State-sensitive monitoring of gold nanoparticle sites on titania and the interaction of the positive Au site with O 2 by Au Lα 1-selecting X-ray absorption fine structure

Abstract

The heterogeneity of gold sites in various Au/TiO 2 catalysts was studied by means of state-sensitive Au L 3-edge X-ray absorption fine structure (XAFS) combined with high energy-resolution X-ray fluorescence spectrometry. A series of Au/TiO 2 catalysts were prepared via deposition-precipitation method on anatase-type or mesoporous (amorphous) TiO 2 added with NaOH (lower Au loading) or urea (higher Au loading). The mean Au particle size ranged between 29 and 87 Å based on high-resolution TEM (transmission electron microscope) measurements. The Au Lα 1 emission peak energy for Au/mesoporous–TiO 2 in air and Au/anatase–TiO 2 in CO (5%) corresponded to Au 0 state. The emission peak energy for Au/anatase–TiO 2 in air shifted toward that of Au I state. For relatively greater Au particles (average 87 Å) dispersed on mesporous TiO 2, the major valence state discriminated by Au Lα 1-selecting XANES (X-ray absorption near-edge structure) spectrum tuned to Au Lα 1 emission peak top was Au 0, but the Au δ− state could be successfully monitored by Au Lα 1-selecting XANES tuned to the emission energy at 9707.6 eV, of which population was relatively small compared to the case of smaller Au particles (average 29 Å) on anatase-type TiO 2. On the other hand, negative charge transfer from Au 5d to support was demonstrated in Au δ+ -state sensitive XANES tuned to 9718.3–9718.7 eV. The Au δ+ -state sensitive XANES spectra resembled theoretically generated XANES for interface Au δ+ sites model on TiO 2 in contact with surface Ti sites. Further charge transfer was demonstrated from Au to adsorbed O 2 for Au/anatase–TiO 2 catalyst.