384703 Illuminating Surface Atoms in Nanoparticles By Differential X-Ray Absorption Spectroscopy

Sunday, November 16, 2014: 5:10 PM
303 (Hilton Atlanta)
Charles S. Spanjers1, Thomas P. Senftle1, Adri C.T. van Duin2, Michael J. Janik1, Anatoly I. Frenkel3 and Robert M. Rioux1, (1)Department of Chemical Engineering, Pennsylvania State University, University Park, PA, (2)Department of Mechanical and Nuclear Engineering, Pennsylvania State University, University Park, PA, (3)Physics Department, Yeshiva University, New York, NY

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.

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See more of this Session: In Situ and Operando Spectroscopy of Catalysts
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