| Popis: |
Location and dispersion of platinum in reduced PtY zeolites were studied by X-ray diffraction, small angle X-ray scattering and chemisorption. In the case of a 300 °C activation most of the Pt2+ ions are found in supercages. On reducing at 300 °C, 6–13 A platinum agglomerates fitting into supercages are formed, and they remain stable up to 800 °C. Hydrogen chemisorption gives H Pt = 1 and the stoichiometry of titration corresponds to that proposed by Wilson and Hall for very small particles. At 900 °C structure breaks down and simultaneously agglomerates are transformed into crystallites of 25–30 A diameter occluded in the bulk of an amorphous solid. In the case of a 600 °C activation, most of the Pt2+ ions are in sodalite cages; on reducing at 300 °C, the platinum is atomically dispersed in these cages and does not chemisorb hydrogen. On evacuating at higher temperatures, Pt-agglomerates and Pt-crystallites are gradually formed at the expense of isolated atoms, and the chemisorption increases up to 800 °C ( H Pt = 0.65 ). At 900 °C the platinum is in the same state of dispersion as in the previous case. |
