Characterisation of cobalt–zinc hydroxycarbonates and their products of decomposition

Abstract

A series of cobalt–zinc hydroxycarbonate precursors with nominal Co/Zn atomic ratios of 0/100, 10/90, 20/80, 30/70, 40/60, 50/50, 70/30, 90/10 and 100/0 have been synthesized from their mixed metal nitrates and ammonium carbonate by a coprecipitation route. X-Ray and electron diffraction studies of the precursors revealed that hydrozincite, Zn₅(CO₃)₂(OH)₆ , was the major phase at Co/Zn ratios≤30/70 and spherocobaltite, CoCO₃ , predominated at Co/Zn ratios of 50/50 to 90/10. The Co/Zn 100/0 precursor formed only the metastable basic carbonate Co(CO₃)_0.5(OH)_1.0 0.1H₂O. UV–VIS–NIR diffuse reflectance spectroscopy revealed that the cobalt was present in the 2+ oxidation state in an octahedral environment in all the precursors. Decomposition of the Co/Zn precursors at 350 °C resulted in the formation of ZnO as the major phase at low Co loadings and Co₃O₄ as the major phase at high loadings. The highest surface areas were attained from the decomposition of basic cobalt carbonate or spherocobaltite containing little or no zinc in solid solution. XPS studies of the oxides revealed that only Co³⁺ and Zn²⁺ ions were present at the surface at Co/Zn ratios≤30/70 indicating the presence of the ‘surface spinel’, ZnCo₂O₄ . Co²⁺ was detected at higher Co loadings.

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