Clarification of photocatalysis induced by iron ion species naturally contained in a clay compound

Abstract

When clay undergoes isomorphic replacement of a cation (e.g. Al³⁺, Mg²⁺) by an Fe²⁺ ion, the Fe ion in the crystal structure of clay is called structural-Fe. The octahedral sites of a structural-Fe are occupied by O²⁻ ions as in Fe(III) oxide (e.g. α-Fe₂O₃); however, the photocatalysis of structural-Fe has not yet been understood in detail. In this study, structural-Fe(II) in clay was oxidized to Fe(III), following which the photocatalytic activity of clay was investigated in terms of the degradation of acetic acid. After the oxidation of structural-Fe(II), hydrous ferric oxide (HFO, Fe₂O₃·nH₂O) as well as structural-Fe(III) coexisted as the products. To clarify the intrinsic photocatalyses of both species, clay compounds with varying amounts of HFO were prepared and examined in terms of the photocatalytic degradation. Kinetic analysis revealed that the photocatalytic activity of HFO is 30 times higher than that of structural-Fe(III), indicating that HFO is the main species that causes active and efficient photocatalysis in clay.