Heterogenized bimetallic clusters: their structures and bifunctional catalysis

Abstract

Tailored bimetallic catalysts have been prepared by using SiO 2-bound RhFe, PtFe and IrFe carbonyl clusters as the precursors. The Fe-containing cluster-derived catalysts exhibited strikingly high activities and improved selectivities for CH 3OH and C 2H 5OH formation in CO + H 2 reaction and for C 4 alcohols in propene hydroformylation, compared with those from the homometallic metal clusters. EXAFS and transmission electron spectroscopy, coupled with 57Fe Mössbauer studies on Fe 2Rh 4, Fe 3Pt 3 and Fe 4Pt carbonyl cluster-derived catalysts suggest that the RhFe and PtFe bimetallic particles have less than a 10 Å diameter, with Rh-Fe 3+-O and Pt-Fe 3+-O bondings. The role of Fe 3+ may be to anchor the ensembles of Rh and Pt atoms to prevent sintering. The iron promotion toward oxygenates is proposed to be associated with the heteronuclear activation of CO with the adjacent Rh-Fe 3+ Pt-Fe 3+ and Ir-Fe 3+ located at the metal cluster/SiO 2 interfaces, enhancing the migratory CO insertion into M-H and M-alkyl groups, reflected in the marked increase of C 1 + C 2 alcohols in CO hydrogenation. A series of bimetallic catalysts has been prepared from hexanuclear carbonyl clusters such as Rh 6− xIr x (CO) 16 ( x = 0–6) which were presynthesized inside NaY cages. EXAFS, FTIR for CO vibrational modes, and 129Xe NMR studies demonstrated that their derived bimetallic catalysts consist of bimetallic particles of less than 10 Å, uniformly distributed inside NaY zeolites and with homogeneous metal compositions similar to their precursor carbonyl clusters. The butane hydrogenolysis activities are remarkably suppressed by increasing a modest Ir content in the Rhir bimetallic catalysts. The activities and selectivities towards scission of the central C-C bond of butane are discussed in terms of breaking down the Rh ensemble size and electron-deficient sites of the Rhir particles inside NaY, based on the 129Xe NMR studies.