Simulation study on modification of reaction performance for ferrite oxygen carrier based on doping with K3FeO4
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摘要: 本研究以密度泛函理论为基础,通过态密度、吸附能和活化能等电子结构性质,研究尖晶石结构的K3FeO4对Fe基载氧体反应性能的影响。结果表明,K3FeO4负载到α-Fe2O3(001)表面后,α-Fe2O3(001)表面微观电子结构发生改变,表面的Fe–O键长伸长,O-p轨道电子朝更高能级方向跃迁,氧原子电子活性提高。负载后,在三个晶格氧位处,CO与表面晶格氧反应的能垒均表现出降低趋势。这是因为负载K3FeO4能够提高表面氧原子活性,Fe–O键伸长使得断键更加容易,所需能量更小;此外,CO与K3FeO4中活性较强的氧原子成键,也与O2位原子形成新的C−O键,以双齿碳酸盐形式吸附在表面α-Fe2O3(001),进而释放并生成CO2。Abstract: Developing the oxygen carriers with large oxygen carrying capacity, high reactivity, and strong cycle stability is one of the research focuses in the chemical looping combustion technology. In this study, the effect of spinel-structured K3FeO4 on the reactivity of Fe-based oxygen carrier was investigated based on the density functional theory involving the electronic structural properties such as the density of states, adsorption energy, and activation energy. The results show that when the K3FeO4 is loaded on the α-Fe2O3(001) surface, the microscopic electronic structure of α-Fe2O3(001) surface is changed, the Fe–O bond on the surface is elongated, the O-p orbital electrons transition to a higher energy level, and the electron activity of oxygen atom is improved. The energy barriers of CO reaction with the surface lattice oxygen show a decreasing trend at the three lattice oxygen sites after the loading of K3FeO4 which can improve the activity of surface oxygen atoms and make the breakage of Fe–O bond via elongation easier with less energy required. In addition, CO can bond with the more active oxygen atom in K3FeO4, and also can combine with the O2 atom to form a new C–O bond, by which CO is adsorbed on the surface in the form of bidentate carbonate that can be decomposed and released as CO2.
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表 1 结构优化和实验得到的α-Fe2O3晶体结构参数
Table 1 Structure optimization and experimental parameters of α-Fe2O3 crystal structure
Structure parameter a = b/nm c/nm α = β/(°) γ/(°) EXPa 0.5035 1.3720 90.0 120.0 GGA/PBE 0.5226 1.4067 90.0 120.0 Relative deviation/% 0.380 0.252 0.00 0.00 a EXP is the abbreviation of the experimental values which are from the Materials Studio database 表 2 K3FeO4团簇负载前后的CO几何参数
Table 2 Geometric parameters of CO before and after loading the K3FeO4 cluster
Position O–CO/nm C–O/nm ∆Eads/eV before loading after loading before loading after loading before loading after loading O1 0.3702 0.3298 0.1155 0.1162 –0.61 –0.73 O2 0.2792 0.1380 0.1159 0.1260 –0.11 –3.57 O3 0.2889 0.2832 0.1155 0.1159 –0.67 –0.81 -
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