Surface functional characteristics (C, O, S) of waste tire-derived carbon black before and after steam activation.

Before the carbon surface is analyzed, the properties of PCB and PCB-PAC were investigated. PCB-PAC with a BET surface area of 526 [m.sup.2]/g was generated at an activation temperature of 900 [degrees]C for 180 min, using a water feed rate of 1 mg [H.sub.2]O/g C/sec. The yield of PCB-PAC was approximately 40 wt%. (20) Two important carbon properties of interest were the specific surface area and the pore structure. Table 1 shows the physical and chemical properties, including BET surface area, average pore radius, pore volume, sulfur content, and pH value. Table 1 shows that the BET surface area and the pore volume of PCB-PAC significantly exceeded those of PCB. In the activation of steam, the BET surface area of PCB increased significantly from 35 to 526 [m.sup.2]/g, whereas the average pore radius decreased drastically from 80 to 18 [Angstrom]. The results indicated that water molecules were effective in promoting the formation of the inner pores of PCB. Our previous research demonstrated that the measured micropore surface area ([S.sub.micro]) and the BET surface area ([S.sub.BET]) of the PCB-PAC samples were 249 and 526 [m.sup.2]/g, respectively. (20) Approximately 53% of the surface area of PCB-PAC was external surface area. The total pore volume of PCB-PAC was 0.47 [cm.sup.3]/g, of which the mesopore volume was 0.40 [cm.sup.3]/g (near 85%), indicating that the mesopores contributed considerably to the total pore volume of PCB-PAC. The main peak of the pore size distribution for PCB-PAC was centered at around 70 nm.

Moreover, steam activation could increase the pH values of carbons, resulting in a basic surface character. (21) In this case, the pH values of PCB and PCB-PAC, measured according to ASTM D 3838-05, showed that their pH values were 8.9 and 9.3, respectively (Table 1). The steam activation of PCB contains both Bronsted- and Lewis-type surface sites. Because the graphitic part of the surface is significant, the [pi] electrons of the graphite planes are of great importance. They may act as Lewis basic sites, accepting protons as the following, (21)

[C.sub.[pi]] + [H.sub.2]O [right arrow] [C.sub.[pi]][H.sub.3]O+ +O[H.sup.-] (1)

In addition, a carbon surface with [pi] electrons may act as a reversible oxygen electrode in the presence of physically adsorbed oxygen, (21)

[C.sub.[pi]] ... [O.sub.2ads] + 2[H.sub.2]O [left and right arrow] [C.sub.[pi].sup.2+] + 4O[H.sup.-] + 2[h.sup.+] (2)

[C.sub.[pi]] ... [O.sub.2ads] + 2[H.sub.2]O [left and right arrow] [C.sub.[pi].sup.2+] + 2O[H.sup.-] + [H.sub.2][O.sub.2] (3)

where [h.sup.+] indicates the generation of holes. PCB-PAC may contain entrapped oxygen due to its highly microporous structure. According to eqs 2 and 3, the more oxygen molecules are adsorbed on the carbon surface, the higher the pH value of PCB-PAC that can be reached during the steam activation process.

Elemental Composition of Carbon Surface by ESCA

Wide-scan spectra in the binding energy range of approximately 0-1000 eV were obtained to identify the surface elements and to support a quantitative analysis (Figure 1). Four elements- oxygen (O), nitrogen (N), carbon (C), and sulfur (S) on the carbon surface were analyzed using an ESCA. Tables 2 and 3 present the bulk and surface elemental composition of PCB and PCB-PAC, respectively. As shown in Table 2, O, N, C, and S contents were measured on the surface of PCB. Among these elements, C was the most abundant element (89.5 wt.%). The amounts of C, O, and S were also measured on the surface of PCB-PAC and their contents were 87.6, 11.9, and 0.5 wt%, respectively. Comparing the elemental composition of PCB with that of PCB-PAC showed that PCB contained more C whereas PCB-PAC contained more O. The decrease in C content was a result of the reaction of carbon black with steam during activation. (22) Oxygen reacted with and/or adsorbed on the carbon surface to form oxygenated functional groups thereon, increasing the oxygen content on PCB-PAC surface. The increase of O content from 9.6% (PCB) to 11.9% (PCB-PAC) might be caused by the adsorption of oxygenated compounds (such as [H.sub.2]O molecules) on the PCB-PAC surface (see eqs 2 and 3). The increase of O content also resulted in the increase of the pH values of PCB-PAC.

[FIGURE 1 OMITTED]

Tables 1-3 present the physicochemical properties of PCB and PCB-PAC measured with an elemental analyzer (EA) and an ESCA. As shown in Tables 1 and 2, the S content of PCB was equivalent to 0.50 wt% using EA and ESCA, respectively. In addition, Tables 1 and 3 show that the S contents of PCB-PAC measured using EA (0.54 wt%) and ESCA (0.50 wt%) were nearly equivalent. The almost equivalent S content of PCB and PCB-PAC suggested that sulfur should be evenly distributed on the internal and external surfaces of carbons because S was added uniformly in the tires during the tire manufacturing process.

Fitting of the Carbon ESCA Spectra

The broad carbon peaks observed in the binding energy of approximately 278-296 eV were attributed to carbon-based surface functional groups with various binding energies (BE). The [C.sub.1s] spectra for PCB and PCB-PAC could be fitted by five peaks: [C.sub.1] (BE = 284.4 eV), [C.sub.2] (BE = 285.9 eV), [C.sub.3] (BE = 288.4 eV), [C.sub.4] (BE = 289.7 eV), and [C.sub.5] (BE = 291.4 eV). These different BE peaks were assigned to C-C at [C.sub.1], hydroxyl (C-O) at [C.sub.2], carbonyl (C=O) at [C.sub.3], carboxyl (O-C=0) at [C.sub.4], and a plasmon peak at [C.sub.5]. The shape of the [C.sub.1] peak depends on the surface characteristics of the sample. The spectra of compounds with large polyaromatic, graphite-like domains on the surface (such as carbon black) have an asymmetric [C.sub.1] peak, whereas the peaks of small aromatic compounds (such as coronene, with seven condensed rings) and of aliphatic compounds are symmetrical. (11,14,16,23) Therefore, the asymmetry of the [C.sub.1] peak yields information about the polyaromatic character of the sample surface. The most intense signal in the spectra of PCB and PCB-PAC was the [C.sub.1] peak. Furthermore, the [C.sub.1] peaks of two samples were asymmetrical.

Figure 2 and Table 4 show the [C.sub.1s] peak region of PCB at 284.4 eV deconvoluted into surface functional group contributions. The [C.sub.1s] peak could be fit with four line shapes at [C.sub.1], [C.sub.2], [C.sub.4], and [C.sub.5]. Therefore, the total area of the [C.sub.1s] peak region of PCB consisted of 35.1 wt% [C.sub.1] 59.6 wt% [C.sub.2], 4.4 wt% [C.sub.4], and 0.9 wt% [C.sub.5]. A similar analysis showed that the total area of the [C.sub.1s] peak region of PCB-PAC consisted of 42.1 wt% [C.sub.1], 46.1 wt% [C.sub.2], 1.5 wt% [C.sub.3], 10 wt% [C.sub.4], and 0.3 wt% [C.sub.5]. The percentages of C=O and O-C=O functional groups in PCB-PAC exceeded those in PCB, suggesting that the oxidation of carbon by steam on the carbon surface was significant during activation.

[FIGURE 2 OMITTED]

Fitting of the Oxygen ESCA Spectra

As shown in Figure 3, the [O.sub.1s] spectra could be fit with three peaks--one for oxygen with a double bond to carbon (C=O, [O.sub.1], BE = 531.6 eV), one for oxygen with a single bond to carbon (C-O, [O.sub.2], BE = 533.7 eV), and another at a BE of approximately 535-536 eV ([O.sub.3]). Among these three peaks, the [O.sub.3] peak is attributable to the adsorbed water and oxygen. (12)

Table 5 shows that the total area of the [O.sub.1s] peak region of PCB consisted of 11.2 wt% [O.sub.1], 82.6 wt% [O.sub.2], and 6.2 wt% [O.sub.3]. The most intensive signal in the spectrum was the [O.sub.2] peak, indicating that C-O was formed preferentially during the tire pyrolysis process. The spectra of PCB-PAC and PCB differed somewhat. The spectra of PCB-PAC consisted of 14.7 wt% [O.sub.1], 33.3 wt% [O.sub.2], and 52 wt% [O.sub.3]. This result showed that the [O.sub.3] peak of PCB-PAC was more intense than the [O.sub.1] and [O.sub.2] peaks.

Because ESCA measurements were used under high vacuum conditions, only strongly adsorbed water and oxygen can be retained on the carbon surface. The high [O.sub.3] peak in the spectrum from PCB-PAC indicated that the sample retained many strong adsorption sites for adsorbing water and/or oxygen. However, the spectrum of PCB did not show the same result.

[FIGURE 3 OMITTED]

As revealed by Tables 2 and 3, PCB-PAC contained more oxygen than PCB, according ESCA. This finding was inconsistent with the PCB-PAC surface having a lower percentage of oxygenated functional groups than PCB (Table 4). The most probable reason for this inconsistency was that water molecules diffused into the interior of PCB and created inner pores in activation, before being further adsorbed on the carbon surface. Therefore, the oxygen detected on the PCB surface was mainly bonded to carbon (C-O), whereas the oxygen on the PCB-PAC surface could be bonded to hydrogen (O-H) and carbon (C-O; Table 5).

Fitting of the Sulfur ESCA Spectra

Figure 4 shows the [S.sub.2p] spectra of PCB and PCB-PAC. The [S.sub.2p] spectra could be fit by two peaks--one for sulfur with double bonds to zinc (zinc sulfide [ZnS], [S.sub.1], BE = 162.1 eV) and one for two sulfur atoms with double bonds to carbon (S=C=S, [S.sub.2], BE = 164 eV). (12)