Hierarchical Nanocomposite by Integrating Reduced Graphene Oxide

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Hierarchical Nanocomposite by Integrating Reduced Graphene Oxide and Amorphous Carbon with Ultrafine MgO Nanocrystallites for Enhanced CO2 Capture Ping Li, and Hua Chun Zeng Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03308 • Publication Date (Web): 04 Oct 2017 Downloaded from http://pubs.acs.org on October 6, 2017

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Environmental Science & Technology

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Hierarchical Nanocomposite by Integrating Reduced

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Graphene Oxide and Amorphous Carbon with

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Ultrafine MgO Nanocrystallites for Enhanced CO2

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Capture

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Ping Li, and Hua Chun Zeng*

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Department of Chemical and Biomolecular Engineering, Faculty of Engineering, National

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University of Singapore, 10 Kent Ridge Crescent, Singapore 119260

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KEYWORDS: MgO-based nanocomposite, graphene, carbon, hierarchical architecture, pyrolysis, CO2 capture

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ABSTRACT: Exploring efficient and low-cost solid sorbents is essential for carbon capture and

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storage. Herein, a novel class of high-performance CO2 adsorbent (rGO@MgO/C) is engineered

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based on controllable integration of reduced graphene oxide (rGO), amorphous carbon and MgO

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nanocrystallites. The optimized rGO@MgO/C nanocomposite exhibits remarkable CO2 capture

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capacity (up to 31.5 wt% at 27 °C, 1 bar CO2 and 22.5 wt% under the simulated flue gas), fast

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sorption rate and strong process durability. The enhanced capture capability of CO2 is the best

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among all the MgO-based sorbents reported so far. The high performance of rGO@MgO/C

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nanocomposite can be ascribed to the hierarchical architecture and special physicochemical

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features, including the sheet-on-sheet sandwich-like structure, ultrathin nanosheets with abundant

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nanopores, large surface area, highly dispersed ultrafine MgO nanocrystallites (ca. 3 nm in size),

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together with the rGO sheet and in-situ generated amorphous carbon which serve as a dual carbon

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support and protectant to prevent MgO nanocrystallites from agglomeration. In addition, the CO2

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uptake capacity at intermediate temperature (e.g., 350 °C) can be further improved by >3 times

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through alkali metal salt promotion treatment. This work provides a facile and effective strategy

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to engineer advanced graphene-based functional nanocomposites with rationally designed

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compositions and architectures for potential applications in the field of gas storage and separation.

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1. Introduction

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With the increase in anthropogenic activity and fast development of modern industry, the

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atmospheric CO2 level has accumulated steadily. The concentration of atmospheric CO2 has

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showed a remarkable change from pre-industrial level of ca. 280 ppm to the present day ca. 400

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ppm in less than 260 years (that is, since the beginning of the Industrial Revolution).1,2 The ever-

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increasing amount of CO2 emission from fossil fuel combustion can cause serious global warming,

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climate change and related environmental issues. Over the last decades, concerns about these

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catastrophic consequences have stimulated the proposal and study of carbon capture, storage, and

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utilization scheme (CCSU).3-5 Numerous new materials and technologies have been developed for

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the carbon capture.6-9 Among them, the use of solid sorbents for CO2 removal, as an attractive


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alternative to the conventional liquid amine scrubbing technique, is regarded as a promising and

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competitive approach.2,10-16

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MgO, one of common alkaline earth metal oxides, is recognized as a practical solid sorbent

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candidate because of its low-cost, abundant earth storage, nontoxic, and more importantly, its high

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theoretical stoichiometric CO2 uptake.17-21 However, up to now, the fact is the CO2 capture

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performance of MgO-based materials is far from satisfactory. On the one hand, despite its high

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theoretical CO2 uptake capacity, the actual uptake capacity of MgO is not competitive. For

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example, the CO2 capture capacity of the commercially available MgO is fairly small (about 2.0

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wt%).22,23 Plus, most of MgO-based sorbents reported in the literature only show moderate CO2

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sorption capacity and relatively slow sorption kinetics.24 The low capture capacity and inadequate

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kinetics by MgO are mainly attributed to the low surface area, and besides, the formation of a rigid,

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CO2-impermeable layer on the surface of MgO particles which severely slow the sorption as well

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as pose a hindrance toward further reaction.17 On the other hand, MgO-based materials usually

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need relatively high temperature (400500 °C) for the regeneration.25 Apart from its required

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energy consumption, such a harsh process is unavoidably accompanied with the sintering and

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agglomeration of MgO species along with the huge loss of surface area, damaging their

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recyclability in the continuous operations. Thus, to fabricate a high-performance MgO-based

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sorbents, the abovementioned two major problems must be settled.

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It is well-documented that the captured CO2 is in a physical and/or adsorbed state on the surface

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of MgO-based sorbents.26,27 Based on this point, it can be anticipated that by dispersing ultrafine

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MgO nanocrystallites on a well-defined support would be a promising way to achieve enhanced

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CO2 capturing performance. Since the sorption active sites of MgO for the acidic CO2 molecule

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are mainly the basic O2− sites in the O2−–Mg2+ bonds, small-sized MgO can provide more edge-


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and corner-located O2− active sorption sites compared with their bulk counterpart, thus more

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efficient utilization of MgO can be achieved. Meanwhile, compositing with support can further

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disperse MgO species to fully expose active sites, and more importantly, enhance their stability,

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preventing them from aggregation and sintering during application. With this design concept,

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herein we propose a combination of two types of carbon, reduced graphene oxide (rGO) and

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amorphous carbon (C), which can serve as excellent dual supports for MgO nanocrystallites, and

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we thus engineer a class of sandwich-like rGO@MgO/C nanocomposite with a hierarchical

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structural architecture.

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Graphene, one of carbon-based nanomaterials, which is known as a indefinitely extended two-

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dimensional (2D) monolayer of sp2-hybridized carbon atoms densely packed into a hexagonal

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lattice resembling a honeycomb, has attracted increasing attention in the past decades owing to its

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many outstanding features, such as large specific surface area, impressive mechanical strength and

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flexibility, unique optical, electronic and thermal properties.28 A broad range of advanced

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functional nanocomposites has been reported by integrating the graphene with other materials.29,30

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For instance, in electrochemical field, the incorporated graphene can afford an excellent

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conductive network for electron transport, and meanwhile serve as a buffer to alleviate the

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aggregation and volume variation of the electroactive components, thus contributing to improved

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electrochemical performance.31,32 Additionally, in catalysis field, graphene can be utilized as a 2D

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support to disperse and stabilize the catalytic active species.33

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In the current study, we present our recent effort in the development of a series of sandwich-like

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nanocomposites based on MgO, rGO and amorphous carbon with a polyol process followed by

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subsequent pyrolysis treatment. The synthesis involves using the graphene oxide (GO) sheet as the

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structure-directing template, on which sheet-like coordination polymer Mg-ethylene glycolate


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(Mg-EG) is anchored, giving rise to a sheet-on-sheet architecture. Through thermal treatment, Mg-

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EG is converted to MgO/C, thus generating rGO supported MgO/C nanocomposite

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(rGO@MgO/C). Featured by its unique physicochemical properties and structural architecture, the

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nanocomposite system exhibits exceptional CO2 capture capacity, fast CO2 sorption kinetics and

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excellent process stability, a superior performance unmatched by other state-of-the-art MgO-based

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sorbents reported in literature to date.

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2. Experimental Section

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2.1. Materials and reagents. The following chemicals were used as received without further

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purification: ethylene glycol (EG, 99.99%, Fluka, Sigma−Aldrich), Mg(CH3COO)2·4H2O (99+%,

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Sigma−Aldrich), polyvinylpyrrolidone (PVP, K30, 99%, Sigma−Aldrich), NaNO3 (99+%,

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Sigma−Aldrich), KNO3 (99.99%, Merck), K2CO3 (99.5%, Merck), commercial MgO powder

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(99+%, Sigma−Aldrich), graphite (99.8%, Alfa Aesar) and ethanol (99.99%, Fisher). Deionized

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water was generated by the Elga Micromeg Purified Water system.

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2.2. Preparation of sandwich-like rGO@MgO/C nanocomposite. The graphene oxide (GO)

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was prepared according to a modified Hummer's method (the FTIR spectrum of the as-obtained

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GO is shown in Figure S1).34 For the synthesis of rGO@Mg-EG precursor, Mg(CH3COO)2·4H2O

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(0.428 g) and PVP (0.32 g) were firstly dissolved in 20 mL of ethylene glycol (EG). Then GO

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(1.210.0 mg) was added and sonicated for 30 min at room temperature to get a homogeneous

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suspension. Afterward, the suspension was transferred to an oil bath and heated at 175 °C for 2 h

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under vigorous stirring. The solid product was centrifuged, rinsed with ethanol for several times,

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and dried in an electrical oven at 80 °C for 12 h. The GO contents in the nanocomposites were 2,


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5, 10 and 15 wt%, respectively. Accordingly, these samples were referred to as rGO@Mg-EG-x

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(x = 2, 5, 10 and 15) in the text.

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The target rGO@MgO/C nanocomposite was prepared through thermal decomposition of

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rGO@Mg-EG-x under an inert gas atmosphere. Typically, the precursor rGO@ Mg-EG-x was

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heated at 550 °C (ramping rate: 10 °C/min) for 3 h under an Ar flow, and then naturally cooled to

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room temperature. The corresponding black products were denoted as rGO@MgO/C-x (x = 2, 5,

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10 and 15).

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2.3. Preparation of MgO/C nanocomposite (control sample). The control sample MgO/C

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nanocomposite was prepared through the aforementioned procedure for rGO @MgO/C, while

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without the addition of GO.

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2.4. Preparation of pure rGO (control sample). The pure rGO was prepared by dispersing GO

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in EG to get a homogeneous suspension through ultrasonication for 30 min at room temperature.

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Then the suspension was stirred and heated in an oil bath at 175 °C for 2 h. The as-obtained

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precipitate was centrifuged, rinsed with ethanol for several times, and then dried in an oven at 80

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°C for 12 h.

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2.5. Preparation of rGO@C (control sample). The rGO@C control sample was prepared by

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treating rGO@MgO/C-5 nanocomposite with an aqueous solution of HCl (2.0 M) overnight to

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etch away the MgO phase, followed by rinsing with deionized water and drying in an oven at 80

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°C for 12 h.

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2.6. Preparation of alkali metal salt promoted rGO@ MgO/C nanocomposite. Alkali metal

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salt promoted rGO@MgO/C nanocomposite was synthesized by a wet impregnation method. In a

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typical synthesis, powder rGO @MgO/C was added to an aqueous solution of alkali metal salt

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under vigorous stirring. The formed homogeneous slurry was then dried in an oven at 60 °C. Three


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types of alkali metal salts including NaNO3, KNO3 and K2CO3 were used in this work. The loading

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of all the alkali metal ions was set at 15 mol %, as this metal content had been previously found to

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be an optimal value for CO2 trapping.35

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2.7. CO2 capture performance test. The performance of the sorbents was investigated using a

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thermogravimetric analyser (TGA/DSC 2 STARe system, Mettler Toledo) at 1 bar. Dried pure CO2

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(99.99%) or CO2 (15.o0 vol%) in N2 was used for the sorption studies and ultrahigh purity N2

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(99.995%) was used as a purging gas for CO2 desorption and sorbent regeneration.

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In a typical experiment, about 10 mg of the sorbent was loaded into an alumina pan. The sorbent

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was firstly heated at 500 °C in N2 flowing at 50 mL/min for 60 min to remove the moisture and

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CO2 adsorbed from the atmosphere during storage and transportation. Then the temperature was

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lowered to the adsorbing temperature, viz. 27, 100 or 200 °C in N2 which was then switched to

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pure CO2 or a mixture of 15 vol% CO2 in N2; in both cases the total flow rate of the reactive gas

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was 50 mL/min. The sorbent was kept at the tested temperature for 120 min for the sorption study.

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The measurement was repeated 4 times for each sample, and the average value was reported in our

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discussion of the main text. The CO2 capture capacity of the sorbent in wt% was calculated from

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the weight gain of the sample in the sorption process.

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For comparison, the CO2 capture performance of the MgO/C nanocomposite and commercially available MgO powder were also measured under identical conditions.

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For the CO2 sorption/desorption cycling tests, the sample was held at 27 °C under a CO2 flow

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(50 mL/min) for 120 min to take up CO2, then the sample was held in N2 (50 mL/min) at 400 °C

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for 30 min for desorption. The adsorption/desorption cycle was repeated 16 times and the weight

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variation of the sample with the time was recorded.


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2.8. Materials characterization. The microscopic features of the samples were characterized by

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scanning electron microscopy (SEM, JEOL-6700F) equipped with an energy-dispersive X-ray

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(EDX) analyzer (Oxford INCA), transmission electron microscopy (TEM, JEOL JEM-2010, 200

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kV), and high resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F, 200 kV).

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The elemental mapping was done by EDX (Oxford Instruments, model 7426). The wide-angle X-

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ray (Cu Kα radiation) diffraction patterns were taken using Bruker D8 Advance system. Nitrogen

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adsorption–desorption isotherms were obtained on Quantachrome NOVA-3000 system at 77 K.

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Prior to the BET measurements, the samples were degassed at 200 °C for 15 h with N2 flow. The

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specific surface area of the samples was measured by the Brunauer–Emmet–Teller (BET) method.

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The pore size distribution curve was obtained using the NLDFT method and the pore volume was

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calculated at P/P0 = 0.9754. Thermogravical analysis (TGA) measurement was carried out under

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a N2 stream (50 mL/min) at a heating rate of 5 °C/min using Shimadzu TGA-50 instrument. Fourier

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transform infrared spectroscopy (FTIR, Bio-Rad FTS-135) was used to obtain chemical bonding

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information of samples using the potassium bromide (KBr) pellet technique.

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The diffuse reflectance infrared Fourier transformed spectroscopy (DRIFT) experiment was

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carried out on Bruker TENSOR II FT-IR spectrometer equipped with a MCT detector. The sample

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was loaded in a DRIFT cell, pretreated at 400 oC for 1 h under a N2 flow (50 mL/min) to clean up

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the surface, and then cooled to 27 oC and exposed to a CO2 flow (50 mL/min) for 1 h, followed by

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N2 purging for 30 min to remove any physically adsorbed CO2. Afterward, the sample was heated

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from 27 to 400 oC in the same N2 flow and FT-IR spectra were recorded at different temperatures;

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the sample was prestabilized at a designated temperature for 10 min before recording the FT-IR

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spectra.

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3. Results and Discussion

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3.1. Preparation and structural characterization. The typical procedure to fabricate

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hierarchically structured sandwich-like rGO@MgO/C nanocomposite is schematically illustrated

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in Scheme 1. Firstly, GO is dispersed in EG solution dissolved with Mg(OAc)2. Mg ions can

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strongly adsorb on the GO sheets which have abundant surface oxygen-containing functional

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groups. With heating at elevated temperatures, the nanosheet-like Mg-EG complex can be

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generated on both sides of the GO sheets through the coordination reaction between EG and the

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Mg ions. Simultaneously, GO is reduced to rGO in reductive EG upon heating. The rGO@Mg-EG

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precursor with sheet-on-sheet sandwich-like architecture is thus obtained. Here by adjusting the

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amount of GO added, a series of rGO@Mg-EG with different GO contents can be prepared. In the

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second step, the rGO@Mg-EG precursor was subjected to a high-temperature pyrolysis under an

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inert gas flow. During the thermolysis, the inorganic Mg species was converted to MgO

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nanocrystallites, and the organic ligands were transformed into carbonaceous moieties via

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chemical decomposition. Meanwhile, accompanied with the release of gaseous products during

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the pyrolysis, numerous nanoscale pores and interstitials were produced, leading to sandwich-like

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rGO@MgO/C nanocomposite which is also highly porous.

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The morphologies of rGO@Mg-EG precursor are first characterized by SEM and TEM

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techniques. The representative SEM and TEM images (Figure 1) of rGO@Mg-EG-5 show that

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numerous twisted thin nanosheets are anchored on the surface and edges of rGO sheets, forming

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sheet-on-sheet sandwich-like structure. Similar morphologies are also observed for other

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rGO@Mg-EG-x (x = 2, 10 and 15) with different rGO amounts (Figure S2). Besides, it is found

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that with a higher rGO content, the nanosheets on the rGO sheet surface become sparser owing to

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the presence of more support to disperse the Mg-EG complex. On the contrary, in the absence of


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rGO, the resulting Mg-EG was obtained with only flowerlike morphology assembled by lots of

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nanosheet building blocks (Figure S3), indicating that the incorporated rGO sheets play a structure-

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directing role for achieving sandwich-like architecture.

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The crystallographic structure of the rGO@Mg-EG-x precursors was examined through X-ray

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diffraction (XRD). As displayed in Figure 2, a strong low-angle reflection around 10° is

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characteristic of stacked metal-oxygen sheets separated by bonded alcoholate anions. Such feature

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is often exhibited in metal alkoxides.26,36-38 Based on this low-angle diffraction peak around 10°,

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the basal spacing of the layered metal alkoxide can be determined. For the rGO@Mg-EG-x in this

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study, it is found that with the increase of rGO content, the low-angle peak shifts to lower angle

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progressively, and accordingly, the basal spacing increases from 0.87 nm in Mg-EG to 1.01 nm in

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rGO@Mg-EG-15. This intriguing trend can be attributed to the dispersing effect of the rGO

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support on the Mg-EG nanosheets. As verified in the SEM and TEM images (Figure 1, S2, and

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S3), the rGO-supported Mg-EG nanosheets become sparser with increased rGO content. With

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more flexible and plentiful space, the sparser Mg-EG nanosheets can adopt looser layered structure,

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thus rendering larger interlayer space. Furthermore, note that no XRD peaks assigned to rGO are

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observed, implying that rGO is highly dispersed and no aggregation of rGO is occurred since Mg-

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EG nanosheets on both sides of GO surface can suppress their restacking.

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We also used FTIR and TGA methods to characterize the precursor samples and their thermal

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conversion. For example, the FTIR spectra of rGO@Mg-EG-x (x = 2, 5, 10, and 15) in Figure 3a

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and S4 show absorption peaks in the range of 28502960 cm–1 and 13051460 cm–1,

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corresponding to the stretching and bending vibrations of C–H, respectively. The bands in the

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range of 10501125 cm–1 are attributed to C–O stretching vibration. The peak at around 565 cm–1

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is assigned to Mg–O stretching vibration.39 Therefore, the FTIR investigation further verifies that


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EG is coordinated with Mg ions to form the Mg-EG complex. Furthermore, the TGA curves of

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rGO@Mg-EG-x (x = 2, 5, 10, and 15) (Figure 3b and S5) exhibit a pronounced weight-loss step

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in the temperature ranging from 400 to 500 °C, which can be ascribed to the thermal decomposition

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of the organic ligands in the precursors.

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According to the TGA result, 550 °C was chosen as the heating temperature to convert

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rGO@Mg-EG precursor to rGO@MgO/C under a stream of N2 flow. The XRD patterns of all the

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solid products (Figure 4) display the characteristic diffraction reflections indexed to periclase

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phase of MgO (PDF card no. 75-0447). Also, no other peaks for the precursor are detected,

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indicating the full conversion of the precursor to MgO. And Debye-Scherrer equation was applied

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to estimate the crystalline size of MgO by using the (200) diffraction peak. For all rGO@MgO/C-

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x, the crystalline size of MgO is calculated to be 2.93.0 nm (Table 1). Moreover, the FTIR

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spectrum of the pyrolyzed solid product shows the disappearance of the peaks attributed to the EG

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ligand, further revealing complete decomposition of the precursor (Figure S6). The SEM and TEM

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images (Figure 5a-c and Figure S7) show that all the pyrolyzed solid products successfully inherit

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the original hierarchical sandwich-like architecture, demonstrating the robustness of rGO@Mg-

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EG precursor which can tolerate the harsh pyrolysis condition without severe destruction or

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aggregation. From the TEM images at high magnifications (Figure 5d, 5e, S7c, S7f and S7i), we

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can see that the nanosheets on both sides of rGO are highly porous and ultrathin with a thickness

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less than 5 nm. Every nanosheet is constructed from particularly ultrafine MgO nanocrystallites

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dispersed in the amorphous carbon moieties. The size of MgO nanocrystallites ranges from 2 to 3

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nm, which agrees well with the XRD results. A representative HRTEM image (Figure 5f) reveals

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clear lattice fringes with a spacing of 0.21 nm, which can be readily indexed to the (200) plane of

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MgO, further confirming the presence of MgO in the pyrolysis product. Furthermore, the EDX


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mappings taken from an individual rGO@MgO/C-5 nanosheet (Figure 5g) show that the

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magnesium, oxygen and carbon elements are uniformly distributed throughout the entire sheet

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structure, indicating the MgO species is homogeneously dispersed on the carbon-based support.

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For the control sample MgO/C, which was converted from the pure MgO-EG precursor (i.e.,

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without rGO incorporation), similar results can be obtained (Figure S8). Besides, based on the

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TGA result, the mass fraction of MgO in the rGO@MgO /C and MgO/C nanocomposites is in the

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range of 64.874.0 wt% (See Table 1 for details).

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Nitrogen adsorption−desorption measurement was carried out to investigate the textural

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properties of the resultant rGO@MgO/C nanocomposites. As shown in Figure S9, all the

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nanocomposites exhibit type IV isotherm with an H3 type hysteresis loop (P/P0 > 0.4), suggesting

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the presence of slit-shaped mesoporous structure. Their BET specific surface areas and total pore

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volumes are summarized in Table 1. As presented, all the samples possess large BET surface areas

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and high total pore volumes. In addition, their pore size distributions were analyzed using the

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NLDFT method (Figure S9). All the samples possess pores with a wide size distribution ranging

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from micropores to mesopores. It is noteworthy that with an appropriate amount of rGO

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incorporated, rGO@MgO/C can display a larger BET specific surface area and a higher total pore

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volume compared with the control sample MgO/C (Figure S10 and Table 1). Particularly, the

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rGO@ MgO/C-5 affords a pore volume of as high as 1.22 cm3/g and a specific surface area of

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478.4 m2/g, the highest value among all these samples. It can be seen that the rGO sheets can

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effectively support and disperse ultrathin nanosheets of MgO/C to prevent them from mutual

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agglomeration, endowing the nanocomposite with high surface area, large pore volume and more

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open structure. Apparently, such features are expected to be beneficial for full exposure of active

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centers, as well as mass transport and diffusion, making the rGO@MgO/C suitable for high-


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performance adsorption of gas molecules, such as CO2. Besides, considering the strong chelation

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ability of EG with a variety of metals (e.g., Ti, V, Mn, Fe, Co, Ni, Cu, etc.), a series of rGO and

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carbon supported metal/metal oxide nanocomposites can be produced via our synthetic protocol

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using different metal salts as precursors. From a reported theoretical calculation, the combination

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of carbon with transition metal species promises viable applications for gas sorption.40 Thus, it

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could be reasonable to anticipate that the work presented here may also serve as a strategy for the

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construction of efficient solid adsorbents with tunable chemical compositions and controlled

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morphologies.

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3.2. CO2 capture performance. Herein we exploit the sorption behavior of the rGO@MgO/C

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nanocomposite in CO2 uptake. We firstly carried out the CO2 capture performance test at 27 °C

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and 1 bar CO2. For comparison, performances of a series of control samples, pure rGO, rGO@C

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(i.e., the MgO-free sample; see Experimental Section), MgO/C (i.e., the rGO-free sample) and the

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commercial MgO powder (the corresponding SEM images are shown in Figure S11), were also

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evaluated. As shown in Figure 6a, the MgO/C exhibits an excellent CO2 capture capacity (27.6

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wt%). When a certain amount of rGO is incorporated, the capacity can be further enhanced (e.g.,

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28.3 wt% for rGO@MgO/C-2 and 31.5 wt% for rGO@MgO/C-5). Particularly, the rGO@MgO/C-

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5 affords as high as 31.5 wt% of capturing capacity. Notably, this value is much higher than those

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of previously reported MgO-based sorbents,20,21,23,41 suggesting that the rGO@MgO/C-5 is a state-

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of-the-art sorbent (see Table S1 for a detailed comparison). In intense contrast, furthermore, the

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commercially available MgO can only offer 2.0 wt% of capacity under the identical testing

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conditions. In other words, through rational design of the sorbent with well-defined nanostructure,

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the resultant rGO@MgO/C-5 can display superior CO2 uptake capacity ca. 15 times more than that

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of the commercial MgO. Besides, to better understand the role of carbon species in rGO@MgO/C-


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5 to the observed high CO2 capture ability, the pure rGO and MgO-free rGO@C samples were

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investigated for their CO2 uptakes. It is found that the pure rGO has a negligible CO2 uptake

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capacity at 27 oC and 1 bar CO2, which is also consistent with the previous report.42 Similarly, the

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rGO@C sample can only display quite limited CO2 capture capacity (ca. 2.3 wt%), thus further

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revealing the critical contribution and high utilization efficiency of ultrafine MgO NPs in

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rGO@MgO/C-5 for CO2 capture. On the other hand, it is observed that too much rGO included in

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the rGO@MgO/C composite results in a decreased CO2 capacity (e.g., 22.0 wt% of capturing

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capacity for rGO@MgO/C-15). This can be attributed to the following two major factors: (i) too

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much rGO incorporation leads to decreased BET surface area and pore volume (See Table 1),

299

thereby less active sites exposed for sorption; and (ii) for the MgO phase, as an active sorption

300

component, a decrease in its content in the nanocomposite reduces its CO2 trapping power. For a

301

better elucidation, therefore, the CO2 uptake capacities are normalized to the MgO phase,

302

considering only a trace contribution of rGO and amorphous carbon for CO2 uptake. As shown in

303

Figure S12, once again, similar trends can be observed, and the rGO@MgO/C-5 can even offer as

304

high as 43.8 wt% of the normalized capturing capacity. This result further demonstrates that the

305

fine integration of rGO and amorphous carbon with MgO nanocrystallites is indeed an effective

306

approach to greatly improve the utilization efficiency of MgO species through the full exposure of

307

the active sites. And the incorporation of optimized rGO amount can boost the utilization

308

efficiency of MgO species further.

309

In addition, to evaluate their potential in practical application, CO2 capture performance of the

310

samples was also investigated in a flow of 15 vol% CO2 in N2, which mimics the dry flue gas from

311

the power plant. As displayed in Figure 6b, similar trends can be observed. Compared with the

312

MgO/C (20.3 wt% of capacity), rGO@MgO/C-x (x = 2, 5 and 10) samples with an appropriate


14

Page 15 of 36


313

rGO content can deliver enhanced capacity (22.0, 22.5 and 21.6 wt%, respectively), which is about

314

19 to 20 times higher than that of the commercial MgO (only 1.1 wt%). More impressively, such

315

high capacities are also significantly higher than those of the reported MgO-based sorbents (Table

316

S1).18,43,44

317

Furthermore, the CO2 capture performance of the rGO@MgO/C-5 at different operating

318

temperature (27, 100 and 200 °C) were also investigated. As displayed in Figure 6c and 6d, the

319

CO2 capture capacity decreases with increasing the sorption temperature, that is, it shows higher

320

activity in CO2 uptake at a lower temperature. It can be explained that both physical adsorption

321

and chemical absorption of CO2 are exothermic processes. Even so, rGO@MgO/C-5

322

nanocomposite is still capable of maintaining a respectable capacity even at 200 °C (7.0 and 5.6

323

wt% at 1 and 0.15 bar CO2, respectively), indicating the versatility of our sorbent for a broad range

324

of working temperature. Furthermore, it is worth mentioning that the kinetics is quite fast from the

325

CO2 capture kinetic curves (Figure 6c and 6d). For the whole sorption process, two-stage sorption

326

kinetic behavior can be observed with a fast first stage followed by a slower sorption process.

327

One important technology in CCSU is pre-combustion capture, which usually operates at

328

intermediate or high temperature. Thus, searching for efficient solid sorbents for trapping CO 2 at

329

intermediate/high operating temperature is particularly attractive.35,45 It has been reported that

330

incorporation of alkali metal salts in the alkaline earth metal oxides is a feasible strategy to enhance

331

CO2 sorption capacity.46-48 In view of the modest CO2 uptake capacity of rGO@MgO/C-5 at

332

intermediate temperature (e.g., 2.4 wt% at 350 °C, 1 bar CO2, Figure 7), here an alkali metal salt

333

promotion strategy was applied to enhance its performance. We modified rGO@MgO/C-5

334

nanocomposite with various alkali metal salts (e.g., NaNO3, KNO3 and K2CO3) by using a wet

335

impregnation method. The TEM images (Figure S13) demonstrate that the morphologies of the as-


15


Page 16 of 36

336

obtained products do not change significantly after impregnation treatment. Their CO2 sorption

337

property was then measured at 350 °C and 1 bar CO2. As displayed in Figure 7, compared with the

338

pristine rGO@MgO/C-5 (2.4 wt% of capacity), all the alkali metal salt-promoted sorbents exhibit

339

improved activity, indicating the positive effects of alkali metal salts on CO2 uptake by the MgO-

340

based materials. Particularly, KNO3 promoted rGO@MgO/C-5 shows dramatically enhanced

341

performance (9.8 wt%), which is above 3 times better than that of the unmodified one.

342

For a practically attractive sorbent, apart from high CO2 capturing capacity, rapid uptake rate

343

and wide operating temperature range, another key performance indicator is the durability of the

344

sorbent over sorption-desorption cycles. Here we select the rGO@MgO/C-5 to perform the cyclic

345

CO2 sorption-desorption test. As shown in Figure 8a, this sample was tested for 16 runs without

346

showing significant decrease in its capture capacity. Moreover, the TEM images of the spent

347

sorbents (Figure 8b, 8c and S14) reveal that the hierarchical sandwich-like morphology and porous

348

nanostructure retain well after 16 continuous cycles, further evidencing the robustness of

349

rGO@MgO/C-5 nanocomposite.

350

To better understand the CO2 adsorption mechanism of rGO@MgO/C in this work, the

351

representative rGO@MgO/C-5 was selected for a further study. In particular, the XRD

352

characterization was carried out on the spent rGO@MgO/C-5 (i.e., the CO2 saturated sample). As

353

displayed in Figure 9a, the spent sample retains the MgO phase, and no peaks are assignable to the

354

MgCO3 phase, indicating no bulk chemical conversion from MgO to MgCO3 took place during

355

the CO2 adsorption. In other words, the CO2 trapping of rGO@MgO/C-5 is through adsorbing CO2

356

on the surface of sorbent, rather than through the chemical phase transformation. To elaborate this

357

claim better, the in situ diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS)

358

was conducted to probe the CO2 adsorption states on the rGO@MgO/C-5 sorbent. The sorbent


16

Page 17 of 36


359

was firstly adsorbed CO2 at 27 oC, then desorbed at increasing temperature under N2 purging, and

360

the DRIFTS spectra were recorded at designated temperatures (27, 100, 200, 300, and 400 oC). In

361

the spectra of Figure 9b and c, there are at least three types of CO2 adsorption modes on the sorbent

362

(i.e., bicarbonate, unidentate carbonate, and bidentate carbonate species).23,26,27 The peaks at

363

around 1215 and 1666 cm−1 can be assigned to the bicarbonate species.27 The formation of

364

bicarbonate is related to the surface hydroxyl groups of the sorbent. Compared with other CO2

365

adsorption states, the bicarbonate is unstable, as revealed from the disappearance of the assigned

366

peaks (1215 and 1666 cm−1) accompanied with the emergence of a peak at ca. 3740 cm−1 which is

367

attributed to hydroxyl groups at elevated temperatures. Moreover, the bands ranging from 1390 to

368

1560 cm−1 correspond to the monodentate carbonate, which involves the interaction with the

369

surface low-coordination O2− ions.23 The peaks at around 1310 and 1648 cm−1 can be assigned to

370

the bidentate carbonate species, and the formation of which requires the involvement of the

371

Mg2+O2− pair sites.26 From the evolution of peaks in DRIFT spectra, we can see that the

372

unidentate carbonate and bidentate carbonate species are more stable; their peaks remain

373

observable even at 400 oC, although their intensities significantly decline. Apart from the bands

374

identified for the above carbonate species, other peaks at around 2800−3100, 1590 and 1246 cm−1

375

belong to the amorphous carbon species in the rGO@MgO/C-5. At elevated temperatures, these

376

bands retain well, exhibiting good stability of the carbon components in the rGO@MgO/C-5.

377

In summary, we have finely engineered a novel class of highly efficient CO2 adsorbent,

378

rGO@MgO/C, with hierarchical sandwich-like architecture composed of reduced graphene oxide

379

(rGO), amorphous carbon and MgO nanocrystallites. The nanocomposite is synthesized through a

380

cost-effective facile strategy involving a polyol-mediated self-assembly and subsequent thermal

381

annealing treatment. The optimized rGO@MgO/C nanocomposite demonstrates ultrahigh uptake


17


Page 18 of 36

382

capacity (up to 31.5 wt% at 27 °C, 1 bar CO2 and 22.5 wt% under the simulated flue gas), fast

383

sorption kinetics, wide working temperature range, and good process durability, outperforming all

384

the previously reported MgO-based sorbents. The achieved performance can be attributed to its

385

unique physicochemical structural features: (i) the sandwich-like sheet-on-sheet architecture, large

386

specific surface area, ultrathin nanosheets with abundant nanopores; (ii) highly dispersed

387

nanoscale MgO crystallites with fully exposed active sites; and (iii) dual supports from rGO sheets

388

and amorphous carbon, which act as protectant to prevent the anchored MgO nanocrystallites from

389

agglomeration. Furthermore, through alkali metal salt promotion, the CO2 uptake capacity of

390

rGO@MgO/C nanocomposite at intermediate temperature (e.g., 350 °C) can be significantly

391

enhanced, which is > 3-fold higher than that of the pristine one. The study presented here may

392

open up a new avenue to develop advanced functional nanocomposites for carbon reduction and

393

general gas sorptive separation technology through designing chemical compositions and micro-

394

/nano-structures.

395 396

ASSOCIATED CONTENT

397

Supporting Information. Additional SEM images, TEM images, EDX patterns, XRD patterns,

398

nitrogen adsorption/desorption isotherms of the samples. This material is available free of charge

399

via the Internet at http://pubs.acs.org.

400

AUTHOR INFORMATION

401

Corresponding Author

402

* E-mail: [email protected]


18

Page 19 of 36


403

Notes

404

The authors declare no competing financial interest.

405

ACKNOWLEDGMENT

406

The authors gratefully acknowledge the financial support provided by the Ministry of Education,

407

Singapore, NUS, and GSK Singapore. This project is also partially funded by the National

408

Research Foundation (NRF), Prime Minister’s Office, Singapore, under its Campus for Research

409

Excellence and Technological Enterprise (CREATE) program.

410 411

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532 533


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Scheme 1. Synthesis procedure for the sandwich-like rGO@MgO/C nanocomposite.

535 536 537


25


Page 26 of 36

538 539

Figure 1. (a, b) SEM and (c, d) TEM images of rGO@Mg-EG-5 precursor.


26

Page 27 of 36


540 541

Figure 2. XRD patterns of rGO@Mg-EG-x (x = 2, 5, 10 and 15) and Mg-EG precursors.

542


27


Page 28 of 36

543 544

Figure 3. (a) FTIR spectrum and (b) TGA curve of rGO@Mg-EG-5 precursor.

545


28

Page 29 of 36


546 547

Figure 4. XRD patterns of the rGO@MgO/C-x (x = 2, 5, 10 and 15) and MgO/C nanocomposites.

548


29


Page 30 of 36

549 550

Figure 5. (a, b) SEM, (c-e) TEM, (f) HRTEM images and (g) EDX elemental mapping of

551

rGO@MgO/C-5 nanocomposite.


30

Page 31 of 36


552 553

Figure 6. The CO2 sorption capacities of the sorbents at (a) 27 °C, 1 bar CO2 and at (b) 27 °C,

554

0.15 bar CO2. The CO2 capture kinetics of the rGO@MgO/C-5 nanocomposite at different

555

temperature and pressure: (c) 1 bar CO2 and (d) 0.15 bar CO2.

556


31


Page 32 of 36

557 558

Figure 7. The CO2 capture kinetics of the alkali metal salt-promoted rGO@MgO/C-5

559

nanocomposites at 350 °C, 1 bar CO2.

560


32

Page 33 of 36


561 562

Figure 8. (a) CO2 sorption/desorption cyclic performance of rGO@MgO/C-5 nanocomposite. (b,

563

c) TEM images of the spent rGO@MgO/C-5 nanocomposite.

564


33


Page 34 of 36

565 566

Figure 9. (a) XRD patterns of the fresh and spent rGO@MgO/C-5 nanocomposite. (b) The

567

DRIFT spectra of CO2 adsorbed on rGO@MgO/C-5 upon desorption under N2 purge at

568

increasing temperature. (c) The enlarged DRIFT spectra of (b) between 1000-2000 cm−1.

569 570 571 572


34

Page 35 of 36

573


Table 1. The textural properties and elemental compositions of the samples. BET surface

Pore volume a

Crystal size

area (m2/g)

(cm3/g)

of MgO b (nm)

MgO content c (wt%)

MgO/C

329.8

0.94

3.0

74.0

rGO@MgO/C-2

357.7

0.88

2.9

73.0

rGO@MgO/C-5

478.4

1.22

2.9

71.9

rGO@MgO/C-10

463.9

1.25

2.9

70.6

rGO@MgO/C-15

318.2

0.58

3.0

64.8

Sample

574

a

The pore volume was calculated at P/P0 = 0.9754. b The crystal size of MgO was calculated from

575

the (200) diffraction peak of XRD patterns using the Scherrer equation. c The mass fraction of

576

MgO in the rGO@MgO/C nanocomposite was determined from the TGA results.

577 578 579 580 581 582 583 584 585


35


586

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587 588 589


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Hierarchical Nanocomposite by Integrating Reduced Graphene Oxide

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