Carbon Dioxide Capture and Chemical Looping

 

Alkali earth metal oxide-based CO2 sorbents

Despite an increasing awareness and concern about climate change, the CO2 concentration in the atmosphere is still increasing. Carbon dioxide capture and storage (CCS) is a technology that can contribute to the mitigation of climate change by removing CO2 from flue gas streams or the atmosphere and storing it in geological formations. As an alternative, the captured CO2 can be converted further into chemicals or fuels in a process that has been termed carbon dioxide capture and utilization (CCU). To reduce further the costs of CO2 capture, the development of solid sorbents that capture CO2 rapidly with full sorbent conversion and excellent cyclic stability, is critical. A practical class of solid CO2 sorbents are the alkaline earth metal oxides MgO and CaO that are in the research focus of the Laboratory of Energy Science and Engineering. These metal oxides capture and release CO2 at elevated temperatures according to:1

 MO + CO2  MCO3     (M = Ca, Mg)                                                                           (ΔH0298 K, MgO = –116.9 kJ mol–1; ΔH0298 K, CaO = –178 kJ mol–1)

Recent advances towards more effective CaO and MgO-based CO2 sorbents made in our laboratory include the fabrication of model structures of metal oxides via atomic layer deposition (ALD) of stabilizing films and the utilization of advanced spectroscopic techniques such as X-ray absorption spectroscopy (XAS) for a detailed characterization of CO2 sorbents under in situ conditions. For example, to probe whether the CO2 uptake of CaO-based sorbents is critically affected by the availability of excess pore volume we fabricated porous CaO-based materials through soft-templating,2 sol-gel or micelle-based evaporation induced self-assembly.3 To minimize the sintering-induced loss of pore volume, stabilizers were introduced into the CaO matrix including ALD deposition of Al2O3 or SiO2 films with a nearly atom level control of their thickness on the CaO surface.3 Our results indicate that mixing between the stabilizer and CaO has to occur on the nano- or even atom scale for effective stabilization. Yet maintaining such level of mixing has been a challenge and recent work using a combination of solid state nuclear magnetic resonance (ss-NMR) and dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP-SENS) (in collaboration with Prof. C. Copéret, ETH Zürich) indicate that de-mixing occurs over multiple CO2 capture and regeneration cycles (Fig. 1).3 Despite of the advances made by several groups working in this area, we still lack a fundamental understanding of the mechanisms controlling carbonation and deactivation and we also do not have effective approaches for structural stabilization and the acceleration of kinetics for CO2 capture. However, such an understanding is critical to develop more effective, yet practical sorbents.

 

Chemical looping

Besides our work on alkali earth metal oxide-based CO2 sorbents, our laboratory also explores emerging CO2 capture and storage (CCS) technologies that could reduce further the costs associated with the capture of CO2. These CCS approaches include chemical looping combustion (CLC) and chemical looping with oxygen uncoupling (CLOU).4 In CLC, a hydrocarbon fuel is combusted with lattice oxygen that is provided by a so-called oxygen carrier, typically a transition metal oxide. For example, the oxidation of methane (CH4) with oxygen provided by a metal oxide (MO) occurs as following:


CH4 + 4 MO → CO2 + 2 H2O + 4 M


Chemical looping with oxygen uncoupling (CLOU) is an extension of CLC to combust efficiently solid fuels. In CLOU a fuel is combusted with molecular oxygen that is derived from a metal oxide via a (thermally-driven) reduction reaction. An example, is the reduction of Cu(II)oxide to Cu(I)oxide:


4 CuO → 2 Cu2O + O2


In both processes, after the condensation of steam, a pure stream of CO2 is obtained. Owing to the high operating temperatures material sintering, leading to a rapid deactivation of the oxygen carriers, is a key challenge. In addition, carbon deposition, the formation of poorly reactive solid solutions and the poor reactivity of a number of metal oxides with methane can reduce significantly the CO2 capture performance of the oxygen carriers.
Recent advances towards more effective oxygen carriers have included the coupling of the methane reforming and reduction reactions on a particle level,5 the exsolution of dopants to improve the reactivity e.g. of iron oxide with methane,6 the deposition of a thin layers of Al2O3 via atomic layer deposition to improve the sintering resistance of oxygen carries and the introduction of promoters to reduce carbon deposition (Fig 2). For example, we could show that the rapid formation of metallic Ni or Cu through exsolution promotes the reducibility of Ca2Fe2O5 with CH4.6 It was also found that the reversible exsolution of Ni or Cu nanoparticles and their reincorporation in the Ca2Fe2O5 structure was key to avoid particle sintering and deactivation.

References

    [1] A.M. Kierzkowska, R. Pacciani, C.R. Müller, ChemSusChem 2013, 6, 1130.
    [2] M. Broda, C.R. Müller, Adv. Mater. 2012, 24, 3059; M.A. Naeem, A. Armutlulu, Q. Imtiaz, F. Donat, R. Schäublin, A. Kierzkowska, C.R. Müller, Nat. Comm. 2018, 9, 2408; A. Armutlulu, M.A. Naeem, H.J. Liu, S.M. Kim, A. Kierzkowska, A. Fedorov, C.R. Müller, Adv Mater. 2017, 29, 1702896.
    [3] S.M. Kim, W.C. Liao, A.M. Kierzkowska, T. Margossian, D. Hosseini, S. Yoon, M. Broda, C. Copéret, C.R. Müller, Chem. Mater. 2018, 30, 1344.
    [4] Q. Imtiaz, D. Hosseini, C.R. Müller, Energy Technol. 2013, 1, 633.
    [5] D. Hosseini, P.M. Abdala, F. Donat, S.M. Kim, C.R. Müller, Appl Cat. B 2019.
    [6] D. Hosseini, F. Donat, P.M. Abdala, S.M. Kim, A.M. Kierzkowska, C.R. Müller, ACS Appl. Mat. Interfaces 2019, 11, 18276.
JavaScript has been disabled in your browser