NANO LETTERS
Imaging of Nearly Uniform Os5C Clusters Dispersed on MgO Powder
2002 Vol. 2, No. 4 381-384
L. F. Allard,† G. A. Panjabi,‡ S. N. Salvi,‡ and B. C. Gates*,‡ Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, and Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, California 95616 Received September 5, 2001; Revised Manuscript Received December 18, 2001
ABSTRACT [Os5C(CO)14]2-
was synthesized in 65% yield on the surface of MgO by treatment of adsorbed [Os3(CO)12] in CO. High-resolution transmission electron microscopy (HRTEM) indicated that the nanoclusters were nearly monodisperse and in the size range consistent with [Os5C(CO)14]2-. The supported [Os5C(CO)14]2- was decarbonylated in helium, giving supported nanoclusters that are well approximated as Os5C, as shown by HRTEM and EXAFS data. Decarbonylation in H2 gave larger clusters of osmium, as shown by HRTEM.
Metal nanoclusters are present in many solid catalysts, being dispersed on stable, high-area porous supports such as metal oxides and zeolites. Conventional preparation methods, whereby metal salts are deposited on supports and then calcined and reduced, lead to nanoclusters and nanoparticles of various sizes and shapes. Because of their nonuniformity, these are difficult to characterize incisively, and thus researchers have been motivated to find methods to prepare uniform metal clusters on supports. One approach involves the high-yield synthesis (or adsorption) of molecular or ionic cluster precursors on supports followed by treatment to remove their ligands without disruption of the cluster frame.1 Nanoclusters in catalysts prepared by such methods have been inferred to be nearly uniform,1,2 but there are hardly any reports of images of the clusters to provide a stringent test of the uniformity. Our goal was to prepare such nanoclusters, characterize them by IR spectroscopy and synchrotron X-ray absorption spectroscopy to determine (indirectly) the presence of individual pentaosmium species, and to image them by high-resolution transmission electron microscopy (HRTEM) to provide direct information about the uniformity of the clusters. We report the preparation of MgO-supported nanoclusters from the supported precursor [Os5C(CO)14]2-, which was treated to remove the CO ligands. Osmium was chosen as the metal because (a) synthetic methods offer the opportunity to make the precursor in high yields on basic supports3 and (b) the heavy Os atoms offer high contrast to the support for imaging by HRTEM. MgO was chosen as the support because it is basic and allows the high-yield synthesis of the precursor3 and because it is advantageous for HRTEM, * Corresponding author: email:
[email protected]. † Oak Ridge National Laboratory. ‡ University of California. 10.1021/nl015621k CCC: $22.00 Published on Web 01/12/2002
© 2002 American Chemical Society
as follows: (a) Mg has a low atomic weight for contrast with the heavy Os in bright-field, phase contrast HRTEM images; (b) although MgO undergoes damage in the electron beam, the damage mechanisms proceed relatively slowly; (c) aggregates of MgO particles supported on holey carbon films can routinely be found that show good stability under the influence of the electron beam (charging effects are minimal), so that particles dangling over the edge of a hole can be photographed with short exposure times that allow the full resolution of the TEM to be achieved; and (d) MgO particles also show similar stability in the scanning transmission electron microscope (STEM), allowing high-angle annular dark-field methods to be used to provide images of the osmium clusters in high contrast. The samples were prepared and handled with standard Schlenk line and glovebox techniques for air exclusion. The MgO powder support (EM Science) was prepared by calcination at 673 K in flowing O2 at 1 atm for 2 h and evacuation at 1.3 × 10-3 mbar at the final temperature (673 K) for 14 h. The BET surface area of the resultant MgO was about 70 m2/g. The MgO-supported samples were prepared by slurrying [Os3(CO)12] with the calcined MgO in n-pentane under N2 for 12 h at 298 K, followed by overnight evacuation at 298 K to remove the solvent. The resultant sample was treated in flowing CO at 548 K and 1 atm for 10 h, under conditions giving a high yield of supported [Os5C(CO)14]2-, which has previously been identified by its infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectra and by extraction into solution and identification by IR spectroscopy.3 The samples were decarbonylated by treatment in flowing helium (1 atm) as the temperature was ramped (3 K/min) from 25 to 573 K and then held at 573 K for 2 h. Alternatively, the samples were decarbonylated in flowing
Figure 1. Tip of TEM specimen holder designed for anaerobic transfer of samples into the microscope. The 3-mm specimen grid is held in a retainer plate, which can be easily installed in the tip in the retracted position (top), selected by movement of a control rod attached to the plate via a keyway, as shown. In the protected position (bottom), the grid retainer plate is sandwiched between a leaf spring (which also serves to stabilize the plate for highresolution observation) and the floor of the tip, allowing the specimen to be blanketed with an inert gas during transfer through the atmosphere.
H2 at 573 K. The samples contained 1 wt % (or occasionally 2 wt %) Os. For HRTEM experiments, the powder samples in dry argon were opened in a glovebag under an argon atmosphere and prepared by dipping a 200-mesh copper grid supporting a holey carbon film into the powder and shaking off the excess. This technique provides samples with excellent dispersions of powder aggregates over the surface and eliminates the possibility of contamination that might result from solvent dispersion techniques. To protect the airsensitive sample from exposure to the atmosphere, the grid was placed into a single-tilt holder (Figure 1) that allowed transfer under an argon blanket into a Hitachi HF-2000 coldfield-emission TEM (see caption of Figure 1 for details). The HF-2000 instrument has a point resolution of 2.3 Å and an instrumental resolution of 1.5 Å. Bright-field images were recorded at Scherzer defocus (-65 nm) with a 1 k × 1 k multiscan CCD camera (Gatan, Inc., Model 794) at instrument magnifications of 300 k× to 500 k×. For imaging, powder particle aggregates dangling over the edges of holes in the carbon film were selected, to avoid any contribution of the support film to image contrast. Gatan’s DigitalMicrograph software was used for image acquisition and processing, and scripting routines provided therein were used to assist in precise microscope alignment and image setup. IR spectra characterizing the sample prepared by adsorption of [Os3(CO)12] on MgO were measured as before.3 Extended X-ray absorption fine structure (EXAFS) spectra were recorded at beamline 2-3 at the Stanford Synchrotron Radiation Laboratory, Stanford, California. The storage ring operated with an electron energy of 3 GeV; the ring current was 60-100 mA. The samples were characterized at the Os LIII edge. Details are as given elsewhere.4,5 Reference files obtained from EXAFS data characterizing materials of known structure were used to analyze the EXAFS data.5 382
Figure 2. (a) High-resolution TEM image of the MgO support material without osmium. (b) MgO support with [Os5C(CO)14]2(1 wt % Os) clusters dispersed over the surface.
The IR spectra are in agreement with what has been reported,3 indicating the formation of [Os5C(CO)14]2-. Furthermore, they also include bands at 2073, 2008, and 1949 cm-1, indicating the formation of MgO-supported [Os3(CO)11]2-,6 as well as minor peaks at 1966, 1950, and 1938 cm-1, indicating the presence of an intermediate in the synthesis of [Os5C(CO)14]2-, [H3Os4(CO)12]- (νCO ) 2085, 2053, 2017, 2009, 1966, 1943 cm-1)6 and some unconverted [Os3(CO)12].7 On the basis of the IR peak intensities, we approximate the yield of [Os3(CO)11]2- from [Os3(CO)12] to be about 98% and the yield of [Os5C(CO)14]2- from [Os3(CO)12] to be about 65%, as expected.3 The EXAFS data characterizing the supported osmium carbonyl clusters match the crystal structure of [Os5C(CO)24]2-,8 within the expected error. The first-shell Os-Os coordination number of 3.2 at a distance of 2.90 Å and a second-shell Os-Os contribution with a coordination number of 0.8 at a distance of 3.97 Å are characteristic of the square-based pyramidal structure of the cluster frame of [Os5C(CO)14]2-.8 A high-resolution TEM image of a typical aggregate of the bare MgO support alone indicates nearly uniform contrast with no discrete strong scattering centers, as shown in Figure 2a. Figure 2b is a micrograph of the MgO-supported [Os5C(CO)14]2- (containing 1 wt % Os). The appearance of a uniform distribution of scattering centers with diameters in the range of about 3-4 Å is consistent with the presence of the supported nanoclusters. Because the low-atomicnumber carbon and oxygen atoms are not resolved in such isolated clusters, the scattering centers are attributed to the osmium cluster frames. The data agree well with the average diameter of the Os5C moiety of [Os5C(CO)14]2-, determined Nano Lett., Vol. 2, No. 4, 2002
Figure 3. MgO-supported [Os5C(CO)14]2- (2 wt % Os). The MgO particle was a single crystal, oriented with the 2.4-Å (111) planes in contrast. This image allows a good measure of cluster sizes. Note: magnification is greater than that in all other images in this letter.
crystallographically8 to be 3.97 Å (second-shell radial interatomic distance). During a test of the effects of the electron beam on the specimen, it was observed that some clusters move about on the surface of the MgO, but there did not appear to be notable cluster aggregation over a 5- to 10-min observation period. This result implies that the images obtained in the first few minutes of observation were representative of the original size and dispersion of the clusters on the support. Furthermore, it was determined from before and after observation that exposure of this sample to air for only 2 min led to the disappearance of the clusters. This observation shows that our method of handling the samples during loading and transfer was successful in protecting them from atmospheric effects. We infer that the air exposure oxidized the clusters and formed the volatile OsO5. (Caution! This compound is toxic.) A micrograph of the sample containing 2 wt % Os is shown in Figure 3. This single-crystal MgO support particle is oriented with the 2.44-Å (111) planes of the structure in contrast, and the image shows clusters with a slightly greater range of diameters (about 2.5-4.5 Å) than shown in Figure 2b. The distribution of diameters is attributed to the various preferential orientations of the adsorbed clusters and/or the possible presence of pairs of clusters, which are expected preferentially for the sample with the higher Os loading. In view of the yield of [Os5C(CO)14]2- and the presence of the slightly smaller triosmium and tetraosmium carbonyls as well, the expected size range observable by HRTEM is in the range observed, 3-4 Å. In summary, the images are consistent with the EXAFS and IR data and show that the nanoclusters were nearly monodisperse and well approximated as [Os5C(CO)14]2- with some triosmium and tetraosmium carbonyl clusters. When MgO-supported [Os5C(CO)14]2- (the sample containing 1 wt % Os, with the slightly smaller clusters) was treated in helium at 573 K, the CO ligands were removed, as shown by the IR spectra. The cluster size range observed by high-resolution TEM was still about 3-4 Å (Figure 4), matching that of the precursor [Os5C(CO)14]2- and consistent with an unchanged cluster frame after decarbonylation. The Nano Lett., Vol. 2, No. 4, 2002
Figure 4. MgO-supported clusters formed by decarbonylation of [Os5C(CO)14]2- (1 wt % Os) in helium at 573 K. The cluster size range is consistent with an unchanged cluster frame after decarbonylation.
Figure 5. (a) Bright-field STEM image showing ambiguity in location of osmium nanoclusters as a consequence of significant phase contrast effects from the MgO support; (b) high-angle annular dark-field (Z-contrast) image from the same sample area showing the heavy metal clusters in bright contrast. Several clusters in each image are denoted with arrows.
EXAFS data, including an Os-Os first-shell coordination number of 3.2, with a distance of 2.89 Å, support this conclusion. Thus, we infer that clusters that were nearly monodisperse and well approximated as Os5C were present on the MgO. This sample was also imaged with high-angle annular dark-field (or Z-contrast) imaging techniques9 in a Hitachi HD-2000 dedicated STEM, to provide further direct image evidence for the uniformity and distribution of the nanoclusters. Figure 5a is bright-field STEM image, in which the unambiguous determination of specific osmium cluster 383
In summary, the data indicate that [Os5C(CO)14]2- (in the presence of some smaller osmium carbonyl clusters) on MgO powder could be decarbonylated to give a high yield of Os5C, which has been imaged by HRTEM. The images reported here clearly demonstrate the complementary information obtained from the field-emission TEM and STEM techniques. The sample is one of the most nearly monodisperse supported nanoclusters.
Figure 6. MgO-supported clusters formed from Os5C after decarbonylation by treatment in H2 at 573 K. The image shows that the decarbonylation was accompanied by migration and aggregation of the osmium, giving clusters that are larger than Os5C and no longer nearly monodisperse.
locations is difficult because of the phase contrast of the support structure. Figure 5b is a Z-contrast image of the same sample area, in which high-atomic-number areas are imaged in bright contrast. This type of image allows observation of the individual clusters, and several are denoted with arrows in each image. The high collection angle (0.3 steradian) of the energy-dispersive spectrometer system on the HD-2000 instrument allowed acquisition of an X-ray spectrum (not shown) from an area containing only a few clusters, and it clearly showed the Os peak from the clusters, confirming their presence. When MgO-supported [Os5C(CO)14]2- was treated in H2 instead of helium, the IR spectra showed that it was fully decarbonylated at 573 K. The EXAFS parameters show that this treatment led to an increase in the first-shell Os-Os coordination number, from 3.2 to 5.9, and a decrease in the Os-Os bond distance, from 2.90 to 2.67 Å; the latter value is characteristic of bulk elemental Os (2.68 Å). The micrograph of the sample formed after treatment in H2 at 573 K (Figure 6), in agreement with the EXAFS results, shows larger nanoclusters, with a size range of about 4.57.5 Å. The near monodispersity of the osmium carbonyl precursor clearly was lost as a result of the H2 treatment; the nonuniformity is typical of conventionally prepared supported metal catalysts.
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Acknowledgment. We thank Wilbur Bigelow of the University of Michigan for his work on the design of the sample holder. This research was supported by the U. S. Department of Energy (DOE), Office of Energy Research, Office of Basic Energy Sciences, Division of Chemical Sciences, Contract FG02-87ER13790. The microscopy research was sponsored by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Transportation Technologies, as part of the High Temperature Materials Laboratory User Program, Oak Ridge National Laboratory, managed by UT-Battelle LLC for the U.S. DOE under contract number DE-AC05-00OR22725. We acknowledge the support of the U. S. DOE, Office of Basic Energy Sciences, for its role in the operation of the Stanford Synchrotron Radiation Laboratory, where the X-ray absorption data were collected. These data were analyzed with the XDAP software.10 References (1) Xu, Z.; Xiao, F.-S.; Purnell, S. K.; Alexeev, O.; Kawi, S.; Deutsch, S. E.; Gates, B. C. Nature (London) 1994, 372, 346. (2) Gates, B. C. AdV. Chem. Eng. 2001, 27, 49. (3) 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. (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, 61, 369. (5) Panjabi, G. A., PhD Dissertation, University of California, Davis, 2000. (6) Psaro, R.; Dossi, C.; Ugo, R. J. Mol. Catal. 1983, 21, 33. (7) D’Ornelas, L.; Choplin, A.; Basset, J.-M.; Puga, J.; Sanchez-Delgado, R. A. Inorg. Chem. 1986, 25, 4315. (8) Johnson, B. F. G.; Lewis, J.; Nelson, W. J. H.; Nicholls, J. N.; Puga, J.; Raithby, P. R.; Rosales, M. J.; Schro¨der, M.; Vargas, M. D. J. Chem. Soc., Dalton Trans. 1983, 2447. (9) Nellist, P. D.; Pennycook, S. J. Science 1996, 274, 5286. (10) Vaarkamp, M. Catal. Today 1998, 39, 271.
NL015621K
Nano Lett., Vol. 2, No. 4, 2002
