Activation of Noncatalytic Surfaces

Noncatalytic surfaces (e.g. nonconductors, noncatalytic metals, noncatalytic semiconductors) have to be activated, that is, made catalytic, prior to the electroless deposition. This activation is performed by generating catalytic nuclei on the surface of a noncatalytic material. Two major types of processes have been used to produce catalytic nuclei electrochemical and photochemical. 

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In this chapter we discuss the electrochemical model of electroless deposition (Sections 8.2 and 8.3), kinetics and mechanism of partial reactions (Sections 8.4 and 8.5), activation of noncatalytic surfaces (Section 8.6), kinetics of electroless deposition (Section 8.7), the mechanism of electroless crystallization (Section 8.8), and unique properties of some deposits (Section 8.9). 

Electrochemical Activation. In the electrochemical method, catalytic nuclei of metal M on a noncatalytic surface S may be generated in an electrochemical oxidation-reduction reaction,... 

Thermally induced deactivation of catalysts is a particularly difficult problem in high-temperature catalytic reactions. Thermal deactivation may result from one or a combination of the following (i) loss of catalytic surface area due to crystallite growth of the catalytic phase, (ii) loss of support area due to support collapse, (iii) reactions/transformations of catalytic phases to noncatalytic phases, and/or (iv) loss of active material by vaporization or volatilization. The first two processes are typically referred to as "sintering." Sintering, solid-state reactions, and vaporization processes generally take place at high reaction temperatures (e.g. > 500°C), and their rates depend upon temperature, reaction atmosphere, and catalyst formulation. While one of these processes may dominate under specific conditions in specified catalyst systems, more often than not, they occur simultaneously and are coupled processes. 

Electroless reactions must be autocatalytic. Some metals are autocatalytic, such as iron, in electroless nickel. The initial deposition site on other surfaces serves as a catalyst, usually palladium on noncatalytic metals or a palladium—tin mixture on dielectrics, which is a good hydrogenation catalyst (20,21). The catalyst is quickly covered by a monolayer of electroless metal film which as a fresh, continuously renewed clean metal surface continues to function as a dehydrogenation catalyst. Silver is a borderline material, being so weakly catalytic that only very thin films form unless the surface is repeatedly cataly2ed newly developed baths are truly autocatalytic (22). In contrast, electroless copper is relatively easy to maintain in an active state commercial film thicknesses vary from <0.25 to 35 p.m or more. 

A gas-solid reaction usually involves heat and mass transfer processes and chemical kinetics. One important factor which complicates the analysis of these processes is the variations in the pore structure of the solid during the reaction. Increase or decrease of porosity during the reaction and variations in pore sizes would effect the diffusion resistance and also change the active surface area. These facts indicate that the real mechanism of gas-solid noncatalytic reactions can be understood better by following the variations in pore structure during the reaction. 

For noncatalytic reactions a null vertex is added to the kinetic graph, while for catalytic reactions in corresponding kinetic graphs no null vertex appears because the role of the null vertex is that of a free active site on the catalyst s surface. 

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