Cluster Studies

Cluster Studies Group

Experimental Cluster Chemistry Studies

S. J. Riley

The chemical reactions of transition metal clusters are studied as models for heterogeneous catalysis. Identification and analysis of reaction products are used to determine such cluster properties as geometrical structure, the nature and energetics of adsorbate binding sites on cluster surfaces, the saturation of clusters with adsorbate molecules, and the kinetics of cluster-molecule reactions. Metals studied include iron, cobalt, nickel, copper, aluminum, rhodium, manganese, paladium, rhodium, and platinum. Adsorbates include hydrogen, carbon monoxide, water, ammonia, nitrogen, and C₆₀. These studies are providing a better understanding of structure-reactivity relationships for clusters.

Chemical probes of cluster structure

Through a systematic determination of the number of adsorbate binding sites on cluster surfaces it is possible to make educated guesses as to the geometrical structures of the clusters. This technique has been applied to a wide range of nickel cluster sizes, and has shown that, for the most part, nickel clusters adopt structures based on icosahedral packing. There are important exceptions, however, such as Ni₃₈ and clusters in the Ni_46-48 size range, where the structure converts to fcc and fcc/hcp packing schemes. The identification of such structural changes gives us a deeper understanding of the nature of metal-metal binding and provides important input for theoretical treatments of cluster structure and properties.

In some cases, the binding of adsorbate molecules on cluster surfaces can change their structure, the cluster analog of surface reconstruction. In the case of Ni₁₉, for example, saturation by hydrogen converts the cluster from a double icosahedron to an fcc (or bcc) cluster. Subsequent coadsorption of ammonia converts the cluster back to the double icosahedron. The evidence suggests that it is the electron donating or withdrawing character of the ligand that is the primary influence on the structure of the metal cluster. Electron donating ligands like ammonia fill the last d orbitals in the nickel atoms, making the Ni-Ni interactions more spherically symmetric, which favors icosahedral or polyicosahedral packing. Electron withdrawing ligands like hydrogen, in contrast, increase the influence of directed d-orbital bonding and favor conversion to fcc or hcp packing. Studies of this sort are providing us with important information about the dynamical nature of cluster-adsorbate interactions.

Alloy clusters

Many practical heterogeneous catalysts are not composed of pure metals, but have heteroatom constituents that modulate and often improve catalytic behavior. We have recently initiated experimental studies of bimetallic clusters with the aim of determining the locations of the different metal atoms within the cluster structure. In the case of nickel-rich nickel/iron clusters in the 13- to 55-atom size range we find that the most stable structures have an internal iron atom. We also see evidence for metastable configurations in which iron atoms are on the outside and which convert to the more stable ones on the millisecond time scale.

In conjunction with extensive theoretical work on nickel-aluminum clusters done in our group (see Theoretical Studies of Metal Clusters and Cluster-Ligand Systems: Unraveling the Complexity), we have looked at the structures of 12-, 13- and 14-atom Ni/Al clusters for all possible compositions. In agreement with the theoretical predictions, we find that for the most part the most stable structures have central nickel atoms, but for some intermediate compositions there can be contributions from structures with central aluminum atoms. These are the first experimental studies of bimetallic clusters to determine such structural details.

Cluster deposition

We have developed several techniques for depositing clusters directly from the beam onto substrates specifically tailored for subsequent investigation. In earlier studies we deposited rhodium clusters onto alumina particles and studied their size distributions via transmission electron microscopy and their catalytic activity for hydrodesulfurization of a model compound. More recently we have deposited films of platinum clusters (average size 100 atoms or ~15 Å dia) on various substrates for analysis by both small-angle X-ray scattering (SAXS) and atomic force microscopy. The SAXS studies show that for low exposure levels on Mylar the platinum forms spherical particles of ~36 Å dia (~1000 atoms). This characteristic size does not appear to be very sensitive to the initial density of the deposited clusters, implying some sort of self-limiting growth process. For higher initial cluster coverages the platinum deposit was found to have a sheet structure, with thicknesses averaging around 22 Å. Again, the thickness did not seem to depend on the initial cluster density. These results have interesting consequences for the dynamics and agglomeration behavior of deposited platinum clusters.

Recent publications

Adsorption of C₆₀ on Nickel Clusters at High Temperature, E. K. Parks, K. P. Kerns, and S. J. Riley Phys. Rev. B 59, 13431 (1999)

The Structure of Nickel-Iron Clusters Probed by Adsorption of Moleuclar Nitrogen, E. K. Parks, K. P. Kerns, and S. J. Riley Chem. Phys. 262, 151 (2000)

The Structure of Ni₄₆, Ni₄₇, and Ni₄₈, E. K. Parks, K. P. Kerns, and S. J. Riley J. Chem. Phys. 114, 2228 (2001)

Investigation of Structural Changes in Ni₁₉ and Ni₂₃ Induced by Adsorption of Hydrogen/Deuterium and Ammonia, E. K. Parks, G. C. Nieman, and S. J. Riley J. Chem. Phys. 115, 4125 (2001)

Theoretical and Experimental Studies of the Structures of 12-,
13- and 14-Atom Bimetallic Nickel/Aluminum Clusters, E. F. Rexer,
J. Jellinek, E. B. Krissinel, E. K. Parks, and S. J. Riley, J. Chem.
Phys. 117 (1), 82-94 (2002)

Locating the Al Atom in Ni₁₄Al-Ni₁₉Al Clusters, E. K. Parks,
E. F. Rexer, and S. J. Riley, J. Chem. Phys. 117 (1), 95-99 (2002)

Analysis of Gas-Phase Clusters made from Laser-Vaporized
Icosahedral Al-Pd-Mn, J. A. Barrow, D. J. Sordelet, M. F. Besser,
C. J. Jenks, P. A. Thiel, E. F. Rexer, and S. J. Riley, J. Phys.
Chem. A 106 (40), 9204-9208 (2002)

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