  Understanding how metal atoms are stabilized on metal oxide supports is
important for predicting the stability of single-atom catalysts. In this study,
we use scanning tunnelling microscopy (STM) and x-ray photoelectron
spectroscopy (XPS) to investigate four catalytically active metals - Platinum,
Rhodium, Nickel and Iridium - on the anatase TiO2(101) surface. The metals were
vapor deposited at room temperature in ultrahigh vacuum (UHV) conditions, and
also with a background water pressure of 2x10-8 mbar. Pt and Ni exist as a
mixture of adatoms and nanoparticles in UHV at low coverage, with the adatoms
immobilized at defect sites. Water has no discernible effect on the Pt
dispersion, but significantly increases the amount of Ni single atoms. Ir is
highly dispersed, but sinters to nanoparticles in the water vapor background
leading to the formation of large clusters at step edges. Rh forms clusters on
the terrace of anatase TiO2(101) irrespective of the environment. We conclude
that introducing defect sites into metal oxide supports could be a strategy to
aid the dispersion of single atoms on metal-oxide surfaces, and that the
presence of water should be taken into account in the modelling of single-atom
catalysts.
