EXAFS as a tool to interrogate the size and shape of mono and bimetallic catalyst nanoparticles

The influence of atom packing and the geometric arrangement of atoms on the coordination number(s) and resultant simulated EXAFS spectra for face centrered cubic (fcc), hexagonal close packed (hcp) and body centered cubic (bcc) monometallic structures and fcc bimetallic structures, has been examined for metal clusters with an isotropic diameter up to 3 nm. We observe clear differences as a function of size and shape for all types of packing; in particular the surface aspect ratios (surface-to-bulk) of the shapes strongly influences the 'growth curves' as a function of the number of atoms for all packing types examined. Discrimination between the different types of structure based on the coordination shell occupancy appears possible, although is dependent on the type of packing. For fcc and hcp packing, structures comprising less than 200 atoms, and particularly less than 100 atoms, exhibit strong variation in the first shell coordination number as a function of shape. For bcc structures this dependency of the coordination shell number for small clusters is much less pronounced and in some cases (for higher shell (>N4) coordination numbers) an opposite trend is observed in that the difference in coordination number as a function of shape becomes more marked with an increasing number of atoms. For the fcc bimetallic systems, model structures possessing distinct bimetal distributions, including both non-random and random alloy types, were simulated and examined. Clear variations in both the 1st shell coordination number and in the simulated spectra were observed as a function of the bimetal structure. A dual edge analysis of the data and subsequent examination of the coordination shell numbers as a function of the bimetal distribution enables for the size, composition and distribution of the each species, to be determined.