Simultaneous hydrogen evolution

The coevolution of H2 gas in electroless deposition processes is a phenomenon that needs to be understood not only to elucidate the mechanism of deposition, but also since it impacts the properties of deposits by H inclusion. Van den Meerakker [51] first proposed a correlation between simultaneous hydrogen evolution in electroless deposition and the heat of adsorption of hydrogen. In this useful endeavor, however, he has been criticized for erroneously calculating the heats of adsorption of H at Cu by Gottesfeld et al. [52], and Group I (or SP type) metals in general by Bindra and Tweedie [53]. 

Cathodic Evolution of Hydrogen. The cathodic evolution of hydrogen is of great scientific and technological imp)ortance. Technological importance stems from the fact that electrodeposition of some metals, such as Ni and Cr, is accompanied by simultaneous hydrogen evolution. 

It was also observed, in 1973, that the fast reduction of Cu ions by solvated electrons in liquid ammonia did not yield the metal and that, instead, molecular hydrogen was evolved [11]. These results were explained by assigning to the quasi-atomic state of the nascent metal, specific thermodynamical properties distinct from those of the bulk metal, which is stable under the same conditions. This concept implied that, as soon as formed, atoms and small clusters of a metal, even a noble metal, may exhibit much stronger reducing properties than the bulk metal, and may be spontaneously corroded by the solvent with simultaneous hydrogen evolution. It also implied that for a given metal the thermodynamics depended on the particle nuclearity (number of atoms reduced per particle), and it therefore provided a rationalized interpretation of other previous data [7,9,10]. Furthermore, experiments on the photoionization of silver atoms in solution demonstrated that their ionization potential was much lower than that of the bulk metal [12]. Moreover, it was shown that the redox potential of isolated silver atoms in water must... 

The mechanism of silicon etching in alkaline solutions is a process of material dissolution with a simultaneous hydrogen evolution. The main soluble product is a silicic anion Si02(0H)2 that can further be condensed to form polysilicic anions. In fact, due to the acido-basic ionization of OH radicals in a highly alkaline solution, Eq. (19) should be modified as follows ... 

Photosensitization of Ti02 powders (Degussa P25 with 0.5% Pt on the surface) treated with Fe(CN) 6, was observed in preliminary experiments where lactic acid was oxidized to pyruvic acid with simultaneous hydrogen evolution, upon visible light irradiation of suspensions of the derivatized powder in dilute lactic acid solutions. 

If the second electrode process does not interfere with the first (e.g., it may be due to the background), the method may still be acceptable in the laboratory (e.g., anodic meth-oxylation or an electrolytic reduction with simultaneous hydrogen evolution). For industrial processes, however, a high current yield (or current efficiency) is generally desirable and waste of current on the background is less tolerable. The current yield is the theoretical amount of electricity divided by the amount actually employed for the production of a particular substance (usually expressed as a percentage). 

These reactions were postulated with an assumption that the alloy electrodeposition was always accompanied by the simultaneous hydrogen evolution (reaction (7.19)). This model has been confirmed by in situ surface Raman spectroscopic studies, by revealing existence of adsorbed intermediate [Ni(C6H507)Mo02]ads at the electrode surface [32]. 

Taking into account that simultaneous hydrogen evolution occurs in all cases, it was necessary to determine the current density for hydrogen evolution (/h), subtract it from the measured (corrected for IR drop) current density values (/tot) given in Fig. 2.25a to obtain current densities for powders electrodeposition (in this case Co powder, jco)- Hence, several values of current density on the polarization curve for Co powder electrodeposition were chosen and the volume of evolved hydrogen was determined in the burette. The current for hydrogen evolution (O) was obtained using the equation [98]... 

It is worth to note that in both acidic and alkaline conditions the simultaneous hydrogen evolution reaction takes place due to aluminium dissolution, which is experimentally seen as bubble formation. 

It seems that in some systems the galvanic deposition produces powders when the simultaneous hydrogen evolution reaction takes place ... 

This is demonstrated for the examples of Al/Ag (Fig. 7.1) or Al/Cu [3] in alkaline solutions. The simultaneous hydrogen evolution (as bubbles) may significantly change the hydrodynamic conditions at the surface at which the deposition of a more noble metal takes place. Changes in the hydrodynamic conditions may lead to growth of particles of various shapes. 

For other systems, where the simultaneous hydrogen evolution is not evident, it seems that to produce powders by this type of deposition it is necessary that the surface of the less noble metal is not completely covered by the more noble metal. An incomplete surface coverage of the less noble metal may form porous (spongy) deposits with a poor adhesion, thus leading to the formation of various shapes of powders. 

Notably, in both cases there is no simultaneous hydrogen evolution as shown by reactions (7.14) and (7.15). The evolution of hydrogen gas bubbles was not observed in the experimental settings. 

Electroless deposition of copper accompanies the simultaneous hydrogen evolution as it is presented with the reaction below ... 

Galvanic displacement deposition of metallic powders is achieved as a consequence of the porosity of a more positive metal or due to a simultaneous hydrogen evolution reaction. Many metallic powders can be produced using this simple process. 

---