| Popis: |
Pt2+ autoreduction during the thermal vacuum decomposition of [Pt(NH3)4]2+Me+ faujasites as well as the reaction of CO+NO were studied in dependence on the zeolite composition (effect of different alkali ions Me+: Li, Na, K, Cs, and zeolite Si/Al ratio: X and Y zeolites). Infrared spectroscopy was employed to characterize the surface species during the decomposition of the tetraamine complex. Gases evolved in this process were identified by mass spectrometry (which was also used in the CO+NO reaction), t.p.r., XPS and TEM assisted in the examination of the metal/zeolite system. It was found that at least four ammonia complexes are formed during the vacuum decomposition of [Pt(NH3)4]2+ and that two of them are more abundant in Y zeolites. Ammonia is evolved from the zeolites in two temperature regions. In the low temperature interval, the higher electropositivity and size of the alkali cations facilitate the disruption of Pt2+-N bonds in the same way in both X and Y zeolites. In the high temperature region, where the autoreduction of Pt2+ is dominant, the cations affect temperature of the NH3 release to a lesser extent. However, the amount of gases evolved from Y zeolites in this temperature region is higher compared with that from X zeolites, which corresponds with a higher fraction of more thermally stable Pt ammonia complexes found in Y zeolites. Under our experimental conditions, about 20–30% of Pt2+ does not undergo autoreduction. Pt clusters are formed inside the zeolite cavities with a size of 1–4 nm. No effect of the faujasite matrix (Si/Al ratio, nature of alkali cations) on the extent of the autoreduction or on the Pt0 size and location is observed in contrast to the reaction of CO+NO, which is substantially accelerated by the increasing electropositivity, diameter, and number (X > Y) of alkali cations. |
Full Text from ScienceDirect