Hydrocarbons chemisorption states

Hydrocarbon Fragments - Modeling by Molecular Orbital and Cluster Chemistry. A basic guideline for metal surface coordination chemistry with respect to hydrocarbon or hydrocarbon derivatives may be formulated as follows If the stereochemistry of the chemisorption state allows C-H hydrogen atoms to closely approach surface metal atoms then the chemisorption state should be further stabilized by the formation of a three-center two-electron C-H-metal bond. This effect should be more pronounced the more electron deficient the metal surface. There should be an activation of the C-H bond and the hydrogen atom should become more protonic in character. If the C-H bond is sufficiently weakened by this interaction then C-H bond cleavage should result. 

If the above generalization is correct, then simple extended Hiickel molecular orbital calculations for the surface chemisorption states of simple hydrocarbons or hydrocarbon fragments should sense this tendency for multicenter C-H-metal bonds to form (these... 

Cluster modeling of possible chemisorption states and of possible intermediate states in surface reactions can to a first approximation be useful in guiding experiments or interpretations of experimental data for surface reactions (23-25). One important and enlightening result (6, 26, 27) in metal carbide cluster chemistry will be used here to illustrate this particular point because it bears directly on the importance of multicenter C-H-M bonding for hydrocarbon fragments in metal chemistry. 

A key aspect of metal oxides is that they possess multiple functional properties acid-base, electron transfer and transport, chemisorption by a and 7i-bonding of hydrocarbons, O-insertion and H-abstraction, etc. This multi-functionality allows them to catalyze complex selective multistep transformations of hydrocarbons, as well as other catalytic reactions (NO,c conversion, for example). The control of the catalyst multi-functionality requires the ability to control not only the nanostructure, e.g. the nano-scale environment around the active site, " but also the nano-architecture, e.g. the 3D spatial organization of nano-entities. The active site is not the only relevant aspect for catalysis. The local area around the active site orients or assists the coordination of the reactants, and may induce sterical constrains on the transition state, and influences short-range transport (nano-scale level). Therefore, it plays a critical role in determining the reactivity and selectivity in multiple pathways of transformation. In addition, there are indications pointing out that the dynamics of adsorbed species, e.g. their mobility during the catalytic processes which is also an important factor determining the catalytic performances in complex surface reaction, " is influenced by the nanoarchitecture. 

This work constitutes a significant achievement if only because it illustrates how the nature of hydrocarbon groupings can be deduced de novo, for states of chemisorption much more nearly like those involved in practical heterogeneous catalysis. 

The van der Waals interaction between cavity and hydrocarbon is expected not to change significantly between the ground and the transition state, because the size of the hydrocarbon part does not change. A comparison of cluster-computed and periodical DFT-computed chemisorption of propylene is shown in Table IV. 

The oxidation state, i.e. the electron density of Pd particles is of significant interest. Recently de Vries et al. have investigated different lanthanum doped automotive catalysts and detected several different Pd species on the catalysts [17-21]. The hi er electron density of the Pd particles has an influence on the chemisorption of hydrocarbon species. 

The concept of chemisorption is a key to the understanding of catalytic reactions. Catalytic events consist of elementary reactions on the catalyst surface in which chemical bonds are formed between surface atoms and an adsorbing molecule. These interactions cause rupture of chemical bonds within the adsorbing molecule and formation of new bonds between the fragments. We will discuss explanations of the selective behavior of metals mainly with respect to three important types of reactions the conversion of synthesis gas, hydrocarbon conversion and selective (metal-catalyzed) oxidation. When particularly relevant, reference to other reactions will be made. We wish to relate proposed reaction intermediates and their chemical change to the electronic properties of the surface site where the surface reaction occurs. One then is interested in the strength of adsorbate-metal chemical bonds before and after chemical change of the reaction intermediate. These values affect the thermodynamics of the elementary steps and hence enable an estimate of the equilibria that exist between different surface species. It is the primary information a chemist requires to rationalize chemical reaction rates. In order to estimate rates, one needs information on transition states. Advanced quantum-chemical calculations can provide such information. 

The particular reactivity of bare Si02 for the production of HCHO is a matter of debate and has not yet been completely rationalized. Parmaliana et al. [113] pointed out that the performance of the silica surface in CH4 partial oxidation is controlled by the preparation method. For several commercial Si02 samples, the following reactivity trend has been established, based on the preparation method precipitation > sol-gel > pyrolysis. The activity of such silicas has been correlated with the density of surface sites stabilized under steady-state conditions acting as O2 activation centers [114], and the reaction rate was the same for all the silicas when expressed as TOF (turnover frequency). Klier and coworkers [115] reported the activity data for the partial oxidation of CH4 by O2 to form HCHO and C2 hydrocarbons over fumed Cabosil and silica gel at temperatures ranging from 903 to 1953 K under ambient pressure. They observed that short residence times enhanced HCHO (and C2 hydrocarbon) selectivity, suggesting that HCHO did not originate from methyl radicals, but rather from methoxy complexes formed upon direct chemisorption. 

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