Electrochemical and UV–visible spectroscopy studies of K6As2W18 − xMoxO62 (x = 0–3) Wells–Dawson heteropolyacid catalysts for oxidative dehydrogenation of benzyl alcohol

Redox behaviors of molybdenum-substituted Wells–Dawson tungstoarsenate heteropolyacid catalysts (α-K₆As₂W_18 − xMo_xO₆₂ (x = 0–3)) were probed by electrochemical and UV-visible spectroscopy measurements. Reduction potential of K₆As₂W_18 − xMo_xO₆₂ (x = 0–3) catalysts shifted to more positive potential with increasing molybdenum content. UV–visible spectroscopy analysis was conducted as a simple diagnostic method for reduction potential. Absorption edge energy determined by UV–visible spectroscopy decreased with increasing reduction potential. Gas-phase oxidative dehydrogenation of benzyl alcohol was carried out as a model reaction. Yield for benzaldehyde increased with increasing electrochemical reduction potential and with decreasing UV–visible absorption edge energy of the catalysts.     

Microwave absorption properties of (1 − x)La0.85Ag0.15MnO3/x graphene (x = 0, 3, and 5 wt.%) composites

Composite of (1 − x)La_0.85Ag_0.15MnO₃/x graphene (x = 0, 3, and 5 wt.%) and epoxy resin with a ratio of 4:1 were prepared to investigate the influence of the addition of graphene in (1 − x)La_0.85Ag_0.15MnO₃/x graphene on real and imaginary parts of permittivity, permeability, as well as microwave reflection loss (RL), using a vector network analyzer in the 8–18 GHz of the frequency range. It is found that the value of RL is smaller at x = 3 wt.% (−20.74 dB at 14.85 GHz) and 5 wt.% (−14.81 at 16.50 GHz) compared to at x = 0 wt.% (−8.89 dB at 15.90 GHz). The result indicates that microwave absorption properties significantly improved as a result of the addition of graphene. It is suggested that the addition of graphene enhanced the dielectric loss–related mechanism such as interfacial polarization and conduction loss resulting in an improvement of microwave absorption performance for both x = 3 wt.% and x = 5 wt.% samples. It also shows that the observed enhanced microwave absorption properties may also be influenced by the resistivity of the sample as x = 3 wt.% sample exhibits enhanced microwave absorption properties and the lowest resistivity among the studied samples.     

Catalytic activity of nanometer La1−xSrxCoO3 (x = 0, 0.2) perovskites towards VOCs combustion

Combustive oxidation of volatile organic compounds (VOCs), such as propyl alcohol, toluene and cyclohexane, were studied. The combustion was catalyzed by nanoparticles of La_1−xSr_xCoO₃ (x = 0, 0.2) perovskites prepared by a co-precipitation method. The results showed high activities of the perovskite catalysts. Compared to LaCoO₃, in particular, La_0.8Sr_0.2CoO₃ was much higher in catalytic ability. The total oxidation of VOCs followed the increasing order: cyclohexane < toluene < propyl alcohol. The T_99% of cyclohexane was 40 °C lower than that of toluene, which appeared to be determined by the bond strengths of the weakest C–H and C–C bonds. The 100-h stability experiments showed that La_1−xSr_xCoO₃ (x = 0, 0.2) perovskite was highly stable.     

New high Q microwave dielectric ceramics with rock salt structures: (1 − x)Li2TiO3 + xMgO system (0 ≤ x ≤ 0.5)

The structural evolution and microwave dielectric properties of (1 ? x)Li₂TiO₃ + xMgO system (0 ≤ x ≤ 0.5) have been investigated in this paper. The ordering degree decreased with the increase of MgO content. The microcracks and cleavage on (0 0 1) due to the weak Li–O bonds disappeared with the increase of MgO content. The dielectric constant and temperature coefficient of resonant frequency decreased with the increase of MgO content. The Q × f value increased with x up to x = 0.2 and then decreases with the further increase of x. An excellent combined microwave dielectric properties could be obtained when x = 0.24, ?_r = 19.2, Q × f = 106,226 GHz and τ_f = 3.56 ppm/°C.     

Improved dielectric strength and energy storage density in Ba6−3xLa8+2xTi18O54 (x = 0.5, 2/3, and 0.75) ceramics

Dielectric strength and energy storage density in Ba_6−3xLa_8+2xTi₁₈O₅₄ (x = 0.5, 2/3, and 0.75) ceramics were investigated as functions of composition and microstructure. With increasing x, although the dielectric constant decreased from 113 to 102, the energy storage density increased from 2.3 J/cm³ to 3.2 J/cm³ due to the increased dielectric strength for ceramics prepared by conventional sintering. The energy storage was further improved to 4.2 J/cm³ in ceramics prepared by spark plasma sintering under an electric field of 1058 kV/cm. Both dielectric strength and energy storage density in the present ceramics indicated the strong processing and microstructure dependence. The optimum dielectric strength and energy storage density were achieved in the dense ceramics with fine grains, while both dielectric strength and energy storage density decreased in the ceramics with coarse columnar grains.     

A study on proton conduction in a layered metal–organic framework,Rb2(adp)[Zn2(ox)3]·3H2O (adp = adipic acid,ox2 − = oxalate)

We newly synthesized a metal–organic framework (MOF) Rb₂(adp)[Zn₂(ox)₃]·3H₂O (adp = adipic acid; ox^2 − = oxalate), where the rubidium ions, carboxylic acid groups, and water molecules are located in an interlayer space of a two-dimensional (2-D) oxalate-bridged network. The structure of this compound was determined using single-crystal X-ray diffraction analysis. Hydrated phases of this compound were examined using thermogravimetry and water vapor adsorption measurements. Proton conductivity in this MOF was investigated by alternating current impedance measurements. Systematic comparison with previously reported isomorphous 2-D compounds A₂(adp)[Zn₂(ox)₃]·3H₂O (A = NH₄ and K) showed that the difference in the ionic radii of the cations leads to a difference in activation energy of proton conductivity and that absence of NH₄⁺ ions causes a significant decrease in proton conductivity, even though the ionic radius of Rb⁺ (1.52 Å) is closer to that of NH₄⁺ (1.61 Å) than that of K⁺ (1.38 Å).     

Electrical characterization of room temperature humidity sensors in La0.8Sr0.2Fe1−xCuxO3 (x = 0, 0.05, 0.10)

Semiconducting oxide gas sensors based on La_0.8Sr_0.2Fe_1?xCu_xO₃ (x = 0, 0.05, 0.10) (LSF, LSFC05, and LSFC10, respectively) were prepared by screen-printing for humidity detection at room temperature.The thick-films were heat-treated at 800, 900 and 1000 °C for 1 h and all the compositions proved to be effective in humidity sensing and presented a good reproducibility between several measurements. However, the best results were obtained with LSFC10 fired at 800 °C which showed a detection limit of 15% relative humidity and a maximum sensor response of about 87%, higher than the previous results. Copper addition to lanthanum strontium ferrites proved to be effective in lowering the sensors’ detection limit.     

Crystal structure,densification, and microwave dielectric properties of Li3Mg2(Nb(1−x)Mox)O6+x/2 (0 ≤ x ≤ 0.08) ceramics

A novel system Li₃Mg₂(Nb_(1−x)Mo_x)O_6+x/2 (0 ≤ x ≤ 0.08) microwave dielectric ceramics were fabricated by the solid-state method. The charge compensation of Mo⁶⁺ ions substitution for Nb⁵⁺ ions was performed by introducing oxygen ions. The X-ray diffraction patterns and Rietveld refinements indicated Li₃Mg₂(Nb_(1−x)Mo_x)O_6+x/2 ceramics with single phase and orthorhombic structure. Micro-structure and density confirmed that the grain of Li₃Mg₂(Nb_(1-x)Mo_x)O_6+x/2 ceramics grew well. In addition, the permittivity of Li₃Mg₂(Nb_(1−x)Mo_x)O_6+x/2 ceramics with the same trend as density decreased slightly with increasing Mo⁶⁺ ions content. However, the Q*f and τ_f were obviously improved with an appropriate amount of Mo⁶⁺ ions. When x ≤ 0.04, the Q*f was closely related to the bond valence of samples, while when x ≥ 0.06, the Q*f was closely related to the density of samples. The variations of τ_f and oxygen octahedral distortion were the opposite. In conclusions, the Li₃Mg₂(Nb_0.98Mo_0.02)O_6.01 ceramic sintered at 1200°C for 6 hours exhibited outstanding properties: ε_r ~ 15.18, Q*f ~ 116 266 GHz, τ_f ~ −15.71 ppm/^oC.     

Mechanical and thermal properties of δ-RE4Hf3O12 (RE = Yb,Lu)

Rare-earth (RE) hafnates are promising thermal and environmental barrier coating (TEBC) materials for SiC_f/SiC ceramic matrix composites. In this study, pure-phase and dense δ-RE₄Hf₃O₁₂ (RE = Yb, Lu) bulk ceramics have been fabricated via a hot-pressing method. The crystal structure, microstructure, mechanical, and thermal properties of δ-RE₄Hf₃O₁₂ were systematically investigated in order to probe their potential application as TEBCs. The high-temperature elastic moduli of δ-Yb₄Hf₃O₁₂ and δ-Lu₄Hf₃O₁₂ are measured to be 185 and 188 GPa at 1673 K, respectively, which are over 85% values of room temperature. The coefficients of thermal expansion are 7.64 × 10⁻⁶ and 7.46 × 10⁻⁶ K⁻¹ for δ-Yb₄Hf₃O₁₂ and δ-Lu₄Hf₃O₁₂, respectively. The relatively low coefficient of thermal expansion and thermal conductivity as well as their excellent high-temperature stability endow these hafnates as potential TEBC candidates.     

The influence of Mo6+ doping on the luminescence properties of red-emitting phosphor Sr9Eu2W4−xMoxO24 (x = 0–4)

A series of red emitting phosphors Sr₉Eu₂W_4?xMo_xO₂₄ (x = 0–4) have been synthesized by solid-state reactions and their crystal structures, photoluminescence properties were studied. The excitation and emission spectra of Sr₉Eu₂W_4?xMo_xO₂₄ phosphors can be modified by Mo⁶⁺ doping. As the molybdate content increased, the Eu³⁺ emission intensity of Sr₉Eu₂W_4?xMo_xO₂₄ (x = 0–4) under 395 nm excitation was found to increase and reached a maximum at x = 2. The excitation spectra, the emission intensities and the chromaticity coordinates of Sr₉Eu₂W_4?xMo_xO₂₄ (x = 2) were compared to those of the conventional red phosphor Y₂O₂S: Eu³⁺. The intense red-emission under near-UV excitation suggests that Sr₉Eu₂W_4?xMo_xO₂₄ (x = 2) could be a potential candidate for white light generation by using near-UV LEDs. In this study, the effects of Mo⁶⁺ doping on the crystal structure and photoluminescence properties of Sr₉Eu₂W_4?xMo_xO₂₄ were discussed.     

Ferroelectric phase transitions studied by dielectric and light scattering spectroscopies on Ba1−xCaxTiO3 single crystals (x = 0.12, 0.20)

The ferroelectric phase transition of Ca-substituted barium titanate single crystals, Ba_1?xCa_xTiO₃ (BCTO-x), with x = 0.12 and 0.20 grown by optical floating zone method were studied by dielectric and Brillouin scattering methods. The X-ray diffraction investigation confirmed the structural phase transition from cubic to tetragonal phase in BCTO-x. The dielectric constant exhibited a sharp peak at the ferroelectric phase transition temperature (T_c) and a deviation from the Curie–Weiss law on approaching T_c from the high-temperature side. Brillouin scattering results showed that, similar to the acoustic properties of BCTO-0.20, dynamic precursor polar clusters are formed in the paraelectric phase of BCTO-0.12 single crystals which interact with acoustic waves via electrostrictive coupling resulting in large acoustic anomalies near T_c. However, the temperature dependence of the dielectric constant indicated that diffuseness of the ferroelectric phase transition of BCTO-x became enhanced with Ca content, which was attributed to compositional disorder induced by random substitution of Ba cations by Ca ions.     

Catalytic performance of NO oxidation over LaMeO3 (Me = Mn,Fe, Co) perovskite prepared by the sol–gel method

The catalytic performance of LaMeO₃ (Me = Mn, Fe, Co) perovskite prepared by a sol–gel method was studied. These catalysts were characterized by X-ray diffraction (XRD), N₂ adsorption (BET), H₂ temperature programmed reduction (TPR), NO temperature programmed desorption (TPD) and CO–O₂ pulse. LaCoO₃ exhibited the best activity than that of LaFeO₃ and LaMnO₃ even after hydrothermal ageing. The activity sequence is in accordance with the reducibility of the samples. The activated oxygen species and adsorbed NO play key roles in the NO oxidation reaction.     

Catalytic decomposition of 2,3-dihydrobenzofuran to monomeric cyclic compounds over Pd/XCs2.5H0.5PW12O40/OMC (ordered mesoporous carbon) (X = 10–30 wt.%) catalysts

A series of Pd/XCs_2.5H_0.5PW₁₂O₄₀/OMC (ordered mesoporous carbon) (X = 10, 15, 20, 25, and 30 wt.%) catalysts with different Cs_2.5H_0.5PW₁₂O₄₀ contents (X, wt%) were prepared by a sequential incipient wetness impregnation method for use in the catalytic decomposition of 2,3-dihydrobenzofuran to monomeric cyclic compounds. 2,3-Dihydrobenzofuran was used as a lignin model compound for representing β-5 linkage of lignin. Acidity of Pd/XCs_2.5H_0.5PW₁₂O₄₀/OMC catalysts served as an important factor determining the catalytic performance in the reaction. Conversion of 2,3-dihydrobenzofuran and total yield for main products (2-ethylphenol and ethylcyclohexane) increased with increasing acidity of Pd/XCs_2.5H_0.5PW₁₂O₄₀/OMC catalysts.     

Lattice structure and microwave dielectric properties of La[Al1−x(Mg0.5Ti0.5)x]O3 (x = 0-0.2)-based ceramics

La[Al_1−x(Mg_0.5Ti_0.5)_x]O₃ (LAMT, x = 0-0.2) ceramics were synthesized by the conventional solid-state reaction method and formed a solid solution. The pure solid solutions were recorded by X-ray diffraction (XRD) in every range. Relative permittivity (ε_r) and structural stability were greatly affected because the Al³⁺ site was replaced by [Mg_0.5Ti_0.5]³⁺. The total ionic polarizability gradually increased with x, and ε_r gradually increased. The trend of τ_f is due to the change in structural stability. The variation in Q × f value increased firstly and then decreased due to the change in the symmetric stretching mode of Al/MgTi–O. The optimum microwave dielectric properties of LAMT were obtained at x of 0.1 after sintering at 1650°C for 5 hours, and ε_r = 24.9, Q × f = 79 956 GHz, and τ_f = −33 ppm/°C. The CaTiO₃ have a large positive τ_f (+800 ppm/°C), thus, the τ_f achieved near zero when CaTiO₃ and LAMT (x = 0.1) ceramics were mixed with a certain molar mass, and the optimum microwave dielectric properties of 0.65CaTiO₃–0.35LaAl_0.9(Mg_0.5Ti_0.5)_0.1O₃ were as follows: ε_r = 44.6, Q × f = 32 057 GHz, and τ_f = +2 ppm/°C.     

Chemical modification of diamond surface with X–(C6H4)–COOH (X = F,Cl, Br,I) using benzoyl peroxide

The chemical reactivity of a hydrogenated diamond surface with X–(C₆H₄)–COOH (X = F, Cl, Br, I) when using benzoyl peroxide was investigated in this study. After the reaction processes the shapes of the IR spectra changed. It was confirmed from the XPS measurements that halogen atoms existed on the samples after the reaction process. The position of the IR peak at ca. 700 cm^? 1 depended on the kind of halogen in X–(C₆H₄)–COOH. Moreover, the peak position depended on the kind of constitutional isomer, that is, ortho-, meta-, or para-. It was confirmed from the experimental results of this study that halogen-containing organic functional groups can be introduced onto a diamond surface.     

Structural,electrical and magnetic properties of Co0.5Zn0.5AlxFe2−xO4 (x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) prepared via sol–gel route

Nanoparticles of Co_0.5Zn_0.5Al_xFe_2?xO₄ (x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) were synthesized by sol–gel method and the influence of Al³⁺ doping on the properties of Co_0.5Zn_0.5Fe₂O₄ was studied. X-ray diffraction studies revealed the formation of single phase spinel type cubical structure having space group Fd-3m. A decreasing trend of the lattice parameter was observed with increasing Al³⁺ concentration due to the smaller ionic radii of Al³⁺ ion as compared to Fe³⁺ ion. TEM was used to characterize the microstructure of the samples and particle size determination, which exhibited the formation of spherical nanoparticles. The particle size was found to be increases up to ～45 nm after annealing the sample at 1000 °C. Electrical resistivity was found to increase with Al³⁺ doping, attributed to the decrease in the number of Fe²⁺–Fe³⁺ hopping. The activation energy decreased with increasing Al³⁺ ion concentration, indicating the blocking of conduction mechanism between Fe³⁺–Fe²⁺ ions. The value of saturation magnetization decreased, when Fe³⁺ ions were doped with Al³⁺ ions in Co_0.5Zn_0.5Fe₂O₄; however, the coercivity values increased with increasing Al³⁺ ion content.     

Redox properties of α2-K8P2W17(M·OH2)O61 (M = MnII,ZnII, FeII,CoII, and NiII) Wells–Dawson heteropolyacids probed by scanning tunneling microscopy and tunneling spectroscopy

Scanning tunneling microscopy (STM) and tunneling spectroscopy investigations of α₂-K₈P₂W₁₇(M·OH₂)O₆₁ (M = Mn^II, Zn^II, Fe^II, Co^II, and Ni^II) Wells–Dawson heteropolyacids (HPAs) were conducted. HPAs formed self-assembled and well-ordered arrays on graphite surface. Negative differential resistance (NDR) phenomena were observed in the tunneling spectra of the HPAs. NDR peak voltage of the HPAs appeared at less negative voltage with increasing reduction potential and with decreasing UV–visible absorption edge energy. HPAs were then applied to the electro-oxidation of methanol as a redox mediator. The oxidation activity for residual intermediates in the reaction increased as NDR peak voltage of HPA appeared at less negative voltage.     

Reduction potentials of H3+xPMo12–xVxO40 and H6+xP2Mo18−xVxO62 heteropolyacid (HPA) catalysts and their catalytic activity for the vapor-phase oxidative dehydrogenation of isobutyric acid

Vanadium-containing H_6+xP₂Mo_18−xV_xO₆₂ (x = 0, 1, 2, and 3) Wells-Dawson heteropolyacid (HPA) catalysts were prepared for use in the oxidative dehydrogenation of isobutyric acid (IBA) to methacrylic acid (MAA). Vanadium-containing H_3+xPMo_12−xV_xO₄₀ (x = 0, 1, 2, and 3) Keggin HPA catalysts were also investigated for the purpose of comparison. The reduction potentials of H_3+xPMo_12−xV_xO₄₀ and H_6+xP₂Mo_18−xV_xO₆₂ catalysts were determined by temperature-programmed reduction (TPR) measurements. The reduction potentials of H_3+xPMo_12−xV_xO₄₀ and H_6+xP₂Mo_18−xV_xO₆₂ catalysts showed volcano-shaped curves and exhibited the same trend with respect to vanadium substitution. The conversions of IBA over H_3+xPMo_12−xV_xO₄₀ and H_6+xP₂Mo_18−xV_xO₆₂ catalysts also showed volcano-shaped curves and exhibited the same trend with respect to vanadium substitution. However, the H_6+xP₂Mo_18−xV_xO₆₂ catalysts showed a higher reduction potential and a higher conversion of IBA than the H_3+xPMo_12−xV_xO₄₀ catalysts at the same level of vanadium substitution. A correlation between reduction potential and catalytic activity of the HPA catalysts clearly demonstrated that the conversion of IBA increased monotonically with increasing reduction potential of the HPA catalysts, across both families of vanadium-containing HPA catalysts examined in this work.