Single step synthesis of activated bio-carbons with a high surface area and their excellent CO2 adsorption capacity

Accepted Manuscript Single step synthesis of activated bio-carbons with a high surface area and their excellent CO2 adsorption capacity Gurwinder Singh, In Young Kim, Kripal S. Lakhi, Prashant Srivastava, Ravi Naidu, Ajayan Vinu PII:

S0008-6223(17)30134-3

DOI:

10.1016/j.carbon.2017.02.015

Reference:

CARBON 11732

To appear in:

Carbon

Received Date: 22 December 2016 Revised Date:

6 February 2017

Accepted Date: 6 February 2017

Please cite this article as: G. Singh, I.Y. Kim, K.S. Lakhi, P. Srivastava, R. Naidu, A. Vinu, Single step synthesis of activated bio-carbons with a high surface area and their excellent CO2 adsorption capacity, Carbon (2017), doi: 10.1016/j.carbon.2017.02.015. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Single step synthesis of activated bio-carbons with a high surface area and their

2

excellent CO2 adsorption capacity†

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Gurwinder Singh,ab In Young Kim,*a Kripal S. Lakhi,a Prashant Srivastava,ab Ravi Naidu*bc

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and Ajayan Vinu*a

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a

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Environment, University of South Australia, Mawson Lakes, South Australia 5095, Australia

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b

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Environment, University of South Australia, Mawson Lakes, South Australia 5095, Australia

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c

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Future Industries Institute, Division of Information technology, engineering and

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Cooperative Research Centre for Contamination Assessment and Remediation of the

Global Centre for Environmental Remediation (GCER), ATC Building, the University of

Newcastle, Callaghan, New South Wales, Australia

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*Corresponding authors;

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E-mail: [email protected]; Tel: +61-8-83025384

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E-mail: Ravi. [email protected]; Tel: +61-2-49138705

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E-mail: [email protected] Tel: +61-8-83026243

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† Electronic Supplementary Information (ESI) available: [carbon and nitrogen contents in

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activated carbons, CO2 adsorption capacity of activated carbons, and textural parameters of

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carbon made using two-step activation.

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ACCEPTED MANUSCRIPT Abstract:

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An effective way to synthesize activated microporous carbons with a high specific surface

3

area is developed by single step reaction between Arundo donax and solid KOH at 600 oC for

4

2 h. The texture and the specific surface area of the activated microporous carbons can be

5

controlled by adjustment of the ratio of Arundo donax and solid KOH. The prepared

6

microporous carbons display larger surface areas and micropore volumes than those of the

7

activated microporous carbon prepared by a two-step reaction. The porous carbon prepared

8

with the KOH/biomass weight ratio of 2 (KLB2) exhibits the largest surface area of 1122

9

m2·g-1 and the highest micropore volume of 0.50 cm3·g-1. Among the materials studied,

10

KLB2 exhibits the highest CO2 adsorption capacity at 273 K of up to 6.3 mmol·g-1 at 1 bar

11

while the adsorption capacity at 273 K is increased to 15.4 mmol·g-1 at 30 bar. Present work

12

demonstrates that highly stable activated microporous carbons can be prepared in a single

13

step for effective CO2 capture for both post- and pre-combustion. As the highly stable

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microporous carbons are prepared from waste biomass, it can provide an efficient way for

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developing cost effective adsorbents for CO2 capture at low and high pressure.

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1. Introduction The emission of CO2 through the burning of fossil fuels and industrial activities is

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one of the major contributors to global warming [1,2]. Therefore, much attention has

4

been given to developing low cost and sustainable methods that would be useful in

5

capturing and storing CO2 in order to reduce its emission into the atmosphere.

6

Currently available technologies for CO2 capture are based on the principles of

7

absorption, adsorption, cryogenic distillation and separation process [3-6]. Among

8

these, amine based adsorption is the most widely used industrial technology for CO2

9

capture. However, it has several disadvantages such as quick corrosion in the

10

equipment, the release of toxic volatile amine vapours and a high energy consumption

11

for regeneration, thereby leading to a high capital and operating costs [7-9]. In the

12

recent past, adsorption using porous materials has arisen as a low cost and high

13

adsorption strategy for capturing CO2. For example, porous materials such as carbon

14

nanofiber webs [10], carbon derived from melamine-formaldehyde resin [11], carbon

15

derived from polyvinylidene fluoride [12], zeolites [13], metal-organic frameworks

16

[14], mesoporous carbon nitrides [15] and polymer-silica nanospheres [16] have

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recently been used as adsorbents for CO2 capture. However, the production of these

18

materials occurs through complicated synthetic procedures and is usually associated

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with a cost barrier [17].

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porous

materials

are

attractive

for

applications

such

as

supercapacitors, fuel cells and lithium-sulfur batteries [18-20]. In the recent years,

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creating nanoporous carbon materials from natural biomass has gained a significant

23

consideration because of its low cost, availability in large quantity, and environment-

24

friendly nature. Various types of natural biomass including wooden and non-wooden

25

have been employed as precursors to prepare activated carbons from biomass, upon

26

calcination at elevated temperatures. Pristine biochar as such is highly non-porous and

27

exhibits lower CO2 capture adsorptivities than those of zeolites, metal-organic

28

frameworks and mesoporous carbon nitrides due to the poor textural parameters

29

including low specific surface area and pore volume. These parameters can be

30

enhanced through modifications such as chemical activation by using moderate to high

31

temperatures. As a result, activated bio-carbons are formed upon activation process of

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biochars, and such materials show enhanced CO2 adsorption capacity. For example,

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activated carbons derived from peanut shell, pine cone, camellia japonica, jujun grass,

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bamboo and coconut shell show CO2 adsorption capacities in the range of 4.4-5.0

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mmol.g-1 at 1 bar and 298 K, which are quite high as compared to those of non-

3

biomass materials such as petroleum pitch (3.8 mmol.g-1 at 1 bar and 298 K) [1,7,17,

4

21-23]. Commonly used chemicals for the activation of carbon derived from natural

6

biomass include K2CO3, H2O2, HNO3, KOH and O2, LaCl3, H3PO4, ZnCl2, CO2-NH3

7

mixture, amine, methanol, and so on [24-33]. Among these, activation with KOH is

8

one of the compelling methods for creating highly microporous structure and

9

functional groups on the surface of the carbons due to the intercalation of potassium

10

between the lattices, combination of oxidation of carbon and activation of carbon with

11

in-situ formed CO2 occurring during the high-temperature treatment [5,34,35]. Carbon

12

materials with a high micropore volumes are regarded as favourable candidates for

13

CO2 sorption [36]. However, KOH activation process generally requires two steps

14

including activation of biomass with KOH solution and subsequent carbonisation

15

through calcination at elevated temperature [7,8,21,22,34,35,37-39]. The number of

16

steps in the activation process can be reduced by replacing KOH solution with solid

17

KOH. Herein we report single step synthesis of activated microporous carbons from

18

Arundo donax through direct activation with solid KOH. To the best of our

19

knowledge, there is no report about the single step synthesis of activated carbons from

20

Arundo donax using solid KOH. Arundo donax is a perennial cane also known as giant

21

reed that can thrive in different climatic regions around the world. The rapidly

22

growing and invasive nature of this plant makes it difficult to cultivate other crops of

23

human use in the region. Consequently, this could ultimately cause an imbalance in

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the natural ecosystem. Therefore, the utilisation of this waste biomass to generate

25

activated carbons will be a decisive technique to counteract the economic losses

26

incurred as a result of eradicating such plants. In this work, the Arundo donax derived

27

activated bio-carbons have been found to be highly stable and used as adsorbents for

28

the capture of CO2 at pressures ranging from 1 bar to as high as 30 bar, which is

29

similar to the pressure condition of the post- and pre-combustion capture of CO2 from

30

flue gas stream. The chemical composition and textural properties of the activated

31

carbons have been controlled by varying the mass ratio of the used biomass and solid

32

KOH. The effect of controlled chemical composition and textural parameters of the

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activated carbons on CO2 adsorption has been investigated in this study.

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2. Experimental

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2.1. Preparation of activated carbons from Arundo donax Arundo donax was obtained from Adelaide, South Australia. The stem part was cut

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into small pieces with the length of 0.5-1 cm. Prior to activation, it was washed several

5

times with deionised water and then dried overnight in an oven at 100 °C to remove

6

any moisture content. The dried material was crushed and sieved into fine biomass

7

particles. Afterwards, 1 g of the biomass was thoroughly mixed with a definite

8

quantity of solid KOH. In order to optimise the effect of activation, different amounts

9

of solid KOH were combined with different KOH/biomass weight ratios of 0:1, 1:1,

10

2:1 and 3:1. Subsequently, the mixtures were calcined at 600 °C for 2 h under a

11

continuous flow of N2 for single step carbonisation and activation. The obtained

12

materials were thoroughly washed with 1 M HCl to completely remove the potassium

13

residues followed by washing with deionised water several times till the pH is ~7.

14

Then, the materials were dried at 100 °C for 6 h. The final black coloured materials

15

were denoted as KLB0, KLB1, KLB2 and KLB3 for the KOH/biomass ratios of 0:1,

16

1:1, 2:1, and 3:1, respectively. For comparison study, activated carbon was also

17

synthesised through a typical two-step reaction involving biomass and KOH solution

18

and the resultant sample was denoted as KLBSOL. Activated carbons were also

19

prepared at 500 °C and 700 °C in order to compare the effect of activation temperature

20

on the textural parameters.

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2.2. Characterization

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Crystal structure of the activated carbons was investigated by powder X-ray

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diffraction (XRD) analysis with PANalytical Empyrean XRD instrument (40 kV, 40

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mA, Cu Kα= 1.5418 Å). Chemical constituents of the activated carbons which were

26

digested with the aqua regia were investigated by inductively coupled plasma mass

27

spectroscopy (ICP-MS). The quantity of C and N in the activated microporous carbons

28

was determined by using a Leco Trumac CNS analyser. The surface morphology of

29

the activated microporous carbons was investigated using scanning electron

30

microscopy (SEM) at 2 kV. The nature of chemical bonding and surface functional

31

groups were investigated through FTIR spectroscopy [40]. The FTIR measurements

32

were done using a Newport FTIR spectrometer. Textural properties of the activated

33

microporous carbons were investigated by measuring N2 adsorption-desorption

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isotherms at 77 K with an Automatic Micrometrics Analyser and results reported

2

through Gemini model 2380 software. Prior to N2 and CO2 gas adsorption analyses,

3

activated microporous carbons were degassed for overnight at 523 K to ensure that all

4

the moisture and gaseous elements have been eliminated. On the basis of the

5

isotherms, the surface area, micropore volume and pore size of the materials were

6

calculated by Brunauer–Emmett–Teller (BET), t-plot and micropore analysis (MP)

7

methods, respectively. CO2 adsorption was carried out using high-pressure

8

Quantachrome Isorb HP1 instrument equipped with water circulation for temperature

9

control. The adsorption experiments were conducted at 273, 283 and 298 K under a

10

pressure variation from 0 bar to 30 bar. Clausius Clapeyron equation was employed to

11

measure the isoelectric heat of adsorption for all the activated microporous carbons.

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Isosteric heat of adsorption was used to ascertain the type of interactions between CO2

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and carbon samples.

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3. Result and Discussion

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3.1. XRD analysis

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All the obtained materials exhibit black colour, indicating a complete carbonisation

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of Arundo donax upon the single step reaction. As shown in Fig. 1, all carbon

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materials exhibit two broad peaks centred around 2θ = 23° and 43°. These two peaks

19

are respectively identical to (002) and (101) reflections of graphite, suggesting that

20

present carbon materials show a partially graphitic crystal structure [41,42].

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Interestingly, intensities of these two reflections are enhanced with increasing the mass

22

ratio of KOH and biomass, revealing that solid KOH promotes the graphitization of

23

Arundo donax during the single step reaction.

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The d-spacing of the (002) plane of KLB0, KLB1, KLB2 and KLB3 is 1.93, 1.98,

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1.99 and 2.01 nm, respectively, which is consistent with the interlayer spacing in

26

graphitic activated carbon. The enhanced d-spacing of (002) reflection with increasing

27

the mass ratio of KOH and biomass may be attributed to the formation of more

28

functional groups in the interlayers of carbon upon the KOH-activation. The absence

29

of any strong peaks indicates that the impurities of crystalline carbons or potassium

30

residues are not present in the activated microporous carbons [43]. However, XRD

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pattern of the sample prepared without KOH activation showed several sharp peaks

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which correspond to crystallised mineral [44]. It is surmised that this mineral

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originated from the contaminants that are present in the land source of biomass.

2

Interestingly, these peaks did not disappear even after washing with HCl solution but

3

are removed completely after activation with KOH. This could be due to the fact that

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KOH activation process decomposes the impurities which are easily soluble in HCl

5

solution.

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Fig. 1 XRD patterns of the Arundo donax derived carbons of (a) KLB0, (b) KLB1, (c)

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KLB2 and (d) KLB3

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3.2. Chemical composition and chemical bonding nature analyses The stem of Arundo donax is composed of ~62% cellulose, ~19% lignin, ~3%

22

protein and ~4% ashes [45]. The chemical composition of Arundo donax derived

23

carbons was analysed by using CNS and ICP-MS analysis. The CNS and ICP-MS

24

results reveal dominant existence of C and N, and negligible presence of N, Na, Mg,

25

Al, P, S, K, Ca, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Cd and Pb in all Arundo donax

26

derived carbons (see Table S1 and S2 in Supplementary Information, SI). Non-

27

activated carbon of KLB0 contains 1.12 wt% nitrogen, which is the highest value

28

among all the present carbons. On the other hand, the activated carbons of KLB1,

29

KLB2 and KLB3 show nitrogen content in the range of 0.53-0.87wt%. Considering

30

that Arundo donax contains inherent nitrogen containing compounds, nitrogen can be

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doped into the lattice of carbon during the single step reaction. The low nitrogen

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contents in activated carbons are ascribed to the depolymerisation of nitrogen-doped

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carbon or the decomposition of nitrogen from the carbon matrix as the thermodynamic

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stability of nitrogen in the carbon matrix is very low at high temperature in the

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presence of trace amount of oxygen. In this case, oxygen is generated from the

3

decomposition of potassium carbonate that is formed during the activation process by

4

reaction with the carbon present in the materials. This is confirmed by the fact that the

5

activation process significantly decreases the nitrogen content in the final activated

6

carbons. It is interesting to note that the carbon content of the non-activated carbon

7

and activated carbons except KLB3 is almost similar (82.5 to 84 wt%) while KLB3

8

exhibited a carbon content much less than 54.9 wt%. This reveals that the content of

9

ashes or minerals or potassium residues in the KLB3 is much higher than those of

10

KLB0, KLB1 and KLB2. In order to reduce the ash content of the KLB3, the sample

11

was washed several times with HCl solution. However, no significant reduction in the

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ash content was observed. These results confirmed that the mass ratio of KOH and

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biomass is a key factor to control the ash and carbon content of the final products.

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Fig. 2 FTIR spectra of the Arundo donax derived carbons (a) KLB0, (b) KLB1, (c)

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KLB2 and (d) KLB3

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The surface functional groups of prepared carbons before and after the activation

30

process were investigated by using FTIR and results are shown in Fig. 2. All carbon

31

materials show a broad peak at 1420 cm-1 corresponding to the aromatic C-C

32

stretching mode. A very broad peak appears at 3500 cm-1 which may be attributed to

33

the O-H and N-H stretching vibration of carboxyl and amine groups, respectively [46].

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The weak C=O stretching mode is observed at 1720-1730 cm-1 in spectra of all carbon

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materials. The existence of peaks at 1010, 1100, 1200 and 1250 cm-1 that correspond

2

to the C-O stretching modes of carboxylic acid, secondary alcohol, phenolic and ether

3

groups, respectively, confirm that several oxygenated functional groups are generated

4

on the surface of the activated carbons. Interestingly, despite similar features between

5

non-activated carbon and activated carbons, activated carbons of KLB1, KLB2, and

6

KLB3 show stronger C-O stretching modes than that of non-activated carbon of

7

KLB0, demonstrating higher concentration of oxygen functional groups in the

8

activated samples. These functional groups can help to enhance the adsorption and

9

separation of different gases. It is interesting to note that C-H peak in aromatic carbon

10

at 770 cm-1 is also enhanced with increasing the mass ratio of KOH and biomass,

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which is well matched with the result of XRD analysis, indicating that KOH-activation

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significantly promotes the graphitization of activated carbons.

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3.3. SEM analysis

The morphology of non-activated and activated microporous carbons is illustrated

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in Fig. 3. KLB0 shows non-porous and flat morphology whereas the carbons of KLB1

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and KLB2 display a flat morphology possessing a large number of pores with the

18

diameter of 0.2-2 µm. The presence of pores in KLB1 and KLB2 provides a clear

19

evidence that Arundo donax is activated by single step reaction with solid KOH. It is

20

found that the morphology of activated materials is greatly affected by varying the

21

mass ratio of KOH and biomass. As can be seen in Fig. 3, the macropore diameter of

22

KLB3 is much larger than that of KLB1 and KLB2. Moreover, the KLB3 displays a

23

shrunk morphology while KLB0, KLB1 and KLB2 show unfolded and flat

24

morphology with uniform macropores throughout the surface of the particles,

25

revealing a high porosity in the later samples. The morphological features of KLB3 are

26

ascribed to a collapse of the matrix of carbon with the highest KOH/biomass ratio of 3.

27

The high content of KOH decomposes the carbon walls connecting continuously

28

oriented porous structure of the carbon through oxidation at a high temperature while a

29

lot of potassium residues that are generated during the activation process completely

30

changes the surface morphology of the KLB3. These results also reveal that controlled

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activation process is a key to obtain activated carbons from biomass with regular

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morphology, continuously oriented porous structure and excellent surface parameters

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including high specific surface area and micropore volume.

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Fig. 3 SEM images of the Arundo donax derived carbons (a) KBL0, (b) KLB1, (c)

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KLB2 and (d) KLB3

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3.4 Mechanism of KOH activation Biopolymers

present

in

biomass

undergo

extensive

changes

such

as

dehydrogenation, cracking, dehydration, polymerisation and aromatisation during the

22

process of carbonisation and activation. KOH is a strong base and its reaction with

23

lignocellulosic biomass such as Arundo donax can be expected to occur via a series of

24

chemical reactions. It is proposed that the initial redox reaction between biomass

25

carbon and KOH starts at low temperatures which leads to oxidation of carbon

26

resulting in the formation of K2CO3 and the hydroxide itself gets reduced to metallic

27

potassium and hydrogen gas. This can be represented as a general reaction as follows:

28

6KOH + 2C → 2K2CO3+ 2K + 3H2↑ ··········· ············································ (1)

29

Initially, as the biomass gets heated up, the functional groups present within the

30

materials decomposes into volatiles such as CO, CO2 and H2O and the rest of the mass

31

undergoes aromatisation to form char. These volatiles will themselves escape through

32

the structure of the carbon leading to the development of porosity. These species can

33

also react with KOH to form K2CO3 and volatiles leading to an intense reaction at a

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high temperature (Reaction 2). The biomass carbon keeps on reacting with the KOH

2

until it is completely consumed and consequently converted into species such as

3

metallic potassium and volatiles (Reaction 3).

4

2KOH + CO2 → K2CO3 + H2O↑ ···························································· (2)

5

2KOH + 2C → 2CO↑ + 2K↑ + H2↑ ············ ············································· (3)

6

K2CO3 + C → K2O + 2CO↑ ································································· (4)

7

K2O + C → 2K + CO↑ ········································································(5)

8

As the temperature is increased to a moderate value of 600 °C, all of KOH is

9

completely consumed as it is confirmed by the absence of any related peaks in the

10

XRD diffraction patterns [47]. K2CO3 formed as a result of redox reactions can further

11

react with biomass carbon to generate K2O, metallic potassium and volatiles (Reaction

12

4-5). The formation of activated carbons with a high porosity is attributed to the

13

above-mentioned reactions of depolymerisation, dehydration and evolution of gases in

14

carbon occurring during the process of carbonisation and activation. The potassium

15

compounds formed during the process are efficiently intercalated into the carbon

16

matrix [48] and their subsequent removal using acid wash results in highly porous

17

carbon structure accompanied with a high specific surface area.

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3.5 N2 adsorption-desorption isotherm and pore size distribution analysis Physical parameters of all samples including specific surface area, micropore

21

volume, total pore volume, and pore diameter are obtained from the N2 adsorption-

22

desorption isotherms. Fig. 4 shows the adsorption-desorption isotherms and textural

23

properties of all carbons are summarised in Table 1. As illustrated in Fig. 4, KLB0

24

does not show significant adsorption of nitrogen while all the activated carbons of

25

KLB1, KLB2, and KLB3 shows high N2 adsorption and display the type-I adsorption

26

isotherm suggesting a microporous nature of activated carbons [50]. According to pore

27

size distribution calculation based on the MP method using adsorption branch of

28

isotherm in Fig. 4, all activated carbon materials possess micropores with a pore

29

diameter of 0.56 nm as shown in Fig. 5. KLB0 exhibits a small BET surface area of 16

30

m2· g-1, indicating the formation of non-porous structure. In contrast to KLB0, KLB1,

31

KLB2 and KLB3 present significantly enhanced surface areas of 637, 1122 and 849

32

m2· g-1, respectively. The micropore volumes of KLB0, KLB1, KLB2 and KLB3 are

33

0.00, 0.25, 0.50 and 0.31 cm3·g-1, respectively.

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Table 1 Textural properties of the Arundo donax derived carbons of KLB0, KLB1,

2

KLB2 and KLB3 Material

SABET

Total pore

Micro pore

Micro-

Average Pore

(m2·g-1)

volume

volume

porosity (%)

diameter

(cm3· g-1)

(cm3·g-1)

(MP method)

16

0.020

0.0

KLB1

637

0.347

0.25

KLB2

1122

0.592

0.50

KLB3

849

0.502

0.31

3 4

6 7 8 9

14 15

0.56

84

0.56

62

0.56

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Fig. 4 N2 adsorption-desorption isotherms of the Arundo donax derived carbons of (a)

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KLB0, (b) KLB1, (c) KLB2 and (d) KLB3

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Higher surface area and greater micropore volume of KLB1, KLB2, and KLB3

20

indicate that single step KOH-activation is successful. The increase in surface area and

21

micropore volume is due to the gasification and oxidation effects occurring through

22

the decomposition of potassium carbonate at a high temperature. Among the activated

23

carbons, KLB2 presents the largest surface area and the highest micropore volume.

24

The lower specific surface area and pore volume of KLB3 are mainly attributed to the

25

over-oxidation of carbon walls with the generated CO2 or CO gases from the

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activating agent and formation of highly insoluble potassium residues or minerals as

2

the amount of KOH added in the activation of KLB3 is much higher than that of

3

KLB2. This result strongly demonstrates that KOH/biomass ratio of 2 is the most

4

effective combination for the creation of extensive microporous structure in the

5

activated carbon while the excessive KOH-treatment with KOH/biomass ratio of 3

6

leads to the diminishment of microporous structure in the activated carbon. To prove

7

the effectiveness of the single step reaction, comparison study is carried out with

8

activated carbons prepared by two-step reaction at same condition of the

9

KOH/biomass ratio of 2. The surface area and micropore volume of activated carbon

10

upon two-step reaction are 849 m2·g-1 and 0.44 cm3·g-1 respectively, which are less

11

than those of KLB2 (see Fig. S1 in SI). This experimental finding provides a strong

12

evidence that the present single step activation is extremely effective in activating

13

biomass as compared to two-step reaction. The role of temperature on the textural

14

parameters was investigated by making activated carbons with three impregnation

15

ratios at 500 °C and 700 °C. When the textural parameters of these carbons are

16

compared with those of carbons made at 600 °C, it is quite evident that 600 °C is the

17

most effective temperature for producing the highest surface area and pore volume

18

among all the activated carbons (see Table S3 in SI).

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19 20 21

24 25 26 27

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Fig. 5 Pore size distribution for Arundo donax derived activated carbons (a) KLB1, (b)

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KLB2 and (c) KLB3

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It is evident from the pore size distribution analysis that all the activated carbons are

2

highly microporous in nature with pores of size 0.56 nm which is quite dominant in

3

the carbon structure. Such extremely small micropores are generally referred to as

4

ultra-micropores and offer opposite space for movement of CO2 molecules inside the

5

activated carbon structure thereby resulting in the greater amount of gas storage.

6

Additionally, the presence of a narrow pore size distribution in the range of 0.5-0.6 nm

7

illustrates the highly tuneable nature of the carbons produced using KOH activation.

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3.6 CO2 adsorption analysis

CO2 adsorptivity of all the carbon materials was investigated under a pressure

11

variation from 0 to 30 bar at 273 K. As plotted in Fig. 6, non-activated carbon of

12

KLB0 shows the CO2 adsorptivity of 2.0 mmol·g-1 at 1 bar and 273 K. The activated

13

carbons of KLB1, KLB2 and KLB3 respectively exhibit CO2 adsorptivities of 4.0, 6.3

14

and 3.7 mmol·g-1 at the same adsorption condition. The higher performance of

15

activated carbons for CO2 capture underscores the merits of KOH activation of

16

biomass for preparing better CO2 adsorbents. The activation process significantly

17

changes the chemical composition and textural properties of carbons. It is surmised

18

that enhancement in functional groups on the surface, increase in surface area and

19

micropore volume upon KOH activation contribute to the improved performance of

20

KLB1, KLB2 and KLB3 for CO2 capture.

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Although the nitrogen content in activated carbons is reduced upon KOH activation,

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it does not affect the enhancement of CO2 adsorptivity of activated carbons. This may

23

be due to combined effect of surface functional groups and the enhanced textural

24

properties including a high specific surface area and micropore volume as they not

25

only enhance the interaction between the CO2 molecules and the adsorbent surface but

26

also provides more number of adsorption sites. As other elements such as P and S exist

27

in extremely low quantities (see Table S2 in SI), it is surmised that such low amounts

28

of these elements play no role in CO2 capture.

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The highest CO2 adsorptivity is observed for KLB2 which has the highest surface

30

area and micropore volume together with a unique and flat morphology, underscoring

31

that a large surface area and a high micropore volume are major factors for increasing

32

CO2 adsorptivity. The CO2 adsorptivities of KLB2 is higher than similar carbon

33

materials at 1 bar and 273 K as presented in Table 2 [34,37-39,50]. Noteworthy, all

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these reported activated carbons are synthesised by two-step reaction, which suggests

2

a merit of present single step reaction for preparing activated carbons with superior

3

CO2 capture activity.

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Fig. 6 CO2 adsorption isotherms of the Arundo donax derived carbons of (a) KLB0,

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(b) KLB1, (c) KLB2 and (d) KLB3 at 273 K

18

Table 2 Experimental conditions and CO2 adsorptivity of the Arundo donax derived

19

carbon and other reported activated carbons

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CO2 adsorption

CO2 adsorption

KOH/biomass

(mmol·g-1)c

(mmol·g-1)d

1 bar/273 K

30 bar/273 K

Refs.

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Biomass

Ratio of

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Type of

Arundo donax

1:2

6.3a

15.4

This work

African palm shell

1:3

6.3b

-

37

Rice husk

1:3

6.2

-

34

Sawdust

1:2

6.1

-

38

Celtuce leaves

1:4

6.0

-

39

Wheat flour

1:3

5.7

-

50

a

Single step reaction, b two-step reaction, c post-combustion and d pre-combustion

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The CO2 adsorptions at 1 bar and 30 bar mimic the conditions of post and pre-

2

combustion CO2 capture, respectively (see Table 2 and Table S4 in SI). Hence, it is

3

challenging to synthesise microporous materials showing good CO2 adsorption both at

4

low and high pressures. KLB1, KLB2 and KLB3 exhibit CO2 adsorptivities of 8.6,

5

15.4 and 10.7 mmol·g-1 at 30 bar and 273 K, suggesting a high CO2 capacity of KLB2

6

at conditions of the pre-combustion. It is very interesting to note that among all the

7

activated carbons, KLB2 shows the best CO2 adsorptivity at both conditions of the

8

post- and pre-combustion at 273 K.

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It is hard to compare the performances of presently activated carbons at 30 bar to

10

those of other activated bio-carbons since there are no reports of CO2 adsorptivity of

11

activated carbons at 30 bar. Interestingly, CO2 adsorptivity of KLB2 at 30 bar and 273

12

K (15.4 mmol·g-1) stands way above the values reported for non-natural materials such

13

as carbon nitride (13.5 mmol·g-1), underscoring the usefulness of the waste biomass of

14

Arundo donax for CO2 capture [2]. The CO2 capturing the behaviour of KLB1, KLB2

15

and KLB3 investigated at 283 and 298 K is shown in Table S4 and Fig. S2 (see SI). A

16

gradual reduction in CO2 uptake is observed when the temperature is increased from

17

273 K to 298 K. This decrease in CO2 uptake is attributed to the exothermic nature of

18

CO2 adsorption process and also increasing the entropy of CO2 molecules at elevated

19

temperature. Even though the adsorptivities of the present materials are slightly

20

decreased with the increase of measurement temperature from 273 K to 298 K, this

21

reduction is weaker or similar to those of other CO2 adsorbents [51,52]. CO2

22

adsorptivity of the KLB2 (3.6 mmol·g-1 at 1 bar and 298 K) is comparable to

23

expensive non-biomass materials such as petroleum pitch (3.8 mmol·g-1 at 1 bar and

24

298 K), suggesting the merit of the cost-effective natural biomass of Arundo donax for

25

CO2 adsorption [23].

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3.7 Isosteric heat of adsorption

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The strength of interactions between adsorbate CO2 molecules and adsorbent carbon

29

materials are examined by the isosteric heat of adsorption analysis [53]. As shown in

30

Fig. 7, the values of the heat of adsorption for KLB1, KLB2 and KLB3 are 26, 31 and

31

22 kJ·mol-1. These values are far below than the energy required for cleavage of the

32

chemical bond of CO2 (749 kJ·mol-1), suggesting that physisorption is the dominant

33

process taking place between CO2 and activated carbons [21].

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After the adsorption process was carried out, materials were reheated to liberate

2

physisorbed CO2 and then again subjected to CO2 adsorption measurements. It was

3

observed that there is no reduction in adsorption capacity, revealing that the adsorption

4

process is highly reversible. These results indicate that activated carbons are highly

5

stable even after their repeated usage as adsorbents for CO2 adsorption. The quantity

6

of the heat of adsorption of KLB1, KLB2 and KLB3 is proportional to their nitrogen

7

content. KLB2 shows the highest quantity of heat of adsorption compared to KLB1

8

and KLB3, which is consistent with its dominant performance in N2 and CO2

9

adsorption.

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Fig. 7 Isosteric heat of adsorption of Arundo donax derived carbons of (a) KLB1, (b)

23

KLB2 and (c) KLB34. Conclusions

24 25

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4 Conclusions

26

In summary, firstly, we have introduced a new approach to design activated carbons using

27

a single step approach which significantly reduces the time and effort used in the whole

28

process. Highly microporous and naturally nitrogen-doped activated carbon materials were

29

developed by single step KOH activation of Arundo donax at 600 °C. It was found that the

30

activation of biomass at this temperature using a KOH/biomass ratio of 2 is the most effective

31

condition in establishing highest degree of porosity and development of high surface areas in

32

activated carbons. The effectiveness of the single step synthesis was compared with a typical

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two-step carbonisation and activation procedure which was carried out using similar

2

temperature (600°C) and activation conditions (KOH/biomass ratio of 2). The surface area

3

and micropore volume of activated carbon made using two-step reaction are lesser than the

4

activated carbons produced by using single-step reaction, providing a strong evidence that the

5

single step carbonisation cum activation is extremely effective in generating highly efficient

6

activated carbons as compared to the two-step reaction.

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Secondly, this is the first report on KOH-activated microporous carbons designed from

8

Arundo donax, which is regarded as an unwanted biomass due to its invasive nature. The pore

9

size distribution of the activated carbons resides in a very narrow region of ultra-micropores

10

(0.5-0.6 nm). As such, waste biomass, Arundo donax can serve as a suitable raw material for

11

developing cost effective and quality adsorbents for various applications including CO2

12

capture.

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Thirdly, we reported for the first time the CO2 adsorption capacity of activated

14

microporous carbons derived from Arundo donax under a varying degree of temperature and

15

pressure conditions. All activated materials showed promising results for CO2 uptake at both

16

low and high-pressure conditions. The enhanced surface area and micropore volume of the

17

activated carbons increased their CO2 adsorptivities even though reduced nitrogen content of

18

the activated carbons diminish the interaction between CO2 and activated carbons. The

19

recorded CO2 uptakes with the optimised KOH/biomass ratio of 2 are 6.3 mmol·g-1 and 15.4

20

mmol·g-1 at 1 bar and 30 bar respectively, measured at 273 K. The fact that these activated

21

carbons are displaying high values for pre- and post-combustion capture of CO2 as compared

22

to other materials makes them ideal candidates for this purpose. This research work is quite

23

significant in terms of our future strategies to design sophisticated activated carbons from

24

biomass.

25

Acknowledgements

26

We are thankful to CRCCARE and the University of South Australia for providing

27

financial support and laboratory facilities.

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