Bimetallic Catalyst Particle Nanostructure. Evolution from Molecular Cluster Precursors

Abstract

A set of supported bimetallic catalysts, designated [Re 7 Ir−N], [Re 7 Ir−P], [Re 5 IrRe 2 −N], and [Re 5 IrRe 2 −P], has been prepared from two structural isomers ( 1 and 2 ) of the cluster compound [Z] 2 [Re 7 IrC(CO) 23 ] (Z + = NEt 4 +, N(PPh 3 ) 2 + ) by deposition onto high surface area alumina (≤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 CH 4 /mol of Re 7 Ir 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 L 3 -edges) has enabled the development of specific models for the catalyst particle nanostructures that correlate with the catalytic activities. The more active catalysts ([Re 7 Ir−N] and [Re 5 IrRe 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 ([Re 7 Ir−P] and [Re 5 IrRe 2 −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.