Atomic Imaging of Carbon-Supported Pt, Pt/Co, and Ir@Pt Nanocatalysts by Atom-Probe Tomography

Atom probe tomography (APT) has been used to characterize commercially prepared Pt, Pt/Co alloy, and Ir@Pt core-shell nanoparticles supported on high-surface-area carbon black. Concentration profiles and 3D atom maps revealing the detailed internal structures and compositions of Pt, Pt/Co alloy, and Ir@Pt core-shell particles have been generated, and the distribution of trace impurity elements, including Na and Cl, has been examined. The observation of retained Na on the support, especially in the Pt nanoparticle system, indicates a more rigorous washing procedure is required. In the Pt/Co alloyed carbon-supported nanopartide system, a marked variation in both compositions and particle sizes is observed. In the case of Ir@Pt, significant intermixing of the Ir core and Pt shell atoms takes place, which would be very difficult to measure by other techniques. All such observations will likely impact the catalytic performance of these materials. We envisage that the single nanoparticle analysis capability of APT, providing atomic-scale structures and chemical mapping, can also act as a means of quality control, identifying differences in the final product compared with the intended specification. Although the catalytic activity of these nanoparticles was not part of current study, the detailed information offered by such studies will permit knowledge-based improvements in nanoscale catalyst preparation methods and will also provide new ways of investigating structure and activity relationships at the nanometer scale.