(15f) Illuminating Surface Atoms in Nanoparticles By Differential X-Ray Absorption Spectroscopy

The adsorption of Ar on Pd nanoparticles (1 nm dia, silica-supported) at 77 K results in restructuring of surface Pd atoms, as evidenced by differential extended x-ray absorption spectroscopy (EXAFS). EXAFS analysis shows that 9 ± 2 nearest-neighbor (NN) Pd-Pd bonds expand by 0.104 ± 0.005 Å as a result of Ar adsorption. This is compared to the total number of NN Pd-Pd bonds in the Pd nanoparticles of ~140. We discuss how the differential EXAFS technique is not limited to a particular set of materials, but instead it can be applied to any system in which modulation causes small changes in local structure. To support our experimental observation of Ar-induced surface restructuring and to differentiate between proposed EXAFS models, we conducted a series of ReaxFF-MD simulations in which model Pd clusters were exposed to both vacuum and an Ar gas phase at 77 K. The simulations demonstrate that Ar may drive surface reconstruction of under-coordinated atoms, thus leading to an increased Pd-Pd bond length of surface Pd atoms, in agreement with the experimental observations. Based on observations of the atomistic simulations, it is likely that under-coordinated atoms are trapped in metastable states at 77 K and Ar provides the kinetic energy needed to overcome the barrier for surface restructuring. We note the possibility of using Ar to probe the number of under-coordinated sites of catalyst nanoparticles which could be compared to selectivity or activity.