Structure of glasses and melts in the Na<Subscript>2</Subscript>O-B<Subscript>2</Subscript>O<Subscript>3</Subscript> system from high-temperature Raman spectroscopic data: II. Superstructural units in melts

The specific features and possible mechanisms of transformation of the intermediate-range order structure in the Na₂O-B₂O₃ system are investigated as a function of the composition and temperature by analyzing the integrated intensities of the characteristic bands at 808, 770–780, and 750–760 cm^?1 (due to the vibrations of boroxol, triborate, and di-triborate rings, respectively) in the Raman spectra of glasses and melts in this system. It is demonstrated that an increase in the Na₂O concentration leads to sequential transformations of boroxol rings into triborate groups and then into di-triborate groups. An increase in the temperature results in a decrease in the fraction of main structural units joined into superstructural units. The concentration of different borate rings can change depending on the temperature due to both the more random distribution of main units in the melt structure as compared to the glass structure and the formation of nonbridging bonds within superstructural units. Moreover, different superstructural units can transform into each other.     

Structure of alkali borosilicate glasses and melts according to Raman spectroscopy data

Using the technique of high-temperature Raman scattering spectroscopy, we studied the structure of glasses and melts of the systems Na₂O-B₂O₃-SiO₂, K₂O-B₂O₃-SiO₂, and Cs₂O-B₂O₃-SiO₂ with the relations ${{X_{M_2 O} } \mathord{\left/ {\vphantom {{X_{M_2 O} } X}} \right. \kern-0em} X}_{B_2 O_3 } = 1$ (M = Na, K, Cs) and ${{X_{SiO_2 } } \mathord{\left/ {\vphantom {{X_{SiO_2 } } X}} \right. \kern-0em} X}_{B_2 O_3 } = 3$ and 4/3. Based on the analysis of the registered spectra, we have shown that, at a high content of SiO₂, the disordered network of glasses is composed of the Q ⁴, Q ³, [BO_4/2]^? tetrahedrons and BO_3/2 triangles. When the content of SiO₂ reduces, asymmetric borate triangles BØ_2/2O^? are formed in the structure in addition. A substantial part of borate tetrahedrons are included into the content of mixed borosilicate rings composed of two silicon-oxygen and two boron-oxygen tetrahedrons. The amount of borate structural units joined into purely borate superstructural groups increases with a decrease of the silicon oxide content, depends on the alkali kation type, and grows in the direction from Cs to Na. An increase in the temperature causes a decrease of various types of rings and growth of the concentration of asymmetric triangles. Both in glasses and melts, the fraction of the BO_2/2O^? triangles depends on the alkali cation type and increases in the succession Cs → K → Na. The results obtained present the basis to suggest that there is some differentiation in the mechanisms of structural reconstruction of the glasses under study by their heating depending on the modifying oxide.     

Effect of electrolyte additives on high-temperature cycling performance of spinel LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> cathode

Lithium difluoroborate (LiDFOB), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFBOP) and lithium difluorophosphate (LiPF₂O₂) are investigated as electrolyte additives to alleviate the severe cycle capacity fading of spinel LiMn₂O₄ cathode of lithium-ion batteries, especially at elevated temperatures. Compared with that of the routine electrolyte, the capacity retention is significantly improved at both room temperature and 55 °C by adding LiBOB and LiDFOB as electrolyte additives. Moreover, surface layer formation processes on the LiMn₂O₄ electrode in the presence of the LiBOB, LiDFOB, LiDFBOP and LiPF₂O₂ are investigated by photoelectron spectroscopy (XPS) and X-ray diffraction. According to the analysis results, BOB^? anions from LiBOB or LiDFOB bond with the dissolved Mn²⁺ to form an insoluble and stable surface layer on the LiMn₂O₄ surface, which is beneficial to the suppression of the LiMn₂O₄ dissolution and electrolyte decomposition, and eventually to the improvement of the cycling performance at elevated temperatures.

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Effect of Potassium on the Physiochemical and Catalytic Characteristics of V<Subscript>2</Subscript>O<Subscript>5</Subscript>/TiO<Subscript>2</Subscript> Catalysts in <Emphasis Type="Italic">o</Emphasis>-Xylene Partial Oxidation to Phthalic Anhydride

Abstract

Vanadia species formed on the surface depend on the K/V atomic ratio. At small K/V ratios, Raman spectra show the formation of the K-doped and K-perturbed monomeric species. At K/V?=?1, kristalline KVO₃ is mainly present on the surface. In situ high temperature XRD-results exhibit a promoting effect on the anatase to rutile phase transformation in the presence of 0.03 and 0.21 wt% potassium. Large amount of K (3 wt%) provides thermal stability of V/Ti/O catalyst and no transformation is found up to 600?°C. Reduction of vanadia K-doped vanadia catalysts is moved to higher temperatures than for the catalyst without potassium. The catalyst having 0.21 wt% K possesses the highest activity in o-xylene oxidation. Furthermore, the K-doped monomeric vanadia species in this catalyst leads to a promoted adsorption or a prevented desorption of phthalide, resulting in a decreased selectivity towards phthalide and CO_x and a increased PA selecticity.

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Influence of Tetraalkylammonium Compounds on Photocatalytic and Physical Properties of TiO<Subscript>2</Subscript>

A wide range of experimental data are reported for the first time on the TiO₂ prepared by hydrolysis of highly concentrated Ti(OⁱPr)₄ in water solutions of quaternary ammonium compounds (QACs). These TiO₂ materials have been shown to be photocatalytically active under visible light irradiation (LED, 450 nm) using acetone as a model substrate oxidized in the gas phase. Five-fold increase in activity in comparison with the commercial photocatalyst KRONOClean 7000 is achieved. Colloidal solutions of hydrolyzed Ti(OⁱPr)₄ have been studied by SAXS method suggesting the way in which QACs solutions may influence the final composition of TiO₂. Phase composition, morphology, texture and surface properties of the modified TiO₂ have been studied using XRD, BET, SEM and low-temperature FTIR with CO probe. The surface elemental composition has been investigated by XPS method. Additional low-energy levels and high concentration of acid surface sites originated from N/C-doping, are likely to be the main reasons for exceptional photocatalytic performance of these samples.

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Preparation and Characterization of an Efficient Nano-Inorganic Composite of CuO/ZnO/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> for the Catalytic Amination of Aryl Halides in Aqueous Conditions

In this research, an efficient recyclable nano-inorganic composite of CuO/ZnO/Al₂O₃ (CuO/ZnO/Al₂O₃ nanocatalyst) is prepared, characterized and used for the amination of aryl halides with aqueous ammonia in water. The catalyst was prepared by co-precipitation method and characterized by various techniques such as the X-ray diffraction, scanning electron microscope, energy dispersive spectroscopy, and brunauer–Emmett–Teller surface area analysis. Various aryl halides reacted with aqueous ammonia and corresponding products were obtained in high yields. CuO/ZnO/Al₂O₃ nanocatalyst as an efficient stable catalyst is recyclable up to five consecutive runs by simple filtration.

Graphical Abstract

An efficient recyclable nano-inorganic composite of CuO/ZnO/Al₂O₃ (CuO/ZnO-Al₂O₃ nanocatalyst) is prepared, characterized and used for the amination of aryl halides with aqueous ammonia in water.

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Density and microhardness of glasses in the SrO-B<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> system

The density d at a temperature of 25°C is measured by the hydrostatic weighing method, the Vickers microhardness H _V is determined, and the fluctuation free volume fraction f _g is calculated for glasses in the SrO-B₂O₃-SiO₂ system with a constant strontium oxide content in the range from 35 to 45 mol %. It is demonstrated that the quantities H _V and f _g decrease and the density d increases with an increase in the SrO content.     

Morphology-Controlled Synthesis of Co<Subscript>3</Subscript>O<Subscript>4</Subscript> Materials and its Electrochemical Catalytic Properties Towards Oxygen Evolution Reaction

In the present study, we present a facile strategy to synthesis Co₃O₄ materials with different morphology. Experimental results show that Co₃O₄ materials with flower-like, fiber, sheet-like and rod morphologies have been successfully prepared by hydrothermal synthesis in different solvent. The effect of the morphology on the electrochemical catalytic properties were also studied. It is found that sheet-like Co₃O₄ exhibits the best activity towards oxygen evolution reaction (η₁₀?=?390 mV) in 1 M KOH, which can be attribute to its short electrolyte infiltration diffusion path lengths and low charge transfer resistant.

Graphical Abstract

LSV curves measured at 5 mV/s in 1 M KOH solution for OER, the inset image is FE-SEM image of prepared Co₃O₄ materials. a Flower, b fiber, c sheet and d rod.

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Electrical conductivity and structure of glasses in the Na<Subscript>2</Subscript>O-Na<Subscript>2</Subscript>S-P<Subscript>2</Subscript>O<Subscript>5</Subscript> and Na<Subscript>2</Subscript>S-P<Subscript>2</Subscript>S<Subscript>5</Subscript> systems

The glasses, in which oxygen was partially replaced with sulfur, have been synthesized in the Na₂O-P₂O₅-Na₂S system. The chemical and chromatographic analyses of the glasses synthesized have been performed. The temperature-concentration dependences of electrical conductivity of the glasses have been studied over a wide temperature range; the glass transition temperatures and the nature of charge carriers have been determined. The IR spectra and Raman spectra have been recorded at room temperature; the density and microhardness of the glasses and ultrasound velocity have been measured. A comparison of the electrical conductivities of the investigated glasses with those of the earlier studied glasses in the Na₂O-P₂O₅ system has shown their fair coincidence. The introduction of sodium sulfide into the Na₂O-P₂O₅ system is accompanied by an approximately threefold increase in electrical conductivity, although the concentrations of charge carriers (sodium ions) in the glasses amount to ∼17 and ∼26 mmol/cm³, respectively. The rise in electrical conductivity has been assumed to be caused by the increase in the degree of dissociation of polar structural chemical units including sulfide ions and by the higher mobility of sodium ions in the oxygen-free matrix.     

CO<Subscript>2</Subscript> methanation and co-methanation of CO and CO<Subscript>2</Subscript> over Mn-promoted Ni/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts

A series of Mn-promoted 15 wt-% Ni/Al₂O₃ catalysts were prepared by an incipient wetness impregnation method. The effect of the Mn content on the activity of the Ni/Al₂O₃ catalysts for CO₂ methanation and the comethanation of CO and CO₂ in a fixed-bed reactor was investigated. The catalysts were characterized by N2 physisorption, hydrogen temperature-programmed reduction and desorption, carbon dioxide temperature-programmed desorption, X-ray diffraction and highresolution transmission electron microscopy. The presence of Mn increased the number of CO₂ adsorption sites and inhibited Ni particle agglomeration due to improved Ni dispersion and weakened interactions between the nickel species and the support. The Mn-promoted 15 wt-% Ni/Al₂O₃ catalysts had improved CO₂ methanation activity especially at low temperatures (250 to 400 °C). The Mn content was varied from 0.86% to 2.54% and the best CO₂ conversion was achieved with the 1.71Mn-Ni/Al₂O₃ catalyst. The co-methanation tests on the 1.71Mn-Ni/Al₂O₃ catalyst indicated that adding Mn markedly enhanced the CO₂ methanation activity especially at low temperatures but it had little influence on the CO methanation performance. CO₂ methanation was more sensitive to the reaction temperature and the space velocity than the CO methanation in the co-methanation process.

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Effects of SiC and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> particles on the electrodeposition of Zn,Co and ZnCo. I. Electrodeposition in the absence of SiC and Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

Electrodeposition of Zn, Co and ZnCo from acid sulfate solutions onto steel was investigated in this first part of a study of the effects of SiC or Al₂O₃ particles on these processes and the formation of ZnCo–SiC and ZnCo–Al₂O₃ electrocomposites. Zn electrodeposition shows a well-defined pre-bulk region, where the hydrogen evolution reaction (HER) and Zn underpotential deposition (upd) compete. Zn bulk electrodeposition begins with primary nucleation and diffusion-controlled growth, strongly dependent on conditions favoring previous Zn upd against HER. It is assumed that this first bulk process takes place over the upd Zn. Zn bulk electrodeposition is followed by secondary nucleation and growth. Co electrodeposition begins with a slow reduction in parallel with HER, followed by a faster reduction. strongly hinders the initial reduction. The ZnCo and Zn electrodeposition curves are initially similar, retaining features of pre-bulk and bulk Zn electrodeposition.     

EPR and optical studies on binary mixed alkali-alkaline earth oxide borate glasses doped with Cu<Superscript>2+</Superscript> ions

Mixed alkali alkaline earth oxide borate glasses of the composition (25 – x)Li₂O–xK₂O–12.5BaO–12.5MgO–49B₂O₃–1CuO (x = 0, 5, 10, 15 and 20 mol %) were prepared by the melt quenching technique. The X-ray diffractograms of all the glass samples were recorded at room temperature. Peak free X-ray spectra revealed the amorphous nature of all the prepared glasses. Modulated differential scanning calorimetry (MDSC) was used to determine the glass-transition temperature (T _g ). The probable mixed alkali effect was investigated using experimental techniques like density, molar volume, MDSC, electron paramagnetic resonance (EPR), and optical absorption studies. From the EPR spectra the spin-Hamiltonian parameters were evaluated. The spin-Hamiltonian parameter values indicated that the ground state of \(C{u^{2 + }}is{\kern 1pt} {d_{{x^2} - {y^2}}}\) orbital (²B_1g) and the site symmetry around Cu² is tetragonally distorted octahedral. The variation of g _|| and A _|| as a function of Li₂O content was found to be nonlinear. A broad optical absorption band was observed in all the glasses containing Cu² ions corresponding to ²B_1g → ²B_2g transition. From the optical absorption studies the values of the optical band gap (E _opt) for indirect, direct transitions and Urbarch energy (ΔE) have been evaluated. By co-relating the EPR and optical absorption data, bonding parameters α², β² and β ₁ ² were evaluated.     

Fabrication of MIL-100(Fe)@SiO<Subscript>2</Subscript>@Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> core-shell microspheres as a magnetically recyclable solid acidic catalyst for the acetalization of benzaldehyde and glycol

Heterogeneous catalysts with convenient recyclability and reusability are vitally important to reduce the cost of catalysts as well as to avoid complex separation and recovery operations. In this regard, magnetic MIL-100 (Fe)@SiO₂@Fe₃O₄ microspheres with a novel core-shell structure were fabricated by the in-situ self-assembly of a metal-organic MIL-100(Fe) framework around pre-synthesized magnetic SiO₂@Fe₃O₄ particles under relatively mild and environmentally benign conditions. The catalytic activity of the MIL-100(Fe)@SiO2@Fe3O4 catalyst was tested for the liquid-phase acetalization of benzaldehyde and glycol. The MIL-100(Fe)@SiO₂@Fe₃O₄ catalyst has a significant amount of accessible Lewis acid sites and therefore exhibited good acetalization catalytic activity. Moreover, due to its superparamagnetism properties, the heterogeneous MIL-100(Fe)@SiO₂@Fe₃O₄ catalyst can be easily isolated from the reaction system within a few seconds by simply using an external magnet. The catalyst could then be reused at least eight times without significant loss in catalytic efficiency.

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Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and CeO<Subscript>2</Subscript>-promoted MgO sorbents for CO<Subscript>2</Subscript> capture at moderate temperatures

A series of Al₂O₃ and CeO₂ modified MgO sorbents was prepared and studied for CO₂ sorption at moderate temperatures. The CO₂ sorption capacity of MgO was enhanced with the addition of either Al₂O₃ or CeO₂. Over Al₂O₃-MgO sorbents, the best capacity of 24.6 mg- CO₂/g-sorbent was attained at 100 °C, which was 61% higher than that of MgO (15.3 mg-CO₂/g-sorbent). The highest capacity of 35.3 mg-CO₂/g-sorbent was obtained over the CeO₂-MgO sorbents at the optimal temperature of 200 °C. Combining with the characterization results, we conclude that the promotion effect on CO₂ sorption with the addition of Al₂O₃ and CeO₂ can be attributed to the increased surface area with reduced MgO crystallite size. Moreover, the addition of CeO₂ increased the basicity of MgO phase, resulting in more increase in the CO₂ capacity than Al₂O₃ promoter. Both the Al₂O₃-MgO and CeO₂-MgO sorbents exhibited better cyclic stability than MgO over the course of fifteen CO₂ sorption-desorption cycles. Compared to Al₂O₃, CeO₂ is more effective for promoting the CO₂ capacity of MgO. To enhance the CO₂ capacity of MgO sorbent, increasing the basicity is more effective than the increase in the surface area.

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Thermal behavior of layered perovskite-like compounds in the Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript>-BiFeO<Subscript>3</Subscript> system

The thermal properties of compounds of the general formula Bi _{m + 1} Fe _{m ? 3} Ti₃O_{3m + 3}, which are layered perovskite-like phases of the Aurivillius type, are investigated as a function of their composition. It is demonstrated that the temperature of decomposition of the Bi _{m + 1} Fe _{m ? 3} Ti₃O_{3m + 3} compounds decreases with an increase in the thickness of perovskite-like layers alternating in the structure and that the composition dependence of the temperature of the structural transition observed in these compounds exhibits a more complex behavior. The linear thermal expansion coefficients of all the compounds under investigation are found to be virtually independent of the composition.     

Embedding Noble-Metal-Free Ni<Subscript>2</Subscript>P Cocatalyst on g-C<Subscript>3</Subscript>N<Subscript>4</Subscript> for Enhanced Photocatalytic H<Subscript>2</Subscript> Evolution in Water Under Visible Light

Photocatalytic hydrogen evolution is considered as one of the promising pathways to settle the energy crises and environmental issues by utilizing solar energy. In this paper, noble-metal-free Ni₂P was used as cocatalyst to enhance g-C₃N₄ for photocatalytic hydrogen production under visible light irradiation (λ?>?420 nm). Characterization results indicated that Ni₂P nanoparticles were successfully loaded onto g-C₃N₄, which can significantly contribute to accelerate the separation and transfer of photogenerated electron. The hydrogen evolution rate reached ～?270 µmol h^?1 g^?1 and the apparent quantum yield (AQY) was ～?2.85% at 420 nm. Meanwhile, there is no obviously decrease of the hydrogen production rate even after 36 h under visible light illumination. In addition, the mechanism of photocatalytic hydrogen evolution was also elaborated in detail.

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Synthesized Hollow TiO<Subscript>2</Subscript>@g-C<Subscript>3</Subscript>N<Subscript>4</Subscript> Composites for Carbon dioxide Reduction Under Visible Light

The hollow TiO₂@g-C₃N₄ composites were synthesized by a facile stirring method. The phase compositions, optical properties, and morphologies of the samples were characterized via X-ray diffraction, scanning electron microscope, transmission electron microscopy, high resolution transmission electron microscopy, fourier transform infrared spectroscopy, N₂ adsorption–desorption, UV–Vis diffuse reflectance spectroscopy and Photoluminescence. The photocatalyitc performance was evaluated by reduction carbon dioxide under visible light irradiation. The results indicated that TiO₂@g-C₃N₄ nanocomposites displayed higher photocatalytic activity compared with pure g-C₃N₄. The increased photocatalytic activity of TiO₂@g-C₃N₄ nanocomposites can be attributed to facilitating the photo-induced electron–hole separation efficiency and enhancing the photo-induced electron migration.

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Inexpensive synthesis of a high-performance Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>-SiO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> photocatalyst: Magnetic recovery and reuse

A sol-gel technique has been developed for the synthesis of a magnetite-silica-titania (Fe₃O₄-SiO₂-TiO₂) tertiary nanocomposite with improved photocatalytic properties based on the use of inexpensive titania and silica precursors. The exceptional photocatalytic activity of the resulting materials was demonstrated by using them to photocatalyze the degradation of methylene blue solution. The best formulation achieved 98% methylene blue degradation. An interesting feature of the present work was the ability to magnetically separate and reuse the catalyst. The efficiency of the catalyst remained high during two reuses. The synthesized nanomaterials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, ultra-violet-visible spectroscopy, diffuse reflectance spectroscopy, and thermogravimetric analysis. XRD analysis revealed the formation of multicrystalline systems of cubic magnetite and anatase titania crystals. SEM and TEM characterization revealed well-developed and homo-geneously dispersed particles of size less than 15 nm. FTIR spectra confirmed the chemical interaction of titania and silica. It was further noticed that the optical properties of the prepared materials were dependent on the relative contents of their constituent metal oxides.

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SHS of Y<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript>-based mineral-like ceramics: Influence of green density

Explored was the influence of compacting pressure (P) and green density (ρ) on the properties of Zr-doped mineral-like pyrochlore ceramics Y₂(Ti_{1 – x} Zr _x )₂O₇ (x ≤ 0.3) prepared by SHS method. The optimal ρ values that provide minimal porosity and maximal mechanical strength of synthesized ceramics were found. An increase in ρ was found to decrease combustion temperature and increase pyrochlore lattice parameter a. Green density was also found to affect phase composition of the SHS-produced ceramics under study.     

AC Conductivity and Dielectric Behavior of Silicophosphate Glass Doped by Nd<Subscript>2</Subscript>O<Subscript>3</Subscript>

Silicophosphate glasses of nominal composition (P₂ O ₅ 50%-SiO₂ 30%-Na₂O 20%) and Nd₂ O ₃ additive (0.5 and 2 wt%) were prepared and dielectric behavior has been studied over a temperature range (302–483 K) in the frequency range (0.5 - 3243 kHz). Frequency dependence of AC conductivity (σ _ac), has been explored using the universal power law. Disparity of the frequency exponent (s) with temperature was examined in terms of diverse conduction mechanisms. The principal conduction mechanisms were found correlated to both barrier hopping (CBH) and quantum mechanical tunneling (QMT) models. Temperature dependence of σ _ac (ω) showed a linear increase with different frequencies. In addition, the capacitance, loss tangent, dielectric loss and dielectric constant were calculated over variable temperature ranges and frequencies.