Mechanosorption of carbon dioxide by Ca- and Mg-containing silicates and alumosilicates. Sorption of CO<Subscript>2</Subscript> and structure-related chemical changes

The discovery of the unusual ability of Ca- and Mg-containing silicates in certain grinding regimes to absorb carbon dioxide from the environment in amounts comparable to the mass of the ground sample has stimulated interest in the study of mechanochemical effects. The range of objects for studying natural minerals, such as labradorite (CaAl₂Si₂O₈)_0.562(NaAlSi₃O₈)_0.438, oligoclase (CaAl₂Si₂O₈)_0.148(NaAlSi₃O₈)_0.852, diopside CaMgSi₂O₆, and akermanite Ca₂MgSi₂O₇, as well as synthetic minerals gehlenite Ca₂Al₂SiO₇ and wollastonite CaSiO₃, is expanded to develop the model of deep mechanosorption of CO₂ and to derive equations that allow the kinetic analysis of the absorption of carbon dioxide by silicates in the course of mechanochemical activation to be performed. Regularities revealed previously and analogies to the processes that occur upon the dissolution of carbon dioxide in silicate melts, are generalized. Data on the absorption of carbon dioxide by Ca- and Mgcontaining silicates and alumosilicates, depending on the duration of mechanochemical activation in an AGO-2 centrifugal planetary mill in the atmosphere of CO₂ at a pressure of 10⁵ Pa, are obtained. Based on the data of X-ray phase analysis and IR spectroscopy, structural chemical changes in minerals and the forms of carbon dioxide in mechanochemically activated samples are discussed. It is shown that the intense penetration of gas molecules in particle bulk and their “dissolution” in structurally disordered silicate matrix in the form of distorted CO ₃ ^2? ions occurs upon mechanosorption.     

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.     

Effect of PEG with different <Emphasis Type="Italic">M</Emphasis><Subscript>W</Subscript> as template direction reagent on preparation of porous TiO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript> with assistance of supercritical CO<Subscript>2</Subscript>

Titania–silica composite have been prepared using polyethylene glycol (PEG) with different molecular weights (M _w), PEG20000, PEG10000, and PEG2000, as template in supercritical carbon dioxide (SC CO₂). The composite precursors were dissolved in SC CO₂ and impregnated into PEG templates using SC CO₂ as swelling agent and carrier. After removing the template by calcination at suitable temperature, the titania–silica composite were obtained. The composite were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and nitrogen sorption–desorption experiment. Photocatalytic activity of the samples has been investigated by photodegradation of methyl orange. Results indicate that there are many Si–O–Ti linkages in the TiO₂/SiO₂ composite; the PEG template has a significant influence on the structure of TiO₂/SiO₂. In addition, the TiO₂/SiO₂ prepared with PEG10000 exhibited high photocatalytic efficiency. So this work supplies a clue to control and obtain the TiO₂/SiO₂ composite with different photocatalytic reactivity with the aid of suitable PEG template in supercritical CO₂.     

Mechanochemical interaction of alkali metal metasilicates with carbon dioxide: 2. The influence of thermal treatment on the properties of activated samples

Processes proceeding during the heating of the samples of lithium, sodium, and potassium metasilicates mechanochemically activated in carbon dioxide are studied using thermal analysis, X-ray phase analysis, and IR spectroscopy. Upon the heating of the samples of sodium and potassium metasilicates, the reversal of phases occurs in the sequence according to their appearance during the mechanical treatment in the mill in the atmosphere of carbon dioxide. Hydrocarbonates are first decomposed to neutral carbonates followed by their interaction with silica to form metasilicates. In the course of thermal treatment of lithium sample, the metastable carbonate-silicate phase, which is formed due to the intense mechanosorption of carbon dioxide, is decomposed first. Afterwards, lithium carbonate formed at the final stage of mechanoactivation reacts with SiO₂.     

Efficiency of CO<Subscript>2</Subscript> Dissociation in a Radio-Frequency Discharge

One possible solution to mitigating the effects of high atmospheric concentrations of carbon dioxide (CO₂) is the use of a plasma source to break apart the molecule into carbon monoxide (CO) and oxygen. This work experimentally investigates the efficiency of dissociation of CO₂ in a 1-kW radio-frequency (rf) plasma source operating at 13.56-MHz in a low-pressure discharge. Mass spectrometry diagnostics are used to determine the species present in the discharge, and these measurements are used to calculate the energy efficiency and conversion efficiency of CO₂ dissociation in the rf plasma source. Experimental results have found that the conversion efficiency of CO₂ to CO can reach values near 90%, however energy efficiency reaches a maximum of 3%. A theoretical energy cost analysis is also given as a method to evaluate the effectiveness of any plasma system designed for CO₂ emissions reduction.     

Hydride lithiation of spinels LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript>

Undoped lithiation of stoichiometric spinel using lithium hydride LiH up to the composition Li_2.25Mn₂O₄ was performed. A homogeneous material with a given Li: Mn ratio was obtained by mechanochemical activation with sequential annealing of a LiMn₂O₄–LiH mixture in a high-purity argon atmosphere and then in air or oxygen at 373–553 K.     

Thermal properties of oxide glasses

A criterion based on the length of induction period of crystallization was used to evaluate the thermal stability of M₂O·SiO₂ (M?=?Li, Na) glasses against crystallization. It was founded out that the stability of studied glasses against crystallization is Li₂O·SiO₂?<?Na₂O·SiO₂. The results coincide with the order determined by stability criteria based on temperatures and the values of activation energy. A criterion based on the length of induction period enables to discriminate among the thermal stabilities of the silicate glass systems.     

Conducting properties of nanocomposite polymer electrolytes based on polyethylene glycol diacrylate and SiO<Subscript>2</Subscript> nanoparticles at the interface with a lithium electrode

Nanocomposite polymer electrolytes based on polyethylene glycol diacrylate and 1 M LiBF₄ solution in γ-butyrolactone with addition of SiO₂ nanoparticles were synthesized and studied. Resistance measurement at the Li/electrolyte and Li/nanocomposite electrolyte interface by the time-resolved electrochemical impedance showed its significant decrease in the presence of SiO₂ nanoparticles. Charge-discharge cycling of prototypes of Li/LiFePO₄ batteries for 50 cycles also showed the advantage of using nanocomposite polymer electrolytes over electrolytes without SiO₂ additives.     

Magnetic and thermal analysis of <Emphasis Type="Italic">M</Emphasis>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> (<Emphasis Type="Italic">M</Emphasis> = Co,Mn, Zn) nanoparticles

Magnetic nanoparticles were prepared by a wet chemical method. Precursors of MFe₂O₄ (M = Co, Mn, Zn) were prepared from a mixture of metal chloride and metasilicate nonahydrate aqueous solutions. The precipitates obtained in the wet chemical method were calcined to obtain MFe₂O₄ nanoparticles encapsulated by amorphous SiO₂. The blocking temperatures T _B’s were between 20 and 320 K, in this temperature range, the anisotropy energy of the particles decreased below their thermal energy. T _B increased with the particle size. In order to clarify the nanoparticle formation process, differential thermal analysis and thermogravimetric (TG-DTA) measurements were performed for the as-prepared samples.     

Solid-phase synthesis of intermetallic compounds Al<Subscript>13</Subscript>Co<Subscript>4</Subscript>, Al<Subscript>13</Subscript>Fe<Subscript>4</Subscript>, and Al<Subscript>13</Subscript>Co<Subscript>2</Subscript>Fe<Subscript>2</Subscript>

Intermetallic compounds Al₁₃Co₄, Al₁₃Fe₄, and Al₁₃Co₂Fe₂ were obtained by solid-phase synthesis in air at temperatures below 600°C using precursor metals subjected to mechanochemical preactivation. The phase composition of the synthesized aluminides and composites Al₁₃Co₄/SiO₂ and Al₁₃Fe₄/SiO₂ was analyzed.     

Synthesis and photocatalytic activity of the Co-containing materials based on amorphous SiO2

Functional materials based on highly dispersed amorphous silica and cobalt compounds have been obtained by solvothermal and mechanochemical methods. The formation of silicate Co₃(Si₂O₅)₂(OH)₂ under hydrothermal conditions has been established. This composition, in comparison with mechano-mixes, provides better photoactivity in the hydroquinone decomposition reaction under ultraviolet irradiation, herewith maintaining the surface characteristics of amorphous SiO₂.     

Phase transition of RbH<Subscript>2</Subscript>PO<Subscript>4</Subscript> and its composite with SiO<Subscript>2</Subscript> studied by thermal analysis

The phase transition at T _p (~109 °C) of RbH₂PO₄ and its composite with SiO₂ has been investigated by thermal analysis here. In the case of neat RbH₂PO₄, there is a linear relationship between endothermic peak temperature (T _m) and square root of heating rate (Φ ^1/2), from which the onset temperature of phase transition can be determined. Besides, Kissinger method and another calculation method were employed to obtain the activation energy of phase transition. The detailed deduction process was presented in this paper, and the estimated activation energies are E ₁ ≈ 126.3 kJ/mol and E ₂ ≈ 129.2 kJ/mol, respectively. On the other hand, the heterogeneous doping of RbH₂PO₄ with SiO₂ as dopant facilitates its proton conduction and leads to the disappearance of jump in conductivity at T _p. The heats of transition in the composites decrease gradually with increasing the molar fraction of SiO₂ additives. In the cooling process, a new and broad exothermic peak appeared between ~95 and ~110 °C, and its intensity also changes with the SiO₂ amount. These phenomena might be related to the formation of amorphous phase of RbH₂PO₄ on the surface of SiO₂ particles due to the strong interface interaction.     

Hydrothermal synthesis of one-dimensional (1D) Na<Subscript>x</Subscript>TiO<Subscript>2</Subscript> nanostructures

Nanorods of sodium titanium dioxide bronze Na_xTiO₂ were synthesized by the hydrothermal treatment of the amorphous TiO₂·nH₂O gel with 10 M NaOH followed by ultrasonication in 0.1 M HCl and thermal treatment (500°C, 10 h). The thermal treatment of the nanorods does not change the morphology of the particles. According to the electron diffraction data, the Na_xTiO₂ nanorods grow along the c axis.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 71–73, January, 2005.     

Photocatalytic reduction of CO<Subscript>2</Subscript> over TiO<Subscript>2</Subscript> based catalysts

At present, carbon dioxide is considered the largest contributor among greenhouse gases. This review covers the current state of problem of carbon dioxide emissions from industrial and combustion processes, the principle of photocatalysis, existing literature related to photocatalytic CO₂ reduction over TiO₂ based catalysts and the effects of important parameters on the process performance including light wavelength and intensity, type of reductant, metal-modified surface, temperature and pressure. Presented at the 34th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 21–25 May 2007.     

Effect of steam and CO<Subscript>2</Subscript> activation on characteristics and desulfurization performance of pyrolusite modified activated carbon

We investigated the effect of activation methods, i.e. steam and carbon dioxide (CO₂) activation, on the physicochemical properties and desulfurization performance of pyrolusite-modified activated carbon. Walnut shell was used as the raw material and three kinds of pyrolusite were used as the additives for the modification of activated carbon. Physicochemical properties of the samples were studied by BET, XPS, FTIR and XRD methods. The results showed that the samples by steam activation (AC-H₂O) had higher S_BET, pore volume and surface oxygen-containing functional groups than those by CO₂ activation (AC-CO₂). Different kinds of pyrolusite had influence on the pore structures and the contents of C–O/C=O groups on the AC-H₂O samples, due to different contents of Mn/Fe in these pyrolusite. All pyrolusite modified samples demonstrated a higher desulfurization activity than blank ones, and the sulfur capacity of the modified samples increased with the increment of Mn/Fe contents in the pyrolusite. Furthermore, the AC-H₂O modified samples performed much higher desulfurization capacity (217 mg/g) than the AC-CO₂ samples (171 mg/g).     

Spectroscopic investigation of structure and mechanism of proton conductivity CsHSO<Subscript>4</Subscript> and composites CsHSO<Subscript>4</Subscript>/SiO<Subscript>2</Subscript>

IR and RS spectra, structural, thermodynamical and transport properties of CsHSO₄ and CsHSO₄/SiO₂ composites have been investigated. It is shown that CsHSO₄ is stabilized in composites in phase II (monoclinic modification), which turns into amorphous state with increasing silicon dioxide content. A formation of disordered highly conducting states of CsHSO₄ in composites at the temperatures noticeably lower than superionic phase transition was stated. Bond length equalization in sulfate-ions, weakening of hydrogen bond system, and as a result easier proton transfer in composites in comparison with pure salt take place in these conducting states. Mechanism for formation of composites and their proton conductivity is proposed.     

Preparation and characterization of waste ion-exchange resin-based activated carbon for CO<Subscript>2</Subscript> capture

Waste ion-exchange resin was utilized as precursor to produce activated carbon by KOH chemical activation, on which the effects of different activation temperatures, activation times and impregnation ratios were studied in this paper. The CO₂ adsorption of the produced activated carbon was tested by TGA at 30 °C and environment pressure. Furthermore, the effects of preparation parameters on CO₂ adsorption were investigated. Experimental results show that the produced activated carbons are microporous carbons, which are suitable for CO₂ adsorption. The CO₂ adsorption capacity increases firstly and then decreases with the increase of activation temperature, activation time and impregnation rate. The maximum adsorption capacity is 81.24 mg/g under the condition of 30 °C and pure CO₂. The results also suggest that waste ion-exchange resin-based activated carbons possess great potential as adsorbents for post-combustion CO₂ capture.     

Some studies on the phase formation and kinetics in TiO<Subscript>2</Subscript> containing lithium aluminum silicate glasses nucleated by P<Subscript>2</Subscript>O<Subscript>5</Subscript>

Lithium aluminum silicate (LAS) glasses of compositions (wt%) 10.6Li₂O–71.7SiO₂–7.1Al₂O₃–4.9K₂O–3.2B₂O₃–1.25P₂O₅–1.25TiO₂ were prepared by the melt quench technique. Crystallization kinetics was investigated by the method of Kissinger and Augis–Bennett using differential thermal analysis (DTA). Based on the DTA data, glass ceramics were prepared by single-, two-, and three-step heat treatment schedules. The interdependence of different phases formed, microstructure, thermal expansion coefficient (TEC) and microhardness (MH) was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), thermo-mechanical analysis (TMA), and microhardness (MH) measurements. Crystallization kinetics revealed that Li₂SiO₃ is the kinetically favored phase with activation energy of 91.10 kJ/mol. An Avrami exponent of n = 3.33 indicated the dominance of bulk crystallization. Based upon the formation of phases, it was observed that the two-stage heat treatment results in highest TEC glass ceramics. The single-step heat treatment yielded glass ceramics with the highest MH.     

A study of the SiO<Subscript>2</Subscript>-aqueous electrolyte (NaCl,CsCl) interface by X-ray photoelectron spectroscopy

The surface of amorphous silicon dioxide in NaCl and CsCl solutions with various values of ionic strength and pH is studied by X-ray photoelectron spectroscopy. Samples are prepared through the fast freezing of wet pastes prepared by the centrifugation of suspensions. It is shown that the X-ray photoelectron spectra of such samples provide experimental information on the composition of the SiO₂-solution interface, the number of electrolyte ions near the solid phase surface, and its chemical modification. The sign of the charge of silicon dioxide particles can be judged from the dependence of a measured Na(Cs)/Cl atomic ratio; the controlled removal of water from the samples in the chamber of an electronic spectrometer makes it possible to directly estimate the surface potential and study the energy effects of counterion hydration using the shifts in the binding energies of the corresponding photoelectron levels. An analysis of the binding energies of Cs 3d^5/2 and Cl 2p_3/2 lines in the X-ray photoelectron spectra for the SiO₂-CsCl solution interface yields additional information on the structure of the formed electrical double layer, thus making it possible to distinguish between the ions adsorbed at the surface and the counterions compensating for its charge.     

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.