Issis C. Romero-Ibarra

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 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.

Alkaline and Alkaline-Earth Ceramic Oxides for CO2 Capture, Separation and Subsequent Catalytic Chemical Conversion

The amounts of anthropogenic carbon dioxide (CO2) in the atmosphere have been raised dramatically, mainly due to the combustion of different carbonaceous materials used in energy production, transport and other important industries such as cement production, iron and steelmaking. To solve or at least mitigate this environmental problem, several alternatives have been proposed. A promising alternative for reducing the CO2 emissions is the separation and/ or capture and concentration of the gas and its subsequent chemical transformation. In that sense, a variety of materials have been tested containing alkaline and/or alkaline-earth oxide ceramics and have been found to be good options.

Efficient cephalexin degradation using active chlorine produced on ruthenium and iridium oxide anodes: Role of bath composition, analysis of degradation pathways and degradation extent

The elimination of cephalexin (CPX) using electro-generated Cl2-active on Ti/RuO2-IrO2 anode was assessed in different effluents: deionized water (DW), municipal wastewater (MWW) and urine. Single Ti/RuO2 and Ti/IrO2 catalysts were prepared to compare their morphologies and electrochemical behavior against the binary DSA. XRD and profile refinement suggest that Ti/RuO2-IrO2 forms a solid solution, where RuO2 and IrO2 growths are oriented by the TiO2 substrate through substitution of Ir by Ru atoms within its rutile-type structure. SEM reveals mud-cracked structures with flat areas for all catalysts, while EDS analysis indicates atomic ratios in the range of the oxide stoichiometries in the nominal concentrations used during synthesis. A considerably higher CPX degradation is achieved in the presence of NaCl than in Na2SO4 or Na3PO4 media due to the active chlorine generation. A faster CPX degradation is reached when the current density is increased or the pH value is lowered. This last behavior may be ascribed to an acid-catalyzed reaction between HClO and CPX. Degradation rates of 22.5, 3.96, and 0.576 μmol L-1 min-1 were observed for DW, MWW and urine, respectively. The lower efficiency measured in these last two effluents was related to the presence of organic matter and urea in the matrix. A degradation pathway is proposed based on HPLC-DAD and HPLC-MS analysis, indicating the fast formation (5 min) of CPX-(S)-sulfoxide and CPX-(R)-sulfoxide, generated due the Cl2-active attack at the CPX thioether. Furthermore, antimicrobial activity elimination of the treated solution is reached once CPX, and the initial by-products are considerably eliminated. Finally, even if only 16% of initial TOC is removed, BOD5 tests prove the ability of electro-generated Cl2-active to transform the antibiotic into biodegradable compounds. A similar strategy can be used for the abatement of other recalcitrant compounds contained in real water matrices such as urine and municipal wastewaters.