Metal oxide overlayers, surface characterization

WILLIAMS ET AL. Surface Characterization of Metal Oxide Overlayers... 

Recent studies of supported vanadium oxide catalysts have revealed that the vanadium oxide component is present as a two-dimensional metal oxide overlayer on oxide supports (1). These surface vanadium oxide species are more selective than bulk, crystalline V2O5 for the partial oxidation of hydrocarbons (2). The molecular structures of the surface vanadium oxide species, however, have not been resolved (1,3,4). A characterization technique that has provided important information and insight into the molecular structures of surface metal oxide species is Raman spectroscopy (2,5). The molecular structures of metal oxides can be determined from Raman spectroscopy through the use of group theory, polarization data, and comparison of the... 

Interest in the composition and structure of submonolayer metal oxide deposits on metals has developed as a consequence of growing evidence that such deposits influence the adsorptive and catalytic properties of the substrate metal [see for example ref. (1)]. In particular, it has been shown that titania deposited on a Ni(l 11) (2) surface and on the surface of Pt and Rh foils (3.41 will enhance the activity of the metal for CO hydrogenation. Similar results have also been reported for niobia deposited on a Pt foil (5). Hie present paper discusses the characterization of titania overlayers deposited on the surface of a polycrystalline Rh foil and a Rh(lll) surface. 

The tltanla-based thin film catalyst models were constructed by first oxidizing the titanium surface In 5 x 10 torr of O2 for approximately 30 minutes at 775 K. This produced an AES llneshape consistent with fully oxidized TIO2. The metal was then vapor deposited onto the oxide support with the latter held at 130 K. The thickness of the metal overlayer and Its cleanliness were verified by AES. After various annealing and adsorption procedures, these thin films were further characterized using SSIMS, AES and TDS. For comparison, some work was done with Pt on Al20s. In this case a Mo foil covered with AI2O3 replaced the Tl(OOOl) substrate. 

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