Langmuir 1994,10,844-854
844
Characterization Study of Carbonaceous Materials. Calorimetric Heat of Adsorption of p-Nitrophenol M. L. Gonzslez-Martin Departamento de m i c a , Universidad de Extremadura, 06071 Badajoz, Spain
C. Valenzuela-Calahorro and V. G6mez-Serrano* Departamento de Qulmica Inorg&nica, Universidad de Extremadura, 06071 Badajoz, Spain Received August 22,1993. In Final Form: October 25,199P Carbonaceous materials (six carbon blacks) were characterizedin terms of their surface area, porosity, pore-size distribution, and surface chemistry. Using p-nitrophenol in aqueous solution as the adsorbate, the calorimetric heat of adsorption was investigated in connection with the studied properties of the adsorbents. Techniques used were gas (Nz, COZ) and p-nitrophenol adsorption, mercury porosimetry, density measurements, FT-IR, and microcalorimetry. The carbon blacks were included in two groups depending mainly on their porosity. The chemical nature of oxygen functional groups or structures was similar in all samples. Significantdifferences were observed in the concentrationof surface oxygen groups. The heat of adsorption depended on the surface area and the microporosity of the samples. The density of oxygen groups was likely an important property in connection with the evolution of heat. The pore-size distribution of the adsorbents might also influence the heat of adsorption. When the amount adsorbed of p-nitrophenoland the heat of adsorptionwere expressed on a per unit surface area basis, good agreement with the FT-IR results was noted. Introduction
Adsorption of the solid/liquid interface is an exothermic process. The amount of heat evolved depends on the extent and the intensity of the surfaceinteractions. These two factors are controlled by properties of the adsorbent including its surface area, porosity, pore-size distribution, and surface chemistry. As earlier reported by Parfitt and Rochester,' this fact suggests that to reliably study the heat of adsorption, both physical and chemical properties of the adsorbent must be considered. Using a number of carbonaceous materials and p-nitrophenol in aqueous solution, an attempt was made in this work to correlate the calorimetric heat of adsorption with characteristics of the adsorbents. With this purpose, the samples were characterized by adsorption from the gas (Nz, COZ)and liquid @-nitrophenol) phases, mercury porosimetry, and density measurements. Information on the surface chemistry of the adsorbents was provided by FT-IR spectroscopy. The calorimetry experiments and the adsorption of p-nitrophenol were carried out using an LKB-8700 microcalorimeter. In recent years, carbon blacks have been extensively used in adsorption s t u d i e ~ . Such ~ * ~ carbon materials have technical and industrial applications due to their high accessible surface area.4 They were employed as catalyst supportsk7 and as model adsorbents for adsorption of
* To whom correspondence should be addressed. Abstract published in Advance ACSAbstracts, January 15,1994. (1)Parfitt, G.D.; Rochester, C. H. In Adsorption from Solution at the
SoZidlLiquidInterface; Parfitt, G. D., Rochester, C. H., Eds.;Academic Press: London, 1983; Chapter 1. (2) Dacey, J. R. In The SoZid-Gas Interface; Flood, A., Ed.; Marcel Dekker, Inc.: New York, 1967; Vol. 2, p 995. (3) Heidenreich, H. D.; Hess, W. M.; Ban, L. L. Appl. Crystallogr. 1968,1, 1. (4) Aric6, A. S.; Antonucci, V.; Minutoly, M.; Giordano, N. Carbon 1989, 27, 337. (5) Venter, J.; Kaminsky, M.; Geoffroy,G. L.; Vannice, M. M. J.Catal. 1987,103,450. (6) Venter, J.;Kaminsky, M.; Geoffroy, G.L.; Vannice, M. M.J. Catal. 1987,105,155.
inorganic compounds.8 The surface chemistry is also an important property of carbon blacks as it reflects their acid-base characteristics.9 p-Nitrophenol was recommended by Giles and NakhwalO as an ideal adsorbate for measuring surface areas of porous solids using adsorption from solution. The method of p-nitrophenol adsorption was followed in characterization studies of carbons including carbon blacks,11-18and further it finds application in the pollution control of surface waters. As inferred from these facts, adsorption studies based on the use of carbon black and p-nitrophenol could be of interest. However, only a few investigations were carried out with this adsorption system as well as with carbon black and other phenolic compounds.lMO (7) Salinas-Martinez de Lecea, C.; Linares-Solano, A,; Vannice, M. A. Carbon 1990,28,467. (8) Groszek, A. J.; Partyka, S.; Cot, D. Carbon 1991,29,821. (9) Rositani, F.; Antonucci, P. L.; Minutoli, M.; Giordano, N.; V i a r i , A. Carbon 1987,25,325. (IO)Giles, C. H.; Nakhwa, S. N. J. Appl. Chem. 1962,12,266. (11) Mattaon, J. S.; Mark, H. B., Jr.; Malbin, M. D.; Weber, W. J.; Crittenden, J. C. J. Colloid Interface Sci. 1969,31, 116. (12) Snoeyink, V. L.; Weber, W. J., Jr.; Marck, H. B., Jr. Enuiron. Sci. Technol. 1969, 3, 918. (13) Coughlin, R. W. Water Pollut. Control Res. Ser. 1970, 70, 29. (14) Marsh, H.; Campbell, H. G. Carbon 1971,9,489. (15) Parkash, S.; Berkowitz, N. Carbon 1976,14, 289. (16) Puri, B. R.; Singh, D. D.; Gupta, U. Indian J. Technol. 1979,17, 458.
(17) Puri, B. R. In Activated Carbon Adsorption of Organice from the Aqueous Phase; Suffet, I. H., McCurie, M. J., Eds.; Ann Arbor Science: Ann Arbor, MI, 1980; p 353. (18) L6pez, GonzBlez, J. d. D.; Valenzuela Calahorro, C.; Navarrete Guijosa, A.; G6mez Serrano, V. An. Quim. 1988,84B, 41. (19) Umeyama, H.; Nagai, T.; Nogami, H. Chem. Pharm. Bull. 1971, 19, 1714. (20) Glushchenko, V. Yu.; Khabalov, V. V. Zh. Fiz. Khim. 1977,51, 1414. (21) Glushchenko, V. Yu.; Khabalov, V. V. Kolloidn. Zh. 1978,40,765. (22) Mamchenko, A. V. Zh. Fiz. Khim. 1990, 64, 2417. (23) GoFBlez Martin, M. L.; Valenzuela Calahorro, C.; G6mez Serrano, V. Langmuir 1991, 7,1296. (24) Gonznez Marth, M. L.;Valenzuela Calahorro, C.; G6mez Serrano, V. An. Qoim. 1991,87, 1036. (25) G6mez Serrano, V.; Beltrh, F. J.; Durh Segovia, A. Chem. Eng. Technol. 1992, 15, 124.
0743-7463/94/2410-0844$04.50/0 0 1994 American Chemical Society
Langmuir, Vol. 10, No. 3, 1994 845
Characterization Study of Carbonaceous Materials
Experimental Section A. Materials. The carbonaceous materials employed in this study were six carbon blacks named Sterling V (SV), Vulcan 3 (V3), Vulcan 6 (V6), Black Pearls 880 (BPBO), Black Pearls 1300 (BP1300), and Black Pearls 2000 (BP2000), provided by Cabot Corp. in Spain. To minimize the effect of particle size on adsorption, the materials were sized and the particle fraction of diameter lower than 0.32 mm was chosen for subsequent studies. The p-nitrophenol (PNP) used as the adsorbate was a Merck analytical grade reagent, and was recrystallized twice from distilled water. B. Procedures. 1. Gas Adsorption. Adsorption isotherms for N2 at -196 OC and for C02 at 0 OC were determined using a Micromeritics ASAP 2000 surface area analyzer. In the N2 adsorption, the desorption branches of the isotherms were also obtained. About 1.5 g of sample was used in each adsorption experiment. Adsorbents were placed in a glass container and outgassed at -10-9 Torr at 120 OC overnight prior to the adsorption measurements. A Carlo Erba Model 200 porosimeter was used to obtain the curves of mercury intrusion. The porosity distribution of the samples was determined down to a pore radius of 3.7 nm. Mercury densities were obtained by the usual method, just before carrying out the experiment of mercury porosimetry. Helium densities were measured with a Quantachrome steropycnometer, following a method described in the literature.31 2. Infrared Spectroscopy. Infrared spectra were recorded on a Perkin-Elmer 1720 FT-IRspectrometer in the 450-4000cm-l frequency range. Disks were prepared by first thoroughly mixing 1mg of carbon with 1000mg of KBr in an agate mortar, and by then pressing the resulting mixture at 10 tons for 3 min with a Perkin-Elmer hydraulic press. After that, disks were ovendried at 110 OC for 1 h for the removal of hygroscopic water adsorbed by KBr. Spectra were recorded with 10 scans at a 2-cm-' resolution. 3. PNP Adsorption. Prior to the adsorption experiments, the stability of PNP in water was checked by periodically measuring the absorbanceof a l(r mol L-l PNP aqueous solution. The results indicated that even after 40 days PNP is a stable substance. Subsequently, the adsorption isotherms for PNP were determined at 30 OC using a LKB-8700 microcalorimeter. The required amount of oven-dried (110 OC, 24 h) sample, which ranged between 0.004 g for BP2000 and 3.440 g for SV, was weighed and placed into the reaction vessel together with 99 mL of a pH 4acetic acidaodium acetate buffer solution. Separately, 1mL of a PNP concentrated solution, prepared using the buffer solution as solvent, was introduced into a glass cell, which was externally isolated by means of wax. The dilution of this solution was accomplished by breaking the cell into the reactor, which resulted in a solution with a concentration ranging between 1.01 X lo-' mol L-l for SV and 1.05 X 1W mol L-I for BP2000. After that, the system was stirred at 600 rpm until the attainment of the adsorption equilibrium, which took less than 15 min for all adsorbents. This was ascertained for each individual sample by monitoring the concentration of the PNP solution with time in a separate test from the adsorption experiment. Finally, the solution was centrifuged and, upon dilution or not according to concentration, analyzed with a Pye-Unicam SP8-250 spectrometer. A previously well-established spectrophotometric analytical method was followed, and the absorbance measurements of the PNP solutions at acidic pH were made at 315 nm.l* 4. Calorimetry. During the adsorption time (i.e., until equilibrium was reached in the adsorption system),the thermistor resistance was periodicallymeasured, usually at 0.01-a intervals. To obtain the heat of adsorption, first the temperature was expressed as a function of the thermistor resistance and the (26)Glushchenko, V. Yu.; Khabalov, V. V. Izu. Akad. Nauk. SSSR, Ser. Khim. 1976,10,2169. (27)Armentrout, D. N.;McLean, J. D.; Long, M. W. Anal. Chem. 1979,51,1039. (28)Di Corcia, A,; Samperi, R.; Sebastiani, E.; Severini, C. Chromatographw 1981,14,86. (29)Borra, C.; Di Corcia, A.; Marchetti, M.; Samperi, R. Anal. Chem. 1986,58,2048. (30)Mangani, F.; Fabbri, A.; Creecentini, G.; Bruner, F. Anal. Chem. 1986,58,3261. (31)Shields, J. E.;Lowell, S. J. Colloid Interface Sci. 1985,103,226.
Table 1. Specific Surface Areas and Microporosities (Na and COa Adsorption) of Carbon Blacks
sv v3 V6
BP880 BP1300 BP2000 O P
= SJS,
37 80 114 224 511 1443
35 77 104 192 358 915
95 96 91 86
70 63
0.001 0.001 0.004 0.016 0.078 0.271
0.005 0.014 0.024 0.083 0.132 0.315
20 7 17 19 59 86
x 100. bP = VdWO x 100.
Fkgnault-Pfaundler method, as described by Barrel,= was applied for temperature corrections. From the resulting temperature values and the previously determined calorimeter calibration constant, the heat of adsorption was then calculated. Dilution corrections were not introduced in the calorimetry resulta since, as shown in a separate test, the process is not accompanied by an appreciable heat release. If this occurs, the amount of heat evolvedis below the sensitivity limit of 0.008J for the calorimeter. Moreover,the stirring in the reactor and the heat exchange with the surrounding thermostated bath, which are other possible causes of error aeaociated with this method of heat determination, were included in the calibration of the calorimeter.
Results and Discussion A. Characterization Study of the Carbonaceous Materials. 1. Surface Area. From the adsorption isotherms for N2 at -196 OC,the specific surface area of the carbonaceousmaterialswas estimated by twomethods, using the software ASAP 2000 provided by Micromeritics. Firstly by applying the Brunauer, Emmett, and Teller (BET) equation to plp" = 0.3 (to this plp" value the BET plots were straight lines)3s and taking A, = 16.2A2,M the SBET values listed in Table 1 were found. They show significant differences in the surface area of the samples which is accessible to N2 at -196 "Cas SBET ranges between 37 m2 g1for SV and 1443 m2 g1for BP2000. Secondly by estimatingthe statistical thickness of the adsorbed layer ( t ) by applying the HarkinsIJura equation,% t=[
1
13.99 0.034- log@/po) lf2 values of the external surface (Sedand the micropore volume (V,) were calculated. The t values used in the least-squares analysis ranged between 3.5 and 5, which correspond toplp" values varying between about 0.07and 0.3. The values thus obtained of Sexand Vfi are given in = Sex Table 1. According to the ASAP 2000 method, SBET + S,i. This method of deriving Sexand S A from S Bcan~ be regarded as a rough approximation (if not controversial) since in the BET model, which was established for nonporous solids, it is assumed that SBET = Sex.In fact, the so high Sexvalue for BP2000 points to an increased external surface in this sample, which might be connected with the estimate method of S , and result from the nondiscrimination properly between the adsorption of N2 on the external surface and the porosity of the material. Moreover, Sexalso varies greatly for the carbonaceous materials, and particularly for SV and BP2000. In comparison with SBET, the magnitude of the difference between SBET and Sexdepends on the sample. While for SV and V3 the SBET and Sexvalues are close (P = 95% and 96%), for BP1300 and BP2000 they are rather different (32)Borrel, P.Thermochim. Acta 1974,9,89. (33)Brunauer, S.;Emmett, P. H.;Teller, E. J. Am. Chem.Soc. 1938, 60,309. (34)McClellan, A. L.;Harnsberger, H.F. J. Colloid Interface Sci. 1967,23,577. (35)Harkins, W . D.; Jura, G. J. Chem. Phys. 1943,11,431.
Gonz6rlez-MartIn et al.
846 Langmuir, Vol. 10, No. 3, 1994 600 '0° 0 Adsorption,
SV
Desorption, Adsorption, Desorption, C. Adsorption, A Desorption,
V3 V3 V6
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P,
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500
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Desorption, Adsorption, Desorption, Adsorption, Desorption,
1800
BP880 BP880 BP1500 BP1300 BP2000 BP2000
1500
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1200
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z:
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400 300
4
P,
c
7 1
0.12
'0
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0.;
0.6
0.'8
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Relative pressure, p/po
Figure 1. Adsorption-desorption isotherms of nitrogen at -196 O C on SV,V3, and V6.
Figure 2. Adsorption-desorption isotherms of nitrogen at -196 O C on BP880,BP1300,and BP2000.
(P = 70% and 63% 1; in the case of BP2000, the P value is only somewhat larger than the value of 60% reported by Dacey for a highly porous carbon black, Carbolac I.2 2. Microporosity. The microporosity (i.e., the microporesare pores with pore widths of less than 20 A)in carbon blacks was investigated by de Boer et al.39who, in a study on the application of the t method to N2 adsorption isotherms,reported the presence of slit-shaped pores of widths varying between 7.0 and 15.0 A. By following the same method, Voet et al.4 suggested that carbon blacks contain pores of width 5.5 A. Using methylene blue and victoria pure blue BO dyes as adsorbates, Lamond and Price'l found that the minimum pore size present in activated carbon blacks is of the order of 20 A. Dacey2pointed out that the pores that do exist in some carbon blacks are micropores ranging from 10 to 15 A in diameter and also that such materials possess a large surface area which stems from the fine particle size. From these results it followsthat the micropores in carbon blacks may vary widely in size between 5.5 and 20 A, according to the source. Further information on the microporosity of carbon blacks is obtained in this work. The study on the porosity of the samples also embraces the mesoporosity and the macroporosity as well as the total porosity. Nitrogen adsorption isotherms at -196 "C (Figures 1 and 2) display an adsorption branch shaped similarly to the isotherms previously reported by Lamond and Price for a series of activated carbon blacks.41 These isotherms resemble the type IV isotherm of the classificationsystem originallyput forward by Brunauer, Deming,Deming,and Teller (abbreviated BDDT).'2 Such an isotherm shape is typical of mesoporoussolids. If the carbonaceousmaterials contain mesopores (Le., pores with widths between 20 and 500 A)together with micropores and the materials also possess an external surface, as suggested by the Vmi
and S,,values in Table 1,the N2 isotherms would then be composite isotherms of types I and IV.4M At low plp" values the adsorption of N2 would occur in micropores by pore filling with the adsorbate in the liquid state and also on the external surface and the mesopore walls by formation of an adsorbate monolayer. At plp" values above the so-called "point B" of the isotherm, the adsorption would proceed on the external surface by multilayer formation and in mesopores by capillary condensation; the contribution of the micropores to the adsorption in this region of the isotherm would be of little significance. An important fraction of the micropores in the samples must be small size pores, as suggested by the absence practically of knees from the isotherms. This means that the filling of such micropores by the adsorbate takes place at very low relative pressures (even smaller than those measured in the adsorption system), presumably as a result of the high adsorption potential in being in the pores because of the overlapping of the potential fields from neighboring walls. In this connection it should be noted that the diameter of the nitrogen molecule at -196 "C is -4 A.46 Concerning the microstructure of carbon blacks, it was assumed that the particles consist of randomly orientated crystallite^^^ and that the constituent layers of carbon atoms are lying parallel to the ~ ~ it - ~was ~ reported that surface of the p a r t i ~ l e . ~Also, spherical particles of carbon black formed by @-Sic decomposition usually have many imperfections such as dislocations, bendings, or nonparallel planes in the particles.sOTherefore, the microporous structure displayed by carbon black might be connected with the orientation of crystallites and of individual carbon layers as well as with imperfectionsof the carbon particles. Adsorption of N2 between well-ordered carbon layers appears less probable unless contraction of the N2 moleculesoccurs, as
-
(36)Dubinin, M. M. Zh. Phys. Chem. 1960,34,959. (37) Dubinin, M. M. Chem. Reu. 1960, 60, 235. (38) IUPAC Manual of Symbols and Terminology, Appendix 2, Pt. 1, Colloid and Surface Chemistry. &re Appl. Chem. 1972,31,578. (39) de Boer,J. H.; Limen, B. G.; van der Plaa, Th.; Zondervan, G. J. J. Catalyais 1966,4,649. (40)Voet,A.; Lamond, T.G.; Sweigart, D. Carbon 1968,6,707. (41) Lamond, T. G.; Price, C. R. J. Colloid Interface Sci. 1969,31,104. (42) Brunauer, S.; Deming, L. S.; Deming, W. S.; Teller, E. J. Am. Chem. SOC.1940,62, 1723.
(43) Pierce, C. J.Phys. Chem. 1960,64, 1184. (44) Sing, K. S. W. Chem. Znd. (London) 1967,830. (45) Gregg, S. J.; Langford, J. F. Trans. Faraday SOC.1969,65,1394. 649.
(46)Carbon Adsorption Handbook; Cheremisinoff,P. N., Ellerbusch, F., Eds.; Ann Arbor Science: Ann Arbor, MI, 1978. (47) Riley, H. L. Chem. Ind. (Berlin) 1939,58, 391. (48) Oberlin,A.; Terriere, G. J. Microscopie 1972,14,1. (49) Auguie, D.; Oberlin, M.; Oberlin, A.; Hyvemat, P. Carbon 1981, 19, 271.
(50) Yamada, H.; Tobisawa, S. Carbon 1989,27,845.
Characterization Study of Carbonaceous Materials suggested by the molecular size of Nz and by the interlayer spacing, which is 3.35 A for graphite.61 From the data of Nz adsorption the micropore volume (V,) was obtained by the above indicated method. In calculating V d , the factor 1.547 X 103 was used, which was derived from the density value of 0.808 g cm3 for liquid Nz? the Vmi values are shown in Table 1. The microporosity is nearly negligible in SV and V3,which is in good agreementwith earlier reported results,23and still very reduced in V6. By contrast, the microporosity is well developed in the remaining samples, and particularly in BP1300 and BP2000. The variation order followed by Vmi is SV EC: V3 < V6 C BP880 < BP1300 C BP2000,and in the direction toward the right in this series Vmi increases rather regularly by about 4-fold, except for the couple made up of SV and V3. The adsorption of COz at about room temperature has been suggested by severalresearchersmVM as the most useful technique for investigating the microporous structure of coals and their carbonized products. Other researched6 concluded that the adsorption of Nz at -196 "Cshould be completed with the adsorption of COZat 0 OC in order to obtain a better knowledge of the whole microporosity of activated carbons. The connection of the adsorption of COz with the microporosity present in coals and carbons appears therefore evident. Accordingly, the micropore was estimated from the adsorption isotherms volume ( WO) for COZat 0 "C by first applying the Dubinin-Radushkevich equationMand by then introducing corrections in the resulting values in order to convert adsorbate volumes in the gaseous state (cm3 g', STP) into volumes in the adsorbed state. The conversion factor used in this case was 1.818X 109,which was obtained using the COz density of 1.08 g ~m-3.5~ The W Ovalues are listed in Table 1. Concerning our adsorbenta, which in some instances are materials with a high external surface,other contributions to the adsorption of COZ, apart from the one of the micropores, are also possible, and this fact, despite the above statement, should be borne in mind when studying W Oand the heat of adsorption. From the Vmi and WOvalues it follows that for all the carbonaceous materials the accessible porosity and the extent of adsorption depend on the adsorbate, increasing with COz. These resulta are likely connected either with the adsorption temperature or with adsorbent-adsorbate interactions. The molecular size of the adsorbate also might influence the extent of adsorption even decisively, although with COZand Nz as adsorbates it seems unlikely due to the similarity in the molecular size of both s u b ~ t a n c e s . ~If~ 6at~ 0 "C COz can enter pores of the samples which are inaccessible to Nz at -196 "C, the behavior is typical of adsorbenta showing an activated entry effect. Accordingly the materials will possess narrow constrictions, the width of which would be very close to (51) Smisek, M.; Cerny, S. Active Carbon: Manufacture, Properties and ADDliCatiOnS: Elsevier: Amsterdam. 1970 D 49. (52j Marsh, H.; Rand, B. J. Colloid Ihterfac; Sci. 1970,33, 110. (53) Marsh, H. Fuel 1966,44, 253. (54) Walker, P. L., Jr.; Austin, L. G.; Nandi, S. P. In Chemietry and Physics of Carbon; Walker, P. L., Jr., Ed.; Arnold London, 1966: Vol. 2, p 279. (65) Rodriguez-Reinoso, F.; Linares-Solano, A. In Chemistry and Physics of Carbon;Thrower, P. A., Ed.;Marcel Dekker, Inc.: New Yolk and Baeel, 1989; Vol. 21, p 61. (56) Dubinin, M. M. In Progress in Surface and Membrane Science: Danielli, J. F., Rosenberg, M. D., Cadenhrad, D. A., Eds.; Academic Press: New York, 1975; Vol. 9, p 1. (57) Toda, Y.; Hatami, M.; Toyoda, S.;Yoshida, Y.; Honda, H. Fuel 1971, 50, 187. (68) Barrer, R. M. Q.Rev. Chem. SOC.1949,3,293. (69)Barrer. R. M. Zeolites and Clay Minerals: Academic Press: London, l97& p 291.
Langmuir, Vol. 10, No. 3, 1994 847 a
I
,
I
,
, , , a , , l
V6
i
10
10'
10
'
?/I Figure 3. Pore size distributions in the regions of macropores and mesopores for SV, V3, and V6.
Hw: BP2000
L
2-
OI
-
P--. o i
5
I
I
BP880
10
10'
?/a
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Figure 4. Pore size distributionsin the regions of macroporee and mesopores for BP880, BP1300, and BP2000.
the diameter of the adsorbate molecule.If not, two possibilities were suggested for COZ due to ita high quadrupolemoment, which is absent from the Nzmolecule, namely, chemisorpotionand interaction with surfacepolar groups or ions of the ads0rbent.a The P values in Table 1 suggest that either the percentage of micropores inaccessible to NZor the extent of interaction of COZwith the adsorbent was very important in V3, and much less significant in BP1300 and particularly in BP2000. 3. Mesoporosity and Macroporoeity, Concerningthe shape of the NZadsorption isotherms (Figures 1 and 2), the main characteristic feature of the type IV isotherm is the hysteresis loop which is associated with capillary condensation taking place in mesopores. If the carbonaceous materials contain mesopores, and although contributions to the adsorption due to interparticle capillary condensationw7 and to external surface are possible, the pip" values of inception of the hysteresis loop suggest that the smallest size of the mesopores present in these samples follows the variation sequence SV > V3 > V6 > BP2000 > BP880 > BP1300. The curves of pore-size distribution in the macropore (i.e., pores with pore widths of more than 500 A)and mesopore ranges are shown in Figures 3 and 4. Except for BP2000, the curves exhibit only one strong maximum ~
(60) Mag@, F. A. P. Research ISMI, 6, 313. (61) Zwietering, P.; van Krevelen, D. W. Fuel 1964,33,331. (62) Gregg, S. J.; Pope, M. I. Fuel 1969,38,601. (63) Marah, H.; Wynne-Jones, W. F. K. Carbon 1%4,1,281. (64) Gregg,S. J.;Smg,K. 9. W.Adsorptron,SurfaceAreaandPorosity; Academic Press: London, 1982. (65) Harkins, W. D.; Jura, G. J. Am. Chem. SOC.1944,136,1362. (66)Harkins, W. D.; Jura, G. J. Am. Chem.SOC.1944,919, 1362. (67) Pierce, c. J. phy8. Chem. 1969,63, 1076.
848 Langmuir, Vol. 10, No. 3, 1994
Gonz&lez-Marttnet al.
2.5
I
5.0
I
m
E
0-
2.0
4.0
v
v
0
-50 eg
0
1.5
3.0
5 >
1.0
2.0
e
0
z
0.5
5
0
0.0
10'
0.0
10'
10'
r/X
Figure 5. Cumulative pore volume against the pore radius. Table 2. Pore Volumes. and Densities of Carbon Blacks
sv
v3 V6 BPSSO BP1300 BP2000
0.03 0.62 0.75 0.72 0.52 0.89
1.52 1.15 1.26 1.07 0.74 3.75
1.97 2.03 2.06 2.05 2.16 2.42
0.49 0.46 0.44 0.47 0.53 0.21
1.53 1.68 1.79 1.64
1.42 4.35
1.55 1.77 2.01 1.81 1.34 4.91
99 95 89 91 106 89
Vm. = V, (at r = 37 A) - V, V, = V, (at r = 250A), V, = cumulative pore volume (mercury porosimetry), VT = 1/p& - l / p ~ . , vm. vm. P' vT/ V'T x 100. V'T Q
vd +
+
'
situated gradually toward lower radius values (these (A) are indicated in parentheses) by the variation sequence SV (375) > V3 (214) > V6 (166) > BP880 (83) > BP1300 (44). According to these results, the carbonaceous materials should contain large pores with a rather uniform pore size, which decreases almost regularly in this series. As the BP2000 curve displays various maxima located throughout the range of pore radii covered by the porosimetry technique, this sample will possess macropores and mesopores of differing sizes. The results on the porosity of the carbonaceous materials obtained by mercury porosimetry corroborate those of Nz adsorption on the mesoporosity. In fact, there is fairly good agreement in the variation of the pore size regardless of the technique used. However, the possibility of simple carbon compaction leading to an increase in the free space by loss of interparticle voids or by breakdown of pore walls owing to the pressure applied in the porosimeter should not be ruled out, at least prior to the analysis of other results. If so, it is obvious that the compacting effect occurs at a different pressure for each individual sample. Compaction was studied by Brown and Lardse using inorganic oxide xerogels. Significant discre ancies in the curves of poresize distribution obtained om mercury intrusion and nitrogen adsorption were noted when the pore volume was large. As BP2000 is the material with the porosity better developed, perhaps compaction should be enhanced in BP2000 with regard to the rest of the samples. In fact, the curve of the cumulative pore volume for BP2000 is clearly different from those for the remaining materials (Figure51, which points at such a possibility. Furthermore, the great macropore and mesopore volumes (V,, Vme, Table 2) of BP2000 and the small mesopore volume of SV should be noted. Moreover, the relative decrease in Vme and V, for BP880 and BP1300 suggests that, if compaction occurs,the micropores present in these two samples are not involved in the genesis of the macropores and the mesopores which are measured by mercury porosimetry. Otherwise, Vme and Vm, for BP880 and BP1300, which in this series of carbonaceous materials are found among the
z
(68) Brown, 5.M.; Lard,E. W. Powder Technol. 1974,9,186.
more microporous solids, should increase in comparison with other samples. 4. Helium and Mercury Densities. Total Porosity. In characterization studies of porous solids the helium density (PHe) is usually taken as the true density, or weight of a unit volume of solid excluding porosity, since helium can penetrate into very fine pores on account of its small molecular diameter, which is equal to P9 or 2.3 A,& according to the source. As previously reported,70J1 the helium density is a function of composition. The mercury density (PHg), termed apparent densitpl or particle is defined as the weight of a unit volume of solid, including pores and cracks. The PHe and p% values for the carbonaceous materials are listed in Table 2. The PHe values exceed those obtained by us for two commercial activated carbons of different particle sizes (1.86 g cm-9, Merck, 1.5mm; 1.90gcm4, Panreac, powder). In the case of BP2000, PHe is even greater than the graphite density, 2.26 g cm3. The high helium densities are compatible with a great exclusion of porosity from the bulk volume of the sample. This suggests an easy access to helium of the pores present in the samples, which is in good agreementwith the results of COSadsorption. In fact, the penetration of He in the samples and the adsorption of COz on the materials took place at close temperatures (namely, room temperature for He and 0 OC for COz). In the adsorption of Nz, the temperature was much lower (-196 "C) and the accessible porosity might be reduced, as above stated. The p ~ gvalues for the samples are generally somewhat larger than the value of 0.40 g cmd for the powder activated carbon and significantly smaller than the value of 0.72 g cm4 for the granular material. This showsthe influence of the particle size on the mercury density, and accordingly the higher PHg for BP1300 than for BP880 and BP2000 (therest of the samplesare different materials) may be due to a certain extent to the particle size of the samples, which should be smaller for BP1300 than for BP880 and BP2000. Another contributory factor to pw is the porosity of the samples, as demonstrated by the P H values ~ for BP1300 and BP2000. They are in line with the fact that BP1300 is the least porous material whereas BP2000 is the most porous one, as seen below. The values of the total pore volume (VT) are given in Table 2. The largest VT value corresponds to BP2000, whereas the smallest one appears for BP1300; this gives one an idea of the development of porosity in these carbonaceousmaterials. The total porosity of the samples has special significance in this study, since assuming that helium is not adsorbed at room temperature and that mercury does not enter the pores significantly (the filling pressure of the penetrometer with mercury was -103 Torr), the VT values will not suffer from the drawbacks of the results of Nz adsorption and mercury porosimetry connected possibly either with diffusion of the adsorbate or with compaction of the adsorbent. The comparison of VT and V'T (the total pore volume obtained by adding up the micro-, meso-, and macropore volumes; its values are also listed in Table 2) proves that compaction of the carbonaceous materials by the effect of the high pressure applied in the porosimeter is more marked for BP2000 and V6 than for the rest of the samples. However, the fact that P" ranges between 89 % for V6 and BP2000 and 99 % for SV (for BP1300 P' is even 106%)indicates that the compaction effect is not very important in these samples. (69) Franklin,R. E. Tram. Faraday SOC.1949,45,274. (70) Franklin, R. E. Fuel 1948,27,46. (71) Neavel, R. C.; Hippo, E. J.; Smith, S. C.; Miller, R. N.Prep. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1980,25,246. (72) Majahan, 0. P. Powder Technol. 1984,40, 1.
hngmuir, Vol. 10, No. 3, 1994 849
Characterization Study of Carbonaceous Materials 32
BP2000
2.
Table 3. Specific Surface Areas (PNP Adsorption, 30 "C) of Carbon Blacks and Specific Heats of Adsorption carbon
sv v3 V6
BP880 BP1300 BP2000
0-
1.:
;0.
0.k
0.6
0.'8
0
1.b
Relative concentration, C/C,
Figure 6. Adsorption isotherms for p-nitrophenol at 30 OC.
It might be argued that the materials possess micropores inaccessibleto N2 at -196 "C and that, as V'T was calculated from V d , the actual V'T values are higher than those used in this study. Although this is so, the use of slightly increased V d values would not change the interpretation of the results since for these samples Vmi is much smaller than Vm, and V., This validates the pore volumes obtained by mercury porosimetry. In comparison with the results of N2 and C02 adsorption, a discrepancy in the variation of Vmi, WO, and VT is observed for BP880 and BP1300. While V~ and W Oincrease for these two samples with regard to V6 and BP880, respectively, VT decreases. This is likely due the poorer developmentof mesoporosity and macroporosityin BP880 and BP1300, as deduced from the Vm, and V, values in Table 2 (or from Figure 5). 6. PNP Adsorption. Surface Area and Porosity. Adsorption isotherms for PNP from aqueous solution at 30 OC on the carbonaceous materials are shown in Figure 6; the amount adsorbed of PNP, X (mol gl),is plotted against the relative concentration, C/Co.The adsorption of PNP generally first undergoes a sharp increase; welldefined knees, which were previously observed for the adsorption also of PNP on carbons including carbon blacks,14J8-18~2s~ appear only for BP880, BP1300, and BP2000. Regarding the initial upward branch of the isotherms, its interpretation is not as straightforward as in the adsorption from the gas phase since the adsorption of solutes from solution may be influenced by the presence in the adsorbents of oxygen functionalgroups, as discussed by a number of r e ~ e a r c h e r s . ~ ~This J ~ Jisotherm ~ ~ ~ region is usually associated with the affinity of the adsorbate toward the adsorbent. The affinity term, when used in connection with the extent of adsorption, not only refers to the adsorbate-adsorbent chemical affinity but also comprises the contributions to the adsorption due to the number of active sites and the adsorption potential, the latter depend on properties of the adsorbent, specifically on the surface area and the porosity distribution, respectively. In addition, it is well known that the effect of pore size on the adsorption merely concerns the pores whose (73)Graham, D.J. Phys. Chem. 1955,59,896. (74)C l a w , A.; Boehm, H. P.;Hofmann, U. 2.Anorg. Allg. Chem. 1967,290,35. (75) Coughlin, R. W.;Ezra, F. S . Enuiron. Sci. Technol. 1968,2,29. (76)Puri, B. R.;Bhardwai, S.S.;Gupta, U.J. Indian Chem. SOC.1976, 53,1095.
SPNP (m*IT') 10 29 48 118 312 996
-AH (J rl) 0.44 1.92 2.97 10.30 29.00 86.00
widths are a few orders of magnitude greater than the adsorbate size. These facts suggest that, even if the chemical nature of the surface functional groups represent in the carbonaceous materials is similar, the slope of such an isotherm branch can only tentatively be correlated with the adsorbent porosity, a greater slope indicating the adsorption in smaller pores. Accordingly, the samples might be ordered by increasing pore size as follows: BP1300 < BP880 < BP2000 < V6 < V3 < SV, which is an almost identical variation sequenceto the above established one on the basis of the results of mercury porosimetry. Nevertheless, in view of the results of gas adsorption and mercury porosimetry, it appears likely that in BP880, BP1300, and BP2000, which are the materials with the greater micropore volumes, at low C/COthe adsorption of PNP will occur mainly in micropores. For SV, V3, and V6, instead, it will take place in large pores or on an external surface, as suggested by the above described results concerning the mesoporosity (except for SV), macroporosity, and fraction of external surface. At high C/Co,the adsorption of PNP increases for all the samples, though showing a trend to the constancy particularly for BP880 and BP1300. This behavior was explained by Puri et al.lS in terms of transitional pore filling or commencement of a second statisticallayer. This statement is corroborated by the above results of Nz adsorption and mercury porosimetry since the increase in the adsorption of PNP is greater for SV, V3, V6, and BP2000, which are the materials that possess a higher fraction of external surface and/or a better developed mesoporosity,macroporosity,or both with regard toBP880 and BP1300. The specific surface area ( S p ~ p )was estimated by application of the modified BET equation1' to C/Co= 1.0 (though with the possible exclusion of one or two experimental points at the lower C/Covalues),with A m = 52.5 A2;77that is, it was assumed a parallel orientation for the PNP molecules on the surface of the adsorbents. The S p ~ pvalues are given in Table 3. They show that SpNp is significantly lower than $BET (Table 1) for all samples. However, Puri et d.16*17 reported close &ET and S p ~ vp ues. These discrepancies in the S (specificsurface area) v ues are probably connected with the C/Co range of application of the BET equation and also with the A,,, value siqce Puri et al.16J7used 55 A2. Finally, it should be point out that, particularly in the region at high C/Co values, the isotherms for PNP display shapes which differ significantly from those exhibited by the isotherms determined using a shakerbath.% Therefore, the adsorption process and mechanism at such C/Covalues were markedly influenced by whether the adsorption system was stirred at 600rpm in the calorimeter reactor or agitated at 120 oscillations/min in the shakerbath. 6. Surface Functional Groups. As earlier reported by Dacey,2carbon blacks contain up to 1%hydrogen and as much as 10% oxygen. The hydrogen is part of the original hydrocarbon and is bonded to crystallites of the
3
(77)Gila, C. H.; D'Silva, A. P.;Trivedi, A. S. J. Appl. Chem. 1970, 20,37.
850 Langmuir, Vol. 10, No. 3, 1994
4000
2000 Wave number, cm'l
Conzdlez-Marttn et al.
4.50
Figure 7. FT-IR spectra of the samples.
carbon, whereas the oxygen is chemisorbed from air on the carbon surface. This suggests that most surface functional groups in carbon blacks are oxygen groups. The FT-IR spectra in Figure 7 display a number of main absorption bands located around 3440 and 1600cm-I, and in the ranges 1400-1600 and 1000-1350 cm-l. The band appearing at higher wavenumbers is ascribed to v(O-H) vibrations (v = stretching) in H-bridged OH groups. The band at 1600 cm-l was also observed by Rositani et al.9 in a study on infrared analysis of carbon blacks. It was attributed to oxygen iono-radicalstructures C,yO,7- to highly conjugated CO groups in a quinone configuration,8l* and to aromatic ring stretching frequencies@jls of an enhanced intensity by the presence of phenol or ether gr0ups.8~Bands between 1600 and 1400 cm-I could also be due to antisymmetric and symmetric stretchings of C02- ions.@ Several overlapped bands between 1350 and 970 cm-l are assigned to u(C-0) vibrations in ethertype structures and in hydroxyl groups. Specifically, the band centered around 1260 cm-l is ascribable to aromatic ether or to epoxygroups,the band at 1210cm-l to phenolic OH groups, and the band at 1130 cm-l to cyclic ethers in five- or six-membered rings. These spectroscopicresults show similarity in the chemical nature of surface groups of the carbonaceous materials. Only the spectrum of BP1300 displays the band at 1730 cm-l (Figure 7), which is associated with v ( C 4 ) vibrations. Some bands in the range 1400-1600cm-l are weaker in (or evenmissingfrom) the spectra of BP1300 and BP2000. The significant differences noted in the intensity of a larger number of bands (e.g., bands in the range 1350-970 cm-l such as those (78) Zawadzki, J. Carbon 1978, 16, 491. (79) Zawadzki, J. Carbon 1980,18,281. (80) Zawadzki, J. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker, Inc.: New York and Basel, 1989; Vol. 21, p 186. (81) OReilly, J. M.; Moeher, R. A. Carbon 1982,21,47. (82) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wiley: New York, 1978.
...
(83)VanDriel. In Activated Carbon A FaucinatingMaterial; Capelle, A., De Vooys, F., E%.; Norit, N. V.: The Netherlands, 1983. (84) Coughlin, R. W.; Kreysa, G. Int. Chem. Eng. 1984,24, 595. (85)Prest, W. M., Jr.; Mwher, R. A. Colloids and Surface in Reprographic Technologies; Hair, M., Croucher, M., Eds.; ASC Symposium Series; American Chemical Society: Washington, DC, 1982. (86)Akhter, M. S.; Keifer, J. R.; Chughtai, A. R.; Smith, D. M. Carbon 1986,23, 589. (87) Painter, P. C.; Snyder, R.
W.;Starsinic, M.; Coleman, M. M.; Kuehn, D. W.; Davis, A. Appl. Spectrosc. 1981,35,475. (88)Pasto, D. J.; Johnson, C. R. Organic Structure Determination; Prentice-Hall, Inc.: Engiewood Cliffs, NJ, 1969.
at 1260and 1210cm-l) indicate an unequal concentration of the surfaceoxygen groups which originate their spectral appearance. The influence of surface oxides on the extent of adsorptionwas studied by a number of researcher^.^^^^^^^^^^ When investigating the adsorption of phenols and nitrophenols, Mattson et al.ll proposed formation of a donoracceptor complex between the adsorbate and surface carbonyl groups. Puri et al.76reported that the surface oxygen complex has a negative or positive effect on the adsorption of phenol depending on whether it comes off as carbon dioxide or carbon monoxide; in the latter case, adsorption might occur by interaction or P electrons of the benzene nucleus with partial positive charge on the carbonyl carbon at0m.8~These findings show that the carbonyl groups may be involved in the adsorption of phenols and nitrophenols, and that the adsorbate-adsorbent interactions being responsible for the adsorption of such substances are of the acid-base type. According to the above study of the carbonaceous materials by FT-IR spectroscopy, surface C=O groups may be found in the samples. In addition to the surface groups absorbing radiation at 1600 cm-l, other predominant groups in all samples are the OH groups and the C - 0 4 structures. Ark6 et al.4 reported tha.t carbon blacks containing OH groups may be both basic and acidic carbons. To explain the dual behavior of the materials, they proposed the following mechanism: C-OH
+ H20
C-OH
c . *
c-*
C-0-
+ H0:
C+ + OH-
(1)
(2)
Under the adsorption conditions of PNP in this work, using PNP solutions at acidic pH, equilibria 1and 2 would shift toward the left and the right sides, respectively. Accordingly, C+carbons might take part in the adsorption process of PNP. As for the ether-type structures, it is well known that these atomic groupings can behave as Lewis basesWlg1and that aromatienitro compoundsinteract with bases with formation of P and u complexes.02 The involvement of basic oxygen groups in the adsorption of PNP appears however less probable not only because of the electronicproperties of this adsorbate, which are dealt with below, but also because of the competition effect between PNP and HsO+. B. Calorimetric Heat of Adsorption of pNitrophenol. Before studying the effect of the surface area, porosity, pore-size distribution, and surface chemistry on the heat of adsorption, let us consider the adsorption possibilities for PNP from aqueous solutions. This adsorbate molecule possesses a high electronic charge at both sides of the aromatic ring, and the NO2 grouping atoms have unshared electrons. Furthermore, the molecule ionizes through the OH group with formation of the p-nitrophenolate anion. The dissociationequilibrium for PNP is very sensitive to temperature and pH variations. The equilibrium is found displaced toward the p-nitrophenolate anion at strongly basic pH and toward the undissociated PNP molecule at pH below 6; in view of this, the acetic acid-sodium acetate buffer solution was used in this work to keep the pH fixed at 4.18 As a result, (89) Bhacca, N. N.; Williams, D. H. TetrahedronLett. 1964,42,3127. (90)March, J. Aduanced Organic Chemistry, 3rd ed.; Wiley: New York, 1985. (91) Leon y Leon, C. A.; Solar, J. M.; Calemma, V.; Radovic, L. R. Carbon 1992,30,797. (92) Buncel, A. R.; Norris, A. R.; Russel, K. E. Q. Reu. 1968,22, 123.
Langmuir, Vol. 10, No. 3, 1994 851
-
-
BP880 B
A
-45'
BP1300 BP2000
1
1
I
Figure 8. Enthalpy of adsorption against the surface coverage. the adsorption of the PNP can occur by interaction of the PNP molecule through ?r electrons, the NO2 group, or the negatively charged 0 atom with Lewis acid type surface groups of the adsorbent. Obviously, the involvement of the aromaticring or functionalgroups and in turn a parallel or perpendicular surfaceorientation of the adsorbate must depend on properties of the adsorbent such as the poresize distribution and the surface distribution or density of the active sites. For a given material containing narrow pores in comparison with the largest molecular dimension of the adsorbate, the parallel orientation would be permittedonly. In pores with a suitable width, this adsorbate might even interact at the same time with active centers or with adsorbate molecules present in the opposite pore walls, which would give rise to an enhanced heat of adsorption. In the absence of restrictions to the adsorbate orientation by pore-size effects, the surface distribution of active sites would be the controllingfactor. If the active centers are sufficiently close to each other as to facilitate the simultaneous covering of a few of them by a single adsorbate molecule, the parallel orientation would be favored because of the increased extent of interaction for the aromatic ring with regard to both functional groups and also of the probable stability gain in the adsorbed state. On the contrary, in the case of an adsorbent with a low density of active sites, the involvementof functional groups would be more likely since in this way the adsorbent-adsorbate interactions would be more confined. 1. Experimental Heat of Adsorption-PNP Adsorption. After the adsorption isotherm, the heat of adsorption is the next most often determined quantity in characterizationstudies on the adsorbent and the adsorbed state.5' In investigations on adsorption calorimetry of N:! on carbon blacks, the variation of the differential enthalpy of adsorption with increasing surface coverage (8 = n/nm; n is the quantity of gas adsorbed expressed in moles of adsorbate per gram of adsorbent and nm is the monolayer capacity) was connected with the surface heterogeneity of the adsorbent93 and with lateral interactions between adsorbate molecules and the completion of the monolayer.g4 In the adsorption of solutes at the solid/liquid interface the enthalpy of adsorption (AH) is an excess quantity. According to the method of AH determination followed in this work, AH represents the enthalpy of PNP adsorption at 30 "C on the adsorbent saturated with the solvent, which in fact was an acetic acid-sodium acetate buffer solution. Figure 8 depicts the variation of AH (kJ mol-l) with 8. In the range studied of 8 up to a value of (93)Joyner, L.G.;Emmett, P. H. J.Am. Chem. SOC.1949,70,2353. (94)Grillet, Y.;Rouquerol, F.; Rouquerol, J. J. Colloid Interface Sci. 1979,70,239.
"
0.0 I 0.0
I
0.2
I
0.4
I
0.6
I
0.8
1.0
Relative concentration, C/Co
Figure 9. Heat of adsorptionagainst the relative concentration. less than 1,the plots show three branches, the slopes of which depend on the sample; the general behavior is two branches at low and high 8 values which present greater slopes than the intermediate one. As these plots resemble in their shape those obtained with Nz also using carbon blacks, they might deserve a similar explanation. As 8 increases PNP would cover adsorption sites of progressively less activity, and once a certain surface coverage is reached, the adsorbate molecules would interact laterally. To facilitate the comparisonwith the adsorption results, the heat of adsorption (-AH, J mol-') was expressed per unit mass of adsorbent (-AH,J gl),as the amount adsorbed of PNP (X,mol gl). Figure 9 illustrates the variation of -AH with C/Co.First, let us describe these -hH plots. They show a steep initial branch and a sharp knee which is followed by a more ore less well-defined "plateau", the length of which depends on the sample and which ends in a small downward-slopingbranch. The heat of adsorption first increases strongly with C/Co, then remains nearly steady, and finally even decreasesslightly. The variation of -AH at low and intermediate C/Covalues is different for the various samples, for BP1300, BP2000, and BP880 being more marked than for V6, V3, and particularly SV, at least during the first adsorption stage. For some samplessuch as SV, BP880, and BP2000 it proves to be fairly similar to that exhibited by X (Figure6) which points to an important role of the surface area in the evolution of heat, as expected. The decrease in -AH at high C/Covalues suggests that a fraction of the adsorption sites showing activity from the standpoint of the release of heat are not involved in the adsorption process. This fact may be connected with a nonordered adsorption of PNP when a relatively concentrated solution of this substance comes into contact with the adsorbent. By contrast, the general trend of the adsorption of PNP is to increase above C/Co= 0.34.4(the increase in X is greater for SV, BP2000, V3, and V6). The discrepancies in the variation of X and -AH at high C / C , values are likely bound up with the adsorption process (specifically, with the adsorption mechanism or with diffusion-activated effects) which depends on the adsorbent.23 2. Variation of Heat of Adsorption with Surface Area. The surface area of an adsorbent solid influences the heat of adsorption through the number of active sites involved in the adsorption process as it must be larger for
852 Langmuir, VoL. 10, No. 3, 1994 i03
Gonz6Lez-Martln et al.
straight lines, the carbonaceous materials can 7 ofberesultant included in two groups. The first one comprises SV,
Specific sbrioce area (rnz g - ' )
Figure 10. Variation of the heat of adsorption with the specific surface area. 100 I
I
0
I
0
5
10
'5
20
I
25
30
35
s-') Figure 11. Variation of the heat of adsorption with the pore volume. Pore voIume/lO-'
(cm3
the material with a higher surface area. Figure 10 depicts the variation of -AH with S; the maximum values of -AH (Table 3) in the plots of -AH against C/Co (Figure 9) were chosen in this study, as they usually appear at intermediate C/Co values between those of application of the BET equation. The continuous increase of -AHwith S (in fact, the fitting by the least-squares method yields linear correlation coefficients higher than 0.998, although this figure may lead to a wrong conclusion), irrespective of the determination and estimate methods of S (N2 adsorption at -196 "C and of PNP at 30 "C; BET and Harkins/Jura equations),shows the-AHdependence on the surface area of the samples. The points fall in two straight lines (one going through SV, V3, and V6 and the other one through BPSSO, BP1300, and BP2000), and this proves the influence of the adsorbent on -AH. As the slope is greater for the straight lines defined by BP880, BP1300, and BP2000, the heat of adsorption per unit surface area of the adsorbents is higher for these samples. Accordingly, any other contribution to the heat of adsorption due the porosity, the pore-size distribution, or the surface chemistry can be regarded as more probable with these samples than with the other ones. 3. Variation of Heat of Adsorption with Porosity. Figure 11illustrates the variation of -AH with the pore volume. Firstly, it should be pointed out that -AH increases with the pore volume. According to the number
V3, and V6. For these samples the slope of the straight lines, which represents the heat of adsorption per unit pore volume of the adsorbents, is greater in the V d plot than in the W Oone. The higher heat of adsorption with respect to the microporosity determined by N2 adsorption is likely connected with the adsorption of PNP on the external surface and in other porosity ranges. In this connection it should be recalled that the -AHvalues chosen in this study correspond to the adsorption of PNP when it had already occurred largely on most samples, whereas the Harkins/Jura equation was applied t o p / p o= 0.3. The decreased slopeof the Woplots is consistent with the higher C02 adsorption (despite being the isotherms for C02 determined to p / p " = 0.03, taking p o = 3.5 MPa) and the larger W Ovalues. The fact that for the second group of samples, which are the materials with a higher development of microporosity, both straight lines are nearly parallel is a quite interesting result as it shows that the heats of adsorption, with the above indicated meaning, are similar regardless of whether V~ or W Ois used. Moreover, good correlation in the variation of -AH with V,,, V, or VT (Table 2) was not noted, as expected. From these results it can be concluded that the heat of adsorption (such as it has been studied as a function of V d and WO) appears to be greatly dependent on whether the adsorption of PNP occurs on the externalsurface and in large pores (mesopores and macropores) or in micropores, which is in good agreement with previously reported results." 4. Influence of Pore-Size Distribution on the Heat of Adsorption. The pore-size distribution of the carbons may influence the heat of adsorption through its incidence on the adsorption process and the adsorbed state as it controls the diffusion of the adsorbate inside adsorbent pores when moving toward the adsorption centers, which for some highly porous carbons such as activated carbon concentrate in micropores, and also the surface orientation of the adsorbate and the separation of this either from active sites or from adsorbed molecules located in facing pore walls. Prevention of mass transport will decrease the adsorption of PNP and hence the heat of adsorption. The last two factors might affect the surface interactions and in turn the heat of adsorption. According to the P values listed in Table 1, the carbonaceous materials with a greater fraction of external surfaceare V3, SV,V6,and BP880. Hence, the adsorption process, which comprises a diffusion stage in adsorbent pores, should be facilitated in these samples. ThePvalues (Table 1)show however that just for these materiale the porosity open to C02 at 0 "C but closed to N2 at -196 OC increases, and these adsorbents might possess pores inaccessible to N2. This would also be possible with a larger molecule such as PNP, althoughwiththis adsorbate the adsorption temperature was higher. The plot of Wo versus S p ~ (Figure p 12) indicates that, on the basis of the results of C02 adsorption and in comparison to other adsorbents, mass transport of PNP is significantly prevented in BP880, whereas it increases for BP2000. If the above suggested adsorption of C02 on the external surface is more important in SV, V3, and V6, which are the samples to which correspond the larger P values (Table 11, the actual values of the micropore volume for these samples would be smaller than WO, and consequently the straight line in Figure 12 might pass through SV, V3, V6, and BP2000. Then, prevention of mass transport of PNP would be greater in BP880 and BP1300. If so, the behavior shown by these samples is in line with the increase
Langmuir, Vol. 10, No. 3, 1994 863
Characterization Study of Carbonaceous Materials
r'
30
0
-E,
20
t,
/'
/
I
I
1 I
t
i
i
i/
ow 0
i I
200
I
400
spNp(m2
I
600
1
800
1000
g-')
Figure 12. W O(COZ,0 "C)againstthe specific surfacearea (PNP, 30 OC).
produced in the adsorption of PNP at high C/Co values by the effect of increasing the adsorption temperature in the 20-40 O C range.29 On the other hand, the low V d , WO, and V,, values for SV (Table 1)rule out the hindrance to the diffusion of PNP by pore-size effects as with this sample the adsorption of PNP must occur largely on the external surface and in large pores. The variation of -AH with S (Figure 10) suggests that the PNP molecule takes up the same orientation on the surface of these carbonaceous materials. Otherwise, the adsorbent-adsorbate interactions and the surface area covered by each adsorbate molecule would be different from some samples, and this should be reflected in the plots of -AH versus S or the pore volume by displaying sudden slope changes. This assumption is supported by the rather similar chemical nature of the oxygen groups in the carbonaceous materials since it would propitiate the same type of specific interactions with the adsorbate molecules. Probably, the adsorption of PNP occurs on the external surface and in large pores or in micropores, but without the pore-size distribution being responsible for differences between the adsorbentsfrom the standpoint of the surface orientation of the adsorbate and of its influence on the heat of adsorption. For BP880, BP1300, and BP2000,which are the samples with a better developed microporosity, such a behavior is of interest since it means either that the adsorption of PNP occurs in micropores which are wide enough in comparison with the largest molecular dimensionof PNP and so the surface orientation of the adsorbate is governed by the density of active sites or that the pores are too narrow, enabling the adsorbate to take up only a determined surface orientation by poresize effects. In the latter case, the simultaneous interaction of the adsorbate with active sites located in opposite pore walls would be possible. In the characterization study of the carbonaceous materials carried out in this work, the information obtained on the pore-size distribution of the sampleswas restricted to the ranges of macropores and large mesopores, and therefore the variation of -AH can not be correlated with awider spectrumof pore sizes. Of particular interest would have been the correlation with the pore-size distribution in the micropore range since for some of these samplesthe adsorption of PNP at low C/Co values probably occurs largely in micropores; however, it was not possible due to the lack of known methods which fulfill the requirements of reliability and easy application. Nevertheless, under certain assumptions -AH might be connected with the size of the pores through the slope of the initial upward branch of the adsorption isotherms. Figures 6 and 9 show that the ascending branch of -AH stands above the one
of X and that the separation between the -AH and X branches varies by the sequence V3 > V6 > BP1300 > BP880 = SV. For V3, V6, and SV, the increased -pH with respects to Xis not attributable to pore-size effects as the presence of micropores in these samplesis very small and consequently differences between -AH and X must be connected with another factor. In the series BP1300, BP2000, and BP880, it is obvious that if the adsorption of PNP occurs in pores progressively larger as C/Co increases, the interaction of the adsorbate with active sites or the adsorbed state present in opposite pore walls should be favored at the lower C/Co values, and therefore the separation between the-AHand X curves should decrease in the direction of increasing C/Co,just the reverse to the behavior noted. Despite this statement, it is possible that the effect of pore size on the heat of adsorption concurs with another predominant one, which may be connected with the lateral interactions among adsorbate molecules, gaining in importance as the adsorption process goes on. In such a case, the effect of the pore size on the heat of adsorption for BP1300 might be greater than for BP880 or BP2000,as suggested by the magnitude of the separation between the plots and by the slopes. 5. Influence of Surface Functional Groups on the Heat of Adsorption. Besides the surface area, the porosity, and the pore-size distribution, the surface chemistry of the adsorbent may also influence the heat of adsorption. When applied to carbons, the term surface chemistry refers to the chemical nature, the number, and the distribution (density or concentration) of the surface groups, and in particular of the oxygen complex which is the majority in carbon materials.95 In relation to the heat of adsorption, the -AH curves (Figure 9) and the -AH values (Table 3) suggestthat the role of the chemical nature of surface oxygen groups is not decisive since changes in the relative position or magnitude are not observed with regard to the adsorption results (namely, the adsorption isotherms and Spw) obtained with PNP. As the number of different oxygen groups in unequal (Figure 7) in samples such as B P W , BP1330,and BP2000,probably only oxygen groups present in all samples (e.g., the -OH groups) are active sites, being responsible for the adsorption of PNP and for the release of heat. If not, the contribution to the adsorption process of other groups would be of little significance. In fact, the weak band at 1730 cm-l in the spectrum of BP1300 denotes a low concentration in this adsorbent of the C 4 groups which may originate its spectral appearance. The number of oxygengroups involved in the adsorption of PNP controls the extent of the adsorbent-adsorbate interactions, which contribute to the evolution of heat during the adsorption of PNP. As shown by Figure 9, -AH varies through the sequence BP2000 > BP1300 > BP880 > V6 > V3 > SV. In principle, this -AH variation does not seem to be consistent with the FT-IR results, even though -AH was expressed per a unit mass of sample and the same mass of sample was always employed in the preparation of carbon/KBr disks used in recording the spectra. Correlation between -AH and the intensity of some individual band is not noted, as illustrated by the couple BP880 and SV. Thus -AH is much higher for BP880 than for SV; however, the spectra of these samples are similarly shaped. As expected, the disagreementwith the FT-IR results applies to the amount adsorbed of PNP, X (Figure 7). This shows that no single oxygen group of the adsorbents is responsible for the whole adsorption of (96) Jankowska, H.; Swiatkoweki, A.; Choma, J. Acta Carbon; Ellie Horwood: New York, 1991; p 81.
;:rp-:;
854 Langmuir, Vol. 10,No. 3,1994
Gonzdrlez-Martln et al.
7 \
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a Of" 00
I
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I
02
04
06
1
08
I 10
0
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a V6
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E 0 32
Relative concentrat o n , C/Co
A
BP2000
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32
0 4
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Figure 13. X/SpNp against the relative concentration.
Figure 14.
PNP. Accordingly, several groups might take part in the uptake of PNP. They could be the C=O and -OHgroups, which is suggested by the intensity of the bands at 1600 and 1210 cm-l in the FT-IR spectra, as seen below. The distribution of active sites influences the heat of adsorption through the possible existence of lateral interactions among adsorbate molecules. The lateral interactions can occur even at low degrees of surface coverage, provided that the active sites already covered by the adsorbate at any adsorption stage are situated in sufficientlynearby positions. By contrast, a high surface coverage does not necessarely imply the proximity of the adsorbate molecules in the adsorbed state and hence the lateral interaction because the real surface of a given solid may differ greatly from the surface showing activity to the adsorption of a particular adsorbate. These facts show the dependence of the lateral interactions on the surface concentration of active sites. If the adsorbate molecules interact laterally, it should contribute to the heat of adsorption in addition to the adsorbent-adsorbate interactions. As these control the extent of adsorption, the occurrence of lateral interactions would give rise to an increased heat of adsorption with respect to the amount adsorbed. In connection with the lateral interactions another interesting point is the surface orientation taken up by the adsorbate since it determines the intensity of interaction in the perpendicular directionon the adsorbent surface by variation of the facing molecular section or functional groups of the adsorbate. In the case of PNP the perpendicular orientation, which means that the PNP molecules stand with either the NO2 or OH group (or the C-O- ion) just on the surface of the adsorbent, would lead to a higher degree of lateral interaction as the facing molecular section increases greatly in comparison with the flat orientation (that is, the PNP molecule is found with the aromaticring on the surface). As indicated above, the surface orientation of the adsorbate is governed by the pore-size distribution and the surface distribution of active sites, although with these carbonaceous materials as adsorbents the latter factor appears to be the prevailing one. Information on the concentration of active sites in the carbonaceous materials has been provided in this study by the amount adsorbed of PNP expressed per unit surface area of adsorbent, X/Spw (mol m-9. The X/Spw variation is BP1300 > BP880 = BP2000 > V3 = V6 > SV (Figure 13)which, obviously,highly resemblesthat of -AH/ Spw (J m-21, the heat of adsorption also per unit surface area (Figure 14). These results correlate approximately with the intensity of the bands at 1210 and 1600 cm-l compared to the band at 1260 cm-l in the FT-IR spectra. The band at 1210cm-l is stronger for BP880, BP1300, and
BP2000 than for the other samples. Furthermore, the relative intensity of the band at 1600 cm-l is greater for BP1300 and for V3 or V6 (these are the samples to which, in the above established groups, correspond the higher X/Spw and -AH/Spw values) and similar for BPS80 and BP2000. Moreover, for samples such as BP1300, V3, and V6 the increased -AHwith regard to X is more significant (Figures 6 and 9). These facta point to the concentration of oxygen groups, and hence to the lateral interactions among adsorbate molecules, as an important factor in connection with the amount of heat evolved during the adsorption of PNP. C. Summary. According to the results obtained in the characterizationstudy, the carbonaceousmaterials can be grouped into two series of samples. The first one comprises SV, V3, and V6, which are the samples with a high external surface and that contain large pores; the microporosity in these materials is almost negligible or very reduced. The adsorption of PNP on these samples will take place on the external surface and on pore walls. The second series is made up of BP880, BP1300, and BP2000, which possess a well-developed surface area and porosity includingmicroporosity. With these samplesthe adsorption of PNP at low C/Covalues will occur mainly in micropores. The chemical nature of surface oxygen groups is similar in all carbonaceous materials, as shown by the FT-IR results. Significantdifferencesin the relative concentration of oxygen groups are observed. The heat of adsorption, when expressed per unit mass of adsorbent, varies similarly to the amount adsorbed of PNP, particularly for SV, BP880, and BP2000. The heat of adsorption depends on the surface area and the microporosityof the samples. The pore-size distribution does not appear to affect -AH through the surface orientation taken up by the adsorbate. However, it does through its influence on the diffusion of PNP in the porosity of adsorbents and also possibly on the interaction of the adsorbate with active sites or with the adsorbed state located in facing pore walls. The chemical nature of surface oxygen groups present in the adsorbents does not seem to be an important property in connection with the heat of adsorption. The variation of -AH is not consistent with the number of each one of the different oxygen groups noted in the samples. When the amount adsorbed on PNP and the heat of adsorption are expressed per unit surface area of adsorbent, there is good agreement with the FT-IR results. This suggests a significant effect of the concentration of oxygen groups on the heat of adsorption.
- A H / s p ~ p against
the relative concentration.
