Hydrocarbons on Metal Surfaces

In this section we deal with hydrocarbons and their adsorption and reaction on metals. The first papers illustrate typical research into bonding of species to surfaces and the units or fragments formed by hydrocarbons upon adsorption. After a brief mention of oxidation and dehydrogenation the remainder of the section is devoted to the appearance of carbonaceous species on the surfaces of catalysts and their role, if any, in catalysis. 

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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. 

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. 

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]). 

Several growth models are proposed for the carbon nanotubes prepared by the pyrolysis of hydrocarbons on metal surfaces. Baker and Harris [100] suggested a four-step mechanism. In the first step, the hydrocarbon decomposes on the metal surface to release hydrogen and carbon, which dissolves in the particle. The second step involves the diffusion of the carbon through the metal particle and its precipitation on the rear face to form the body of the filament. The supply of carbon onto the front face is faster than the diffusion through the bulk, causing an accumulation of carbon on the front face, which must be removed to prevent the physical... 

The thermalization stage of this dissociation reaction is not amenable to modelling at the molecular d5mamics level because of the long timescales required. For some systems, such as O2 /Pt(l 11), a kinetic treatment is very successful [77]. However, in others, thermalization is not complete, and the internal energy of the molecule can still enhance reaction, as observed for N2 /Fe(l 11) [78, 79] and in the dissociation of some small hydrocarbons on metal surfaces [80]- A detailed explanation of these systems is presently not available. 

Hydrocarbons on metal surfaces provide greater challenges in spectral interpretation and we choose the example of ethene chemisorbed on different metal surfaces. Here the relevant model compounds are inorganic binuclear or trinuclear metal clusters with the hydrocarbon ligand of interest and additional... 

The effect of the presence of alkali promoters on ethylene adsorption on single crystal metal surfaces has been studied in the case ofPt (111).74 77 The same effect has been also studied for C6H6 and C4H8 on K-covered Pt(l 11).78,79 As ethylene and other unsaturated hydrocarbon molecules show net n- or o-donor behavior it is expected that alkalis will inhibit their adsorption on metal surfaces. The requirement of two free neighboring Pt atoms for adsorption of ethylene in the di-o state is also expected to allow for geometric (steric) hindrance of ethylene adsorption at high alkali coverages. 

The decarbonylation of oxide-supported metal carbonyls yields gaseous products including not just CO, but also CO2, H2, and hydrocarbons [20]. The chemistry evidently involves the support surface and breaking of C - O bonds and has been thought to possibly leave C on the clusters [21]. The chemistry has been compared with that occurring in Fischer-Tropsch catalysis on metal surfaces [20] support hydroxyl groups are probably involved in the chemistry. 

Kineti cs. To date only addition reactions have been reported. These reactions produce products or adducts that are the result of complete addition, or addition and subsequent elimination. An example of the later reaction is dehydrogenation of hydrocarbons on platinum clusters. These addition reactions are in many ways analogs to the chemisorption process on metal surfaces. 

Carbonaceous species on metal surfaces can be formed as a result of interaction of metals with carbon monoxide or hydrocarbons. In the FTS, where CO and H2 are converted to various hydrocarbons, it is generally accepted that an elementary step in the reaction is the dissociation of CO to form surface carbidic carbon and oxygen.1 The latter is removed from the surface through the formation of gaseous H20 and C02 (mostly in the case of Fe catalysts). The surface carbon, if it remains in its carbidic form, is an intermediate in the FTS and can be hydrogenated to form hydrocarbons. However, the surface carbidic carbon may also be converted to other less reactive forms of carbon, which may build up over time and influence the activity of the catalyst.15... 

In addition to a large catalytically active surface and good capacity for adsorption, an efficient catalyst requires selectivity, that is, preferential affinity for the appropriate reactants. Nonselectivity is a source of significant problems in catalysis, particularly in the petrochemical industry, which has to deal with hydrocarbons of various types and isomers in a single stream. This situation has provided a strong incentive for the development of artificial (man-made) catalysts that offer the type of selectivity unthinkable on metal surfaces discussed in the last section of Chapter 9 (see, for example, Ball 1994) and illustrates another example of molecular design (or molecular engineering ) of advanced materials for use in science and industry. 

The existence of several adsorbed states of an olefin on metal surfaces is shown by infrared spectroscopic studies [68]. This technique has the advantage that it yields direct information regarding the chemical identity of the various adsorbed species, although there are limitations to its use. One of the main limitations is that the presence of surface intermediates may not be revealed if the appropriate band intensities are too weak [69]. In this context, it has been suggested [70] that the C—H bands associated with carbon atoms which are multiply bonded to the surface are too weak to be observed. Pearce and Sheppard [71] have also proposed the operation of an optical selection rule, similar to that found with bulk metals [72], in determining the bands observed with adsorbed species on supported metal catalysts. In spite of these limitations, however, the infrared approach has contributed significantly to the understanding of the nature and reactivity of adsorbed hydrocarbons. 

This finding nicely models the lability of C—H bonds generally observed with hydrocarbons chemisorbed on metal surfaces (139-141). 

Scheme I. Possible C, hydrocarbon species on metal surfaces. On metal surfaces the M atoms are usually bonded together. These ate alternative to the upper row of structures. They are most likely to occur on /-election-deficient metals to the left of the Iransition-metal periods, or on coordinatively unsaturated sites on metals to the right of these periods.

Figure 5 collects together information on bands of medium (m) or strong (s) intensity expected on metal surfaces for most of the possible types of C and C2 hydrocarbon ligands. Relationships between the latter structures are set out systematically in Scheme 2, (M = cr-bonded metal atom M = -bonding to metal). In this scheme the parent adsorbate hydrocarbons are indicated by solid rectangular outlines, and dashed rectangles encompass those surface species that can be derived from the parent without breaking the CH bonds.

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