Materials Chemistry and Physics 127 (2011) 495–500
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Materials Chemistry and Physics
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simple and highly effective process for the preparation of activated carbonsith high surface area
ing Li, Xuefeng Ding, Yupeng Guo, Lili Wang, Chunguang Rong, Yuning Qu, Xiaoyu Ma, Zichen Wang ∗
ollege of Chemistry, Jilin University, Changchun 130012, China
r t i c l e i n f o
rticle history:eceived 17 September 2010eceived in revised form 10 January 2011
a b s t r a c t
Activated carbons with high surface area were prepared by phosphoric acid as activation agent and ricehusks as precursors. It was found that the characteristics of the activated carbons were influenced not
ccepted 17 February 2011
eywords:ctivated carbonice huskhosphoric acid
only by the preparation but also by the post-processing method. The high surface area of the activatedcarbons was prepared under the optimum condition (50% H3PO4 with impregnation ratio of 5:1, activationtemperature of 500 ◦C, activation time of 0.5 h, wash water temperature of 100 ◦C). SiO2 content couldaffect the surface area of activated carbons, either. The lower SiO2 content of the activated carbons, thehigher pore volume the carbons had. The SiO2 content was 11.2% when used the optimum condition. Theexplanation was that silicon element in rice husks reacted with H3PO4 to form silicon phosphate (SiP2O7),
rther
ilicon phosphate and it could be proved fu
. Introduction
Activated carbons are of interest due to their extensive sur-ace area, high degree of surface reactivity, and favorable pore sizeistribution [1]. Among the various activation agents, phosphoriccid is a well-established agent for preparation of activated car-ons, which has many advantages. Nowadays, the raw material ofhosphoric acid activation, usually stone [2–6], shell [7,8], poly-er [9], wood [10–12] and coal [11,13], is limitative or expensive,
r nonrenewable, which restrict their application. Thus lignocel-ulosic agricultural wastes are investigated as precursors due toheir renewability and huge amount. Rice husk, a by-product ofhe rice milling industry, with an annual output 40 million tonsn China [14,15], usually burned in open heaps as waste, can besed as the precursor of the activated carbon. However, due to theigh ash content of rice husk, which is difficult to remove, acti-ated carbon prepared from rice husk by H3PO4 activation withesirable surface area and pore volume often contains two proce-ures: alkali-leaching process to remove SiO2 followed by H3PO4ctivation, surface area and pore volume reach 1741 m2 g−1 and.315 ml g−1 [16].
In this work, we prepared activated carbons with high surface
rea (1820 m2 g−1) using rice husks as raw materials by H3PO4ctivation without alkali-leaching process. The mechanism of phos-horic acid activation was enriched through this research. Theharacteristics of the activated carbon were greatly influenced
by post-processing method. To our best knowledge, the carbonswashed by water with different temperatures had different surfacearea, pore volume and ash content, which was first discussed in ourmanuscript. The whole process was simple and economical, whichnot only enlarge the scope of the raw materials through H3PO4activation, but also make it possible to produce high surface areaactivated carbons in industry.
2. Experimental
2.1. Precursors
Rice husk is obtained from a rice mill nearby Changchun city. Rice husk waswashed thoroughly with distilled water to remove soil and dust, then dried at 100 ◦Covernight and porphyrized to 60 mesh before employment.
2.2. Preparation of activated carbons
The precursors were impregnated with phosphoric acid solution (40–85 wt.%)with impregnation ratio of (the weight ratio of phosphoric acid to rice husk) 3:1–6:1,and stirred for at least 2 min at room temperature, then the mixtures were heatedup to different temperatures: 300, 400, 500, 600 ◦C for 0.5–2 h. The products werewashed with hot distilled water (100 ◦C) for several times until the filtrate wasneutral, then dried at 120 ◦C for 12 h before further characterization.
2.3. Characterization
2.3.1. Nitrogen adsorptionPore structure analysis was performed using a Micromeritics ASAP 2010 Sur-
face Analyzer through nitrogen adsorption at 77 K in the relative pressure rangeof 10−6–1 atm. Prior to the analysis, all the samples were degassed at 200 ◦C to3 �m Hg under vacuum. Surface area was calculated by the Brunauer, Emmett, andTeller (BET) equation using the data obtained from nitrogen adsorption isotherm.The relative pressure range used for the calculation was 0.05–0.2. The singlepoint total pore volume was obtained from the amount of nitrogen adsorbed
496 Y. Li et al. / Materials Chemistry and Physics 127 (2011) 495–500
ith 5
atm
2
(s
2
sta
3
3
bstnbt
C
Fpp
Fig. 1. (a) TEM and (b) HRTEM images of activated carbon prepared by activation w
t a relative pressure around 0.95. The pore size distribution was derived fromhe adsorption branches of the isotherms by the Density Functional Theory (DFT)
ethod.
.3.2. X-ray diffractionThe crystal structures of samples were characterized by X-ray diffraction (XRD)
SHIMADZU XRD-6000 diffractometer employing Ni-filtered Cu K� radiation, at acanning rate of 6◦ min−1 with 2� ranging from 15◦ to 60◦).
.3.3. Transmission electron microscopy (TEM)The surface morphology of the as-prepared samples was examined by transmis-
ion electron microscope (JEOL 2000EX) operating at 200 kV and high-resolutionransmission electron micrograph (HRTEM FEI TECNAI G2). HRTEM equipped withn energy dispersive X-ray (EDX) analyzer (Phoenix).
. Results and discussion
.1. Characteristics of activated carbons
Fig. 1 shows the TEM and HRTEM images of the activated car-ons. It can be clearly conformed the nature of the amorphoustructure from Fig. 1a. The structure of the activated carbon wasurbostratic with a pore structure consisting of inter-connected
anochannels, which were formed by the disordered packing of tur-ostratic carbon sheets and clusters [17]. Most pore size is smallerhan 1 nm from HRTEM image (Fig. 1b).
According to the IUPAC (International Union of Pure and Appliedhemistry) classification, the isotherms in this work were classi-
1.00.80.60.40.20.0
0
200
400
600
800
1000
1200
1400
1600
a-- 500 oC, 0.5 h
b-- 500 oC, 1.5 h
c-- 600 oC, 1.5 h
c
b
a
Vo
lum
e a
dso
rbed
(m
l/g
)
Relative pressure (P/P0)
ig. 2. Adsorption/desorption isotherms of nitrogen at 77 K of activated carbon pre-ared by activation with 50% H3PO4 impregnation ratio of 4:1 and a carbonizationrocess at 500 ◦C or 600 ◦C for 0.5 h or 1.5 h.
0% H3PO4 impregnation ratio of 5:1 and a carbonization process at 500 ◦C for 0.5 h.
fied into type IV, H4. Fig. 2 shows adsorption/desorption isothermsof nitrogen at 77 K of activated carbons. It illustrates that theexistence of microporous structure where pore filling occurs atlow relative pressures. The presence of adsorption hysteresis loopindicates the presence of mesoporosity according to type IV,and the porous carbon contains slit-shaped pores according toH4.
In this work, 2 and 50 nm were taken as the boundary sizesbetween micro and mesopore, and meso and macropore, respec-tively. From Fig. 3, we can see that the pore diameter of theprepared activated carbon contain 1.3–2.4 nm, 2.7–4.5 nm, 6.8 nmand 9.7–11.0 nm, respectively, which indicates the porous carbonhave micro and mesopore structure.
3.2. Effect of different factors on activated carbons
3.2.1. Influence of activation temperatureThe BET surface areas and pore volumes of activated carbons
prepared at different activation temperatures using the impregna-tion ratio of 4:1, activation time of 1.5 h are summarized in Fig. 4aand b. The concentration of H3PO4 was 50, 60 and 85%, respectively.As seen in Fig. 4a and b, the BET surface areas and pore volumesof activated carbons increase gradually with activation temper-
ature increasing from 300 to 500 ◦C, then decrease with furtherincreasing in the final activation temperature. With activation tem-perature of 300 ◦C, the activation extent of rice husk was relativelylow demonstrated by the small surface area. With increasing tem-
2018161412108642
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
dV
/dD
(m
l/g
.nm
)
pore diameter (nm)
Fig. 3. Pore size distribution of activated carbons prepared at 500 ◦C for 0.5 h with50% H3PO4 impregnation ratio of 5:1.
Y. Li et al. / Materials Chemistry and Physics 127 (2011) 495–500 497
Fig. 4. Effect of activation temperature on (a) the BET surface areas, (b) the pore vol-ur
potdbaa
161412108642
0.000
0.005
0.010
0.015
0.020
0.025
0.030
dV
/dD
(m
l/g
.nm
)
pore diameter (nm)
b
aa-- 500
oC, 1.5 h
b-- 600 oC, 1.5 h
Fig. 5. Pore size distribution of activated carbons prepared at 500 ◦C and 600 ◦C withother parameter constant.
mes and (c) the ash contents of carbons prepared from rice husks at impregnationatio of 4:1, reaction time of 1.5 h.
erature to 600 ◦C, violent gasification reactions might cause partf the pore structure to be destroyed by collapsing which causedhe small surface area [18]. From Fig. 5 we can see the pore volume
◦
eclined when the temperature increased to 600 C, which coulde deduced from the collapsing of pores. So the optimum temper-ture for activation was 500 ◦C. The ash content is shown in Fig. 4cnd Table 1.
Fig. 6. XRD patterns of activated carbons.
The yield of activated carbon was calculated based on the weightof rice husk on a dry basis from the following equation:
Yield of activated carbon (wt.%)
= Weight of activated carbonWeight of rice husk
Ash content of rice husk was around 18%, the yield of activatedcarbons was about 40%, so the ash content of activated carbonsshould be around 32% theoretically, but it decreased when the acti-vation temperature was 400 or 500 ◦C, which could be explainedthat silicon element existed in rice husk reacted with H3PO4 to formphosphate in this temperature range, and most of the phosphatecould be removed by post-processing. However, as the tempera-ture increased to 600 ◦C, the ash content of activated carbon becamehigher, which was due to the decreasing of the product yield.
Fig. 6 shows the XRD patterns of activated carbons. As seen inFig. 6, all diffraction peaks are similar to those of silicon phosphate(SiP2O7) and the diffraction data is in good agreement with JCPDS
file no. 22-1320. As seen from Fig. 7, the EDX shows the activatedcarbons have the element of C, Si, P, O, which prove further theexistence of silicon phosphate in activated carbons.
498 Y. Li et al. / Materials Chemistry and Physics 127 (2011) 495–500
Table 1Yield and ash content of activated carbon prepared at different activation conditions.
.2.2. Influence of activation timeFig. 8a and b shows the BET surface areas and pore volumes
f activated carbons prepared at different activation times withther factors are fixed at 500 ◦C, impregnation ratio of 4:1. Theoncentration of H3PO4 was 50, 60 and 85%, respectively.
As observed from Fig. 8, the BET surface area and pore volume ofctivated carbon reach maximum value when the activation times 0.5 h. The ash content of activated carbons increased with thencrease of activation time, which was due to the soluble siliconompound may convert to other format that could deposit into theores of activated carbons. It blocked the pores and resulted in theurface area decreasing and ash content increasing. From Fig. 9 wean deduce that when the duration time extending to 1.5 h, the poreiameter of 6.5–7.0 nm and 10–12 nm decreased rapidly.
For the activated carbon prepared at 500 ◦C, 50% H3PO4 impreg-ation ratio of 4:1, reaction time of 0.5 h, the BET surface area andore volume reached 1712 m2 g−1 and 1.968 ml g−1, the ash contentas 11.7%.
.2.3. Influence of the concentration of H3PO4 and impregnationatio
The effect of the different concentration of H3PO4 on the sur-
ace area and pore volume of activated carbon were investigatedt impregnation of 4:1 and reaction temperature of 500 ◦C. 40%3PO4 was also used as the activation agent, but the characteristicsf the activated carbons were worse than 50% H3PO4 as the activa-ion agent. 50% H3PO4 was better than others with the activation
ivated carbons.
time of 0.5 h. When used 85% H3PO4, pore volume was larger andthe surface area was smaller than the activated carbons producedby 50% H3PO4. The reaction was rigorous when using 85% H3PO4,caused some of the pores combining together and produced poorpore size distribution, therefore, the pore volume was large. Thereaction was relatively moderate when using 50% H3PO4, resultingin narrow micropore size distribution and high surface area.
The carbons with the highest surface area were obtained whenthe raw materials were impregnated with 50% H3PO4 and then acti-vated at 500 ◦C for 0.5 h, different impregnation ratios were usedto find the optimum condition with other parameters constant. Asseen from Fig. 10, the optimal impregnation ratio for activationagent: rice husk is 5:1 (g/g). The BET surface area was 1820 m2 g−1,pore volume was 2.01 ml g−1 and ash content was 11.2%. Excessiveimpregnation was detrimental to activation, which was becauseH3PO4 as flame retardant prevented the porosity development.Moreover, the excess of H3PO4 and phosphates blocked the poresof the activated carbons.
3.2.4. Influence of the post-processing temperatureThe post-processing temperature had great influence on the
characteristic of the activated carbon. The carbons (prepared at
500 ◦C, 50% H3PO4 impregnation ratio of 5:1, reaction time of 0.5 h)washed by the water with different temperature had different sur-face area, pore volume and ash content.
As seen in Table 2, the ash content is high when the wash wateris below 40 ◦C. As this study showed before that the silicon element
Y. Li et al. / Materials Chemistry and Physics 127 (2011) 495–500 499
Fig. 8. Effect of activation time on (a) the BET surface areas, (b) the pore volumesand (c) the ash contents of carbons prepared from rice husks at impregnation ratioof 4:1, activation temperature of 500 ◦C.
Table 2Effect of post-processing temperature on the characteristics of the activated carbons.
Fig. 9. Pore size distribution of activated carbons prepared at 0.5 h and 1.5 h withother parameter constant.
Fig. 10. Effect of H3PO4 impregnation ratio on the BET surface areas and the porevolumes of carbons prepared from rice husks at 50% H3PO4, activation temperatureof 500 ◦C and activation time of 0.5 h.
could react with H3PO4 to form SiP2O7. SiP2O7 had a good solubilityin hot water, but could not dissolve in cold water. If it could not beremoved it deposited onto the carbons again, resulted in the lowsurface area and pore volume. Thus, the activated carbons charac-teristics were greatly improved by changing the post-processingtemperature.
4. Conclusions
This study had shown that rice husk could be used as raw mate-rial for preparation of activated carbons with high surface area anduniform pore size. The activated carbons contained micropores andmesopores. The pore diameter of the prepared activated carboncontain 1.3–2.4 nm, 2.7–4.5 nm, 6.8 nm and 9.7–11.0 nm, respec-tively. The ash content decreased when the optimum condition wasused to prepare activated carbons. It could be deduced that siliconelement in rice husks reacted with H3PO4 to form silicon phosphate(SiP2O7), and proved further the existence of SiP2O7 in activated
carbons by XRD analysis, SiP2O7 had a good solubility in hot water,but was difficult to dissolve in cold water. Activated carbons withdifferent surface areas and pore volumes were obtained when wechanged reaction factors such as activation temperature, activationtime and impregnation ratio. The post-processing temperature had
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great influence on the characteristics of the activated carbons.hese results provided us with a good method for preparation ofctivated carbon with rice husk and gave us a further understand-ng of the formation mechanism of carbon materials. This methodould be extended to variety of materials such as rice bran and otherusk shells.
cknowledgements
This research was supported by the financial support of thehina National Key Technology R&D Programme under Contracto. 2008BAE66B00 and Jilin Provincial Environmental Protectionepartment under Contract no. 200917.
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