Metals exposed

Sodium, like every reactive element, is never found free in nature. Sodium is a soft, bright, silvery metal which floats on water, decomposing it with the evolution of hydrogen and the formation of the hydroxide. It may or may not ignite spontaneously on water, depending on the amount of oxide and metal exposed to the water. It normally does not ignite in air at temperatures below llSoC. 

Oxygen concentration is an especially important parameter to metals exposed to aqueous environments. Temperature and temperature gradients should also be reproduced as closely as possible. Concentration gradients in solutions also should be reproduced closely. Careful attention should be given to any movement of the corrosive medium. Mixing conditions should be reproduced as closely as possible. 

Little scientific examination of the deterioration of materials at depth has been undertaken except that by the US Naval Civil Engineering Laboratory and Naval Research, Laboratory. The results of this work were reported by Reinhart in 1966 and more recently the work has been reviewed by Kirk . Typical corrosion data for a selection of metals exposed in the Pacific Ocean at several sites and for different times are shown in Table 2.19 and are compared with results obtained in surface waters at Wrightsville Beach by International Nickel Inc. 

Painted metal exposed at coastal areas, ports and docks often suffers most from such hazards, which may be aggravated by high levels of u.v. radiation and the erosive action of blowing sand. Such conditions can prevail up to approximately 3 km inland. 

Nail sickness Nail sickness is chemical decay associated with corroded metals in marine situations. Chemical degradation of wood by the products of metal corrosion is brought about by bad workmanship or maintenance, or unsuitable (permeable) timber species, all of which permit electrolyte and oxygen access which promotes corrosion. Chemical decay of wood by alkali occurs in cathodic areas (metal exposed oxygen present). Softening and embrittlement of wood occurs in anodic areas (metal embedded oxygen absent) caused by mineral acid from hydrolysis of soluble iron corrosion products. 

Fouling deposition of flora and fauna on metals exposed to natural waters, e.g. sea-water. 

Another way to protect a metal uses an impervious metal oxide layer. This process is known as passivation, hi some cases, passivation is a natural process. Aluminum oxidizes readily in air, but the result of oxidation is a thin protective layer of AI2 O3 through which O2 cannot readily penetrate. Aluminum oxide adheres to the surface of unoxidized aluminum, protecting the metal from further reaction with O2. Passivation is not effective for iron, because iron oxide is porous and does not adhere well to the metal. Rust continually flakes off the surface of the metal, exposing fresh iron to the atmosphere. Alloying iron with nickel or chromium, whose oxides adhere well to metal surfaces, can be used to prevent corrosion. For example, stainless steel contains as much as 17% chromium and 10% nickel, whose oxides adhere to the metal surface and prevent corrosion. 

The most important metals for catalysis are those of the groups VIII and I-B of the periodic system. Three crystal structures are important, face-centered cubic (fee Ni, Cu, Rh, Pd, Ag, Ir, Pt, Au), hexagonally dose-packed (hep Co, Ru, Os) and body-centered cubic (bcc Fe). Figure 5.1 shows the unit cell for each of these structures. Note that the unit cells contain 4, 2, and 6 atoms for the fee, bcc, and hep structure, respectively. Many other structures, however, exist when considering more complex materials such as oxides, sulfides etc, which we shall not treat here. Before discussing the surfaces that the metals expose, we mention a few general properties. 

Clean, polycrystalline metals expose mostly their most densely packed surface, because, to create this surface, the minimum number of bonds have to be broken (see Fig. 5.3). 

In a series of studies of carefully prepared catalysts of Pt on silica gel (7,10-12) we have shown that the Pt particles are equi-axed, (and de-finitely not cuboidal as is often assumed) that the size (or percent metal exposed) agrees with results from hydrogen chemisorption, and that the particles are free of microstrain faults or twins, except when the average size is similar to the pore size of the support. In this latter case, the particles are elongated, and there is microstrain, probably due to differ-... 

Figure la. Diffraction pattern, Pt on Ajtj 0 (34. wt. pet.) 40 percent metal exposed. Incident beam energy, llSOScv (solid line) and 11558 cv (dashed line). 

It is noteworthy that surface carbon did not come from those CO molecules responsible for the HT peak but from sites that are able to disproportionate CO and correspond to the LT peak. Because the latter sites are important only on quite small particles, it is tempting to associate them with low coordination number surface metal atoms, the relative concentration of which increases rapidly as the particle size decreases below 2 nm (8). Thus, these atoms may be the sites responsible for the relatively weakly adsorbed state of CO. Results similar to our work were found on other Group VIII metals. In the case of a Ru/Si02 sample, Yamasaki et al. (9) have shown by infrared spectroscopy that the deposition of carbon occurs rapidly by CO disproportionation on the sites for weakly held CO. The disproportionation also occurred on a Rh/Al20 sample with 66% metal exposed so that appreciable concentrations of low coordination atoms are expected (10). 

Pd/Si02 metal exposed metal exposed determined by... 

Pd/Si02 Catalyst wt/% Pd Percentage metal exposed/% sijch/10 4 S 1 4 Reference... 

Many metals are fairly easily oxidized. The atmosphere contains a powerful oxidizing agent oxygen. Because metals are constantly in contact with oxygen, they are vulnerable to corrosion. In fact, the term corrosion is sometimes defined as the oxidation of metals exposed to the environment. In North America, about 20% to 25% of iron and steel production is used to replace objects that have been damaged or destroyed by corrosion. However, not all corrosion is harmful. For example, the green layer formed by the corrosion of a copper roof is considered attractive by many people. 

The necessary condition for hydrogen storage is that the thermodynamic and kinetic conditions are fulfilled. In that case, a metal exposed to hydrogen gas absorbs hydrogen until equilibrium is established. There are a number of reaction steps that kinetically may hinder a hydrogen-storing system from reaching its... 

These data showed that the isolectric point (IEP) of Si02 was around pH 2, as expected. The binding of phosphate ions to the chrome surface were also estimated as a function of pH and ionic strength (Choi et al 2004). Further, both STM and AFM have been used to investigate the corrosion mechanisms of metals exposed to the aqueous phase. Since both STM and AFM can operate under water, this provides a variety of possibilities. 

A variety of industrial catalytic processes employ small metal-particle catalysts on porous inorganic supports. The particle sizes are increasingly in the nanometre size range which gives rise to nanocatalysts. As described in chapter 1, commonly used supports are ceramic oxides, like alumina and silica, or carbon. Metal (or metallic) catalysts in catalytic technologies contain a high dispersion of nanoscopic metal particles on ceramic oxide or carbon supports. This is to maximize the surface area with a minimum amount of metal for catalytic reactions. It is desirable to have all of the metal exposed to reactants. 

Highly dispersed metals expose atoms to the gas phase in unusual geometric arrangements, which leads to particular low coordination (unsaturated) etc. (84-93). 

A second type of surface reaction involves bonding of the organic compounds with atoms (e.g., metals) exposed on the surface of the solid (Table 11.4). In these cases a water or a hydroxyl bound to a metal on the solid is displaced by the organic sorbate ... 

Metals exposed lo air pollution can be placed into three groups ... 

Plutonium monoxide occasionally appears on the surface of metal exposed to atmospheric oxidation, but is prepared more conveniently by treating the oxychloride with barium vapor at about 1,250°C. The oxide is classified with the interstitial compounds rather than with the typical metal oxides. 

Burwell and coworkers (ref. 15) studied the transformation of methylcyclo-propane on the same series of Pt catalysts, and found it to be mildly structure-sensitive. The TOF in the hydrogenolysis of methylcyclopropane increased continuously as a function of the dispersion. The total TOF varied by a factor of two, while the activation energy of the reaction was independent of the percentage of metal exposed.These facts offered a simple geometric expla-... 

What is the number of 60Co atoms produced in a 10-mg sample of cobalt metal exposed for 2 min to a thermal neutron flux of 2 x 1013 n/cm2-s in a reactor The cross section for producing 10.5-min 60Com is 16 bams, while the cross section for producing 5.3 y 60Co is 20 bams. What is the disintegration rate of the cobalt sample 4 h after the end of the irradiation ... 

Electron transfer from bare iron metal exposed by pitting of the oxide layer, while the pitting mechanism involves localized corrosion and possible catalytic dissolution pathways... 

Conceptual models of electron transfer (ET) mechanisms at Fe°-oxide-water interface (A) ET from bare iron metal exposed by pitting of the oxide layer (B) ET from conduction bands in the oxide layer (C) ET from adsorbed or lattice Fe(II) surface sites. (From Scherer, M.M. et al., CRC Crit. Rev. Environ. Sci. Technol., 30(3), 363 11, 2000. With permission.)... 

Grill, E., Winnacker, E.-L. Zenk, M.H. (1986n). Synthesis of seven different homologous phytochelatins in metal-exposed Schizosac-charomyces pombe cells. FEBS Letters 197, 115-20. 

---