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
Pt-TiO, Catalyst in Ethane Hydrogenolysis
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The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1511
m i n sputtering-
Figure 1. Sputteringsystem for platinum deposition on Ti02 powder: (a) target platinum foil bonded to copper; (b) grounded screen; (c) container for TiOz powder; (d) electromechanical drive unit; (e) vacuum pumping port; (f) power supply to drive unit; (g) glass cylindrical vacuum vessel; (h) dc supply to cathode, normally 400 V, 20 mA; (i) insulating dc
feedthrough. been very sparsely reported in patents6bor scientific articles.” The aim of our novel and simple sputtering device was to provide uniform deposition throughout a powdered sample and was achieved in an improved dc sputtering system by mechanical vibration of the powder. In order to characterize the catalysts obtained and relate their mode of intervention in a chemical reaction, we have carried out an investigation of the kinetics of ethane hydrogenolysis. Until now, such evidence has been available for Ni, Co, Cu, and Pt on Si02.’ For the Pt/SiO, catalyst, only the activation energy was measured and the effect of C2H6 and H2pressures on rates of C2H6 hydrogenolysis. Other work by those authors8 involved the relationship of Ni crystallite size (in Ni/Si02) to ethane hydrogenolysis. We report here the first study of Pt/TiO, catalysts obtained by sputtering methods. This catalyst was used in ethane hydrogenolysis. This study also presents electron microscopy, photoelectron spectroscopy, diffuse reflectance spectroscopy, and chemisorption methods to allow a characterization of the microstructure of these catalysts with up-to-date techniques.
Experimental Section The sputtering system was a Vacotec (Switerland) device, in which a rotary vacuum pump was backed by an Alcatel turbomolecular pump capable of attaining a vacuum of lo4 mbar. The target was an 80 mm diameter Pt foil bonded to the copper cathode disk by electrically conductive silver-loaded epoxy adhesive. In the sputter-down geometry necessary for deposition on powder, the dish containing the powder (60 mm diameter) was 25 mm vertically below the Pt target. Vibration was affected about the vertical axis of symmetry in the system, as shown in Figure 1, by an electromechanical transducer. The analysis for Pt content of the sputtered samples was carried out by dissolving the Pt/Ti02 samples in a mixture of H N 0 3 and H F and boiling and digesting the solution repeatedly, almost until dry. This procedure has been previously reported.’O The final Pt content was complexed with SnC12.2H20 solution in an acid medium. Electron microscopy (TEM) was carried out with a Philips 300s instrument at 100 kV having a resolution of -2 A. The magnification attained was 450 000. Diffuse reflectance (6) (a) Husson, S. IBM J . Res. Deu. 1979, 23, 2. (b) Cairns, J.; Nelson, R.; Barafield, R. U S . Patent 4,046,712. Sept. 7, 1977. (c) Takeuchi, A,; Wise, H. J. Catal. 1983, 83, 477. (7) Sinfelt, J.; Taylor, W.; Yates, D. J. Phys. Chem. 1965, 69, 95. (8) (a) Taylor, W.; Sinfelt, J.; Yates, D. J. Phys. Chem. 1965, 69, 3857. (b) Carter, J.; Cusumano, J.; Sinfelt, J. J. Phys. Chem. 1966, 70, 2257.
Figure 2. Pt % on Ti02vs sputtering time using the installation shown in Figure 1.
spectroscopy (DRS) used a Perkin Elmer-Hitachi Model 40, equipped with an integrating sphere. The ESCA measurements were conducted using a Leybold EA 1 1/ 100 energy analyzer and a double-anode X-ray source (Mg Ka, A1 Ka) at a power of 150 W. The spectra were recorded in the AE = constant detection mode, pass energy 75 eV. The mbar and during base pressure of the spectrometer was 4 X the measurements 2 X lo4 mbar. The analyzed area was 1.5 cm2, For data acquisition and evaluation, a HP 1000 computer system and a DS 100 set (Leybold) were utilized. In the measurement of the hydrogen adsorption isotherms, the procedure adopted was to admit a known quantity of hydrogen to the adsorption cell and then wait for a period of about 1 h before reading the equilibrium p r e s ~ u r e . ~The hydrogen adsorption isotherms for the various catalysts studied are plotted in Figure 6 . Determination of CH4 and C2H6 were carried out in a Carlo Erba 5300 Mega gas chromatograph equipped with a Carbosieve column and thermal conductivity detector at 200 OC with He as carrier gas. H, was followed during the reaction also using a T C detector and a 5A molecular sieve column at 70 “ C with Ar. The C content of the catalyst before and after reaction was analyzed via a Perkin Elmer 240-C instrument.
Results and Discussion ( i ) Preparation and Determination of Pt in Powdered Sputtered Samples. The sputtering device used in the present work is shown in Figure 1. The novel design used ensures uniform exposure of the TiOz substrate placed on an A1 dish and vibrated electromechanically during the time of sputtering. The applied vibration produces a constant exchange between the bulk and the exposed powder surface. A satisfactory mass-transfer rate was attained by this conventional planar diode dc sputtering as will be shown later by electron microscopy imaging of the homogeneous Pt deposit. It was not necessary, therefore, to use rf or magnetron&* sputtering techniques to attain a high dispersion of Pt on the substrate. The sputtering medium was Ar (high purity) at 1 mbar pressure. A dc potential of -400 V and plasma current of 20 mA was used on the Pt target throughout the experiments. Figure 2 shows the results found for the weight of Pt deposited on Ti02 as a function of sputtering time. Analysis of Pt on the substrate was carried out as outlined in the experimental part.1° It is readily seen that the amount of Pt deposited increases in an almost linear fashion with the sputtering time while the TiO, becomes dark grey. (ii) Surface Characterization of P t / Ti4 Powdered Samples. Figure 3 presents the results of electron microscopy studies on (9) Whyte, T. Catal. Rev. 1973, 8, 117. (10) (a) Ayres, H.; Meyer, S. Anal. Chem. 1951, 23, 299. (b) Gu, B.; Kiwi, J.; Gratzel, M. N o w . J . Chim. 1985, 9, 539.
1512 The Journal of Physical C h e m i s t r y . Vol. 93. No. 4. 1989
Albers et al.
Figure 3. Electron micragrapns 01 Y t I I tu2sample sputterea 1 bU mtn. u ~ . ~ . . i amagnification l was 450 000 here I cm = 35 A. Two halves have been used to show different aspects of this material. Inset: I cm = 120 A.
Pt/TiO, catalysts sputtered for 160 min. Highly uniform deposits were obtained with over 80% of the Pt clusters having sizes between 22 and 29 A. The size distribution of the particles was determined by counting spherically shaped particles which exhibited high contrast against the support. The atomic planes of Ti are readily seen in Figure 3 as being 3.5 A apart. Pt clusters obtained with 40, 80, 160, and 320 min sputtering showed Pt clusters -25 A in size. This figure also shows the TiO, (P-25 Degussa) with sizes 1%2-20 A. The insert in Figure 3 indicates a very regular topology for the clusters on the TiO, substrate. On a few TiO, grains the deposit is almost absent. It is also not possible to know if a part of the implanted Pt atoms and ions is buried in the bulk of the substrate. This will be examined later in Figure 6 via chemisorption experiments. It is generally accepted for spherical packing of Pt atoms that clusters of this material of IO-A size contain 2 2 0 atomsl’ and the electronic structure of the cluster is identical with that of the bulk metal.I2 In figure ( I I ) Boudart, M.: Topsoe, H.; Dumcsic, J. A. In The Physieol Basis Jor Hcrerogeneous Catoly*is; Drauglis. E., Jaffe, R., Eds.; Plenum Press: New York. 1975.
3 we report Pt clusters which are metallic in character with a certain amount of Pt(I1) and Pt(1V) ionic species. This will be seen by in situ ESCA experiments later (Figure 5a,b). To estimate the surface of the TiO, covered by Pt clusters in a 160 min sputtered sample, it is possible to say that about IO” siteslcm’ are available on TiO, Degussa P-25.I1J4 Knowing that the BET area of TiO, P-25 Degussa is 50 mz/g (50 X 1OI9 sites/g), it follows that, for I .2% Pt loading, a coverage of 15 X IOl9 atoms of Pt reside on 50 X IOI9 siteslg of TiO,. Then about 30% of the available area is covered by Pt, assuming 100% dispersion. Clusters as shown in Figure 3 of 22-29 A afford a -34-44% dispersion as seen by TEM.” Therefore, about 12.5% of the available area is covered in real terms by Pt. Electron micrographs in Figure 3 roughly confirm this estimate. (12) Anderson, R. The Slrueture OJ Merollic Cotolyrs; Academic Press: New York, 1975. ( I ? ) Each surface site is equated to a surface hydroxyl group; c.8.. see: Mormon, R.S. The Chemieol Physics oJSurJoces: Plenum P m : New York.
1977.
(14) Boehm, P. Discuss. Faraday Sm. 1971.52, 164
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1513
Pt-Ti02 Catalyst in Ethane Hydrogenolysis
TABLE I: Fit Results (%) 5 - 1
3
600
300
0
900 nm-
Figure 4. Diffuse reflectance spectroscopy (DRS) for samples: (1) TiO,; (2) 10 min Pt sputtered; (3) 40 min Pt sputtered: (4) 160 min Pt sputtered: ( 5 ) 320 min P t sputtered. I
I
I
I
77 6
75 6
I
300
250
200 :50
:oo 50
786
//
,
71 C
716
sample
Pt(0)
Pt(I1)
Pt(IV)
320 min sputt 160 min sputt 80 min sputt 40 min high agit 40 min low agit
9.3
20.3 32.8 50.4 43.2 37.3
70.3
67.1 18.3 29.8 15.9
31.2 26.8 46.6
surface stoichiometry a t % Pt 1.64 0.88 0.18 0.30 0.16
referencing to the 0 1s and Ti 2p signals. The Pt 4f doublet was treated by Gauss-Lorentz fitting routines to obtain the amounts of the different chemical states of the platinum. Standard spectra of bulk and of finely dispersed Pt were measured when comparing the peak shapes, intensity ratios, asymmetry, and half-widths. The dispersion shift to be expected for the actual particle sizes17 is rather small, but it has to be considered in data analysis by setting the reference values of the highly dispersed zerovalent Pt to 71.2 eV instead of 70.9 eV for metallic bulk Pt, with respect to typical literature values1*and to calibration measurements on Pt particles of different dispersion states. Therefore, the shift in the observed Pt 4f7/2-5/2 doublet will reflect fairly well the binding energies associated with different oxidation By TEM investigations on different Pt/Ti02 samples (Figure 3) we have seen that the sputtered particles have an average particle size of 25 f 4 A. Since Pt-cluster size effects can, therefore, by neglected, the shift in the observed Pt 4f7/2-5/2 doublet will reflect fairly well the binding energies associated with different oxidation states. For the evaluation of the ESCA spectra obtained, we used the values referenced to the Au 4f7/2 level at 83.8 eV.I9 Figure 5a presents the ESCA spectrum (5) for a 40 min sputtered sample. For zerovalent Pt, the Pt 4f7/2-5/2 doublet has the binding energies of 70.9 (71.2) and 74.2 (74.5) eV.Z0,21The experimental curve (5) was then deconvoluted by a GaussianLorentzian generator. Curve 1 represents the zerovalent Pt component of the generated ESCA spectrum (4) which is in fair agreement with the experimental data shown in spectrum (5). The shift of the experimental peaks (5) toward higher binding energies reflects the existence of Pt species having higher oxidation states. Curve 2 represents the Pt(I1) 4f7/2-5/2 doublet having binding energies of 72.4-73.7 eV and 75.7-77.0 eV, respectively. Curve 3 represents the Pt(1V) 4f7/2-5/2 doublet with binding energies of 74.5 and 77.8 eV, respectively.22 The addition of curves 1, 2, and 3 gives curve 4 which is close to the experimental spectrum
(5). \
( 1 5 ) Kiwi, J.; Gratzel, M. J. Phys. Chem. 1984, 88, 1302. (16) (a) Wang, L.; Qiao, G . ;Ye, H.; Kuo, K. Proc. 9th Inf. Conf. Caral. Calgary 1988.3, 1253. (b) Beard, B.; Ross, P. J . Phys. Chem. 1986.90.681 1.
Figure 5b presents the ESCA spectrum of a 160 min Pt/TiO, sputtered sample. Spectrum 4 is obtained experimentally. Note the considerable shift of the spectrum when compared with the values reported in Figure 5a for the 40 min sputtered catalyst. As in Figure 5a, trace 1 presents the doublet for the zerovalent Pt component and trace 2 represents the Pt(I1) 4f7/2-5/2 doublet. The addition of both is shown by trace 3. There is no Pt(1V) component. The results obtained for the studied sputtered samples are outlined in Table I, showing the percentage of Pt(O), Pt(II), and Pt(1V) in the different samples. Results obtained for the Ti peak and distance between Pt and Ti peaks indicate a considerable amount of Ti3+present. All these results indicate stable ionic species consisting of Pt clusters with different valences, e.g., Pto, Pt+, Pt2+, Pt4+. The present experimental results do not allow us to state the exact composition or structure of these Pt species. (17) (a) Ertl, A.; Kiippers, J. Low Energy Electrons and Surface Chemistry; VCH: Weinheim, 1985; pp 65-85. (b) Cheung, T. Surf. Sci. 1984, 140, 15 1. (c) Photoemission and the Electronic Properties of Surfaces; Feuerbacher, B., Fitton, B., Willis, R., Eds.; Wiley: Chichester, U.K., 1978. (1.8) (a) Wagner, D. Handbook of X-Ray Photoelectron Spectroscopy; Perkin Elmer, Physical Electronics Division, Eden Prairie, MN, 1979. (b) Paal, Z.; Tetenyi, P.; Prigge, D.; Wang, X.; Ertl, G. Appl. Surf.Sci. 1982, 14,83. (c) Huizinga, T. SurJ Sci. 1983, 135, 580. (d) Peuckert, M.; Bonzel, H. Surf. Sci. 1984, 145, 239. (19) Whyte, T. Catal. Reu. 1973, 8, 117. (20) Kim, K.; Winograd, N.; Davis, R. J . Am. Chem. SOC.1971,93,6292. (21) Hoffmann, W.; Gratzel, G.; Kiwi, J. J . Mol. Catal. 1987, 43, 183. (22) Cuksdahl, K.; Houston, R. J . Phys. Chem. 1961, 65, 1464.
1514
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
Albers et al. 3000b T 300°C
'
I
2 o
I
C
2 36
4
13
6 32
E q u 11 3r i u m p r e s s u r e
I
8 27
? O 33
(mbarl
Figure 6. Moles of H2 vs equilibrium pressure in chemisorption experiments using 40, 160, and 320 min Pt/Ti02-sputtered samples.
Chemisorption results obtained to assess quantitatively Pt dispersion on TiOz are shown in Figure 6. The technique used H2 a d s o r p t i ~ n . Samples ~~ were evacuated for 2 h at 300 OC and reduced for 2 h at this temperature with H z previously passed through a deoxo trap and dried. Vacuum was then maintained for 15 h at 200 "C and 1 h at 20 OC, and the Hz adsorption was performed between 0 and 11 mbar. Taking the adsorption a t 0 mbar as the monolayer point:*23 and assuming that one H atom is adsorbed on each Pt atom on the surface, one then has a value for the fraction of total Pt atoms that are present on the surface of the Pt crystallites. As seen in Figure 6, the dispersions reported, from 34.7% to 46.0% for these catalysts, agree quantitatively with the dispersion values reported (Figure 3) by TEM for these materials. This observation has a twofold importance: (a) since the values for the particle size obtained by TEM and chemisorption agree fairly well, the spherical shape assumed for the clusters (H/Pt SI)is valid; (b) sputtering with energies of < 12 eV-the ionization energy of Ar-will not cause a deep penetration of Pt into the substrate. Therefore, our deposition is superior to ion implantation with its deep penetration profile as reported elsewhere.24 (iii) Kinetics of Ethane Hydrogenolysis over Ptl Ti02-Sputtered Catalysts. Hydrogenolysis of ethane was carried out in a 20" quartz cell heated at 300 OC over a 24-h period, mixing 3 cm3 of C2H6and 3 cm3 of H2. Results for a typical experiment using a 160-min sputtered catalyst are shown in Figure 7. Detection of gases was carried out using gas chromatography as described in the experimental part. Since this experiment was carried out at 300 OC, the dispersion of the catalyst Pt state is identical with the one reported in the chemisorption experiments in Figure 6. Ethane hydrogenolysis has been r e p ~ r t e das ~,~ CzH6
+ H z * 2CH4
Reaction 1 proceeds with a high initial rate involving the rupture of C-H ethane bond in this molecule.8a Similar observations have been previously reported for Ni/SiOZ, Co/SiO,, Cu/SiOz, and Pt/Si02.'t8 After an initial steep growth of CHI, the reaction proceeds almost to completion in 24 h. C,H6 and H2 react with 87% conversion over 24 h following the stoichiometry indicated in eq 1. These results also show the amount of gases adsorbed at 300 "C on the catalyst surface is low. A 40-min highly agitated sputtered sample followed the same kinetic profile (Figure 7) but had a 76% conversion in 24 h. A standard catalyst prepared by exchange technique^'^ having 96% dispersion and reduced at 300 OC (2.0% Pt/TiOz, Pt cluster 10 A) was also tried in a kinetic run under the same conditions. A 92% conversion to CH4 was obtained over 24 h. Therefore, the sputtering technique employed compares favorably with the catalyst having a higher Pt content (23) Benson, J.; Boudart, M. J . Catal. 1965, 4 , 704. (24) (a) Cairns, A. IEEE Trans. Nucl. Sci. 1981, NS-28, 1804. (b) Haining, B.; Rabette, P.; Che, M . Proc. 7th I n t . Congr. Caral. A 1981, 317.
lOOOt/#
1 -
1 ia
0
6
12
24
hours reaction-
Figure 7. Ethane hydrogenolysis showing the amount of ethane and hydrogen consumed and methane concomitantlygenerated vs time at 300 "C; 60 mg of Pt/Ti02 sputtered 160 min was used.
and maximal metal dispersion. It was also found that only Pt/ Ti02 materials sputtered 40 min or more were effective in catalysis. Therefore, a minimum of -0.2% Pt on Ti02 (Figure 2) was needed to favorably affect the reaction shown by eq 2. The specific activities after 1 h found for ethane hydrogenolysis (mol/mz of Pt)/h for a 160 (Figure 7 as shown) were 22 X min sputtered catalyst and 14 X 7X and 4 X (mol/m2 of Pt)/h for catalysts sputtered 40, 80, and 320 min, respectively. This initial value (which is an estimate for the observed rate of the reactions) falls to (1-2) X (mol/m2 of Pt)/h after 24 h in all cases (e.g., see Figure 7). The initial specific activities observed for ethane hydrogenolysis correlate to some extent to the content in zerovalent Pt of these catalysts as reported in Table I. This observation is consistent with the fact that the 160 min sputtered catalyst was the most active as shown in Figure 7. Since Pt/TiOz catalyst presented clusters -25 8, in size, the different activities for different loads of Pt can be attributed to different surface species (see Table I) in contrast to the size dependence of Ni clusters reported for the SiOz c a ~ e . Sinfelt ~,~ found that the activity per unit Ni surface area decreased as the crystallite size increased.8b The observed ethane hydrogenolysis rates were not the same for the three most active catalysts (40, 160, and 320 min sputtering) having the same metal dispersion. As can be seen from Table I, one can suggest that Pt loading and oxidation state play a determining role in the catalyst efficiency. A 160 min sputtered material shows a highly reduced state (only Pto and Pt2+ are present) while the 40-min high agitation and 320 min sputtered materials show a relative high content of active Pto, besides Pt(I1) and Pt(IV) species. It seems, therefore, that Pt ionic species in Table I affect the electrophilic character of platinum which results in a decrease of the hydrogenolysis activity.'2v2S One can only suggest that each component in the Pt/TiO2 "complex" has a specific role in the reaction mechanism shown by eq 1. To test the catalytic nature of the Pt/TiOz catalysts, we recycled this material a few times over 24-h periods at 300 OC. Very small loss of activity (- 10%) was observed. The surface carbon ap( 2 5 ) Rylander, P. Catalytic Hydrogenation over Platinum Metols; Academic Press: New York, 1967.
J. Phys. Chem. 1989, 93, 1515-1521 TABLE 11: Carbon Content no. of recycles 0 1 ~~
(%) of a 160 min Sputtered Catalyst
~
2 3
C 0.4 0.1 0.07 0.05
H
N
0.25
0.06
0.17 0.19 0.21
0.0 0.0 0.0
parently reacted with H 2 at 300 O C during the process and this is shown in Table 11. In conclusion, this work has presented a novel setup for simple metal deposition on powders, via dc sputtering. Deposition of Pt on Ti02 is quick and uncomplicated and can be carefully controlled. Improvement in the mode of agitation of the powder would render a more homogeneous coverage. Work is under way in our laboratory in this direction. In situ ESCA techniques show that sputtered Pt/Ti02 materials have different oxidation states formed
1515
and stabilized at the catalyst surface. The most active catalyst contained only two oxidation states. For charge transfer this condition is therefore sufficient to affect the fluctuating electron density for catalytic activity.26 The present work has combined hydrogen chemisorption on supported metals, electron microscopy (TEM), and ESCA with kinetic data. Acknowledgment. Financial support for the study was given by the Swiss National Science Foundation. We thank I. Rodicio and P. Ruterana of the EPFL for their help with electron microscopy and chemisorption work. Discussions with J. Highfield during the course of this work are appreciated. Registry No. Pt, 7440-06-4;Ti02, 13463-67-7;C2H6, 74-84-0; H2. 1333-74-0. (26) Falicov, L.; Somorjai, G. Proc. Narl. Acad. Sci. USA 1985,82,2207.
Electron Spin Resonance and Diffuse Reflectance Spectroscopy of the Reduction of Ni2+with H, in Zeolites X and Y, Exchanged with La3+ and NH,' Robert A. Schoonheydt,* Ivan Vaesen, and Hugo Leeman Laboratorium voor Oppervlaktechemie, K. U. Leuuen, K. Mercierlaan, 92, B- 3030 Leuven, Belgium (Received: May 18, 1988)
After an oxidative pretreatment the thermal reduction of Ni2+ with H2 to Ni(0) goes over Ni+. In NiLa-Y two types of ESR-active Ni+ were found. Signal I consists of three g,values at 2.67, 2.51, and 2.33 and one g, value at 2.096. It is due to Ni+ in the cubooctahedra. Up to 11% of the Ni2' can be converted to this type of Ni+, which is thermally stable up to 773 K. Signal I1 with g values of 2.16 and 2.064 is due to a thermally unstable Ni+ species and is present only in trace amounts. The stabilization of ESR-active Ni+ requires a low Ni content, polyvalent cations, such as La3+and Ca2+, and trigonally coordinated Ni2+. It is almost absent in NiLa-X, because of the absence of trigonally coordinated Ni2+. In NiH-Y the mobility of Ni+ is too high to stabilize it as a mononuclear ESR-active species, but Ni clusters can be stabilized. The stabilization of Ni+ and Ni clusters is also much less pronounced in NiH-X, due to a combination of factors such as lattice instability and mobility of Ni'. Ni clusters are formed simultaneously with ESR-active Ni+. They are characterized by DRS bands at 13 500,27 500,34000, and 38 000 cm-'. The first two bands are characteristic for molecularly sized clusters. The latter two may be cluster bands.
Introduction The reduction of Ni2+ in dehydrated zeolites depends on the structure type, the coexchanged cation, and the loading.I4 Some representative data are given in Table I. It is shown that under identical experimental conditions the degree of reduction follows the order zeolite X > zeolite Y and mordenite > zeolite Y.2,4 While the coexchanged cations determine the overall chemical properties of the zeolites through the average ele~tronegativity,~ they also compete with Ni2+ for exchange sites. Thus, under the same experimental conditions the degree of reduction of Ni2+ in alkaline-earth-exchanged Y exceeds that of NiNa-Y and follows the order Mg < Ba < Sr < Ca.4 However, the degree of reduction in NiNaX exceeds that of NiCaX.6 In X-type zeolites Ce3+ is known to promote the reduction of NiZ+,while La3+retards (1) Lausch, H.; Morke, W.; Vogt, F.; Bremer, H. Z . Anorg. Allg. Chem. 1983, 499, 213.
(2) Briend-Faure,M.; Jeanjean, J.; Kermarec, M.; Delafosse, D. J . Chem.
SOC.,Faraday Trans. I 1978, 7 4 , 1538. ( 3 ) Briend-Faure, M.; Jeanjean, J.; Delafosse, D.; Gallezot, P . J . Phys. Chem. 1980, 84, 875. (4) Suzuki, M.; Tsutsumi, K.; Takahashi, H. Zeolites 1982, 2, 51. (5) Mortier, W. J.; Schoonheydt, R. A. Prog. Solid Srate Chem. 1985, 16, 1. ( 6 ) Contarini, S.; Michalik, J.; Narayana, M.; Kevan, L. J . Phys. Chem. 1986, 90, 4586. (7) Guilleux, M. F.; Delafosse, D.; Martin, G. A.; Dalmon, J. A. J . Chem. SOC.,Faraday Trans. I 1979, 75, 165. (8) Djemel, S.;Guilleux, M. F.; Jeanjean, J.; Tempere, J. F.; Delafosse, D. J . Chem. Soc., Faraday Trans. 1 1982, 78, 835.
0022-3654/89/2093-1515$01.50/0
TABLE I: Degree of Reduction of NiZ+in Zeolites
reduction degree, % 90 60
zeolite NiNa-Y-10" NiNa-Y-72 NiLa-Y- 14
873 K, 643 K, 873 K, 643 K, 643 K,
70 11
Ni H-Y -3 3
NiNa-X-71 NiNa-M-37* NiH-M-28
reduction conditions 870 K, 2 h 873 K, 25 h
100 100 13
25 h 10 h 20 h 10 h 10 h
ref 4 2 3 4 2 4 4
"Degree of Ni exchange in percent of the CEC. b M stands for mordenite.
In the presence of acidic O H groups the reduction of Ni2+ is difficult, because the equilibrium Ni2+
+ H2
Ni(0)
+ 2H+
(1)
is driven to the left.4
Equation 1 represents a complicated set of reactions, involving several intermediates. One intermediate, Ni', has been isolated in small quantities in NiCa-X and NiCa-Y and studied in great detail by electron spin resonance (ESR).11-'4 (9) Briend-Faure, M.; Jeanjean, J.; Spector, G.; Delafosse, D.; BozonVerduraz, F. J . Chim. Phys. 1982, 79, 489. (10) Sauvion, G.-N.; Tempere, J. F.; Guilleux, M. F.; Djega-Mariadassou, G.; Delafosse, D. J . Chem. Soc., Faraday Trans. 1 1985, 81, 1357.
0 1989 American Chemical Society
