In situ tem and iss studies of supported metal ‘SMSI’ catalysts

Journal of Molecular Catalysis, 20 (1983) 235 - 249

235

IN S1Z’U TEM AND ISS STUD~S OF SUPPOSED CATALYSTS

MICHAEL J. KELLEY*, Ex~er~~e~ta~ Station, (U.S.A.)

METAL ‘SMSI’

DAVID R. SHORT* and DENNIS G. SWARTZFAGERt

E. 1. du Pont de Nemours

and Co. Inc., Wilmington, DE 19898

Summary Supporting catalytic metals on reducible oxides may give rise to a special state termed ‘strong metal-support interaction’ (SMSI) after severe reduction. It is characterized by sharply reduced ~hem~o~tion, enhanced thermal sintering resistance and, in at least some instances, altered metal particle morphology, Proposed causes include electron donation from the support to the metal and en~ro~hment of the support over the metal We have sought evidence for its cause, applying ion beam surface analysis and in sihc transmission electron microscopy (TEM) combined with electron energy loss spectroscopy (EELS) to Pt:TiO*. The support was prepared by hydrolyzing titanium isopropoxide; the Pt was applied by ion exchange from the t&ran-&e hydroxide. Specimens were reduced from 473 K to 873 K in flowing, purified E-I2 and examined without intervening air exposure; some were also reduced in a special reaction cell on the TEM. Results of the ion beam surface analysis contradict the notion of support encroachment over the metal or metal migration into the support. However, in the SMSI state Pt becomes less ‘visible’. Both in situ and ex sites TEM showed no change of particle size or morphology accompanying SMSI. The EELS showed a decrease in the ratio of the titanium L3 edge to the Lz at the onset of SMSI. This result and the Pt ‘visibility’ change point to underlying electron structure changes, which presumedly are also responsible for the loss of chemisorption capacity. Further work is needed to elucidate just what these changes might be.

Introduction It is now well established that reduction temperature profoundly affects the chemisorption capacity of metals supported on certain oxides. For example, Pt:TiOz reduced at 200 “C adsorbs H2 or CO much as Pt on *Engineering Technology 0304”5102/83/$3.00

Laboratory ; *Central Research Department. @ Elsevier Sequoia/Printed in The Netherlands

other supports, but when reduced above about 400 “C, chemisorption is essentially absent though it can be largely restored by oxygen exposure [ l] . The effect has gained the name ‘SMSI’ strong metal-support interaction. So far as we are aware, none of the many investigation of Pt:TiOz has contradicted this general pattern, though differences as to details have been reported; what is at issue is the underlying physical mechanism(s). Reduction of at least the surface of the TiO? is believed to play a key role. Transmission electron microscopy (TEN) showed that TiOz is reduced to Ti407 in the presence of Pt under conditions favorable to SMSI [2, 31 accompanied by a change in the Pt particle morphology. Electron spin resonance (ESR) showed Ti3+ formation when Pt:TiOz was exposed to Hz and that the Ti3+ signal was not removed by subsequent evacuation if the Hz exposure was in the SMSI temperature range [4, 51. Recent studies using X-ray pho~ele~~on spectroscopy (XPS) are in conflict as to whether Ti4+ is reduced to Ti3+ under SMSI conditions [6 - 81. However, Pt:Ti103 did not chemisorb Hz or CO [ 81, lending additional support to the notion that SMSI is associated with reduction of the TiOz support. It is not evident why the probable reduction of the TiOZ surface under SMSI conditions should affect the chemisorption behavior of platinum. X-SW-SCF molecular orbital calculations applied to a Pt:TiOz model cluster indicated that the electron density gained by Ti as a consequence of reduction is transferred to the platinum [ 91. SMSI was thus held to be a consequence of direct cation-cation bonding, leading to increased electron density on Pt, reducing its chemisorption capacity [9]. XPS studies detected binding energy shifts which were interpreted as showing the 0.6 electron transfer predicted by the calculations [6, lo] . No shift was detected for large (100 A) Pt particles, though they still seemed to exhibit SMSI [6]. All shifts required considerable adjustment for particle size changes which also accompanied the reduction treatments [6, 71. A more recent XPS study detected no change and also showed that Pt does not diffuse into the TiOz under SMSI conditions [ 81. An X-ray absorption spectroscopy investigation of the Pt L3 and L2 edges concluded from analysis of the extended fine s~cture (EXAFS) that no change in Pt-Pt bond length occurs with the onset of SMSI behavior [ll] . Analysis of the edges themselves led to the additional conclusion that no net change in Pt electron density in excess of 0.05 electrons takes place. Therefore, SMSI does not seem to be associated with either net electron transfer or conversion of the Pt into some welldefined compound. A more subtle electron structure change involving little net transfer is not excluded, however. Experimental Materials The TiOZ was prepared by hydrolyzing titanium isopropoxide (Tyzor; Du Pont). Water-10% isopropanol was added drop-wise to 50% titanium iso-

237

propoxide-50% isopropanol at room temperature while stirring vigorously. After standing overnight, the precipitate was washed repeatedly, dried for 24 h at 120 “C and calcined in air at 400 “C. X-Ray diffraction patterns showed only anatase. The Pt was applied by ion exchange. The TiOz was stirred for 50 h in a platinum t&amine nitrate solution at pH 5.5. The pH was raised to 10 by ammonium hydroxide addition and stirring continued for another 24 h. The TiOz was filtered off, dried overnight at 120 “C and calcined in flowing O2 for 2 h at 350 “C. Material for adsorption measurements and electron microscopy received no further treatments. The Pt loading was determined by ICP (Inductively Coupled Plasma Emission Spectroscopy) to be 1.06 wt.%. Material for ion scattering spectroscopy was reduced for 2 h at 600,400 or 200 “C and cooled in flowing hydrogen which had been purified by diffusion through a palladium membrane. After the treatment, the reduction vessel was sealed off and unloaded in a Nz dry box, where the Hz0 and O2 partial pressures were continuously monitored. The powder was pressed into indium foil, mounted into the specimen holder, transported in a Nz filled container and loaded into the spectrometer with the aid of a Nz glove bag. Adsorption Total surface areas were measured by conventional Nz BET; typical reproducibility in our system was +5% over this range of values. Metal surface areas were determined by Hz/O2 titration in a flow system using established procedures. Our typical reproducibility for highly dispersed Pt in this loading range was + 10%. Ion sea ttering spectroscopy The spectrometer was a modified 3M Corporation Model 525B with a fixed laboratory scattering angle of 137” and used a cylindrical mirror analyzer with an on-axis ion gun. Measurements were performed with a normally incident beam at 2.0 keV ion energies with 20Ne or 4He as the probe ion. The ion beam was rastered over 0.112 cm2 and the scattered ion signal was electronically gated such that only the center portion (0.079 cm2) of the rastered area was observed. Electron microscopy The electron microscope was a modified Philips EM 400 transmission electron microscope (TEM) described previously [ 121. Briefly, the specimen stage had been equipped with a doubly differentially pumped reaction cell permitting normal TEM operation in excess of 100 Torr H2 and 1000 “C. Analytical information was obtained from a computer-operated electron energy loss spectrometer located below the camera [ 131. Energy resolution was cu. 3 eV, limited by the energy spread of the electron beam from the tungsten filament. The calcined material was dispersed ultrasonically in methanol and deposited on to a nickel grid covered with a graphitized carbon film. The

238

grid was mounted in the specimen rod, inserted into the microscope and outgassed for cu. 1 h at 300 “C and 5 X lo-’ Torr before hydrogen was introduced. To make observations, a TiOz particle sufficiently thin to give a good EELS spectrum was photographed and the Ti Ls,s (461 eV, 455 eV) and 0 K (532 eV) edges recorded. The Hz pressure was then raised to 100 Torr at a flow rate of 200 cm3 min- ’ (S.T.P.). After 30 min, the particle was photographed and an EELS spectrum recorded. While maintaining H2 flow and pressure, this sequence was repeated after successive temperature increases to 300, 400 and 500 ‘C, all for the same Ti02 particle. A total of six particles on three grids were thus characterized. No differences were found between them so that the results are believed to be meaningful even though not statistically well established. Electron microscope images were also obtained for the ex situ reduced material used in the ISS experiments.

Results Adsorption Table 1 presents the adsorption results. The small apparent change when the reduction temperature was raised from 200 “C to 400 “C is less than the reproducibility of the measurement and is best understood as no change. The substantial decrease after 600 “C reduction is expected since the TiOz was originally calcined to only 400 “C. The metal dispersion after 200 “C reduction was near unity, indicating small particles or monolayer rafts. No titration runs were made for material reduced at higher temperature, since the titration procedure is expected to eliminate the SMSI effect. Previous work has established that Pt:TiOz prepared in this way adsorbs essentially no Hz or CO after reduction at 400 “C or higher [ 111. TABLE 1 Adsorption results Reduction temperature W)

Hz/O* titration

200 400 600

1.1 H/Pt None None

BET area (m2K1)

Pt size by TEM

118 115 46

1 1 2.5

(nm)

Ion scattering spectroscopy ISS detects the number of ions back-scattered at various energies when a surface is bombarded with monoenergetic noble gas ions such as 4He+ or *ONe? The very high probability that these ions will be neutralized during any scattering event makes the technique sensitive to the first monolayer of atoms only. The dominant scattering events are adequately described as

239

simple binary elastic collisions so that the energy distribution of the backscattered ions reflects the distribution of atomic masses (and therefore elements) in the first monolayer. The intensity of scattering for any given element depends on the neutralization probability and the scattering crosssection for that particular combination of ion and element. Values of these parameters vary substantially among the elements, but the intensity of a given spectral peak is always proportional to element concentration, though the proportionality constant is different for each element. The details of ISS have been described [ 141; its use for analyzing metal surfaces compared to the more familiar electron spectroscopies has been reviewed [15] and its application to highly dispersed supported Pt catalysts has been reported

[=I. Figure 1 shows the spectrum obtained from He scattering after 200 “C reduction; the scattered intensity is an arbitrary but linear scale. Note that detecting oxygen required the use of He rather than Ne, since elements lighter than the bombarding ion cannot be detected. In addition to the peaks identified in the caption, there is a small feature at approximately E/E, = 48 due to He scattering from residual Ne in the system. The next four figures present another kind of data: sputter profiles. In each case, the surface was analyzed by obtaining a spectrum such as Fig. 1, bombarded heavily and another spectrum obtained. The location of each peak was redetermined and its intensity measured each time a spectrum was taken. These intensities are plotted uersus the total ion dose preceding the spectrum. He ions were used for Figs. 2 and 3 so that all three elements (Pt, Ti, 0) could be monitored; Figs. 4 and 5 were obtained using Ne to better measure the Pt peak intensity. In displaying these data we have resisted-the temptation to apply smoothing procedures to reduce the scatter; we believe that the scatter itself is a significant result since it permits readers to judge

Fig. 1. 4Hei. scattering from Pt:TiOz, Hz-reduced 2 h at 200 “C.

240

.w

4e.m

24.W

72.W

‘.W

ION DOSE / Ct12x 10 -I5

Fig. 2. 4He+ sputter profiles of ‘Pi(X) and O(0) after 2 h Hz reductions at 200 “C (lower set) and 400 “C (upper set).

0.08

24.08

ION DOSE / CM2 x IO -15

Fig. 3. 4He+ sputter profiles of Ti(0) and Pt(X) after 2 h Hz reductions at 200 “C (lower set) and 400 “C (upper set).

241

f

B s T

14.28

7.82

0.w

21.88

28.88

ION DOSE / Cf12x 10 -I5

profiles Fig. 4. 20Ne+sputter set) and 400 “C!(upper set).

of Ti(0) and Pt(X) after 2 h Hz reductions

(2.6 X Scale)

at 200 “C (lower

Q

QQ

QQ QQQQQ Q

Q

Q

ION DOSE / CM2 x IO-"

Fig. 5. 20Ne+sputter (bottom).

profiles

of Pt after 2 h Hz reductions

at 200 “C (top) and 600 “C

242

the quality of the data upon which conclusions are based. The vertical scales are again linear, but not arbitrary. Each element profile has been adjusted so that its maximum is full scale on the plot, permitting comparison of the different shapes. The numerical values of these maxima were used to obtain the ratios in Table 2. TABLE 2 ISS maximum peak intensity ratio.?

O/Ti Pt/Ti pt/Ti

Bombarding ion

Reduction at 200 “C, 2 h

Reduction at 400 “C, 2 h

Intensity ratio 400 “C/200 “C

4 He’ 4Het 20Net

0.439 0.0551 0.338

0.434 0.0335 0.183

0.99 0.608 0.540

*These values are the ratios of the intensity maximum for the E/E0 peak corresponding to the given element on the sputter profiles.

In Table 2, surface compositions are described as the ratio of the actual full-scale intensities. The ratio of 0 to Ti is the same within experimental uncertainty for 200 “C and 400 “C reduction, but it falls more rapidly upon sputtering after 400 “C reduction (Fig. 2). This indicates that the near-surface titania contains less oxygen, i.e. is somewhat reduced, in the SMSI state. The Pt/Ti ratio decreases by about 40% by He scattering or 45% by Ne scattering going from 200 “C! to 400 “C reduction. The Ne results are probably the better measure. Since the O/Ti ratio is the same for both reductions, this really represents less Pt visible, though not necessarily less Pt present as discussed later. The sputtering behavior after the two reductions is also different. For He scattering, 200 “C reduction gives a well-defined peak which falls off rather rapidly with little scatter in the data, typical of small metal particles (Fig. 3). After 400 “C reduction, the Pt sputter profile rises at about the same rate, reaching 75% of its maximum after five sputter cycles. However, it shows no well-defined peak and gives considerably more scatter in the data. The increase in scatter is proportionately greater than the decrease in Pt signal. The results from Ne bombardment show the same trends, but less pronounced (Fig. 4). Particularly the effect of reduction temperature on the difference between the rate decrease of the Pt and Ti signals is seen to be substantially less. Ne sputter profiles after 200 and 600 “C reduction show a flatter peak and slower fall after the higher temperature (Fig. 5). Transmission electron microscopy Figures 6 and 7 show the effect of 400 “C and 600 “C ex-situ reduction; 400 “C ex situ reduction gave the same appearance as in situ reduction at 200 “C and 400 “C. The notable features are the lack of change between the two lower reduction temperatures and the extensive change at the higher

243

Fig. 6. Pt:TiOz after 2 h Hz reduction

at 400 “C.

Fig. 7. Pt:TiOz after 2 h Hz reduction

at 600 “C.

one. The latter is to be expected since the original support calcination temperature was greatly exceeded. The fractional increase in metal particle diameter was about the same as the fractional decrease in support surface area, consistent with all the original metal being consolidated into the larger par-

244

I_

,o Tot-r

LO TOtT

ENERGYLOSS (IO EV

-

80

1

mrr

Fig. 8. Effect of Hz pressure on electron energy loss near the no-loss peak in the absence of any specimen.

400

450 ENERGY Loss

500

550

(EV)

Fig. 9. Electron energy loss at 300 “C under 100 Torr Hz pressure.

titles. It is not clear from these images whether the metal particles are rafts or hemispheres after 600 “C reduction. Figures 8 and 9 present electron energy loss spectra; they and others not shown are summarized in Table 3. In the absence of any specimen (Fig. 8), modest Hz pressure gives a single loss feature centered at 16.5 eV, the energy to convert Hz to H;. The relationship of this feature’s intensity to that of the no-loss peak is described by simple exponential attenuation until a second feature appears at about 31 eV loss. The relationship between the no-loss peak intensity and that of the loss features can be described by the same simple exponential attenuation as at lower pressure only if twice the intensity of the 31 eV feature is added to the one at 15.5 eV. This relationship and its energy suggest that the 31 eV feature represents double scattering by the same process as the 15.5 eV feature. These features are convoluted into all other loss information obtained under Hz.

246 TABLE Electron

3 energy

loss peak heightsa

Treatment

Ti L3

Ti Lz

OK

300 300 400 500

12 74 64 65

77 78 77 77

14 15 14 14

“C/vacuum “C/H? “C/H2 “C/H?

‘In arbitrary

units.

Figure 9 shows the spectral region containing the Ti L, and LZ, and 0 K edges under 100 Torr Hz at 300 “C. Table 3 presents the intensities of the edges after various reduction treatments. The substantial noise was due at least in part to the signal decrease brought about by the 8 mrad limit imposed on the collection angle by the reaction cell differential pumping apertures. No effect of multiple scattering by the Hz is evident. There is, however, a broad feature seen centered about 25 eV above the LZ edge whose amplitude increases with reduction. This could be a multiple scattering feature composed of the Ti core loss and a plasmon. A separate experiment with Pt black showed loss features at 519 and 609 eV; none are evident above background here probably because only 1.06 wt.% Pt was present. Discussion Sputter profiles form an important group of results in the present study and it is worthwhile to begin by reflecting on their meaning. It was once believed (hoped?) that sputter profiles represent nothing more than the layer-by-layer peeling away of the surface to reveal what lies beneath. It is now well agreed that sputtering represents a complex radiation damage process with atomic displacements extending a considerable depth into the material. Generally speaking, the depth of the damage zone increases with the energy and decreases with the mass of the bombarding species and increases with the atomic weight of the target [ 171. In metal alloys at 25 ‘C, this damage zone extends perhaps 4 nm below the surface, making possible sufficient diffusional mixing even at room temperature that the equilibrium concentration profile is destroyed [ 181. We are aware of no careful comparable studies of supported metal catalysts, but caution seems advisable especially at high total ion doses where damage has built up. The 200 “C profile has been included to doses well beyond the fall-off of the initial peak as an illustration. Beyond a total dose of about 20 X 1015 ions cme2 the signal begins to rise rather than fall and more or less levels off at about 50 X 1015 ions cmv2. The comparable profile in Fig. 4 is not carried to sufficiently high doses to show similar features. A sputter damage depth comparable to that in the metal alloys would permit

246

damage-assisted diffusion to proceed from the sides and back of a TiOz particle at high doses. Further careful study is needed to establish if this is in fact the case, but the present results serve to emphasize the need to use caution when interpreting sputter profiles of supported catalysts at doses above that needed to penetrate the original metal particles. Turning now to the profiles themselves, Figs. 2 and 3 were obtained with He bombardment and Figs. 4 and 5 with Ne; the heavier ion is expected to sputter more rapidly. In Fig. 5 consider the 2.6 scale factor between the two profiles and their shapes in the low dose region. The greater intensity and the very rapid rise of the 600 “C reduced sample together are evidence against any kind of overlayer on top of the Pt, even after a treatment so severe as to make the support sinter. If the support were to creep over the metal, it is hard to see how it would not do so under these circumstances. The substantial Pt intensity increase after this severe reduction deserves further comment, especially in view of the 2.5-fold decrease in support surface area. The amount of Pt in the sample is not expected to have been altered by reduction at 600 “C rather than 200 ‘C, so that it must be accommodated on less support surface area after the high temperature reduction. The amount of metal per unit area of the support would change, thereby changing the amount of metal seen when viewing the catalyst surface. The observation that the increase in metal seen was the same as the decrease in support area is evidence that the true metal surface area (m2 of metal per g of metal) was approximately constant. This could come about if the original Pt particles simply moved about as the support sintered so as to become more numerous per unit area of support. The TEM images obtained after 600 “C reduction, however, show twoto three-fold larger particles. For these particles to provide the same Pt area as the original small particles requires that their morphology present the same fraction of atoms exposed on the surface. Particles with any substantial thickness would not do this (e.g. hemispheres). Thin rafts would, however, and their observation after high ~mpera~re reduc~on of Pt:TiOz has been reported previously [ 2,3] . Now consider the details of the sputter profiles in Fig. 5. If the Pt atoms are present as monolayer particles of whatever diameter, removing one Pt atom will not expose another so that the Pt intensity will fall. Since the sputter yield (Pt atoms out per ion in) may be expected to remain constant in the low dose region, a plot of Pt intensity uersus ion dose should be initiahy linear. In contrast, for multilayer particles, removing one Pt atom exposes more P&though Pt at the bottom of the vacancy emnot be expected to have the same ‘visibility’ as Pt in the first monolayer. Moreover, we have already noted that the atoms remaining in the solid are highly mobile under sputtering conditions and it is not clear that the sputter-created vacancy would persist for any substantial time. Even though the atomic details are difficult to describe precisely, it seems clear that particles having multilayer thickness may be expected to show an initial region in a sputter profile where the Pt peak intensity is substantially independent of ion dose, in contrast with the linear decrease expected for monolayers.

247

The Ne profiles in Fig. 5 show just these features. No point is higher than the first after 200 “C reduction, white there is a finite width to the peak top after 600 “C reduction. The region is small enough and the data scatter large enough that it seems unreasonable to attempt fitting a mathematical model, but it is reasonable to conclude that there are some regions of more (but not much more) than monolayer thickness. The data scatter is much more evident in the low total dose profile presented in Fig. 4 for 200 “C reduction than in Fig. 5. If the scatter is comparable to that in the very slowly varying, high ion dose region, then a straight line fits the first two-thirds of the fall on both profiles. The same is true for the He profiles in Fig. 3, where the sputtering rate is slower because of the lighter ion. Here the four points at approximately the same intensity at the top of the peak cannot be understood as indicating more than monolayer thickness because similar groupings of points appear all along the profile; they simply reflect scatter. Taken together with the unity dispersion found in the titrations and the small particle size seen by TEM, the ISS results indicate that by far the best description of the metal particles after 200 “C reduction is monolayer rafts. Now consider the results obtained after 400 “C reduction by He bombardment in Fig. 3 and Ne bombardment in Fig. 4. The striking features here are the decreased Pt peak intensity, the essentially flat profiles and the increased data scatter, The decreased peak intensity is another con~adiction to the notion that the onset of SMSI is associated with the conversion of multilayer particles into monolayer rafts, since this would have led to more, not less Pt visible after reduction to the SMSI state. The peak profiles on the low-dose side are also of interest, but are partialIy masked by the data scatter. To see better where the rise ends and steady-state sputtering begins, imagine a single straight line beginning at the high dose end of the profile and drawn through the data. The rising portion of the profile would reach this line after five sputter cycles with Ne and seven with He following 400 “C reduction, and again five with Ne but four with He following 200 “C reduction. As noted earlier, Ne scattering is expected to better measure the metal surface area. Considering the Ne results, the same ion dose has uncovered Pt atoms at the same rate after both reductions, indicating that the sputter yield of whatever covered the Pt was the same in both cases. This is evidence (though not proof) that the same species covered the Pt in both instances. The rates are typical of our experience with adsorbed gases. This contradicts the notion that SMSI is associated with the support covering the metal in some way after high temperature reduction. Moreover, the similar rates of change of the difference between the Pt and Ti signals after the 200 “C and 400 “C reductions argues against metal migration into the support. None of the foregoing accounts for why Pt is so much less visible after 400 “C reduction. The differences in spectral intensities may arise from changes in the Pt itself. The intensity scattered into a given peak is the product of the number of bombarded atoms having the correct mass, the scattering cross-section for atom-ion collisions and the probability that the incoming ion will be neu-

248

tralized rather than scattered back as an ion; changes in any one of these would alter the peak intensity. The latter two factors are essentially electronic ~~ractions, especially the neu~~ization probab~i~ describing the ease of transferring an electron from a Pt atom to an ion. An electron structure change can therefore be reasonably expected to affect ion scattering as well as chemisorption. However, X-ray absorption spectroscopy at the Pt L3 and LZ edges provides no support for a simple electron structure change such as a net gain or loss of electrons [ 111. Alternatively, we might conjecture that Pt atoms ‘nestle down’ slightly in the reduced TiOz surface, reducing their visibility. In the EELS results, the very large Ti L,,, edge relative to the 0 K is expected since the cross-sections are in the ratio of 2O:l [ 191. Comp~ng spectra obtained after the successive reduction treatments for a given particle shows a change in Ti at the onset of SMSI: a decreased relative L, edge intensity. This change cannot be interpreted simply as a measure of oxidation state as in fifth row metals, since the 3d levels do not experience splitting as do the 5d levels 1201 , These same Ti edges have been examined previously by EELS using thin film model specimens in a special high-resolution instrument [20], In TiOz it was found that an additional small peak was split off the low-loss side of each edge, but no splitt~g was seen in Ti metal. The edges were separated using a special deconvolution routine and ratios of the L,/L2 areas determined to be 0.7 and 0.8 for the metal and oxide, respectively. The present resolution was not sufficient to make this same separation, though it could be conjectured that the misshapen low-loss side of the peaks indicates that the spli~~g is there. Comparing the oxide and metal on the basis of the height of the large peak at the high-loss side of each edge gives 0.88 and 1.0, respectively, for the high-resolution study, and these are the values which should be compared to those obtained here. The important point is that the difference between the oxide and metal seen previously is about the same difference between the normal and SMSI states seen here. It would be especially valuable to determine in a future study if this difference is mostly associated with the first reduction step to T&O,. The difference seen in both studies certainly represents some change in the electronic structure, but it is not clear just what it is. It is appealing to believe that the TiOZ has acquired some metal-like character in the SMSI state so that SMSI Pt:TiOz might have some of the character of a Pt-Ti intermetallic. However, even if this is proven by further investigation to be true, it is not obvious why this should so sharply impair chemisorption.

Conclusion We conclude that, although SMSI is firmly es~b~~ed as a real effect, the same cannot be said of explanations put forward for it. Particularly, the present findings provide no support for a connection between metal particle

249

morphology and SMSI for highly dispersed catalysts. There is evidence of electron structural changes in both Pt and Ti upon entering the SMSI state, but their detailed nature and catalytic consequences remain to be understood.

Acknowledgement We are indebted to the National Science Foundation for support of previous portions [ll] of this work carried out at the Center for Catalytic Science and Technology at the University of Delaware under grant CPE8016212 for industry-university cooperative research. We are grateful to Mr. Robert D. Nicholls of the Engineering Technology Laboratory, E. I. du Pont de Nemours and Co., Inc. for his able assistance in the transmission electron microscopy. Dr. Henry Shuman of the Pennsylvania Muscle Institute of the University of Pennsylvania provided several helpful and stimulating discussions of the electron energy loss spectroscopy.

References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

S. J. Tauster, S. C. Fung and R. L. Garten, J. Am. Chem. Sot., 100 (1978) 170. R. T. K. Baker, E. B. Frestridge and R. L. Garten, J. Catal., 56 (1979) 390. R. T. K. Baker, E. B. Frestridge and R. L. Garten, J. Catal., 59 (1979) 293. T. Huizinga and R. Prins, J. Phys. Chem., 85 (1981) 2156. J. C. Conesa and J. Soria, J. Phys. Chem., 86 (1982) 1392. S. C. Fung, J. Catal., 76 (1982) 225. B. A. Sexton, A. E. Hughes and K. Foger, J. Catal., 77 (1982) 85. Bor-Her Chen and J. M. White, J. Phys. Chem., 86 (1982) 3534. J. A. Horsely, J. Am. Chem. Sot., 101 (1979) 2870. M. K. Bahl, S. C. Tsai and Y. W. Chung, Phys. Rev. B, 21 (1980) 1344. D. R. Short, A. N. Mansour, J. W. Cook, D. E. Sayres and J. R. Katzer, J. Catal., in press. R. D. Nicholls, E. D. Nicholson, J. R. Toran and M. J. Kelley, Proc. 40th Annu. Mtg., Elect. Microsc. Sot. Am., 1982,~. 648. H. P. Shuman, Ultramicroscopy, 5 (1980) 45. E. Taglauer and W. Heiland, Appl. Phys., 9 (1976) 261. M. J. Kelley and V. Ponec, Prog. Surf. Sci., 11 (1981) 139. M. J. Kelley, R. L. Freed and D. G. Swartzfager, J. Catal, 78 (1982) 445. S. A. Schwartz and C. R. Helms, J. Appl. Phys., 50 (1979) 5492. D. G. Swartzfager, S. B. Ziemiecki and M. J. Kelley, J. Vat. Sci. Technol, 19 (1981) 185. R. D. Leapman, P. Rez and D. F. Mayers, J. Chem. Phys., 72 (1980) 1232. R. D. Leapman and L. A. Grunes, Phys. Rev. Lett., 45 (1980) 397.