Effect of additives on redox behavior of iron oxide for chemical hydrogen storage

doi:10.1016/j.jiec.2007.10.003

Journal of Industrial and Engineering Chemistry

Volume 14, Issue 2, March 2008, Pages 252-260

https://doi.org/10.1016/j.jiec.2007.10.003 Get rights and content

Abstract

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. The modified iron oxides were prepared by co-precipitation method using urea precipitant. The role of additives was also examined using XRD and SEM analysis in detail. As a result, Pd, Pt, Rh and Ru additives have an effect on promoting the reduction and lowering the re-oxidation temperature of iron oxide. Especially, it is revealed that the effect of Rh species on lowering the reduction temperature is attributed to decrease of activation energy for H₂ reduction according to Fe₂O₃ → Fe₃O₄ course. Meanwhile, Al, Ce, Ti and Zr additives played an important role in prevention of deactivation of iron oxide by repeated redox cycles. Redox performances of iron oxides were also enhanced due to cooperative effects by co-addition of Rh and another species such as Al, Ce and Zr. Finally, Fe–O/(Rh, Ce, Zr) sample exhibited good performance for H₂ evolution by water-splitting through synergistic effect of component additives.

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:

•
Step 1: Chemical hydrogen storage (reduction of magnetite)Fe₃O₄ + 4H₂ → 3Fe + 4H₂O
•
Step 2: Hydrogen recovery (water-splitting)3Fe + 4H₂O → Fe₃O₄ + 4H₂

The first step (Step 1) is reduction of iron oxide with hydrogen, and the second step is subsequent oxidation with steam to release equivalent hydrogen. Through the reaction, the amounts of hydrogen storage, theoretically, are calculated at 4.8 wt.%/g-Fe [4]. High quality hydrogen without carbon monoxide is produced from this reaction. In addition, the oxidation temperature with water vapor for hydrogen production was preferred as low as possible (<573 K) in order to put this technology to practical uses such as a polymer electrolyte fuel cell. Moreover, it is well known that iron oxide without any metal additives could not be applied in the repeated cycles because of deactivation caused by sintering of the reduced Fe metals [4], [5]. Therefore, the additives must have the effects on prevention of the deactivation as well as lowering the water-splitting temperature of iron oxide [4], [5], [8], [9], [10].

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

•
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.
•

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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Effect of additives on redox behavior of iron oxide for chemical hydrogen storage

Abstract

Introduction

Section snippets

Experimental

Pd, Pt, Rh and Ru additives

Conclusions

Acknowledgement

Int. J. Hydrogen Energy

Int. J. Hydrogen Energy

Mater. Sci. Eng. B

Int. J. Hydrogen Energy

J. Power Sources

J. Power Sources

J. Power Sources

J. Catal.