Sayle Thi X T, Parker Stephen C, Sayle Dean C
Dept. Environmental and Ordnance Systems, Cranfield University, Defence Academy of the United Kingdom, Shrivenham, Swindon, UK.
Faraday Discuss. 2007;134:377-97; discussion 399-419. doi: 10.1039/b601521b.
Ceria, CeO2, based materials are a major (active) component of exhaust catalysts and promising candidates for solid oxide fuel cells. In this capacity, oxygen transport through the material is pivotal. Here, we explore whether oxygen transport is influenced (desirably increased) compared with transport within the bulk parent material by traversing to the nanoscale. In particular, atomistic models for ceria nanocrystals, including perfect: CeO2; reduced: CeO1.95 and doped: Rh0.1Ce0.9O1.95, have been generated. The nanocrystals were about 8 nm in diameter and each comprised about 16,000 atoms. Oxygen transport can also be influenced, sometimes profoundly, by microstructural features such as dislocations and grain-boundaries. However, these are difficult to generate within an atomistic model using, for example, symmetry operations. Accordingly, we crystallised the nanocrystals from an amorphous precursor, which facilitated the evolution of a variety of microstructures including: twin-boundaries and more general grain-boundaries and grain-junctions, dislocations and epitaxy, isolated and associated point defects. The shapes of the nanocrystals are in accord with HRTEM data and comprise octahedral morphologies with {111} surfaces, truncated by (dipolar) {100} surfaces together with a complex array of steps, edges and corners. Oxygen transport data was then calculated using these models and compared with data calculated previously for CeO1.97/ YSZ thin films and the (bulk) parent material, CeO197. Oxygen transport was calculated to increase in the order: CeO2 nanocrystal < (reduced) CeO1.95 nanocrystal approximately Rh0.1Ce0.9O1.95 nanocrystal < CeO1.97/YSZ thin film < (reduced) CeO1.97 (bulk) parent material; the mechanism was determined to be primarily vacancy driven. Our findings indicate that reducing one- (thin film) or especially three- (nanocrystal) dimensions to the nanoscale may prove deleterious to oxygen transport. Conversely, we observed dynamic evolution and annihilation of surface vacancies via surface oxygens migrating to the bulk of the nanocrystal; the vacancies left are then filled by other oxygens moving to the surface. Coupled with previous simulation studies, in which we calculated that oxygen extraction from the surface of a ceria nanocrystal was energetically easier compared with the bulk surface, our calculations predict that ceria nanocrystals would facilitate effective oxidative catalysis. This study describes framework simulation procedures, which can be used in partnership with experiment, to explore transport in nanocrystalline ionic systems, which include complex microstructures. Such data can provide predictions for experiment or help reduce the number of experiments required.
二氧化铈（CeO₂）基材料是废气催化剂的主要（活性）成分，也是固体氧化物燃料电池的潜在候选材料。在这种应用中，氧在材料中的传输至关重要。在此，我们探讨与母体块状材料内部的传输相比，通过深入到纳米尺度，氧的传输是否会受到影响（理想情况下是增加）。特别是，已经生成了二氧化铈纳米晶体的原子模型，包括完美的：CeO₂；还原的：CeO₁.₉₅和掺杂的：Rh₀.₁Ce₀.₉O₁.₉₅。这些纳米晶体直径约为8纳米，每个约由16000个原子组成。氧的传输也会受到诸如位错和晶界等微观结构特征的影响，有时影响还很显著。然而，使用例如对称操作在原子模型中很难生成这些微观结构。因此，我们从非晶态前驱体中使纳米晶体结晶，这促进了包括孪晶界、更普遍的晶界和晶界结、位错和外延、孤立和相关的点缺陷等各种微观结构的演化。纳米晶体的形状与高分辨率透射电子显微镜（HRTEM）数据一致，由具有{111}面的八面体形态组成，并被（偶极）{100}面截断，同时还有复杂的台阶、边缘和角落阵列。然后使用这些模型计算氧传输数据，并与先前为CeO₁.₉₇/YSZ薄膜和（块状）母体材料CeO₁.₉₇计算的数据进行比较。计算得出氧传输按以下顺序增加：CeO₂纳米晶体＜（还原的）CeO₁.₉₅纳米晶体≈Rh₀.₁Ce₀.₉O₁.₉₅纳米晶体＜CeO₁.₉₇/YSZ薄膜＜（还原的）CeO₁.₉₇（块状）母体材料；确定其机制主要是由空位驱动的。我们的研究结果表明，将一维（薄膜）或特别是三维（纳米晶体）尺寸减小到纳米尺度可能对氧传输有害。相反，我们观察到表面氧迁移到纳米晶体本体时，表面空位会动态演化和湮灭；留下的空位随后由其他迁移到表面的氧填充。结合我们之前的模拟研究，在该研究中我们计算得出从二氧化铈纳米晶体表面提取氧在能量上比块状表面更容易，我们的计算预测二氧化铈纳米晶体会促进有效的氧化催化作用。本研究描述了框架模拟程序，该程序可与实验结合使用，以探索包括复杂微观结构的纳米晶离子系统中的传输。这些数据可为实验提供预测或有助于减少所需的实验数量。
