Specificity of the Thermal Stability and Reactivity of Two-Dimensional Layered Cu-Fe Sulfide-Mg-Based Hydroxide Compounds (Valleriites).

Abstract

We recently synthesized prospective new materials composed of alternating quasi-atomic sheets of brucite-type hydroxide (Mg, Fe)(OH)2 and CuFe1-xS2 sulfide (valleriites). Herein, their thermal behavior important for many potential applications has been studied in inert (Ar) and oxidative (20% O2) atmospheres using thermogravimetry (TG) and differential scanning calorimetry (DSC) analyses and characterization with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX). In the Ar media, the processes are determined by the dehydroxylation of the hydroxide layers forming MgO, with the temperature of the major endothermic maximum of the mass loss at 413 °C. Sulfide sheets start to degrade below 500 °C and melt at nearly 800 °C, with bornite, chalcopyrite, and troilite specified as the final products. In the oxidative atmosphere, the exothermic reactions with the mass increase peaked at 345 and 495 °C, corresponding to the partial and major oxidations of Cu-Fe sulfide layers. Sulfur oxides captured in magnesium hydroxide layers to form MgSO4 compromised the layer integrity and promoted the oxidation of the sulfide entities. The final products also contained minor MgO, Cu2MgO3, Fe3O4, and MgFe2O4 phases. Samples doped with Al, which decreases the content of Fe in hydroxide layers, show notably impeded decay of valleriite in argon but facilitated the oxidation of Cu-Fe sulfides, while the impact of Li (it slightly increases the number of the Fe-OH sites) was less expressed. The mutual stabilization of the two-dimensional (2D) hydroxide and sulfide layers upon heating in an inert atmosphere but not in oxygen as compared with bulk brucite and chalcopyrite was suggested to explain high thermal resistance across the stacked incommensurate sheets, which slows down the endothermic reactions and accelerates the exothermic oxidation; the high number of Fe atoms in the hydroxide sheets are expected to promote the phonon exchange and heat transfer between the layers.