13408
J. Phys. Chem. 1996, 100, 13408-13415
Supported Ir and Pt Clusters: Reactivity with Oxygen Investigated by Extended X-ray Absorption Fine Structure Spectroscopy S. E. Deutsch,† J. T. Miller,‡ K. Tomishige,§ Y. Iwasawa,§ W. A. Weber,† and B. C. Gates*,† Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, California 95616, Amoco Oil Company, Research and DeVelopment Department, P.O. Box 3011, NaperVille, Illinois 60566, and Department of Chemistry, Faculty of Science, The UniVersity of Tokyo, Bunkyo-ku, Tokyo 113, Japan ReceiVed: February 20, 1996; In Final Form: June 3, 1996X
Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to investigate the effects of oxidation and reduction on supported noble metal clusters. After long-term exposure to laboratory air at room temperature, platinum clusters with an average diameter of approximately 7 Å inside the pores of BaKLTL zeolite were almost completely fragmented. The EXAFS data indicate small, highly disordered platinumplatinum contributions but no bulk platinum oxides. Treatment in H2 at 450 °C led to the formation of platinum clusters about 10 Å in average diameter, and exposure of these to air at room temperature led to the formation of a disordered platinum-oxide-like phase covering a core of platinum atoms. The reactivity of platinum clusters with oxygen is size-dependent, and platinum cluster growth by oxidation-reduction cycles may be inhibited by the size of the zeolite pores. Similarly, iridium clusters on MgO, approximated as Ir4, were fragmented by treatment in O2, but treatment of the oxidized iridium clusters with H2 at 573 K resulted in the formation of clusters with an Ir-Ir coordination number of 2.7 (and an Ir-Ir distance of 2.68 Å), indicating slightly less than four Ir atoms per cluster. These data are nearly the same as those characterizing the original sample prior to oxidation, suggesting that the clusters had been reformed into almost their original state. This is the first evidence of supported metal clusters being oxidized and then regenerated nearly intact. The near reversibility of the oxidation-reduction process may be unique to iridium, being related to its slight but non-negligible oxophilicity and its resistance to sintering.
Introduction diameter1
Supported metal clusters smaller than about 10 Å in are applied commercially as alkane dehydrocyclization catalysts; the clusters are platinum, and the support is basic LTL zeolite.2 The catalyst is highly selective for production of aromatics.3 The platinum clusters have been shown by extended X-ray absorption fine structure (EXAFS) spectroscopy to consist of about 5-12 atoms each, on average.1,3-5 Because of their smallness, nonuniformity, and interactions with supports, supported clusters are difficult to characterize precisely.1 EXAFS spectroscopy has provided the most incisive characterization data.1 Because almost all the atoms in an extremely small supported cluster are in contact with the support, the support may act as a ligand that substantially affects the reactivity of the cluster. Here, we report an EXAFS investigation of the reactivity of supported platinum and iridium clusters with O2. This reactivity is important because the catalysts are routinely handled in air prior to application and prior to characterization by physical methods such as transmission electron microscopy and because they may be treated in O2 for regeneration to burn off carbonaceous deposits. The samples were characterized before and after exposure to air and after reduction with H2. Experimental methods Preparation of Samples. Preparation of LTL-ZeoliteSupported Platinum Clusters. Platinum clusters in BaKLTL * To whom correspondence should be addressed. † University of California, Davis. ‡ Amoco Oil Company. § The University of Tokyo. X Abstract published in AdVance ACS Abstracts, July 15, 1996.
S0022-3654(96)00507-2 CCC: $12.00
zeolite were prepared as follows. KLTL zeolite (Linde) was ion exchanged with aqueous barium nitrate, washed, and dried at 127 °C. The resultant BaKLTL zeolite was impregnated with aqueous tetraamine platinum(II) nitrate so that the platinum content of the resultant dried material was 1.2 wt %. The resultant Pt/BaKLTL sample was reduced in flowing H2 for 1 h at 500 °C. A portion of the reduced sample was allowed to stand in laboratory air for 4 years. EXAFS experiments were done beginning with this sample, referred to as the aged sample. Preparation of MgO-Supported Iridium Clusters. MgOsupported iridium clusters were prepared according to published methods.6 MgO (EM Science) was calcined at 700 °C under flowing O2 for 2 h and then at 700 °C under vacuum for 14 h. The BET surface area of the MgO was 18 m2/g, and the surface was about 5% hydroxylated.7 Hexane was dried by distilling over Na/benzophenone and degassed by repeated freezepump-thaw cycles. [Ir4(CO)12] (Strem) (0.029 g) and MgO (2.00 g) were slurried in hexane for 8 h, and the solvent was removed by evacuation overnight. The resultant sample contained 1 wt % Ir. EXAFS Spectroscopy. EXAFS measurements were carried out at beamline 2-3 at the Stanford Synchrotron Radiation Laboratory (SSRL), Stanford, CA. Incoming X-rays were monochromatized by a Si(111) double-crystal monochromator. Higher harmonics were rejected by detuning the monochromator by 15%. The ring current was at least 50 mA for all measurements. The ring energy was 3 GeV. Samples were scanned in the region of the Pt LIII edge (11 564 eV) or the Ir LIII edge (11 215 eV). Measurements were done with the sample at about -150 °C. EXAFS measurements were done similarly with one of the Pt/BaKLTL samples at beamline 10B at the Photon Factory in Tsukuba, Japan. Incoming X-rays were monochromatized by © 1996 American Chemical Society
Ir and Pt Clusters
J. Phys. Chem., Vol. 100, No. 32, 1996 13409
TABLE 1: Structural Parameters Characterizing Reference Compounds Used in EXAFS Data Analysisa crystallographic data
Fourier transform
sample
shell
N
R, Å
∆k, Å-1
∆r, Å-1
n
Pt foil Na2Pt(OH)6
Pt-Pt Pt-O
12 6
2.77 2.05
1.9-19.8 1.4-17.7
1.9-3.0 0.5-2.0
3 3
a Notation: N, coordination number for absorber-backscatterer pair; R, radial distance from absorber to backscatterer; ∆k, limits used for forward Fourier transformation (k is the wave vector); ∆r, limits used for inverse Fourier transformation (r is the distance); n, power of k used for Fourier transformation.
a Si(311) channel-cut monochromator. The ring energy was 2.5 GeV. Samples were scanned in the region of the Pt LIII edge. The sample temperature was about -170 °C. The Pt/BaKLTL sample (140 mg) was prepared by pressing into a self-supporting wafer and loading into an airtight EXAFS cell. Treatments of the sample prior to EXAFS measurements were carried out in situ as follows. The cell was evacuated and cooled to -150 °C, whereupon the sample was scanned six times. The sample in the cell was then treated in flowing H2 at 500 °C for 30 min, cooled in flowing H2, and evacuated before cooling to -150 °C; it was then scanned five times. Following these scans, the cell was warmed to room temperature, and the sample was exposed to air at room temperature for 15 min before the cell was evacuated and cooled to -150 °C for eight more scans. The Ir/MgO sample was prepared as follows. Approximately 175 mg of the MgO-supported iridium sample was pressed into a self-supporting wafer and loaded into an airtight EXAFS cell. It was treated in flowing He at 300 °C for 2 h followed by flowing H2 at 300 °C for 2 h. The cell was then evacuated and the sample scanned. It was then opened to the air (in a matter of seconds), heated to 50 °C, held at this temperature for 30 min, cooled to -150 °C, and scanned again. Then it was treated in flowing H2 at 300 °C for 1 h, cooled to room temperature, evacuated, cooled to -150 °C, and scanned again. EXAFS Data Analysis. The analysis of the data obtained at SSRL was carried out with the Vaarkamp-Koningsberger software on the basis of experimentally determined phase shifts and backscattering amplitudes characterizing materials with known crystal structures. The Pt-Pt and Ir-Ir interactions were analyzed with phase-shift and backscattering amplitude data obtained from EXAFS spectra measured for Pt foil. The Pt-O and Ir-O contributions arising from interactions of the clusters with the support or with adsorbed oxygen were analyzed with phase shifts and backscattering amplitudes obtained from EXAFS data for Na2Pt(OH)6. EXAFS parameters for the reference materials are summarized in Table 1. The raw EXAFS (χ) data were obtained by using standard techniques.8 Normalization of the EXAFS data was done by dividing the absorption intensity by the height of the background
absorption at 50 eV past the absorption edge. The main contributions of the EXAFS were isolated by Fourier filtering of the final EXAFS function (extracted EXAFS data) with k3 weighting (k is the wave vector). The Fourier filtering parameters for each sample are given in Table 2. Data analysis was performed on the filtered data. Best-fit parameters were obtained by using the Koningsberger difference file technique.8,9 Low-Z and high-Z contributions were fitted in both r space (r is the distance from absorbing atom to backscattering atom) and k space, with both k1 and k3 weighting so as not to overemphasize either the low-Z or high-Z backscatterers. The data obtained at the Photon Factory were analyzed similarly with XDAP,10 a recent version of the VaarkampKoningsberger software. The methods are almost the same as those described above; details are summarized elsewhere.11 For comparison, these data were also analyzed with the program EXAFSH, with the fitting done only in k space by using theoretical parameters determined with the program FEFF5.12 Results EXAFS Spectra of Pt/BaKLTL Zeolites. Oxidized Platinum Clusters. The best-fit EXAFS parameters for the Pt/ BaKLTL zeolite samples are given in Table 3. The raw EXAFS data characterizing the samples are shown in Figure 1, and Fourier transforms of the filtered data are shown in Figure 2. Only low-amplitude oscillations are evident for k > 8 Á-1 for the samples exposed to O2. A small peak near 2.7 Å in the Fourier transform indicates a Pt backscatterer. These samples are characterized by a Pt-Pt contribution with a low coordination number at a distance shorter than that characterizing bulk platinum (2.77 Å), indicating the presence of small platinum clusters. Parameters determined from the two analyses of the data obtained at the Photon Factory are summarized in Table 3. These parameters are similar but not identical to those cited above. Reduced Platinum Clusters. The raw EXAFS data characterizing the sample that had been oxidized and then reduced by treatment in flowing H2 at 500 °C for 30 min are shown in Figure 1. Oscillations were evident at values of k as large as 14 Å-1. The magnitude of the Fourier transform of the filtered data is shown in Figure 2b. A large peak in the Fourier transform at about 2.8 Å indicates Pt-Pt bonding. This sample is characterized by a Pt-Pt contribution with a coordination number of 5.8 at a distance of 2.76 Å, indicative of the metalmetal bonding typically observed in supported metals. The coordination number shows that the cluster size increased as a result of the H2 treatment. EXAFS Spectra of MgO-Supported Iridium Clusters. Supported Iridium Clusters Formed by Decarbonylation of Iridium Carbonyl Precursor. The raw EXAFS data characterizing the decarbonylated clusters, Ir/MgO, prior to treatment in
TABLE 2: Parameters Used for Fourier Filtering of Raw EXAFS Data Characterizing Oxidized and Reduced Pt/BaKLTL Samplesa sample b
Pt/BaKLTL Pt/BaKLTLc Pt/BaKLTLb Pt/BaKLTLb Ir/MgOb Ir/MgOb Ir/MgOb
treatment
∆k, Å-1
∆r, Å-1
n
p
air, 4 years air, 4 years H2, 723 K, 30 min air, room temp, 15 min He, 300 °C, 2 h air, 50 °C, 30 min H2, 300 °C, 1 h
2.83-13.37 2.00-13.10 3.62-14.39 2.67-11.73 3.07-15.71 3.04-12.33 2.54-16.08
1.10-2.40 1.20-2.50 1.57-3.25 1.07-3.24 1.21-3.18 1.15-2.92 1.34-3.35
3 3 2 3 3 3 3
9.0 10.2 12.5 13.5 16.9 11.5 18.3
a Notation: ∆k, range used for forward Fourier transformation; ∆r, range used for inverse Fourier transformation; n, power of k weighting used in forward Fourier transformation; p, maximum number of free parameters given by the Nyquist theorem, p ) 2∆k∆r/π + 1. b Data acquired at SSRL. c Data acquired at Photon Factory.
13410 J. Phys. Chem., Vol. 100, No. 32, 1996
Deutsch et al.
TABLE 3: Best-Fit EXAFS Parameters Characterizing Pt/BaKLTL Samplesa treatment
backscatterer
N
R (Å)
∆σ2 (Å2)
∆E0 (eV)
H2, 450 °C, 1 h (data from ref 4)
Pt O O Ba Pt O O Pt Os
3.7 0.6 1.1 1.4 2.5 2.4 0.3 (1.2) [2.9] (3.8) [0.6] (0.22) (1.2) 5.8 0.5 0.1 4.0 1.7 0.4
2.75 2.14 2.71 3.76 2.69 2.06 2.61 (2.694) [2.02] (2.05) [2.55] (2.58) (3.77) 2.76 2.13 2.74 2.72 2.05 2.61
0.000 32 0.000 67 0.000 95 0.000 90 0.009 90 0.000 09 -0.004 27 0.005 50 [0.004 99] (0.007 02) [-0.000 53] (-0.007 53) (-0.003 85) 0.003 41 -0.005 31 -0.008 59 0.009 85 0.003 03 -0.007 70
1.4 1.8 2.1 6.6 -1.14 0.86 3.79 -0.16 [9.12] (2.9) [5.79] (-8.5) (-1.05) -3.71 -1.61 -5.60 -2.12 2.82 2.71
air, room temp, 4 years (SSRL) air, room temp, 4 years (PF)
Ol H2, 450 °C, 30 min (SSRL) air, room temp, 15 min (SSRL)
Ol Pt O O Pt O O
aNotation: N, Ir backscatterer coordination number; R, distance from Ir to backscatterer; ∆σ2, Debye-Waller factor; ∆E , inner potential correction; 0 SSRL, data acquired at Stanford Synchrotron Radiation Laboratory; PF, data acquired at Photon Factory. Parameter values shown neither in parentheses nor brackets were determined with the Vaarkamp-Koningsberger Eindhoven University data analysis software. Parameter values shown in parentheses were determined with the XDAP software,10 with the data analysis method being virtually the same as that described by Zhao and Gates.11 Parameter values shown in brackets were determined with the EXAFSH software, with fitting done only in k space.
Figure 1. Raw EXAFS data characterizing the Pt/BaKLTL samples: (a) platinum clusters after exposure to air for 4 years (data acquired at SSRL); (b) platinum clusters after exposure to air for 4 years (data acquired at Photon Factory); (c) platinum clusters after treatment of (a) in H2 at 450 °C for 30 min; (d) platinum clusters after treatment of (c) in air at room temperature for 15 min.
O2, are shown in Figure 3a, and the Fourier transform of the filtered data is shown in Figure 4a. The EXAFS parameters, determined by a fit in both k space and r space, are given in Table 4. The data are characterized by oscillations at values
of k as large as 16 Å-1, indicative of metal-metal interactions, and there is a prominent peak near 2.7 Å in the Fourier transform indicative of iridium backscatters. The Ir-Ir coordination number (3.2) indicates the presence of small clusters.
Ir and Pt Clusters
Figure 2. Fourier transforms (k3-weighted and phase and amplitude corrected using Pt foil reference data) of filtered EXAFS data characterizing the Pt/BaKLTL samples: (a) platinum clusters after exposure to air for 4 years (data acquired at SSRL); (b) platinum clusters after treatment of (a) in H2 at 450 °C for 30 min; (c) platinum clusters after treatment of (b) in air at room temperature for 15 min.
Supported Iridium Clusters Following Exposure to Air. The raw EXAFS data characterizing the Ir/MgO sample after exposure to air at 50 °C for 30 min (Figure 3b) are characterized by almost no oscillations for values of k > 8 Å-1, indicating a lack of high-Z backscatterers such as iridium. The magnitude of the Fourier transform of the filtered EXAFS data is shown in Figure 4b. There is a large peak around 2.0 Å and a smaller peak near 2.7 Å, consistent with the loss of Ir-Ir bonds resulting from oxidation of the clusters. The EXAFS parameters are given in Table 4.
J. Phys. Chem., Vol. 100, No. 32, 1996 13411
Figure 3. Raw EXAFS data characterizing the Ir/MgO samples: (a) iridium clusters formed by treatment of [Ir4(CO)12] adsorbed on MgO with He and the H2 at 300 °C for 2 h; (b) iridium clusters after treatment in air at 50 °C for 30 min; (c) iridium clusters in (b) after further treatment in H2 at 300 °C for 1 h.
Supported Iridium Clusters Formed by Treatment of Oxidized Sample in H2. The raw EXAFS data characterizing the sample after it had been oxidized and then treated in H2 are shown in Figure 3a; the Fourier transform of the filtered data is shown in Figure 4c. Calculated best-fit EXAFS parameters, determined by a fit in both k space and r space, are given in Table 4. The data are characterized by oscillations at values of k as large as 16 Å-1, indicative of metal-metal interactions, and a prominent peak near 2.7 Å in the Fourier transform indicates iridium backscatterers. The Ir-Ir coordination number (2.7) indicates small clusters. Discussion Oxidation and Reconstruction of Supported Iridium Clusters. Small metal clusters may have unique catalytic
13412 J. Phys. Chem., Vol. 100, No. 32, 1996
Figure 4. Fourier transforms (k3-weighted and phase and amplitude corrected using Pt foil reference data) of the filtered EXAFS data characterizing the Ir/MgO samples: (a) iridium clusters formed by treatment of [Ir4(CO)12] adsorbed on MgO with He and the H2 at 300 °C for 2 h; (b) iridium clusters after treatment in air at 50 °C for 30 min; (c) iridium clusters in (b) after further treatment in H2 at 300 °C for 1 h.
properties. The high selectivity for aromatization of n-hexane catalyzed by platinum clusters in LTL zeolite is consistent with this suggestion (but the small cluster size is likely not the sole explanation for the high selectivity).1 The performance of this supported cluster catalyst has stimulated interest in the properties of metal clusters that are stabilized by dispersion on supports. Small platinum clusters ( 8 -1, and the Fourier transform of the filtered EXAFS data (Figure 2) shows only small peaks in the region from 2.5-3.0 Å. These results indicate that there were few Pt-Pt bonds in the sample, consistent with the presence of broken-up clusters resulting from oxidation. The oxidation leads to disordering of the clusters, evidenced by the high Debye-Waller factors and the short PtPt distances. Reduction of the aged sample led to the formation of larger platinum clusters than were present in the original sample.4 After treatment of the aged sample in H2 at 450 °C, the Pt-Pt coordination number had increased to 5.8 from the original value reported earlier to be 3.7.4 This coordination number of 5.8 indicates platinum clusters of roughly 15-20 atoms each on average. Such clusters are approximately 10 Å in diameter.32 They are small enough to fit in the ellipsoidal cages of LTL zeolite, which have dimensions of 4.8 Å × 12.4 Å × 10.7 Å. We suggest that the zeolite may help to confine the platinum in approximately 10 Å clusters. Because the EXAFS data show that the Pt-Pt coordination number increased markedly as a result of the H2 treatment, we infer that even with the confinement by the zeolite cages, there was still some cluster growth associated with the O2 and H2 treatments. This growth may be associated with the nobility of platinum, and it may be related to the presence of water in the pores of the zeolite, resulting either from adsorption from air or from reaction of platinum oxide with H2. Water in the form of steam or surface hydroxyl groups has been observed to facilitate sintering of supported metals.33,34 The EXAFS data give evidence of Pt-Pt interactions, indicating the presence of 7 Å Pt clusters on average in the sample exposed to air for 15 min. The sample had initially contained approximately 10 Å clusters on average. This decrease in cluster size may have resulted from fragmentation of the approximately 10 Å clusters by oxidation. Alternatively, the EXAFS data could be interpreted as the result of disruption of surface Pt-Pt bonds in the approximately 10 Å clusters by oxidation, leading to approximately 7 Å platinum clusters covered by oxide overlayers. Comparison of the two reductionoxidation cycles suggests that the latter explanation is more plausible. The original clusters were roughly the same size as the clusters after the second oxidation. The original clusters showed evidence of fragmentation after exposure to air. We expect that the behavior of 7 Å clusters in air would be the same after reduction as after some exposure to air. The reason that the approximately 7 Å clusters remaining after exposure to air do not fragment further may be that they are covered by a shell of oxidized platinum. A thin shell of disordered platinum
13414 J. Phys. Chem., Vol. 100, No. 32, 1996 covered by oxygen may be responsible for the large DebyeWaller factor characterizing the sample. The results are consistent with the nature of noble metals like platinum, which are not easily oxidized. The composition of the oxidized platinum phase is unknown, but the EXAFS coordination numbers characterizing the shorter Pt-O distance (the bonding distance, Table 3) suggest a Pt:O atomic ratio of roughly 1:2. Structures with this composition could be formed either by dissociative chemisorption of O2 on platinum or by formation of surface Pt-O-Pt bridged species. Consistent with our interpretation, Fukushima et al.35 reported EXAFS data showing that oxygen adsorption on γ-Al2O3supported platinum aggregates about 10 Å in diameter at -196 °C followed by warming to room temperature caused severe disruption of the metal-metal bonds. In contrast, the same treatment had much less effect on crystallites about 26 Å in average diameter. The authors also reported a Pt-Pt interaction at 3.15 Å after oxygen adsorption on the approximately 10 Å aggregates (which they attributed to platinum oxide) and a decrease of 0.2 Å in the Pt-Pt distance as the aggregates were oxidized. An effect of platinum particle size on the stoichiometry of platinum oxide particles formed by treatment in O2 has been reported as well. Nandi et al.36 and Huizinga et al.37 reported that platinum particles with greater than 40% dispersion (percentage of Pt atoms exposed) exist as Pt3O4 after exposure to air, as determined by EXAFS spectroscopy and oxygen chemisorption. Nandi et al.36 reported that particles with a lower dispersion, after exposure to air, exist as Pt3O4 covering a platinum core. McCabe et al.38 concluded from chemisorption, X-ray diffraction, and EXAFS data that oxidation of supported platinum particles occurs only on the surface of the platinum, regardless of treatment temperature or time of exposure. Aggregates smaller than about 8 Å in diameter are completely oxidized to PtO2, but the nature of oxygen interacting with surface platinum could not be determined for aggregates or particles larger than 8 Å. No distinction could be made between chemisorbed oxygen and an oxide film.38 These results and the small size of the original supported platinum clusters reported here before exposure to air4 suggest that long-term exposure to air caused complete oxidation of the clusters. Reduction of the sample after oxidation caused an increase in cluster size so that further exposure to air was not sufficient to oxidize the clusters completely. Rather, it only led to disruption of the platinum on the cluster surfaces. Comparison of EXAFS Data from Two Synchrotrons. The data of Table 3 provide the basis for at least a preliminary evaluation of the consistency of EXAFS data determined for equivalent samples at different synchrotrons. The consistency of the Ir-Ir and Ir-O distances is good; the uncertainties in the values are about (1-2%, as has been commonly assumed. The uncertainties in the Pt-Pt and Pt-O coordination numbers and Debye-Waller factors are much larger, about (30-40% (Table 3); there are too few data here for a thorough comparison. Conclusions The differences in behavior of iridium and platinum clusters in O2 and H2 are related to the relative oxophilicities (or nobilities) of the metals. The EXAFS results demonstrate that iridium and platinum clusters about 6-7 Å in diameter, on average, are highly susceptible to fragmentation by exposure to air, even at room temperature for short times (minutes). The clusters of the more oxophilic iridium (on MgO) were fragmented to a greater extent than those of the more noble platinum
Deutsch et al. (in BaKLTL zeolite). Because supported metal catalysts are routinely handled in air prior to application or characterization by electron microscopy or other techniques, care must be taken to account for the loss of cluster integrity. Exposure of such small iridium clusters on MgO to air is reversible; treatment of the oxidized clusters in H2 leads to regeneration of clusters of nearly the original size. Long-term exposure to air resulted in almost complete fragmentation of the small platinum clusters in BaKLTL zeolite, and treatment in H2 at 450 °C led to clusters that were about 10 Å in average diameter, markedly larger than the clusters present before oxidation but still small enough to fit in the zeolite pores. Exposure of the reduced platinum clusters to air for only 15 min resulted in a disordering of the clusters, giving structures similar to those formed after long-term air exposure, but this exposure is not sufficient to break all the Pt-Pt bonds. The EXAFS results characterizing the oxidized sample suggest the formation of clusters of platinum with shells of disordered platinum covered by oxygen. Acknowledgment. This work was supported by the U.S. Department of Energy, Office of Energy Research, Office of Basic Energy Sciences (Contract FG02-87ER13790), and the international collaboration was supported by the National Science Foundation (INT-8815553) and the Japan Society for the Promotion of Science. We thank the Stanford Synchrotron Radiation Laboratory and the Photon Factory for beam time. X-ray absorption data were analyzed with the Eindhoven University EXAFS Data Analysis Program, developed by M. Vaarkamp and D. C. Koningsberger, and with the XDAP software.10 References and Notes (1) Gates, B. C. Chem. ReV. 1995, 95, 511. (2) Hughes, T. R.; Mohr, D. H.; Wilson, C. R. In Proceedings of the 7th International Zeolite Conference; Elsevier Publishers, B.V.: Amsterdam, 1986; p 725. (3) Lane, G. S.; Modica, F. S.; Miller, J. T. J. Catal. 1991, 129, 145. (4) Vaarkamp, M.; Grondelle, J. V.; Miller, J. T.; Sajkowski, D. J.; Modica, F. S.; Lane, G. S.; Gates, B. C.; Koningsberger, D. C. Catal. Lett. 1990, 6, 369. (5) Vaarkamp, M.; Modica, F. S.; Miller, J. T.; Koningsberger, D. C. J. Catal. 1993, 144, 611. (6) van Zon, F. B. M.; Maloney, S. D.; Gates, B. C.; Koningsberger, D. C. J. Am. Chem. Soc. 1993, 115, 10317. (7) Anderson, P. J.; Horlock, R. F.; Oliver, J. F. Trans. Faraday Soc. 1965, 61, 2754. (8) Kampers, F. W. H. Thesis, University of Eindhoven, The Netherlands, 1988. (9) Kampers, F. W. H.; Engelen, C. W. R.; van Hooff, J. H. C.; Koningsberger, D. C. J. Phys. Chem. 1990, 94, 8574. (10) Vaarkamp, M.; Linders, J. C.; Koningsberger, D. C. Physica B 1995, 209, 159. (11) Zhao, A.; Gates, B. C. J. Am. Chem. Soc. 1996, 118, 2458. (12) Rehr, J. J.; de Leon, J. M.; Zabinsky, S. I.; Albers, R. C. J. Am. Chem. Soc. 1991, 113, 5135. (13) Folefoc, G. N.; Dwyer, J. J. Catal. 1992, 136, 43. (14) Kuba, T.; Arai, H.; Tominaga, H.; Kunugi, T. Bull. Chem. Soc. Jpn. 1972, 45, 607. (15) Kobayashi, M.; Inoue, Y.; Takahashi, N.; Burwell, R. L.; Butt, J. H.; Cohen, J. B. J. Catal. 1980, 64, 74. (16) Jaeger, N. I.; Jourdan, A. L.; Schulz-Ekloff, G. J. Chem. Soc., Faraday Trans. 1991, 87, 1251. (17) McVicker, G. B.; Kao, J. L.; Ziemiak, J. J.; Gates, W. E.; Robbins, J. L.; Treacy, M. M. J.; Rice, S. B.; Vanderspurt, T. H.; Cross, V. R.; Ghosh, A. K. J. Catal. 1993, 139, 48. (18) Fiedorow, R. M. J.; Chahar, B. S.; Wanke, S. E. J. Catal. 1978, 51, 193. (19) Koningsberger, D. C.; Gates, B. C. Catal. Lett. 1992, 14, 271. (20) Tauster, S. J.; Fung, S. C. J. Catal. 1978, 55, 29. (21) McVicker, G. B.; Garten, R. L.; Baker, R. T. K. J. Catal. 1978, 54, 129. (22) Brooks, C. S. J. Colloid Interface Sci. 1970, 34, 419.
Ir and Pt Clusters (23) Da Silva, P. N.; Guenin, M.; Leclercq, C.; Frety, R. Appl. Catal. 1989, 54, 203. (24) Frety, R.; Da Silva, P. N.; Guenin, M. Appl. Catal. 1990, 58, 175. (25) Falconer, J. L.; Wentrcek, P. R.; Wise, H. J. Catal. 1976, 45, 248. (26) Alexeev, O.; Xiao, F.-S.; Gates, B. C. To be published. (27) Purnell, S. K.; Sanchez, K. M.; Patrini, R.; Chang, J.-R.; Gates, B. C. J. Phys. Chem. 1994, 98, 1205. (28) Eisenberger, P.; Brown, G. S. Solid State Commun. 1979, 29, 481. (29) Cotton, F. A.; Walton, R. A. Struct. Bonding (Berlin) 1985, 62, 1. (30) Marques, E. C.; Sandstrom, D. R.; Lytle, F. W.; Greegor, R. B. J. Chem. Phys. 1982, 77, 1027. (31) Robota, H. J.; Cohn, M. J.; Ringwelski, A. Z.; Eades, R. A. Mater.
J. Phys. Chem., Vol. 100, No. 32, 1996 13415 Res. Soc. Symp. Proc. 1988, 111, 201. (32) Kip, B. J.; Duivenvoorden, F. B. M.; Koningsberger, D. C.; Prins, R. J. Catal. 1987, 105, 26. (33) Triantafillou, N. D.; Gates, B. C. J. Phys. Chem. 1994, 98, 8431. (34) Ruckenstein, E.; Hu, X. D. J. Catal. 1986, 100, 1. (35) Fukushima, T.; Katzer, J. R.; Sayers, D. E.; Cook, J. In Proceedings of the 7th International Congress on Catalysis; Seiyama, T., Tanabe, K., Eds.; Elsevier: Amsterdam, 1981; p 79. (36) Nandi, R. K.; Molinaro, F.; Tang, C.; Cohen, J. B.; Butt, J. B.; Burwell, R. L., Jr. J. Catal. 1982, 78, 289. (37) Huizinga, T.; van Grondelle, J.; Prins, R. Appl. Catal. 1984, 10, 199. (38) McCabe, R. W.; Wong, C.; Woo, H. S. J. Catal. 1988, 114, 354.
JP960507P
