Ledges and Corners

Joints between hopper and extruder and also in a sliding or swivelling cut-off can result in ledges and corners where pellets may collect and fall out at some future date (Figure 5.7). This is possibly less likely with round hoppers. 

The important feature is the formation of a coordinatively unsaturated site (cus), permitting the reaction to occur in the coordinative sphere of the metal cation. The cus is a metal cationic site that is able to present at least three vacancies permitting, in the DeNOx process, to insert ligands such as NO, CO, H20, and any olefin or CxHyOz species that is able to behave like ligands in its coordinative environment. A cus can be located on kinks, ledges or corners of crystals [16] in such a location, they are unsaturated. This situation is quite comparable to an exchanged cation in a zeolite, as studied by Iizuka and Lundsford [17] or to a transition metal complex in solution, as studied by Hendriksen et al. [18] for NO reduction in the presence of CO. 

Metals - Although many catalytic processes occur on metals, it is not often known unambiguously whether these involve extended metal surfaces (which can be modelled as infinite two dimensional planes), defective metal surfaces (with ther catalysis occurring mainly at ledges or corners) or smaller clusters of metal atoms. Furthermore in the case of clusters it is not clear how many atoms must be included to model a solid adequately, and indeed it appears to depend on which property one wishes to calculate. There are a... 

Lattice relaxation, though, affects these A static lattice simulation of the surface shows that relaxation at the corners and at ledges serves where possible to contract the ions at these sites so that they lie closer to their neighbours , Kinks, on the other hand, relax... 

Also, ledges, corners, and crevices, e.g., should be avoided as far as possible to prevent dust layer formation. 

Definition of atomic crystallographic sites of significance in anodic dissolution. Schematic on a square 2D lattice kinks at steps (edges or ledges), corners, and terraces positions. 

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