Aberration-corrected imaging of active sites on industrial catalyst nanoparticles.

The active components of industrial heterogeneous catalysts are often small metallic particles, whose reactivity and selectivity depend on the presence of steps and kinks on their surfaces. Although scanning probe microscopy can be used to identify the roles of active sites on the extended surfaces of model catalysts, little is known about atomic arrangements on the surfaces of commercial catalyst nanoparticles, as highlighted by a recent study of ammonia synthesis in a Ru-catalyzed reaction. High-resolution transmission electron microscopy (HRTEM) has been used extensively to characterize such particles. However, conventional HRTEM images are affected by electron microscope objective lens aberrations, limiting spatial resolution, and complicating interpretation. The visibility of nanoparticles can also be affected by strong phase contrast from their support. Here, we show that spherical-aberration-corrected transmission electron microscopy (TEM) can be used to provide atomic-resolution information about the local topologies of active sites on commercial nanoparticles with greatly improved sensitivity. We used two advanced TEM techniques, which are based on recent developments in hardware (aberration correction) and computation (exit wavefunction restoration), to examine a powder of Pt nanoparticles on carbon black, which had been heated to 900 8C in a N2-rich atmosphere. These particles are used to both electrooxidize H2 and electroreduce O2 at the anode and cathode, respectively, of a polymer electrolyte membrane fuel cell. Figure 1A shows the measured phase of the specimen exit surface wavefunction of a representative Pt nanoparticle, determined in the present study from a focal series of aberrationcorrected images acquired using a JEM-2200FS transmission electron microscope, viewed close to a h110i direction. Despite the presence of a graphitic carbon support, terraces and steps around the edge of the particle are visible directly with a sensitivity, visibility, and spatial resolution that are improved greatly over those in a conventional high-resolution TEM image. The monatomic steps that are visible in Figure 1A are conventionally termed A or B type, with either {100} or {111} microfacet risers that have different catalytic properties. The heights of the peaks in Figure 1A are related to the number of atoms in each atomic column and can be used to obtain information about the atomic arrangement on each of the surfaces that is viewed in cross section. In contrast to the atomic arrangements that have been suggested by homoepitaxial growth experiments on extended Pt surfaces, our results, which are consistent between different Pt particles (Figure 2), provide compelling evidence that the outermost atomic layers consist of irregular islands of atoms. Figure 3 illustrates the improvement obtained by recovering the specimen exit wavefunction over the information present in a single HRTEM image. The power spectra shown below each image in Figure 3 show the presence of higher-spatialresolution lattice fringe information in the reconstructed image and the decreased contribution to the power spectrum from amorphous carbon. Figure 1B shows a simulated phase image, calculated from the model shown in Figure 1C, which is consistent with the experimental image shown in Figure 1A. Although the relative alignment of adjacent columns in the electron beam direction cannot be determined uniquely from Figure 1A alone, the nature of the roughness of the terraces shown in Figure 1B,C is clear. Multislice simulations of the amplitude and phase of the exit wavefunction, in which the surface of the particle was modified by adding or removing atoms, were performed for a range of models of the particle (Figure 4) until a best fit to the experimental image was obtained. In the simulations, the phase images were band-pass-filtered using a cut-off frequency equal to the maximum spatial frequency present in the experimental image. The best-fitting model shown in Figure 1C suggests that steps on the (100) facets of this particle are largely straight, while kinks and step vacancies are present on the (111) facets. We anticipate that the ability to obtain results such as those shown in Figures 1–3, especially in a gas reaction environment and in combination with electron tomography, will contribute to the design and development of improved [*] L. C. Gontard, Prof. R. E. Dunin-Borkowski Department of Materials Science and Metallurgy University of Cambridge Pembroke Street, Cambridge CB23QZ (UK) Fax: (+44)1223-33-4567 E-mail: rdb@cen.dtu. dk Homepage: http://www.rafaldb. com/