Nickel catalytic etching

Historically, catalytic etching has been a poorly understood phenomenon, and it has not always been clearly distinguished from thermal etching. Moreover, the largest number of studies have focused on platinum (3, 27, 34, 124-130) although the catalytic etching of silver (122, 131, 132), copper (16, 28, 133), nickel (134), and some platinum alloys (51, 121, 135-138) has also been investigated. 

Chemical reduction is used extensively nowadays for the deposition of nickel or copper as the first stage in the electroplating of plastics. The most widely used plastic as a basis for electroplating is acrylonitrile-butadiene-styrene co-polymer (ABS). Immersion of the plastic in a chromic acid-sulphuric acid mixture causes the butadiene particles to be attacked and oxidised, whilst making the material hydrophilic at the same time. The activation process which follows is necessary to enable the subsequent electroless nickel or copper to be deposited, since this will only take place in the presence of certain catalytic metals (especially silver and palladium), which are adsorbed on to the surface of the plastic. The adsorbed metallic film is produced by a prior immersion in a stannous chloride solution, which reduces the palladium or silver ions to the metallic state. The solutions mostly employed are acid palladium chloride or ammoniacal silver nitrate. The etched plastic can also be immersed first in acidified palladium chloride and then in an alkylamine borane, which likewise form metallic palladium catalytic nuclei. Colloidal copper catalysts are of some interest, as they are cheaper and are also claimed to promote better coverage of electroless copper. 

These results led the workers to suggest that catalysis actually leads to the removal of surface nickel atoms, primarily due to local heating which takes place at the reaction site. Furthermore, during the catalytic process, the nickel atom is temporarily part of a liquid- or gas-phase intermediate. Once the catalytic process is complete, the authors postulated that the free nickel atom readsorbed onto the bulk nickel, adsorbed onto the inert support, remained as nickel sol in the liquid, or continued to act as a catalyst. It was claimed that this model explained several observations, such as the differences between unsupported and supported nickel. The supported metal has a greater surface area upon which the metal can readsorb, so it tends to leave fewer atoms in the product liquid. The model also explains the observation that the reaction vessel became coated with a thin film of nickel after lengthy use. This postulated etching mechanism is similar to the recent model discussed above, whereby etching results from free-radical-surface interactions. 

Complex surface processes during HC1 etching (acidic dissolution of oxides, electrochemical oxidation) lead to the formation of a porous, chloride-containing iron oxide layer while nickel remains in the zero-valent state. Subsequent reduction, facilitated also by hydrogen atoms formed on nickel sites, results in an increased number of surface iron and nickel atoms and an enhanced catalytic activity. The larger concentration of atomic hydrogen on the surface and the presence of surface Ni are observations that are supported by the decreased selectivity of olefin formation. 

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