Elucidating the CO2 Chemisorption Process on Sodium Cobaltate (NaxCoO2±d)

Carbon dioxide (CO₂) is a major greenhouse gas that is located naturally in the atmosphere. However, many human activities have considerably raised the concentration levels of this gas, approximately 40 ppm in the last ten years ^[1]. In order to reduce the emissions of CO₂ into the atmosphere and to storage it, many ceramic materials in which a reversible chemisorption occurs have been developed ^[2].

Among these materials, lithium ceramics have shown high capacities to capture CO₂ at high temperatures ^[3]. The replacement of lithium by sodium in some of these materials have notoriously improved its capture capacity ^[4]. The CO₂ capture process in these materials consists of two stages, first there is a superficial reaction of the material with the CO₂, where a fine carbonate surface is formed. Once this reaction is completed, the sodium atoms diffusion to the surface occurs and they continue to react with the CO₂.

Sodium cobaltate (NaCoO₂) has a lamellar structure where the sodium atoms are located between (CoO)⁺¹ layers^[5], therefore the sodium atoms may easily diffuse through them. The aim of this work is to test and elucidate the CO₂ capture process in a NaCoO₂phase.

The Na_xCoO_2±δ was synthesized via solid state reaction and later characterized structural and microstructurally by different techniques such as X-ray diffraction (XRD), elemental analysis (Inductively couple plasma ICP), nitrogen (N₂) adsorption-desorption and scanning electron microscopy (SEM).

XRD results showed a pure Na_xCoO_2±δ phase, and in order to accurately measure the concentrations of sodium (Na) and cobalt (Co) in the sample, an elemental analysis was performed. Therefore, the real chemical sample composition was Na_0.69CoO_1.84. The N₂ adsorption-desorption isotherm corresponded to a type II isotherm, associated to a non-porous material and the BET model provided a small area of 0.6 m²/g. This result is in good agreement with synthesis method, solid-state reaction. Secondary electron images showed particles with a planar agglomerated morphology.

The synthesized material was thermogravimetrically analyzed dynamically from 30 to 900 °C under a CO₂ flow. The dynamic TG curve evidenced two temperatures ranges where weight increments were produced. From this experiment different temperatures were selected to perform isothermal analyzes. According to these results, the Na_0.69CoO_1.84 is able to capture CO₂ in a large interval of temperatures: between 300 and 550 °C, the CO₂ is superficially captured, and after 550 °C the CO₂ is captured due to diffusion processes. Finally, at 750 °C the desorption process begins. As it was previously mentioned, the isothermal analyzes were performed between 400 and 765 °C. It was found that the Na_0.69CoO_1.84 presents the highest CO₂ capture between 600 and 725 °C. At 700 °C the material reaches its highest capture capacity, achieving to capture up to 2.15 mmol of CO₂per gram of ceramic, corresponding to a 22.4% efficiency of capture.

After the isothermal treatment some of these products were analyzed by SEM and XRD. Diffractograms performed over the 550 and 700 °C isotherms evidenced the formation of new phases: sodium carbonate (Na₂CO₃) and different cobalt oxides (Co₃O₄ and CoO). These phases corroborated the CO₂ reactivity with Na_0.69CoO_1.84. Moreover, both cobalt oxides indicated a change of valence in the cobalt, which implies an electron exchange during the CO₂ capture process. All these factors may be of interest in further analysis such as the CO₂conversion to added value products.

Backscattered electron images confirmed the formation of different phases and it was very evident that those particles presented a much higher densification than the pristine one.

Finally a kinetic analysis of CO₂ chemisorption was performed. Considering that isothermal experiments were performed in a CO₂ saturated atmosphere it could be assumed that the capture process corresponds to a first-order reaction with respect to the Na_0.69CoO_1.84. Hence, the first seconds of the isothermal TG curves, before the diffusion process begins, were fitted to first-order equations. From this analysis, the reaction rate constants from the superficial capture process were obtained. As it would be expected the kinetic constant values increased as a function of temperature. Then, these values were fitted to the Eyring’s model. In this case, only the kinetics constants between 450 and 765 °C fitted, linearly, to the model. Thus, in this temperature range the activation enthalpy determined for the CO₂chemisorption process was ΔH=82.01 kJ/mol.

Summarizing, these results show that Na_0.69CoO_1.84 is able to trap CO₂ chemically in a wide range of temperatures. While the superficial process are important, the CO₂ chemisorption is mainly associated to diffusion processes. Although the Na_0.69CoO_1.84shows a low capture capacity, the change in the cobalt valence indicates a complex capture process in which not only superficial and diffusion are present but also electronic exchanges are taking place. The kinetic analysis also showed a complex capture process which presents a high activation enthalpy.

References

1. - Monthly Mean Concentrations at the Mauna Loa Observatory (PPM), National Oceanic & Atmospheric Administration, Earth System Research Laboratory, Trends In Carbon Dioxide.

2.- Markewitz P., Kuckshinrichs W., Leitner W., Linssen J., Zapp P., Bongartz R., Schreiber A., Müller T.E.. Worlwide Innovations in the Development of Carbon Technologies and the Utilization of CO₂. Ener. Environ. Sci. 2012, 5, 7281-7305.

3.- Ávalos-Rendón T., Casa-Madrid J., Pfeiffer H., J. Phys. Chem. A 2009, 113, 6919-6923.

4. - Pfeiffer H. Advances in CO₂Conversion and Utilization; ACS Symposium Series; Hu, Y. H., Ed.; American Chemical Society: Washington, DC, 2010; Vol. 1056, pp 233−253.

5.- Li Z., Yang J., Hou J.G., Zhu Q., First-principles lattice dynamics of NaCoO₂, Physical Review B 2004, 70, 144518.

Keywords

CO₂ capture, sodium cobaltate, thermogravimetric analysis.