The temperature-dependent oxidation of Pt nanoclusters on a thin film of Al2O3 on NiAl(100), in the absence of a gaseous oxidizing agent, was investigated with various surface probe techniques. The Pt clusters (of mean diameter 2.2 nm and height 0.4 nm) grown from vapor deposition on the thin film of Al2O3 on NiAl(100) at 300 K became partially oxidized, as charge transfer from the Pt clusters to the oxide was indicated by a significant negative shift (0.4−0.5 eV) of binding energy (BE) of Alox 2p and O 1s states from Al2O3. The oxidation of the cluster proceeded to a further level when the sample was annealed above 450 K; the Pt 4f7/2 core level moved positively from BE 72.0 eV with increasing annealing temperature and eventually attained 72.6 eV above 650 K, which indicates a state of Pt2+. Accompanying this further oxidation, signals of both Alox 2p and O 1s shifted back to greater BE. The valence spectra indicate that the Pt−Al2O3 interaction was sustained whereas a new Pt−O bond was formed. Formation of a Pt x Al y O z complex is proposed to explain the observations. The new Pt oxide binding was substantially stronger than the initial one, as was evident from the oxidized clusters being resistant to sintering induced in electrochemical processes. The oxidation was associated also with a migration of oxide materials onto the Pt clusters, as both Al2O3(100) and NiAl(001) facets roughened after the annealing, and a probe of methanol adsorption showed no bare Pt clusters exposed but alumina-like structures on the surface.

No abstract

Adsorption and decomposition of methanol on Au nanoclusters supported on an ordered ultrathin film of Al 2 O 3 /NiAl(100) have been investigated by means of their photoelectron spectra excited with synchrotron radiation and scanning tunneling microscopy. The results show adsorption sites on the clusters of at least two types: one catalyzes the decomposition of methanol via scission of the O-H bond into CO at a temperature as low as 120 K, whereas the other activates dehydrogenation only into methoxy. The CO from the decomposition remains on the surface up to 250 K, begins to desorb above 250 K, and decomposes further into elemental carbon around 350 K. All carbon species are removable from the surface at 450 K, a temperature much lower than on other metals studied. Regarding effects of morphology of the clusters on the reactivity, smaller Au clusters, of height 1-2 atomic layers, exhibit inactivity toward the decomposition of methanol.