From solid solution towards pyrochlore and kappa phases: introducing configurational entropy in ordered ceria-zirconia systems

Abstract

Following the example of well-known ceria-zirconia pyrochlore and kappa structures, the high-entropy rare-earth counterparts were synthesized. To synthesize ceria-zirconia-based solid solutions, the modified aqueous citrate sol-gel method was applied. In order to obtain pyrochlore phases, reduction by hydrogen (3% H2 in Ar) was needed at a high temperature of around 1500 °C. The last step included mild re-oxidation at around 600 °C under atmospheric conditions to accomplish kappa phases of a high degree of ordering. A direct comparison of different phases of CeZrO and its high-entropy counterpart LCPGY compounds was studied. Their structural similarities and differences were investigated using powder X-ray diffraction (PXRD), Raman spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), and physisorption measurements. Using thermogravimetric analysis (TGA), the thermal behaviour was inspected, which showed direct transformation of pyrochlore to kappa phase due to oxygen uptake, i.e. mass gain. The surfaces of the compounds were analysed by X-ray photoelectron spectroscopy (XPS) to investigate Ce3+/Ce4+, Oads/Ototal, and Pr3+/Prtotal ratios. Temperature programmed desorption (TPD) using 5% CH4/Ar was conducted to show the catalytic activity of the synthesized compounds towards methane oxidation. Introducing configurational entropy in ordered ceria-zirconia systems showed that to obtain single-phase pyrochlore and kappa phases, it is also necessary to pay attention to the reduction/oxidation time and temperature, in addition to the radius ratio, oxidation states, and atomic size disorder. Guided by this theory and experimental findings, we propose that the synthesized pyrochlore high-entropy compound is dual-phase pyrochlore and fluorite, while its kappa form is actually partially oxidized single-phase pyrochlore. High-entropy forms of synthesized compounds showed much better catalytic performance towards methane oxidation than their non-high-entropy counterparts.