Effect of additives on redox behavior of iron oxide for chemical hydrogen storage
Introduction
Hydrogen as an energy carrier is a clean energy system with long-term potential because it can produce near-zero emission from combustion and can be derived from various energy sources such as solar, nuclear, and other renewable energies. However, existing technologies for hydrogen production, storage and utilization are still not economical for large-scale application, although those are currently used commercially in chemical and refining industries. Therefore, much research in fields of hydrogen production, storage and utilization is necessarily required [1], [2].
Various methods for hydrogen storage, as one of the major hydrogen-related technologies, are currently being proposed and developed [3]. Hydrogen can be stored physically as a compressed gas/liquid phase or in chemically stored on chemical compounds. Compressed gas storage is dominant technology available in present, but it is inconvenient to apply for practical use due to relatively low energy density compared with storing volume. Liquid storage, on the other hand, is profitable to less storing volume than gas storage, but requires high energy for storage of hydrogen to liquid phase and expensive containers. In chemical storage method, materials such as metal hydrides and carbon-nanotubes are under developing and not yet clearing up of the prices of main materials and storing amounts.
As one of the promising alternatives for the hydrogen storage, the iron oxide redox cycle system had been proposed by Otsuka et al. [4], [5], which is based on the old steam iron process to produce hydrogen rich fuel gas [6], [7]. The system consists of very simple redox reaction of magnetite as following procedure:
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Step 1: Chemical hydrogen storage (reduction of magnetite)Fe3O4 + 4H2 → 3Fe + 4H2O
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Step 2: Hydrogen recovery (water-splitting)3Fe + 4H2O → Fe3O4 + 4H2
To suppress the deactivation of iron oxide with redox cycles, many studies have been trying to improve the reactivity in the repeated redox cycling. Otsuka et al. [4], [5] and Takenaka and co-workers [8], [9] reported that the sintering during repeated cycles of Fe/Fe3O4 system was suppressed by an addition of Al, Mo or Ce and the rate of H2O decomposition in an oxidation step was remarkably increased by an addition of Rh or Ir. Urasaki et al. [11] reported that very small amounts of palladium or zirconia to Fe/Fe3O4 system resulted in enhancement of reduction and/or oxidation rates. Addition of one or more elements to Fe/Fe3O4 system resulted in marked enhancement of its activity during repeated cycles due to cooperative effects [4], [5], [11]. In other reaction systems using the redox reaction of metal oxides, we also found that the metal oxides with an additive of Zr or Al had a good regeneration ability for cyclic tests of the redox reaction [12], [13].
The objectives of this study are to develop and improve a medium for chemical hydrogen storage using redox cycles of iron oxide. Iron oxides modified with Pd, Pt, Ru, Rh, Al, Ce, Ti and Zr as additives were prepared by co-precipitation using urea method. Their redox behaviors during repeated cycles were characterized and investigated using temperature-programmed reaction (TPR), XRD and SEM analyses. The roles of a single addition and co-addition of additives were discussed in detail.
Section snippets
Experimental
The iron oxides which were modified with Pd, Pt, Rh, Ru, Al, Ce, Ti and Zr as additives were prepared by co-precipitation method using urea precipitant as described in previous work [4]. Iron nitrate enneahydrate (Waco, 99%) was used as the starting material for iron oxide and palladium(II) acetate (ACROS, 47.5%), platinum(II) chloride (Aldrich, 98%), rhodium(III) chloride (Aldrich, 99.98%), ruthenium(III) chloride hydrate (Aldrich, 99.98%), aluminum isopropoxide (Aldrich, 98%), ammonium
Pd, Pt, Rh and Ru additives
The H2-TPR patterns of iron oxides, which added with Pd, Pt, Rh and Ru, are exhibited in Fig. 1. As exhibited in Fig. 1(a), the TPR pattern of Fe–O/(n) sample showed two reduction steps, consisting of one peak in the range of ca. 620–700 K due to H2 reduction of Fe2O3 → Fe3O4 course and another peak in the next temperature range to 823 K due to H2 reduction of Fe3O4 → Fe course. For TPR patterns of second and third cycle, it was observed that the reduction peak due to Fe2O3 → Fe3O4 course disappeared
Conclusions
The redox behaviors of iron oxides, which were modified with Pd, Pt, Rh, Ru, Al, Ce, Ti and Zr as additives, were investigated using temperature-programmed reaction (TPR) technique. From the results, the following conclusions were drawn out:
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Pd, Pt, Rh and Ru additives have the effects on promoting the reduction and lowering the re-oxidation temperature of iron oxide. Meanwhile, Al, Ce, Ti and Zr additives played an important role in prevention of deactivation of iron oxide by repeated cycles.
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Acknowledgement
This research was performed for the Hydrogen Energy R&D Center, one of the 21st Century Frontier R&D Programs, funded by the Ministry of Science and Technology of Korea.
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