Synthesis of Cu-Rich, Al₂O₃-Stabilized Oxygen Carriers Using a Coprecipitation Technique: Redox and Carbon Formation Characteristics

Chemical looping combustion (CLC) is an emerging, new technology for carbon capture and storage (CCS). Copper-based oxygen carriers are of particular interest due to their high oxygen carrying capacity and reactivity, low tendency for carbon deposition, and exothermic reduction reactions. In this work, CuO-based and Al₂O₃-stabilized oxygen carriers with high CuO loadings were developed using a coprecipitation technique. The cyclic redox performance of the synthesized oxygen carriers was evaluated at 800 °C in a laboratory-scale fluidized bed reactor using a reducing atmosphere comprising 10 vol. % CH₄ and 90 vol. % N₂. The CuO content in the oxygen carrier was found to increase with the pH value at which the coprecipitation was performed. The oxygen carrying capacity of the oxygen carrier containing 87.8 wt % CuO was found to be high (5.5 mmol O₂/g oxygen carrier) and stable over 25 redox cycles. Increasing the CuO content further, i.e. > 90 wt %, resulted in materials which showed a decreasing oxygen carrying capacity with cycle number. It was also shown that the incorporation of K⁺ ions in the oxygen carrier can avoid the formation of the spinel CuAl₂O₄ and significantly reduce carbon deposition

Highly Efficient CO₂ Sorbents: Development of Synthetic, Calcium-Rich Dolomites

The reaction of CaO with CO₂ is a promising approach for separating CO₂ from hot flue gases. The main issue associated with the use of naturally occurring CaCO₃, that is, limestone, is the rapid decay of its CO₂ capture capacity over repeated cycles of carbonation and calcination. Interestingly, dolomite, a naturally occurring equimolar mixture of CaCO₃ and MgCO₃, possesses a CO₂ uptake that remains almost constant with cycle number. However, owing to the large quantity of MgCO₃ in dolomite, the total CO₂ uptake is comparatively small. Here, we report the development of a synthetic Ca-rich dolomite using a coprecipitation technique, which shows both a very high and a stable CO₂ uptake over repeated cycles of calcination and carbonation. To obtain such an excellent CO₂ uptake characteristic it was found to be crucial to mix the Ca²⁺ and Mg²⁺ on a molecular level, that is, within the crystalline lattice. For sorbents which were composed of mixtures of microscopic crystals of CaCO₃ and MgCO₃, a decay behavior similar to natural limestone was observed. After 15 cycles, the CO₂ uptake of the best sorbent was 0.51 g CO₂/g sorbent exceeding the CO₂ uptake of limestone by almost 100%

Redox-Driven Restructuring of FeMnZr-Oxygen Carriers Enhances the Purity and Yield of H₂ in a Chemical Looping Process

Chemical looping-based approaches allow for the production of high purity hydrogen from methane with an inherent separation of the coproduced carbon dioxide. In such a process, first methane is oxidized using lattice oxygen from a solid oxygen carrier. In the second half-cycle, the reduced oxygen carrier is reoxidized with steam to yield hydrogen. In this work, we report on the development of an iron-based oxygen carrier with an excellent redox stability and probe the synergistic effect of adding Mn₂O₃ and ZrO₂, leading to an enhancement of the reactivity of iron oxide with methane and increasing the hydrogen yield over multiple redox cycles. The promoting effects of Mn₂O₃ and ZrO₂ in the oxygen carrier were elucidated by combining TPR, STEM-EDX, reactivity tests, and conductivity measurements complemented by SIMS analysis. The addition of ZrO₂ promoted the oxidation reactivity of the material, whereas the addition of Mn₂O₃ accelerated the reduction of iron oxide. Conductivity measurements revealed that the addition of Mn₂O₃ lowers the activation energy for charge transfer, providing an explanation for the improved cyclic redox performance of the oxygen carriers. A redox-driven surface modification that results in the formation of an (Fe,Mn)O phase was found to retard effectively the cracking of methane on surface iron, leading to a high resistance to carbon deposition and in turn a high purity of the hydrogen produced. The observations reported here illustrate the importance of charge transfer characteristics for chemical looping based redox processes and open new perspectives for the design of more efficient oxygen carriers

ZrO₂‑Supported Fe₂O₃ for Chemical-Looping-Based Hydrogen Production: Effect of pH on Its Structure and Performance As Probed by X‑ray Absorption Spectroscopy and Electrical Conductivity Measurements

Chemical looping is a promising process to produce high purity H₂ while simultaneously capturing CO₂. The key requirement for this process is the availability of oxygen carriers that possess a high cyclic redox stability, resistance to carbon deposition, and thermal sintering. In this study, ZrO₂-supported Fe₂O₃-based oxygen carriers were developed using a coprecipitation technique. We assess in detail the influence of the key synthesis parameter, i.e., the pH value at which the precipitation was performed, on the morphological properties, chemical composition, local structure, and cyclic redox stability. The performance of the new oxygen carriers was compared to unsupported Fe₂O₃ and Al₂O₃-supported Fe₂O₃. A higher degree of disorder in the local structure of oxygen carriers precipitated at low pH values was confirmed by X-ray absorption spectroscopy (XAS) measurements. Electrical conductivity measurements showed that supporting Fe₂O₃ on ZrO₂ lowered significantly the activation energy for charge transport when compared to pure Fe₂O₃. In line with this observation, ZrO₂-supported oxygen carriers displayed a very high and stable H₂ yield over 15 redox cycles when precipitation was performed at pH > 5

<i>In Situ</i> XRD and Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy Unravel the Deactivation Mechanism of CaO-Based, Ca₃Al₂O₆‑Stabilized CO₂ Sorbents

CaO is an effective high temperature CO₂ sorbent that, however, suffers from a loss of its CO₂ absorption capacity upon cycling due to sintering. The cyclic CO₂ uptake of CaO-based sorbents is improved by Ca₃Al₂O₆ as a structural stabilizer. Nonetheless, the initially rather stable CO₂ uptake of Ca₃Al₂O₆-stabilized CaO yet starts to decay after around 10 cycles of CO₂ capture and sorbent regeneration, albeit at a significantly reduced rate compared to the unmodified reference material. Here, we show by a combined use of <i>in situ</i> XRD together with textural and morphological characterization techniques (SEM, STEM, and N₂ physisorption) and solid-state ²⁷Al NMR (in particular dynamic nuclear polarization surface enhanced NMR spectroscopy, DNP SENS) how microscopic changes trigger the sudden onset of deactivation of Ca₃Al₂O₆-stabilized CaO. After a certain number of CO₂ capture and regeneration cycles (approximately 10), Ca₃Al₂O₆ transformed into Ca₁₂Al₁₄O₃₃, followed by Al₂O₃ segregation and enrichment at the surface in the form of small nanoparticles. Al₂O₃ in such a form is not able to stabilize effectively the initially highly porous structure against thermal sintering, leading in turn to a reduced CO₂ uptake