Reduction displacement deposition

This is obtained via combination of the two partial electrode reactions, oxidation and reduction, reaction (9.5) and (9.6), respectively. Thus, in the displacement deposition of Cu on a Zn substrate, a layer of metallic Cu is deposited on the zinc while Zn dissolves into solution (Fig. 5.11). We stated that this reaction is possible since the Zn/Zn system has an electrode potential lower than that of the Cu/Cu system (Table 5.1 and Fig. 5.10). The overall displacement deposition reaction according to Eq. (9.7) can be considered as the reaction of the electrochemical cell... 

When Er is larger than Ei, reduction occurs at the right-hand electrode [Eqs. (9.4) and (9.6)]. Thus, when S > 0, the overall displacement deposition reaction [Eqs. (9.1) and (9.7)] will occur from left to right. The reaction is spontaneous (feasible) in a direction from left to right since AG is negative for pKJsitive values of [Eqs. (9.8) and (9.9)]. This is in agreement with earlier discussions in Chapter 5 and Eigure 5.10. 

Electroless plating should not be confused neither with the electrochemical (galvanic) displacement deposition - process involving the oxidation (dissolution) of the metallic substrate and concomitant reduction of metallic ions in solution - nor with the homogeneous chemical reduction process - indiscriminate deposition over all objects in contact with the solution. 

In the galvanic displacement deposition, the electrons produced due to oxidation or dissolution of the metal Mi, as shown by the reaction (9.1), are further used for the reduction of the metallic ions of the metal M2 (reaction (9.2)). Consequently, a combination of the reactions (9.1) and (9.2) leads to ... 

In these reactions, metallic nickel is formed under conditions of net carbon deposition, indicative of mobility of one (or both) elements during the catalytic process. When small amounts of hydrogen were included in the gaseous reactant mixtures, the values of (log A, E) were significantly closer to the compensation line characteristic of reactions on the metal, as expected for displacement of catalyst equilibria in the direction of an increase in the area of the active metal phase present, presumably due to surface reduction. 

For further explanation, see Fig. 42.) For propane, the reduction of the root mean square displacement due to deposition of n-hexane coke (4.3 wt% C) is indicated (J34). 

When a metal is in contact with a solution containing its own ions, it attains a state of dynamic equilibrium where the rate of dissolution of metals from the surface is equal to the rate of deposition from solution. If the reaction is to proceed in one direction or another, the equilibrium must be displaced in an anodic sense to promote the dissolution reaction (oxidation, see Equations 10.2 and 10.3) and in a negative sense to promote the deposition reaction (reduction). The extent to which the potential of an electrode is displaced from its equilibrium value is termed the polarisation of the electrode. In the corrosion situation the difference between the equilibrium potential for metal dissolution and for the cathodic reaction on the same metal provides the driving force for the dissolution of the metal. 

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