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The enhancement of H2O on the carbonation reaction of CaO with CO2 is now widely recognized in the calcium-looping systems. However, the microscopic origins of steam-enhanced reactions remain unclear. A new insight into this issue from the atomic level is provided. We performed molecular dynamics (MD) simulations using a recently developed ReaxFF reactive force field to study the role of H2O on the carbonation reaction of CaO for enhancing CO2 capture. First, the effects of H2O on the carbonation reaction of CaO with CO2 were investigated by MD simulations combined with thermogravimetric analysis (TGA) experiments. Our calculation results well-supported by the TGA experiments showed that H2O just enhances CaO carbonation at the diffusion-controlled stage, whereas there is little influence on the kinetic stage. Then, we analyzed the properties of ion/gas diffusion and the solid product layer to deeply understand the role of H2O in the diffusion-controlled stage. It was found that the ion/gas diffusion could be significantly improved by H2O. In addition, H2O promotes the formation and growth of the solid product layer. We believed that it is attributed to the following facts. The existence of OH derived from H2O dissociation promotes CO3 2– formation and increases the thickness of the CaCO3 product layer. The protons can penetrate toward the center of the CaO particle by forming new hydroxyl groups with the solid oxygen, thereby destroying the reactant structure and altering the reaction activity, which leads to improved carbonation.

Chemical looping hydrogen (CLH) production, or hydrogen production combined with CO2 capture, is becoming an emerging alternative technology that has attracted much attention. Iron-based oxygen carriers were most widely used in CLH. In this study, bi-active component composite metal oxides with Cu-doped Fe2O3/Al2O3 were synthesized. The oxygen carriers were characterized using different methods such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET), and their redox and cycle properties in the CLH process were investigated. The results showed that copper doping enhanced the activities of iron-based oxygen carriers, but excessive copper would reduce their reactivity due to melting and agglomeration. Hydrogen temperature-programmed reduction (H2-TPR) analysis, fixed-bed experiments, and density functional theory (DFT) calculations all confirmed that Cu promoted the deep reduction of Fe2O3. Among them, the oxygen carrier doped with 1 wt % CuO was the most suitable material for CLH, where H2 yield was the highest and sustained in a high and stable level in multiple redox cycles.

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