Rh films of 5−50 monolayers (ML) were grown on TiO 2 (110)−(1 × 1) surface by physical 8 vapor deposition (PVD) at 300 K followed by annealing at max. 1050 K. In the coverage range of 5−15 ML,

No abstract

Gold, rhodium, and their coadsorbed layers were prepared on a nearly stoichiometric titania surface by physical vapor deposition (PVD) and were characterized by low-energy ion scattering (LEIS) and scanning tunneling microscopy (STM). It was found that because of the strong tendency of Au segregation in the Au-Rh bimetallic system, Rh atoms that impinged onto Au clusters pregrown on TiO 2 (110) became covered by gold atoms by place exchange or surface diffusion even at room temperature. The incorporation of rhodium led to a slight enlargement of gold clusters indicated by STM and to an increase in the number of Au atoms on the outermost layer of metal clusters evidenced by LEIS. At the same time, separate Rh clusters were also formed on the free oxide surface. The observed effect on the bimetallic nanoclusters is attributed to the different surface free energies of metals. For Au, this value is much smaller than for Rh, which provides a driving force for the bimetallic clusters to be covered by gold atoms. Annealing experiments revealed that monometallic Rh clusters are encapsulated at ∼750 K by the oxide. Encapsulation by titania is negligible up to 900 K for the bimetallic nanoparticles almost completely covered by Au.

The interaction of CO with K-free and K-covered Mo 2 C/Mo(100) surfaces has been investigated by HREELS, AES, and TPD. CO adsorbed molecularly on the clean Mo 2 C/Mo(100) surface at 140 K. A part of the adsorbed CO desorbed from the saturated adlayer with T p ) 330 K, which was accompanied by dissociation of the molecule at 300-350 K. The C and O adatoms formed recombined at higher temperatures, giving a CO desorption peak at 960 K. Preadsorbed potassium at intermediate coverages (Θ K ) 0.5-0.8 ML) led to a considerable weakening of the C-O bond even at 140 K, as evidenced by the HREELS feature at 1320-1375 cm -1 . The appearance of this loss suggested a short-range interaction, while the development of losses at 1670-1685 and 1860 cm -1 is attributed to long-range interaction in the coadsorbed layer. Potassium adatoms decreased the amount of CO desorbed and converted it into a more reactive form, whichsinstead of desorptions underwent dissociation at 350-550 K. The recombinative desorption of CO occurred at 970 K; its amount was 3 times higher than that measured for K-free Mo 2 C. Potassium was stabilized in the adsorbed layer mainly by O adatoms arising from the decomposition of CO.