Cation-deficient nano-dimensional particle size cobalt–manganese spinel mixed oxides

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Abstract

Cobalt–manganese binary spinel oxides of high-dispersity are synthesized by thermal decomposition of hydroxycarbonate precursors in the limits 0.33≤Mn/Co≤2. The mixed oxides are cation-deficient and contain a high surface concentration of non-stoichiometric oxygen. The oxygen-to-metal surface ratios, determined by ESCA are higher than the stoichiometric value for dioxides. The spinel lattice is tetragonally distorted for atomic ratios Mn/Co>1. The samples synthesized below 400 °C are composed from spherical-shaped particles with diameters in the nano-scale range 6–12 nm. Reduction of the binary spinel oxides with hydrogen proceeds in multiple steps that correspond to the reduction steps Co(III)⇒Co(II); Co(II)⇒Co reduction and Mn(IV), Mn(III)⇒Mn(II). In the temperature range 100–1200 °C reduction to metallic manganese has not been observed.

Introduction

Mixed oxides of divalent and trivalent cations form stable spinel structures of the type AB2O4. These materials have found a wide and increasing application as oxidation catalysts and oxide electrode materials [1], [2], [3], [4], [5], [6]. In normal spinels the divalent cation A occupies one-eighth of the tetrahedral sites and the trivalent cation B occupies one-half of the octahedral sites. Due to differences in crystal field stabilization energy for the transition metal cations, divalent cations may occupy octahedral sites, resulting in an inverse spinel structure. The existence of cations in multiple oxidation states Fe(II)/Fe(III), Co(II)/Co(III), Mn(II)/Mn(III)/Mn(IV) with close values of the crystal field stabilization energies for octahedral and tetrahedral sites is the main reason for the abundance of defective spinel structures and non-stoichiometry. Spinel cobaltites are intensively studied—a number of divalent and trivalent cations may occupy tetrahedral and octahedral positions substituting cobalt in the spinel Co3O4. Trivalent cobalt is diamagnetic in spinels, in a low-spin state 1I (t2g6) and has a strong preference to the octahedral sites. Divalent cobalt cations 4F (e4 t23) occupy the tetrahedral sites and they give rise to magnetic properties. Divalent manganese in spinels is stable on tetrahedral sites, in a high-spin 6S (e2 t23) state. Trivalent manganese however has strong preference to octahedral coordination. Above certain concentration of trivalent manganese cations, tetragonal spinel structure is formed due to a cooperative effect of Jahn–Teller Mn(III)–(O)6 local distortions [7]. Sesquioxides M2O3, where MFe, Mn, Cr crystallize also in metastable spinel structures, although corundum structure is the thermodynamically stable. Non-stoichiometric cation-deficient spinel structures AB2O4+δ are known for many transition metal oxides. The ordering of defects on tetrahedral and octahedral sites determines specific properties of the oxide material. From spinel manganites, a λ-MnO2 ([ ]A[Mn2]BO4) phase can be obtained and this oxide is a promising cathode material for rechargeable cells [8]. Certain steps of the synthesis of the mixed oxides play decisive role in the formation of the desired structure [9], [10], [11]. Precursor compounds may direct the synthesis to a desired composition, particle size and morphology of the oxide material. Hydroxy-salts proved to be promising among the precursors, obtained by co-precipitation [12]. Their thermal decomposition results in a high-dispersity material with Lewis-base properties, related to cation deficiency [13].

Section snippets

Synthesis

The hydroxycarbonates were coprecipitated from a solution of Co(II) and Mn(II) chlorides, taken in a desired molar ratio and total Mn(II) concentration 0.6 mol/dm3 with a sodium carbonate solution at pH 9. The initially formed precipitates were kept under continuous stirring for 1 h at 30–40 °C. After filtration and washing, they were dried at 60 °C. The spinel oxides were obtained by thermal decomposition of hydroxycarbonate precursors at temperatures between 400 and 600 °C.

Characterization

Atomic absorption

Precursors

The CO2/H2O ratio increases with increasing the Mn/Co ratio in the precursors (see Table 1). The infrared spectra of hydroxycarbonates show the typical bands for carbonate groups, coordinated as monodentate ligands to the metal cations [17] (see Table 2). The vibrations of hydroxyl groups and weakly bound water molecules are also registered. Monodentate coordination can be inferred from the lack of significant splitting of the doubly degenerate asymmetric stretching vibration ν3(E′) of the

Conclusion

The high-dispersity spinel Co–Mn oxides, synthesized from carbonate precursors, contain an excess of surface oxygen, related to surface cation deficiency. The tetragonally deformed spinel structure is the thermodynamically stable one for manganese-rich compositions and it is of high thermal stability. Reduction with hydrogen proceeds in multiple steps and very high temperature is needed for the complete reduction of manganese. The hydroxycarbonate precursor route is highly favorable for the

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