Accurate Control of Cage-Like CaO Hollow Microspheres for

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Energy and the Environment

Accurate Control of Cage-Like CaO Hollow Microspheres for Enhanced CO2 Capture in Calcium Looping via a Template-assisted Synthesis Approach Jian Chen, Lunbo Duan, and Zhao Sun Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b06138 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 18, 2019

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Environmental Science & Technology

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Accurate Control of Cage-Like CaO Hollow

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Microspheres for Enhanced CO2 Capture in Calcium

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Looping via a Template-assisted Synthesis

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Approach

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Jian Chen, Lunbo Duan*, Zhao Sun

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Key Laboratory of Energy Thermal Conversion and Control, Ministry of Education, School of

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Energy and Environment, Southeast University, Nanjing 210096, China

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ABSTRACT: Herein we report the development of synthetic CaO-based sorbents for enhanced

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CO2 capture in calcium looping via a template-assisted synthesis approach, where carbonaceous

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spheres (CSs) derived from hydrothermal reaction of starch are used as the templates. Cage-like

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CaO hollow microspheres are successfully synthesized only using urea as the precipitant, and the

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formation mechanism of this unique hollow microsphere structure is discussed deeply. Moreover,

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cage-like CaO hollow microspheres possess an initial carbonation conversion of 98.2% and 82.5%

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under a mild and harsh conditions, respectively. After the fifteen cycles, cage-like CaO hollow

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microspheres still possess a carbonation value of 49.2% and 39.7% under the corresponding

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conditions, exceeding the reference limestone by 85.7% and 148.1%, respectively. Two kinetic

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models are used to explore the mechanism of carbonation reaction for cage-like CaO hollow

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microspheres, which are subsequently proved to be feasible for analysis of chemical-controlled

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stage and diffusion-controlled stage in the carbonation process. It is found the unique hollow

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microsphere structure can significantly reduce the activation energy of carbonation reaction

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according to the kinetic calculation. Furthermore, the energy and raw material consumptions

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related to the synthesis of cage-like CaO hollow microspheres are analyzed by the life cycle

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assessment (LCA) method.

24 25 26

INTRODUCTION

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Calcium looping, based on a reversible carbonation/calcination reaction of CaO-based

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sorbents, is recognized as one of the most promising CO2 capture technologies.1-12 However, the

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CO2 uptake capacity of naturally occurring CaO-based sorbents decreases significantly with the

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repeated carbonation/calcination cycles.13,14 Thus, to stabilize the CO2 capture performance of

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CaO-based sorbents, different strategies such as hydration13,15 and thermal pre-treatment16 have

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been proposed. Unfortunately, these methods can only partially mitigate the dramatic decline in


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the CO2 uptake of CaO-based sorbents.17,18 The development of synthetic CaO-based sorbents has

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received considerable attention recently.19 It has been reported CaO-based sorbents synthesized

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with a nanostructure possessed a high and stable CO2 uptake capacity, which can minimize the

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diffusion lengths of CO2 through the freshly formed CaCO3 product layer.20-22 Furthermore, the

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CaO-based sorbents should possess a certain level of porosity in order to be able to compensate

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for the pronounced volume variations during the repeated cycles, as the molar volume of CaCO3

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is more than twice as high as that of CaO.23

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Hollow structures refer to materials with well-defined boundaries and interior cavities, which

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have attracted tremendous attention due to their unique structure and controllable morphology.24-

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26

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assisted synthesis approach, using different templates such as carbonaceous spheres (CSs),27

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polystyrene microbeads,28 sulfonated polystyrene,29 and carbon gel.30 Recently, CaO hollow

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microspheres have been proposed for calcium looping process due to their significant potential to

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enhance CO2 capture performance. On one hand, a hollow microsphere structure can provide a

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high specific surface area with numerous active sites for the occurrence of carbonation reaction,

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enabling rapid kinetics and high sorption capacity. On the other hand, the central void can provide

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the required volume to buffer against the pronounced volume variations accompanied with the

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repeated carbonation/calcination cycles, thus leading to improved cycling stability. Ping et al.

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synthesized CaO hollow microspheres using CSs as the template, and found that CaO hollow

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microspheres with a cavity diameter of 1.62 μm reached the maximal theoretical value (i.e., 0.786

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g CO2/g CaO).27 Noted that CS templates are more environmental-friendly compared to the

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polymer sphere templates such as polystyrene microbeads28 and sulfonated polystyrene.29 It is

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because the synthetic procedure of CS templates involves none of the toxic organic solvents,

The most widely employed method for the production of hollow structures is the template-


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initiators or surfactants, which are commonly used in the fabrication of polymer sphere

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templates.31,32 However, the work conducted by Ping et al.27 studied the CO2 sorption capacity and

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sorption rate of CaO hollow microspheres only in the first cycle, which is lack of research for

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cyclic stability. And their experiments were conducted under an unrealistic operating condition,

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i.e., regeneration in pure N2 atmosphere at a low temperature of 800 °C. Besides, the essential

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conditions of fabrication of CaO hollow microspheres were not clearly obtained in their work.

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Although CaO hollow microspheres exhibit great potential in enhancement of CO2 capture

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performance, yet little work has been conducted concerning the fabrication of CaO hollow

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microspheres. Thus, here we use the template-assisted synthesis approach to yield a highly

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effective CaO-based sorbent. CSs, formed from a hydrothermal reaction of soluble starch, are used

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as the templates. The influence of different precipitants, Ca2+ concentration, and the size of CS

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templates on the morphologies of the as-synthesized CaO-based sorbents are systematically

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investigated. Cage-like CaO hollow microspheres featuring highly porous shells can be accurately

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synthesized, and the formation mechanism of this unique hollow microsphere structure is

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discussed deeply. Moreover, the cyclic CO2 capture performance of the synthetic CaO-based

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sorbents is assessed critically under both mild and harsh conditions, which was also compared to

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the reference limestone. Kinetic study using two different models is conducted to understand the

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reaction mechanism at different stages in the carbonation process of the cage-like CaO hollow

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microspheres. Furthermore, the energy and raw material consumptions related to the synthesis of

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cage-like CaO hollow microspheres are analyzed by the life cycle assessment (LCA) method.

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EXPERIMENTAL SECTION


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For simplicity, here the abbreviations of CaO-U, CaO-SC, and CaO-OA are used to denote

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CaO-based sorbents prepared using urea, sodium carbonate and oxalic acid as the precipitant,

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respectively. The detailed information concerning preparation of CS templates, synthesis of CaO-

80

based sorbents, characterization and performance test can be found in Supporting Information (SI).

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RESULTS AND DISCUSSION

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Characterizations of CS templates. CS templates are fabricated via the hydrothermal

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reaction of soluble starch. It is reported by Sun et al. that the diameters of CSs were influenced by

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hydrothermal temperature, duration and concentration of the starting material,32 which would, in

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turn, affect the morphologies of the resultant materials.27,33 Therefore, we systematically

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investigate the influence of synthesis parameters, including hydrothermal temperature, duration

87

and concentration of starch, on the morphologies and sizes of CS templates, aiming to synthesize

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monodispersed ones with controllable sizes.

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As shown in Figure S1 (SI), the starting material, i.e., soluble starch, exhibits very irregular

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morphologies. After hydrothermal treatment at 180 °C with a duration of 3 h, CSs with smooth

91

surfaces and narrow particle size distributions can be obtained. However, for a short hydrothermal

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duration of 1.5 h, no CSs form; whereas CSs with non-uniform particles size can be observed for

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the prolonged durations, i.e., over 3 h. Moreover, a hydrothermal temperature below or over 180

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°C will lead to coarse surface or wide particle size distribution (Figure S2). It is worth mentioning

95

that no CSs form when the hydrothermal temperature is as low as 140 °C. Similar results can be

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found in other literature.32,34,35 Furthermore, it seems that soluble starch concentration plays an

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important role in particle size distributions of CS templates. The particle sizes of templates increase

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with the increase of starch concentrations, as given in Figure 1 and Table 1. Thus, we can


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accurately control the size of CS templates by adjusting the soluble starch concentrations under a

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hydrothermal temperature of 180 °C with a duration of 3 h. For simplicity, the CS templates

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derived from starch concentrations of 50, 100, and 200 g starch/L·H2O are denoted as T-1, T-2,

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and T-3, respectively.

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The XRD pattern of T-3 (Figure 2a) exhibits only a broad peak between 20° and 30°, which

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corresponds to indicative of amorphous carbon and is a characteristic of colloidal CSs.36-38

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Moreover, the adsorption band present in Figure 2b at 1709 and 1613 cm−1 can be assigned to C=O

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and C=C vibrations, respectively, suggesting the dehydrative aromatization of starch during

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hydrothermal treatment.32 The bands observed at 3400 and 1000-1300 cm-1 are assigned to

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stretching and bending vibrations of C=O and OH groups, implying the existence of large numbers

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of residual hydroxy groups.39 These results indicate that the surface of CS templates is hydrophilic

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and enriched with OH, C=O, C=C groups, which will be able to bind metal cations through

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coordination or electrostatic interactions.32,40,41 Furthermore, the Raman spectra of T-3 (Figure S3)

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shows two broad overlapping bands at around 1360 and 1587 cm-1, which is typical of carbonized

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material.42 The G band (1587 cm-1) is assigned to the in-plane bond-stretching motion of pairs of

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C sp2 atoms both in aromatic and olefinic molecules,43 whereas the D band (1360 cm-1) is assigned

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to ring-breathing vibrations in benzene or condensed benzene rings in amorphous (partially

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hydrogenated) carbon films.44 Therefore these results confirm the existence of small aromatic

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clusters in T-3, which agrees well with the aromatization of the materials observed by FTIR

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spectra.


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Figure 1. SEM images and particle size distributions of the CS templates prepared under

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different concentration of starch with a hydrothermal temperature of 180 °C and a duration of 3

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h. (a), (d): T-1; (b), (e): T-2; (c), (f): T-3.

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Table 1. Particle size, zeta potential, and pH value of different CS templates Sample

T-1

T-2

T-3

Mean particle size (μm)

0.44

0.84

2.19

Zeta potential (mV)

-16.0

-21.5

-33.0

pH value

7.2

7.3

7.3

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Figure 2. Characterizations of CS templates and CaO-based sorbents. (a): Typical XRD patterns

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of sample a’: T-3, sample b’: T-3 after Ca2+ ions deposition in the presence of urea, and sample

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c’: sample b’ after template removal by calcination. (b): Typical FTIR spectra of T-3 before and

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after Ca2+ ions deposition.

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Formation mechanism of synthetic CaO-based sorbents with different structures. Zeta

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potential analysis on the CS templates indicates that the surfaces of the templates are negatively

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charged in ethanol (given in Table 1). However, this value is changed to -3.65 mV (pH=8.0) after

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Ca2+ ions deposition on T-3 with the addition of urea. Moreover, as clearly shown in Figure S4,

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the surface of T-3 after Ca2+ ions deposition becomes much coarser, which is coated by a product

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layer. On the basis of the XRD result for T-3 after Ca2+ ions deposition (Figure 2a), except for the

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broad peak corresponding to indicative of amorphous carbon, CaCO3 is also detected. The product

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layer is thus identified as CaCO3, and the increase of zeta potential value after Ca2+ deposition

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results from the Ca2+ ions deposition on the surface of T-3, i.e., CaCO3 product layer.

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Consequently, the increase of zeta potential value after Ca2+ ions deposition in this work suggests

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Ca2+ ions are deposited into the surface layer of the templates. For the case of CS template after

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Ca2+ ions deposition, FTIR results reveal that the absorption bands weaken in general, especially


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for the OH group at 3400 cm-1 (Figure 2b). This result keeps consistent with the work conducted

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by Armutlulu et al.39 and Sun et al..32 After template removal, only CaO is detected, suggesting

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the complete removal of the CS templates by calcination (Figure 2a).27

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Figure 3 gives the surface morphologies of CaO-based sorbents prepared using different

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precipitants, including sodium carbonate, oxalic acid, and urea. It is interesting to find that the

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precipitant exerts an important role in the morphologies of synthetic CaO-based sorbents. When

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sodium carbonate or oxalic acid is used as the precipitant, loose structure, rather than hollow one,

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can be obtained (Figure 3a-d). However, cage-like hollow microsphere structures featuring highly

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porous shells can be obtained with the addition of urea (Figure 3e, f). The presence of highly

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porous shells is critical due to it allows a rapid transport of CO2 to and from the material. Moreover,

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the porous shell is consisted of CaO nanoparticles (Figure S5), thus satisfying an essential

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requirement to avoid (or at least minimize) diffusion limitations during the CO2 capture reaction.23

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Figure 3g shows a typical TEM image of the cage-like CaO hollow microspheres, further

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confirmed the hollow interiors clearly by the obvious electron-density difference between the dark

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edge and the pale center. And the cage-like CaO hollow microspheres are polycrystalline

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according to the diffraction pattern (Figure 3h).

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Figure 3. SEM images of (a), (b): CaO-SC, (c), (d): CaO-OA, and (e), (f): CaO-U; TEM image

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(g) and SAED pattern (h) of CaO-U. Each CaO-based sorbent was prepared from 0.5 M

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Ca(NO3)2 solution and used T-3 as the template.

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The entirely different morphologies of synthetic CaO-based sorbents prepared using different

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precipitants can be probably ascribed to both the formation route and the amount of the precipitated

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CaCO3. When sodium carbonate or oxalic acid is added into the Ca(NO3)2 solution, the vast

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majority of Ca2+ ions are precipitated as CaCO3 or CaC2O4 immediately, which subsequently

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agglomerate and wrap around the templates randomly, as shown in Figure 4B. Hence, CaO-based

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sorbent with loose structure can be obtained after removal of templates. Despite hollow

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microsphere structure is not obtained with the addition of sodium carbonate or oxalic acid, the

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removal of CS templates results in an enhancement of porosity, which is beneficial to CO2 capture.

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Duan et al.45 synthesized CaO-based sorbent using commercial flour as the biomass-template, and

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found that the CO2 capture capacity was significantly enhanced due to the improved porosity with

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high specific surface area and pore volume. Similar conclusion was also drawn by Ridha et al.,46

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who prepared four types of biomass-templated CaO-based sorbents and found that biomass

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materials improved the porosity of sorbents. Whereas with the addition of urea at room

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temperature, a clean solution can be obtained without any precipitated CaCO3. This is due to the

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+ 2gradually hydrolyses of urea, i.e., CO  NH 2 2 +2H 2 O  2NH 4  CO3 , occurs only at a little

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higher temperature (>75 °C).27,33,39 Once the solution is transferred into the water bath (90 °C),

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Ca2+ ions are precipitated gradually as CaCO3 with the progressive hydrolysis of urea. At the same

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time, the amount of the precipitated Ca2+ ions using urea as the precipitant is significantly reduced

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compared to the conditions where sodium carbonate or oxalic acid is used as the precipitant, mainly

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because of the incomplete hydrolysis of urea.47 Both of them are beneficial to the uniform coating


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of CaCO3 around the templates, as shown in Figure 4C. It should be noted that this synthesis

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approach is distinctively different from the surface adsorption method of metal cations,48 since the

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precipitation process is not limited by the adsorption capacity of the surface (compare to Figure

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4A and Figure 4B, C). Finally, the removal of carbon cores, and densification, cross-link, and

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phase transformation of CaCO3 in the surface layer via calcination lead to the formation of cage-

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like CaO hollow microspheres.

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Figure 4. Schematic illustrations of the formation mechanism of CaO-based sorbents

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synthesized (A): without any precipitants, with the addition of (B): sodium carbonate or oxalic

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acid and (C): urea.

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Further investigation demonstrates that the formation of cage-like CaO hollow microspheres

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is strongly dependent on experimental parameters such as Ca(NO3)2 concentration, the presence

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of template, the particle size of template, the addition of urea, and the heating rate during template

195

removal process. When the Ca(NO3)2 concentration is reduced to 0.2 M, cage-like CaO hollow


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microspheres with a smaller particle size can be obtained, as shown in Figure 5a, b. This means

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we can accurately control the particle size of CaO hollow microspheres via the adjusting of

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Ca(NO3)2 concentration. Similar results were also obtained in the preparations of SnO241 and

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Fe2O3.33 Moreover, it seems that the addition of both urea and template are essential to obtain cage-

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like CaO hollow microspheres. In the absence of urea, loose structure rather than cage-like hollow

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microsphere one can be obtained (Figure 5c). This can be explained by the fact that the amount of

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Ca2+ ions deposited on the surface of templates is significantly decreased, mainly due to the limit

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of the adsorption capability of Ca2+ ions by the hydrophilic surface within such a short deposition

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duration (i.e., 4 h). As a result, the shell walls of hollow microspheres are so thin, which easily

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collapsed during template removal process.33 This result also further proves that the synthesis

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method using precipitants is the precipitation strategy instead of the surface adsorption route. In

207

the absence of template (Figure 5d, e), it is interesting to find that CaO-based sorbent exhibits long

208

strip shapes rather than cage-like hollow microspheres. Besides, a worse porosity is observed

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compared to the sorbents prepared with the addition of template, mainly due to the lack of gaseous

210

product release during the template removal process. It thus also implies that the addition of

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template is beneficial to CO2 capture, whether cage-like hollow microsphere structures are formed

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finally.

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The heating rate during the template removal process also plays an important role in the

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formation of cage-like CaO hollow microspheres. As shown in Figure 5f, when the template

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removal process is performed directly at 800 °C, there are many fragments and broken cage-like

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CaO hollow microspheres, rather than complete microspheres. It results from the significant

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thermal stress and intense gas release caused by the fast calcination of templates. Furthermore, it

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is interesting to find that when T-2 or T-1 is used as the template, i.e., a reduction of template


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particle size from 2.19 μm to 0.84 μm or 0.44 μm, respectively, a loose structure with great porosity

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is obtained (Figure S6c-f). This result can be explained that both the particle size of T-2 and T-1

221

are much smaller than that of T-3, thus, the amount of precipitated CaCO3 is relatively larger than

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that required for the uniform coating of CaCO3 around the templates, as shown in Figure S7. This

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is similar to the condition when sodium carbonate or oxalic acid is used as the precipitant, in which

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the templates are embedded in precipitated CaCO3 or CaC2O4, subsequently leading to the

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significant enhancement in porosity after the removal of templates via calcination.

226 227

Figure 5. SEM or TEM of CaO-based sorbents prepared from 0.2 M Ca(NO3)2 solution under

228

different conditions. (a): SEM image of CaO-U; (b): TEM image of CaO-U; (c): SEM image of

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CaO-based sorbent prepared in the absence of urea; (d): SEM image of CaO-based sorbent

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prepared without the presence of template (×1K); (e): High resolution SEM image of marked

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area in (d) (×10K); (f): SEM image of CaO-based sorbent prepared using urea as the precipitant,

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subsequently calcined at 800 °C directly for 1 h.


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CO2 capture performance. Figure 6 and Table 2 show the cyclic carbonation conversions

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of synthetic CaO-based sorbents as well as the reference limestone over the tested fifteen cycles

235

under both mild and harsh conditions, respectively. It is clear that all the synthetic CaO-based

236

sorbents show a significant enhancement in initial CO2 uptake compared to the reference

237

limestone, whether the hollow microsphere structure is formed. For example, the reference

238

limestone possesses an initial carbonation conversion of 74.5% and 63.2% under a mild and harsh

239

condition, respectively. While cage-like CaO hollow microspheres possess an initial one of 98.2%

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and 82.5% under the corresponding conditions, exceeding that of the reference limestone by 31.8%

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and 30.5%, respectively. The significant improvement in CO2 uptake capacity of the synthetic

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CaO-based sorbents results from the higher specific surface area compared to the limestone (Table

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S1), since surface area plays a critical role in CO2 capture performance of sorbents.12 Moreover, it

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is well known that carbonation reaction consists of two stages: an initially rapid chemical-

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controlled stage, followed by a substantially slower diffusion-controlled stage.7,8,14,49 As shown in

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Figure 7a, a carbonation conversion of 69.4% can be obtain for cage-like CaO hollow

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microspheres with only three min under a harsh condition, accounting for 84.1% of the total initial

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CO2 uptake capacity. This result suggests cage-like hollow microsphere structure can significantly

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improve the CO2 sorption kinetics, especially at the chemical-controlled stage.

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During the tested fifteen repeated carbonation/calcination cycles, decays in CO2 uptake are

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observed for all the sorbents. However, it should be noted that all the synthetic CaO-based sorbents

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possess higher final CO2 uptake capacities compared to the limestone. Under a harsh condition,

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cage-like CaO hollow microspheres possess a final carbonation conversion of 39.7%, exceeding

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the reference limestone by 148.1%. While CaO-OA and CaO-SC have a final one of 22.8% and

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18.8%, exceeding the reference limestone by 42.5% and 17.5%, respectively. And the


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corresponding carbonation curves in Figure 7b confirm that all the synthetic CaO-based sorbents

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still possess better CO2 sorption kinetics than the limestone. For the best material, i.e., cage-like

258

CaO hollow microspheres, an additional test of 30 repeated carbonation/calcination cycles under

259

a harsh condition is conducted. A carbonation conversion of 37.7% can be obtained after 30

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repeated cycles (Figure S8), suggesting the good cyclic stability of CO2 capture performance for

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cage-like CaO hollow microspheres. Besides, it is interesting to find that cage-like CaO hollow

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microspheres possess better cyclic stability of CO2 capture than CaO-OA and CaO-SC. It is due

263

to not only the cage-like CaO hollow microspheres have unique hollow microsphere structure

264

compared to CaO-OA and CaO-SC, but also they can maintain this unique structure during the

265

repeated carbonation/calcination cycles even under a harsh condition. As clearly shown in Figure

266

8 and Figure S9, cage-like CaO hollow microspheres can maintain its unique hollow structure after

267

the repeated cycles, whereas the loose structures disappear and only compact surfaces with low

268

porosity are observed for both CaO-OA and CaO-SC.

269 270

Figure 6. CO2 capture performance of CaO-based sorbents and reference limestone over the

271

tested fifteen repeated cycles under (a) mild and (b) harsh conditions, respectively.


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Table 2. Carbonation conversions of synthetic CaO-based sorbents and limestone under mild

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and harsh condition Carbonation conversion sample

under a mild condition (%)

Carbonation conversion under a harsh condition (%)

1st cycle

15th cycle

1st cycle

15th cycle

Limestone

74.5

26.5

63.2

16.0

CaO-U

98.2

49.2

82.5

39.7

CaO-OA

85.1

33.9

71

22.8

CaO-SC

77.8

28.5

67.7

18.8

274

275 276

Figure 7. Carbonation curves of the synthetic CaO-based sorbents and the limestone under a

277

harsh condition in the (a): 1st cycle and (b): 15th cycle, respectively.


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Figure 8. SEM images of (a): CaO-U, (b): CaO-OA, and (c): CaO-SC after the fifteen repeated

280

carbonation/calcination cycles under a mild condition.

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Kinetic of carbonation reaction. In order to understand the reaction mechanism at different

282

stages in the carbonation process of cage-like CaO hollow microspheres, kinetic study using two

283

different models is conducted in this work. At the same time, the kinetic study of the reference

284

limestone is also done and used as the reference. Here the grain model and three-dimensional

285

diffusion model are used to build the relationship between carbonation conversion and time, which

286

are commonly used to analyze the reaction rate constants in chemical-controlled stage and

287

diffusion-controlled stage, respectively.50-54 The two models can be expressed as:

288

Grain model for chemical-controlled stage:

289

1- 1-Cn 

13

 kc t

(1)

290

Three-dimensional diffusion model for diffusion-controlled stage:

291 292

1- 1-Cn 1 3   kd t (2)   where Cn is the carbonation conversion at the n cycle (in %), kc and kd are the reaction rate

293

constants during chemical-controlled stage and diffusion-controlled stage, respectively.

2

294

The results of kinetic calculation with models at two reaction stages for cage-like CaO hollow

295

microspheres and limestone are given in Figure 9. It can be found that the calculated data obtained


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by kinetic models agree well with the fitting data, suggesting that both the grain model and the

297

three-dimensional diffusion model can be applied to analyze the two stages in the carbonation

298

process. The reaction rate constants of different stages can be obtained from the linear plot slope

299

and the results are given in Table S2. It can be found that the reaction rate constant of chemical-

300

controlled stage is always larger than that of diffusion-controlled stage, which agrees well with the

301

initial rapid increase of carbonation conversion followed by slow increase in Figure 7. And the

302

reaction rate constants of both chemical-controlled stage and diffusion-controlled stage in the 15th

303

cycle are lower than those in the 1st cycle, indicating the occurrence of decline in CO2 capture

304

performance. Moreover, it is interesting to find that the reaction rate constants for cage-like CaO

305

hollow microspheres are much higher than those of limestone, suggesting the good CO2 sorption

306

kinetics of cage-like CaO hollow microspheres. This can be related to the unique hollow

307

microsphere structure.

308 309

Figure 9. Kinetic calculation with two different models for cage-like CaO hollow

310

microspheres and limestone at chemical-controlled stage and diffusion-controlled stage in the 1st

311

and 15th cycles, respectively.


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Here the kinetic parameters for carbonation conversion are calculated below by fitting the

313

reaction rate equation based on the grain model.50,53,54

314

The reaction rate of CaO-CO2 can be expressed as:

315 316

317 318

319 320

n dX 1 3  3r 1  X   56ks  PCO 2  PCO 2,eq  S dt 1  X  At the initial time t = 0, Eq. (3) becomes:

R' 

R' 

n dX  3r0  56k s  PCO 2  PCO 2,eq  S0 dt

(3)

(4)

In logarithmic form, with ks  A  exp   E R gT  , Eq. (4) becomes:

 56  E ln  r0   n ln  PCO 2  PCO 2,eq  + ln  S0   ln  A    3  R gT

(5)

Assuming zero reaction order when PCO 2  PCO 2,eq  10kPa , Eq. (5) can be rearranged as:

322

 56  E ln  r0   l n  AS0   (6)  3  R gT where r0 is the initial specific reaction rate in the carbonation reaction (in s-1), A is the pre-

323

exponential factor (in mol/(m2·s)), S0 is the initial surface area of sorbent (in m2/g), Rg is gas

324

constant (8.314 J/(mol·K)), T is carbonation temperature (in K), and E is the activation energy (in

325

kJ/mol). Thus, E and A can be determined by the slope and intercept of a plot of ln(r0) versus

326

1000/T, as shown in Figure 10. Considering the difficulty to determine the specific surface area of

327

sorbent before the 15th cycle because of the small amount of the sorbent sample for TGA test, the

328

kinetic parameters for both cage-like CaO hollow microspheres and limestone are calculated only

329

in the 1st cycle, as given in Table 3. The activation energy of limestone is determined as 39.2

330

kJ/mol, which is in the range obtained by other researchers in similar experimental conditions.50-

321


19


Page 20 of 32

331

54

332

lower than that of limestone, which can be caused by the unique hollow microsphere structure.

It is interesting to find that the activation energy of cage-like CaO hollow microspheres is much

333 334

Figure 10. Arrhenius plots for carbonation at 550-750 °C in 15 vol.% CO2 (N2 balance) for (a):

335

cage-like CaO hollow microspheres and (b): limestone in the 1st cycle, respectively.

336

Table 3. Kinetic Parameters of the carbonation reaction of cage-like CaO hollow microspheres

337

and limestone sample

E (kJ/mol)

A (mol/(m2·s))

CaO-U

28.1

4.7×10-3

Limestone

39.2

0.12

338 339

Life cycle assessment. A “cradle-to-factory gate” LCA approach is used in this work, which

340

has been reported in previous work.55,56 Figure 11 outlines the system boundary for this LCA study.

341

It should be noted that some parameters are not considered due to the lack of source data of relevant


20

Page 21 of 32


342

chemicals/processes. Therefore, it should be treated with caution when applying for other synthetic

343

processes or materials.

344 345

Figure 11. System boundary for LCA of cage-like CaO hollow microspheres via a template-

346

assisted synthesis approach. There are three main steps (1-3) in the material synthesis.

347

The material and energy consumption data during the synthesis are summarized in Tables S3

348

-S5, respectively. Therefore, we can obtain the production cost distribution of raw materials,

349

electricity and biomass (as fuel) in each step for the synthesis of one kg cage-like CaO hollow

350

microspheres according to Tables S3-S5, as shown in Figure 12. The calculated production cost

351

for synthesis of one kg cage-like CaO hollow microspheres is 24.95 RMB, 86.1% of which is spent

352

on raw material. Although this production cost seems high, it should be noted that the cost here is

353

based on our laboratory data, and can be reduced by a large extent by scaling up. For example,

354

recycling the chemicals such as urea and Ca(NO3)2 is important to reduce the production cost,

355

since these two raw materials account for 51.4% of the total cost. According to the material

356

balance, 80% of the urea and 10.7% of the Ca2+ ions can be recycled. Moreover, Ca(NO3)2 can be

357

replaced by other cheap calcium based material like carbide slag.51 The use of CS templates


21


Page 22 of 32

358

derived from the hydrothermal treatment of other cheap starting material such as glucose41 will

359

also largely reduce the production cost. And the heat derived from the removal of CS templates

360

via combustion process can be recovered in a large-scale synthesis, which can further lower the

361

production cost. Through all the improvement, the cost of one kg cage-like CaO hollow

362

microspheres can be reduced to 7.53 RMB.

363 364

Figure 12. Production cost distribution of raw materials, electricity and biomass in each step for

365

the synthesis of one kg cage-like CaO hollow microspheres.

366

The unit price of limestone is 700 RMB/t, which is lower than production cost of CaO hollow

367

microspheres. However, the significant decline in CO2 capture performance of limestone should

368

be taken into account when conducting the economic analysis. In terms of limestone, a relative

369

makeup (fresh limestone/sorbent circulation) rate is generally determined as 0.04, as reported in

370

previous work.57,58 As a result, for one kg limestone, the overall CO2 captured can be calculated

371

as 2.6 kg based on its capacity curve of the raw limestone over recycle numbers. Whereas this

372

makeup rate can be greatly reduced when using our sorbent, due to the significant enhanced CO2


22

Page 23 of 32


373

capture performance of cage-like CaO hollow microspheres. The carbonation conversion rate of

374

our sorbent can be stable at 37.7% after 30 cycles under a harsh condition, while the carbonation

375

conversion rate of the parent limestone is only 16% after 15 cycles. In this case, the decreased cost

376

of CO2 capture for cage-like CaO hollow microspheres can offset the production cost by reducing

377

the relative makeup rate. The overall cost of cage-like CaO hollow microspheres is expected to be

378

lower than the raw limestone capturing the same amount of CO2. Furthermore, biomass is used to

379

supply the heat that required for the synthesis process. And the templates used in this work are

380

derived from biomass. Therefore, the synthesis of cage-like CaO hollow microspheres is carbon

381

neutral in nature.

382

ASSOCIATED CONTENT

383

Supporting Information.

384

This information is available free of charge via the Internet at http://pubs.acs.org.

385

Experimental preparation of CS templates; Experimental synthesis of CaO-based sorbents;

386

Experimental characterization; Experimental performance test; SEM images of the CS templates

387

prepared under different hydrothermal duration; SEM images of the CS templates prepared under

388

different hydrothermal temperature; Raman spectra of T-3; SEM images of T-3 before and after

389

Ca2+ ions deposition in the presence of urea; High-resolution SEM image of cage-like CaO hollow

390

microspheres prepared using T-3 as template and from 0.5 M Ca(NO3)2 solution; SEM images of

391

CaO-based sorbents using T-3, T-2 and T-1 as the template; SEM images of CaO-based sorbents

392

using T-2 and T-1 as the template before template removal process; Cyclic carbonation conversion

393

of cage-like CaO hollow microspheres over the tested 30 cycles under a harsh condition; TEM

394

image of cage-like CaO hollow microspheres after fifteen cycles under a harsh condition; TEM


23


Page 24 of 32

395

image of cage-like CaO hollow microspheres after fifteen repeated carbonation/calcination cycles

396

under a harsh condition; Reaction rate constant during chemical-controlled and diffusion-

397

controlled stages in the 1st cycle and 15th cycle carbonation process of CaO hollow microspheres

398

and limestone; material, electricity and biomass consumptions for the synthesis of cage-like CaO

399

hollow microspheres.

400 401

AUTHOR INFORMATION

402

Corresponding Author

403

*Email: [email protected]

404

Author Contributions

405

The manuscript was written through contributions of all authors. All authors have given

406

approval to the final version of the manuscript.

407

Funding Sources

408

The authors wish to acknowledge the financial support from the National Key Research and

409

Development Program of China (2016YFE0102500-06-01).

410

Notes

411 412 413

The authors declare no competing financial interest. ACKNOWLEDGMENT The authors wish to acknowledge the financial support from the National Key Research and

414

Development Program of China (2016YFE0102500-06-01).

415

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Accurate Control of Cage-Like CaO Hollow Microspheres for

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