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.