Corner sites

The process requires the interchange of atoms on the atomic lattice from a state where all sites of one type, e.g. the face centres, are occupied by one species, and the cube corner sites by the other species in a face-centred lattice. Since atomic re-aiTangement cannot occur by dhect place-exchange, vacant sites must play a role in the re-distribution, and die rate of the process is controlled by the self-diffusion coefficients. Experimental measurements of the... 

In lead zh conate, PbZrOs, the larger lead ions are displaced alternately from the cube corner sites to produce an antifeiToelectric. This can readily be converted to a feiToelectric by dre substitution of Ti" + ions for some of the Zr + ions, the maximum value of permittivity occumirg at about the 50 50 mixture of PbZrOs and PbTiOs. The resulting PZT ceramics are used in a number of capacitance and electro-optic applicahons. The major problem in dre preparation of these solid soluhons is the volatility of PbO. This is overcome by... 

Borasiloxane 92 with boron and silicon atoms at alternate corners and the general formula B4Si40io for the central cage is composed of two dib-oratetrasiloxane rings, that are joined by two Si-O-Si bridges [129]. Unlike the related and extensively studied silasesquioxanes 93 (Fig. 25), in which the silicon atoms can be substituted by various combinations of group 13 and 15 elements [137], this molecule contains alternately three-coordinate boron and four-coordinate silicon corner sites. 

Perimeter interfaces around gold NPs as reaction sites for CO and O2 [12,39] AI2O3, Si02, Ti02, MnOz, FejOj, C03O4, NiO, ZnO, ZrOj, CeOj. Edge and corner sites of gold NPs [40]. 

A hypothesis that edge and corner sites work as active sites can explain why turn over frequency (TOF), which is defined as the reaction rate per one active site, in the case of metal catalysts, per surface exposed metal atom, increases with a decrease in the diameter of gold particles. However, it fails to explain the significant contribution of support materials and the contact structure of gold NPs. It seems to be reasonable that those edges and corners act as the sites for adsorption of one of the reactants, for example, CO in its oxidation. 

Consequently, we favor a method that shows a view of organic molecules on a surface drawn as close as possible to their relative sizes. Two such surfaces, a face centered cubic (fee) (111) and an fee (100), are shown in Fig. 1.9, in which M, 2M, and 3M represent plane, edge, and corner sites, respectively, according to the nomenclature invented by Siegel and associates.21 These are... 

In view of the mean particle size of Ir-8 we have good reason to expect that with this catalyst the influence of corner sites becomes noticeable. Obviously, the specific rate of deuteration on these corner sites must be lower than that on the edge sites in addition, the cyclohexane formed on them must contain a larger proportion of the more highly deuterated species than the deuteration product coming from either the edge sites or the sites at the faces. 

Topspe proposed that corner sites are responsible for direct sulfur extraction (A Do) (53-60), but the exact nature of corner sites is not known. What is known is that the active sites for sulfur removal constitute only about 10% of all of the Co(Ni)-Mo-S sites as identified by Mossbauer emission spectroscopy (MES) (57). Thus, there is something special about some of the Co-Mo-S sites. Further study in this area is greatly needed to clarify this issue, and it is recommended that, in the future, authors use terminology in a uniform manner. Some suggestions for standardization are made in later discussions. 

To estimate how much improvement may be possible, we can take as a first estimate the conclusions of Burch and Collins 68) and/or the modified data shown in Fig. 20—only one active site per MoS2 crystallite. The worst scenario would be that today s catalysts now have the maximum possible activity per site. In such a case, the only improvement possible is to increase the number of MoS2 crystallites on the catalyst surface or to increase the number of the special corner sites noted by Topspe. In the Burch and Collins model, 33 Mo atoms per crystallite were assumed. However, improved preparation procedures have reduced crystal sizes in experimental catalysts to less than 10 A or only about seven Mo atoms per crystallite. The maximum improvement would be to reduce the number of Mo atoms in an active crystallite to two (assuming the SBMS structures proposed by Startsev [2]). Thus, an improvement by a factor of only about 3 might be expected, relative to the most active reported catalysts. 

Tsang and Falicov (49) have calculated the charge density distribution at corner sites in ionic and rare gas crystal surfaces. For ionic solids, low coordination number surface sites should have large charge density variations... 

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