Structural investigation of Au catalysts on TiO ₂-SiO ₂ supports: Nature of the local structure of Ti and Au atoms by EXAFS and XANES

X-ray absorption spectroscopy (XAS) was utilized to investigate both the Au particle size on several supports composed by silica and titanium oxide, and the coordination of TiO ₂ in the support phase. Particularly, we wanted to utilize the technique to probe mixing in the support phase, by using different synthetic methods such as by functionalizing silica or by ALD (atomic layer deposition) techniques as prepared in our laboratories, and the growth and stability of Au nanoparticles deposited on these supports. The study using cosynthesis techniques to dope bulk mesoporous SiO ₂ with TiO ₂ resulted in TiO ₂ being dispersed in the SiO ₂ matrix; however, a second phase starts forming as the TiO ₂ content increases as indicated by the EXAFS Ti-O shell shift in position and increase of coordination number. On the supports prepared by cosynthesis, Au particles were smaller and more stable. The study using the surface sol-gel technique for deposition of single monolayers of an oxide such as TiO ₂ produced Ti environments in which the Ti-O shell and the next two Ti-Ti shells lie on the same position as expected for an anatase structure. Although undercoordinated, the presence of the Ti-Ti shells indicate that the titania species are not molecularly dispersed on the SiO ₂ surface as hypothesized, but there is indeed a cross-linking of the titania moieties.