Interactions of Oxide Surfaces with Water Revealed with Solid-State NMR Spectroscopy

Abstract

Hydrous materials are ubiquitous in the natural environment and efforts have previously been made to investigate the structures and dynamics of hydrated surfaces for their key roles in various chemical and physical applications, with the help of theoretical modelling and microscopy techniques. However, an overall atomic-scale understanding of the water-solid interface, including the effect of water on surface ions, is still lacking. Herein, we employ ceria nanorods with different amounts of water as an example and demonstrate a new approach to explore the water-surface interactions by using solid-state NMR in combination with density functional theory. NMR shifts and relaxation time analysis provide detailed local structure of oxygen ions and the nature of water motion on the surface: the amount of molecularly adsorbed water decreases rapidly with increasing temperature (from room temperature to 150 °C), whereas hydroxyl groups are stable up to 150 °C; dynamic water molecules are found to instantaneously coordinate to the surface oxygen ions. The applicability of dynamic nuclear polarization for selective detection of surface oxygen species is also compared to conventional NMR with surface selective-labeling: the optimal method depends on the feasibility of enrichment and the concentration of protons in the sample. These results provide new insight into the interfacial structure of hydrated oxide nanostructures which is important for relevant applications.

Oxygen Vacancy Distribution in Yttrium-Doped Ceria from 89Y–89Y Correlations via Dynamic Nuclear Polarization Solid-State NMR

“…Common strategies for sensitivity enhancement consist of introducing paramagnetic agents to shorten relaxation times or isotopic ( 17 O) enrichment. We have recently shown how metal ion-based dynamic nuclear polarization (MIDNP) under magic angle spinning (MAS) can lead to significant sensitivity enhancements of low-sensitivity nuclei in inorganic materials. , The possibility of measuring the bulk of inorganic samples differentiates this approach from conventional exogeneous DNP, which has been applied on similar materials to study their surface properties. − …”

CeTiO2N oxynitride perovskite: paramagnetic 14N MAS NMR without paramagnetic shifts

“…Proton signal at 4.9 ppm corresponds to physisorbed H2O, whereas remaining resonances originate from different types of bridging (>OHB) and terminal hydroxyl groups (−OHT). 35 In contrast, surface proton signals from paramagnetic CeTiO2N (Figure 1(a), red trace) are strongly affected by the presence of unpaired electrons from Ce 3+ ions. Spectrum is severely broadened as a result of both paramagnetic NMR shift interactions and relaxation enhancement.…”

Effect of Adsorbed Water Molecules on the Surface Acidity of Niobium and Tantalum Oxides Studied by MAS NMR

“…The currently used water-resistant solid acid catalysts include niobium oxides, titanium oxides, molecular sieves, zirconium-containing composite oxides, and so on . Much research − has studied the water interaction with solid acid surfaces at the atomic level. ,,− The catalytic activity of TiO 2 /SiO 2 in an isopropanol dehydration reaction was enhanced in the presence of water, attributing to the newly generated Brønsted acid sites and the remaining Lewis acid sites . In addition, the formation of additional Brønsted acid sites in Ga-HZSM-5 and Sn-BEA , were also observed after water was introduced.…”