Structured overlayers on metals

Modifications of Surface Reactivity by Structured Overlayers on Metals... 

Overall reaction, 32 281, 285-287 see also Simple overall reactions Overlayer structure, adsorbates on metal surfaces, 29 7-8... 

A breakdown of the structural results by type of surface shows results for nearly 50 clean, unreconstructed metal surfaces and about 10 alloys and reconstructed metal surfaces. The structures of about 65 atomic overlayers on metal surfaces have been determined, some 40 of these involving chalcogen atoms. Just over 20 molecular structures have been determined for metal surfaces, half of these being overlayers of undissociated carbon monoxide and the others various hydrocarbons. Turning to semiconductors, some 13 clean, usually reconstructed structures were determined, against nearly 10 atomic overlayer structures. In addition, about 15 insulator surface structures have been investigated. 

Table 8.1 Hexagonal structures identified in a variety of metal and metal-oxide overlayers on metallic and alloy single-crystal substrates... 

The atomic resolution of STM is widely exploited for the structural investigation of overlayers on metal substrates. Achieving atomic resolution requires not only a good instrument, but also a carefid elimination of vibrations and thermal gradients as well as the preparation of single crystals with a minimiun of surface defects. 

The electron-rich oxygen anions at the top of oxide overlayers on metals show basic (electron donor) properties the metal cations exposed in coordinately unsaturated sites (cus) show acidic (electron acceptor) character. This is depicted by structure I in Scheme 1. It is expected that the electron donors and acceptor orbitals are more localized on metal oxide surfaces than on clean metal surfaces. This forms the basis for the interpretation of the surface chemistry occurring on oxide surfaces... 

Only very recently it has become possible to also use laboratory sources for unraveling the structure of organic overlayers on metal surfaces. For example the ordering and the adsorption-induced intramolecular distortions have been determined in adlayers of end-capped quarterthiophene [99MEYa] and 2-thiouracil [99MEYb] on Ag(lll). 

The underpotential deposition (UPD) of metals on foreign metal substrates is of importance in understanding the first phase of metal electrodeposition and also as a means for preparing electrode surfaces with interesting electronic and morphological properties for electrocatalytic studies. The UPD of metals on polycrystalline substrates exhibit quite complex behavior with multiple peaks in the linear sweep voltammetry curves. This behavior is at least partially due to the presence of various low and high index planes on the polycrystalline surface. The formation of various ordered overlayers on particular single crystal surface planes may also contribute to the complex peak structure in the voltammetry curves. 

Thin oxide Aims formed on metal electrodes are of widespread technical importance for passivation and/or catalysis of certain electrode reactions. For example, the stability of most engineered metallic structures towards environmental degradation, i.e., corrosion, is largely dependent upon the formation of thin protective oxide overlayers. Alternatively, electrosynthe-... 

Selected Overlayer Structures for Adsorbates on Metal Surfaces... 

It appears that the stronger metal-carbon interaction on iridium surfaces imposes the periodicity on the carbon atoms in the overlayer, while the structure of the graphite overlayer on the Pt( III) face is independent of the substrate periodicity and rotational symmetry. Ordering of the dehydrogenated carbonaceous residue on the stepped iridium surface is absent when the surface is heated to above 1100 K. Atomic steps of (100) orientation appear to prevent the formation of ordered domains that are predominant on the Ir(lll) crystal face. The reasons for this are not clear. Perhaps the rate of C-C bond breaking on account of the steps is too rapid to allow nucleation and growth of the ordered overlayer. On the (111) face, the slower dehydro-... 

For organic thin films on metals, one might be able to monitor the order of the underlying substrate if the metallic response dominates over the overlayer nonlinearity. One generally assumes that the overlayer does not perturb the underlying surface of the substrate but there are currently no other direct experimental probes to verify this. As with the reconstruction data, it is always important to consider how the electronic properties of the surface may be altered by the overlayer in addition to the geometrical structure as measured by the symmetry of the response. 

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. 

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... 

---