New Insights into the Dynamics That Control the Activity of Ceria–Zirconia Solid Solutions in Thermochemical Water Splitting Cycles

Abstract

The reactivity of a ceria-rich Ce0.85Zr0.15O2 solid solution towards the thermochemical water splitting process (TWS) was studied over repeated H2/H2O redox cycles. The structural and surface modifications after treatment at high temperature under air or N2 atmospheres were characterized by High-Resolution Transmission Electron Microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray Photoemission Spectroscopy (XPS) and Positron Annihilation Lifetime Spectroscopy (PALS). Samples treated under nitrogen resulted more active due to phase segregation with formation of a zirconyl oxynitride phase in catalytic amount. Insertion of N 3into the structure contributes to increase the numbers of oxygen vacancies that preferably arrange in large clusters, and to stabilize Ce 3+ centers on the surface. In comparison, treatment under air resulted in a different arrangement of defects with less Ce 3+ and smaller and more numerous vacancy clusters. This affects charge transfer and H-coupling processes, that play an important role in boosting the rate of H2 production. The behavior is found to be only slightly dependent on the starting ceria-zirconia composition and it is related to the development of a similar surface hetero-structure configuration, characterized by the presence of at least a ceria-rich solid solution and a (cerium-doped) zirconyl oxynitride phase, which is supposed to act as a promoter for TWS reaction. The above findings confirm the importance of a multi-phase structure in the design of ceria-zirconia oxides for water splitting reaction and allow a step forward to find an optimal composition. Moreover, the results indicate that doping with nitrogen might be a novel approach for the design of robust, thermally resistant and redox active materials. All these findings suggest new approaches for the development and design of ceria based materials for the two-step water splitting reaction and highlight the importance of engineering the surface defect structure/configuration of the material to obtain an efficient catalyst. In this regard, the role and the impact of nitridation process need to be further investigated.

Nanostructured Equimolar Ceria-Praseodymia for Total Oxidations in Low-O2 Conditions

“…Five "u" and five "v" peaks were identified, related to 3d 5/2 and 3d 3/2 states, respectively. The v • , v', u • and u' peaks were ascribed to Ce 3+ ions, while the other six peaks were assigned to the 4+ oxidation state [33,34]. A similar deconvolution was performed on the Pr 3d XP spectrum, in the 968-923 eV region (Figure 5b), ascribing four peaks (herein denoted as a • , a', b • and b') to Pr 3+ ions and other six peaks to the 4+ oxidation state [35].…”

Syngas Production Through H2O/CO2 Thermochemical Splitting Over Doped Ceria-Zirconia Materials

“…Previous research has reported on H 2 /H 2 O cycling, outlining that it can be used to define the splitting properties of materials (Landi, 2019). Results of thermogravimetric (TG) measurements (reduction under inert Ar atmosphere; oxidation under 40 vol.% CO 2 /Ar) and H 2 O splitting tests on H 2 -reduced samples were successfully compared (Al-Shankiti et al, 2013;Pappacena et al, 2016Pappacena et al, , 2017Luciani et al, 2018).…”

“…As previously reported (Luciani et al, 2018(Luciani et al, , 2019Portarapillo et al, 2019), the samples were pre-treated in a hydrogen flow (TPR; 10 l(SLT)/h; 5 vol.% H 2 /N 2 ) up to approximately 1,000 • C, heating rate 10 • C/min (Al-Shankiti et al, 2013;Pappacena et al, 2016Pappacena et al, , 2017 and cooled down to 60 • C under a nitrogen flow. TPO was then carried out by flowing (i) a 3 vol.% H 2 O/N 2 mixture (10 l(SLT)/h), (ii) a 6 vol.% CO 2 /N 2 mixture (10 l(SLT)/h), (iii) a (6 vol.% H 2 O + 6 vol.% CO 2 )/N 2 mixture (10 l(SLT)/h); the temperature raised up to about 1,000 • C (heating rate: 10 • C/min; time at the maximum temperature: 20 min).…”

“…The improved splitting activity of ceria-zirconia solid solutions has been related to the formation of a Zr 2 ON 2 likephase at the nanoscale level during thermal treatment in N 2 atmosphere (Pappacena et al, 2016(Pappacena et al, , 2017, which is associated with a decrease of surface Ce/Zr ratio (Luciani et al, 2019). Good splitting performance has been related also to the formation of both bulk and surface oxygen vacancies, enhancing surface reactivity (oxygen evolution during self-reduction and splitting reactivity during oxidation) and oxygen diffusivity from the bulk to the surface and vice-versa (Luciani et al, 2019).…”

Recent advances in titanium dioxide/graphene photocatalyst materials as potentials of energy generation

“…During recent decades, research on photocatalytic water-splitting and CO 2 reduction started to draw more attention. It should be noted that there are numerous publications reporting 'achievement' of H 2 production via water splitting using various kinds of sacrificial reagents, such as methanol, amine and amide among others [25,[32][33][34][35][36][37][38][39][40][41][42]. However, it is worth noting that H 2 production reactions are not likely to be considered as a genuine 'water-splitting' reaction.…”