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 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.

Automated summary

This study reveals the atomic-scale structure and chemical properties of alumina films prepared by ALD on silica surface using organometallic chemistry and NMR techniques. The aluminum content and nature of acidic sites in the alumina films change with increasing ALD cycles, impacting the catalytic activity of heterogeneous catalysts.