State-sensitive monitoring of gold nanoparticle sites on titania and the interaction of the positive Au site with O2 by Au Lα1-selecting X-ray absorption fine structure

The heterogeneity of gold sites in various Au/TiO2 catalysts was studied by means of state-sensitive Au L3-edge X-ray absorption fine structure (XAFS) combined with high energy-resolution X-ray fluorescence spectrometry. A series of Au/TiO2 catalysts were prepared via deposition-precipitation method on anatase-type or mesoporous (amorphous) TiO2 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 Lα1 emission peak energy for Au/mesoporous–TiO2 in air and Au/anatase–TiO2 in CO (5%) corresponded to Au0 state. The emission peak energy for Au/anatase–TiO2 in air shifted toward that of AuI state. For relatively greater Au particles (average 87 Å) dispersed on mesporous TiO2, the major valence state discriminated by Au Lα1-selecting XANES (X-ray absorption near-edge structure) spectrum tuned to Au Lα1 emission peak top was Au0, but the Auδ− state could be successfully monitored by Au Lα1-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 TiO2. On the other hand, negative charge transfer from Au 5d to support was demonstrated in Auδ+-state sensitive XANES tuned to 9718.3–9718.7 eV. The Auδ+-state sensitive XANES spectra resembled theoretically generated XANES for interface Auδ+ sites model on TiO2 in contact with surface Ti sites. Further charge transfer was demonstrated from Au to adsorbed O2 for Au/anatase–TiO2 catalyst.