AVS 64th International Symposium & Exhibition
    Fundamental Discoveries in Heterogeneous Catalysis Focus Topic Wednesday Sessions
       Session HC+NS+SS-WeM

Paper HC+NS+SS-WeM12
Understanding the Growth and Chemical Activity of Pt-Re Clusters on HOPG and Titania Surfaces

Wednesday, November 1, 2017, 11:40 am, Room 24

Session: Nanoscale Surface Structures in Heterogeneously-Catalyzed Reactions
Presenter: Donna Chen, University of South Carolina
Authors: D.A. Chen, University of South Carolina
T.D. Maddumapatabandi, University of South Carolina
A.J. Brandt, University of South Carolina
G. Seuser, University of South Carolina
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The growth and chemical activity of bimetallic Pt-Re clusters supported on highly-oriented pyrolytic graphite (HOPG) and TiO2(110) surfaces are studied as model systems for understanding Pt-Re catalysts in the water gas shift (WGS) and methanol oxidation reactions. STM experiments demonstrate that both Pt and Re atoms are mobile on unmodified HOPG surfaces and form large clusters that preferentially located at step edges. However, Ar+ sputtering creates defect sites that serve as nucleation sites and result in smaller clusters with uniform size distributions. At high metal coverages, exclusively bimetallic clusters are formed from both the deposition of Pt on Re or Re on Pt. The cluster surfaces are Pt-rich even for the Re on Pt clusters due to the lower surface free energy of Pt compared to Re and the high mobility of atoms within clusters. Similarly, Pt-rich Pt-Re clusters have also been grown on titania supports. The activities of the Pt-Re bimetallic clusters are investigated in a microreactor coupled to an ultrahigh vacuum chamber so that the surfaces can be characterized by X-ray photoelectron spectroscopy (XPS) before and after reaction. In the WGS reaction, Re remains metallic, and Pt on Re surfaces exhibit higher activity than pure Pt. In methanol oxidation, oxygen-diffusion of Re to the surface occurs under reaction conditions, and the formation of volatile Re2O7 results in Re sublimation from the surface. Furthermore, the titania support appears to stabilize Re2O7 against sublimation, whereas Re oxidation is enhanced by the presence of Pt in the bimetallic clusters.