Intrazeolite large palladium clusters prepared by organometallic chemical vapour deposition

Abstract

Allyl(cyclopentadienyl)palladium was selectively introduced inside the cages of the zeolite NaY via chemical vapour deposition in Ar at 25 °C. Palladium clusters were then formed by thermal removal of the volatile ligands under H₂ at 300 °C. High-resolution transmission electron microscopy measurements indicated the formation of large palladium clusters with diameters ranging from 7 to 25 Å. Indirect characterization by IR spectroscopy confirmed that most of the metal is located inside the cages. The existence of entrapped clusters of 25 Å, i.e. larger than the zeolite cages, can be explained by a local disruption of the zeolite lattice. The sintering process observed in H₂ at 500 °C is caused by the coalescence of clusters in adjacent cavities, together with a limited migration of palladium atoms resulting in larger external particles. The electronic state and location of the large palladium clusters were studied by Fourier-transform infrared spectroscopy of adsorbed CO molecules. The marked downward shift of the v_CO bands is attributed to the increase in electron density on these clusters. This electron-rich character is due to charge-transfer interactions with the negatively charged oxygen atoms of the cage walls.