Atomic-Scale Structure and Its Impact on Chemical Properties of Aluminum Oxide Layers Prepared by Atomic Layer Deposition on Silica
Atomic-Scale Structure and Its Impact on Chemical Properties of Aluminum Oxide Layers Prepared by Atomic Layer Deposition on Silica
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DOI：
10.1021/acs.chemmater.1c00516
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发表时间：
2021-04-23
影响因子：
8.6
通讯作者：
Florian, Pierre
中科院分区：
文献类型：
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作者：
Kaushik, Monu;Leroy, Cesar;Florian, Pierre
氧化铝和硅酸铝在不同的合成条件下制备，在多相催化中发挥着核心作用，具有广泛的工业应用前景。本文报道了三甲基铝在部分脱羟基二氧化硅上原子层沉积(ALD)得到的氧化铝层的原子级结构。通过涉及Al-27、H-1和Si-29核的各种一维和二维固体核磁共振(NMR)实验，获得了对随着Al含量增加而形成的物种的原子结构的如此详细的洞察。一维和二维实验数据集的多组分拟合表明，当沉积Al的质量分数为3.4wt%时，SiO_2表面形成了包含Al-[4]((3Si))、Al-[4]((4Si))和Al-[5]((2Si))物种的亚单层，其中大部分位置带有羟基。经过额外的ALD循环(沉积9.2或15.4wt%Al)后得到的薄膜具有高Al-[5]物种和丰富的OH基团的非晶态氧化铝相的特征。在SiO_2和Al_2O_3的界面上，最可能的物种是Al-[4]((2Si))、Al-[4]((3Si))和Al-[5]((2Si))。N-15动态核极化表面增强核磁共振谱(N-15 DNP SENS)和以N-15标记的吡啶为探针分子的红外光谱表明，无定形二氧化硅上的氧化铝氧化层同时含有强Bronsted和强Lewis酸中心，因此这些中心的相对丰度和性质以及表面的酸性都随着氧化铝薄膜的厚度(由ALD循环次数控制)而变化。这项研究首次对ALD制备的(亚)纳米级氧化铝薄膜在部分脱羟基二氧化硅载体上的生长情况进行了深入的原子尺度描述，为在分子水平上理解这种具有定制酸性的多相催化剂的催化活性开辟了道路。
Alumina and aluminosilicates, prepared under various synthesis conditions, play a central role in heterogeneous catalysis with a broad range of industrial applications. We report herein the atomic-scale structure of alumina layers obtained by atomic layer deposition (ALD) of trimethylaluminum onto partially dehydroxylated silica. Such a detailed insight into the atomic structure of the species formed with increasing Al content was gained using a variety of one- and two-dimensional solid-state nuclear magnetic resonance (NMR) experiments involving Al-27, H-1, and Si-29 nuclei. Multicomponent fittings of the 1D and 2D experimental data sets allowed us to show that at 3.4 wt % of deposited Al, a submonolayer containing Al-[4]((3Si)), Al-[4]((4Si)), and Al-[5]((2Si)) species forms on the silica surface, with most of these sites carrying OH groups. The films obtained after additional ALD cycles (depositing 9.2 or 15.4 wt % Al) feature characteristics of an amorphous alumina phase with a high concentration of Al-[5] species and abundant OH groups. The most probable species at the interface between silica and alumina are Al-[4]((2Si)), Al-[4]((3Si)), and Al-[5]((2Si)). N-15 dynamic nuclear polarization surface-enhanced NMR spectroscopy (N-15 DNP SENS) and infrared spectroscopy using N-15-labeled pyridine as a probe molecule reveal that aluminum oxide layers on amorphous silica contain both strong Bronsted and strong Lewis acid sites, whereby the relative abundance and nature of these sites, and therefore the acidity of the surface, evolve with increasing thickness of the alumina films (controlled by the number of ALD cycles). This study provides the first in-depth atomic-scale description of (sub-)nanometer-scale aluminum oxide films prepared by ALD as a function of their growth on a partially dehydroxylated silica support, opening the way to molecular-level understanding of the catalytic activity of such heterogeneous catalysts with tailored acidity.