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Weekly UpdatesABSTRACT
The effects of steam activation on the surface functional characteristics of waste tire-derived carbon black were investigated. Two carbon-based materials, powdered carbon black (PCB) and PCB-derived powdered activated carbon (PCB-PAC), were selected for this study. A stainless steel tubular oven was used to activate the PCB at an activation temperature of 900 [degrees]C and 1 atm using steam as an activating reagent. X-ray photoelectron spectroscopy (XPS) was adopted to measure the surface composition and chemical structure of carbon surface. Various elemental spectra (C, O, and S) of each carbon sample were further deconvoluted by peak synthesis. Results showed that the surfaces of PCB and PCB-PAC consisted mainly of C-C and C-O. The PCB-PAC surface had a higher percentage of oxygenated functional groups (C=O and O-C=O) than PCB. The [O.sub.1s] spectra show that 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). Sulfur on the surface of PCB consisted of 58.9 wt% zinc sulfide (ZnS) and 41.1 wt% S=C=S, whereas that on the surfaces of PCB-PAC consisted mainly of S=C=S. Furthermore, the increase of oxygen content from 9.6% (PCB) to 11.9% (PCB-PAC) resulted in the increase of the pH values of PCB-PAC after steam activation.
INTRODUCTION
Several studies have investigated the pyrolytic kinetics of waste tire pyrolysis and the distribution of pyrolytic products, particularly carbon black. (1-5) Converting waste tires or pyrolytic carbon black into activated carbons and further applying them as adsorbents for controlling air pollution and treating wastewater has also been developed. (6-11) However, the change in sulfur functional characteristics of waste tire-derived carbon black before and after steam activation has seldom been investigated in the past. (12) Theoretically, the surface functional characteristics of activated carbon affected its adsorptive capacity. Leng and Pinto (13) elucidated the effects of surface oxygen complexes and metals on the adsorption of aromatics. A higher concentration of surface oxygen groups on the surface of activated carbon can adsorb more water vapor compared with the activated carbon with a lower concentration of surface oxygen groups. (14) Furthermore, mercury can be captured by bonding to I, Cl, S, or O anionic species on the surfaces of carbonaceous and other sorbents, but only in the form of [Hg.sup.2+]. (15)
X-rays photoelectron spectroscopy (XPS), more commonly known as electron spectroscopy for chemical analysis (ESCA), has been extensively applied to analyze the surface chemistry and the structure of carbon-based materials. Darmstadt et al. (12) used ESCA to investigate the characteristics of pyrolytic carbon black ([CB.sub.P]) prepared either in vacuo or at atmospheric pressure. (12) They found that [CB.sub.P] produced by vacuum pyrolysis was chemically closer in chemical nature to commercial carbon black than [CB.sub.P] produced by atmospheric pyrolysis. Additionally, small amounts of sulfur in the carbon disulfide ([CS.sub.2]) were detected on the surface (<1.7 wt%). C-H/C-C was the major carbon functional component on the surface of [CB.sub.P] but other functional forms were also present, such as C=O and C-O. (14) Moreover, C-C was the major carbon functional component on the surface of charcoal obtained by the vacuum pyrolysis of bark. Hardly any nitrogenated content was obtained on the surface of charcoal. (16) Additionally, the surface chemical composition of some non-wood pulps has been investigated. (17) They found that C-O was the major carbon functional component on the surface of non-wood pulps but other functional forms, such as C-C, C=O, O-C-O, and O-C=O, were also present. The surface characteristics of activated carbon fibers (ACFs) have been investigated using ESCA, indicating that the carbon surface of ACFs consisted of approximately 78-84 wt% C-C, approximately 6-8 wt% C-O, and approximately 9-16 wt% O-C=O. (18)
Although previous researches, numerous articles, book chapters, and books have been published about the surface chemistry of waste tire-derived carbon black, (12,14) few studies were performed to investigate the variation of the surface functional characteristics of waste tire-derived carbon black before and after steam activation. To further consider the potential application of activated carbon produced from waste tire-derived carbon black, this study analyzed the surface functional characteristics of powdered activated carbon derived from waste tires using ESCA and investigated the variation of carbon black prepared from the pyrolysis of waste tires before and after steam activation. The results were further compared with those of several previously published studies to elucidate the variations among carbon surfaces obtained from various carbon sources and operating parameters.
EXPERIMENTAL METHODS
Materials
Two carbon-based materials, including powdered carbon black (PCB) and PCB-derived powdered activated carbon (PCB-PAC), were investigated. A waste tire pyrolysis plant (Pro-Research Pyrolysis Technology Corp. Ltd.) supplied PCB. In this plant, the pyrolysis of waste tires was controlled at 1 atm pressure and 420 [degrees]C. PCB-PAC was prepared from the steam activation of PCB in a laboratory-scale activation system. The experiments were conducted in two steps. The first step was to prepare PCB-PAC from PCB using deionized water (DI [H.sub.2]O) as an activating reagent. The second step was to analyze the elemental composition and the structure of carbon surface. In this work, the operating parameters for producing PCB-PAC were 900 [degrees]C for 180 min at the water feed rate of 1 mL [H.sub.2]O/g C/sec.
Preparation of Powdered Activated Carbon
A tubular oven made of stainless steel with an internal diameter (ID) of 7 cm and a length of 100 cm was designed to activate PCB. Approximately 2.5 g of PCB was placed in a ceramic crucible at the center of the tubular oven in the absence of oxygen. PCB was activated at 900 [degrees]C and 1 atm. During the activation, approximately 2.4-6.5 mL/sec of DI [H.sub.2][O.sub.(1)] was continuously injected at the upstream of the tubular oven using a peristaltic pump (Gilson, Model Minipuls II) and then evaporated to steam as the activating reagent. Highly pure nitrogen gas (99.995%) was introduced at a flow rate of 0.5 L/min into the tubular oven as the carrier gas.
Physicochemical Analysis
In this study, ESCA was adopted to analyze the elemental concentration and chemical structures of PCB and PCB-PAC on the basis of the difference between the kinetic energies of photoelectrons. The surface characteristics of PCB and PCB-PAC were detected with a Fison (VG) ESCA 210 spectrometer using a monochromic Al K-[alpha] X-ray source. All survey spectra, scans of 1000 eV or more, were taken at a pass energy of 50 eV, providing an instrumental resolution of 1 eV. The narrow scans of strong lines were just wide enough to encompass the peaks of interest and were obtained with a pass energy of 25 eV. Before the experiments, the binding energy was calibrated against the following lines: [Au.sub.4f7/2](83.8 eV), Cu([L.sub.3][M.sub.5][M.sub.5])(567.9 eV), and [Cu.sub.2p3/2](932.4 eV). (19) During the data treatment of the spectra, the shift of the binding energy scale due to charging was corrected by referencing the [C.sub.1] peak in the [C.sub.1s] signal to 284.4 eV.
Moreover, a Brunauer-Emmett-Teller (BET) surface analyzer (Micrometritics Instrument Corp., Model ASAP 2000) was used to measure the specific surface area, the average pore diameter, and the pore volume of PCB and PCB-PAC by nitrogen gas adsorption at 77 K. The micropore surface area and the external surface area ([S.sub.micro] and [S.sub.ext], respectively) of the samples were obtained by a t-plot method. The amount of [N.sub.2] adsorbed at relative pressures (P/[P.sub.0]) of near unity (0.98 herein) corresponds to the total amount of [N.sub.2] adsorbed ([V.sub.total]) in both the micropores and the mesopores. PCB and PCB-PAC samples were degassed at 150 [degrees]C for 8 hr before micropore volume measurements were made to clean the surface of the chars. The sulfur content of PCB-PAC was measured using an elemental analyzer (Fisons, Model EA 1108). In addition, the pH values of PCB and PCB-PAC were determined according to the ASTM D 3838-05 test method (standard test method for pH of activated carbon).
Spectra Fitting Analysis
To further analyze the possible functional characteristics from the spectra fitting of carbon, the narrow scans were taken from 279.6 eV to 294.6 eV. After subtraction of a nonlinear background from the raw spectra, the peak of each carbon sample was deconvoluted by peak synthesis, which fitted the numerous Gaussian peaks, each with an average full width half maximum (FWHM) value of 0.2 eV and a fixed binding energy, to the measured peaks. (14) The best fit between the measured and the synthesized spectra was obtained by simulating the intensity of each functional component peak using a computer simulation. In addition, the possible functional characteristics of PCB and PCB-PAC were also investigated by the spectra fitting analysis of oxygen and sulfur. Their narrow scans were taken from 528 to 540 eV and from 159.8 to 167.6 eV, respectively. Following the above-mentioned method, the [O.sub.1s] and [S.sub.2p] spectra of PCB and PCB-PAC were also deconvoluted, respectively.
RESULTS AND DISCUSSION
Properties of PCB and PCB-PAC
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.
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