Surface of a metal

Schematic diagram showing the development of a dipolar field and ionization on the surface of a metal filament, (a) As a neutral atom or molecule approaches the surface of the metal, the negative electrons and positive nuclei of the neutral and metal attract each other, causing dipoles to be set up in each, (b) When the neutral particle reaches the surface, it is attracted there by the dipolar field with an energy Q,. (c) If the values of 1 and <() are opposite, an electron can leave the neutral completely and produce an ion on the surface, and the heat of adsorption becomes Q,. Similarly, an ion alighting on the surface can produce a neutral, depending on the values of I and <(), On a hot filament the relative numbers of ions and neutrals that desorb are given by Equation 7.1,which includes the difference, I - <(), and the temperature, T,...

Schematic diagram showing how placing a thin layer of highly dispersed carbon onto the surface of a metal filament leads to an induced dipolar field having positive and negative image charges. The positive side is always on the metal, which is much less electronegative than carbon. This positive charge makes it much more difficult to remove electrons from the metal surface. The higher the value of a work function, the more difficult it is to remove an electron. Effectively, the layer of carbon increases the work function of the filament metal. Very finely divided silicon dioxide can be used in place of carbon.

An especially insidious type of corrosion is localized corrosion (1—3,5) which occurs at distinct sites on the surface of a metal while the remainder of the metal is either not attacked or attacked much more slowly. Localized corrosion is usually seen on metals that are passivated, ie, protected from corrosion by oxide films, and occurs as a result of the breakdown of the oxide film. Generally the oxide film breakdown requires the presence of an aggressive anion, the most common of which is chloride. Localized corrosion can cause considerable damage to a metal stmcture without the metal exhibiting any appreciable loss in weight. Localized corrosion occurs on a number of technologically important materials such as stainless steels, nickel-base alloys, aluminum, titanium, and copper (see Aluminumand ALUMINUM ALLOYS Nickel AND nickel alloys Steel and Titaniumand titanium alloys). 

Using 2eohte catalysts, the NO reduction takes place inside a molecular sieve ceramic body rather than on the surface of a metallic catalyst (see Molecularsieves). This difference is reported to reduce the effect of particulates, soot, SO2/SO2 conversions, heavy metals, etc, which poison, plug, and mask metal catalysts. ZeoHtes have been in use in Europe since the mid-1980s and there are approximately 100 installations on stream. Process applications range from use of natural gas to coal as fuel. Typically, nitrogen oxide levels are reduced 80 to 90% (37). 

Pitting corrosion is a form of localized corrosion in which large pits are formed in the surface of a metal usually in contact with an aqueous solution. 

The segregation process of graphite on the surface of a metal particle is similar to that proposed by Ober-lin and Endo[35] for carbon fibers prepared by thermal decomposition of hydrocarbons. Flowever, the... 

Filiform corrosion is characterised by the formation of a network of threadlike filaments of corrosion products on the surface of a metal coated with a transparent lacquer or a paint him, as a result of exposure to a humid atmosphere. This phenomenon first attracted attention because of its formation on lacquered steel, and for this reason it is sometimes referred to as underfilm corrosion, but although it is most readily observed under a transparent lacquer it can also occur under an opaque paint film or on a bare metal surface. Filiform corrosion has been observed on steel, zinc, magnesium and aluminium coated with lacquers and paints, and with aluminium foil coated with paper. Surface treatment of the metal by phosphating or chromating lessens the tendency for filiform corrosion to occur, but it is not completely... 

Surfactants are probably the materials which most affect the performance of alkali cleaners. Surfactants are complex chemicals which modify the solubility of various materials in, and their surface affinity for, oil and water. The diverse composite which makes up the surface of a metal object must be fully wetted out if the cleaner is to perform properly. Surfactants lower the surface tension to allow wetting out to occur. Oils and greases must either be dissolved off the surface or lifted from it surfactants assist in both areas. 

Critical Pitting Potential the most negative potential required to initiate pits in the surface of a metal held within the passive region of potentials (it varies with the nature of solution, temperature, time, etc.). 

Pickle, Pickling a solution (usually acidic) used to remove or loosen corrosion products from the surface of a metal. 

Passivating agents are chemicals that promote the formation of a passivating film on the surface of a metal or alloy, such that the electrochemical behavior of the metal or alloy then approaches that of an appreciably more noble metal. 

A form of corrosion exhibited by the throwing off of thin slivers or chips from the surface of a metal such as cupro-nickel. 

XM measures the changes occurring at the surfaces of a metal and water as the two phases are brought in contact to create an interface. In surface science concepts, Xm corresponds to the decrease in work function... 

Based on the preceding discussion on spillover-backspillover and in anticipation of Chapter 4 it is worth to briefly examine the thermodynamic driving force for ion (e.g. O2 ) backspillover between a solid electrolyte (e.g. YSZ) and the gas-exposed surface of a metal (e.g. Pt) electrode. 

Example H2(g) + Cl2(g) 2 HCl(g). photoelectric effect The emission of electrons from the surface of a metal when electromagnetic radiation strikes it. 

Consider first the situation for a particle approaching the surface of a metal in a vacuum (Fig. 11). If we neglect the spread of energy levels in... 

The previous sections have set the stage for describing the essentials of what happens when a molecule approaches the surface of a metal. The most important features of chemisorption are well captured by the Newns-Anderson model [D.M. Newns, Phys. Reu. 178 (1969) 1123 P.W. Anderson, Phys. Rev. 124 (1961) 41], which we describe in Section 6.4.1. Readers who are not particularly fond of quantum mechanics and its somewhat involved use of mathematics, but merely want to learn the outcome of this model, may skip this section and go directly to Section 6.4.2, where we present a summary in qualitative terms. The same readers may also want to consult Roald Hoffmann s Solids and Surfaces [(1988), VCH, Weinheim], abook we warmly recommend. 

The Newns-Anderson approximation successfully accounts for the main features of bonding when an adsorbate approaches the surface of a metal and its wave functions interact with those of the metal. It can also be used to describe features of the dynamics in the scattering of ions, atoms and molecules on surfaces. In particular the neutralization of ions at surfaces is well understood in this framework. The subject is beyond the scope of this book and the reader is referred to the literature [J.K. N0rskov, J. Vac. Sci. Technol. 18 (1981) 420],... 

In 1902, Wagner published an analysis, based on diffusion reactions, of the oxidation of the surface of a metal. His interpretation has remained a classic in solid state diffusion analysis. The surface of a metal consists of metal atoms bound to the inner structure by a series of hybrid-bonds. If oxygen gas is present (air), the metal can form an oxide coating ... 

Active catalyst sites can consist of a wide variety of species. Major examples are coordination complexes of transition metals, proton acceptors or donors in a solution, and defects at the surface of a metallic, oxidic, or sulphidic catalyst. Chemisorption is one of the most important techniques in catalyst characterization (Overbury et al., 1975 Bartley et al, 1988 Scholten et at, 1985 Van Delft et al, 1985 Weast, 1973 and Bastein et al., 1987), and, as a consequence, it plays an essential role in catalyst design, production and process development. 

If V is localized, say, near the origin, then for locations far from the origin, this behaves like j 2kFr)/r2, which means as cos(2kFr)/ r3. These damped oscillations of frequency 2kF are the Friedel oscillations, which always arise when an electron gas is perturbed the frequency of oscillation comes from the kink in the dielectric function at 2kF. We see the Friedel oscillations (in planar rather than in spherical geometry) for the electron gas at a hard wall [Eq. (12) et seq.] and for the electron density at the surface of a metal. 

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