Mechanosorption of carbon dioxide by Ca- and Mg-containing silicates and alumosilicates: Kinetic regularities and correlations with the dissolution of CO<Subscript>2</Subscript> in silicate melts

The kinetics of the deep mechanosorption of carbon dioxide by natural and synthetic silicates and aluminosilicates (labradorite (CaAl₂Si₂O₈)_0.562(NaAlSi₃O₈)_0.438, oligoclase (CaAl₂Si₂O₈)_0.148(NaAlSi₃O₈)_0.852, diopside CaMgSi₂O₆, akermanite Ca₂MgSi₂O₇, gehlenite Ca₂Al₂SiO₇, and wollastonite CaSiO₃ in the course of mechanochemical activation in an AGO-2 centrifugal planetary mill at CO₂ pressure of 10⁵ Pa is studied. Equations are proposed that make it possible to calculate coefficients of mechanosorption that characterize the ability of CO₂ molecules to penetrate into the disordered silicate matrix of minerals under intense mechanical actions with the formation of carbonate ions. Solubilities and diffusion coefficients of CO₂ at temperature of 1900 K and pressure of 1500 MPa in silicate melts, the compositions of which coincide with those of studied materials, are calculated using published data. Correlations are revealed between the degree of silicate carbonization upon mechanochemical activation in carbon dioxide and the solubility of carbon dioxide in silicate melts with analogous compositions, as well as between coefficients of mechanosorption and diffusion of CO₂ in melts.     

Mechanochemical interaction of alkali metal metasilicates with carbon dioxide: 1. Absorption of CO<Subscript>2</Subscript> and phase formation

Processes proceeding during the mechanochemical activation of alkali metal metasilicates M₂SiO₃ (where M is Li, Na, K) are studied in the air and in an atmosphere of carbon dioxide. At the initial stage of activation in a centrifugal planetary mill in an atmosphere of carbon dioxide, the main portion of supplied mechanical energy is expended for grinding and the mechanosorption of CO₂ occurs in the regime of cleavage, i.e., on the freshly formed surfaces of particles. As the time of activation increases, the specific surface area becomes constant, which, however, does not substantially affect the rate of interaction between carbon dioxide and silicates. The absorption of CO₂ occurs in the regime of friction on the active sites of already formed surfaces and is accompanied by the tribodiffusion of gas molecules into structurally disordered layers of particles. With identical amounts of supplied energy, the CO₂/M₂SiO₃ molar ratio in the samples activated in the medium of carbon dioxide increases in the Li < Na < K series. The main product of mechanically induced interactions between Li₂SiO₃ and CO₂ is the X-ray amorphous carbonate-silicate phase. In the case of sodium and potassium metasilicates, the reaction of mechanochemical substitution occurs to form corresponding carbonates, hydrocarbonates, and amorphous silica. It is shown that the character of mechanochemical interaction between M₂SiO₃ and CO₂ depends on the change in the Gibbs energy of the transformation of silicate into corresponding carbonate, as well as on the melting temperature and the hygroscopicity of silicate.     

The mechanosorption of carbon dioxide by magnesium, calcium, and strontium metasilicates

Processes that occurred in the mechanical activation of MSiO₃ silicates (M = Mg, Ca, Sr) in a centrifugal planetary mill in the atmosphere of carbon dioxide (p(CO₂) = 10⁵ Pa) were comparatively studied. The interaction of carbon dioxide with silicates had the character of deep mechanosorption with massed penetration of gas molecules into the volume of particles and their “solution” in structurally disordered silicate matrices in the form of distorted CO₃²⁻ ions. No individual carbonate phases were formed. The absorbing ability of silicates with respect to carbon dioxide decreased in the series SrSiO₃ > CaSiO₃ > MgSiO₃. The carbon dioxide mechanosorption coefficients K _MS were calculated for Mg, Ca, and Sr silicates according to the earlier suggested kinetic model. A linear dependence of logK _MS on Δ_r G ₂₉₈^o for the transformation of silicates into the corresponding carbonates was observed.     

Kinetics of carbon dioxide mechanosorption by Ca-containing silicates

Kinetics of mechanically induced CO₂ extensive sorption by silicate minerals (labradorite, diopside, okermanite, ghelenite and wollastonite) was considered. Mechanical activation of the silicates was carried out in a planetary mill in CO₂ at atmospheric pressure. Carbon dioxide was consumed by the silicates in the form of carbonate ions and its content in the samples after 30 min of mechanical treatment reached 3–12 mass% CO₂ depending on mineral composition. Equations that reasonably good represent kinetics of CO₂ mechanosorption by silicates were proposed. These equations enable to calculate mechanosorption coefficients that characterize the diffusivity of CO₂ into disordered silicate matrix under intensive mechanical impact. Thermal analysis of the mechanically activated silicates showed that there was positive correlation between temperature of complete carbonate decomposition and mechanosorption coefficient.     

Single-phased white-light emitting CaAl<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>8</Subscript>: Eu<Superscript>2+</Superscript>, Mn<Superscript>2+</Superscript> phosphors prepared by a sol–gel method

CaAl₂Si₂O₈: Eu²⁺, Mn²⁺ phosphors have been prepared by a sol–gel method. X-ray diffractometer, spectrofluorometer and UV–Vis spectrometer were used to characterize structural and optical properties of the samples. The results indicate that anorthite (CaAl₂Si₂O₈) directly crystallizes at 1000 °C in the sol–gel process. CaAl₂Si₂O₈: Eu²⁺, Mn²⁺ phosphors show two emission bands excited by ultraviolet light. Blue (around 415 nm) and yellow (around 575 nm) emissions originate from Eu²⁺ and Mn²⁺, respectively. With appropriate tuning of Mn²⁺ content, CaAl₂Si₂O₈: Eu²⁺, Mn²⁺ phosphors exhibit different hues and relative color temperatures.     

Photocatalysis enhancement of CaAl<Subscript>2</Subscript>O<Subscript>4</Subscript>:Eu<Superscript>2+</Superscript>, Nd<Superscript>3+</Superscript>@TiO<Subscript>2</Subscript> composite powders

CaAl₂O₄:Eu²⁺, Nd³⁺@TiO₂ composite powders were synthesized by a sol–gel method under mild conditions (i.e. low temperature and ambient pressure). The as-prepared powders were characterized by transmission electron microscopy (TEM) and analyzed by X-ray diffraction (XRD). The photocatalytic behavior of the TiO₂-base surfaces was evaluated by the degradation of nitrogen monoxide gas. It suggested that CaAl₂O₄:Eu²⁺, Nd³⁺@TiO₂ composite powders were composed of anatase titania and that CaAl₂O₄:Eu²⁺, Nd³⁺. TiO₂ particles were deposited on the surface of CaAl₂O₄:Eu²⁺, Nd³⁺ to form uniform film. CaAl₂O₄:Eu²⁺, Nd³⁺@TiO₂ composite powders exhibited higher photocatalytic activity compared with pure TiO₂ under visible light. And the result also clearly indicated that the long afterglow phosphor absorbed and stored lights for the TiO₂ to remain photocatalytic activity in the dark.     

Chemistry of the CARBEX process: Identification of absorption bands of the ligands in the electronic spectra of aqueous solutions of Na<Subscript>4</Subscript>[UO<Subscript>2</Subscript>(O<Subscript>2</Subscript>)CO<Subscript>3</Subscript>)<Subscript>2</Subscript>]

On the basis of consideration of dissociation, hydration, association, and ligand exchange, the assignment of absorption bands in the electronic spectra of aqueous solutions of the Na₄[UO₂(O₂)CO₃)₂] complex has been performed. It has been demonstrated that the absorption in the range 190–400 nm is caused by the oxygen atoms of the O₂^2- and CO₃^2- groups and hydration water molecules of dissociated and neutral complex species Na₃[UO₂(O₂)(CO₃)₂]^–, Na₂[UO₂(O₂)(CO₃)₂]^2–, and Na₄[UO₂(O₂)(CO₃)₂].     

Untersuchungen zum Existenzgebiet des CaAl2Si2-Strukturtyps bei ternären Siliciden

Investigations about the Stability Range of the CaAl₂Si₂ Type Structure in the Case of Ternary Silicides Five compounds LnAl₂Si₂ (Ln: trivalent rare-earth metal, Y) were synthesized by heating the elements at 800°–1000 °C. They are isotypic and crystallize in the CaAl₂Si₂ type structure (P 3 m1; Z = 1) (lattice constants see “Inhaltsübersicht”). The electronic structures (LMTO band structure calculations) of CaAl₂Si₂ and YAl₂Si₂, the latter one is in accordance to Ln³⁺(Al³⁺)₂(Si^4–)₂ not electrovalent, are discussed with regard to the bondings and the electrical conductivity respectively. Investigations of GdAl_2–xMn_xSi₂ mixed crystals showed, that the structure type already at low Mn content (x ≈ 0,3) changes from CaAl₂Si₂ (GdAl₂Si₂) to ThCr₂Si₂ type structure (GdMn₂Si₂).     

Über Chloridsilicate des Calciums,Strontiums und Bariums

On Chloride Silicates of Calcium, Strontium, and Barium Alkaline earth chloride silicates are synthesized by heating of mixtures of CaCO₃, SrCO₃ or BaCO₃, and SiO₂ with different molar ratios in the melt of the relating alkaline earth chloride. They can be separated from an excess of chloride by methanol extraction. The structures of their silicate anions were investigated by the molybdate method and paper chromatographic separation. The compounds Sr₅[SiO₄]Cl₆, Sr₅[Si₂O₇]Cl₄, and Sr₈[Si₄O₁₂]Cl₈, hitherto unknown, were synthesized and compared with the compounds of calcium and barium in regard of their chloride contents, silicate anion structure and crystal structure.     

Zum Anionenaufbau von Tetra-n-butylammoniumsilicaten und ihren wäßrigen Lösungen

On the Anion Constitutions of Tetrabutylammonium Silicates and their Aqueous Solutions The anion distribution of tetra-n-butylammonium-(TBA)-silicate solutions with molar TBA/SiO₂ ratios between 0.6 and 4 and silica concentrations between 0.1 M and 2.2 M has been investigated by trimethylsilylation and ²⁹Si NMR techniques. In contrast to concentrated tetramethylammonium- and tetraethylammonium silicate solutions in TBA silicate solutions a preference of double ring silicate anions does not occur. In TBA silicate solutions a broad distribution of silicate anions consisting of monomeric, oligomeric chain and ring, as well as polymeric silicate anions has been observed. Crystalline TBA silicates with TBA/SiO₂ ratios of 0.78 to 1 contain mainly double five-ring silicate anions Si₁₀O₂₅^10? whereas for TBA/SiO₂ ratios higher than 1.4 the double three-ring anion Si₆O₁₅^6? predominates. A recently prepared TBA silicate with low TBA content (TBA/SiO₂ = 0.23) has been found to consist of double four-ring silicate anions with 6 SiOH groups per double four-ring.     

The synthesis and thermochemical study of (Mg,Fe)<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>5</Subscript>(OH)<Subscript>4</Subscript> nanotubes

Nanotubular (Mg,Fe²⁺,Fe³⁺)₃Si₂O₅(OH)₄ hydrosilicates with a chrysotile structure were synthesized under hydrothermal conditions. The phases prepared were studied thermochemically on a high-temperature Tian-Calvet microcalorimeter by solution calorimetry. The standard enthalpies of formation of magnesium-iron nanotubular hydrosilicates were determined. The formation of iron-containing nanotubes was shown to be lass favorable energetically than the formation of magnesium nanotubes.     

High temperature thermal expansion of BaAl2Si2O8, CaAl2Si2O8, and Ca2Al2SiO7 studied by high-temperature X-ray diffraction (HT-XRD)

Sealing of components for high temperature applications with coefficients of linear thermal expansion (CTE) > 10·10⁻⁶ K⁻¹ can be achieved by glasses from which crystalline phases with high CTE are precipitated. Many sealing glasses also contain further components as e.g. aluminium and hence, not only the desired phase is crystallized, but also additional phases. For this purpose, high temperature XRD was performed in order to determine the CTE of BaAl₂Si₂O₈, CaAl₂Si₂O₈, and Ca₂Al₂SiO₇. In the case of BaAl₂Si₂O₈ and Ca₂AlSi₂O₇ the CTEs were 7.8·10⁻⁶/K and 7.9·10⁻⁶/K, respectively. In the case of CaAl₂Si₂O₈ the CTE is 4.4·10⁻⁶/K. Especially the formation of the latter phase should be avoided for a sealing material of high temperature fuel cells. Sintered specimens of the respective compounds were also characterized by dilatometry.     

Adsorption equilibria of CO<Subscript>2</Subscript> and CH<Subscript>4</Subscript> in cation-exchanged zeolites 13X

Ion-exchange with different cations (Na⁺, NH₄ ⁺, Li⁺, Ba²⁺ and Fe³⁺) was performed in binderless 13X zeolite pellets. Original and cation-exchanged samples were characterized by thermogravimetric analysis coupled with mass spectrometry (inert atmosphere), X-ray powder diffraction and N₂ adsorption/desorption isotherms at 77 K. Despite the presence of other cations than Na (as revealed in TG-MS), crystalline structure and textural properties were not significantly altered upon ion-exchange. Single component equilibrium adsorption isotherms of carbon dioxide (CO₂) and methane (CH₄) were measured for all samples up to 10 bar at 298 and 348 K using a magnetic suspension balance. All of these isotherms are type Ia and maximum adsorption capacities decrease in the order Li > Na > NH₄–Ba > Fe for CO₂ and NH₄–Na > Li > Ba for CH₄. In addition to that, equilibrium adsorption data were measured for CO₂/CH₄ mixtures for representative compositions of biogas (50 % each gas, in vol.) and natural gas (30 %/70 %, in vol.) in order to assess CO₂ selectivity in such scenarios. The application of the Extended Sips Model for samples BaX and NaX led to an overall better agreement with experimental data of binary gas adsorption as compared to the Extended Langmuir Model. Fresh sample LiX show promise to be a better adsorption than NaX for pressure swing separation (CO₂/CH₄), due to its higher working capacity, selectivity and lower adsorption enthalpy. Nevertheless, cation stability for both this samples and NH₄X should be further investigated.     

Temperature programmed desorption of F-doped SnO<Subscript>2</Subscript> films deposited by inverted pyrosol technique

Fluorine-doped tin dioxide (FTO) films were deposited on silicon wafers by inverted pyrosol technique using solutions with different doping concentration (F/Sn=0.00, 0.12, 0.75 and 2.50). The physical and electrical properties of the deposited films were analyzed by SEM, XRF, resistivity measurement by four-point-probe method and Hall coefficient measurement by van der Pauw method. The electrical properties showed that the FTO film deposited using the solution with F/Sn=0.75 gave a lowest resistivity of 3.2·10^–4 ohm cm. The FTO films were analyzed by temperature programmed desorption (TPD). Evolved gases from the heated specimens were detected using a quadruple mass analyzer for mass fragments m/z, 1(H⁺), 2(H₂ ⁺), 12(C⁺), 14(N⁺), 15(CH₃ ⁺), 16(O⁺), 17(OH⁺ or NH₃ ⁺), 18(H₂O⁺ or NH₄ ⁺), 19(F⁺), 20(HF⁺), 28(CO⁺ or N₂ ⁺), 32(O₂ ⁺), 37(NH₄F⁺), 44(CO₂ ⁺), 120(Sn⁺), 136(SnO⁺) and 152(SnO₂ ⁺). The majority of evolved gases from all FTO films were water vapor, carbon monoxide and carbon dioxide. Fluorine (m/z 19) was detected only in doped films and its intensity was very strong for highly-doped films at temperature above 400°C.     

Reduction of Eu to Eu in MAl2Si2O8 (M=Ca, Sr, Ba) in air condition

In general, the reduction of Eu³⁺ to Eu²⁺ in solids needs an annealing process in a reducing atmosphere. In this paper, it is of great interest and importance to find that the reduction of Eu³⁺ to Eu²⁺ can be realized in a series of alkaline-earth metal aluminum silicates MAl₂Si₂O₈ (M=Ca, Sr, Ba) just in air condition. The Eu²⁺-doped MAl₂Si₂O₈ (M=Ca, Sr, Ba) powder samples were prepared in air atmosphere by Pechini-type sol-gel process. It was found that the strong band emissions of 4f⁶5d¹-4f⁷ from Eu²⁺ were observed at 417, 404 and 373 nm in air-annealed CaAl₂Si₂O₈, SrAl₂Si₂O₈ and BaAl₂Si₂O₈, respectively, under ultraviolet excitation although the Eu³⁺ precursors were employed. In addition, under low-voltage electron beam excitation, Eu²⁺-doped MAl₂Si₂O₈ also shows strong blue or ultraviolet emission corresponding to 4f⁶5d¹-4f⁷ transition. The reduction mechanism from Eu³⁺ to Eu²⁺ in these compounds has been discussed in detail.     

钙镁磷肥(FCMP)阴离子构型的硅烷化气相色谱(TMS-GC)研究

通过对钙镁磷肥(FCMP)硅烷化气相色谱(TMS-GC)的研究证明:FCMP中硅以一聚、二聚、三聚链及环四构型为主,同时还有少量其他一维、二维、三维构型存在;而磷则以单个四面体与硅聚合或独立存在。     

Zwitterionic λ5Si‐Silicates with SiO2S2C Skeletons: Syntheses and Structural Characterization in the Solid State

The zwitterionic λ⁵Si‐silicates [(dimethylammonio)methyl]bis[methanecarboxylatothiolato(2–)‐O,S]silicate ( 9 ) and bis[benzene‐1‐carboxylato‐2‐thiolato(2–)‐O,S][(dimethylammonio)methyl]silicate ( 10 ) were synthesized by treatment of the zwitterionic λ⁵Si‐tetrafluorosilicate F₄SiCH₂NMe₂H with two molar equivalents of Me₃SiSCH₂C(O)OSiMe₃ and 1,2‐Me₃SiS–C₆H₄–C(O)OSiMe₃, respectively (formation of four molar equivalents of Me₃SiF). Compounds 9 and 10 were characterized by elemental analyses (C, H, N, S) and solid‐state NMR studies (¹³C, ²⁹Si). In addition, compound 10 was structurally characterized by single‐crystal X‐ray diffraction.     

Synthesis and X-ray diffraction study of the LaMg<Superscript>I</Superscript>Mg(CrO<Subscript>3</Subscript>)<Subscript>2</Subscript> (M<Superscript>I</Superscript> = Li,Na, K) compounds

Ternary chromites of the composition LaM^IMg(CrO₃)₂ (M^I = Li, Na, K) were synthesized for the first time by ceramic technology from stoichiometric amounts of high purity grade La₂O₃; pure for analysis grade Li₂CO₃, Na₂CO₃, K₂CO₃, and MgCO₃; and chemically pure grade Cr₂O₃. Using X-ray diffractometry, it has been established that compounds are crystallized in cubic and tetragonal crystal systems, and parameters of their crystal lattices have been determined.     

Enhanced electrochemical properties of Li<Subscript>2</Subscript>ZnTi<Subscript>3</Subscript>O<Subscript>8</Subscript>/C nanocomposite synthesized with phenolic resin as carbon source

Li₂ZnTi₃O₈/C nanocomposite has been synthesized using phenolic resin as carbon source in this work. The structure, morphology, and electrochemical properties of the as-prepared Li₂ZnTi₃O₈ samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), Raman spectroscopy (RS), galvanostatic charge–discharge, and AC impedance spectroscopy. SEM images show that Li₂ZnTi₃O₈/C was agglomerated with a primary particle size of ca. 40 nm. TEM images reveal that a homogeneous carbon layer (ca. 5 nm) formed on the surface of Li₂ZnTi₃O₈ particles which is favorable to improve the electronic conductivity and inhibit the growth of Li₂ZnTi₃O₈ during annealing process. The as-prepared Li₂ZnTi₃O₈/C composite with 6.0 wt.% carbon exhibited a high initial discharge capacity of 425 and 159 mAh g^?1 at 0.05 and 5 A g^?1, respectively. At a high current density of 1 A g^?1, 95.5 % of its initial value is obtained after 100 cycles.     

Synthesis of La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> by thermal treatment of mechanically activated salt mixture

A lanthanum zirconate La₂Zr₂O₇ was synthesized by soft mechanochemical method using zirconium oxynitrate ZrO(NO₃)₂·6H₂O and lanthanum carbonate La₂(CO₃)₃·8H₂O as reagents. Mechanical activation of the reagents was carried out in a centrifugal planetary ball mill. The processes occurring during calcination of the jointly and the separately mechanically activated salt mixture were studied using DSC, TG coupled with mass spectrometry, XRD analysis, and FTIR spectroscopy. It was shown that in the course of joint mechanical activation in the mill alongside with intimate mixing of the reagents and their amorphization exchange reaction occurred, producing lanthanum nitrate, basic lanthanum nitrate, basic zirconium carbonate, and hydrated zirconium oxide. The DSC curve of the jointly mechanically activated salt mixture showed a strong exothermic peak at 878 °C which was not associated with mass loss. This peak was attributed to La₂Zr₂O₇ crystallization in agreement with XRD data. Nanocrystalline lanthanum zirconate synthesized by annealing of the jointly mechanically activated salt mixture was characterized using XRD analysis, scanning, and transmission electron microscopy.