Article pubs.acs.org/JPCC
Understanding the Growth, Chemical Activity, and Cluster−Support Interactions for Pt−Re Bimetallic Clusters on TiO2(110) Randima P. Galhenage,† Kangmin Xie, Hui Yan,‡ Grant S. Seuser, and Donna A. Chen* Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States S Supporting Information *
ABSTRACT: The growth and chemical activity of Re, Pt, and Pt−Re bimetallic clusters supported on TiO2(110) have been studied. Pure Re clusters interact strongly with the titania support, resulting in the reduction of the titania surface, and the Re clusters also appear to be partially covered by TiOx at Re coverages as high as 13 ML. Bimetallic clusters can be grown from sequential deposition of Pt and Re in either order at high metal coverages (3.7 ML), where the number of initial nucleation sites is large; in contrast, at lower coverages (0.24 ML), pure Re clusters coexist with Pt−Re clusters for Re deposited on Pt due to the higher nucleation density of Re compared with Pt. The surface composition of the high coverage Pt on Re clusters is ∼100% Pt, but the Re on Pt clusters contain both Pt and Re at the surface after diffusion of some fraction of Re atoms in the bulk. The lower surface free energy of Pt compared to Re makes it thermodynamically favorable for Pt to remain at the surface when Pt is deposited on Re, whereas Re atoms deposited on the Pt clusters will diffuse into the clusters. Isotopic labeling experiments that incorporate 18O into the titania lattice demonstrate that lattice oxygen participates in both CO oxidation on the Pt on Re bimetallic clusters and recombination of carbon and oxygen to form CO on the Re-containing clusters.
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weaker binding of CO33,34 and the decreased activation energy for H2 desorption on Pt−Re compared to Pt(111) have been ascribed to electronic effects.35 Our group’s recent investigations show that oxygen dissociates more readily at 500 K on a Pt−Re alloy surface compared to Pt(111), but in this case, it is believed that Re diffuses to the alloy surface to facilitate oxygen dissociation.36 Another effect of the addition of Re to the Pt catalysts is increased Pt dispersion, particularly on oxide supports.5,14,29,37,38 Moreover, the Pt−Re particles are more resistant to sintering compared to Pt alone.16,24,39 For example, after 20 h onstream for WGS at 300 °C, the Pt/TiO2 particles lose 36% of their initial dispersion, whereas the Pt−Re/TiO2 particles lose only 8%.16 However, the increased activity for Pt−Re in the WGS reaction cannot in general be attributed solely to increased dispersion.16,40 Despite the many investigations of Pt−Re catalysts, the details of Pt−Re bimetallic interactions, Re oxidation states, and interactions of the metal particles with the oxide support are still not completely understood. This is largely because the Re oxidation state and extent of Pt−Re interactions are known to be strongly influenced by exact preparation conditions, including catalyst pretreatment conditions.13,14,41−47 Although it is possible to control the Pt−Re particle size to
