Water splitting reaction on Ce_0.15Zr_0.85O₂ driven by surface heterogeneity

Abstract

The compositional and structural heterogeneity of a sample of Ce0.15Zr0.85O2 subjected to a two-step thermochemical water splitting reaction was investigated by means of X-ray powder diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS) and High-Resolution Transmission Electron Microscopy (HRTEM) analysis. High temperature treatment under N2 resulted in segregation of a Zr-rich monoclinic phase on one side and a Ce-rich cubic phase on the other. The treatment also led to a higher reducibility of the material compared to similar studies on Ce-rich compositions. HRTEM revealed the presence of a zirconia based superstructure that it is identifiable with an oxynitride phase Zr2ON2, while ceria surface enrichment was detected via XPS. H2 yield, investigated at 800°C by pulsing water over several redox cycles, showed a six-fold increase after the first cycle remaining constant after at least three subsequent cycles. The presence of zirconia oxynitride was found to be beneficial for both the oxidation and reduction steps. Solar thermochemical water splitting cycles (WSC) are an attractive carbon-free approach to H2 production from water and sunlight.1 Two-step metal oxide based cycles generate H2 through high temperature (~1500-2000°C) reduction in inert atmosphere and the subsequent water oxidation at a lower (~400-1300°C) temperature making water splitting (WS) possible at temperatures lower than the thermodynamic value (2300°C). 2 Among many metal oxides investigated in literature, ceria is one of the most viable candidates, 3 and it can deliver pure oxygen and hydrogen according to the following two-step redox cycleThe main drawback of this cycle is that a significant reduction of ceria occurs only at temperatures higher than 1800°C, where sublimation can occur with a decrease of the yield over cycles. 5 It follows that studies on ceria-based systems have been focused on lowering the reduction temperature of the Ce 4+ /Ce 3+ redox couple, while maintaining the high reactivity of reduced Ce 3+ species towards water. 3,6 The addition of high valence dopant cations, such as Zr 4+ , proved effective in increasing the thermodynamic driving force of CeO2 reduction at lower temperatures, 7-9 and the effect of zirconium content in the two step water splitting reaction has been widely studied. [10][11][12][13][14] Its presence favours CeO2 reduction under inert atmosphere at temperatures lower than 1500°C preventing sublimation and the consequent yield loss; moreover, the increased oxygen storage of ceria-zirconia positively affects O2yield. On the other hand, the H2 productivity depends on the number of exposed surface redox sites, and thus on the textural, morphological and structural properties of these materials. It was reported that in these materials the kinetics of water splitting is often hampered by gassolid diffusion limitations due to the simultaneous occurence of sintering processes. Several efforts have been made to overcome this issue, and the change of the morphological and structural properties o...

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

“…This study allows also to clarify and explain the behavior of Zr-rich compositions like Ce0.15Zr0.85O2. 27 Despite this latter composition is less thermally stable and shows a more significant surface segregation under nitrogen flow, the final composition at the outer part of the particles is similar to that observed for Ce0.85Zr0.15O2. Figure 7 summarizes the main steps of the transformation for both compositions trying to elucidate the reasons why ceria-rich and zirconia-rich materials show similar water splitting reactivity after thermal treatment in nitrogen and redox activation.…”

“…The main component (peak 1) at about 529.5 eV is associated with the cerium-zirconium oxide matrix, while the component centered at 531.5 eV (peak 2) can be ascribable mainly to surface hydroxyl groups. 44 The signal N1s at 399.9 eV ( Figure 5C) confirmed the insertion of nitrogen into the surface of the CZ85_N2 and the formation of a zirconyl oxynitride phase 27,46,47 In addition, the evidence of the same nitrogen peak on the CZ85_N2 surface after WS cycles (CZ85_N2_ws, Figures 5D) in the starting material (500 °C) is due to the nanometric nature of the powder and to the presence of extrinsic defects induced by the zirconium doping. 38 Besides, a small quantity of Ce 3+ could be generated by the reduction of Ce 4+ under the X-ray irradiation during XPS analysis.…”

“…A comparison with results obtained on Ce0.15Zr0.85O2 indicates that regardless of initial composition H2 yields reach close values for the two nitrogen-pretreated compositions after a few cycles. 27 To exclude the role of oxygen in the promotion of H2 production observed from the 2 nd cycle, the same experiment was repeated without oxidizing the samples with air at the end of each cycle, and the results are compared in Figure S1.…”

CeO2 - ZrO2 Catalysts for the Use of Biogas in IT-SOFC

“…XRD structural analysis indicates that supports present a cubic Fm-3m symmetry and the structure was retained during subsequent calcinations, not showing any phase segregation (spectra not shown). In Table I the catalyst crystal size, calculated over the (111) peak of ceria-zirconia solid solution using the Scherrer's equation, remains constant, thus confirming a high thermal stability of powders due to the preparation method and to the effect of lanthanum and neodymium co-doping (16,17).…”

“…The supports Ce 0.8 Zr 0.2 O 2 (CZ80) and Ce 0.8 Zr 0.13 La 0.5 Nd 0.2 O 2-x (LN_CZ80) were prepared with a surfactant assisted method as reported in (16) and calcined at 500°C/4h (fresh catalysts). Successively the powders were calcined at 800°C/3h (aged catalysts), impregnated with a nitrate nickel salt solution up to a metal loading of 3.5 or 7 wt.% and finally calcined at 800°C/3h.…”