Abstract
The detailed information on the surface structure and binding sites of oxide nanomaterials is crucial to understand the adsorption and catalytic processes and thus the key to develop better materials for related applications. However, experimental methods to reveal this information remain scarce. Here we show that 17O solid-state nuclear magnetic resonance (NMR) spectroscopy can be used to identify specific surface sites active for CO2 adsorption on MgO nanosheets. Two 3-coordinated bare surface oxygen sites, resonating at 39 and 42 ppm, are observed, but only the latter is involved in CO2 adsorption. Double resonance NMR and density functional theory (DFT) calculations results prove that the difference between the two species is the close proximity to H, and CO2 does not bind to the oxygen ions with a shorter O···H distance of approx. 3.0 Å. Extensions of this approach to explore adsorption processes on other oxide materials can be readily envisaged.
Highlights
The detailed information on the surface structure and binding sites of oxide nanomaterials is crucial to understand the adsorption and catalytic processes and the key to develop better materials for related applications
We showed that 17O solid-state nuclear magnetic resonance (NMR) spectroscopy provides very high resolution and oxygen species at different facets or different layers in oxide nanostructures can be clearly distinguished based on 17O NMR shifts[12,13,14,15]
High-resolution transmission electron microscopy (HRTEM) images show the nanosheet morphology of the MgO materials and the nanosheets are denoted as NS-773 and NS-1073 (NS for nanosheets), according to the corresponding thermal treatment temperatures (Supplementary Fig. 2)
Summary
The detailed information on the surface structure and binding sites of oxide nanomaterials is crucial to understand the adsorption and catalytic processes and the key to develop better materials for related applications. Despite the very high-resolution microscopy can possibly achieve these days, the images may not be representative of the whole structure and quantitative analysis cannot be performed[8] Spectroscopic investigations, such as IR, often require probe molecules, e.g., CO, making such methods indirect[9] and unsuitable for in situ studies, in which probe molecules may interfere. We showed that 17O solid-state NMR spectroscopy provides very high resolution and oxygen species at different facets or different layers in oxide nanostructures can be clearly distinguished based on 17O NMR shifts[12,13,14,15] This approach does not introduce any external species that may potentially alter the structure or properties of oxide surface, such as probe molecules[12]. The approach, which is potentially extendable to other oxides, can distinguish two surface oxygen species only slightly different on the third coordination shell, identify the sorption sites of CO2, and reveal the nature of these sites as well as the reasons behind the differences in adsorption properties
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