Shape-selective catalysts:: quasi-two-dimensional Pd-particles encapsulated in graphite

Pd-graphites were prepared by reduction of PdCl2-graphite intercalation compounds (PdCl2-GICs). Transmission electron microscope (TEM) measurements on the samples indicated that Pd nanoparticles with a wide size distribution were formed, mostly encapsulated inside the graphite host. The quasi-two-dimensional character of the Pd nanoparticles was confirmed by H-2 sorption studies, which revealed the absence of P-hydride formation over a broad range of pressure. Although the structures of the three Pd-graphites studied were very similar, differences were found between them in terms of their catalytic performance in the gas-phase reactions of 1-butene, cis-2-pentene and cyclohexene. It was established that the transformations of both 1-butene and cis-2-pentene took place in the interlamellar space of the graphite, whereas, in consequence of the the non-planar character of the substrate, the diffusion of cyclohexene between the graphite layers was sterically hindered. This molecular sieving effect implies that the Pd-graphites may be regarded as shape-selective catalysts. Pretreatment at 473 K was found to lower the catalytic activities considerably. The experimental evidence indicated that restructuring of the Pd crystallites resulted in a moderate aggregation of the active Pd-particles available for the reactant molecules. (C) 2001 Elsevier Science B.V. All rights reserved.