Role of Cobalt

One of the more important and intriguing questions posed is the role which cobalt plays in catalysis over molybdena catalysts. Its promoting effect is unquestioned in HDS reactions and is a large reason for the commercial success of these catalysts. Also, it is probably the subject which has received the greatest attention in the study of these catalysts. Despite this effort, controversy still attends the primary function of cobalt. Many theories have been proposed based on limited results, many of these being in disagreement with other results. 

Some of the more important theories are presented in Table X together with pertinent references. These depend on either alteration of catalyst phases or specific kinetic effects of the cobalt. The merits of each are briefly discussed below. 

Increases Mo monolayer area. The addition of Co causes the Mo to better disperse on the support. Activity is then proportional to Mo areas. Ben-Yaacov s (61) findings disagree, showing no effect of Co on Mo dispersion. 

Increases Mo reduction. The molybdena catalyst reduces more in the presence of Co and activity depends on extent of reduction. As discussed earlier, there is considerable disagreement on the former point and the latter is unproved, especially for sulfided catalysts. 

Due to CoMoS2 intercalation. As has been discussed previously (see Section VII.B), this assigns the Co to a very specific site in the MoS2 

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Promotional effects of sulfide can evidently be explained, because exposure of reduced metals Is Increased on reduced sulfided catalysts. The role of cobalt Is less clear. It Is normally not fully reduced. It apparently does not promote greater exposure of Mo In any form detected, either In the presence or absence of sulfide. On the contrary. It evidently only decreases the concentration of exposed Mo atoms, although, at concentrations typically used, most. Mo atoms are unaffected by Co. Either some property of Co alone or some local cooperative effect of adjacent Co and Mo must explain promotion. Simple mechanical mixtures will not give the synergism observed, however (1-4). 

Ionic and free-radical reactions leading to the formation of ketonic products can occur simultaneously in the copper-catalyzed reaction of a Grignard reagent with a sterically hindered acid halide. These reactions have been studied by Dubois and co-workers 102-108, 194). Contrary to the report of Percival et al. 229), ferric chloride inhibits the catalytic role of copper and does not favor the formation of ketones 106, 108). The scheme depicted in Fig. 4 is proposed 103) to account for the products of the reaction. Similar radical reactions were suggested by Khar-asch et al. 169) to explain the role of cobalt chloride in like reactions. 

Miller, J.T., Reagan, W.J., Kaduk, J.A., Marshall, C.L., and Kropf, AJ. Selective hydrodesulfurization of FCC naphtha with supported MoS2 catalysts The role of cobalt. Journal of Catalysis, 2000, 193, 123. 

Kazantzis G. 1981. Role of cobalt, iron, lead, manganese, mercury, platinum, selenium, and titanium in carcinogenesis. Environ Health Perspect 40 143-161. 

Cobalt is found in vitamin Bn, its only apparent biological site. The vitamin is a cyano complex, but a methyl or methylene group replaces CN in native enzymes. Vitamin-Bi2 deficiency causes the severe disease of pernicious anemia in humans, which indicates the critical role of cobalt. The most common type of reaction in which cobalamin enzymes participate results in the reciprocal exchange of hydrogen atoms if they are on adjacent carbon atoms, yet not with hydrogen in solvent water ... 

Gladstones JS, Loneragan JF and Goodchild NA (1977) Field responses to cobalt and molybdenum by different legume species with interferences on the role of cobalt in legume growth. Aust J Agric Res 28 619-628. 

New Zealand, Kenya, Russia, Florida, and in Germany s Black Forest, giving rise to cobalt deficiency syndromes in farm animals. To protect sheep and cattle in Co-deficient regions, 1 - 2 kg of cobalt sulfate must be added per hectare every 3-5 years. A soil is regarded as Co-deficient if the cobalt level is below 5 mg kg dry matter only about 6% of the total cobalt present is bioa-vailable (Young 1979). For a description of the role of cobalt in animal nutrition, see Suttle (1999). 

The role of Cobalt-dioxygen complexes in autooxidations other than phenol oxidation is less certain, and ostensibly similar reactions appear to follow radically different pathways. Thus, in the oxidation of thiols to disulfide catalyzed by Co species catalysis by the phthalocyanine complex [Co"(TSPc)] apparently proceeds via a Co intermediate and without participation of Co—Oj species, whereas catalysis by [Co"(TPP)] appears to involve initial formation of an / cobalt-dioxygen complex from which Of is displaced by thiolate. Several reviews giving extensive coverage to oxidations catalyzed by cobalt(II) complexes are available. ... 

Cobalt is essential for animal nutrition, but it has not been established as essential for plant growth. Ruminant animals require cobalt for the synthesis of vitamin Bi2 by their rumen microflora. This was established about 1935, but an essential role of cobalt in plants was not demonstrated until 1960 [33]. Cobalt has been found to be essential for the growth of legumes which rely on symbiotic nitrogen fixation. 

Putting aside for the moment the question of the role of a pro-tic substance, such as water, it is evident, according to Sinn et al. (1961), that the important role of cobalt is to orient the monomer in the cis conformation. This conclusion is supported by complexing studies with aromatic ir-electron donors, such as mesit-ylene and durene. These aromatic compounds compete with the butadiene for the cobalt in an equilibrium reaction, thus decreasing the polymerization rate (Van de Kamp, 1962). 

Thus, the role of cobalt(III) as template is associated with control of the behaviour of iminium ion by the activation of an imine to nucleophilic attack as a result of its coordination to the central ion [3]. 

Barroso M, Cowan AJ, Pendlebury SR et al (2011) The role of cobalt phosphate in enhancing the photocatalytic activity of a-Fc203 toward water oxidation. J Am Chem Soc 133 14868-14871... 

If the compound contains an active cobalt-containing adhesion promoter, metallic cobalt precipitates onto the brass surface and also forms the Me-S bond, in addition to copper. Zinc can also form this bond, but does not bond to rubber, as the zinc sulphide growth rate is low (see stage IV). The role of cobalt in NR skim stocks is thus to activate (or... 

Cobalt is an essential precursor for the microbial biosynthesis of B12. Pining in sheep which feed on cobalt-deficient pastures can be cured by the ingestion of cobalt, which increases the biosynthesis of B12 in the rumen (Marston, 1952), or by the injection of vitamin B12 (Koch and Smith, 1951). The role of cobalt in biosynthesis in the intestine has been studied by feeding radioactive cobalt, with or without aureomycin, to rats (Davis and Chow, 1951). 

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