Electron microscopy of Pt, Pd and Ni particles in a NaX zeolite matrix

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Abstract
The morphology, size distribution and crystallography of metal precipitates up to 10 nm in diameter in an NaX zeolite (Si/Al = 1.2) loaded with Pt, Pd and Ni have been studied by electron microscopy. Using special techniques to avoid specimen damage by the electron beam, it was shown by bright and dark field microscopy, selected area diffraction, microdiffraction and lattice imaging that all three metals precipitate as equiaxial single crystals having the normal crystal structure of the bulk metal (f.c.c.) in an intact zeolite matrix. Pt and Pd showed a preferred orientation relationship, with greater scatter in the case of Pd. The thermal treatments used led to monodispersed metal phases with uniform particle size distributions, in the case of Ni to a double distribution. Increasing the degree of ion exchange led to an increase in the density rather than the size of the particles. The results are discussed with regard to possible nucleation and growth mechanisms, and an explanation for the observed orientation relationship is proposed.

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