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Three-Dimensional Graphene-Based Porous Adsorbents for Post-Combustion CO2 Capture Shamik Chowdhury, and Rajasekhar Balasubramanian Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04052 • Publication Date (Web): 19 Feb 2016 Downloaded from http://pubs.acs.org on February 26, 2016
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Industrial & Engineering Chemistry Research
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Three-Dimensional Graphene-Based Porous
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Adsorbents for Post-Combustion CO2 Capture
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Shamik Chowdhury, Rajasekhar Balasubramanian
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Department of Civil & Environmental Engineering, National University of Singapore,
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1 Engineering Drive 2, Singapore 117576, Republic of Singapore
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* Corresponding author. Tel.: +65 65165135. Fax: +65 67744202. E-mail:
[email protected] 1 ACS Paragon Plus Environment
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ABSTRACT
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Three-dimensional (3D) crumpled graphene-based porous adsorbents with highly
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interconnected networks were synthesized through a facile one-step physical activation
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method using reduced graphene oxide (RGO) as precursor, for separation of carbon dioxide
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(CO2) from post-combustion flue gas mixtures. Due to their large surface area (above 1300
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m2 g−1), high pore volume (over 1 cm3 g−1), and well-defined bimodal microporous–
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mesoporous structure, these hierarchical porous graphene-based materials demonstrate
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rapid, stable, reversible and high CO2 uptake of 2.45 mmol g−1 at 25 oC and 1 bar.
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Importantly, the CO2 over nitrogen (N2) adsorption selectivity is among the best at partial
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pressures relevant to capturing CO2 emanating from post-combustion power plants fueled
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either with coal or natural gas. Moreover, the isosteric heat of adsorption of only –27.42 kJ
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mol−1 at zero-coverage reflects the ease of adsorbent regeneration with significantly low
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energy consumption, which in turn might reduce the costs of carbon capture and
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sequestration.
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Keywords: reduced graphene oxide, physical activation, CO2 adsorption, kinetics,
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selectivity, isosteric heat 2 ACS Paragon Plus Environment
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1. INTRODUCTION
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Fossil sources of energy, particularly coal and natural gas, are likely to remain the most
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dominant resources for securing energy provision in both developed and developing nations
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for decades to come.1 As CO2 is an inherent product of fossil fuel combustion, its selective
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removal from the flue stack of power plants is critical to global climate change mitigation.
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However, the technologies that are currently available to separate CO2 from flue gas
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mixtures, such as the use of an absorption column loaded with alkanolamines (or variants),
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are too expensive for widespread deployment.1 Recently, adsorption-based separation
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processes have received considerable attention due to their potential to satisfy the targets of
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energy-efficient and cost-effective CO2 remediation.2 As a result, many different types of
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solid adsorbents have been explored by the scientific community, among which graphene-
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based porous adsorbents are of current interest.1 Because of their porous structure together
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with the exceptional intrinsic properties of monolayer graphene, nanoporous graphene
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materials have high specific surface areas and large pore volumes, which ensure a high and
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rapid adsorption of CO2 from flue gas.3 They also have adequate robustness and mechanical
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strength to remain stable upon repeated exposure to hot and humid conditions,4 and are
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therefore extremely suitable to be applied on an industrial scale for reducing CO2 emissions.
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Accordingly, a wide range of techniques have been developed to obtain graphene-based
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porous adsorbents for enhanced separation of CO2 from fossil fuel combustion systems. Of
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the various methods available, chemical activation with potassium hydroxide (KOH) is
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frequently employed because of its lower temperature requirements, shorter activation time
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and greater development of porosity.5,6 For example, Srinivas et al. prepared highly porous
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graphene-based adsorbents with large specific surface area (1900 m2 g−1) and high pore
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volume (1.65 cm3 g−1) through KOH activation of exfoliated graphene oxide (GO)
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precursors.7 The GO-derived carbons proved to be promising for separating CO2 from high
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pressure flue gas: 16.38 mmol g−1 at 27 oC and 20 bar. Chandra et al. reported the synthesis
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of nitrogen-doped graphene-based porous carbon materials via chemical activation of
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polypyrrole functionalized graphene sheets.8 The synthesized materials had high surface
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area (1360 m2 g−1), large pore volume (0.59 cm3 g−1), and profound microporosity, resulting
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in an excellent selective adsorption of CO2 (4.30 mmol g−1) over N2 (0.27 mmol g−1) at 25
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o
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large scale production of porous graphene materials, such as the high cost of the activating
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agents, the need for an additional washing stage to remove the residual chemical agent and
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thus contributing essentially to environmental pollution, and the corrosiveness of the
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process.6,9
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Alternatively, physical activation in the presence of a suitable oxidizing gas, such as air,
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CO2, steam, or their mixtures, is an effective and convenient procedure for the preparation
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of porous graphene materials because of: (i) its simplicity and ease of operation with no
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additional processing such as washing of the end products; (ii) its environmental benign
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approach since it avoids the use of toxic and harmful chemicals; and (iii) its scalability and
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low-cost.10,11 Sui et al. have recently developed porous graphene-based carbons through
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physical activation of graphene aerogel (GA) using steam as the activating agent.12 The
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resultant steam activated GA (SAGA) showed a high specific surface area (1230 m2 g−1),
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large pore volume (3.67 cm3 g−1), and a well-connected pore network with pore diameter
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centered at about 2.5 nm. Volumetric gas adsorption measurements revealed that SAGA
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could selectively adsorb up to 2.45 mmol CO2 per gram at 0 oC and 1 bar. Further
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improvements in the porosity characteristics and CO2 uptake properties of such porous
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graphene materials can be achieved through a proper choice of precursor, method of
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activation, and control of processing conditions.
C and 1 bar. Nevertheless, the chemical activation process has several disadvantages for
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Herein, we describe a facile one-stage physical activation method to produce 3D porous
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graphene-based materials with large accessible surface area and a well-defined
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interconnected pore structure. RGO, prepared by the thermal reduction of GO, was used as
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precursor, while CO2 was the activating agent of choice as it can produce larger pore volume
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and narrower micropores than steam.9
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highly efficient for selective separation of CO2 from flue gas mixtures, especially under
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conditions relevant to post-combustion capture from power plants fueled either with coal or
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natural gas. Because of their hierarchical porous morphology, these RGO-derived graphene
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materials may also be favorably considered for other energy related applications, including
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supercapacitor electrodes, lithium-ion adsorption/desorption in batteries, solar cells, and
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energy gas (hydrogen/methane) storage.
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2. EXPERIMENTAL
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2.1. Materials. Graphite powder (
