes (a glassy carbon electrode, Ag/AgCl electrode and platinum wire electrode) and a potentiostat. Additionally, the potential shifts were proportional to the logarithm of the antioxidant concentration, which obeyed the Nernstian equation. Moreover, the antioxidant capacities of extracts from vegetables (green pepper, ginger and eggplant) were measured with a handheld electrochemical system. Each measurement was finished in only ca. 3 min. The electrochemically obtained antioxidant data were comparable to those from DPPH free-radical scavenging assays and superoxide anion scavenging activity (SOSA) assays, as well as the total phenolic compound content.

Ce(iv)-centered charge-neutral perovskite layers topochemically derived from anionic [CeTa2O7]- layers.

Layered perovskites have been extensively investigated in many research fields, such as electronics, catalysis, optics, energy, and magnetics, because of the fascinating chemical properties that are generated by the specific structural features of perovskite frameworks. Furthermore, the interlayers of these structures can be chemically modified through ion exchange to form nanosheets. To further expand the modification of layered perovskites, we have demonstrated an advance in the new structural concept of layered perovskite "charge-neutral perovskite layers" by manipulating the perovskite layer itself. A charge-neutral perovskite layer in [CeIVTa2O7] was synthesized through a soft chemical oxidative reaction based on anionic [CeIIITa2O7]- layers. The Ce oxidation state for the charge-neutral [CeIVTa2O7] layers was found to be tetravalent by X-ray absorption fine structure (XAFS) analysis. The atomic arrangements were determined through scattering transmission electron microscopy and extended XAFS (EXAFS) analysis. The framework structure was simulated through density functional theory (DFT) calculations, the results of which were in good agreement with those of the EXAFS spectra quantitative analysis. The anionic [CeIIITa2O7]- layers exhibited optical absorption in the near infrared (NIR) region at approximately 1000 nm, whereas the level of NIR absorption decreased in the [CeIVTa2O7] charge-neutral layer due to the disappearance of the Ce 4f electrons. In addition, the chemical reactivity of the charge-neutral [CeIVTa2O7] layers was investigated by chemical reduction with ascorbic acid, resulting in the reduction of the [CeIVTa2O7] layers to form anionic [CeIIITa2O7]- layers. Furthermore, the anionic [CeIIITa2O7]- layers exhibited redox activity which the Ce in the perovskite unit can be electrochemically oxidized and reduced. The synthesis of the "charge-neutral" perovskite layer indicated that diverse features were generated by systematically tuning the electronic structure through the redox control of Ce; such diverse features have not been found in conventional layered perovskites. This study could demonstrate the potential for developing innovative, unique functional materials with perovskite structures.

Preparation of Silicon Hydroxyapatite Nanopowders under Microwave-Assisted Hydrothermal Method.

The synthesis of partially substituted silicon hydroxyapatite (Si-HAp) nanopowders was systematically investigated via the microwave-assisted hydrothermal process. The experiments were conducted at 150 °C for 1 h using TMAS (C4H13NO5Si2) as a Si4+ precursor. To improve the Si4+ uptake in the hexagonal structure, the Si precursor was supplied above the stoichiometric molar ratio (0.2 M). The concentration of the TMAS aqueous solutions used varied between 0.3 and 1.8 M, corresponding to saturation levels of 1.5-9.0-fold. Rietveld refinement analyses indicated that Si incorporation occurred in the HAp lattice by replacing phosphate groups (PO43-) with the silicate (SiO4-) group. FT-IR and XPS analyses also confirmed the gradual uptake of SiO4- units in the HAp, as the saturation of Si4+ reached 1.8 M. TEM observations confirmed that Si-HAp agglomerates had a high crystallinity and are constituted by tiny rod-shaped particles with single-crystal habit. Furthermore, a reduction in the particle growth process took place by increasing the Si4+ excess content up to 1.8 M, and the excess of Si4+ triggered the fine rod-shaped particles self-assembly to form agglomerates. The agglomerate size that occurred with intermediate (0.99 mol%) and large (12.16 mol%) Si contents varied between 233.1 and 315.1 nm, respectively. The excess of Si in the hydrothermal medium might trigger the formation of the Si-HAp agglomerates prepared under fast kinetic reaction conditions assisted by the microwave heating. Consequently, the use of microwave heating-assisted hydrothermal conditions has delivered high processing efficiency to crystallize Si-HAp with a broad content of Si4+.

Polyoxometalates in Imidazolim-based Ionic Liquids: Acceptor Number and Polarity Estimated from Their Voltammetric Behaviour.

The selection of an appropriate solvent is essential for achieving high yields and selectivity in chemical reactions. The chemical and physical parameters of organic solvents have been classified into several groups, and solvents can be compared with each other with respect to these properties. The acceptor number (AN), donor number (DN) and polarity (ETN) have been widely accepted and used for theoretically and quantitatively evaluating the properties of organic solvents. In a similar manner, the AN, DN and ETN of room temperature ionic liquids (RTILs) have been estimated from spectral changes in solvatochromic compounds. In this paper, the AN and ETN of eight types of imidazolium-based RTILs were estimated from the relationship between the AN and ETN values and the first redox potential obtained from the voltammograms of polyoxometalates (POMs) in various organic solvents. The obtained parameters were compared with those estimated by spectrophotometric methods reported previously by several groups. This new method for estimating the AN and ETN of RTILs using the voltammetric behaviour of POMs with low charge density and high symmetry could provide the other path to obtain more reliable AN and ETN of RTILs.

Polyoxometalates in Analytical Sciences.

Polyoxometalates (POMs) have been used for spectrophotometric determinations of silicon and phosphorus under acidic conditions, referred to as the molybdenum yellow method and molybdenum blue method, respectively. Many POMs are redox active and exhibit fascinating but complicated voltammetric responses. These compounds can reversibly accommodate and release many electrons without exhibiting structural changes, implying that POMs can function as excellent mediators and can be applied to sensitive determination methods based on catalytic electrochemical reactions. In addition, some rare-earth-metal-incorporated POMs exhibit fluorescence, which enables sensitive determination by the enhancement and quenching of fluorescence intensities. In this review, various analytical applications of POMs are introduced, mainly focusing on papers published after 2000, except for the molybdenum yellow method and molybdenum blue method.

Impact of the Lithium Cation on the Voltammetry and Spectroscopy of [XVM11O40]n- (X = P, As (n = 4), S (n = 3); M = Mo, W): Influence of Charge and Addenda and Hetero Atoms.

Polyoxometalates (POMs) have been proposed as electromaterials for lithium-based batteries because they provide access to multiple electron transfer reactions coupled to fast lithium ion transport processes and high stability over many redox cycles. Consequently, knowledge of reversible potentials and Li+ cation-POM anion interactions provides a strategic basis for their further development. In this study, detailed cyclic voltammetric studies of a series of [XVVM11O40]n- (XVM11n-) POMs (where X (heteroatom) = P (n = 4), As (n = 4), and S (n = 3) and M (addenda atom) = Mo, W) have been undertaken in CH3CN in the presence of LiClO4, with n-Bu4NPF6 also present when required to keep the ionic strength close to constant value of 0.1 M. An analysis of the data has allowed the impact of the POM charge, and addenda and hetero atoms on the reversible potentials and the interaction between Li+ and the oxidized XVVM11n- and reduced XVIVM11(n+1)- forms of the VV/IV redox couple to be determined. The SVV/IVM113-/4- process is independent of the Li+ concentration, implying the absence of the association of this cation with either SVVM113- or SVIVM114- redox levels. However, lithium-ion association constants for both VV and VIV redox levels were obtained from a comparison of simulated and experimental cyclic voltammograms for the reduction of the more negatively charged XVVM114- (X = P, As; M = Mo, W), since the Li+ interaction with these more negatively charged POMs is much stronger. The interaction between Li+ and the oxidized, XVVM11n-, and reduced, XVIVM11(n+1)-, forms was also investigated by 51V NMR and EPR spectroscopy, respectively, and it was confirmed that, due to their lower charge density, SVVM113- and SVIVM114- interact significantly less strongly with the lithium ion than XVVM114- and XVIVM115- (X = P, As). The lithium-POM association constants are substantially smaller than the corresponding proton association constants reported previously, which is attributed to a smaller surface charge density. The much stronger impact of Li+ on the WVI/V- and MoVI/V-based reductions that occur at more negative potentials than the VV/IV process also has been qualitatively evaluated.

New layered perovskite family built from [CeTa2O7]- layers: coloring mechanism from unique multi-transitions.

A Ce3+-based layered perovskite compound, Dion-Jacobson type RbCeTa2O7, has been discovered for the first time by introducing only a trivalent cerium ion to form [CeTa2O7]- layers. The unique green color was experimentally and theoretically ascribed to two charge transfer transitions of ligand-to-metal O2p → Ce4f and metal-to-metal Ce4f → Ta5d.

Metal-substituted tungstosulfates with Keggin structure: synthesis and characterization.

Simple synthetic procedures for accessing novel metal-substituted tungstosulfates [SMW11O39]4- with Keggin-type structures were developed based on the reaction of metal ions (M = Mn2+, Co2+, Ni2+, and Cu2+) with lacunary tungstosulfate, [SW11O39]6-, which was obtained by treating [SW12O40]2- with a weak base in acetone. All metal-substituted tungstosulfates were characterized by elemental analysis, X-ray crystallography, ESI-MS, IR, Raman, UV-Vis and cyclic voltammetry analyses.

Quantitative Determination of the Effective Mn4+ Concentration in a Li2TiO3:Mn4+ Phosphor and Its Effect on the Photoluminescence Efficiency of Deep Red Emission.

Obtaining highly efficient photoluminescence with Mn4+-activated phosphors, which have been extensively studied in diverse lighting devices, requires the precise control of the manganese valence states. However, this control is difficult to achieve because manganese ions can have various valence states ranging from divalent to heptavalent. Additionally, the concentrations of Mn ions in each valence state, especially the effective Mn4+ concentration, have never been quantitatively determined in a phosphor crystal lattice. The relationship between the effective Mn4+ concentration and the luminescence properties of Mn4+-activated phosphors is of current interest for improving the phosphor properties. In the present study, the effective Mn4+ concentration in Li2TiO3:Mn4+ (LTO:Mn) phosphors prepared by the sol-gel method with heating at various temperatures was quantitatively analyzed by X-ray absorption near-edge spectroscopy. Moreover, the effect of the existence of Mn2+, Mn3+, and Mn4+ ions on the photoluminescence efficiency was investigated. The effective Mn4+ concentration was found to be over 60% in all phosphor samples. The quantum efficiencies (QEs) of all LTO:Mn phosphors strongly depend on the effective Mn4+ concentration. In particular, the LTO:Mn phosphor prepared by heating at 800 °C (LTO:Mn@800) contained the highest effective Mn4+ concentration of 98.1% and exhibited the highest internal QE of 31.6%. The results of this work provide new and important insights for the development of Mn4+-activated phosphors with high efficiency.

Radio frequency alternating electromagnetic field enhanced tetraruthenium polyoxometalate electrocatalytic water oxidation.

Application of radio frequency (RF) electromagnetic waves enhances the electrocatalytic water oxidation by protonated tetraruthenium polyoxometalate ([Ru4(µ-O)4(µ-OH)2(H2O)4(γ-SiW10O36)2]10-). In particular, an enhancement factor of 3.6 is achieved when the level of proton substitution of protonated tetraruthenium polyoxometalate is changed from zero to 9 under application of RF at 200 MHz.

Proton-Enhanced Dielectric Properties of Polyoxometalates in Water under Radio-Frequency Electromagnetic Waves.

Electromagnetic waves, such as microwaves, have been used to enhance various chemical reactions over polyoxometalates. The dielectric properties of catalysts are among the relevant parameters facilitating catalytic reactions under electromagnetic radiation. This study describes the dielectric properties of polyoxometalate catalysts in aqueous and organic solutions to understand the mechanism of interactions between polyoxometalates and electromagnetic waves. Specific loss factors of polyoxometalates were observed at lower frequencies (<1 GHz) by the ionic conduction of the polyoxometalate solution. The evolution of ionic conduction depended strongly on cations rather than anions. Proton-type polyoxometalates exhibited significantly higher loss factors than other cations did. The activation energy for ionic conduction in protonated silicotungstic acid (H4SiW12O40) was significantly low in water (7.6⁻14.1 kJ/mol); therefore, the high loss factor of protonated polyoxometalates in water was attributed to the proton relay mechanism (i.e., Grotthuss mechanism). The results suggested that the proton relay mechanism at the radio-frequency band is critical for generating selective interactions of polyoxometalates with applied electromagnetic fields.

Bluish-White Luminescence in Rare-Earth-Free Vanadate Garnet Phosphors: Structural Characterization of LiCa3MV3O12 (M = Zn and Mg).

Extensive attention has been focused toward studies on inexpensive and rare-earth-free garnet-structure vanadate phosphors, which do not have a low optical absorption due to the luminescence color being easily controlled by its high composition flexibility. However, bluish emission phosphors with a high quantum efficiency have not been found until now. In this study, we successfully discovered bluish-white emitting, garnet structure-based LiCa3MV3O12 (M = Zn and Mg) phosphors with a high quantum efficiency, and the detailed crystal structure was refined by the Rietveld analysis technique. These phosphors exhibit a broad-band emission spectra peak at 481 nm under near UV-light excitation at 341 nm, indicating no clear difference in the emission and excitation spectra. A very compact tetrahedral [VO4] unit is observed in the LiCa3MV3O12 (M = Zn and Mg) phosphors, which is not seen in other conventional garnet compounds, and generates a bluish-white emission. In addition, these phosphors exhibit high quantum efficiencies of 40.1% (M = Zn) and 44.0% (M = Mg), respectively. Therefore, these vanadate garnet phosphors can provide a new blue color source for LED devices.

Voltammetric and Spectroscopic Studies of α- and ß-[PW12O40]3- Polyoxometalates in Neutral and Acidic Media: Structural Characterization as Their [(n-Bu4N)3][PW12O40] Salts.

The structure of the Keggin-type ß-[PW12O40]3- (PW12) polyoxometalate, with n-Bu4N+ as the countercation, has been determined for the first time by single-crystal X-ray analysis and compared to data obtained from a new determination of the structure of the α-PW12 isomer, having the same countercation. Analysis of cyclic voltammograms obtained in CH3CN (0.1 M [n-Bu4N][PF6]) reveals that the reversible potential for the ß-PW12 isomer always remains ca. 100 mV more positive than that of the α-PW12 isomer on addition of the acid CF3SO3H. Simulations of the cyclic voltammetry as a function of acid concentration over the range 0-5 mM mimic experimental data exceptionally well. These simulation-experiment comparisons provide access to reversible potentials and acidity constants associated with α and ß fully oxidized and one- and two-electron reduced systems and also explain how the two well-resolved one-electron W(VI)/W(V) processes converge into a single two-electron process if sufficient acid is present. 183W NMR spectra of the oxidized forms of the PW12 isomers are acid dependent and in the case of ß-PW12 imply that the bridging oxygens between the WI and WII units are preferentially protonated in acidic media. EPR data on frozen solutions of one-electron reduced ß-[PWVWVI11O40]4- indicate that either the WI or the WIII unit in ß-PW12 is reduced in the ß-[PWVI12O40]3-/ß-[PWVWVI11O40]4- process. In the absence of acid, reversible potentials obtained from the α- and ß-isomers of PW12 and [SiW12O40]4- exhibit a linear relationship with solvent properties such as Lewis acidity, acceptor number, and polarity index.