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二氧化钚(PuO2)作为一种重要的核燃料材料, 表面特性直接影响放射性元素的稳定性和迁移行为, 在能源储存领域收到广泛关注. 本文通过第一性原理方法研究水分子在二氧化钚(111)和(110)表面的吸附行为以及氧空位过量电子对这些表面的影响. 模拟表明, PuO2(111)表面比(110)表面表现出更高的稳定性, 具有更高的氧空位形成能. 在化学计量的PuO2(111)和(110)表面上, 水分子的解离吸附构型是最稳定的. 利用轻推弹性带方法, 研究发现在PuO2(111)和(110)表面上, 第1个氢原子的解离仅需0.11 eV和0.008 eV的能垒, 而第2步的完全解离则需要更高的能垒, 分别为0.85 eV和1.02 eV. 在还原的PuO2(110)表面存在氧空位的情况下, 可以促进水分子解离成位于氧空位上方的羟基和与表面氧原子结合的氢原子; 在一定条件下, 克服3.31 eV的能垒, 2个氢原子即可形成H2, 在PuO2(111)过氢表面产生H2能垒下降到1.92 eV. 本文研究对于改进核燃料储存技术、延长储存寿命和降低潜在风险具有重要的实际意义.Plutonium dioxide, as one of the primary materials for nuclear fuel, serves as a critical component in fast neutron reactor fuel and mixed oxide (MOX) fuel due to its distinctive physical and chemical properties. It can significantly enhance the utilization efficiency of uranium and diminish the demand for natural uranium resources. Moreover, plutonium dioxide constitutes an essential component of spent nuclear fuel. However, during long-term storage, oxygen vacancies on its surface can facilitate hydrogen release under the influence of water molecules, thereby posing potential risks to nuclear safety. Therefore, it is crucial to have a deep understanding of the interaction mechanism between water molecules and the plutonium dioxide surface. Such insights provide valuable theoretical guidance for ensuring the safe storage of spent nuclear fuel., The adsorption behavior of H2O molecules on the PuO2 (111) and (110) surfaces, as well as the effects of oxygen vacancies and excess electrons on these surfaces, is investigated numerically based on the first-principles calculations in this work. The simulation results show that the PuO2 (111) surface is very stable compared with the PuO2 (110) surface, indicating that PuO2 (110) is more prone to oxygen vacancies. For the adsorption of water molecules on PuO2 (111) and (110) surfaces, the plutonium atom vertex site is identified as the only stable adsorption site, with one hydrogen atom of the water molecule preferentially bonding to a surface oxygen atom. Due to the higher reactivity of the PuO2 (110) surface than that of the stoichiometric PuO2 (111) surface, water molecules exhibit molecular adsorption configurations on the latter, while dissociative adsorption configurations are favored on the former. Using the CI-NEB method, the energy barriers for the dissociation of the first hydrogen atom on stoichiometric surfaces of PuO2 (111) and (110) are determined to be 0.11 eV and 0.008 eV, respectively. In contrast, the energy barriers for complete dissociation are 0.85 eV and 1.02 eV, respectively, which are significantly higher. For reduced PuO2 (111) surfaces containing surface oxygen vacancies, the energy barrier for H2 production via water decomposition is calculated to be 3.31 eV. On the over-hydrogenated PuO2 (111) surface, the energy barrier for H2 production decreases markedly to 1.92 eV, providing theoretical insights into the mechanism of hydrogen release during nuclear fuel storage.
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Keywords:
- plutonium dioxide /
- first-principles method /
- water molecular /
- oxygen vacancy /
- adsorption behavior
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图 1 二氧化钚(PuO2)晶体(111)和(110)表面(钚原子和氧原子分别用青色和黄色表示, 白色方框表示氧空位) (a)化学计量表面; (b)第1层氧空位的还原表面; (c)第2层氧空位的还原表面
Fig. 1. Stoichiometric models and reduction of PuO2 crystals on the (111) and (110) surfaces: (a) The stoichiometric surface; (b) the reduced surface with first-layer oxygen vacancies; (c) the reduced surface with second-layer oxygen vacancies. Plutonium atoms and oxygen atoms are represented in cyan and yellow, respectively. The white boxes indicate oxygen vacancies.
图 2 化学计量(111)表面(a)和(110)表面(b)、第1层氧空位还原(110)表面(c)和(111)表面(d)的PDOS, 其中费米能量设为零
Fig. 2. Projected density of states (PDOS) of stoichiometric (111) surface (a) and (110) surface (b), first-layer oxygen vacancy reduced (110) surface (c) and (111) surface (d). The Fermi energy is set to zero.
图 4 水分子在化学计量(a), (c)和还原(b), (d)PuO2(111)表面和(110)表面吸附的最稳定结构, 其中白色方框表示氧空位, 距离单位为Å, 颜色编码与图1相同
Fig. 4. The most stable structures of water molecule adsorption on stoichiometric (a), (c) and reduced (b), (d) PuO2 (111) and (110) surfaces. The white box indicates oxygen vacancy. Distances are in Å. The color coding is the same as in Fig. 1.
图 5 水分子在PuO2 (a), (b), (e), (f) 化学计量和(c), (d)还原(111)和(110)表面部分解离吸附和完全解离吸附的最稳定结构, 其中白色方框表示氧空位, 黑色方框表示水解离产生的羟基自由基, 颜色编码与图1相同
Fig. 5. The most stable structures of partially dissociative and fully dissociative adsorption of water molecules on PuO2 (a), (b), (e), (f) stoichiometric and (c), (d) reduced (111) and (110) surfaces. The white box indicates oxygen vacancy, and the black box indicates the hydroxyl radical from water dissociation. The color coding is the same as in Fig. 1.
图 6 计算得到的水分子在化学计量(a), (b) (111)和(c), (d) (110)表面的水解离能量剖面 (a), (c) 第1次脱氢; (b), (d)第2次脱氢; 插图分别对应初始态(左)、过渡态(中)和最终态(右)的优化结构, 距离单位为Å, 颜色编码如图1所示
Fig. 6. Dissociation energy profiles of a single water molecule on stoichiometric (a), (b) (111) and (c), (d) (110) surfaces: (a), (c) The first dehydrogenation on the surface; (b), (d) the second dehydrogenation. The insets correspond to the optimized structures of the initial state (left), transition state (middle), and final state (right). Distances are measured in ÅColor codes are as shown in Fig. 1
图 7 计算得到的在还原的PuO2(111)表面上水分子解离路径和两种H2形成的能量, 其中白色方框表示氧空位, TS是相对于前一状态的过渡态能量, 距离单位为Å, 能量单位为eV, 颜色编码与图1相同
Fig. 7. Calculated dissociation pathways of water molecules and the energies of two types of H2 formation on the reduced PuO2(111) surface. The white box indicates an oxygen vacancy, and TS represents the transition state energy relative to the previous state Distances are in Å, and energies are in eV. The color coding is shown in Fig. 1.
表 2 PuO2(111)和(110)表面第1层-Top和第2层-Sub氧空位的形成能(以eV为单位)
Table 2. Formation energies of top-surface and subsurface oxygen vacancy in PnO2 (111) and (110) surfaces (in units of eV).
Refs. Methods Surface Top Sub Wang et al.[13] DFT+U γ(111) 2.81 2.43 γ(110) — — This work γ(111) 2.66 2.58 γ(110) 1.17 1.39 
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