CO2 Chemisorption and Structural Analyses of the Na2(Zr1-xAlx)O3-x/2 Solid Solution

Different alkaline ceramics, mainly lithium or sodium, show excellent CO₂ capture properties.^1-3 Among them, zirconates have been reported as good candidates as CO₂ solid sorbents. In 2004 it was reported that Na₂ZrO₃ is able to absorb CO₂ at 600 °C. Since then, several works have reported that Na₂ZrO₃ presents much better characteristics as CO₂ captor in comparison with Li₂ZrO₃.^4-6 It has been explained because Na₂ZrO₃ has a lamellar structure, where sodium atoms are located among the (ZrO₃)^2‑ layers, which favors sodium diffusion porcesses.⁴               

On the other hand, different structural and/or chemical modifications have been carried out in order to enhance the CO₂ chemisorption of this kind of ceramics. For example, different solid solutions such as Li_2−xNa_xZrO₃, Li_3.7Al_0.1SiO₄, Li_3.7Fe_0.1SiO₄ and Li_4−xNa_xSiO₄ have been evaluated as possible CO₂ captors.^5,7-9 In general, all these solid solutions have shown an improvement on different properties of the CO₂ chemisorption reaction, in comparison to their respective pure alkaline ceramics. The improvements observed on these solid solutions have been attributed to the point defects formation into the lattice or to the secondary phase production.

            Therefore, the aim of the present work was to analyze both structural and microstructural characteristics of Na₂(Zr_1−xAl_x)O_3-x/2 solid solution as a function of the CO₂ chemisorption capacities.

To determine the influence of the aluminum content in the sodium zirconate during the CO₂ chemisorption process, specific compositions in the Na₂(Zr_1−xAl_x)O_3-x/2 solid solution were synthesized via a solid-state reaction. Samples were characterized using X-ray diffraction (D8 Bruker diffractometer), N₂ adsorption-desorption (Minisorp II equipment from BEL Japan) and ²⁷Al solid-state nuclear magnetic resonance (Bruker Avance II spectrometer). Then, samples were tested as CO₂ captors using dynamic and isothermal analyses with Q500HR equipment, from TA Instruments. The samples were dynamically heated from room temperature to 800 at 5 °C/min. These analyses were carried out under a saturated CO₂ atmosphere (60 mL/min). For the isothermal analysis, the samples were initially heated to 850 °C using a N₂ flow. This initial thermal step was performed to eliminate any previous sample carbonation. Then, each sample was cooled down to its respective isothermal temperature (between 300 and 700 °C) to perform independent CO₂ chemisorption processes. As the sample reached the corresponding temperature, the gas flow was switched from N₂ to CO₂.

            Additionally, ionic conduction experiments were performed in different samples to determine the effect of these phases during the CO₂ chemisorption process. For these experiments, fine grain powders of the samples were pressed in the form of pellets of about 1 cm in diameter and 0.1cm in thickness. Then, each pellet was heated at the sintering temperature of each compound, and the top and bottom pellet surfaces were coated with gold (as oxygen-ion blocking electrode), using a sputtering technique. The measurements were performed in a quartz cell coupled to a vacuum system (800-700 mTorr) and heated between 400 and 800 °C. Both sample sets were analyzed using the 2-point DC technique, and platinum wires as electrodes.

XRD results show that Na₂ZrO₃ and Na₂(Zr_1−xAl_x)O_3-x/2 solid solution corresponds to the monoclinic Na₂ZrO₃ crystalline phase (35-0770 JCPDS diffraction file). After the crystalline verification of the samples, some microstructural properties were determined by N₂ adsorption-desorption. The samples showed N₂ adsorption-desorption type II isotherms, with very narrow H3-type hysteresis loop, according to the IUPAC classification. This behavior corresponds to non-porous materials frequently obtained by solid state synthesis. Then, the surface areas were measured using the BET method, obtaining very similar values, ≤ 1 m²/g, in all cases. Besides, the ²⁷Al solid-state NMR results showed that aluminum atoms are located at the zirconium or sodium crystalline positions in the Na₂ZrO₃ structure (tetrahedral or octahedral positions, respectively). Thus, the aluminum dissolution is compensated by different structural defects. The CO₂ capture evaluation shows that the aluminum presence into the Na₂ZrO₃ structure improves the CO₂ chemisorption within certain aluminum content and under specific thermal conditions. Samples of the Na₂(Zr_1−xAl_x)O_3-x/2 solid solution were able to chemisorb CO₂ in a wide temperature range (200−700 °C), exhibiting higher CO₂ chemisorptions than Na₂ZrO₃. These results were corroborated kinetically. In fact, isothermal experiments shows that the Na₂ZrO₃ is the sample that chemisorbed CO₂ at a slower rate in comparison with the compounds of the Na₂(Zr_1−xAl_x)O_3-x/2 solid solution. Isotherms were fit to double or triple exponential models; where the observed reaction steps are: (1) CO₂ chemisorption over the surface of the Na₂(Zr_1−xAl_x)O_3-x/2 particles, (2) CO₂ chemisorption kinetically controlled by diffusion processes, and (3) CO₂ desorption process. The last process was detected only between 200 and 400 °C. The kinetic constant values indicated that the CO₂ chemisorption kinetically controlled by diffusion processes is the rate-limiting step for the whole process. Additionally, ΔH^‡ values were calculated using the Eyring’s model and they tended to increase when the aluminum was increased. This result indicated a higher thermal dependence of the CO₂ chemisorption as a function of the aluminum content. Additionally, the cyclic experiments indicate that Na₂(Zr_1−xAl_x)O_3-x/2 solid solution samples exhibited high and stable CO₂ capture behaviors.¹⁰

Finally, the sodium ionic conductivity, of different phases, was analyzed to complement the CO₂ chemisorption mechanism in these materials. The analyzed phases were; Na₂ZrO₃ and Na₂(Zr_0.9Al_0.1)O_2.95 as initial chemisorbents, as well as Na₂CO₃ and NaAlO₂ as the different sodium phases produced in the external shell. The ionic conductivity and activation energy of the Na₂(Zr_0.9Al_0.1)O_2.95 are smaller than those in Na₂ZrO₃. This result was is in good agreement with the initial ΔG_f thermodynamic theory, establishing that Na-O bond must become weaker in the aluminum containing compounds. However, if the ionic conductivity activation energy of NaAlO₂ and Na₂CO₃ (sodium phases present at the external shell) are compared, the activation energy in the NaAlO₂ case is higher than that in Na₂CO₃. Thus, the ionic conductivity processes in the external shell are limited by the NaAlO₂ presence.

Therefore, the results presented here confirm that aluminum atoms are incorporated into the Na₂ZrO₃ structure, occupying zirconium and sodium atom sites. The samples of the Na₂(Zr_1−xAl_x)O_3-x/2 solid solution, with aluminum atoms localized at octahedral sites, were able to chemisorb CO₂ in a wide temperature range (200−700 °C), exhibiting higher CO₂ chemisorptions than Na₂ZrO₃. The CO₂ chemisorption differences were attributed to several factors such as the presence or absence of different structural vacancies and the presence of different secondary phases in the carbonated external shell. Finally, NaAlO₂ formation must be mainly responsible for the drop in the CO₂ chemisorption that is experimentally observed.

References

1. Kumar, S.; Saxena, S.K. A Comparative Study of CO₂ Sorption Properties for Different Oxides. Mater. Renew. Sustain. Ener. 2014, 3, 1–15.
2. Wang, J.; Huang, L.; Yang, R.; Zhang, Z.; Wu, J.; Gao, Y.; Wang, Q.; O'Hare, D.; Zhong, Z. Recent Advances in Solid Sorbents for CO₂ Capture and New Development Trends. Ener. Environ. Sci. 2014, DOI: 10.1039/C4EE01647E
3. Ortiz-Landeros, J.; Ávalos-Rendón, T.; Gómez-Yáñez, C.; Pfeiffer, H. Analysis and Perspectives Concerning CO₂ Chemisorption on Lithium Ceramics Using Thermal Analysis. J. Therm. Anal. Calorim. 2012, 108, 647−655.
4. Alcerreca-Corte, I.; Fregoso-Israel, E.; Pfeiffer, H. CO₂ Absorption on Na₂ZrO₃: A Kinetic Analysis of the Chemisorption and Diffusion Processes. J. Phys. Chem. C 2008, 112, 6520−6525.
5. Pfeiffer, H.; Vazquez, C.; Lara, V. H.; Bosch, P. Thermal Behavior and CO2 Absorption of Li_2‑xNa_xZrO₃ Solid Solutions. Chem. Mater. 2007, 19, 922−926.
6. Zhao, T.; Ochoa-Fernández, E.; Rønning, M.; Chen, D. Preparation and High-Temperature CO₂ Capture Properties of Nanocrystalline Na₂ZrO₃. Chem. Mater. 2007, 19, 3294−3301.
7. Mejia-Trejo, V.; Fregoso-Israel, E.; Pfeiffer, H. Textural, Structural, and CO₂ Chemisorption Effects Produced on the Lithium Orthosilicate by its Doping with Sodium (Li_4−xNa_xSiO₄). Chem. Mater. 2008, 20, 7171−7176.
8. Gauer, C.; Heschel, W. Doped Lithium Orthosilicate for Absorption of Carbon Dioxide. J. Mater. Sci. 2006, 41, 2405−2409.
9. Pfeiffer, H.; Lima, E.; Bosch, P. Lithium-Sodium Metazirconate Solid Solutions, Li_2-xNa_xZrO₃ (0 ≤ X ≤ 2), a Hierarchical Architecture. Chem. Mater. 2006, 18, 2642.
10. Alcántar-Vázquez, B.; Diaz, C.; Romero-Ibarra, I. C.; Lima, E.; Pfeiffer, H. Structural and CO₂ Chemisorption Analyses on Na₂(Zr_1−xAl_x)O₃ Solid Solutions. J. Phys. Chem. C 2013, 117, 16483−16491.