Nanometre Tomography: Visualising the Smallest Catalyst Particles

Researching the Mobile Fuel Cell by Electrons and Computers

For the first time a BAM and Berlin Helmholtz Centre (HZB) research team has measured small ruthenium catalyst particles, about two nanometres in diameter used for fuel cells, using three-dimensional electron tomography to assess their chemically active surface. By distinguishing the open particle surface from the fraction buried in the carbon carrier, the efficiency of the catalyst can be optimised for future ”electricity from the fuel tank“.

Catalysts stimulate chemical reactions without consuming themselves. In the case of fuel cells they produce charge carriers from hydrogen for the electric current. Ruthenium replaces the conventional platinum, which is considered uneconomical for the production of mobile fuel cells and currently hampers investments. For an efficient performance the catalytic effect of ruthenium, which is smaller than that of platinum, requires an exact knowledge of its reactive surface by spatial imaging in the nanometre range.

The scientists have solved this task in the BMBF project RuN-PEM and reported it in the renowned “Journal of the American Chemical Society“ (JACS) ^[1]. For this purpose catalysts were prepared on a functional carbon matrix and irradiated in a transmission electron microscope. In contrast to the conventional (two-dimensional) microscopy, the tomographic 3D method requires irradiation at many rotational angles. The resulting projections are however inevitably “distorted“ and incomplete. Even after rectification of the projections by means of fiducial gold particles, the reconstruction of the tomographic slices using conventional mathematical methods is too blurred. The algorithm DIRECTT (direct iterative reconstruction of computer tomographic trajectories) being currently developed at BAM is for now the only method that enables the 3D imaging by computer tomographic reconstruction with an accuracy of about one nanometre and enables the determination of the precise area of the specific surface of the elliptic catalyst particles.

Electron tomography, particularly using the reconstruction algorithm DIRECTT, may effectively push the research of the new type of technical catalyst forward.

^[1] R. Grothausmann, G. Zehl, I. Manke, S. Fiechter, P. Bogdanoff, I. Dorbandt, A. Kupsch, A. Lange, M.P. Hentschel, G. Schumacher, J. Banhart: Quantitative Structural Assessment of Heterogeneous Catalysts by Electron Tomography, J. Am. Chem. Soc. 133 (45) (2011), 18161-18171, DOI: 10.1021/ja2032508.

Contact:
Dr. rer. nat. Andreas Kupsch
Working group X-Ray Topography
Division 8.5 Micro NDE
Email: andreas.kupsch@bam.de

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