Alkane Chemisorption

In the calculations of methane interaction with nickel or titanium surfaces, five different modes of coordination have been considered (Figure III.2) [6c], It has been concluded that acts of methane activation by a separate molecule of a metal complex on the one hand and the metal surface on the another hand are very similar. In both processes the C-H bonds of methane are ruptured and new M-H bonds are formed. Intermediate species formed during alkane chemisorption on oxide catalysts have been investigated both by MO calculations and spectroscopic methods [7]. 

Many molecules undergo partial oxidation on adsorption and many alkanes and alkenes are believed to yield an adsorbed CHO group on adsorption (Petrii, 1968). These processes usually lead to the complete oxidation of the organic molecule to carbon dioxide and few workers have attempted to halt the reaction at an intermediate stage. Hence, although there are undoubtedly possibilities for using dissociative chemisorption for synthetic reactions, this chapter will not consider these processes further. 

In summary a few "generalizations" have been found. First, size selective chemistry is strongly associated with chemisorption that requi res bond-breaking. Second, metal clusters react rapidly with ligands that molecularly chemisorb even when the eventual products involve dissociation of the ligand. Dehydrogenation of Cg-alkanes on small platinum clusters take exception to this. 

It is proposed that hydrated or dehydrated titaniumperoxo compounds are formed in TS-1 by H2O2 chemisorption on the titanyl (Ti 0) group, and that these complexes constitute the actual oxidants [96]. In the particular case of alkane oxidation, a homolytic reaction mechanism is proposed, as is tentatively represented in scheme 6 [114]. 

Room-temperature adsorption on finely divided metal catalysts has been shown in several cases (Ni/Si02 and Pt/Si02) to give rise to dissociative adsorption as alkylidynes and other products. It is therefore very clear that, contrary to often-expressed views, C-H bonds within the alkanes can readily be broken by interaction with metal catalyst surfaces. Methane is a very important feedstock and, although this may be the most resistant to chemisorption, there is clearly much further of interest to be discovered in this area involving interactions of light alkanes with different metals. 

During the past decade a very considerable literature has developed concerning the generation and reactivity of alkyl and alkylidene groups adsorbed on metal single-crystal surfaces produced via the photochemical or thermal decomposition of adsorbed alkyl halides or nitrogen-substituted alkanes. In this review, we concentrate on publications which exhibit VEEL or RAIR spectra of the hydrocarbon groupings that can be used as reference spectra for the identification of such species in spectra of species derived from hydrocarbon chemisorption. Reviews of such work cover the kinetic as well as spectroscopic aspects of this area of research (142-144). 

Nanocrystalline MgO and CaO with high surface area are able to absorb large amounts of chlorine, which undergo dissociative chemisorption. These can serve as rather selective, catalytic alkane chlorination reagents, which suggests that trapped Cl atoms are involved in the reaction.295 Liquid-phase low-temperature chlorination of alkanes is also possible in the presence of various alkenes as inductors and AIBN [azobis(isobutyronitrile)] 296... 

In Figure 5 the generally accepted reaction path (14) for hydroisomerization of n-alkanes has been represented along with different possibilities for the cracking step. The n-alkane molecules are adsorbed at a dehydrogenation/hydrogenation site where n-alkenes are formed. After desorption and diffusion to an acidic site chemisorption yields secondary carbenium ions that rearrange... 

Second, we shall present our approach as to how to probe the palladium surfaces in more complex Pd/support catalysts, especially when the so-called metal-support interactions are expected. We shall develop our idea of how to use such chemical probes as a catalytic reaction (alkane catalytic conversion) or chemisorption in order to see important changes in the catalytic behavior. When possible, an adequate reference to available data from more sophisticated physical techniques is made. 

Several important studies have been conducted by various research groups regarding the chemisorption of various saturated hydrocarbons on the Ir(l 1 0) and Pt(l 11) surfaces, providing much insight into the dynamics of alkane activation on transitional metal surfaces. 

The dissociative adsorption of alkanes on Ir(l 1 0) surface was investigated in a series of studies performed by the Madix group [13, 32, 33]. A study performed by Hamza et al. [32] probed the dynamics of the dissociative chemisorption of methane, ethane, propane, and n-butane on the Ir(l 1 0)-(l x 2) surface. These investigations were complemented by a later study of propane dissociation on the Ir(l 1 0)-(l x 2) surface by Soulen and Madix [13]. Shown in Figs 6 and 7 are plots of S0 vs. E obtained for propane at various surface temperatures [32] and a plot of experimental and theoretical values of S0 for propane (at an incident translational energy of 50kcal/mol) on Ir(l 1 0)-(l x 2) as a function of surface temperature [13]. 

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