The reduction process of catalyzed metal ions is very complex: it consists of many stages, and its mechanism is not understood in detail. Currently, only an accurate description of the fundamental stages of catalytic processes is possible. The localization of the reduction reaction on the metal-catalyst surface (reason for catalysis) is often attributed to the requirement of the catalyst surface for one or more stages of the catalytic process. According to the previous explanations, active intermediates are obtained only on the catalytic surface, followed by the reduction of metal ions. First, hydrogen atoms, and later, negative hydrogen ionhydrides were considered such products. An intermediate hydride reaction scheme well explains the relationship between nickel and copper plating. 5 However, there is no direct evidence that hydride ions are indeed formed during these processes. Furthermore, hydride theory only explains reactions containing strong hydrogen reducing agents, which may actually be hydrogen donors.
A more comprehensive explanation of the cause of catalysis in these processes is based on electrochemical reactions. The suggestion is that the anodic redox agent gets electron transfer from the metal ion on the catalyst surface, which is the cathodic reduction. The catalytic process involves two simultaneous, mutually compensating electrochemical reactions. In this interpretation of the catalytic process, electrons are active intermediates. However, the conversational intermediates of electrons and reactions are fundamentally different. They can be easily transferred along the catalyst without mass transfer, for this reason, the reaction of the catalyst, with all other possible mechanisms (which is often called "chemical interaction") occurs not due to direct contact between the reactants, Or reactions, or reactants and intermediates, but due to "anonymous" electrons exchange through metals.
On metal surfaces, when anodizing the reducer.
Red → + Ox ne
Cathodic reduction of metal ions
Me n+ +ne
At the same time, the steady state of the electroless plating catalytic system is obtained, and the metal catalyst obtains a mixed potential under the condition that the two electrochemical reaction rates are equal. The magnitude of this potential is between the equilibrium potential e of the reducer and the metal. The specific value e depends on the kinetic parameters of the two electrochemical reactions.
Studies of electrochemically catalyzed metal deposition reactions have shown that electrochemical mechanisms are realized in all electroless plating systems.
At the same time, it is clear that the process is often not that simple. Appears to be anodic.
Simultaneous cathodic reactions often do not remain kinetically independent but influence each other. For example, reduction of copper ions increases with anodization of formaldehyde. eight
Cathodic reduction of nickel ions and electroless nickel phosphoric acid anodic oxidation plating solution occurs faster than these electrochemical reactions alone. This interactive electrochemical reaction may be related to the change of the metal-catalyst surface state.
Electrochemical reactions can also interfere with each other: for example, the reduction of silver ions with hydrazine in cyanide solution is slower than that of silver-silver(1) and redox systems alone.
The discussion of the electrochemical properties of most of the autocatalytic processes enables us to apply electrochemical methods to study them. However, they need to be applied throughout the electroless plating system without the need for anode and cathode processes in separate spaces. 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 (both in the laboratory and in industry). The polarization resistance Rp is inversely proportional to the process rate:
where A and BC are the coefficients of the Tafel equation (b≈1/αNF), α is the transfer coefficient, n is the number of electrons in one molecule of the reactant participating in the reaction, and F = F/RT (F = Faraday number) .
Catalyzed metal reduction reactions are also not possible electrochemically. Two courses of reactions have been demonstrated: (i) the formation of an intermediate metal hydride decomposition, pair and hydrogen (borohydride reduction of copper ions); and (b) hydrolysis of the metal complex, thereby oxidation of the metal on the surface The precipitate is then reduced to the metal by the presence of a reducing agent in solution (tartrate reduces silver ions).
