Growth of nanocrystalline platinum in a zeolite host - An in situ X-ray powder diffraction study

The growth of nanocrystalline platinum particles in zeolite [Pt(NH3)(4)]NAX was investigated by in situ Xray diffraction in combination with Rietveld refinement at T = 509, 519, 530 and 562 K. The reaction was directly followed by the gradual decrease of the Pt2+-occupation and the simultaneous increase of the metallic platinum particles, which grow up to an average size of 4 nm. An increase of the zeolite lattice constant and a gradual decomposition of the framework up to 20% were simultaneously observed. The process of crystal growth can be influenced by the preparation conditions of the zeolite. After dehydration up to 383 K at 10(-7) bar more than 90% of the platinum cations are located near the centre of the 12-ring-window. Dehydration in the temperature range from 403 to 443 K under vacuo results in a partial shift of the platinum cations to a second position and to an increase of the reactivity.