Electrochemical behavior of acid activated clays

Smectites are phyllosilicates of 2:1 type, meaning that octahedral sheet in sandwiched between two tetrahedral sheets. Due to isomorphous substitutions (Al3+ or Fe3+ for tetrahedral Si4+ and Mg2+, Fe2+ or Fe3+ for octahedral Al3+) these sheets have resulting negative charge. Hydrated cations in interlayer region neutralize negative charge. Acid activation of clays consists of mixing clay with acid solution. During acid treatment of clay interlayer cations are replaced by H+ ions what is followed by dissolution of structural cations. The extent of acid activation can be controlled by several factors: the concentration of acid, the ratio between clay and acid solution, temperature and duration of contract between acid and clay. In this paper is to investigate acid activation of smectite under very mild conditions. The purpose of this activation is exposing of presence Fe2+/Fe3+ ions to the environment and subsequent investigation of obtained material as an electrode material. The obtained materials were characterized by XRD and FTIR technique. According to X-ray diffraction patterns smectite is the main constituent of the investigated clay. The obtained basal spacing d001=1.52 indicates that Ca2+ is the main exchangeable cation, defining raw clay as Ca-bentonite. The XRD analysis confirmed that conditions applied for acid activation were mild enough not to disturb crystal structure of smectite. The lines corresponding to smectite (011 and 060) are present at sample treated with the highest used concentration although in lower intensity indicating that dissolution of clay mineral was partial as intended. The three bending vibrations of hydroxyl groups associated with Al3+, Fe2+ and Mg2+ (915 cm-1, 875 cm-1 and 848 cm-1) were present in the FTIR spectra of starting and acid activated samples confirming that acid activation did not completely removed iron ions from smectite framework. The acid activated clay samples were tested as electrode materials in acidic electrolyte, 0.1 H2SO4. Aside from current rise at potential range limits due to hydrogen and oxygen evolution, presented CV’s have only one feature at potential around 0.4 V. This peak is ascribed to Fe3+/Fe2+ oxidation/reduction process. The current maximum are obtained for samples treated with 1 M and 4.5 M HCl. The observed potential for oxido-reduction for Fe3+/Fe2+ couple are lower than the standard potential for aqueous Fe3+/Fe2+ couple (+0.77 vs. SHE). The reduction potential is lower for structural Fe3+ than for solution Fe3+. The shift in reduction potential observed for different samples is driven by change of the ratio of Fe(II) to Fe(III) species. The Fe3+/Fe2+ ratio can significantly influence the application of material as catalyst. The acid activation provides an easy way to modify clay to desir