In-situ Transmission Electron Microscopy Study of Carbon Nanotubes Growth over Platinum Nanoparticles

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EMAG - In-situ EM Techniques & Analysis
Dr. Milivoj Plodinec (1), Dr. Hannah C. Nerl (2)
1. ETH Z├╝rich
2. Humboldt University Berlin

In-situ TEM, Pt nanoparticles, CNT growth, Metastable platinum carbide

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Carbon deposition, also known as coking, is a common problem in many industrial catalytic processes, such as the dehydrogenation of propane and dry or steam reforming of methane. This leads to a decrease in catalytic activity and selectivity.1-2 The formation of carbon deposits is a complex process influenced by various factors, such as reaction conditions, catalyst properties, and gas feed composition.1-2 The most widely accepted model for carbon nanotubes (CNTs) growth suggests that carbon atoms from the gas dissolve into the metal, and once the metal is saturated, graphitic carbon emerges from the catalyst.3-4 However, there is no consensus on the active structure of catalysts. It has been found that a carbide structure forms in the most commonly used catalysts for CNTs synthesis, such as nickel and iron.5-6 However, the formation of CNTs over noble metals like platinum is still under debate.

Recent progress in the development of in-situ electron microscopy techniques has revolutionized the study of catalysts, allowing unprecedented insights into their dynamic behavior at the atomic and molecular levels. Moreover, in-situ gas-phase transmission electron microscopy (TEM) has enabled the observation of structural changes and chemical transformations of catalysts at high spatial resolution, in real-time and under reaction conditions.7 Thus providing, important information on the mechanisms and kinetics of catalytic reactions, as well as changes in the morphology and composition of catalyst materials.

In this study, we used a commercially available DENSolutions "Climate" system and a double aberration-corrected JEOL JEM-ARM300F Grand ARM transmission electron microscope operated at 300 kV to observe the real-time, atomically resolved growth of carbon nanotubes (CNTs) on the surface of platinum nanoparticles (Pt NPs) in a gaseous mixture of methane and hydrogen. High-resolution TEM and electron diffraction were employed to track changes in Pt NP shape and identify the active structure for CNT growth. We observed the formation of a metastable phase of platinum carbide (PtC) several atomic layers thick during CNT growth, which was confirmed by both electron diffraction (ED) and high-resolution TEM.8 Additionally, we found that Pt undergoes slight changes in shape during CNT growth. This new insight supports the hypothesis that the outer layer of Pt atoms plays an active role in shaping the graphitic carbon layers.

In summary, the results obtained in this study reveal that the growth mechanism of CNTs on Pt nanoparticles involves the formation of Pt-PtC-CNT interfaces, where carbon atoms dissolve into the Pt nanoparticle, form metastable PtC, and subsequently nucleate CNTs on the surface of PtC. Overall, in-situ gas phase TEM provides important insights into the active structure of Pt catalysts for CNT growth and opens up new avenues for suppressing catalyst coking, a major source of catalyst deactivation in important industrial catalytic processes.



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