It is a well-observed phenomenon that carbon forms and deposits on the surface of platinum nanoparticles when they are used to catalyze the transformation of hydrocarbons at an elevated temperature. The carbonaceous species, or coke, accumulated on Pt surface can block the active sites of Pt for many reactions, causing catalyst deactivation, or leads to changes in product distribution. A good understanding of the processes by which coke forms on Pt can thus help finding ways to suppress its formation and develop more durable and stable catalysts. In situ TEM has proven to be a promising tool in understanding such dynamic processes. By directly imaging a working catalyst, both structural and dynamic information of reacting particles as well as real-time profiles of carbon growth can be achieved. In order to benefit from in situ observations made by TEM, a careful investigation of the reaction and imaging conditions is necessary. First, the compatibility of the mbar pressure range in the microscope with reactor conditions has to be confirmed. Second, an optimization of low electron dose conditions while preserving image resolution and sensitivity is important to avoid invasive effects of the electron beam on the observations. Here, such a careful investigation is presented in order to enable monitoring of coking on platinum by ethylene decomposition. Furthermore, in situ TEM experiments (773K, 1 mbar ethylene) on Pt/MgO reveal the mechanisms by which carbon grows on platinum. We observe the nucleation of carbon on the surface of platinum followed by the growth of multiple layers of graphene that encapsulate the particle. Shape changes of nanoparticles also illustrate the dynamic transformation that accompanies this phenomena. A closer examination of the metal surface reveals restructuring of platinum that is supported by scanning tunneling microscopy (STM) studies using a Pt(111) model surface.

Understanding coking on Platinum catalysts by in situ TEM<i>
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248th National Meeting (2014)

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