(735g) Growth of Encapsulating Carbon on Supported Pt Nanoparticles Studied By in Situ TEM
In contrast to studies of carbon growth on 3d transition metals, only a few in situ studies have been reported for Pt . Encapsulation of 3d metal nanoparticles by multiple carbon layers has been proposed to occur via precipitation of carbon dissolved in the bulk of the nanoparticle at lower temperatures. However, in view of the low solubility of carbon in Pt, it remains an open question pertaining to how multiple carbon layers form on top of each other and what the role of the Pt surface is during carbon encapsulation. To address this issue, the onset of carbon growth and the initial encapsulation of Pt nanoparticles by multiple layers of carbonaceous matter are examined by means of in situ TEM . Such in situ HRTEM experiments were done to investigate the formation of carbon layers on MgO-supported Pt nanoparticles upon exposure of the catalyst to ethylene at elevated temperature. Despite the impressive capability of in situ TEM to monitor carbon formation on working catalysts with atomic scale resolution, beam-induced processes remain challenging. An imaging strategy was carefully established in order to suppress beam-assisted carbon growth and to minimize beam-induced sputtering of grown carbon layers. Hereby, in situ TEM reveals via time-resolved image series that multiple partial layers of carbon formed without the Pt particle being completely encapsulated by the first carbon layer. Additionally, particles became rounder as a function of reaction time, suggesting that coking caused significant restructuring on Pt nanoparticles. A closer look at the surface of the nanoparticles revealed that, in some cases, steps formed spontaneously in the surface and assisted in nucleation and growth of carbon layers.
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