Reactive and selective redox system of Ni(II)-ferrite for a two-step CO and H 2 production cycle from carbon and water

Abstract

A number of redox systems composed of carbon and bivalent metal-ferrites, MFe 2O 4(M = bivalent metal ions), were studied to find the most reactive and selective working materials for a thermochemical water-decomposition cycle combined with CO production from a carbon compound. Magnetite and Mg(II)-, Mn(II)-, Co(II)-, Ni(II)-, and Zn(II)-ferrites mixed with carbon powder have been screened for reactivity and selectivity in the CO-production step (first step) and subsequent water-decomposition step (second step). The Ni(II)-ferrite showed the most reactivity and selectivity for CO formation from carbon in the first step at temperatures above 700°C. The Ni(II)-ferrite could be completely reduced with carbon to the metallic phase of the NiFe alloy and Ni in the first step although only a small portion of magnetite was reduced to wustite under similar conditions. The ferrites reduced in the first step were oxidized with water vapor to generate H 2 in the second step. The highest conversion of H 2O to H 2 was obtained using the Ni(II)-ferrite. The total amount of evolved H 2 using Ni(II)-ferrite was 10 times larger than for the magnetite at 500°C. The processes could be repeated using the phase transition between Ni(II)-ferrite and NiFe alloy in the temperature range 700–800°C, with the highly efficient net reaction H 2O + C → H 2 + CO.