Atoms edge/corner

The activity of a catalyst often depends on the type of sites that are present on the catalytically active particles. Hence, ideally one wants to know how atoms are distributed over terraces, edges, corners, etc. - a question that has long occupied the thoughts of many investigators of catalysis. Van Hardeveld and Hartog [17] addressed this problem by analyzing the different atom environments on a number of different crystal geometries, such as cubes and truncated octahedrons, as a function of their size, and their report from 1969 has subsequently become a true citation classic. 

The third concern of those who are searching for better catalysts might be the metal crystallite size effect. As shown in Figure 5, with a decrease in the diameter of metal particles the fractions of edges, corners, and surface exposed atoms increase, markedly in the range below 2 nm. The electronic state of metal particles may also... 

When Pd is deposited on an MgO site like an ion at a terrace, a step, or a corner site the activation of C2H2 is much more efficient than for an isolated Pd atom. The structural distortion of adsorbed C2H2 follows the trend Pd-0 3c > Pd-0 4c > Pd-O sc >free Pd atom [215]. The donor capability of Pd increases as a function of the adsorption site on MgO in the order terrace < edge < corner. However, even on the low-coordinated anions the Pd atom is not an active catalyst for the cyclotrimerization. The increase of electron density on Pd due to the bonding with the MgO substrate at these irregular sites does not account for the observed reactivity. 

These are most likely edge-corner atoms. Additional poisoning destroys all activity eventually. In cases such as this, selective poisoning of undesired activity gives improved yields of desired products. 

Researchers in the area of heterogeneous catalysis have recently focussed considerable attention to the relationships among catalytic activity, product selectivity and the size and shape of metal particles for reactions catalyzed by metals (15). Reactions that are influenced by the size and shape of metal particles or electronic interactions of the metal particles with the support are known as structure sensitive reactions. Theoretical calculations of various crystallographic structures (16) have shown that the number of specific type of surface atoms (face, corner, edge) change as a function of particle size. For example, for a face centered cubic system, the number of face atoms decreases as particle size decreases. If, therefore, a reaction is catalyzed on a face and there are a substantial number of face atoms necessary for catalysis to occur, then as particle size decreases catalytic activity will decrease. This idea often runs counter to principles discussed in general science texts (17). 

On the other hand the change in selectivity for carvone hydrogenation could be explained by assuming that the atoms of Au are deposited preferentially on edges, corners, leaving free large planes, on which a rapid desorption of carvotanecetone can occur. 

Y 8 atoms x atom corner = 1 Y atom Ba 8 atoms x atom edge" = 2 Ba atoms Cu 3 Cu atoms completely inside unit cell 3 Cu atoms... 

A quick method to count the number of atoms in a unit cell is to displace the unit cell outline to remove all atoms from corners, edges and faces. The atoms remaining, which represent the unit cell contents, are all within the boundary of the unit cell and count as 1. 

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