A Nanoscale Model Catalyst Preparation:  Solution Deposition of Phosphine-Stabilized Gold Clusters onto a Planar TiO₂(110) Support

Abstract

Agglomeration of noble metal particles has been a long-standing obstacle in the preparation of planar, finely dispersed, metal cluster−oxide support model catalyst systems. A solution deposition method was devised. Six-atom gold clusters, in the form of [Au 6 (PPh 3 ) 6 ][BF 4 ] 2 (Au 6 L 6 ), were deposited onto a TiO 2 (110) single crystal after an acetone pretreatment of the substrate and examined with scanning tunneling microscopy (STM), high-resolution electron energy loss spectroscopy (HREELS), and X-ray photoelectron spectroscopy (XPS). STM revealed single-unit entities of clusters ca. 1 nm in height, corresponding to the six-atom Au center with the triphenyl phosphine ligands attached. As a check on the success of the homogeneous dispersion, electron-stimulated desorption (ESD) was performed, irradiating the TiO 2 (110) surface with a 0.12 C cm - 2 electron beam flux followed by STM, HREELS, and XPS. Evidence for ligand removal was shown by a pronounced reduction in height in the STM, a disappearance of the aromatic ν(C−H) intensity at ca. 3000 cm - 1 in the HREEL spectra, and a +0.4 eV shift in the XPS Au 4f 7/2 core level.