Observation of Low-γ Quadrupolar Nuclei by Surface-Enhanced NMR Spectroscopy

We introduce a novel NMR approach that extends the capabilities of indirect dynamic nuclear polarization (DNP) under magic-angle spinning to probe the local environment of half-integer spin quadrupolar nuclei. Compared to cross-polarization, this novel method based on the refocused INEPT scheme with adiabatic dipolar recoupling is easier to optimize and does not distort the quadrupolar line shapes. Furthermore, the use of this technique, instead of the PRESTO (Phase-shifted Recoupling Effects a Smooth Transfer of Order) scheme or direct DNP, greatly improves the sensitivity of DNP-NMR for the detection of quadrupolar isotopes with small dipolar couplings to protons, including notably those located in the subsurface of inorganic materials or with low gyromagnetic ratio (γ). This technique has been applied to identify the atomic-level structure of Brønsted acid sites of hydrated titania-supported MoO₃, MoO₃/TiO₂, a widely used heterogeneous catalyst. The spectra of protonated and unprotonated ¹⁷O sites, acquired in natural abundance, indicate the presence of various oxomolybdate species as well as HOMo₂ and HOMo₃ Brønsted acid sites. The enhanced sensitivity of this new method has also enabled the acquisition of the first DNP-enhanced spectra of ⁹⁵Mo and ^47,49Ti low-γ quadrupolar isotopes. This possibility has been demonstrated by detecting the signals of these nuclei near the surface of MoO₃/TiO₂. This technique has allowed the observation of ⁴⁹Ti surface sites, which are absent from the bulk region of TiO₂. Furthermore, both ⁹⁵Mo and ^47,49Ti DNP spectra have shown an increased structural disorder of TiO₂ and MoO₃ phases near the surface of the particles and notably the preferential location of the amorphous TiO₂ phase at the surface of the particles. The proposed polarization transfer is also employed to acquire the first DNP-enhanced spectrum of ⁶⁷Zn, another low-γ quadrupolar isotope. This possibility is demonstrated for Al-doped ZnO nanoparticles used in optoelectronic devices. The obtained ¹⁷O, ²⁷Al, and ⁶⁷Zn DNP-NMR data prove that the surface region of these nanoparticles contains ZnO phase as well as secondary phases, such as α-Al₂O₃ and partially inverse ZnAl₂O₄ spinel.