Insight Into Oxygen Release/uptake Mechanism And CuO-support Interactions Of Cu-based Oxygen Carrier

CO2 emission in fossil fuel combustion has been accepted to be a major factor contributing to the climate change. Chemical-looping combustion(CLC) is a highly promising approach that allows for the inherent separation of CO2 with low cost and energy penalties. Chemical looping with oxygen uncoupling(CLOU) is a novel CLC process much suitable for solid fuels. Oxygen carrier(OC) is the key issue of CLOU. Although some high performance OCs have been obtained in massive trial-and-error experiments, a fundamental understanding of the oxygen release characteristics of the OC with or without supports is rare. A clear understanding of these characteristics can help rationalize the design of high performance OC for CLOU.Using periodical density functional theory(DFT) calculations and thermogravimetric analysis(TGA) experiments, this research first investigated the decomposition mechanisms of CuO, CuAl2O4 and CuFe2O4. Isothermal experiments showed that CuAl2O4 had different oxygen release features with the CuO. DFT calculation suggested that the surface reaction(including formation and desorption processes of O2) were the rate-limiting step for CuO decomposition. While for the CuAl2O4, O anion diffusion was also a rate-limiting step. The calculation can reasonable explain the experimental results. For the CuFe2O4, both surface reaction and O anion diffusion had high energy barrier, implying the low reactivity of CuFe2O4.Next, DFT calculation and temperature programmed decomposition(TPD) experiments were conducted to gain insight into the effect of CuAl2O4 on the OC performance in terms of resistance to sintering and reactivity. Strong interaction between the CuAl2O4 surface and CuO clusters were observed, which prevented the agglomeration of CuO clusters. DFT calculation suggested that the CuAl2O4 addition did not favor the reactivity of OC, which was consistent with the TPD experiments. Next, Cu-Ti, Cu-Zr and Cu-MgAl were studied using the same method. Based on the calculated adsorption energy of CuO cluster on different support, we obtained that the resistance to sintering increased in the order of Cu-Ti < Cu-CuAl < Cu-Zr < Cu-MgAl. It was agreed in general with the previous experimental researches. Based on the calculated energy barrier of CuO cluster decomposition on different support, we obtained that the reactivity increased in the order of Cu-Ti > Cu-Zr > Cu-CuAl > Cu-MgAl. It was agreed in general with the oxygen release rates obtained from the TPD experiments at different heating rates. It was found that the OC reactivity was in some extent contrary to the absorption energy order. Therefore, it is necessary to gain a compromise between the oxygen release energy barrier and the absorption energy to attain favourable reactivity and sintering resistance of the Cu-based OCs.Finally, using DFT calculation, this research studied the oxygen uptake mechanisms of Cu2 O, CuAlO2 and CuFeO2, especially the surface reaction of O2. It was concluded that the adsorbed O2 on Cu2 O was diffused to O3 f and O4 f, which resulted in the new Cu2 O surface was similar to that of the CuO(111). The surface reaction of O2 was exothermic for the three OCs, implying the surface reaction was not the rate-limiting step for oxygen uptake of these Cu-based OCs.