Synthesis, characterization and stability performance of CoWO4 as an oxygen carrier under redox cycles towards syngas production

Abstract

A redox reaction scheme consisting of two steps as an alternate to the partial oxidation of methane (POX) for synthesis gas production is proposed. With the use of a thermodynamic analysis the CoWO4 spinel was introduced as a new oxygen carrier material, capable to be reduced with methane (15% CH4/Ar) and reoxidized with steam (30% H2O/Ar) to produce syngas and hydrogen in each redox step, respectively. H2 production is feasible during the regeneration of CoWO4 by oxidation of reduced species (Co and W) with steam. CoWO4 synthesis was performed using the solid state reaction. Characterization included: XRD, SEM and particle size determination. Redox-cycle performance was followed by TGA. Thermal stability of CoWO4 was tested by exposing the material to ten consecutive reduction/oxidation (H2/Ar, H2O/Ar) cycles at 850°C and after. Results indicate that CoWO4 showed no significant loss of activity. This stable behavior is associated to its crystalline structure, which allows the fixation of the active metals (Co + W) through the spinel structure, thus inhibiting particle nucleation and migration, which are the main causes of sintering after exposure to steam at high temperatures. The addition of Ni to CoWO4 as a POX catalyst produced an enhancement of the reduction step of 20% in metal oxide conversion with respect to CoWO4. Copyright © 2007 The Berkeley Electronic Press. All rights reserved.