Heriberto Pfeiffer

Thermokinetic Analysis of the CO2 Chemisorption on Li4SiO4 by Using Different Gas Flow Rates and Particle Sizes

Lithium orthosilicate (Li(4)SiO(4)) was synthesized by solid-state reaction and then its CO(2) chemisorption capacity was evaluated as a function of the CO(2) flow rate and particle size. Initially, a Li(4)SiO(4) sample, with a total surface area of 0.4 m(2)/g, was used to analyze the CO(2) chemisorption, varying the CO(2) flow between 30 and 200 mL/min. Results showed that CO(2) flows modify the kinetic regime from which CO(2) capture is controlled. In the first moments and at low CO(2) flows, the CO(2) capture is controlled by the CO(2) diffusion through the gas-film system, whereas at high CO(2) flows it is controlled by the CO(2) chemisorption reaction rate. Later, at larger times, once the carbonate-oxide external shell has been produced the whole process depends on the CO(2) chemisorption kinetically controlled by the lithium diffusion process, independently of the CO(2) flow. Additionally, thermokinetic analyses suggest that temperature induces a CO(2) particle surface saturation, due to an increment of CO(2) diffusion through the gas-film interface. To elucidate this hypothesis, the Li(4)SiO(4) sample was pulverized to increase the surface area (1.5 m(2)/g). Results showed that increasing the surface particle area, the saturation was not reached. Finally, the enthalpy activation (DeltaH(double dagger)) values were estimated for the two CO(2) chemisorption processes, the CO(2) direct chemisorption produced at the Li(4)SiO(4) surface, and the CO(2) chemisorption kinetically controlled by the lithium diffusion, once the carbonate-oxide shell has been produced.

Synthesis of lithium silicates

Abstract Lithium silicates were synthesized by three techniques: (1) solid state reaction, (2) the precipitation method and finally, (3) the sol–gel method. Reactions were performed with different Li:Si molar ratios: 0.5, 1, 2 and 4. The obtained products were Li2SiO3, Li4SiO4 and Li2Si2O5. According to the synthesis method the composition of the samples changed as well as the morphology of the particles. The sol–gel method using CH3OLi provided the highest content of Li2SiO3 (94%); the solid state and precipitation methods provided pure Li4SiO4.

Thermochemical Capture of Carbon Dioxide on Lithium Aluminates (LiAlO2 and Li5AlO4): A New Option for the CO2 Absorption

Lithium aluminates (LiAlO(2) and Li(5)AlO(4)) were synthesized, characterized, and tested as possible CO(2) captors. LiAlO(2) did not seem to have good qualities for the CO(2) absorption. On the contrary, Li(5)AlO(4) showed excellent behavior as a possible CO(2) captor. Li(5)AlO(4) was thermally analyzed under a CO(2) flux dynamically and isothermically at different temperatures. These results clearly showed that Li(5)AlO(4) is able to absorb CO(2) in a wide temperature range (200-700 degrees C). Nevertheless, an important sintering effect was observed during the thermal treatment of the samples, which produced an atypical behavior during the CO(2) absorption at low temperatures. However, at high temperatures, once the lithium diffusion is activated, the sintering effect did not interfere with the CO(2) absorption. Eyrings model was used to determine the activation enthalpies of the CO(2) absorption (15.6 kJ/mol) and lithium diffusion (52.1 kJ/mol); the last one is the limiting process.

CO2 capture properties of lithium silicates with different ratios of Li2O/SiO2: An ab initio thermodynamic and experimental approach

The lithium silicates have attracted scientific interest due to their potential use as high-temperature sorbents for CO2 capture. The electronic properties and thermodynamic stabilities of lithium silicates with different Li2O/SiO2 ratios (Li2O, Li8SiO6, Li4SiO4, Li6Si2O7, Li2SiO3, Li2Si2O5, Li2Si3O7, and α-SiO2) have been investigated by combining first-principles density functional theory with lattice phonon dynamics. All these lithium silicates examined are insulators with band-gaps larger than 4.5 eV. By decreasing the Li2O/SiO2 ratio, the first valence bandwidth of the corresponding lithium silicate increases. Additionally, by decreasing the Li2O/SiO2 ratio, the vibrational frequencies of the corresponding lithium silicates shift to higher frequencies. Based on the calculated energetic information, their CO2 absorption capabilities were extensively analyzed through thermodynamic investigations on these absorption reactions. We found that by increasing the Li2O/SiO2 ratio when going from Li2Si3O7 to Li8SiO6, the corresponding lithium silicates have higher CO2 capture capacity, higher turnover temperatures and heats of reaction, and require higher energy inputs for regeneration. Based on our experimentally measured isotherms of the CO2 chemisorption by lithium silicates, we found that the CO2 capture reactions are two-stage processes: (1) a superficial reaction to form the external shell composed of Li2CO3 and a metal oxide or lithium silicate secondary phase and (2) lithium diffusion from bulk to the surface with a simultaneous diffusion of CO2 into the shell to continue the CO2 chemisorption process. The second stage is the rate determining step for the capture process. By changing the mixing ratio of Li2O and SiO2, we can obtain different lithium silicate solids which exhibit different thermodynamic behaviors. Based on our results, three mixing scenarios are discussed to provide general guidelines for designing new CO2 sorbents to fit practical needs.

Analysis and perspectives concerning CO2 chemisorption on lithium ceramics using thermal analysis

CO2 removal from flue gas has been proposed as one of the most reliable solutions to mitigate global greenhouse emissions. Lithium ceramics are among several materials that have potential applications in CO2 removal. Lithium ceramics are able to chemisorb CO2 in a wide temperature range, presenting several interesting properties. All lithium ceramics present a similar CO2 chemisorption reaction mechanism that has been described at the micrometric scale. However, there are several issues that have not been fully elucidated. The aim of this study is to re-analyze different experiments related to the CO2 chemisorption on lithium ceramics and to propose how different factors control this process. This study focuses on diffusion controlled CO2 chemisorption, which has been shown to be the limiting step of the CO2 chemisorption process. Diffusion controlled CO2 chemisorption appears to be mainly influenced by the chemical composition of a product’s external shell.

Structural and Thermochemical Chemisorption of CO2 on Li4+x(Si1–xAlx)O4 and Li4–x(Si1–xVx)O4 Solid Solutions

Different Li(4)SiO(4) solid solutions containing aluminum (Li(4+x)(Si(1-x)Al(x))O(4)) or vanadium (Li(4-x)(Si(1-x)V(x))O(4)) were prepared by solid state reactions. Samples were characterized by X-ray diffraction and solid state nuclear magnetic resonance. Then, samples were tested as CO(2) captors. Characterization results show that both, aluminum and vanadium ions, occupy silicon sites into the Li(4)SiO(4) lattice. Thus, the dissolution of aluminum is compensated by Li(1+) interstitials, while the dissolution of vanadium leads to lithium vacancies formation. Finally, the CO(2) capture evaluation shows that the aluminum presence into the Li(4)SiO(4) structure highly improves the CO(2) chemisorption, and on the contrary, vanadium addition inhibits it. The differences observed between the CO(2) chemisorption processes are mainly correlated to the different lithium secondary phases produced in each case and their corresponding diffusion properties.

Analysis of the CO2 chemisorption reaction mechanism in lithium oxosilicate (Li8SiO6): a new option for high-temperature CO2 capture

Lithium oxosilicate (Li8SiO6) was successfully synthesized via a solid-state reaction. The samples structure and microstructure were characterized using X-ray diffraction, scanning electron microscopy and N2 adsorption. The CO2 chemisorption capacity was evaluated dynamically and isothermally. Li8SiO6 was found to chemisorb CO2 over a wide temperature range with a maximum weight increase of 52.1 wt%, which corresponds to 11.8 mmol CO2 per gram ceramic. Using different thermogravimetric analyses with some structural and microstructural analyses, a CO2 chemisorption mechanism could be proposed, and the chemical species formed (Li4SiO4, Li2SiO3 and Li2CO3) during the CO2 capture process in Li8SiO6 could be elucidated. The kinetic parameter values (k) obtained for the Li8SiO6–CO2 reaction were higher than the k values previously reported for the Li4SiO4–CO2 reaction system. Additionally, ΔH‡ was found to be 53.1 kJ mol−1. According to these results, the Li8SiO6–CO2 chemisorption mechanism depends on the reaction temperature. Thus, Li8SiO6 may find potential applications as an alternative for CO2 capture because of its wide temperature range, CO2 chemisorption capacity and kinetic parameters.

Structural Analysis and CO2 Chemisorption Study on Nonstoichiometric Lithium Cuprates (Li2+xCuO2+x/2)

Lithium cuprate (Li(2)CuO(2)) was prepared by solid state reaction, using different quantities of lithium excess, which produced nonstoichiometric ceramics, Li(2+x)CuO(2+x/2). These ceramics were characterized by X-ray diffraction, transmission and scanning electron microscopies, solid state nuclear magnetic resonance, and atomic absorption. The results obtained showed that lithium excess is located mainly into the Li(2)CuO(2) interlayers forming nanoparticles of a different phase, perhaps lithium oxide. Additionally, the lithium excess produced morphological changed at a micrometric and nanometric levels. As lithium excess increased, the particle size increased as well and it formed some kind of filament-like structures. It was explained in terms sintering, due to the high mobility of lithium atoms. On the other hand, all these ceramics were tested as CO(2) captors, presenting encouraging properties through a chemisorption process. As expected, the CO(2) absorption increased as a function of total lithium contained into the ceramics. Finally, it was performed a kinetic analysis of the CO(2) absorption.

Thermokinetic study of the rehydration process of a calcined MgAl-layered double hydroxide.

The rehydration process of a calcined MgAl-layered double hydroxide (LDH) with a Mg/Al molar ratio of 3 was systematically analyzed at different temperatures and relative humidity. Qualitative and quantitative experiments were done. In the first set of samples, the temperature or the relative humidity was varied, fixing the second variable. Both adsorption and absorption phenomena were present; absorption process was associated to the LDH regeneration. Of course, in all cases the LDH regeneration was confirmed by other techniques such as TGA, solid state NMR, and SAXS. In the second set of experiments, a kinetic analysis was performed, the results allowed to obtain different activation enthalpies for the LDH regeneration as a function of the relative humidity. The activation enthalpies varied from 137.6 to 83.3 kJ/mol as a function of the relative humidity (50 and 80%, respectively). All these experiments showed that LDH regeneration is highly dependent on the temperature and relative humidity.

Synthesis of advanced ceramics by hydrothermal crystallization and modified related methods

The present article aims to give a brief overview about the advantages of the hydrothermal crystallization method for the synthesis of advanced ceramics. Emphasis is given, not only on the conventional hydrothermal crystallization, but also on some of its variants; such as ultrasound-assisted, electrochemical-assisted, microwave-assisted and surfactant-assisted hydrothermal methods which open up new opportunities for the synthesis of ceramic materials with novel properties demanded for advanced applications. In the current work the synthesis of barium titanate (BaTiO3), lithium metasilicate (Li2SiO3) and sodium-potassium niobate (Na, K)NbO3 powders are reported as cases of study.