Mechanistic investigation on self-redox decompositions of cobalt-hydroxide-nitrate compounds with different nitrate anion configurations in interlayer space

Abstract

Self-redox decompositions of two cobalt-hydroxide -nitrates [CoII 0.80CoIII 0.20 (OH)2.00(NO3)0.14(CO3) 0.03·0.77H2O and CoII(OH)1.50(NO3)0.40 CO3)0.05·0.05H2O] have been investigated with various analytical methods under an oxygen-free atmosphere. The investigated compounds all have a lamellar structure in which an oxidant layer (nitrate) and a reductant layer (Co2+) are stacked in an alternate manner. In addition to the "molecular" level mixing of reagents, these compounds are purposely prepared with variations of reactant concentration and anion configuration in the interlayer space. It is found that redox decompositions within the compounds take place according to the following sequence: (i) decomposition of carbonate anions, (ii) dehydroxylation of brucite-like layers, and (iii) decomposition of nitrate anions, although actual reaction temperatures and gas-evolving patterns are different in the two cases. The nitrate decomposition can be ascribed to an attachment of nitrate anion to divalent cobalt of forming cobalt-oxides after the dehydroxylation. More complete decomposition has been observed for the compound that "stores" more redox reagents with direct contact in its lamellar structure, although the final decomposed products of both compounds are all in nanophase C0304 (12-15 nm; formed at 400 °C). Surface compositions of the studied samples have also been examined for the decompositions.