Kinetics and Mechanism of Turanite Reduction by Hydrogen

Abstract

Turanite, Cu5(VO4)2(OH)4, is a naturally occurring mineral whose physical and chemical properties are of interest for magnetic data storage, battery applications, and catalysis. In this study, the reduction of nanostructured Cu5(VO4)2(OH)4 by H2 is investigated using a combination of scanning transmission electron microscopy–electron energy-loss spectroscopy, X-ray photoelectron spectroscopy, in situ diffuse reflectance infrared Fourier transform spectroscopy, in situ X-ray absorption spectroscopy, first-principles calculations, and kinetic analysis based on thermogravimetry. A two-step mechanism is proposed, in which Cu5(VO4)2(OH)4 is first reduced, following exponential growth kinetics, to form a stable intermediate consisting of a mix of Cu3VO4, Cu2O, and CuV2O5. The intermediate phase is then subsequently reduced in a 3-D diffusion-limited process to form two segregated phases consisting of Cu metal and V2O3 particles. The apparent energy barriers for these two steps, obtained from the kinetic analysis, were found to be 52.5 and 71.2 kJ mol–1 for Stages 1 and 2 of the reduction, respectively. These results highlight the importance of developing a rigorous understanding of the reduction processes involved in metal oxide catalyst preparation.