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Thermochemical cycles received renewed interest as CO 2 and H 2 O 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 CO 2 and/or H 2 O 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 Ce 0.75 Zr 0.25 O 2 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), N 2 physisorption, Scanning Elecron Microscopy (SEM), X-Ray Photoelectron Spectroscopy (XPS), and Electron Paramagnetic Resonance (EPR); their self-reducibility and CO 2 splitting activity were tested in a Thermogravimetric (TG) balance, while H 2 O splitting properties were studied in an ad-hoc fixed bed reactor on H 2 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. Image 1 • Ce 0.75 Zr 0.25 O 2 materials were prepared by hydrothermal synthesis and co-precipitation. • Hydrothermal materials showed improved splitting properties towards both CO 2 and H 2 O. • Hydrothermal materials showed larger Ce3+ fractions both on the surface and in the bulk. • Redox properties can be tuned by the opportune choice of the preparation method. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Perovskites with Mn and Fe as B cation successfully prepared and tested in STS. • Mixed materials show structural features intermediate between pure Mn and Fe perovskites. • The presence of Fe improves self-reduction and, therefore, the producible H 2. • The presence of Mn improves the oxidation kinetics, lowering typical splitting temperature. • Use of diverse B ions as an effective strategy to manage self-reducibility and splitting activity. Abstract Mixed perovskites, described by the formula La 0.6 Sr 0.4 Mn 1−x Fe x O 3−δ with x ranging from 0 to 1, were prepared and tested as potential catalysts for the solar thermochemical water and carbon dioxide splitting. The increase of iron amount promotes the reduction step according to the trend of the starting reduction temperatures; on the other hand, the reactivity towards H 2 O and CO 2 seems to be fostered by high Mn contents. The highest amounts of oxygen evolved during thermogravimetric analysis and hydrogen produced during splitting tests occur to the sample with x = 1 even if a non-monotonic trend with iron substitution is appreciated. Instead, CO amount progressively decreases by increasing iron content. The results reported in this work indicate that the formulation of bi-functional systems with two active sites, one showing good self-reduction the other good reactivity towards splitting reactions, is the best route to prepare materials with larger intrinsic H 2 and CO productivity as well as lower reduction/oxidation temperature. [ABSTRACT FROM AUTHOR]

The solar thermochemical water and carbon dioxide splitting, mediated by ceria, has a great potential to produce "green" syngas. Chromium was added to ceria to improve the syngas production. Three preparation methods were applied, resulting in different morphologies allowing to investigate the role of chromium. The samples were characterized by X-ray diffraction, Raman and X-ray spectroscopy, and electron microscopy. Materials made by polymerized-complex-method and dry-impregnation consisted of two crystal phases: ceria and chromia. In contrast, materials made by flame-spray pyrolysis exhibited a homogeneous Cr-doped ceria phase, and chromia was found only at a chromium-content higher than 25 mol%. The chromium-doped ceria released additional oxygen during the formation of CeCrO3 perovskite, which did not enhance hydrogen or carbon monoxide production. All chromia-containing samples exhibited improved oxygen exchange capacity, possibly due to a redox cycle of chromia itself, and significantly improved the activity of water and carbon dioxide splitting. Hydrogen production increased from 3.2 to 6.7 mL/g and the time to reach redox equilibrium was shorten from 41 to 3 min. The best hydrogen and carbon dioxide production rates were up to 20 and 500 times higher than pure ceria, respectively. The presence of chromium is therefore crucial as a catalyst, promoter, and oxygen storage enhancer. This work emphasises the importance of a catalysed re-oxidation reaction and demonstrates that a metal oxide, becoming active in situ, can catalyse water and carbon dioxide splitting. [ABSTRACT FROM AUTHOR]