Interdisciplinary Research Center for Catalytic Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan.
Univ. Lille, CNRS, Centrale Lille, Univ. Artois, UMR 8181, UCCS - Unité de Catalyse et de Chimie du Solide, F-59000 Lille, France.
J Am Chem Soc. 2020 Jun 17;142(24):10659-10672. doi: 10.1021/jacs.9b13838. Epub 2020 Jun 4.
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 MoO, MoO/TiO, a widely used heterogeneous catalyst. The spectra of protonated and unprotonated O sites, acquired in natural abundance, indicate the presence of various oxomolybdate species as well as HOMo and HOMo Brønsted acid sites. The enhanced sensitivity of this new method has also enabled the acquisition of the first DNP-enhanced spectra of Mo and Ti low-γ quadrupolar isotopes. This possibility has been demonstrated by detecting the signals of these nuclei near the surface of MoO/TiO. This technique has allowed the observation of Ti surface sites, which are absent from the bulk region of TiO. Furthermore, both Mo and Ti DNP spectra have shown an increased structural disorder of TiO and MoO phases near the surface of the particles and notably the preferential location of the amorphous TiO phase at the surface of the particles. The proposed polarization transfer is also employed to acquire the first DNP-enhanced spectrum of Zn, another low-γ quadrupolar isotope. This possibility is demonstrated for Al-doped ZnO nanoparticles used in optoelectronic devices. The obtained O, Al, and Zn DNP-NMR data prove that the surface region of these nanoparticles contains ZnO phase as well as secondary phases, such as α-AlO and partially inverse ZnAlO spinel.
我们介绍了一种新的 NMR 方法，该方法扩展了魔角旋转下间接动态核极化 (DNP) 的能力，可用于探测半整数自旋四极核的局部环境。与交叉极化相比，这种基于重新聚焦 INEPT 方案并采用绝热偶极重新耦合法的新型方法更容易优化，并且不会扭曲四极线谱。此外，与 PRESTO（相移再耦合法）方案或直接 DNP 相比，使用该技术可大大提高 DNP-NMR 对具有小偶极耦合到质子的四极同位素的检测灵敏度，包括那些位于无机材料的亚表面或具有低回旋磁比 (γ) 的同位素。该技术已应用于鉴定水合二氧化钛负载的 MoO、MoO/TiO 中 Brønsted 酸位的原子级结构，MoO/TiO 是一种广泛使用的多相催化剂。在天然丰度下获得的质子化和未质子化 O 位谱表明存在各种氧钼酸盐物种以及 HOMo 和 HOMo Brønsted 酸位。该新技术的高灵敏度还能够获得 Mo 和 Ti 低γ四极同位素的第一个 DNP 增强谱。通过检测 MoO/TiO 近表面这些核的信号证明了这种可能性。该技术还可以观察到 TiO 颗粒体相中不存在的 MoO/TiO 表面位。此外，Mo 和 Ti 的 DNP 谱均表明 TiO 和 MoO 相在颗粒表面附近的结构无序度增加，尤其是无定形 TiO 相优先位于颗粒表面。所提出的极化转移也用于获得另一个低γ四极同位素 Zn 的第一个 DNP 增强谱。这对于用于光电设备的掺 Al ZnO 纳米粒子是证明了这种可能性。所获得的 O、Al 和 Zn 的 DNP-NMR 数据证明了这些纳米粒子的表面区域包含 ZnO 相以及 secondary phases，如α-AlO 和部分反向 ZnAlO 尖晶石。
