Catalytic etching models

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... 

The above model of catalytic etching is not universally accepted. Several older mechanistic models exist. For example, it was suggested that localized temperature gradients, induced by surface reactions, might lead to uneven rates of diffusion or volatilization, and hence catalytic etching. 

In sum, according to one postulated mechanism of catalytic etching, particularly of platinum, etching occurs via the transport of volatile metal oxide species. It has always been understood, even by the model s proponents, that it does not fully explain the observations. It has been repeatedly suggested that other volatile species may exist, or that other unaccounted-for processes are responsible for the observed behavior. Indeed, as discussed below, several recent studies suggest that other volatile species and other unaccounted-for processes may very well explain some or all catalytic etching. 

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. 

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. 

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