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J. Phys. Chem. B 2005, 109, 12738-12741
Intact and Fragmented Triosmium Clusters on MgO: Characterization by X-ray Absorption Spectroscopy and High-Resolution Transmission Electron Microscopy Vinesh A. Bhirud,† Hakim Iddir,‡ Nigel D. Browning,*,†,‡ and Bruce C. Gates*,† Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, California 95616, and National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California 94720 ReceiVed: March 29, 2005; In Final Form: May 7, 2005
Oxidative fragmentation of the clusters Os3(CO)12 adsorbed on MgO powder was investigated by X-ray absorption spectroscopy and scanning transmission electron microscopy (STEM). Exposure of the clusters to air leads to their fragmentation, oxidation of the osmium, and formation of ensembles consisting of three Os atoms. X-ray absorption near-edge spectra demonstrate the oxidative nature of the fragmentation process. Extended X-ray absorption fine structure (EXAFS) spectra indicate an average Os-Os distance of 3.33 Å and an Os-Os coordination number of 2, consistent with the formation of ensembles of three Os atoms on the support. STEM images confirm the presence of such trinuclear ensembles, and the diameters of the observed scattering centers (6.0 Å) match that indicated by the EXAFS results.
Introduction Metal nanoclusters on high-area metal oxide supports are important catalysts.1 Key questions about such clusters, especially when they are extremely small and nearly two-dimensional, are how their properties differ from those of the bulk metal and what the metal oxidation states are. Among the most incisive methods for characterization of supported metal nanoclusters are high-resolution transmission electron microscopy (HRTEM), which determines sizes of individual clusters, and X-ray absorption spectroscopy (XAS), including (a) extended X-ray absorption fine structure (EXAFS) spectroscopy, which provides evidence of cluster sizes and interactions with the support, and (b) X-ray absorption near-edge structure (XANES) spectroscopy, which provides evidence of the metal oxidation state. Among the best-investigated, smallest, and most nearly uniform supported metal nanoclusters are those approximated as triosmium, synthesized from the precursor Os3(CO)12. The synthesis chemistry of osmium carbonyls on the basic surface of MgO is analogous to that occurring in basic solutions;2,3 similar chemistry also occurs on less strongly basic metal oxides (γ-Al2O3, SiO2, and TiO2).4,5 Oxide-supported osmium carbonyl clusters heated in inert atmospheres have been inferred to undergo breaking of the Os-Os bonds, with support OH groups participating as oxidizing agents. The products have been represented as isolated OsII(CO)n complexes (n ) 2 and 3) bonded to the oxide.6-8 The evidence for these species consists largely of νCO infrared (IR) and EXAFS spectra. High-resolution electron micrographs showed scattering centers about 6 Å in diameter after fragmentation of triosmium carbonyl clusters on γ-Al2O39 (consistent with the presence of ensembles of three Os atoms). Dossi et al.10 inferred the existence of such ensembles on SiO2 on the basis of IR spectra, indicating that triosmium carbonyl clusters were reconstructed upon CO treatment of the sample, consistent with the hypothesis that the Os atoms in the † ‡
University of California, Davis. Lawrence Berkeley National Laboratory.
fragmented clusters had not moved far from their original positions on the support. Scanning transmission electron microscopy (STEM) and Z-contrast microscopy provide especially good images of clusters of heavy metals on MgO, a support consisting only of light elements. We investigated fragmented osmium clusters on this support in an attempt to provide an accurate comparison of the average structural information provided by EXAFS spectroscopy with the evidence of individual clusters provided by STEM. The EXAFS and STEM results show Os3 ensembles of essentially the same size. Experimental Methods and Data Analysis Triosmium carbonyl clusters supported on MgO (EM Science) were prepared from Os3(CO)12 in dried, deoxygenated n-pentane slurried with MgO powder that had been calcined at 673 K and then evacuated at room temperature.2 All manipulations were done with the exclusion of air and moisture in a N2-filled glovebox. The solid samples contained 1 wt % Os. IR and XA spectroscopies were used to characterize the species formed on MgO after removal of the solvent and following treatments in air at 298 K for various times. Each IR spectrum is the average of 128 scans. EXAFS data representing each sample are the averages of five scans; details of the EXAFS data collection and analysis are as described.7,8,11 The samples were also characterized by Z-contrast electron microscopy with the 200 kV JEOL 2010F STEM at the University of Illinois at Chicago. The features of this microscope and the experimental settings for achieving high-resolution STEM are described in detail elsewhere.12,13 The probe size was 2 Å. Each specimen was prepared for microscopy by dipping a holey carbon-coated copper grid into the powder sample and dusting off the excess. The specimen was exposed to air for about an hour during the handling and transfer to the microscope column. The size distributions of the supported osmium ensembles were calculated by measuring the sizes of 50 ensembles in the micrographs. The measurements were made by considering the
10.1021/jp051601l CCC: $30.25 © 2005 American Chemical Society Published on Web 06/10/2005
Intact and Fragmented Triosmium Clusters on MgO
J. Phys. Chem. B, Vol. 109, No. 26, 2005 12739 TABLE 2: Sample Formed by Adsorption of Os3(CO)12 on MgO Calcined at 673 K, Followed by Air Exposure for 6 h at Room Temperaturea,b EXAFS parameters
Figure 1. IR spectra in the νCO region characterizing the MgOsupported sample prepared from Os3(CO)12 after treatment in air at room temperature, following exposure for (A) 15, (B) 45, (C) 120, and (D) 360 min.
TABLE 1: EXAFS Parameters Representing Samples Formed by Adsorption of Os3(CO)12 Dissolved in n-Pentane on MgO Calcined at 673 Ka,b EXAFS parameters backscatterer
N
R (Å)
103∆σ2 (Å2)
∆E0 (eV)
EXAFS ref
Os CO ligands Cterminal O* support contributions O
2.2
2.89
3.30
10.0
Os-Os
3.2 3.2
1.89 3.03
3.45 3.92
5.6 2.6
Os-C Os-O*
0.9
2.12
-1.54
-4.5
Re-O
a
Notation: N, coordination number; R, distance between absorber and backscatterer atom; ∆σ2, Debye-Waller factor; ∆E0, inner potential correction. b Errors: N, (20%; R, (0.02 Å; ∆σ2, (20%; ∆E0, (20%.
ensemble edge to be defined by a threshold intensity 15% greater than that of the background (support). Results Adsorption of Os3(CO)12 on MgO. The νCO IR spectrum of the as-prepared sample, including bands at 2075(s), 2006(vs), 1960(w), and 1930(s) cm-1 (Figure 1), is consistent with the formation of chemisorbed triosmium carbonyls.2 The spectrum suggests a high yield of the expected [Os3(CO)11]2tightly ion-paired to MgO.2,14 EXAFS results characterizing this sample are summarized in Table 1. The Os-Os coordination number of nearly 2 indicates the intact Os3 core. The Os-C and Os-O contributions are consistent with the IR spectra, indicating largely [Os3(CO)11]2(i.e., coordination numbers of approximately 3 (3.2) and bond distances consistent with terminal CO ligands on the clusters). The EXAFS data also include a contribution from the support oxygen, with an Os-O coordination number of 0.9 at a distance of 2.12 Å, which is typical of bonding of group 8 metals to metal oxides and suggests some distortion of the clusters so that Os atoms in some clusters interact with the support directly and not just through CO ligands.8,15 Cluster Fragmentation. The IR spectra of Figure 1 show that exposure of the sample to air at room temperature led to the formation of mixtures of osmium di- and tricarbonyls.16 As the exposure time increased, the intensity of the 2075 cm-1 band decreased; after 6 h, the spectrum, with νCO bands at 2121(w), 2087(w), 2018(s), and 1925(s) cm-1, shows that most of the
EXAFS ref
8.05
4.7
Os-Os
1.87 3.04
2.36 3.78
12.1 1.8
Os-C Os-O*
2.10
1.62
-8.2
Re-O
N
Os CO ligands Cterminal O* Support contributions O
2.0
3.33
1.9 1.9
a
2.2
Notation: as in Table 1.
b
103∆σ2 (Å2)
∆E0 (eV)
backscatterer
R (Å)
Errors: as in Table 1.
osmium was present in mononuclear osmium dicarbonyls; this result indicates the breakup of the cluster frame.2,4,6,16 EXAFS data recorded after 6 h exposure to air no longer show the Os-Os contribution originally observed at 2.89 Å, which is indicative of Os-Os bonds in the cluster. Thus, these data confirm the cluster fragmentation. The EXAFS data of the air-treated sample instead show an Os-Os contribution at 3.33 Å, with a coordination number of 2.0 (Table 2 and Figure 2), demonstrating that the osmium species formed by cluster fragmentation were present in groupings consisting on average of approximately three Os atoms. The EXAFS data also give evidence of CO ligands on the Os atoms, with an Os-CO coordination number of about 2 (Table 2), consistent with the IR evidence of osmium dicarbonyls. The Os-O EXAFS contribution (Table 2) shows that Os atoms interact with support surface oxygen atoms, with a coordination number of about 2 at a distance of 2.10 Å, consistent with bonding of the osmium dicarbonyls to the support, as expected on the basis of similar results for γ-Al2O3-supported samples.7 Figure 3 shows XANES spectra of the sample before and after fragmentation of the clusters. Fragmentation of the clusters led to an Os LIII edge energy shift of about +2 eV, consistent with oxidation of the osmium. Furthermore, the white line intensity was higher for the fragmented sample than the precursor, pointing to the electron-deficient nature of the osmium in the oxidized sample. Figure 4 is a scanning transmission electron micrograph showing the Z-contrast image of the air-exposed sample captured at magnification of 2 M×. It shows nearly uniform scattering centers distributed over the MgO surface. Some of the scattering centers are nearly triangular, with some being elongated. The elongated images depict the ensembles on the MgO face oriented along the direction of the beam, so the longer dimension was chosen for the size determination. The ensemble size distribution data are summarized in Figure 5. The images show scattering centers with an average diameter of 6.8 Å with a standard deviation of 0.9 Å. Considering the probe size of 2 Å, the deconvolution of the observed diameter leads to an estimate of the actual ensemble diameter of 6.0 ( 0.9 Å. Discussion Previous reports of oxidative fragmentation of triosmium carbonyl clusters on oxide supports indicate such fragmentation resulting from treatment in inert gas or under vacuum at temperatures ranging from 423 to 473 K, depending on the support (γ-Al2O3,4 TiO2,4 or SiO2 6). The presence of O2 and moisture has been found to accelerate the fragmentation of the cluster Ru3(CO)12 supported on SiO2,17 and the reactivity of this cluster is similar to that of Os3(CO)12. In the present work, we correspondingly found that, in the presence of moist air, the trinuclear osmium clusters on MgO fragmented at a
12740 J. Phys. Chem. B, Vol. 109, No. 26, 2005
Bhirud et al.
Figure 3. XANES characterizing the MgO-supported sample prepared from Os3(CO)12: (solid line) as-is sample; (dotted line) sample after treatment in air at room temperature for 6 h.
Figure 2. Results of analysis of Os LIII edge EXAFS data and comparison with fit obtained with the best calculated coordination parameters characterizing MgO-supported triosmium ensembles: (a, top) k3-weighted EXAFS function, k3χ (solid line), and sum of the calculated contributions (dotted line); (b, bottom) imaginary part and magnitude of the Fourier transform of data (solid line) and sum of the calculated contributions (dotted line).
Figure 4. Z-contrast image of sample formed from Os3(CO)12 supported on MgO after treatment in air at room temperature for 1 h.
much lower temperature (room temperature) than that required for fragmentation under vacuum or in an inert atmosphere (423-473 K). The fragmentation products were predominantly osmium dicarbonyls, as indicated by the IR and EXAFS results. The XANES data provide the first direct evidence that the fragmentation process leads to an increase in the oxidation state of the osmium, as indicated by the shifts of the Os LIII edge and the increase in white line intensity. Thus, the XANES results are consistent with the oxidation of Os during the fragmentation process. However, because the XANES signature is dependent both on the oxidation state of the Os and on its ligand environment, the XANES is not sufficient to determine the oxidation state exactly. Thus, although XANES provides clear evidence of oxidation of the osmium, we also rely on the chemistry of the process referred to above as a basis for describing the oxidative fragmentation. The EXAFS data characterizing the fragmented clusters show that the Os atoms remained on the support in groups of nearly three, as shown by the Os-Os coordination number of 2.0. Thus, the data point to ensembles consisting of approximately three Os atoms each. The Os-Os distance of 3.33 Å is markedly greater than the Os-Os bonding distance of about 2.7 Å in
osmium carbonyl clusters or aggregates of osmium metal, consistent with the presence of Os atoms in ensembles of individual complexes and not bonded together in clusters. The comparatively high value of the Debye-Waller factor characterizing this shell (Table 2) indicates a high degree of disorder in the shell. Assuming a triangular arrangement of the Os atoms in three-atom ensembles, this distance implies the presence of ensembles that are about 6 Å in diameter, calculated on the basis of an atomic radius of Os of 1.33 Å18 with the average Os-Os distance of 3.33 Å determined by the EXAFS data. The essential result is the comparison of this average ensemble diameter with that determined by the TEM data, 6.0 ( 0.9 Å.19,20 Thus, within the expected errors, the EXAFS and TEM data give the same ensemble size, and the TEM images show that the ensembles were nearly uniform, consistent with the inference that they formed from the triosmium carbonyl clusters on the surface (formed by chemisorption of Os3(CO)12) in an oxidative fragmentation process that evidently left the Os atoms in nearly their original positions on the MgO surface. The variation in the shapes of the ensembles as shown by the micrograph is inferred to arise from irregularities in the positioning of Os atoms on the support surface, as would be
Intact and Fragmented Triosmium Clusters on MgO
J. Phys. Chem. B, Vol. 109, No. 26, 2005 12741 Acknowledgment. This research was supported by the Petroleum Research Fund, administered by the American Chemical Society (Grants 39484-AC3 [B.C.G.] and 37552-AC5 [N.D.B.]), and by the U.S. Department of Energy (DOE), Office of Energy Research, Office of Basic Energy Sciences, Division of Chemical Sciences, Contract FG02-04ER15600. We acknowledge the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the DOE, Division of Materials Sciences and Division of Chemical Sciences, under Contract No. DE-AC02-98CH10886; the staff at beamline X18B; and the Research Resources Center at the University of Illinois at Chicago for the use of microscopy facilities. References and Notes
Figure 5. Size distribution of osmium ensembles obtained from the Z-contrast images of the sample after treatment in air. The results are based on measurements of 50 ensembles.
expected because of nonuniformity of the MgO surface. It is clear that the Os cations do not migrate far from their initial positions where the clusters were anchored, finding positions of relative energy minima nearby on the MgO and bonding there. The supported triosmium ensembles are among the most nearly monodisperse nanostructures. The new information presented here resulting from the highquality high-angle annular dark field images gives the first evidence of the variety of shapes resulting from the fragmentation of metal clusters on a nonuniform support surface. The agreement between the ensemble sizes determined by TEM and EXAFS spectroscopy provides a good basis for comparison of the size estimates based on the averaging technique (EXAFS) and a technique (TEM) that determines spatially resolved images of individual ensembles. The data are a strong demonstration of the uniformity of the ensembles in the sample. Conclusions Adsorption of Os3(CO)12 on MgO gives supported triosmium carbonyls, which, upon exposure to air at room temperature, undergo oxidative fragmentation to form ensembles of three Os atoms each, with an average spacing of 3.33 Å. XANES indicates that the Os atoms are oxidized, confirming the oxidative nature of the cluster fragmentation. The average ensemble size of 6.0 ( 0.9 Å observed by TEM agrees well with the EXAFS results. This comparison demonstrates the uniformity of the clusters in the sample.
(1) Guzman, J.; Gates, B. C. J. Chem. Soc., Dalton Trans. 2003, 17, 3303. (2) Lamb, H. H.; Fung, A. S.; Tooley, P. A.; Puga, J.; Krause, T. R.; Kelley, M. J.; Gates, B. C. J. Am. Chem. Soc. 1989, 111, 8367-73. (3) Gates, B. C. J. Mol. Catal. 1994, 86, 95-108. (4) Deeba, M.; Gates, B. C. J. Catal. 1981, 67, 303-7. (5) Dossi, C.; Fusi, A.; Psaro, R.; Ugo, R.; Zanoni, R. In Structure and ReactiVity of Surfaces; Morterra, A., Zacchina, A., Costa, G., Eds.; Elsevier: Amsterdam, 1989. (6) Psaro, R.; Ugo, R.; Zanderighi, G. M.; Besson, B.; Smith, A. K.; Basset, J. M. J. Organomet. Chem. 1981, 213, 215-247. (7) Deutsch, S. E.; Chang, J. R.; Gates, B. C. Langmuir 1993, 9, 12849. (8) Duivenvoorden, F. B. M.; Koningsberger, D. C.; Uh, Y. S.; Gates, B. C. J. Am. Chem. Soc. 1986, 108, 6254-62. (9) Schwank, J.; Allard, L. F.; Deeba, M.; Gates, B. C. J. Catal. 1983, 84, 27-37. (10) Dossi, C.; Fusi, A.; Grilli, R.; Psaro, R.; Ugo, R.; Zanoni, R. Catal. Today 1988, 2, 585-594. (11) van Zon, J. B. A. D.; Koningsberger, D. C.; van’t Blik, H. F. J.; Sayers, D. E. J. Chem. Phys. 1985, 82, 5742. (12) James, E. M.; Browning, N. D.; Nichols, A. N.; Kawasaki, M.; Xin, Y.; Stemmer, S. J. Electron Microsc. 1998, 47, 561-574. (13) James, E. M.; Browning, N. D. Ultramicroscopy 1999, 78, 125139. (14) Psaro, R.; Dossi, C.; Ugo, R. J. Mol. Catal. 1983, 21, 33. (15) Lamb, H. H.; Gates, B. C.; Kno¨zinger, H. Angew. Chem., Int. Ed. Engl. 1988, 100, 1162-80. (16) Deeba, M.; Scott, J. P.; Barth, R.; Gates, B. C. J. Catal. 1981, 71, 373-80. (17) Zanderighi, G. M.; Dossi, C.; Ugo, R.; Psaro, R.; Theolier, A.; Choplin, A.; D’Ornelas, L.; Basset, J. M. J. Organomet. Chem. 1985, 296, 127-146. (18) Shriver, D.; Atkins, P. Inorganic Chemistry; 3rd ed.; W. H. Freeman and Co.: New York, 1999. (19) Previously, comparisons of EXAFS and HRTEM measures of cluster size were reported for MgO-supported [Os5C(CO)14]2-, but the comparison is not as exact as ours because these clusters were present in a mixture with intermediates formed in the synthesis, such as [H3Os4(CO)12]-.20 (20) Allard, L. F.; Panjabi, G. A.; Salvi, S. N.; Gates, B. C. Nano Lett. 2002, 2, 381-384.
