Clean metal structures, surface structure

Lack of structural data for clean metal oxide surfaces (point (iv)), has also hindered quantitative structural determinations of adsorbates on these surfaces. This is particularly true for those techniques providing a rather complete description of the adsorption geometry (e.g. quantitative LEED (LEED-IV)), since at least some knowledge of the clean surface geometry is usually necessary for structure elucidation. The reasons for the lack of structural data for clean metal oxide surfaces are basically identical to the previous three points (i-iii) for adsorbate covered surfaces. 

Because LEED theory was initially developed for close packed clean metal surfaces, these are the most reliably determined surface structures, often leading to 7 p factors below 0.1, which is of the order of the agreement between two experimental sets of 7-V curves. In these circumstances the error bars for the atomic coordinates are as small as 0.01 A, when the total energy range of 7-V curves is large enough (>1500 eV). A good overview of state-of-the-art LEED determinations of the structures of clean metal surfaces, and further references, can be found in two recent articles by Heinz et al. [2.272, 2.273]. 

The very new techniques of scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) have yet to establish themselves in the field of corrosion science. These techniques are capable of revealing surface structure to atomic resolution, and are totally undamaging to the surface. They can be used in principle in any environment in situ, even under polarization within an electrolyte. Their application to date has been chiefly to clean metal surfaces and surfaces carrying single monolayers of adsorbed material, rendering examination of the adsorption of inhibitors possible. They will indubitably find use in passive film analysis. 

A catalyst used for cleaning exhaust gases from automobiles consist, among other things, of Rh particles on an AI2O3 support material. The Rh particles expose primarily Rh(lll) and secondarily Rh(lOO) surface structures. Rh is a FCC metal with a lattice distance of a = 0.381 nm. 

Figure 1,2 Atomic arrangement on various clean metal surfaces. In each of the sketches (a) to (h) the upper and lower diagrams represent top and side views, respectively. Atoms drawn with dashed lines lie behind the plane of those drawn with thick lines, Atoms in unrelaxed positions (i.e. in the positions they occupy in the bulk) are shown as dotted lines. From G.A. Somorjai, Chemistry in Two Dimensions, Cornell University Press, London, 1981, p. 133, For the Miller index convention in hexagonal close-packed structures, see also G.A. Somorjai loc. cit, Used by permission of Cornell University Press,...

Field emission microscopy was the first technique capable of imaging surfaces at resolution close to atomic dimensions. The pioneer in this area was E.W. Muller, who published the field emission microscope in 1936 and later the field ion microscope in 1951 [23]. Both techniques are limited to sharp tips of high melting metals (tungsten, rhenium, rhodium, iridium, and platinum), but have been extremely useful in exploring and understanding the properties of metal surfaces. We mention the structure of clean metal surfaces, defects, order/disorder phenomena,... 

The experimental approach discussed in this article is, in contrast, particularly amenable to investigating solvent contributions to the interfacial properties 131. Species, which electrolyte solutions are composed of, are dosed in controlled amounts from the gas phase, in ultrahigh vacuum, onto clean metal substrates. Sticking is ensured, where necessary, by cooling the sample to sufficiently low temperature. Again surface-sensitive techniques can be used, to characterize microscopically the interaction of solvent molecules and ionic species with the solid surface. Even without further consideration such information is certainly most valuable. The ultimate goal in these studies, however, is to actually mimic structural elements of the interfacial region and to be able to assess the extent to which this may be achieved. 

The structure and dynamics of clean metal surfaces are also of importance for understanding surface reactivity. For example, it is widely held that reactions at steps and defects play major roles in catalytic activity. Unfortunately a lack of periodicity in these configurations makes calculations of energetics and structure difficult. When there are many possible structures, or if one is interested in dynamics, first-principle electronic structure calculations are often too time consuming to be practical. The embedded-atom method (EAM) discussed above has made realistic empirical calculations possible, and so estimates of surface structures can now be routinely made. 

If absorption or solution into the interior of the crystal structure is involved, the measurement of adsorption is still more complicated, as has been shown in Section IV. It is often very difficult to discern absorption from adsorption. The literature on adsorption of hydrogen is abundant with examples in which absorption has vitiated the measurements and conclusions in one or another way. Chemisorption on clean metal surfaces is very fast. If slow activated sorption of hydrogen is observed, absorption or solution may be involved. 

The adsorption (chemisorption) of hydrogen on clean metal surfaces is almost always accompanied by absorption of hydrogen into the interior of the structure. This absorption is a slow activated process and has in the past been mistaken for activated adsorption of hydrogen on the surface. 

Fig. 7.2). The second structure, however, was found to play an important role in the overall phase diagram. This structure is called a surface oxide since the outermost layers of the material are in an oxide form while the bulk of the material is a pure metal. It can be seen from Fig. 7.5 that at most temperatures, there is a range of pressures spanning several orders of magnitude for which this surface oxide structure is more stable than either the bulk oxide or the clean metal surface. This phase diagram strongly suggests that the working catalyst under industrial conditions is a surface oxide rather than bare Ag.

A cleaning treatment used to be applied to artifacts with a good metallic structure, whose surface is generally covered with a thin layer consisting in a mixture of corrosion products and grime, sometimes called tarnish. Cleaning aims to remove this undesired superficial layer, without (or with minimal) loss of the metallic substrate. In many cases, such a goal is more easily achieved by electrochemical methods than by mechanical and chemical methods [282]. 

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