Codeposition of Cobalt and Molybdenum

The aforementioned systems are usually obtained by such conventional methods as fusion or condensation of required components on various supports. However, it is possible to use electrolysis of solutions or melts. Both these electrochemical methods have their advantages and disadvantages. 

In the case of acidic or neutral media, citric acid can be presented by the symbol of LHg that points to the presence of three -COOH groups in the molecule of this ligand capable of generating hydrated protons. As follows from analysis of the literature [17, 18, 135], in the case of pH 7, one may restrict oneself to consideration of the following reversible processes  

The required values of standard potentials were substituted into the corresponding Nernst equations that took the following form at 25 C  

The obtained potential diagrams allow selectingthe optimum solution compositions. As potentials of the redox systems ofCo Co(ll) and H2 H30 draw together at an increase in pH, the most attractive media are weakly acidic ones. Herewith, an excess of ligand is undesirable, but its concentration must not be too low. Otherwise, low-soluble hydroxides may be formed in the solution bulk due to loss of the system stability. Similarly, the composition of the Co(II) and Mo(Vl) containing malic or tartaric acid was considered. As we found no available data in 

Co(II) complexes are reduced at more negative potentials simultaneously with hydrogen evolution. Therefore, cathodic voltammograms obtained in Co(II) and malic acid solutions contain poorly defined plateau of limiting current density (fn ) [136]. The rate of this partial process grows at an increase in cathodic polarization. Besides, decreases at an increase in the ligand concentration. If an 

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Gomez et al. " electrodeposited Co-Mo magnetic alloys from a sulfate-citrate bath on carbon electrodes. Although the focus of their paper was not on elucidating the mechanism of induced codeposition, it was suggested that hydrogen could not be responsible for the deposition of Mo in the Co-Mo system, because its concentration was fairly low and because another mechanism should explain the need for citrate or polycarboxylate anions in solution. The deposition process was foimd to be favored when molybdate was present in solution, even at very low concentrations. Hence, the authors adopted the model of Podlaha and Landolt, according to which a mixed-metal complex of cobalt(II), citrate and molybdenum dioxide is adsorbed on the surface and promotes Mo reduction. 

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