Cobalt hexamine complex

Figure 8.18 Schematic view of the structure of the cobalt hexamine complex.

The hexamine cobalt (II) complex is used as a coordinative catalyst, which can coordinate NO to form a nitrosyl ammine cobalt complex, and O2 to form a u -peroxo binuclear bridge complex with an oxidability equal to hydrogen peroxide, thus catalyze oxidation of NO by O2 in ammoniac aqueous solution. Experimental results under typical coal combusted flue gas treatment conditions on a laboratory packed absorber- regenerator setup show a NO removal of more than 85% can be maitained constant. 

Recent years, the authors have innovatively proposed a method by using the aqueous ammonia liquor containing hexamine cobalt (II) complex to scrub the NO-containing flue gases[6-9], since several merits of this complex have been exploited such as (1) activation of atmospheric O2 to a peroxide to accelerate the O2 solubility, (2) coordination of NO, as NO is a stronger ligand than NH3 and H2O of Co( II) complexes to enhance the NO absorption and (3), catalysis of NO oxidation to further improve the absorption both of O2 and NO. Thus, a valuable product of ammonium nitrate can be obtained. 

Since the cobalt(II) complex is inert, its decomposition (114) even in acidic medium k = 1.2 x 10-2 mol T s ) is slow compared with the electron-transfer rate ()e = 5.0 mohU s" ). Therefore, process (113) dominates, and no photoeffect has been observed. The inertness of the [Co(sep)] + cation in photochemical processes is caused by complete encapsulation of the cobalt ion. In the system with the nonmacrocyclic hexamine [Co(NHs)6] trication, photoreduction was found to occur in relatively high quantum yield ( = 0.16). In this case, the photocomposition rate of [Co (NH3)5(NH3)+] + cation formed on irradiation k > 10 s >) is higher than that of electron transfer k = 10-5 mol-T s-i). 

This paper presents the experimental results, with a focus on studies of the regeneration of the hexamine cobalt complex additive by using the activated carbon in a laboratory packed-bed absorber. 

The simple but rather useful molecular mechanics calculations were applied to both electron transfer kinetics and reduction potentials for a wide range of hexamine cobalt(III/II) complexes with primary, secondary, tertiary, and macrobicyclic amine ligands [237]. The redox potentials of the Co +/2+ couples varied from... 

So far, we have generally assumed that the complexation reaction is fast and that equilibrium is attained. This is often, but not always, the case. The differentiation of the thermodynamic terms, stable and unstable, from the kinetic terms, labile and inert (or robust), should be made. The classical example of a kinetically inert complex is the hexamine cobalt(III) cation in acid solution ... 

Cobalt(II), like other transition metal ions, forms a stable complex with EDTA. The suitable pH for the titration, adjusted by hexamine, is pH6.0 and the indicator is xylenol orange. 

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