In-situ generation of maximum trivalent cobalt in synthesis of hydrotalcite-like compounds Mg(x)Co(II)(1-x-y)Co(III)(y)(OH)2(NO3)(y)·nH2O

Abstract

In-situ generation of trivalent cobalt cations has been investigated for the hydrotalcite-like compounds Mg(x)Co(II)(1-x-y)Co(III)(y)(OH)2(NO3)(y)·nH2O at 25-40 °C under oxygen-containing atmospheres. It is noted that with more involvement of Mg2+ in the compounds, less Co2+ cations are needed to maintain the hydrotalcite-like structure. Because of the presence of the Mg2+, more Co2+ can be oxidized under the current experimental conditions and the highest mole ratios of Co3+ to total cobalt cations (Co3+:Co) observed in this work is 57%. The mole ratio of Co3+ to total metal cations (Co3+:(Mg+Co)) achieved in this work is 31% after 4 days of oxidation reaction, which is close to its upper charge limit to produce a single-phase hydrotalcite-like structure (33%). Two major thermal events are observed when the compounds are heated. The first one at 113-134 °C is attributed to the removal of interlayer water molecules while the second at 274-333 °C to the dehydroxylation and decomposition of intercalated anions. Higher catalytic activity for nitrous oxide (N2O) decomposition is observed for the Mg-Co oxides with the same cobalt content but lower mole ratios of Co3+: Co in their hydrotalcite-like precursors. The reason for this activity variation has also been addressed.