Iron-group metals, deposition

In the last 10 years, the electrodeposition of the new Zn-Me alloys and ternary alloys was studied. The influence of plating conditions and different additives for deposition of alloys of Zn with iron group metals was studied in detail. 

Depolarization of metal deposition sometimes occurs when two metals which separate simultaneously form compounds or solid solutions. The reversible potential of a solid solution generally lies in between those of the pure constituents hence an alloy containing both metals may be deposited at a potential that is less cathodic than that necessary for the less noble constituent in the pure state. This probably accounts for the fact that zinc and nickel are deposited simultaneously at a potential of about — 0.6 volt, whereas that required for pure zinc is nearly 0.2 volt more cathodic. The simultaneous deposition of the iron-group metals is partly due to the similarity of the discharge potentials, but the formation of solid solutions also plays an important part. Although the deposition potentials of cobalt and nickel are lower than that of iron, the cathodic deposit almost invariably contains relatively more of the latter metal. ... 

The term anomalous codeposition (ACD) was first introduced by Abner Brenner/ to describe an electrochemical deposition process m which the less noble metal is deposited preferentially under most plating conditions. This behavior is typically observed in codeposition of iron-group metals (i.e. Fe, Co and Ni) or in codeposition of an iron-group metal with Zn or Cd, with either inhibition or acceleration of the rate of deposition of one of the alloying elements by the other. ... 

It was applied first to describe the electroless deposition of Ni-P alloys, and later for electroplating of alloys of W and Mo with the iron-group metals. 

A phenomenon of induced codeposition, similar to that discussed above for W, is observed when Mo is codeposited with iron-group metals. Similarly to tungsten, molybdenum cannot be deposited alone from aqueous solutions. Electrodeposition of Mo alloys exhibits similar dependencies on experimental variables as that of W. It should be noted that, although the two systems are very similar, some differences are found in the literature, as described bellow. 

NiCit] to the cathode surface. Because the deposition rate of the two metals is coupled, the alloy composition does not vary with rotation rate. In contrast, if the concentration of Ni in solution is comparable to or higher than that of MoO4 , the rate of formation of the Ni-Mo intermediate is limited by the transport of molybdate, while Ni can be deposited in parallel—its rate of deposition being independent of the rate of mass transport. Increasing either the rotation rate or the molybdate concentration—the partial current density of Mo deposition will also increase, while that of Ni deposition will not be affected. Thus, it was proven beyond any doubt that the induced codeposition of Mo with Ni and other iron-group metals was dependent on the existence of the iron-group metal ions in solution. 

The difficulty in attempting to determine the mechanism of alloy deposition from the current-potential relationship observed in complex solutions, which sometimes contain more than one ligand, was alluded to in the introduction to this chapter (cf., Section 1.2.2). The comments made here are not meant to criticize the experimental work presented in these papers in the field of induced codeposition of Mo with iron-group metals. It is only given to show the limits of validity of mathematical models, particularly when the solution is complex and the number of freely adjustable parameters is large. 

All the above-mentioned methods are expensive in comparison with the electrodeposition of Mo-Ni alloy coatings. Although molybdenum cannot be separately deposited from aqueous solutions, it can be codeposited with the iron-group metals (Fe, Co, Ni) in the presence of appropriate complexing agents, by the type of alloy electrodeposition defined by Brenner [116] as induced codepositiOTi. 

This chapter deals with aspects of the synthesis of fuel cell catalysts. Practical catalysts for low-temperature fuel cells are typically in the nano-size range and are frequently formed or deposited on high-surface-area supports. Pt is the most eommonly used eatalyst for both cathode and anode in proton exchange membrane fuel eells (PEMFCs). In the case of the cathode, combined catalyst systems such as Pt nanoparticles supported on Au or Pt alloy catalysts, as well as Pt-skin catalysts formed in combination with the iron group metals have also attracted attention. Much work has been carried out on the development of non-noble metal (Pt-ffee) catalysts, the synthesis of which will be discussed in Section 9.5. In the case of the anode, bi-metallie eatalysts are typically employed unless the fuel is neat H2. Pt-Ru is the state-of-the-art catalyst for both methanol and reformate fuel cells. For the latter, other anode catalysts such as Pt/MoOx and Pt/Sn are also considered promising. 

When the deposition of one metal makes possible the deposition of another metal, which cannot be reduced alone, the so-called induced codeposition is observed (e.g., deposition of W and Mo in the presence of an iron-group metal). 

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. 

Experience shows that in the deposition of a number of metals (mercury, silver, lead, cadmium, and others), the rate of the initial reaction is high, and the associated polarization is low (not over 20 mV). For other metals (particularly of the iron group), high values of polarization are found. The strong inhibition of cathodic metal deposition that is found in the presence of a number of organic substances (and which was described in Section 14.3) is also observed at mercury electrodes (i.e., it can be also associated with the initial step of the process). 

The model based on metal-hydroxide ions ([MOH]+) was further developed by Grande and Talbot [71]. Sasaki and Talbot [72] demonstrated the extendibility of this model to the electrodeposition of Co—Fe and Ni—Co alloys. They found that there is a slight inhibition of the more positive metal deposition and a promotion (acceleration) of the less positive metal deposition for all binary iron-group alloys. 

When compared with electrodeposition, in the example of iron group of metals or their alloys, the proposed mechanism explaining the autocatalytic deposition must take into consideration the following aspects. 

There is an obvious difference in the kinetic behavior between the electrodeposition and autocatalytic deposition of metals such as Pb, Cd, and Zn. These metals are easily electrode-posited at low overvoltages and high exchange currents. However, they act as inhibitors or stabilizers when added in very small amounts to solutions from which an autocatalytic deposition of iron group of metals or alloys is carried out. 

In the electrodeposition of alloys from the iron group of metals, anomalous deposition is observed, i.e., the more electronegative metal tends to be deposited preferentially. In the autocatalytic deposition this behavior is not observed. 

The formation of a monovalent species in deposition and dissolution of divalent ions of the iron-group transition metals is commonly assumed in the literature. Since the monovalent ions (such as Fe ) are unstable in solution, they are assumed to be adsorbed on the surface, either as the ion itself or as a hydroxide, such as FeOHads- This could stabilize the monovalent form of the element. Moreover, since a monovalent hydroxide is not known to exist in solution, one does not know its solubility product, and the possibility of the existence of this adsorbed species, even in solutions of low pH, cannot be ignored a priori. Unfortunately, the nature of the adsorbed intermediate, or even the evidence for its existence on the surface, is at best circumstantial. 

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