(735g) Growth of Encapsulating Carbon on Supported Pt Nanoparticles Studied By in Situ TEM

Authors: 
Wu, J., University of California - Berkeley
Helveg, S., Haldor Topsoe Research Laboratories
Peng, Z., The University of Akron
Bell, A. T., University of California, Berkeley
Supported platinum nanoparticles are used extensively to promote the transformation of alkanes to a variety of products via reactions such as dehydrogenation, dehydroaromatization, and isomerization. These processes are often accompanied by formation of carbonaceous deposits that lead to blockage of the active sites and ultimately to catalyst deactivation [1]. Because coke formation on Pt is an important problem, considerable effort has been devoted to characterizing coke formed under different conditions and developing an understanding of the factors that affect coke formation on Pt. Transmission electron microscopy (TEM) has proven to be a particularly informative technique for this purpose because it enables visualization of the relationship of the coke with respect to the Pt nanoparticles with atomic resolution and can provide insights into the processes involved in the nucleation and growth of carbon. The development of techniques for obtaining TEM images of catalysts in the presence of a low pressure of hydrocarbon gas (1-10 mbar) and at elevated temperatures has opened the way for making in situ observations of carbon growth as a function of time [2].

In contrast to studies of carbon growth on 3d transition metals, only a few in situ studies have been reported for Pt [3]. 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 [4]. 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.

[1] J. Barbier, G. Corro, Y. Zhang, Appl. Catal. 13 (1985) 245-255.

[2] S. Helveg, C. Lopez-Cartes, J. Sehested, P.L. Hansen, B.S. Clausen, J.R. Rostrup-Nielsen, F. Abild-Pederson, J.K. Nørskov, Nature 427 (2004) 426-429.

[3] Z. Peng, F. Somodi, S. Helveg, C. Kisielowski, P. Specht, A.T. Bell, J. Catal. 286 (2012) 22-29.

[4] J. Wu, S. Helveg, S. Ullmann, Z. Peng, A.T. Bell, J. Catal. 338 (2016) 295-304.

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