Abstract
Detailed vibrational (IR, Raman, far-IR) and thermal (TGA, TG–MS, DSC) analysis has been performed on di[κ1O,κ2O-carbonatotetraamminecobalt(III)] sulfate trihydrate, ([Co(NH3)4CO3]2SO4·3H2O (1). Its isothermic heating at 100 °C leads to formation of [Co(NH3)4CO3]2SO4 (compound 2). UV and IR studies showed that the distorted octahedral arrangement around cis-O2CoN4 core in compound 1 does not change during dehydration, which explains the reversible water loss and ability of compound 2 to rehydrate into compound 1. Compound 2 decomposes at ~ 240 °C in inert atmosphere giving final decomposition products, which are two modifications of nanosized metallic cobalt (hcp-15 nm, fcc-250 nm) and CoO (55 nm). The redox reaction results in N2 as an ammonia oxidation product. The decomposition intermediate is a cobalt(II) compound, Co2O1,14+δ(SO4)0.86 (δ = the oxygen surplus due to the presence of 2.8% of Co(III) ion). The same reaction in air atmosphere resulted in Co2O1.25+δ(SO4)0.75 (δ = the oxygen surplus due to the presence of 5.3% of Co(III) ion (compound 3a). Compound 3a is oxidized in air at 793 °C into Co3O4. The compound 3a exhibits catalytic activity in photodegradation in Congo red. The photodegradation process follows pseudo-first-order kinetic (kapp = 1.0 and 7.0. at pH = 3.4 and 5.25, respectively).
Highlights
To continue our studies on the solid-phase quasi-intramolecular redox reactions between complex cations having redoxactive ligands or cations [1,2,3,4,5,6,7,8], our target was the preparation of [Co(NH3)4CO3](XO4)n (X = oxidizing tetrahedral anion) type compounds as precursors for the preparation of various Co oxide catalysts [9, 10]
Amigo et al [21] studied the decomposition of compound 1 in air until 800 °C, while Onodera et al [22] combined TG, DSC and GC studies performed in inert atmosphere till 400 °C
We found 45.6% and 44.7% of mass loss in air and N2, respectively, which is somewhat more than the theoretical value (%)
Summary
The thermal decomposition characteristics of compound 1 in air and inert atmosphere with DSC and TG–MS were studied in detail. The compound 1, its dehydrated form ([Co(NH3)4CO3]2SO4, compound 2) and the earlier unknown thermal decomposition intermediate formed in the decomposition of compound 1 in air (basic cobalt(II) sulfate, Co2O1.25+δ(SO4)0.75, δ = the oxygen surplus due to presence of 5.3% of Co(III) ion in the compound), compound 3a) were characterized by spectroscopic methods. The solution left back contains cobalt(III) hydroxide precipitate and sodium carbonate was acidified, and the liberated C O2 was precipitated with Ba(OH) as BaCO3 and measured according to the usual way [30]. The cobalt content of the solution was measured as Co3O4, oxidizing the cobalt content of the sample (in case of compound 3 after dissolution in perchloric acid) with sodium peroxodisulfate precipitating the Co(OH) and heating that at 950 °C for 2 h.
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