Probing the interactions of Pt, Rh and bimetallic Pt-Rh clusters with the TiO2(110) support

Abstract

Pt, Rh, and Pt-Rh clusters on TiO2(110) have been investigated by scanning tunneling microscopy (STM), soft X-ray photoelectron spectroscopy (sXPS), and low energy ion scattering (LEIS). The surface compositions of Pt-Rh clusters are Pt-rich (66-80% Pt) for room temperature deposition of both 2 ML of Pt on 2 ML of Rh (Rh + Pt) and 2 ML of Rh on 2 ML of Pt (Pt + Rh). Pt and Rh atoms readily diffuse within the clusters at room temperature, and although diffusion is slower at 240 K, intermixing of Pt and Rh still occurs. The binding energies of surface and bulk states for Rh(3d(5/2)) and Pt(4f(7/2)) can be distinguished in sXPS studies, and an analysis of these spectra indicates that the surface compositions of the Pt + Rh and Rh + Pt clusters are similar at room temperature but not identical. In addition to sintering, the pure Pt, pure Rh and Pt-Rh clusters become completely encapsulated by titania upon heating to 700 K. sXPS investigations show that annealing the clusters to 850 K induces reduction of titania support to Ti+2 and Ti+3, with the extent of reduction being the greatest for Pt, the least for Rh and intermediate for Pt-Rh. We propose that TiO2 is reduced at the metal-titania interface on top of the clusters, not at the base of the clusters. Furthermore, the extent of titania reduction is greater for metal clusters with weaker metal-oxygen bonds because oxygen atoms are less likely to migrate to the top of the clusters, and therefore the encapsulating titania is oxygen-deficient. (C) 2007 Elsevier B.V. All rights reserved.