Autocatalytic metal ion reduction processes are very complex: they contain many stages, and their mechanisms are not very detailed. At present, only the basic stages that accurately describe the catalytic process can be given. Localized surfaces for reduction reactions on metal catalysts (cause of catalysis) are often attributed to the requirement for one or more catalytic surfaces for further stages of the process to proceed. According to one of the previous explanations, only at the catalytic surface are active intermediates obtained, which then reduce metal ions. One, atomic hydrogen and later negative hydrogen ion hydrides are thought to be such products. A scheme for reactions using intermediate hydrides nicely explains the relationship observed in nickel and the copper plating process. However, there is no direct evidence that hydride ions are indeed formed in these processes. Furthermore, hydride theory only explains reactions with strong hydrogen-containing reducing agents, which may actually be H-donors.
A more general explanation for the cause of catalysis in these processes is based on electrochemical reactions. It is suggested that the reducing agent is anodically oxidized on the surface of the catalyst, and the obtained electrons are transferred to the metal ions, which are reduced by the cathode. The catalytic process involves two simultaneous and mutually compensating electrochemical reactions. In this explanation , electrons are active intermediates in the catalytic process. However, electrons are fundamentally different from the conversational intermediates of reactions. They may easily transfer along the catalyst without mass transfer, therefore, the catalyst reacts, with all other possible mechanisms (often called "chemical mechanisms") not as a result of direct contact of reactants or reactants or reactants with intermediates matter, but because of the exchange of "anonymous" electrons through metals.
On the metal surface, when the anodic oxidation of the reducing agent proceeds simultaneously, the electroless plating catalytic system reaches a steady state in which the rates of the two electrochemical reactions are equal and the metal catalyst obtains a mixed potential . The magnitude of this potential is between the equilibrium potential Ec of the reducer and the metal. The specific value Em depends on the kinetic parameters of these two electrochemical reactions. Electrochemical studies of catalyzed metal deposition reactions have shown that the electrochemical mechanism is realized in all systems of electroless plating.
Cathodic reduction of metal ions
At the same time, it's clear that the process is often not that simple. It appears that simultaneous anodic and cathodic reactions do not generally remain kinetically independent but rather influence each other. For example, reduction of copper ions increases with anodization of formaldehyde. 8 The cathodic reduction of nickel ions in electroless nickel plating and the anodic oxidation of hypophosphite in the plating solution are faster than those electrochemical reactions occurring alone. The interaction of this electrochemical reaction may be related to the change of electrode state. metal catalyst surface.
Electrochemical reactions may also hinder each other: for example, in the reduction of silver ions by hydrazine from cyanide solutions, their rate is lower than that of Ag-Ag(1) and redox systems alone.
The electrochemical nature of most of the autocatalytic processes discussed allows us to apply electrochemical methods for their investigation. However, they need to be applied to the entire electroless plating system plating without separating the anode and cathode processes in the space. One suitable method is based on the measurement of polarization resistance. It can provide information on process mechanisms and can be used to measure metal deposition rates (in laboratory and industry).
The polarization resistance Rp is inversely proportional to the process rate i:
where ba and bc are the Tafel equation coefficients (b ≈ 1/αnf), α is the transfer coefficient, n is the number of electrons in a reactant molecule participating in the reaction, and f = F/RT (F = Faraday number).
Autocatalytic metal reduction reactions may also not proceed electrochemically. The course of such reactions has been shown to be: (a) an intermediate metal hydride is formed, which decomposes into meta and hydrogen (borohydride reduction of copper ions); (b) the metal complex is hydrolyzed, resulting in the precipitation of the metal oxide on the surface , which is then reduced to the reducing agent present in the metal solution (tartrate reduces silver ions).
