Production of pure hydrogen from methane mediated by the redox of Ni- and Cr-added iron oxides

Methods for the formation of pure hydrogen from methane were proposed based on the reduction of Fe3O4 with methane into iron metal followed by the oxidation of iron metal with water vapor into Fe3O4. Iron oxides without additives were deactivated quickly for the redox reaction due to the sintering. Addition of Cr cations to iron oxides prevented the sintering of iron species during the redox, but the reduction with methane of iron oxides containing Cr cations needed high temperatures (1023 K). Addition of Ni to iron oxides enhanced the reduction with methane and the subsequent oxidation with water vapor at low temperatures, but Ni species promoted the sintering of iron species during the redox. In contrast, the iron oxides containing both Ni and Cr species (denoted as Ni-Cr-FeOx) could produce pure hydrogen repeatedly with high reproducibility through the redox at temperatures < 923 K. XANES and EXAFS studies showed that Ni species in Ni-Cr-FeOx were present as Ni-Fe alloys after the reduction with methane, whereas they were present as Ni metal crystallites after the oxidation with water vapor. Cr species in Ni-Cr-FeOx were always stabilized as octahedral Cr3+O6 on the B sites in the ferrites CrxFe3-x O-x(4) during the redox reactions. (C) 2004 Elsevier Inc. All rights reserved.