Mechanism of formation of the complex oxide Na<Subscript>2</Subscript>Nd<Subscript>2</Subscript>Ti<Subscript>3</Subscript>O<Subscript>10</Subscript>

The processes of phase formation in the Nd₂O₃-TiO₂-Na₂CO₃ system have been investigated in the temperature range 500–1100°C. The mechanism of the high-temperature solid-phase reaction of formation of the complex oxide Na₂Nd₂Ti₃O₁₀ has been studied. It has been established that the Na₂Nd₂Ti₃O₁₀ compound is formed from the intermediate product Na_0.5Nd_0.5TiO₃ with a perovskite structure in the temperature range 830–890°C and from the NaNdTiO₄ oxide with a perovskite-like layered structure in the temperature range 960–1100°C.     

Kinetics and mechanism of the formation of hollandites in the BaO(Cs<Subscript>2</Subscript>O)-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript> system from initial mixtures prepared by different methods

The mechanism and kinetics of formation of solid solutions based on hollandite in the BaO(Cs₂O)-Al₂O₃-TiO₂ system are investigated using the initial mixtures prepared by two methods: (i) mechanical grinding and mixing of the initial components and (ii) coprecipitation from aqueous solutions of the salts. It is established that the mechanism of formation of hollandite in the system under investigation depends on the degree of dispersion of the initial mixtures used in the synthesis. When the synthesis is performed with the initial mixture prepared by mechanical grinding and mixing of the initial reactants (the particle size is equal to 50–300 nm), hollandite is formed at temperatures in the range 1100–1250°C in the presence of the accompanying phase Cs₂Al₂Ti₂O₈. When the synthesis is performed with the initial mixture prepared by coprecipitation from aqueous solutions of salts (the particle size is equal to 10–12 nm), hollandite is formed at temperatures in the range 850–1050°C. The investigation into the kinetics of formation of the hollandite phase from the above mixtures made it possible to determine the temperature-time conditions for the synthesis of this titanate in the form of a powder with a particle size of approximately 50 nm or in the form of a dense ceramic material with a particle size of ～200 nm.     

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: I. Influence of temperature on the local structure of glasses and melts

The local structure of glasses and melts in the Na₂O-B₂O₃ system in the composition range 10–30 mol % Na₂O is investigated by high-temperature Raman spectroscopy in the temperature range 20–1150°C. The recorded spectra are analyzed by simulating the experimental spectra in the form of a superposition of the set of Gaussian lines. The results of the simulation are used to determine the concentrations of the main structural units of borates as a function of the composition and temperature. The dynamic equilibrium of the interaction between the structural units in melts has the form

$B_{4/2}^ - \rightleftarrows B_{2/2} O^ - $

and shifts toward the right, i.e., toward the formation of BØ₂O^? metaborate triangles (where Ø is a bridging oxygen atom), with an increase in temperature. The enthalpy of the disproportionation process depends on the melt composition and increases with an increase in the oxide modifier concentration.

     

A Novel Process for Renewable Methane Production: Combining Direct Air Capture by K<Subscript>2</Subscript>CO<Subscript>3</Subscript>/Alumina Sorbent with CO<Subscript>2</Subscript> Methanation over Ru/Alumina Catalyst

CO₂ methanation over supported ruthenium catalysts is considered to be a promising process for carbon capture and utilization and power-to-gas technologies. In this work 4% Ru/Al₂O₃ catalyst was synthesized by impregnation of the support with an aqueous solution of Ru(OH)Cl₃, followed by liquid phase reduction using NaBH₄ and gas phase activation using the stoichiometric mixture of CO₂ and H₂ (1:4). Kinetics of CO₂ methanation reaction over the Ru/Al₂O₃ catalyst was studied in a perfectly mixed reactor at temperatures from 200 to 300 °C. The results showed that dependence of the specific activity of the catalyst on temperature followed the Arrhenius law. CO₂ conversion to methane was shown to depend on temperature, water vapor pressure and CO₂:H₂ ratio in the gas mixture. The Ru/Al₂O₃ catalyst was later tested together with the K₂CO₃/Al₂O₃ composite sorbent in the novel direct air capture/methanation process, which combined in one reactor consecutive steps of CO₂ adsorption from the air at room temperature and CO₂ desorption/methanation in H₂ flow at 300 or 350 °C. It was demonstrated that the amount of desorbed CO₂ was practically the same for both temperatures used, while the total conversion of carbon dioxide to methane was 94.2–94.6% at 300 °C and 96.1–96.5% at 350 °C.     

Synthesis and thermal behavior of Fe<Subscript>3</Subscript>O<Subscript>2</Subscript>(BO<Subscript>4</Subscript>) oxoborate

Iron oxoborate Fe₃O₂(BO₄) has been first produced in solid-phase chemical reactions. Its thermal behavior in the temperature range 20–900°C is studied with the use in-situ high-temperature powder X-ray diffraction. It is shown that Fe₃O₂(BO₄) begins decomposing with the formation of Fe₂O₃ in the temperature range 660–900°C. Thermal expansion is sharply anisotropic at room temperature (α_max/α_min = 7) and becomes more isotropic with an increase in the temperature (α_max/α_min = 1.2). The degree of oxidation of Fe³⁺ has been confirmed by Mössbauer spectroscopy (at a room temperature), and two nonequivalent positions in the structure have been detected, which are occupied by iron atoms with the octahedral environment of the oxygen atoms.     

Thermal behavior of reedmergnerite NaBSi<Subscript>3</Subscript>O<Subscript>8</Subscript> and searlesite NaBSi<Subscript>2</Subscript>O<Subscript>5</Subscript>(OH)<Subscript>2</Subscript>

The results of studying the thermal behavior of natural boron albite–reedmergnerite NaBSi₃O₈ and aqueous boricilicate–searlesite NaBSi₂O₅(OH)₂ obtained by the method of hydrothermal synthesis are presented. In the investigated temperature range, reedmergnerite (30–780°C) does not undergo phase transformations. Thermal expansion is sharply anisotropic, which is a characteristic feature of the expansion of feldspars. Aqueous borosilicate searlesite (25–750°C) is amorphized above 400°C as a result of dehydration and cristobalite crystallizes from the amorphous phase. The expansion of searlesite is relatively isotropic.     

Synthesis and properties of binary spinels in a NiO–CuO–Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>–Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> system

The conditions for the formation of a spinel structure from a NiO–CuO–Fe₂O₃–Cr₂O₃ oxide mixture using several technological approaches have been examined. Addition of KCl is accompanied with the formation of two spinel-like phases, whereas in the absence of KCl just one solid solution of nickel–copper ferrite–chromite with the structure of a cubic spinel is formed. At the temperature of thermal treatment of 900°C, the presence of an admixture phase of the delafossite (CuCrO₂) type was established. The conditions for the fabrication of samples containing two spinel phases (cubic and tetragonal) characterized with the most developed surface and manifesting = increased catalytic activity in the reaction of the decomposition of an organic substance by hydrogen peroxide have been formulated. The studied features of spinel synthesis can be of interest for developing materials with an active surface promising for application as adsorbents of catalysts and sensors.     

Molten salt synthesis of La2Zr2O7 ultrafine powders

Ultrafine powders of pyrochlore-type La₂Zr₂O₇ were synthesized via a simple molten salt mediated process using zirconium oxide and lanthanum oxide as raw materials, and sodium chloride, potassium chloride and sodium fluoride to form a reaction medium. The effects of reaction temperature, salt/reactant ratio and salt type on the La₂Zr₂O₇ formation were investigated. Among the three attempted salt assemblies (KCl–LiCl, Na₂CO₃–K₂CO₃, and NaCl–KCl–NaF), NaCl–KCl–NaF showed the best accelerating effect on the La₂Zr₂O₇ formation. At a given temperature, the La₂Zr₂O₇ content in the final products increased with the increase in the salt amount. Phase pure submicron sized La₂Zr₂O₇ ultrafine powders were obtained after 3 h firing at 1100 °C with the salt/reactant weight ratio of 5:1 or at 1200 °C with salt/reactant weight ratio of 3:1. The synthesis temperature (1100 °C) was much lower than that required by the conventional solid-state mixing method or a wet chemical method. The “dissolution–precipitation” mechanism had dominated the synthesis process.     

Adsorption of CO<Subscript>2</Subscript> by iron-exchanged heteropolyacid Fe<Subscript>1.5</Subscript>PMo<Subscript>12</Subscript>O<Subscript>40</Subscript> supported on mesoporous cellular foams

Novel low-temperature swing adsorbents that preferably adsorb CO₂ were synthesized by varying loading of heteropolyacid Fe_1.5PMo₁₂O₄₀ (Fe–PMA) supporting on mesoporous cellular foams (MCFs) by wetting impregnation. The synthesized materials were characterized by various physicochemical, thermal and spectral techniques and the CO₂ adsorption capacity of the materials were evaluated. Solid adsorbents showed a significantly high adsorption capacity toward CO₂ due to the chemisorptions of CO₂. The CO₂ adsorption capacities of the materials decreased as the temperature increased. The results showed that the adsorption capacity reached a level of 81.8 mg CO₂/g-adsorbent at 25 °C for the 20 wt% Fe–PMA–MCFs. These results indicated that the iron (Fe²⁺) complexes acted as efficient catalysts for the separation of CO₂. The as-synthesized adsorbents were selective, thermally stable, long-lived, and could be recycled at a temperature of 110 °C.     

Formation of ultradispersed systems based on sodium polyaluminates from thermal plasma flows

The process of synthesis of sodium β-alumina in a low-temperature nitric plasma is investigated. It is established that the main product of the plasmachemical reaction is sodium polyaluminate Na₂O · nAl₂O₃ of an unknown structure, whose homogeneity range is within the interval of n = 3.5–8.0. The specifics of phase transformations in heating products of plasmachemical synthesis in the temperature interval of 600–1400°C are considered. The formation of the phase with the β-alumina structure (β-and β-Al₂O₃) proceeds in several stages with the formation of intermediate metastable sodium polyaluminates.     

Effect of calcination temperature on catalyst reducibility and hydrogenation reactivity in rice husk ash–alumina supported nickel systems

Nickel catalysts supported on rice husk ash–alumina (Ni/RHA–Al₂O₃) were prepared by an incipient wetness impregnation method. Characterization included TGA, DSC, TPR, XRD, and BET. Results show that the decomposition of the nickel compound to nickel oxide was complete above 500 °C. The TPR analysis revealed a strong interaction between nickel and support, and a decrease in reducibility of NiO with increasing calcination temperature. The XRD analysis of Ni/RHA–Al₂O₃ catalyst precursors demonstrated the presence of spinel. It also showed that the size of crystallites in the supported NiO first decreased with increase in calcination temperature up to 700 °C, and then increased due to phase transformation of nickel oxide to spinel. The pores are mesopores and their meshy surface structure was not affected by calcination temperature in the range investigated. The catalytic activity was tested by CO₂ hydrogenation with an H₂/CO₂ ratio of 4/1 at 500 °C. The CO₂ conversion and CH₄ yield for CO₂ hydrogenation over 15 wt% Ni/RHA–Al₂O₃ catalyst were almost independent of calcination and reduction temperatures. Copyright © 2004 Society of Chemical Industry     

Electrochemical promotion of CO<Subscript>2</Subscript> hydrogenation on Rh/YSZ electrodes

The electrochemical promotion of the CO₂ hydrogenation reaction on porous Rh catalyst–electrodes deposited on Y₂O₃-stabilized-ZrO₂ (or YSZ), an O²⁻ conductor, was investigated under atmospheric total pressure and at temperatures 346–477 °C, combined with kinetic measurements in the temperature range 328–391 °C. Under these conditions CO₂ was transformed to CH₄ and CO. The CH₄ formation rate increased by up to 2.7 times with increasing Rh catalyst potential (electrophobic behavior) while the CO formation rate was increased by up to 1.7 times with decreasing catalyst potential (electrophilic behavior). The observed rate changes were non-faradaic, exceeding the corresponding pumping rate of oxygen ions by up to approximately 210 and 125 times for the CH₄ and CO formation reactions, respectively. The observed electrochemical promotion behavior is attributed to the induced, with increasing catalyst potential, preferential formation on the Rh surface of electron donor hydrogenated carbonylic species leading to formation of CH₄ and to the decreasing coverage of more electron acceptor carbonylic species resulting in CO formation.     

Investigation of the mechanism of formation of BaTi<Subscript>4</Subscript>O<Subscript>9</Subscript> from initial mixtures of different dispersion

The mechanism of solid-phase interaction in synthesis of BaTi₄O₉ is investigated using initial mixtures characterized by different degrees of dispersion and prepared by three methods: (i) mechanical mixing and grinding of initial components, (ii) coprecipitation from aqueous solutions of salts, and (iii) the citrate-nitrate sol-gel technique. The use of initial mixtures consisting of nanoparticles in the synthesis makes it possible to decrease the sintering temperature by 100–300°C, which ensures the preparation of single-phase samples, and to reduce several times the heat treatment time. Dilatometric investigations of the sintering process in the synthesis of BaTi₄O₉ from the initial mixtures under study indicate that the maximum change in the linear sizes of samples occurs in the temperature range 1000–1250°C. It is shown that, in this temperature range, the concurrently formed impurity phases undergo decomposition and the growth rate of BaTi₄O₉ crystals increases. The optimum conditions for synthesis of BaTi₄O₉ in the form of a powder with a particle size of approximately 77 nm and in the form of a ceramic material are determined, which is necessary for use of this compound as different functional materials.     

Electrical properties of lead-free 0.98(Na<Subscript>0.5</Subscript>K<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript>-0.02Ba(Zr<Subscript>0.52</Subscript>Ti<Subscript>0.48</Subscript>)O<Subscript>3</Subscript> piezoelectric ceramics by optimizing sintering temperature

Lead-free 0.98(Na_0.5K_0.5)NbO₃-0.02Ba(Zr_0.52Ti_0.48)O₃ [0.98NKN-0.02BZT] ceramics were fabricated by the conventional mixed oxide method with sintering temperature at 1,080°C to 1,120°C. The results indicate that the sintering temperature obviously influences the structural and electrical properties of the sample. For the 0.98NKN-0.02BZT ceramics sintered at 1,080°C to 1,120°C, the bulk density increased with increasing sintering temperature and showed a maximum value at a sintering temperature of 1,090°C. The dielectric constant, piezoelectric constant [d ₃₃], electromechanical coupling coefficient [k _p], and remnant polarization [P _r] increased with increasing sintering temperature, which might be related to the increase in the relative density. However, the samples would be deteriorated when they are sintered above the optimum temperature. High piezoelectric properties of d ₃₃ = 217 pC/N, k _p = 41%, dielectric constant = 1,951, and ferroelectric properties of P _r = 10.3 μC/cm² were obtained for the 0.98NKN-0.02BZT ceramics sintered at 1,090°C for 4 h.     

Low temperature synthesis of plate-like Na0.5Bi0.5TiO3 via molten salt method

In this study, plate-like Na_0.5Bi_0.5TiO₃ (BNT) templates with perovskite structure were obtained by two-step molten salt synthesis (MSS) method at a low temperature. Firstly, Bi₄Ti₃O₁₂ precursors were synthesized at 1030 °C in NaCl–KCl molten salt. Secondly, plate-like Na_0.5Bi_0.5TiO₃ particles with perovskite structure were obtained from plate-like layer-structured ferroelectric ceramic of Bi₄Ti₃O₁₂ by topochemical microcrystal conversion method. Result showed that excessive Na₂CO₃ was beneficial to facilitate the low temperature synthesis. In the case of an excess of 30 mol% Na₂CO₃, plate-like BNT particles could be obtained by synthesis at temperatures ranging from 760 °C to 800 °C, which indicated a flexible processing route. Also, it has been observed that plate-like BNT particles show a high aspect ratio with 1 μm in thickness and 10–20 μm in length. These Na_0.5Bi_0.5TiO₃ plate-like particles can be good candidates for the preparation of lead-free BNT-based piezoelectric ceramics with oriented grain microstructure.     

Oxychlorination of methane over FeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/CeO<Subscript>2</Subscript> catalysts

Methane activation through oxychlorination is in the spotlight due to the relatively mild reaction conditions at atmospheric pressure and in the temperature range of 450–550 °C. Although CO₂ is known to exhibit good activity for methane oxychlorination, significant amounts of by-products such as CO₂, CO and carbon deposits are produced during the reaction over CO₂. We investigated the effect of iron in FeO_x/CO₂ catalysts on methane oxychlorination. FeO_x/CO₂ with 3 wt% iron shows the maximum yield at 510 °C with 23% conversion of methane and 65% selectivity of chloromethane. XRD and H₂ TPR results indicate that iron-cerium solid solution was formed, resulting in the production of more easily reduced cerium oxide and the suppression of catalysts sintering during the reaction. Furthermore, the selectivity of by-products decreased more significantly over FeO_x/CO₂ than cerium oxide, which can be attributed to the facilitation of HCl oxidation arising from the enhanced reducibility of the former sample.     

Effects of alkali-metal carbonates and nitrates on the CO<Subscript>2</Subscript> sorption and regeneration of MgO-based sorbents at intermediate temperatures

The effects of alkali-metal carbonates and nitrates on the CO₂ sorption and regeneration of MgO-based sorbents were investigated in the presence of 10 vol% CO₂ and 10 vol% H₂O in an intermediate temperature range, 300 to 450 °C. The CO₂ capture capacities of the MgO-based sorbents promoted with Na₂CO₃ and K₂CO₃ were 9.7 and 45.0 mg CO₂/g sorbent, respectively. On the other hand, a MgO-based sorbent promoted with both Na₂CO₃ and NaNO₃ exhibited the highest CO₂ capture capacity of 97.4mg CO₂/g sorbent at 200 °C in 10 vol% CO₂, which was almost ten-times greater than that of the MgO-based sorbent promoted with Na₂CO₃. The CO₂ sorption rate of these sorbents was higher than that of the MgO-based sorbents promoted with alkali-metal nitrates due to the formation of Na₂Mg(CO₃)₂ or K₂Mg(CO₃)₂ by the alkali-metal carbonate and the eutectic reaction of the alkali-metal nitrates. In addition, the reproducibility problem of double-salt sorbents obtained by the precipitation method was completely resolved by impregnating MgO with alkali-metal carbonates and nitrates. Furthermore, we found that their desorption temperatures are lower than those of the MgO-based sorbents promoted with alkali-metal carbonates due to the eutectic reaction during the regeneration process.     

Influence of microwave and ultrasonic treatment on the formation of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> under hydrothermal conditions

The formation of CoFe₂O₄ nanocrystals under hydrothermal conditions at a temperature of 130°C is investigated. The average size of CoFe₂O₄ particles varies from 6 to 11 nm depending on the synthesis time. The hydrothermal medium is heated by two different methods, i.e., the microwave (with a synthesis time from 1 min to 2 h) and conventional (with a synthesis time from 30 min to 45 h) methods. It is demonstrated that the use of microwave heating considerably accelerates the formation of CoFe₂O₄ particles. Preliminary ultrasonic treatment for 3 min increases the phase formation rate in the case of microwave heating and hardly affects the occurrence of the process upon conventional heating. It is revealed that the exposure of the initial mixture (preliminarily treated with ultrasound) to room temperature for 2 h or longer almost completely reduces the efficiency of the action of ultrasonic treatment on the phase formation process under hydrothermal conditions.     

Influence of Calcination Temperature on Activity and Selectivity of Ni–CeO<Subscript>2</Subscript> and Ni–Ce<Subscript>0.8</Subscript>Zr<Subscript>0.2</Subscript>O<Subscript>2</Subscript> Catalysts for CO<Subscript>2</Subscript> Methanation

Herein, we studied the influence of calcination temperature (500–800 °C) of Ni/CeO₂ and Ni/Ce_0.8Zr_0.2O₂ catalysts on the specific surface area, pore volume, crystalline size, lattice parameter, chemical bonding and oxidation states, nickel dispersion and CH₄/CO production rate in CO₂ methanation. In general, the catalytic performance revealed that Zr doping catalysts could increase the CH₄ production rate. Combined with the production rate and the characterizations results, we found that the combination of nickel dispersion, peak area of CO₂–TPD and O_II/(O_II + O_I)) play the critical role in increasing the CH₄ production rate. It is well to be mentioned that the CO production rate is strongly influenced by the nickel dispersion. Furthermore, the in-situ DRIFTS confirmed that the CO originates from the decomposition of H-assisted formate species.     

Glass formation and electrical conductivity of glasses in the Na<Subscript>2</Subscript>Se-P<Subscript>2</Subscript>Se<Subscript>5</Subscript> system in a wide temperature range

The glass formation region in the Na₂Se-P₂Se₅ system and the temperature-concentration dependences of the electrical conductivity of glasses have been investigated over a wide range of temperatures. The densities and glass transition temperatures T _g of glasses have been determined. A comparison of the electrical conductivity of glasses in the Na₂Se-P₂Se₅ and Na₂O-P₂O₅ systems has demonstrated that the conductivity of selenium-containing glasses (at 25°C) is approximately three orders of magnitude higher than the electrical conductivity of oxide glasses. The assumption has been made that an increase in the electrical conductivity of glasses with selenium is caused by the increase in the degree of dissociation of Na⁺[Se⁻PSe_3/2] polar structural chemical units and the higher mobility of sodium ions in the oxygen-free matrix.