Han Qiao, Gao Pan, Liang Lixin, Chen Kuizhi, Dong Aiyi, Liu Zhengmao, Han Xiuwen, Fu Qiang, Hou Guangjin
State Key Laboratory of Catalysis, National Laboratory for Clean Energy, 2011-Collaborative Innovation Center of Chemistry for Energy Materials, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China.
University of Chinese Academy of Sciences, Beijing 100049, China.
Anal Chem. 2021 Dec 21;93(50):16769-16778. doi: 10.1021/acs.analchem.1c02759. Epub 2021 Dec 8.
Hydroxyl groups are among the major active surface sites over metal oxides. However, their spectroscopic characterizations have been challenging due to limited resolutions, especially on hydroxyl-rich surfaces where strong hydroxyl networks are present. Here, using nanostructured InO as an example, we show significantly enhanced discrimination of the surface hydroxyl groups, owing to the high-resolution H NMR spectra performed at a high magnetic field (18.8 T) and a fast magic angle spinning (MAS) of up to 60 kHz. A total of nine kinds of hydroxyl groups were distinguished and their assignments (μ, μ, and μ) were further identified with the assistance of O NMR. The spatial distribution of these hydroxyl groups was further explored via two-dimensional (2D) H-H homonuclear correlation experiments with which the complex surface hydroxyl network was unraveled at the atomic level. Moreover, the quantitative analysis of these hydroxyl groups with such high resolution enables further investigations into the physicochemical property and catalytic performance characterizations (in CO reduction) of these hydroxyl groups. This work provides insightful understanding on the surface structure/property of the InO nanoparticles and, importantly, may prompt general applications of high-field ultrafast MAS NMR techniques in the study of hydroxyl-rich surfaces on other metal oxide materials.
羟基是金属氧化物表面主要的活性位点之一。然而，由于分辨率有限，对其进行光谱表征具有挑战性，尤其是在存在强羟基网络的富羟基表面。在此，以纳米结构的InO为例，我们展示了表面羟基辨别能力的显著增强，这得益于在高磁场（18.8 T）下进行的高分辨率H NMR光谱以及高达60 kHz的快速魔角旋转（MAS）。总共区分出了九种羟基，并借助O NMR进一步确定了它们的归属（μ、μ和μ）。通过二维（2D）H-H同核相关实验进一步探索了这些羟基的空间分布，利用该实验在原子水平上解开了复杂的表面羟基网络。此外，对这些羟基的高分辨率定量分析能够进一步研究这些羟基的物理化学性质和催化性能表征（在CO还原中）。这项工作为InO纳米颗粒的表面结构/性质提供了深刻的理解，重要的是，可能会促使高场超快MAS NMR技术在研究其他金属氧化物材料的富羟基表面方面得到广泛应用。
