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Phenol adsorption on zeolite-templated carbons with different structural and surface properties
Carbon 43 (2005) 1156–1164 www.elsevier.com/locate/carbon
Phenol adsorption on zeolite-templated carbons with different structural and surface properties Fabing Su, Lu Lv, Tee Meng Hui, X.S. Zhao
*
Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Received 16 September 2004; accepted 8 December 2004
Abstract The adsorption behaviors of phenol in aqueous solution on zeolite-templated porous carbons with different pore structures and surface properties were studied. The structural and surface properties of the carbon samples were characterized using N2 adsorption, water vapor adsorption, X-ray photoelectron spectrometer (XPS), and thermogravimetric analysis (TGA). It was observed that thermal treatment under nitrogen can significantly eliminate surface oxygen-containing groups, thus altering phenol adsorption behaviors. The adsorption capacity of phenol on the porous carbons after thermal treatment was markedly increased in spite of reduced surface area and pore volume. Possible reasons behind the experimental observations including the formation of water molecular cluster, p–p dispersive interaction, electron donor–acceptor mechanism, and micropore structure were discussed. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Porous carbon; Surface treatment; Surface oxygen complexes, Adsorption properties
1. Introduction Porous carbons have been widely applied in liquidphase adsorption, separation, and purification processes [1]. The use of porous carbons for adsorptive removal of phenols and their derivatives frequently presented in many wastewater streams has been a recent research focus [1,2]. On the other hand, as phenol can serve to indicate the adsorption characteristics of small polar aromatic compounds [3], its adsorption behaviors on various commercial activated carbons have been widely investigated, aimed to clarify adsorption mechanism. Generally, the surface and pore structure properties of a porous carbon are believed to be the key factors determining the adsorption behaviors of the carbon–phenol system [1,2]. The surface chemistry of a porous carbon depends on the presence or absence of heteroatoms such as oxygen, *
Corresponding author. Tel.: +65 68744727; fax: +65 67791936. E-mail address: [email protected] (X.S. Zhao).
0008-6223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.12.034
nitrogen, hydrogen, and sulfur, which exist as surface functional groups at the edges of graphene layers [4]. Oxygen-containing functional groups such as carboxyls, carbonyls, phenols, lactones, aldehydes, ketones, pyrones, quinones, hydroquinones, and anhydrides [4–8] have a significant effect on the interactions between an adsorbate and the carbon surface [2]. The oxygen-containing groups and the delocalized electrons of the graphitic structure can, to a large extent, determine the charge and acidity/basicity of the porous carbon surface [2,6,9]. Carboxyl, phenolic hydroxyl and lactonic groups are acidic while carbonyl, pyrone, chromene and quinone are basic [2,7,10–14]. pH and temperature could lead to different effects of the surface functional groups on phenol adsorption [1,2,15]. Although chemisorption is insignificant in a phenol–carbon adsorption system [16], it always occurs due to the presence of oxygen in the phenol solution and/or on the surface of the carbon. Such oxygen molecules may facilitate the irreversible adsorption and oxidative coupling reactions via catalysis [15,17]. In addition, oxygen-containing basic groups
F. Su et al. / Carbon 43 (2005) 1156–1164
such as the chromene-type and pyrone-type are likely to promote irreversible adsorption of phenolic compounds [18]. Due to the complicated surface properties of porous carbon, several mechanisms of phenol adsorption on porous carbon have been proposed, such as the p–p dispersion interaction mechanism [19], the hydrogenbonding formation mechanism [19], the electron donor–acceptor complex mechanism [20], and the ‘‘solvent effect’’ mechanism [15,21]. It must be noted that solution pH and temperature can change the general adsorption mechanisms [15,16]. The pore structure of a porous carbon containing micropores, mesopores and macropores can also greatly influence phenol adsorption because the pores of different sizes play different roles in the adsorption process. Three stages of phenol adsorption on porous carbon involving adsorption at infinite dilution, micropore filling, and adsorption in larger micropores and in mesopores [15] are believed to be cooperative. Recently, template synthesis of novel porous carbons with tailored pore structures and surface properties has been successfully demonstrated [22–27]. These templatesynthesized porous carbons are potential adsorbents and provide a good opportunity for fundamental study of adsorption. This work attempted to investigate the influence of structural and surface properties of zeolite Y-templated porous carbons on phenol adsorption characteristics in the aqueous solution.
2. Experimental 2.1. Template synthesis of porous carbons In this study, carbon samples with different pore sizes were prepared with two carbon precursors (sucrose and furfuryl alcohol) in the presence of zeolite Y template. To prepare a carbon sample with substantial mesopores, sucrose (98%, Fluka) and sodium-form zeolite Y (Na-form, SiO2/Al2O3 = 5.1, Zeolyst International Company, USA) were used as the carbon source and template, respectively. 10 g of zeolite Y powder was dried at 200 °C for 4 h before adding into a solution containing 10 g sucrose and 0.001 L sulfuric acid (98%, Fisher Scientific, UK) in 0.050 L deionized water. The mixture was stirred at room temperature for 4 h, followed by drying at 100 °C for 15 h and 160 °C for 6 h to form a zeolite/sucrose mixture. This mixture was carbonized at 1000 °C for 4 h in a quartz tube under nitrogen flow (99.9995%, 30 cm3/min) and subsequently treated with 46% aqueous HF solution to etch away the zeolite framework. The black carbon was washed with copious deionized water and dried at 150 °C. The carbon sample thus prepared is designed CS-1, where C means porous carbon and S refers to sucrose precursor. Another
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carbon sample also with substantial mesopores, denoted as CS-2, was similarly prepared except a carbonization temperature of 800 °C was used instead of 1000 °C. To prepare a carbon sample with substantial micropores, liquid furfuryl alcohol (98%, ACROS ORGANICS, USA) and ammonium-form zeolite Y (NH4Y-form, SiO2/Al2O3 = 5.1, Zeolyst International Company, USA) were used as the carbon precursor and template, respectively. A carbonization temperature 1100 °C was used. The detailed preparation can be found elsewhere [27]. The carbon thus prepared is denoted CF-1, where F means furfuryl alcohol precursor. To modify the surface properties of the carbon samples, thermal treatment was conducted at 900 °C for 4 h under pure nitrogen to obtain samples designed as CS-1N, CS-2N and CF-1N, respectively. 2.2. Characterization The structures of the samples were investigated with physical adsorption of nitrogen at 196 °C on an automatic volumetric sorption analyzer (Quantachrome, NOVA1200). Prior to the measurements, samples were degassed at 200 °C for 5 h under vacuum of 101 Torr. The specific surface areas (SBET) were determined using the Brunauer–Emmett–Teller (BET) method in the relative pressure range of 0.05–0.2. The micropore volumes (Vmi) were obtained with the accumulative pore volume of density-functional-theory (DFT) method. The total pore volumes (Vt) were obtained from the volumes of nitrogen adsorbed at a relative pressure of 0.95. The mesopore volumes (Vme) were calculated by subtracting Vmi from Vt. The ratio of Vmi/Vt was used to indicate microporosity rate Rmi. The pore size distribution curves and cumulative pore volume plots were obtained using the DFT method. The thermal behaviors of the carbons were evaluated using a thermogravimetric analyzer (TGA 2050, Thermal Analysis Instruments, USA) under highly pure nitrogen (99.9995%). The surface chemical compositions of the samples were determined using X-ray photoelectron spectroscopy (XPS) (Kratos Analytical, AXIS HSI 165 spectrometer) with a monochromatized AlKa X-ray source (1486.71 eV). Adsorption of water vapor was conducted on a home-built adsorption system. Prior to adsorption measurements, samples were dried at 150 °C overnight under vacuum (101 Torr). The equilibrium adsorption amount at 21.0 ± 0.5 °C was determined by the weight gain after adsorption equilibrium was attained. 2.3. Adsorption of phenol in aqueous solution Stock solution of phenol (Merck, 99.5%) with a concentration of 200 mg/L (pH = 7, unbuffered) was prepared using deionized water. A carbon sample dried at
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F. Su et al. / Carbon 43 (2005) 1156–1164
150 °C overnight under vacuum of 101 Torr was added into a flask containing 0.02 L of a phenol solution. After shaken at 30 °C for 3 days, the phenol solution was filtered and the concentration was analyzed using a UV– visible spectrophotometer (UV-1601, Shimadzu, Japan) at a wavelength of 269.5 nm. In order to reduce measurement errors, the UV absorption intensity of each equilibrium solution sample was measured three times and the average value was used to calculate the equilibrium concentration based on a standard calibration curve, whose correlation coefficient square (R2) was 0.999. The amount adsorbed at equilibrium, qe (mg/g), was calculated using the following equation: qe ¼
ðC 0 C e ÞV W
ð1Þ
where C0 and Ce are the initial and equilibrium concentrations of phenol (mg/L), respectively, V (L) is the volume of the solution and W (g) is the mass of the carbon, qe (mg/g) is the amount adsorbed at equilibrium concentration Ce (mg/L). The adsorption equilibrium data were fitted to the Langmuir equation: Ce 1 1 ¼ þ Ce qe K L q0 q0
ð2Þ
where q0 (mg/g) is the maximum adsorption capacity, KL (L/mg) is the adsorption equilibrium constant, characteristic of affinity between the adsorbent and adsorbate, KL andq0 can be obtained by linear regression of (Ce/qe) against Ce.
3. Results and discussion 3.1. Adsorption isotherms of phenol
Table 1 Adsorption parameters of Langmuir equation in phenol solution Sample
Langmuir equation
CS-1 CS-1N CS-2 CS-2N CF-1 CF-1N
400 350
1.6
(a)
CS-1 CS-2 CF-1
250 200 150
CS-1 CS-2 CF-1
CS-1N CS-2N CF-1N
0.0
0
40
80 120 Ce (mg/L)
160
200
0.060 0.111 0.070 0.151 0.023 0.050
0.993 0.996 0.997 0.999 0.982 0.987
Ce
þq qeN K q 0 ¼ 1L 0 Ce qe þ K q q LN 0N
ð3Þ
0N
Using the parameters of the Langmuir model in Table 1, Eq. (3) becomes qeN 16:7 þ C e ¼ qe 6:01 þ 0:67C e
ð4Þ
ð5Þ
for carbons CS-2N and CS-2, and
0.8
qeN 43:5 þ C e ¼ qe 12:40 þ 0:62C e
50 0
138.9 208.3 128.2 192.3 200.0 322.6
qeN 14:3 þ C e ¼ qe 4:42 þ 0:67C e
0.4
100
R2
for carbons CS-1N and CS-1, (b)
1.2 Ce/qe (g/L)
qe (mg/g)
300
CS-1N CS-2N CF-1N
KL (L/mg)
CF-1 is higher than those on samples CS-1 and CS-2. Similarly, for thermally treated carbons, CF-1N also displays a higher adsorption capacity than CS-1N and CS-2N. Moreover, the Langmuir linear fitting to experiment data seems very well in Fig. 1b. The parameters of fitting the experimental adsorption equilibrium data are given in Table 1. The data calculated from the Langmuir equation reveal a pronounced increase in the maximum adsorption capacity (q0) upon thermal treatment under nitrogen and a twofold increase in the adsorption equilibrium constant (KL), which is an indication of the affinity between phenol and carbon surface. In any cases, when considering the value of correlation coefficient square (R2), it seems that the Langmuir model is better fitted to the experimental data of the treated carbons than to the original carbons, possibly implying that the thermal treatment under nitrogen had resulted in a more homogeneous surface of the carbons. The ratios of phenol equilibrium adsorption capacity on the thermally treated carbons (qeN) over that on the original carbons (qe), i.e. qeN/qe, given in Eq. (3), were used to help understand the adsorption behaviors of phenol on the carbon samples. 1
Fig. 1 shows the equilibrium adsorption isotherms and Langmuir fitting cures of phenol on the different porous carbons. The higher adsorption capacities of phenol on the thermally treated samples than on their original counterparts are obviously seen. For original carbons, the amount of phenol adsorbed on sample
q0 (mg/g)
0
50
100 150 Ce (mg/L)
200
Fig. 1. Experimental isotherms (a) and Langmuir fitting cures (b) of phenol adsorption on different porous carbons.
ð6Þ
for carbons CF-1N and CF-1. Fig. 2 shows the plots of qeN/qe against equilibrium concentrations. It can be seen that that as phenol concentration increases, the ratio of qeN/qe decreases dramatically and finally approaches constant, indicating
F. Su et al. / Carbon 43 (2005) 1156–1164
3.5
CS-1 series CS-2 series CF-1 series
qeN /qe
3.0
2.5
2.0
1.5 0
50
100
150
200
Ce (mg/L) Fig. 2. The curves of adsorption capacity ratio for original and treated carbons as a function of equilibrium concentration.
the different effects of pore structure and surface chemistry of the porous carbons at different equilibrium concentrations of phenol. 3.2. Effect of pore structure Shown in Fig. 3 is the nitrogen adsorption/desorption isotherms of the carbon samples before and after thermal treatment. It can be seen that the adsorption isotherms of sample CS-1, CS-1N, CS-2 and CS-2N are all of type IV with an H4 type hysteresis loop, characterizing mesoporous materials with slit-like pores [28]. For samples CF-1 and CF-1N in Fig. 3, the isotherms are of type I, indicating a microporous material [28]. The amount of nitrogen adsorbed on sample CF-1 gradually increased in the whole relative pressure range, whereas the amount of nitrogen adsorbed on sample CF-1N
1600
Volume (cm3/g, STP)
CF-1N CF-1 1200
CS-1N CS-1 CS-2N
800
CS-2 400
0 0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0) Fig. 3. Nitrogen adsorption/desorption isotherms of porous carbons (for clarity, the isotherms of CS-1, CS-1N, CS-2N, CF-1 and CF-1N were vertically shifted for 100, 450, 250, 200 and 800 cm3/g, respectively).
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reached a plateau after relative pressure reached about 0.2, implying that carbon CF-1 contains a more amount of mesopores. Fig. 4 presents the DFT pore size distribution curves and cumulative pore volume plots (inset) of the porous carbons. The pore size distribution curve of sample CS-1 in Fig. 4a displays two broad peaks, centered at around 1.5 and 2.7 nm, respectively. However, after thermal treatment, three peaks located at about 0.5, 1.2 and 2.7 nm, respectively, are seen. From the inset in Fig. 4a it is seen that the cumulative pore volume of CS-1N is larger than that of CS-1 in the pore size less than 1.0 nm. Possibly due to the similar preparation of carbon CS-2 to that of carbon CS-1, these two samples display similar pore size distribution. Nitrogen treatment of carbon CS-2 also resulted in the appearance of an additional peak at about 0.5 nm on the pore size distribution curve of CS-2N, while the cumulative pore volume of CS-2N is larger than that of CS-2 in the pore size less than 1.4 nm in Fig. 4b. For samples CF-1 and CF-1N in Fig. 4c. The pore size distribution curves of the two carbons show the presence of a mainly shape peak centered at 1.5 and 1.2, respectively, together with other small peaks in the lager micropore and mesopore range. The cumulative pore volume of CF-1N is also larger than that of its original carbon in the pore size less than 1.3 nm in Fig. 4c-inset. Thus, it can be concluded that thermal treatment of the carbon samples had led to the generation of a considerable amount of smaller micropores. The pore structure parameters derived from the isotherms are compiled in Table 2. As can be seen, compared with the original carbons, thermal treatment at 900 °C under nitrogen for 4 h caused a decrease in BET surface area and pore volume, which might be attributed to partial collapses of the pore structures. The samples CS-1 and CS-2 have the same values as CS-1N and CS-2N in microporosity Rmi, respectively. On the contrary, the microporosity of CF-1N is obviously higher than that of CF-1, further suggesting the formation of small micropores at the expense of large micropores and mesopores. It is observed that the maximum phenol adsorption capacities (q0) of the original and treated porous carbons in Table 1 increase with their surface areas and micropore volumes, respectively. In addition, assuming monolayer adsorption of phenol molecules on carbon surface, the number of phenol molecules adsorbed on per unit surface area (Xm, number/ nm2) can be calculated from the maximum adsorption capacities in Table 1 and BET surface areas (SBET) in Table 2. It is seen that the values of Xm estimated data in Table 2 for the original carbons is comparable, namely in the range Xm = 0.4–0.6, so as is for the thermally treated carbons, namely in the range Xm = 1.0– 1.2. The Xm values of the treated carbons are all at least doubled in comparison with their original counterparts.
0.6 0.4 0.2 0.0
0
1 2 3 4 Pore size (nm)
CS-1 CS-1N
0.2 0.0
0.6 0.4
0.4 0.2 0.0
0 1 2 3 4 Pore size (nm)
CS-2 CS-2N
(c)
2.5 2.0 1.5
1.5
3
3
0.6
0.2 0.0
0 1 2 3 4 5 6 7 Pore size (nm)
3.0
0.8
dV/dD (cm3/nm/g)
0.4
(b)
0.8
Pore volume (cm /g)
0.6
0.8
1.0
dV/dD (cm3/nm/g)
dV/dD (cm3/nm/g)
(a)
3
0.8
Pore volume (cm /g)
F. Su et al. / Carbon 43 (2005) 1156–1164
Pore volume (cm /g)
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1.0 0.5 0.0
0 1 2 3 4 Pore size (nm)
1.0
CF-1 CF-1N
0.5 0.0
0 1 2 3 4 5 6 7 Pore size (nm)
0 1 2 3 4 5 6 7 Pore size (nm)
Fig. 4. DFT pore size distribution curves and cumulative pore volume plots (inset) of (a) CS-1 and CS-1N, (b) CS-2 and CS-2N, and (c) CF-1 and CF-1N.
Table 2 Pore structure parameters of porous carbons Samples
SBETa (m2/g)
Vtb (cm3/g)
Vmic (cm3/g)
Vmed (cm3/g)
Rmie
Xm f
CS-1 CS-1N CS-2 CS-2N CF-1 CF-1N
1752 1339 1354 1062 3054 1917
1.32 1.05 0.95 0.80 1.69 0.91
0.48 0.38 0.41 0.34 1.04 0.75
0.84 0.67 0.54 0.46 0.65 0.16
0.36 0.36 0.43 0.42 0.62 0.82
0.5 1.0 0.6 1.2 0.4 1.1
a b c d e f
BET surface area. Total pore volume. Micropore volume. Mesopore volume. Microporosity. Number of phenol molecules adsorbed on per unit surface area.
In general, adsorption capacity depends, to a large extent, on the accessibility of the organic molecules to the inner surface of carbon adsorbent. Small molecules can access micropores driven by the strong adsorption potential near the micropore wall. It has been known that to some extent, the adsorption of phenol is mainly due to micropore filling [29–33]. It has been noted that during thermal treatment, the creation of small micropores occurred because of opening of the micropores blocked by surface oxygen atoms [2,29], especially in the case of carbon CF-1. This possibly enhanced the adsorption potential on the carbon surface, thus collectively increasing phenol adsorption. In phenol solutions, water molecules are preferentially adsorbed by the oxygen groups and then the remainder of the surface and/or the micropore volume is available to the phenol molecules [29]. Thus, the large ratio of qeN/qe at the low equilibrium concentration observed in Fig. 2 is partially pertinent to the smaller micropore space available due to lower content of functional groups, which occupy the entrances or walls of micropores. On the other hand, because the three original carbons were subjected to HF washing and the three treated carbons also underwent same thermal treatment, they had a similar surface property, respectively. Therefore, it can be reasoned that
the maximum adsorption capacities of phenol on original and treated porous carbons are correlated with the surface area and micropore volume of the carbons, respectively. Also, it can be concluded that the value of Xm with two different levels of 0.4–0.6 (original carbons) and 1.0–1.2 (treated carbons) in Table 2 is correlated to the surface chemistry, not pore structure. Additionally, although mesopores of a porous carbon, which may shorten the diffusion path of phenol molecules to the carbon interior surface with adsorptive sites of higher adsorption energy [3], play an important role in adsorption process, no direct correlation of the phenol adsorption capacity with mesopore volume was observed in this study. 3.3. Effect of surface chemistry The thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of porous carbons CS-1, CS-2 and CF-1 under nitrogen atmosphere are shown in Fig. 5. For carbons CS-1, CS-2 and CF-1, there was about 13–15% weight loss when temperature was increased to 900 °C in Fig. 5a. Three peaks can be seen from the DTG curves shown in Fig. 5b. The peak below 200 °C is due to weight loss of physically adsorbed spe-
F. Su et al. / Carbon 43 (2005) 1156–1164
Weight (%)
(b) Deriv.Weight (%/oC)
(a)
100
95
90
85
CS-1 CS-2 CF-1
0
200 400 600 800 1000 o Temperature ( C)
0
CS-1 CS-2 CF-1
200 400 600 800 1000 o Temperature ( C)
Fig. 5. (a) TG and (b) DTG curves of original porous carbons under nitrogen.
cies like water. The one from 200 to 400 °C with the strongest intensity for samples CS-1 and CS-2 is most probably due to the evolution of CO2 from the decomposition of carboxylic and carboxylic-group derivatives [34–37]. The broad peak located in the temperature range of 400–900 °C is most likely related to the evolution of CO because of the decomposition of these oxygen-containing groups such as carbonyls, ethers, ketones, quinones and hydroquinones [34,37]. The weight loss events suggest the removal of some oxygen-containing groups. Table 3 shows the elemental atomic concentrations (%) calculated from the individual peak area divided by the appropriate sensitivity factor in C1s XPS spectra of porous carbons fitted with five component peaks. The ratio of total oxygen to total carbon, Ot/Ct, which indicates the degree of surface oxidation [37], is also included. The individual peaks represent aromatic and aliphatic carbons C–C and C–H with a binding energy (BE) of 284.5 eV (C1), carbon species in hydroxyl or alcohol or ether groups (C–OH or C–O–C) with a BE of 285.8 eV (C2), carbon species in carbonyl or quinone groups (C@O and O–C–O) with a BE of 287.0 eV (C3), carbon species in carboxyl or ester groups (COOH) with a BE of 288.8 eV (C4), and a satellite signal due to p ! p* transitions in aromatic rings with a BE of 290.3 eV (C5), respectively [38–41].
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In Table 3, the slightly higher carbon atomic concentration, less oxygen atomic concentrations and lower Ot/ Ct values observed on the treated carbons (CS-1N, CS2N and CF-1N) than that on the original carbons (CS-1, CS-2 and CF-1) indicate that thermal treatment in nitrogen removed some non-carbon elements such as oxygen-containing groups. Table 3 also lists the atomic concentrations of C1s individual peak calculated from the peak areas of the various carbon species. Compared to the original carbons, the thermally treated carbons have relatively higher concentrations of hydroxyl groups (C2), carbonyl groups (C3) and p ! p* transitions (C5), but a lower concentration of carboxylic groups (C4). The increase of the p ! p* peak area (C5) after thermal treatment may be a result of the decrease in C1 peak, suggesting an increased polyaromatic nature of the carbon surface [41]. Although oxygen functional groups possibly react with phenol molecules via an ester formation route [42], causing chemisorption on the carbon surface, physical adsorption of phenol has been shown to be predominant [16]. Based on the p–p dispersion interaction between the benzene ring of phenol and the carbon basal planes [19], it is well know that acidic groups on porous carbon surface, especially carboxyl, can remove electrons from the p-electron system of the carbon basal planes, creating positive holes in the conducting p-band of the graphitic planes, leading to weaker interactions between the p-electrons of the phenol aromatic ring and the p-electrons of the basal planes [33,43–46], thus lowering phenol adsorption amount. The data in Table 3 show that after thermal treatment, reduction in the amount of carboxyl oxygen groups and increase in polyaromatic characters for treated carbon surface can increase the p-electron density in the graphene layers, giving a rise to an enhancement of carbon dispersive adsorption. In addition, the increase of carbonyl groups may attribute to the improvement of phenol adsorption due to an electron donor–acceptor mechanism [20], which suggests an interaction between the carbonyl oxygen groups and the phenol aromatic ring, where
Table 3 Atomic concentrations (%) of elements and C1s individual peaks Sample
CS-1 CS-1N CS-2 CS-2N CF-1 CF-1N
Atomic concentration of element (%) C
O
89.2 91.6 89.1 90.6 86.6 94.7
10.2 7.8 9.9 8.5 12.2 4.3
Ot/Ct
0.11 0.09 0.11 0.09 0.14 0.05
Atomic concentration of individual peak (%)
C1 C–C, C–H
C2 C–OH
C3 C@O
C4 COOH
C5 p ! p*
54.8 55.5 64.4 58.1 62.3 58.3
16.9 17.3 13.5 16.5 10.6 14.6
7.4 7.6 3.2 6.2 3.6 7.0
5.7 4.3 6.2 5.2 6.9 4.4
4.4 6.9 1.8 4.6 3.3 10.4
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the carbonyl oxygen groups of the carbon surface act as an electron donor and the aromatic ring of the phenol act as an electron acceptor. As discussed earlier in Table 3, the thermal treatment under nitrogen increased the carbonyl groups and led to more carbonyl sites for the donor–acceptor interactions [15,16]. Therefore, it is believed that p–p dispersive interactions and the donor– acceptor mechanism are the key parameters determining the increase in phenol adsorption in this study. 3.4. Effect of water molecular clusters The adsorption isotherms of water vapor on different porous carbons are shown in Fig. 6. It can be seen that the isotherms of mesoporous carbons CS-1 and CS-2 before and after thermal treatment all display a type III isotherm and those of microporous carbon CF-1 and CF-1N are of type V [47]. The steep rise in the relative pressure range of 0.5–0.7 for carbons CF-1 and CF1N is caused by the growth of water clusters [47]. It is also seen that the adsorption amounts of water vapor on the thermally treated carbons at low relative pressures were decreased, in particular for samples CS-1N and CS-2N, indicating an enhancement of surface hydrophobicity due to the removal of oxygen functional groups [48]. When a porous carbon comes into contact with a phenol solution, water will first adsorb on the hydrophilic polar oxygen groups including those located at the micropore entrances [49,50] because water molecules can form H-bonding with surface oxygen groups [19,51] and water molecules are more competitive than phenol towards the adsorption sites [46], especially at neutral pH and extremely low concentration [16]. The adsorbed water molecules will be further associated with each other to form water clusters [48,52],which are remarkably stabilized in micropores [53], causing partial blockage of the micropores, reducing the accessible surface area [2,46], and impeding or even preventing phenol adsorption [50,54]. As demonstrated by the data shown in Fig. 5 and Table 3, thermal treatment can substan-
Phenol adsorption in dilute solution on template-synthesized porous carbons depends on both surface oxygen groups and pore structures. Thermal treatment of porous carbons under nitrogen can markedly facilitate phenol adsorption because of the substantial removal of the oxygen groups. At low phenol concentrations, the surface chemistry of the carbons is of most importance and affects the adsorption behaviors of phenol to a large extent. For porous carbons with a similar surface chemistry, the maximum phenol adsorption capacity increases with the increases in surface area and micropore volume of the porous carbons. The enhancement of phenol adsorption capacity after thermal treatment under nitrogen can be explained by (1) fewer water clusters formed on the carbon surface due to the removal of oxygen groups on carbon surface, (2) the stronger dispersive interactions between the benzene ring of phenol and the carbon basal planes because of the reduction of carboxylic groups and an increase of polyaromatic characters on carbon surface, (3) more carbonyl groups that pro-
800 CS-1 CS-1N
40 200 0 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (P/Po)
1000
(b)
800 600
CS-2 CS-2N
400 200 0 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (P/Po)
Adsorbed amount (mg/g)
(a)
Adsorbed amount (mg/g)
Adsorbed amount (mg/g)
4. Conclusions
1000
1000
600
tially remove oxygen-containing groups, especially carboxyl groups, dramatically reducing the adsorption of water molecules on the surfaces of the treated samples (see Fig. 6), lessening the possibility of the formation of water clusters, enhancing the accessibility of the pores for phenol molecules, and improving the interaction potential between phenol molecules and the carbon surface as a whole. Particularly, the modification of surface oxygen groups resulted in an enhancement in phenol adsorption capacity at low equilibrium concentrations than that at high concentrations because water molecules are preferentially bound by H-bonds with surface oxygen groups in competition with phenol [16,34,55]. Therefore, the adsorption capacities of the treated carbons were largely increased, despite that their surface areas and pore volumes (see the data in Table 2) were decreased to some different extent because of the thermal treatment.
(c) 800 600
CF-1 CF-1N
400 200 0 0.0 0.2 0.4 0.6 0.8 1.0
Relative pressure (P/Po)
Fig. 6. Adsorption isotherms of water vapor on (a) CS-1 and CS-1N, (b) CS-2 and CS-2N, (c) CF-1 and CF-1N at 21 °C.
F. Su et al. / Carbon 43 (2005) 1156–1164
vide more sites for the donor-accepter interactions, (4) creation of more smaller micropores in pore structure.
Acknowledgment We thank NUS for financial support (grant number R279000124112).
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Phenol adsorption on zeolite-templated carbons with different structural and surface properties Fabing Su, Lu Lv, Tee Meng Hui, X.S. Zhao
*
Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Received 16 September 2004; accepted 8 December 2004
Abstract The adsorption behaviors of phenol in aqueous solution on zeolite-templated porous carbons with different pore structures and surface properties were studied. The structural and surface properties of the carbon samples were characterized using N2 adsorption, water vapor adsorption, X-ray photoelectron spectrometer (XPS), and thermogravimetric analysis (TGA). It was observed that thermal treatment under nitrogen can significantly eliminate surface oxygen-containing groups, thus altering phenol adsorption behaviors. The adsorption capacity of phenol on the porous carbons after thermal treatment was markedly increased in spite of reduced surface area and pore volume. Possible reasons behind the experimental observations including the formation of water molecular cluster, p–p dispersive interaction, electron donor–acceptor mechanism, and micropore structure were discussed. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Porous carbon; Surface treatment; Surface oxygen complexes, Adsorption properties
1. Introduction Porous carbons have been widely applied in liquidphase adsorption, separation, and purification processes [1]. The use of porous carbons for adsorptive removal of phenols and their derivatives frequently presented in many wastewater streams has been a recent research focus [1,2]. On the other hand, as phenol can serve to indicate the adsorption characteristics of small polar aromatic compounds [3], its adsorption behaviors on various commercial activated carbons have been widely investigated, aimed to clarify adsorption mechanism. Generally, the surface and pore structure properties of a porous carbon are believed to be the key factors determining the adsorption behaviors of the carbon–phenol system [1,2]. The surface chemistry of a porous carbon depends on the presence or absence of heteroatoms such as oxygen, *
Corresponding author. Tel.: +65 68744727; fax: +65 67791936. E-mail address: [email protected] (X.S. Zhao).
0008-6223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.12.034
nitrogen, hydrogen, and sulfur, which exist as surface functional groups at the edges of graphene layers [4]. Oxygen-containing functional groups such as carboxyls, carbonyls, phenols, lactones, aldehydes, ketones, pyrones, quinones, hydroquinones, and anhydrides [4–8] have a significant effect on the interactions between an adsorbate and the carbon surface [2]. The oxygen-containing groups and the delocalized electrons of the graphitic structure can, to a large extent, determine the charge and acidity/basicity of the porous carbon surface [2,6,9]. Carboxyl, phenolic hydroxyl and lactonic groups are acidic while carbonyl, pyrone, chromene and quinone are basic [2,7,10–14]. pH and temperature could lead to different effects of the surface functional groups on phenol adsorption [1,2,15]. Although chemisorption is insignificant in a phenol–carbon adsorption system [16], it always occurs due to the presence of oxygen in the phenol solution and/or on the surface of the carbon. Such oxygen molecules may facilitate the irreversible adsorption and oxidative coupling reactions via catalysis [15,17]. In addition, oxygen-containing basic groups
F. Su et al. / Carbon 43 (2005) 1156–1164
such as the chromene-type and pyrone-type are likely to promote irreversible adsorption of phenolic compounds [18]. Due to the complicated surface properties of porous carbon, several mechanisms of phenol adsorption on porous carbon have been proposed, such as the p–p dispersion interaction mechanism [19], the hydrogenbonding formation mechanism [19], the electron donor–acceptor complex mechanism [20], and the ‘‘solvent effect’’ mechanism [15,21]. It must be noted that solution pH and temperature can change the general adsorption mechanisms [15,16]. The pore structure of a porous carbon containing micropores, mesopores and macropores can also greatly influence phenol adsorption because the pores of different sizes play different roles in the adsorption process. Three stages of phenol adsorption on porous carbon involving adsorption at infinite dilution, micropore filling, and adsorption in larger micropores and in mesopores [15] are believed to be cooperative. Recently, template synthesis of novel porous carbons with tailored pore structures and surface properties has been successfully demonstrated [22–27]. These templatesynthesized porous carbons are potential adsorbents and provide a good opportunity for fundamental study of adsorption. This work attempted to investigate the influence of structural and surface properties of zeolite Y-templated porous carbons on phenol adsorption characteristics in the aqueous solution.
2. Experimental 2.1. Template synthesis of porous carbons In this study, carbon samples with different pore sizes were prepared with two carbon precursors (sucrose and furfuryl alcohol) in the presence of zeolite Y template. To prepare a carbon sample with substantial mesopores, sucrose (98%, Fluka) and sodium-form zeolite Y (Na-form, SiO2/Al2O3 = 5.1, Zeolyst International Company, USA) were used as the carbon source and template, respectively. 10 g of zeolite Y powder was dried at 200 °C for 4 h before adding into a solution containing 10 g sucrose and 0.001 L sulfuric acid (98%, Fisher Scientific, UK) in 0.050 L deionized water. The mixture was stirred at room temperature for 4 h, followed by drying at 100 °C for 15 h and 160 °C for 6 h to form a zeolite/sucrose mixture. This mixture was carbonized at 1000 °C for 4 h in a quartz tube under nitrogen flow (99.9995%, 30 cm3/min) and subsequently treated with 46% aqueous HF solution to etch away the zeolite framework. The black carbon was washed with copious deionized water and dried at 150 °C. The carbon sample thus prepared is designed CS-1, where C means porous carbon and S refers to sucrose precursor. Another
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carbon sample also with substantial mesopores, denoted as CS-2, was similarly prepared except a carbonization temperature of 800 °C was used instead of 1000 °C. To prepare a carbon sample with substantial micropores, liquid furfuryl alcohol (98%, ACROS ORGANICS, USA) and ammonium-form zeolite Y (NH4Y-form, SiO2/Al2O3 = 5.1, Zeolyst International Company, USA) were used as the carbon precursor and template, respectively. A carbonization temperature 1100 °C was used. The detailed preparation can be found elsewhere [27]. The carbon thus prepared is denoted CF-1, where F means furfuryl alcohol precursor. To modify the surface properties of the carbon samples, thermal treatment was conducted at 900 °C for 4 h under pure nitrogen to obtain samples designed as CS-1N, CS-2N and CF-1N, respectively. 2.2. Characterization The structures of the samples were investigated with physical adsorption of nitrogen at 196 °C on an automatic volumetric sorption analyzer (Quantachrome, NOVA1200). Prior to the measurements, samples were degassed at 200 °C for 5 h under vacuum of 101 Torr. The specific surface areas (SBET) were determined using the Brunauer–Emmett–Teller (BET) method in the relative pressure range of 0.05–0.2. The micropore volumes (Vmi) were obtained with the accumulative pore volume of density-functional-theory (DFT) method. The total pore volumes (Vt) were obtained from the volumes of nitrogen adsorbed at a relative pressure of 0.95. The mesopore volumes (Vme) were calculated by subtracting Vmi from Vt. The ratio of Vmi/Vt was used to indicate microporosity rate Rmi. The pore size distribution curves and cumulative pore volume plots were obtained using the DFT method. The thermal behaviors of the carbons were evaluated using a thermogravimetric analyzer (TGA 2050, Thermal Analysis Instruments, USA) under highly pure nitrogen (99.9995%). The surface chemical compositions of the samples were determined using X-ray photoelectron spectroscopy (XPS) (Kratos Analytical, AXIS HSI 165 spectrometer) with a monochromatized AlKa X-ray source (1486.71 eV). Adsorption of water vapor was conducted on a home-built adsorption system. Prior to adsorption measurements, samples were dried at 150 °C overnight under vacuum (101 Torr). The equilibrium adsorption amount at 21.0 ± 0.5 °C was determined by the weight gain after adsorption equilibrium was attained. 2.3. Adsorption of phenol in aqueous solution Stock solution of phenol (Merck, 99.5%) with a concentration of 200 mg/L (pH = 7, unbuffered) was prepared using deionized water. A carbon sample dried at
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150 °C overnight under vacuum of 101 Torr was added into a flask containing 0.02 L of a phenol solution. After shaken at 30 °C for 3 days, the phenol solution was filtered and the concentration was analyzed using a UV– visible spectrophotometer (UV-1601, Shimadzu, Japan) at a wavelength of 269.5 nm. In order to reduce measurement errors, the UV absorption intensity of each equilibrium solution sample was measured three times and the average value was used to calculate the equilibrium concentration based on a standard calibration curve, whose correlation coefficient square (R2) was 0.999. The amount adsorbed at equilibrium, qe (mg/g), was calculated using the following equation: qe ¼
ðC 0 C e ÞV W
ð1Þ
where C0 and Ce are the initial and equilibrium concentrations of phenol (mg/L), respectively, V (L) is the volume of the solution and W (g) is the mass of the carbon, qe (mg/g) is the amount adsorbed at equilibrium concentration Ce (mg/L). The adsorption equilibrium data were fitted to the Langmuir equation: Ce 1 1 ¼ þ Ce qe K L q0 q0
ð2Þ
where q0 (mg/g) is the maximum adsorption capacity, KL (L/mg) is the adsorption equilibrium constant, characteristic of affinity between the adsorbent and adsorbate, KL andq0 can be obtained by linear regression of (Ce/qe) against Ce.
3. Results and discussion 3.1. Adsorption isotherms of phenol
Table 1 Adsorption parameters of Langmuir equation in phenol solution Sample
Langmuir equation
CS-1 CS-1N CS-2 CS-2N CF-1 CF-1N
400 350
1.6
(a)
CS-1 CS-2 CF-1
250 200 150
CS-1 CS-2 CF-1
CS-1N CS-2N CF-1N
0.0
0
40
80 120 Ce (mg/L)
160
200
0.060 0.111 0.070 0.151 0.023 0.050
0.993 0.996 0.997 0.999 0.982 0.987
Ce
þq qeN K q 0 ¼ 1L 0 Ce qe þ K q q LN 0N
ð3Þ
0N
Using the parameters of the Langmuir model in Table 1, Eq. (3) becomes qeN 16:7 þ C e ¼ qe 6:01 þ 0:67C e
ð4Þ
ð5Þ
for carbons CS-2N and CS-2, and
0.8
qeN 43:5 þ C e ¼ qe 12:40 þ 0:62C e
50 0
138.9 208.3 128.2 192.3 200.0 322.6
qeN 14:3 þ C e ¼ qe 4:42 þ 0:67C e
0.4
100
R2
for carbons CS-1N and CS-1, (b)
1.2 Ce/qe (g/L)
qe (mg/g)
300
CS-1N CS-2N CF-1N
KL (L/mg)
CF-1 is higher than those on samples CS-1 and CS-2. Similarly, for thermally treated carbons, CF-1N also displays a higher adsorption capacity than CS-1N and CS-2N. Moreover, the Langmuir linear fitting to experiment data seems very well in Fig. 1b. The parameters of fitting the experimental adsorption equilibrium data are given in Table 1. The data calculated from the Langmuir equation reveal a pronounced increase in the maximum adsorption capacity (q0) upon thermal treatment under nitrogen and a twofold increase in the adsorption equilibrium constant (KL), which is an indication of the affinity between phenol and carbon surface. In any cases, when considering the value of correlation coefficient square (R2), it seems that the Langmuir model is better fitted to the experimental data of the treated carbons than to the original carbons, possibly implying that the thermal treatment under nitrogen had resulted in a more homogeneous surface of the carbons. The ratios of phenol equilibrium adsorption capacity on the thermally treated carbons (qeN) over that on the original carbons (qe), i.e. qeN/qe, given in Eq. (3), were used to help understand the adsorption behaviors of phenol on the carbon samples. 1
Fig. 1 shows the equilibrium adsorption isotherms and Langmuir fitting cures of phenol on the different porous carbons. The higher adsorption capacities of phenol on the thermally treated samples than on their original counterparts are obviously seen. For original carbons, the amount of phenol adsorbed on sample
q0 (mg/g)
0
50
100 150 Ce (mg/L)
200
Fig. 1. Experimental isotherms (a) and Langmuir fitting cures (b) of phenol adsorption on different porous carbons.
ð6Þ
for carbons CF-1N and CF-1. Fig. 2 shows the plots of qeN/qe against equilibrium concentrations. It can be seen that that as phenol concentration increases, the ratio of qeN/qe decreases dramatically and finally approaches constant, indicating
F. Su et al. / Carbon 43 (2005) 1156–1164
3.5
CS-1 series CS-2 series CF-1 series
qeN /qe
3.0
2.5
2.0
1.5 0
50
100
150
200
Ce (mg/L) Fig. 2. The curves of adsorption capacity ratio for original and treated carbons as a function of equilibrium concentration.
the different effects of pore structure and surface chemistry of the porous carbons at different equilibrium concentrations of phenol. 3.2. Effect of pore structure Shown in Fig. 3 is the nitrogen adsorption/desorption isotherms of the carbon samples before and after thermal treatment. It can be seen that the adsorption isotherms of sample CS-1, CS-1N, CS-2 and CS-2N are all of type IV with an H4 type hysteresis loop, characterizing mesoporous materials with slit-like pores [28]. For samples CF-1 and CF-1N in Fig. 3, the isotherms are of type I, indicating a microporous material [28]. The amount of nitrogen adsorbed on sample CF-1 gradually increased in the whole relative pressure range, whereas the amount of nitrogen adsorbed on sample CF-1N
1600
Volume (cm3/g, STP)
CF-1N CF-1 1200
CS-1N CS-1 CS-2N
800
CS-2 400
0 0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0) Fig. 3. Nitrogen adsorption/desorption isotherms of porous carbons (for clarity, the isotherms of CS-1, CS-1N, CS-2N, CF-1 and CF-1N were vertically shifted for 100, 450, 250, 200 and 800 cm3/g, respectively).
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reached a plateau after relative pressure reached about 0.2, implying that carbon CF-1 contains a more amount of mesopores. Fig. 4 presents the DFT pore size distribution curves and cumulative pore volume plots (inset) of the porous carbons. The pore size distribution curve of sample CS-1 in Fig. 4a displays two broad peaks, centered at around 1.5 and 2.7 nm, respectively. However, after thermal treatment, three peaks located at about 0.5, 1.2 and 2.7 nm, respectively, are seen. From the inset in Fig. 4a it is seen that the cumulative pore volume of CS-1N is larger than that of CS-1 in the pore size less than 1.0 nm. Possibly due to the similar preparation of carbon CS-2 to that of carbon CS-1, these two samples display similar pore size distribution. Nitrogen treatment of carbon CS-2 also resulted in the appearance of an additional peak at about 0.5 nm on the pore size distribution curve of CS-2N, while the cumulative pore volume of CS-2N is larger than that of CS-2 in the pore size less than 1.4 nm in Fig. 4b. For samples CF-1 and CF-1N in Fig. 4c. The pore size distribution curves of the two carbons show the presence of a mainly shape peak centered at 1.5 and 1.2, respectively, together with other small peaks in the lager micropore and mesopore range. The cumulative pore volume of CF-1N is also larger than that of its original carbon in the pore size less than 1.3 nm in Fig. 4c-inset. Thus, it can be concluded that thermal treatment of the carbon samples had led to the generation of a considerable amount of smaller micropores. The pore structure parameters derived from the isotherms are compiled in Table 2. As can be seen, compared with the original carbons, thermal treatment at 900 °C under nitrogen for 4 h caused a decrease in BET surface area and pore volume, which might be attributed to partial collapses of the pore structures. The samples CS-1 and CS-2 have the same values as CS-1N and CS-2N in microporosity Rmi, respectively. On the contrary, the microporosity of CF-1N is obviously higher than that of CF-1, further suggesting the formation of small micropores at the expense of large micropores and mesopores. It is observed that the maximum phenol adsorption capacities (q0) of the original and treated porous carbons in Table 1 increase with their surface areas and micropore volumes, respectively. In addition, assuming monolayer adsorption of phenol molecules on carbon surface, the number of phenol molecules adsorbed on per unit surface area (Xm, number/ nm2) can be calculated from the maximum adsorption capacities in Table 1 and BET surface areas (SBET) in Table 2. It is seen that the values of Xm estimated data in Table 2 for the original carbons is comparable, namely in the range Xm = 0.4–0.6, so as is for the thermally treated carbons, namely in the range Xm = 1.0– 1.2. The Xm values of the treated carbons are all at least doubled in comparison with their original counterparts.
0.6 0.4 0.2 0.0
0
1 2 3 4 Pore size (nm)
CS-1 CS-1N
0.2 0.0
0.6 0.4
0.4 0.2 0.0
0 1 2 3 4 Pore size (nm)
CS-2 CS-2N
(c)
2.5 2.0 1.5
1.5
3
3
0.6
0.2 0.0
0 1 2 3 4 5 6 7 Pore size (nm)
3.0
0.8
dV/dD (cm3/nm/g)
0.4
(b)
0.8
Pore volume (cm /g)
0.6
0.8
1.0
dV/dD (cm3/nm/g)
dV/dD (cm3/nm/g)
(a)
3
0.8
Pore volume (cm /g)
F. Su et al. / Carbon 43 (2005) 1156–1164
Pore volume (cm /g)
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1.0 0.5 0.0
0 1 2 3 4 Pore size (nm)
1.0
CF-1 CF-1N
0.5 0.0
0 1 2 3 4 5 6 7 Pore size (nm)
0 1 2 3 4 5 6 7 Pore size (nm)
Fig. 4. DFT pore size distribution curves and cumulative pore volume plots (inset) of (a) CS-1 and CS-1N, (b) CS-2 and CS-2N, and (c) CF-1 and CF-1N.
Table 2 Pore structure parameters of porous carbons Samples
SBETa (m2/g)
Vtb (cm3/g)
Vmic (cm3/g)
Vmed (cm3/g)
Rmie
Xm f
CS-1 CS-1N CS-2 CS-2N CF-1 CF-1N
1752 1339 1354 1062 3054 1917
1.32 1.05 0.95 0.80 1.69 0.91
0.48 0.38 0.41 0.34 1.04 0.75
0.84 0.67 0.54 0.46 0.65 0.16
0.36 0.36 0.43 0.42 0.62 0.82
0.5 1.0 0.6 1.2 0.4 1.1
a b c d e f
BET surface area. Total pore volume. Micropore volume. Mesopore volume. Microporosity. Number of phenol molecules adsorbed on per unit surface area.
In general, adsorption capacity depends, to a large extent, on the accessibility of the organic molecules to the inner surface of carbon adsorbent. Small molecules can access micropores driven by the strong adsorption potential near the micropore wall. It has been known that to some extent, the adsorption of phenol is mainly due to micropore filling [29–33]. It has been noted that during thermal treatment, the creation of small micropores occurred because of opening of the micropores blocked by surface oxygen atoms [2,29], especially in the case of carbon CF-1. This possibly enhanced the adsorption potential on the carbon surface, thus collectively increasing phenol adsorption. In phenol solutions, water molecules are preferentially adsorbed by the oxygen groups and then the remainder of the surface and/or the micropore volume is available to the phenol molecules [29]. Thus, the large ratio of qeN/qe at the low equilibrium concentration observed in Fig. 2 is partially pertinent to the smaller micropore space available due to lower content of functional groups, which occupy the entrances or walls of micropores. On the other hand, because the three original carbons were subjected to HF washing and the three treated carbons also underwent same thermal treatment, they had a similar surface property, respectively. Therefore, it can be reasoned that
the maximum adsorption capacities of phenol on original and treated porous carbons are correlated with the surface area and micropore volume of the carbons, respectively. Also, it can be concluded that the value of Xm with two different levels of 0.4–0.6 (original carbons) and 1.0–1.2 (treated carbons) in Table 2 is correlated to the surface chemistry, not pore structure. Additionally, although mesopores of a porous carbon, which may shorten the diffusion path of phenol molecules to the carbon interior surface with adsorptive sites of higher adsorption energy [3], play an important role in adsorption process, no direct correlation of the phenol adsorption capacity with mesopore volume was observed in this study. 3.3. Effect of surface chemistry The thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of porous carbons CS-1, CS-2 and CF-1 under nitrogen atmosphere are shown in Fig. 5. For carbons CS-1, CS-2 and CF-1, there was about 13–15% weight loss when temperature was increased to 900 °C in Fig. 5a. Three peaks can be seen from the DTG curves shown in Fig. 5b. The peak below 200 °C is due to weight loss of physically adsorbed spe-
F. Su et al. / Carbon 43 (2005) 1156–1164
Weight (%)
(b) Deriv.Weight (%/oC)
(a)
100
95
90
85
CS-1 CS-2 CF-1
0
200 400 600 800 1000 o Temperature ( C)
0
CS-1 CS-2 CF-1
200 400 600 800 1000 o Temperature ( C)
Fig. 5. (a) TG and (b) DTG curves of original porous carbons under nitrogen.
cies like water. The one from 200 to 400 °C with the strongest intensity for samples CS-1 and CS-2 is most probably due to the evolution of CO2 from the decomposition of carboxylic and carboxylic-group derivatives [34–37]. The broad peak located in the temperature range of 400–900 °C is most likely related to the evolution of CO because of the decomposition of these oxygen-containing groups such as carbonyls, ethers, ketones, quinones and hydroquinones [34,37]. The weight loss events suggest the removal of some oxygen-containing groups. Table 3 shows the elemental atomic concentrations (%) calculated from the individual peak area divided by the appropriate sensitivity factor in C1s XPS spectra of porous carbons fitted with five component peaks. The ratio of total oxygen to total carbon, Ot/Ct, which indicates the degree of surface oxidation [37], is also included. The individual peaks represent aromatic and aliphatic carbons C–C and C–H with a binding energy (BE) of 284.5 eV (C1), carbon species in hydroxyl or alcohol or ether groups (C–OH or C–O–C) with a BE of 285.8 eV (C2), carbon species in carbonyl or quinone groups (C@O and O–C–O) with a BE of 287.0 eV (C3), carbon species in carboxyl or ester groups (COOH) with a BE of 288.8 eV (C4), and a satellite signal due to p ! p* transitions in aromatic rings with a BE of 290.3 eV (C5), respectively [38–41].
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In Table 3, the slightly higher carbon atomic concentration, less oxygen atomic concentrations and lower Ot/ Ct values observed on the treated carbons (CS-1N, CS2N and CF-1N) than that on the original carbons (CS-1, CS-2 and CF-1) indicate that thermal treatment in nitrogen removed some non-carbon elements such as oxygen-containing groups. Table 3 also lists the atomic concentrations of C1s individual peak calculated from the peak areas of the various carbon species. Compared to the original carbons, the thermally treated carbons have relatively higher concentrations of hydroxyl groups (C2), carbonyl groups (C3) and p ! p* transitions (C5), but a lower concentration of carboxylic groups (C4). The increase of the p ! p* peak area (C5) after thermal treatment may be a result of the decrease in C1 peak, suggesting an increased polyaromatic nature of the carbon surface [41]. Although oxygen functional groups possibly react with phenol molecules via an ester formation route [42], causing chemisorption on the carbon surface, physical adsorption of phenol has been shown to be predominant [16]. Based on the p–p dispersion interaction between the benzene ring of phenol and the carbon basal planes [19], it is well know that acidic groups on porous carbon surface, especially carboxyl, can remove electrons from the p-electron system of the carbon basal planes, creating positive holes in the conducting p-band of the graphitic planes, leading to weaker interactions between the p-electrons of the phenol aromatic ring and the p-electrons of the basal planes [33,43–46], thus lowering phenol adsorption amount. The data in Table 3 show that after thermal treatment, reduction in the amount of carboxyl oxygen groups and increase in polyaromatic characters for treated carbon surface can increase the p-electron density in the graphene layers, giving a rise to an enhancement of carbon dispersive adsorption. In addition, the increase of carbonyl groups may attribute to the improvement of phenol adsorption due to an electron donor–acceptor mechanism [20], which suggests an interaction between the carbonyl oxygen groups and the phenol aromatic ring, where
Table 3 Atomic concentrations (%) of elements and C1s individual peaks Sample
CS-1 CS-1N CS-2 CS-2N CF-1 CF-1N
Atomic concentration of element (%) C
O
89.2 91.6 89.1 90.6 86.6 94.7
10.2 7.8 9.9 8.5 12.2 4.3
Ot/Ct
0.11 0.09 0.11 0.09 0.14 0.05
Atomic concentration of individual peak (%)
C1 C–C, C–H
C2 C–OH
C3 C@O
C4 COOH
C5 p ! p*
54.8 55.5 64.4 58.1 62.3 58.3
16.9 17.3 13.5 16.5 10.6 14.6
7.4 7.6 3.2 6.2 3.6 7.0
5.7 4.3 6.2 5.2 6.9 4.4
4.4 6.9 1.8 4.6 3.3 10.4
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the carbonyl oxygen groups of the carbon surface act as an electron donor and the aromatic ring of the phenol act as an electron acceptor. As discussed earlier in Table 3, the thermal treatment under nitrogen increased the carbonyl groups and led to more carbonyl sites for the donor–acceptor interactions [15,16]. Therefore, it is believed that p–p dispersive interactions and the donor– acceptor mechanism are the key parameters determining the increase in phenol adsorption in this study. 3.4. Effect of water molecular clusters The adsorption isotherms of water vapor on different porous carbons are shown in Fig. 6. It can be seen that the isotherms of mesoporous carbons CS-1 and CS-2 before and after thermal treatment all display a type III isotherm and those of microporous carbon CF-1 and CF-1N are of type V [47]. The steep rise in the relative pressure range of 0.5–0.7 for carbons CF-1 and CF1N is caused by the growth of water clusters [47]. It is also seen that the adsorption amounts of water vapor on the thermally treated carbons at low relative pressures were decreased, in particular for samples CS-1N and CS-2N, indicating an enhancement of surface hydrophobicity due to the removal of oxygen functional groups [48]. When a porous carbon comes into contact with a phenol solution, water will first adsorb on the hydrophilic polar oxygen groups including those located at the micropore entrances [49,50] because water molecules can form H-bonding with surface oxygen groups [19,51] and water molecules are more competitive than phenol towards the adsorption sites [46], especially at neutral pH and extremely low concentration [16]. The adsorbed water molecules will be further associated with each other to form water clusters [48,52],which are remarkably stabilized in micropores [53], causing partial blockage of the micropores, reducing the accessible surface area [2,46], and impeding or even preventing phenol adsorption [50,54]. As demonstrated by the data shown in Fig. 5 and Table 3, thermal treatment can substan-
Phenol adsorption in dilute solution on template-synthesized porous carbons depends on both surface oxygen groups and pore structures. Thermal treatment of porous carbons under nitrogen can markedly facilitate phenol adsorption because of the substantial removal of the oxygen groups. At low phenol concentrations, the surface chemistry of the carbons is of most importance and affects the adsorption behaviors of phenol to a large extent. For porous carbons with a similar surface chemistry, the maximum phenol adsorption capacity increases with the increases in surface area and micropore volume of the porous carbons. The enhancement of phenol adsorption capacity after thermal treatment under nitrogen can be explained by (1) fewer water clusters formed on the carbon surface due to the removal of oxygen groups on carbon surface, (2) the stronger dispersive interactions between the benzene ring of phenol and the carbon basal planes because of the reduction of carboxylic groups and an increase of polyaromatic characters on carbon surface, (3) more carbonyl groups that pro-
800 CS-1 CS-1N
40 200 0 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (P/Po)
1000
(b)
800 600
CS-2 CS-2N
400 200 0 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (P/Po)
Adsorbed amount (mg/g)
(a)
Adsorbed amount (mg/g)
Adsorbed amount (mg/g)
4. Conclusions
1000
1000
600
tially remove oxygen-containing groups, especially carboxyl groups, dramatically reducing the adsorption of water molecules on the surfaces of the treated samples (see Fig. 6), lessening the possibility of the formation of water clusters, enhancing the accessibility of the pores for phenol molecules, and improving the interaction potential between phenol molecules and the carbon surface as a whole. Particularly, the modification of surface oxygen groups resulted in an enhancement in phenol adsorption capacity at low equilibrium concentrations than that at high concentrations because water molecules are preferentially bound by H-bonds with surface oxygen groups in competition with phenol [16,34,55]. Therefore, the adsorption capacities of the treated carbons were largely increased, despite that their surface areas and pore volumes (see the data in Table 2) were decreased to some different extent because of the thermal treatment.
(c) 800 600
CF-1 CF-1N
400 200 0 0.0 0.2 0.4 0.6 0.8 1.0
Relative pressure (P/Po)
Fig. 6. Adsorption isotherms of water vapor on (a) CS-1 and CS-1N, (b) CS-2 and CS-2N, (c) CF-1 and CF-1N at 21 °C.
F. Su et al. / Carbon 43 (2005) 1156–1164
vide more sites for the donor-accepter interactions, (4) creation of more smaller micropores in pore structure.
Acknowledgment We thank NUS for financial support (grant number R279000124112).
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