Real Time Isothermal Raman Spectroscopy of Solid-State Kinetics: Goethite Dehydration in N2 Atmosphere

A novel, real-time, in-situ, micro-Raman technique was developed for the monitoring and physicochemical kinetics characterization of solid-state reactions. The technique comprises the acquisition and analysis of a set of time-dependent, isothermal Raman spectra. The pertinence of the technique is demonstrated by the contributions to the study of the dehydration of synthetic goethite (FeOOH) to hematite (Fe₂O₃). The process was monitored for up to 16 hrs in N₂ atmosphere at temperatures of 220, 230 and 240 ^oC.  The Raman spectral range of 200 to 700 cm^-1 was deconvoluted using an iterative fitting algorithm. The isothermal, time-dependent mode analysis yields values for the reaction rates, which exhibit first-order kinetic behavior. The energy barriers of a two-step dehydration process are estimated to be 105 ± 19 kJ/mol and 74 ± 7 kJ/mol in agreement with molecular dynamic simulations (Wen-Juan Zhang et al, J. Mol. Struct.: THEOCHEM, 950 (2010) 20–26).  Our study concurs with other reports on the activation energy of the same process in air, employing infrared spectroscopy, thermogravimetry, and differential scanning calorimetry (Walter D., Thermochim. Acta 445 (2006) 195 -199; Prasad et al., J. Asian Earth Sci. 27 (2006) 503-511). This in-situ, Raman spectroscopic experiment is a promising, non-destructive, real-time analysis technique with applications to various kinetic solid-state processes such as oxidation, dehydration, and phase transformations.