characterisation of activated carbons with high surface area and variable porosity produced from...

ORIGINAL PAPER

Characterisation of Activated Carbons with High Surface Areaand Variable Porosity Produced from Agricultural Cotton Wasteby Chemical Activation and Co-activation

Mohamad Anas Nahil • Paul T. Williams

Received: 23 November 2011 / Accepted: 6 February 2012 / Published online: 19 February 2012

� Springer Science+Business Media B.V. 2012

Abstract Activated carbons were prepared by phosphoric

acid activation of cotton stalks in nitrogen (chemical acti-

vation) and steam/nitrogen atmospheres (co-activation)

at various temperatures in the 500–800�C range and at

different H3PO4 acid to cotton stalk impregnation ratios

(0.3–3). The BET surface area and the pore structure of

activated carbons were evaluated from nitrogen adsorption

data at 77 K in relation to process conditions. The results

showed that the surface area and the porosity of the acti-

vated carbons were strongly dependent on the impregna-

tion ratio, temperature and the atmosphere. Increasing

impregnation ratio favours the development of mesopores,

whereas the temperature has a negative effect on the sur-

face area and porosity up to 700�C and thereafter it pro-

motes the creation of micropores at 800�C. Steam promotes

formation of narrower micropores in the activated carbons

prepared at higher temperatures and the formation of

mesopores at lower temperatures. Transmission electron

microscopy analysis also identifies mesopore structures in

the mesoporous activated carbon and microporous struc-

tures in the microporous activated carbons. The effect of

activation temperature and atmosphere on surface chem-

istry of activated carbon, in terms of carbon, oxygen and

phosphorus functionalities, was extensively studied using

X-ray photoelectron spectroscopy. The results showed that

carbon content increased as the activation temperature was

increased from 500 to 800�C in both atmospheres and as

the atmosphere was changed from nitrogen to steam/

nitrogen mixture in both temperatures, while the oxygen

and phosphorus contents decreased due to the removal of

phosphorus compounds.

Keywords Cotton stalks � Agriculture waste �Activated carbon � XPS � Steam

Introduction

Activated carbon is a versatile adsorbent because of its

good adsorption properties. It is used extensively in dif-

ferent environmental applications for removal of chemical

species by adsorption from liquids or gases. The applica-

tions in liquid phase form roughly 80% of the total use of

activated carbon. Recently, important applications of acti-

vated carbons have been reported in the automotive sector,

catalysis and the storage of fuels [1–3]. Activated carbon

can be made from a wide range of source materials. The

most commonly used precursors are coal, wood and

coconut shells. However, many studies have shown that

the activated carbons obtained from agriculture waste can

be favourably compared with other commercial activated

carbons used in industry with respect to their adsorptive

properties [4–9].

Large tonnages of cotton stalk waste are generated after

the harvesting season in the countries in which the cotton is

being planted on a large scale. For example, the annual

production of cotton stalks in Turkey, which is considered

as one of the major cotton producing countries, is estimated

at 3.24 million tonnes and gradually will increase to 4.86

million tonnes by 2020 [10]. Many environmental prob-

lems are associated with this stalk waste since much of the

waste throughout the world is disposed of by either burning

or dumping. Conversion of cotton stalks waste to activated

carbons would increase its economic value, produce low

M. A. Nahil � P. T. Williams (&)

Energy Research Institute, The University of Leeds,

Leeds LS2 9JT, UK

e-mail: [email protected]

123

Waste Biomass Valor (2012) 3:117–130

DOI 10.1007/s12649-012-9109-7

cost adsorbent materials and encourage diversion of the

waste away from landfill and open burning.

Activated carbons are produced by physical and chem-

ical activation. Physical activation involves two stages,

firstly carbonisation of the precursor (typically below

700�C) followed by the partial gasification of the resulting

char with oxidising gases such as air, carbon dioxide or

steam, usually at high temperature (700–1,100�C). Chem-

ical activation consists of the pyrolysis of the precursor in

the presence of a chemical activating agent at a tempera-

ture (400–800�C). Among the activating agents, phospho-

ric acid, potassium hydroxide and zinc chloride are widely

used [4, 5, 9]. According to some studies reported in the

literature, potassium hydroxide and zinc chloride produce

microporous activated carbons, while phosphoric acid

produce mixed microporous/mesoporous activated carbons.

Most commercially activated carbons are microporous

with high surface area. These adsorbents can be success-

fully used in the adsorption of gaseous pollutants which

usually have small size molecules. For removing contami-

nants with large size molecules from aqueous solutions,

activated carbons with high mesoporous texture is required

[11]. Physical activation of lignocellulosic materials pro-

duces microporous activated carbons. Using phosphoric

acid allows the development of both micropores and mes-

opores in the resulting carbon structure [12]. Usually

phosphoric acid activation is performed under nitrogen or

air atmospheres [8, 9, 13, 14]. Fewer studies have investi-

gated the effect of a steam atmosphere on the porosity of the

activated carbon in the presence of the chemical activating

agent, i.e. co-activation [15–17]. Klijanienko et al. [17]

concluded that phosphoric acid activation of wood under an

atmosphere of steam produces activated carbon with better

developed mesopores than that produced under nitrogen

atmosphere. In addition, it is well known that surface

chemistry of activated carbon plays an important role in

specific adsorption and surface reactions [18, 19]. Surface

chemistry of carbon materials depends on the presence of

heteroatoms like hydrogen, oxygen, nitrogen, phosphorus,

chlorine etc. and inorganic ash components that may come

from carbon precursor or activating agent [18, 20, 21].

Heteroatoms affect acid–base characteristics of carbons and

modify their electrochemical properties [22, 23]. The rea-

son is likely to be due the electronegativity of heteroatoms

being different from that of carbon [24]. For example,

oxygen-containing surface groups confer hydrophilic and

cation exchange properties and nitrogen-containing carbons

show enhanced anion exchange properties [18]. Phospho-

rus-containing carbons show a number of specific charac-

teristics that range from acid surface groups and cation

exchange properties to enhanced oxidation stability [25–

27]. Many studies have shown that phosphoric acid acti-

vation results not only in well developed porosity of the

product carbons but also leads to inclusion of a significant

amount of phosphorus into carbon structure [16, 17, 28, 29].

It has been shown that phosphorus compounds are respon-

sible for enhanced cation exchange properties and other

chemical properties of phosphoric acid activated carbons

which are necessary for specific applications [30, 31]. On

the other hand, activation of lignocellulosic materials by

phosphoric acid produces acidic carbons due to the phos-

phorus content which precludes the use of such materials in

some applications such as the pharmaceutical applications

[15]. Therefore, activated carbons with no or very low

phosphorus content is demanded in these applications.

Consequently, production of activated carbons with or

without phosphorus content using the phosphoric acid

activation method is required depending on the application.

This study was carried out to investigate the effect of

H3PO4 activation in nitrogen (chemical activation) or

steam atmospheres (co-activation) on the properties cotton

stalk derived activated carbon prepared at different

impregnation ratios and temperatures through chemical

activation using phosphoric acid. These properties include

the pore characteristic and surface chemistry which deter-

mine the adsorption behaviour of activated carbons.

Materials and Methods

Waste Materials

Cotton stalks were used as raw material for the preparation

of activated carbons and were air-dried and crushed, to

produce a size fraction of between 1 and 3 mm. The

proximate and elemental analyses of the sample are given

in Table 1. The elemental analysis (C, H, N, S, O) was

carried out using a Carlo Erba Flash EA 1112 elemental

analyser. A Shimadzu TGA-50H analyser was used to

perform the proximate analysis. This was carried out by

heating of the sample in a nitrogen atmosphere to 110�C to

obtain the weight loss associated with moisture. Secondly,

the sample was heated to 925�C, again under a nitrogen

atmosphere, and maintained at this temperature for 40 min

to obtain a constant weight. The weight loss between 110

and 925�C corresponds to the loss of volatile matter from

the sample. The final stage involved the substitution of

nitrogen by air to burn off the fixed carbon in the sample

leaving a mass of ash within the alumina crucible.

Preparation of Activated Carbons

Impregnation of Cotton Stalks

Ten grams of dried cotton stalks (1–3 mm) were soaked

in hundred millilitres of phosphoric acid solution. The

118 Waste Biomass Valor (2012) 3:117–130

123

phosphoric acid was obtained as an 85% solution. A known

mass of activating agent was mixed with distilled water,

and the cotton waste was then impregnated in the acidic

solution to provide H3PO4:cotton stalk ratios of 0.3:1,

0.75:1, 1.5:1 and 3:1 by weight. The liquid/solid mixture

was stirred continuously at ambient temperature for 1 h to

allow penetration of the H3PO4 into the cotton agriculture

waste [32]. After mixing, the slurry was oven-dried at

110�C for 48 h and weighed.

Activation

Normal chemical activation was carried out under nitrogen

atmosphere. A weighed amount of impregnated samples

was placed in static bed batch reactor, 250 mm in length by

30 mm internal diameter and was externally heated by a

tube furnace 1.2 kW as shown in Fig. 1a. The furnace was

controlled to produce the desired heating rate, final tem-

perature and final activation temperature hold time. The

impregnated precursor material was carbonised in the static

bed reactor under a nitrogen atmosphere at temperatures of

500, 600, 700, 800�C for 2 h.

Co-activation of the same samples was carried out under

a steam/nitrogen atmosphere. The activation reactor was

adapted to introduce the steam to the reaction zone

(Fig. 1b). The steam generator furnace was maintained at

300�C. A Sage instrument model 255-2-syringe pump was

used to inject deionised water into the steam generator. The

same conditions were used, including the heating rate and

final temperature, and once the activation temperature was

attained, the steam was introduced for a period of 2 h.

Following activation, the samples were cooled in nitrogen

overnight, removed from the reactor at the end of each

experiment and weighed. The samples were then thor-

oughly washed sequentially with solutions of sodium

hydroxide (0.1 N), hot water and finally cold distilled water

until the washing water reached a stable pH of 6–7. The

samples were then dried at 110�C for 24 h prior to analysis.

The yield of activated carbons was calculated from the

following relationship:

Yield wt % ¼W1

W0

� 100

where W0 = initial dry mass of cotton stalks (10 g) and

W1 = dry mass of activated carbon after washing (g).

Characterisation of Activated Carbons

The BET surface area (SBET) and the porosity of the acti-

vated carbons were determined by the adsorption of N2 at

77 K, using a Micromeritics TriStar 3,000 apparatus. The

system operates by measuring the quantity of nitrogen

adsorbed onto or desorbed from a solid sample at different

equilibrium vapour pressures. The dry, weighed, samples

Table 1 Elemental and proximate analysis of cotton stalks

Elemental analysis (daf) Proximate analysis

C (wt%) H (wt%) N (wt%) O (wt%) Moisture (wt%) Fixed carbon (wt%) Volatiles (wt%) Ash (wt%)

43.6 5.8 0.8 49.8 4.9 18.3 70.5 6.3

Daf dry and ash free

Fig. 1 Schematic diagram of (a) the static bed pyrolysis reactor and

(b) the co-activation reactor

Waste Biomass Valor (2012) 3:117–130 119

123

(*0.5 g) were outgassed at 300�C for 2 h, under vacuum.

The relative pressure range used while applying the BET

method was between 0.05 and 0.2. The micropore volume

(Vmic) was obtained by the Dubinin-Radushkevich (DR)

method [33]. The DR method should give a straight line

between log Vads (cm3 STP g-1) and log2(P0/P) with an

intercept log (V0), from which the volume of adsorbate gas

adsorbed in the micropores can be calculated. Samples

investigated in this study showed extensive linearity over a

wide range of relative pressures and therefore this method

was applied for the current study.

For the low pressure region, microporous region of

nitrogen adsorption, typically 15 incremental measure-

ments were taken over the range of 0.0–0.1 relative

pressure. The mesopore volume (Vmes) was deduced from

the adsorption isotherm of nitrogen as equivalent to the

volume calculated by the subtraction of the micropore

volume (DR-N2) from the volume of N2 adsorbed at a

relative pressure 0.95. The total pore volume (Vtotal) was

determined from the quantity of gas adsorbed at a rela-

tive pressure of 0.98, by assuming that pores are filled

with liquid adsorbate. Pore size distribution was deter-

mined by the density functional theory (DFT) method

[34].

The X-ray photoelectron spectroscopy (XPS) technique

using an ESCA Lab 250 XPS system was applied to four

samples of activated carbon to provide complementary

information on the chemical properties of their surfaces

and how these properties change with the atmosphere and

temperature. The samples were placed onto a carbon tape

then degassed and transferred into the analysis chamber.

XPS measurements were performed in an ultrahigh

vacuum (3 9 10-9 Pa). An accelerating voltage of 12 kV

and a current of 10 mA were used and the binding energies

were corrected using the value of 284.7 eV for the C1s

level. The energy step at the survey spectra was 0.25 eV

and the step at the detailed spectra was 0.025 eV. The XPS

analysis depth, in the case of carbon matrix, was about

5 nm. The P 2p curves were fitted taking into account the

spin–orbit splitting and ratio of 2p1/2 :2p3/2 components as

0.5 [18]. The samples were analysed under identical con-

ditions and the resulting spectra were fitted using the same

procedure by applying Shirley type background subtraction

using CasaXPS 2.3.15 software.

The activated carbons were also examined using trans-

mission electron microscopy (TEM). A Philips CM200

FEGTEM was used to attain high magnification micro-

graphs of the activated carbon samples. The instrument was

operated at an accelerating voltage of 200 kV. The samples

were ground to a fine powder, dispersed with acetone, and

then mounted on a perforated carbon film supported by a

copper grid.

Results and Discussion

Influence of the Atmosphere on the BET Surface Area

and Porosity of Activated Carbon

Co-activation of cotton stalk samples impregnated with

phosphoric acid was carried out under steam/nitrogen

atmosphere. Usually phosphoric acid activation is per-

formed under nitrogen atmosphere. Therefore, this work

has compared the activated carbons produced by H3PO4

activation in two different atmospheres, under nitrogen

and steam/nitrogen mixture, in terms of porous texture and

later on the surface chemistry of activated carbons. All

experiments were carried out in the same temperature range,

between 500 and 800�C for impregnation ratio of 1.5, and at

different impregnation ratios at temperature of 500�C.

The porosity characteristics of the activated carbons

prepared at different impregnation ratios and activated

at 500�C are shown in Fig. 2. It can be seen that the

impregnation ratio and the atmosphere have a significant

impact on the porous texture. In the presence of nitrogen

atmosphere, as the impregnation ratio was increased from

0.3 to 3, the surface area of activated carbons prepared

at 500�C increased from 327 m2 g-1 at an impregnation

ratio of 0.3 and reached a maximum of 1,718 m2 g-1, at a

ratio of 1.5 and then decreased to 1,160 m2 g-1 at the

H3PO4:cotton stalk ratio of 3. The micropore volume and

the total pore volume followed the same trend, while the

mesopore volume continually increased as the impregna-

tion ratio was increased. For impregnation ratios up to 1.5,

the increase in the adsorption of the resultant activated

carbons is attributed to the increase in contact area

between the cotton stalks and phosphoric acid which pro-

motes the diffusion of phosphoric acid into the structure

[35]. The dramatic increase in the micropore volume at

lower impregnation ratios (0.3 and 0.75) and mesopore

volume at higher impregnation ratios (1.5 and 3) can be

linked to the polymeric species length of dehydrated acid,

polyphosphoric acid, involved in the activation process.

For narrower pores, the length of the polymeric species

might be small which is more possible at low impregnation

ratio, while this length increases at high impregnation ratio

and therefore promotes the formation of wider porosity

[36]. When the ratio increases to 3, the porosity of the

resultant activated carbon decreases. This observation is

probably due to the dehydrated acid, polyphosphoric acid,

forming an insulating layer around the particles that partly

prevents the penetration of the activating agent into the

cotton stalks [37].

The use of steam/nitrogen mixture compared to nitrogen

atmosphere leads to a considerable increase in both the

BET surface area and porosity including micropore


123

volume, mesopore volume and subsequently total pore

volume (Fig. 2). An increase in micropore volume is

observed under a steam/nitrogen atmosphere for samples

prepared at lower impregnation ratio of 0.3 and 0.75, while

considerable increase in mesopore volume, more than

double, is observed under a steam/nitrogen atmosphere for

samples prepared at higher impregnation ratio of 1.5 and 3.

This indicates that the presence of steam promotes the

formation of narrower porosity at low impregnation ratios

and wider porosity at high impregnation ratios. This can be

linked to the polymeric species length of dehydrated acid,

polyphosphoric acid, that mentioned previously.

The increase in the porous texture of activated carbons

produced under a steam/nitrogen atmosphere may be

attributed to that the steam eliminates the formation of

condensate phosphates due to the protogenic character of

H3PO4 that enriches the medium with protons resulting in

the elimination of phosphate groups previously embodied

in the carbon [15, 17]. This diminishes the blockage of

pores by phosphorous compounds giving the activated

carbons with more developed porosity. In addition, it is

also known that the steam has the ability to penetrate into

solid materials and facilitate desorption and efficient

removal of volatiles from them [38]. The removal of vol-

atiles in the steam pyrolysis results in significant increase

in the pore volume, the surface area and the adsorption

capacity of the activated carbons obtained in presence of

steam [16].

Figure 3 shows a comparison of BET surface area and

porosity of activated carbons produced at different tem-

peratures at an impregnation ratio of 1.5 in the presence

of nitrogen and steam/nitrogen atmospheres. The results

show that, under nitrogen atmosphere, the surface area

decreased from 1,718 to 1,164 m2 g-1 as the temperature

was increased from 500 to 700�C and then increased to

1,399 m2 g-1 at 800�C. The micropore volume and total

pore volume followed the same trend, while the mesopore

volume stayed almost stable. The decrease in the surface

area might be due to contraction or collapse of pores at

high temperature which leads to the reduction in porosity

development [39]. While the increase in the surface area

at 800�C and the porosity is likely to be due to the

0.3:1 0.75:1 1.5:1 3:1 0.3:1 0.75:1 1.5:1 3:1

0.3:1 0.75:1 1.5:1 3:1 0.3:1 0.75:1 1.5:1 3:1

0

200

400

600

800

1000

1200

1400

1600

1800

2000

BE

T S

urf

ace

Are

a (m

2 g-1)

H3PO

4:Cotton stalks Ratio (g g-1)

Nitrogen atmosphere Steam/nitrogen atmosphere

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Mic

rop

ore

Vo

lum

e (c

m3 g

-1)

H3PO

4:Cotton stalks Ratio (g g-1)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Mes

op

ore

Vo

lum

e (c

m3 g

-1)

H3PO

4:Cotton stalks Ratio (g g -1)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

To

tal P

ore

Vo

lum

e (c

m3 g

-1)

H3PO

4:Cotton stalks Ratio (g g -1)

Fig. 2 Comparison of BET surface area and the porous texture of activated carbons produced at an different impregnation ratios at 500�C in the

presence of nitrogen and steam/nitrogen atmospheres


123

volatilisation of phosphorus compounds which can produce

two competing effects on the porous texture: (1) a decrease

in porosity due to contraction of the material caused by the

rupture of phosphate and polyphosphate bridges such as

H2PO4-1 or/and H2P2O7

-2 that keep the structure expanded;

and (2) an increase in porosity, as phosphorus compound

volatilisation can produce new channels (pores) when

leaving the material. In addition, the partial gasification of

the carbon by small phosphorus species, melted P2O5,

which results from the decomposition of the phosphoric

acid at high temperature [32, 40, 41]. This reaction,

between melted P2O5 and carbon, might result in formation

of new micropores and widening of the existing pores

which leads eventually to an increase in micropore and

mesopore volumes [41].

By applying the steam/nitrogen atmosphere at these

temperatures, the results also show that the activated car-

bons prepared by both impregnation with phosphoric acid

and steam activation have larger surface areas and pore

volumes, Vmicro, Vmeso, and Vtotal, compared to those pre-

pared by pyrolysis only in nitrogen (Fig. 3). These com-

parison results suggest that steam activation promotes

formation of narrower micropores in the activated carbons

prepared at higher temperatures (700 and 800�C), while it

promotes formation of mesopores in the activated carbons

prepared at lower temperatures (500 and 600�C). This

could be attributed to the fact that at high temperatures the

reaction between the carbon and steam is more abundant

than the elimination of phosphate groups by steam since

this reaction is endothermic. This enhances the creation of

new pores (micropores) and development of the existing

pores. At low temperatures, the elimination of phosphate

groups from the carbon matrix by steam becomes more

abundant. This enhances the formation of mesopores in the

activated carbons. The enhancement of the formation of

micropores and mesopores by the activation with steam

are favourable for the adsorption properties of the carbons.

The presence of a well-developed meso structure ensures

sufficient access to the micropores [16].

Figure 4a and b shows a comparison of the DFT pore

size distributions of activated carbons produced at activa-

tion temperatures of 500 and 800�C at an impregnation

ratio of 1.5 in the presence of nitrogen and steam/nitrogen

atmospheres. The data confirm that steam enhances the

500 600 700 800 500 600 700 800

500 600 700 800 500 600 700 800

0

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

2400

BE

T S

urf

ace

Are

a (m

2 g-1)

Activation Temperature (°C)


0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Mic

rop

ore

Vo

lum

e (c

m3 g

-1)


0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

Mes

op

ore

Vo

lum

e (c

m3 g

-1)


0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

To

tal P

ore

Vo

lum

e (c

m3 g

-1)


Fig. 3 Comparison of BET surface area and the porous texture of activated carbons produced at different temperatures at an impregnation ratio

of 1.5 in the presence of nitrogen and steam/nitrogen atmospheres


123

formation of micropores (\2 nm) at 800�C and mesopores

([2 nm) at 500�C.

The yields of activated carbon produced were relatively

high and they ranged from 30.3 to 56.8% by weight,

depending on the impregnation ratio, the activation tem-

perature and the atmosphere. The activated carbon yields

decreased as the impregnation ratio and the activation

temperature was increased and as the atmosphere was

changed from nitrogen to steam/nitrogen mixture. The high

yield of activated carbon is due to phosphoric acid acti-

vation which promotes dehydration and redistribution of

biopolymers, and also favours the conversion of aliphatic

to aromatic compounds thus increasing the yield of acti-

vated carbon [42]. In addition, it should be noted that some

insoluble phosphorous species may be present in the acti-

vated carbon structure after the washing stage which leads

to an apparent increase in the carbon yield.

Porosity Characteristics of Activated Carbons

by Transmission Electron Microscopy

TEM was employed to investigate the porosity of the

activated carbons at higher magnification than the scanning

electron microscope was able to investigate. Activated

carbons are classified as disordered materials in compari-

son to the high order achievable in carbon in the form of

graphite or diamond. Two different activated carbon sam-

ples were selected by the information generated from the

porosity results. The samples investigated were the acti-

vated carbon sample produced at an impregnation ratio of

0.3, at 500�C in the presence of nitrogen atmosphere and

the activated carbon sample produced at an impregnation

ratio of 3, at 500�C in the presence of steam/nitrogen

atmosphere. The micrograph of these samples was also

compared with that of amorphous carbon film.

According to the porosity results, the activated carbon

sample produced at an impregnation ratio of 0.3, at 500�C

in the presence of nitrogen atmosphere had a surface area

of 327 m2 g-1, micropore volume of 0.15 cm3 g-1 which

forms about 93% of the total pore volume. On the other

hand, the activated carbon sample produced at an

impregnation ratio of 3, at 500�C in the presence of steam/

nitrogen atmosphere had a surface area of 1,751 m2 g-1,

micropore volume of 0.77 cm3 g-1 and mesopore volume

of 0.89 cm3 g-1 which forms 52% of the total pore vol-

ume. Figure 5a and b shows transmission electron micro-

graphs of these two samples respectively. Figure 5a shows

that the structure of the activated carbon sample, micro-

porous activated carbon, produced at an impregnation ratio

of 0.3, at 500�C in the presence of nitrogen atmosphere

seems very similar to the confused amorphous carbon

structure given a homogeneous morphology [43]. The

activated carbon sample produced at an impregnation ratio

of 3, at 500�C in the presence of steam/nitrogen also

exhibited the microporous type features as exhibited in

Fig. 5a, but also showed the presence to a limited extent

of larger mesopore sized features as shown in Fig. 5b. The

TEM image shows that the mesopores mainly range from

just over 2 to about 10 nm in size.

Influence of Activation Temperature and Atmosphere

on Surface Chemistry of Activated Carbons

In this section the effect of activation temperature and

atmosphere on the surface chemistry of activated carbons

was investigated. The chemical state of selected elements

and surface composition of the activated carbon samples

produced at 500 and 800�C at an impregnation ratio of 1.5

in the presence of nitrogen and steam/nitrogen atmospheres

were determined by X-ray photoelectron spectroscopy.

Figure 6 shows the typical XPS survey scans of these

(a)

(b)

0 1 2 3 4 5 60.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0.11

Po

re V

olu

me

(cm

3 nm

-1 g

-1)

Pore Width (nm)


0 1 2 3 4 5 60.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

Po

re V

olu

me

(cm

3 nm

-1 g

-1)

Pore Width (nm)


Fig. 4 Comparison of the DFT pore size distribution of activated

carbons produced at activation temperature of 500�C (a) and 800�C

(b) at an impregnation ratio of 1.5 in the presence of nitrogen and

steam/nitrogen atmospheres


123

samples respectively, while Table 2 presents the elemental

composition of the surface, XPS data. The XPS data show

that the surface of the activated carbons contains mainly

carbon, oxygen, phosphorus and lower percentages of

nitrogen and calcium. The atomic% of these elements was

obtained by determining the contribution to the total area

under the curve for all of the elements that was made by the

area under the curve for each element. These values reflect

the sample composition only over a depth of about a few

nanometres at which the X-rays can penetrate to.

The results of elemental analysis suggest that the acti-

vation temperature and the atmosphere have a significant

effect on the contents of elements of the sample surface. As

expected all activated carbon samples have high contents

of carbon and relatively low contents of oxygen and

phosphorus. By comparison of the elemental composition

of these samples, it can be seen that the carbon content

increased as the activation temperature was increased from

500 to 800�C in both atmospheres and as the atmosphere was

changed from nitrogen to steam/nitrogen mixture in both

temperatures, while the oxygen and phosphorus contents

decreased. This can be explained by the removal of phos-

phorus compounds, H2PO4-1, H2P2O7

-2 and P2O5 which

mainly consist of oxygen and phosphorus, that enhances

at both higher temperature and steam/nitrogen atmosphere

as shown previously. This leads finally to a decrease in the

content of oxygen and phosphorus and an increase in the

content of carbon. Similar trends were observed by Budi-

nova et al. [16] who studied the effect of the atmosphere on

elemental composition of the activated carbons prepared

from woody biomass birch at 600�C. They found that the

oxygen content of the sample treated only by pyrolysis in

inert atmosphere is the highest and the residual phosphorous

content in this sample is also the highest. The sample sub-

jected to the combination of chemical and physical activa-

tion has accelerated the chemical changes in the material and

facilitated the removal of hydrogen, phosphorus and oxygen

resulting in increased carbon content.

Table 2 also shows the O/C, P/C and O/P atomic ratios

for the same samples. The O/C and P/C atomic ratios

decreased as the activation temperature was increased from

500 to 800�C in both atmospheres and as the atmosphere

was changed from nitrogen to steam/nitrogen in both

temperatures. This indicates the increase in carbon content

at the expense of oxygen and phosphorus contents as a

result of the removal of phosphorus compounds. For O/P

atomic ratio, it is clear that almost the same atomic ratio of

about 4 is observed in the surface of activated carbon

samples produced at 500�C in both atmospheres and in

the surface of activated carbon sample produced at

800�C under nitrogen atmosphere. The actual O/P ratio of

P-containing species is obviously lower when it is con-

sidered that other than oxygen atoms is bound to phos-

phorus. It is intriguing that linear polyphosphates of

general formula Hn?2PnO3n?1 show an O/P atomic ratio

from 4 for n = 1 (orthophosphoric acid) to 3 for n = ?.

This fact argues for polyphosphates as probable chemical

structure of phosphorus species in phosphoric acid acti-

vated carbons as also confirmed by Puziy et al. [26]. The

increase in the O/P atomic ratio from around 4–6 for the

activated carbon sample produced at 800�C under steam/

nitrogen atmosphere could be attributed to evaporation of

phosphorus compounds and the endothermic reaction

between the carbon and steam that is enhanced at high

temperature and incorporates some oxygen atoms on the

carbon matrix as this reaction is an oxidation reaction [26].

Figure 7 shows the typical high resolution XPS spectra

of the C1s region of the activated carbon samples produced

Fig. 5 Transmission electron micrograph of activated carbon pro-

duced at an impregnation ratio of 0.3, at 500�C in the presence of

nitrogen atmosphere (a) and at an impregnation ratio of 3, at 500�C in

the presence of steam/nitrogen atmosphere (b)


123

at 500 and 800�C at an impregnation ratio of 1.5 in the

presence of nitrogen and steam/nitrogen atmospheres. The

spectra were deconvoluted into six components with fixed

positions adopted from the standard analysis of carbon

materials [29, 44]. The components shown in the Table 3

represent graphitic carbon (peak A); carbide carbon (peak

B); carbon species in alcohol, ether groups and/or C–O–P

linkage (peak C); carbon in carbonyl groups (peak D);

carboxyl and/or ester groups (peak E); and shake-up

satellite due to p–p* transitions in aromatic rings (peak F)

[18, 45, 46]. The atomic% of these components composing

the activated carbon surfaces was obtained by determining

the contribution to the total area under the C1s curve made

by the area under the curve for each carbon group. It can be

seen that about 59.40% of carbon atoms, which were

composed of 77.77 atom% of the total elemental compo-

sition, present in the surface of activated carbon produced

at 500�C at an impregnation ratio of 1.5 in the presence

of nitrogen are graphitic, 1.36% are carbidic and about

32.15% belong to oxidised carbon. These percentages

200 400 600 800 1000 1200 1400 1600 200 400 600 800 1000 1200 1400 1600

200 400 600 800 1000 1200 1400 1600 200 400 600 800 1000 1200 1400 1600

00

60000

P2p

Inte

nsi

ty (

arb

itra

ry u

nit

s)

Binding Energy (eV)

C1s

Ca2pN1s

O1s

30000

120000

90000

180000

(a) (b)

(c) (d)

150000

P2p

Inte

nsi

ty (

arb

itra

ry u

nit

s)

Binding Energy (eV)

C1s

Ca2pN1s

O1s

60000

30000

120000

90000

180000

150000

P2p

Inte

nsi

ty (

arb

itra

ry u

nit

s)

Binding Energy (eV)

C1s

Ca2pN1s

O1s

P2p

Inte

nsi

ty (

arb

itra

ry u

nit

s)

Binding Energy (eV)

C1s

Ca2pN1s

O1s

0

60000

30000

90000

120000

150000

180000

0

60000

30000

90000

120000

150000

180000

Fig. 6 Typical XPS survey scan of activated carbons produced at an

impregnation ratio of 1.5, (a) at 5008C in the presence of nitrogen

atmosphere (b) at 8008C in the presence of nitrogen atmosphere (c) at

5008C in the presence of steam/nitrogen atmosphere and (d) at 8008C in

the presence of steam/nitrogen atmosphere

Table 2 Elemental composition of the surface (XPS data)

Sample Atmosphere Atomic%

C O P N Ca O/C P/C O/P

Activated carbon produced at 500�C Nitrogen 77.77 16.86 4.06 0.68 0.63 0.22 0.05 4.15

Steam/nitrogen 90.23 7.40 1.76 0.39 0.22 0.08 0.02 4.20

Activated carbon produced at 800�C Nitrogen 84.64 11.52 2.84 0.37 0.63 0.14 0.03 4.06

Steam/nitrogen 92.85 5.32 0.87 0.25 0.71 0.06 0.009 6.11

ND not detected


123

altered to 62.81, 1.58 and 26.22% in the surface of acti-

vated carbon produced at 500�C in the presence of steam/

nitrogen atmosphere, 67.66, 1.63 and 25.37% in the surface

of activated carbon produced at 800�C in the nitrogen

atmosphere, 62.95, 1.98 and 22.6% in the surface of acti-

vated carbon produced at 800�C in the presence of steam/

nitrogen atmosphere respectively. It should be noted also

that phosphorus compounds cannot be clearly determined

from C1s region because binding energy of C–O–P bond-

ing is similar to binding energy in alcohol and ether groups

(peak C) [18].

To conclude, C-oxygen functionalities decreased as the

activation temperature was increased from 500 to 800�C in

both atmospheres and as the atmosphere was changed from

300 298 296 294 292 290 288 286 284 282 280 300 298 296 294 292 290 288 286 284 282 280

300 298 296 294 292 290 288 286 284 282 280 300 298 296 294 292 290 288 286 284 282 280

Data points

(a) (b)

(c) (d)

Final curve fit

Inte

nsi

ty (

arb

itra

ry u

nit

s)

Binding Energy (eV)

FE D

B

C

A

Inte

nsi

ty (

arb

itra

ry u

nit

s)

Binding Energy (eV)

FE D

B

C

A

10000

15000

20000

25000

30000

5000

0

Inte

nsi

ty (

arb

itra

ry u

nit

s)

Binding Energy (eV)

FE D

B

C

A

0

10000

5000

15000

20000

25000

30000

Inte

nsi

ty (

arb

itra

ry u

nit

s)

Binding Energy (eV)

FE D

B

C

A

10000

15000

20000

25000

30000

5000

0

10000

15000

20000

25000

30000

5000

0

Fig. 7 Typical high resolution XPS spectra of the C(1s) region of

activated carbons produced at an impregnation ratio of 1.5, (a) at

5008C in the presence of nitrogen atmosphere (b) at 8008C in the

presence of nitrogen atmosphere (c) at 5008C in the presence of

steam/nitrogen atmosphere and (d) at 8008C in the presence of steam/

nitrogen atmosphere

Table 3 Deconvolution results of the C(1s) region of activated carbons

Region Peak Position Assignment Relative content (atomic%)

Activated carbon produced at 500�C Activated carbon produced at 800�C

Nitrogen Steam/nitrogen Nitrogen Steam/nitrogen

C1s A 284.7 Graphite 59.40 62.81 67.66 62.95

B 284 Carbide 1.36 1.58 1.63 1.98

C 286 R-OH, C–O–C, C–O–P 17.98 11.38 13.56 8.24

D 287.2 C=O 8.72 9.86 6.80 9.88

E 289.3 COOH, –C(O)–O–C 5.45 4.98 5.01 4.48

F 291.1 p–p* in aromatic rings 7.09 9.39 5.34 12.47


123

nitrogen to steam/nitrogen mixture in both temperatures,

while the graphitic carbon increased. This can be explained

by that many oxygen atoms were removed at high tem-

perature or/and under steam/nitrogen atmosphere which

accounted for the high content of graphitic carbon and low

amount of oxygenated carbon [47].


of the O1s region of the activated carbon samples produced

at 500 and 800�C at an impregnation ratio of 1.5 in

the presence of nitrogen and steam/nitrogen atmospheres.

The optimum fitting was achieved by resolving each of the

O1s spectra into three peaks [29, 45]. Table 4 presents the

corresponding functional groups assigned to each peak.

Peak A was attributed to oxygen double bonded to carbon

(C=O) groups (carbonyl, carboxyl, oxygen of quinone) and

oxygen double bonded to phosphorus (O=P) in phosphates

and polyphosphates, peak B to singly bonded oxygen (–O–)

in C–O groups in esters, phosphates (P–O–C) and oxygen

atoms in hydroxyls or ethers and peak C to chemisorbed

oxygen and water [18, 29, 48].

The singly bonded oxygen is the main component in all

samples (–O–, peak B) while the second most abundant

form is double bonded oxygen (=O, peak A) (Table 4). The

contribution of other forms of oxygen (peaks C) is less than

10 atomic%. By comparison the singly bonded oxygen and

double bonded oxygen structures existed in the surface of

all activated carbon samples after taking into account the

total oxygen content of each sample present in Table 2, it is

clear that the singly bonded oxygen and double bonded

oxygen structures decreased as the activation temperature

was increased from 500 to 800�C in both atmospheres and

as the atmosphere was changed from nitrogen to steam/

nitrogen in both temperatures. This can be linked directly

to the elimination of phosphorus compounds at high tem-

perature or/and steam/nitrogen atmosphere which mainly

contain oxygen double bonded to phosphorus (O=P) and

singly bonded oxygen to phosphorus (P–O–C) as explained

previously.


of the P2p region of the activated carbon samples produced

540 538 536 534 532 530 528 526 524 522 540 538 536 534 532 530 528 526 524 522

540 538 536 534 532 530 528 526 524 522 540 538 536 534 532 530 528 526 524 522

C

Inte

nsi

ty (

arb

itra

ry u

nit

s)

Binding Energy (eV)

A

B

5000

7500

10000

12500

15000

(a) (b)

(c) (d)

2500

0

Data points Final curve fit

Inte

nsi

ty (

arb

itra

ry u

nit

s)

Binding Energy (eV)

C

AB

5000

7500

10000

12500

15000

2500

0

Inte

nsi

ty (

arb

itra

ry u

nit

s)

Binding Energy (eV)

A

B

C5000

7500

10000

12500

15000

2500

0

Inte

nsi

ty (

arb

itra

ry u

nit

s)

Binding Energy (eV)

A

B

C5000

7500

10000

12500

15000

2500

0

Fig. 8 Typical high resolution XPS spectra of the O1s region of





nitrogen atmosphere


123

at 500 and 800�C at an impregnation ratio of 1.5 in the

presence of nitrogen and steam/nitrogen atmospheres. P2p

spectra of all activates carbon samples were successfully

fitted to one peak with BE = 133.6 eV suggesting the

existence of sole type of phosphorus compounds in all

the samples structure but it is known, in the case of P2p

that the resulting line observed is core-line doublets

corresponding to P2p l/2 and P2p 3/2 [18, 49]. The binding

energy of this peak, as present in Table 5, is characteristic

of [PO4] as in phosphates (Na2HPO4, binding ener-

gy = 133.4 eV) or/and in condensed phosphates (Na4P2O7,

binding energy = 133.5 eV) and no peaks in the P2p

region corresponding to neutral phosphorus (binding

energy = 130.2 - 130.9 eV) or phosphorus pentoxide

Table 4 Deconvolution results of the O1s region of activated carbons

Region Peak Position Assignment Relative content (atomic%)

Activated carbon produced

at 500�C

Activated carbon produced

at 800�C


O1s A 531 =O in carbonyl, carboxyl, phosphates or

polyphosphates (O=P) and oxygen of quinone

44.47 41.88 47.46 42.93

B 532.6 O–C, –O– in phosphates (P–O–C),

oxygen atoms in hydroxyl groups

52.35 51.82 48.38 50.85

C 534.8 Chemisorbed O and water 3.18 6.30 4.16 6.22

140 138 136 134 132 130 128 126 124 122 140 138 136 134 132 130 128 126 124 122

140 138 136 134 132 130 128 126 124 122 140 138 136 134 132 130 128 126 124 122

1200

Data points Final curve fit

Inte

nsi

ty (

arb

itra

ry u

nit

s)

Binding Energy (eV)

2p1/2

2p3/2

400

600

800

1000

1400

(a) (b)

(c) (d)

200

0

2p1/2

Inte

nsi

ty (

arb

itra

ry u

nit

s)

Binding Energy (eV)

2p3/2

1200

400

600

800

1000

1400

200

0

Inte

nsi

ty (

arb

itra

ry u

nit

s)

Binding Energy (eV)

2p1/2

2p3/2

1200

400

600

800

1000

1400

200

0

Inte

nsi

ty (

arb

itra

ry u

nit

s)

Binding Energy (eV)

2p1/2

2p3/2

1200

400

600

800

1000

1400

200

0

Fig. 9 Typical high resolution XPS spectra of the P2p region of





nitrogen atmosphere


123

(binding energy = 135.6 eV) [18, 29]. The way that

polyphosphates are bound to carbon is P–O–C as the C1s

and O1s spectra suggest that. This is in agreement with

FTIR study of phosphoric acid activated carbon carried out

by Puziy et al. [25]. Their study did not reveal any bands in

the region 795–650 cm-1, where C–P bond gives a med-

ium to strong absorption. In addition, Lee and Radovic [50]

analysed the stability of C–O–P and C–P–O bonding in

context of oxidation inhibition of carbon materials. They

found that the structure with C–O–P bonding is more stable

and able to survive longer than the structure with C–P–O

bonding. Figure 9 also shows that phosphorus content in

the activated carbon samples decreased as the activation

temperature was increased from 500 to 800�C in both

atmospheres and as the atmosphere was changed from

nitrogen to steam/nitrogen in both temperatures. This is due

to the elimination of phosphorus compounds at high tem-

perature or/and steam/nitrogen atmosphere.

Conclusions

This study has shown that the appropriate selection of the

experimental conditions of H3PO4 chemical activation or

co-activation processes could be used to manipulate the

surface area and porosity of the activated carbons to pro-

duce microporous, mesoporous or mixed microporous/

mesoporous activated carbons. Low impregnation ratio and

high activation temperature lead to microporous carbons.

Activated carbons with strongly developed mesoporosity

(up to 0.89 cm3 g-1) can be obtained when high impreg-

nation ratio, low activation temperature and steam/nitrogen

atmosphere are used. The steam/nitrogen atmosphere

compared to nitrogen atmosphere led to a considerable

increase in both the surface area and porosity. Steam pro-

motes the formation of narrower porosity at low impreg-

nation ratios and wider porosity at high impregnation

ratios, while it promotes formation of narrower micropores

in the activated carbons prepared at higher temperatures

(700 and 800�C) and the formation of mesopores in the

activated carbons prepared at lower temperatures (500 and

600�C). The XPS results revealed that the phosphorus

species existed in all activated carbons produced by

phosphoric acid activation of cotton stalks as phosphate-

like structures. This species decreased as the activation

temperature was increased from 500 to 800�C in both

nitrogen and steam/nitrogen atmospheres and as the

atmosphere was changed from nitrogen to a steam/nitrogen

mixture in both temperatures. This is due to the elimination

of phosphorus compounds at high temperature and/or

steam/nitrogen atmosphere. The decrease in the phospho-

rus species is accompanied by a decrease in oxygen content

in the surface of activated carbon, as the phosphate struc-

ture contains a high atomic% of oxygen, with a corre-

sponding increase in the carbon content.

Acknowledgments We would like to thank the Syrian Ministry of

Higher Education and Al Baath University, Homs, Syria and also to

Prof. Saad Kherfan for support for Mohamad Anas Nahil. We would

also like to thank Dr Jude Onwudili for help with analyses.

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