Active state, iron

Electroless reactions must be autocatalytic. Some metals are autocatalytic, such as iron, in electroless nickel. The initial deposition site on other surfaces serves as a catalyst, usually palladium on noncatalytic metals or a palladium—tin mixture on dielectrics, which is a good hydrogenation catalyst (20,21). The catalyst is quickly covered by a monolayer of electroless metal film which as a fresh, continuously renewed clean metal surface continues to function as a dehydrogenation catalyst. Silver is a borderline material, being so weakly catalytic that only very thin films form unless the surface is repeatedly cataly2ed newly developed baths are truly autocatalytic (22). In contrast, electroless copper is relatively easy to maintain in an active state commercial film thicknesses vary from <0.25 to 35 p.m or more. 

In de-aerated 10sulphuric acid (Fig. 3.45) the active dissolution of the austenitic irons occurs at more noble potentials than that of the ferritic irons due to the ennobling effect of nickel in the matrix. This indicates that the austenitic irons should show lower rates of attack when corroding in the active state such as in dilute mineral acids. The current density maximum in the active region, i.e. the critical current density (/ ii) for the austenitic irons tends to decrease with increasing chromium and silicon content. Also the current densities in the passive region are lower for the austenitic irons... 

In the polarization curve for anodic dissolution of iron in a phosphoric acid solution without CP ions, as shown in Fig. 3, we can see three different states of metal dissolution. The first is the active state at the potential region of the less noble metal where the metal dissolves actively, and the second is the passive state at the more noble region where metal dissolution barely proceeds. In the passive state, an extremely thin oxide film called a passive film is formed on the metal surface, so that metal dissolution is restricted. In the active state, on the contrary, the absence of the passive film leads to the dissolution from the bare metal surface. The difference of the dissolution current between the active and passive states is quite large for a system of an iron electrode in 1 mol m"3 sulfuric acid, the latter value is about 1/10,000 of the former value.6... 

An example of the process of a passivating metal is the reaction of tetravalent cerium with iron (see Fig. 5.54D). Iron that has not been previously passivated dissolves in an acid solution containing tetravalent cerium ions, in an active state at a potential of Emix2. After previous passivation, the rate of corrosion is governed by the corrosion current ya and the potential assumes a value of Emixl. 

Ramsey A., Hillas P., Fitzpatrick P. (1996). Characterization of the active site iron in tyrosine hydroxylase redox states of the iron. J. Biol. Chem. 271, 24395-400. 

The transition from the active state to the passive state is the passivation, and the transition in the reverse direction is the activation or depassivation. The threshold of potential between the active and the passive states is called the passivation potential or the passivation-depassivation potential. Similarly, the transition from the passive state to the transpassive state is the transpassivation, and the critical potential for the transpassivation is called the transpassivation potential. Further, a superficial thin film formed on metals in the passive state is often called the passive film (or passivation film), the thickness of which is in the order of 1 to 5 nm on transition metals such as iron and nickel. 

Anodic passivation can be observed easily and clearly with iron group metals and alloys as shown in Fig. 11-10. In principal, anodic passivation occurs with most metals. For instance, even with noble metals such as platinum, which is resistant to anodic dissolution in sulfuric acid solutions, a bare metal surface is realized in the active state and a superficial thin oxide film is formed in the passive state. For less noble metals of which the affinity for the oxide formation is high, the active state is not observed because the metal surface is alwa covered with an oxide film. 

It is weD known that metallic iron corrodes violently in dilute nitric acid solutions, but metallic iron is passivated in concentrated nitric add solutions as shown in Fig. 11-14(a). This passivation of metallic iron results from a strong oxidizing action of concentrated nitric add that changes the iron electrode irom the active state to the passive state. 

The intersection of the anodic polarization curve of iron dissolution with the cathodic polarization ctuve of nitric add reduction occurs in the range of potential of the active state in dilute nitric acid, but it occurs in the range of potential of... 

Early attempts to purify the enzyme brought the quick realization that aconitase is easily inactivated (6,7). In the early 1950 s Dickman and Qoutier (8,9) found that inactivated aconitase could be reactivated by incubation with iron and a reduc-tant. From kinetic analyses of the iron and reductant effects on enzyme activity, Morrison argued that both formed Michaelis-Menten complexes wiA the enzyme (10). This refuted the earlier idea that the sole role of the reductant was to maintain iron in a reduced state (9). Of several metal cations tried, only ferrous ion was found to be capable of this reactivation process (8,11). Because of the absolute requirement for iron in activation, the known chelation properties of citrate, and Ogston s 3-point attachment proposal, Speyer and Dickman proposed that the active site iron provides three coordination sites for substrate binding - one for hydroxyl and two for carboxyl groups (12). 

Gehring, A.U. (1985) A microchemical study of iron ooids. Eclogae Geol. Helv. 78 451-457 Gehring, A.U. Karthein, R. Reller, A. (1990) Activated state in the lepidocrocite structure during thermal treatment. Naturwissenschaf-ten 77 177-179... 

Calmodulin binds to and activates neuronal NOS (Bredt and Snyder, 1990 Schmidt et al., 1991), and also functions as a tightly bound prosthetic group to keep macrophage NOS in its active state (Cho et al., 1992). Work with the neuronal NOS has uncovered the basis for its calmodulin activation Calmodulin binding triggers electrons to transfer onto the NOS heme iron (Abu-Soud and Stuehr, 1993) (Fig. 10). Because this transfer is associated with initiation of... 

It was shown by these authors that the amount of nitrogen present during pretreatment of a catalyst affects the ultimate activity for ammonia synthesis (206). Specifically, it was found that treating H2-reduced small particles with ammonia at 670 K, followed by re-reduction of the catalyst with a H2 N2 gas mixture, gave rise to an increase in the catalytic activity compared to the activity measured after H2 reduction alone. However, when the catalyst in this high-activity state was further treated with H2 alone at 670 K, the catalytic activity was found to decrease to that value observed before the above ammonia treatment. Subsequent ammonia treatment returned the catalyst to its high-activity state. No such effects were observed for metallic-iron particles greater than 10 nm in size. 

O-PO4 is often used in combined preoperational cleaning and passivation programs and acts, in the presence of oxygen, to promote passivity on iron surfaces, changing it from an active state to a passive state by forming a barrier to corrosive ions (although the starting material may actually be P-PO4). 

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