AVS 52nd International Symposium
    Surface Science Wednesday Sessions
       Session SS2-WeA

Paper SS2-WeA7
Growth and Reactivity of Bimetallic Pt-Rh Nanoclusters on Titanium Dioxide

Wednesday, November 2, 2005, 4:00 pm, Room 203

Session: Reactions on Nanoclusters
Presenter: D.A. Chen, University of South Carolina
Authors: S. Ma, Brookhaven National Laboratory
J.S. Ratliff, University of South Carolina
J.B. Park, University of South Carolina
D.A. Chen, University of South Carolina
Correspondent: Click to Email
Bimetallic Pt-Rh nanoclusters have been grown on rutile TiO@sub 2@ by vapor deposition under ultrahigh vacuum conditions. Low energy ion scattering (LEIS) experiments show that Pt tends to segregate to the surface of the nanoclusters, regardless of whether Rh is deposited on top of Pt or Pt is deposited on top of Rh. Pt diffusion into the clusters occurs readily at room temperature and is only slightly slower when the deposition temperature is decreased to 220 K. Scanning tunneling microscopy (STM) experiments indicate that for 2 monolayers of Pt deposited on 2 monolayers of Rh, the resulting clusters have an average height of 0.9 nm and diameter of 5 nm. Although heating to 850 K significantly increases the Pt-Rh cluster size (1.3 nm height, 7 nm diameter), LEIS experiments indicate that the clusters become completely encapsulated with TiO@sub 2@ at temperatures above 700 K. In situ STM experiments demonstrate that during the deposition of Pt on Rh clusters, existing clusters grow bigger, new Pt clusters are nucleated, and many growing Pt-Rh clusters merge with neighbors to form larger clusters. The reaction of NO on pure Pt or Rh clusters results in production of N@sub 2@ as well as desorption of NO. However, NO chemistry on bimetallic Pt-Rh clusters is qualitatively different since a new product, N@sub 2@O, is also observed. The reduction of NO with CO to produce N@sub 2@ and CO@sub 2@ does not occur on pure Pt or Rh clusters but is observed on the bimetallic Pt-Rh clusters.