纳米CeO2-xNx固溶体的光谱特征对其催化性能影响研究

二氧化铈(CeO₂)具有储量丰富，价格低廉，催化性能优异等特性而得到广泛应用。通过在其晶格中掺杂其他离子制得CeO₂固溶体，可以进一步调控CeO₂的晶格大小，增加晶格缺陷浓度，从而有效提高催化性能。目前研究较多的掺杂离子多为金属阳离子，而对非金属阴离子掺杂的研究尚有待深入探索。本文以CO(NH₂)₂为N源，采用水热法合成不同N掺杂浓度的纳米CeO_2-xN_x固溶体(x=0.00，0.05，0.10，0.15，0.20)，系统对固溶体的微观结构及光谱特征进行表征。X射线衍射(XRD)结果表明，所有掺杂浓度的CeO_2-xN_x固溶体均呈萤石立方单相结构。与纯CeO₂相比，N含量为0.05时样品的晶胞参数显著增大，而随掺杂浓度的进一步增加，晶胞参数又呈现出逐渐减小的趋势。拉曼(Raman)测试表明，N掺杂样品的F_2g振动模式峰向高波数移动，其原因是由于当N3-取代部分O2-后，Ce4+周围出现Ce—N键，Ce—N键长因静电引力变强而缩短，从而引起峰位的移动。通过紫外可见吸收光谱(UV-Vis)分析掺杂所引起样品电子跃迁状态的改变，发现N元素的掺杂使CeO₂在可见光区域具有了吸光性能，CeO_2-xN_x固溶体的能隙明显减小，这是由于N(2p)与O(2p)的电子轨道发生交互作用而形成中间能级，使得电子跃迁所需能量降低，从而引起能隙的红移。荧光光谱(PL)测试表明，发射峰强度随N掺杂浓度的增大而增大，其原因一方面是由于N掺杂会引起晶格缺陷及氧空位比例的提升，发生带间跃迁的几率变大，进而提高发射峰的相对强度；另一方面，N的掺杂在价带O(2p)与导带Ce(4f)间形成中间能带，同样会导致发射峰变强。为表征纳米固溶体的催化特性，分别选取N掺杂量最小的CeO_1.95N_0.05与N掺杂量最高的CeO_1.80N_0.20以及纯CeO₂作为典型催化剂，采用球磨法制备Mg₂Ni/Ni/CeO_2-xN_x复合材料，系统分析了复合材料电极的储氢动力学性能。交流阻抗(EIS)测试发现，催化剂可以有效提高储氢合金的表面电荷转移活性，N掺杂量越高，CeO₂基固溶体的催化活性越强；动电位极化曲线测试表明，掺杂催化剂也能显著提高H原子在合金内部的扩散速率，且CeO_1.95N_0.05较CeO_1.80N_0.20具有更好的催化活性。催化机理主要从催化剂的微观结构及光谱特征进行分析，如前所述，随着N含量的提高，CeO₂固溶体晶格中的氧空位比例增大，晶格畸变程度提高，N的掺杂还使固溶体的电子跃迁能隙降低，从而有利于电子在合金表面的迁移；同时，纳米材料的晶粒尺寸越小，表明晶粒表面缺陷比例越大，说明催化剂的活性增强，因此表现为N掺杂浓度越高，复合材料电极交流阻抗弧半径的越小，即CeO_1.80N_0.20可以更加有效提高复合材料的表面活性；另一方面，若催化剂的晶胞体积增大，可使H原子在穿过材料表面的传输过程中具有更大的空间，由于CeO_1.95N_0.05的晶胞参数大于CeO_1.80N_0.20催化剂，故H原子通过催化剂进入合金内部的传输更加容易。H原子在合金内部的扩散速率与催化剂的晶胞参数或晶胞体积的大小密切相关。

Research on the Influence of Spectrum Characteristics on the Catalysis Effect of Nanosized CeO2-xNx Solid Solutions

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 O2- was substituted by N3-. 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 CeO₂ were 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.