In situ kinetics studies of Zn–Al LDH intercalation with corrosion related species

Abstract

Kinetic parameters for three anion exchange reactions – Zn-LDH-NO₃ → Zn-LDH-Cl, Zn-LDH-NO₃ → Zn-LDH-SO₄ and Zn-LDH-NO₃ → Zn-LDH-VO_x – were obtained by in situ synchrotron study. The first and the second ones are two-stage reactions; the first stage is characterized by the two-dimensional diffusion-controlled reaction following deceleratory nucleation and the second stage is a one-dimensional diffusion-controlled reaction also with a decelerator nucleation effect. In the case of exchange NO₃⁻ → Cl⁻ host anions are completely released, while in the case of NO₃⁻ → SO₄²⁻ the reaction ends without complete release of nitrate anions. The exchange of Zn-LDH-NO₃ → Zn-LDH-VO_x is a one-stage reaction and goes much slower than the previous two cases. The latter is characterized by a one stage two-dimensional reaction with an instantaneous nucleation. As a result, at the end of this process there are two crystalline phases with different polyvanadate species, presumably V₄O₁₂⁴⁻ and V₂O₇⁴⁻, nitrate anions were not completely released. The rate of replacing NO₃⁻ anions by guest ones can be represented as Cl⁻ > SO₄²⁻ > VO_x^y−.