Alkanes on metal surfaces

Luntz A C and Harris J 1992 The role of tunneling in precursor mediated dissociation Alkanes on metal surfaces J. 

Isomerization of alkanes on metal surfaces occurs via the intermediate formation of a,y-adsorbed, metallacyclobutane, carbene and it-bounded species (Scheme III.2). 

Some reference has already been made to the chemisorption of alkanes on metal surfaces (Sections 4.8 and 6.1) it is a difficult process requiring a significant activation energy, and probably two sites , although as we have seen they may not... 

In some catalytic processes, it is necessary to avoid carbon-carbon bond cleavage. For example, isobutane is mainly transformed into its lower alkane homologues (hydrogenolysis products) on metal surfaces, while it can be converted more and more selectively into isobutene when the Pt catalysts contain an increasing amount of Sn (selective dehydrogenation process) [131]. 

Metal alkylidyne fragments are frequently invoked as intermediates in the transformation of hydrocarbons on metal surfaces. These species are usually formulated as triply bridging alkylidynes however, terminal surface alkylidynes may be considered as reactive surface intermediates (30). Evidence for metal carbyne intermediates on Pt—Co bimetallic surfaces was found in a study of the isomerization and hydrogenolysis of alkanes (3]). 

Nevertheless, the metal-catalyzed isomerization reaction is of interest from the point of view of understanding the nature of hydrocarbon transformations on metal surfaces. It has been suggested that carbonium-ion-like intermediates are involved in alkane isomerization reactions on platinum (23), and a specific mechanism has been proposed by Anderson and Avery (24). 

Contact potential difference is a valuable means of demonstrating the existence and nature of adsorbed monolayers on metal surfaces. Bewig and Zisman [14] investigated horizontal adlineation of n-alkanes on a... 

Alkanes adsorb very poorly on metallic surfaces. Thus they do not modify the adsorption of CO, O2 and NO. As would be expected CO oxidation is practically insensitive to the presence of alkanes. 

Similar mechanisms are postulated for commercial alkene/arene, carbonyl and nitrile hydrogenations on metal surfaces in particular, individual metal atoms are involved. In contrast hydrogenolysis, the cleavage of C—C or C—O (N, S, etc.) bonds, appears to need two or more adjacent sites and can sometimes be reduced by alloying the main component (addition of copper to nickel, for example). The stability of supported metal (especially platinum) catalysts permits their use at high temperatures, to promote hydrogen transfers between alkanes, alkenes and arenes or dehydrogenation processes. 

Alkanes may be associatively adsorbed on metal surfaces. Figure 26 shows the ordering and structures of alkanes deposited on metal surfaces as a function of temperature [23]. As the adsorption temperature is decreased, these molecules form first a disordered, then an ordered monolayer. As the temperature is lowered still further in the presence of the organic vapor, condensation occurs leading to crystal growth of an organic single crystal. In this way, not only ad-... 

Kao CL, Madix RJ The adsorption dynamics of small alkanes on (111) surfaces of platinum group metals. Surf Sci 557(1-3) 215-230, 2004. 

Sheppard N and De La Cruz C 1998 Vibrational spectra of hydrocarbons adsorbed on metals. Part II. Adsorbed acyclic alkynes and alkanes, cyclic hydrocarbons including aromatics and surface hydrocarbon groups derived from the decomposition of alkyl halides, etc Adv. Catal. 42 181-313... 

Precious Meta.1 Ca.ta.lysts, Precious metals are deposited throughout the TWC-activated coating layer. Rhodium plays an important role ia the reduction of NO, and is combiaed with platinum and/or palladium for the oxidation of HC and CO. Only a small amount of these expensive materials is used (31) (see Platinum-GROUP metals). The metals are dispersed on the high surface area particles as precious metal solutions, and then reduced to small metal crystals by various techniques. Catalytic reactions occur on the precious metal surfaces. Whereas metal within the crystal caimot directly participate ia the catalytic process, it can play a role when surface metal oxides are influenced through strong metal to support reactions (SMSI) (32,33). Some exhaust gas reactions, for instance the oxidation of alkanes, require larger Pt crystals than other reactions, such as the oxidation of CO (34). 

The actual spacings of the metal atoms in the surface will clearly be of importance in making one face of a metal crystal catalytically effective, and another not, depending on how closely the actual atom spacings approximate to the bond distances in alkene and hydrogen molecules. In practice only a relatively sma l proportion of the total metal surface is found to be catalytically effective—the so-called active points . These adsorb alkene strongly, and then desorb immediately the resultant alkane, thus becoming free for further alkene adsorption. 

The titanosilicate version of UTD-1 has been shown to be an effective catalyst for the oxidation of alkanes, alkenes, and alcohols (77-79) by using peroxides as the oxidant. The large pores of Ti-UTD-1 readily accommodate large molecules such as 2,6-di-ferf-butylphenol (2,6-DTBP). The bulky 2,6-DTBP substrate can be converted to the corresponding quinone with activity and selectivity comparable to the mesoporous catalysts Ti-MCM-41 and Ti-HMS (80), where HMS = hexagonal mesoporous silica. Both Ti-UTD-1 and UTD-1 have also been prepared as oriented thin films via a laser ablation technique (81-85). Continuous UTD-1 membranes with the channels oriented normal to the substrate surface have been employed in a catalytic oxidation-separation process (82). At room temperature, a cyclohexene-ferf-butylhydroperoxide was passed through the membrane and epoxidation products were trapped on the down stream side. The UTD-1 membranes supported on metal frits have also been evaluated for the separation of linear paraffins and aromatics (83). In a model separation of n-hexane and toluene, enhanced permeation of the linear alkane was observed. Oriented UTD-1 films have also been evenly coated on small 3D objects such as glass and metal beads (84, 85). 

In heterogeneous metal catalysis alkanes, alkenes, and aromatics adsorbed on the metal surface rapidly exchange hydrogen and deuterium. The multiple adsorption of reactants and intermediates lowers the barriers for such exchange processes. Hydrogenation of unsaturated aliphatics and isomerisation can be accomplished under mild conditions. Catalytic dehydrogenation of alkanes to alkenes requires temperatures >200 °C, but this is because of the thermodynamics of this reaction. 

The activation of alkanes on transition metal surfaces is an important step in many catalytic reactions. Hydrogenolysis, steam reforming and isomerization of alkanes all involve alkane dissodation. Thus, much interest exists in the mechanistic and kinetic aspects of alkane dissociation. 

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