Storage and supply of hydrogen by means of the redox of the iron oxides modified with Mo and Rh species
Introduction
The redox property of a metal oxide is one of the factors which determine its catalytic activity and selectivity for a catalytic reaction. Frequently, various foreign metals or metal oxides, which are often termed as promoters, are added into the metal oxide catalysts in order to improve their redox performances [1], [2], [3]. In general, the surface areas of metal oxides decrease due to the sintering when they are used continuously in the catalytic reactions. The promoters sometimes prevent the sintering of the host metal oxide catalysts [4], [5], [6]. Therefore, the promoters are indispensable for the design of the catalysts for many catalytic reactions.
Previously, we have proposed a new method for the storage and supply of pure hydrogen by applying the redox between Fe3O4 and iron metal [7]: Fe3O4 + 4H2 ⇆ 3Fe + 4H2O. Fe3O4 is reduced with hydrogen into iron metal by eliminating water from the system and subsequently the metal is oxidized with water vapor into Fe3O4 to form hydrogen. Hydrogen is stored chemically as Fe metal in this method. By means of this method, 1 mol of Fe can store and regenerate 1.33 mol of H2, which corresponds to 4.8 wt% of Fe metal. However, iron oxide sample without any promoters was deactivated quickly for the hydrogen formation through the oxidation of iron metal with water vapor due to the sintering of Fe3O4 and/or iron metal [8]. In addition, the redox reaction of iron oxide sample without any promoters required relatively high temperatures (). It is desirable to perform the redox reactions of iron oxide samples at temperatures as low as possible from an economical viewpoint. Thus, the redox performances of the iron oxide samples added with various metal species were investigated [8], [9]. Iron oxides added with Mo cations could form hydrogen repeatedly through the redox, while iron oxide without any promoters was deactivated quickly for the redox. The addition of Rh into iron oxides enhanced hydrogen formation through the oxidation of iron metal with water vapor at low temperatures. However, the role of these promoters on the redox of iron oxides has been unclear. The role of these promoters on the redox should be investigated on the bases of the local structures and the electronic states of metal species added into the iron oxides.
In the present study, the local structures and the electronic states of Rh and Mo species added into the iron oxide samples were investigated by Rh and Mo K-edge XANES and EXAFS. On the bases of these results, the roles of Rh and Mo species added into the iron oxide samples on the redox reactions between Fe3O4 and Fe metal will be discussed.
Section snippets
Experimental
Iron oxide without any promoters (denoted as FeOx hereafter) was prepared by precipitation of Fe(OH)3 from an aqueous solution of Fe(NO3)3 by using hydrolysis of urea at 363 K. The iron oxides added with Rh and/or Mo species (denoted as Rh–FeOx, Mo–FeOx and Rh–Mo–FeOx hereafter) were prepared by coprecipitation of the corresponding metal hydroxides from mixed aqueous solutions containing these metal cations by using hydrolysis of urea at 363 K. RhCl3 and (NH4)6Mo7O24 were used as metal sources.
Redox of iron oxides
The redox performances of FeOx, Rh–FeOx, Mo–FeOx, and Rh–Mo–FeOx were examined. For the reduction of these samples, hydrogen was contacted with these samples at 573 K and the temperatures at the reactor were increased to 823 K. After the reduction, the reduced samples were contacted with water vapor at 373 K and the temperature increased to 873 K. Five redox cycles were repeated for all the samples except for Rh–FeOx. Fig. 1 shows change of the formation rate of hydrogen as a function of
Conclusion
We concluded as follows based on the results described above:
- 1.
The addition of Rh species to iron oxides enhanced the formation of hydrogen at low temperatures through the oxidation of iron metal with water vapor. The addition of Rh to the iron oxide samples decreased apparent activation energy for the hydrogen formation. However, Rh species in iron oxides promoted sintering of iron species during the redox. The addition of Mo cations to Rh–FeOx prevented the sintering of iron species during the
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