Interconversion of Atomically Dispersed Platinum Cations and Platinum Clusters in Zeolite ZSM-5 and Formation of Platinum gem-Dicarbonyls

Catalysts composed of platinum dispersed on zeolite supports are widely applied in industry, and coking and sintering of platinum during operation under reactive conditions require their oxidative regeneration, with the platinum cycling between clusters and cations. The intermediate platinum species have remained only incompletely understood. Here, we report an experimental and theoretical investigation of the structure, bonding, and local environment of cationic platinum species in zeolite ZSM-5, which are key intermediates in this cycling. Upon exposure of platinum clusters to O-2 at 700 degrees C, oxidative fragmentation occurs, and Pt2+ ions are stabilized at six-membered rings in the zeolite that contain paired aluminum sites. When exposed to CO under mild conditions, these Pt2+ ions form highly uniform platinum gem-dicarbonyls, which can be converted in H-2 to Pt delta+ monocarbonyls. This conversion, which weakens the platinum-zeolite bonding, is a first step toward platinum migration and aggregation into clusters. X-ray absorption and infrared spectra provide evidence of the reductive and oxidative transformations in various gas environments. The chemistry is general, as shown by the observation of platinum gem-dicarbonyls in several commercially used zeolites (ZSM-5, Beta, mordenite, and Y).

Automated summary

This study investigates the structure and properties of cationic platinum species in zeolite ZSM-5 through experimental and theoretical research. The results show that platinum clusters undergo oxidative fragmentation and form Pt2+ ions, which are stabilized in six-membered rings containing paired aluminum sites. These Pt2+ ions form uniform platinum gem-dicarbonyls in the presence of CO, which can be converted to Pt delta+ monocarbonyls in H-2. The conversion weakens the platinum-zeolite bonding and serves as the first step towards platinum migration and aggregation into clusters.