Pt nano-particles from about 1 to 10 nm have been prepared on silica, alkali-silica, alumina, silica-alumina, carbon and SBA-15 supports. EXAFS spectra of the reduced catalysts in He show a contraction of the Pt–Pt bond distance as particle size is decreased below 3 nm. The bond length decreased as much as 0.13 Å for 1 nm Pt particles. Adsorption of CO and H2 lead to a increase in Pt–Pt bond distance to that near Pt foil, e.g., 2.77 Å. In addition to changes in the Pt bond distance with size, as the particle size decreases below about 5 nm there is a shift in the XANES to higher energy at the L3 edge, a decrease in intensity near the edge and an increase in intensity beyond the edge. We suggest these features correspond to effects of coordination (the decrease at the edge) and lattice contraction (the increase beyond the edge). At the L2 edge, there are only small shifts to higher energy at the edge. However, beyond the edge, there are large increases in intensity with decreasing particle size. At the L1 edge there are no changes in position or shape of the XANES spectra. Adsorption of CO and H2 also lead to changes in the L3 and L2 edges, however, no changes are observed at the L1 edge. Density Functional Theory and XANES calculations show that the trends in the experimental XANES can be explained in terms of the states available near the edge. Both CO and H2 adsorption result in a depletion of states at the Fermi level but the creation of anti-bonding states above the Fermi level which give rise to intensity increases beyond the edge.
Pt nanoparticles Bond length contraction Particle size effect in XANES spectra Particle size effect in Pt bond length Pt XANES EXAFS
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RJM would like to acknowledge the generous grants for computational time on Jazz and Fusion at Argonne National Lab. Use of the Advanced Photon Source is supported by the U. S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357. MRCAT operations are supported by the Department of Energy and the MRCAT member institutions. In addition, RJM would like to acknowledge the Department of Energy for use of Advanced Photon Source at Argonne National Lab associated with GU-8689. RJM also acknowledges the National Science Foundation for their partial support of this work through CBET grant #0747646. Finally, RJM, JJ and JTM would like to thank Suljo Linic and Hongliang Xin of the University of Michigan for the thoughtful discussions of these results and without whom this work would not have been possible.