Thermal Stability of Au Nanoparticles in O2 and Air on Fully Oxidized TiO2(110) Substrates at Elevated Pressures. An AFM/XPS Study of Au/TiO2 Model Systems

The thermal stability of gold nanoparticles (2.5−4 nm diameter) on fully stoichiometric TiO 2 (110) rutile substrates in an O 2 atmosphere and in air at up to ambient pressure, and for comparison under ultrahigh-vacuum (UHV) conditions, was investigated by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Substrate material and annealing parameters were chosen similar to those used for the pretreatment of realistic disperse Au/TiO 2 catalysts. Under UHV conditions, the particles are rather stable even after annealing at 400 °C, and only minor changes in the particle size are detected after annealing at 500 °C. In 50 mbar O 2, the particles are relatively stable at room temperature but grow significantly after annealing to 400 °C. In air, particle growth starts already at room temperature, accompanied by the formation of Au−OH species. The relevance of these results for the understanding of the pretreatment of disperse Au/TiO 2 catalysts is discussed.