Mass Balance and Performance Analysis of Potassium Hydroxide

Article pubs.acs.org/IECR

Mass Balance and Performance Analysis of Potassium Hydroxide Activated Carbon R. Hilton,* P. Bick, A. Tekeei, E. Leimkuehler, P. Pfeifer, and G. J. Suppes Department of Chemical Engineering, University of Missouri, W2065 Lafferre Hall, Columbia, Missouri 65203, United States ABSTRACT: Highly nanoporous carbon with surface areas in excess of 3,000 m2/g can be produced via potassium hydroxide (KOH) activation of a high surface area (1,150 m2/g) carbon intermediate. These materials have exhibited methane storage capabilities in excess of 20 wt % at ambient temperature with interest toward commercial production. In preparation for commercial production, detailed mass balances are needed to quantify yield and waste streams, understand the propensity to recycle the KOH, and to provide a benchmark for further optimization. Analytical processes used to evaluate produced carbon performance are detailed in addition to a mass balance on the reaction of KOH with carbon and a KOH balance. Carbon balances revealed that increasing activation time and activation temperature produce lower yields of carbon.

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INTRODUCTION Activated carbon is a proven, high-performance material for a range of applications from water purification to catalyst supports. In the process of chemical activation of carbon, chemical additives are used to etch nanopores into the carbon matrix. These nanopores significantly increase the overall surface area of the carbon, thereby increasing the carbon’s gravimetric storage capacity. Brunauer−Emmett−Teller (BET) surface area, as reported in m2/g, is a standard for performance evaluation of the carbon. Surface areas of 300 m2/g are considered good for many materials. Such 300 m2/g surface areas are low compared to BET surface areas in excess of 3,000 m2/g capable from processes like the ALL-CRAFT (The Alliance for Collaborative Research in Alternative Fuel Technology) process (see Table 1).1 Several different carbon

carbon with a higher surface area than in either of the aforementioned processes. In the ALL-CRAFT procedure, phosphoric acid is first used to pyrolyze and char ground corncob, and then potassium hydroxide is used to etch nanopores into the carbon, further increasing the surface area as well as the purity of the carbon. The importance of this two-step process, as well as the mechanisms that drive this process are discussed in the Background section. An activation temperature between 750 and 800 °C6 and a KOH/C ratio between 3 and 47 have been shown to produce the highest BET surface area carbons. The high BET surface areas of the ALL-CRAFT carbons rival those of the Maxsorb III created by Rahman et al.8 as well as Activated Carbon Fibers created by Zhou et al.9 Table 1 has sample BET surface area measurements for some of the ALLCRAFT activated carbons. A constructive comparison between these carbons and known literature values is viewable in the Results section. Activated carbon is a promising medium for the reversible storage and transport of hydrogen and natural gas. Storage capacities with an activated carbon medium allow for a much greater storage capacity than compressed gas in an empty vessel. A primary reason for the lack of commercialization of the higher surface area activated carbons is the high complexity and costs of their synthesis. In particular, high surface area carbons often have low yields (kg of activated carbon divided by the kg of biomass feed stock). Also, the use of reagents like KOH can increase reagent costs as well as waste disposal costs. This publication details the mass balance for the production of the ALL-CRAFT carbon which has exhibited high surface areas and methane storage capacities. Optimum methane storage occurs at activation temperatures ranging from 700 to 800 °C.10 Storage capability generally increases as BET surface area increases.11 In addition, it has been shown that experimental methane delivery is maximized at a KOH/C ratio of 3:112

Table 1. Sample BET Measurements for ALL-CRAFT Carbonsa sample no.

KOH ratio

activation temp (°C)

activation time (h)

specific surface area (m2/g)

porosity from isotherm %

1 2 3 4

0 2 3 4

480 790 790 790

1 1 1 1

1150 1880 2750 3210

0.59 0.69 0.78 0.81

a

3K Surface Area (standard deviation = 200), 3K Porosity (standard deviation = 0.025).

activation methods exist, and dozens of precursors can be used. Huang et al.2 use potassium carbonate to chemically activate the carbon in water bamboo leaves to create carbons with a surface area as high as 2481 m2/g. Ip et al.3 use phosphoric acid with bamboo to obtain a carbon with a surface area as high as 2100 m2/g. Teng et al.4 use potassium hydroxide (KOH) with used tires to create a carbon with a surface area around 500 m2/ g. The ALL-CRAFT process uses a similar procedure to that of Pechyen et al5 with phosphoric acid used in the first part of the activation, and KOH used in the second. The ALL-CRAFT carbon is the only carbon to use this procedure with ground corncob as a precursor, and in doing so, is able to create a © 2012 American Chemical Society

Received: September 26, 2011 Accepted: June 15, 2012 Published: June 15, 2012 9129

dx.doi.org/10.1021/ie301293t | Ind. Eng. Chem. Res. 2012, 51, 9129−9135

Industrial & Engineering Chemistry Research

Article

Figure 1. Lignin molecule.

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BACKGROUND Carbon can be produced from a wide range of biomass and fossil fuels. Corncobs are the preferred feedstock for the ALLCRAFT project because they are an abundant agricultural waste product containing low amounts of minerals, and create a carbon with high-density pores with diameters between 7 and 12 Å.13 Analysis from Anderson’s Inc. (Maumee, OH) shows a 43.5% carbon content in corncob feedstock, which makes it a promising precursor for the production of nanoporous carbon. The corncobs also contain 48.4% oxygen, 7.9% hydrogen, and 0.21% nitrogen. If it is assumed that half the oxygen leaves as water and the other half as carbon dioxide, and that the hydrogen and nitrogen both dissolve into the exiting streams, then a theoretical yield of 43.5% pure carbon should be obtainable from pyrolysis of the corncobs.1a The properties of activated carbon can vary depending on the initial carbon source as well as the method of activation. In this study, activated carbon is produced from corncobs in two chemical activation steps: an initial phosphoric acid charring process, which produces the char, followed by a KOH activation process, which produces the activated carbon. Figure 2 in the Experimental Section summarizes the synthesis process including a phosphoric acid charring process and KOH activation process. Mechanisms for the charring step as well as the activation step are as follows: In the charring step, a 40−60 mesh corncob is mixed with dilute phosphoric acid, and it hydrolyzes overnight. After the initial acid soak, the mixture is heated to 480 °C under a nitrogen purge, and this temperature is held for 1 h. During both the overnight hydrolysis and the 1 h pyrolysis, the

phosphoric acid reacts with lignin (see Figure 1 Lignin molecule), the most abundant component of the mesh corncob. The phosphoric acid protonates many of the oxygen-containing sites of the molecule, making them more prone to detach from the carbon atoms. After the charring has occurred, the char product is washed to eliminate all watersoluble and noncarbon constituents, thereby isolating the carbon.14 Guo and Gao15 have proposed a mechanism for the activation step. The KOH melts around 400 °C, and at a slightly higher temperature, it dehydrates and yields K2O, which reacts with the carbon to produce K2CO3 (Reaction 2). Both the K2O and the K2CO3 react with the carbon matrix (Reactions 2 and 3), bending and shaping the graphene sheets, causing them to spread out more, creating some metallic potassium. As the temperatures go even higher, the Kcontaining compounds are also reduced to metallic potassium, which is smaller, and diffuses easier into the carbon matrix, yielding graphite intercalation compounds,16 which cause a separation of the lamellar layers. These mechanisms are important in understanding why the surface areas of activated carbon can reach such high values. The theoretical maximum surface area of a graphene sheet is 2,630 m2/g.17 From all of the bending and twisting that happens to these graphene layers during activation, edge components on these contorted sheets can add significant surface area, increasing the total surface area to above 3,000 m2/g. The following chemical reactions provide the necessary mass balance for the process.18 2C + 6KOH → 2K + 3H 2 + 2K 2CO3 9130

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2C + 3K 2O + 3H 2O → 2K + 3H 2 + 2K 2CO3

(2)

2K 2CO3 + 4C → 4K + 6CO

(3)

2K 2CO3 + C → 4K + 3CO2

(4)

(KOH) is added to the char (in a ratio of 2:1 for 2K, 3:1 for 3K, or 4:1 for 4K by mass). The K-value of the activated carbon refers to the mass to mass ratio of the KOH to the char during activation. The char-KOH mixture is ball-milled for 2.5 h. Each individual ball milled sample is added to a 50 mL alumina crucible with a drop of water to aid diffusion, and the sample is placed in a high temperature oven using the single set point mode. Parameters of investigation included the ratio of activating agent (KOH) to carbon(K-value), activation temperature, and activation time. When the oven temperature cooled to 100 °C, the samples were removed and weighed. A similar KOH activation process was used by Wu et al.21 The activated mixtures were transferred to and washed in centrifuge tubes with distilled water to remove the potassium from the carbon. During the washing process, the water− carbon mixture was mixed in the tube and then the phases were separated via centrifuge and the water was decanted from the carbon. This washing process was repeated three times. The tubes containing the washed carbon were kept in a vacuum oven overnight at 140 °C to dry. The mass of the centrifuge vial containing carbon was taken before washing and after drying to determine the overall percent of carbon lost. The following masses were recorded: mass of char, mass of activating agent, mass of mixture added to the crucible, mass in crucible after activation, and mass of solid carbon remaining after washing. The product of this procedure is activated carbon. For the purposes of this publication, the end product of the process will always be referred to as “activated carbon.” Figure 2 summarizes the entire process visually.

Similar reaction mechanisms have been proposed by Tay et al.19 and Lillo-Rodenas et al.20 After the activation process, the KOH is washed from the crevices creating pores suitable for gas adsorption. If carbon dioxide is produced from reactions 1 and 3, then 3 mols of carbon (MW = 12) are consumed for every 6 mols of KOH (MW = 56) consumed. In other words, KOH is consumed in a 2:1 molar ratio to carbon, which is a 9.3:1 mass ratio. If carbon monoxide is produced, 6 mols of carbon are consumed for every 6 mols of KOH consumed, a 1:1 molar ratio. In this case, KOH is consumed at a 4.7:1 mass ratio to carbon. It is unlikely that reaction 2 can ultimately produce CO2 via the water-gas shift reaction, as these reactions do not begin until temperatures are above 400 °C, and the water that was initially mixed with the activated carbon has already vaporized and left the vessel.

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EXPERIMENTAL SECTION Materials and Equipment. Corncob feedstock (4060 mesh) was obtained from The Andersons, Inc. Agricultural Company (Maumee, Ohio). This material is from the middle pith layer of the corncob. Phosphoric acid (85%) was obtained from Fisher Scientific Research Company. KOH (>90%) was obtained from Aldrich Chemical Co. (Milwaukee, WI). Caution! Extreme care must be taken when handling the KOH, as it can cause severe burns to the skin. Gloves must be worn. Nitrogen gas (99.99%) was supplied by PraxAir Inc., industrial gas provider. Samples were milled in a ball mill provided by United Nuclear Scientific Equipment and Supplies Company. A model FD1545 M Furnace from ThermoScientific was used to perform the activation. A nitrogen purge was directed at the reactor in the oven with a purge rate of 5 L/min and adequate ventilation. Aluminum oxide crucibles (50 mL) from Cole Parmer were used for charring and activation. Carbon washing was carried out on a physician’s compact centrifuge from BD Adams Biological Research Company. BET surface areas were found using a Quantachrome Quadrasorb SI surface area analyzer. Experimental isotherms of methane adsorption were determined by the measurement of equilibrium uptake of methane in different pressures (50−600 psi) at ambient temperature (298 K). Uptake analysis was performed for each carbon sample. Sample Preparation. The 40−60 mesh corncob provided by Andersons, Inc. is used as the precursor for activated carbon production. Dilute phosphoric acid at a mass ratio of 0.8:1 acid to corncob is added to the ground corncob gradually and mixed thoroughly. The mixture is then left to soak in an oven at 45 °C for 12 h. After the initial acid soak, the mixture is heated to 480 °C under a nitrogen purge, and this temperature is held for 1 h. These products are washed to eliminate all water-soluble and noncarbon constituents, thereby isolating the carbon.14 The product of this step is the char. For the purposes of this publication, the phosphoric acid-pyrolyzed corncobs will always be referred to as “char.” The char is dried under vacuum at 140 °C overnight. The char is ground in a commercial blender for 5 min, or until it can be sieved through holes of 425 μm diameter. One gram of char is used for each sample. The activating agent

Figure 2. Summary of activated carbon production process.

Similar phosphoric acid activation methods were employed by Vernersson et al.22 and Goméz-Serrano et al.14a A large batch of char was produced from which all the activated carbon of this paper was produced. The char stock was kept in a vacuum oven at 140 °C. 9131



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RESULTS Charring. Table 2 shows representative mass losses during the charring process. The carbon yield is similar to a theoretical

of mixing the carbon with water, centrifuging the mixture, and decanting of the filtrate. Table 4 summarizes representative results for washing the potassium compounds (KOH, K, K2CO3) after activation. For the 3K carbon activated at 790 °C there was an average of 87.03% mass loss in the washing step with a standard deviation of ±0.39%. Figures 3−5 show sample mass loss and carbon mass loss during activation for all 9 experiments. “Sample mass loss”

Table 2. Mass Lost in Pyrolysis/Phosphoric Acid Activation Process total mass (g)

mass phosphoric acid (g)

mass corn cobs (g)

mass after charring (g)

mass after wash (g)

% carbon remaining

% carbon lost

100 100 151 200 253 253

0 0 90.7 120 153 153

100 100 60.3 80 100 100

25.7 24.5 72.35 96.1 114 108

25.7 24.5 25.2 33.1 41.9 40.9

25.7 24.5 41.8 41.4 41.9 40.9

74.3 75.5 58.2 58.6 58.1 59.1

carbon yield of 43% where all the oxygen in the cellulosic feedstock combines to form water and carbon dioxide. Significantly lower yields of carbon were realized when phosphoric acid was not used for pretreatment, which suggests that carbon from the corncob is leaving the acid-corncob mix in the form of carbon dioxide. Similar yield values for the phosphoric acid activation were observed by Rosas et al.23 and Goméz-Serrano et al.14a Activation. Table 3 outlines the different parameters that were varied in the activation step, as well as the resulting BET

Figure 3. Carbon mass loss and sample mass loss as functions of activation temperature.

refers to the quantity (mass of unwashed sample)/(initial mass of char + mass of KOH), and “carbon mass loss” is (mass of washed and dried sample)/(initial mass of char + mass of KOH). Figure 3 illustrates how increasing temperatures lead to increasing loss of mass during activation with the 3K activation for 1 h. The mass loss is from the evolution of gases from the mixture of KOH and char. A comparison of 2K, 3K, and 4K is summarized by Figure 4. Again, the K-value of an activated

Table 3. Parameters Modified in Activation Stepa experiment no.

activation temperature (°C)

KOH/C ratio

activation time (h)

BET surface area (m2/g)

1 2 3 4 5 6 7 8 9

700 750 800 790 790 790 790 790 790

3 3 3 2 3 4 3 3 3

1 1 1 1 1 1 1 2 3

2470 ± 250 2730 ± 270 2750 ± 280 1880 ± 190 2750 ± 280 3210 ± 330 2750 ± 280 2680 ± 270 2620 ± 260

a

Control experiments in bold.

surface areas. The 3K 790 °C 1 h activation is considered the control case. The KOH/C ratio has the most significant impact on the BET surface area as experiments 4−6 have a much greater variance than the others. The yield of carbon is typically of greater interest than the loss of mass from the mixture of KOH and char during activation. Several processing steps are involved between the production of char and the final activated carbon. To obtain statistically significant results an emphasis was placed on developing a carbon washing method based on repeated steps

Figure 4. Sample mass loss and carbon mass loss as a function of KOH/C ratio.

carbon is the mass ratio of KOH to char during activation. The lower mass loss of the 4K carbon is a result of the higher loading of potassium which is not significantly volatile in any of its possible derivatives at 790 K.

Table 4. Mass Loss in Washing Percentages on Single Batch of 3K Carbon tube no.

mass empty tube (g)

mass tube plus unwashed 3K (g)

mass unwashed 3K (g)

mass dry tube and 3K (g)

mass loss (g)

mass remaining (g)

% mass loss

1 2 ... 12

9.38 9.33

11.4 11.3

1.98 2.01

9.63 9.60

1.74 1.74

0.252 0.270

87.3 86.5

9.31

11.3

2.00

9.57

1.74

0.260

87.0

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DISCUSSION Table 6 provides an elemental analysis of the char (as provided by Elemental Analysis, Inc.), and while a char yield of 41% (see

The results of Figures 3 and 4 were for 1 h of activation. As illustrated by Figure 5, increasing activation time leads to increasing % mass loss. All carbon is lost at an activation time of 4 h at 790 °C.

Table 6. Elemental Analysis of Char

The following table (Table 5) compares char along with ALL-CRAFT activated carbons activated at six different KOH/ Table 5. Comparison of ALL-CRAFT Carbon Surface Areas and Pore Volumes with Literature Values char 2K 2.5K 3K 3.5K 4K 4.5K maxsorb III8 ACF (A-20)9

BET surface area [m2/g]

total pore volume [cm3/g]

± ± ± ± ± ± ±

0.53 0.907 0.945 1.342 1.397 1.783 2.909 1.783 1.1

1150 2240 2600 2750 2770 3210 3470 3280 2200

120 220 260 280 280 320 350

wt % in char 72.2 1.69 1.04 21.5 1.03 0.55 2.47

Table 2) is near the theoretical maximum for complete consumption of oxygen, not all of the oxygen was driven from the carbon. The carbon was washed until neutral pH was achieved, and if all phosphorus is assumed to be from H3PO4, about 5% of the total weight is attributed to oxygen in the phosphate, leaving the remaining 16.5% (21.5%−5%) as oxygen chemically bonded to the carbon. The char yield decreased from 41% to 25% when phosphoric acid was not used. Since this additional mass loss is above the theoretical amount from reaction with bound oxygen, it must be a result of tar vapors. Tar vapors are the same as the vapors emitted during phosphoric acid-catalyzed pyrolysis, but they are more carbon-rich, and they have lower oxygen content. A mechanism in which the phosphoric acid associates with the biomass would result in decreased tar vapor emissions. Table 7 shows the composition data of the 3K activated carbon at an activation time and temperature of 1 h and 790

Figure 5. Sample mass loss and carbon mass loss as a function of activation time.

sample

element carbon hydrogen nitrogen oxygen sodium potassium phosphorus

Table 7. Elemental Analysis of Activated Carbona element carbon iron chromium calcium magnesium sodium silicon potassium other

char ratios with the established literature values of Maxsorb III carbon and A-20 Activated Carbon Fibers. All of the ALLCRAFT activated carbons for this data set are activated for 1 h at 790 °C. Pore volumes are determined by the QSDFT method, as outlined in refs 24 and 25. Figure 6 displays the pore size distribution of some of the ALL-CRAFT carbon samples activated for 1 h at 790 °C.

a

composition by mass

994 924 628 593 278 262

99.1% 0.432% ppm ppm ppm ppm ppm ppm 0.1%

3K, 790°C, 1 h.

°C, respectively, as provided by Elemental Analysis Inc. The activation step is important in improving the purity of the product, as the high temperature vaporizes many of the undesired components of the activated carbon. Kinetic reaction theories predict that as temperature increases, both the rate of reaction increases and new, more aggressive reactions may become possible (a manifestation of increasing reaction rates with increasing temperature). These trends are consistent with the increased mass loss with increasing temperature of Figure 3. Evans et al.26 reported similar trends in yields. Tseng reported similar trends as that of Figure 4 where increasing loadings of KOH resulted in slightly reduced loss of carbon.11 Others21,26 have observed increased carbon loss with increasing KOH loadings. It should be noted that, within the standard deviation, the carbon mass loss is fairly constant for

Figure 6. Pore size distributions of ALL-CRAFT carbons. 9133



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volume, the pore size distribution is more important. From Burress,1b 11 Å is the ideal size for uptake of methane molecules, as it allows enough space for methane molecules to bind to the pore walls 2 molecules wide, without excessive empty space. The 3K carbons activated at temperatures close to 800 °C had the highest methane uptake, eclipsing the 20 wt % excess adsorption at the 500 psi mark. This is fairly consistent with the aforementioned importance of pores of 11 Å diameter. The 3K tends to have the highest combined pore volume in the areas around 11 Å. For methane uptake at lower temperatures, wider pores can become more vital, but that is a point to be investigated in later research.

the data of Figure 4. Multiple competing phenomena impact the rate of carbon loss which requires a more in-depth discussion. Larger sample sizes during activation lead to reduced rates of carbon loss, and at a constant carbon loading the sample sizes of Figure 4 did increase with increasing KOH/C ratios. Heat transfer into the core of larger samples leads to temperature differences in excess of 50 °C.27 In simulations of activation the temperature gradients translated to larger sample sizes exhibiting delayed mass loss, and the impact of sample size on mass loss is particularly evident as shorter activation times like the 1 h activation times of Figure 5. Another mechanism through which larger samples exhibit reduced mass loss is associated with the condensation of tar vapors, which is directly proportional to tar vapor concentration. Larger samples have increased tar vapor concentrations for longer periods leading to greater yields of carbon. In view of systematic sample size trends superimposed over the KOH/C trends of Figure 4 and in view of the standard deviation, it is likely that increased KOH/C ratios do lead to increased rates of mass loss, but this phenomenon was obscured by trends where increased sample sizes decreased rates of mass loss. In a reaction environment of solid carbon (unit activity) and molten KOH, higher KOH loading will tend to maintain higher reaction rates because the diluting impact of soluble products (e.g., carbonate salts) is reduced. At lower KOH loadings, the hydroxide oxidant is consumed quicker which will also result in decreased carbon mass loss rates at lower KOH loadings. The data trends of Figures 3−5 are consistent with these fundamental phenomena related to sample size and KOH stoichiometries. A 3K carbon has a carbon to hydroxide molar ratio of 1 to 0.642. This translates to a 36% yield if carbon monoxide (or K2CO3) is formed and a 68% yield if carbon dioxide is formed. The best yields of Figure 5 (3K, 1 h, 790 °C) are consistent with 36% yield and formation of carbon monoxide (or K2CO3). The 36% yield is consistent with either carbon monoxide gas evolution or washing away of potassium carbonate. A combination of the release of these two products is consistent with the 17% mass loss (Figure 5) of the 3K mixture during activation. The resulting BET surface areas indicate a strong correlation between KOH/C ratio and surface area. The 4K carbon had a significantly higher surface area than both the 2K and 3K carbons. This is due to an increase in KOH concentration in the sample, which etches nanopores into the carbon matrix when exposed to intense heat. The BET surface area data also indicate that a higher activation temperature within the range 750−800 °C also yields a higher surface area, although this trend is not as significant as the trend produced by the KOH/C ratio. This speaks to the importance of high heat in aiding the KOH in activation of the carbon. The least significant BET surface area trend is the one found in the activation time. Two and three hour activation times yield only a slightly lower surface area than the 1 h activation, but they are all within the margin of error. During the long activations, the KOH is eventually all released from the carbon matrix, and small amounts of this high surface area, exposed carbon may begin to combust, causing nanopores to collapse, impeding the surface area. Figure 6 displays the pore size distributions of the ALLCRAFT carbons, found using the QSDFT calculation method. While methane uptake is linked to surface area and pore

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CONCLUSIONS

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AUTHOR INFORMATION

The two-step ALL-CRAFT carbon synthesis process has exhibited the ability to produce carbons with ultrahigh surface areas with methane uptake above 20 wt % at ambient temperature. The phosphoric acid char synthesis (first step) achieves near-theoretical yields of 41% carbon because of the ability of the phosphoric acid to suppress tar vapor losses. Following the phosphoric acid charring, the char is activated with varying ratios of potassium hydroxide (KOH) at temperatures ranging from 700 to 800 °C, and between 1 and 3 h. Higher activation temperature ultimately yields a higher mass loss in the activated carbon, while higher KOH loading yields lower mass loss, and increased activation time yields higher mass loss. A 3K carbon that was evaluated in greater detail exhibited 100% carbon loss in 4 h which is consistent with carbon loss through parallel mechanism of oxidation by the hydroxide and tar vapor evolution. While the impact of sample size on carbon yield complicates the ability to scale the results to what would be realized in commercial production, stoichiometric consumption of oxygen to yield carbon monoxide with a yield of 36% for a 3K carbon are consistent with the best of yields reported in this work. This translates to an overall yield of 15% activated carbon from cellulose. The BET surface areas of 4K and 4.5K activated carbon, activated at 790 °C for 1 h, rival the BET surface area of the Maxsorb-III8 activated carbon. The ultimate goal of this carbon is to achieve volumetric methane storage capacities consistent with standards set forth by the U.S. Department of Energy (180 v/v storage). Additionally, this storage medium must be able to desorb in a manner that can be controlled easily enough for the carbon to be used in a vehicle powered by natural gas. This high surface area activated carbon is an important benchmark to achieving these goals.

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was made possible through funding from the National Science Foundation (Award Number 0438469), the Department of Energy (Contract Number DE-FG3608GO18142), and the Department of Defense (Contract Number N00164-08-C-GS37), the California Energy Commission (Contract Number 500-08-022), and the Southern 9134



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(17) Ahn, C. Henry’s Law and Isosteric Heats; Caltech, Pasadena, CA. (18) Marsh, H. Activated Carbon, 1st ed.; Elsevier: Amsterdam, The Netherlands, 2006. (19) Tay, T.; Ucar, S.; Karagöz, S. Preparation and characterization of activated carbon from waste biomass. J. Hazard. Mater. 2009, 165 (1− 3), 481−485. (20) Lillo-Ródenas, M. A.; Cazorla-Amorós, D.; Linares-Solano, A. Understanding chemical reactions between carbons and NaOH and KOH: An insight into the chemical activation mechanism. Carbon 2003, 41 (2), 267−275. (21) Wu, F.-C.; Tseng, R.-L.; Juang, R.-S. Preparation of highly microporous carbons from fir wood by KOH activation for adsorption of dyes and phenols from water. Sep. Purif. Technol. 2005, 47 (1−2), 10−19. (22) Vernersson, T.; Bonelli, P. R.; Cerrella, E. G.; Cukierman, A. L. Arundo donax cane as a precursor for activated carbons preparation by phosphoric acid activation. Bioresour. Technol. 2002, 83 (2), 95−104. (23) Rosas, J. M.; Bedia, J.; Rodríguez-Mirasol, J.; Cordero, T. HEMP-derived activated carbon fibers by chemical activation with phosphoric acid. Fuel 2009, 88 (1), 19−26. (24) Ravikovitch, P.; Neimark., A. Density Functional Theory Model of Adsorption on Amorphous and Microporous Silica Materials. J. Am. Chem. Soc. 2006, 22 (26), 11171−11179. (25) Neimark, A.; Lin, Y.; Ravikovitch, P.; Thommes, M. Quenched solid density functional theory and pore size analysis of micromesoporous carbons. Carbon 2009, 47 (7), 1617−1628. (26) Evans, M. J. B.; Halliop, E.; MacDonald, J. A. F. The production of chemically-activated carbon. Carbon 1999, 37 (2), 269−274. (27) Tanoue, K.; Wijayanti, W.; Yamasaki, K.; Nishimura, T.; Taniguchi, M.; Sasauchi, K. Numerical Simulation of Heat Transfer through the Pyrolysis of Woody Biomass. In Proceedings of the AIChE Annual Meeting, Nashville, TN, November 8−13, 2009.

California Gas Company (Contract Number SCG 5660020662). Their support is greatly appreciated.

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REFERENCES

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