Catalyst-assisted chemical looping for CO2 conversion to CO

Abstract

A bifunctional 5% Ni/CeO2–Fe2O3 (1:1) sample was prepared for the transformation of CO2 to CO by catalyst-assisted chemical looping. Generation of CO from CO2 through CH4+CO2/CO2 redox cycles was investigated between 873K and 973K. In the reduction step, CH4+CO2 is converted over Ni into syngas, which then reduces the CeO2–Fe2O3 oxygen storage material, yielding CO2 and H2O. CO is produced upon re-oxidation of the oxygen storage material by CO2. The addition of Ni to CeO2–Fe2O3 has a beneficial effect upon the activity of the material during periodic cycling. The crystallographic structure of the sample was examined using time-resolved in situ X-ray diffraction under H2 reduction and CO2 oxidation. H2-TPR reduced the Fe2O3 phase to Fe3O4, FeO and Fe. From 600K onwards NiO reduction is initiated. Interaction between Ni and Fe leads to Ni3Fe alloy formation from 850K onwards. This alloy remains stable up to 873K during CO2 re-oxidation but is completely decomposed into Ni and Fe3O4 at 973K. This re-oxidation of iron follows a one-step pathway to Fe3O4 between 773K and 873K, but a two-step pathway above 873K. The methane reforming on the Ni-based catalyst is accompanied with formation and accumulation of surface carbon. This carbon is removed by oxidation with lattice oxygen from CeO2–Fe3O4 during or after re-oxidation with CO2.