LDH/MgCO₃ hybrid multilayer on an aluminium substrate as a novel high-temperature CO₂ adsorbent

Abstract

A LDH/MgCO₃ hybrid multilayer thin film as a high-temperature CO₂ adsorbent has been fabricated in situ on an aluminium foil/mesh substrate, which possesses high CO₂ capture capacity (0.56 mmol g⁻¹) and may offer practical advantages such as higher thermal and mechanical stability, and a more flexible solution for making robust solid porous structures.

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There is a growing awareness that global warming is challenging the structure of our global society, and the anthropogenic emission of greenhouse gases (GHGs) such as CO₂ resulting from the burning of fossil fuels and various chemical processes is the major cause.^1,2 Unfortunately, it is predicted that the concentration of atmospheric CO₂ will continue to increase in the next several decades.³ To mitigate climate change, a large reduction in CO₂ emission is highly desired in the coming decades.⁴

In recent years, there has been increasing research interest in improving fossil energy utilization efficiency and novel technologies for CO₂ capture, utilization, and storage (CCUS).⁵ In order to remove CO₂ on a large scale, several different approaches have been proposed.⁶ Among them, the sorption-enhanced water gas shift (SEWGS) process, which is a combination of the WGS reaction and CO₂ adsorption, has been widely recognized as a very promising pre-combustion CO₂ capture technology. By adsorbing and removing CO₂ from the reaction mixture, the reaction maybe driven to the right-hand side, thereby completely converting CO and maximizing the production of H₂. SEWGS produces a hot stream of hydrogen that can be directly fed to a gas turbine, and a cooled stream of pure CO₂ that can be compressed and transported for further storage and utilization. To ensure the success of this technology, it is crucial to choose a highly efficient and suitable CO₂ adsorption material.^7–9 Among many different solid adsorbents, layered double hydroxides (LDHs) known as a class of ionic lamellar compounds which made up of positively charged brucite-like layers with an interlayer region containing charge compensating anions and solvation molecules have been regarded as one of the most promising CO₂ adsorbents for the SEWGS process.^10,11

To date, many studies have been performed on LDH-derived CO₂ adsorbents. For instance, the effects of divalent cations,¹² trivalent cations,¹⁰ charge compensating anions,¹¹ Mg–Al ratio,¹³ synthesis method,¹⁴ the presence of SO₂ and H₂O,^15,16 and particle size.¹⁷ Despite numerous studies, one critical issue that cannot be bypassed is their stability under real operating conditions. During the CO₂ adsorption/desorption cycles, LDH granules will gradually change to slurry (LDH pasting issue) due to the sintering at high operating temperature and the presence of steam, which consequently leads to a significant reduction in CO₂ adsorption capacity. To solve this problem, we propose the fabrication of a multilayer MgAl–CO₃ LDH based thin film on an aluminium substrate as novel high-temperature CO₂ adsorbent. Using this approach, the formed LDHs are strongly adhered to the metal surface. The anchoring effect together with the vertical alignment of the LDH platelets in the LDH thin film should be able to prevent the sintering and pasting issues of LDHs. In addition, these uniform aligned three-dimensional (3-D) arrays of LDHs should produce a high surface area.^18–20

In this work, a multilayer LDH/MgCO₃ hybrid based thin film on an aluminium substrate was fabricated using a urea hydrolysis method, in which aluminium foil/mesh was used as the substrate as well as the sole source of aluminium. By varying the crystallisation time, crystallisation temperature, the Mg/urea ratio, the formation process of MgAl–CO₃ LDH thin film was investigated in detail. The X-ray diffraction (XRD) patterns of MgAl–CO₃ LDH thin film crystallized for different time were compared in Fig. 1, together with the bare aluminium substrate. After the urea hydrolysis treatment in the presence of Mg(NO₃)₂, in addition to the characteristic peaks of the bare aluminium substrate, some new Bragg reflections were observed. When the crystallisation time was 6–9 h, the Bragg reflections corresponding to both the 00l and 0kl classes, together with the 110 and 113 Bragg reflections of a typical LDH (JCPDS 70-2151) phase appeared, which demonstrated the successful formation of MgAl–CO₃ LDH on the aluminium substrate.

Fig. 1 (a) XRD patterns of samples prepared at 90 °C for different time (0–48 h), (▼) aluminium, (●) aluminium hydroxide, (◆) MgCO₃. SEM images of samples prepared at 90 °C for different time: (b) cross-sectional microstructures, (c) 6 h, (d) 9 h, (e) 48 h, and (f) the enlarged image of the selected area in (e).

Upon increasing the crystallisation time to 12 h, the characteristic Bragg reflections of MgCO₃ (JCPDS 70-1177) were observed. As reported in literature, the deposition of a colloidal suspension of LDHs on substrates such as glass or silicon generally leads to the LDH nanoplatelets having preferred orientation with their c-axis perpendicular to the substrate.²¹ The fact that the MgAl–CO₃ LDH nanoplatelets are perpendicularly attached to the surface via their edges suggests that they are grown onto the substrate via a strong chemical interaction.²² Fig. S1† demonstrated that the after calcination, both LDHs and MgCO₃ decomposed and transformed into MgO (JCPDS 71-1176). Fresh material showed a low BET surface area of 10.8 m² g⁻¹. While after being calcined at 400 °C for 5 h, the BET specific surface area was greatly increased to 72.4 m² g⁻¹. It is reasonable since the calcined LDHs could result in a much higher specific surface area, which is favourable for CO₂ capture.

The morphology and alignment of obtained LDH thin films on the aluminium substrate were examined using SEM analysis, as shown in Fig. 1(b–f). Fig. 1(b) clearly shows that the LDH platelets within the films were perpendicularly attached to the surface of the substrate. Fig. 1(c–f) illustrated the surface morphology evolution as a function of crystallisation time. After hydrothermal synthesis at 90 °C for 6 h, a single-layer film with nanoplate-like LDH crystallite was observed on the aluminium substrate (Fig. 2(a)). With the increase in crystallisation time, the growth of the LDH was accelerated and a dense thin film of LDH platelets with their (00l) planes perpendicular to the aluminium gradually formed. Surprisingly after 9 h, some new rosette-shaped LDH crystallites formed on the top of the first LDH film (Fig. 2(b)). With a further extension of the crystallisation time to 48 h, some larger flower-like impurities were observed (Fig. 2(c)). According to the XRD analysis, such big flowers could be ascribed to MgCO₃. Fig. 1(e–f) indicates that the particle size of the flower-like MgCO₃ (300 μm) was much bigger than the rosette-shaped LDHs (2 μm). Thus we proposed that with the increase in crystallisation time, the LDH first formed a monolayer film, followed by the formation of another layer consisting of rosette-shaped LDHs on the top of the first layer. At sufficiently long reaction time, a third layer consisting of large flower-like MgCO₃ particles begins to appear.

Fig. 2 Schematic diagram of the fabrication process for the Mg/Al–LDH thin film grown directly on Al foil.

It is well known that the formation process of the LDH thin films on substrates includes both nucleation and growth.²³ For the growth of the multilayer thin film, a mechanism containing three steps was proposed, as shown in Fig. 2. Firstly, the Mg²⁺, CO₃²⁻, and OH⁻ ions in solution migrated to the surface of the aluminium substrate and were then adsorbed onto the active sites of the aluminium surface to form a single layer of plate-like LDH crystallite.²⁴ Secondly, under supersaturation conditions, the excess Al³⁺ and Mg²⁺ ions continued to form another “petal-like” LDH type crystallites upon the first layer. Finally at sufficiently long reaction time, the aluminium surface is not able to provide the necessary Al³⁺, which then triggered a switch to the formation of MgCO₃ on the surface.

In order to evaluate the high-temperature CO₂ capture performance of these LDH thin film based adsorbents, thermal gravimetric sorption of CO₂ was measured using a Q50 TGA analyzer. It is well known that a fresh LDH itself does not possess any CO₂ capture capacity. Upon thermal treatment, LDHs gradually lose their interlayer water, and then dehydroxylate and decarbonate to a large extent, leading to the formation of a mixed metal oxide with a poorly defined 3D network. The resultant partly-amorphous solid possesses a high surface area and high surface basicity, which is suitable for CO₂ adsorption.²⁵ Before adsorbing CO₂, samples were first calcined at 400 °C for 5 h to decompose the LDH's layered structure into mixed metal oxides to obtain more absorption surface area. To eliminate the error caused by memory effect,²⁵ all experiments were done immediately after the pretreatment. CO₂ adsorption experiments were carried out at 1 atm using a constant flow of CO₂ (20 ml min⁻¹). The CO₂ capture capacity of samples was tested at 200 °C, with an adsorption time of 2 h.

The effect of crystallisation time on CO₂ capture capacity was first investigated by fixing the crystallisation temperature at 90 °C. As shown in Fig. 3(a), the CO₂ capture capacity increased from 0.05 to 0.14 mmol g⁻¹ with the increase in crystallisation time from 6 to 48 h and thereafter it reached an equilibrium plateau. It suggested that the CO₂ capture performance matched well with the XRD and SEM analysis in Fig. 1. When the crystallisation time was short, only one monolayer LDH was attached on the surface of aluminium. The relatively low quantity of CO₂ adsorption can be attributed to the small amount of LDH. When the reaction time was long enough, the impurity of MgCO₃ was generated. After being calcined at 400 °C, MgCO₃ might decarbonate into MgO (Fig. S1†), which enhanced the CO₂ capture capacity. The influence of synthesis temperature on the CO₂ capture capacity of the samples was studied, as shown in Fig. 3(b). During the synthesis of LDHs, increasing the aging temperature was known to be beneficial in terms of increasing both the crystallinity and the size of the LDHs platelets.²³ Here we fixed the synthesis time at 48 h, and changed the synthesis temperature in the range of 90–180 °C. The results showed that the maximum capture capacity of 0.18 mmol g⁻¹ was happened at 140 °C.

Fig. 3 (a) The effect of synthesis time (90 °C, Mg/urea = 1 : 8), (b) the effect of synthesis temperature (48 h, Mg/urea = 1 : 8), (c) the effect of Mg/urea ratio (140 °C, 48 h), (d) the effect of synthesis time with the optimal synthesis temperature of 140 °C and the optimal Mg/urea ratio of 1 : 1 on the CO₂ adsorption capacity.

Fig. 3(c) and Table S1† showed the influence of Mg/urea ratio, when fixing the synthesis time at 48 h and temperature at 140 °C. The CO₂ capacity increased with the increase in Mg/urea ratio. The maximum CO₂ adsorption capacity of 0.33 mmol g⁻¹ was achieved with the Mg/urea ratio was 1 : 1. In order to find the best CO₂ capture synthesis condition, we chose the optimal Mg/urea ratio of 1 : 1 and the optimal synthesis temperature of 140 °C, and evaluated the effect of synthesis time again on CO₂ capture capacity. As shown in Fig. 3(d). We found that the CO₂ capture capacity can be further increased to as high as 0.56 mmol g⁻¹ when the synthesis time was 6–12 h. Fig. S2† shows that the CO₂ capture gradually decreased with the increase in adsorption temperature, which are 0.56, 0.48, 0.31, and 0.25 mmol g⁻¹ at 200, 250, 300, and 350 °C, respectively.

In addition to the CO₂ capture capacity, the continuous adsorption/desorption cycling stability of this novel adsorbent was also evaluated using the typical temperature swing adsorption (TSA) process, as shown in Fig. 4(a). The adsorption was performed at 200 °C for 30 min with pure CO₂, and the desorption was performed at 400 °C for 30 min with pure N₂. Fig. 4(a) shows that the adsorption capacity only slightly decreased during the first cycles and then levels off from the 15th cycle on with a value of 0.39 mmol g⁻¹. This result indicated that it has good CO₂ adsorption/desoprtion cycling stability and potential for practical applications.

Fig. 4 (a) The CO₂ capture capacity of LDH thin film based adsorbent during the adsorption/desorption cycling tests, (b) a possible scheme for the practical use the Al foil supported LDH thin film adsorbent. (c) A possible scheme for the practical use the Al mesh supported LDH thin film adsorbent.

For the practical application of high-temperature CO₂ adsorbents in the SEWGS process, many attempts have been tried to protect the materials from sintering and pasting. While the Al-substrate supported LDH thin film based adsorbents reported in this contribution provides an alternative option.²⁶ And both the Al-foil and Al-mesh based LDH thin film can be further fabricated into certain shapes for practical applications, as proposed in Fig. 4(b and c). For instance, the Al-foil can be made in to any shapes such as pellets for practical use. Before growing the LDH and MgCO₃ films on its surface, some holes can be made on the Al-foil, which creates more gas diffusion pathways which will enhance gas diffusion within the solid (Fig. 4(b)). After growing the LDH and MgCO₃ films on the surface of an Al-wire, it can be packed into an Al-mesh (Fig. 4(c)). In this way, the surface formed LDH and MgCO₃ films can be separated by the Al-foil and Al-wire substrates, preventing them from sintering and pasting, as shown in Fig. S3.† In addition, the anchoring effect between LDH film and Al substrate also improves the stability of the adsorbents.

In conclusion, a LDH/MgCO₃ hybrid multilayer thin film type CO₂ adsorbent has been fabricated in situ using an aluminium foil/mesh as both the substrate and the sole aluminium source by means of urea hydrolysis. Under the optimal synthesis condition, the LDH thin film based adsorbent showed a CO₂ adsorption capacity as high as 0.56 mmol g⁻¹, which is comparable to that of the conventional Mg–Al–CO₃ LDH powder derived adsorbent. It also demonstrated many practical advantages such as higher thermal and mechanical stability, and a more flexible solution for making robust solid porous structures.

Acknowledgements

This work is supported by the Program for New Century Excellent Talents in University (NCET-12-0787), the Beijing Nova Programme (Z131109000413013), the National Natural Science Foundation of China (51308045), and the Fundamental Research Funds for the Central Universities (TD-JC-2013-3).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra19170j

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