DOI: 10.1002/chem.201000860 Triosmium Clusters on a Support: Determination of Structure by X-ray Absorption Spectroscopy and High-Resolution Microscopy Shareghe Mehraeen, [a] Apoorva Kulkarni, [a] Miaofang Chi, [b] Bryan W. Reed, [c] Norihiko L. Okamoto, [a] Nigel D. Browning,* [a, c] and Bruce C. Gates* [a] Introduction Stabilized nanoparticles [1] exemplified by oxide-supported metal clusters, [2, 3] have generated wide interest because they offer new and unique structures, reactivities, and catalytic properties that originate from their small sizes and interac- tions with the support; clusters with as few as three metal atoms have been reported. [4–7] Supported metal clusters and particles are important catalysts; the metals are typically Group 8 metals, and the supports are typically porous oxides or carbon. The support surfaces and the clusters themselves are almost always nonuniform in structure. The nonuniformity limits the structural information that can be obtained by methods such as spectroscopy or chemisorption of hydrogen or CO, and so determination of structure–reac- tivity relationships and the nature of the cluster–support in- terface has been challenging. In attempts to overcome these limitations, researchers have worked to synthesize structurally uniform supported metal clusters. [2, 8–15] One synthesis method involves the use of molecular metal clusters (such as metal carbonyls) as pre- cursors, with the aim of retaining the metal frame in the supported clusters. [2, 16–17] Determination of the structures of the metal frames in supported metal clusters has benefited from recent advances in transmission electron microscopy (TEM); now, even the smallest metal clusters can be imaged. [16, 18, 19] The most infor- mative images of metal clusters have been obtained with high-resolution scanning transmission electron microscopy (STEM). [6, 20–24] Recent advances in STEM instruments and image analysis methods [25–27] have allowed determination of Abstract: The structures of small, robust metal clusters on a solid support were determined by a combination of spectroscopic and microscopic meth- ods: extended X-ray absorption fine structure (EXAFS) spectroscopy, scan- ning transmission electron microscopy (STEM), and aberration-corrected STEM. The samples were synthesized from [Os 3 (CO) 12 ] on MgO powder to provide supported clusters intended to be triosmium. The results demonstrate that the supported clusters are robust in the absence of oxidants. Convention- al high-angle annular dark-field (HAADF) STEM images demonstrate a high degree of uniformity of the clus- ters, with root-mean-square (rms) radii of 2.03  0.06 . The EXAFS Os Os coordination number of 2.1  0.4 con- firms the presence of triosmium clus- ters on average and correspondingly determines an average rms cluster radius of 2.02  0.04 . The high-reso- lution STEM images show the individ- ual Os atoms in the clusters, confirming the triangular structures of their frames and determining Os  Os distances of 2.80  0.14 , matching the EXAFS value of 2.89  0.06 . IR and EXAFS spectra demonstrate the presence of CO ligands on the clusters. This set of techniques is recommended as optimal for detailed and reliable structural characterization of supported clusters. Keywords: aberration-corrected STEM · EXAFS spectroscopy · HAADF-STEM · MgO-supported clusters · osmium carbonyls [a] S. Mehraeen, A. Kulkarni, N. L. Okamoto, Prof. N. D. Browning, Prof. B. C. Gates Department of Chemical Engineering and Materials Science University of California, One Shields Avenue Davis, California, 95616 (USA) Fax: (+ 1) 530-752-1031 E-mail: nbrowning@ucdavis.edu bcgates@ucdavis.edu [b] Dr. M. Chi Materials Science Division, Oak Ridge National Laboratory Oak Ridge, Tennessee, 37830 (USA) [c] Dr. B. W. Reed, Prof. N. D. Browning Condensed Matter and Materials Division Lawrence Livermore National Laboratory, Livermore California 94550 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000860.  2011 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem. Eur. J. 2011, 17, 1000 – 1008 1000