Bimetallic Catalyst Particle Nanostructure. Evolution from Molecular Cluster Precursors

Abstract

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.