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Langmuir 1997, 13, 2354-2358
Effect of Oxygen Surface Groups on the Immersion Enthalpy of Activated Carbons in Liquids of Different Polarity F. Rodrı´guez-Reinoso,* M. Molina-Sabio, and M. T. Gonza´lez Departamento de Quı´mica Inorga´ nica, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain Received October 28, 1996. In Final Form: January 30, 1997X A series of carbons, prepared by nitric acid oxidation of an activated carbon and subsequent heat treatment at increasing temperatures to selectively reduce the oxygen surface groups, have been used to determine the enthalpy of immersion in liquids with different polarity (benzene, methanol, and water). The areal enthalpy of immersion for the carbon with low oxygen content follows the order benzene > methanol . water, similar to that found for nonporous carbons. On the other hand, the presence of oxygen surface groups which evolve as CO2 upon heat treatment does not affect much the enthalpy of immersion in methanol or water, the groups evolving as CO being mainly responsible for the evolution of enthalpy of immersion. For nonpolar molecules such as benzene the enthalpy of immersion of the carbons is independent of the chemical nature of the carbon surface.
1. Introduction The properties and application of activated carbon are based on both the porosity and the chemical nature of the surface. In fact the presence of oxygen surface groups modifies the wettability of the carbon surface, moisture content, adsorptive behavior, catalytic properties, and, in general, the acid-base character.1 When activated carbon is prepared by a thermal process (e.g. gasification of a char with carbon dioxide or steam at high temperature, up to 900 °C), the resulting activated carbon usually possesses a low amount of oxygen surface groups.2 As a consequence, it is a normal practice to increase the amount of oxygen surface groups by roomtemperature oxidation with oxidizing solutions such as nitric acid, hydrogen peroxide, and sodium hypochlorite3 without substantially modifying the porous texture of the carbon. Upon oxidation, the oxygen becomes bound to the carbon network, thus forming structures with different nature and thermal stability; the controlled heat treatment of the oxidized carbon under an inert atmosphere produces the selective decomposition of the oxygen groups,4 thus allowing for the quantification of the amount and nature of these groups. This is the base of the temperature programmed decomposition (TPD) method. The comparison of this method with the traditional selective titration proposed by Boehm et al.5 for the case of activated carbons with different porosity and oxygen content has already been discussed.6 The important role of the oxygen surface groups in the adsorption of molecules with different degrees of polarity by activated carbon has been shown by some authors.6,7 In general terms, an increase in the amount of oxygen X
Abstract published in Advance ACS Abstracts, April 1, 1997.
(1) Van Driel, J. In Activated Carbon...A Fascinating Material; Norit, N. V., Ed.; Amersfoort: Holland, 1983. (2) Mattson, J. B.; Mark, H. B., Jr. Activated Carbon Surface Chemistry and Adsorption from Solution; Marcel Dekker: New York, 1971. (3) Puri, B. R. In Chemistry and Physics on Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1970; Vol. 6. (4) Tremblay, G.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1978, 16, 35. (5) Boehm, H. P. Adv. Catal. 1964, 16, 179. (6) Rodrı´guez-Reinoso, F.; Molina-Sabio, M.; Mun˜ecas, M. A. J. Phys. Chem. 1992, 96, 2707. (7) Matsumura, Y.; Yamabe, K.; Takahashi, H. Carbon 1985, 23, 263.
S0743-7463(96)01047-5 CCC: $14.00
surface groups will not modify the adsorption of a nonpolar molecule such as nitrogen, but will progressively influence the adsorption of polar molecules such as sulfur dioxide, methanol, and water. For polar molecules the amount adsorbed at low relative pressures is larger for the oxidized carbon, thus indicating that not only the nonspecific carbon-adsorbate interactions but also specific interactions with the oxygen surface groups participate in the adsorption process. Direct evidence of the interactions governing the adsorption process could be obtained by immersion calorimetry since this technique measures the heat evolved when a liquid is brought into contact with the adsorbent. This technique has been used to determine the surface area8-10 and pore size distributions of activated carbons using liquids of different molecular dimensions.10-13 On the other hand, the enthalpy of immersion in polar liquids such as methanol and water shows important differences in a series of carbons prepared by physical and chemical methods in relation to nonpolar liquids such as benzene.9,14,15 For the case of water, a predominant role of the chemical nature of the carbon has been suggested for both adsorption and immersion processes; Stoeckli and Kraehenbuehl16 have established a relationship between the number of primary centers deduced from the adsorption isotherm of water and the volume of micropores with the enthalpy of immersion in water. The objective of the present work is to establish the relative importance of the porosity and the nature of oxygen surface groups of activated carbon on the enthalpy of immersion in liquids of different polarity (benzene, (8) Denoyel, R.; Ferna´ndez-Colinas, J.; Grillet, Y.; Rouquerol, J. Langmuir 1993, 9, 515. (9) Ferna´ndez-Colinas, J.; Denoyel, R.; Grillet, Y.; Vadermeersch, J.; Reymonet, J. L.; Rouquerol, F.; Rouquerol, J. In Fundamentals of Adsorption; Mersmann, A. B., Scholl, S. E., Eds.; Engineering Foundation: New York, 1991; p 261. (10) Gonza´lez, M. T.; Sepu´lveda-Escribano, A.; Molina-Sabio, M.; Rodrı´guez-Reinoso, F. Langmuir 1995, 11, 2151. (11) Stoeckli, H. F.; Kraehenbuehl, F. Carbon 1981, 19, 353. (12) Kraehenbuehl, F.; Stoeckli, H. F.; Addoun, A.; Ehrburger, P.; Donnet, J. B. Carbon 1985, 24, 483. (13) Stoeckli, H. F. Carbon 1990, 28, 1. (14) Ferna´ndez-Colinas, J.; Denoyel, R.; Grillet, Y.; Rouquerol, F.; Rouquerol, J. Langmuir 1989, 5, 1205. (15) Barton, S. S.; Evans, M. J. B.; Harrison, B. J. Colloid Interface Sci. 1972, 45, 542. (16) Stoeckli, H. F.; Kraehenbuehl, F.; Morel, D. Carbon 1983, 21, 589.
© 1997 American Chemical Society
Immersion Enthalpy of Activated Carbons
Langmuir, Vol. 13, No. 8, 1997 2355
Table 1. Oxygen Surface Groups and Surface (BET) of Activated Carbons oxygen surface groups (mmol/g) carbon
SBET (m2/g)
CO
CO2
H0 H110 H200 H300 H400 H500 H600 H700 H800 H900
926 852 860 882 946 950 950 956 971 979
0.82 3.32 3.30 3.18 3.09 2.94 2.42 1.76 0.78 0.25
0.12 1.17 0.95 0.75 0.36 0.28 0.16 0.12 0.06 0.04
methanol, and water), using a carbon prepared by physical activation of a char with steam and subsequently oxidized with nitric acid; the oxidized carbon is then subjected to different heat treatments to eliminate selectively the oxygen surface groups introduced by oxidation. 2. Experimental Section The starting carbon, H0, was prepared by activation with steam at 750 °C (burn-off, 37%) of carbonized olive stones.17 Around 10 g of carbon H0 were oxidized with 100 mL of a 6 N solution of nitric acid (1 h at the boiling point). The oxidized carbon was washed with distilled water to zero acid removal and dried at 110 °C (carbon H110). Different fractions of this carbon were heat treated under a flow of helium (50 mL/min, heating rate of 10 °C/min) at temperatures ranging from 200 to 900 °C (residence time at the final temperature, 15 min). The temperature of the treatment is included in the nomenclature of the samples. The freshly prepared carbons were kept under an atmosphere of highpurity nitrogen to prevent contact with air. Characterization of the samples and the measurement of the enthalpy of immersion were carried out immediately after the heat treatment. The BET surface area of the carbons has been determined from the adsorption isotherms of nitrogen at 77 K. Previous outgassing of the samples was carried out for 10 h at 100 °C under high vacuum to prevent partial decomposition of oxygen surface groups present in the carbons. TPD was used to measure the oxygen surface groups of the carbons by determining the amounts of CO and CO2 evolved upon heating under a flow of helium up to 1050 °C; a quadrupolar mass spectrometer was used to analyze the gases evolved during TPD. The experimental conditions (heating rate, gas flow, etc.) used in TPD runs were the same as those used during the heat treatments of the oxidized carbon. The enthalpies of immersion in benzene, methanol, and water at 30 °C were measured in a Tian-Calvet type differential microcalorimeter (Setaram, C80D), using the procedure described in a previous work.10
3. Results and Discussion The adsorption isotherms of N2 at 77 K on activated carbons prepared by steam activation of carbonized olive stones, published in previous works,18 show that the carbon with 37% burn-off chosen for this work (BET surface area, 926 m2/g) exhibits a uniform microporosity, the contribution from meso- and macroporosity being relatively small. When the carbon is oxidized with nitric acid (carbon H110), the surface area slightly decreases (see Table 1) since the oxygen bonded to the carbon network partially blocks the microporosity. Progressive elimination of oxygen groups as a consequence of the heat treatment (samples H200H900) has two effects: (i) elimination of the constrictions of the microporosity, and (ii) slight activation of the carbon (the oxygen atoms leave the surface together with carbon (17) Rodrı´guez-Reinoso, F.; Molina-Sabio M.; Gonza´lez, M. T. Carbon 1995, 33, 15. (18) Gonza´lez, M. T.; Molina-Sabio, M.; Rodrı´guez-Reinoso, F. Carbon 1994, 32, 1407.
Figure 1. CO and CO2 profiles for (a) carbons H0 and H110 and (b) carbons H400, H600, and H800.
atoms, to form CO2 and CO) so that the surface area of carbon H900 is somewhat larger than that of the original carbon H0. As could be expected, oxidation produces a considerable increase in the oxygen content of the carbon in relation to the change in porosity mentioned above. Figure 1 includes the TPD profiles for CO2 and CO for some of the carbons. The amount of CO2 evolved for carbon H0 is almost nil since the decomposition of groups to CO2 is produced at a temperature lower than that used during steam activation (750 °C). Consequently, carbon H0 exhibits only surface groups decomposing as CO, with a maximum at 880 °C, a temperature higher than activation. Similar TPD profiles have been found for other activated carbons prepared under similar conditions as H0 but with different residence time (different burn-off), for which the amount of groups evolving as CO2 is kept almost constant but those evolving as CO increase with activation, the shape of the profile and the maximum temperature of evolution remaining almost constant.19 Oxidation with nitric acid produces a variety of structures with a range of thermal stability. At low temperature (200-500 °C) there is predominance of the so-called “low-temperature CO2 groups”, mainly attributed to carboxylic groups.4 In the temperature range 600-800 °C there is an appreciable amount of CO2 + CO (anhydride groups, usually called “high-temperature CO2 groups”) overlapping with the CO from other types of structures (phenol, quinone, etc.) up to temperatures around 1000 °C. This behavior has been observed by other authors when oxidizing activated carbons with nitric acid.20,21 (19) Molina-Sabio, M.; Gonza´lez, M. T.; Rodrı´guez-Reinoso, F.; Sepu´lveda-Escribano, A. Carbon 1996, 34, 505. (20) Molina-Sabio, M.; Mun˜ecas, M. A.; Rodrı´guez-Reinoso, F. In Characterization of Porous Solids; Rodrı´guez-Reinoso, F., et al., Eds.; Elsevier: Amsterdam, 1991; p 329.
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Figure 2. Evolution of the enthalpies of immersion in benzene, methanol, and water as a function of temperature. Filled symbols: original carbon H0. Half-filled symbols: oxidized carbon H110. Open symbols: heat-treated oxidized carbon.
The TPD profiles shown in Figure 1b indicate that there is a progressive elimination of groups evolving as CO2 and CO with increasing temperature of the heat treatment. Thus, there are no low-temperature CO2 groups for carbon H400, only CO and high-temperature CO2 groups remaining. For H600 only the fraction of CO groups with high thermal stability remain, and for H800 the amount of oxygen remaining in the surface is very small. The amount of CO2 and CO groups evolved upon TPD has been determined by integration of the area under the curves of the profiles of the type shown in Figure 1. The results of Table 1 show, in addition to the increase in CO2 and CO groups with oxidation (H110), that the groups evolving as CO2 have decomposed up to 500 °C, whereas there is a small decrease of CO groups, which decompose at higher temperatures. On the other hand, the amount of oxygen surface groups of the original carbon H0 (activated with steam at 750 °C) is somewhat in between those of carbons H700 and H800. Figure 2 includes the plots of the enthalpy of immersion of carbons in benzene, methanol, and water at 30 °C as a function of the temperature of the heat treatment. For the case of the nonpolar molecule, benzene, there is a slight decrease followed by an increase at higher temperatures, an evolution which is very similar to that shown by the adsorption of nitrogen (BET surface area values of Table 1). This behavior indicates that the effect of oxygen surface groups is almost negligible and that the enthalpy of immersion is a function of only the surface area of the carbon. The values of inmersion enthalpy of the carbons in methanol are similar to those obtained with benzene, thus indicating the important role of the surface area of the carbon. However, the increase in ∆Hi with oxidation (H110) and the higher value of the enthalpy (with respect to the benzene value) for carbons with high oxygen content (up to H400), and the lower values thereafter, reflect the role of the oxygen surface groups in the enthalpy of immersion, especially the groups decomposing at low to medium temperature. The greatest relative importance of the carbon surface-methanol interactions in relation to the oxygen group-methanol interactions has also been (21) Otake, Y.; Jenkins, R. G. Carbon 1993, 31, 109.
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Figure 3. Evolution of the enthalpies of immersion in water as a function of CO and CO2 groups. Half-filled symbols: oxidized carbon H110. Open symbols: heat-treated oxidized carbon.
shown for the adsorption of methanol in the gas phase on carbons with different degrees of oxidation.6 There was in the gas-phase adsorption an increase in the amount adsorbed at low relative pressures (up to about P/P0 ) 0.04) for oxidized carbons with respect to the unoxidized carbon, reflecting the extra interaction with the oxygen surface groups of the carbon. It is in the heat of the immersion in water (Figure 2) where the more important role of the oxygen surface groups is clearly shown, considering the high increase in the enthalpy for the oxidized carbon with respect to the original carbon and the important decrease for carbons treated above 500 °C. In fact, the enthalpy of immersion for the carbon heat treated at 800 °C is very similar to that of the original carbon, which was activated at a similar temperature (750 °C). It is important to note the parallelism between the evolution of the enthalpy of immersion in water and the evolution of oxygen surface groups (Table 1), especially for CO groups. Whereas the important loss of CO2 groups during the heat treatment up to 500 °C does not modify the enthalpy of immersion in water (samples H110-H500), there is an important decrease for both ∆Hi(H2O) and the amount of CO groups for samples treated at higher temperatures. This is clearly shown in Figure 3. When the amount of CO2 groups is large, the enthalpy of immersion in water remains constant, in contrast with the oxygen surface groups decomposing to CO, for which there is a practically linear relationship with the values of ∆Hi(H2O). The extrapolated value of ∆Hi(H2O) is near 22 J/g, an important fraction of the total enthalpy registered for each carbon, thus indicating that the interaction of the carbon surface with the water molecule is not exclusively due to oxygen surface groups, the interaction for the oxygen-free carbon surface being very important. In fact, this later interaction must be responsible for the adsorption on activated carbons reduced in hydrogen at 900 °C, for which the adsorption of water starts at around P/P0 ) 0.46 in a type V isotherm. However, for oxidized carbons the adsorption process starts at a relative pressure below 0.1, due to the contribution of the interaction of the water molecule with oxygen surface groups. This effect has also been shown for a nonporous carbon such as Elftex 120 carbon black.22
Immersion Enthalpy of Activated Carbons
Figure 4. Evolution of the areal enthalpies in benzene, methanol, and water as a function of temperature for: (a) activated carbons and (b) graphite.15 Filled symbols: original carbon H0. Half-filled symbols: oxidized carbons. Open symbols: heat-treated oxidized carbons.
The effect of oxygen surface groups on the heat of immersion for graphite (BET surface area of 163 m2/g) outgassed at different temperatures in benzene, methanol, and water has been studied by Barton and Harrison,15 and consequently the comparison of the two materials (graphite and activated carbon) seems to be appropriate. For this purpose the areal enthalpies of immersion (J/m2) for the three liquids as a function of the heat treatment of the carbon can be found in Figure 4a,b. The similarity of the two figures is remarkable, despite the different structural characteristics of the two carbon materials. Thus, not only is the enthalpy per unit area for benzene almost constant (and consequently independent of the chemical nature of the carbon surface) but in addition the two sets of values of parts a and b of Figure 4 are very similar (∼0.13 J/m2), thus indicating that the energetic contribution due to the interactions of benzene with the surface of the micropores is the same as with the nonmicroporous surface of the carbon. This in turn strengthens the idea of determining the surface area of activated carbons from immersion microcalorimetry in benzene, taking a nonporous carbon as reference, as described previously,8-10 irrespective of the chemical nature of the carbon. (22) Carrott, P. J. M. Carbon 1992, 30, 201.
Langmuir, Vol. 13, No. 8, 1997 2357
The enthalpy per unit area of a carbon heat treated at 900 °C for methanol is similar for both carbon materials (∼0.12 J/m2), and it increases slightly (up to ∼0.14 J/m2) when the carbon has oxygen surface groups. The decrease in the enthalpy of immersion when the CO2 groups are eliminated (at temperatures near 400 °C, Table 1) indicates that the methanol molecule interacts with these groups very probably throughout the formation of hydrogen bonds, given the very slight increase in enthalpy with oxidation, chemisorption being ruled out. The enthalpy per unit area for water (Figure 4a,b) in the two carbon materials is somewhat lower than for methanol. For the oxidized carbon (H110) the ∆Hi(H2O)/ ∆Hi(CH3OH) ratio is 0.68, whereas for the two carbons treated at 800 °C (a temperature normally used for steam activation) it is 0.30, values that are similar to those found by other authors9 for charcoals prepared by steam activation (0.29-0.30 J/m2) or phosphoric acid activation (0.64 J/m2). On the other hand, the enthalpy in water for an activated carbon, H900, with a small amount of oxygen groups is 0.032 J/m2, a value coincident with Graphon23 and very similar to the value given for graphite in Figure 4b (0.040 J/m2). There are two consequences of this similarity. First, the enthalpy of immersion in water is not affected by the presence of micropores, except from the point of view that micropores contribute more to the surface area. Consequently, as in the case of benzene, one could use a nonporous carbon heat treated to eliminate the oxygen surface groups as a reference surface for the determination of the surface area of physically activated carbons (with a low content of oxygen groups). Second, the enthalpy per unit area of a graphitic surface (where the presence of hydrophobic basal planes is predominant) is the same as that of the surface of an activated carbon free from oxygen surface groups. If one assumes that in an activated carbon the structural units are at least on the order of 100 times smaller than in graphite, the proportion of carbon atoms in the edges in relation to those in the interior of the basal planes is much larger in activated carbons than in Graphon or graphite. Since the enthalpy per unit area is the same in the two cases, one has to assume that once the oxygen groups have been eliminated there are no preferential sites for the interaction of water molecules with the carbon atoms according to the position in the structure. In any case, this interaction is somewhat less than the that produced by the water molecule with quinonic, phenolic, and other groups decomposing to CO. 4. Conclusions Oxidation with nitric acid produces a slight decrease of the porosity and surface area of activated carbon, although drastic changes in the chemical nature of the surface take place. Heat treatment at 500 °C of the acidtreated carbon produces almost complete elimination of the oxygen surface groups evolving as CO2 and increases the surface area up to that of the original carbon. However, this treatment does not modify the groups evolving as CO, which decompose almost completely at around 800 °C. The enthalpy of immersion of the carbons studied here in a nonpolar liquid such as benzene is not modified by the chemical nature of the surface, being sensitive to only the available porosity, so that the enthalpy per unit area is kept independent of the oxygen surface groups and is similar to that described for nonporous carbons. This behavior contrasts with that observed for the immersion (23) Wade, W. H. J. Colloid Interface Sci. 1969, 31, 111.
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in water, for which the role of oxygen surface groups is relevant, in particular the groups with higher thermal stability (less acidic). However, there is an important contribution (minimum of 25%) to the total value of ∆Hi(H2O) due to the proper surface of the carbon (free from oxygen), which is similar to that found for nonporous carbons and graphite, thus indicating that the ∆Hi(H2O)/ unit surface area ratio does not depend on the proportion of carbon atoms placed at the basal planes with respect to the total. In the case of methanol immersion, the situation is halfway between benzene and water, although
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in this case the groups with lower thermal stability contribute to the value of ∆Hi; their contribution to the total registered value is small in relation to that due to the interaction of the methanol molecule with the oxygenfree carbon surface. Acknowledgment. Financial support from DGICYT (Project PB94-1500) is gratefully acknowledged. LA961047U
