Ultraviolet-Visible Spectroscopy and Temperature-Programmed Techniques as Tools for Structural Characterization of Cu in CuMgAlOx Mixed Metal Oxides

Ultraviolet-visible (UV-vis) spectroscopy was used in combination with temperature-programmed reduction (TPR) methods to provide information about the Cu structure in CuMgAlOx mixed oxides. UV-vis spectra revealed the presence of highly dispersed, isolated and oligomeric, CuO species in a distorted octahedral environment. From these spectra, optical absorption edge energies were determined and correlated with the number of nearest CuO neighbors (a measure of CuO domain size) and Cu content in the mixed oxides. Increasing the Cu content from 4 to 21 at. % increased the number of nearest CuO neighbors in oligomeric CuO from bout 2 to 4 and produced materials that were more easily reducible, as inferred from TPR of untreated and N2O-passivated CuMgAlOx. A further increase in Cu content to 38 at. % increased the number of nearest CuO neighbors to 4.5 but resulted in a decrease of reducibility because of the evolution of an amorphous CuO phase in the bulk of the mixed oxides. This work represents the first demonstration of combining UV-vis spectroscopy with TPR of untreated and N2O-passivated CuMgAlOx as relatively simple and inexpensive methodology for in-depth structural characterization of Cu in mixed metal oxides from which the composition, domain size, and relative fraction of total (surface + bulk) and surface oligomeric CuO species can be determined. A methodology that allows assessment of the extent of CuO bulk enrichment in CuMgAlOx materials is also resented.