A set of supported bimetallic catalysts, designated [Re7Ir−N], [Re7Ir−P], [Re5IrRe2−N], and [Re5IrRe2−P], has been prepared from two structural isomers (1 and 2) of the cluster compound [Z]2[Re7IrC(CO)23] (Z+ = NEt4 +, N(PPh3)2 +) by deposition onto high surface area alumina (≤1% Re) and activation in H2 at 773 K. The specific activities of the catalysts for ethane hydrogenolysis at 500 K vary significantly (3−63 mmol of CH4/mol of Re7Ir per s) and depend on both the metal framework structure and the counterion present in the precursor. Interpretation of EXAFS data (from both Re and Ir L3-edges) has enabled the development of specific models for the catalyst particle nanostructures that correlate with the catalytic activities. The more active catalysts ([Re7Ir−N] and [Re5IrRe2−N]) are modeled by a hemisphere of close-packed (hcp) metal atoms (average diameter 1 nm) with Ir at the core. On the other hand, the less active catalysts ([Re7Ir−P] and [Re5IrRe2−P]) are better described as two-dimensional layer structures. A combination of techniques, TPDE, IR, XANES, and EXAFS, applied under temperature-programmed conditions, has demonstrated that evolution of the final catalyst particle nanostructure depends on significant initial fragmentation of the cluster framework followed by preferential nucleation at iridium centers.

Using a refined quantitative TEM technique, we measure the relative mass of individual organometallic rhenium clusters on a graphite support. We observe that whereas 6-Re atom species preserve integrity on precipitation from solution, fragmentation and sintering occur in 7-and 8-Re atoms cluster species. These organometallics are candidates as catalytic materials. The technique is based on high-angle dark-field scanning transmission electron microscope (STEM) imaging. The emphasis in this analysis is on using very high angles of scattering to suppress coherence effects from these clusters and on reliably measuring their relative experimental elastic scattering cross sections.

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