纳米CeO2-xNx固溶体的光谱特征对其催化性能影响研究

doi:10.3964/j.issn.1000-0593(2018)10-3192-07

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Abstract CeO₂ has been widely used because of its abundant resources on earth, price advantages and excellent catalysis properties in many fields. The lattice size and the concentration of the lattice defects of CeO₂, which can be deemed as the key features to improve the catalysis properties, can be adjusted by doping foreign ions to form CeO₂ based solid solution. At present, numerous researches mainly focus on doping metal cations into the lattice of CeO₂, while introducing non-metal anions is still under exploration. In this paper, nanosized CeO_2-xN_x solid solutions with different N doped contents (x=0.00, 0.05, 0.10, 0.15, 0.20) were synthesized by using CO(NH₂)₂ as the N source via hydrothermal method. In addition, the microstructure and spectral characteristics of the solid solutions were analyzed systematically. The XRD results showed that all of the CeO_2-xN_x solid solutions exhibited cubic fluorite single phase structure. Compared with the pure CeO₂, the cell parameters of the sample with N 0.05 increased obviously, while it decreased gradually with the further increasing N content. The Raman spectrum indicated that the vibration mode of F_2g peak shift to higher wavenumbers. This could ascribe to the enhanced electrostatic attraction of Ce—N with the shorten bond length, which was formed in the lattice of CeO₂ when the O^2- was substituted by N^3-. The change of the electron transition state of the samples was illustrated by UV-Vis spectra. It was found that the doping of N element into the CeO₂ gives rise to the absorption in the visible light region, and the band gap energies decreased obviously. It could be explained that the formed intermediate energy level, which was caused by the interaction effect of N(2p) and O(2p) electron orbits, induced the decreased energy of the electron transition. The photoluminescence spectra indicated that the intensity of the emission peak was enhanced by increasing the N doped content. This could be illustrated from two aspects. On the one hand, the promotion of the concentrations of the lattice defects and the oxygen vacancies precipitate the increased rate of the transition between the bands, and then improve the relative intensity of the emission peak; on the other hand, the intermediate energy level formed between the valence band of O(2p) and the conduction band of Ce(4f) because of the introduction of N element also resulted in the strength of the emission peak. In order to characterize the catalysis properties of the nanosized solid solutions, the sample of CeO_1.95N_0.05 with the minimum N doped content, the sample of CeO_1.80N_0.20 with the highest N content and the pure CeO₂were chosen as the typical catalysts to synthesize the Mg₂Ni/Ni/CeO_2-xN_x composites via the ball milling method. The cell kinetic properties of the composites were measured systematically. The electrochemical impedance spectrum (EIS) test found that the solid solutions catalysts could enhance the charge transfer abilities on the surface of the Mg₂Ni hydrogen storage alloy electrodes effectively. And the more the N content was doped, the higher the catalysis activity of the CeO₂-based solid solutions showed. The potentiodynamic polarization curves measurement displayed that the diffusion rates of the H atom in the bulk of Mg₂Ni were also improved by adding the doped catalysts, and the catalysts of CeO_1.95N_0.05 has better catalysis effect than that of CeO_1.80N_0.20. The catalysis mechanism of the solid solutions was investigated from the point of the microstructure and the spectra features of the nanosized catalysts. As discussed above, it was found that the concentration of the oxygen vacancies and the degree of distortion of the lattice were increased by improving the N content, and the band gap energies of the solid solutions were decreased by N doping, which made the catalysts in favor of the electrons exchange interactions on the alloy surface. Meanwhile, the more refining of the crystalline size indicated the higher content defects on the particle surface, and further illustrated the improvement of the effect of the catalysts. Thus, these features of the catalysts could be used to explain why the catalysts with higher N content, the smaller radius of the AC impedance of the composites electrode showed. Or in other words, the catalysts of CeO_1.80N_0.20 could enhance the surface activity of the composites more obviously. What’s more, the enlargement of the cell volume of the catalyst would provide larger space for the H atoms when they were moving across the surface of composites. Under this condition, the transition of H atoms became easier, for the composite with CeO_1.95N_0.05 has larger cell parameter than CeO_1.80N_0.20. Therefore, diffusion rates of H atoms in the bulk of the alloys were closely related to the cell volumes of the catalysts.

Key words：CeO_2-xN_x solid solutions; N doped; Hydrothermal; Catalysis; Hydrogen storage kinetic

Received: 2017-10-04 Accepted: 2018-02-11

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2018)10-3192-07 OR https://www.gpxygpfx.com/EN/Y2018/V38/I10/3192