J . Phys. Chem. 1986, 90, 6811-6817 transfer mechanism. We suggest that a plausible solute-solvent conformation which could undergo this reaction is the cyclically hydrogen-bonded “monosolvate” structure depicted below: R
monosolvate
H
polysolva l e
Similar structures have been proposed in the ESIPT reaction of 3-hydro~yflavone.~ Similar time-resolved measurements show that the room-temperature risetime of the tautomeric emission is < l o ps. The interpretation of the rapid ESIPT reaction in room-temperature ethanol is somewhat more ambiguous. The rapid rate can be interpreted in terms of an activated ESIPT mechanism involving solvated structures such as the monosolvate. The presence of a considerable energetic barrier could result in rapid ESIPT at room temperature, which slows to 100 ps at 77 K. Alternatively, a variety of hydrogen-bonded solute-solvent structures (including the above monosolvate) could be in equilibrium with a higher energy intramolecularly hydrogen-bonded form, which undergoes rapid ESIPT. As mentioned above, the rate of equilibration, and/or the equilibrium constant, could render this mechanism inefficient at low temperatures. We emphasize that the present data do not allow us to discriminate between these two mechanisms. The room-temperature normal emission decay time is 40 ps. As indicated above, there is no 40-ps risetime component of the tautomeric emission. This indicates that the hydrogen-bonding conformations responsible for normal emission result in little or no ESIPT on the 40-ps time scale. We suggest that polysolvate-type structures depicted above inhibit proton transfer and therefore result in normal emission. Similar assignments have been made for 3-hydro~yflavone~ and 2-hydroxy-4,5-naphthotropane.* The normal emission decays nonexponentially, with a “half-life” (time required for the emission to decay to half of its maximum intensity) of about 700 ps at 77 K. We assign this
6811
nonexponential behavior to luminescence from many different polysolvate-type structures, each having different fluorescence lifetimes. The relative abundances of the solute-solvent conformation yielding normal and tautomeric emission (presumably the monasolvate and polysolvate forms discussed above) can be estimated by the intensities and lifetimes of the normal and tautomeric emissions. The normal and tautomeric emission lifetimes at room temperature are 40 and 75 ps, respectively, and at 77 K are 1 and >> 10 ns, respectively. The corresponding emission intensity ratios (norma1:tautomeric) at room temperature and 77 K are about 1:2 and l:lO, respectively. We therefore conclude that the monosolvate- and polysolvate-type forms are of roughly equal abundance in room-temperature solution. However, polysolvate structures are much more abundant in the 77 K ethanol glass.
-
Conclusions The central conclusions which result from this work can be summarized as follows: 1. Photoexcitation of matrix-isolated HBT results in rapid proton transfer across a preexisting hydrogen bond. The proton-transfer rate can be bracketed by time-resolved and line-width measurements and is between 25 fs and 10 ps. This time scale indicates facile proton tunneling through an energetic barrier. 2. Static spectra of HBT in ethanol solutions and glasses can be interpreted in terms of ”monosolvate” and “polysolvate” hydrogen-bonded conformations. The time-resolved measurements indicate that the monosolvate exhibits proton transfer on the 100-ps time scale at 77 K, while little or no proton transfer is observed in the polysolvate. We have recently demonstrated the selective production and detection of specific hydrogen-bonded HBT solvent complexes. Detailed dynamical studies are currently in progress and will be reported in a later paper.
Acknowledgment. We thank Prof. 0. L. Chapman for the sample of HBT. We also thank IBM Corporation and NSF for support of this work. Registry No. 2-Hydroxy-4,5-benzotropone, 3 144-47-6
Pt-Ti Alloy Formation from Hlgh-Temperature Reduction of a Titanla-Impregnated Pt Catalyst: Implications for Strong Metal-Support Interaction Bruce C. Beard and Philip N. ROSS* Materials and Molecular Research Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 (Received: October 9, 1985)
Highly dispersed Pt supported on carbon black was impregnated with titania and heated in an inert atmosphere. The amount of titania used was twice the stoichiometric amount required for the formation of the ordered alloy phase Pt3Ti. The chemical state of Ti at various stages of heating was determined by X-ray photoelectron spectroscopy and by extended X-ray absorption fine structure (EXAFS) analysis. The formation of alloy phases could also be determined by conventional X-ray diffraction methods. These characterizations showed unambiguously the conversion of separate Pt and TiO, dispersed phases into Pt-Ti alloy phases at progressively higher heat treatment temperatures, eventually resulting in the formation of crystallites of the ordered (intermetallic) phase Pt3Ti. On the basis of this observation and other related work in the literature, we present a model for the chemistry occurring during the formation of the SMSI state of Pt/TiO, catalysts.
Introduction We present the observation of Pt-Ti alloy formation when Pt on carbon black is impregnated with T ~ Qand heated in an inert atmosphere. This observation is of particular interest in its implications for the chemistry occurring in the strong metal-support interaction (SMSI) between TiOz and the group VI11 metals. In the seminal SMSI paper by Tauster,
Fung, and Garten,’ three different mechanisms were suggested for the SMSI effect: Pt encapsulation by the support, poisoning by impurities, and alloy formation. Alloy formation was shown by Tauster et al. to be at least thermodynamically possible under the high-temperature reduction treatment used to produce the (1) Tauster, S.; Fung, S.; Garten, R. J . Am. Chem. Sor. 1978, 100, 170.
0022-3654/86/2090-6811$01.50/00 1986 American Chemical Society
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Beard and Ross
The Journal of Physical Chemistry, Vol. 90, No. 26, 1986
SMSI state. Following Brewer,2 they suggested that d-orbital bonding between the metals would be expected to change the adsorption properties of the Pt atoms. Horsley attempted to quantify the intermetallic bonding with SCF-Xa calculations of bonding of Pt to Ti3+through oxygen va~ancies.~Experimentally, however, it has not been possible to determine whether in fact alloy formation occurs during the high-temperature reduction of Pt dispersed on Ti02. Both X-ray photoelectron spectroscopy (XPS) and extended X-ray adsorption fine structure spectroscopy (EXAFS) are capable, in principle, of determining whether an alloy state forms, but when applied to the Pt/Ti02 system there arise fundamental problems in detectability. In forming a Pt-Ti alloy, the largest changes in chemistry occur for the Ti atoms, e.g., from +4 with all oxygen nearest neighbors to near zerovalent with all Pt nearest neighbors, and chemical shifts in Ti core levels and/or Ti K-edge EXAFS shifts are correspondingly large for these changes in the state of Ti. But the fraction of all Ti atoms present in the catalyst undergoing this change in chemistry is extremely small (about 1OOO:l for 1% Pt on Ti02), and the signal from the atoms undergoing the change either is too low to detect (by Ti(2p) XPS) or is lost in the background of the signal from the majority of the Ti atoms present (in Ti K-edge EXAFS). The Pt core level shift for the alloy state is small,4 and Ti is a weak scatterer of electrons relative to Pt, so that Pt L-edge EXAFS is dominated by Pt-Pt coordination. However, these problems in detectability can be circumvented by studying the chemical interaction between Pt and T i 0 2 phases in an “inert” matrix. Here, the matrix is inert in the Sense that it does not react directly with either phase itself. The changes in Ti chemistry are then easily followed, but there is a loss in direct applicability to the real Pt on TiOz catalyst system. We have chosen to use carbon black as the inert matrix for this study rather than one of a variety of non-SMSI oxidess one might have chosen for this purpose. Many of the possible oxides do react with TiOz to form ternary oxides, and Pt is more mobile on carbon black than on oxides, thereby enhancing the rate of the interphase reaction under study. The chemical state of Ti and the identification of phases present were made by using a combination of XPS, EXAFS, and conventional X-ray powder diffraction (XRD). These characterizations showed unambiguously the conversion of separate Pt and T i 0 2 dispersed phases into Pt-Ti alloy phases at progressively higher heat treatment temperatures, eventually resulting in the formation of crystallites of the ordered (intermetallic) phase Pt3Ti. On the basis of this observation, together with the conclusions of numerous recent reports of encapsulation of Pt supported on TiOz, we present a model for the chemistry occurring during the formation of the SMSI state. Preparation
All catalysts were prepared starting from a commercially available carbon-supported R catalyst obtained from the Prototech Co. (Newton Highlands, MA). The Pt loading of the material used was 10% by weight with a narrow particle size distribution of 15-30 A. T i 0 2was impregnated into the catalyst from aqueous solution by hydrolysis of TiC14. High-purity (3 N) liquid pure TiC14was used as received from Alfa. A series of catalysts were prepared by addition of a calibrated volume of TiC14to an agitated 1:l methanol/water solution containing the Pt catalyst in suspension. The volume of T i c 4 added was sufficient to yield about 1.5 wt % Ti in the final catalyst or about twice the stoichiometric amount for complete conversion of all Pt to Pt,Ti. The catalyst was brought to dryness in an ultrasonic mixer. Large chunks of catalyst were then ground with a mortar and pestle to obtain a fluffy fine powder we designate here as the as-prepared catalyst. Subsequent heat treatments were all run for 2 h under flowing high-purity He. The 700 and 900 OC heatings were done in a Pt boat, while the 1200 O C heating was performed in a graphite (2) Brewer, L. In Phase Stability in Metals and Alloys; Rudman, P., Stringer, J., Jaffee, R.,Eds.; McGraw-Hill: New York, 1967; pp 39-61. (3) Horsley, J. J. Am. Chem. SOC.1979, 101, 2870. (4) Derry, G.; Ross,P. Solid State Commun. 1984, 52, 1 5 1 . (5) Tauster, S . ; Fung, S . J . Catal. 1978, 55, 29.
TABLE I: Binding Energies‘ for Pt and Ti in Standard Compounds and Chemical Shifts Pt(4f,/,) Ti(2~,i,) ABE Pt foil Pt3Ti Ti foil Ti0 Tic TiPt, TiO, TiC13
rep +0.3
70.9 71.2 453.8 454.6 454.7 455.1 458.5 459.4
refb +0.8 +0.9 +1.3 +4.7 +5.6
“Binding energy scale referenced to Au(4f7,*) at 83.8 eV. Reference state for chemical shifts.
boat. The samples were cooled to ambient temperature in flowing helium and stored in nitrogen-purged sealed containers prior to analysis. In mounting the powders for the various analytical instruments, the samples were exposed to air, and in the case of X-ray diffraction analysis the samples were continuously exposed to air. Instrumentation
X-ray photoelectron spectroscopy (XPS) was performed in a Physical Electronics 548 Auger/ESCA system. The spectrometer uses a Mg Ka (1253.6 eV) X-ray source and a double-pass cylindrical mirror analyzer. The system is ion pumped with a base pressure following bake out of 4 X Pa. Specimens were introduced into the analysis chamber by a rapid introduction probe. Binding energies were referenced to 83.8 eV for the Au 4f,,, photoelectron peak. For analysis the catalyst specimens were mixed with Teflon powder (10 wt %) as a binder and then pressed into a 5-mm hole drilled through a 2 cm X 1 cm X 1 mm aluminum holder. The aluminum holder was then mounted onto the probe tip. The data collection and analysis system used for XPS have been described elsewhere.6 X-ray diffraction (XRD) was performed using Cu K a radiation in a Siemens D-500 diffractometer. Powder specimens were held in a 2.5 cm X 2.5 cm X 1 mm trough cut into the surface of a Lucite block. Powders were pressed into the trough with a glass slide to obtain an even, smooth distribution of the powder. X-ray fluorescence (XRF) spectra were collected with a Tracor X-ray Spectrace 4020. An Intel 8086 microprocessor controls data acquisition and analysis. A rhodium x-ray source and a Si (Li) detector are used for excitation and collection of the fluorescence data. Extended X-ray absorption fine structure (EXAFS) experiments were performed on the new beam line VI at SSRL. The experiments were performed with beam conditions of 3 GeV and 60 mA. The powders were mounted on a porous graphite substrate again with Teflon as a binder. The EXAFS specimen holder and fluorescence detector were obtained from F. W. Lytle.’ Fluorescence detection was chosen owing to the superior S / N capability in the analysis of metals in the highly dispersed state.8 Data evaluation was performed using software provided by Sand~trom.~ Results
XPS Analysis. X-ray photoelectron spectroscopy was-employed to identify the chemical state of Ti as a function of heat treatment. Titanium (2p), platinum (4f), carbon (Is), and oxygen (1s) photoelectron spectra were collected from each specimen. Survey scans were also collected to observe the presence of any contaminants. Bulk polycrystalline Pt3Ti was analyzed to obtain standard Ti(2p) and Pt(4f) binding energies for the alloyed state, while (6) Beard, B.; Dahlgren, D.; Ross,P. J . Vac. Sci. Technol., A , in press. (7) Lytle, F.; Greegor, R. B.; Sandstrom, D. R.;Marques, E. C.; Wong, J.; Spiro, C. L.;Hoffman, G. P.; Huggins, F. E. Nucl. Instrum. Methods Phys. Res., Sect. A 1984, 226, 542. (8) Hastings, J. In EXAFS Spectroscopy, Techniques and Applications; Teo, B. K., Joy, D. C., Eds.; Plenum: New York, 1981; Chapter 12. (9) Sandstrom, D. R. Nuouo Cimento SOC.Ital. Fis., D 1984, 3, 825.
The Journal of Physical Chemistry, Vol. 90, No. 26, 1986 6813
Pt-Ti Alloy Formation
1200~C
86 0 900°C
TABLE III: XRD Analysis of TiO,-Impregnated Pt on Carbon Catalyst Following Heat Treatment to Higher Temperature
catalyst as-prepared
As prepared
700 O
C
900 “C 450.5
455.1
1200 oc
441
Binding energy (eV)
Figure 1. XPS spectra from the Ti(2p) region as a function of heat
treatment temperature. TABLE II: Integrated XPS Intensity Ratios of Ti(2p3,*) to Pt(4f74 Signals for Heat-Treated Catalysts
chemical state of Ti as TiO, as ‘alloy” total Ti
as-prepared 1.89 0
1.89
700 OC 0.49 0.09 0.58
900 O C 0.48 0.22 0.70
61 4
Figure 2. XPS spectra from the Pt(4f) region of standard Pt on carbon catalyst after air exposure.
700°C
)
Binding energy
1200 OC 0.47 0.55
1.02
the Ti(2p) and Pt(4f) binding energies for TiOz and metallic Pt were obtained from oxidized Ti foil and Pt foil, respectively. Binding energies of T i c , TiO, and TiC13were obtained by analysis of these powders mixed with graphite. A summary of the binding energies for these standard compounds is given in Table I. Titanium spectra from the catalysts are shown in Figure 1. With increasing temperature the peak characteristic of intermetallic Ti (ca. 455 eV) grew dramatically. Titanium to platinum atomic ratios calculated from fitted XPS spectra quantitatively represent the changing form of the titanium (Table 11). As temperature increased, the intermetallic Ti to Pt ratio rose to a level equal to that of the TiOz to Pt ratio, as expected, since twice the stoichiometric amount of Ti for formation of Pt3Ti was used in the preparation. A more surprising observation is the apparent “loss” of total Ti on going from the as-prepared to the 700 O C specimen. We know from the X R F elemental analysis that Ti is not lost in heat treatment. The apparent “loss” indicated by XPS data can be explained in terms of the relative escape depths of the Ti and Pt photoelectrons, making cerain assumptions about the distribution of T i 0 2 in the catalyst matrix following impregnation. The Ti(2p) photoelectrons have lower kinetic energy, and thus shallower escape depth, than the Pt(4f) photoelectrons. If it is assumed that impregnated T i 0 2 is deposited preferentially on the “exterior” of carbon aggregates (containing Pt uniformly dispersed), then the apparent “loss” can be rationalized in terms of Ti diffusion into the “interior” of carbon aggregates and resultant attentuation of Ti(2p) photoemission (relative to the uniformly distributed Pt). In this context, “interior” and “exterior” refer to geometries relative to electron energy analysis, i.e., as “seen” by the spectrometer. The platinum spectra had small but reproducibly observable changes in response to the heat treatments. In all cases, deconvolution of the Pt(4f7/,) spectra indicated the presence of a minor Pt state with a binding energy ca. 2.5 eV higher than the metallic peaks, as indicated by Figure 2. While we have not assigned this
lattice parameter, A 3.926 3.916 3.907 3.906
particle size, A 48 110 125 144
superlattice no no Yes Yes
state to a specific Pt compound, it is clear that this state is not related to the formation of the intermetallic phase. The Pt(4f7,~) binding energies exhibited small ( 3 x 105 cop02 > lo6 3Ti02 + CO T i 3 0 5+ C 0 2 H2/H20 > 3 X l o 5 TiOl H2 T i 0 H20
+
--
+ +
to 1200 OC under helium. Even at temperatures as low as 700 OC, some alloying was detected. It is interesting to note the absence of Tic as an intermediate phase forming in the reduction of Ti02. While this reaction might be expected intuitively, the reduction of TiO, by carbon in an inert atmosphere is not thermodynamically favorable. Using the JANAF Thermochemical Tables,26 one finds that considering the equilibria TiOz + 2C = T i c + CO,, AGO(llOO K) = +192.5 kJ/mol Tic
+ O2 = TiO, + CO,,
\\\\\\
Pt
A
HZ
"TiO"
"TiO"
n n
\\\\\\\' \ \ \ ' Pt3Ti
AGo(llOO K) = -983.2kJ/mol
the equilibrium partial pressures of oxygen and/or COz for carbide formation are impractically low, considering that the carbon black we are using contains ca. 5 atom % oxygen and the inert gas at least a few ppm oxygen. If the equilibria are changed to reflect the formation of Pt3Ti as the reduced state instead of T i c , the equilibrium pressures for oxygen and/or Cot become more realizable, since the free energy of formation of Pt3Ti is ca. 100 kJ/mol more negative than for T i c . In practice, the actual reducing reactions are not known, but considering that the carbon black we are using releases significant quantities of hydrogen and carbon monoxide when heated in an inert atmosphere to 1100 K,27 the most probable reactions are T i 0 2 + 2H2 + 3Pt = Pt3Ti + 2 H 2 0 , AGo(l 100 K) = +68.2 kJ/mol
TiO,
TiOZ I 'V2
+ 2CO + 3Pt = Pt3Ti + 2C02,
AGO( 1100 K) =
+61.9 kJ/mol where the equilibrium favors the formation of intermetallic if CO/COz > 41:l and/or H2/H20> 60:l (at 1100 K). Other possible reduction products would be suboxides of Ti02, for example, T i 0 and Ti305. The thermodynamic favorability of formation of these solid phases may be compared to that for alloy formation by calculation of the reducing atmosphere partial pressure ratios required for their formation (Table IV). Compared to the thermodynamics for the formation of the intermetallic, the reduction of the Ti02 to a suboxide is much less favorable. It is quite likely that the partial reduction of the TiO, will create small concentrations of various "suboxides" as intermediates; however, the energetics favor complete reduction and alloying to Pt3Ti given sufficient temperature and time. Horsley's suggested model of Pt-(TiO# bonding could very well be the first step in the reduction of the TiO2, as indicated by the XPS data of FungI4 for hydrogen-reduced Pt/TiO,. The results of this study provide definitive evidence that alloy formation occurs when Pt and TiO, are in contact and heated in a reducing atmosphere. It is certain that alloy formation will occur in various Pt/Ti02 phase configurations provided the atmosphere is sufficiently reducing and the temperature is high enough. Tauster et al. have already shown, by use of the same thermochemical calculations as made above, that alloy formation is at least thermodynamically favored at the standard SMSI reduction conditions. The temperatures at which we observed the onset of alloy formation were approximately 200 OC higher than the usual SMSI reduction temperature (500 "C). We suggest that this higher temperature is due to the fact that impregnation of T i 0 2 in the Pt/carbon does not necessarily result in physical contact between all the Pt and T i 0 2crystallites, and a higher temperature is necessary to promote Pt migration to "find" the TiO,. Given that the thermochemical calculations indicate alloy formation is thermodynamically favored at 500 OC and that alloy formation has been observed directly at 700 OC (and higher), it seems reasonable to reconsider the possible role of alloy formation in SMSI chemistry. We suggest here a modified picture of SMSI
Figure 6. Proposed model for chemistry occurring in the formation of the SMSI state for (a) Pt thin films on T i 0 2 substrates, (b) T i 0 2 thin films on Pt, and (c) dispersed Pt on TiO, powder.
chemistry that incorporates alloy formation and that is still consistent both with the characteristics of the SMSI state' and with recent results interpreted at "encapsulation" of Pt by Ti0,.17-22 In Figure 6a, we depict a conceptual model of Pt encapsulation occurring when a thin Pt film deposited on Ti02 is annealed in ultrahigh vacuum at 500 OC, e.g., from the report by Belton et al.,, that TiO, ( x 1) moieties are observed (by electron spectroscopy) "on top of" the Pt after annealing. Supporters of the encapsulation concept usually invoke surface diffusion of TiO, moieties on the Pt surface as the mechanism of the encapsulation. We suggest a refinement of this concept, in which surface diffusion of TiO, (if it really occurs) is concomitant with Ti dissolution into the Pt film to form the alloy. Figure 6b depicts the dissolution process derived from our previous study of T i 0 2 overlayer reduction and Ti atom dissolution into Pt.Is Reduction of a Ti02 multilayer was found to proceed via reduction to a suboxide (TiOl,5)multilayer accompanied by Ti dissolution into the Pt substrate and then further reduction of the suboxide to a discontinuous (island) monolayer of nominal composition "TiO". The "TiO" islands were very stable to any further reduction and appeared to be an equilibrium configuration, possibly representative of an equilibrium between oxygen on the surface and oxygen in the bulk, as suggested earlier by Spencer2*and KO and G ~ r t e . , ~It is, therefore, not necessary to invoke surface diffusion of TiO, moieties on Pt to explain the appearance in A B spectra of TiO, "on top of" Pt. Rather, "TiO" surface segregation is concomitant with alloy formation from the reduction of TiO, (17) Meriandeau, P.; Dutel, J.; Dufaux, M.; Naccache, C. In MefulSupport and Metal-Additive Effecrs in Catalysis; Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1982; p 95 ff. (18) Santos, J.; Phillips, J.; Dumesic, J. J . Curd. 1983, 82, 147. (19) Resasco, D.; Haller, G. J . Coral. 1983, 82, 279. (20) Sadeghi, H.; Henrich, V. J . Cural. 1984, 87, 279. (21) Chung, Y.; Takatani, S. J. Carol. 1984, 90, 75. (22) Belton, D.; Sun,Y.; White, J. J . Phys. Chem. 1984, 88, 5172. (23) Belton, D.; Sun, Y.; White, J. J . Phys. Chem. 1984, 88, 1690. (24) Tanaka, K.; White, J. J . Coral. 1983, 79, 81. (25) Baker, R.; Prestridge, E.; McVicker, G. J . Cural. 1984, 89, 422. Baker, R.; Prestridge, E.; Garten, R. J. Curd. 1979, 59, 253. (26) Stull, D.; Prophet, H. JANAF Thermochemicul Tables, 2nd ed.; National Bureau of Standards: Washington, DC, 1971; NSRDS-NBS37. (27) Redmond, J.; Walker, P. Nurure (London) 1960, 186, 72; J . Phys. Chem. 1960,154. 1093. (28) Spencer, M. J . Curd 1985, 93, 216. (29) KO,C.; Gorte, R. J . Card 1984, 90, 59. (30) In this paper the periodic group notation in parentheses is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transitionelements comprise groups 3 through 12, and the pblcck elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e.g., I11 3 and 13.)
-
J . Phys. Chem. 1986, 90,6817-6821 by Pt. Figure 6c depicts our suggested chemistry for the SMSI state for Pt crystallites dispersed on a T i 0 2 support embodying the foregoing concepts. T i 0 2 reduction occurs locally around Pt crystallites simultaneously with alloy formation; the alloy phase becomes partially imbedded in a TiO, ( x = 1) matrix as a consequence of the counterdiffusion of Pt and Ti atoms that occur with these simultaneous processes. The imbedding of the alloy phase causes the Pt/Ti surface ratio to decrease and “ItO” character to appear at the surface, phenomena which give the appearance in AES analysis that there has been migration of “TiO” moieties onto the Pt surface. The extent to which the alloy becomes totally imbedded (“encapsulated”) depends on the temperature and time of reduction. The metal surface that is left exposed is the Pt3Ti alloy surface and not a pure Pt surface. The adsorptive properties of the exposed alloy surface for CO are consistent with the C O adsorption results of White and cow o r k e r ~ ~for ~ - Pt * ~films on T i 0 2 and the effect of thermal annealing on C O bonding. We found in our study of C O chemisorption on Pt3TiI6that CO is less strongly bound on Pt3Ti than on Pt, with a TDS desorption peak temperature for Pt3Ti 40-50 OC lower than for pure Pt. A similar shift in TDS peak temperature of ca. 40 OC was reported by Belton et al.22Tanaka and White24investigated CO stretching frequencies for CO adsorbed on Pt/Ti02 catalysts. Upon high-temperature reduction, they observed a preferential loss of a CO stretching frequency they attributed to C O bound at step sites on Pt. We suggested this shift could also be interpreted as loss of CO bound at Pt-Pt bridge sites due to the formation of Pt3Ti, since we have shownI6 that the structure of the surface of Pt3Ti is such that Pt-Pt bridge
6817
bonding sites are missing. The model in Figure 6c also accounts for the observation of an overall decrease in the volume of CO adsorbed in the SMSI state, since the exposed area of Pt3Ti is much lower than the original Pt area. The model is consistent with the changes in the Pt crystallite morphology observed by Baker et al.25 with electron microscopy of Pt/Ti02 catalysts subjected to high-temperature reduction. They reported the apparent “spreading” of Pt into thin “pillbox” structures, which we suggest is the formation of the alloy phases concurrent with local T i 0 2 reduction; i.e., “spreading” of Pt is actually Pt-Ti interdiffusion in alloy formation. As we have shown in our study of the oxidation of Pt3Ti,I5 heating Pt,Ti in oxygen will cause the formation of separate Pt and T i 0 2 phases in contact, Le., heating the systems on the right-hand side of Figure 6 restores the original Pt/Ti02 phases, consistent with the reversibility of the SMSI state. We do not suggest that Figure 6c depicts the only SMSI state, but rather it is the thermodynamically favored state if the reduction conditions of the starting Pt/Ti02 are sufficiently strong. Intermediate states consistent with the chemistry illustrated in Figure 6 are easily envisioned, where the basic reactions occurring to form the alloy start but the reduction process is incomplete; e.g., Horsley’s model3 of Pt atoms bonded to Ti3+ centers is consistent with this chemistry.
Acknowledgment. This work was supported by the Assistant Secretary for Fossil Energy, Office of Fuel Cells, Advanced Concepts Division of the U S . Department of Energy, under Contract No. DE-AC03-76SF00098. Registry No. Pt, 7440-06-4; Ti02, 13463-67-7; C, 7440-44-0.
Vapor Pressure Measurements of an 011 In Water Microemulsion System John L. Cavalla* and Henri L. Rosano Department of Chemistry, City College, City University of New York, New York, New York 10031 (Received: March 4, 1986; In Final Form: August 14, 1986)
The vapor pressure of an oil in water microemulsion system consisting of octane/sodium cetyl sulfate (SCS)/5% NaCl + 0.01 N NaOH (pH 11.2) titrated to clarity with dodecyldimethylamine oxide (DDAO) was determined as a function of increasing volume of hydrocarbon in the dispersed phase. At low volume fractions of hydrocarbon, the microemulsion droplets behave like large molecules in solution. The size and molecular weights of these aggregates were found to be in agreement with particle size measurements determined by photon correlation spectroscopy. Beyond a certain hydrocarbon volume fraction, the droplets are less stable. The energy of activation corresponding to the breaking and reforming of the interfacial sheath surrounding each droplet was found to be 35.2 kJ/mol. The interfacial bending energy of the surfactant/ol sheath surrounding the individual droplets was calculated to be 10-ll-lO-’o erg. Larger volumes of octane in the microemulsion formulation appear to increase the interfacial film flexibility; droplets then start to break and re-form simultaneously. Eventually, a bicontinuous structure or phase separation results.
1. Introduction
Mixtures of oil and water in the presence of surface-active agents usually form coarse emulsions that are optically opaque, or nearly so, which usually separate on standing. In some cases, transparent mixtures of oil and water can be prepared when the initial emulsion is titrated to clarity with a cosurfactant.Ia The free energy, enthalpy, and entropy changes accompanying microemulsion formation have been determined.’J Relatively small negative free energy changes may explain why the method of preparation affects the final result.’ The importance of the method of preparation had previously led us to conclude that some of these systems were kinetically table;^ nevertheless, our latest results lead us to conclude that these systems are essentially swollen micellar solutions that are thermodynamically stable’ and that ‘hesent address: &tee Lauder, 125 Pinelawn Road, Melville, NY 11747.
energy barriers must be overcome in order to obtain spontaneous clearing. The proper method of adding the ingredients is responsible for a transitory low interfacial tension due to diffusion of the cosurfactant across the oil in water (o/w) interface, during which time maximum production of interface is possible. It has always been assumed that microemulsions are composed of discrete spherical particles.2 WeatherfordShas shown, using (1) Rosano, H. L.; Cavallo, J. L.; Lyons, G. Presented at the 5th International Conference on Surface and Colloid Science, Potsdam, NY, June 25-28, 1985. (2) Schulman, J. H. Nature (London) 1943, 152, 102. (3) Rosano, H. L.; Lyons, G . B. J . Phys. Chem. 1985,89, 363. (4) Rosano, H.L.; Lan, T.; Weiss, A.; Gerbacia, W. E. F.;Whittam, J. H.J . Colloid Interface Sci. 1979, 72, 233. ( 5 ) Weatherford, W. D.J. Dispersion Sci. Technol. 1985, 6, 467. (6) Rosano, H.L.J. Cosmetic Chem. 1974, 25, 609419.
0022-3654/86/2090-68 17%01.50/0 0 1986 American Chemical Society
