Nanoparticle size evaluation of catalysts by EXAFS: Advantages and limitations

In this article we determine particle size of nanocatalysts using the first-shell fitting results of Extended X-ray Absorption Fine Structure (EXAFS) measurements. The EXAFS technique measures the average coordination number of nanoparticles in the path of X-ray beam. Since nanoparticles can be found in variety of cluster structures with varying coordination number of surface atoms, the discussion is limited to the structures of face centered cubic (fcc) lattice in which most metals of interest for catalysis crystalize. Two nanoparticle structures, namely cuboctahedron and icosahedron, were analyzed and their calculated average coordination numbers compared to those determined by EXAFS. It was found that the particle size determined using EXAFS corresponds best to the diameter of the sphere that has the same volume as the nanoparticle. This volume-corrected sphere was calculated for a number of platinum group metals. It is further shown that the model for particle size evaluation can be extended to bimetallic and trimetallic nanoparticles. Advantages and limitations of the technique in assessing the particle size are discussed.