2704
J . Phys. Chem. 1992, 96, 2104-2101
of the depletion width. But if the same kinds of sites are used on the two faces, the binding equilibrium constant, which is independent of the number of these sites, should be the same, as observed. A diagram of the electronic structures of the two surfaces that illustrates this situation is shown in Figure 9. A set of surface states straddles the same energy range for the two surfaces but with a smaller density and less capacity for additional electrons on the Se-rich surface.
Acknowledgment. We are grateful to Dr. Joel Miller of Du Pont for gifts of the TCNQ derivatives used in these studies and to Professor George Lisensky of Beloit College and Dr. Miller for helpful discussions. The Office of Naval Research is thanked for their generous support of this work. Registry No. TCNQ, 1518-16-7; TCNQ(i-Pr),, 21004-01-3; TCNQCI,, 21004-03-5; TCNQBr,, 56403-70-4; TCNQF4, 29261-33-4; q-CdSe, 1306-24-7.
Ejection of CH, from Adsorbed CH,N, into the Gas Phase S. Serghini Monim and P. H. McBreen* Departement de Chimie, Universite Laval, Ste-Foy, Quebec, Canada G l K 7P4 (Received: October 1 . 1991)
Gas-phase methylene was detected during temperature-programmeddesorption of diazirine from Pd( 1 10). Coincident detection of CHI, N2, and an m / e = 14 signal was observed for low initial surface coverages of diazirine. Correction for the contribution of nitrogen and methane to the mass = 14 signal indicates that the m / e = 14 peak is partially due to gas-phase methylene. The observation of gas-phase :CH2is consistent with the known thermolysis chemistry of diazirine and of its isomer, diazomethane. We propose that the chemisorption bond lowers the activation energy for CN bond cleavage and results in the formation of nitrogen and free methylene. The detected methylene does not make contact with the surface at any time during the ejection process. The observed methane is due either to a parallel surface process or to CH4 formed through the interaction of :CH, with the chamber walls. The upper limit for the amount of :CHI generated is 0.9 X 1014 radicals cm-*.
Introduction Thermal decomposition measurements performed using temperature-programmed desorption (TPD) contribute enormously to an understanding of the surface chemistry of adsorbed molecules. TPD studies show that adsorbed hydrocarbons are usually progressively dehydrogenated to yield surface species which may be identified by other techniques such as EELS. The overall process is the transformation of the adsorbed hydrocarbon into H2, which desorbs from the surface, and a carbon deposit which remains on the surface.’ In a few cases the adsorbed hydrocarbon fragments are observed to form new bonds and desorb as molecules such as methane2 or benzenea3 There are, however, reports in the literature of thermal desorption of CH2from Al(11 1),4the desorption of CH2A12following exposure of Al( 111) to CH212,5 and the desorption of aluminum hydride from hydrogen exposed Al( 11 1).6 Smentkowski et al.’ have reported the thermal desorption of the :CC12 diradical from an adsorbed layer of CCll on Fe(ll0). The observation of gas-phase methyl during the thermal decomposition of methoxy on oxygen-dosed Mo( 1 has been rationalized in a recent article by Shiller and Anderson? Zhou et al. have recently reported the desorption of methyl radicals during the adsorption of CH,Br on K-predosed Ag( 1 11).8b In this paper, we report the detection of gas-phase methylene during a thermal desorption experiment. The adsorbate under study is cyclic CH2N2,diazirine. As in the case of its linear isomer, diazomethane, diazirine is a source of free methylene. Thermolysis ( I ) Bent, B. E.; Mate, C. M.; Crowell, J. E.; Koel, B. E.; Somorjai, G.A. J . Phys. Chem. 1987, 91, 1493. (2) Yagasaki, E.; Masel, R. I. J . Am. Chem. SOC.1990, 112, 8746. (3) Yang, Q.Y.; Johnson, A. D.;Maynard, K. J.; Ceyer, S.T. J . Am. Chem. SOC.1989, 111. 8748. (4) Domen, K.; Chuang, T. J. J . Am. Chem. SOC.1987, 109, 5288. ( 5 ) Hara, M.; Domen, K.; Kato, M.; Onishi, T.; Nozoye, H. J . Chem. SOC., Chem. Commun. 1990, 1717. (6) Hara, M.; Domen,K.; Onishi, T.; Nozoye, H. J . Phys. Chem. 1991, 95, 6. (7) Smentkowski, V. S.;Cheng, C. C.; Yates, Jr., J. T. SurJ Sci. 1989, 215, L279. (8) (a) Serafin, J. G.; Friend, C. M. J . Am. Chem. SOC.1989,111, 8967. Coon, S. R.; White, J. M. J. Chem. Phys. 1991, 94, 1612. (b) Zhou, X.-L.; (9) Shiller, P.; Anderson, A. B. J . Phys. Chem. 1991, 95, 1396.
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TABLE I: Mass Spectra of Diazirine mle SMM PEI6 GI7 mle 12 13 14 15 26
10 21 100 3 3
11 23 100
11 23 100
2
2
27 28 40 41 42
SMM 6 17 2 10
PE16 GI7 7 8 2 11 48
6 18 2 11 43
and photolysis are the most effective means for decomposing CH2N2into :CH2and N2.10-11Apart from being a carbene source, diazirine is an interesting adsorbate because it may adopt a number of different bonding configurations12J3and because it shows potential for surface photochemistry studies.14 Furthermore, as detailed elsewhere, the high symmetry and the chemical inertness of diazirine makes it very suitable for surface spectroscopic studies of CH2N2under ultrahigh-vacuum (UHV) condition^.'^
Experimental Section The experimental system and the preparation of diazirine are described in more detail e1~ewhere.l~Diazirine should be handled with extreme care as it is both highly toxic and highly explosive. We confronted this problem by preparing a quantity below the PEL (permissible exposure limit) limit prior to each day’s experiment. The entire gas handling line was shielded by plexiglass and pumped on by ventilators. The actual synthesis of diazirine was performed under vacuum in a stainless steel line fitted with kovar-glass tubes and pumped on by a turbomolecular pump. A much more detailed description of the experimental procedure is given in two separate p ~ b l i c a t i o n s . ~The ~ J ~purity of the diazirine was verified using a mass spectrometer. The observed cracking pattern for diazirine is of importance in the discussion (IO) Liu, M. T. H.; Stevens, I. D.R. In Chemistry ojDiuzirines; Liu, M. T. H., Ed.; CRC Press: Boca Raton. FL, 1987; Vol. 1, Chapter 5. (11) Engel, P. S. Chem. Reu. 1980,80, 109. (12) Kisch, H. In ref IO, Vol. 11, Chapter IO. ( I 3) McBreen, P. H.; Serghini Monim, S.; Ayyoob, M. J . Am. Chem. Soc., in press. (14) McBreen, P. H.; Serghini Monim, S. Submitted for publication in Chem. Phys. Lett. (IS) Serghini Monim, S.;Venus, D.;Roy, D.;McBreen, P. H. J . Am. Chem. SOC.1987, 1 1 1 , 4108.
0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96,No. 6,1992 2705
Ejection of CH2 from Adsorbed CHzN2 Thermal desorption of diazirine
I
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A
I
c
P
v) v)
I
0
100
400
700
100
200
Temperature (K)
300 400 500 600 Temperature (K)
700
800
Figure 2. Multiplexed thermal desorption data are shown in the top panel. From the top toward the bottom, the desorption traces shown are for m / e = 2 (Xl),m / e = 28 (Xl),m / e = 16 (Xl),m / e = 14 (XI),and m / e = 42 (XIO). The numbers in parentheses are the magnification factors. The lower panel shows the m/e = 14 trace following subtraction of 9% of the m / e = 14 signal, 15% of the m / e = 16 signal, and the m / e = 42 signal multipled by 10. The resulting spectrum still displays a m / e = 14 peak magnified by 3. The data in this figure are for low surface coverage of diazirine on Pd(ll0). amu
Figure 1. The upper panel displays thermal desorption data for low-
coverage diazirine on Pd(l10). Heating rate = 5 K/s. The lower panel shows the principal peaks in the mass spectrum of diazirine as measured in the UHV chamber using a quadrupole mass spectrometer. The m / e = 14 and the m / e = 42 traces in the top panel are measured data and are characteristic of molecular diazirine. The middle desorption trace, which was obtained by subtracting 15% of the CH4signal and 9% of the N2 signal from the m / e = 14,signal displays an extra peak relative to the m / e = 42 trace. Data for CH4 and N2 desorption are shown in Figure 2. of the results presented in this paper. Hence, as displayed in Table I, we list the relative abundances of the ions observed under our experimental conditions (SMM) for comparison with the results obtained by Paulett and Ettinger (PE)16 and by Graham.17 The characteristic peaks for diazirine are the parent peak at 42 amu and the CH2 fragment at 14 amu. The relative abundance of the various ions is in excellent agreement with the previously reported values in the range m / e I28. Both Paulett and EttingeP and Graham” report a relative abundance of the m / e = 42 peak, which is greater by a factor of about 4.0 than that found in our experiments. We calibrated the mass spectrometer with Ar and N, and observed the appropriate ratio for the two Ar peaks and for the nitrogen-derived mass 28 and 14 peaks. The cracking pattern (Table I) in the region m / e = 26-28 rules out the possibility that C2H4or H C N is present in significant quantities. The X-ray photoelectron spectra obtained on exposing Pd( 110) at 107 K to diazirine indicate a carbon-to-nitrogen atomic ratio of 1:1.8, which is reasonably close to the value expected for molecularly adsorbed diazirine, considering the difficulties involved in background subtraction from the complex N(ls)spectra. Thus, we conclude that we are working with essentially pure diazirine.
Results and Discussion The results displayed in the top panel of Figure 1 for the desorption of low coverage diazirine from Pd( 110) show that there is an extra desorption feature in the mass = 14 spectrum as compared to the mass = 42 spectrum. By comparison to the lower panel of Figure 1 which displays the relative abundances, as measured in our UHV system, of the three major ions in the mass spectrum of diazirine, one may conclude that the low-temperature desorption peak corresponds to molecular diazirine. The desorption traces displayed in Figure 1, which are characteristic of all of our low-coverage TPD measurements, are presented in greater detail in Figure 2. The latter Figure includes desorption traces for the (16) Paulett, G . S.;Ettinger. R. J . Chem. Phys. 1963, 39, 825. (17) Graham, W . H. J . Am. Chem. SOC.1962,84, 1063.
-r 100
200
300
400
500
600
700
800
Temperature (K) Figure 3. Thermal desorption data for multilayer diazirine on Pd(1 IO).
Heating rate = 0.5 K/s. The topmost panel shows, from top to bottom, desorption traces for m / e = 2 (XlO),m / e = 28 (X2),m / e = 27 (XZ), m / e = 16 (X2),m / e = 14 (Xl), and m / e = 42 (X5). The numbers in parentheses are the magnification factors. The lower panel shows the m / e = 14 trace obtained following subtraction of the contribution of N,, CH2N2,C2H4,and CH4 to the mass = 14 signal. The corrected trace still displays an m / e = 14 peak. The latter peak is multiplied by 2. following masses: 42,28, 16, 14, and 2. Other TPD measurements for low-coverage diazirine on Pd( 1lo), which are not shown in this paper, reveal that HCN desorbs at approximately 350 K and that the mass = 28 peak is primarily due to N2 rather than C2H4 desorption. The m / e = 16 peak is attributed to methane. Note that both the CH4 and the N 2 peaks are structured, consisting of a coincident sharp feature, at the same temperature as the extra m / e = 14 peak, superimposed on the high-temperature side of a broader feature. The spectrum displayed in the lower panel of Figure 2 was obtained by subtracting out the contribution of the cracking patterns of diazirine, nitrogen, and methane to the m / e = 14 trace. A peak remains at 250 K. We attribute this peak to gas-phase methylene. The m / e = 14 trace following subtraction of the N2and CH, contribution, but not the CH2N2contribution, is shown in Figure 1. The situation is somewhat more complicated in the case of high coverages of diazirine, as in this case a strong ethylene desorption peak is also observed (Figure 3). However, as shown in the lower panel of Figure 3, a peak at m / e = 14 also remains above 200 K on subtracting out the contribution of N,, C2H4, CHI, and diazirine. Again, we attribute this peak to gas-phase methylene. In calculating the spectra shown in the lower panels of Figures 2 and 3, we used to cracking patterns for CHI, N,, C2H4, and diazirine as measured in our experimental system.
2706 The Journal of Physical Chemistry, Vol. 96, No. 6, 1992 However, it is conceivable, for example, that N2 exiting from adsorbed CH2N2 displays a different cracking pattern than ground-state nitrogen. We found that it was necessary to use a relative abundance of 2896,rather than 9% for the mass = 14 peak in the N, spectrum in order to remove the m / e = 14 peak shown in the lower panel of Figure 1. The same holds for all the other lowcoverage data. The ejection of methylene from adsorbed CH2N2is consistent with the known chemistry of this molecule. Therefore, it is unreasonable to invoke a vastly changed cracking pattern for N2 as the origin of the observed m/e = 14 peak (Figure 1, lower panel). There is another factor which argues against the possibility that the extra m / e = 14 peak is simply due to the fragmentation of CH,, N2, and, in the case of high surface coverage, CzH4. One would not expect the detection of :CH2 with high efficiency. A highly reactive species such as the methylene diradical would be unlikely to survive collisions with the chamber or the mass speckmeb enclosure walls. Thus, the detected CH2+ signal a lower limit. XPS measurementsZSof the total N(I,) intensity as a function of anneal temperature enable us to put an upper limit on the number of adsorbed CHzN2 species which decompose to yield gas-phase :CH2. The integrated N(ls) intensity decreases to 190 K after which it levels out. If we associate the region 170-190 K with the production of gas-phase :CH2, then the upper limit is 0.9 X lo1, molecules cmv2. TPD measurements as a function of coverage, in the low-coverage region, show that the relative intensity of the :CH2signal is greatest at the lowest surface coverd.ge. Note that the thermal decomposition of diaziiine on Pd( I 10) yields adsorbed CHI also (- 2 X lo1,CH2 ~ m - 9However, .~ as discussed below, this paper does not concern the chemistry of adsorbed methylene. The detection of a free radical, though not unprecedented, is a rare Occurrence in thermal desorption studies of molecules on metal surfaces. One would not expect desorption of a species such as CHzd from a metal surface. CH2,ads,18J9 in contrast to adsorbed C H and CH3 species, is not detected very often in surface spectroscopic studies of adsorbed hydrocarbons. This is partially because methylene is a very reactive species. Adsorbed CH2 may under suitable conditions ufidergo dehydrogenation to yield Cads and Had, hydrogenation to produce methane, polymerization to yield species containing CC bonds, or transformation into CH,& or CHSaB. Thus, it is highly unlikely that a CHI group bonded to a metal surface would desorb intact. The predicted site-dependent adsorption energies for methylene adsorbed on late transition metal surfaces range between 1.50 and 5.55 eY.20-23 Desorption of CH2, according to the Redheadz4 analysis for first-order desorption with E, = 1.5 eV and u = lo-" s, would display a rate maximum above 500 K, whereas the observed maximum is slightly above 200 K. Furthermore, as we have shown adsorbed CI12 reacts on Pd( 1 10) at temperatures above 160 K to yield methane and ethylene desorption peaks. The CH, and C2H4desorption peaks are present as a shoulder at 170 K in Figure 3. In addition, XPS C(,s)data, which show a shift in the C binding energy from 283.6 to 284.1 eV on heating the surface !k;m 130 to 190 K, indicate that some of the adsorbed methylene reacts to yield a carbonaceous deposit.25 Species such as C2H, acetylide, display a C(ls)binding energy in the range 284.0-284.5eV.26 The chemistry which we observe for methylene adsorbed on Pd(ll0) is in excellent agreement with that observed by Berlowitz and Kung2' for methylene on Pt( 1 1 1) in that we also observe methane and ethylene desorption close to 200 K and the (18) Hills, M. M.; Parmenter, J . E.; Mullins, C. B.; Weinberg, W. H. J . Am. Chem. Soc. 1986,108, 3554. (19) McBreen, P. H.; Erley, W.; Ibach, H. Surf Sci. 1984, 148, 292. (20) de Koster, A.; van Santm, R. A. J . Catal. 1991, 127, 141. (21) Zheng. C.; Apelog, Y.; Hoffmann, R. J . Am. Chem. SOC.1988,110, 749. (22) Baetzold, R. C. J . Phys. Chem. 1984.88, 5583. (23) Yang,H.; Whittcn, J. L. J . Chem. Phys. 1989, 91, 126. (24) Redbad, P. A. Vacuum 1962, 12, 203. (25) Scrghini Monim, S.;McBreen, P. H. Surf. Sci., in press. (26) Levis, R. J.; DeLouise, L. A.; White, E.J.; Winograd, N . Surf. Sci. 1990,230, 35. (27) Berlowitz, P.; Yang, B. L.; Butt, J. B.; Kung, H. H. Surf Sci. 1985, 159, 540.
Monim and McBreen deposition of some residue of general formula C a t on the surface. In summary, we observe a desorption rate maximum for CH, slightly above 200 K, a temperature high enough to cause the reaction of adsorbed CH2 but not high enough to cause the desorption of CHz. Therefore, the detected gas-phase methylene which is the subject of this paper does not originate from methylene bound to the surface. This apparent contradiction may be resolved by assuming that the CH, group is ejected into the gas phase directly from an adsorbed molecule. The process is then analogous to the thermal decomposition of gas-phase CH2N2with the major exception that the surface system consists of an array of aligned molecules which are activated by the chemisorption bond. Diazirines display activation energies and preexponential factors for thermal decomposition of approximately 31 kcal mol-' and s, respectively.28 The activation energy for the thermal decomposition of diazomethane is in the range 32-35 kcal mol-'?9 and the activation energy for the isomerization of diazirines to diazoalkanes is approximately 27-30 kcal/mol-1.28 The detailed mechanism for the gas- or liquid-phase thermolysis of diazirines is not completely understood. Liu and Stevenslo conclude that all the data on the decomposition of diazirines may best be understood in terms of a rate-determining step which forms an [CHI: - - -N2] encounter complex and subsequent product determining steps. Bigot et al.m have provided theoretical justification for the existence of a stable exciplex [CH2:- - -N2]complex in the photochemical decomposition of diazirine. Several authors have pointed out that it is most probable that the CN bonds are cleaved c"tive1y in the decomposition of diazirines.'OJIJz Preliminary HREELS results from our laboratory provide support for ring opening of diazirine on Pd( 1 lo)." A loss at 1920 cm-' develops on heating the diazirine exposed surface from 90 to 170 K. Coordinated diam groups are characterized by a strong band at approximately 1950 cm-' in their IR spectra.33 The EELS loss at 1920 cm-' disappears on heating to 300 K. More EELS data in the temperature range 150-250 K are needed to definitively link the detection of gas-phase :CH2 with the decomposition of an adsorbed diazomethane intermediate. Nevertheless, we make the assumption that diazirine decomposes on Pd( 1 lo), as it does in the gas phase, to yield free methylene via an open-ring intermediate. Note that this assumption need not be made explicitly since, as outlined above, the thermolysis of diazirine probably occurs via ring apening in any case. The homogeneous-phase deoomposition of diazomethane is a first-order unimolecular process, and thus the activation energy for dissociation provides an upper limit for the band dissociation, D(W2-N), energy. The activation energy is approximately 32 k c a l / m ~ l ? ~and hence the bond dissociation energy is 132 k a l / m ~ l . The ~ ~ calculated value is 29 kcal/m01.~~The desorption rate maximum at approximately 210 K implies an activation energy of about 13 kcal/molu for the decomposition of adsorbed diazomethane. The adsorbtd species diffdrs from the free molecule in that detachment of N2 could lead to a transient adsorbed N2 species, the actml formation of which would lower the energy of the transition state for decomposition. With reference to the adsorption energy of approximately 8 kcal mol-' for N2 on Pd(1 lo)?' one a u l d estimate the latter contribution at 18 kcal mol-'. The remaining activation energy could easily be accounted for in terms of activation of adsorbed diazomethane via the chemisorption bond. The highest occupied orbital, the b2 orbital, in (28) Meier, H . In ref 10, Vol. 11. Chapter 6. (29) Setser, D. W.; Rabinovitch, R. S. Can. J . Chem. lW2, 40, 1425. (30) Bigot, B.; Ponec, R.; %in, A.; Devaquet, A. J. Am. Chem. Soc. 1978, 100,6575. (31) Baird, N . C. In ref 10, Vol. I, Chapter 1. (32) Moffat, J. B. In Chemistry of Diazonium and Diazo Compounds; Patai, S.,Ed.; John WGiley'& Sons: New York, 1978. (33) Menu, M. J.; Desrosiers, P.; Dartiguenave, M.; Dartiguenave, Y.; Bertrand, G. Organometallics 1987, 6, 1822. (34) Loggenberg, P.M.; Carlton, L.; Copperthwaite,R. G.; Hutchings, G. J. Surf Sci. 1987, 184, L339. (35) Laufer, A. H.; Okabe, H. J . Am. Chem. SOC.1971, 93, 4137. (36) Bmulet, J.; Lievin, J. Theor. Chim. Acta 1982, 61, 59. (37) Kuwahara, Y.;Jo, M.; Tsuda, H.; Onchi, M.; Nishijima, M. Surf. Sci. 1987, 180, 421.
J. Phys. Chem. 1992,96, 2707-2713 diazomethane is C N bonding and N N antibonding.38 Linearly adsorbed diazomethane would likely involve removal of electron density from the HOMO orbital and therefore lead to a reduction in the C N bond order. Considering the fact that the bond energy of a C N (single) bond is 78 kcal mol-', it is evident that even a weak reduction in C N bond order would significantly lower the activation energy for dissociation and thereby permit the observed release of free methylene. In effect, chemisorption would serve to weaken the C N bond and strengthen the N N bond and thereby push the system along a reaction coordinate leading to free methylene and molecular nitrogen. Note that the interaction of diazomethane with Cu, Ni, and Fe atoms in cryogenic (12 K) matrices leads in each case to the spontaneous splitting of the molecule and the generation of metal atom-methylene species.3w1 In our experiments there is no solid matrix present on the vacuum side to capture the free methylene, and a fraction is detected by the mass spectrometer. Furthermore, if the amplitudes of the adsorbate torsional modes are sufficiently large, some of the CH2 groups may impinge on the surface and desorb as methane or ethylene or decompose to yield surface carbon species. It is also possible that some of the detected methane and ethylene arises from the interaction of methylene with the chamber walls and (38) Hoffmann, R. Tetrahedron 1966, 22, 539. (39) Chang, S.-C.; Kafafi, Z. H.; Huage, R. H.; Billups, W. E.; Margrave, J. L.J . Am. Chem. SOC.1987, 109, 4508. (40) Chang, S.-C.; Huage, R. H.; Kafafi, Z. H.; Margrave, J. L.; Billups, W. E. Inorg. Chem. 1990, 29, 4373. (41) Chang, S.-C.; Huage, R. H.; Kafafi, Z. H.; Margrave, J. L.; Billups, W. E. J. Am. Chem. SOC.1988, 110, 7975.
2707
the stainless steel enclosure of the mass spectrometer. We have shown elsewhere that diazirine on Pd(ll0) displays other unanticipated forms of beha~ior.'~Notably, the third adsorption state of diazirine to be populated at 107 K is the first to decompose on heating the sample. The latter behavior, and the formation of gas-phase methylene detailed in this paper, may be in large part due to the intramolecular dynamics of the molecule. Diazirine is a strained species which contains a nascent nitrogen molecule. The release of the strain energy and the formation of the very stable nitrogen molecule may be the dominant driving forces in the chemistry of adsorbed diazirine. In the case of most simple adsorbates, the reactivity of the metal surface plays a crucial role in the decomposition of the adsorbed species. However, in the case of a molecule such as diazirine the surface may simply serve to activate the adsorbate which is then driven by intramolecular factors to yield products such as the free methylene observed in this study. Acknowledgment. This research was made possible by a grant funded by the Network of Centres of Excellence Programme in association with the Natural Sciences and Engineering Research Council of Canada, by an NSERC Operating Grant, an FCAR &pipe grant, and a University Research Grant from Imperial Oil Ltd. Registry No. CH2, 2465-56-7; CH2N,, 157-22-2; Pd, 7440-05-3. Supplementary Material Available: Figure showing surface concentration of carbon and nitrogen atoms on Pd(ll0) as a function of anneal temperature (1 page). Ordering information is given on any current masthead page.
Effect of Microporosity and Oxygen Surface Groups of Activated Carbon in the Adsorption of Molecules of Different Polarity F. Rodriguez-Reinoso,* M. Molina-Sabio, and M. A. Muiiecas Departamento de Quimica Inorgrinica e Ingenieria Qdmica, Universidad de Alicante, Alicante, Spain (Received: October 17, 1991)
This work describes the adsorption of molecules with different polarity (N2,SO2,H20, and CH,OH) on microporous activated carbons with different amounts of oxygen surface groups. The results presented show that both the microporosity and the chemical nature of the carbon surface affect the adsorption process: for nonpolar molecules (Le., N2) the adsorption is mainly influend by the porous structure,but the nature and amount of oxygen surface groups are extremely important in the adsorption of polar molecules, the more important the higher the polarity of the molecule. On the other hand, there is a noticeable change in the adsorption mechanism of polar molecules at low relative pressures, the change being more drastic for carbons with wide micropores and low content of oxygen surface groups.
Introduction The adsorbentadsorbate interaction in the physical adsorption of gases by a solid is a function of the polarity of the solid and the adsorptive.' Activated carbon, with a mainly nonpolar surface, is very useful for the adsorption of molecules of low polarity such as hydrocarbons but is not very adequate for the adsorption of polar molecules.2 Thus, a previous study on the adsorption of SO2 on activated carbons with a low amount of oxygen surface groups3has shown that the amount of SO2adsorbed at low relative pressures is lower than the amount of N, adsorbed, the former being lower the wider is the micropore size distribution of the carbon. It is then easy to understand the need for a modification of the chemical nature of the carbon if one seeks to increase the adsorption capacity for polar molecules. Thus, Davini4 has recently *To whom all correspondence should be addressed.
0022-3654/92/2096-2107$03.00/0
shown that the basic surface groups increase the adsorption of SO2 at room temperature and their effect on the strength with which the molecules are adsorbed by the carbon. Beebe and Dell5 have also described that the adsorption of SO2 at 273 K by a carbon black increased with its content in oxygen. Matsumura et aL6 have shown that the suppression of hydrophilic structures (oxygen surface groups and inorganic impurities) of activated carbon decreases the adsorption capacity toward methanol and water but not toward benzene. In this sense, it has been postulated ~
(1) Gregg, S. J.; Sing, K. S. W.Adsorption, Surface Area and Porosity; Academic Press: London, 1982. (2) Kaneko, K.; Inouye, K. Carbon 1986,8, 772. ( 3 ) Mufiecas-Vidal, M. A.; Rodriguez-Reinoso, F.; Molina-Sabio, M.. to be published. (4) Davini, P. Carbon 1990, 28, 565. ( 5 ) Beebe, R. A.; Dell, R. M. J . Phys. Chem. 1955.59, 746. ( 6 ) Matsumura, Y.; Yamabe, K.; Takahashi, H. Carbon 1985, 23, 263.
0 1992 American Chemical Society
