Atomic Imaging of Carbon-Supported Pt, Pt/Co, and Ir@Pt Nanocatalysts by Atom-Probe Tomography Tong Li,* ,†,‡ Paul A. J. Bagot, † Elvis Christian, § Brian R. C. Theobald, § Jonathan D. B. Sharman, § Dogan Ozkaya, § Michael P. Moody, † S. C. Edman Tsang, ⊥ and George D. W. Smith † † Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom ‡ Australian Center for Microscopy and Microanalysis, and School of Aerospace, Mechanical & Mechatronic Engineering, The University of Sydney, Sydney, New South Wales 2006, Australia § Johnson Matthey Technology Centre, Blount’s Court, Sonning Common, Reading RG4 9NH, United Kingdom ⊥ Wolfson Catalysis Centre, Department of Chemistry, University of Oxford, Oxford OX1 3QR, United Kingdom * S Supporting Information ABSTRACT: 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 nanoparticle 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. KEYWORDS: atom probe tomography, platinum, platinum-cobalt alloyed particles, platinum-iridium core-shell nanoparticles, catalysts 1. INTRODUCTION Specific structures and morphologies of active phases can catalyze chemical reactions selectively, enabling the formation of desired products at high production rates. 1-3 The majority of commercial catalysts are solid state, taking the form of nanoparticles dispersed onto high-surface-area porous solids. In a large proportion of these catalysts, bimetallic systems containing two different metal species are particularly utilized. These demonstrate significantly improved activity, selectivity, and resistance to poisoning. 4-6 For example, in direct methanol fuel cells, the Pt catalyst component is highly effective in breaking C-O and C-H bonds, but Ru is also included, which greatly improves the resistance of the catalyst to carbon monoxide poisoning. 7,8 Bimetallic nanoparticles can be roughly divided into two types of structures: alloyed structures, which incorporate a homogeneous chemical distribution of atoms within the nanoparticle, and core-shell structures, in which one metal forms the core of the nanoparticle and is surrounded by a shell of atoms of a different type. The final structures produced are dependent on synthesis methods and environment. This core- shell geometry shows particular tunable activity and selectivity. It is believed that electronic effects between the core and shell materials, which depend on core-shell dimensions, can markedly alter the electronic and adsorptive properties of the shell layer. 9 This suggests the possibility of nanoengineering of highly active morphologies if the core and shell structure and morphology can be controlled. Atomic-scale characterization of these core-shell interfaces will thus be critical to producing optimized nanoparticles for catalysis. In addition, catalyst nanoparticles are hosted on a support material, which not only maximizes the surface area of the particles but also actively contributes to the overall efficacy through metal-support interactions. The adsorptive properties of the support may also Received: July 15, 2013 Revised: January 13, 2014 Research Article pubs.acs.org/acscatalysis © XXXX American Chemical Society 695 dx.doi.org/10.1021/cs401117e | ACS Catal. 2014, 4, 695-702