Identification of different oxygen species in oxide nanostructures with ¹⁷ O solid-state NMR spectroscopy

Abstract

Nanostructured oxides find multiple uses in a diverse range of applications including catalysis, energy storage, and environmental management, their higher surface areas, and, in some cases, electronic properties resulting in different physical properties from their bulk counterparts. Developing structure-property relations for these materials requires a determination of surface and subsurface structure. Although microscopy plays a critical role owing to the fact that the volumes sampled by such techniques may not be representative of the whole sample, complementary characterization methods are urgently required. We develop a simple nuclear magnetic resonance (NMR) strategy to detect the first few layers of a nanomaterial, demonstrating the approach with technologically relevant ceria nanoparticles. We show that the 17O resonances arising from the first to third surface layer oxygen ions, hydroxyl sites, and oxygen species near vacancies can be distinguished from the oxygen ions in the bulk, with higher-frequency 17O chemical shifts being observed for the lower coordinated surface sites. H217O can be used to selectively enrich surface sites, allowing only these particular active sites to be monitored in a chemical process. 17O NMR spectra of thermally treated nanosized ceria clearly show how different oxygen species interconvert at elevated temperature. Density functional theory calculations confirm the assignments and reveal a strong dependence of chemical shift on the nature of the surface. These results open up new strategies for characterizing nanostructured oxides and their applications.

Probing Interactions of γ‐Alumina with Water via Multinuclear Solid‐State NMR Spectroscopy

“…No sharp component at 0 ppm can be observed, indicating the complete conversion of water to hydroxyl species. Even with a short dipolar recoupling time of only 0.14 ms and a relatively fast spinning speed of 14 kHz, the REDOR NMR spectral intensity of the peak at −23 ppm decreases by about 65 %, confirming this resonance is associated with a very short O−H distance and can be readily assigned to hydroxyl species (“Double resonance” in Figure ) . The spectrum with dephasing pulses clearly shows two shoulders at 68 and 40 ppm, while these peaks are less obvious in the control experiment and are not observed in the difference spectrum, implying they can be ascribed to oxygen species with a much longer O−H distance than the hydroxyl species.…”

Understanding Surface and Interfacial Chemistry in Functional Nanomaterials via Solid‐State NMR

“…In more recent work, Peng's group developed a systematic SSNMR approach to study ceria nanoparticles. Their approach, in our opinion, is generally adaptable for nanoporous materials.…”

“…A) 17 O NMR of ceria nanoparticle at variable temperatures and B) the 17 O chemical shifts assigned to different layers according to DFT calculations. Reproduced under the terms of the CC‐BY‐NC‐4.0 licence . Copyright 2015, The Authors; published by the American Association for the Advancement of Science.…”

Surface-selective direct 17O DNP NMR of CeO2 nanoparticles

“…Nuclear magnetic resonance (NMR) spectroscopy can reveal a wealth of chemical and structural information on the atomic scale, and previous work has shown that 17 O solid-state NMR (ssNMR) spectroscopy is a powerful tool in investigating the structure and activity of zeolites, 6 metal oxide nanoparticles, 7,8 and other functionally relevant oxides. 9 However, the inherent difficulty of attaining sufficient signal to noise in NMR spectroscopy is exacerbated for experiments on 17 O, the only NMRactive nucleus of oxygen, as its low natural abundance (0.037%) leads to lower intensity and its quadrupolar character (I = 5/2) can result in additional spectral broadening.…”

“…Nonetheless, in recent work by Wang et al, 17 O ssNMR spectra of nanoparticulate CeO 2 have been recorded and assigned to specific surface environments via a combination of surface-selective enrichment (with H 2 17 O) and density functional theory (DFT) calculations. 8 However, in this case the surface-selective enrichment is only possible due to the high reactivity of ceria. A more general approach to overcome the sensitivity problems inherent to ssNMR is the use of dynamic nuclear polarisation (DNP), 10 which has seen a significant resurgence in recent years.…”