Transmission Electron Microscopy (TEM) or Scanning Transmission Electron Microscopy (STEM) can provide qualitative information on the distribution of metal nanoparticles over a support, as in the example of Fig 1 showing a STEM image of a Pt/CeO₂ catalyst. Nevertheless, as the obtained electron microscopy images are simply 2D projections of the object, they do not provide information on the 3D distribution of the particles which is an essential parameter controlling the catalytic activity. In this particular system, nanopores exist at the surface and a 3D imaging technique such as electron tomography is necessary in order to determine whether metal nanoparticles are anchored on the support surface or, e.g., trapped inside the oxide nanopores.

A 1 wt% Pt-CeO₂ catalyst was prepared by one-step solution combustion synthesis (SCS) [1], a fast and simple method favoring metal-support interaction. The sample was analyzed by electron STEM-HAADF tomography in a FEI environmental TEM (Fig. 2). The analysis was performed at two complementary scales: (i) at a relatively low resolution to gain insight into the ceria pore size and shape distributions, and (ii) at higher resolution to determine the Pt particle size distribution and location with respect to the support [2].

It is found that ceria has a bimodal pore size distribution, with small pores about 10 nm in size between the crystallites and bigger pores around 150 nm formed by winding up of ceria layers. It is highly probable that the synthesis route promotes this peculiar and usnusal microstructure. Noticeably, the present nano-tomography analysis further demonstrates that only ca. 50% of the 6 nm-sized Pt particles are located at the surface of the ceria, the other 50% being embedded in the support. This implies that only half the particles contribute to catalytic activity, which may have important consequences on the interpretation of catalytic data.

[1] F. Morfin, T.S. Nguyen, J. L. Rousset, L. Piccolo, Appl. Catal. B 2016, in press, doi: 10.1016/j.apcatb.2016.01.056.

[2] L. Roiban, S. Koneti, T.S. Nguyen, M. Aouine, F. Morfin, T. Epicier, L. Piccolo, in preparation.

Acknowledgements

Thanks are due to CLYM (Consortium Lyon – St-Etienne de Microscopie, www.clym.fr) for the access to the microscope funded by the Region Rhône-Alpes, the CNRS and the ‘GrandLyon’.

Acknowledgments are also due to BQR project SEE3D granted by Insa-Lyon, ANR project 3DClean, Labex iMUST and IFP Energies Nouvelles for the financial support.

Figures:

Fig. 1. STEM-HAADF image of the Pt particles supported on ceria. The material was synthesized in one step by glycine-nitrate solution combustion.

Fig.2. Reconstructed 3D model of a Pt/ceria catalyst, the ceria support is represented in green and the Pt particles with an average size of 6 nm diameter are depicted in blue.

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EMC Abstracts - https://emc-proceedings.com/abstract/electron-tomography-analysis-of-ptceo2-catalyst-powders-synthesized-by-solution-combustion/