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Preparation and ozone-surface modification of activated carbon. Thermal stability of oxygen surface groups
Applied Surface Science 256 (2010) 5232–5236
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation and ozone-surface modification of activated carbon. Thermal stability of oxygen surface groups J. Jaramillo a, P.M. A´lvarez a, V. Go´mez-Serrano b,* a b
Dpto. de Ingenierı´a Quı´mica y Quı´mica Fı´sica, UEx, Badajoz 06071, Spain Dpto. de Quı´mica Orga´nica e Inorga´nica, UEx, Badajoz 06071, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 28 December 2009
The control of the surface chemistry of activated carbon by ozone and heat treatment is investigated. Using cherry stones, activated carbons were prepared by carbonization at 900 8C and activation in CO2 or steam at 850 8C. The obtained products were ozone-treated at room temperature. After their thermogravimetric analysis, the samples were heat-treated to 300, 500, 700 or 900 8C. The textural characterization was carried out by N2 adsorption at 77 K, mercury porosimetry, and density measurements. The surface analysis was performed by the Bohem method and pH of the point of zero charge (pHpzc). It has been found that the treatment of activated carbon with ozone combined with heat treatment enables one to control the acidic– basic character and strength of the carbon surface. Whereas the treatment with ozone yields acidic carbons, carbon dioxide and steam activations of the carbonized product and the heat treatment of the ozonetreated products result in basic carbons; the strength of a base which increases with the increasing heat treatment temperature. pHpzc ranges between 3.6 and 10.3. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Cherry stones Activated carbon Ozone treatment Oxygen surface groups Thermal stability
1. Introduction Activated carbon is characterized by its porous structure and surface chemistry. Surface chemistry significantly influences the wettability, adsorptive, electrical, electrochemical, catalytic, acid– base, redox, hydrophilic–hydrophobic, and other properties [1,2]. It is determined by the type, quantity and bonding of the various heteroatoms, in particular oxygen which constitutes 2–25% by weight of activated carbon [3]. Heteroatoms are believed to adopt the character of the functional groups typical of aromatic compounds. Owing to great importance of oxygen groups of activated carbon, the surface composition of this material has been frequently modified via oxidation to increase the contents of such groups. With such an aim, a large number of oxidizing agents both in gas phase and aqueous solution has been used [4,5]. Ozone oxidation has been occasionally used in the surface modification of activated carbon [6–9] and also as a chemical method for its regeneration [10]. Because ozone is a very strong oxidant (standard reduction potential, E0 = +0.076 V), it has been successfully used in the surface modification of glassy carbon, which is a very stable chemically carbonaceous material [11]. In some instances, however, it may be of interest to decrease or even to totally eliminate the content of oxygen surface groups in activated carbon, for example to confer it a more specific
* Corresponding author. Tel.: +34 924 289300; fax: +34 924 271149. E-mail address: [email protected] (V. Go´mez-Serrano). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.12.109
behaviour in adsorption or catalysis processes. Selective removal of oxygen surface groups of activated carbon can be achieved simply by heat treatment of the material as oxygen starts giving off as CO and CO2 at temperatures generally above 227 8C and its complete removal requires temperatures as high as 977 8C [12]. Between 227 and 977 8C, depending on heat treatment temperature, the type and number of oxygen surface functional groups undergoing thermal decomposition can differ over a fairly wide range. The primary object of this research work was to propose a method for the control of surface composition of activated carbon. By taking into account that the properties of this material (i.e., its texture and surface composition and reactivity) depend on the starting material, method and conditions used in its preparation; here, in a series of successive steps, activated carbon was prepared from cherry stones, ozone-treated for modification of its surface composition, analyzed from the standpoint of the thermal behaviour, and subjected to heat treatment at various temperatures, prior to analyzing the oxygen surface groups present in the resultant products by titration methods and pH of the point of zero charge. Most samples were characterized in terms of texture and surface chemistry. Emphasis was put not only on the formation of oxygen surface groups by ozone-treatment of activated carbon but also on the thermal stability of such groups in connection with their presence in the final heat-treated carbons. In earlier study [8], the carbon Filtrasorb was treated with ozone for different time periods and the oxygen surface groups were analyzed by TPD and other tools. Furthermore, the textural changes produced as a result
J. Jaramillo et al. / Applied Surface Science 256 (2010) 5232–5236
of the ozone-oxidation treatment of the carbon were also investigated. 2. Materials and methods 2.1. Starting material and reagents The cherry stones (CS, hereafter) used as the starting material were furnished by the Agrupacio´n de Cooperativas Valle del Jerte (Ca´ceres Province, Spain). As received, CS were thoroughly washed with distilled water, air-dried, ground, and sized, the fraction of particle sizes between 1.6 and 2.0 mm being chosen for subsequent studies. Pure grade NaHCO3, Na2CO3, NaOH, and HCl (Merck) were used for titration analyses. 2.2. Methods The experimental methods used in the preparation of activated carbon, modification of its surface, and analysis are briefly described below. 2.2.1. Preparation of activated carbon The heat treatments were carried out in a vertical electrical furnace, provided with a system for temperature control. Using a steel reactor, 60 g of CS were first heated from ambient temperature to 900 8C at 20 8C min1 in N2 atmosphere (flow = 20 L min1). The time of isothermal heating at the maximum heat treatment temperature (MHTT) was 2 h. Then, the carbonized product (C900, 15 g) was activated in CO2 (CD) or steam (WV) atmosphere. In these treatments, the system was heated from room temperature to 850 8C at 20 8C min1 in N2 atmosphere (flow = 20 L min1). After that, the N2 atmosphere was switched to the activation atmosphere (i.e. CO2, flow = 20 L min1; steam carried by N2, flow = 20 L min1), which was held at 850 8C only for the selected residence time of 2 h for CO2 and 3 h for steam. Once this time had elapsed, the furnace was cooled down to room temperature in N2 atmosphere. CD and WV were stored in a desiccator until use. The steam generation system consisted of an HP (series 1050) pump and of a water vaporizer. A water flow of 0.4 mL min1 was continuously furnished to the vaporizer at 400 8C. 2.2.2. Surface modification of activated carbon by ozone treatment Approximately 10 g of CD or WV were placed in the steel reactor and treated with ozone at 25 8C for 1 h in an ozone-air stream (flow = 20 L min1), containing a prefixed ozone concentration ðP O3 ¼ 2 kPaÞ. Ozone was produced from air in a Constrema SLO generator and analyzed in the gas stream with a GM-19 Anseros Ozomar analyzer at 254 nm. The notations used for the ozonetreated products are CDO and WVO. 2.2.3. Textural characterization The textural characterization of the samples was accomplished by N2 adsorption at 196 8C, mercury porosimetry, and helium and mercury density measurements. The N2 isotherms were determined in a semiautomatic equipment, Quantachrome (Autosorb1). The carbon sample (0.10 g) was oven-dried at 120 8C overnight and outgassed at 250 8C also overnight under a pressure lower than 103 Torr, prior to the N2 adsorption measurements. The experiments of mercury intrusion were carried out in a porosimeter, Quantachrome (Autosacan-60), using 0.3 g of sample. The mercury density (rHg) was measured as usual. Approximately 0.3 g of carbon was first weighed and placed in the glass holder, which was then filled with mercury in the Filling Apparatus (Quantachrome) and weighed. From the resultant mass datum the volume of sample was eventually calculated, knowing
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the mercury density at the working temperature and the sample holder calibration volume. The helium density (rHe) was measured in a Quantachrome stereopycnometer, using 3 g of sample. From the N2 adsorption isotherm, the specific surface area of the samples was evaluated by the BET method [13], SBET. Also, the external surface area (Sex) and micropore volume (Vmi) were calculated by the as method [14]; Vmi was expressed as liquid volume. From the values of cumulative pore volume (Vcu) against pore radius (r) (mercury porosimetry) the macropore volume, Vma = Vcu (at r = 25 nm), and a mesopore volume, Vme = Vcu (at r < 2 nm) Vma, were obtained. The total pore volume was calculated as VT = Vmi + Vme + Vma and by expression V T 0 ¼ ð1=rHg Þ ð1=rHe Þ. 2.2.4. Thermogravimetric analysis The TG–DTG curves for the samples were obtained in a thermogravimetric apparatus, Mettler TA-3000, consisting of a TG50 thermobalance (precision: 2 8C in temperature; 1 mg in weight) and a TG-10 processor. About 120 mg of sample were heated from room temperature to 900 8C in N2 atmosphere (flow = 200 mL min1). The heating rate in the dynamic treatment was 10 8C min1. 2.2.5. Heat treatments Approximately 10 g of CD, WV, CDO or WVO were heated from room temperature to the MHTT, i.e. to 900 8C for CD and WV and to 300, 500, 700 or 900 8C for CDO and WVO, at 20 8C min1 in N2 atmosphere (flow = 20 L min1). The soaking time at MHTT was 1 h. The resultant products are designated as CDT, WVT, CDOT, and WVOT, T being MHTT. 2.2.6. Surface chemistry analysis The surface chemistry of a large number of samples was analyzed by the Boehm method [15] and pH of the point of zero charge [16] (pHpzc). A series of different strength bases in aqueous solution were used in the titration of various acidic oxygen surface groups of the carbons. Such solutions were 0.05 M NaHCO3 for carboxylic groups and carboxylic acid anhydrides, 0.05 M Na2CO3 for lactones and lactols, 0.05 M NaOH for phenolic hydroxyl groups, and 0.25 M NaOH or 0.1 M NaOC2H5 for acidic carbonyl groups. Corrections were introduced in the measured titration volumes by allowing the base of a given strength to neutralize those surface functional groups which are more acidic. On the other hand, a 0.05 M HCl solution was used for measuring the total content of basic groups. pHpzc was measured using 0.1 M NaNO3 aqueous solutions at pH 3, 5 or 11. These pH values were fixed by adding HNO3 or NaOH aqueous solution. Using a set of test tubes, 25 mL of each of such solutions were brought into contact with 25 g of carbon and the system was maintained under continuous stirring for 24 h. After that, the supernatant was separated by filtration and its pH was measured. The average value of the three pH measurements was taken as pHpzc. 3. Results and discussion 3.1. Preparation of the samples: mass changes The preparation of samples was carried out in three successive stages of carbonization, activation, and surface modification. The yield of the carbonization process of CS was as low as 24.8 wt.%, which indicates that the pyrolysis of CS caused the release of a great amount of volatile matter. As is well known, it is accompanied by an increase in the degree of aromatization of the residual carbonized product. For other lignocellulosic materials, also heated at 900 8C, the mass loss is slightly higher than for CS. As a guide, the carbonization yield was 21.4 wt.% for rockrose
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Fig. 1. N2 adsorption isotherms at 196 8C.
Fig. 2. Curves of mercury intrusion.
[17] and 22.0 wt.% for sawdust obtained from holm-oak wood [18]. For the activation of C900, the burn-off percentage was 22.0 in CO2 and 39.4 in steam. Accordingly, a larger amount of disorganized matter and of carbon atoms was gasified in the case of the steam activation process. As the activation time was not very different in both atmospheres (i.e. 2 h in carbon dioxide and 3 h in steam), it is attributable to a higher reactivity of the water molecule as compared to the CO2 molecule rather to the unequal diffusion of the substances in the incipient porosity of C900 as the molecular geometry is angular for water and linear for CO2. Finally, the mass increase produced as a result of the treatment of CD and WV with ozone was 2.1 and 2.0 wt.%, respectively. These close values of the mass increase do not mean at all that the amount of oxygen chemisorbed on the carbon surface was similar for both activated carbons as the chemisorption process could go on further and this causes mass loss.
increasing exposure time of a carbon Filtrasorb 400 to ozone has been previously reported by Valde´s et al. [8]. The curves of mercury intrusion (Fig. 2) show that all tested samples possess a well-developed porosity in the range of macropores. Nevertheless, the size of these pores is more uniform and their volume is higher (also see textural data in Table 1) by the order WV > CD > C900. Accordingly, macroporosity not only is different for two kinds of carbonaceous materials but is also conditioned by the activating agent. As regards mesoporosity, its degree of development is poor for C900 and also, though less, for CD. However, it is much larger for WV, which is in line with the higher burn-off percentage produced in the steam activation treatment of C900 and also with N2 adsorption results. The treatments of CD and WV with ozone to a certain extent developed mesoporosity, mostly for CD. However, the effect on macroporosity in relative terms was of little significance. The calculated values of pore volumes for the activated carbons and ozone-treated products (Table 1) are similar to those previously reported for typical activated carbons. For different activated carbons the volume of micropores is approximately 0.15–0.50 cm3 g1, the volumes of mesopores lie between 0.02 and 0.10 cm3 g1, and the volume of macropores is between 0.2 and 0.8 cm3 g1 [4]. For WVO SBET and Sex are as high as 968 and 180 m2 g1, respectively.
3.2. Textural analysis The N2 adsorption isotherms measured for the samples are shown in Fig. 1. At a glance it can be seen that the adsorption isotherm obtained for C900 is a typical type I isotherm of the BDDT classification system [19]. The isotherm is practically parallel to the x-axis in the entire P/P0 interval to P/P0 = 1. This means that the micropores present in C900 are narrow pores as they are filled with liquid adsorptive at very low relative pressures. From the relative position and shape of the adsorption isotherms obtained for C900, CD, and WV, it can be deduced that carbon dioxide and steam activations produced great development of microporosity. The effect is stronger for steam than for carbon dioxide. WV also contains an important fraction of mesopores of a wide range of pore sizes. On the other hand, it is worth noting that the effect of the ozone treatment on microporosity is opposite depending on the activated carbon sample. Microporosity increases for WV and decreases for CD. A gradual reduction of microporosity with
3.3. Ozone treatment: surface modification of activated carbon The values of the content of surface groups and of pHpzc obtained for CD, VW, CDO, and WVO are set out in Table 2. It is seen that the composition of the carbon surface and hence its acidic– basic character and strength depend on the activating agent used in the preparation of the carbons and also on whether they were subsequently treated with ozone or not. Firstly, it is seen that CD and WV contain surface groups which are acidic or basic in nature. The acidic groups include carboxyl, lactone, hydroxyl and carbonyl
Table 1 Calculated values of textural parameters. Sample
SBET (m2 g1)
Sex (m2 g1)
Vmi (cm3 g1)
Vme (cm3 g1)
Vma (cm3 g1)
VT (cm3 g1)
V T0 (cm3 g1)
C900 CD WV CDO WVO
204 604 901 603 968
19 35 122 46 180
0.084 0.299 0.383 0.278 0.380
0.035 0.055 0.134 0.073 0.142
0.238 0.341 0.473 0.356 0.503
0.36 0.70 0.99 0.71 1.03
0.37 0.78 0.92 – –
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Table 2 Functional group contents (mequiv./g) and pHpzc for the activated carbons and ozone-treated products. Sample
CG
L
HG
CaG
TAG
TBG
pHpzc
CD WV CDO WVO
16 4 487 680
6 12 14 45
74 48 98 78
42 28 63 103
138 92 662 906
385 369 302 231
8.8 9.8 4.3 3.6
Abbreviations: CG, carboxylic groups; L, lactones; HG, hydroxyl groups; CaG, carbonyl groups; TAG, total acidic groups; TBG, total basic groups; pHpzc, pH of the point zero charge.
oxygen functionalities [20,21]. The postulated basic surface groups are chromene-type structures [22] and pyrone-type oxides [23]. The existence of pyrone-type structures is supported by the studies of Leo´n y Leo´n et al. [24] and Papirer et al. [25]. For CD and WV the concentration of various acidic oxygen surface groups is low and the concentration of basic groups is higher. Since the latter groups are predominant, both virgin activated carbons possess a surface with basic character and hence pHpzc > 7. As a rule the content of acidic oxygen groups is higher for CD than for WV. Notice that the content of carboxylic groups is very low in both carbons, in particular for WV. Secondly, the treatment of CD and WV with ozone gave rise to the formation of a large amount of oxygen surface groups, mostly carboxylic ones. In view of these results, it appears that the chemical nature of the oxygen surface groups is influenced by the activated carbon and the chemical agent used in the oxidation treatment of the material. Thus, using the NORIT activated carbon and O2 and N2O (both in N2) in gas phase and HNO3 and H2O2 in aqueous solution, it was concluded earlier by Figueiredo et al. [5] that gas phase oxidation of the carbon increases mainly the concentration of hydroxyl and carbonyl groups while oxidations in liquid phase increase especially the concentration of carboxylic acids. The surface of CDO and WVO is acidic in character, pHpzc being 4.3 and 3.6 respectively. These pHpzc values also show the influence of the activating agent used in the preparation of the activated carbons on the surface properties of the ozone-treated products.
Fig. 4. TG–DTG curves. Samples: WV and WVO.
3.4. Thermal analysis From the TG–DTG curves obtained for CD, WV, CDO, and VWO (Figs. 3 and 4) it becomes clear that the thermal behaviour of the tested sample is strongly dependent on whether it is a virgin activated carbon or an ozone-treated product. As inferred from the TG curves, firstly, the total mass loss produced to 900 8C is much higher for CDO and WVO (10–11 wt.%) than for CD and WV (2– 3 wt.%). Accordingly, as expected, the amount of surface oxygen chemisorbed in CD and WV during the preparation of CDO and WVO greatly exceeds the mass increase of 2 wt.%. It indicates that an important amount of activated carbon was gasified during the treatment with ozone. Secondly, the DTG curves show the presence of five very well defined maxima of weight loss which are associated with the thermal decomposition of various oxygen surface groups. According to literature [1], in the course of heating a carbon sample under vacuum or in a neutral gas, particular surface compounds decompose at different characteristic temperatures yielding reaction products such as CO2 for carboxylic and lactone groups from ca. 200 to 700–800 8C; CO for quinone, phenol and ether groups from 200–300 to 400–500 8C; H2O for phenolic groups in the range from 200–300 to 400–500 8C; and H2 as a result of splitting of C–H and O–H bonds above 500–700 8C. In the case of CDO and WVO, the thermal effects on the mass of sample are assigned as described in Table 3. Notice that two peaks overlap between 500 and 700 8C for CDO. Also, peaks shift their position towards slightly higher temperatures for WVO.
Table 3 Ozone treatment of activated carbon. Thermal stability of oxygen surface groups.
Fig. 3. TG–DTG curves. Samples: CD and CDO.
Effect number
Temperature range (8C)
Oxygen surface group
Gas evolved
I II, III
150–300 300–400, 400–500
IV, V
500–800
Carboxylic Acid anydride Lactone Phenolic hydroxyl Carbonyl Ether
CO2 CO, CO2 CO2 CO
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Table 4 Effect of heat treatment on the functional group contents (mequiv./g) and pHpzc. Sample
CG
L
HG
CaG
TAG
TBG
pHpzc
CD900 WV900 CDO300 CDO500 CDO700 CDO900 WVO300 WVO500 WVO700 WVO900
– – 12 – – – 13 – – –
– – 36 – – – 51 – – –
– – 79 45 12 – 75 49 – –
12 – 71 72 16 11 121 102 35 –
12 12 198 132 28 11 260 151 35 –
436 461 345 425 479 558 355 389 503 588
9.5 10.1 8.5 9.0 9.5 9.9 7.3 8.9 9.6 10.3
Abbreviations: CG, carboxylic groups; L, lactones; HG, hydroxyl groups; CaG, carbonyl groups; TAG, total acidic groups; TBG, total basic groups; pHpzc, pH of the point zero charge.
The DTG curves (Figs. 3 and 4) show that the effects of mass loss are much stronger for CDO and WVO than for CD and WV, which denotes that the treatment of CD and WV with ozone is a very effective method to introduce surface oxygen and to form various oxygen groups. As shown by peak intensities, the effectiveness of the aforesaid method to modify the surface chemistry of the carbons is significantly influenced by the activating agent used in the preparation of CD and WV, as a larger amount of oxygen groups is formed in the case of steam. Because WV possesses better developed porosity in the regions of meso- and macropores (see data in Table 1) and the surface area is also larger for this activated carbon, the access of ozone to a larger number of active sites was possible. Finally, it must be highlighted that as a rule the thermal stability of the oxygen surface groups is different for the virgin activated carbons as compared to their respective ozone-treated products, as reflected by the change in peak positions of the DTG curves. 3.5. Thermal treatment: analysis of oxygen surface groups In view of the results obtained in the previous study on the thermal stability of the oxygen surface groups, CD, WV, CDO, and WVO were first subjected to heat treatment. CD and WV were heated from room temperature to 900 8C, whereas for CDO and WVO the heat treatment was conducted to 300, 500, 700 or 900 8C, by taking into account the temperature ranges in which the removal of oxygen surface groups occurred. In this way, a number of products were prepared and their surface chemistry was then analyzed. The values of the content of oxygen surface groups and of pHpzc obtained for the heat-treated products are listed in Table 4. It follows (also see Table 2 for comparison purposes) that the basic strength of the carbon surface is markedly influenced by heat treatment temperature. By simply heating the virgin carbons to 900 8C, the surface basic strength increases. pHpzc is 8.8 for CDO and 9.5 for CD900. For the ozone-treated products, the basic strength increases significantly with the increase in heat treatment temperature. pHpzc ranges between 8.5 and 9.9 for the CDOT samples and between 7.3 and 10.3 for the WVOT samples. Thus, the variation of pHpzc is greater for the steam activation-derived ozone-treated products. Also, pHpzc is slightly higher for CDO900 and WVO900 as compared in turn to CD900 and WV900. Accordingly, the treatment of the activated carbons with ozone yields a more basic surface after heat treatment at 900 8C. In an earlier study on the oxidation of activated carbon with nitric acid and on the thermal stability of the oxygen surface groups it was found that the total amount of pyrone-type groups is proportional to the initial degree of oxidation of the material and that the variation of pyrone content with heat treatment temperature follows similar trends as the temperature increased [25]. At high temperatures pyrone-type structures are generated by thermal decomposition of oxygenated acidic groups which create active sites capable of fixing oxygen in ether form, and rearrangement with
existing carbonyl groups which resist pyrolysis [26]. Also, the complete evolution of oxygen as CO, CO2 and H2O [27] delocalizes carbon basal plane electrons and results in surface sites with greater ability to donate electron pair and behave as Lewis bases [28]. The basic character of the activated carbon surface is an important property in connection with its use as, for example, oxygencontaining basic surface functional groups appear to be primarily responsible for the catalytic properties of the material towards oxidative coupling of phenolic compounds [28]. As a final comment it should be pointed out that the oxygen-containing surface complexes present on both HNO3- and air-oxidized carbons are essentially completely decomposed by heat treatment to 1000 8C [2]. 4. Conclusions From the above-results it may be concluded that by simply preparing activated carbon by CO2 or steam activation of a cherry stones-derived carbonized product, ozone-treated activated carbons, and effecting an eventual thermal treatment, the control of the acidic–basic character and strength of the surface chemistry of activated carbon is possible. The ozone treatment of the activated carbons changes the character of their surface from basic to acidic. pHpzc is as low as 3.6. However, the opposite applies to the heat treatment of the ozone-treated products. In this case, the basic strength of the carbon surface increases with the increasing heattreatment temperature. pHpzc is as high as 10.3. Acknowledgments This work has been supported by the Junta de Extremadura through project PDT08-A012. Dr. J. Jaramillo also thanks the Consejerı´a de Educacio´n of Junta de Extremadura of Spain for providing her a sabbatical research stay at the Departamento de Ingenierı´a Quı´mica of the University of Extremadura. References [1] H. Jankowska, A. Swiatkowski, J. Choma, Active Carbon, Ellis Horwood, New York, 1991. [2] Y. Otake, R.G. Jenkins, Carbon 31 (1993) 109. [3] W.F. Wolff, J. Phys. Chem. 63 (1959) 653. [4] J.T.P. Cookson Jr., in: N. Cheremisinoff, F. Ellerbusch (Eds.), Carbon Adsorption Handbook, Ann Arbor Science, Ann Arbor, 1980, pp. 241–279. [5] J.L. Figueiredo, M.F.R. Pereira, M.M.A. Freitas, J.J.M. O´rfa˜o, Carbon 37 (1999) 1379. [6] V. Go´mez-Serrano, P.M. Alvarez, J. Jaramillo, F.J. Beltra´n, Carbon 40 (2002) 513. [7] V. Go´mez-Serrano, P.M. Alvarez, J. Jaramillo, F.J. Beltra´n, Carbon 40 (2002) 523. [8] H. Valde´s, M. Sa´nchez-Polo, J. Rivera-Utrilla, Z.A. Zaror, Langmuir 18 (2002) 2111. [9] P.M. Alvarez, J.F. Garcı´a-Araya, F.J. Beltra´n, I. Gira´ldez, J. Jaramillo, V. Go´mezSerrano, Carbon 44 (3102) (2006). [10] P.M. A´lvarez, F.J. Beltra´n, V. Go´mez-Serrano, J. Jaramillo, E.M. Rodrı´guez, Water Res. 38 (2004) 2155. [11] F. Lo´pez-Garzo´n, M. Domingo-Garcı´a, M. Pe´rez-Mendoza, P.M. Alvarez, V. Go´mezSerrano, Langmuir 19 (2003) 2838. [12] N.R. Laine, F.J. Vastola, P.L. Walker Jr., J. Phys. Chem. 67 (1963) 2030. [13] S. Brunauer, P. Emmett, E. Teller, Am. Chem. Soc. 60 (1938) 309. [14] K.S.W. Sing, Surface Area Determination, Butterworths, London, 1970. [15] H.P. Boehm, Carbon 32 (1994) 759. [16] J.S. Noh, J.A. Schawrz, J. Colloid Interface Sci. 130 (1989) 157. [17] J. Pastor-Villegas, C. Valenzuela-Calahorro, A. Bernalte-Garcı´a, V. Go´mez-Serrano, Carbon 31 (1993) 1061. [18] C. Valenzuela-Calahorro, A. Bernalte-Garcı´a, V. Go´mez-Serrano, M .J. BernalteGarcı´a, J. Anal. Appl. Pyrol. 12 (1987) 61. [19] S. Brunauer, L.S. Deming, W.S. Deming, E. Teller, J. Am. Chem. Soc. 62 (1940) 1723. [20] H.P. Boehm, E. Diehl, W. Heck, R. Sappok, Angew. Chem. Int. Ed. 3 (1964) 669. [21] H.P. Boehm, High Temp. High Pressures 22 (1990) 275. [22] V.A Garten, D.E. Weiss, Aust. J. Chem. 10 (1957) 309. [23] H.P. Boehm, M. Voll, Carbon 8 (1970) 227. [24] C.A. Leo´n y Leo´n, J.M. Solar, V. Calemma, L.R. Radovic, Carbon 30 (1992) 797. [25] A. Polania-L, E. Papirer, J.B. Donnet, G. Dagois, Carbon 31 (1993) 473. [26] E. Papirer, S. Li, J. Donnet, Carbon 25 (1987) 243. [27] B.R. Puri, in: P.L. Walker, Jr. (Ed.), Chemistry and Physics of Carbon, vol. 6, Marcel Dekker, New York, 1970, pp. 191–282. [28] R.D. Vidic, C.H. Tessmer, L.J. Uranowski, Carbon 35 (1997) 1349.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation and ozone-surface modification of activated carbon. Thermal stability of oxygen surface groups J. Jaramillo a, P.M. A´lvarez a, V. Go´mez-Serrano b,* a b
Dpto. de Ingenierı´a Quı´mica y Quı´mica Fı´sica, UEx, Badajoz 06071, Spain Dpto. de Quı´mica Orga´nica e Inorga´nica, UEx, Badajoz 06071, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 28 December 2009
The control of the surface chemistry of activated carbon by ozone and heat treatment is investigated. Using cherry stones, activated carbons were prepared by carbonization at 900 8C and activation in CO2 or steam at 850 8C. The obtained products were ozone-treated at room temperature. After their thermogravimetric analysis, the samples were heat-treated to 300, 500, 700 or 900 8C. The textural characterization was carried out by N2 adsorption at 77 K, mercury porosimetry, and density measurements. The surface analysis was performed by the Bohem method and pH of the point of zero charge (pHpzc). It has been found that the treatment of activated carbon with ozone combined with heat treatment enables one to control the acidic– basic character and strength of the carbon surface. Whereas the treatment with ozone yields acidic carbons, carbon dioxide and steam activations of the carbonized product and the heat treatment of the ozonetreated products result in basic carbons; the strength of a base which increases with the increasing heat treatment temperature. pHpzc ranges between 3.6 and 10.3. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Cherry stones Activated carbon Ozone treatment Oxygen surface groups Thermal stability
1. Introduction Activated carbon is characterized by its porous structure and surface chemistry. Surface chemistry significantly influences the wettability, adsorptive, electrical, electrochemical, catalytic, acid– base, redox, hydrophilic–hydrophobic, and other properties [1,2]. It is determined by the type, quantity and bonding of the various heteroatoms, in particular oxygen which constitutes 2–25% by weight of activated carbon [3]. Heteroatoms are believed to adopt the character of the functional groups typical of aromatic compounds. Owing to great importance of oxygen groups of activated carbon, the surface composition of this material has been frequently modified via oxidation to increase the contents of such groups. With such an aim, a large number of oxidizing agents both in gas phase and aqueous solution has been used [4,5]. Ozone oxidation has been occasionally used in the surface modification of activated carbon [6–9] and also as a chemical method for its regeneration [10]. Because ozone is a very strong oxidant (standard reduction potential, E0 = +0.076 V), it has been successfully used in the surface modification of glassy carbon, which is a very stable chemically carbonaceous material [11]. In some instances, however, it may be of interest to decrease or even to totally eliminate the content of oxygen surface groups in activated carbon, for example to confer it a more specific
* Corresponding author. Tel.: +34 924 289300; fax: +34 924 271149. E-mail address: [email protected] (V. Go´mez-Serrano). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.12.109
behaviour in adsorption or catalysis processes. Selective removal of oxygen surface groups of activated carbon can be achieved simply by heat treatment of the material as oxygen starts giving off as CO and CO2 at temperatures generally above 227 8C and its complete removal requires temperatures as high as 977 8C [12]. Between 227 and 977 8C, depending on heat treatment temperature, the type and number of oxygen surface functional groups undergoing thermal decomposition can differ over a fairly wide range. The primary object of this research work was to propose a method for the control of surface composition of activated carbon. By taking into account that the properties of this material (i.e., its texture and surface composition and reactivity) depend on the starting material, method and conditions used in its preparation; here, in a series of successive steps, activated carbon was prepared from cherry stones, ozone-treated for modification of its surface composition, analyzed from the standpoint of the thermal behaviour, and subjected to heat treatment at various temperatures, prior to analyzing the oxygen surface groups present in the resultant products by titration methods and pH of the point of zero charge. Most samples were characterized in terms of texture and surface chemistry. Emphasis was put not only on the formation of oxygen surface groups by ozone-treatment of activated carbon but also on the thermal stability of such groups in connection with their presence in the final heat-treated carbons. In earlier study [8], the carbon Filtrasorb was treated with ozone for different time periods and the oxygen surface groups were analyzed by TPD and other tools. Furthermore, the textural changes produced as a result
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of the ozone-oxidation treatment of the carbon were also investigated. 2. Materials and methods 2.1. Starting material and reagents The cherry stones (CS, hereafter) used as the starting material were furnished by the Agrupacio´n de Cooperativas Valle del Jerte (Ca´ceres Province, Spain). As received, CS were thoroughly washed with distilled water, air-dried, ground, and sized, the fraction of particle sizes between 1.6 and 2.0 mm being chosen for subsequent studies. Pure grade NaHCO3, Na2CO3, NaOH, and HCl (Merck) were used for titration analyses. 2.2. Methods The experimental methods used in the preparation of activated carbon, modification of its surface, and analysis are briefly described below. 2.2.1. Preparation of activated carbon The heat treatments were carried out in a vertical electrical furnace, provided with a system for temperature control. Using a steel reactor, 60 g of CS were first heated from ambient temperature to 900 8C at 20 8C min1 in N2 atmosphere (flow = 20 L min1). The time of isothermal heating at the maximum heat treatment temperature (MHTT) was 2 h. Then, the carbonized product (C900, 15 g) was activated in CO2 (CD) or steam (WV) atmosphere. In these treatments, the system was heated from room temperature to 850 8C at 20 8C min1 in N2 atmosphere (flow = 20 L min1). After that, the N2 atmosphere was switched to the activation atmosphere (i.e. CO2, flow = 20 L min1; steam carried by N2, flow = 20 L min1), which was held at 850 8C only for the selected residence time of 2 h for CO2 and 3 h for steam. Once this time had elapsed, the furnace was cooled down to room temperature in N2 atmosphere. CD and WV were stored in a desiccator until use. The steam generation system consisted of an HP (series 1050) pump and of a water vaporizer. A water flow of 0.4 mL min1 was continuously furnished to the vaporizer at 400 8C. 2.2.2. Surface modification of activated carbon by ozone treatment Approximately 10 g of CD or WV were placed in the steel reactor and treated with ozone at 25 8C for 1 h in an ozone-air stream (flow = 20 L min1), containing a prefixed ozone concentration ðP O3 ¼ 2 kPaÞ. Ozone was produced from air in a Constrema SLO generator and analyzed in the gas stream with a GM-19 Anseros Ozomar analyzer at 254 nm. The notations used for the ozonetreated products are CDO and WVO. 2.2.3. Textural characterization The textural characterization of the samples was accomplished by N2 adsorption at 196 8C, mercury porosimetry, and helium and mercury density measurements. The N2 isotherms were determined in a semiautomatic equipment, Quantachrome (Autosorb1). The carbon sample (0.10 g) was oven-dried at 120 8C overnight and outgassed at 250 8C also overnight under a pressure lower than 103 Torr, prior to the N2 adsorption measurements. The experiments of mercury intrusion were carried out in a porosimeter, Quantachrome (Autosacan-60), using 0.3 g of sample. The mercury density (rHg) was measured as usual. Approximately 0.3 g of carbon was first weighed and placed in the glass holder, which was then filled with mercury in the Filling Apparatus (Quantachrome) and weighed. From the resultant mass datum the volume of sample was eventually calculated, knowing
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the mercury density at the working temperature and the sample holder calibration volume. The helium density (rHe) was measured in a Quantachrome stereopycnometer, using 3 g of sample. From the N2 adsorption isotherm, the specific surface area of the samples was evaluated by the BET method [13], SBET. Also, the external surface area (Sex) and micropore volume (Vmi) were calculated by the as method [14]; Vmi was expressed as liquid volume. From the values of cumulative pore volume (Vcu) against pore radius (r) (mercury porosimetry) the macropore volume, Vma = Vcu (at r = 25 nm), and a mesopore volume, Vme = Vcu (at r < 2 nm) Vma, were obtained. The total pore volume was calculated as VT = Vmi + Vme + Vma and by expression V T 0 ¼ ð1=rHg Þ ð1=rHe Þ. 2.2.4. Thermogravimetric analysis The TG–DTG curves for the samples were obtained in a thermogravimetric apparatus, Mettler TA-3000, consisting of a TG50 thermobalance (precision: 2 8C in temperature; 1 mg in weight) and a TG-10 processor. About 120 mg of sample were heated from room temperature to 900 8C in N2 atmosphere (flow = 200 mL min1). The heating rate in the dynamic treatment was 10 8C min1. 2.2.5. Heat treatments Approximately 10 g of CD, WV, CDO or WVO were heated from room temperature to the MHTT, i.e. to 900 8C for CD and WV and to 300, 500, 700 or 900 8C for CDO and WVO, at 20 8C min1 in N2 atmosphere (flow = 20 L min1). The soaking time at MHTT was 1 h. The resultant products are designated as CDT, WVT, CDOT, and WVOT, T being MHTT. 2.2.6. Surface chemistry analysis The surface chemistry of a large number of samples was analyzed by the Boehm method [15] and pH of the point of zero charge [16] (pHpzc). A series of different strength bases in aqueous solution were used in the titration of various acidic oxygen surface groups of the carbons. Such solutions were 0.05 M NaHCO3 for carboxylic groups and carboxylic acid anhydrides, 0.05 M Na2CO3 for lactones and lactols, 0.05 M NaOH for phenolic hydroxyl groups, and 0.25 M NaOH or 0.1 M NaOC2H5 for acidic carbonyl groups. Corrections were introduced in the measured titration volumes by allowing the base of a given strength to neutralize those surface functional groups which are more acidic. On the other hand, a 0.05 M HCl solution was used for measuring the total content of basic groups. pHpzc was measured using 0.1 M NaNO3 aqueous solutions at pH 3, 5 or 11. These pH values were fixed by adding HNO3 or NaOH aqueous solution. Using a set of test tubes, 25 mL of each of such solutions were brought into contact with 25 g of carbon and the system was maintained under continuous stirring for 24 h. After that, the supernatant was separated by filtration and its pH was measured. The average value of the three pH measurements was taken as pHpzc. 3. Results and discussion 3.1. Preparation of the samples: mass changes The preparation of samples was carried out in three successive stages of carbonization, activation, and surface modification. The yield of the carbonization process of CS was as low as 24.8 wt.%, which indicates that the pyrolysis of CS caused the release of a great amount of volatile matter. As is well known, it is accompanied by an increase in the degree of aromatization of the residual carbonized product. For other lignocellulosic materials, also heated at 900 8C, the mass loss is slightly higher than for CS. As a guide, the carbonization yield was 21.4 wt.% for rockrose
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Fig. 1. N2 adsorption isotherms at 196 8C.
Fig. 2. Curves of mercury intrusion.
[17] and 22.0 wt.% for sawdust obtained from holm-oak wood [18]. For the activation of C900, the burn-off percentage was 22.0 in CO2 and 39.4 in steam. Accordingly, a larger amount of disorganized matter and of carbon atoms was gasified in the case of the steam activation process. As the activation time was not very different in both atmospheres (i.e. 2 h in carbon dioxide and 3 h in steam), it is attributable to a higher reactivity of the water molecule as compared to the CO2 molecule rather to the unequal diffusion of the substances in the incipient porosity of C900 as the molecular geometry is angular for water and linear for CO2. Finally, the mass increase produced as a result of the treatment of CD and WV with ozone was 2.1 and 2.0 wt.%, respectively. These close values of the mass increase do not mean at all that the amount of oxygen chemisorbed on the carbon surface was similar for both activated carbons as the chemisorption process could go on further and this causes mass loss.
increasing exposure time of a carbon Filtrasorb 400 to ozone has been previously reported by Valde´s et al. [8]. The curves of mercury intrusion (Fig. 2) show that all tested samples possess a well-developed porosity in the range of macropores. Nevertheless, the size of these pores is more uniform and their volume is higher (also see textural data in Table 1) by the order WV > CD > C900. Accordingly, macroporosity not only is different for two kinds of carbonaceous materials but is also conditioned by the activating agent. As regards mesoporosity, its degree of development is poor for C900 and also, though less, for CD. However, it is much larger for WV, which is in line with the higher burn-off percentage produced in the steam activation treatment of C900 and also with N2 adsorption results. The treatments of CD and WV with ozone to a certain extent developed mesoporosity, mostly for CD. However, the effect on macroporosity in relative terms was of little significance. The calculated values of pore volumes for the activated carbons and ozone-treated products (Table 1) are similar to those previously reported for typical activated carbons. For different activated carbons the volume of micropores is approximately 0.15–0.50 cm3 g1, the volumes of mesopores lie between 0.02 and 0.10 cm3 g1, and the volume of macropores is between 0.2 and 0.8 cm3 g1 [4]. For WVO SBET and Sex are as high as 968 and 180 m2 g1, respectively.
3.2. Textural analysis The N2 adsorption isotherms measured for the samples are shown in Fig. 1. At a glance it can be seen that the adsorption isotherm obtained for C900 is a typical type I isotherm of the BDDT classification system [19]. The isotherm is practically parallel to the x-axis in the entire P/P0 interval to P/P0 = 1. This means that the micropores present in C900 are narrow pores as they are filled with liquid adsorptive at very low relative pressures. From the relative position and shape of the adsorption isotherms obtained for C900, CD, and WV, it can be deduced that carbon dioxide and steam activations produced great development of microporosity. The effect is stronger for steam than for carbon dioxide. WV also contains an important fraction of mesopores of a wide range of pore sizes. On the other hand, it is worth noting that the effect of the ozone treatment on microporosity is opposite depending on the activated carbon sample. Microporosity increases for WV and decreases for CD. A gradual reduction of microporosity with
3.3. Ozone treatment: surface modification of activated carbon The values of the content of surface groups and of pHpzc obtained for CD, VW, CDO, and WVO are set out in Table 2. It is seen that the composition of the carbon surface and hence its acidic– basic character and strength depend on the activating agent used in the preparation of the carbons and also on whether they were subsequently treated with ozone or not. Firstly, it is seen that CD and WV contain surface groups which are acidic or basic in nature. The acidic groups include carboxyl, lactone, hydroxyl and carbonyl
Table 1 Calculated values of textural parameters. Sample
SBET (m2 g1)
Sex (m2 g1)
Vmi (cm3 g1)
Vme (cm3 g1)
Vma (cm3 g1)
VT (cm3 g1)
V T0 (cm3 g1)
C900 CD WV CDO WVO
204 604 901 603 968
19 35 122 46 180
0.084 0.299 0.383 0.278 0.380
0.035 0.055 0.134 0.073 0.142
0.238 0.341 0.473 0.356 0.503
0.36 0.70 0.99 0.71 1.03
0.37 0.78 0.92 – –
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Table 2 Functional group contents (mequiv./g) and pHpzc for the activated carbons and ozone-treated products. Sample
CG
L
HG
CaG
TAG
TBG
pHpzc
CD WV CDO WVO
16 4 487 680
6 12 14 45
74 48 98 78
42 28 63 103
138 92 662 906
385 369 302 231
8.8 9.8 4.3 3.6
Abbreviations: CG, carboxylic groups; L, lactones; HG, hydroxyl groups; CaG, carbonyl groups; TAG, total acidic groups; TBG, total basic groups; pHpzc, pH of the point zero charge.
oxygen functionalities [20,21]. The postulated basic surface groups are chromene-type structures [22] and pyrone-type oxides [23]. The existence of pyrone-type structures is supported by the studies of Leo´n y Leo´n et al. [24] and Papirer et al. [25]. For CD and WV the concentration of various acidic oxygen surface groups is low and the concentration of basic groups is higher. Since the latter groups are predominant, both virgin activated carbons possess a surface with basic character and hence pHpzc > 7. As a rule the content of acidic oxygen groups is higher for CD than for WV. Notice that the content of carboxylic groups is very low in both carbons, in particular for WV. Secondly, the treatment of CD and WV with ozone gave rise to the formation of a large amount of oxygen surface groups, mostly carboxylic ones. In view of these results, it appears that the chemical nature of the oxygen surface groups is influenced by the activated carbon and the chemical agent used in the oxidation treatment of the material. Thus, using the NORIT activated carbon and O2 and N2O (both in N2) in gas phase and HNO3 and H2O2 in aqueous solution, it was concluded earlier by Figueiredo et al. [5] that gas phase oxidation of the carbon increases mainly the concentration of hydroxyl and carbonyl groups while oxidations in liquid phase increase especially the concentration of carboxylic acids. The surface of CDO and WVO is acidic in character, pHpzc being 4.3 and 3.6 respectively. These pHpzc values also show the influence of the activating agent used in the preparation of the activated carbons on the surface properties of the ozone-treated products.
Fig. 4. TG–DTG curves. Samples: WV and WVO.
3.4. Thermal analysis From the TG–DTG curves obtained for CD, WV, CDO, and VWO (Figs. 3 and 4) it becomes clear that the thermal behaviour of the tested sample is strongly dependent on whether it is a virgin activated carbon or an ozone-treated product. As inferred from the TG curves, firstly, the total mass loss produced to 900 8C is much higher for CDO and WVO (10–11 wt.%) than for CD and WV (2– 3 wt.%). Accordingly, as expected, the amount of surface oxygen chemisorbed in CD and WV during the preparation of CDO and WVO greatly exceeds the mass increase of 2 wt.%. It indicates that an important amount of activated carbon was gasified during the treatment with ozone. Secondly, the DTG curves show the presence of five very well defined maxima of weight loss which are associated with the thermal decomposition of various oxygen surface groups. According to literature [1], in the course of heating a carbon sample under vacuum or in a neutral gas, particular surface compounds decompose at different characteristic temperatures yielding reaction products such as CO2 for carboxylic and lactone groups from ca. 200 to 700–800 8C; CO for quinone, phenol and ether groups from 200–300 to 400–500 8C; H2O for phenolic groups in the range from 200–300 to 400–500 8C; and H2 as a result of splitting of C–H and O–H bonds above 500–700 8C. In the case of CDO and WVO, the thermal effects on the mass of sample are assigned as described in Table 3. Notice that two peaks overlap between 500 and 700 8C for CDO. Also, peaks shift their position towards slightly higher temperatures for WVO.
Table 3 Ozone treatment of activated carbon. Thermal stability of oxygen surface groups.
Fig. 3. TG–DTG curves. Samples: CD and CDO.
Effect number
Temperature range (8C)
Oxygen surface group
Gas evolved
I II, III
150–300 300–400, 400–500
IV, V
500–800
Carboxylic Acid anydride Lactone Phenolic hydroxyl Carbonyl Ether
CO2 CO, CO2 CO2 CO
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Table 4 Effect of heat treatment on the functional group contents (mequiv./g) and pHpzc. Sample
CG
L
HG
CaG
TAG
TBG
pHpzc
CD900 WV900 CDO300 CDO500 CDO700 CDO900 WVO300 WVO500 WVO700 WVO900
– – 12 – – – 13 – – –
– – 36 – – – 51 – – –
– – 79 45 12 – 75 49 – –
12 – 71 72 16 11 121 102 35 –
12 12 198 132 28 11 260 151 35 –
436 461 345 425 479 558 355 389 503 588
9.5 10.1 8.5 9.0 9.5 9.9 7.3 8.9 9.6 10.3
Abbreviations: CG, carboxylic groups; L, lactones; HG, hydroxyl groups; CaG, carbonyl groups; TAG, total acidic groups; TBG, total basic groups; pHpzc, pH of the point zero charge.
The DTG curves (Figs. 3 and 4) show that the effects of mass loss are much stronger for CDO and WVO than for CD and WV, which denotes that the treatment of CD and WV with ozone is a very effective method to introduce surface oxygen and to form various oxygen groups. As shown by peak intensities, the effectiveness of the aforesaid method to modify the surface chemistry of the carbons is significantly influenced by the activating agent used in the preparation of CD and WV, as a larger amount of oxygen groups is formed in the case of steam. Because WV possesses better developed porosity in the regions of meso- and macropores (see data in Table 1) and the surface area is also larger for this activated carbon, the access of ozone to a larger number of active sites was possible. Finally, it must be highlighted that as a rule the thermal stability of the oxygen surface groups is different for the virgin activated carbons as compared to their respective ozone-treated products, as reflected by the change in peak positions of the DTG curves. 3.5. Thermal treatment: analysis of oxygen surface groups In view of the results obtained in the previous study on the thermal stability of the oxygen surface groups, CD, WV, CDO, and WVO were first subjected to heat treatment. CD and WV were heated from room temperature to 900 8C, whereas for CDO and WVO the heat treatment was conducted to 300, 500, 700 or 900 8C, by taking into account the temperature ranges in which the removal of oxygen surface groups occurred. In this way, a number of products were prepared and their surface chemistry was then analyzed. The values of the content of oxygen surface groups and of pHpzc obtained for the heat-treated products are listed in Table 4. It follows (also see Table 2 for comparison purposes) that the basic strength of the carbon surface is markedly influenced by heat treatment temperature. By simply heating the virgin carbons to 900 8C, the surface basic strength increases. pHpzc is 8.8 for CDO and 9.5 for CD900. For the ozone-treated products, the basic strength increases significantly with the increase in heat treatment temperature. pHpzc ranges between 8.5 and 9.9 for the CDOT samples and between 7.3 and 10.3 for the WVOT samples. Thus, the variation of pHpzc is greater for the steam activation-derived ozone-treated products. Also, pHpzc is slightly higher for CDO900 and WVO900 as compared in turn to CD900 and WV900. Accordingly, the treatment of the activated carbons with ozone yields a more basic surface after heat treatment at 900 8C. In an earlier study on the oxidation of activated carbon with nitric acid and on the thermal stability of the oxygen surface groups it was found that the total amount of pyrone-type groups is proportional to the initial degree of oxidation of the material and that the variation of pyrone content with heat treatment temperature follows similar trends as the temperature increased [25]. At high temperatures pyrone-type structures are generated by thermal decomposition of oxygenated acidic groups which create active sites capable of fixing oxygen in ether form, and rearrangement with
existing carbonyl groups which resist pyrolysis [26]. Also, the complete evolution of oxygen as CO, CO2 and H2O [27] delocalizes carbon basal plane electrons and results in surface sites with greater ability to donate electron pair and behave as Lewis bases [28]. The basic character of the activated carbon surface is an important property in connection with its use as, for example, oxygencontaining basic surface functional groups appear to be primarily responsible for the catalytic properties of the material towards oxidative coupling of phenolic compounds [28]. As a final comment it should be pointed out that the oxygen-containing surface complexes present on both HNO3- and air-oxidized carbons are essentially completely decomposed by heat treatment to 1000 8C [2]. 4. Conclusions From the above-results it may be concluded that by simply preparing activated carbon by CO2 or steam activation of a cherry stones-derived carbonized product, ozone-treated activated carbons, and effecting an eventual thermal treatment, the control of the acidic–basic character and strength of the surface chemistry of activated carbon is possible. The ozone treatment of the activated carbons changes the character of their surface from basic to acidic. pHpzc is as low as 3.6. However, the opposite applies to the heat treatment of the ozone-treated products. In this case, the basic strength of the carbon surface increases with the increasing heattreatment temperature. pHpzc is as high as 10.3. Acknowledgments This work has been supported by the Junta de Extremadura through project PDT08-A012. Dr. J. Jaramillo also thanks the Consejerı´a de Educacio´n of Junta de Extremadura of Spain for providing her a sabbatical research stay at the Departamento de Ingenierı´a Quı´mica of the University of Extremadura. References [1] H. Jankowska, A. Swiatkowski, J. Choma, Active Carbon, Ellis Horwood, New York, 1991. [2] Y. Otake, R.G. Jenkins, Carbon 31 (1993) 109. [3] W.F. Wolff, J. Phys. Chem. 63 (1959) 653. [4] J.T.P. Cookson Jr., in: N. Cheremisinoff, F. Ellerbusch (Eds.), Carbon Adsorption Handbook, Ann Arbor Science, Ann Arbor, 1980, pp. 241–279. [5] J.L. Figueiredo, M.F.R. Pereira, M.M.A. Freitas, J.J.M. O´rfa˜o, Carbon 37 (1999) 1379. [6] V. Go´mez-Serrano, P.M. Alvarez, J. Jaramillo, F.J. Beltra´n, Carbon 40 (2002) 513. [7] V. Go´mez-Serrano, P.M. Alvarez, J. Jaramillo, F.J. Beltra´n, Carbon 40 (2002) 523. [8] H. Valde´s, M. Sa´nchez-Polo, J. Rivera-Utrilla, Z.A. Zaror, Langmuir 18 (2002) 2111. [9] P.M. Alvarez, J.F. Garcı´a-Araya, F.J. Beltra´n, I. Gira´ldez, J. Jaramillo, V. Go´mezSerrano, Carbon 44 (3102) (2006). [10] P.M. A´lvarez, F.J. Beltra´n, V. Go´mez-Serrano, J. Jaramillo, E.M. Rodrı´guez, Water Res. 38 (2004) 2155. [11] F. Lo´pez-Garzo´n, M. Domingo-Garcı´a, M. Pe´rez-Mendoza, P.M. Alvarez, V. Go´mezSerrano, Langmuir 19 (2003) 2838. [12] N.R. Laine, F.J. Vastola, P.L. Walker Jr., J. Phys. Chem. 67 (1963) 2030. [13] S. Brunauer, P. Emmett, E. Teller, Am. Chem. Soc. 60 (1938) 309. [14] K.S.W. Sing, Surface Area Determination, Butterworths, London, 1970. [15] H.P. Boehm, Carbon 32 (1994) 759. [16] J.S. Noh, J.A. Schawrz, J. Colloid Interface Sci. 130 (1989) 157. [17] J. Pastor-Villegas, C. Valenzuela-Calahorro, A. Bernalte-Garcı´a, V. Go´mez-Serrano, Carbon 31 (1993) 1061. [18] C. Valenzuela-Calahorro, A. Bernalte-Garcı´a, V. Go´mez-Serrano, M .J. BernalteGarcı´a, J. Anal. Appl. Pyrol. 12 (1987) 61. [19] S. Brunauer, L.S. Deming, W.S. Deming, E. Teller, J. Am. Chem. Soc. 62 (1940) 1723. [20] H.P. Boehm, E. Diehl, W. Heck, R. Sappok, Angew. Chem. Int. Ed. 3 (1964) 669. [21] H.P. Boehm, High Temp. High Pressures 22 (1990) 275. [22] V.A Garten, D.E. Weiss, Aust. J. Chem. 10 (1957) 309. [23] H.P. Boehm, M. Voll, Carbon 8 (1970) 227. [24] C.A. Leo´n y Leo´n, J.M. Solar, V. Calemma, L.R. Radovic, Carbon 30 (1992) 797. [25] A. Polania-L, E. Papirer, J.B. Donnet, G. Dagois, Carbon 31 (1993) 473. [26] E. Papirer, S. Li, J. Donnet, Carbon 25 (1987) 243. [27] B.R. Puri, in: P.L. Walker, Jr. (Ed.), Chemistry and Physics of Carbon, vol. 6, Marcel Dekker, New York, 1970, pp. 191–282. [28] R.D. Vidic, C.H. Tessmer, L.J. Uranowski, Carbon 35 (1997) 1349.