3226
Langmuir 1999, 15, 3226-3231
On the Adsorption of Formaldehyde at High Temperatures and Zero Surface Coverage M. Domingo-Garcı´a, I. Ferna´ndez-Morales, F. J. Lo´pez-Garzo´n,* C. Moreno-Castilla, and M. Pe´rez-Mendoza Grupo de Investigacio´ n en Carbones, Dpto. de Quı´mica Inorga´ nica, Facultad de Ciencias, 18071 Granada, Spain Received October 27, 1998. In Final Form: January 25, 1999 The adsorption of formaldehyde (HCHO) at very low vapor concentration (zero surface coverage) is studied on several carbon materials by using inverse gas-solid chromatography (IGC). The adsorption process is carried out in the Henry’s law region. Therefore, the specific retention volumes, Vs, allow the determination of the standard enthalpy of adsorption, ∆HoA, using the linear relationship between ln Vs vs 1/T. Nevertheless two linear plots are obtained for the adsorption of HCHO on these carbon materials. Several experiments have been designed in order to determine the physical meaning of these linear plots. The general conclusion is that they are produced by a temperature-dependent mechanism which allows the adsorption to occur in smaller pores at lower temperatures and in larger ones at higher temperatures.
Introduction Formaldehyde is a major intermediate product which is industrially produced from methanol. It is one of the most versatile chemicals and is, therefore, used by many industries to manufacture a large number of indispensable products used in daily life.1 However, it is also a byproduct of many chemical processes. In some of these it is released to the atmosphere, producing environmental pollution. A major source of atmospheric formaldehyde is the incomplete combustion of motor fuel. As a consequence, formaldehyde appears in car and aircraft exhaust gases and also in significant amounts in off-gases from heating plants, incinerators, and petroleum refineries. In addition, a significant amount of formaldehyde is released to the atmosphere by production plants (around 1% of the total amount1). In confined areas, formaldehyde is released in cigarette smoke, from phenol-formaldehyde resin foam insulation and open fireplaces, and during disinfection and sterilization of large surfaces. For these reasons the adsorption of formaldehyde on carbon surfaces is of potential interest from two viewpoints. First, there is the possibility of recovering and concentrating the product for industrial applications. The second point concerns the elimination of formaldehyde from polluted atmospheres for environmental protection. In both cases the adsorption process should be studied under dynamic conditions in order to closely reproduce real conditions. For this purpose, inverse gas solid chromatography (IGC or IGSC) has the advantage over conventional adsorption methods in that measurements can be made at relative high temperatures, at very low gas-phase concentration, and under dynamic conditions.2-5 Activated carbons are frequently used to concentrate products or to remove pollutants from different environ* To whom all correspondence should be sent. E-mail: FLOPEZ@ GOLIAT.UGR.ES. (1) Ullman’s Encyclopedia of Industrial Chemistry; VCH Verlagsfesellschft mbH: Weinheim, Germany, 1988, Vol. A11 and references therein. (2) Jagiello, J.; Bandosz, T.; Schwarz, J. A. Carbon 1992, 30, 63. (3) Vidal, A.; Papirer, E.; Wang, M. J.; Donnet, J.-B. Chromatographia 1987, 23, 121. (4) Domingo-Garcı´a, M.; Ferna´ndez-Morales, I.; Lo´pez-Garzo´n, F. J.; Pyda, M. Chromatogarphia 1992, 34, 564. (5) Vukov, A. Y.; Gray, D. G. Langmuir 1988, 4, 743.
ments.6-8 Their value for these purposes lies in the great versatility of their textural (surface area and porosity) and chemical properties (chemical surface groups). Due to these characteristics, the adsorption of a polar molecule such as formaldehyde on activated carbons can be controlled by the porosity of the adsorbent (textural factors), by the chemical groups (chemical factors), or by both factors. The shape and size of the adsorbate is important in relation to the textural factors, while the dipole moment is of relevance to the chemical factors. The aim of this work is to study the dynamic adsorption of formaldehyde on carbon materials. Inverse gas-solid chromatography has been used for this purpose. The process was carried out at very low (zero) surface coverage on several carbon materials with different textural characteristics. Experimental Section Several carbon materials of different origins were used as adsorbents. A commercial activated carbon manufactured by CECA (GAe) was used. H25 is an activated carbon obtained by carbonization of olive stones in N2 flow at 1273 K and further activation9 with CO2 at 1263 K. C0 is a char obtained by pyrolysis of almond shells in N2 flow at 1273 K. S700 and S700-ox are carbon materials obtained from Saran.10 The former is produced by pyrolysis in N2 flow at 973 K and the latter by oxidation of S700 with H2O2 and further pyrolysis at 623 K. Spheron-6 is a carbon black manufactured by Union Carbide, and Graphon is a graphitized carbon black obtained from Spheron-6 and also manufactured by Union Carbide. The textural characteristics of the samples were obtained by mercury porosimetry and N2 and CO2 adsorption at 77 and 273 K, respectively. The former was used to obtain macro- (V3) and mesopores volumes (V2) g 3.6 nm (6) Golden, T. C.; Sircar, S. Carbon 1990, 28, 683. (7) Rao, M. B.; Sircar, S. J. Membr. Sci.1993, 85, 253. (8) Louie, D.; Goin, J.; Jaeckel, M.; Takei, N. Activated Carbon, Chemical Economic Handbook-SRI International; SRI International: Menlo Park, CA, 1992; p 731.2000a. (9) Salas-Peregrı´n, M. A.; Carrasco-Marı´n, F.; Lo´pez-Garzo´n, F. J.; Moreno-Castilla, C. Energy Fuels 1994, 8, 239. (10) Ferna´ndez-Morales, I.; Guerrero-Ruı´z, A.; Lo´pez-Garzo´n, F. J.; Rodrı´guez-Ramos, I.; Moreno-Castilla, C. Carbon 1984, 22, 301.
10.1021/la9815190 CCC: $18.00 © 1999 American Chemical Society Published on Web 04/02/1999
Adsorption of Formaldehyde
Langmuir, Vol. 15, No. 9, 1999 3227
Table 1. Textural Characteristics
GAe H25 S700 S700-ox C0
V3 (cm3/g)
V2 (cm3/g)
Wo (cm3/g)
SN2 (m2/g)
SCO2 (m2/g)
0.146 0.060 0.000 0.039 0.053
0.171 0.200 0.050 0.027 0.079
0.293 0.391 0.380 0.320 0.167
1026 910 914 858 400
714 1012 956 846 602
in width. The surface areas were obtained from the BET and Dubinin-Radushkevich (DR) methods applied to the adsorption data of N2 and CO2 respectively. For these purposes a molecular area of 0.162 (N2) and 0.235 nm2 (CO2) were used.11 Carbon, nitrogen, hydrogen, and sulfur contents were determined by elemental analysis. The oxygen content was calculated from the difference between these. The adsorption of formaldehyde was studied at very low vapor concentration (zero surface coverage) using IGC. Prior to the adsorption runs, the adsorbent was conditioned in the chromatographic column for 12 h at 593 K in N2 flow. The adsorption experiments were carried out using N2 as carrier gas at a flow rate ranging between 15 and 40 cm3/min. The adsorption temperature was between 393 and 553 K. Formaldehyde was obtained by heating paraformaldehyde at 358 K in a sampling bulb from which different amounts of vapor were withdrawn with a syringe and injected into the column containing the adsorbent. At least three different amounts were injected at each temperature. The adsorption process was followed by a flame ionization detector (FID). The chemical composition of the vapor to be injected in the chromatograph was checked in several blank experiments in N2 and air flows. A thermogravimetric system on line with a Fourier transform infrared instrument (TGA-FTIR) was used for these experiments. In addition, the adsorption of methanol, methyl formate, benzene, and cyclohexane was studied using IGC. This was carried out under the same experimental conditions as those used for formaldehyde adsorption. Further details are given in the next section to explain the reasons for studying the adsorption of these molecules. Results and Discussion The textural characteristics of the samples have been compiled in Table 1. The samples are mainly microporous (Wo, obtained from DR plots) although significant volume of mesopores (V2) is found in GAe and H25, and a significant volume of macropores (V3) in GAe. Concerning the surface areas, both H25 and C0 have lower SN2 than SCO2, which means that there are constrictions11 to the access of N2 at 77 K, which appears to be more important in C0. However, GAe has larger SN2 than SCO2 which is normally explained as being due to capillary condensation in supermicropores12 in the N2 adsorption process. The results of the elemental analysis are given in Table 2. The most interesting feature is the significant increase in oxygen content in sample S700-ox with respect to S700 due to treatment with H2O2. The retention time of the experimental chromatographic peaks was independent of the amount of adsorbate injected, and the peaks were in most cases symmetrical. In these cases, retention times were measured at the peak maxima. In some cases (mainly in sample C0 due to the pore constrictions) the peaks showed some asymmetry (11) Rodrı´guez-Reinoso, F.; Linares-Solano, A. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1989; Vol. 21. (12) Gregg, S. J.; Sing, K. S. W. Adsorption Surface Area and Porosity, 2nd ed.; Academic Press: New York, 1982.
Figure 1. Variation of the net retention volumes of formaldehyde vs. the CO2 surface area. Table 2. Elemental Analyses GAe H25 S700 S700-ox C0
%N
%C
%H
%O
0.8 0.4 nil nil 0.4
91.3 91.4 91.3 87.1 95.6
0.4 0.5 1.0 1.2 0.4
7.5 7.7 7.7 11.7 3.6
and the retention times were calculated from the firstorder moment method.13 The retention times can be transformed into net retention volumes, VN, which are a measure of the adsorption capacity of the samples. The values of VN vs the surface area of the carbon materials (measured by CO2 adsorption) are plotted at two temperatures in Figure 1. The adsorption capacity for HCHO increases for surface areas (SCO2) higher than 850 m2/g, whereas it remains almost unchanged with smaller surface areas in the range 600-850 m2/g. This is probably due to the larger number of micropores similar in size to the molecular size of formaldehyde in the samples with large CO2 surface areas (S700 and H25) which can be easily reached by the adsorptive. The adsorption of HCHO molecules is, therefore, greater in these samples. The importance of the accessibility of the molecules into the carbon porosity is more clearly seen in samples S700 and S700-ox. As mentioned above the latter is obtained by treatment of the S700 with H2O2. This treatment fixed oxygen chemical functionalities at the entrance of the micropores,10 producing constrictions which hindered the adsorption of the adsorbate molecules. The consequence of this is that SCO2 and SN2 (Table 1), and VN (Figure 1) decrease from sample S700 to S700-ox. However, while the decrease in CO2 and N2 surface areas are 12 and 6% respectively, a clearly larger decrease in HCHO adsorption was observed (21% at 433 K and 44% at 533 K). To understand these differences it is necessary to bear in mind the differences in the experimental conditions between HCHO, CO2, and N2 adsorption measurements. The former was carried out using IGC, and only very small amounts of HCHO, sufficient to fill only a few micropores (zero surface coverage), were injected. The relative pressure of the adsorbate injected was close to 10-6. Therefore, HCHO will be adsorbed in the narrower micropores, i.e., in pores of similar size to that of the molecular size of HCHO, which are largely affected by the H2O2 treatment. However, the adsorption of CO2 and N2 was carried out in a gravimetric system and the amount of adsorbate is greater than in the HCHO adsorption experiment. Under these conditions, CO2 and N2 filled the micropores, covering a (13) Conder, J. R.; Young, C. L. Physicochemical Measurements by Gas Chromatography; John Wiley & Sons: New York, 1979.
3228 Langmuir, Vol. 15, No. 9, 1999
Domingo-Garcı´a et al.
Figure 2. Plot of ln Vs vs 1/T for formaldehyde adsorption.
much larger range of pore sizes than HCHO did and as a result, the adsorption of CO2 and N2 was less affected by the H2O2 treatment. It is necessary to point out that the adsorption process occurs in the Henry’s law region when the maxima of the chromatographic peaks are independent of the amount of adsorptive injected. Then, the adsorption equilibrium constant14 is the specific retention volume, Vs, i.e., the net retention volume per unit of surface area of the adsorbate inside the column. The parameter Vs can be used to determine the standard enthalpy of adsorption ∆H°A. For this purpose the slope of the plot of ln Vs vs 1/T permits this determination13-15 using the equation
ln Vs ) -
∆H°A +C RT
(1)
These plots are recorded in Figure 2 for GAe and C0 as examples. It is worth noting that in all cases these plots have two straight lines with two different slopes, although eq 1 makes the assumption that ∆H°A is independent of T. There is a great deal of information in the literature2-5,13-15 in relation to the use of eq 1 for the adsorption of different molecules. In most cases a straight line is recorded over the entire temperature range studied. Two or more straight lines are obtained when changes in the surface characteristics of the adsorbent are produced.13,16 This is the case for bulk polymers or solids coated with polymers which are subject to temperature-dependent phase transitions. Few references, apart from these, are found17 which report this type of plots when eq 1 is used. The two slopes obtained for all the carbons were used to (14) Domı´ngo-Garcia, M.; Ferna´ndez-Morales, I.; Lo´pez-Garzo´n, F. J.; Moreno-Castilla, C.; Pyda, M. J. Colloid Interface Sci. 1995, 176, 128. (15) Domı´ngo-Garcia, M.; Ferna´ndez-Morales, I.; Lo´pez-Garzo´n, F. J.; Pyda, M. Chromatographia 1992, 34, 568. (16) Rayss, J. Adsorption on New and Modified Inorganic Sorbents; Elsevier: New York, 1996; p 503. (17) Bautista, F. M.; Campelo, J. M.; Garcia, A.; Luna, D.; Marinas, J. M. J. Colloid Interface Sci. 1987, 117, 347.
calculate the standard enthalpies of adsorption in the two temperature ranges. Their values are recorded in Table 3. Several hypotheses can explain these kinds of plots which have two straight lines of different slopes. One of these can be a change in the surface characteristics of the carbon materials. This is not plausible because these materials are obtained at temperatures much higher than those used in the adsorption experiments. Moreover, they have been conditioned at higher temperatures than those used in the adsorption runs. It can, therefore, be assumed that a change that could be chemical or physical is produced in the adsorbate molecule, HCHO. The former would concern changes in HCHO produced as a consequence of the adsorption temperature or as a catalytic behavior of the carbon material inside the column. It is known that HCHO is a very versatile molecule which is easily capable of several chemical changes, including polymerization.1,18-21 Experiments were carried out in order to determine whether the injected molecule was pure HCHO or whether some other molecule was produced because of the injection temperature. These experiments consisted in heating paraformaldehyde in a TGA-FTIR system in order to monitor the product. The study was carried out in both oxygen and nitrogen flow, under isothermal conditions (at 358 K) and also in ramping temperature up to 498 K. Only the monomer HCHO appeared in all cases, resulting in the rejection of the first hypothesis. The second possible explanation is that a chemical change is catalyzed by the carbon placed in the column. In this case formaldehyde would be adsorbed in one of the temperature ranges not as HCHO but as another molecule produced as a result of a catalytic effect of the carbon material inside the column. Formaldehyde can polymerize to the cyclic trimer known as trioxymethylene or 1,3,5trioxycyclohexane1,20
or it is easily transformed into methyl formate19-21
2HCHO f HCOOCH3 In particular, the catalytic effect seems to arise from the oxygen groups on the surface of the solids. In this case, it is necessary to accept that the catalytic effect, if it existed, will produced conversions of almost 100% since only a single chromatographic peak appeared in the temperature range studied. This is not unrealistic if one considers that the amount of HCHO injected is very low (zero surface coverage, P/P0 < 10-6). The adsorption of benzene, cyclohexane, methyl formate and methanol on C0 was studied by IGC in order to determine whether one of these chemical products was produced catalytically. If one of these molecules is produced its Vs, VN and ∆HoA values should be very close or equal to those recorded in Table 3. Benzene and cyclohexane were used as molecular models for trioxymethylene because they have similar shape and size. In particular, cyclohexane is geometrically very similar to 1,3,5-trioxyciclohexane. Benzene, which is also (18) Millar, G. J.; Rochester, C. H.; Waugh, K. C. J. Catal. 1995, 155, 52. (19) Yamakawa, T.; Hiroi, M.; Shinoda, S. J. Chem. Soc., Dalton Trans. 1994, 15, 2265. (20) Kern, F.; Ruf, St.; Emig, G. Appl. Catal. A: Gen. 1997, 150, 143. (21) Hung, W. H.; Bemasek, S. L. Surface Science 1996, 346, 165.
Adsorption of Formaldehyde
Langmuir, Vol. 15, No. 9, 1999 3229
Table 3. Specific Retention Volumes, Standard Enthalpies of Adsorption and Crossing Temperature for the Adsorption of HCHO Vs(cm3/m2) GAe H25 S700 S700-ox C0
433 K
533 K
-∆H°A (kJ/mol)a
-∆H°A (kJ/mol)b
Tc (K)
0.067 ( 0.003 0.066 ( 0.002 0.071 ( 0.005 0.056 ( 0.002 0.076 ( 0.006
0.015 ( 0.002 0.026 ( 0.001 0.027 ( 0.003 0.017 ( 0.001 0.020 ( 0.003
32.0 ( 0.3 20.6 ( 1.5 27.6 ( 1.2 33.3 ( 0.1 30.0 ( 0.8
25.6 ( 0.3 13.2 ( 0.5 15.6 ( 0.6 22.7 ( 1.3 20.3 ( 1.5
450 ( 6 497 ( 10 444 ( 3 431 ( 4 463 ( 9
a Standard enthalpies of adsorption in the low range of temperature. b Standard enthalpies of adsorption in the high range of temperature.
Table 4. Adsorption Parameters on C0 VN CH3OH HCOOCH3 C6H6 C6H12
(cm3/g)
Vs (cm3/m2)
433 K
533 K
433 K
533 K
-∆H°A (kJ/mol)
154.9 ( 4.9 948.8 ( 19.0 2145.5 ( 50.1 1262.9 ( 42.8
15.2 ( 1.0 49.2 ( 1.5 184.8 ( 2.1 40.4 ( 1.2
0.257 ( 0.012 1.576 ( 0.032 35.304 ( 0.904 2.097 ( 0.083
0.025 ( 0.003 0.082 ( 0.002 0.307 ( 0.005 0.067 ( 0.006
44.5 ( 1.2 56.6 ( 0.5 91.0 ( 1.6 67.3 ( 0.9
different slopes. HCHO is known to be able to adopt several modes of adsorption. The different specific modes of adsorption of this molecule have been reported in a large number of studies. Some of these examine this aspect from a theoretical point of view22-25 and others are essentially experimental.18,26-29 The latter relate the different specific modes of adsorption to the oxygen functionalities. In particular, among other forms,18,22-29 HCHO can be adsorbed through one atom (oxygen) or through two atoms (oxygen and carbon). The former can be on top or bridged (in the pictures A represents the atoms of the solid):
The latter can be produced (i) with the oxygen and carbon atoms in two of the surface sites
(ii) in two sites producing formate species. Figure 3. Plot of ln Vs vs 1/T for the adsorption on C0.
a six-membered ring, can be adsorbed specifically in a manner similar to that for 1,3,5-trioxyciclohexane. The results obtained for cyclohexane and methyl formate are plotted in Figure 3. The net and specific retention volumes and the standard enthalpies of adsorption of these molecules on C0 are recorded in Table 4. Comparison of these data with those in Table 3 shows large differences. In fact, the adsorption capacity, VN, of C0 for these molecules is much larger than for HCHO. Moreover, the adsorption equilibrium constants, Vs, are also larger for these molecules than for HCHO. Similarly, the absolute value of the standard enthalpies of adsorption are higher for these molecules than for HCHO. To sum up, none of these molecules show a similar adsorption behavior to that of formaldehyde. Therefore, the hypothesis that one of the two slopes is produced by a catalytic effect of the carbon material inside the column must be discarded. The change in the HCHO adsorption form could also be considered to be responsible for the two straight lines with
In the latter case one of the oxygen atoms belongs to a chemical surface group of the solid. If a change in adsorption form occurred, the difference between the standard enthalpies of the two ranges of (22) Delbecq, F.; Sautet, P. Surf. Sci. 1993, 295, 353. (23) Delbecq, F.; Sautet, P. Langmuir 1993, 9, 197. (24) Delbecq, F.; Sautet, P. Catal. Lett. 1994, 28, 89. (25) Delbecq, F.; Sautet, P. J. Catal. 1995, 152, 217. (26) Millar, G. J.; Rochester, C. H.; Waugh, K. C. J. Faraday Trans. 1991, 87, 2785 (27) Millar, G. J.; Rochester, C. H.; Waugh, K. C. J. Faraday Trans. 1992, 88, 1033 (28) Popova, G. Ya.; Budneva, A. A.; Andrushkevich, Kinet. Catal. Lett. 1997, 61, 353. (29) Popova, G. Ya.; Budneva, A. A.; Andrushkevich, Kinet. Catal. Lett. 1998, 62, 97.
Chem. Soc., Chem. Soc., T. V. React. T. V. React.
3230 Langmuir, Vol. 15, No. 9, 1999
temperature should be the same for all the samples. For instance, this would be the case if HCHO is adsorbed on top and then after is adsorbed bridged. This is not the case as can be deduced from the data in Table 3. Moreover, the temperature at which the change in the slope is produced, Tc, should also be the same. Nevertheless, this temperature was different for each carbon (Table 3). This suggests that factors other than a change in the form of adsorption of HCHO produce the two different straight lines and that these are probably related to the textural properties of the adsorbents It has been reported30,31 that the adsorption of polar molecules on porous carbon materials at zero surface coverage is produced by a mixed mechanism with two components: nonspecific and specific. The specific interaction is mainly produced on the chemical surface groups and on electronically rich regions of the surface. The nonspecific interaction is controlled by dispersion forces and is produced in the micropores when adsorption is carried out at zero surface coverage. Although the former can be significant for molecules with a large dipole moment, in many cases the latter can be very important, especially for the adsorption of small molecules on porous materials.30 In these cases porosity plays a significant role from both a thermodynamic and a kinetic point of view in the adsorption process at zero surface coverage.31,32 This suggest that the two straight lines with different slopes appear because the adsorption is produced in micropores of different sizes. The adsorption of HCHO at zero surface coverage on Spheron-6 and Graphon has been studied in order to confirm this hypothesis. Both have low surface area (99 and 78 m2/g, respectively) compared to the other carbon materials. In addition, the porosity is almost negligible and for Spheron-6 mainly appears as irregularities, gaps, and crevices on the surface,33 and for Graphon this is almost negligible as a consequence of the graphitization process. Spheron-6 and Graphon differ from each other in the amount of oxygen functionalities and in the energy of their surfaces. Graphon has essentially a homogeneous surface composed of (001) planes whereas the surface of Spheron-6 is heterogeneous in nature, composed of both (001) and (100) planes.33 The plots of ln Vs vs 1/T for the adsorption of HCHO on these carbon blacks are recorded in Figure 4. Neither of these plots shows two straight lines, suggesting that the existence of the two straight lines with different slopes could be related to the porosity of the other samples. The standard enthalpies of adsorption obtained from the slopes of the plots in Figure 4 are -6.3 and -30.6 kJ/mol for Graphon and Spheron-6, respectively. The absolute value for Graphon is very low because adsorption is produced on a very homogeneous surface, whereas Spheron-6 exhibits a high absolute value similar to the largest enthalpies in Table 3, which is due to the nature of this surface.33 It has been reported31,34 that the adsorption at zero surface coverage of different molecules on porous carbon materials is mainly produced in pores close to the molecular dimension of the adsorbate. This is possible because the concentration in the vapor phase is very low and therefore, the probability of finding pores of similar (30) Domingo-Garcı´a, M.; Lo´pez-Garzo´n, F. J.; Moreno-Castilla, C.; Pyda, M. J. Phys. Chem. 1997, 101, 8191. (31) Lo´pez-Garzo´n, F. J.; Domingo-Garcı´a, M. Adsorption on New and Modified Inorganic Sorbents; Elsevier: New York, 1996; p 517. (32) Foley, H. C.; Lafyatis, D. S.; Mariwala, R. K.; Sonnischen, G. D.; Brake, L. D. Chem. Eng. Sci. 1994, 49, 4771. (33) Mahajan, O. P.; Moreno-Castilla, C.; Walker, P. L., Jr. Sep. Sci. Technol. 1980, 15, 1733. (34) Lo´pez-Garzo´n, F. J.; Pyda, M.; Domingo-Garcı´a, M. Langmuir 1993, 9, 531.
Domingo-Garcı´a et al.
Figure 4. Plot of ln Vs vs 1/T for the adsorption of formaldehyde on non porous carbons.
size to that of the adsorbate is very high. Moreover, the influence of the pore and the molecule dimensions on the thermodynamic parameter of adsorption is such that closer molecular dimension and pore size produce higher absolute values of the standard enthalpy of adsorption.30,31,34 From this point of view the ∆H°A values compiled in Table 3 mean that, at lower temperatures, adsorption is produced in pores of smaller dimension (higher slope and absolute value of ∆H°A) than at higher temperatures (lower slope and absolute value of ∆H°A). In relation to Tc, the temperature at which the change in slope is produced (Table 3), it is interesting to note that this is dependent on the carbon material involved. It is, therefore, possible to accept that if the change in slope were produced by a modification in the textural or physical characteristics of the carbon the values of Tc should be almost the same for all the samples. The general conclusion obtained from all these data is that the change in the standard enthalpy of adsorption is temperature-dependent, which allows the adsorption of HCHO to be produced in smaller pores at lower temperatures and in larger ones at higher temperatures. It is plausible to consider that this behavior is not a particular characteristic of HCHO alone but rather a common performance of adsorptives. This fact is supported by data reported17 for the adsorption of several benzene derivatives. For all these adsorptives derived from benzene, two straight lines with different slopes were produced for the adsorption on a number of phosphate compounds. One could expect other molecules to show a similar behavior to HCHO. For this reason this possibility has been studied for the adsorptives previously used on C0: namely, methanol, benzene, cyclohexane, and methyl formate. Only methanol shows two straight lines with two different slopes in the temperature interval used. As explained above these two straight lines are probably
Adsorption of Formaldehyde
Langmuir, Vol. 15, No. 9, 1999 3231
adsorption of smaller molecules (monomethyl (MMA), dimethyl (DMA), trimethylamine (TMA), and n-butane) has been studied in carbon C0 in the same experimental conditions as those used for HCHO adsorption. The results obtained for MMA and n-butane are plotted in Figure 5. It is worth noting that both plots show two straight lines with different slopes, which supports the above explanation; i.e., adsorption in the lower temperatures range occurs in smaller pores whereas at higher temperatures it occurs in larger pores. To the best of our knowledge, no explanation has been reported in the literature for this kind of adsorption behavior at zero surface coverage. However, a similar treatment of the adsorption process is considered by Dubinin and Isotova35 at high surface coverage. In fact, the application of the Dubinin-Radushkevich equation, which predicts the fit of experimental data to a straight line, frequently shows a deviation of linearity. Dubinin and Isotova considered that this type of plot is produced by the superimposition of two ranges of microporosity. These two ranges have experimental data which can be fitted to two straight lines, and different heats of adsorption and micropore sizes can be determined from them. These data demonstrate that it is not possible to predict the amount and the heat of adsorption of a particular adsorbate (HCHO) by linear extrapolation (ln Vs vs 1/T) over a wide range of temperatures. Therefore, the behavior of an activated carbon in the concentration or elimination of small amounts of HCHO at relatively high temperatures is not easily predicted from its adsorption at lower temperatures. Figure 5. Plot of ln Vs vs 1/T for the adsorption of MMA and n-butane on C0.
produced as a result of a kinetic effect. This therefore, depends on the temperature and the molecular weight of the adsorbate, where the larger the molecular weight the higher the temperature Tc. Therefore, higher temperatures would be needed to produce a change in the adsorption of the molecules with a larger molecular weight, i.e., benzene, cyclohexane, and methyl formate. For these reasons, the
Acknowledgment. This work has been supported by the DGYCIT under Projects PB94-0754 and PB97-0831. M.P.M. acknowledges financial support from Junta de Andalucı´a. LA9815190 (35) Dubinin, M. M. In Chemistry and Physics of Carbon, Vol 2, Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1966; Vol. 2.
