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
2708 The Journal of Physical Chemistry, Vol. 96, No. 6, 1992
Rodriguez-Reinoso et al.
TABLE I: Micropore Volume, V , and Oxygen Surface Groups Evolved on TPD of Reduced and Oxidized Carbons
BO B4N B15N MO M8A M24A M4N M7N M15N MP1 MP2 MP3 MPlT M4NT M7NT Ml5NT A0 A4N A15N RO R8A R24A R4N R7N R15N
4N 15 N
HNOj HNO,
l h l h
air air 4N 7N 15 N 5M 5M 5M
573 K 573 K HNO, HNOj HNOj H202 H202 H202
8h 24 h l h l h l h l h 2h 3h
4N 15 N
HNO, HNOj
l h l h
air air 4N 7N 15 N
573 K 573 K HNO, HNOj HNO,
8h 24 h l h l h l h
that the adsorption of water by the carbon surface takes place through the formation of hydrogen bonds between the water molecule and the oxygen surface groups.‘ However, the relative influence of both microporosity and type and amount of oxygen surface groups of activated carbon in the adsorption of polar molecules has not been yet established. This is the main objective of this work. Experimental Section The oxidation of porous carbon with reagents such as air, nitric acid, hydrogen peroxide, etc., is rather simple and, if carried out under adequate conditions, may introduce noticeable changes in the chemical nature of the carbon surface without appreciably changing its porosity.**9 In this way it is possible to prepare series of carbons with essentially the same microporosity but different degree of oxidation and to carry out a systematic study of the adsorption of molecules with different polarity. To achieve this objective, four activated carbons were prepared by carbonization (N2, 1123 K) of peach stones followed by activation in CO, at 1098 K to different burnoffs: BO (ll%), MO (24%), and A0 (53%); the fourth carbon, RO, was prepared by carbonization (N2, 1123 K) of a very similar precursor,I0 plum stones, followed by activation in a N2-water vapor mixture ( p H p = 12.4 kPa) at 1093 K to 48% bumoff. The four carbons were heat treated in hydrogen (1223 K, 90 min) to reduce the number of oxygen surface groups to a minimum. The reduced carbons were oxidized in air (573 K) or with aqueous solutions of H N 0 3 or H202,the details being described in ref 8 and summarized in Table I. Some of the oxidized carbons were heat treated in N, at 923 K to selectively reduce the number of oxygen surface groups (the letter T is then added to the nomenclature). The porosity and chemical nature of the carbans were determined by adsorption of N, (77 K) and COz (273 K), selective titration and temperature-programmed desorption (TPD). The micropore volume (V,) and the amounts of CO and C02evolved (7) Dubinin, M. M. Carbon 1980, 18, 355. (8) Molina-Sabio, M.; Mutiecas-Vidal, M. A.; Rodriguez-Reinoso, F. Characterization of Porous Solids II; Rodriguez-Reinoso, F., et al., Ed.; Elsevier: Amsterdam, 1991; p 329. (9) Prado-Burguete, C.; Linares-Solano, A,; Rod6guez-Reinoso. F.; Salinas-Martinez de Lecea, c. J . Cutal. 1989, 115, 98. (IO) Rodriguez-Reinoso, F.; Linares Solano, A. In Chemistry and Physics ofcarbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1988; Vol. 21,
P
1.
0.23 0.03 0.08 0.35 0.35 0.37 0.36 0.35 0.35 0.36 0.38 0.38 0.38 0.41 0.42 0.46 0.55 0.54 0.51 0.53 0.52 0.54 0.47 0.46 0.49
0.24 0.25 0.23 0.34 0.34 0.34 0.33 0.32 0.3 1 0.34 0.33 0.3 1 0.33 0.34 0.34 0.33 0.43 0.41 0.40 0.36 0.37 0.37 0.32 0.33 0.34
0.09 1.12 1.63 0.14 0.37 0.44 1.38 1.93 2.90 0.64 1.02 1.45 0.20 0.30 0.41 0.45 0.14 1.59 2.81
0.13 1.82 2.55 0.25 1.02 1.48 3.04 3.45 4.02 1.29 1.65 2.39 0.87 1.88 2.10 2.56 0.28 3.14 4.42 0.20 0.85 1.20 2.15 2.71 3.35
0.08 0.24 0.29 0.93 1.28 1.69
0.6
0.6
$0.4
0. 4
> 0.2
0.2
0
05 PI e
10
0
05
10
PI p,
Figure 1. Adsorption isotherms of (a) N 2 a t 77 K and (b) SO2 a t 273 K for reduced carbons: ( 0 )BO, (0) MO, (A)AO, (m) RO.
in TPD are included in Table I. A full description of samples characterization is given in ref 8. The adsorption isotherms of SO2 (262,273, and 299 K), H 2 0 (298 K), and CH,OH (298 K) were determined in a conventional gravimetric system; the high-purity adsorptives were further purified using the freeze-thaw technique. All samples were outgassed for 12 h at 373 K under high vacuum. To calculate the micropore volume, the following adsorbate densities were taken: 1.465 g/cm3 for SO2 a t 262 K 1.442 g/cm3 for SO2 at 273 K; 1.392 g/cm3 for SO2 a 299 K; 0.998 g/cm3 for H 2 0 at 298 K; 0.790 g/cm3 for C H 3 0 H at 298 K.
Results and Discussion 1. Adsorption of N,. The N2 (77 K) adsorption isotherms for the reduced original carbons shown in Figure 1 are of type I, as corresponds to essentially microporous carbons, but increasing activation in CO, from carbon BO to A0 produces a gradual development of microporosity of increasing width. Carbon RO, prepared by steam activation, exhibits. a wider microporosity and a more important mesoporosity contribution than carbon AO, with similar burnoff in C02. The application of the Dubinin-Radushkevich (DR) equation to all N2 adsorption isotherms leads to the micropore volumes, V, listed in Table I, together with those deduced from the adsorption of C 0 2 at 273 K. As shown in previous worh,loJ1the micropore volumes deduced from adsorption of Nz (77 K) and CO, (237 K) can provide a good information on the microporosity of porous carbons since C 0 2 (273 K) gives the volume of only narrow micropores (up to ( 1 1 ) Rodriguez-Reinoso. F. Pure Appl. Chem. 1989, 61, 1859.
The Journal of Physical Chemistry, Vol. 96, No. 6, 1992 2709
Adsorption of Molecules of Different Polarity 1
1
1
14
/ A/A /O
0.4 vocso,
/
O.*i
- 0
1
I
0
0.2
0.4 VO(NZ
0.6
1
Figure 3. Micropore volumes (cm3/g) deduced from adsorption of SO2 a t 273 K and N 2 at 77 K: (0)B, (0) MA, (A)MP, (0)M N , (v)T, (A)A.
\
( 1 2) Garrido,-J.; Linares-Solano, A,; Martin-Martinez, J. M.; MolinaSabio, M.; Rodriguez-Reinoso, F.; Torregrosa, R. Lungmuir 1987, 3, 76.
carbon. In general terms both parameters follow a sequence similar to that of the adsorption of N 2 at 77 K, although the amount of SO2 adsorbed, specially at low relative pressures (below PIPo = O.l), is lower. It is just this the portion of the adsorption isotherm being modified by the oxidation of the carbon, as shown below. Figure 2 includes the adsorption isotherms (as DR plots) of SO, at 273 K for the nonoxidized (XO) and the more oxidized (X15N) carbon of each series. The DR plots for carbons BO, MO, and A0 are not linear in the whole range of relative pressure, showing deviations of type A in the classification of Marsh and Rand,I3 in agreement with other reports.I4 The change in slope is more marked and displaced to larger relative pressures the wider is the microporosity of the carbon and in fact, for carbon RO-the one with wider microporositysuch deviation cannot be observed. Similar results have been found when studying the adsorption of SO, in a series of carbons prepared by activation of carbonized apricot stones in CO, and covering the 10-96% burnoff range.3 When the carbon is oxidized,the deviations become less marked and for carbons with a large number of oxygen surface groups (e&, X15N in Figure 2) are nonexistent. However, the amount of SO,absorbed at higher relative pressures and the subsequent extrapolation (see Figure 2) to calculate Voare slightly lower than in the nonoxidized carbons; the lower extrapolation may be attributed to the percentage of oxygen atoms occupying the porosity of the carbon.* For carbon M15NT and the rest of the series of carbons heat-treated in N 2 after oxidation the change produced in the DR plot in all the range of relative pressures is large and is then due to both the decrease in the number of oxygen groups and the subsequent increase in microporosity produced by elimination of surface groups, mainly as CO,. Plots similar to those of Figure 2 are found for the adsorption of SO, at 262 and 299 K on the same carbons, with type A deviations (except for series RO) which become the less marked the more uniform is the microporosity and the larger the degree of oxidation. The extrapolation of the linear portion of the DR plots at low log2 (Po/P)values gives the micropore volume, Vo, of the carbons. Figure 3 includes a plot of the values deduced from the adsorption of SO,a t 273 K as a function of the values deduced from the adsorption of N 2 at 77 K for series B, M, and A. The slope of the plot, 0.95, indicates the similarity of the values deduced from the two adsorptives. Carbons of series B deviate from linearity as a consequence of the restricted access of the N, molecule to the very narrow microporosity at the low temperature of adsorption (77 K), as discussed above. The similarity of the Vovalues deduced from N 2 and SO, and the increase of the slopelow log2 (Po/P)values-of the DR plots of Figure 2 with the widening of microporosity from carbon BO to MO, AO, and RO indicate that the model of micropore volume (13) Marsh, H.; Rand, B. J . Colloid Inferfuce Sci. 1970, 33, 101. (14) Kaneko, K.; Nakahigashi, Y . ; Nagaka, K . Carbon 1988, 26, 327.
Rodriguez-Reinoso et al.
2710 The Journal of Physical Chemistry, Vol. 96, No. 6, 1992
TABLE 11: Relative hessure of Change in Slope of D-R Plots and Correspondiug Amouob Adsorbed (cm3/g) SO, at 273 K
SO, at 262 K ~
carbon MO M8A M24A M4N M7N M15N
- A
CHIOH at 298 K
SO, at 299 K
(PIP0)C
VC
(PIP0)C
VC
(PIP0)C
VC
(PIP,),
VC
0.024
0.25
0.25
0.044
0.21
0.25 0.26
0.045 0.043
0.26
0.027
0.027 0.034 0.037
0.046
0.21
0.019
0.25 0.23
0.027
0.038
0.25
NA
NA
0.26
0.052
NA 0.21
0.028 0.030
MPl MP2 MP3
0.035 0.043 0.030 0.037 0.039
0.26 0.26 0.26
0.25
0.26 0.27 0.24
0.043 0.063
0.24 0.26
0.039 NA 0.049 0.047 NA
0.26 0.26 0.24
0.25 NA
00 0
2 4 6 qo+ nCo,h"elg 1
8
Figure 4. Relationship between the slope of D-R plots (SO2at 273 K) at low relative pressure and the amount of CO and C02evolved in TPD for carbons of the series (0)MA, (A)MP, (0)MN, (m) RA, ( 0 )RN. filling given by Dubinin is also valid for the adsorption of S0215716 However, since the deviations at low relative pressures (large log2 (Po/P))cannot be attributed to activated diffusion (they are more marked in carbons with wide microporosity and the amount of SO2adsorbed-expressed in mmol/g-increases with decreasing temperature), one has to admit that the adsorption mechanism at low relative pressures is different from the conventional micropore filling, the oxygen surface groups playing an important role. The role played by both the chemical nature of the surface and the microporosity in the deviation of the DR plots may be quantified by calculating the slope, D,of the portion at low relative pressures of the plots in Figure 2. D values have been plotted in Figure 4 as a function of the number of oxygen groups of the carbons deduced from TPD experiments. In all cases D decreatm with increasing degree of carbon oxidation (Le,, the amount adsorbed at low relative pressures increases and the slope approaches that of the portion at high relative pressures corresponding to the filling of the micropores) and all points for a given series (Mor R in Figure 4) fit the same straight line. On the other hand, D decreases more drastically with the amount of oxygen surface groups for carbons of series R, with more heterogeneous micropore size distributions. Similar results are obtained when analyzing D values for the adsorption of SO2at 262 and 299 K. The basic differences found between the three temperatures lay in the relative pressure (PIPo), at which the change in slope of the DR plot (and subsequent change in adsorption mechanism to micropore fhing) is produced. The analysis of the isosteric heats of adsorption of SO2at the three temperatures show that at low relative pressures the heat of adsorption slightly increases with the degree of oxidation and that the specific interactions between SO2 molecules and oxygen complexes are significant up to a point near (P/Po)c The increase in (PIP,),with adsorption temperature (see Table 11) and the constancy of the amount adsorbed V, at such relative pressures, suggests that at (PIP,,),the filling of the micropores begins with condensation of the adsorbate as liquid (in fact, there is a linear relationship between log (PIP,),and 1/T, as expected in a gas (15) Marsh, H. Carbon 1987, 25, 49. (16) Tomkov, K.; Siemieniewska, T.; Baldyga, A.; Guerin, H.; Grillet, Y.; Francois, M. J . Chim. Phys. 1974, 7, 1062.
o 02
0 4 06 08 100 PIP,
02 a4 06 08
io
PIP.
Figure 5. Adsorption isotherms of H 2 0 at 298 K on some activated
carbons. condensation process); this process will continue up to the complete filling of the micropores, as deduced from Figures 2 and 3 (the experimental observation of a longer time needed to reach equilibrium in the vicinity of (P/Po),-l h when 15 min is enough for the rest of the experimental points-may be a confirmation of the condensation taking place at that relative pressure). The negative deviation at low relative pressures found in the DR plots of Figure 2 can be explained in terms of the polarity of the SO2molecule16since the interaction adsorptiveadsorptive is larger than in nonpolar molecules such as N2and, consequently, at low relative pressures the energy of adsorption for SO2 will be lower than for N2.5 Two extreme situations can be described: (i) In carbons with narrow and uniform microporosity of dimensions similar to those of the SO2molecule, the enhancement of the adsorption potential in the micropores is high and the deviation in the DR plot is not large, ending at very low relative pressure (as in the case of carbon BO in Figure 2). The isotherm will be type I and similar to that of N2 at 77 K. (ii) In carbons with wide microporosity, the interaction adsorptive-adsorptive for SO2 in the gas phase at low relative pressures is large in respect to the adsorbentadsorptive interaction, and there will be an important deviation in the DR plot ending at higher relative pressures (as in the case of carbon A0 in Figure 2). The isotherm for SO2is not as well-defined type I as that of N2 at 77 K. For a given porosity then, any mofication of the polarity of the carbon surface will increase the amount adsorbed at low relative pressures and, as shown in Figure 4, the increase will be a function of the amount of such oxygen surface groups. Consequently, in the first stages (low relative pressures) of the adsorption of SO2 on an activated carbon, there is both unlocalized adsorption due to dispersion forces and looalized adsorption due to interactions of SO2molecules with the oxygen surface groups of the carbon. Once the first few molecules of SO2 are adsorbed, the next molecules will be easily adsorbed by a cooperative mechanism. As the relative pressure increases, the number of SO2molecules is enough to produce the filling of the microporosity in a state similar to liquid (as in the case of N2). 3. Admpthu of H20. The adsorption of water has been camed out at 298 K for all carbons of series M and R. Some of the isotherms are plotted in Figure 5 where it is seen that the shape
The Journal of Physical Chemistry, Vol. 96, No. 6, 1992 2711
Adsorption of Molecules of Different Polarity
nco2h“elg )
TABLE 111: Some Adsorption Characteristics of Carbons H,O at 298 K N 2 at 77 K
M8A M24A MPl MP2 MP3 M4N M7N Ml5N MPlT M15NT RO R8A R24A R4N R7N R15N
0.57 0.54 0.52 0.50 0.48 0.43 0.39 0.33 0.57 0.57 0.70 0.62 0.60
0.51 0.48 0.45
0.23 0.24 0.23 0.23 0.26 0.23 0.23 0.24 0.24 0.29 0.24 0.23 0.23 0.24 0.24 0.26
0.29 0.31 0.30 0.30 0.36 0.30 0.30 0.32 0.31
0.38 0.38 0.41 0.41 0.45 0.47 0.47
0.37 0.38 0.38 0.40 0.41 0.38 0.36 0.37 0.40 0.48 0.64 0.63 0.66 0.56 0.54 0.58
changes from type V (reduced carbons, MO and RO) to an hybrid between type I and type V for oxidized carbons. Type V isotherms are characteristic of water adsorption on activated carbons’,’ and show the low interaction of the adsorbate with the carbon surface at low relative pressures. The effect of porosity in the adsorption process can be analyzed by comparing the isotherms for carbons MO and RO since both have a very small amount of oxygen groups and different porosity. In both cases, the amount of water adsorbed up to PIPo = 0.5 is very low, increasing considerably thereafter for a small change in relative pressure. The more noticeable differences are found in the relative pressure range above 0.6-0.7: for carbon RO-with larger micropore volume and wider microporosity-the total amount of water adsorbed is larger and the “plateau” is worse defined. The introduction of oxygen surface groups modifies the adsorption isotherm (Figure 5) in the following way: (i) The amount of water adsorbed at low relative pressures increases with the degree of oxidation, becoming appreciable even at PIP, near 0.1 for the more oxidized carbons (M15N and R15N). (ii) The increase in amount adsorbed at medium relative pressures is not as steep but the “plateaun starts at the same value of amount adsorbed ( 5in Table 111) for a given series of carbons. (iii) The “plateau” at high relative pressures is almost parallel to that of the nonoxidized carbon. All these changes indicate that the region of low relative pressures, below the “plateau” of the water isotherm, is controlled by the chemical nature of the carbon surface. A good example showing the combined effect of the chemical nature and the microporosity is carbon M15NT. The heat treatment of carbon M l 5 N in Nz at 923 K produces a decrease in oxygen surface groups and an increase in microporosity (Table I), inducing the corresponding change in adsorption isotherm shown in Figure 5. The importance of the carbon surface groups in the region of the steep portion of the isotherm a t low and medium relative pressures has been recognized by several a~thors.~’J*The results shown here indicate that for carbons with the same porositysamples withing series M or R-there is a good relationship between the relative pressure at which the “plateau” startsin Table 111-and the total number of oxygen surface groups of the carbon (expressed as mmoles of CO + C 0 2evolved during TPD experiments). On the other hand, the slope of the portion of the isotherm at medium relative pressure (up to (P/Po)i) is similar for the nonoxidized carbons and those oxidized in air (with predominantly oxygen groups being evolved as CO, e.g., M24A and R24A in Figure 5), but the “plateau” starts at lower (17) Evans, M. .I.B. Carbon 1987, 25, 81. (18) Youssef, A. M.; Ghazi, T.M.; ECNabarawy, T. H. Carbon 1982, 20, 113.
0
0
1
2
3
2
3
4
5
nco( mmde l g 1 Figure 6. Relationship between primary adsorption centers deduced from H 2 0 adsorption and the CO evolved in TPD for carbons of the series ( 0 ) MA, (a) RA, (V)MT, or C 0 2evolved for carbons of the series (0)MN, ( 0 )RN, (A) MP.
value of for the later. For carbons oxidized with nitric acid-Ml5N and R15N in Figure 5-with a larger amount of C 0 2groups, that slope is not as steep and (PIP,), is much lower. This seems to indicate that the total amount of surface groups controls the value of (P/P& whereas the type of oxygen surface groups conditions the slope of the isotherm up to ( P / P o ) l .The evolution in isotherm shape from carbon M15N to M15NT confirms this, since the latter has a similar slope to the carbon oxidized in air (M24A in Figure 5) and in both cases most of the surface groups evolve as CO. Dubinin and Serpinski, assuming that the adsorption of water up to a point around what is has been defined here as (PIP,), takes place by direct interaction of the adsorptive with the surface groups or, in general terms, with the active centers, gave a method to quantify the so-called primary adsorption centers, ao.19 The calculated values of a, for all carbons have been plotted in Figure 6 as a function of the amount of CO groups for the carbons oxidized in air (series MA and RA, with a small content in C 0 2 groups) and the number of C 0 2groups for carbons oxidized with H N 0 3 and H202 (series MN, RN, MP, and RP). The good relationship indicates that, independently of the porosity of the carbon, the groups evolving in TPD as C 0 2 (mainly carboxilic and lactonic structuresz0) interact more strongly with water molecules than groups evolving in TPD as CO (mainly carbonyls and ethers). In the first stages of the adsorption process (at low relative pressure), the higher is the number of oxygen surface groups and the higher the polarity (e.g., carboxylic) the lower is the relative pressure at which the water molecules are retained in the surface groups; further molecules will then be adsorbed by a cooperative mechanism, favored by the high heat of condensation of the adsorbate, forming islands of adsorbed water. These islands merge and cover a fraction of the carbon surface, similar for the nonoxidized carbon and the resulting oxidized samples of Table 111). Dubinin suggested that this value could be used to calculate the geometrical surface area of the carbon.’ The similarity of both the and VO(CO2) values for series R and M seems to indicate that the narrow microporosity conditions somewhat the value of V,. Further increases in relative pressure above (PIP,),lead to the filling of the whole microporosity of the carbon and reveal the differences in micropore size distribution of carbons in series M and R (low slope for series M and high slope for series R). However, since the “plateau” for the carbons of a given series are parallel the amount of water adsorbed at PIP, = 0.95 (near saturation) is larger the larger is the degree of oxidation of the carbon, specially when the carbon microporosity is wide (series R) but, in any case, lower than the micropore volume of the
(c
vi
(19) Dubinin, M. M.; Serpinski, V. V. Carbon 1981, 19, 402. (20) Tremblay, G.; Vastola, F. J.; Walker, P. L., Jr. Curbon 1978, 16, 35.
2712 The Journal of Physical Chemistry, Vol. 96, No. 6 . 1992
Rodriguez-Reinoso et al.
-0.6
.
h
>
0
- -1.0
0
0
E
0
V
v
>
Y H 3 O H -1.4
0 0
0.5 P I p,
1.0
Figure 7. Adsorption isotherms of N2 (77 K), SOz (262 K), CH30H (298 K), and H 2 0 (298 K) for carbon MO.
’
0,05w 0
0
M15N
0 02
0.04
PI Po Figure 8. Adsorption isotherms of methanol at 298 K for some carbons.
carbon-measured
by N2 adsorption (see Tables I and 111).
4. Adsorption of CH30H. The adsorption of methanol was
considered interesting because this molecule has polar moment and heat of condensation intermediate between SO2 and H 2 0 , with the possibility of forming hydrogen bonds. The adsorption isotherms of methanol determined for some carbons of series M are of type I although the amount adsorbed is lower than for N 2 at 77 K, specially at low relative pressure. This is clearly shown in Figure 7 which includes the adsorption isotherms for all adsorptives on carbon MO which has a very small amount of surface groups. As in the case of SO2,the amount of methanol adsorbed at low relative pressures increases with the degree of oxidation, as shown in Figure 8 where the low relative pressure portion of the isotherms for carbons MO, M8A, MP1, and M 15N are included. This increase is a consequence of a larger carbon-methanol interaction through the oxygen surface groups. On the other hand, the amount adsorbed of methanol is similar for all oxidized carbons at relative pressures larger than PIPo = 0.045. Consequently, the DR plots would be very similar to thme of Figure 2, and it is possible to calculate the micropore volume which in all cases is 92%of the value deduced from the adsorption of N2 at 77 K. 5. Comparioon of Adsorptives. The basic differences found for the adsorption of N2,SO2,CH30H, and H 2 0 (see Figure 7 for a nonoxidized carbon MO) lies in the low relative pressure region of the isotherms, the amount adsorbed decreasing in the order N2 > SOz > CH30H > H20. These differences may be associated to a larger adsorptiveadsorptive interaction due to the permanent polar moment (N2 = 0, SO2= 1.6 D, CH30H = 1.7 D, H 2 0 = 1.8 D) and, additionally, to the presence of intermolecular hydrogen bonds. This joint effect makes the difference between SO2 and methanol larger than the expected from only the polar moment. In the case of water, besides the larger association by hydrogen bonds, one has to consider the weak nonspecific interactions with the carbon surface due to its low polarizability. On the other hand, the amount adsorbed near saturation (Figure 7) follows a similar trend, although the differences are smaller
6
4
log*p,IP Figure 9. D-R plots for carbon MO (open symbols) and M15N (closed symbols). TABLE I V Influence of Polarity of the Adsorptive and Extent of Oxidation on the W R Plots extent of oxidation
polarity of the adsorptive
’ l/z
2
I
h
I
N,: /AD 0 SOZ: PD = 1.6 D CH,OH po = 1.7 D hydrogen bonds = 1.8 D
(
1.00
1.00
0.62 0.65 0.36 0.59
1.00
0.83
0.06 0.25
0.42
1.00
J . Phys. Chem. 1992, 96,27 13-27 17
DR plots are given for three carbons of series M with increasing degree of oxidation. These results may be summarized as follows: (i) The polarity of the adsorptive prevents the filling of the micropores at low relative pressures as noted by the decrease in slope ratios from N2 to HzO. (ii) The polar moment is not the only factor to explain the deviations (relative to N2)found for SO2,CH30H, and H20 (they have very similar values); neither is the hydrogen bond by itself because is similar for water and methanol. The most important factor seem to be a low nonspecific adsorbatecarbon interaction and a high adsorptive-adsorptive interaction.
2713
(iii) The differences between C H 3 0 H and SO2 suggest that hydrogen bond causes a larger deviation than the simple polar moment interaction. (iv) Increasing degree of oxidation increases the carbon-adsorbate interaction, the adsorption mechanism approaching then that of N2, micropore filling with the adsorbate as liquid.
Acknowledgment. This work was supported by CICYT (Project No. PB 86/279). Registry No. N2, 7727-37-9; SO2, 7446-09-5; H20, 7732-18-5; CH3OH, 67-56-1; C, 7440-44-0.
Addition of Ferrocene Derivatives to the Surface of Quantum-Confined Cadmium Sulflde Clusters: Steady-State and lime-Resolved Photophysical Effects Robin R. Chandler, Jeffery L. Coffer,* Department of Chemistry, Texas Christian University, Fort Worth, Texas 76129
Stephen J. Atherton, and Paul T. Snowden Center For Fast Kinetics Research, University of Texas at Austin, Austin, Texas 78712 (Received: October 24, 1991; In Final Form: December 3, 1991)
The effect of substituted ferrocene complexes on the surface of quantum-confined (Q) cadmium sulfide clusters in inverse micelles was examined by steady-state and time-resolved photoluminescence (PL) spectroscopy. Addition of (dimethylamin0)methylferrocene (DMAMF) to Q-CdS enhanced cluster PL peak areas by 120%, in contrast to the carboxylic acid derivatives ferrocenecarboxylic acid (FCA) and ferrocenedicarboxylic acid (FDCA), which quenched PL by 60% and 80%, respectively. Unsubstituted ferrocene, hydroxymethylferrocene,and ferrocenecarboxaldehyde had no effect on the PL intensity of these clusters. The induced PL changes fit a Langmuir-type adsorption isotherm from which formation constants for the Q-CdSsurface adducts were calculated. The average log Kfvalue calculated for DMAMF adsorption was 4.12. FCA addition gave an average log Kf of 5.17, and the average log Kf value for FDCA addition was 6.14. Binding of the amino group of DMAMF to shal!ow trap states arising from Cd2+sites is postulated as its mechanism for PL enhancement, while the quenching mechanism for FCA and FDCA is attributed to proton transfer induced from ionization of the acids in the micelle water pools. “Competition” experiments show that quenching by the carboxylic acid derivatives can be reversed by the addition of amino-substituted ferrocene, and vice versa. The results also suggest a possible DMAMF-induced surface reconstruction. These steady-state observations are supported by measurements of the PL decay via subnanosecond timecorrelated single photon counting. Fits of the decay monitored at 620 nm to a modified stretched exponential model indicate that the average distributed decay time ( T 2 ) of nanosecond duration can be either lengthened or shortened, depending on the nature of the added ferrocene.
Introduction Efforts designed to understand the physics and chemistry of quantum-confined semiconductor clusters continue with extensive scrutiny.’ These materials, often possessing a hybrid of molecular/buk properties, are of interest not only from the fundamental perspective of quantum-size effects but also with respect to photocatalysis1bq2and nonlinear optic^.^ Of great significance is the spatial confinement of charge camers of these semiconductor particles in three dimensions, giving rise to their description as “quantum dots” or “zero-dimensional exciton^".^ A reoccurring theme discussed in recent reviews of these Q-state materials is the potential for “tailoring” of excited-state photophysics via surface modification.If For the case of bulk, singlecrystal II-VI surfaces, Ellis and co-workers have demonstrated that changes in semiconductor photoluminescence (PL) are quite sensitive to the chemical nature of an ad~orbate.~ However, such studies directed at deliberate surface modification of quantumconfined clusters are fewer in number.6 Layers of Cd(OH)2 on CdS6band ZnS on CdSek do remove the defect sites responsible for lower energy trap emission, while the binding of aminesMand lanthanide p-diketonate complexes6e to the surface of Q-CdS * To whom correspondence should be addressed.
enhances its overall PL. Other accounts note the ability of aliphatic thiols’ and ammonia8 to reduce the number of surface (1) (a) Brus, L. E. J . Phys. Chem. 1986,90,2555. (b) Henglein, A. Top. Curr. Chem. 1988, 143, 133. (c) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990,23, 183. (d) Stucky, G. D.; MacDougall, J. E. Science 1990,247, 669. (e) Wang, Y.; Herron, N. J . Phys. Chem. 1991, 95, 525. (f) Steigerwald, M. L.; Brus, L. E. Annu. Rev.Mater. Sci. 1989, 19, 471. (2) Bard, A. J. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 1187. (3) (a) Nozik, A. J . Phys. Chem. 1986, 90, 12. (b) Hilinski, F.; Lucas, P.; Wang, Y. J . Chem. Phys. 1988, 89, 3435. (c) Wang, Y.;Herron, N.; Wahler, W.; Suna, A. J . Opt. SOC.Am. A 1989,6, 808. (d) Jain, R.; Lind, R. J. Opt. SOC.Am. 1983, 75, 647. (4) Brus, L. E. IEEE J. Quantum Electron. 1986, QE22, 1909. (5) (a) Meyer, G. J.; Lisensky, G. C.; Ellis, A. B. J . Am. Chem. Soc. 1988, 110, 4914. (b) Lisensky, G. C.; Penn, R. L.; Murphy, C. J.; Ellis, A. B. Science 1990, 248, 840. (c) Meyer, G. J.; Leung, L. K.; Yu,J. C.; Lisensky, G. C.; Ellis, A. B. J. Am. Chem. SOC.1989, 111, 5146. (d) Murphy, C. J.; Ellis, A. B. J. Phys. Chem. 1990, 94, 3082. ( e ) Murphy, C. J.; Ellis, A. B. Polyhedron 1990,9, 1913. (f) Murphy, C. J.; Lisensky, G. C.; Leung, L. K.; Kowach, G. R.; Ellis, A. B. J . Am. Chem. SOC.1990, 112, 8344. (6) (a) Ramsden, J. J.; Gratzel, M. J. J . Chem. SOC.,Faraday Trans. 1 1984, 80, 925. (b) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J . Am. Chem. SOC.1987, 109, 5654. (c) Kortan, A. R.; Hull, R.; Opila, R. L.; Bawendi, M. G.; Steigerwald, M. L.; Carroll, P. J.; Brus, L. E. J . Am. Chem. SOC. 1990, 112, 1327. (d) Dannhauser, T.; ONeil, M.; Johansson, K.; Whitten, D.; McLendon, G. J. Phys. Chem. 1986, 90,6074. ( e ) Chandler, R.; Coffer, J. L. J . Phys. Chem. 1 9 9 1 , 95, 4.
0022-365419212096-27 13$03.00/0 0 1992 American Chemical Society
