Preparation of Activated Carbons with Large Specific Surface Areas

ARTICLE pubs.acs.org/IECR

Preparation of Activated Carbons with Large Specific Surface Areas from Biomass Corncob and Their Adsorption Equilibrium for Methane, Carbon Dioxide, Nitrogen, and Hydrogen Yong Sun and Paul A. Webley* Department of Chemical Engineering, Monash University, Wellington Road, Clayton, Victoria 3800 Australia

bS Supporting Information ABSTRACT: Activated carbons were produced from agricultural waste corncob using K2CO3 and H3PO4 as activators. The optimal activation temperature for producing the largest BET specific surface area and pore volume of the carbon was 800 °C for K2CO3 activation (sample CK-800) and 500 °C for H3PO4 activation (sample CP-500). The maximum BET specific surface area and pore volume of the resultant carbons were 1450 m2/g and 1.1 cm3/g for K2CO3 activation and 1069 m2/g and 1.0 cm3/g for H3PO4 activation, respectively. The two produced carbons for high pressure carbon dioxide, methane, and nitrogen separation and hydrogen storage were closely investigated. The adsorption isotherm model based on the Toth equation together with the BenedictWebbRubin (BWR) equation of state for determination of the gas phase fugacity provides a satisfactory representation of the high pressure carbon dioxide, methane, and nitrogen equilibrium adsorption. The adsorption isotherm model based on the LangmuirFreundlich equation together with the BenedictWebbRubin (BWR) equation of state for determination of the gas phase fugacity provides a satisfactory representation of the high pressure hydrogen adsorption. The preferential adsorption of CO2 on the two carbons indicates its selectivity in separation of CO2/CH4 and CO2/N2 mixtures. The hydrogen adsorption capacity of the CK-800 attained 0.34 wt % at 298 K and 60 bar, while CP-500 attained 0.29 wt % at room temperature when the full coverage of the solid surface by hydrogen is reached. The isosteric enthalpies of CK-800 for CO2, CH4, N2, and H2 are around 17, 18, 8, and 9 kJ/mol, respectively; those for CP-500 for CO2, CH4, N2, and H2 are around 13, 12, 9, and 6.5 kJ/mol, respectively.

’ INTRODUCTION Physical adsorption on activated carbon (AC) and carbonaceous adsorbent is widely used for separation and purification of gases1 due to AC's well-developed porosity, low density, good chemical stability, ready availability, and low cost.2 High pressure adsorption has often been applied in industrial adsorption processes.3 One significant commercial application is the separation of methane and carbon dioxide during natural gas processing.4 Another important technology is the high pressure adsorption of sludge digestion gas, biogas, which contains mainly carbon dioxide and methane.5 The detailed adsorption data of methane, carbon dioxide, nitrogen, and hydrogen are very important for those technologies. The development of such adsorption systems requires basic adsorption equilibrium data across a wide range of pressures and temperatures. Although there are several reviews of adsorption equilibrium available,6 there are very limited numbers of isotherm data for various adsorbents at high pressure. Preparation of AC with large specific surface area from biomass such as lignin, corncob, cornstalk, and dates has attracted much attention. Among these carbon sources, corncob is a suitable precursor for preparing carbon with high specific surface area.7 The carbons prepared from corncob have been used in wastewater treatment such as removal of organic pollutants.8 In our previous work, we used H3PO4 and K2CO3 to produce high surface area carbon from corncob for hydrogen storage at 77 K and low pressure investigations.9 However, a comprehensive study of the carbon produced from K2CO3 and H3PO4 chemical activation strategies, and their subsequent gas adsorption performance r 2011 American Chemical Society

with different gases, at different temperatures, and at different pressures, has not to our knowledge been reported. Therefore, in this study we report the carbon from corncob with large surface area carbon materials using a one-step H3PO4 and a two-step K2CO3 chemical activation procedures, and their low and high pressure adsorption equilibria of adsorbing different gases at different temperatures were closely studied.

’ EXPERIMENTAL SECTION Preparation of AC. Two kinds of carbons were prepared in this research. The first, designated “CK”, was produced through a two-step activation approach. Corncobs were precarbonized at 450 °C for 4 h with nitrogen gas as the ambient atmosphere. Thereafter, 1 g of precarbonized char was soaked with K2CO3 solutions at a concentration of 0.5 mol/L (16 mL) for 2 h at room temperature. These char-containing solutions were then treated with ultrasound for 2 h at room temperature. Afterward, the chars were separated by filtration and activated from 600 to 900 °C for 120 min. After the activation, all the samples were washed with hot distilled water. This was continued until the pH value of the washing effluent reached approximately 7. The wet samples were then dried at 105 °C overnight. The second carbon type was Received: November 29, 2010 Accepted: June 12, 2011 Revised: June 12, 2011 Published: June 12, 2011 9286

dx.doi.org/10.1021/ie1024003 | Ind. Eng. Chem. Res. 2011, 50, 9286–9294

Industrial & Engineering Chemistry Research produced by a typical one-step activation process. In this process, 2 g of corncob was soaked with 50 wt % H3PO4 (with a ratio of 1 by weight) for 2 h at room temperature. The mixture was then activated from 400 to 700 °C for 120 min. The washing procedure was the same as that described above. The carbon samples produced from phosphoric acid activation are designated “CP”. The carbon with the largest BrunauerEmmettTeller (BET) specific surface area from two-step K2CO3 activation is denoted CK-800, and that with the largest BET specific surface area from H3PO4 activation is denoted CP-500. High and Low Pressure Adsorption Equilibria of Different Gases on Carbons. High pressure adsorption measurements were performed on a magnetic suspension balance (Rubotherm, Bochum, Germany). A setup of the equipment is shown in Figure 1. The balance has the advantage of noncontact weighing of

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samples. The unit consists of a conventional high pressure stainless sample cell, which is connected to a gas reservoir via an air-driven gas booster. The sample of adsorbent is weighed and placed in the basket suspended by a permanent magnet through an electromagnet. The sample basket was loaded with around 1 g of the sample, and the system was evacuated for 24 h at 2 103 Torr. Equilibrium was achieved in 10 min. The equilibrium pressure was recorded by the digital pressure transducer. At each point, the sample weight was measured three times and averages were taken. High-purity grades of methane up to 50 bar (99.99%), nitrogen up to 50 bar (99.999%), carbon dioxide (99%) up to 30 bar, and hydrogen up to 60 bar (99.999%) supplied by Linder Gas were used in this experiment. The specific surface areas and porosities of the AC samples were determined by nitrogen gas adsorptiondesorption at 77 K with a saturation pressure of 106.65 kPa using an ASAP 2020 automated gas sorption system. The BET surface area was assessed within the range of relative pressures from 0.05 to 0.3. The micropore volume was measured by the DubininRadushkevich (DR) method. The total pore volume was calculated by measuring the amount of liquid or N2 adsorbed at a relative pressure of 0.99. The average pore width was calculated on the basis of the BarrettJoynerHalenda (BJH) method. The low pressure adsorption equilibria at 273, 283, and 293 K of CH4, CO2, and N2, respectively, were obtained from the ASAP 2010 automated gas sorption system. All experimental results are within about 5% uncertainty.

’ RESULTS AND DISCUSSION

Figure 1. Experimental setup of the magnetic suspension balance.

Optimization AC Preparation Condition. Among various parameters, activation temperature is the most important factor in determining the specific surface area of the resultant carbons.79 In this paper we explored the effect of activation temperature upon the porosity of resultant carbons. The specific surface area and pore volume of the AC under different activation temperatures are shown in Figures 2 and 3, respectively. In terms of CK carbons, the specific surface area (the micropore and mesopore surface areas) and pore volume (micropore and mesopore) of the AC increase with an increase of activation temperature, and the maximum BET specific surface area and pore volume of 1450 m2/g and 1.06 cm3/g are observed at 800 °C. This result indicates that the simultaneous pore opening and pore widening (especially the micropores)

Figure 2. Surface area as a function of activation temperature of two different carbons. 9287

dx.doi.org/10.1021/ie1024003 |Ind. Eng. Chem. Res. 2011, 50, 9286–9294

Industrial & Engineering Chemistry Research

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Figure 3. Pore volume as a function of temperature of two different carbons.

Figure 4. Average pore diameter as a function of temperature of two different carbons.

take place with increase of activation temperature. With further increase of activation temperature, the pore widening begins to dominate, and this leads to the decrease of the micropore specific surface area and micropore volume and the increase of mesopore specific surface area and mesopore pore volume. The changes of average pore diameter of different carbons at different temperatures also agree with the changes of surface areas and pore volumes. Also, this result agrees with the results reported by McKee that K2CO3 was reduced by the carbon and metal K and CO were formed:10 2C + K 2 CO3 ¼ 2K + 3CO In terms of CP carbons, the specific surface area (mesopore surface area) and pore volume (mesopore) of AC increase with an increase of activation temperature, and the maximum BET specific surface area and pore volume of 1020 m2/g and 1.0 cm3/g are observed at 500 °C. This activation temperature is much lower than that of the CK carbons, indicating their different thermal degradation behaviors. With increase of the activation temperature, the pore opening and widening (mainly mesopores) take place with increase of activation temperature below 500 °C. With further increase of activation temperature, the surface area (mesopore) and pore volume (mesopore

volume) begin to decrease quickly while the micropore surface area and micropore volume begin to increase, indicating that the increase of activation temperature together with the chemical activator H3PO4 contracts the carbon skeleton and the widened and opened pores begin to shrink. This results in the decrease of mesopore surface area and pore volume and the increase of micropore surface area and micropore volume. This might be due to the fact that the phosphorus ester, which bridges the phosphoric acid and cellulose during infiltration and low temperature activation, becomes unstable when the temperature exceeds 500 °C, and also due to a substantial structural rearrangement that would require a reduction in the carbon structure cross-link density.11 These results are in close agreement with the results of the research on flame retardance.12,13 In Figure 4, the changing pattern of average pore diameter with increase of activation temperature of CP agrees with that of surface area and pore volume. After activation, the carbon yields of both CP-500 and CK-800 carbons could reach around 40% with 5% uncertainty, respectively. The carbon and hydrogen elements in both CP500 and CK-800 are around 72 ( 2% (C%) and 0.5 ( 0.1% (H%), respectively, which is comparable to the element content of the commercial AC. The results also indicate that there are 9288



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no significant elementary differences among these two types of carbons. In Table 1, different carbons with different biomass precursors and their gas adsorption data are compared. The results show that different precursors and sdifferent preparation conditions will lead to different gas adsorption performance. In addition, without carbonation and activation, the raw corncob shows a very poor gas adsorption performance due to poor pore development in the corncob matrix. Characterization of Resulting Carbons. After chemical activation, the carbons with the largest specific surface areas and pore volumes from CK and CP (CK-800 and CP-500, respectively) were selected for detailed gas separation assessment. The nitrogen adsorption isotherms and the corresponding density functional theory (DFT) pore size distributions of different carbons are shown in Figure 5. In terms of nitrogen adsorption isotherms, according to BrunauerDeming DemingTeller (BDDT) classification,18 CK-800 exhibits the typical type I isotherm. The major uptake occurs at a relatively low pressure, indicating the formation of highly microporous

materials with a narrow pore size distribution—these materials are essentially entirely microporous. On the other hand, CP500 exhibits a type IV isotherm. The hysteresis effect and slope of the plateau with significant increase in the nitrogen uptake through the entire pressure range indicate the presence of mesopores. The porosity parameters obtained from N2 adsorption isotherms of different carbons are summarized in Table 2. In terms of DFT pore size distribution, the carbons activated by K2CO3 (CK-800) show good pore development in the microporous region, while CP-500 has relatively poor pore development in this region. A large proportion of micropores of CK-800 distributes in the range 0.51.3 nm. In the case of CP-500, pore development occurs in both the microporous and mesoporous regions (>2 nm). The morphologies of CK-800 and CP-500 are shown in Figure 6. Both carbons are porous with honeycomb shaped and irregular holes. The carbon samples CK-800 and CP-500 both show a tubular-like channel with different pore sizes. The pore width of CK-800 is from 10 to 100 μm, and that of CP-500 is around 40 μm. It can be inferred that the existence of these macropores facilitates soaking of activator such as K2CO3 during the infiltration process, which facilitates the reaction between chemical activator and the carbon skeleton during the activation process, and this in turn results in good pore development. High Pressure Adsorption of CO2, N2, CH4, and H2 at Different Temperatures. High pressure adsorption equilibria of methane, carbon dioxide, nitrogen, and hydrogen on CK-800 and CP-500 were measured. We adopted the eight-parameter BenedictWebbRubin (BWR) equation to determine the fugacity and compressibility factor for each of the different gases: C0 p ¼ RTF + B0 RT A0 2 F2 + ðbRT aÞF3 T

Table 1. Comparison of Different Carbons and Their Gas Adsorption Performance precursors and activation

N2 adsorption at 77 K and

methods

1 bar [mL/g (STP)]

ref

850

14

rice husk with steam activation olive kernels with steam activation

140 250

15 16

almond shell with steam activation

350

17

corncob with H3PO4 activation

700

this work

corncob with K2CO3 activation

500

this work

corncob

nitrogen > hydrogen, while the ΔH0 of CP-500 is in the order carbon dioxide > nitrogen > methane > hydrogen. As can be seen from the equilibrium isotherms, the adsorption is selective to carbon dioxide for both CK-800 and CP-500. The preferential adsorption of carbon dioxide indicates that these two materials can be used for the separation of CO2 from carbon dioxide, methane, and nitrogen gas mixtures. The variation of ideal selectivity (nCO2/ni) of carbon dioxide relative to methane and nitrogen is shown in Figure S3 in the Supporting Information. In addition, we also compare the ideal selectivity between methane and nitrogen for these two carbons; the result is shown 9291


Industrial & Engineering Chemistry Research in Figure S4 in the Supporting Information, indicating that CP-500 has a better performance in the ideal selectively adsorbing methane from nitrogen compared with that of CK-800. In the cases of CK and CP in absorbing hydrogen, we adopted the LangmuirFreundlich equation: bPq ð10Þ ¼ n + va Fg nΩ ¼ nm 1 + bPq where b, q, and nm* are the constants of the equation. By taking the same assumption of monolayer arrangement of adsorbate molecules adsorbed above the critical temperature, we obtain ! bf q S3=2 n¼ Fg nm ð11Þ 1 + bf q ðnm Av Þ1=2 By fitting the hydrogen excess adsorption data of CK-800 and CP-500 to eq 11, we obtained the constants of the Langmuir Freundlich equation, and results are shown in Table S2 in the Supporting Information. From nm*, which is a parameter that corresponds to full coverage of the solid surface, results indicate that the hydrogen adsorption capacity of the CK-800 can attain 0.34 wt %, while CP-500 can only attain 0.29 wt %, respectively, at room temperature when the full coverage of the solid surface by hydrogen is reached. These adsorption capacities are still far below the US Department of Energy target of 6.5 wt % at room temperature. The preference of adsorbing carbon dioxide and methane of CP-500 indicates that the existence of not only micropores but also mesopores facilitates the diffusion of carbon dioxide and methane into the skeleton of carbons. In terms of hydrogen adsorption, the existence of micropores plays the most significant role in determining hydrogen adsorption capacity, which also agrees with that of literature reports.9 The enthalpy of adsorption is a significant property for characterization of the type of adsorption and degree of heterogeneity of a surface. The isosteric enthalpies of adsorption were not measured experimentally in this paper but were estimated from equilibrium data by use of the ClausiusClapeyron equation: d ln P ΔH ¼ RT 2 ð12Þ dT n Figure S5 in the Supporting Information shows that the isosteric enthalpy of adsorption of CK-800 in adsorbing N2 decreases with the increase of loading. This is a typical profile for heterogeneous adsorbents and is consistent with more energetic sites for nitrogen filled preferentially at low loading and less active sites filled as adsorption proceeds, resulting in a decrease in isosteric heat. Sample CK-800 absorbing hydrogen shows a similar trend. The slight increase or decrease of the isosteric enthalpy of CK-800 in adsorbing CO2 and CH4 indicates the relatively heterogeneous adsorption system for CO2 and CH4 although the trends are well within experimental uncertainty. The isosteric enthalpies of different gases on CK-800 are approximately 925 kJ/mol, which clearly indicates physisorption. In the case of CP-500 in Figure S6 in the Supporting Information, the isosteric enthalpies of carbon dioxide and methane increase gradually as the loading increases. This indicates that the adsorbateadsorbate interactions are becoming more significant than adsorbateadsorbent interactions as the loading increases. The isosteric enthalpy of adsorption of hydrogen for CP-500 decreases with an increase of loading as expected for this weakly adsorbing gas. All these results indicate

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that both CK-800 and CP-500 have energetically heterogeneous surfaces. In a ranking of the adsorbates on the basis of isosteric enthalpies of adsorption at low loading, the adsorption on CP-500 is always lower than that of CK-800. This is because the heat of adsorption at low surface coverage is related to the interaction between the adsorbent surface and adsorptive molecular forces. The value of the isosteric enthalpy of adsorption at low surface coverage is often related to the pore size distribution of the AC, and the value of CP-500 is always lower than that of CK-800 due to the fact that it has the larger average pore diameter. In Table S3 in the Supporting Information, we compare different carbon samples with detailed physical characters and their corresponding isosteric enthalpies. The results indicate that our results fall in the region of the reported values from those in the literature.2325 Low Pressure Adsorption of CO2, N2, CH4 at Different Temperatures. In addition to the calculations based on the high pressure adsorption equilibrium data, we also conducted adsorption equilibrium experiments of the two carbons for nitrogen, methane, and carbon dioxide adsorption at different temperatures at low pressure in order to make a complementary comparison for the isosteric enthalpy calculation results based on the high pressure adsorption data. In the low pressure region (Henry’s law), the isotherm should be linear: q ¼ hHenry P

ð13Þ

where q is the amount adsorbed (mmol/g) and hHenry is the Henry’s law constant. Since hHenry is a function of temperature only, the isosteric heat of adsorption of hydrogen will represent a characteristic of the adsorbateadsorbent system. By substituting eq 13 into 12, the isosteric heat of adsorption at zero loading limit can be expressed in terms of Henry’s law constant: ∂ ln hHenry Hst0 ¼ RT 2 ð14Þ ∂T q The isosteric heat of adsorption calculated from the Clapeyron equation is sensitive to the choice of the isotherm equation. Here, we choose the most frequently used Toth and Unilan isotherm equations to yield the relevant parameters at different temperatures at low pressure. The Unilan equation is the following: qm 1 + bes P ln q¼ ð15Þ 1 + bes P 2s where qm is the adsorption saturation capacity (mmol/g); b and s are the affinity and the heterogeneity parameters of the adsorbent. The Henry’s law constant for the Unilan equation is es es 2s The Toth equation is the following: q m P q¼ ðb + Pt Þ1=t hHenry ¼ qm b

ð16Þ

ð17Þ

where qm is the adsorption saturation capacity (mmol/g); b and t are the constants in the Toth equation. The Henry’s law constant for the Toth equation is hHenry ¼ qm b1=t

ð18Þ

The different gases absorbing on CK-800 and CP-500 below 1 atm at different temperatures are shown in Figure S7 in the 9292


Industrial & Engineering Chemistry Research Supporting Information. By applying the Unilan and Toth equations to those low pressure adsorption data, we obtain the parameters of the Unilan and Toth equations of CK-800 and CP-500 at different temperatures. The results are shown in Tables S4 and S5 in the Supporting Information, respectively. Both the Toth and Unilan equations give very good fits for the experimental data. In the case of CK-800 using the Toth equation, the isosteric heat of adsorption at zero loading for different gases is in the order carbon dioxide (25.2 kJ/mol) > methane (20.1 kJ/mol) > nitrogen (18.2 kJ/mol), which generally agrees with that of the calculation based on the high pressure adsorption, indicating the reliability of the high pressure adsorption data. In the case of CP-500 using the Toth equation, the isosteric heat of adsorption at zero loading for different gases is in the order carbon dioxide (16.8 kJ/mol) > nitrogen (12.7 kJ/mol) > methane (10.3 kJ/mol), which agrees well with that of the calculation based on the high pressure adsorption data as well. Comparing the calculated isosteric heat of adsorption at zero loading for different gases derived from the Toth and Unilan equations, the values that were obtained from the Toth equation are generally a little bigger than the values from Unilan. This might be due to the different hypothesis and approximation approaches of different models. In terms of relative variance of using the Unilan and Toth equations for modeling different gas adsorptions on CK-800 and CP-500, the Unilan equation gives a relatively better performance than the Toth equation for modeling low pressure gas adsorptions on CK-800 and CP-500.

’ CONCLUSIONS Activated carbons were produced from agricultural waste corncob using K2CO3 and H3PO4 as activators. The BET specific surface area and pore volume of the carbon were 1450 m2/g and 1.1 cm3/g for K2CO3 activation, and they were 1069 m2/g and 1.0 cm3/g for H3PO4 activation. The two produced carbons for high pressure carbon dioxide (up to 3 MPa), methane (up to 5 MPa), nitrogen (up to 5 MPa), and hydrogen (up to 6 MPa) were closely investigated. The adsorption isotherm model based on the Toth equation together with the BenedictWebbRubin (BWR) equation of state for determination of the gas phase fugacity provided a satisfactory representation of the high pressure carbon dioxide, methane, and nitrogen data. The adsorption isotherm model based on the LangmuirFreundlich equation together with the BenedictWebbRubin (BWR) equation of state for determination of the gas phase fugacity provided a good representation of the high pressure hydrogen adsorption. The preferential adsorption of CO2 on the two carbons indicates its selectivity in separation of CO2, and CP-500 is better than CK-800 in the preferential adsorption of CO2 and CH4. The hydrogen adsorption capacity of the CK-800 can reach 0.34 wt %, while CP-500 can only reach 0.29 wt %, respectively, at room temperature when the full coverage of the solid surface by hydrogen is reached. The results of the isosteric enthalpies of adsorption for different gases on the two carbons indicate their energetically heterogeneous surfaces. Our research also indicates that preparation of AC from corncob with high specific surface area is a promising approach for high value conversion of abundant agricultural waste. ’ ASSOCIATED CONTENT

bS

Supporting Information. Figures showing Toth, LangmuirFreundlich, and Unilan model fits for CK-800 and CP-500

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in adsorbing N2, CH4, CO2, and hydrogen; ideal selectivities of carbon dioxide relative to methane and nitrogen and of methane relative to nitrogen for CK-800 and CP-500; isosteric enthalpies of adsorption with respect to surface loading on CK-800 and CP-500. Tables listing constants of Toth and Unilan models from CK-800 and CP-500; constants of LangmuirFreundlich model for hydrogen adsorption; comparison of adsorption equilibrium measurement from different activated carbons. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support for this work was provided by Monash University. The authors also want to thank the Adsorption Engineering Laboratory at Monash University. The critical comments from four anonymous reviewers are highly appreciated. ’ REFERENCES (1) Yang, R. T. Gas Separation by Adsorption Processes; Imperial College Press: Boston, 1987. (2) Sahaym, U.; Norton, M. G. Advances in the Application of Nanotechnology in Enabling a Hydrogen Economy. J. Mater. Sci. 2008, 43, 5395. (3) Sicar, S.; Golden, T.; Rao, M. B. Activated Carbon for Gas Separation and Storage. Carbon 1996, 34, 1. (4) Cavenati, S.; Grande, C. A.; Rodrigues, A. E. Adsorption Equilibrium of Methane Carbon Dioxide and Nitrogen on Zeolite 13X at High Pressure. J. Chem. Eng. Data 2004, 49, 1095. (5) Sarapatka, B. Factors Influencing Biogas Production during Fullscale Anaerobic Fermentation of Farmyard Manure. Bioresour. Technol. 1994, 49, 17. (6) Yang, R. T.; Doong, S. J. Gas Separation by Pressure Swing Adsorption: A Pore Diffusion Model for Bulk Separation. AIChE. J. 1985, 31, 1829. (7) Sun, Y.; Zhang, J. P.; Yang, G.; Li, Z. H. An Improved Process for Preparing Activated Carbon with Large Specific Surface Area from Corncob. Chem. Biochem. Eng. Q. 2007, 21, 169. (8) Sun, Y.; Zhang, J. P.; Yang, G.; Li, Z. H. Removal of Pollutants with Activated Carbon Produced from K2CO3 Activation of Lignin from Reed Black Liquors. Chem. Biochem. Eng. Q. 2006, 20, 429. (9) Sun, Y.; Webley, P. A. Preparation of Activated Carbons from Corncob with Large Specific Surface Area by a Variety of Chemical Activators and their Application in Gas Storage. Chem. Eng. J. 2010, 162, 883. (10) McKee, D. W. Mechanisms of the Alkali Metal Catalyzed Gasification of Carbon. Fuel 1983, 62, 170. (11) Solum, M. S.; Pugmire, R. J.; Jagtoycn, M.; Derbyshire, F. Evolution of Carbon Structure in Chemically Activated Wood. Carbon 1995, 33, 1247. (12) Bourbrigot, S.; Bras, M. L.; Delobel, R.; Breant, P.; Tremillon, J. M. Carbonization Mechanisms Resulting from Intumescence—part II. Association with an Ethylene Terpolymer and the Ammonium Polyphosphate-pentaerythritol Fire Retardant System. Carbon 1995, 33, 283. (13) Jagtoyen, M.; Derbyshire, F. Activated Carbons from Yellow Polar and White Oak by H3PO4 Activation. Carbon 1998, 36, 1085. (14) Liou, T. H. Development of Mesoporous Structure and High Adsorption Capacity of Biomass-based Activated Carbon by Phosphoric Acid and Zinc Chloride Activation. Chem. Eng. J. 2010, 158, 129. (15) Amaya, A.; Medero, N.; Tancredi, N.; Silva, H.; Deiana, C. Activated Carbon Briquettes from Biomass Materials. Bioresour. Technol. 2007, 98, 1635. 9293



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Preparation of Activated Carbons with Large Specific Surface Areas

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