Effects of morphology on the electrochemical performance of NiFe2O4 nanoparticles with battery-type behavior

We report the synthesis of NiFe₂O₄ nanoparticles by the complexation EDTA-citrate method under acidic (pH = 3) and basic (pH = 9) conditions. The structural, optical, vibrational, magnetic, and electrochemical properties were studied. The samples have crystallite sizes of 21 nm (pH 3) and 73 nm (pH 9), with rounded particles and layered structures. The ⁵⁷Fe Mössbauer spectra at 12 K showed that both samples had an inverse spinel cation distribution. At 5 K, the sample prepared at pH 9 showed saturation magnetizations of about 50 emu/g. Raman spectra showed typical bands of NiFe₂O₄ phase. The materials were tested as electrodes under alkaline condition. The cyclic voltammetry and charge-discharge experiments indicated a battery-type behavior, with maximun capacities of 65 and 5 C/g (at specific currents of 3 and 10 A/g) for samples prepared at pH 9 and 3, respectively. This work offers a route for obtaining NiFe₂O₄ nanoparticles with different morphologies and sizes tuned by the synthesis conditions.     

Modeling and optimization of combustion synthesis for hydroxyapatite production

In this study, the combustion synthesis of hydroxyapatite was studied and optimized under controlled experimental conditions. The hydroxyapatite content, crystallinity and crystallite size were monitored under changes in type of fuel, pH or red/ox ratio, muffle temperature, and reaction time. The products were characterized by X-ray diffraction, refinement of crystalline phases by the Rietveld method, scanning electron microscopy, and infrared spectroscopy with the Fourier transform. The decomposition of hydroxyapatite, which was the major phase, produced beta tricalcium phosphate in all samples, as indicated by their diffractograms. Infrared spectroscopy analysis revealed the presence of b-type carbonate groups in the structure; thus, the synthesized compound has the following chemical formula: Ca_10-x (PO₄)_6-x (CO₃)_x (OH)_2-x, where 0 < x < 2, which corresponds to carbonated hydroxyapatite (CHAp). The combustion process was modeled using a first-degree polynomial, and we found that the interaction between time and temperature to be the most influential parameter. Optimization indicated that processing conditions at 650 °C for 30 min, using urea at pH = 2 and ϕ_e = 1.134, produced the best result in terms of HAp composition, yielding 89.25% wt.     

The kinetics and mechanism of combusted Zr–B–Si mixtures and the structural features of ceramics based on zirconium boride and silicide

The study focuses on investigation of the combustion kinetics and mechanisms, as well as the phase- and structure formation processes, during elemental synthesis of ceramics based on zirconium diboride and silicide doped with aluminum. The effect of the degree of dilution with an inert component and initial temperature T₀ on the combustion kinetics of the Zr–Si–Al–B mixture is studied. An increase in T₀ in the range of 298–700 K causes a directly proportional rise in the combustion temperature T_c and rate U_c, which demonstrates that staging of the reactions of formation of zirconium boride and silicide remains invariant. The effective activation energy E_eff of the combustion process is 225 kJ/mol, suggesting that the liquid-phase processes have a decisive effect on the reaction kinetics. The interaction of zirconium with boron and silicon runs through the Zr–Si–Al–B melt that is formed in the combustion zone. Staging of chemical transformations during phase and structure formation of SHS products is studied. The primary ZrB₂ grains crystallize from the melt in the combustion zone; the ZrSi silicide phase is formed with a delay of no longer than 0.5 s. Compact ceramics with composition ZrB₂–ZrSi–ZrSi₂–ZrSiAl₂ synthesized by forced SHS- pressing showing a great potential for high-temperature applications both as a construction material and as a precursor for ion-plasma deposition of coatings.     

Synthesis of γ-Al2O3 nanowires through a boehmite precursor route

By regulating the pH values of the reaction solution, the boehmite (γ-AlOOH) nanowires and nanoflakes were successfully synthesized with a simple hydrothermal route using anhydrous AlCl₃, NaOH and NH₃·H₂O as starting materials. Crystalline γ-Al₂O₃ nanowires with diameter of 10–30 nm and length of several hundreds of nanometer have been prepared by thermal decomposition of γ-AlOOH precursor. X-ray diffraction (XRD), transmission electron microscope (TEM), selected area electron diffraction (SAED), and high resolution transmission electron microscope (HRTEM) were used to characterize morphology and structure feathers of the synthesized samples. The pH values of the solution play important roles in the formation of γ-AlOOH nanowires. After calcinated at 500 °C for 2 h, the obtained γ-Al₂O₃ with a linear structure is similar to the γ-AlOOH precursor.     

Influences of pH value on the microstructure and phase transformation of aluminum hydroxide

The effects of pH value on the composition, structure, morphology, and phase transformation of aluminum hydroxides prepared by chemical precipitation were studied. Aluminum hydroxide precipitated at the pH values of 5 and 6 is amorphous and transforms to α-Al₂O₃ at 950 °C via the amorphous aluminum hydroxide → amorphous Al₂O₃ → α-Al₂O₃ transformation path. Aluminum hydroxide precipitated at pH = 7 is boehmite and transforms to α-Al₂O₃ at 950 °C via the γ-AlOOH → γ-Al₂O₃ → α-Al₂O₃ path. Aluminum hydroxide precipitated at pH values in the 8 to 11 range is bayerite and transforms to α-Al₂O₃ at 1000 °C via the α-Al(OH)₃ → γ-Al₂O₃ → ε-Al₂O₃ + θ-Al₂O₃ → α-Al₂O₃ path. Moreover, the pH value affects not only the morphology of aluminum hydroxide particles which changes from ultrafine floccules through 50 nm blowballs then to 150 nm irregular agglomerates with increasing pH value but also the microstructures of final decomposition products of aluminum hydroxides.     

Investigation on microstructure and nonanal sensing properties of hierarchical Sb2WO6 microspheres

Various metal oxides with unique microstructures have been hopeful in fabricating gas sensors applications. This paper reports the room-temperature sensing properties of hierarchical Sb₂WO₆ (antimony tungstate) microspheres for nonanal, which is a dominant aromatic aldehyde compound in cooked rice. The hierarchical Sb₂WO₆ microspheres were obtained by pH control (from pH 1 to 5) through a one-step hydrothermal reaction. The experiment indicated that the optimal response of the Sb₂WO₆ (pH = 4) gas sensor to 30 ppm nonanal reached 62 at room temperature, which was about 8 times that of Sb₂WO₆ (pH = 1). The improved sensing performance resulted from the finer microstructure and larger specific surface area of Sb₂WO₆ with a pH value of 4, providing abundant adsorption sites for nonanal molecules. Meanwhile, the practicability of cooked rice stored for various periods was verified through gas detection, and the results indicated that the sensor might recognize the deterioration of cooked rice and ready-to-eat rice. Therefore, the as-prepared sensor can be used in electronic nose systems to detect cooked rice and ready-to-eat rice deterioration and improve rice cooking methods.     

Influence of particle size on magnetic and electromagnetic properties of hexaferrite synthesised by sol-gel auto combustion route

Magnetoplumbite barium hexaferrite (BaFe_11.8Co_0.2O₁₉) is synthesised through sol-gel auto-combustion method under two pH conditions of precursor solutions (acidic i.e. pH < 1 and neutral i.e. pH = 7). The XRD analysis followed by Reitveld refinement indicates the formation of phase pure samples in both cases but the barium hexaferrite obtained from acidic precursor solution has smaller crystallite sizes. The Transmission Electron Microscopy (TEM) analysis followed by High Resolution Transmission Electron Microscopy (HRTEM) confirms a lower particle size of ～20 nm for barium hexaferrite synthesised from acidic pH precursor solution. The shift in Raman peak (520-540 cm^?1) by 20 cm^?1, represents the whole structural block and further confirms the differences in the distribution of particle sizes due to the method of synthesis. The magnetic studies display a lower coercive field for the samples with smaller particle sizes. This is due to the crystalline size-induced microstrain that controls the magneto-crystalline anisotropy, shape anisotropy and stress anisotropy. The electromagnetic characterisation confirms broader absorption in the range of 8–18 GHz (X-band) with R_L ≤ ?7 db for the entire range for the samples with smaller particle sizes.     

Ball milling assisted hydrothermal synthesis of ZrO2 nanopowders

The formation of ZrO₂ nanopowders under various hydrothermal conditions such as temperature, time, autoclave rotation speed, heating rate and particularly assistance of ball milling during reaction was investigated. Full ZrO₂ formation (with monoclinic phase) from zirconium solution was completed at shorter times with increasing temperature such as after 4 h at 150 °C, 2 h at 175 °C and less than 2 h at 200 °C. Crystallite size increased from 2.9 to 4 nm with increasing reaction temperature from 125 °C to 200 °C, respectively. Ball milling assisted hydrothermal runs were performed to understand the effect of mechanical force on phase formation, crystallinity and particle size distribution. Monoclinic ZrO₂ was formed in both milled and non-milled runs when zirconium solution was used. Mean particle size for the 2 M solution was measured to be 94 nm for the milled and 117 nm for the non-milled powders. However, when amorphous aqueous zirconia gels (precipitated at pH 5.8) were used, tetragonal phase was also formed in addition to monoclinic phase. Mean particle size was measured to be 0.7 μm (d₉₀≅1.3 μm) for the milled and 7.9 μm (d₉₀≅13 μm) for the non-milled powders. Ball milling during hydrothermal reactions of both zirconium solution and aqueous zirconium gel resulted in smaller crystallite size and mean particle size and, at the same time, effectively controlled particle size distribution (or agglomeration) of nanopowders.     

Research and Development of the Coprecipitation Process for Lanthanum Germanate Oxyapatite

Although lanthanum germanate oxyapatite (La–Ge–O) has shown good potential for use as a solid electrolyte in energy storage applications, its synthesis has been challenging by either solid‐ or solution‐state methods. In this study, a new synthesis of La–Ge–O was developed through a coprecipitation technique, in which a highly concentrated homogeneous aqueous solution of La and Ge was prepared from aqueous ammonium germanate and lanthanum nitrate solutions with the addition of dilute nitric acid. Several precipitates were formed by pH manipulation, including an amorphous material obtained at pH > 3. Compared to the individual precipitation behaviors of the parent compounds, the amorphous precipitate was formed only from the aqueous two‐component mixture, and appeared to contain both metals. This material was transformed into crystalline mixtures upon heating at 1273 K. The crystalline phases were La₂Ge₃O₉ and hexagonal‐type GeO₂ when the precipitate was formed below pH 8, and the La–Ge–O and La₂Ge₂O₇ phases when the precipitate was formed around pH 8. Product formation from the coprecipitate was discussed based on X‐ray diffraction and thermal analyses. The improved availability of La–Ge–O will allow more extensive investigations of its useful properties.     

Densification and toughening mechanisms in spark plasma sintered ZrB2-based composites with zirconium and graphite additives

The densification behavior and toughening mechanisms of ZrB₂-based composites with in-situ formed ZrC were investigated. The composites were spark plasma sintered at 1700 °C for 7 min under the applied pressure of 40 MPa. Metallic zirconium and graphite flakes were used as precursors to achieve ZrC reinforcement. Microstructural and phase analyses as well as mechanical characterizations were carried out on the near fully-dense composite samples. Results indicated ZrC as the only secondary phase in composite with 5 vol% of metallic Zr and graphite flakes. However, higher volume fractions of precursor materials led to the formation of ZrO₂ as the dominant secondary phase. Whereas decreasing trend of the hardness number versus volume fraction of the precursors was observed, the highest indentation fracture toughness was achieved in sample with 15 vol% metallic Zr/graphite flakes. Finally, the formation of secondary phases and their effects on densification, and mechanical behavior of the composites were discussed.     

The Effects of pH and Alkaline Earth Ions on the Formation of Nanosized Zirconia Phases Under Hydrothermal Conditions

The phase composition and crystallite size of zirconia formed under hydrothermal conditions is strongly dependent on crystallization conditions, in particular, pH and a mineralizer. Monoclinic ZrO₂ formed easily in the strong acid and basic media. When pH<1 or >14, monoclinic ZrO₂ was exclusively obtained and its crystallite size increased with increasing pH. In the range of pH 1–14, products of hydrothermal reaction are a mixture of monoclinic and tetragonal ZrO₂. The effects of Mg²⁺, Ca²⁺ and Sr²⁺ ions on the formation of zirconia under hydrothermal conditions were investigated. The presence of these bivalent M²⁺ ions was in favour of the formation of tetragonal (or cubic) ZrO₂. The nanosized ZrO₂ crystallites of cubic and tetragonal symmetry were obtained at pH 10, 220°C for 2 h and in the case of Ca²⁺ and Sr²⁺ as mineralizer, respectively.     

<Emphasis Type="Italic">Phytolacca americana</Emphasis> from Contaminated and Noncontaminated Soils of South Korea: Effects of Elevated Temperature,CO<Subscript>2</Subscript> and Simulated Acid Rain on Plant Growth Response

Chemical analyses performed on the invasive weed Phytolacca americana (pokeweed) growing in industrially contaminated (Ulsan) and noncontaminated (Suwon) sites in South Korea indicated that the levels of phenolic compounds and various elements that include some heavy metals (Al, As, B, Cd, Co, Cu, Fe, Mn, Ni, Pb, and Zn) were statistically higher in Ulsan soils compared to Suwon soils with Al being the highest (>1,116 mg/l compared to 432 mg/l). Analysis of metals and nutrients (K, Na, Ca, Mg, Cl, NH₄, N, P, S) in plant tissues indicated that accumulation occurred dominantly in plant leaves with Al levels being 33.8 times higher in Ulsan plants (PaU) compared to Suwon plants (PaS). The ability of PaU and PaS to tolerate stress was evaluated under controlled conditions by varying atmospheric CO₂ and temperature and soil pH. When grown in pH 6.4 soils, the highest growth rate of PaU and PaS plants occurred at elevated (30°C) and non-elevated (25°C) temperatures, respectively. Both PaU and PaS plants showed the highest and lowest growth rates when exposed to atmospheric CO₂ levels of 360 and 650 ppm, respectively. The impact of soil pH (2–6.4) on seed germination rates, plant growth, chlorophyll content, and the accumulation of phenolics were measured to assess the effects of industrial pollution and global-warming-related stresses on plants. The highest seed germination rate and chlorophyll content occurred at pH 2.0 for both PaU and PaS plants. Increased pH from 2–5 correlated to increased phenolic compounds and decreased chlorophyll content. However, at pH 6.4, a marked decrease in phenolic compounds, was observed and chlorophyll content increased. These results suggest that although plants from Ulsan and Suwon sites are the same species, they differ in the ability to deal with various stresses.     

Steady-state fluorescence and NMR study on self-association behavior of poly(methacrylamides) bearing hydrophobic amino acid residues

Self-association properties of poly(N-methacryloylphenylalanine) and poly(N-methacryloyltryptophan) (pMPhe and pMTrp, respectively) were investigated by several characterization techniques, including steady-state fluorescence and NMR. These characterization data revealed similarities and distinctions of their self-association properties.The pH dependencies of association properties of pMPhe and pMTrp are practically the same. Apparent pK_a values for pMPhe and pMTrp were determined to be 5.7 and 5.8, respectively, by potentiometric titration. Steady-state fluorescence measurements at varying pH using pyrene as fluorescence probe indicated that, at pH≈5 (< apparent pH), hydrophobic microdomains were formed, while, at pH≈7 and 9 (> apparent pK_a), hydrophobic microdomains were not formed significantly. ¹H NMR spectra for both the polymers measured in D₂O exhibited that a significant fraction of aromatic rings in amino acid residues were located close to the polymer main chain, and that, at pH≈5, the mobility of the polymer main chain and the aromatic ring was extremely restricted.The polymer concentration (C_p) dependencies of association properties of pMPhe and pMTrp at pH≈5 are distinct. Steady-state fluorescence data at varying C_p indicated that pMPhe was more hydrophobic microscopically than pMTrp. Dynamic light scattering data indicated that pMTrp had a stronger tendency for interpolymer association than pMPhe did at pH≈5. It is concluded that the distinction in C_p dependency of the self-association properties of pMPhe and pMTrp is due to the differences in the bulkiness and the hydrophobicity of the substituents of amino acid residues.     

Electrochemical degradation of the Acid Blue 62 dye on a β-PbO<Subscript>2</Subscript> anode assessed by the response surface methodology

The electrochemical degradation of the anthraquinonic dye Acid Blue 62 in a filter-press reactor on a Ti/Pt/β-PbO₂ anode was investigated using the response surface methodology with the variables: current density, pH, [NaCl], and temperature. The system’s modeling was carried out with the charge required for 90% decolorization (Q ⁹⁰) and the chemical oxygen demand removal percentage after a 30 min electrolysis (COD ³⁰), with good correlations between predicted and observed values. Best conditions for decolorization were attained in acidic solutions (pH = 4) with medium to high [NaCl] (1.0–2.0 g L⁻¹) and lower temperature due to the prevalent oxidant species HOCl and Cl₂. Optimal conditions for COD ³⁰ removal were attained at high current densities in pH > 5 solutions with high [NaCl], when the prevalent oxidant species are HOCl and OCl⁻. The lowest charge per unit volume of the electrolyzed solution necessary for total mineralization was attained at pH 11.     

Hydraulic reactivity and cement formation of baghdadite

In this study, the hydraulic reactivity and cement formation of baghdadite (Ca₃ZrSi₂O₉) was investigated. The material was synthesized by sintering a mixture of CaCO₃, SiO₂, and ZrO₂ and then mechanically activated using a planetary mill. This leads to a decrease in particle and crystallite size and a partial amorphization of baghdadite as shown by X-ray powder diffraction (XRD) and laser diffraction measurements. Baghdadite cements were formed by the addition of water at a powder to liquid ratio of 2.0 g/ml. Maximum compressive strengths were found to be ~2 MPa after 3-day setting for a 24-h ground material. Inductively coupled plasma mass spectrometry (ICP-MS) measurements showed an incongruent dissolution profile of set cements with a preferred dissolution of calcium and only marginal release of zirconium ions. Cement formation occurs under alkaline conditions, whereas the unground raw powder leads to a pH of 11.9 during setting, while prolonged grinding increased pH values to approximately 12.3.     

The Impact of Water on CO Oxidation with Au/TiO<Subscript>2</Subscript> Catalysts: Poison or Promotor? A Study with an Au–TiO<Subscript>2</Subscript>/MCM-48 Model Catalyst

Abstract  

CO oxidation was studied with a model catalyst containing Au and TiO _x nanoaggregates confined in a siliceous MCM-48 host. With this material, which has a particular small ratio between the TiO _x and Au components, activities well comparable to those of unconfined Au/TiO₂ catalysts were obtained in particular when a thermal activation in inert gas at temperatures between 523 and 673 K was applied. When the subsequent catalytic tests were performed in a feed containing ca. 20 ppm H₂O, strong deactivation phenomena were observed which could be reverted by repeated thermal treatment and are most likely caused by carbonate deposition. This deactivation was strongly attenuated when the water content of the feed was decreased to ca. 6 ppm, which suggests that water plays an important role in the formation of the poisoning species. With unconfined Au/TiO₂ catalysts, a promoting role of water on the formation of catalyst poison was observed as well, but to a much lower extent. The effect may therefore have escaped undetected so far as a contribution to the well-known catalyst deactivation by carbonate species.     

Role of sodium hexametaphosphate in MgO/SiO2 cement pastes

The extent of reaction between magnesium oxide (MgO) and silica fume (SiO₂) is normally limited and mixes require high water contents to give suitable rheology. The use of considerably lower water contents and the formation of magnesium silicate hydrate (M-S-H) gel as a binding phase is made possible by adding sodium hexametaphosphate (Na-HMP) to the mix water prior to the addition of MgO and SiO₂. This results in the formation of extensive reaction products and cured samples with high compressive strength and low porosity. In this work, the effect of Na-HMP on the hydration of MgO/SiO₂ mixes is investigated using high water to solids ratio samples to allow monitoring of pH and the solution chemistry during hydration. It is shown that a relatively small amount of Na-HMP inhibits the formation of Mg(OH)₂ when MgO is hydrolyzed. It is proposed that this is due to adsorption of phosphate species on the MgO which inhibits the nucleation of the Mg(OH)₂. This gives rise to high Mg^2 + species in solution and elevated pH (> 12) conditions relative to when Mg(OH)₂ forms. In contrast, the phosphate does not suppress formation of M-S-H gel. In combination with the enhanced dissolution rate of SiO₂ at high pH, M-S-H gel can form quickly without competition for Mg^2 + ions by Mg(OH)₂ precipitation. Incorporating the optimum concentration of Na-HMP into the mix water therefore transforms the properties of cement paste and mortar samples formed by reacting MgO and SiO₂.     

Characterization of Poly(Aminosiloxane) Surface Chemistry on Aluminum Oxide

N-2-aminoethyl-3-aminopropyltrimethoxysilane (AAPS) was shown to react with aluminum oxide powder to form an amine/carbonate salt, as observed by diffuse reflectance infrared spectroscopy (DRIFT) and thermogravimetric analysis with mass spectroscopy (TGA-MS). TGA-MS, together with electron spectroscopy for chemical analysis (ESCA), reveal that the stoichiometric ratio of amine salt to free amine is higher on the surface of aluminum oxide powder than in a comparable neat film. In addition, TGA-MS shows that a nonstoichiometric ratio of CO₂H₂O is evolved upon heating the surface-treated powder (4.5/1), whereas the neat film evolves CO₂/H₂O at a ratio near unity. The high fraction of protonated amines, together with the higher ratio of CO₂/H₂O in the presence of alumina, is consistent with a proposed bonding mechanism which involves carbonate bridging between protonated amines and hydroxyl sites on the aluminum oxide surface.     

Solvent effect on synthesis of zirconia support for tungstated zirconia catalysts

Solvothermal reaction of zirconium n-butoxide (ZNB) in different solvent media, such as 1,3-pentanediol, 1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol resulted in the formation of zirconium dioxide (ZrO₂) nanostructure. Then, the 15%W/ZrO₂ (WZ) catalysts using different zirconia supports were prepared by impregnation method. The effects of solvent on preparation of zirconia on the catalytic performance of WZ catalysts in esterification of acetic acid and methanol at 60 °C were investigated. The experimental results showed that ZrO₂ particles prepared in 1,4-butanediol (ZrO₂-BG) have a spherical shape, while in other glycols the samples were irregularly-shaped particles. The reaction results of esterification illustrated that the W/ZrO₂-BG catalysts had high surface acidity and showed high acetic acid conversion. The W/ZrO₂-PeG catalysts (ZrO₂ particles prepared in 1,5-pentanediol, PeG) exhibited the lowest surface acidity among other samples due to strong interaction of proton species and the zirconia supports as proven by TGA. One of the possible reasons can be attributed to different amounts of carbon residue on the surface of catalysts.     

The deposition of coke from methane on a Ni/MgAl2O4 catalyst

Temperature-programmed reaction techniques and Raman spectroscopy were used to characterize coke species deposited on a 5% Ni/MgAl₂O₄ catalyst for dry reforming of methane. The CH₄ temperature-programmed decomposition profiles showed that the ignited decomposition temperatures of CH₄ increased from 273 to 378 °C when MgAl₂O₄ replaced the catalyst support γ-Al₂O₃. The temperature-programmed oxidation, temperature-programmed hydrogenation and temperature-programmed CO₂ reaction profiles showed that there were three carbon species (i.e. C_α, C_β and C_γ) on the catalyst surface. Raman spectroscopy showed that C_γ was graphite-like carbon species, which was responsible for catalyst deactivation. The C_γ species was the most inactive species toward H₂ and O₂, while it was unexpectedly more active toward CO₂. The unique reactivity of CO₂ with different coke specie could be ascribed to the carbonate, bidentate and formate species formation on MgAl₂O₄ surface. These surface species enhanced the oxidation of C_γ species and thus contributed to the high stability of Ni/MgAl₂O₄ catalyst.