Nickel 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 methanation reaction is carried out over a catalyst at operating conditions of 503—723 K, 0.1—10 MPa (1—100 atm), and space velocities of 500—25,000 h . Although many catalysts are suitable for effecting the conversion of synthesis gas to methane, nickel-based catalysts are are used almost exclusively for industrial appHcations. Methanation is extremely exothermic (AT/ qq = —214.6 kJ or —51.3 kcal), and heat must be removed efficiently to minimise loss of catalyst activity from metal sintering or reactor plugging by nickel carbide formation. 

Four pilot plant experiments were conducted at 300 psig and up to 475°C maximum temperature in a 3.07-in. i.d. adiabatic hot gas recycle methanation reactor. Two catalysts were used parallel plates coated with Raney nickel and precipitated nickel pellets. Pressure drop across the parallel plates was about 1/15 that across the bed of pellets. Fresh feed gas containing 75% H2 and 24% CO was fed at up to 3000/hr space velocity. CO concentrations in the product gas ranged from less than 0.1% to 4%. Best performance was achieved with the Raney-nickel-coated plates which yielded 32 mscf CHh/lb Raney nickel during 2307 hrs of operation. Carbon and iron deposition and nickel carbide formation were suspected causes of catalyst deactivation. 

X-ray analysis of the spent catalyst (Table XI) revealed metallic nickel and nickel carbide, Ni3C, in the catalyst near the gas inlet and only metallic nickel near the gas outlet. 

Nickel carbide, detected on the catalyst in experiment HGR-14, is another compound suspected of deactivating Raney nickel catalyst. However, the shutdown involved purging with hydrogen while the catalyst... 

Wm. Haynes The nickel carbide formation has been reversed. That is,nickel carbide has been eliminated by hydrogen treatment in some of the laboratory tests at the Bureau of Mines, and catalyst activity has been restored that way. In the pilot plant, however, we have not been able to achieve any such regeneration of the catalyst. 

Dr. Moeller We have done this, and we compared an iron catalyst used for the Fischer-Tropsch plant and a nickel catalyst used in the methanation plant. By the same x-ray techniques, we found no nickel carbide on the used methanation catalyst, but we did find iron carbide on the used Fischer-Tropsch catalyst. 

Schouten, S.C., Gijzeman, O.L., and Bootsma, G.A., Reaction of methane with nickel single crystal surfaces and the stability of surface nickel carbides, Bull. Soc. Chim. Belg., 88, 541,1979. 

Fig. 3. A comparison of AES carbon signals on a NiflOO) crystal with those from single stal graphite and nickel carbide, (a) Following 1000s heating at 600 K in 24 torr CO. (b) Nickel carbide, (c) Following 1000s heating at 700 K in 24 torr CO. (d) Single crystal graphite. (From Rrf. 3.)...

Nickel Carbide, NijC solid, stable up to 380-400decompd by dil acids or by. super heared steam to methane other products. Was prepd by interaction of Ni pdr with CO ar 200-300°... 

Compensation Behai ior for Reactions on Nickel and on Nickel Carbide ... 

Fig. 3. Compensation plot for cracking reactions on nickel carbide (see text, Table I, B), line calculated by least squares method (see Appendix II). Points for reactant mixtures containing hydrogen, O line for cracking reactions on nickel metal (from Fig. 2) shown dashed.

Cracking reactions on nickel carbide in the absence of added hydrogen (22), hydrogenation of nickel carbide (162), and the reactions of water and of sulfur dioxide with this solid (163) exhibited a different compensation line (Fig. 3, full line, and Table I, B) from that for cracking reactions on the metal (Fig. 3, dashed line). When data for the reactions of propane on nickel carbide in the presence of some added hydrogen (O on Fig. 3) (22) are included in the calculation, the position of the line is almost unchanged, but the values of a are significantly increased (by the factor x 2, Table I, C). 

Arrhenius parameters for nickel carbide hydrogenation 162) is close to both lines on Fig. 3. Compensation behavior for reactions on the carbide phase must include an additional feature in the postulated equilibria, to explain the removal of excess deposited carbon, if the active surface is not to be poisoned completely. The relative reduction in the effective active area of the catalyst accounts for the lower rates of reaction on nickel carbide, and the difference in the compensation line from that of the metal (Fig. 3) is identified as a consequence of the poisoning-regeneration process. After any change in reaction conditions, a period of reestablishment of surface equilibria was required before a new constant reaction rate was attained (22). 

We conclude, therefore, that the mechanisms of catalytic cracking reactions on nickel metal and nickel carbide are closely comparable, but that the latter process is subject to an additional constraint, since a mechanism is required for the removal of deposited carbon from the active surfaces of the catalyst. Two phases are present during reactions on the carbide, the relative proportions of which may be influenced by the composition of the gaseous reactant present, but it is not known whether the contribution from reactions on the carbide phase is appreciable. Since reactions involving nickel carbide yielded products other than methane, surface processes involved intermediates other than those mentioned in Scheme I, although there is also the possibility that if cracking reactions were confined to the metal present, entirely different chemical changes may proceed on the surface of nickel carbide. 

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). 

NiC8 (c). Roth s5 data on the heat of combustion of nickel carbide yield Qf= —9.2. See also Ruff and Gersten.2... 

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