Redox behavior of potassium doped and transition metal co-doped Ce_0.75Zr_0.25O₂ for thermochemical H₂O/CO₂ splitting

CeO₂ slow redox kinetics as well as low oxygen exchange ability limit its application as a catalyst in solar thermochemical two-step cycles. In this study, Ce_0.75Zr_0.25O₂ catalysts doped with potassium or transition metals (Cu, Mn, Fe), as well as co-doped materials were synthesized. Samples were investigated by X-ray diffraction (XRD), N₂ sorption (BET), as well as by electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) to gain insight into surface and bulk features, which were connected to redox properties assessed both in a thermogravimetric (TG) balance and in a fixed bed reactor. Obtained results revealed that doping as well as co-doping with non-reducible K cations promoted the increase of both surface and bulk oxygen vacancies. Accordingly, K-doped and Fe-K co-doped materials show the best redox performances evidencing the highest reduction degree, the largest H₂ amounts and the fastest kinetics, thus emerging as very interesting materials for solar thermochemical splitting cycles.