J . Phys. Chem. 1989, 93, 2058-2065
2058
2 928
1
2.9275
4
2 925
n
4
?
~
~
2 9245
--I
0
20
40
60
00
100
parcent d-4 mdhanol
Figure 8. Peak position of methylene antisymmetric stretch vs percent methanol-d, in solvent.
8. They also indicate a trend of gradually changing environment of the alkane chains. Previously, experiments on conductivity and light scattering of SDS solutions at lower concentrations indicated that aggregates were not present in solvents with methanol content above 30%.16 This study demonstrates that surfactant molecules can form aggregates in methanol-rich solutions.
Conclusions The data indicate that the alkane chains of SDS have approximately the same proportions of bent end-methyl, double(16) (a) Ward, A. F. H. Proc. R. SOC.London, A 1940, 176, 412. (b) Parfitt, G. D.; Wood, J. A. Kolloid Z . 2.Polym. 1968, 229, 5 5 .
gauche, and kink states as tridecane. Although these results support the models of disordered micellar systems, it is not possible to predict long-range order by using these experimental methods. Since this FTIR study provides an average estimate of conformational states, information about the gradient of conformational order along the chain (such as that provided by N M R order parameters) cannot be confirmed. Increasing methanol content of the solvent affects the environment of the hydrocarbon chains. The chains are gradually being exposed to a more polar environment, but there is no "critical point" beyond which the aggregates are no longer present. If the aggregation of surfactant molecules was completely disrupted, the chains would see a fairly constant polar environment, and further addition of methanol to the solvent should not significantly affect their environment or their conformational state. Technical advances in FTIR spectroscopy in recent years, along with better data-handling capabilities, make this an appropriate tool for the acquisition of important experimental data in this field. The study of conformations of alkane systems could help to elucidate the internal structure of a micelle with respect to chain organization and order. It has also been demonstrated that the technique could be used to study solvent interactions with micellar systems. Acknowledgment. The authors thank Perkin-Elmer for donating the PE Model 1800 FTIR for this research. Thanks are due to Dr. David Burns for assistance with the computer programming and data analysis. F.H. thanks the Center for Process Analytical Chemistry at the University of Washington for the graduate research assistantship. Registry No. SDS, I5 1-21-3;methanol, 67-56-1;heptadecane, 62978-7; tridecane, 629-50-5.
Effects of Preparation Variables on Particle Size and Morphology for Carbon- and Alumina-Supported Metallic Iron Samples Scott A. Stevenson,? Scott A. Goddard, Masuhiko Arai,t and J. A. Dumesic* Department of Chemical Engineering, University of Wisconsin, Madison. Wisconsin 53706 (Received: June 28, 1988)
Triiron dodecacarbonyl, Fe3(C0)12,was used to prepare highly dispersed metallic iron on carbon black, graphite, and dehydroxylated alumina. Magnetic susceptibility measurements were used to estimate particle size distributions;these experiments indicate that after reduction at 673 K much of the iron is present in particles containing only a few iron atoms. The high dispersions obtained for the cluster-derivedFe/A1203and Fe/carbon black catalysts were resistant to sintering during treatment in hydrogen at up to 773 K. Carbon monoxide chemisorption measurements are in agreement with the dispersions estimated from the magnetic susceptibility data for the alumina- and graphite-supported catalysts. Mossbauer spectroscopy suggests that the cluster-derived, carbon black supported iron particles may be more spherical and possibly more widely separated than conventionally prepared iron particles on the same support.
Introduction promising for organometallic cluster oneof the compounds in the field of heterogeneous catalysis is for the preparation of highly dispersed zerovalent metal particles. of catalyst genesis, which involve the deConventional position of metal salts, require reduction of the metal cations to the zerovalent state. Highly dispersed cations of elements such as iron, however, are difficult to reduce completely. In contrast,
* Author
to whom correspondence should be addressed. Present address: Institut fiir Physikalische Chemie der Universitat
Miinchen, Sophienstrasse 11, 8000 Miinchen 2, West Germany. 'Present address: Chemical Institute of Non-Aqueous Solutions, Tohoku University, Sendai 980, Japan. 0022-3654/89/2093-2058$01.50/0
the metallic atoms in many organometallic compounds are already in the zerovalent state, so that reduction is not required. Early work d e " m a t e d the value of this concept by the production of highly dispersed zerovalent an element that is difficult to reduce once supported in an oxidized state. Because of the difficulty of reducing isolated ferrous cations,3d organo(1) Brenner, A,; Burwell, R. L., Jr. J . Cafal. 1978, 52, 353. (2) Brenner, A.; Burwell, R. L., Jr. J . Cafal. 1978, 52, 364. (3) Garten, R. L.; Ollis, D. F. J . Cafal. 1974, 35, 232. (4) b u d a r t , M.; Delbouille, A,; Dumesic, J . A.; Khammouma, S.;Topsw, H.J . Coral. 1975, 37, 486. (5) Raupp, G. B.; Delgass, W. N. J . Cafal. 1979, 58, 337. (6) Yuen, S.; Chen, Y . ;Kubsh, J. E.; Dumesic, J. A,; Topsae, N.; Topsae, H . J . Phys. Chem. 1982, 86, 3022.
0 1989 American Chemical Society
Carbon- and Alumina-Supported Iron Samples metallic compounds are a natural route for the preparation of metallic iron catalysts. Indeed, a number of studies have prepared supported iron by using carbonyl c o m p o ~ n d s . ~ - 'These ~ studies have indicated that the nature of the initial interaction between the support and the precursor compound varies with the type and pretreatment of the support, and that changes in this interaction can have significant consequences for the resulting catalytic properties. The goal of this work was to prepare and characterize highly dispersed zerovalent iron particles. Because of the importance of the initial interaction between the support and the cluster compound, we chose to compare catalysts prepared by using three supports that might interact differently with iron carbonyl clusters: graphite, carbon black, and alumina. Graphite, which has a low surface area and little surface functionality, would be expected to adsorb the clusters only weakly. Carbon black, however, which has a high surface area as well as a significant concentration of defects and reactive surface species, might be thought to adsorb iron clusters more strongly, altering the nature of the resulting catalyst. In fact, carbon black has been used to prepare highly dispersed iron particles, often with unusual catalytic properties, by conventional deposition of ferric salts.1b23 Some work has also been performed with iron carbonyl compounds supported on carbon black.2e26 The behavior of iron particles on carbon black is different from that of alumina-supported iron; this may be related to differences originating during preparation and can best be examined by comparing similar preparations using the two different supports. In this study, interaction of cluster and support, metal particle size and particle size distribution, particle morphology, and adsorptive and reactive properties have been examined for all samples. The influence of preparation and support on particle size and morphology will be considered in this paper; subsequent papers will discuss the impact of these properties on the adsorptive and reactive properties of
(7) Brenner, A. J . Chem. Soc., Chem. Commun. 1979, 251. (8) Brenner, A.; Hucul, D. A. Inorg. Chem. 1979, 18, 2836. (9) Hugues, F.; Smith, A. K.; Ben Taarit, Y.; Basset, J. M. J . Chem. Soc., Chem. Commun. 1980, 68. (10) Hugues, F.; Bussiere, P.; Basset, J. M.; Commereuc, D.; Chauvin, Y.; Bonneviot, L.; Olivier, D. Stud. Surf.Sci. Catal. 1980, 7, 418. (1 1) Hugues, F.; Dalmon, J. A.; Bussiere, P.; Smith, A. K.; Basset, J. M. J . Phys. Chem. 1982,86, 5136. (12) Guglielminotti, E.; Zecchina, A. J . Mol. Catal. 1984, 24, 331. (13) Iwasawa, Y.; Yamada, M.; Ogasawara, S.; Sato, Y.; Kuroda, H. Chem; Lett. 1983, 621. (14) Santos, J.; Dumesic, J. A. Stud. Surf. Sci. Caral. 1982, 11, 43. (15) Santos, J.; Phillips, J.; Dumesic, J. A. J . Caral. 1983, 81, 147. (16) Vannice, M. A,; Walker, P. L., Jr.; Jung, H. J.; Moreno-Castilla, C.; Mahajan, 0. P. Stud. Surf. Sci. Catal. 1981, 7, 460. (17) Jung, H. J.; Walker, P. L., Jr.; Vannice, M. A. J . Catal. 1982, 75, 416. (18) Jung, H. J.; Vannice, M. A.; Mulay, L. N.; Stanfield, R. M.; Delgass, W. N . J . Catal. 1982, 76, 208. (19) Niemantsverdriet, J. W.; van der Kraan, A. M.; Delgass, W. N . ; Vannice, M. A. J . Phys. Chem. 1985, 89, 67. (20) Guerrero-Ruiz, A,; Moreno-Castilld, C.; Rodriguez-Ramos, I. React. Kinet. Caral. Lett. 1985, 27, 283. (21) Rddguez-Reinoso, F.; Lbpez-Gonzdlez, J. D.; Moreno-Castilld, C.; Guerrero-Ruiz, A.; Rodriguez-Ramos, I. Fuel 1984, 63, 1089. (22) Christensen, P. H.; Mplrup, S.; Niemantsverdriet, J. W. J . Phys. Chem. 1985,89, 4898. (23) Christensen, P. H.; Marup, S.; Niemantsverdriet, J. W. Hyperfine Interact. 1986, 28, 91 1 . (24) Rodriguez-Reinoso, F.; Rodriguez-Ramos, I.; Guerrero-Ruiz, A,; Lbpez-Gbnzalez, J. D. Appl. Catal. 1986, 21, 251. (25) Guerrerc-Ruiz, A.; Roddguez-Ramos, I.; Romero-Sgnchez, V. React. Kinet. Catal. Lett. 1985, 28, 419. (26) Chen, A. A.; Vannice, M. A.; Phillips, J . J . Catal., in press. (27) Stevenson, S . A,; Dumesic, J. A,, submitted for publication in J . Mol. Catal. (28) Stevenson, S. A.; Arai, M.; Gddard, S. A.; Dumesic, J. A,, submitted for publication in J . Coral.
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2059
Experimental Methods Sample Preparation and Handling. Because of the high susceptibility of zerovalent iron compounds and particles to oxidation, all cluster-derived catalysts were prepared and characterized without exposure to air. These catalysts were prepared inside a Vacuum Atmospheres glovebox connected to a purification and recirculation system; the concentration of oxygen and water was maintained at less than 1 ppm. Catalysts prepared with conventional methods were prepared and stored in air; following pretreatment, however, all characterization was performed without further exposure to oxygen or moisture. Four catalysts were prepared and examined in this study. Triiron dodecacarbonyl, Fe3(C0),2, was dispersed on a highsurface-area carbon black, a high-purity, low-surface-area graphite, and dehydroxylated y-alumina. For comparison, a carbon black supported, conventionally prepared catalyst was also studied. The preparation of each of these catalysts is described below. FelCarbon Black, Wet Impregnation. This catalyst was prepared by wet impregnation of Cabot CSX-203 carbon black with triiron dodecacarbonyl. Prior to use, the carbon black was treated in hydrogen for 12 h at 1223 K to remove sulfur;29 chemical analysis showed the residual sulfur content to be approximately 0.18 wt %. Treatment in hydrogen at 1273 K for 7 days did not further reduce the sulfur content. The carbon black was then soaked in 1 M nitric acid to remove inorganic impurities, washed in distilled water, dried overnight at 383 K, and stored in air until use. The surface area as measured by Brunauer-Emmett-Teller (BET) analysis was approximately 1400 m2/g. A saturated solution of Fe3(C0)12in hexane was combined with the desulfurized carbon in a stoppered flask in the glovebox. The flask was left, with occasional agitation, until the color of the solution had changed from deep green, characteristic of the triiron cluster, to clear, indicating that all of the cluster material had adsorbed on the carbon. This process required between 10 and 14 days. Once adsorption was complete, the hexane was decanted, and the catalyst was dried at room temperature and stored in the glovebox until use. After complete drying and decomposition of the cluster, this catalyst contained 0.95% iron by weight (Galbraith Laboratories). FelAlumina, Wet Impregnation. y-Alumina (American Cyanamid, 99.99%) was dehydroxylated for 2 h a t 1223 K in flowing argon,' after which infrared spectroscopy showed no detectable absorption in the OH stretching region. Following this treatment, the alumina was passed into the glovebox without contact with air. The alumina surface area as measured by BET analysis was 172 m2/g. A saturated hexane solution of Fe3(CO)12 was combined with the alumina in a stoppered flask, and the mixture was allowed to stand for 2 h; at the end of this time the solution was clear and the support was light green. The remaining solution was decanted, and the catalyst was dried in the glovebox at room temperature. Following drying, its color had changed to a dull brown. Chemical analysis indicated that this catalyst contained 0.36% iron by weight. FelGraphite, Incipient Wetness Impregnation. This catalyst was prepared by using UCP-1-200 ultra "F" purity graphite (Ultra Carbon Corp.). This graphite contained no impurities in concentrations greater than 0.3 ppm, and its BET surface area was approximately 3 m2/g. The graphite was not pretreated before use. Because of the low surface area of this support, the catalyst was prepared by incipient wetness impregnation of Fe3(C0)12from hexane solution. Cluster-containing solution was added to the graphite until it was wet; the sample was then dried for approximately 30 min, after which time more solution was added. This process was repeated until the desired weight loading was reached. This catalyst contained 0.36 wt % iron. Fe/ Carbon Black, Incipient Wetness Impregnation Using Fe(NU,),. This catalyst was prepared by conventional incipient wetness impregnation of carbon black, pretreated as described above, with an aqueous solution of ferric nitrate. The resulting (29) Kaminsky, M.; Yoon, K. J.; Geoffroy, F. L.; Vannice, M. A. J . Caral. 1985, 91, 338.
2060 The Journal of Physical Chemistry, Vol. 93. No. 5. 1989 catalyst was dried in air for 48 h at 383 K and stored in air until further use. This catalyst contained 4.67 wt % iron. Prior to characterization, all of the cluster-derived catalysts were subjected to the following slow reduction program: 1 h a t 373 K, 1 h a t 473 K, 1 h a t 573 K, 2 h at 673 K, and, if required, 2 h at 113 K, all in purified hydrogen flowing at a rate of a p proximately 300 mL/min. More severe conditions were required to reduce the ferric cations present in the conventionally prepared catalyst; this catalyst was treated in flowing hydrogen for 1 h at 383 K, 1 h at 473 K, 1 h a t 573 K, 16 h at 673 K, and, if required, 2 h at 723 K. Magnetic Surceptibi/ity Measurements. Magnetic susceptibility measurements were made by the Faraday using an electromagnet (Alpha 4800) and a vacuum microbalance (Cahn) connected to a diffusion-pumped vacuum system capable of a dynamic vacuum of ca. 100 pPa. The microbalance sensitivity was approximately 1 pg, and 30 mg of sample was typically used. The applied field was varied between zero and 6 kG, fields were calibrated with a Thomas and Skinner (Model 7305) gaussmeter. The vertical magnetic field gradient was determined from measurements on the compound Hg[Co(SCN),]. Samples were transferred from the glovebox to the magnetic susceptibility apparatus by using a nitrogen-purgedglovehag. (To minimize the possible effects of any traces of oxygen that may be present in the glovehag, the samples were transferred to the magnetic susceptibility apparatus prior to reduction.) Data were collected at more than one temperature to confirm that the samples behaved superparamagnetically. Because of the small size of the iron particles, the samples could not be saturated even at 77 K; however, Mossbauer spectroscopy indicates that the iron is completely reduced by hydrogen treatment. Metal particle size distributions were estimated from a least-squares analysis of the data using the sum of a series of between six and nine Langevin equations.'2 Hydrogen (Airco, 99.995%) was purified by passage through a Deoxo unit (Engelhard) followed by a 13X molecular sieve trap m l e d to 77 K. Purge nitrogen (Badger Welding Supplies, 99.6%) was purified by passage over copper turnings maintained at a p proximately 550 K followed by a 13X molecular sieve trap m l e d to 77 K. Mossbauer Spectroscopy Measurements. Mossbauer spectra were collected by using an Austin Sciences spectrometer and electronics. The source consisted of 50 mCi of nCo diffused into a palladium matrix. Doppler velocities were calibrated with a 6-pm metallic iron foil and sodium nitroprusside for the velocity ranges of *I0 and f4 mm/s, respectively; isomer shifts are reported relative to metallic iron at mom temperature. The resulting spectra were computer-fit with a modified version of the program
Stevenson et al. 0.4
7
0.3
0.2
A
0.1
0.0 Clustersize 0.5-2.0 2.0-4.0 4 . 0 - 7 0 7.0-10.0 0.5
10.01
1
0.4
0.3 0.2
B
0.1 0.0
40-70
'70-10.0
10.01
C:uilcrim05~2.0 2 . 0 ~ 4 0 4.0.7.0
7.0-10.0
10.01
ClvsterrireO5-20
20-40
MFIT?3
Samples were pretreated in a quartz reactor. Hydrogen (Airw, 99.995%) was purified by passage through a palladium thimble
(Serfass). Following treatment, the closed reactor was passed into the glovebox, where the sample could be transferred to an air-tight Mossbauer spectroscopy cell. Spectra were wllected at liquid nitrogen temperature by using an Air Pmducts m l i n g system and cold finger. During collection, the region surrounding the sample cell was maintained under a vacuum of approximately IO Pa. A temperature of 78 K was routinely obtained. Carbon Monoxide Chemisorption. Chemisorption studies were performed volumetrically in a diffusion-pumped glass system with an ultimate pressure of ca. IO @Pa. Pressure measurements were made with a Texas Instruments precision pressure gauge. Because of the large amount of physical adsorption on the carbon black at 195 K, chemisorption measurements were performed at 298 (30) Mulay, L. N.Phyrienl Methods in Chemistry, Port IY; Wcisskger, A., Rwiter, 8.W.. Eds.; Wiley: New York, 1972; p 431. (31) Sclwccd. P. W. Chemisomtion and Mamctization: Academic Press: (32) Richardson, J. T. Appl. Phys. 1978, 49, 1781. (33) Ssrenscn. K. Internal Repart No. 1. Laboratory of Applied Physics 11, Technical University of Denmark, 1972.
Clurrerrirr0.5-2.0
2.0.4.0 4 0 ~ 7 . 0 '7.0-10.0 10.0+
Diameter (nm) Figwe 1. Particle size (nm) distributions for supported iron samples: (A) Felgraphite reduced at 673 K, (B) Fe/carbon black (iron carbonyl impregnation) reduced at 673 and 773 K, (C) Fc/alumina reduced at 673 and 773 K, and (D) Fe/carbon black (ferric nitrate impregnation) reduced at 673 and 723 K.
K. Tests with the alumina-supported catalyst demonstrated that the irreversible uptakes were the same at both 195 and 298 K. Metallic iron dispersions were calculated by using an adsorption stoichiometry of one CO molecule per two surface iron atoms.' The hydrogen used in these experiments was purified by flowing through a palladium thimble. Carbon monoxide (Matheson, 99.9%) was purified by passage over molecular sieve beads maintained at 573 K followed by a molecular sieve trap woled to 195 K. Results Magnetic Susceptibility. Particle size distributions estimated from the magnetic susceptibility data have been condensed to a
The Journal of Physical Chemistry, Vol. 93, No. 5, I989
Carbon- and Alumina-Supported Iron Samples
f
2061
#
Y+ .io
.10.0
s.0
0.0
10.0
Velocity ("/d Figure 2. Mossbauer spectra collected at 295 K of Fe/carbon black catalyst prepared by aqueous incipient wetness of Fe(N03)3: (A) as prepared, (B) following reduction at 673 K. TqBLE I: Comparison of Metallic Iron Dispersions Estimated from Magnetic Susceptibility and Chemisorption Data
sample Fe/graphite Fe/AI2O3 Fe/carbon black, cluster-derived Fe/carbon black, ferric nitrate
Ta, K 673 473 673 773 673 773 673 723
magnetic data dispersion 0.38 0.68 0.64 0.68 0.62 0.41 0.23
chemisorption data dispersion 0.34 0.38" 0.44" 0.52 b b b
" Cluster
not completely decomposed; see text. lated to be > 1.
-5.0
0.0
-2.5
2.5
5.0
Velocity ( m d s ) Figure 3. Mossbauer spectra collected at 79 K of Fe/graphite catalyst: (A) as prepared, (B) following reduction at 673 K.
Dispersion calcu-
consistent set of intervals and are presented in Figure 1, where the mass fraction is shown as a function of particle diameter. The uncertainty in the computer-fit mass fraction is about 15%. The portion of the distribution labeled "cluster size" corresponds to particles of 0.4 nm or less in diameter. The fit obtained for the alumina-supported catalyst reduced at 673 K is adjusted for the amount of undecomposed cluster present, as discussed below. CO Chemisorption. Metallic iron dispersions estimated from CO chemisorption are presented in Table I, where they are compared to dispersions estimated from the particle size distributions determined by magnetic susceptibility. The dispersion, D, of a particle of diameter d (in nanometers) was estimated by the relation D = 0.85/d (ref 4). For both carbon black supported catalysts, the dispersions calculated from chemisorption data were greater than 1; this behavior is due to the formation of Fe(CO)S,
as discussed in greater detail elsewhere.*' Mossbauer Spectroscopy. Room temperature Mossbauer spectra collected for the conventionally prepared carbon black supported catalyst are shown in Figure 2, and the spectral parameters determined by the fitting procedure are tabulated in Table 11. Figure 2A is a room-temperature spectrum of the sample as prepared. The sharp doublet with an isomer shift of 0.35 mm/s and quadrupole splitting of 0.70 mm/s is indicative of ferric cations. After treatment of the sample in hydrogen at 673 K, spectrum 2B was obtained. Two major features are present: a broad, nearly symmetric singlet near zero velocity and two outer pairs of peaks of similar width and dip at approximately *3 and f5.2 mm/s. The latter peaks are the outermost four peaks of magnetically split zerovalent a-Fe; the two inner peaks of this sextuplet are hidden within the singlet. Computer fitting gives the magnetic hyperfine field of this sextuplet as 321.9 kOe; the actual hyperfine field is approximately 3 15 kOe, after subtraction of 7 kOe for the presence of the demagnetizing field.19,34.3sThe
TABLE 11: Miissbauer Spectroscopy Parameters of Carbon-Supported Iron Catalysts
sample
treatment
figure
Fe/carbon black (ferric nitrate impregnation)
as prepared 673 K, H2
2A 2B
Fe/graphite (iron carbonyl impregnation)
Fe/carbon black (iron carbonyl impregnation) a
(s) denotes superparamagnetic species.
T, K 295 295
as prepared
3A
79
673 K, H2
3B
79
773 K, H2
4B
79
species Fe3+ Fea Fe0(s)" Fe" a-Fe3(CO),, b-Fe,(CO)12 Feo Feo(s) Feo(s) Fe2+
8, mm/s 0.350 0.016 0.121 0.382 0.110 0.053 -0.003 -0.043 0.144 1.807
A, mm/s 0.704 0.019
H, kOe
re1 spectral area, 7%
321.87
100 31 58 11
1.078 1.100 0.134 -0.105
1.578
339.81
58 42 93 7 78 22
2062
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
Stevenson et al. TABLE III: Recoil-Free Fractions (f) of Supported Iron Particles sample spectrum T, K f Fe/carbon black, ferric nitrate 2A 295 0.12 Fe/graphite, cluster-derived Fe/carbon black, cluster-derived
B
[o.oos
I
4.0
-2.5
I
0.0
2.5
5.0
Velocity (mm/s) Figure 4. Mossbauer spectra collected at 79 K of Fe/carbon black catalyst prepared by wet impregnation of Fe,(CO),>: (A) as prepared, (B) following reduction at 773 K.
large singlet is due to particles that are superparamagnetic and hence do not show magnetic splitting. As indicated in Table 11, the asymmetry of this peak is probably caused by the presence of Fe3+ cations, representing at most 10% of the spectral area. Spectra collected for the graphite-supported catalyst are shown in Figure 3, and the corresponding spectral parameters are presented in Table 11. These spectra were collected at 79 K to increase the iron recoil-free fraction. Spectrum 3A is of the as-prepared catalyst and consists of three clearly resolved peaks of approximately equal dip. This spectrum can be fit with two doublets, one of an isomer shift of 0.1 1 mm/s and a quadrupole splitting of 1.10 mm/s, and a second with an isomer shift of 0.05 mm/s and a very small quadrupole splitting, estimated to be 0.13 mm/s. These values agree closely with the parameters of bulk Fe3(CO),* of 0.1 1 and 1.13 mm/s for the outer doublet and 0.05 and 0.13 mm/s for the inner doublet.36 Following treatment in hydrogen at 673 K, the triiron dodecacarbonyl is converted to metallic iron, as shown by Figure 3B. The four peaks shown are the innermost four peaks of the ferromagnetic a-Fe spectrum. The total spectral area of the two peaks not visible was estimated by using the peak area ratios of 3:2:1: 1:2:3 expected for randomly oriented iron crystallite^.^^^^' The magnetic hyperfine field of this sextuplet corresponds to 339.8 kOe. This value is larger than the bulk value of 337 kOe at 77 K;38the difference can again be attributed to the demagnetizing field. A small feature near zero velocity is also present, corresponding to approximately 7% of the spectral area. This is due (34) von Eynatten, G.; BBmmel, H. E. Appl. Phys. 1977, 14,415. (35) Knudsen, J. E.; Morup, S. J . Phys., Colloq. 1980, 41, C1-155. (36) Greenwood, N. N.; Gibb, T. C. M6ssbauer Spectroscopy; Chapman and Hall: London, 1971. (37) Dumesic, J. A.; Topsoe, H. Ado. C a r d 1977, 26, 121. (38) Preston, R. S.; Hanna, S. S.; Heberle, J. Phys. Reu. 1962, 128, 2207.
2B 3A 3B 4A 4B
295 79 79 79 79
0.08 0.28
0.60 0.29 0.35
to small, superparamagnetic iron particles. Spectra collected at 77 K for the cluster-derived carbon black supported catalyst are presented in Figure 4, and the spectral parameters are summarized in Table 11. The spectrum collected of the untreated catalyst, spectrum 4A, consists of a very broad peak that cannot be meaningfully fit (the line drawn through the data points in the figure was generated by computer fitting with a series of unconstrained singlets). Some of the iron must be oxidized, since the center of mass of the spectrum is at 0.46 mm/s. In particular, the shoulders at approximately +1 and +2 mm/s are probably due to ferric and ferrous cations, respectively. The shoulder seen near -1.2 mm/s is probably due to Fe(CO)5 formed by the decomposition of the initial cluster. Reduction of this sample at 773 K results in Figure 4B. Even at 79 K, the temperature at which this spectrum was Collected, all of the iron remains in the superparamagnetic state. The shoulder at 2.5 mm/s is part of an Fe2+ doublet comprising 22% of the spectral area. At least some of this Fez+ is an experimental artifact due to sample oxidation during the course of the collection of the spectrum, since the intensity of this peak increased with time. In any case, this spectrum indicates that most of the iron is in the zerovalent state. Spectra of the alumina-supported catalyst are not presented because the higher mass density of the alumina, as compared to that of the carbon, increased the beam attenuation and hence decreased the signal-to-noise ratio. Estimates of the recoil-free fractions of the samples are presented in Table 111. These estimates were calculated by assuming that the iron foil standard had a recoil-free fraction of 0.79.39 The ratio of the spectral area of the sample and the iron foil multiplied by the ratio of their cross sectional iron densities and multiplied by 0.79 gave the sample recoil-free fraction estimate. The increase of the fraction of nonresonant y-rays caused by the presence of the sample and the sample cell was accounted for by collecting the spectrum of a sodium nitroprusside standard behind each sample.
Discussion Interaction of Cluster with Support Materials. The adsorption of cluster carbonyl compounds on fully dehydroxylated alumina is generally believed to be via interaction with Lewis acid sites on the support, i.e., by bonding between exposed alumina cations and oxygen atoms in the carbonyl ligands, as observed for W(CO)6,40R u ~ ( C O ) , and ~ , ~ [$-(C5H5)Fe(C0)]4,42 ~ or by partial decarbonylation and interaction with surface oxygen atoms, as suggested for M o ( C O ) ~and ~ ~Ni(C0)4.M An indication of the strength of interaction between the cluster and the alumina is found in the chemisorptive behavior of these samples. As can be seen from Table I, the extent of C O adsorption increases as the pretreatment temperature is increased. Temperature-programmed decomposition and thermogravimetric studies have indicated that bulk Fe3(CO),, decomposes between 353 and 453 K in flowing (39) Hanna, S. S.;Preston, R. S. Phys. Reu. A 1965, 139, A722. (40) Bilhou, J. L.; Theolier, A.; Smith, A. K.; Basset, J. M. J . Mol. Catal. 1977, 3, 245. (41) Kuznetsov, V. L.; Bell, A. T.; Yermakov, Y. I. J. Cafal.1980,65, 374. (42) Tessier-Youngs, C.; Correa, F.; Pioch, D.; Burwell, R. L., Jr.; Shriver, D.F. Organometallics 1983, 2, 898. (43) Laniecki, M.; Burwell, R. L., Jr. J . Colloid Interface Sci. 1980, 75, 95. (44) Bjorklund, R. B.; Burwell, R. L., Jr. J. Colloid Interface Sci. 1979, 70, 383.
Carbon- and Alumina-Supported Iron Samples
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2063
sample, reduction at 473 K is sufficient to entirely decompose the inert gas, depending on the heating rate.8,45,46 It might thus be clusters on graphite. expected that 1 h in hydrogen at 473 K would be sufficient to One reason for the weak interaction between Fe3(C0)12and decompose completely the supported cluster, and that higher graphite is the lack of functionality on the graphite surface. A pretreatment temperatures would cause sintering and hence desecond reason is the high loading of the cluster in relation to the crease the amount of CO chemisorption. That this does not occur low surface area of the support, Le., the loading in this catalyst, suggests that some of the iron is stabilized in a state that will not while small on a weight percent basis, represents more than a adsorb CO, presumably either as a carbonyl cluster or as ferric monolayer of cluster. or ferrous cations. Previous work with Fe3(C0)12supported on Particle Size Measurement. All measurements of this study hydroxylated alumina indicated that the presence of hydroxyl agree that the iron is well-dispersed on all supports, especially on groups stabilized zerovalent subcarbonyl species to approximately the alumina- and carbon black supported samples prepared with 423 K; further treatment oxidized the iron to Fe2+.8 It might, therefore, be suggested that the increase in CO adsorption is due Fe3(C0)12. The iron dispersion estimated from the magnetic susceptibility data for these two samples is greater than 60%, and to a reduction of ferric cations. However, several observations a significant fraction of the iron is present in particles containing argue against this suggestion. Firstly, IR examination of the OH only a few iron atoms. The dispersions calculated from the CO stretching region showed the absence of hydroxyl groups in dechemisorption measurements for the alumina-supported sample tectable amounts. Secondly, if ferric or ferrous cations were agree well with those estimated by magnetic susceptibility, and formed by reaction with support hydroxyl groups, it is unlikely the lack of a ferromagnetic iron species in the Mossbauer spectrum that treatment in hydrogen at 773 K would reduce them to the of the reduced, cluster-derived Fe/carbon black catalyst provides zerovalent state, since it is generally agreed that isolated ferrous added evidence that the iron is present in very small particles. cations supported on alumina cannot be reduced a t this temFurthermore, the particles on these catalysts are resistant to p e r a t ~ r e . ~Finally, IR spectra of this sample following presintering, since treatment at 773 K converts some of the clustreatment at progressively higher temperature show that although ter-sized particles to slightly larger particles, but otherwise changes a significant decrease in the original CO band intensity is observed the overall distributions little. Presumably the slow deposition following treatment a t 473 K, the bands persist with decreasing of the cluster by wet impregnation ensures that the iron precursor intensity to a treatment temperature of approximately 700 K. This indicates that some of the clusters are stabilized against decomis uniformly spread across the support surface, allowing these species to react with the strongest adsorption sites. Following position on the alumina support. The adsorption sites of the clusters on the carbon black support decomposition, the iron atoms are apparently unable to migrate, may be oxygen-containing surface groups. To test this hypothesis, or they do not come in contact with other iron particles due to the low number of atoms per support area. Earlier work has carbon black that had been pretreated at 1223 K in hydrogen was brought into the glovebox without exposure to air. This carbon suggested that the presence of surface heterogeneity can increase was then added to a solution of triiron dodecacarbonyl in hexane. metal dispersion and decrease sintering of carbon-supported metal catalyst^.^^-^^ Even after 3 weeks, the amount of cluster adsorbed by the carbon The situation is somewhat different on the conventionally gave only a 0.31 wt 7% iron catalyst, a lower loading than when the carbon was exposed to air prior to impregnation. prepared catalyst. The high surface area of the carbon black allows some small particles to be formed, but reduction a t 723 Further indication that adsorption is occurring on oxygenK is sufficient to sinter all iron into particles greater than 2 nm containing sites comes from Mossbauer spectroscopy. Figure 4A shows the spectrum collected following the initial adsorption of in diameter. It is possible that the higher loading of this sample and the incipient wetness preparation used with this catalyst result the cluster. The complexity of the spectrum prevents detailed analysis, although shoulders indicative of Fe2+and Fe(C0)5 can in most of the iron being present near the outer surface area of be seen. Work on 10 wt % Fe/carbon black samples prepared the carbon black. The local concentration of iron would therefore be higher, increasing the probability of sintering. Moreover, the by incipient wetness impregnation of Fe3(C0)12demonstrates that surface of carbon black is more graphitic than the pore triiron dodecacarbonyl can decompose to iron p e n t a ~ a r b o n y l . ~ ~ , ~outer ~ That these authors observed less oxidation and more Fe(CO)5 s t r ~ c t u r e ?and ~ would hence have fewer defects and surface groups to stabilize small iron particles. formation than in the present study is due to the order-of-magPrevious workers have studied similar conventionally prepared nitude higher loading that they used, which makes interaction with other carbonyl clusters more likely than interaction with the carbon-supported iron catalysts and have reported similar particle sizes. Vannice and co-workers1618 prepared 2.54% iron catalysts support. As can be seen from Table 111, some of the species present on several types of high-surface-area carbon. No X-ray diffraction have a low recoil-free fraction, indicating either a small size or a weak interaction with the support. The broad peak seen at 78 lines were visible, while CO chemisorption at 195 K indicated that the average particle size was near 2.5 nm for the carbon black K collapses to a Fe3+doublet at 295 K, indicating that some ferric supported catalyst. Mossbauer spectroscopy studies of a similar species are also present. Probably the iron is present as a complex mixture of ferric and ferrous species formed by interaction with 5% Fe/carbon black sample indicated that the iron particles were approximately 2 nm in diameter. Christensen et al.22923collected adsorbed water, as well as Fe(CO)5 and Fe3(C0)12associated with Miissbauer spectra in the presence of a magnetic field to determine surface oxygen. The interaction between the support and the iron carbonyl the iron particle size in a 3.5 wt % iron/carbon black sample. cluster is weaker in the case of the graphite-supported catalyst. From the magnitude of the splitting observed as a function of the The Mossbauer spectrum presented in Figure 3A is essentially applied field strength, the average particle size was calculated to the same as that of bulk Fe3(C0)12,and it is likely that the cluster be 2.5 nm. is only physically adsorbed on the graphite surface. The only The graphite-supported catalyst, as might be expected from significant spectral difference is an increase in the area of the its lower surface area and small number of defects and functional central doublet, which is due to iron atoms with only terminal groups, contains mostly large iron particles between 5 and 12 nm ligands, with respect to the area of the outer doublet, which is due to iron atoms bridged by CO ligands;36 it is possible that adsorption shifts some of the bridging carbonyls to the terminal (48) Ehrburger, P.; Mahajan, 0.P.; Walker, P. L., Jr. J. Catal. 1976,43, 61. position. Evidence for only a weak support-cluster interaction (49) Ehrburger, P.; Walker, P. L., Jr. J . Catal. 1978, 55, 63. is also provided by the fact that, unlike the alumina-supported (45) Psaro, R.; Fusi, A.; Ugo, R.; Basset, J. M.; Smith, A. K.; Hugues, F. J . Mol. Catal. 1980, 7, 51 1 . (46) Fillman, L. M.; Tang, S. C. Thermochim. Acta 1984, 75, 71. (47) Chen, A. A.; Vannice, M. A.; Phillips, J. 9th North American Catalysis Society Meeting, San Diego, CA, 1987.
(50) Ehrburger, P.; Mongilardi, A.; Lahaye, J. J . Colloid Interface Sci. 1983, 91, 151. (51) Ehrburger, P.; Lahaye, J. ACS Symp. Ser. 1986, No. 303, 310. (52) Arai, M.; Nishiyama, Y . Prepr. Am. Chem. SOC.,Diu. Pet. Chem. 1985, 30, 173. ( 5 3 ) Kirk-Othmer Encyclopedia of Science and Technology, 3rd ed.; Grayson, M., Eckroth, D., Eds. Wiley: New York, 1978.
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in diameter. Even on this catalyst, however, almost 20% of the iron is present in cluster-sized particles. Carbon monoxide chemisorption data agree well with dispersions calculated from magnetic susceptibility data for this catalyst. The low recoil-free fractions observed with Mossbauer spectroscopy are also indicative of the presence of small iron particles. The values estimated for these catalysts are in reasonable agreement with those estimated by other workers for small iron partic1es.lgJ4 Particle Morphology. The magnetic properties observed by using Mossbauer spectroscopy are dependent both on particle size and magnetic anisotropy. For particles whose relaxation time is longer than that required for observation of the Mossbauer effect, s,54 the magnetic hyperfine splitting is given by ca. 2.5 X
where H is the hyperfine field at a given particle volume, V, and temperature, T, K is the magnetic anisotropy constant, and k is the Boltzmann constant.55 Observation of a superparamagnetic spectrum requires that the superparamagnetic relaxation time be s. The relaxation time, 7,can be estimated less than 2.5 X by the relation 7
=
r0 exp(KV/kT)
(2)
where T~ is a constant on the order of sS6 Therefore, if the particle size distribution is known for a given sample (e.g., from magnetic susceptibility), eq 1 or 2 can be used to provide an estimate of the magnetic anisotropy constant. Consider first spectrum 3B, collected from the graphite-supported sample following reduction at 673 K. Computer fitting of this spectrum determined that approximately 93% of the iron was ferromagnetic at 79 K, with a measured hyperfine field of 333 kOe, after adjustment for the presence of the demagnetizing field. The particle size distribution obtained from magnetic susceptibility data suggests that the average diameter of the particles larger than 2.5 nm (Le., the approximate size above which the particles are expected to be ferromagnetic) is 6.6 nm; application of eq 1 yields an anisotropy constant of approximately 3 X IO5 J/m3. For comparison, the magnetocrystalline anisotropy of metallic iron is approximately 5 X IO4 J/m3,55and Marup et al.57 have estimated the anisotropy constant of 6-nm silica-supported iron particles to be approximately 1 X lo5 J/m3. The slightly larger value estimated for our Fe/graphite catalyst may be the result of shape anisotropy, due to the presence of nonspherical particles, or to magnetic interactions between iron particles in close proximity.55 The magnetic susceptibility data indicate that approximately 26% of the particles are smaller than 2.5 nm. That the spectral fraction of the superparamagnetic singlet is only about 7% can be attributed to the lower recoil-free fraction of the smaller particles. After correction for the demagnetizing field, the hyperfine splitting observed at 295 K for the conventionally prepared carbon black supported catalyst was 314.9 kOe. This value is large in light of the magnetic susceptibility data, which indicate that no particles are larger than about 6 nm. Agreement between the two sets of data requires a magnetic anisotropy constant of approximately 7 X lo5 J/m3. This value is probably due to significant contributions from shape and surface anisotropy. Christensen et aLZ2also reported a magnetic anisotropy of 7 X lo5 J/m3 for a 3.5% Fe/carbon black catalyst prepared in a similar manner. In addition, interaction anisotropy between neighboring particles may be more significant for the conventionally prepared carbon black supported catalyst. However, this contribution could (54) Wickman, H. H. Mossbauer Eflect Methodology; Gruverman, I. J., Ed.; Plenum Press: New York, 1966; Vol. 11, p 39. (55) Merup, S.;Dumesic, J. A.; Topsee, H. Applications of Mossbauer Spectroscopy; Cohen, R. L., Ed.; Academic Press: New York, 1980; Vol. 11, P 1. (56) NeBl, L. Ann. Geophys. 1949, 5 , 99. ( 5 7 ) Merup, S.; Clausen, B. S.;Topsee, H. J . Phys., Colloq. 1980, 41, C1-331.
Stevenson et al. at most be 1 X lo5 J/m3,S5and therefore would not fully account for the difference in the total anisotropy of the conventionally prepared catalyst and the Fe/graphite catalyst. In contrast to the samples discussed above, the sample prepared by the wet impregnation of triiron dodecacarbonyl into carbon black shows no ferromagnetic splitting, even at 79 K, following reduction at 773 K. The magnetic susceptibility data indicate that a significant fraction of the iron particles in this catalyst are in the range 3-3.6 nm in diameter. Application of eq 2 indicates that the magnetic anisotropy constant cannot exceed approximately 1.5 X lo5 J/m3. ‘This value is smaller than those observed for the other samples of this study, and it is approximately equal to the value of 1 X lo5 J/m3 found by Marup et aLS7for 6-nm silica-supported iron particles. Given that the small particle size requires an increased contribution from surface anisotropy, this result suggests that the iron particles in this sample are more spherical and possibly more widely spaced than in the other carbon-supported catalysts. It is now possible to interpret and summarize the different behavior of iron in the three different carbon-supported catalysts. The rapid deposition of the iron during aqueous incipient wetness impregnation of carbon black using ferric nitrate and the relatively high loading of this sample likely concentrate the iron near the outer surfaces of the carbon. These outer surfaces tend to he more graphitic and to have fewer defects than the carbon black pores. In contrast, the slow impregnation of carbon black with Fe3(C0)12 allows the iron to penetrate the pore structure, and, moreover, deposits the clusters on the most favorable adsorption sites, which are probably oxygen-containing defects. Thus, the morphologies of the iron particles present following treatment in hydrogen are different for these two samples, the cluster-derived iron particles being more spherical, and possibly more widely separated than those on the conventionally prepared catalysts. The graphitesupported samples would be expected to show a high magnetic anisotropy, as did the conventionally prepared carbon black supported samples, since the above arguments would suggest that the iron in both samples is present on graphitic surfaces. Also, the iron particles on this support may be in close proximity due to the low surface area.
Summary Fe3(CO)12was used to prepare small iron particles supported on graphite, carbon black, and alumina; a conventionally prepared carbon black supported catalyst was also studied. Wet impregnation of the carbonyl cluster onto the alumina and carbon black materials resulted in highly dispersed iron, a significant fraction of the iron being present in cluster-sized particles containing only a few iron atoms. The presence of surface oxygen and/or water was required for adsorption of the cluster onto the carbon black surface. A fraction of the clusters was stabilized against decomposition on the alumina support; treatment at 700 K was required to decompose all of the cluster material. These interactions between the cluster and the surfaces of the carbon black and the alumina prevent sintering and maintain a high dispersion. Indeed, these catalysts were resistant to sintering even during treatment in hydrogen at 773 K. Due to the much lower surface area and lack of surface functionality, the interaction of the iron cluster with the graphite support was much weaker, with only weakly adsorbed Fe3(C0)12 being observed by Mossbauer spectroscopy. Following treatment at 673 K, the particle size distribution showed that about 25% of the particles were smaller than 1 nm in diameter, with the remainder of the particles broadly distributed between 3 and 1 1 nm. Some small particles were also present in the carbon black catalyst prepared from ferric nitrate, but these particles sintered at 723 K. Estimates of magnetic anisotropy constants obtained from the Mossbauer spectroscopy and magnetic susceptibility data indicated that the particles on the Fe/graphite and conventionally prepared Fe/carbon black catalysts were present in a less spherical morphology and possibly were more closely spaced than in the cluster-derived Fe/carbon black catalyst. Differences in particle size,
J. Phys. Chem. 1989, 93, 2065-2068 shape, and location have significant consequences for the chemisorptive and catalytic properties of these catalysts, as discussed else~here.~~,*~ Acknowledgment. We acknowledge the funding of the National Science Foundation that supported this work. We are also
2065
thankful to the National Science Foundation for providing a Graduate Fellowship to one of the authors (S.A.S.). In addition, we thank the government of Japan for providing financial support to M.A. that allowed him to work at the University of Wisconsin. Registry No. Fe, 7439-89-6;CO, 630-08-0; graphite, 7782-42-5.
Replacement of Sodium Ions by Cryptate (C222) Complexed Barium Ions in Sodium Dodecyl Sulfate Micelles Studied by Electron Spin Resonance and Electron Spin-Echo Modulatlon of 5-Doxylstearlc Acid and N,N,N’,N’-Tetramethylbenzidlne Photoionization and by Viscoslty Measurements Thomas Wolff Physikalische Chemie, Universitat Siegen, 0 - 5 9 0 0 Siegen, West Germany
and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: July 6, 1988)
Ultraviolet irradiation of solubilized N,N,N’,N’-tetramethylbenzidine (TMB) and viscosity measurements have been carried out at room temperature in mixed micellar solutions of sodium dodecyl sulfate (SDS) and barium dodecyl sulfate [Ba(DS)*]. The barium ions were complexed by the macrocyclic cryptate C222. The irradiations did not lead to constant photoionization yields in the barium/C222-containing solutions because of secondary reactions. The viscosity as a function of the Ba2+/2 mole fraction (X) changed from 1.47 mPa s at X = 0 to 1.33 mPa s at X = 1, passing through a maximum at X = 0.2. At 77 K in frozen solutions the TMB photoionization yield as measured by electron spin resonance increased by a factor of 1.8 in the range X = 0-0.2 and leveled off at larger X. Deuterium modulation depths of electron spin-echo decay curves measured at 4.2 K for TMB’ and 5-doxylstearic acid increase as a function of X in a similar way as the photoionization yields. The results indicate a higher local concentration of water molecules in the micellar surface region when the C222-complexed barium ions are present.
Introduction The size, shape, and properties of ionic micelles in aqueous solutions are known to depend on the identity of the counterions to a great extent. This dependence has been observed in a variety of colloid chemical, thermodynamic, and spectroscopic investigations’-” and is discussed in terms of the intermicellar and intramicellar electrostatic forces that are governed by micellecounterion dissociation equilibria. For instance, large and strongly binding counterions induce growing of micelles that often change in shape from spheres to rods;S*6the structure of the micellar (1) Lindman, B.; Wennerstrom, H. Top. Curr. Chem. 1980, 87, 1. (2) WennerstrBm, H.; Lindman, B. Phys. Rep. 1980, 52, 1. (3) Berr, S. S.;Coleman, M. J.; Jones, R. R.; Johnson, J. S., Jr. J . Phys. Chem. 1986, 90,6492. (4) Almgren, M.; Swamp, S. J . Phys. Chem. 1983,87, 876. ( 5 ) Angel, M.; Hoffmann, H.; LBbl, M.; Reizlein, K.; Thurn, H.; Wunderlich, I. Prog. Colloid Polym. Sci. 1984, 69, 12. (6) Wolff, T.; Suck, T. A.; Emming, C.-S.;von Biinau, G. Prog. Colloid Polym. Sci. 1987, 73, 18. (7) Szajdinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M. J . Am. Chem. SOC.1984, 106, 4675. (8) Szajdinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M.; Coleman, M. J. J . Am. Chem. SOC.1985, 107, 784. (9) Jones, R.R.M.; Maldonado, R.; Szajdinska-Pietek, E.; Kevan, L. J . Phys. Chem. 1986, 90, 1126. (10) Wolff, T.; von BUnau, G. Eer. Bunsen-Ges. Phys. Chem. 1982, 86, 225. ( 1 1) Wolff, T.; von Biinau. G. Ber. Bumen-Ges. Phys. Chem. 1984, 88, 1098.
0022-3654/89/2093-2065$01.50/0
interface is affected,’-I0 and, macroscopically, the bulk viscosity of micellar solutions may vary.6V1’J2 In several recent studies the influence of complexing cationic counterions by macrocyclic ligands such as crown ethers and cryptates has been investigated.l3-I6 It was shown for sodium dodecyl sulfate (SDS) micelles that upon crown ether complexation of sodium counterions the aggregation number decrea~es,’~ while the degree of di~sociation~~ and the photoionization yield of N,N,N’,N’-tetramethylbenzidine (TMB) in~rease.’~.’~ Simultaneously, water penetration into the micellar surface is decreased as revealed by spin-probe experiments with 5-doxylstearic acid (5-DSA).13 Similar but quantitatively distinct trends were found for lithium dodecyl sulfate (LiDS) micelle^.'^^'^ The increase of the TMB+ yield was ascribed to an average location of T M B closer to the micellar surface due to TMB-crown ether interactions. Thereby, the distance between TMB and bulk water as an electron acceptor is decreased. Since the TMB-crown ether interactions as revealed by ESR line-shape variation^'^ may interfere with the investigation of micellar properties by the TMB probe, it was desirable to find a micellar system containing a macrocyclic ligand that does not (12) (13) 467. (14) 4726. (15)
Wolff, T.; von Biinau, G. J . Photochem. 1986, 25, 239. Baglioni, P.; Kevan, L. J . Chem. Soc., Faraday Trans. 1 1988, 84, Baglioni, P.; Rivava-Minten, E.; Kevan, L. J . Phys. Chem. 1988, 92,
Evans, D. F.; Sen, R.; Warr, G. G. J . Phys. Chem. 1986,90, 5550. (16) Evans, D. F.; Evans, J. B.;Sen, R.; Warr, G. G. J . Phys. Chem. 1988, 92, 784.
0 1989 American Chemical Society
