O2–CO2 Mixed Gas Production Using a Zr-Doped Cu-Based Oxygen Carrier

Experimental Study of Transition Metal-Doped Cu-Based Oxygen Carrier for Chemical Looping with Oxygen Uncoupling

The Cu-based oxygen carrier exhibits promising potential in the chemical looping with oxygen uncoupling (CLOU) process, wherein its performance is significantly influenced by the characteristics of oxygen release and cyclic reaction stability. In this work, a series of Cu-based oxygen carriers doped with transition metals (Ti, Y, Zr, and Ce) were synthesized by using the sol-gel combustion method and impregnation method. The microstructure, phase structure, O2 desorption, and cyclic reaction of Cu-based oxygen carriers were studied by experiments. The results show that (1) the doped element loading rate, specific surface area, average pore size, and pore volume of the oxygen carrier prepared by the sol-gel combustion method surpass those achieved through the impregnation method; (2) Ce doping formed a solid solution with CuO and CuAl2O4 without altering the main crystal phases, while Zr, Ti, and Y doping resulted in the formation of distinct phases; (3) the doped samples exhibit superior oxygen release performance compared to the undoped ones, with Ce-doped carriers demonstrating the highest level of performance; and (4) the cycling stability of Cu-based oxygen carriers is significantly enhanced by Ce doping. The aforementioned results collectively demonstrate the remarkable efficacy of Ce doping as a highly effective modification technique for Cu-based oxygen carriers. Furthermore, the sol-gel combustion method emerges as a superior method for preparing doped oxygen carriers.

Theoretical Study of the Reactive Mechanisms of Li-Doped Ni-Based Oxygen Carrier during Chemical Looping Combustion

Ni-based oxygen carriers (OCs) are considered promising materials in the chemical looping combustion (CLC) process. However, the reactivity of Ni-based OCs still offers the potential for further enhancement. In this work, the Li doping method has been employed for the modification of Ni-based OCs. The reactivity and microreaction mechanisms of different concentrations of Li-doped Ni-based OCs with CO in CLC are clarified using density functional theory (DFT) simulation. The structures, energy, and density of states are obtained through computational investigation of the reaction path in elementary reactions. The results show that (1) the adsorption energies of CO molecules on NiO surfaces with 4, 8, and 12% Li doping concentrations are -0.53, -0.48, and -0.54 eV, respectively, demonstrating an enhanced reactivity compared to that of pure NiO (-0.41 eV); (2) the calculation of the transition state indicates that the most favorable pathway for CO oxidation takes place on the surface of NiO with an 8% Li doping concentration, exhibiting the lowest energy barrier of 0.51 eV; and (3) the oxygen vacancy formation energies on the surface of NiO are 3.05, 2.30, and 2.10 eV for 4, 8, and 12% doping concentrations, respectively. Additionally, the decrease in oxygen vacancy formation energies exhibits a gradual decline with an increasing Li doping concentration. By comprehensive analysis, 8% is considered to be the optimal doping concentration of NiO for chemical looping combustion.

A Theoretical Study of the Oxygen Release Mechanisms of a Cu-Based Oxygen Carrier during Chemical Looping with Oxygen Uncoupling

The Cu-based oxygen carrier is a promising material in the chemical looping with oxygen uncoupling (CLOU) process, while its performance in the CLOU is significantly dependent on the oxygen release properties. However, the study of oxygen release mechanisms in CLOU is not comprehensive enough. In this work, the detailed oxygen release mechanisms of CuO(110) and CuO(111) are researched at an atomic level using the density functional theory (DFT) method, including the formation of O2, the desorption of O2 and the diffusion of O anion, as well as the analysis of the density of states. The results show that (1) the most favorable pathway for O2 formation and desorption occurs on the CuO(110) surface of O-terminated with energy barriers of 1.89 eV and 3.22 eV, respectively; (2) the most favorable pathway for O anion diffusion occurs in the CuO(110) slab with the lowest energy barrier of 0.24 eV; and (3) the total density of states for the O atoms in the CuO(110) slab shifts to a lower energy after an O vacancy formation. All of the above results clearly demonstrate that the CuO(110) surface plays a significantly important role in the oxygen release reaction, and the oxygen vacancy defect should be conducive to the reactivity of oxygen release in a Cu-based oxygen carrier.

Modification of Metal (Fe, Al) Doping on Reaction Properties of a NiO Oxygen Carrier with CO during Chemical Looping Combustion

Oxygen carriers can significantly enhance the performance of chemical looping combustion at low energy-cost CO₂ capture. Based on the density functional theory, a microscopic model of the metal Fe, Al-doped NiO oxygen carrier was established. The results indicate that the intermediate state energy and the reaction energy reduce due to electronic interaction of the Al-doped surface. With the progress of the reaction, the NiO-Al surface promotes the oxidation process of CO, indicating that the activity of the NiO surface enhanced, which is attributed to the electronic and steric effects of the Al-O structure. For the decomposition of CO on the OC surface, doping with other atoms is beneficial to suppress the carbon deposition, which is related to the steric hindrance caused by doping with other atoms. Besides, doping with iron and aluminum atoms is more conducive to the movement of OC bulk crystal lattice oxygen to the surface, thereby promoting subsequent reactions. Therefore, it is feasible to improve the reactivity of the Ni-based OC by doping metal Al, and its modification effect is closely related to the characteristics of the components.