Hydrocarbons on metals

The thennalization stage of this dissociation reaction is not amenable to modelling at the molecular dynamics level becanse of the long timescales required. For some systems, snch as O2 /Pt(l 11), a kinetic treatment is very snccessfiil [77]. However, in others, thennalization is not complete, and the internal energy of the molecnle can still enliance reaction, as observed for N2 /Fe(l 11) [78, 79] and in tlie dissociation of some small hydrocarbons on metal snrfaces [M]- A detailed explanation of these systems is presently not available. 

The effect of electronegative additives on the adsorption of ethylene on transition metal surfaces is similar to the effect of S or C adatoms on the adsorption of other unsaturated hydrocarbons.6 For example the addition of C or S atoms on Mo(100) inhibits the complete decomposition (dehydrogenation) of butadiene and butene, which are almost completely decomposed on the clean surface.108 Steric hindrance plays the main role in certain cases, i.e the addition of the electronegative adatoms results in blocking of the sites available for hydrocarbon adsorption. The same effect has been observed for saturated hydrocarbons.108,109 Overall, however, and at least for low coverages where geometric hindrance plays a limited role, electronegative promoters stabilize the adsorption of ethylene and other unsaturated and saturated hydrocarbons on metal surfaces. 

Hydrogenolysis reactions of hydrocarbons on metal catalysts have been investigated in some detail. Extensive studies have been conducted on both alkanes and cycloalkanes. While a number of questions still remain with regard to mechanistic and kinetic details of the reactions, the general features seem reasonably clear. 

The articles by J. R. Anderson, J. H. Sinfelt, and R. B. Moyes and P. B. Wells, on the other hand, deal with a classical field, namely hydrocarbons on metals. The pattern of modem wTork here still very much reflects the important role in the academic studies of deuterium exchange reactions and the mechanisms advanced by pioneers like Horiuti and Polanyi, the Farkas brothers, Rideal, Tw igg, H. S. Taylor, and Turkevich. Using this method, Anderson takes ultrathin metal films with their separated crystallites as idealized models for supported metal catalysts. Sinfelt is concerned with hydrogcnolysis on supported metals and relates the activity to the percentage d character of the metallic bond. Moyes and Wells deal with the modes of chemisorption of benzene, drawing on the results of physical techniques and the ideas of the organometallic chemists in their discussions. 

Cu in solid solution, 31 259-260 Scanning tunneling microscopy chemisorbed hydrocarbons on metals, 41 30 heteropoly compounds, 41 137 molecular precursors for tailored metal catalysts, 38 303-305... 

The modification of theoretical gas-phase reaction techniques to study gas-surface reactions continues to hold promise. In particular, the LEPS formalism appears to capture a sufficient amount of realistic bonding characteristics that it will continue to be used to model gas-surface reactions. One computational drawback of the LEPS-style potentials is the need to diagonalize a matrix at each timestep in the numerical integration of the classical equations of motion. The size of the matrix increases dramatically as the number of atoms increases. Many reactions of more direct practical interest, such as the decomposition of hydrocarbons on metal surfaces, are still too complicated to be realistically modeled at the present time. This situation will certainly change in the near future as advances in both dynamics techniques and potential energy surfaces continue. 

In typical stereochemical experiments, the reactivity of two or more compounds of the same structure but of different configuration is compared either in separate or competitive experiments. The method has been reviewed several times for heterogeneous catalytic reactions, mostly with respect to reactions of hydrocarbons on metals (16-18). The results concerning eliminations on acidic catalysts have been summarized in an article dealing with the mechanism of this type of reactions (12). Clarke and Rooney (17) have broadened the notion of the stereochemical approach to heterogeneous catalysis when they included into it all work in which mechanistic conclu-... 

Metal catalysed skeletal reactions of hydrocarbons on metal catalysts. Dispersed metal catalysts. 

Complexes Operating upon Reforming of Hydrocarbons on Metals ... 

Mechanisms of Skeletal Isomerization of Hydrocarbons on Metals F. G. Gault... 

The adsorption of saturated hydrocarbons on metallic substrates is typically considered as an example of a weak physical interaction, which is dominated by van der Waals forces. The classification of this type of interaction, denoted physisorption where no direct chemical bonds are formed between the adsorbate and substrate, has been based on the heat of adsorption. A physisorbed state is considered to be one in which the heat of adsorption is comparable to the heat of vaporization or... 

R. L. Burwell, Jr., "Deuterium as a Tracer in Reactions of Hydrocarbons on Metallic Catalysts, Accounts of Chemical Research 2, 289 (1969). 

Groups probably still active in research on hydrocarbons on metals. (acac) = acetylacetate. VEELS. 

---