Cobalt-carbides

In the nickel—carbon and cobalt—carbon systems, the nickel carbide (3 1) [12012-02-1], Ni C, and cobalt carbide (3 1) [12011-59-5] C03C, are isomorphous with Fe C and exist only at low temperatures. The manganese—carbon system contains manganese carbide (3 1) [12121 -90-3] Mn C, isomorphous with Fe C, and manganese carbide (23 6) [12266-65-8] isomorphous with chromium carbide (23 6) [12105-81 -6] These... 

The presence of sulfur is found to enhance the formation of graphitic carbon shells around cobalt-containing particles, so that cobalt or cobalt carbide particles encapsulated in graphitic polyhedra are found throughout the soot along with the single-layer nano-... 

Cobalt Carbonitrides, Co2(C, N), and of Cobalt Carbide, Co2C. Chem. week 68, 1004 (1951)-... 

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. 

The formation of bulk cobalt carbide is quite a slow process since it requires the diffusion of carbon into the cobalt bulk. It was reported that the full conversion of unsupported and reduced Co to Co2C only occurred after 500 h of exposure to pure CO at 230°C. Increasing the reaction temperature resulted in a faster rate of carburization.81 Bulk cobalt carbides are considered to be thermodynamically metastable species, and therefore Co2C will decompose to hep cobalt and graphite, while Co3C will decompose to fee cobalt and methane. Thermal decomposition of bulk carbides under an inert atmosphere is believed to occur at 400°C.81 Hydrogenation of the bulk carbides is believed to be a fast process and occurs around 200°C.82 83... 

An Overview of Reported Claims of Bulk Cobalt Carbide Being Observed after/when Performing Fischer-Tropsch Synthesis over Supported Cobalt-Based Catalysts... 

Pennline et al.32 used bifunctional Co/ThO/ZSM-5 catalysts at 280°C, 21 bar, H2/CO = 1 in the FTS. XRD of the used catalyst indicated that cobalt carbide is present. They found that the relative amount of the carbide species is larger on the used catalyst operated at 280°C than on the used catalyst operated at 320°C. They argued that this is because cobalt carbide begins to decompose around 300°C. Since this catalyst lacked high water gas shift activity, and a low feed gas ratio of... 

To summarize, from literature there does not seem to be much consensus on whether bulk cobalt carbide forms during realistic FTS conditions. Bulk carbide is generally considered a metastable species. However, it is clear that it may form under upset conditions. Furthermore, there is strong evidence to show that if bulk cobalt carbide is present, it is deleterious in terms of both catalyst activity and selectivity. With this in mind, it would be prudent to operate the catalyst in a regime (sufficiently high H2/CO ratio) where bulk carbide formation is avoided. 

Graver, V., Zhan, X., Engman, J., Robota, H. J., Suib, S. L., and Polverejan, M. 2004. Deactivation of a Fischer-Tropsch catalyst through the formation of cobalt carbide under laboratory slurry reactor conditions. Prepr. Pap.-Am. Chem. Soc. Div. Pet. Chem. 49 192-94. 

Cobalt-base wear-resistant alloys, 7 221t Cobalt borates, 4 282 Cobalt(II) bromide, uses, 7 239t Cobalt by-product, 16 129 Cobalt carbide, 4 690—692 lattice, 4 652... 

Cobalt carbide (3 1), 4 649t, 691 Cobalt carbonate, analysis, 7 237t CobaltCII) carbonate, 7 230 uses, 7 239t... 

We also found that addition of methane did not yield any SiNW at up to 1100°C. This may be caused by an increase in the melting point of cobalt carbide nanoparticles when methane was added. 

Since the completion of this review (mid-1982), the chemistry of carbidocarbonyl clusters has continued to expand rapidly. The task of the reviewer is made even more difficult as fascinating results continue to appear. In resisting the temptation to make a comprehensive update of the field, it would be remiss of me not to direct the reader s attention to the continued investigations of Lewis, Johnson, and co-workers in the chemistry of ruthenium and osmium carbidocarbonyls (89), the report by Longoni and coworkers (90) of the syntheses of the first nickel carbide clusters and some mixed nickel-cobalt carbides, the syntheses by Shapley of a new ruthenium dicarbide cluster [Ru,oC2(CO)24]- (91) and of Os6C(CO)l7 (92), and the work of Shriver which implies the existence of a very reactive tri-iron carbide cluster (93). 

Finally, cobalt carbide-carbonyl clusters have recently been isolated through a two-step synthesis. First of all, the well-known Co3(CO)9CCl is prepared from Co2(CO)8 and CCh, and then the hexanuclear carbide dianion [Co6(CO)i5C]2- is obtained in good yields (9) by further reaction with Xa[(, o(CO)4] in diisopropylether [see Eqs. (18) and (19)]. Further redox condensation between [Oo6(CO)i5C]2-and Co4(CO)i2 [see Eq. (11)] gives the square antiprismatic octanuclear cluster [Cdo8(Cdt )i8 C]2- (13). Both these carbide derivatives, as well as all of the other cobalt high nuclearity clusters, are sensitive to air and react with carbon monoxide at atmospheric pressure. 

In considering the effect of the electronic structure of catalysts on activity, Dowden (33) suggested that carbides, and similarly nitrides and carbonitrides, should be less active for synthesis than the corresponding metal since the interstitial atoms may contribute electrons to the unfilled d-shells of the metal, which are believed to be essential for the catalytic activity of transition metals in hydrogenation reactions. This hypothesis is supported by the low activity of cobalt carbide compared with that of reduced cobalt (28,29). For iron catalysts the hypothesis... 

Cobalt Carbide.—Carbon dissolves in molten cobalt, and at very high temperatures the carbide, Co3C, analogous to cementile, FegC, appears to be formed. It decomposes so rapidly on cooling into cobalt and graphite that the carbide cannot be detected even m quenched specimens.6... 

In the case of cobalt, unstable cubic cobalt was identified as the product of the reduction of standard cobalt catalysts, while hexagonal cobalt was found as a product of the hydrogenation of cobalt carbide. Used cobalt catalysts show no carbide by x-ray examination. Bulk phase carbide decreases the activity of cobalt catalysts. Surface area measurements show no appreciable change when the cobalt of cobalt catalysts was converted to cobalt carbide. Carburization at conditions where free carbon is formed increases the area considerably. 

Weller (133) passed carbon monoxide over a cobalt catalyst at synthesis conditions and found high carbon monoxide consumption during the very first moment of the treatment. The carbon monoxide consumption diminished rapidly to a much lower but steady rate. Evidently a flash carbiding of the surface occurred, followed by a slower reaction of the bulk metal. The rate of synthesis is comparable with the initial carbiding rate. Hydrogenation of the cobalt carbide proceeds faster than the carbiding (in contrast to the experiences with iron catalysts). 

Active cobalt catalysts can be converted to carbide too. Cobalt-carbide however, reacts easily with hydrogen. Spent cobalt catalysts, therefore, contain only small amounts of carbide. 

It should be noted that this inhibitory effect of precarbiding before synthesis is not explainable as a poisoning due to free carbon deposited during the carbiding, because the effect is completely reversed by hydrogenation at temperatures where free carbon is not affected. These experiments show that bulk cobalt carbide is not a catalyst for the synthesis this conclusion, furthermore, is contrary to that which would be expected if bulk carbide were an intermediate in the synthesis. 

Further evidence which indicates that bulk cobalt carbide is not of importance in the synthesis is furnished by x-ray analysis of catalysts which have been used in the sjmthesis. It has been showm (91) that on low-temperature carburization with carbon monoxide, reduced cobalt... 

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