Cerium is the most abundant rare earth element in the Earth’s crust. Its most stable oxide, cerium dioxide (CeO₂, ceria), is increasingly utilized in the field of catalysis. It can catalyze redox and acid-base reactions, and serve as a component of electrocatalysts and even photocatalysts. As one of the most commonly used in situ/operando characterization methods in catalysis, infrared (IR) spectroscopy is routinely employed to monitor reaction intermediates on the surface of solid catalysts, offering profound insight into reaction mechanisms. Additionally, IR vibrational frequencies of probe molecules adsorbed on solid catalysts provide detailed information about their structure and chemical states. Numerous studies in the literature have utilized carbon monoxide and methanol as IR probe molecules on ceria particles. However, assigning their vibrational frequencies is often highly controversial due to the great complexity of the actual surface of ceria particles, which include differently oriented crystal facets, reconstructions, defects, and other structural variations. In our laboratory, taking bulk ceria single crystals with distinct orientations as model systems, we employed a highly sensitive ultrahigh vacuum (UHV) infrared spectroscopy system (THEO) to study the adsorption of CO and methanol. It turns out that the theoretical calculations adopting hybrid functionals (HSE06) can bring the theoretical predictions into agreement with the experimental results for the CO frequencies on ceria single crystal surfaces. The obtained frequencies serve as reliable references to resolve the long-standing controversial assignments for the IR bands of CO and methanol adsorbed on ceria particles. Furthermore, these characteristic frequencies allow for the determination of orientations, oxidation states and restructuring of exposed crystal facets of ceria nanoparticles, which is applicable from UHV conditions to industrially relevant pressures of up to 1 bar, and from low temperatures (∼65 K) to high temperatures (∼1000 K). We also used molecular oxygen as a probe molecule to investigate its interaction with the ceria surface, crucial for understanding ceria’s redox properties. Our findings reveal that the localization of oxygen vacancies and the mechanism of dioxygen activation are highly sensitive to surface orientations. We provided the first spectroscopic evidence showing that the oxygen vacancies on ceria (111) surfaces tend to localize in deep layers. In addition, we employed N₂O as a probe molecule to elucidate the origin of the photocatalytic activity of ceria and showed that the photocatalytic activity is highly sensitive to the surface orientation (i.e., surface coordination structure). This Account shows that probe-molecule infrared spectroscopy serves as a powerful in situ/operando tool for studying the surface structure and chemistry of solid catalysts, and the knowledge gained through the “Surface Science” approach is essential as a crucial benchmark.

中文翻译：

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通过表面配体红外光谱 （SLIR） 测定 CeO2 粉末催化剂的表面结构和化学性质


铈是地壳中含量最丰富的稀土元素。其最稳定的氧化物二氧化铈（CeO₂，二氧化铈）越来越多地用于催化领域。它可以催化氧化还原和酸碱反应，并用作电催化剂甚至光催化剂的组分。作为催化中最常用的原位/原位表征方法之一，红外 （IR） 光谱通常用于监测固体催化剂表面的反应中间体，从而深入了解反应机理。此外，吸附在固体催化剂上的探针分子的红外振动频率提供了有关其结构和化学状态的详细信息。文献中的许多研究都使用一氧化碳和甲醇作为氧化铈颗粒上的 IR 探针分子。然而，由于铈粒子的实际表面非常复杂，包括不同方向的晶体刻面、重建、缺陷和其他结构变化，因此分配它们的振动频率通常极具争议。在我们的实验室中，以具有不同取向的块状铈单晶作为模型系统，我们采用了高灵敏度的超高真空 （UHV） 红外光谱系统 （THEO） 来研究 CO 和甲醇的吸附。事实证明，采用混合泛函 （HSE06） 的理论计算可以使理论预测与氧化铈单晶表面 CO 频率的实验结果一致。获得的频率可作为可靠的参考，以解决吸附在铈颗粒上的 CO 和甲醇的 IR 波段的长期争议分配。 此外，这些特征频率允许确定铈纳米颗粒暴露晶体面的取向、氧化态和重组，适用于从 UHV 条件到高达 1 bar 的工业相关压力，以及从低温 （∼65 K） 到高温 （∼1000 K）。我们还使用分子氧作为探针分子来研究它与铈表面的相互作用，这对于了解铈的氧化还原特性至关重要。我们的研究结果表明，氧空位的定位和双氧活化的机制对表面取向高度敏感。我们提供了第一个光谱证据，表明 ceria （111） 表面上的氧空位倾向于定位在深层。此外，我们采用 N₂O 作为探针分子来阐明二氧化铈光催化活性的来源，并表明光催化活性对表面取向（即表面配位结构）高度敏感。本说明表明，探针分子红外光谱是研究固体催化剂表面结构和化学的强大原位/原位工具，通过“表面科学”方法获得的知识作为关键基准至关重要。