Improvement of splitting performance of Ce0.75Zr0.25O2 material: Tuning bulk and surface properties by hydrothermal synthesis

Abstract

Thermochemical cycles received renewed interest as CO2 and H2O energy-upgrading processes using solar energy as source. The two-step cycles, based on self-reduction in a solar reactor at high temperature (above 1300–1400 °C) and re-oxidation by CO2 and/or H2O flow, are the most interesting due to their simplicity and high theoretical solar-to-fuel efficiency. In the two-step cycle, ceria has been recognized as the benchmark material but it suffers from high reduction temperature, low re-oxidation kinetics as well as low stability, thus hindering practical application. In this work, the redox properties of two Ce0.75Zr0.25O2 materials prepared by hydrothermal synthesis were compared with those of a co-precipitated sample with the same nominal composition used as reference. Samples were characterized by X-Ray Diffraction (XRD), N2 physisorption, Scanning Elecron Microscopy (SEM), X-Ray Photoelectron Spectroscopy (XPS), and Electron Paramagnetic Resonance (EPR); their self-reducibility and CO2 splitting activity were tested in a Thermogravimetric (TG) balance, while H2O splitting properties were studied in an ad-hoc fixed bed reactor on H2 pre-reduced samples. Characterization results and activity tests agreed that the Ce3+ fraction both on the surface and in the bulk of ceria-zirconia can be increased by hydrothermal synthesis, thus providing improved redox properties and higher splitting activity with respect to the co-precipitated sample. So, hydrothermal synthesis, providing a controlled nucleation and growth of crystallites, appears as a promising route for the preparation of ceria-based materials with tuned oxygen vacancies.