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.

Na2SiO3 was synthesized by two different routes: solid-state reaction and combustion method. It was determined that Na2SiO3 sample prepared by the combustion method presented a surface area 3 times larger than the solid-state reaction sample. Different water vapor sorption experiments were performed using N2 or CO2 as carrier gases. If N2 was used as carrier gas, it was evidenced that Na2SiO3 is able to trap water in two different ways: physically and chemically producing Na–OH and Si–OH species. Moreover, when CO2 was used, Na2SiO3 continued trapping water, as in the previous case, but in this case CO2 was trapped, forming Na2CO3 and NaHCO3 phases. Additionally, as it could be expected, the surface area resulted to be a very important factor controlling the CO2 capture efficiency. The Na2SiO3 sample prepared by the combustion method captured up to 8.5 mmol of CO2 per gram of ceramic (efficiency of 52%), a considerably high CO2 amount among different materials. Therefore, the presence of water vapor strongly favored the CO2 chemisorption on Na2SiO3.

Potassium carbonate is a highly hygroscopic salt, and this aspect becomes important for CO2 capture from ambient air. Moreover, CO2 capture from ambient air requires adsorbents with a very low pressure drop. In the present work an activated carbon honeycomb monolith was coated with K2CO3, and it was treated with moist N2 to hydrate it. Its CO2 capture capacity was studied as a function of the temperature, the water content of the air, and the air flow rate, following a factorial design of experiments. It was found that the water vapor content in the air had the largest influence on the CO2 adsorption capacity. Moreover, the deliquescent character of K2CO3 led to the formation of an aqueous solution in the pores of the carrier, which regulated the temperature of the CO2 adsorption. The transition between the anhydrous and the hydrated forms of potassium carbonate was studied by means of FT-IR spectroscopy. It can be concluded that hydrated potassium carbonate is a promising and cheap alternative for CO2 capture from ambient air for the production of CO2-enriched air or for the synthesis of solar fuels, such as methanol.

No abstract

No abstract