The Journal of
Physical Chemistry
0 Copyright, 1986, by the American Chemical Society
VOLUME 90, NUMBER 9 APRIL 24,1986
LETTERS EXAFS Evidence for Direct Metal-Metal Bonding in Reduced Rh/Ti02 Catalysts Stanley Sakellson, Martin McMillan, and Gary L. Haller* Department of Chemical Engineering, Yale University, New Haven, Connecticut 06520 (Received: April 8. 1985; In Final Form: March 7, 1986)
An EXAFS analysis of a reduced Rh/Ti02 catalyst prepared by ion exchange demonstrates that direct Rh-Ti bonding is produced by reduction at 7 7 3 K. The bond length for Rh-Ti formed in the catalyst is significantly shorter than that observed in the most stable intermetallic between the two elements, RhTi. This is taken as evidence that Ti (and perhaps Rh) has some cationic character, presumably as a result of association with oxygen.
The discovery that group VI11 (groups 8-10)24 metals supported on Ti021and other reducible oxides2 can produce dramatic, reversible effects on chemisorption and catalysis has stimulated widespread interest in these ~ysterns.~This effect has been labeled a strong metal-support interaction (SMSI), and initial interpretations attributed the chemisorption and catalytic effects to an electronic i n t e r a ~ t i o n . ~ Because of the parallel behavior between group VIII-group Ib (groups 8-1 1) and Rh-Ti02 interaction effects on structure-sensitive and structure-insensitive reactions, we have proposed a model involving migration of a reduced species of the support onto metal particles which results in a geometric effect as weL5 Several model system studies of Fe/Ti02,6 Ni/Ti02,7-9 Pt/Ti02,10-15 and Rh/Ti0210*16 have provided direct physical evidence for migration of a (reduced) (1) Tauster, S. J.; Fung, S. C.; Garten, R. L. J. Am. Chem. Soc. 1978, ZOO, 170. ( 2 ) Tauster, W.J.; Fung, S. C. J. Catal. 1978, 55, 29. (3) Imelik, B., Naccache, C., Coudutier, G., Praliaud, H., Meriqudeau, P., Gallezot, P., Martin, G. A., Vedrine, J. C., Eds., Mefal-Support and Metal-Additive Effecfs in Catalysis; Elsevier: New York, 1982. (4) Tauster, S. J.; Fung, S. C.; Baker, R. T. K.; Horsley, J. S. Science 1981, 211, 1121. (5) Resasco, D. E.; Haller, G.L. J. Cutul. 1983, 82, 279.
0022-3654/86/2090-1733$01.50/0
titania species onto metal particles during heating and reduction by H2. Such a migration clearly implies a chemical interaction or bonding which provides the thermodynamic driving force for the movement of the titanium entity from the surface of Ti02 to the surface of the metal particle. This migration must result in both a geometric and electronic perturbation of the metal surface. Thus, two of the most important unanswered questions concern the nature of the titanium species which migrates (metallic or
(6) Tatarchuk, B. J.; Dumesic, J. A. J. Catal. 1981, 70, 308-322, 323-334, 335-346. (7) Takatani, S.; Cheng, Y.-W. J. Cafal. 1984, 90, 75. (8) Raupp, G. B.; Dumesic, J. A. J . Phys. Chem. 1984, 88, 660. (9) Simoens, A. J.; Baker, R. T. K.; Dwyer, D. J.; Lund, C. R. F.; Madon, R.J. J . Catal. 1984, 86, 359. (10) Cairns, J. A,; Baglin, J. E. E.; Clark, G. J.; Ziegler, J. F. J. Catal. 1983, 83, 301. (11) Belton, D. N.; Sun, Y.-M.; White, J. M. J . Phys. Chem. 1984, 88, 1690. (12) Belton, D. N.; Sun, Y.-M.; White, J. M. J. Phys. Chem. 1984, 88, 5172. (13) Vannice, M. A,; Sudhakar, C. J. Phys. Chem. 1984, 88, 2429. (14) KO,C. S.; Gorte, R. J. J. Catal. 1984, 90, 59. (15) Dryer, D. J.; Cameron, S. D.; Gland, J. Surf. Sci. 1985, 1-79, 430. (16) Sadeghi, H . R.; Henrich, V. E. J. Catal. 1984, 87, 279.
0 1986 American Chemical Society
1734 7 h r Journal of Physical Chemistry, Vol. 90, No. 9, 1986
oxidized) and the kind of bonding between this species and the group VI11 (group 8-10) metal. We report here the first evidence for direct Rh-Ti bonding for a highly dispersed Rh on TiO, catalyst. The catalyst was prepared by ion exchange. The TiO, was first solution at pH 11 for 15 h while stirring immersed in ".,OH continuouly with a magnetic stirring bar. It was then washed with distilled water until a pH near 7.5 was obtained, filtered, and dried for 24 h at 373 K. This produced a discoloration of the TiO,. The ammonia-treated 'T'iO, was placed in a round bottom flask with water in the ratio of 60 cm3/g of TiO,. The temperature was raised to about 323 K and, with vigorous stirring, 1 cm3/g of TiOz of stock Rh(NO,), solution (0.0066 g of Rh/cm3) was added over a period of 5 h. The solution was allowed to stir overnight at room temperature. The resulting catalyst was centrifuged and washed several times with hot distilled water, allowed to dry at room temperature for several days, and then dried for 5 h at 383 K. After reduction in flowing H, at 773 K for 2 h and oxidation at 673 K for 2 h. the catalyst was stored in a desiccator. The metal loading measured by HCI extraction and atomic absorption was 0.47% and the H/Rh chemisorption measured by our standard procedure was I f 0.0S.17 X-ray absorption spectra were obtained at the Cornell high energy synchroton source (CHESS). All spectra reported here were obtained at liquid N, temperature. The catalyst was pressed into self-supporting wafers of approximately 20 mg/cm2. Two such wafers were placed in the inner chamber of a cell which allows in situ reduction. 'The cell is designed with a double set of Ai windows; one pair is mounted directly onto the heatable Cu block holding the catalyst sample and the second pair is mounted on the outer AI chamber which is continuously flushed with He. The cell can only be operated at atmospheric pressure because the seal between the windows and both chambers is obtained by mechanical compression of the AI foil windows. However, any small leak into the catalyst chamber will be pure He, and the flow rate through ) no significant dilution. the catalyst (>50 ~ n : ~ / m i nassures The reference materials were Rh foil and the intermetallic compound RhTi. The RhTi was provided by Dr. D. E. Peterson, Los AIamos National Laboratory. It was prepared by arc melting of the elements. X-ray diffraction indicated a single phase and spectrochemical analyses showed no major impurities and Fe, the element of highest impurity, at a concentration of 300 ppm. The RhTi compound is extremely hard. Samples for X-ray absorption were prepared by crushing in a special hardened steel motar and sieving the powder to less than 400 mesh. This powder was dispersed in Duco cement following the procedure outlined by Wong'* and two samples were stacked in the cell to niinimi7e pinholes. The extended X-ray absorption fine structure (EXAFS) above the K edge of Rh at 23.224 keV was measured. The absorbance was normalized to the atomic absorption of Rh after subtraction of the atomic absorption. The resulting interference function was weighted by the wave vector k = (2mE/h2)'/2 or k3 and Fourier transformed over the range of wave vectors 3 to 15 A-'. Transforms shown in Figures 1-3 were k3 weighted while those in Figures 5 and 6 are k weighted; all transforms are presented without phase correction. The magnitude of the Fourier transforms of the EXAFS of the oxidized catalyst is shown in Figure 1. In Figure 2, a-c, we show, respectively, the magnitude of the Fourier transforms after in situ reduction at 494 K for 90 min, 628 K for 60 min, and 775 K for 90 min (the catalyst had been exposed to CO at room temperature and oxidized at 373 K for 30 rnin between the 628 and 775 K reductions). The major peak in Figure 1 corresponds to the Rh-0 bond at 2.05 A (after phase correction) in the completely oxidized catalyst. However, the peak at about 3 A. (not corrected for phase shift) in Figure 1 does not correspond to any Rh-O-Rh distance in Rhol or Rh203. We believe this peak is associated with Rh bonding to TiO,, i.e., a Rh-0-Ti linkage. After reduction at 494 K a (17) Resasco. D. E.: Haller. G.L. J . Phys. Chem. 1984, 88, 4552. (18) Wong, 1. Nucl. Insrrrim. Methodr 1985, A238, 554.
Letters
Figure 1. The k3-weightedFourier transform ( k = 3-15 k') of the EXAFS spectra for the Rh/TiO, catalyst oxidized at 673 K.
pRh-Ti,
2.53i
4.0 6.0 8.0 10.0 12.0 R IN ANGSTROMS Figure 2. (a) The k3-weightedFourier transform ( k = 3-15 kl)of the EXAFS spectra for the Rh/Ti02 catalyst after in situ reduction at 494 K for 90 min (---). (b) The k3-weightedFourier transform ( k = 3-1 5 .&I) of the EXAFS spectra for the Rh/Ti02 catalyst after in situ reduction at 628 K for 60 min (-.-). (c) The k3-weighted Fourier transform ( k = 3-1 5 K-l) of the EXAFS spectra for the Rh/TiO, catalyst after in situ reduction at 775 K for 90 rnin (-). 0.0
2.0
second nearest-neighbor Rh peak, which represents Rh-Rh bonding at 2.69 A (after phase correction), appears and grows in as a result of reduction. This is the same bond distance as found in bulk Rh metal. The peak we associate with Rh bonding to TiOz disappears although Rh-O bonding is still very prominant. The relative magnitudes under the Rh-0 peaks in Figures 1 and 2a suggest that not more than one third of the Rh has been reduced. This is unusual compared to well-dispersed Rh supported on SiO, where complete reduction can be accomplished at 494 K. Reduction remains inconiplete at 628 K, Figure 2b, and there is evident the development of another peak as a shoulder on the Rh-Rh peak, which becomes a distint peak after reduction at 775
The Journal of Physical Chemistry, Vol. 90, No. 9, 1986 1735
Letters
12'01 9.0
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9 v)
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Figure 4. The inverse transform of the doublet between 1.5 and 3.0 is shown by 0 and the smooth curve is the best fit which produces the interatomic distances and coordination numbers shown in the row labeled Figure 3. The k3-weighted Fourier transform (k = 3-15
A-') of the
k'.
EXAFS spectra of RhTi. 5.0
TABLE I: Summary of Interatomic Distances, R , and Coordination Numbers. N . for Rh/TiO, Reduced at 775 K k' k k'
2 63 2.65 2 62
4.8 18 3.2
2.53 2.56 2.56"
1
1.9 1.9 1.9"
"These values (as well as the Debye-Waller term and Eo) were constrained to be equal to those values obtained for Rh-Ti k fit of the first peak in Figure 5.
K (see Figure 2c). We assign this peak to a Rh-Ti metallic bond. In order to estimate the internuclear bond distance Rh-Ti, we use the intermetallic compound RhTi. This compound probably has a monoclinic structure with a = 2.96 A, b = 2.86 A, c = 3.41 A, and @ = 90" 37'.19 For this structure there would be two Rh-Ti and two Rh-Rh distances, e.g., 2.86 and 2.96 for the latter. Thus the average Rh-Ti distance is 2.67 A and the average Rh-Rh distance is 2.91 A in the RhTi compound. These two peaks are obvious in the EXAFS modified radial distribution function (uncorrected for phase shifts) in Figure 3. Because @ is so close to 90", the structure is often reported (and may be) a tetragonal AuCuI structure with a = 4.17 and c = 3.354.2O This simpler structure would result in only a single Rh-Ti and Rh-Rh distance of 2.68 and 2.95 A, respectively. Fortunately, the average Rh-Ti bond length in the monoclinic structure and its equivalent are nearly identical. When Rh foil is used as a reference, the phase-corrected Rh-Rh distance calculated from a fit of the EXAFS of RhTi is 2.90 A, in excellent agreement with the diffraction measured crystallographic average distance reported for the monoclinic structure. In order to estimate the Rh-Ti distance in the catalyst, the doublet in Figure 2 was isolated by a window function and back-transformed. This interference function which represents the backscattering in the first coordination sphere of an average Kh was fit with a program that assumes two different elements with associated interatomic distances, coordination numbers, and Debye-Waller terms. The two elements were assumed to be Rh and Ti and Rh foil and RhTi intermetallic compound were used (19) Eremenko, V. N.; Shtepa, T. D.; Sirotenko, V. G . Poroshk. Mefall., Adad. Nauk Ukr. SSR 1966, 6, 68. ( 2 0 ) Raman, A,; Schubert, K. Z . Merallk. 1964, 55, 704.
E
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R IN ANGSTROMS Figure 5. The k-weighted Fourier transform (k = 3-15 A-') of the EXAFS spectra for the Rh/TiO, catalyst after in situ reduction at 7 7 5 K for 90 min.
as references for the Rh-Rh and Rh-Ti distances, amplitudes, and phase shifts, respectively. The resulting best fit is shown in Figure 4 and the interatomic distances and coordination numbers are listed in Table I. Because of the possibility of local minima in the multiparameter fit that can only be discriminated against by intuition and experience, we sought an objective check. When the EXAFS function used to produce the transforms exhibited in Figure 2 are weighted by k (instead of k3 as used in Figure 2) the doublet of Figure 2c becomes nearly resolved so that the peaks can be separately isolated and back-transformed, see Figure 5. This allows one to use a simplier fitting routine with fewer floating parameters and only a single reference, Rh foil or RhTi intermetallic compound. The results of this k weighted fit are also summarized in Table I. Were there no artefacts introduced by the fitting procedure, the two different fits of the same data should be
Letters
1736 The Journal of Physical Chemistry, Vol. 90, No. 9, 1986
10.0 p
4b--
4:O 6.0 8:O 10.0 R IN ANGSTROMS Figure 6. The k-weighted Fourier transform ( k = 3-15 A-') of the E X A F S spectra of RhTi.
0.0
2.0
identical. Within experimental uncertainty, that is the case for the interatomic distances and for the average coordination number for Ti around Rh. However, the average coordination number for Rh around Rh is significantly different. This is in part due to the fact that the k fit was performed on a windowed backtransform of one of the doublet peaks between 2.4 and 3.2 8, while comparison with Figure 2c suggests that both of these peaks as well as the peak attributed to Rh-Ti are under the doublet in Figure 2c. It may be that there are two different Rh-Rh distances, a result that could be anticipated if there was a contribution from both a bulk RhTi and Rh,Ti phases. We discount this (see below) because we believe the average coordination number for Ti around Rh indicates that Ti exists only as a surface phase. It should be noted that better agreement could be forced by moving the window off the minimum between the two peaks which could be rationalized if there were some interference from Rh and Ti backscattering. However, this cannot be done objectively without performing a more sophisticated analysis. Because the k fit to the Rh-Ti is simple and very good we have repeated the two-element k3 fit of the doublet in Figure 2c fixing the parameters associated with Rh-Ti to the values obtained from the k fit, the third row of Table I. This does not significantly change the Rh-Rh bond distance but does decrease the average Rh-Rh coordination number to 3.2. It seems reasonable to conclude that the average Ti coordination is about 2 and that the Rh coordination number is between 2 and 3. We believe that the observation that the Rh-Ti interatomic distance for the catalyst reduced at 775 K is 2.53-2.56 A, a bond significantly shorter than the 2.68-A bond in the intermetallic, indicates that the Ti (and perhaps the Rh) has cationic character. This is to be expected based on the model studies, e.g., see ref 16, which have always detected oxygen associated with the Ti that migrates over the surface of Rh particles. the metal-oxygen linkage suggested from the model studies where the Ti species has migrated onto bulk metal is reported to have a Ti/O ratio of about As noted earlier, we see what appears to be a Rh-0-metal distance in the oxidized catalyst which cannot be fitted to any Rh-0-Rh in the oxides. If we assume this is a Rh-0-Ti linkage, the Rh-Ti distance would be 3.83 8, which
suggests that the oxygen may bridge the metals, Rho Ti. While this peak disappears during low temperature reduction, Figure 2a, a peak in the same position returns after high temperature reduction, Figure 2c. Further work will be required to complete the picture, but the nature of the driving force which causes migration of a Ti species from TiOz to a metal surface, Le., metal-metal bonding, appears to be unequivocally established. To interpret the coordination number of Ti around the average Rh we can again use the RhTi intermetallic compound for our model. The AuCuI structure is an ordered layer structure with Ti layers between Rh layers. Noting that the number of Ti around Rh in the intermetallic is 8 and that, based on our hydrogen chemisorption, every Rh is exposed, then a complete overlayer of Ti on the Rh particles would give an average coordination number for Ti around Rh of one-half that of a Ti layer in the bulk of the intermetallic compound, Le., four. Our measured coordination number for Ti is only two suggesting approximately one-half a monolayer of Ti on the average Rh particle. This is in good agreement with the estimate made for similar catalysts based on the decrease in the activity for a structure insensitive reaction, the dehydrogenation of cyclohexane. We have previously reported that the rate of this reaction is decreased by about a factor of two when reaction rates of a 473 and 773 K reduced catalyst are compared,2' Le., the depression of the rate of the structureinsensitive reaction is consistent with about half coverage of Rh bq Ti. An appealing picture for the Rh interaction with Ti02 has been presented by Galicia et a1.22based on their electron microscopy of Rh/Ti02 prepared by an ion-exchange method very similar to ours. After reduction at 473 K, they observe two-dimensional structures of Rh with about 10-8, cross section and one-dimensional rows of Rh along the [OOl] direction of the (1 10) surface of rutile. Presumably these rows of Rh atoms lie in the valleys between bridging oxygen and cover the exposed fivefold Ti cations (see H e n r i ~ hfor~ ~a diagram and description of the (1 10) rutile structure). Galicia et al. have not yet performed electron microscopy after a high-temperature reduction which introduces surface Ti3+ions and 02-vacancies. Two types of 0-vacancy point defect can be formed by removal of bridging 02-or in-plane 02-.In the former case two of the sixfold surface cations have their coordination reduced to fivefold, while in the later case two fourfold cations are formed in the surface plane. Presumably one of these sites is the position taken up by Rh after high temperature reduction to form Rh-Ti bonds.
Acknowledgment. We thank D. E. Peterson and B. Roof of the Los Alamos National Laboratory for the preparation and X-ray analysis of the intermetallic compound RhTi and to V. E. Henrich for helpful discussion. We are grateful to B. M. Kincaid of Bell Laboratories for providing the EXAFS analysis programs and CHESS for providing beam time. This work was supported by the Department of Energy, Office of Basic Energy Sciences under Contract No. DE-AC02-81ER10829. (21) Haller, G. L.; Resasco, D. E.; Rouco, A. J. Trans. Faraday Disuss. 1982, 72, 109. (22) Galicia, E.; Cordoba, G.; Fuentes, S. IX North American Meeting, The Catalysis Society of North America, abstract P15, Houston, March 17-21, 1985. (23) Henrich, V . E. Rep. Progr. Phys. 1985, 48, 1481. (24) 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-transition elements comprise groups 3 through 12, and the p-block 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.. 111 3 and 13.)
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