(459c) Influence of Percolation Threshold on the Redox Performance of CeO2-Supported Cu-Based Materials

A mid-term solution to stabilize the concentration of CO₂ in the atmosphere is the implementation of CO₂ capture and storage technologies (CCS). Chemical looping combustion (CLC) is a promising CCS technology and conceptually linked to oxy-fuel combustion. In CLC, the combustion of a hydrocarbon takes place in the presence of an oxygen carrier, typically a transition metal oxide. In CLC, the oxygen required for the combustion of a hydrocarbon comes from an oxygen carrier, viz.

C_nH_2m (g) + (2n + m)Me_xO_y (s) → nCO₂ (g) + mH₂O (g) + (2n+m)Me_xO_y-1 (s)

Desorption of the reaction product(s) generates an oxygen vacancy on the surface which is filled subsequently by oxygen from the bulk. Therefore, the CLC performance depends critically on the activation energy for transport of ions and electrons through the oxygen carrier. Cu is a very attractive material for CLC owing to its (i) high oxygen carrying capacity of 0.25 g O₂/g Cu and (ii) low tendency for carbon deposition. However, Cu has one main disadvantage, i.e. a low Tammann temperature of 405 °C and, thus, requires stabilization by a high Tammann temperature support e.g. CeO₂. The advantage of using CeO₂ as a support over other conventional ceramics (e.g. MgAl₂O₄) is that CeO₂-based cermets can contribute to the storage and release of oxygen via the Ce⁴⁺ - Ce³⁺ transition. Here, we investigate the influence of the percolation threshold on the redox kinetics of CeO₂-supported Cu. We used a co-precipitation technique to synthesize CeO₂-stabilized, Cu-based materials containing 20 - 60 wt. % Cu. The synthesized oxygen carriers were characterized using 4-point conductivity measurements, energy dispersive X-ray (EDX) spectroscopy, temperature programmed reduction (TPR) and scanning electron microscopy (SEM). The cyclic redox stability of the synthesized materials was evaluated in a thermo-gravimetric analyzer (TGA) at 900 °C using 10 vol. % H₂ in N₂ as the fuel and air for re-oxidation.

For Cu-rich materials, EDX mapping revealed the presence of CeO₂ islands in a continuous Cu network, therefore, the transport of oxygen ions and electrons predominately occurred via Cu/CuO conduction pathways. Below the percolation threshold of Cu, CeO₂ formed a continuous network containing CuO islands. The conductivity measurements showed that in this regime the conduction of electrons and ions occurred through CeO₂ grains. Moreover, conductivity and performance measurements showed that the activation energy for charge transport through the material increased with decreasing Cu content, which in turn decreased the rate of oxidation of the material. Owing to the low activation energy for charge transport and fast rate of oxidation, the material containing 60 wt. % Cu was identified as a promising material for CLC.