17O solid-state NMR studies of oxygen-containing catalysts

Can Subsurface Oxygen Species in Oxides Participate in Catalytic Reactions? An 17O Solid-State Nuclear Magnetic Resonance Study

The impacts of subsurface species of catalysts on reaction processes are still under debate, largely due to a lack of characterization methods for distinguishing these species from the surface species and the bulk. By using 17O solid-state nuclear magnetic resonance (NMR) spectroscopy, which can distinguish subsurface oxygen ions in CeO2 (111) nanorods, we explore the effects of subsurface species of oxides in CO oxidation reactions. The intensities of the 17O NMR signals due to surface and subsurface oxygen ions decrease after the introduction of CO into CeO2 nanorods, with a more significant decrease observed for the latter, confirming the participation of subsurface oxygen species. Density functional theory calculations show that the reaction involves subsurface oxygen ions filling the surface oxygen vacancies created by the direct contact of surface oxygen with CO. This new approach can be extended to the study of the role of oxygen species in other catalytic reactions.

Identification of CO2 adsorption sites on MgO nanosheets by solid-state nuclear magnetic resonance spectroscopy

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.

Application of solid-state NMR techniques for structural characterization of metal-organic frameworks

Solid-state NMR can afford the structural information about the chemical composition, local environment, and spatial coordination at the atomic level, which has been extensively applied to characterize the detailed structure and host-guest interactions in metal-organic frameworks (MOFs). In this review, recent advances for the structural characterizations of MOFs using versatile solid-state NMR techniques were briefly introduced. High-field sensitivity-enhanced solid-state NMR method enabled the direct observation of metal centers in MOFs containing low-γ nuclei. Two-dimensional (2D) homo- and hetero-nuclear correlation MAS NMR experiments provided the spatial proximity among linkers, metal clusters and the introduced guest molecules. Moreover, quantitative measurement of inter-nuclear distances using solid-state NMR provided valuable structural information about the connectivity geometry as well as the host-guest interactions within MOFs. Furthermore, solid-state NMR has exhibited great potential for unraveling the structure property of MOFs containing paramagnetic metal centers.

17O Solid-State NMR Studies of Ta2O5 Nanorods

17O solid-state NMR spectroscopy was used to study the structure of Ta2O5 nanorods for the first time. Although the observations of oxygen ions in the "bulk" part of the Ta2O5 nanorods can be achieved with conventional high-temperature enrichment with 17O2, low-temperature isotopic labeling with H2 17O generated samples whose surfaces are selectively enriched, leading to surface-only detection of oxygen species. By applying 17O-1H double-resonance NMR techniques and 1H NMR spectroscopy, surface hydroxyl species and adsorbed water can also be studied. The results form the basis for further understanding of the structure-property relationship of Ta2O5 nanomaterials.

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

Interaction of γ-alumina with water are important in controlling its structure and catalytic properties. We apply solid-state multinuclear NMR spectroscopy to investigate this interaction by monitoring ¹H and ¹⁷O spectra in real-time. Surface-selective detection is made possible by adsorbing ¹⁷O-enriched water on γ-alumina nanorods. Structural evolution on the surface was selectively probed by ¹H/¹⁷O double resonance NMR and ²⁷Al NMR at ultrahigh 35.2 T magnetic field. Formation of hydroxyl species on the surface of nanorods is rapid upon the exposure of water, which involves low coordinated aluminum ions with doubly bridging and isolated hydroxyl species being generated first. Fast exchange occurs between oxygen atoms in the water molecules and bare surface sites, indicating high reactivity of these oxygen species. These results provide new insights into the structure and dynamics on the surface of γ-alumina and the methods applied here can be extended to study the interaction of other oxides with water.

Proton detection of MAS solid-state NMR spectra of half-integer quadrupolar nuclei

Fast magic angle spinning (MAS) and proton detection has found widespread application to enhance the sensitivity of solid-state NMR experiments with spin-1/2 nuclei such as ¹³C, ¹⁵N and ²⁹Si, however, this approach is not yet routinely applied to half-integer quadrupolar nuclei. Here we have investigated the feasibility of using fast MAS and proton detection to enhance the sensitivity of solid-state NMR experiments with half-integer quadrupolar nuclei. The previously described dipolar hetero-nuclear multiple quantum correlation (D-HMQC) and dipolar refocused insensitive nuclei enhanced by polarization transfer (D-RINEPT) pulse sequences were used for proton detection of half-integer quadrupolar nuclei. Quantitative comparisons of signal-to-noise ratios and the sensitivity of proton detected D-HMQC and D-RINEPT and direct detection spin echo and quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) solid-state NMR spectra, demonstrate that one dimensional proton detected experiments can provide sensitivity similar to or exceeding that obtainable with direct detection QCPMG experiments. 2D D-HMQC and D-RINEPT experiments provide less sensitivity than QCPMG experiments but proton detected 2D hetero-nuclear correlation solid-state NMR spectra of half-integer nuclei can still be acquired in about the same time as a 1D spin echo spectrum. Notably, the rarely used D-RINEPT pulse sequence is found to provide similar, or better sensitivity than D-HMQC in some cases. Proton detected D-RINEPT benefits from the short longitudinal relaxation times (T₁) normally associated with half-integer quadrupolar nuclei, it can be combined with existing signal enhancement methods for quadrupolar nuclei, and t₁-noise in the indirect dimension can easily be removed by pre-saturation of the ¹H nuclei. The rapid acquisition of proton detected 2D HETCOR solid-state NMR spectra of a range of half-integer quadrupolar nuclei such as ¹⁷O, ²⁷Al, ³⁵Cl and ⁷¹Ga is demonstrated.

Evidence for Core Oxygen Dynamics and Exchange in Metal Oxide Nanocrystals from In Situ 17O MAS NMR

Long-term stability of the properties of nanocrystals (NCs) is of paramount importance for any applicative development. However, these are jeopardized by chemical and structural alterations of the NCs induced by the environment and the working conditions. Among the species that alter the NCs properties, water molecules are of tremendous importance. We used ¹⁷O solid-state NMR spectroscopy to follow this process and the dynamics of O atoms in metal oxide NCs. Using ZnO as reference material, different chemical environments for the O atoms are characterized and a dynamic exchange process between the NCs and the O atoms from water is evidenced. The exchange does not involve only surface atoms but also ones located deeper inside the ZnO core of the NCs. Finally, a postsynthesis process based on watering/drying cycles is proposed that may greatly improve the long-term stability of metal oxide NCs.