Surface carbyne

CC-coupling reactions of carbon units on the catalyst surface occur either through radical species or through "unsaturated" carbon intermediates such as surface bound carbenes (CKy or carbynes (CH). 

Both the long C-C bond distance (1.50 A) and the very short Pt—C distances (2.0 A) indicate the strong interaction between the adsorbed molecule and the three platinum surface atoms. The covalent Pt—C distance would be 2.2 A. The shorter metal-carbon distances indicate multiple metal-carbon bonding that may be carbene or carbyne-like. Compounds with these types of bonds exhibit high reactivity in metathesis and in other addition reactions The carbon-carbon single bond distance indicates that the molecule is stretched as much as possible without breaking of this chemical bond. 

The rapid development of the chemistry of transition metal complexes containing terminal carbene (A) or carbyne (B) ligands (7) has been followed more recently by much research centered on bridged methylene compounds (C) (2). The importance of /t-methylidyne complexes, whether in recently established binuclear examples (D), the well-known trinuclear derivatives (E), or the unusual complexes (F), has also become apparent. All are based on one-carbon (C,) fragments, and considerable interest is centered on their possible significance as models for intermediates in surface-catalyzed reactions between carbon monoxide and hydrogen (Fischer -Tropsch reactions) and related processes. These topics have been extensively ... 

FIGURE 5.4 Schematic representation of the surface chemistry of a graphene layer, including carbene- and carbyne-type free edge sites, carboxyl-, lactone-, quinone-, pyrone-, and phenolic-type oxygen functionalities, delocalized unpaired 7t electrons, as well as nondissociatively adsorbed 02 on the carbene-type sites. 

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]). 

M. Green, S. J. Porter, and F. G. A. Stone, J. Chem. Soc., Dalton Trans., 1983, 513. 83Bridging carbynes are important ligands in metal cluster compounds (Chapter 13) and may play a role in surface-catalyzed reactions. Methine (C-H), bound to more than one metal atom, is considered to be involved in intermediate stages of the Fischer-Tropsch reaction, which is used to convert basic hydrocarbon starting materials, such as those from coal, into gasoline and diesel fuel (M. J. Overett, R. O. Hill, and J. R. Moss, Coord. Chem. Rev., 2000, 206-207, 581). 

FIGURE 3.1 Proposed epitaxial relationship between a non-reconstructed (111)-(1x1) diamond surface (triangles) and a 2-D hexagonal (00.1) net of carbyne... 

Owing to the compressed topology of the reconstructed diamond surface the lattice mismatch is thought to decrease. The carbon atom chains 1 and 2 of carbyne coincide rather closely with the atom positions A of the top layer... 

As shown in Figure 3.7b the carbyne layer deposited consists of small columnar crystallites about 1 pm long that are arranged more or less perpendicular to the substrate surface, i.e. (Ill) of diamond. These structures closely resemble those reported by Onuma et al. [17] and Kawai et al. [18], and attributed to chaoite, a carbyne polytype with an assumed chain length of = 11 carbon atoms [12]. 

FIGURE 3.9 (a) Raman spectrum (A. = 514.5nm) of the carbyne deposit shown in Figure 3.7. Top spectrum as-measured, bottom spectrum baseline subtracted and deconvoluted. (b) UV-Raman spectrum (X = 325nm) recorded at different surface points of the carbyne deposit shown in Figure 3.7. The inset shows the down-shifting of the diamond peak position from 1332 to 1328 cm with decreasing crystallite size. 

Demyashev et al. [49] have introduced a different approach to electrochemical carbyne. They have electropolished titanium carbide, TiC, in acid electrolyte containing acetic acid and HCIO4. At certain voltages, the Ti atoms were found to be removed preferentially from the TiC surface, which in turn became enriched with carbon. TEM and electron diffraction of the surface revealed a possible existence of carbyne in this layer, and it was predicted that such a production of carbyne might be generally adopted for some other carbides. 

Demyashev, G,M, Taube, A.L. Siores, E, Surface modification of titanium carbide with carbyne, J. Nanosci. Nanotechnol. 2002, 2, 133-137,... 

FIGURE 5.10 AFM analysis of a carbyne crystal, (a) On the surface (b) analysis of a layered structure. 

Carbyne identification is still a matter of contention since not all identifications are complete and rigorous. They are generally identified by color, hardness, fracture surface, density and XRD, a bulk technique that produces a list of interplanar (hkl) or (/-values [23]. Subtle combinations of (/-values and x-ray line intensity are used for carbyne identification [23] but this approach will fail when using electron diffraction data with an up to... 

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