(140c) The Role of Support in Increasing Agglomeration Resistance of Cu-Based Oxygen Carriers

Chemical looping combustion (CLC) is a promising alternative to the conventional, i.e. combustion based, fossil fuel conversion processes. In CLC, a solid oxygen carrier is used to transfer oxygen from air to a carbonaceous fuel. This indirect combustion route allows for effective CO₂ capture since a sequestrable stream of CO₂ is inherently produced without any need of energy intensive CO₂ separation step. From a thermodynamic point of view, CuO is arguably the most promising oxygen carrier candidate for CLC. However, the main challenge associated with the use of CuO for CLC is its structural instability as the Tammann temperatures of Cu (405 ºC), Cu₂O (481 ºC) and CuO (526 ºC) are lower than the typical operating temperatures of the CLC process. Thus, irreversible microstructural changes are expected for CuO-based oxygen carriers during CLC operation. To avoid the thermal sintering and agglomeration of CuO, it is commonly stabilized by a high Tammann temperature ceramic, e.g. Al₂O₃, MgAl₂O₄, MgO or ZrO₂. However, the role of support in suppressing agglomeration remains unclear to date because a high Tamman temperature of the support does not always ensure a high resistance to agglomeration. For example, the Tammann temperatures of ZrO₂ and MgAl₂O₄ are 1214 ºC and 929 ºC, respectively. Gayán et al.¹ identified that compared to MgAl₂O₄ (40 wt. %) a higher weight fraction of ZrO₂ (60 wt. %) was required to manufacture agglomeration-resistant oxygen carriers. In this work, we investigate the mechanism through which a support decreases the agglomeration tendency of Cu/CuO. For this, well-defined surfaces comprising thin films of Cu/CuO and a support (Al₂O₃, Y₂O₃ ZrO₂, MgO, SiO₂) were prepared via magnetron sputtering. The cyclic redox stability of the synthesized materials was evaluated in a thermo-gravimetric analyser (TGA) at 400 °C or 900 °C using 10 vol. % H₂ in N₂as the fuel and air for re-oxidation. Energy dispersive X-ray (EDX) spectroscopy on the cross-sections of the thin films revealed that Cu atoms have the tendency to diffuse outward through the films of the support material during the redox reactions. The support that inhibits the outward movement of Cu/CuO is postulated to provide the highest agglomeration resistance.

1. Gayán P, Adánez-Rubio I, Abad A, de Diego LF, García-Labiano F, Adánez J. Fuel. 2012;96:226-38.