Hydrogen corner sites

As discussed previously, this reaction was also run under these same conditions over the series of specifically cleaved platinum single erystals shown in Fig. 3.2. 3 The results of these experiments show that it was the corner atoms on these crystals that promoted C-H bond breaking. Thus, the saturation sites on the dispersed metal catalysts are also comer atoms. Since this saturation site description agrees with that proposed on the basis of the butene deuteration described previously,5 -62 it is likely that the isomerization sites, M, are edge atoms and the hydrogenation inactive sites, M, are face atoms. A similar approach can be used to determine the nature of the active sites responsible for promoting almost any type of reaction. 5.70... 

Upon increasing tin content the number of accessible Pt sites decreases The H/Pt ratio decreases much faster than the CO/Pt ration. This is an indication that tin strongly blocks kink and corner sites involved in hydrogen activation ... 

Dehydrogenation is considered to occur on the corners, edges, and other crystal defect sites on the catalyst where surface vacancies aid in the formation of intermediate species capable of competing for hydrogen with ethylbenzene. The role of the potassium may be viewed as a carrier for the strongly basic hydroxide ion, which is thought to help convert highly aromatic by-products to carbon dioxide. 

In the face-centred cubic structure tirere are four atoms per unit cell, 8x1/8 cube corners and 6x1/2 face centres. There are also four octahedral holes, one body centre and 12 x 1 /4 on each cube edge. When all of the holes are filled the overall composition is thus 1 1, metal to interstitial. In the same metal structure there are eight cube corners where tetrahedral sites occur at the 1/4, 1/4, 1/4 positions. When these are all filled there is a 1 2 metal to interstititial ratio. The transition metals can therefore form monocarbides, niU ides and oxides with the octahedrally coordinated interstitial atoms, and dihydrides with the tetrahedral coordination of the hydrogen atoms. 

Addition of very small amounts of Bu4Sn can completely transform the performance of these catalysts by poisoning the hydrogenation sites. For example, when a Ni°/Si02 catalyst is used, the best result corresponds to a 2-carene yield of 30%, with at least 30% of the carenes transformed into by-products. Addition of 0.04 mole of Bu4Sn/Nis results in an increase of the yield of 2-carene, up to 37%, and a decrease of the amount of by-products to less than 10%. In this case, tin is present as adatoms on the most hydrogenating sites (very hkely those situated on the faces rather than on corners and edges). 

Preferred adsorption of the unsaturated bond of the substrate occurs at that face which presents the least steric interactions between the adsorbed substrate and the surface. Since some amazingly sterically hindered molecules can be hydrogenated, at least some active sites must look like corners or edges or some other protuberances. 

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