Atomic-Scale Structure and Its Impact on Chemical Properties of Aluminum Oxide Layers Prepared by Atomic Layer Deposition on Silica

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 27Al, 1H, and 29Si 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 [4]Al(3Si), [4]Al(4Si), and [5]Al(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 [5]Al species and abundant OH groups. The most probable species at the interface between silica and alumina are [4]Al(2Si), [4]Al(3Si), and [5]Al(2Si). 15N dynamic nuclear polarization surface-enhanced NMR spectroscopy (15N DNP SENS) and infrared spectroscopy using 15N-labeled pyridine as a probe molecule reveal that aluminum oxide layers on amorphous silica contain both strong Brønsted 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.