Tracking sodium cobaltate formation pathways and its CO₂ capture dynamics in real time with synchrotron X-ray diffraction

Abstract

Na-based high temperature CO₂ solid sorbents hold great potential for capturing the CO₂ emitted by large stationary sources. However, to benefit from these materials in schemes of CO₂ capture, simple synthesis procedures together with a comprehensive understanding of their behaviour under operative conditions is essential. In this work, we use time-resolved in situ synchrotron X-ray diffraction coupled with Rietveld analysis to investigate the synthesis and high temperature CO₂ capture dynamics of NaCoO₂ solid sorbent. NaCoO₂ was synthesized via two different routes, from CaCO₃·H₂O and Na₂CO₃ and from Co₃O₄ and Na₂CO₃ reactants, and a comparative analysis of the temperature-dependent phase transformations occurring during each synthesis reaction and their effect on the final product allowed to identify the most efficient synthesis route. The reaction mechanism between NaCoO₂ and CO₂ in the temperature range between 50 °C and 750 °C at 1 bar of CO₂ is also provided. The results on the fundamental aspects underpinning NaCoO₂ synthesis and its CO₂ capture dynamics under realistic operative conditions are key for the design and development of affordable CO₂ solid sorbents.

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Greenhouse gas emissions are pinpointed as the key driver of global warming and therefore responsible for the changes observed in the climate, which are unprecedented in thousands of years and which have set in motion far-reaching massive effects on ecosystems, bringing immeasurable ecological, economical and social negative impacts.^1–5 Among greenhouse gases of varying lifetimes contributing to climate change, the great persistence displayed by CO₂ renders its warming practically irreversible for more than 1000 years.⁶ Because of this, strong and sustained reductions in emissions of CO₂ are an imperious need.

Society is confronted with the important objective of achieving carbon neutrality on a global scale. While there is widespread agreement that electromobility is leading the charge in decarbonizing transportation, finding viable solutions for other industrial sectors proves to be a significant challenge. In cases where electrification is not feasible, it becomes essential to explore technological advancements that can effectively prevent CO₂ emissions from being released into the atmosphere. Considering that power plants contribute to 40% of human-caused CO₂ emissions,⁷ the development and implementation of engineering solutions capable of capturing CO₂ from high-temperature flue gases in large stationary sources, such as power stations and industrial plants, emerge as a promising avenue for mitigating CO₂ gas emissions.

Many ceramic compounds are able to react with CO₂ at high temperatures, and this occurs via a two-step temperature dependent gas–solid reaction. At low temperatures, CO₂ reacts at the surface of the material producing a carbonated shell, and at high temperatures the reaction proceeds through the bulk of the particles. The captured carbon can be later utilized as a valuable feedstock for industrial chemical production. Within ceramic compounds, Lithium containing materials have shown high CO₂ sorption capacity, together with good reversibility over a wide temperature operation range.⁵

Li₄SiO₄ is one of the most studied systems for high temperature CO₂ capture applications,^8–10 exhibiting a high CO₂ sorption capacity of 0.367 g_CO2 g_sorbent⁻¹, low regeneration temperatures (<750 °C), and remarkable sorption/desorption cycling stability, which ensures a long operational lifetime under realistic conditions.¹¹ However, despite Li₄SiO₄ excellent CO₂ capture properties, the growing demand of Li to fulfil the needs of other technological applications, such as Lithium-ion batteries, along with the fact that it is not highly abundant has given rise to a serious concern about Li long-term availability¹² and creates the need to explore more accessible, cheap and sustainable materials for CO₂ sorbents.

Partial replacements of lithium by sodium in Li₄SiO₄ ceramic in the form of solid solutions of Li_4−xNa_xSiO₄ have shown an enhancement of CO₂ absorption kinetics compared to pure Li₄SiO₄, together with increased CO₂ absorption.¹³ This indicates that Na atoms act as active sites for CO₂ capture reactions. Furthermore, complete replacements of Li by Na, leading to the Na₄SiO₄ structure, have shown improved CO₂ capture performance in terms of high temperature sorption and desorption compared with Li₄SiO₄.¹⁴

Na₂CO₃, Na₂ZrO₃ and NaCoO₂ are promising options for obtaining lithium-free CO₂ solid sorbents.^15–19 A recent study showed the great ability of NaCoO₂ to catalyze the conversion of CO to CO₂ and to subsequently chemisorb the latter.²⁰ NaCoO₂ compounds are comprised of hexagonal CoO₂ blocks and a Na layer, forming a layered oxide material. Notably, the Na layer exhibits a high degree of vacancy, leading to variations in the crystal structure depending on the Na content. Such unique structure consisting of layered hexagonal blocks with alkaline elements is expected to allow for easy diffusion and improved chemisorption of CO₂, and the introduction of structural defects can be used as an strategy to modify the diffusion processes. Therefore, the crystalline structure of the material plays a crucial role in enhancing the diffusion of sodium during the CO₂ chemisorption process. Furthermore, the presence of cobalt in this ceramic may induce catalytic activity in the CO oxidation reaction. Based on these findings, efforts have been dedicated to understand the chemical reactions that this material undergo at high temperatures.^18–20

Results from ex situ X-ray diffraction studies suggest that upon carbonation NaCoO₂ would transform to Na₂CO₃ and cobalt oxides, Co₃O₄ and CoO. While these findings provide support for the reactivity of NaCoO₂ towards CO₂, the presence of such cobalt oxides suggests a change of cobalt valence, indicating a complex CO₂ capture process involving not only surface and diffusion reactions but also electronic exchanges. This ex situ evidence provokes the need of a comprehensive understanding on how NaCoO₂ behaves in the presence of CO₂ in the temperature range that is of interest for CO₂ capture applications, which is critical to evaluate its performance under real working conditions and to determine its regeneration potential. This requires the use of in situ advanced characterization techniques.

X-ray diffraction analysis (XRD) has been widely used for identifying crystallographic phases after high-temperature CO₂ experiments, and in situ XRD experiments have proven to be particularly beneficial for monitoring the chemical reactions and morphological changes that sorbents undergo upon carbonation.^21–25 Moreover, the advanced capabilities of synchrotron facilities, including their high brilliance and detection capabilities, have enabled the performance of time-resolved synchrotron XRD studies. This cutting-edge approach allows for the dynamic behavior of materials at elevated temperatures to be tracked with a resolution of seconds, allowing to reveal the appearance of intermediate phases and the occurrence of re-conversion processes which would not be possible to detect using standard laboratory sources.^26–29 Hence, time-resolved high temperature synchrotron analysis on the carbonation reactions of NaCoO₂ sorbent would bring unique insights on its reaction mechanism with CO₂ under operative conditions and would provide solid grounds to analyze its potential use as CO₂ solid sorbent for real applications.

Owing to its promising properties as a CO₂ sorbent and its excellent performance as cathode for rechargeable sodium batteries,^30–32 there is also a general interest in improving NaCoO₂ synthesis conditions. Among the many reported synthesis methods, the solid-state route is a well-established and simple procedure to synthesize NaCoO₂ structures. Reddy and co-workers have shown to obtain hexagonal P2-NaCoO₂ through the calcination of mixed stoichiometric amounts of CH₃COONa, Co(CH₃COO)₂·4H₂O and glycine at 800 °C. Also, in a recent work,³³ P2-NaCoO₂ was synthesized by solid state reaction from Na₂CO₃ and Co₃O₄ heat treated at 850 °C for 24 h under air flow. Other works used reactive Na₂O₂ and Co₃O₄ as starting reactants and performed different heat treatments at temperatures ranging from 450 °C to 750 °C.³⁴ Hence, due to the multiple methods that have been reported for the synthesis of NaCoO₂ and the need to use cheap and benign precursors, more thorough investigations are required.

To provide a comprehensive assessment of the potential use of NaCoO₂ as CO₂ sorbent material, this study focuses on two main aspects. Firstly, we perform a comparative investigation of two thermally driven synthesis routes for obtaining this material at the lower possible temperature from simple reactants using time-resolved synchrotron powder X-ray diffraction coupled with Rietveld analysis. Later, we analyze in real time the structural changes of NaCoO₂ under CO₂ flow in the temperature range from 30 °C to 750 °C. In situ X-ray diffraction coupled with Rietveld analysis allowed to identify reaction products and to propose a simple reaction mechanism, which is different from the one proposed on the basis of previous ex situ XRD results. The results from this work are crucial for the efficient synthesis of NaCoO₂ and provide solid grounds of its reaction mechanism as CO₂ sorbent and its regeneration capability.

1. Experimental

1.1. In situ synthesis of NaCoO₂

NaCoO₂ was synthesized via two solid-state routes. In the first synthesis route, powders of Na₂CO₃ (99.5%, Sigma Aldrich) and CoCO₃·xH₂O (99.9%, Sigma Aldrich) were used as reactants, whereas in the second synthesis NaCoO₂ was produced from Na₂CO₃ and Co₃O₄ (99.9%, Sigma Aldrich). Based on preliminary results, 20 wt% excess of the sodium source was added to the mixture to compensate Na losses due to sublimation, which is in line with previous publications.²⁰ Reactant powders were mechanically mixed in a Retsch shaker for 5 minutes, then loaded into a 1 mm quartz capillaries and heated up under synthetic air flow from room temperature to 800 °C at a ramp rate of 5 °C min⁻¹. Temperature inside the capillary was measured constantly during the experiment using a thermocouple in direct contact with the sample. Another thermocouple was located on the outside of the capillary of control the temperature of the hot nitrogen stream provided by the gas blower to heat the system. In the first synthesis 30 min isothermal conditions were kept after each 50 °C increment, whereas in the second synthesis the temperature was constantly increased without any isothermal step.

Time-resolved in situ synchrotron X-ray powder diffraction measurements were performed during both synthesis reactions. A sketch of the experimental setup can be found elsewhere.²⁷ Experiments were performed at the High-energy X-ray Diffraction Beamline ID31 of the European Synchrotron Radiation Facility (ESRF). Data collection time was 2 seconds, and the time resolution was of 30 seconds. Data was acquired using a Pilatus Dectris 2 M CdTe detector, at a wavelength was of 0.1771 Å and 0.1589 Å, and with a beamsize of 0.6 mm × 0.6 mm (V × H). Data was processed using pyFAI package³⁵ and Rietveld analysis was performed using Fullprof suite software.³⁶

1.2. In situ NaCoO₂ CO₂ capture

NaCoO₂ powder was loaded into a 1 mm quartz capillary. The inlet of the capillary was connected to a gas rig system that allowed the feed of a 10 mL min⁻¹ flow of CO₂ : N₂ (50 : 50) gas mixture to the sample and a continuous monitoring of the gas pressure. The capillary was horizontally placed on top of a gas blower that enabled the controlled heating and cooling of the sample in the temperature range from 30 °C to 750 °C at a ramp rate of 5 °C min⁻¹.

2. Results

2.1. NaCoO₂ from Na₂CO₃ and CoCO₃·xH₂O

In Fig. 1a is displayed a contour plot with the X-ray diffraction patterns collected during the high-temperature synthesis reaction of NaCoO₂ from monoclinic Na₂CO₃ (m-Na₂CO₃) and CoCO₃·xH₂O. The miller indices of the main phases present at different stages of the synthesis process are indicated within the graph. The evolution of temperature during the synthesis is depicted in Fig. 1b. The vertical axis of Fig. 1b is scaled in order to match the evolution of the diffractograms presented in Fig. 1a.

Fig. 1 a) Contour plot of the XRD patterns collected during the synthesis of NaCoO₂ from CoCO₃ and m-Na₂CO₃ precursors, b) evolution of the temperature and time during the synthesis process.

As temperature increases during the initial heating process, the reflections associated with the starting phases undergo a noticeable shift towards lower angles. This shift is a result of the thermal expansion of the respective lattices due to the temperature rise.

A first phase transition is observed at 240 °C. At this temperature, CoCO₃ decomposes into Co₃O₄ and CoO. This reaction must be accompanied by a release of CO₂. Interestingly, CoO rapidly reacts with oxygen coming from the synthetic air stream and is converted to Co₃O₄. According to previous reports,³⁷ at 240 °C this intermediate reaction that results in the formation of CoO is not thermodynamically favoured against the one that results on its complete transformation into Co₃O₄. However, the presence of CoO is clear in the diffraction patterns. This is an indication that the conversion is being affected by kinetic and/or diffusion limitations, which introduce this intermediate reaction. In this regard, it is worth noting that previous thermogravimetric analysis conducted on the heating of CoCO₃ (ref. 38) in air atmosphere based on the assumption of the chemical reaction 6CoCO₃ + O₂ → 2Co₃O₄ + 6CO₂ showed weight losses higher than the theoretical ones. These results could be explained considering the intermediate transition of CoCO₃ to Co₃O₄ + CoO + CO₂.

At 240 °C a coexistence of four phases, m-Na₂CO₃, CoCO₃·xH₂O, CoCO₃ and CoO, is evidenced. At slightly higher temperatures, the reflections belonging to the CoCO₃ phase completely vanish. Further heating until 480 °C provokes a phase transition of Na₂CO₃ from monoclinic to an hexagonal crystal structure (h-Na₂CO₃). This high temperature phase transition was previously reported to occur above 400 °C (ref. 39) and has shown reversibility upon cooling.⁴⁰ At temperatures above 480 °C, the only phases present are h-Na₂CO₃ and Co₃O₄.

At 770 °C, both phases are expected to react to form NaCoO₂ through the following reaction:

2Na₂CO₃ + 4CoCO₃ + O₂ → 4NaCoO₂ + 6CO₂

However, the in situ XRD results showed that the phase Na_0.74CoO₂ is formed, as it can be observed both from the refined crystal structure (Fig. 2f) and from comparison with previous results,⁴¹ indicating the following reaction:

0.74Na₂CO₃ + 2CoCO₃ + 0.63O₂ → 2Na_0.74CoO₂ + 2.74CO₂

Fig. 2 Experimental X-ray diffraction patterns and results of the fittings performed using the Rietveld method for data collected at: a) 50 °C, b) 300 °C, c) 500 °C, d) 700 °C, e) 770 °C and f) 780 °C during NaCoO₂ synthesis process. Values of the refined parameters are presented at Tables S1A–F in ESI.†

This synthesis occurs immediately, the formation of a phase with vacancies in the Na site has been widely reported in previous experiments.²⁰ At 800 °C no further phase transformations are observed, and no residues from h-Na₂CO₃ and Co₃O₄ are evidenced, meaning that the reaction is completed. Once the synthesis is completed, the sample was cooled down at a rate of 10 °C min⁻¹. The shifts to the right of the peaks corresponding to the Na_0.74CoO₂ phase are due to cooling down of the sample.

In Fig. 2 are shown selected XRD patterns corresponding to six different stages of the synthesis process, together with their respective Rietveld refinement fitting results. Starting Na₂CO₃ and CoCO₃·xH₂O exhibit a monoclinic C12/m1 (JCPDS 01-077-2082) and rhombohedral R3̄c (JCPDS 01-078-0209) structures, respectively. At 300 °C these two structures are still preserved, but the cobalt carbonate suffers a disproportion to cubic Co₃O₄Fd3̄m (JCPDS 01-076-1802) and cubic CoO Fm3̄m (JCPDS 00-009-0402). At this stage the phase percentages obtained by Rietveld analysis yielded the following results: 28.8 wt% Na₂CO₃, 31.6 wt% CoCO₃, 16.9 wt% Co₃O₄ and 22.7 wt% CoO. Further heating leads to the complete transformation of cobalt-based phases to Co₃O₄, with a phase composition of 48.4 wt% Na₂CO₃ and 51.6 wt% Co₃O₄, and Na₂CO₃ transforms from monoclinic C12/m1 to hexagonal P63/mmc structure (JCPDS 01-009-8623).

Further heating up to 600 °C results in a significant growth h-Na₂CO₃ grain domains, as it can be observed from the more defined and pronounced diffraction peaks of Fig. 2d. In Fig. 2e are depicted the XRD pattern corresponding to a phase coexistence between Co₃O₄, h-Na₂CO₃ and NaCoO₂, with a phase percentage of 64.0 wt%, 15.6 wt% and 20.4 wt%, respectively. Fig. 2f shows the state of the system at 780 °C, where the reaction is not fully completed but most of the system has transformed to Na_0.74CoO₂. The reported experimental results demonstrate that NaCoO₂ (Na_0.74CoO₂) can be synthesized at 780 °C, which is a temperature 70 °C lower than the reported in the literature using the same reactants.²⁰

2.2. In situ NaCoO₂ synthesis from Na₂CO₃ and Co₃O₄

NaCoO₂ was also synthesized from Na₂CO₃ and Co₃O₄ powders. This synthesis route was previously used by Krasutskaya and co-workers.⁴² In the proposed method, powder precursors were first mixed and grinded using an agate mortar and the resulting powder mixture was pressed at 40 MPa to produce pellets, which were then fired at 860 °C in air for 12 h. In their work, the authors used a Na : Co ratio of 1.2× : 1. This excess of Na₂CO₃ in the starting mixture compensates for the Na₂O loss during the high temperature heat treatment. In another work,⁴³ NaCoO₂ was synthesized from the same reactants, but using a procedure in which the powder mixture was grinded manually in an agate mortar for 2 h, then loaded into an alumina crucible and subsequently sintered three times at 860 °C in air. In such synthesis procedure, intermediate grinding was performed in a moisture-free atmosphere inside a glove box to obtain an homogeneous composition. Therefore, the total sintering time was of 36 h.

In this work, the synthesis was performed by heating the mixture of oxides with a ramp of 5 °C min⁻¹ without any isothermal step. In Fig. 3 it can be observed that m-Na₂CO₃ and Co₃O₄ are the main phases present from room temperature until 200 °C, temperature at which Co₃O₄ evolves towards the formation of Co₂O₃. The initial composition is presented in Fig. 4a and the coexistence of the three mentioned phases is clearly observed in Fig. 4b. The presence of Co₂O₃ phase was not expected, since this phase is thermodynamically unstable, however, Fig. 4c indicates a significant amount of this phase at 450 °C. As can be seen from Fig. 3, the temperature range at which this particular phase exists is narrow, and Co₂O₃ swiftly reverts to Co₃O₄ upon further heating. At 550 °C, Fig. 4d this reversion is fully completed, leading to Co₃O₄ becoming the dominant phase in the system. From this temperature onwards, Na₂CO₃ and Co₃O₄ are the main components, although now m-Na₂CO₃ has transitioned to h-Na₂CO₃, as it was reported in the previous synthesis process. Finally, reflections corresponding to Na_0.74CoO₂ phase emerge at 690 °C as observed in Fig. 4e. At 700° only 12 wt% of Na_0.74CoO₂ is present on the sample, and after 20 min of isothermal conditions at this temperature, Fig. 4f, the synthesis reaction reaches completion. After 1 h of isothermal conditions the system remains stable, with 3.6 wt% of h-Na₂CO₃ detectable as impurity.

Fig. 3 a) Contour plot of the XRD patterns collected during the synthesis of NaCoO₂, starting from m-Na₂CO₃ and Co₃O₄ powders, b) temperature evolution during the solid-state synthesis of NaCoO₂.

Fig. 4 Experimental X-ray diffraction patterns and results of the fittings performed using the Rietveld method for data collected at: a) 50 °C, b) 350 °C, c) 450 °C, d) 550 °C, e) 700 °C and f) 700 °C after 1 h of isothermal conditions during NaCoO₂ synthesis process. Values of the refined parameters are presented at Tables S2A–F in ESI.†

According to the results, Na_0.74CoO₂ is formed according to the following chemical reaction:

0.74Na₂CO₃ + 0.667Co₃O₄ + 0.297O₂ → 2Na_0.74CoO₂ + 0.74CO₂

Which reveals again the presence of Na vacancies on the reaction product.

The synthesis of Na_0.74CoO₂ from m-Na₂CO₃ and Co₃O₄ reactants shows a Na_0.74CoO₂ formation temperature of about 100 °C lower compared with the solid state route involving the use of Na₂CO₃ and CoCO₃·H₂O precursors. Comparing the in situ XRD data of both synthesis processes, it becomes clear that the difference observed in the synthesis temperature of Na_0.74CoO₂ comes from the need of an intermediate stage in the first synthesis method, in which the CoCO₃ transitions towards Co₃O₄, a step that requires higher temperature and energy input.

Overall, the lower synthesis temperature observed in the reaction between Na₂CO₃ and Co₃O₄ suggests a more efficient and potentially cost-effective route for the production of sodium cobaltate compared to the solid-state method involving m-Na₂CO₃ and CoCO₃·H₂O as precursors. It must also be emphasized that the reported synthesis temperature for a sodium cobalate from m-Na₂CO₃ and Co₃O₄ is 150 °C lower than temperatures reported in the literature using the same reactants.

2.3. CO₂ capture

Previous research on sodium and lithium ceramics suggests that a reaction between NaCoO₂ and CO₂ would occur according to the following route:^19,20

2NaCoO₂ + CO₂ → Na₂CO₃ + Co₂O₃

However, Co₂O₃ reaction product is a highly unstable compound according to thermodynamics and has been rarely observed outside very particular conditions,^44,45 so its formation during the absorption process is highly unlikely, although it may be possible for it to appear as an intermediate product for shorts amounts of time before decomposing into O₂ and Co₃O₄.

In a study conducted by Vera et al.,²⁰ the dynamic thermogram of NaCoO₂ under CO₂ flow was measured and displayed a continuous weight gain between 150 °C and 740 °C. This weight gain was divided into two temperature ranges: (i) 180–415 °C, corresponding to the superficial CO₂ chemisorption process, and (ii) 415–740 °C, corresponding to the bulk CO₂ chemisorption process. The authors analyzed ex situ XRD samples of CO₂NaCoO₂ isothermal reaction products and observed significant structural modifications of NaCoO₂ at 400 °C, which were attributed to partial sodium release and subsequent formation of Co₃O₄. As the temperature increased further, between 450 °C and 700 °C, reflections belonging to NaCoO₂ phase gradually disappeared, diffraction peaks corresponding to Na₂CO₃ phase emerged and reflections corresponding to Co₃O₄ phase became stronger. Interestingly, at temperatures exceeding 700 °C, the authors evidenced the presence of CoO phase. These findings contradicted previous literature proposing the presence of a different cobalt oxide, Co₂O₃, based on the assumption that cobalt retains its +3 valence state. Hence, Vera and coworkers claim that the valence of cobalt changes from +3 to +2, as indicated by the presence of Co₃O₄ and CoO detected by XRD analysis and that therefore, cobalt would reduce with increasing temperature, which must be accompanied by an oxygen release.

The carbonation reaction mechanism of NaCoO₂ provided by Vera et al. is presented in the following:

3Co₂O₃ → 2Co₃O₄ + 1/2O₂ 400 °C < T < 700 °C

Co₃O₄ → 3CoO + 1/2O₂T > 700 °C

In this work, dynamical chemical reactions occurring during standard thermograms in CO₂ sorption analysis were analyzed. For this, synthesized NaCoO₂ was subjected to continuous heating at a ramp of 5 °C min⁻¹ under a CO₂ : N₂ gas flow. Fig. 5a displays the evolution of the X-ray diffraction patterns upon heating, and Fig. 5b shows the corresponding temperature evolution.

Fig. 5 a) Contour plot displaying XRD data corresponding to NaCoO₂ dynamic heating under CO₂ gas flow until 750 °C, b) temperature evolution during NaCoO₂ carbonation process.

The contour plot of Fig. 5a sheds light on the high-temperature reactions taking place during CO₂ absorption, and reveals the occurrence of a single step process in which NaCoO₂ reacts to form Co₃O₄ and Na₂CO₃. Importantly, no Co₂O₃ is evidenced. Therefore, it can be stated that Co₂O₃ would not form as an intermediate compound previous to the formation of Co₃O₄, as it was claimed by previous authors. These findings are consistent with theoretical results,⁴⁴ which indicate that Co₂O₃ compound is thermodynamically highly unstable. Also, it must be remarked that the appearance of CoO phase is not observed from our experimental data, which corresponds to temperatures up to 750 °C. It is worth noting that the presented results do not contradict the previous reports by Vera et al.²⁰ since they have reported the presence of CoO only in samples that had been subjected to temperatures higher than 750 °C, which is the maximum temperature reached in this experiment.

Rietveld refinement results allowed to track the composition of the system in real time. The analysis of the XRD patterns during the CO₂ capture process was performed and is presented in Fig. 6. As it is possible to observe, Na_0.74CoO₂ remains stable until approximately 100 °C, temperature at which the absorption reaction starts. At this temperature, marked with a vertical dashed line in the figure, the formation of Co₃O₄ and Na₂CO₃ is observed. From Fig. 6 it is possible to see a previous jump in the reaction at t = 37 min. This is an artifact caused by the need to keep the temperature on hold for a long time due to a beam loss during the experiment (in Fig. 5 and 6 the time loss caused by the beam drop is compressed to make the figures more readable). The reaction keeps advancing very slowly until the system reaches 400 °C, temperature at which the reaction rate starts to grow. To make it easier to follow with the reaction, weight fractions were transformed into molar fractions using the following equation:

Fig. 6 Evolution of temperature and sample weight percent during NaCoO₂ carbonation reaction according to results from Rietveld analysis.

where x_a, w_a and M_a are the mole fraction, weight percent and molecular mass of the phase a respectively.

At 400 °C, the CO₂ capture rate increases until reaching 550 °C. At this temperature, the system CO₂ capture dynamics slows again showing an asymptotic behaviour. Previous works have reported that at temperatures under 400 °C there is process of surface adsorption, however our results indicate that at these temperatures there is already some formation of Co₃O₄ and Na₂CO₃. This is an indication that the actual process occurring is actually a chemisorption and that the reason why the reaction does continue is because of kinetic limitations.

Based on the time-resolved in situ results provided in this work, the proposed temperature-dependent reaction mechanism for Na_0.74CoO₂ carbonation process in the temperature range of 50 °C < T < 750 °C is the following:

4Na_0.74CoO₂ + 1.48CO₂ → 1.48Na₂CO₃ + 1.33Co₃O₄ + 0.59O₂

The main difference between the proposed reaction mechanism and the one that has been proposed by previous authors is the absence of Co₂O₃ phase, which was not observed at any point during the absorption process.

3. Conclusions

In this work, we analyzed the solid-state synthesis of NaCoO₂via two different routes. In the first route, the starting reactants were Na₂CO₃ and CoCo₃·nH₂O, while in the second case powders of Co₃O₄ and Na₂CO₃ were used as precursors. In both cases, the starting powder mixtures were heated up from room temperature to 700–800 °C and the evolution of the chemical reactions taking place at high temperatures was tracked time-resolved synchrotron powder X-ray diffraction.

Results revealed that the synthesis of NaCoO₂ from Na₂CO₃ and CoCo₃·nH₂O involves a two step process, in which CoCo₃·nH₂O first decomposes to Co₃O₄ and CoO, and the latter readily reacts with oxygen, resulting in its rapid conversion to Co₃O₄. Then, Co₃O₄ reacts with Na₂CO₃ to form NaCoO₂. In the second route, NaCoO₂ is formed following a single step process from Na₂CO₃ and Co₃O₄. However, a reversible reaction of Co₃O₄ to form Co₂O₃ was evidenced. Co₂O₃ reaction product is unstable thermodynamically and therefore was rapidly converted to Co₃O₄. Overall, the second synthesis route allowed to obtain NaCoO₂ at the lowest temperature of 700 °C.

The CO₂ capture dynamics of the resulting NaCoO₂ was also analyzed in the temperature range from 30 °C to 750 °C by heating the sample under controlled CO₂ flow. Time-resolved in situ X-ray diffraction data allowed to propose a reaction mechanism for the gas–solid reaction in which NaCoO₂ reacts with CO₂ to form Na₂CO₃ and Co₃O₄ as solid reaction products. These results contradict the previously proposed reaction mechanism for NaCoO₂ carbonation reaction.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank the European Synchrotron Radiation Facility for beamtime allocation, Florian Rusello and Tiago Couthinio are acknowledged for their help with the preparation of the experimental setup.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3re00459g

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