Free radicals, catalytic etching

Catalytic etching, 41 359, 383-384 definition, 41 360-361 in low earth orbit, 41 414—415 models, 41 359, 360-362 plasma etching, 41 407-414 thermally generated free radicals, 41 406-407... 

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. 

In recent years a new model of catalytic etching has emerged. This model attributes etching to the interactions between homogeneously generated free radicals and metal surfaces. As discussed below, this model is very similar to models devised to explain etching encountered in other (non-reaction gas) environments. 

In sum, the mechanism of plasma etching is often similar to the new postulated mechanism for catalytic etching. In virtually all cases free radicals play a key role in creating volatile species that remove mass from the surface. Plasma-generated free radicals interact with the surface to form volatile intermediates which then desorb, leading to a net loss of mass from the solid Sometimes radicals form volatile intermediates with surface atoms, but the... 

In the final chapter, Wei and Phillips tie together old and new results characterizing the processes of surface etching. They summarize evidence that chemical etching takes place by reactions of gas-phase free radicals. This subject pertains to catalyst redispersion and regeneration, and the chapter links the catalysis literature and literature less often consulted by catalytic scientists and engineers. 

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