Metal deposition parallel reactions

V to +0.7 V vs. RHE for a Pd surface. Normally, this is anodic, or positive, with respect to the Em value of the electroless reaction (Fig. 1). Following removal of the oxide species from the catalyst surface, whether deposition subsequently initiates or not depends on the interplay between the kinetics of the parallel metal ion and O2 reduction reactions, and oxidation of the reducing agent. Once an appropriate Em value is reached, metal deposition will occur. 

In the deposition of certain metals (e.g., Fe), H2 is produced in a parallel reaction. The amount of H2 formed can be determined by holding the disk at a potential for the co-deposition of Fe and H2 and the ring at a potential for the oxidation of H2. The current produced on the ring can be combined with a knowledge of N to give the total current of H2 evolution on the disk, and thus deduce the rate of the cathodic deposition of Fe, free from interference by co deposition of H2. 

In summary, the preparation of bimetallic catalysts by surface redox reaction using a reductant preadsorbed on the parent monometallic catalyst has been studied in detail. Unfortunately, the method is intricate and time consuming, especially if several successive operations are required. Furthermore, when the modifier has a standard electrochemical potential higher than that of the parent metal (AUCI4 deposited on Pt°), the overall reaction is a complex one involving a reduction by adsorbed reductant but also direct oxidation of the metallic parent catalyst. The relative rate of the two parallel reactions determines the catalytic properties of the resulting bimetallic catalyst. 

It is often possible to describe the mechanism of coke (elementary carbon) deposition during the process over metal catalysts using a system of linearly independent parallel reactions ... 

Considerations of the mechanism of charge transfer discussed for metal deposition apply also to alloys, but there are some differences. First, it must be realized that alloy deposition is a complex process, in which at least two parallel reactions take place simultaneously (i.e., the deposition of the two metals constituting the alloy), and in many cases hydrogen evolution constitutes a third parallel reaction. 

If the mixed-metal complex is the precursor for deposition of the alloy, then it can be seen that adding either of the two ions would increase the rate of deposition of the other. This simplistic interpretation could lead only to alloys having equal concentrations of the two elements. However, if the concentration of the two elements in solution is not equal, one could have a parallel reaction, in which one of the metals would also be deposited in parallel from its complex with citrate, giving rise to a whole range of alloy compositions. 

EXAMPLE 2.10. Parallel Reactions of Metal Deposition and Hydrogen Evolution... 

THE PROBLEM A batch laboratory reactor with an electrolyte volume of 700 cm and an electrode area of 30 cm is used to deposit a divalent metal from an aqueous solution in a potentiostatic mode. Initial concentration of the metal is O.lkmol/m. The reactor mass transfer coefficient has been measured as 3.3 x 10" m/s. Hydrogen evolution occurs as a parallel reaction according to the equation % = H p [ — ], where kn = 1.30 X 10" A/m and = 12 If the metal deposition is operated at its limiting current density at an electrode potential of —0.9 V (SCE), determine how conversion, total current density, and current efficiency vary with time, in a potentiostatic mode. What will be the current efficiency at the final... 

Wetting agents. These are added to accelerate the release of hydrogen gas bubbles from the surface. In their absence, the hydrogen which is often evolved in a parallel reaction to metal deposition can become occluded in the deposit causing, for example, hydrogen embrittlement. 

Eiectrodeposition runs parallel with the process of electrolysis. Redox reactions taking place in the bath solution simultaneously result in the metal deposition on the cathode, also known as the working electrode. Various steps involved in the eiectrodeposition include (i] oxidation at anode on the application of external current, (ii] dissolution of metal ions in electrolyte solution, (iii] metal ion transportation from electrolytic solution to the cathode surface, (iv] reduction of ions at the cathode, and (v] continuous metal layer formation on the cathode surface. The amount of metal deposition depends on deposition time and other parameters determined by Faraday s law, described by the following equation ... 

Induced co-deposition is observed for deposition of metals that cannot be deposited at all from an aqueous solution, such as W, or can barely be deposited, with a low current efficiency and poor adherence of the deposit, such as Re. However, alloys of W with the iron-group metals can readily be formed, using, for example, a solution of NiS04 and Na2W04, with citric acid added as a complexing agent. In this particular case it was shown that a Ni-W alloy is deposited from a complex containing both metals, while Ni is also deposited in parallel reactions from its complex with citrate. Very similar behavior is observed for deposition of alloys of molybdenum. 

The spatial distribution of deposited Ni and V in the reactor bed is determined by the activity of the catalyst and phenomenologically parallels that for profiles in individual pellets. Metals will tend to deposit near the reactor inlet with a highly active catalyst. A more even distribution or one skewed toward the reactor outlet is obtained for catalyst with less activity, as shown by Pazos et al. (1983). Generally with a typical small-pore (60-A), high-surface-area desulfurization catalyst, metals will concentrate near the inlet (Sato et al., 1971 Tamm et al., 1981). Fleisch et al. (1984) observed concentration maximums a short distance into the catalyst bed, as a probable consequence of the consecutive reaction path. 

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