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Hydrogenation crystal-face specificity

Interesting information exists about the crystal-face specificity/reactivity [75]. In ethene hydrogenation the (001) face of Ni is inactive, because this face, under the standard reaction conditions applied, is completely covered by carbonaceous deposits. The (111) and (Oil) faces are both active and differ by less than a factor of 2 in their activity. The data just mentioned are just another example of a frequently encountered phenomenon the crystal face specificity or the particle size sensitivity of a reaction is induced by a side reaction and is not caused by the reaction in question [76]. [Pg.179]

Reactions which may occur on sites consisting of one or two atoms only on the surface of the catalyst are generally known as facile reactions. Reactions involving hydrogenation on metals are an example. Eor such reactions, the state of dispersion or preparation methods do not greatly affect the specific activity of a catalyst. In contrast, reactions in which some crystal faces are much more active than others are called structure sensitive. An example is ammonia synthesis (discovered by Fritz Haber in 1909 (Moeller 1952)) over Fe catalysts where (111) Fe surface is found to be more active than others (Boudart 1981). Structure-sensitive reactions thus require sites with special crystal structure features, which... [Pg.152]

The reason for the difference in the effectiveness between each of the crystal face to catalyze the ammoxidation of alkyl aromatics selectively is a result of the specific electronic character of the oxygen atoms associated with the vanadium atoms of the V2 O5 structure. As was learned about the role of lattice oxygen (0 ) in the selective ammoxidation of propylene to acrylonitrile (see above), hydrogen abstraction and oxygen insertion require oxygen atoms with nucleophilic character (79). On the other hand, nonselective oxidation is affected by electrophilic oxygen species, O2 and O . These are the intermediate species in the dissociative chemisorption and reduction of O2 to lattice (80). [Pg.265]

The carbon backbone of the sucrose structure (Figure 3) is almost completely shielded by the o gen and hydrogen atoms and is expected to play only a minor role, if any, in solvent interactions. The relative contact areas of oj gens and hydrogens are orientation dependent and may impart a specific character to the different faces of the sucrose crystal. [Pg.62]

Above we have outlined the Importance of the structure of the crystal when we want to make a correlation between the kinetic data and the surface of all the faces. So we considered the PBC s analysis as a necessary tool to obtain the maximum of information on all sites of each crystal surface (3). The PBC s analysis specifically allows us to determine the polarity of the complementary forms. As an example we consider the complementary interface q and q (Figure 10). The two opposite interfaces show complementary behavior with respect to the hydrogen bond (HB) pointing toward the mother solution. The q interface exposes 3 HB donors and 4 acceptors whereas the opposite situation is set up on the q face. We fixed the ratio K between the number of donors and the number of acceptors over one unit cell. Hence for q face K ... [Pg.79]

Samples of a-TiCls, prepared by reduction of TiCIa with hydrogen, contain a low number of propagation centers. The Cp value of these well crystallized samples (the specific surface area according to BET is 3 m /g) is several per cent of the number of surface titanium ions. The low number of ACs is in agreement with the Cossee and Arlman concept and the experimental data of Rodrigues et al. on the localization of the ACg on the lateral faces and outlets of spiral dislocations on TiCls crystals. [Pg.69]

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... [Pg.45]

A four-component capsule with a structure that conforms to a tetrahedron has been recently described by Venkataraman66 Specifically, triphenylamine ortho-tricarboxylic acid self-assembled via 12 O H-O hydrogen bonds to form a molecular tetrahedron (Fig. 36). The hydroxyl groups of the polyhedron participated in hydrogen bonds with single ethanol molecules embedded within each triangular face of each tetrahedron. The assembly crystallized in the rare cubic crystal system. The ability of the acid to form a tetrahedron was reminiscent of the ability of triphenyl-methanol to form a tetrahedron, which also forms inclusion compounds with solvent guests that occupy voids between the polyhedra.32... [Pg.45]

Use of a less bulky guest such as ethyl acetate induces a dimeric lattice pattern 65, which exclusively emits excimer fluorescence from the face-to-face dimeric anthracene units [53]. Segregated anthracene (A) and anthraquinone (Q) columns (66) are found in the charge-transfer molecular crystals of adduct 29 2 (anthraquinone) [66]. This is also the case for the 1 1 complex of the diresorcinol derivative of anthracene (29) and the dipyrimidine derivative of anthraquinone (67) as a specific hydrogen-bond donor and an acceptor, respectively [66]. [Pg.148]


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See also in sourсe #XX -- [ Pg.179 ]




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