Copper ion reduction

The discussion in Section 4.2.1 suggested that titanium dissolution may be driven by the reduction of H, the reduction of Oj, or the reduction of oxidizing agents. When titanium is polished in the presence of copper ions, copper ion reduction leads to the oxidation of the titanium. A galvanic couple is formed between the Cu/Cu and the Ti/Ti systems such that the copper ions are reduced and plated onto the titanium surface. At the same time, titanium is dissolved into solution. Thus, copper replaces the titanium on the surface. The reactions governing such a process are ... 

In a previous investigation, we showed that feeding NO at low temperature (80°C) to Cu over-exchanged (640%) ZSM5 after He pre-treatment at elevated temperatures, a transient production of N2O is observed as result of catalyst re-oxidation [14]. Similar results have been obtained in the present study. Fig.3 shows that, after catalyst pre-treatment of 2 h in He flow at 550°C, causing copper ions reduction from Cu to Cu", the response to a step change of NO gaseous concentration (0-600 ppm) at 80°C, results in the transient formation of N2O due to the re-oxidation of copper sites. 

Hydrazine oxygen copper ion Reductions with diimide s. 17, 22... 

The effects of these ligands on the second-order rate constants for the Cu (ligand) catalysed reaction of Ic with 2 are modest In contrast, the effects on IC2 are more pronounced. The aliphatic Oramino acids induce an approximately two-fold reduction of Iv relative to for the Cu" aquo ion. For the square planar coordinated copper ions this effect is expected on the basis of statistics. The bidentate ligands block half the sites on the copper centre. 

When dezincification occurs in service the brass dissolves anodically and this reaction is electrochemically balanced by the reduction of dissolved oxygen present in the water at the surface of the brass. Both the copper and zinc constituents of the brass dissolve, but the copper is not stable in solution at the potential of dezincifying brass and is rapidly reduced back to metallic copper. Once the attack becomes established, therefore, two cathodic sites exist —the first at the surface of the metal, at which dissolved oxygen is reduced, and a second situated close to the advancing front of the anodic attack where the copper ions produced during the anodic reaction are reduced to form the porous mass of copper which is characteristic of dezincification. The second cathodic reaction can only be sufficient to balance electrochemically the anodic dissolution of the copper of the brass, and without the support of the reduction of oxygen on the outer face (which balances dissolution of the zinc) the attack cannot continue. 

The potentials of film-free a-brass and /3-brass in solutions comparable to those existing inside the alloy at the advancing front of attack were found to be —0-38V and —0-56V (v. S.H.E.), respectively. It was also established, taking into account the activities of copper ions in equilibrium with the sparingly soluble corrosion product CU2CI2, that whereas Cu ions can be reduced to copper at —0 -16 V the reduction of Cu ions is possible only at potentials more negative than —0-41 V. Thus whereas the /3-phase of an a/3-brass can reduce both Cu and Cu ions, the a-brass can reduce only the Cu ion. 

Many other metal displacement reactions can be visualized, but not all of them occur. Some metals are oxidized readily, but others are highly resistant to oxidation. Likewise, some metal cations are highly susceptible to reduction, but others resist reduction. Zinc displaces copper ions from aqueous solutions, but copper will not replace zinc ions, because Cu is easier to reduce than Zn . Zinc will not displace ions, because... 

Ishihara, T., Kagawa, M., Hadama, F. et al. (1997) Copper ion-exchanged SAPO-34 as a thermostable catalyst for selective reduction of NO with C3H6, J. Catal., 169, 93. 

These solution NMR and X-ray crystallographic findings have been contradicted by X-ray structures solved by Rypniewski et al.32 The results show a reduced active site unchanged from the oxidized state and let these authors to propose a five-coordinate copper ion that exists throughout the oxidation and reduction process. In 2001 the Protein Data Bank listed 39 X-ray crystallographic and NMR solution structures for CuZnSOD, including oxidized, reduced, genetically modified, and other species with or without attached substrates or substrate mimics such as azide ion. The reader is advised to search the Protein Data Bank for additional and more up-to-date structural depositions and search the literature for further discussion of mechanism. 

One very fast and reliable method for the reduction of double bonds is that of transfer hydrogenation with diimine (Scheme 20.30). Under the influence of traces of copper ion and oxygen from air, hydrazine is rapidly transformed into diimine. This compound is able to hydrogenate double bonds with great success under the formation of nitrogen [120],... 

Phthalic anhydride initially reacts with ammonia, which in turn is liberated, for instance, by decomposition of urea. Diiminophthalimide is then produced via phthalimide and monoiminophthalimide. Subsequent self-condensation (as in the phthalonitrile process) under cleavage of ammonia affords polyisoindolenines, which form complexes with copper ions. Ring closure is achieved through further release of ammonia, and copper phthalocyanine is finally obtained by reduction. 

Metal ion reduction can also occur during surface studies. One of the best documented examples is that of the reduction of copper(II) to copper(I), a process that has been reviewed extensively in the research literature (11). Certain copper(II) minerals, such as CuO (tenorite), are quite susceptible to photoreduction (11), and care must be taken in conducting surface studies on them. 

Let us continue with the example of copper ions in contact with copper metal and zinc ions in contact with zinc metal. This combination is usually referred to as the Darnell cell or zinc/copper couple(Fig. 6.5a). For this electrochemical cell the reduction and oxidation processes responsible for the overall reaction are separated in space one half reaction taking place in one electrode compartment and the other takes place in the other compartment. 

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