Surface carbide

Carbides may also be prepared, either by dhect carburizing, as in the case of steel, in which a surface carbide film dissolves into the subsuate steel, or by refractoty metal carbide formation as in die cases when one of the refractory metal halides is mixed with methane in the plasma gas. 

The surface carbide is hydrogenated in subsequent steps, which leads to an appreciable coverage of the surface by CHx(ads) intermediates. Chain growth processes are assumed to occur by radical pathways such as Eqs. V and VI. 

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... 

Figure 4.1 summarizes the different routes that can potentially lead to carbon deposition during FTS (a) CO dissociation occurs on cobalt to form an adsorbed atomic carbon, which is also referred to as surface carbide, which can further react to produce the FT intermediates and products. The adsorbed atomic carbon may also form bulk carbide or a polymeric type of carbon. Carbon deposition may also result (b) from the Boudouard reaction and (c) due to further reaction and dehydrogenation of the FTS product (what is commonly called coke), a reaction that should be limited at typical FT reaction conditions. Carbon formed on the surface of cobalt can also spill over or migrate to the support. This is reported to readily occur on Co/A1203 catalysts.43 The chemical nature of the carbonaceous deposits during FTS will depend on the conditions of temperature and pressure, the age of the catalyst, the chemical nature of the feed, and the products formed. 

It would be fitting at this stage to define in detail the various carbon species for this review, as often different terms are used in the literature. A representation of the various carbon species is shown in Figure 4.2. Surface carbide or atomic carbon can be defined as isolated carbon atoms with only carbon-metal bonds, resulting from CO dissociation or disproportionation, the latter of which is not favored on cobalt at normal FTS conditions. Recent theoretical and experimental work has indicated that the CO dissociation is preferred at the step sites, so absorbed surface carbide is expected to be located near these sites.44-46... 

FIGURE 4.2 Representation of different carbon types on cobalt, (a) Atomic carbon/ surface carbide in a threefold hollow site, (b) CHX species located in threefold hollow sites, (c) Subsurface carbon lying in octahedral positions below the first layer of cobalt, (d) Cobalt carbide (Co2C) with an orthorhombic structure, (e) Polymeric carbon on a cobalt surface, (f) A sheet of graphene lying on a cobalt surface. The darker spheres represent carbon atoms in all the figures. 

CHX and hydrocarbon wax are, respectively, the active intermediates formed by the hydrogenation of surface carbide and products of FTS formed by chain growth and hydrogenation of CHX intermediates. The hydrocarbon wax can contain molecules with the number of carbon atoms in excess of 100. Bulk carbide refers to a crystalline CoxC structure formed by the diffusion of carbon into bulk metal. Subsurface carbon may be a precursor to these bulk species and is formed when surface carbon diffuses into an octahedral position under the first surface layer of cobalt atoms. 

Polymeric carbon refers to chains of carbon monomers (surface carbide) that are connected by covalent bonds. It has been shown recently47 that the barrier for C-C coupling on flat surfaces (1.22 eV) is half that for a step site (2.43 eV), and may indicate that the growth of these polymeric species is favored on terraces. Polymeric carbon may also refer to carbon chains that contain hydrogen. In the case of CO hydrogenation on ruthenium catalysts, polymeric carbon has been identified as a less reactive carbon that forms from polymerization of CHX and has an alkyl group structure.48... 

Temperature plays an important role in determining the amount and type of the carbon deposit. Generally during FTS at higher temperatures the amount of carbon deposited will tend to increase,30-31 but the case is often not so straightforward. An example of temperature dependence on the rate of carbon deposition and deactivation is the case of nickel CO hydrogenation catalysts, as studied by Bartholomew.56 At temperatures below 325°C the rate of surface carbidic carbon removal by hydrogenation exceeds that of its formation, so no carbon is deposited. However, above 325°C, surface carbidic carbon accumulates on the surface... 

Surface carbide can be a product of both CO dissociation and disproportionation and can be formed from a mixture of Hj/CO as well. 

It has been shown that it is favorable for surface carbon to go into the first subsurface layer of cobalt.71 Diffusion to octahedral sites of the first subsurface layer is thermodynamically preferred by 50 to 120 kJ/mol and the corresponding activation energy is low. Theoretical calculations on the conversion of surface carbidic to subsurface carbon on Co (0001) found that the electron withdrawing power, and therefore the poisoning effect on potential CO adsorption, is maximal for subsurface carbon.41 Metal dx/ orbitals are less likely to accept electrons from the CO 5a orbital, and thus metal-CO bonding will weaken. The dxz orbital will in turn be less able to back-donate into the CO 2n orbital, resulting in additional... 

From the work reported in literature it can be thus concluded that there will be various forms of carbonaceous species, which vary in reactivity, that exist on the catalyst or support during FTS. Some forms of this carbon are active (atomic surface carbide and CHX species) and even considered as intermediate species in FTS. However, it is also clear that especially during extended runs there may be a build up/transformation to less reactive forms of carbon (e.g., polymeric carbon). The amounts of these species may be small, but depending on their location, they may be responsible for a part of deactivation observed on cobalt-based FTS catalysts. The electronic interaction of carbon with the catalyst surface may also result in decreased activity. 

A selectivity model was proposed based on a theory that the product spectrum consists of two separate products formed on two different catalytic surfaces. Chemical reactivity arguments were used to show that the two surfaces (carbide and oxide) are responsible for the production of different groups of products. Separation of the product spectrum in this way enabled us to explain and sometimes predict changes in selectivity and gave us a handle on tracking the full selectivity changes by following the C02 and CH4 selectivity. 

Most of the ethylene that interacts with Ru(001) at 323 K produces a nondesorbable carbon layer. This result is similar to that for the interaction of C2H4 with Ni, which produces a surface carbide at temperatures between about 300-600 K (14). SIMS results suggest, however, the presence of small amounts of molecularly adsorbed ethylene, acetylenic and other hydrocarbon complexes in addition to the nondesorbable carbon layer. 

The first scheme was ruled out by showing that, at room temperature, a surface formed by very brief exposure of the oxygen-saturated surface to carbon monoxide is stable after removal of the carbon monoxide from the reaction chamber. In other words, no further surface carbide formed by lateral reactions of adsorbed carbon monoxide with surface oxygen atoms. The second scheme was ruled out by showing that exposure of the surface formed in the latter experiment to oxygen had no effect. Consequently the third scheme is believed to represent the mechanism of oxidation of carbon monoxide at the step and kink sites of platinum. 

Fig. 2. Real space and LEED structures for surface carbides of W(IOO).

Similar results were found for the decomposition on surface carbides of tungsten. The W(IOO) surface reconstructs to form a W(IOO) (5 x 1)C structure, as shown in Fig. 2. This structure has been interpreted to be a hexagonal layer of W2C on top of the (100) surface 110) with the carbon atoms in interstitial positions under the first layer of tungsten atoms 111). The presence of the carbon atoms strongly modified the reactivity of the surface. Rather than dissociating formic acid completely like W(IOO), the carbide surface selectively formed the formate. The decomposition proceeded at 540 K with... 

A point which has not been examined is the nature of the surface during exchange reactions carried out at high temperatures such as those required for the exchange of methane. Surface carbides may be formed under these conditions. The inactivity of iron films and the comparatively small activity of cobalt films at 300° for the exchange of ethane 19) may possibly be due to the tendency of these metals to form not only surface but also bulk carbides. 

A new mechanism to interpret alkene formation in Fischer-Tropsch synthesis has been presented 499-501 There is a general agreement that hydrocarbon formation proceeds according to the modified carbene mechanism. Specifically, CO decomposes to form surface carbide and then undergoes hydrogenation to form surface methine (=CH), methylene (=CH2), methyl and, finally, methane. Linear hydrocarbons are formed in a stepwise polymerization of methylene species. When chain growth is terminated by p-hydride elimination [Eq. (3.61)], 1-alkenes may be formed,502 which is also called the alkyl mechanism ... 

Further increase in temperature results in carbon—carbon bond fission and the formation of a surface carbide containing single carbon units. 

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