Probing the surface of γ-Al2O3 by oxygen-17 dynamic nuclear polarization enhanced solid-state NMR spectroscopy†,Physical Chemistry Chemical Physics

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Probing the surface of γ-Al2O3 by oxygen-17 dynamic nuclear polarization enhanced solid-state NMR spectroscopy†
Physical Chemistry Chemical Physics ( IF 2.9 ) Pub Date : 2018-06-01 00:00:00 , DOI: 10.1039/c8cp03132k
Wenzheng Li _{1,

2,

3,

4,

5} , Qiang Wang _{1,

2,

3,

4,

5} , Jun Xu _{1,

2,

3,

4,

5} , Fabien Aussenac _{6,

7,

8} , Guodong Qi _{1,

2,

3,

4,

5} , Xingling Zhao _{1,

2,

3,

4,

5} , Pan Gao _{1,

2,

3,

4,

5} , Chao Wang _{1,

2,

3,

4,

5} , Feng Deng _{1,

2,

3,

4,

5}

Affiliation

γ-Al₂O₃ is an important catalyst and catalyst support of industrial interest. Its acid/base characteristics are correlated to the surface structure, which has always been an issue of concern. In this work, the complex (sub-)surface oxygen species on surface-selectively labelled γ-Al₂O₃ were probed by ¹⁷O dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP-SENS). Direct ¹⁷O MAS and indirect ¹H–¹⁷O cross-polarization (CP)/MAS DNP experiments enable observation of the (sub-)surface bare oxygen species and hydroxyl groups. In particular, a two-dimensional (2D) ¹⁷O 3QMAS DNP spectrum was for the first time achieved for γ-Al₂O₃, in which two O(Al)₄ and one O(Al)₃ bare oxygen species were identified. The ¹⁷O isotropic chemical shifts (δ_cs) vary from 56.7 to 81.0 ppm and the quadrupolar coupling constants (C_Q) range from 0.6 to 2.5 MHz for the three oxygen species. The coordinatively unsaturated O(Al)₃ species is characterized by a higher field chemical shift (56.7 ppm) and the largest C_Q value (2.5 MHz) among these oxygen sites. 2D ¹H → ¹⁷O HETCOR DNP experiments allow us to discriminate three bridging (Al_n)-μ₂-OH and two terminal (Al_n)-μ₁-OH hydroxyl groups. The structural features of the bare oxygen species and hydroxyl groups are similar for the γ-Al₂O₃ samples isotopically labelled by ¹⁷O₂ gas or H₂¹⁷O. The results presented here show that the combination of surface-selective labelling and DNP-SENS is an effective approach for characterizing oxides with complex surface species.

中文翻译：

探测的表面的γ-Al ₂ ö ₃通过氧-17动态核极化增强的固态NMR光谱†

的γ-Al ₂ ö ₃是一种重要的催化剂和催化剂载体的工业利益的。其酸/碱特性与表面结构相关，这一直是一个令人关注的问题。在这项工作中，在复杂的（子）表面氧种表面选择性标记的γ-Al ₂ ö ₃通过探查¹⁷ ö动态核极化表面增强NMR光谱（DNP-SENS）。直接的¹⁷ O MAS和间接的¹ H – ¹⁷ O交叉极化（CP）/ MAS DNP实验可以观察（亚）表面裸露的氧种类和羟基。特别是二维（2D）¹⁷ø3QMAS DNP光谱为用于在γ-Al取得的第一次₂ Ó ₃，其中两个O（Al）的₄和一个O（Al）的_3种裸氧物质进行了鉴定。在^17个Ô各向同性化学位移（δ _CS）改变从56.7到81.0 ppm的和四极耦合常数（C ^ _Q）的范围从0.6至2.5兆赫的三个氧物种。配位不饱和的O（Al）₃物种的特征是在这些氧位置中具有较高的场化学位移（56.7 ppm）和最大C _Q值（2.5 MHz）。2D ¹小时→ ¹⁷øHETCOR DNP实验允许我们区分3个桥接（铝_Ñ）-μ ₂ -OH和两个终端（铝_Ñ）-μ ₁ -OH羟基基团。裸氧种类物质和羟基的结构特征是对于类似的γ-Al ₂ ö ₃通过同位素标记的样品¹⁷ Ò ₂气体或H _2个¹⁷ O的结果这里介绍表明，表面选择性标记和DNP的组合-SENS是表征具有复杂表面物种的氧化物的有效方法。

更新日期：2018-06-01

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