Nanostructured Carbon Xerogels by Super-Fast Carbonization

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Nanostructured carbon xerogels by super-fast carbonization Ahmad Mohaddespour, Saeid Atashrouz, and Seyed Javad Ahmadi Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 11 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Nanostructured carbon xerogels by super–fast carbonization Ahmad Mohaddespour,∗,†,‡ Saeid Atashrouz,¶ and Seyed Javad Ahmadi† †Nuclear Science and Technology Research Institute, North Karegar Ave., Tehran, Iran, P.O. Box 14399-1113 ‡Chemical Engineering Department, McGill University, Montreal, QC, Canada H3A 0C5 ¶Amirkabir University of Technology (Tehran Polytechnic), Mahshahr Campus, Mahshahr, Iran., P.O. Box 15875-4413 E-mail: [email protected] Phone: +1 (778) 838–1362 Abstract Traditional synthesis methods of highly porous carbon xerogels impose many limitations on production in large scale such as low heat and mass transfer and long processing time. In this study, for the first time, recorsinol–formaldehyde (RF) xerogels were carbonized by fast heating rates (σ) in a fluidized bed reactor. The specific surface area (Φ) and pore volume of carbon xerogels were examined in terms of particle size (≈ 100 and 297 µm), carbonization temperature (298 to 1273 K), and σ (5 to 50 K min−1 ). The temperature above which Φ decreases by increasing temperature was shifted to lower values for larger particles. Moreover, Φ and volume of micro– and mesopores increased by increasing σ. Possible mechanisms to interpret the effects of carbonization temperature and σ on physical properties of carbon xerogels were finally furnished. Carbonization time was found to be ≈ 25 times faster by fluidization while maintaining the quality of xerogels.

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Introduction Porous carbon materials are widely used in a large variety of applications including supporting materials for catalysts, adsorbents, chromatography packing, 1–3 and, especially, as electrode materials for double layer capacitors and energy storage devices 4–11 because of their large surface area (Φ) and excellent thermal and chemical stability. 12 The performance of supercapacitors is mainly influenced by the specific area and pores size distribution which should be adapted to the size of ions in electrolyte. 13 In general, micropores which are inaccessible by electrolyte will result in low energy storage capacity below 200 F g−1 . 12 Therefore, careful attention should be made to hold a controllable hierarchical structure in order to achieve supercapacitor electrode materials with high electric double-layer capacitance of ≈ 200–350 F g−1 . 14 However, the considerably high cost of production of these porous carbon materials has limited their applications in large scales. Resorcinol–formaldehyde (RF) gels are the commonly used materials to produce porous carbon because of their high porosity (> 80%), surface area (400 − 1200 m2 g−1 ), and pore volumes. 15,16 However, these characteristics markedly depend on synthesis and processing conditions. Overall, RF gels undergo two main stages during synthesis: (I) sol mixture preparation, gelation, and curing and (II) drying of the wet gel. The next step to produce porous carbon is to carbonize the obtained dried RF xerogel. The conditions for carbonization (pyrolysis) such as the carbonization temperature and thermal activation time are the most important factors determining the final characteristics of porous carbon. 15–20 Not only the final carbonization temperature is important in achieving the desired porosity, but also the heating rate (σ) during the carbonization step can influence the structure and characteristics of the final product. 21 In general, the influence of σ on the final properties of porous carbon has rarely been reported in the literature. 22–25 Inagaki et al. 22 studied the effect of σ during carbonization on graphitization of carbon films derived from commercially available aromatic polyimide films. Higher shrinkage of coke at higher σ was also reported without in-depth discussion 2 ACS Paragon Plus Environment

Page 2 of 25

Page 3 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


about pore size and pore volume. 23 Marcilla et al. 24,25 showed that low σ ≈ 5 K min−1 in the pyrolysis step (from 548 to 673 K) slightly affects the porosity development of chars from almond shells when d¯p ≈ 1.13 mm, while high σ (flash) caused an increase in Φ from 1173 to 1217 cm2 g−1 having two activation times of 4 and 5 h, respectively. In the latter, inevitably, the difference in Φ can also be attributed to the different activation times rather than solely high σ. The authors also found that all the activated carbons obtained after ≈ 5 h were essentially microporous. Nevertheless, the influence of σ on RF xerogel’s has not been reported in the literature. Moreno et al., 21 however, studied the impact of σ on Φ in a fixed bed reactor when the size of xerogel particles was smaller than 212 µm, where no significant change in Φ was perceived and no interpretation was provided by the authors for the trend observed. The speculation is that since small carbon xerogels have significantly low thermal conductivity, an increase in σ, specially when it is large (in this case, up to 50 K min−1 ), noticeably hinders thermal diffusion through the fixed bed. 26,27 Therefore, when handling moderate quantities of aerogels or xerogels (tens of grams), the temperature profile might not be even uniform in the bed, leading to non-uniformity in the final gel structure. The speculation is that limitations upon thermal diffusion might have hindered obtaining reliable data at large σ. For instance, the thermal conductivity of porous carbon having pores of ≈ 100 nm obtained by pyrolysis of RF aerogels is ≈ 0.05 W m−1 K−1 27 which is a factor of five to ten lower than that of established high–temperature thermal insulation materials such as carbon fiber felts or carbon foams currently available. 28 Decrease in the pore size also suppresses porous carbon’s thermal conductivity 27 which can happen during carbonization at higher temperatures. 24,25 Therefore, this low thermal conductivity leads to a non-uniform temperature profile in the bed which is intensified in larger scales. The essential reason is morphology of highly porous carbon which allows suppressing the thermal conductivity because of small pores. These pores limit the mean free path of gas molecules at high temperatures which effectively diminishes the thermal heat transfer in the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

reactor. 28 The relatively high heat capacity of carbon aerogels also requires higher heating flux and time so that all carbon xerogel particles can reach the desired temperature. 27 As a result, particles at the vicinity of the bed’s surface might suffer a higher temperature than desired to allow particles inside the bed reaching their final temperature. 29 Therefore, Φ at the vicinity of the bed surface might decrease due to the collapse of micopores. 21 This inhomogeneity in the temperature profile is expected to be more challenging when producing carbon xerogels in large scales using conventional fixed bed furnaces. To conceptually show the feasibility of overcoming limitations in large scale carbonization and to significantly reduce the carbonization time, in this study, a fluidized bed reactor is used for the first time to carbonize RF xerogels prepared by polycondensation. Gas–solid fluidized beds are used extensively in industries because of their excellent mixing ability and high heat and mass transfer rates between particles and the fluidizing medium. 30–33 It is shown that applying fluidization to pyrolyse RF xerogels results in a much faster carbonization process compared to that of traditional fixed bed (≈ 25 times) by increasing the heat and mass transfer rates between the inert gas and xerogel particles while maintaining quality in terms of Φ, pore size, and pore volume. Moreover, the effects of final carbonization temperature and σ on Φ for different particle sizes are studied and possible mechanisms to interpret the observed trends are provided.

Experimental Synthesis of the organic xerogels Aqueous organic xerogels were synthesized by the polycondensation of resorcinol (R) and formaldehyde (F) using deionised water as solvent and a sodium hydroxide solution (1 M) as basification agent. To prepare precursor solutions, resorcinol (Sigma Aldrich, 99%) was first dissolved in deionised water under magnetic stirring. Then, formaldehyde (Sigma Aldrich F1635, 10–15% methanol as stabilizer, 37 wt.% in H2 O) was added to the mixture. The 4 ACS Paragon Plus Environment

Page 4 of 25

Page 5 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


resorcinol/formaldehyde molar ratio (R/F) and the dilution ratio (D) were fixed at 0.5 and 1:6, respectively. The dilution ratio is the total solvent/reactant molar ratio where solvent is deionised water and methanol solution and reactant is RF. The pH of solution was set to 5.8 to yield carbon xerogels with a good textural development. 34 The solutions were placed in beakers and, then, in an electrical oven at 358 K to undergo gelation, curing, and drying. Once gelation had taken place, the mass loss of the sample was measured in order to determine whether the synthesis had ended. In this case, the time required to reach constant mass was approximately 108 h. Finally, RF xerogels were milled and sieved into three categories: (I) dp,[3,2] ≈ 171 µm and dp,[4,3] ≈ 304 µm used for measuring the effects of temperature on fluidization characteristics, where dp,[3,2] and dp,[4,3] are Sauter and P P DeBroukere means of particles, respectively, defined as dp,[3,2] = ni=1 d3i νi / ni=1 d2i νi and Pn 4 Pn 3 dp,[4,3] = i=1 di νi where d and ν are particle size and volume percentage of i=1 di νi / particles with diameter d, respectively; (II) d¯p ≈ 100 µm; and (III) d¯p ≈ 297 µm, where d¯p is the mean particle size. The latter two categories were used to measure the effects of temperature on Φ while only the second category was undertaken for the effect of σ on Φ and pore volumes.

Carbonization The dried RF xerogel was carbonized using a fluidised bed reactor (ID= 3 cm, height= 30 cm) when u/um ≈ 3, where u and um are gas velocity and the minimum gas velocity required for fluidization, respectively. The scheme of this fluidized bed reactor is illustrated in figure 1. Before heating the fluidized bed reactor, nitrogen flow was sent to the reactor for 5 min in order to purge any remaining air in RF xerogel. Then, the carbonization step was carried out within three categories: (I) to obtain the relation between um and temperature, samples were carbonized from ambient temperature to 473, 673, 873, and 1073 K when σ = 20 K min−1 ; (II) to assess the effects of carbonization temperature on Φ, samples were carbonized from ambient temperature to 923, 1023, 1073, 1123, 1173, 1223, and 1273 K when σ = 20 K 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

min−1 ; and (III) to examine the effects of σ on Φ, size, and volume of pores, samples were carbonized from ambient temperature to 1173 K when σ = 5, 10, 20, 30, 40, and 50 K min−1 . The oven used was not capable of providing σ > 50 K min−1 . For all experiments, the reactor was allowed to cool down to the room temperature under nitrogen flow. After each experiment, samples were collected by removing the reactor’s cap and using rotation. The gas velocity was expected to increase by increasing temperature in the reactor. Note that at large gas velocities (u > 10um ), discharge of particles occurs. 35 Therefore, in all experiments, the gas velocity in the reactor was maintained unchanged with increasing temperature at ≈ 3um by synchronizing the temperature inside the bed with heat flux and gas flow using a programmed heating system connected to a digital flow meter (Sierra, SmartTrak 100). The temperature inside the reactor was measured using a thermocouple inserted from the top of reactor through the bed next to the distributor. To measure the pressure drop (∆P ) at different positions, three pressure transducers were used below and above the distributor as well as before the reactor’s outlet. Measuring the pressure drop assists us in understanding the fluidization quality as well as finding um with regard to the designated particle characteristics. The nominal maximum pore size of the porous quartz distributor (disk) at the bottom of the fluidized bed was 10–16 µm much less than the minimum RF xerogel particle size preventing small particles penetrating through and blocking the available pores. A similar distributor (as a filter) was placed on the top of the fluidized bed to prevent very fine particles leaving the reactor. However, a hole were made through the top filter for thermocouple insertion. Before starting the fluidization experiments, in order to recognize different stages of carbonization, a single temperature–programmed carbonization experiment was performed in a thermogravimetric analyzer (TA Instruments, TGA 550), where 20 mg of the RF xerogel (dp,[3,2] ≈ 171 µm and dp,[4,3] ≈ 304 µm) was placed in an alumina crucible and heated at σ = 5 K min1 from ambient temperature to 1073 K under a nitrogen flow rate of 20 mL min1 .


Page 6 of 25

GAS OUTLET

VENT

FILTER

ELECTRICAL HEATING

DAQ LAB VIEW COMPUTER

HEATING ZONE

TEMPERATURE CONTROLLER

TC

PT 3

HEATING ZONE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ICE & WATER

Page 7 of 25

PT 2

PT 1

MFC

GAS

DIGITAL MFC

NITROGEN

Figure 1: Scheme of the fluidized bed reactor used for fast carbonization of RF carbon xerogels (not to be scaled).

Structure and physical properties The textural properties of organic and carbon xerogels were measured from nitrogen adsorption– desorption isotherms at 77 K (ASAP 2400). All samples were heated to 293 K for 2 h under vacuum to remove all of the physisorbed species before measuring the adsorption isotherms. The micropore volume was calculated from the amount of N2 adsorbed at a relative pressure of 0.1, and the mesopore volume was calculated by subtracting the amount adsorbed at a relative pressure of 0.1 from that at a relative pressure of 0.95. 36 Calculation of φ was based on BET and the pore size distribution analysis in the mesopore range was performed using the BJH method applied to the desorption branch, and the full micro-meso pore size distribution was calculated using the 2D–NLDFT–HS model assuming surface heterogeneity of carbon pores. 37 True density of xerogel particles was measured by a pycnometer (micro-Ultrapyc 1200e). Before measuring the helium density, xerogel particles were heated to 473 K for 2 h under vacuum to clean their surface.



Results and Discussion Fluidization characteristics The challenge in using a fluidized bed reactor is to find the critically important gas velocity for minimum fluidization conditions. This velocity is a function of particle properties such as size and density as well as gas properties such as viscosity and density where all are functions of temperature changing during the carbonization process. Fluidization cannot be achieved with low gas velocities while having large gas velocities leads to the discharge of particles, thereby adversely influencing the structural properties of carbon xerogels such as Φ and pore size distribution. More profoundly, shape of particles affects the packing properties of the fixed bed, the associated void spaces, and velocity of fluid through them. To find um , experimental and theoretical approaches can be used. However, some methods rely on a direct correlation: 38 3 2.5 2 ξ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 25

1.5 1 0.5 0 1

2

3

4

5

6

T /T∞

Figure 2: Influence of temperature on the properties of fluidization gas (N2 ) 39 and carbon xerogel particles in this study. ξ is a normalized parameter representing nitrogen density (ρN2 , •), nitrogen viscosity (µN2 , N), carbon xerogel’s particle size (dp,[3,2] , •; dp,[4,3] , N) and density (ρp , N). Each parameter is normalized with its corresponding value at ambient temperature (T∞ = 298 K): ρN2 ,∞ = 1.17 kg.m−3 , µN2 ,∞ = 1.76 × 10−5 Pa.s, dp,[3,2],∞ = 171 µm, dp,[4,3],∞ = 304 µm, and ρp,∞ = 1200 kg m−3 . Note that dp,[3,2] and dp,[4,3] are Sauter mean andÂă DeBroukere mean of particles, respectively (Adapted in part with permission from 39 Copyright 2010 AIP).


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


um =

0.9 9.0 × 10−4 d1.8 p [(ρp − ρg )g] ρg0.066 µ0.87 g

(1)

where ρp and ρg are particle and gas densities, respectively, dp is the particle diameter, µg is the gas dynamic viscosity, and g is gravitational acceleration. Note that in Equation 1, ρg and µg depend on temperature. In the case of RF xerogel pyrolysis, dp and ρp also depend on temperature, because RF xerogel particles undergo shrinkage during pyrolysis, hence directly influencing these two factors. 25 20

15

V(%)

Page 9 of 25

10

5

0 0 10

1

10

10

2

3

10

dp (µm)

Figure 3: Particle size distribution of organic and carbon xerogels at different temperatures: RF xerogel at 298 K (blue, dp,[3,2] ≈ 171 and dp,[4,3] ≈ 304 µm), carbon xerogels at 473 K (red, dp,[3,2] ≈ 160 and dp,[4,3] ≈ 276 µm), 673 K (green, dp,[3,2] ≈ 65 and dp,[4,3] ≈ 150 µm), 873 K (magenta, dp,[3,2] ≈ 47 and dp,[4,3] ≈ 110 µm), and 1073 K (black, dp,[3,2] ≈ 41 and dp,[4,3] ≈ 75 µm), using a fluidized bed reactor when u/um ≈ 3, σ = 20 K min−1 , and isothermal stage for 5 min. The influence of temperature on the size and density of carbon particles and also on the properties of the fluidization gas (ρN2 and µN2 ) are illustrated in Figure 2. The density of particles increased by increasing carbonization temperature almost by 60% while their size decreased due to the shrinkage. Xerogel’s particle size distributions (PSD) for initial organic and carbon xerogels at different temperatures are shown in Figure 3. As illustrated, increasing the carbonization temperature yields a decrease in the carbon xerogels’ particle size while increasing the particles’ density which can be due to the opening of closed micropores 9 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 25

and shrinkage. 40,41 By experimentally measuring the particle properties in terms of ρp and dp and knowing the gas properties at different temperatures, um can be calculated from Equation 1. The normalized fluidization velocity um /um,T∞ , where um,T∞ is the minimum fluidization velocity at T∞ = 298 K, at different carbonization temperatures is shown in Figure 4. Obviously, considering dp,[4,3] results in a larger um . Thus, the average of um from Equation 1 for dp,[3,2] and dp,[4,3] should be considered at each carbonization temperature. Determining um as a function of temperature is important because u needs to be adjusted according to the particles’ characteristics at different temperatures. Otherwise, at large u, discharge of particles will occur, hence ultimately influencing the final characteristics of carbon xerogels such as surface area and pore size distribution. For example, in the case of this study, um decreases by temperature. Therefore, fixing um at um,T∞ will result in the discharge of particles at elevated temperature if only the effect of temperature in the gas phase is considered to control the gas flow rate. Thus, u in experiments should be synchronized by the bed temperature using the digital flow meter such that u/um ≈ 3 can be always respected in order to have fluidization and, simultaneously, augmented heat and mass transfer without losing very fine particles. The experimental data shown in Figure 4 was fitted with an exponential expression:

um /um,T∞ = 1 − exp(−α × β T /T∞ ),

(2)

where α ≈ −77 and β ≈ 0.092. This expression is used to synchronize the gas flow with the bed temperature according to the gas and particle properties such as dp , ρp , ρg , and µg while all are functions of T . In addition to finding um by Equation 2 obtained by using a combination of material properties at different temperatures and an empirical expression (Equation 1), um can be obtained experimentally by direct measurement of the relationship between ∆P (the dif-


Page 11 of 25

1 0.8 um /um,T∞

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6 0.4 0.2 0 1

1.5

2

2.5 T /T∞

3

3.5

4

Figure 4: Minimum fluidization velocity (um ) of RF carbon xerogels versus carbonization temperature normalized by um at ambient temperature (T∞ = 298 K) obtained by replacing experimental data presented in Figure 2 into Equation 1 in a fluidized bed reactor with two initial average particle sizes at ambient temperature: dp,[3,2] (N, ≈ 171 µm) and dp,[4,3] (•, ≈ 304 µm). The blue curve was obtained by fitting the data to an exponential expression um /um,T∞ = 1 − exp(−α × β T /T∞ ) where α ≈ −77, β ≈ 0.092, and um,T∞ ≈ 0.0096 and 0.027 m s−1 when dp,[3,2] and dp,[4,3] ≈ 171 and 304 µm, respectively. ference between the pressure measured by PT2 and PT3 as shown in Figure 1) and u. As diagrammed in Figure 5, ∆P increases with flow rate until the bed expands and increases the porosity. Note that the relationship between ∆P and u is not necessarily linear. Upon further increase in u, ∆P remains approximately constant. If the process is reversed by steadily lowering u, different data are obtained due to the different voidage resulting from the rearrangement of the particles. This method defines dashed blue and red lines in Figure 5 for increasing and decreasing u, respectively. The minimum fluidization velocity is the velocity at which these two lines intercept. 42 The results illustrated in Figure 5 were obtained at ambient temperature. To adjust the gas flow through the fluidized bed and, hence u, the um obtained by this method at ambient temperature is used to program the digital flow meter using Equation 2. Note that um obtained experimentally at T∞ using Figure 5 also accords with the results obtained by Equation 1.



0.9 A 0.8 0.7 0.6 ∆P/Pmax

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.5 0.4 0.3 0.2 0.1 0 0

um 0.005

0.01

0.015 0.02 u (m s−1 )

0.025

0.03

0.035

Figure 5: Minimum fluidization velocity by direct measurement of ∆P with increasing (N) and decreasing (•) gas flow in the fluidized bed reactor for RF xerogel (see Figure 3) at ambient temperature where Pmax = mg/A (m and A are mass (≈ 30 g) and cross sectional area of the bed (ID = 3 cm), respectively. Point A (um ≈ 0.017 m s−1 ) represents the intercept of dashed blue and red lines fitted to increasing and decreasing flow patterns, respectively.

Thermogravimetric analysis In order to design the carbonization experiments by the fluidized bed reactor, thermogravimetric analysis of RF xerogel was performed and the results are depicted in Figure 6. Mass loss ≈ 50% occurs up to ≈ 1000 K below which three stages of degradation can be recognized: 43 (I) below 373 K: extraction of the remaining solvent and/or the elimination of H2 O formed from the condensation of OH groups; (II) 600–750 K: release of organic molecules, elimination of hydrogen and oxygen atoms from the polymer network as CO2 and CH4 , and desorption of adsorbed organic compounds; and (III) above 750 K: continuing elimination of carbon, oxygen, and hydrogen. In conventional carbonization processes using tube furnace reactors, it is necessary to have isothermal steps in the heating process for acquiring complete carbonization due to the aforementioned, limited heat and mass transfer. Using a fluidized bed reactor by markedly eliminating these limitations is expected to remove the necessity of having these isotherms in the conventional carbonization process and to reduce the time required for the carbonization step .


Page 12 of 25

100

80

TG (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20

0 300

400

500

600

700 T (K)

800

900

1000

900

1000

(a)

0.25 DTG (mg min−1 )

Page 13 of 25

0.2 0.15 0.1 0.05 0 300

400

500

600

700 T (K)

800

(b)

Figure 6: Thermogravimetric analysis of RF xerogel: (a) mass loss and (b) mass loss rate at σ = 5 K min−1 .



Particle size effects The carbonization temperature from 973 to 1223 K was shown to reduce Φ for carbon xerogels when dp > 212 µm while it was almost constant (from 653 to 656 m2 g−1 ) when dp < 212 µm. 21 However, PSD for these two ranges of particle size were not provided and the original organic xerogel was sieved to separate particle above and below 212 µm. 21 Therefore, dp in their study represented particles having smaller diameters than a particular particle size (212 µm) rather than the mean or average value. The results in Figure 7 manifest that when T > 1073 K and d¯p ≈ 297 µm, Φ for carbon xerogels decreased with increasing temperature in the fluidized bed reactor. One possible explanation is that by increasing the carbonization temperature, the structure is reorganized leading to a partial collapse of micropores, and hence to a smaller Φ. 850 800 750 700 Φ (m2 g−1 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Tc

650 600 550 500 450 400

900

1000

1100 T (K)

1200

1300

Figure 7: The effects of carbonization temperature on Φ for carbon xerogels when d¯p ≈ 100 (N) and ≈ 297 µm (•) at σ = 20 K min−1 followed by an isothermal step for 5 min. Blue and red curves are fitted to experimental data to demonstrate the trends of data. Tc (≈ 1173 and 1025 K when d¯p ≈ 100 and ≈ 297 µm, respectively) represents the temperature where Φ is maximum. Not only Φ decreased when d¯p ≈ 297 µm at T > 1073 K, but also it decreased when d¯p ≈ 100 µm, however, at T > 1173 K. Note that Φ for organic xerogels is inherently smaller than that of carbon xerogels except from carbon xerogels carbonized at very high temperatures where partial micropores’ collapsing dominates the mechanism of changes in 14 ACS Paragon Plus Environment

Page 14 of 25

Page 15 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Φ. Thus, in tracking changes to Φ by increasing temperature, it is common to expect an increase, primarily before 873 K 16 due to elimination of organic groups. It is also reported that Φ for carbon aerogels is adversely decreased when T > 873 K. 40 The results of Moreno et al. 21 for small particles (dp < 212 µm) also do not accord with the trend observed for carbon aerogels having d¯p ≈ 27.5 µm, where Φ increased from ≈ 670 to 800 m2 g−1 when temperature increased from 773 to 1073 K, respectively, while Φ decreased to 740 m2 g−1 at T = 1173 K. 44 Carbon xerogel

mesopores

micropores

Increasing carbonization 𝑇 Increasing 𝜎 (increasing 𝑝𝑖 ) Mechanism I: Micropores collapse to mesopores

Mechanism II: Increase of micropores due to burnout of organic compounds (H and O elimination)

Decrease in Φ Increase in Φ

Figure 8: Possible mechanisms of changes in the surface area and pore volume of carbon xerogels observed in Figure 7 and Table 1. Increase in two different driving forces (carbonization temperature or σ) can influence Φ by two different mechanisms. pi is the partial pressure of species i in the gas phase inside the pore.

The results of fluidized bed’s experiments reveal that the critical temperature Tc above which Φ is declined is shifted to higher temperature when d¯p ≈ 100 µm compared to that of d¯p ≈ 297 µm. The two possible mechanisms to explain changes in Φ (Figure 7) by increasing carbonization temperature is illustrated in Figure 8. When T < Tc , increasing the temperature results in proliferation of micropores due to the removal of H and O atoms and burnout 15 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 25

of organic groups. On the other hand, by increasing temperature, micropores collapse and form mesopores which, in turn, reduce Φ when T > Tc . Since larger particles inherently have lower Φ, Tc is expected to occur earlier as confirmed by fluidized bed experiments. Table 1: Characteristic pore properties of RF carbon xerogels at different σ from ambient temperature up to 1173 K followed by an isothermal step for 5 min when d¯p ≈ 100 µm. SD is standard deviation from three replicates. σ (K min−1 ) 5 10 20 30 40 50

Φ g−1 ) 678 701 792 815 859 871

(m2

SD g−1 ) ±15 ±17 ±18 ±21 ±23 ±17

(m2

Vmic (cm3 g−1 ) 0.29 0.31 0.33 0.34 0.37 0.38

Vmes (cm3 g−1 ) 0.40 0.41 0.44 0.45 0.48 0.50

¯ D (nm) 8.1 7.2 6.8 6.7 6.1 5.8

The effects of heating rate σ (K min−1 ) on Φ for carbon xerogels carbonized from ambient temperature to 1173 K in the fluidized bed reactor when d¯p ≈ 100 µm are summarized in Table 1. The experimental data in the literature for the effects σ on Φ is really rare. However it was shown that Φ increases by σ from 5 to 20 K min−1 while it almost remains constant up to σ = 50 K min−1 21 and no interpretation was provided by the authors to elaborate on the trend observed. Note that according to the existing limitations in terms of insulation characteristics of carbon xerogels in large scale, thermal diffusion to the fixed bed might be effectively hindered, especially when σ is large. However this ostensible reason might not interpret the unchanged Φ observed by increasing σ from 20 to 50 K min−1 in , 21 many points of ambiguity remain untouched. Changes in Φ by increasing σ for RF carbon xerogels are summarized in Table 1 and adsorption-desorption profiles of N2 are shown in Figure 9. Applying σ from 5 to 50 K min−1 resulted in increasing Φ from 678 to 871 m2 g−1 . It is speculated that by increasing the carbonization temperature, H and O atoms are detached from carbon xerogel particles. These atoms at elevated temperature can collide with micro– and mesopores’ wall on their paths to


Page 17 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


exit the particle’s tortious porous structure and form CO2 and CH4 , thereby increasing Φ. By increasing σ, it is expected that large amounts of H and O atoms are abruptly released and, thus, partial pressure of these species increases in the produced pores. Therefore, the number of collisions to the pores’ walls is escalated, thereby augmenting formation and, on the other hand, partial collapse of micropores. Figure 8 illustrates these possible mechanisms though which Φ and pore volume are influenced by increasing carbonization temperature and σ. The pore volume of carbon xerogels when d¯p ≈ 100 µm at different σ is also summarized in Table 1 obtained by the BJH method from desorption branch of the isotherm. The results ¯ decreases. exhibit that the volume of micro– and mesopores increases by increasing σ while D Hypothetically, higher heating rates give rise to the organic materials’ partial pressure inside pores which, accordingly, increases the volume of pores. Increase in the partial pressure of species can lead to two phenomena: (I) patrial collapse of micropores to mesopores, and (II) formation of more micropores. While the former diminishes Φ, the latter vice versa. These two possible mechanisms are depicted in Figure 8 while the driving force in the case of increasing σ is the elevated partial pressure of species rather than increasing temperature. Note that to use carbon xerogels in supercapacitors, an activation step, e.g. by CO2 , can considerably increase Φ and control the size of micro– and mesopores. 29 Increasing σ in the carbonization step when CO2 is used for activation might lead to more interesting results. Therefore, this study only focused on the effects of σ, dp , and carbonization temperature on Φ and volume of pores in the pyrolysis step as preliminary experiments in a fluidized bed reactor inspiring researchers for further studies in this field.

Conclusions A fluidized bed reactor which has many advantages over conventional tube furnaces was used for the first time to carbonize resorcinol–formaldehyde (RF) xerogels prepared by poly-



1600 1400 1200 V (cm3 g−1 ) STP

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 25

1000 800 600 400 200 0 0

0.2

0.4

0.6

0.8

1

P/P◦

Figure 9: Influence of carbonization heating rate σ on the adsorption-desorption profiles of N2 at 77 K on the RF carbon xerogels from ambient temperature up to 1173 K followed by an isothermal step for 5 min when d¯p ≈ 100 µm and σ = 5 (♦ and ), σ = 10 (◦ and •), 20 (M and N), 30 (♦ and ), 40 (O and H), and 50 K min−1 ( and ). Note that filled symbols represent desorption isotherms. condensation. Minimum fluidization velocity was evaluated using empirical expressions and direct experimental measurements. Therefore, all fluidization experiments were performed when u/um ≈ 3 for all carbonization temperatures up to 1273 K. The effects of particle size, carbonization temperature, and heating rate on the surface area and pore volume of carbon xerogels were examined. The surface area increased up to a critical temperature while decreased thereafter. Increasing the particle size from d¯p ≈ 100 to 297 µm shifted this critical temperature from 1173 to 1073 K. Moreover, the surface area increased from 678 to 871 m2 g−1 when heating rate increased from 5 to 50 K min−1 . Possible mechanisms for these trends were provided. It is speculated that the increase in partial pressure of gaseous species at higher heating rates can augment collisions of atoms to micro– and mesopores’ walls and, hence increase the surface area. Moreover, the volume of micro– and mesopores increased ≈ 20 and 17% by increasing heating rate from 5 to 50 K, respectively. The results of this study suggests that the carbonization process of organic gels can be carried out at much higher heating rates up to 50 K min−1 in the carbonization step while maintaining or improving the product’s quality. Fluidization of organic gels seems to be a promising start 18 ACS Paragon Plus Environment

Page 19 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


for synthesis of porous carbon materials in large scale. However, the effects of much higher heating rates on the carbonization of organic gels might be examined in further research.

Acknowledgement This work has been funded by Iran Nanotechnology Initiative Council and Iran National Elites Foundation. The authors thank Nuclear Science and Technology Research Institute of Iran for its strong support allowing the authors to perform experiments and complete all the required characterizations.

References (1) Moreno-Castilla, F., C. andMaldonado-Hodar Carbon aerogels for catalysis applications: An overview. Carbon 2005, 43, 455 – 465. (2) Job, N.; Pereira, M. F. R.; Lambert, S.; Cabiac, A.; Delahay, G.; Colomer, J.-F.; Marien, J.; Figueiredo, J. L.; Pirard, J.-P. Highly dispersed platinum catalysts prepared by impregnation of texture-tailored carbon xerogels. J. Catal. 2006, 240, 160 – 171. (3) Wu, Z.; Zhao, D. Ordered mesoporous materials as adsorbents. Chem. Commun. 2011, 47, 3332–3338. (4) Kakunuri, M.; Vennamalla, S.; Sharma, C. S. Synthesis of carbon xerogel nanoparticles by inverse emulsion polymerization of resorcinol–formaldehyde and their use as anode materials for lithium–ion battery. RSC Adv. 2015, 5, 4747–4753. (5) Pekala, R. W. Organic aerogels from the polycondensation of resorcinol with formaldehyde. J. Mater. Sci. 1989, 24, 3221–3227. (6) Mayer, S. T.; Pekala, R. W.; Kaschmitter, J. L. The aerocapacitor: An electrochemical double–layer energy–storage device. J. Electroch. Soc. 1993, 140, 446–451. 19 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(7) Frackowiak, E.; Beguin, F. Carbon materials for the electrochemical storage of energy in capacitors. Carbon 2001, 39, 937 – 950. (8) Tonanon, N.; Wareenin, Y.; Siyasukh, A.; Tanthapanichakoon, W.; Nishihara, H.; Mukai, S. R.; Tamon, H. Preparation of resorcinol formaldehyde (RF) carbon gels: Use of ultrasonic irradiation followed by microwave drying. J. Non-Cryst. Solids 2006, 352, 5683 – 5686. (9) Zhu, Y.; Hu, H.; Li, W.; Zhang, X. Resorcinol-formaldehyde based porous carbon as an electrode material for supercapacitors. Carbon 2007, 45, 160 – 165. (10) Zhao, X.; Wang, A.; Yan, J.; Sun, G.; Sun, L.; Zhang, T. Synthesis and electrochemical performance of heteroatom-incorporated ordered mesoporous carbons. Chem. Mater. 2010, 22, 5463–5473. (11) Hasegawa, G.; Aoki, M.; Kanamori, K.; Nakanishi, K.; Hanada, T.; Tadanaga, K. Monolithic electrode for electric double-layer capacitors based on macro/meso/microporous S-containing activated carbon with high surface area. J. Mater. Chem. 2011, 21, 2060– 2063. (12) Zhang, L. L.; Zhao, X. S. Carbon-based materials as supercapacitor electrodes. Chem. Soc. Rev. 2009, 38, 2520–2531. (13) Largeot, C.; Portet, C.; Chmiola, J.; Taberna, P.-L.; Gogotsi, Y.; Simon, P. Relation between the Ion Size and Pore Size for an Electric Double-Layer Capacitor. J. Am. Chem. Soc. 2008, 130, 2730–2731. (14) Lee, J.; Kim, J.; Hyeon, T. Recent progress in the synthesis of porous carbon materials. Adv. Mater. 2006, 18, 2073–2094. (15) Job, N.; Panariello, F.; Marien, J.; Crine, M.; Pirard, J.-P.; Leonard, A. Synthesis


Page 20 of 25

Page 21 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


optimization of organic xerogels produced from convective air–drying of resorcinol– formaldehyde gels. J. Non-Cryst. Solids 2006, 352, 24 – 34. (16) Al-Muhtaseb, S.; Ritter, J. Preparation and properties of resorcinol–formaldehyde organic and carbon gels. Adv. Mater. 2003, 15, 101–114. (17) Tamon, H.; Ishizaka, H.; Mikami, M.; Okazaki, M. Porous structure of organic and carbon aerogels synthesized by sol-gel polycondensation of resorcinol with formaldehyde. Carbon 1997, 35, 791 – 796. (18) Hasegawa, G.; Kanamori, K.; Nakanishi, K. Facile preparation of macroporous graphitized carbon monoliths from iron–containing resorcinol–formaldehyde gels. Mater. Lett. 2012, 76, 1 – 4. (19) Xu, J.; Wang, A.; Zhang, T. A two–step synthesis of ordered mesoporous resorcinol– formaldehyde polymer and carbon. Carbon 2012, 50, 1807 – 1816. (20) Czakkel, O.; Marthi, K.; Geissler, E.; Laszlo, K. Influence of drying on the morphology of resorcinol–formaldehyde-based carbon gels. Micropor. Mesopor. Mat. 2005, 86, 124 – 133. (21) Moreno, A.; Arenillas, A.; Calvo, E.; Bermudez, J.; Menendez, J. Carbonisation of resorcinol–formaldehyde organic xerogels: Effect of temperature, particle size and heating rate on the porosity of carbon xerogels. J. Anal. Appl. Pyrol. 2013, 100, 111 – 116. (22) Inagaki, M.; Hishiyama, Y.; Kaburagi, Y. Effect of heating rate during carbonization on graphitization of carbon films derived from aromatic polyimides. Carbon 1994, 32, 637 – 639. (23) Rosenblatt, U.; Schmidt, P. The influence of the heating rate on the properties of carbon materials. Carbon 1982, 20, 146 – 157.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(24) Marcilla, A.; Asensio, M.; Martin-Gullon, I. Influence of the carbonization heating rate on the physical properties of activated carbons from a sub–bituminous coal. Carbon 1996, 34, 449 – 456. (25) Marcilla, A.; Garcia-Garcia, S.; Asensio, M.; Conesa, J. Influence of thermal treatment regime on the density and reactivity of activated carbons from almond shells. Carbon 2000, 38, 429 – 440. (26) Kim, S.; Yeo, D.; Lim, J.; Yoo, K.; Lee, K.; Kim, H. Synthesis and characterization of resorcinol–formaldehyde organic Aerogel. J. Chem. Eng. Jpn. 2001, 34, 216–220. (27) Hemberger, F.; Weis, S.; Reichenauer, G.; Ebert, H.-P. Thermal transport properties of functionally graded carbon aerogels. Int. J. Thermophys. 2009, 30, 1357–1371. (28) Wiener, M.; Reichenauer, G.; Braxmeier, S.; Hemberger, F.; Ebert, H.-P. Carbon aerogel–based high–temperature thermal insulation. Int. J. Thermophys. 2009, 30, 1372–1385. (29) Rasines, G.; Macias, C.; Haro, M.; Jagiello, J.; Ania, C. O. Effects of CO2 activation of carbon aerogels leading to ultrahigh micro–meso porosity. Micropor. Mesopor. Mat. 2015, 209, 18 – 22. (30) van Ommen, J. R.; Valverde, J. M.; Pfeffer, R. Fluidization of nanopowders: a review. J. Nanopart. Res. 2012, 14, 737. (31) Hong, S.; Lee, D.; Oh, C.; Kim, Y. Fluidized bed drying effect on the aerogel powder synthesis. J. energy eng. 2012, 21, 43–46. (32) Wang, D.; McLaughlin, E.; Pfeffer, R.; Lin, Y. Aqueous phase adsorption of toluene in a packed and fluidized bed of hydrophobic aerogels. Chem. Eng. J. 2011, 168, 1201 – 1208.


Page 22 of 25

Page 23 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(33) Hao, Z.; Zhu, Q.; Lei, Z.; Li, H. CH4–CO2 reforming over Ni/Al2O3 aerogel catalysts in a fluidized bed reactor. Powder Technol. 2008, 182, 474 – 479. (34) Calvo, E.; Juarez-Perez, E.; Menendez, J.; Arenillas, A. Fast microwave-assisted synthesis of tailored mesoporous carbon xerogels. J. Colloid Interf. Sci. 2011, 357, 541 – 547. (35) Pinchbeck, P.; Popper, F. Critical and terminal velocities in fluidization. Chem. Eng. Sci. 1956, 6, 57 – 64. (36) Rodriguez-Reinoso, F.; de D. Lopez-Gonzalez, J.; Berenguer, C. Activated carbons from almond shells. Carbon 1982, 20, 513 – 518. (37) Jagiello, J.; Olivier, J. P. Carbon slit pore model incorporating surface energetical heterogeneity and geometrical corrugation. Adsorption 2013, 19, 777–783. (38) Abrahamsen, A.; Geldart, D. Behaviour of gas–fluidized beds of fine powders part I. Homogeneous expansion. Powder Technol. 1980, 26, 35 – 46. (39) Span, R.; Lemmon, E. W.; Jacobsen, R. T.; Wagner, W.; Yokozeki, A. A reference equation of state for the thermodynamic properties of nitrogen for temperatures from 63 to 1000 K and Pressures to 2200 MPa. J. Phys. Chem. Ref. Data 2000, 29, 1361– 1433. (40) Zhang, S.; Wang, J.; Shen, J.; Deng, Z.; Lai, Z.; Zhou, B.; Attia, S.; Chen, L. The investigation of the adsorption character of carbon aerogels. Nanostruct. Mater. 1999, 11, 375 – 381. (41) Pekala, R.; Alviso, C.; Kong, F.; Hulsey, S. Aerogels derived from multifunctional organic monomers. J. Non-Cryst. Solids 1992, 145, 90 – 98. (42) Ye, M.; van der Hoef, M.; Kuipers, J. The effects of particle and gas properties on the fluidization of Geldart A particles. Chem. Eng. Sci. 2005, 60, 4567 – 4580. 23 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(43) Job, N.; Pirard, R.; Marien, J.; Pirard, J.-P. Porous carbon xerogels with texture tailored by pH control during sol–gel process. Carbon 2004, 42, 619 – 628. (44) Horikawa, T.; Hayashi, J.; Muroyama, K. Size control and characterization of spherical carbon aerogel particles from resorcinol–formaldehyde resin. Carbon 2004, 42, 169 – 175.


Page 24 of 25

Page 25 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Only 247x142mm (300 x 300 DPI)

ACS Paragon Plus Environment

Nanostructured Carbon Xerogels by Super-Fast Carbonization

Recommend Documents