Comer atoms hydrogenation

We have used our Single Turnover (STO) reaction sequence to characterize dispersed metal catalysts with respect to the numbers of alkene saturation sites, double bond isomerization sites, and hydrogenation inactive sites they have present on their surfaces (ref. 13). Comparison of the product composition observed when a series of STO characterized Pt catalysts were used for cyclohexane dehydrogenation with those observed using a number of instrumentally characterized Pt single crystal catalysts has shown that the STO saturation sites are comer atoms of one type or another on the metal surface (ref. 10). 

It appears that supported metal catalysts can be used to promote synthetically useful organometallic reactions. The utilization of such reactions can be of practical, economic, and environmental importance to the fine chemical industry. Frontier Molecular Orbital and mechanistic considerations indicate that these reactions, along with hydrogenations and, presumably, oxygenations, take place on the coordinately unsaturated comer atoms present on the surface of these dispersed metal catalysts. 

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... 

While the FMO treatment depicted in Fig. 4.12 fully describes the surface interactions taking place when a double bond is hydrogenated, this approach is more complex than is needed for most purposes. The reaction sequences shown in Schemes 3.2, 3.4 and 3.5, which are based on an octahedral species however, are incorrect. Scheme 4.1 shows the reaction sequence for alkene hydrogenation over a MH site that is analogous to Scheme 3.2 but uses a more descriptive surface site model. Instead of the depiction of the MH site as a comer atom as... 

As the extent of saturation decreased, the amount of isomerization increased, showing that adsorption of the solvent molecules on these comer atoms and adatoms results in a decrease in the surface coordination. This effectively converts those saturation sites that are capable of adsorbing two hydrogen atoms and an alkene into the less unsaturated isomerization sites that can only hold one hydrogen and the alkene molecule as shown in Fig. 5.2 for ethanol adsorption on an adatom. This latter species is analogous to the isomerization sites described in Scheme 4.4. 

It is sometimes possible to estimate the extent of hindrance by an examination of a molecular model of the substrate. While the sites that promote hydrogenation reactions are the comer atoms or adatoms on the catalyst siuface, models of such surface species are not commonly available. The classic procedure for determining the relative steric hindrance to adsorption has been to place a model of the substrate on a flat surface with the It cloud of the alkene perpendicular to the surface as depicted in Fig. 14.3. In this case adsorption from side B is clearly favored. A similar conclusion can be drawn on examining the adsorption of the n cloud on a surface comer atom as depicted in Fig. 14.4. Here adsorption from the B direction is favored because of the interference of the 3,5 diaxial hydrogens to adsorption from side A, but the difference between the two modes of adsorption does not appear to be as great as that assumed from consideration of Fig. 14.3. 

On the other hand, 1,4-addition must involve a diadsorbed sp)ecies such as 64. The surface atoms can be MH, MH, or 3mH2, but only one can be the MH2 type. The transfer of one hydrogen atom from each surface site gives the adsorbed olefin with its stereochemistry determined by the mode of adsorption of the diene. The s-cis form of butadiene (65) is about 2.3 kcal/mol less stable than the s-trans form (66). This corresponds to about a 30 70 ratio, which agrees quite well with the observed cis/trans-2-butene ratios formed over Type B catalysts. The likelihood of 1,4-addition is increased when larger catalyst particles are present because more neighboring edge and comer atoms are also present. 

A simplified structure called the skeletal formula shows the carbon skeleton in which carbon atoms are represented as the end of each line or as comers. The hydrogen atoms are not shown, but each carbon is understood to have bonds to four atoms. For example, in the skeletal formula of hexane, each line in the zigzag drawing represents a single bond. The carbon atoms on the ends are bonded to three hydrogen atoms. However the carbon atoms in the middle of the carbon chain are each bonded to two carbons and two hydrogen atoms (see Figure 11.2). 

In each of the following structures, each comer and endpoint represents a carbon atom. Hydrogen atoms are only drawn if they are connected to heteroatoms (such as... 

Figure 8.16 shows the B l spectmm of the B5FI9 molecule. The boron atoms are situated at the comers of a square pyramid. There are four B-FI-B bridging hydrogen atoms and... 

Beta radiation Electron emission from unstable nuclei, 26,30,528 Binary molecular compound, 41-42,190 Binding energy Energy equivalent of the mass defect measure of nuclear stability, 522,523 Bismuth (m) sulfide, 540 Blassie, Michael, 629 Blind staggers, 574 Blister copper, 539 Blood alcohol concentrations, 43t Body-centered cubic cell (BCC) A cubic unit cell with an atom at each comer and one at the center, 246 Bohrmodd Model of the hydrogen atom... 

In the face-centred cubic structure there are four atoms per unit cell, 8x1/8 cube comers 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 comers 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, nitrides and oxides with the octahedrally coordinated interstitial atoms, and dihydrides with the tetrahedral coordination of the hydrogen atoms. 

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

It is well established that commercially important supported noble metal catalysts contain small metal crystallites that are typically smaller than a few nanometers. The surface of these crystallites is populated by different types of metal atoms depending on their locations on the surface, such as comers, edges, or terraces. In structure sensitive reactions, different types of surface metal atoms possess quite different properties. For example, in the synthesis of ammonia from nitrogen and hydrogen, different surface crystallographic planes of Fe metal exhibit very different activities. Thus, one of the most challenging aspects in metal catalysis is to prepare samples containing metal particles of uniform shape and size. If the active phase is multicomponent, then it is also desirable to prepare particles of uniform composition. 

The four hydrogen atoms occupy equivalent positions with respect to the central carbon atom in methane. The C-H bond length in methane is 1.107 A (1 A [angstrom] = 1 x 10 ° meters), a very short distance indeed. The four atoms of hydrogen are arranged about the central carbon atom at the comers of a regular tetrahedron. 

The structure of the methane molecule is that of a regular tetrahedron, with the carbon atom in the center and the four hydrogen atoms at the four comers. 

The composition of white phosphorus is P4. Each of the phosphorus atoms lies at the comer of a regular tetrahedron, just as the four atoms of hydrogen in methane do. The phosphoms atoms feel strain reacting to form more stable stmctures relieves that strain and releases energy. People behave in basically the same way finding ways to relieve strain. 

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