Bimetallic catalyst particle nanostructure. Evolution from molecular cluster precursors

A set of supported bimetallic catalysts, designated [Re7Ir-N], [Re(7)lr-P], [Re(5)lrRe(2)-N], and [Re5IrRe2-P], has been prepared from two structural isomers (1 and 2) oi the cluster compound [Z](2)[Rr(7)IrC(CO)(23)] (Z(+) = NEt(4)(+), N(PPh(3))(2)(+)) by deposition onto high surface area alumina (less than or equal to 1% Re) and activation in H-2 at 773 K. The specific activities of the catalysts for ethane hydrogenolysis at 500 K vary significantly (3-63 mmol of CH4/mol of Re-Ir per s) and depend on both the metal framework structure and the counterion present in the precursor. interpretation of EXAFS data (from bath Re and Ir L(3)-edges) has enabled the development of specific models for the catalyst particle nanostructures that correlate with;he catalytic activities. Thr more active catalysts ([Re(7)lr-N] and [Re(5)lrRe(2)-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 technique, 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.