For a long time, humans have applied charcoals as adsorbents and medical remedies. The earliest documented use was in 1777, when Fontana [1] reported that freshly fired charcoal cooled by mercury captured several types of gases many times its own volume. Scheele (1780) [2] also reported that air emitted by heating charcoal was recaptured once it had cooled. In 1814, Saussure [3] proposed that the ability of these solids to trap gases depended on the exposed surface area, while in 1843 Mischerlich [4] emphasized the important role of the pores within charcoal. These two factors, surface area and porosity, exert complementary roles in the adsorption capacity of charcoal as well as many other solids. In 1900, activated carbon was produced industrially instead of bone char during the sugar refining process [5]. Today, various types of activated carbon are mass-produced, and their application is developing in large city utilities, such as water purification, air purification, refrigerator deodorization, and military toxic gas removal. Additionally, many researchers are studying the development and application of high-performance activated carbon by controlling the size distribution of pores that develop on the surface and are developing new applications through the addition of catalysts. Therefore, while charcoal has been applied in various sectors throughout human history, it has now degenerated into a fuel source. Nonetheless, the recent discovery of new carbon materials such as carbon nanotubes (CNTs) [6] and graphene [7] has resulted in the development of new products with an emphasis on the properties of carbon materials, new fields of application, and the modification of carbon materials such as graphite and carbon black increased along with the need to develop new roles for activated charcoal [8-10].

Charcoal is categorized into black and white forms depending on the method of burning in a charcoal kiln. When black charcoal is baked at the carbonization temperature of 600–700°C the entrance and chimney passages of the kiln are sealed with stones and clay. When the remaining fire in the kiln is cooled, the product has no ash on its surface and presents a black color. White charcoal is produced by blowing air into the kiln at the stage of complete carbonization, and the gas generated during wood pyrolysis is burned. At 1,000°C, the almost complete charcoal is rested for a certain period and the red charcoal is removed rapidly to extinguish the fire. The ash adheres to the surface of the charcoal giving it a white color. The carbon structure of charcoal consists of microcrystals similar to graphite, surrounded by amorphous parts. The size of the microcrystals increases as the degree of carbonization increases, and if a crystal develops, the electric conductivity increases and the electric resistance decreases [11]. Recently, studies in Japan documented changes in internal structure and surface morphology during high-temperature treatment to produce artificial graphite from charcoal [12-15]; however, information is lacking due to few subsequent studies. In this study, white charcoal produced from traditional South Korean kilns was thermally treated up to 2,400°C to analyze changes in the fixed carbon content, surface morphology, and porosity, and to explore new fields of application.

Materials

An oak white charcoal lump was used in this study, as shown in Figure 1A, which was produced from a traditional charcoal kiln in Sangdong-eup, Taebaek-gun, Gangwon-do. This lump was broken into pieces as shown in Figure 1B, and then finely ground with a ball mill to prepare a powder, as shown in Figure 1C after passing through a 325 mesh (<44 ????m) standard sieve.

Figure1: Oak tree-based white charcoal: (A) As received, (B) flake, and (C) powder.

A self-built graphite-moderated reactor is shown in Figure 2. White charcoal was subjected to high-temperature treatment in two steps. The first step involved 1 h carbonization of fragmented and powdered white charcoal in nitrogen gas up to 1,500°C at each temperature. The second step involved high vacuum treatment of the graphite-moderated reactor at 1,500°C to remove residual oxygen and gases before switching to argon gas. The process was based on the graphitization of white charcoal powder for 1 h at 1,800°C, 30 min at 2,000°C, and 10 min at 2,400°C.

Figure 2: Graphite furnace.

Analysis

The samples used in the experiment were weighed on a precision electronic balance before thermal treatment to determine the yield. (1) General analysis: the yield was measured with a thermal analyzer (Mettler-Toledo TGA/DSC1), and the fixed carbon was measured by elemental analysis with an automatic elemental analyzer II (C, H, N, S) (ThermoFisher Scientific FLASH 2000, SEM/EDS). (2) Surface characterization: the surface morphology was observed and analyzed with a scanning electron microscope (SEM) ion beam sectioning machine (CP: Cross section Polisher JEOL SM-09010), and the porosity characteristics was analyzed by specific surface area (measured from surface area measuring device, Micromeritics ASAP 2400, USA, from nitrogen adsorption at 77 K), total pore volume (determined through the Brunauer-Emmett-Teller (BET) equation [15]), the micropore volume (determined by the t-plot [16]), and the average pore size (determined through the Horvath-Kawazoe (HK)-plot [17]).

General Analysis

Thermal Analysis: The gaseous atmosphere thermogravimetric analysis (TGA) results for the raw white charcoal fragments are shown in Figure 3. White charcoal produced at about 1,000°C included a small amount of moisture and adsorbents. There was a 5.51 wt% weight loss with thermal treatment up to 200°C, followed by 7.62 wt% weight loss up to 1,520°C according to the first-order expression, and the residue was 86.87 wt%. After about 400°C, the functional groups remaining in the white charcoal were slowly decomposed in the air.

Figure 3: Thermal gravimetric analysis of white charcoal in the air (40–1,520°C, 10°C min-1.).

The change in compositional elements following high-temperature treatment of white charcoal is summarized in Table 1. The degree of carbonization of as-received white charcoal was low, having a carbon content of 79.36 wt%; however, thermal treatment at 1,100°C or above increased the degree of carbonization. It reached 99.88 wt% under 1,800°C treatment and 100 wt% under 2,000°C treatment. Hydrogen was completely removed under a 1,500°C nitrogen atmosphere, and nitrogen was completely removed under a 1,800°C argon atmosphere.

Table 1: Elemental analysis of high-temperature treated white charcoals(powder).

(O): obtained from EA(II) : 100-C-H-N-S.

Surface Morphology

White charcoal carbonization up to 1,500°C in a nitrogen atmosphere, and graphitization up to 2,400°C in an argon atmosphere were observed with SEM (Figure 4). The surface morphology of the cross-section and the side of the powder sample are shown. Comparing the cross-section and side of the white charcoal revealed that the raw material was activated due to the development of pores following the generation of gas along the axis of the oak tree pillar. Micropores or mesopores contracted, reduced in size, or disappeared when carbonization proceeded for 1 h at 1,500°C. Simultaneously, micropores combined to form macropores, and the density of interlaminar tissue increased. Carbonization for 60 min at 1,800°C resulted in a greater reduction in micropores, or formation of macropores, and the morphology resembled that of a graphite lump. Carbonization at 2,000°C resulted in micropore decay, increasing surface roughness. Carbonization at 2,400°C somewhat reduced surface roughness, and this surface morphology was similar to that observed under SEM following graphitization of cedar charcoal by Takeshi et al. (1999) [12] and Toshimitsu et al. (1998) [13].

Figure 4: SEM observations of white charcoal following heat treatment at 1000, 1100, 1300, 1500°C in N2, and 1800, 2000, and 2400°C in Ar environments.

Porosity Characteristics

Porosity characteristics, such as pore size distribution, on the solid surface or specific surface area were analyzed by measuring the adsorption isotherms due to nitrogen adsorption at 77 K. In general, pores developed on carbon adsorbent are classified into micropores for those ≤2 nm, mesopores for those between 2-50 nm, and macropores for those ≥50 nm, depending on the width between the layers [18]. The mechanisms of adsorption occurred according to the micropore filling theory [19] in micropores, and the capillary filling theory [20] in mesopores. Macropores play a role on the external surface. Condensation and evaporation proceeded according to the diffusion theory [21], and the adsorption isotherms manifest as Types I, IV, and II according to the surface tissue and pore size distribution.

The adsorption isotherms of the as-received white charcoal (produced at about 1,000°C) carbonized at (A) 1,000°C, (B) 1,100°C, (C) 1,300°C, and (D) 1,500°C for 1 h each in a nitrogen atmosphere, the white charcoal powder (< 44 ????m) carbonized for 1 h at (E) 1,500°C, and those that were graphitized after for 10 min in an argon atmosphere for 1 h at (F) 1,800°C, 30 min at (G) 2,000°C, and 10 min at (H) 2,400°C are shown in Fig 5. The images revealed that the raw white charcoal flakes in (A) presented about 60 cc m^-3 nitrogen adsorption around the lowest relative pressure (P/Po=0), which increased rapidly thereafter. They followed a typical Type I curve, which approximated the maximum adsorption amount, and pores in the white charcoal were identified as mostly micropores. This was because macropore or mesopore volumes that develop in white charcoal are negligible compared to the micropore volume. In particular, the desorption curve did not follow the adsorption curve, and a significant adsorption state was maintained at desorption under significantly low relative pressure, which was attributed to the unique characteristic of charcoal, as previously reported [20]. This phenomenon is known as low-pressure hysteresis of the adsorption isotherms. In short, the desorption isotherm of some adsorbents does not follow the adsorption curve under significantly low pressures, as shown in the Fig.5(A-C). For example, Cadenhead et al. (1958) [21] reported that charcoal obtained by activating coconut, or active carbon obtained from anthracite coal showed low-pressure hysteresis under relatively low pressures even if they were sufficiently degassed (10^-4 torr). McEnaney (1974) [22] reported a similar phenomenon when adsorbing and desorbing carbon tetrachloride at room temperature on inactivated polyacrylonitrile carbons. Deitz et al. (1973) [23] also reported similar phenomenon when adsorbed and desorbed krypton on thinly exfoliated graphite. Additionally, Pope and Gregg (1960) [24] reported low-pressure hysteresis as adsorbate swelled when n-butane was adsorbed on coal powder pressed at room temperature. Arnell and McDermott (1957) [25] considered this low-pressure hysteresis to be due to swelling of the adsorbed particle during isothermal adsorption, and that the swollen particle changed the structure of the adsorbent. Thus, they believed that the swollen adsorbate particle pushed the weak parts of the adsorbent micropores, created gaps, and became stuck, thus inhibiting the acceptance of other adsorbate particles. In addition, adsorbate particles escaped from gaps very slowly during isothermal desorption, or did not desorb at all. However, if the desorption temperature increased, the researchers thought it was possible that the particles would desorb. Therefore, Bailey et al. (1971) [26] investigated low-pressure hysteresis by increasing the desorption temperature for various types of adsorbents, and discovered that low-pressure hysteresis disappeared upon deaeration at high temperatures. Therefore the micropores that developed in charcoal had smooth gaps, allowing nitrogen atoms to be trapped in the pores, preventing their escape during desorption. Such low-pressure hysteresis was also experienced in charcoals thermally treated at 1,100 and 1,300°C, as shown in (B) and (C). However, measuring the adsorption isotherms of nitrogen after 1 h of carbonization at 1,500°C resulted in a curve similar to that shown in (D). Near the lowest relative pressure (P/Po=0), the amount of nitrogen adsorption was about 2.8 cc m^-3. Furthermore, the typical Type IV curve shows normal hysteresis where low-pressure hysteresis disappeared and mesopores coexist. This indicated that carbonization at high temperatures eliminated existing micropores and only some mesopores remained, while the gaps filled with nitrogen, as reported by Arnell and McDermott (1957) [25] and Bailey et al. (1971) [26], were desorbed during isothermal desorption. Thus, carbonization at 1,500°C alters the surface structure of the white charcoal and strengthens the pore openings and passages, with no risk of adsorbents becoming stuck.

Measuring the nitrogen adsorption isotherms after pulverizing white charcoal into powder and carbonizing for 1 h at 1,500°C results in (E), similar to (D). Near the lowest relative pressure (P/Po=0), nitrogen adsorption increased to about 20 cc m^-3, and followed the typical Type IV curve. Hysteresis, in which mesopores exist, was shown. Meanwhile, thin hysteresis, even under a relative pressure lower than P/Po=0.4, was attributed to the hardened powder and reduced removal of adsorbates. Compared to flakes, powder has a high specific surface area, even before carbonization. Even when the specific surface was significantly reduced following carbonization, the amount of nitrogen adsorption near the lowest relative pressure (P/Po=0) was greater than that of the flake (D). Measuring the nitrogen adsorption isotherms after 1 h of graphitization of white charcoal powder at 1,800°C resulted in a curve similar to that shown in (F), where nitrogen adsorption near to the lowest relative pressure (P/Po=0) significantly decreased to about 5.8 cc m^-3. This indicated that most micropores were removed and the specific surface area was significantly decreased; however, a typical Type IV curve similar to that shown in (E), which shows low-pressure hysteresis, was found.

Graphitizing white charcoal for 30 min at 2,000°C and measuring the nitrogen adsorption isotherms resulted in a curve similar to that shown in (G), where nitrogen adsorption near the lowest relative pressure (P/Po=0) is reduced to about 3.5 cc m^-3, and the low-pressure hysteresis due to micropores is lost. However, hysteresis due to mesopores at P/Po=0.4 or above was attributed to the adsorbate being adsorbed to the graphitized white charcoal surface, which became unevenly rough, as observed by SEM. Measurement of nitrogen adsorption isotherms after 10 min graphitization of the above white charcoal at 2,400°C resulted in a curve similar to that shown in (H), where nitrogen adsorption near the lowest relative pressure was reduced to 1.0 cc m^-3 or below. Furthermore, the curve changed to a Type II curve, which is seen for graphite. Here, the rough surface shown at 2,000°C smoothened, as in the graphite surface, implying that the adsorbate was easily desorbed.

Figure 5: Adsorption isotherms of heat-treated white charcoals.

Figure 6 shows the pore size distribution of the high-temperature white charcoal obtained through the Barrett- Joyner-Halenda (BJH)-plot method from the adsorption isotherms. The pore size of the graphitized and carbonized white charcoal from the images was mostly distributed at 2.5 nm or below, as also shown by SEM (Figure 4). Meanwhile, as the temperature of the treatment increased, the micropores contracted and were lost, most mesopores were transformed into micropores, but some remained, and the existence of macropores increased the average pore size. However, if graphitization proceeded, the mesopore or macropore surface became graphitized, such that the inside of the pores in carbon nanotube (CNT) was non-porous.

Figure 6: Pore size distribution of heat-treated white charcoals.

Table 2 shows the characteristics of high temperature carbonized and graphitzed white charcoals obtained from the BET equation [28], the total pore volume, the micropore volume obtained through the t-plot [29], the average pore size (dav) obtained through the HK method [27], etc. Meanwhile, the Table 2 includes the monolayer capacity (nm) obtained from the amount of nitrogen adsorbed on a single surface area of the high- temperature treated white charcoal. The monolayer capacity of the solid obtained based on the adsorption isotherms of nitrogen indicates the actual surface area of the solid. It is determined by applying the Langmuir model equation [30] from the nitrogen volume adsorbed on a monolayer of the solid, which differs from the specific surface area (SBET) value obtained from the amount of nitrogen adsorbed on all pores of the solid. The monolayer capacity is proportional to the volume of nitrogen adsorbed onto the single surface; therefore, this section introduces the method of obtaining nitrogen adsorbed (nm) on a single surface. This can be seen in Fig. 5(A), where the adsorption isotherm presents a rapid increase at the initial low relative pressure (P/Po), then bends to show a knee, followed by a gentle increase. A tangential line denotes the gradual increase. The start of the tangential line is termed the B point, and the value on the Y-axis that meets the horizontal line drawn from the B point is the single surface area nitrogen adsorption, which is divided by the standard-state volume of 1 mole, 22,414 cc mol^-1, to obtain the monolayer capacity (nm). Then, the monolayer capacity can be obtained from A = nmamL. Here, am is the area occupied by the projection of one molecule of nitrogen (am = 16.2 × 10^-20 m² molecule^-1 at 77 K) and L is the Avogadro constant.

As shown in the Table 2, the specific surface area of the as-received white charcoal flake produced at about 1,000°C is 256.46 m² g^-1, while the specific surface area due to micropores is 220.52 m² g^-1, and the average pore size is smaller than 2.0 nm; therefore, it was considered a white charcoal with well-developed micropores. The specific surface area rapidly decreased as the carbonization temperature of this white charcoal increased, because the micropores had significantly decreased. However, the increase in average pore size was due to the remaining macropores as well as the transformation of mesopores into micropores, as observed via SEM (Figure 4). Carbonizing powder white charcoal at 1,500°C increased the specific surface area to 136.26 m² g^-1 compared to carbonizing white charcoal flakes. This was because, despite the reduction in micropores, the powder itself increased the surface area. Graphitizing power white charcoal at 1,800°C for 1 h significantly reduced the specific surface area to 26.32 m² g^-1 However, the average pore size increased to 5.85 nm; despite the micropores having almost disappeared and the inner surface of the pores beginning to be graphitized, mesopores are also present as macropores (Figure 4). Then, when the white charcoal powder is graphitized at 2,000°C for 30 min, the specific surface area decreased to 16.04 m² g^-1, but the average pore size was 3.47 nm because mesopores still remain. However, a 10 min treatment at 2,400°C decreased the specific surface area to 8.10 m² g^-1, which is similar to that of pure graphite powder. This is because the pores that penetrate the insides have mostly disintegrated and been graphitized. The average pore size was 1.29 nm, based on measurements from the surface curves rather than the actual pores. The monolayer capacity values shown in Table 2 show that as the carbonization and graphitization temperature increases, the pores of the white charcoal flakes rapidly decreased, and the monolayer surface area decreased. Despite the large surface area of powder, the surface area of the monolayer decreased as the processing temperature increased, and graphitization occurred.

Table 2: Characteristics of heat-treated white charcoals.

SBET: measured by BET eq. [28], Vmi : t-plot [29], dav : HW-method [27], nm, : Langmuir eq.[30].

• High temperature treatment of oak white charcoal leads to the gradual dense packing of axial tissues, leading to pore contraction, micropores are lost or merge into mesopores or macropores, and some remain at 2,400°C; however, the internal organization of white charcoal becomes irregular, lamellar graphite.
• Measuring nitrogen adsorption isotherms during high-temperature treatment of white charcoal flakes shows unique forms where Types I and IV are combined according to the low-pressure hysteresis; however, low-pressure hysteresis is lost as the thermal treatment temperature is increased or the flakes are finely ground. At 2,400°C, a Type II curve similar to that obtained for graphite is shown.
• Graphitization begins when white charcoal is treated at 1,800°C or above, and treatment for 10 min at 2,400°C could result in lamellar graphitized carbon with 100 wt% fixed carbon.

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