Formation and thermal stability of Au2O3 on gold nanoparticles:: Size and support effects

Gold nanoparticles with two different size distributions (average sizes of similar to 1.5 and similar to 5 nm) have been synthesized by inverse micelle encapsulation and deposited on reducible (TiO2) and nonreducible (SiO2) supports. The thermal and chemical stability of oxidized gold species formed upon cluster exposure to atomic oxygen have been investigated in ultrahigh vacuum using a combination of temperature-, time- and CO dosing-dependent X-ray photoelectron spectroscopy (XPS), as well as temperature-programmed desorption (TPD). Our work demonstrates that (a) low-temperature (150 K) exposure to atomic oxygen leads to the formation of surface as well as subsurface gold oxide on Au nanoparticles, (b) the presence of the reducible TiO2 substrate leads to a lower gold oxide stability compared to that on SiO2, possibly because of a TiO2 oxygen vacancy-mediated decomposition process, (c) heating to 550 K (Au/SiO2) and 300 K (Au/TiO2) leads to a near-complete reduction of small (similar to 1.5 nm) NPs while a partial reduction is observed for larger clusters (similar to 5 nm), and (d) the desorption temperature of O-2 from preoxidized Au clusters deposited on SiO2 depends on the cluster size, with smaller clusters showing stronger O-2 binding.