(622a) Synthesis and Redox Pathways of CuO-Al2O3 Oxygen Carriers for Chemical Looping Combustion

Fossil fuels are the dominating energy carriers for the electricity generation and transportation sectors. The combustion of fossil fuels accounts for approximately one third of the total global CO₂ emissions, therefore, it is imperative to find ways to capture CO₂ from the flue gases of fossil-fuel fired power plants, oil refineries and large industrial facilities at low costs. Chemical looping combustion (CLC) is an emerging carbon dioxide capture and storage (CSS) technology that offers an efficient and economical way to produce a pure stream of CO₂ and to reduce the emission of NO_x. CLC is a two stage process comprised of a reduction and an oxidation stage. In the reduction step, lattice oxygen of the oxygen carrier is used to oxidize a (hydro-)carbonaceous fuel, say CH₄, viz: CH₄ + 4CuO → 4Cu + CO₂ + 2H₂O. Subsequently, Cu is re-oxidized with air, 2Cu + O₂ → 2CuO, to close the cycle. The CLC process produces inherently a pure stream of CO₂ (after the condensation of steam). Consequently, the high costs associated with the capture of CO₂ from flue gases, mostly arising in the regeneration step of “conventional” CO₂ sorbents, are substantially reduced. Copper oxide is a very promising oxygen carrier for CLC owing to its (i) high oxygen carrying capacity, (ii) exothermic reduction reactions, (iii) low tendency for carbon deposition, and (iv) high reactivity for both the reduction and oxidation reactions. However, because of the low Tammann temperatures of CuO and Cu of only 558 °C and 405 °C, respectively, unsupported CuO is prone to agglomeration and thermal sintering. Hence, CuO is typically supported on a high melting point material, e.g. Al₂O₃.

The aim of this study was to investigate the synthesis steps and reaction pathways of Cu-based, Al₂O₃-supported, oxygen carriers. Here, Cu-rich (82 wt. % CuO) oxygen carriers were synthesized using a co-precipitation technique. A detailed characterization of the synthesized oxygen carriers with regards to their morphological properties, chemical composition and surface topography was carried out using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), attenuated total reflection Fourier transform infra-red (ATR-FTIR) spectroscopy, Raman spectroscopy, N₂ adsorption and force gauge measurements. The redox stability of the oxygen carriers was evaluated at 800 °C in a fluidized bed reactor using methane as the fuel.^1-2 In addition, in-situ X-ray absorption spectroscopy (XAS) was performed at 500 °C using H₂and air as the reducing and oxidizing gases, respectively, to probe the reduction and oxidation pathways of the new oxygen carrier.

Raman spectroscopy revealed that Al³⁺ was precipitated as gibbsite at a pH value of 8.5 and as boehmite at all other pH values investigated. Furthermore, the Raman spectra showed that Cu²⁺ precipitated out as gerhardtite at all pH values studied. The scanning electron micrographs showed that the surface morphology of the synthesized oxygen carriers was strongly influenced by the pH value at which the precipitation was performed. Furthermore, an Al-rich oxygen carrier was formed at pH = 3.8 owing to (i) an insufficient amount of NaOH to achieve complete precipitation of Cu²⁺ and Al³⁺ ions from the solution and (ii) the smaller solubility constant of Al(OH)₃ (K_sp = 4.6 ×10⁻³³)³ compared to Cu(OH)₂ (K_sp = 2.2 × 10⁻²⁰)³. On the other hand, the chemical composition of the oxygen carriers synthesized at pH values ≥ 5.5 was similar. All oxygen carriers showed a stable cyclic oxygen carrying capacity. XRD analysis revealed that CuO, present in the form of a mixed oxide (CuAl₂O₄) in the calcined oxygen carriers, is fully reducible. Therefore, the formation of CuAl₂O₄ did not reduce the oxygen carrying capacity of the synthesized oxygen carriers. With respect to reaction pathways, previous, low-temperature (200 – 300 °C) in-situ XAS measurements by Kim e. al. revealed that the reduction of commercial, pure CuO using a gas mixture containing 5 % H₂ (flow rate > 15 mL/min) proceeded via the direct CuO – Cu transition.⁴ In contrast, the preliminary in-situ XAS measurements reported here, indicate that at 500 °C the reduction of Al₂O₃-stabilized CuO proceeded via the sequential transition, i.e. CuO – Cu₂O – Cu. Also, using in-situ XAS measurements the induction period for the reduction reaction of co-precipitated CuO was found to be negligible at 500 °C. Furthermore, we found that under the operating conditions studied here, the Cu₂O intermediate did not fully reduce to Cu (even after a reaction time of 15 mins.). Similar observations, however at lower operating temperatures, were reported by Kim et al. who explained this observation by the formation of an impermeable film of Cu on Cu₂O grains. With regards to the oxidation reaction, it was found that at 500 °C also the oxidation of Cu/Al₂O₃ proceeded via the Cu₂O intermediate. Thus, in-situ XAS measurements demonstrated experimentally the formation of Cu₂O in both the reduction and oxidation step at operating conditions relevant for the chemical looping process. This finding has important consequences with regards to the numerical modeling of the chemical looping process.

[1] Q. Imtiaz, A. M. Kierzkowska and C. R. Müller, ChemSusChem 2012, DOI: 10.1002/cssc.201100694

[2] Q. Imtiaz, A. M. Kierzkowska, M. Broda and C. R. Müller, Environ. Sci. Technol. 2012, 46, 3561.

[3] R. W. Clark and J. M. Bonicamp, J. Chem. Ed. 1998, 75, 1182.

[4] J. Y. Kim, J. A. Rodriguez, J. C. Hanson, A. I. Frenkel and R. L. Lee. J. Am. Chem. Soc. 2003, 125, 10684.