Thermal decomposition and spectral characterization of di[carbonatotetraamminecobalt(III)] sulfate trihydrate and the nature of its thermal decomposition products
Fernanda Paiva Franguelli, Berta Barta Holló, Vladimir M. Petruševski, István E. Sajó, Szilvia Klébert, Attila Farkas, Eszter Bódis, Imre Miklós Szilágyi, Rajendra P. Pawar, László Kótai
Abstract
Abstract Detailed vibrational (IR, Raman, far-IR) and thermal (TGA, TG–MS, DSC) analysis has been performed on di[κ 1 O,κ 2 O-carbonatotetraamminecobalt(III)] sulfate trihydrate, ([Co(NH 3 ) 4 CO 3 ] 2 SO 4 ·3H 2 O ( 1 ). Its isothermic heating at 100 °C leads to formation of [Co(NH 3 ) 4 CO 3 ] 2 SO 4 (compound 2 ). UV and IR studies showed that the distorted octahedral arrangement around cis-O 2 CoN 4 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 N 2 as an ammonia oxidation product. The decomposition intermediate is a cobalt(II) compound, Co 2 O 1,14+δ (SO 4 ) 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 Co 2 O 1.25+δ (SO 4 ) 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 Co 3 O 4 . The compound 3a exhibits catalytic activity in photodegradation in Congo red. The photodegradation process follows pseudo-first-order kinetic ( k app = 1.0 and 7.0. at pH = 3.4 and 5.25, respectively).