Journal of Catalysis, Vol.228, No.1, 66-74, 2004
Storage and supply of hydrogen by means of the redox of the iron oxides modified with Mo and Rh species
The redox performance of iron oxides containing Mo and/or Rh species (Fe3O4 + 4H(2) reversible arrow 3Fe + 4H(2)O) was investigated. The redox cycle of the reduction of Fe3O4 with hydrogen and the subsequent oxidation of iron metal with water vapor can be applied for the storage and supply of pure hydrogen. Addition of Rh to the iron oxide samples improved the reactivity for the redox at a low temperature. The addition of Rh to iron oxide samples decreased apparent activation energy for the oxidation of iron metal with water vapor. However, Rh species added to iron oxides promoted sintering of iron species during the redox. The addition of Mo cations to iron oxides containing Rh prevented the sintering of iron species during the redox. Therefore, iron oxides containing both Rh and Mo species could form hydrogen repeatedly through the redox. The local structures of Mo and Rh species added to the iron oxide samples were investigated by Mo and Rh K-edge XANES and EXAFS. Mo cations added to the iron oxide samples were always stabilized in the ferrites MoxFe3-xO4 during the repeated redox reactions. Rh species added to the iron oxide samples were present as Rh-Fe alloys after the reduction with hydrogen, while they were present as Rh metal after the oxidation with H2O. (C) 2004 Elsevier Inc. All rights reserved.