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

Wang M, Wu XP, Zheng S, Zhao L, Li L, Shen L, Gao Y, Xue N, Guo X, Huang W, Gan Z, Blanc F, Yu Z, Ke X, Ding W, Gong XQ, Grey CP, Peng L - Sci Adv (2015)

Bottom Line: We show that the (17)O 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 (17)O chemical shifts being observed for the lower coordinated surface sites.H2 (17)O can be used to selectively enrich surface sites, allowing only these particular active sites to be monitored in a chemical process. (17)O NMR spectra of thermally treated nanosized ceria clearly show how different oxygen species interconvert at elevated temperature.These results open up new strategies for characterizing nanostructured oxides and their applications.

View Article: PubMed Central - PubMed

Affiliation: Key Laboratory of Mesoscopic Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China.

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 (17)O 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 (17)O chemical shifts being observed for the lower coordinated surface sites. H2 (17)O can be used to selectively enrich surface sites, allowing only these particular active sites to be monitored in a chemical process. (17)O 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.

No MeSH data available.


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Solid-state NMR spectra of reduced ceria in comparison with bulk and nanosized ceria.(A) 17O NMR spectra of bulk ceria enriched in 17O2 at 1073 K (bottom) and reduced in H2 atmosphere at 1073 K (top). (B) 17O NMR spectra of nanosized ceria enriched in 17O2 at 773 K (bottom), reduced in H2 atmosphere at 773 K (top), and reoxidized in air reduced ceria (middle). Inset shows the full width of the three 17O spectra. A rotor-synchronized Hahn-echo sequence (π/6 - τ - π/3- τ - acquisition) was used. Data were obtained at 9.4 T under a MAS rate of 20 kHz. One hundred thousand (A, bulk), 300,000 (A, reduced), 1024 (B), and 40,000 scans (B, inset) were averaged, and recycle delays of 0.1 s (A), 1 s (B), and 0.01 s (B, inset) were used. Asterisks denote spinning sidebands.
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Figure 5: Solid-state NMR spectra of reduced ceria in comparison with bulk and nanosized ceria.(A) 17O NMR spectra of bulk ceria enriched in 17O2 at 1073 K (bottom) and reduced in H2 atmosphere at 1073 K (top). (B) 17O NMR spectra of nanosized ceria enriched in 17O2 at 773 K (bottom), reduced in H2 atmosphere at 773 K (top), and reoxidized in air reduced ceria (middle). Inset shows the full width of the three 17O spectra. A rotor-synchronized Hahn-echo sequence (π/6 - τ - π/3- τ - acquisition) was used. Data were obtained at 9.4 T under a MAS rate of 20 kHz. One hundred thousand (A, bulk), 300,000 (A, reduced), 1024 (B), and 40,000 scans (B, inset) were averaged, and recycle delays of 0.1 s (A), 1 s (B), and 0.01 s (B, inset) were used. Asterisks denote spinning sidebands.

Mentions: Oxygen vacancies and Ce3+ are often present as defects in CeO2-based materials, and their concentrations are significant in nanoparticles (17, 26), which can be determined by x-ray photoelectron spectroscopy (XPS) (27, 28) (figs. S14 and S15). These species play an important role in controlling the physical properties of this material for a variety of applications, such as heterogeneous catalysis of redox reactions (29), and are thus investigated here with NMR. H2 temperature programmed reduction (TPR) experiments were first carried out on bulk ceria and ceria nanoparticles to determine the temperature to generate Ce3+ and oxygen vacancy in H2 atmosphere (fig. S16). For bulk ceria, a small and relatively narrow H2 consumption peak can be observed at a temperature lower than 800 K, whereas a much broader and more intense peak centered at 773 K is seen for ceria nanoparticles calcined at 773 K. These peaks can be ascribed to reaction of H2 with surface oxygen of ceria (30). Again, the large peak width associated with the nanoparticles can be ascribed to the wide size distribution of the nanosized sample (fig. S1). The consumption of H2 increased again at a temperature above 1050 K, which should be associated with the bulk oxygen species. Thus, the 17O NMR spectrum were collected for bulk ceria enriched at 1073 K and then reduced in H2 atmosphere at 1073 K (Fig. 5A). A shoulder to lower frequency of the OCe4 peak (877 ppm) and a more distinct lower frequency resonance at about 845 ppm with a full width at half maximum of 12 ppm can be observed. The lower frequency components are tentatively assigned to oxygen ions near the oxygen vacancies or in the Ce3+ second cation coordination shell. The spectrum of ceria nanoparticles initially enriched at 773 K and then reduced at 773 K in H2 also contains a relatively broad resonance with a maximum intensity at about 870 ppm and a broader shoulder at lower frequencies, different from the case of the original nanoparticles (without reduction) or that obtained on reoxidation in oxygen environment (Fig. 5B). The resonance assigned to surface three-coordinated oxygen ions disappears after reduction. Careful examination of these spectra (inset of Fig. 5B) shows that a broader component is present underneath the resonances of the reduced materials. Fermi contact (hyperfine) 17O shifts of more than 728 ppm have been observed for oxygen atoms directly bound to the paramagnetic Ce3+ ion, and similarly, large contact shifts are expected in this system (31). We ascribe the broad component to oxygen ions in the first coordination cell of one or more Ce3+ ions, with the broadening arising from the distribution in local environments and the dipolar interactions between the unpaired electrons centered on Ce3+ and 17O.


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

Wang M, Wu XP, Zheng S, Zhao L, Li L, Shen L, Gao Y, Xue N, Guo X, Huang W, Gan Z, Blanc F, Yu Z, Ke X, Ding W, Gong XQ, Grey CP, Peng L - Sci Adv (2015)

Solid-state NMR spectra of reduced ceria in comparison with bulk and nanosized ceria.(A) 17O NMR spectra of bulk ceria enriched in 17O2 at 1073 K (bottom) and reduced in H2 atmosphere at 1073 K (top). (B) 17O NMR spectra of nanosized ceria enriched in 17O2 at 773 K (bottom), reduced in H2 atmosphere at 773 K (top), and reoxidized in air reduced ceria (middle). Inset shows the full width of the three 17O spectra. A rotor-synchronized Hahn-echo sequence (π/6 - τ - π/3- τ - acquisition) was used. Data were obtained at 9.4 T under a MAS rate of 20 kHz. One hundred thousand (A, bulk), 300,000 (A, reduced), 1024 (B), and 40,000 scans (B, inset) were averaged, and recycle delays of 0.1 s (A), 1 s (B), and 0.01 s (B, inset) were used. Asterisks denote spinning sidebands.
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Figure 5: Solid-state NMR spectra of reduced ceria in comparison with bulk and nanosized ceria.(A) 17O NMR spectra of bulk ceria enriched in 17O2 at 1073 K (bottom) and reduced in H2 atmosphere at 1073 K (top). (B) 17O NMR spectra of nanosized ceria enriched in 17O2 at 773 K (bottom), reduced in H2 atmosphere at 773 K (top), and reoxidized in air reduced ceria (middle). Inset shows the full width of the three 17O spectra. A rotor-synchronized Hahn-echo sequence (π/6 - τ - π/3- τ - acquisition) was used. Data were obtained at 9.4 T under a MAS rate of 20 kHz. One hundred thousand (A, bulk), 300,000 (A, reduced), 1024 (B), and 40,000 scans (B, inset) were averaged, and recycle delays of 0.1 s (A), 1 s (B), and 0.01 s (B, inset) were used. Asterisks denote spinning sidebands.
Mentions: Oxygen vacancies and Ce3+ are often present as defects in CeO2-based materials, and their concentrations are significant in nanoparticles (17, 26), which can be determined by x-ray photoelectron spectroscopy (XPS) (27, 28) (figs. S14 and S15). These species play an important role in controlling the physical properties of this material for a variety of applications, such as heterogeneous catalysis of redox reactions (29), and are thus investigated here with NMR. H2 temperature programmed reduction (TPR) experiments were first carried out on bulk ceria and ceria nanoparticles to determine the temperature to generate Ce3+ and oxygen vacancy in H2 atmosphere (fig. S16). For bulk ceria, a small and relatively narrow H2 consumption peak can be observed at a temperature lower than 800 K, whereas a much broader and more intense peak centered at 773 K is seen for ceria nanoparticles calcined at 773 K. These peaks can be ascribed to reaction of H2 with surface oxygen of ceria (30). Again, the large peak width associated with the nanoparticles can be ascribed to the wide size distribution of the nanosized sample (fig. S1). The consumption of H2 increased again at a temperature above 1050 K, which should be associated with the bulk oxygen species. Thus, the 17O NMR spectrum were collected for bulk ceria enriched at 1073 K and then reduced in H2 atmosphere at 1073 K (Fig. 5A). A shoulder to lower frequency of the OCe4 peak (877 ppm) and a more distinct lower frequency resonance at about 845 ppm with a full width at half maximum of 12 ppm can be observed. The lower frequency components are tentatively assigned to oxygen ions near the oxygen vacancies or in the Ce3+ second cation coordination shell. The spectrum of ceria nanoparticles initially enriched at 773 K and then reduced at 773 K in H2 also contains a relatively broad resonance with a maximum intensity at about 870 ppm and a broader shoulder at lower frequencies, different from the case of the original nanoparticles (without reduction) or that obtained on reoxidation in oxygen environment (Fig. 5B). The resonance assigned to surface three-coordinated oxygen ions disappears after reduction. Careful examination of these spectra (inset of Fig. 5B) shows that a broader component is present underneath the resonances of the reduced materials. Fermi contact (hyperfine) 17O shifts of more than 728 ppm have been observed for oxygen atoms directly bound to the paramagnetic Ce3+ ion, and similarly, large contact shifts are expected in this system (31). We ascribe the broad component to oxygen ions in the first coordination cell of one or more Ce3+ ions, with the broadening arising from the distribution in local environments and the dipolar interactions between the unpaired electrons centered on Ce3+ and 17O.

Bottom Line: We show that the (17)O 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 (17)O chemical shifts being observed for the lower coordinated surface sites.H2 (17)O can be used to selectively enrich surface sites, allowing only these particular active sites to be monitored in a chemical process. (17)O NMR spectra of thermally treated nanosized ceria clearly show how different oxygen species interconvert at elevated temperature.These results open up new strategies for characterizing nanostructured oxides and their applications.

View Article: PubMed Central - PubMed

Affiliation: Key Laboratory of Mesoscopic Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China.

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 (17)O 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 (17)O chemical shifts being observed for the lower coordinated surface sites. H2 (17)O can be used to selectively enrich surface sites, allowing only these particular active sites to be monitored in a chemical process. (17)O 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.

No MeSH data available.


Related in: MedlinePlus