1506
J . Phys. Chem. 1989, 93, 1506-1510
Solubilization of Phenol and Benzene in Cationic Micelles: Binding Sites and Effect on Structure Kazuhiko Kandori,? Robert J. McCreevy, and Robert S. Schechter* Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 7871 2 (Received: May 12, 1988; In Final Form: July 12; 1988)
The effect of phenol and benzene additives on micellar structure in aqueous solutions of dodecyltrimethylammonium bromide has been studied by means of various experimental measurements. The solution properties studied include additive solubilities, tracer diffusion coefficients, electrical conductivity, viscosity, and ultraviolet absorbance. From the tracer diffusion measurements the degree of partitioning of phenol into the micelles was calculated as a function of phenol concentration. A procedure for determining surfactant aggregation number and micelle concentration in ionic surfactant systems by means of diffusion and electrical conductivity data is presented. The solubilization of phenol and benzene in this system causes the micelles to swell, and it was observed that phenol addition leads to a greater increase in the size of aggregates than addition of benzene. Ultraviolet absorbance measurements revealed that the site of solubilization within the micelles is different for the two additives: Benzene solubilizes in the central core, while at low concentrations phenol is taken up in the outer palisade layer. However, the site of phenol solubilization, the shape of the micelles, and the physical properties of DTAB/phenol solutions change at a concentration of 1 mol of phenol solubilizate per mole of surfactant. Due to the saturation of the palisade layer with surfactant and additive molecules, phenol added to the DTAB system beyond this transition point most likely binds to the exterior of the micelles.
Introduction The effect of additives such as alcohols on the properties of aqueous surfactant solutions has been a subject of intense research by many investigators. Studies have been conducted on aggregate lifetimes,l on partitioning of alcohols between micellar and aqueous phase^,^^^ and on properties such as aggregation number and molecular eight.^-^ In this study, two additives, phenol and benzene, were added to solutions of dodecyltrimethylammonium bromide with the purpose of elucidating their effects on micellar size and structure. These additives provide an instructive focus for comparison because, while their molecular structure is similar, phenol is much more soluble in water and would be expected to partition between aqueous and micellar phases, while benzene is almost completely solubilized inside the micelles. Furthermore, while benzene is almost certainly solubilized into the interior core of the micelle, the site of phenol binding is far from established. Recently, it has been suggested that phenol may in fact be positioned at any one of three possible sites6 Based on this work, it is possible to conclude that the mode of solubilization changes rather abruptly when the phenol to surfactant ratio is unity. This interesting transition has not been observed because previous studies have not considered the proportion of the added phenol which remains as monomer in the aqueous phase. Materials and Methods Dodecyltrimethylammonium bromide (DTAB) (Aldrich Chemical Co.) was of 99% purity and was used as received. Reagent grade phenol (Fisher Scientific) was also used as received. For tracer diffusion experiments the radioactive species used was ''C-labeled phenol obtained from ICN Radiochemicals, and 3Hlabeled sodium p - ( 1-propylnonyl)benzenesulfonate,specially prepared by Ashland Chemical Co. Although this tracer surfactant is not identical with the untagged DTAB, the radioactive solutions used for making diffusion measurements contained only a very small fraction of labeled surfactant compared to the total amount present, so that micellar structure is unchanged with the use of the tracer species. Tracer diffusion coefficients of labeled species were determined by the Taylor dispersion technique, which is described in earlier Solubility measurements were performed on solutions of 0.1 M DTAB (additive-free basis), since this system was the *To whom all correspondence should be addressed. 'On leave from Science University of Tokyo, Tokyo, Japan. 0022-365418912093-1506$01.50/0
one used for all experimental measurements in this study. Five milliliters of surfactant solution was titrated with added phenol and benzene, the solutions were agitated during each titration, and measurements were done in a constant temperature bath at 25 OC. Solubility data for this system were determined by visual inspection. Ultraviolet (UV) spectrometry was also used in order to estabish the site of within the micelles for both phenol and benzene. These measurements were performed with a Beckman Model DU-40 spectrophotometer. Electrical conductivity measurements were made using a Cole-Partner Model 148 1-00 conductivity meter, while viscosity of the solutions was measured with a Ubbelohde viscometer. All experiments for this study were performed at 25 OC.
Theory 1, Tracer Diffusion and Partitioning of Components. The effective tracer diffusivity denotes the movement of labeled surfactant and additive in both the monomer and micellar forms. Assuming local equilibrium, the effective tracer diffusivity for a tagged surfactant can be represented aslZ Deff
=
%Df+
(1 - 4 D n l
(1)
where asis the fraction of tagged sulfonate surfactant existing in monomer form, Dfis the tagged surfactant monomer diffusivity (which in general is not equal to the unlabeled surfactant monomer diffusivity), and D, is the diffusivity of the micelles. A similar expression exists for the observed movement of radioactively labeled phenol (1) Leung, R.; Shah, D. J. Colloid. Interface Sci. 1986, 113, 484. (2) Hayase, D.; Hayano, S. J. Colloid Interface Sci. 1978, 63, 446. (3) Rao. I.: Ruckenstein. E. J . Colloid Interface Sci. 1986. 113. 375. (4) Zana, R.; Yio, S.;Strazielle, C.; Lianos, 6. J . Colloid Interfaace Sci. 1981. 80. 208. (5) Aimgren, M.; Swarup, S. J. Colloid Interface Sci. 1983, 91, 256. (6) Bunton, C. A.; Cowell, C. P. J . Colloid Interface Sci. 1988, 122, 154. (7) Taylor, G. I. Proc. R. Soc. London A 1953, 219, 186. (8) Weinheimer, R.; Evans, D.; Cussler, E. J. Colloid Interface Sci. 1981, 80. 357. (9) Lam, A. C. Ph.D. Dissertation, The University of Texas, Austin, TX, 1986. (10) Rehfeld, S. J. J. Phys. Chem. 1970, 74, 117. ( 1 1) Rehfeld, S.J. J. Phys. Chem. 1971, 75, 3905. (12) Kamenka, N.; Lindman, B.; Brun, B. Colloid Polym. Sci. 1974, 252, 144.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1507
Solubilization in Micelles
where a pis defined as the fraction of tagged phenol residing in monomer form, and where DPf and DPcffare defined in the same manner for phenol as the above nomenclature is for tagged surfactant. The experimental values for DJr and DPf,the monomer diffusion coefficients of labeled surfactant and phenol in water, have been determined as 4.7 X 10” and 7.5 X 10“ cm2/s, respectively. The objective of using tracer diffusion experiments is to calculate the parameter ap,which expresses the relative partitioning of phenol additive between aqueous and micellar phases. Thus, tracer diffusion experiments provide a means of calculating partition coefficients for additives in micellar solutions. In order to calculate the partitioning factor a p as a function of added phenol concentration in DTAB solutions, it is first necessary to calculate the dependence of micellar diffusion coefficient D , on concentration of additive. The determination of D , requires the labeled surfactant partitioning factor a,,also as a function of additive concentration. A method for calculating tracer surfactant partitioning in micellar solutions in the presence of additives is given in McGreevy and Schechter13 which involves the assumption of ideal mixing of the tracer species between aqueous and micellar phases.14 However, the ideal mixing assumption used in that paper is not valid here because the labeled surfactant is anionic, while the bulk species DTAB is cationic. Because of ionic attractive effects between labeled and bulk surfactant, it is expected that essentially all of the labeled species partitions into the micelles in the solutions used for this study, which makes the partition factor a,equal to zero for all additive concentrations studied. The validity of this reasoning was confirmed by diffusion measurements in which trace amounts of 14C-labeledhexadecane solubilizate were added to aqueous DTAB solutions. The observed tracer diffusion coefficients for labeled hexadecane and surfactant are nearly identical, confirming that both species are completely within the micellar phase. Thus for each additive concentration used in this study the observed surfactant diffusion coefficient Der and micellar diffusion coefficient D , are equal. With these simplifications to the above equations, the fraction of phenol partitioning in DTAB solutions can be calculated knowing the observed tracer diffusivities of surfactant and phenol. 2. Determination of Aggregation Number of DTAB Micelles Based on Electrical Conductivity and Degree of Micellar Dissociation. Analysis in this section concerns the surfactant aggregation number in the micelles as a function of additive present in solution. A method will be presented for calculating changes in aggregation number for ionic surfactant systems containing additives by means of electrical conductivity measurements, when used together with diffusion data on the same system. When surfactant concentration is above the cmc, the material balance for surfactant is C, = cmc nC, (3)
+
where C, and cmc are defined as above and n and C,,, represent the surfactant aggregation number and the number concentration of micelles in solution, respectively. The electrical conductivity of dispersed ionic surfactant systems has been shown to be9
L.J
0.0
1 .o
2.0
3.0
[ Solubilizate]/[DTAB] Figure 1. Effect of solubilizates on electrical conductivity.
related to aggregation number n through the mass balance (3). For varying amounts of added phenol, electrical conductivity measurements which are made on solutions where the surfactant concentrations are below the cmc show that C Z ; D i is constant cm2/s. This term can be inserted into and equal to 1.16 X the right-hand side of eq 4 to eliminate an unknown. The micellar diffusion coefficient D , is determined from surfactant diffusion measurements and is a function of added phenol concentration. The only unknown term left in (4) is the net valence Z,, defined as
Z,=n-q
(5)
where q is the number of counterions (Br- in this case) bound to the micelle. 2, can also be related to aggregation number n by the degree of micellar dissociation a,the value of which has been determined for aqueous solutions of DTAB by a variety of methods.15 Other measurements of dissociation in trimethylammonium bromides4-16show that the effect of alcohols of less than 10 vol % on the value of a are small, being within *lo% in most cases. The highest phenol concentration used in our study is 7 vol %, and for this analysis we assume a to be constant and equal to 0.24 over the range of phenol concentrations. Our expression for net valence of the aggregate is thus Z , = 0.24n
(6)
Finally, the mass balance (3) can be inserted into the conductivity equation (4), and an expression for electrical conductivity k is obtained for ionic surfactant systems in terms of aggregation number n, with all other factors known. Calculated values of surfactant aggregation number for various concentrations of additives are shown in the next section.
where e is the fundamental unit of charge, kB is Boltzmann’s constant, Z,is the valence of ionic species i, 2, is the net valence of the aggregates, Ci is the concentration of species i in the continuous phase, and Di is the molecular diffusivity of component i in the continuous phase. Our scheme for determining surfactant aggregation number by conductivity measurements centers on the elimination of all unknown terms in (4) except C,, which can be
Results A summary of experimental results and calculations can be found in Figures 1-6. Measurements of electrical conductivity and viscosity for DTAB solutions with phenol and benzene additives are plotted in Figures 1 and 2. The abscissa used in these plots is the moles of additive contained in the micelles (equal to total moles of additive multiplied by 1 - a,, where the subscript a refers to either phenol or benzene) relative to total moles of surfactant. Phenol addition causes a slight initial decrease in electrical conductivity followed by a significant increase at higher concentrations. With benzene addition a gradual decrease in conductivity was measured across the range of solubilizate concentrations. In Figure 2 benzene solubilizate causes a monotonic
(13) McGreevy, R. J.; Schechter, R. S . J . Colloid Interface Sci., in press. (14) Scamehom, J. F.; Schechter, R. S.; Wade, W. H. J . Colloid Interface Sci. 1982, 85, 463.
(1 5 ) Attwood, D.; Florence, A. T. Surfactant Systems, Their Chemistry, Pharmacy and Biology; Chapman and Hall: London, 1985; p 19. (16) Larsen, J. W.; Tepley, L. B. J . Colloid Interface Sci. 1974, 49, 113.
1508 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
Kandori et al. 0.8
0.7 h
e 0
Y
0
4
$. 0.6
e ._ * C
l
e,
0
v
0.5
x .e u
e 0 0 e
0.4
.e
>
1.0‘
u.0
1 .o
0.3 u.0
3.0
2.0
1 .o
2.0
3.0
[Phenol solubilizate]/[DTAB] Figure 4. Change in micellar partitioning of phenol.
[Solubilizate]/[DTAB] Figure 2. Effect of solubilizates on viscosity.
300
200
C
x
P
101
100
I1
e, 0 0
s .-0e ¶
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1 0
20
Dm(benzene) Dp,eff
1
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a
[Solubilizate]/[DTAB] Figure 3. Effect of solubilizates on tracer diffusion coefficients.
0
u.0
1 .o
2.0
3.0
[Solubilizate]/[DTAB] Figure 5. Effect of solubilizates on surfactant aggregation number. lo5
increase in solution viscosity, but the effect for phenol is quite different. An initial sharp increase and a maximum in viscosity are the result of increasing phenol solubilization in the micelles. Tracer diffusion coefficients for the system obtained from Taylor dispersion are given in Figure 3. The notation D, refers to observed surfactant diffusion as a function of additive, which has been demonstrated to be the diffusion coefficient of the micelles. Tracer diffusion coefficients of labeled phenol additive in solution are given as DPcff.As benzene is solubilized inside DTAB micelles the micellar diffusion coefficient gradually decreases across the range of benzene concentrations. For the case of phenol addition to DTAB solutions, labeled phenol diffusion coefficients monotonically increase with increasing solubilizate concentration. The most interesting feature of this plot, however, is the minimum observed for D, in the presence of phenol solubilizate. This phenomenon of micellar diffusion occurs at the same phenol concentration as the dramatic changes in electrical conductivity and viscosity for DTAB/phenol systems shown in Figures 1 and 2. We will return to this point in the next section. The calculated partitioning factor apfor phenol in this system is plotted in Figure 4. It can be understood from this figure that, while the amount of phenol solubilizate increases with total phenol added, the fraction existing in the micelles decreases due to the considerable solubility of phenol in bulk water. Figure 5 shows calculated surfactant aggregation numbers with increasing additive solubilization, which are obtained from conductivity and diffusion
0
1
2
3
4
[Phenol solubilizate]/[DTAB] Figure 6. Calculated volumes of micellar core using two different methods: (m) calculated from aggregation number and known volume per surfactant. (+) calculation from known surfactant tail length, assuming spherical shape. measurements. At zero additive concentration, the aggregation number n is equal to 67.1, which is in excellent agreement with the value of 65 found in the literature.” Phenol has the largest
Solubilization in Micelles effect on both properties and a transition occurs at the same phenol concentration noted previously for the experiments in Figures 1-3. Finally, a set of comparative calculations for the micellar core18 with phenol addition is given in Figure 6. The bottom set of points gives the volume calculated when assuming that the chain length of surfactant is equal to the core radius and that the core is spherical. The upper set of points gives the volume calculated by using surfactant aggregation numbers presented in Figure 5, and assuming a known volume per surfactant molecule of 323.3 A3.The disparity between the volumes predicted by these two methods shows the extent to which DTAB micelles are nonspherical.
Discussion These experimental data and calculations for the DTAB system in the presence of phenol and benzene can be used to explain the changes in micellar structure which occur at varying additive or solubilizate concentrations. Before discussing changes in the nature of the aggregates, let us first consider the site of solubilization for these additives within the micelles, which can be established by UV spectrometry measurements performed in this study. Absorbance of benzene was measured in three solvents while scanning over wavelengths ranging from 230 to 300 nm. Absorbance peaks were found for benzene dissolved in water, decane, and 0.1 M DTAB in water. Four different benzene peaks were found for the latter two solvents, and their wavelengths at 242.6, 248.2, 253.8, and 259.8 nm were the same. In the former case, benzene and water were mixed and separated into two phases. The saturated water phase absorbance was measured and found to contain trace amounts of benzene. Four benzene peaks were observed in the water solvent, but their wavelengths were smaller than for the case of benzene absorbed in the decane and surfactant solutions. From these results we conclude that in DTAB solutions benzene is solubilized in the core or center portion of the micelles, since this oillike environment gives the same absorbance peaks as found in decane. Phenol was dissolved in the same three solvents at a concentration of 0.01 vol %, and its absorbance was measured over the same scan. The number of peaks and the wavelengths observed for phenol absorbance differed in each solvent, and from this we infer that phenol is solubilized in a location within the DTAB micelles that is neither completely hydrophobic nor hydrophilic. Jacobs et used ‘HN M R chemical shift measurements to investigate the solubilization of phenol in SDS micelles. The conclusion of these investigations was that phenol was oriented in the micelles in such a manner that the hydroxyl group was close to the polar micellar surface. Bunton and Cowel16 also observed N M R shift spectra for phenol added to aqueous solutions of CTAB, which is very similar to DTAB used in this study. They concluded likewise that for cationic micelles phenol is bound near the surface. When our observations of phenol absorbance are considered in this light, it appears that in DTAB micelles the solubilization of phenol molecules in the hydrophobic core can be ruled out. In order to demonstrate more fully the effect of the additives on micellar structure, let us first examine the case of benzene addition, referring to Figures 1-3. The addition of increasing amounts of benzene to the DTAB system results in a monotonic decrease in electrical conductivity and micellar diffusion coefficient and an increase in solution viscosity. As benzene is solubilized, it seems reasonable that the aggregates will become swollen and increase in size. The diffusion data in Figure 3 support this claim. Referring to the right-hand side of the expression for electrical conductivity in eq 4, it is evident that k is proportional to D,, so a drop in electrical conductivity is consistent with larger swollen micelles, and this is in fact observed in Figure 1. Figure 5 il( 1 7 ) Milliaris, A,; Le Moigne, J.; Strum, J.; Zana, R. J . Phys. Chem. 1985, 89, 2709. (18) Tanford, C . The Hydrophobic Effecr: Formarion of Micelles and Biological Membranes; Wiley: New York, 1980; pp 51-57. (19) Jacobs, J. J.; Anderson, R. A.; Watson, T. R. J . Pharm. Pharmacol. 1971, 23, 148.
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1509 lustrates that benzene addition has little effect on surfactant aggregation number except at the highest concentrations. One can conclude from these data and calculations that the primary effect of benzene solubilizate on the DTAB system is to increase the size of the micelles. Now the effect of phenol addition must be considered. The low-concentration region of phenol on the left-hand side of Figures 1-3 ranges up to 1 mol of phenol solubilizate per mole of DTAB. While the viscosity of the system increases sharply over this region, a gradual decrease was observed for both electrical conductivity and the micellar diffusion coefficient. These results correspond to an increase in the uptake of phenol by the micelles and an increasing hydrodynamic radius. The increase in micellar size seems to be particularly great for phenol addition compared to benzene addition if the D, for each case is considered. This difference could be related to the site of solubilization within the micelles and suggests that solubilization of additive in the palisade layer results in a greater aggregate size increase than solubilization in the core. With further phenol addition to DTAB solutions, a dramatic change in properties was observed beginning at a solubilizate concentration of approximately 1 mol of phenol per mole of DTAB. The electrical conductivity and diffusion measurements go through a minimum and then increase as more phenol is solubilized. Over the same concentration range a maximum is observed for viscosity, which then drops slightly at higher concentrations of phenol. Using the same reasoning as given above for the dilute phenol region, in the concentrated region these experimental measurements are all consistent with each other and lead to the conclusion that in the concentrated phenol region (near 1 mol of phenol solubilized per mole of surfactant) the hydrodynamic radius of the aggregates reaches a maximum and then begins to decrease with further addition of phenol. The reason for this unusual change in the size of micelles in this system is far from obvious. Kinematic viscosity measurements of aqueous solutions with no surfactant containing phenol over the concentration range used in this study reveal that viscosity increases no more than 15% compared to that of pure water, while solutions containing 0.1 M DTAB increase in viscosity up to 77% with phenol addition. Calculations of the phenol partitioning factor apand surfactant aggregation number n shown in Figures 4 and 5 can be used to help explain the phenomena seen in Figures 1-3. Concerning Figure 4 it was observed in the previous section that as more phenol is added to the system the fraction of phenol partitioned into the micelles 1 - cyp steadily declines, suggesting that increasing amounts of phenol are present in the aqueous phase. In Figure 5 the trend in calculated aggregation number levels off abruptly at a “saturation” point where there is 1 mol of phenol solubilized per mole of surfactant. This is in fact the transition concentration noted previously, and it appears that the palisade layer inside the micellar surface is saturated with additive. Further phenol solubilization by DTAB micelles must occur at a different location. Bunton and Cowel16 suggested three possible modes or sites of solubilization of phenols near the surface of a cationic micelle. The first two possiblities involve solubilization inside the micelle, wedged between surfactant molecules. This appears to be the mechanism in our system at low concentrations of added phenol. The other mode of phenol solubilization occurs by binding to the exterior of the micelle, and we hypothesize this mechanism of binding to occur at high phenol concentrations above the transition point. Refer now to Figure 6, which gives the volume of the micellar core calculated from aggregation numbers (upper set), compared with the case of spherical aggregates (lower set). The interpretation of this figure is that, if DTAB micelles were completely spherical, the volume calculated from aggregation numbers would equal that calculated from surfactant tail lengths, and all points would fall on the bottom line. It is evident that, even without added phenol, the micelles are nonspherical at the surfactant concentration used in this study. However, as phenol solubilization in this system increases the micelles become more and more nonspherical up to the transition point. The exact shape cannot
1510
J . Phys. Chem. 1989, 93, 1510-1515
be determined by our measurements. The change of the core away from a spherical shape levels off as phenol is added beyond the transition point, and its shape appears not to be affected further. This evidence presented for phenol solubilization in DTAB solutions allows a greater understanding of the changing structure of the micelles than was previously possible. As phenol is solubilized in low concentrations up to the transition point, phenol binding occurs in the palisade layer, the micelles become more rodlike and less spherical, and the surfactant aggregation number increases. Further solubilization beyond the transition poiht results in phenol binding to the micelle exterior, while the shape of the core and the aggregation number remain unchanged under these conditions. Increasing phenol addition also affects the aqueous phase by decreasing its dielectric constant, causing it to be more hydrophobic. Under these circumstances we would expect the hydration layer surrounding the micelles to shrink in thickness. Such an effect would give rise to smaller hydrodynamic radii and increased micellar diffusion and electrical conductivity. This is in fact observed for the high-concentration region of phenol in Figures 1-3. Conclusions Based on the experiments and calculations performed in this study, we make the following conclusions concerning phenol and benzene addition to aqueous solutions of DTAB: 1. Partition coefficients for additives to surfactant solutions can be calculated by means of tracer diffusion experiments where the additive and amphiphilic components are radioactively labeled. Partition results for phenol in the DTAB system reveal that the fracton of phenol residing in the micelles declines as its concentration in solution increases. 2. Ionic surfactant aggregation numbers can be calculated by means of electrical conductivity and micellar diffusion measurements for a given surfactant system. Aggregation numbers calculated by this method were found to be in excellent agreement with those in the literature.
3. Both of the additives used in this study, phenol and benzene, are solubilized by the micelles in DTAB solutions and cause them to swell with increasing additive concentration. This effect seems to be more pronounced for the case of phenol, and probably is due to the fact that phenol solubilizes in the palisade layer and not in the micellar core. 4. Phenol addition has a much greater effect on surfactant aggregation number than does benzene. The site of phenol solubilization by the micelles changes at a transition concentration of one phenol molecule per surfactant molecule, which has a dramatic effect on the physical properties of this surfactant system. 5. Phenol solubilization below the transition point occurs in the palisade layer between surfactant molecules; further addition above the transition point results in phenol binding to the exterior of the micelle. 6. Phenol addition up to the transition point causes DTAB micelles to be more rodlike and less spherical, and causes surfactant aggregation to increase; solubilization beyond the transition point does not have any further effect on shape or aggregation number. 7. Increasing phenol solubilization in the bulk water makes the aqueous phase more hydrophobic, and causes the hydration layer surrounding the micelles to shrink at high phenol concentrations. This effect leads to smaller hydrodynamic radii of aggregates at high phenol concentrations.
Acknowledgment. This research was supported by the US. Department of Energy, Celanese, Chevron Oil Field Research, Elf-Aquitaine, Exxon Production Research, Imperial Chemical Industries, Norsk Hydro Research Centre, Shell Development, Standard Oil Production, Sun Exploration and Production, Texaco, and the National Science Foundation. We thank Julijanto Tanudjaja for performing the diffusion experiments. Dr Schechter holds the Getty Oil Company Centennial Chair in Petroleum Engineering. Registry NO. DTAB, 1119-94-4; PhOH, 108-95-2; C6H,5, 71-43-2.
High-Dispersion Direct Current Sputtered Platinum-TiO, Powder Catalyst Active in Ethane Hydrogenolysis P. Albers, K. Seibold, Degussa AG, Abt. FCPh, 0 - 6 5 4 0 Wolfgang, Hanau I , FRG
A. J. McEvoy, and J. Kiwi* Institut de Chimie Physique, Ecole Polytechnique Fcdsrale, CH- 1015 Lausanne, Switzerland (Received: May 12, 1988; In Final Form: July 19, 1988) The effects of Pt-cluster size, Pt loading, oxidative state of the metal surface species, and surface metal dispersion have been investigated on Pt/TiO, catalysts by using ethane hydrogenolysis as the test reaction. High Pt dispersion has been obtained by using a novel arrangement for dc sputtering on powders. This technique allows for Pt-cluster deposition with sizes 22-29 8, irrespective of Pt loading up to 2.2%Pt on Ti02 The surface characteristicsof the most active catalyst (160 min sputtering) are markedly different from those of higher and lower Pt content. These sputtered catalysts have been thoroughly examined by transmission microscopy (TEM), photoelectron spectroscopy (ESCA), diffuse reflectance spectroscopy (DRS),elementary analysis, and hydrogen chemisorption, making it possible to compare the observed activities on a more fundamental basis since the difference in the degree of dispersion of the metal cluster is not the controlling factor in the dynamic process. Introduction Platinum-loaded titania is known as one the the most active materials used in a number of catalytic’ and photocatalyticZreactions. It has usually be obtained by impregnati~n,~ exchange: (1) Tauster, S.; Fung, S.; Garten, R. J . Am. Chem. SOC.1978, ZOO, 170. (2) (a) Kraeutler, B.; Bard, A. J . Am. Chem. SOC.1978. ZOO, 4318. (b) Kiwi, J.; Gratzel, M. Nature 1979, 281, 657. (3) Heise, M.; Schwarz, J. In Prepararion of Caralysrs IV; Delmon, B., et al., Eds.; Elsevier: Amsterdam, 1978; p 1.
or phot~platinization.~We have chosen dc sputtering to deposit highly dispersed Pt on TiOz without the introduction of foreign ions and solvents as encountered in wet impregnation3 or exchange with metal salt solutions. Sputtering metals as thin films is a well-established method in research and industry.6a Sputtering on powdered solids has (4) Benesi, H.; Curtis, M.; Studer, P. J . Carol. 1968, 10, 328. (5) Kiwi, J.; Gratzel, M. J . Mol. Catal. 1987, 39, 63.
0022-3654/89/2093-15 10$01.50/0 0 1989 American Chemical Society
