A thermogravimetric study of thermal dehydration of copper hexacyanoferrate by means of model-free kinetic analysis

Abstract

The kinetics of thermal dehydration of K2x/3Cu[Fe(CN)6]2/3·nH2O was studied using thermogravimetry for x = 0.0 and 1.0. Data from both non-isothermal and isothermal measurements were used for model-free kinetic analysis by the Friedman and KAS methods. The water content was determined to be n = 2.9–3.9, plus an additional ~10% of water, likely surface adsorbed, that leaves very fast when samples are exposed to a dry atmosphere. The determined average activation energy for 0.2 ≤ α ≤ 0.8 is 56 kJ mol−1. The dehydration is adequately described as a diffusion-controlled single-step reaction following the D3 Jander model. The determined dehydration enthalpy is 11 kJ (mol H2O)−1 for x = 0.0 and 27 kJ (mol H2O)−1 for x = 1.0, relative to that of water. The increase with increasing x is evidence that the H2O molecules form bonds to the incorporated K+ ions.