Langmuir 1988,4, 827-830
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Direct Observation of the Surfaces of Small Metal Crystallites: Rhodium Supported on TiOa A. David Logan, Ehrich J. Braunschweig, and Abhaya K. Datye* Department of Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico 87131
David J. Smith* Center for Solid State Science and Department of Physics, Arizona State University, Tempe, Arizona 85287 Received November 12, 1987. I n Final Form: February 19, 1988 Surfaces of small rhodium crystallites, ca. 5-8 nm in diameter, have been imaged in a 400-kV highresolution transmission electron microscope by using the profile-imagingmode. The observations were facilitated by supporting the metal on nonporous titanium oxide particles of simple geometric shape. Since all the metal was located on the exterior surfaces of the oxide, the formation of surface overlayersduring catalytic pretreatments could be studied. Surfaces which were clean after 473 K reduction in hydrogen were covered with amorphous deposits after treatment in H2 at 773 K, and there was a concurrent drop in the chemisorption uptake of H2.Subsequent oxidation at 473 K did not lead to the complete removal of these deposits, even though the chemisorption uptake by the Rh was partially restored. These results confirmed that migration of Ti suboxides was responsible for altered catalytic behavior during the SMSI state.
Introduction The interest in titania as a support for heterogeneous catalysts centers around its ability to alter the chemisorptive and catalytic properties of the supported metal after high-temperature reduction in Hz. This effect was first identified by Tauster et al.l and was termed a "strong metal support interaction" (SMSI). Later work showed that other reducible oxides also affected the behavior of the supported metal in a similar manner.2 The mechanism underlying the SMSI effect is not completely understood, but two possible explanations have emerged. The first is an electronic effectg5 which postulates electron transfer between the metal and the support leading to altered chemisorptive behavior. The second explanation is morphological; Le., a migration of a sub-oxide onto the metal takes place6-10and/or the metal spreads out onto the oxide."J2 These possibilities are not mutally exclusive and can be considered simultaneously as encapsulation and local electronic perturbation.13J4 Previous evidence from transmission electron microscopy suggests that, in the case of Pt/TiOz, TiOz is reduced to Ti40, in the vicinity of the metal and that the metal crystallites are flattened and assume raftlike morphologies in the SMSI state.ll These observations were based on micrographs of small Pt crystallites on thin planar films of titania, but only the projected images of the metal crystallites could be observed through the oxide support with this configuration. When high surface area porous oxides (e.g., ref 9) are used, the metal crystallite is also only (1) Tauster, S. J.; Fung, S. C.; Garten, R. L. J. Am. Chem. SOC. 1978, 100,170. (2)Tauster, S.J.; Fung, S. C. J. Catal. 1978,55,29. (3)Horsley, J. A. J. Am. Chem. SOC.1979,101,2870. (4)Chen, B. H.; White, J. M. J. Phys. Chem. 1982,86,3534. (5)Herman, J. M. J. Catal. 1984,89,404. (6)Santos, J.; Phillips, J.; Dumesic, J. A. J. Catal. 1983,81, 147. (7)Simoens, A. J.; Baker, R. T. K.; Dwyer, D. J.; Lund, C. R. F.; Madon, R. J. J. Catal. 1984,86,359. (8)Sadeghi, H. R.; Henrich, V. E. J. Catal. 1984,87,279. (9)Singh, A. K.;Pande, N. K.; Bell, A. T. J. Catal. 1985,94,422. (10)Huizinga, T.; Van't Blik, H. F. J.; Vis, J. C.; Prins, R. Surf. Sci. 1983,135,580. (11)Baker, R. T. K.; Prestridge, E. B.; Garten, R. L. J. Catal. 1979, 56,390. (12)Tatarchuk, B.J.; Dumesic, J. A. J. Catal. 1981,70,335. (13)Belton, D. N.;Sun, Y. M.; White, J. M. J.Am. Chem. SOC.1984, 106,3059. (14)Resasco, D. E.;Haller, G. L. J. Catal. 1983,82,279.
0743-7463/88/2404-0S27$01.50/0
Table I. ChemisorDtion UDtake of H, at Room TemDerature uptake, pmol/g of Dretreatment
catalvst
LTR (H,at 473 K overnight) HTR (H, at 773 K overnight) O2 at 473 K for 2 h/LTR 0,at 473 K overnight/LTR
17.5 2.9
6.1 14.7
rarely observed edge-on. In order to examine the metal surfaces we have proposed the use of nonporous oxide particles of simple geometric shape as supports.15 However, a major problem which still remains when attempting observations of surfaces is the necessity for in situ pretreatments as well as U H V environments in order to keep the surfaces clean. In this paper we report our observations of Rh/Ti02 catalysts that were handled in air after removal from the reactor but before insertion into the microscope. Within the available resolution limit (1.7 A point to point), our observations with the profile-imagingmodel6 indicated that the surfaces were remarkably clean." This prompted us to examine the surfaces after different pretreatments and then to relate the observed surface structure with the experimental chemisorptive uptake of Hz.
Methods and Materials The TiOzsupport consisted of submicron spherical particles that are crystalline and nonporous. The particles were generated by striking a discharge between two high-purity Ti electrodes in a controlled O2atmosphere. The technique is quite general and has been used for generating a variety of metal oxide particle^.^^^^^ The titania powder had a nominal surface area of 10-15 m2/g. X-ray diffraction showed the presence of anatase and rutile phases with 7030 relative peak intensities. The Rh/TiOz was made by impregnation using rhodium(II1)2,4-pentanedionate (commonly called Rh(acac)B)in acetonitrile (4 mL/g of catalyst)with continuous stirring until dry. The catalyst was then air-dried at 383 K. Reduction was carried out in flowing H2(20 sccm) using a (15)Datye, A.K.;Logan, A. D. Roc.-Annu. meet., Electron Microsc. SOC. Am. 1986,44th,772. (16)Marks, L. D.; Smith, D. J. Nature (London) 1983,303,316. (17)Our own experience,also confirmed by image simulationfor C on Au (Marks, L. D. Surf. Sci. 1983,139,281),is that even a monolayer of carbonaceous material would be visible in a through-focalseries, because its contrast behavior is different from that of the substrate or metal. (18)Uyeda, R. J . Cryst. Growth 1974,24/25,69. (19)Iijima, S.J. Appl. Phys. Jpn. 1984,23,L347.
0 1988 American Chemical Society
828 Langmuir, Vol. 4, No. 4,1988
Logan et al.
Figure 1. Electron micrograph of Rh/TiO, catalyst after low-temperaturereduction (LTR)in H, at 473 K. Rutile (110) d = 3.25-A and Rh (111) d = 2.19-A lattice fringes are well resolved. two-step proeess: first, the Rh(acac), wan allowed to decompose at 423 K for 5 h, and then the temperature was raised to 473 K to complete the reduction. Chemisorption measurementswere made in a static volumetric chemisorption apparatus with gas uptake being detected hy a Baratron capacitance manometer. Low-temperature reduction (LTR) was carried out overnight in 250 Torr of H, at 473 K. High-temperaturereduction (HTR) was carried out overnight in 250 Torr of H,at 773 K. Oxidation wan carried out at 473 K and 200 Torr of H, to break the SMSI state.
Results and Discussion The chemisorption uptake after the different pretreatmenta is shown in Table I. It is evident that high-temperature reduction causes a marked loss in the amount of H, chemisorbed at room temperature. Treatment in oxygen at 473 K for 2 h WBS not sufficient to break the SMSI state. Oxidation a t 473 K Overnight (16 h) did restore more than 80% of the original uptake of H2 However, oxidation at 773 K overnight appears to be necessary to restore completelythe catalytic behavior or the Rh to ita pre-SMSI state.* Figure 1 shows a micrograph of the catalyst after the LTR in Hz. The structure of the small Rh crystallites is quite well resolved, and the Rh surfaces appear clean with no evidence of any amorphous surface f h s due to carbon contamination or surface oxidation of the Rh due to handling in air. The (111)surfaces of the Rh crystallites are preferentially exposed. The lattice fringes corresponding to (110) planes of rutile are well resolved and extend to the edge of the particle with no evidence of any amorphous layers at the oxide surface. The surface-averaged diameter of the Rh crystallites is 5.3 nm, which is consistent with a dispersion of 18% derived from the chemisorption uptake and a 2 w t % metal loading. (20) Brsunsehweig,E.J.; Lagan. A. D.; Datye, A. K.Motor. Res. Sac. Symp. Roe. 1988, in press.
Figure 2 shows a micrograph of this catalyst after HTR in H,. The Rh metal crystallites as well as the surface of the TiOl spheres are now covered by an amorphous layer. Although the layer is not uniform, it appears to be between 2 and 4 A in thickness. In the Drofde view, the amorphous layer appears to encapsulate the metal crystallites: The appearance of this layer correlates well with the drop in hydrogen chemisorption uptake. Spectroscopic evidence from previous work has shown that suboxides of titania can migrate over the metal surface upon high-temperature reduction.8 Figure 2 shows that when titania in proximity to the metal is reduced, the suboxides tend to spread over the metal as well as the rest of the titania support. In contrast to previous TEM observations on Pt," the Rh crystallites do not appear to spread over the support after this high-temperature treatment. There is no wetting of the oxide by the metal, while the suboxide certainly weta the metal. Figure 3 shows a micrograph of the catalyst after an Overnight oxygen treatment followed by LTR. While 80% of the hydrogen uptake has been recovered, the amorphous surface film is still evident on the surface of the Rh crystallites. The thickness of the film has, however, decreased considerably. Surprisingly, the amorphous film on the oxide surfaces is also intact, and the surface of the titania looks quite irregular. It has been suggested in previous work that oxidation of the sample may allow the suboxide film to ball up due to altered surface energies6 and allow the metal surface to be exposed. Figure 3 does not provide any direct evidence for the formation of small TiOz crystallites after the oxidation treatment. Incorporation of the titania by dissolution into the metal has also been suggested by Demmin et al.2I The lattice fringes in the metal crystallites are well resolved, but any changes (21) Demmin, R. A,; KO,C. S.; Go&, R. J. Strong Metal Support Znteroctiom; Baker, R. T.K.;Tauter, S. J. Domesie, J. A,, Ed&; ACS Symposium Series 298; American Chemical Smiety: Washingbn, DC, 1986; p 48.
Rhodium Supported on TiO,
Langmuir, Vol. 4, No. 4, 1988 829
Figure 2. Micrograph of the catalyst after high-temperature reduction (HTR)at 713 K i n H,. The presence of amorphous surface layers (mowed) of approximately 2-4 A is seen both on the metal and the oxide surface.
Figure 3. Micrograph of the catalyst after oxidation with 200 Torr of O2at 473 K overnight followed by LTR. The surface overlayers are still evident despite the partial restoration of the chemisorption uptake. in the metal lattice spacings of a few percent due to oxide dissolution would be difficult to quantify. Thus, the mechanism for removal of the oxide overlayers cannot specified at this time. The bulk of the anatase or rutile spheres is unaffected by the high-temperature treatments. This study has provided direct evidence for the encapsulation of Rh by an amorphous film in the SMSI state.
In the profile images, the overlayer appears to completely cover the metal surface and cause the suppression of Hz chemisorption uptake. The reactivity of these catalysts in the CO hydrogenation reaction increased 2-3 times in the SMSI state while the butane hydrogenolysis activity declined by over 2 orders of magnitude;2othis behavior is similar to that reported earlier.22~23One possible expla-
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nation for the reactivity of the encapsulated Rh surface is that these amorphous surface overlayers are sufficiently porous for the reactants to diffuse through. The TEM micrographs do not provide any information on the continuity of these films in a direction normal to the micrographs. Another possibility is that the film is discontinuous, exposing bare patches of Rh and thereby accounting for the small but measurable chemisorption uptake in the SMSI state. The reaction may proceed on these bare patches and at the metal-oxide interface. In conclusion, we have demonstrated that high-resolution transmission electron microscopy can be used to ob(22) Vannice, M. A.; Garten, R. L.J. Catal. 1979, 56,236. (23) KO.E.I.; Garten, R. L.J. Catal. 1981, 68,233.
serve the surfaces of small metal crystallites in heterogeneous catalysts. Subnanometer overlayers that are catalytically significant can be detected in edge-on views of the small metal particles.
Acknowledgment. Financial support for this work from the NSF via Grant CBT-8707693 is gratefully acknowledged. ADL acknowledges receipt of a scholarship from the American Vacuum Society-NM chapter. Electron microscopy was performed at the Electron Microbeam Analysis Facility at the Department of Geology, University of New Mexico, and at the NSF national HREM facility within the Center for Solid State Science, Arizona State University (supported by Grant DMR-86-11609). Registry No. Rh, 7440-16-6; Ti02, 13463-67-7;H2, 1333-74-0.
Photohydrogenation of Acetylene in Titanium Dioxide Based Colloidal Aqueous Solutionst Zun-Sheng Cai and Robert R. Kuntz* Department of Chemistry, University of Missouri-Columbia, Columbia, Missouri 65211 Received January 14, 1988. I n Final Form: March 2, 1988 The photohydrogenation of acetylene on bare and modified colloidal TiOz surfaces has been studied in aqueous solutions containing a sacrificial electron donor. Yields of CzH4 and CzHe as a function of pH, loading with Mo, particle size, and light intensity were determined. The two-electron reduction to C2H, occurs on the colloidal surface by interaction with adjacent Ti(II1) centers. Mood2-,photoreduced on the colloidal particle, transfers electrons through Ti(II1)centers to the acetylene substrate, resulting in a direct four-electron reduction to CzH6 without the intermediacy of CZH4. The efficiency of the individual Mo catalytic center is quite high, but the net quantum efficiency is generally
