Alumina and aluminosilicates, prepared under various synthesis conditions, play a central role in heterogeneous catalysis with numerous industrial applications, thanks to their unique surface properties, mainly Brønsted and Lewis acidities. The amorphous aluminosilicates are currently less explored, despite the fact that they display controllable surface acidities that are particularly interesting in catalysis. Thus, the detailed understanding of surface and bulk sites in amorphous aluminosilicates remains elusive.
In this work, we have used Atomic Layer Deposition (ALD) to control the growth of an alumina layer on top of silica nanoparticles (coatings from 1 to 10 nm in thickness), and investigate with state-of-the-art NMR methodologies (1D along with 2D ¹H/²⁷Al, ²⁹Si/²⁷Al and ²⁷Al/²⁷Al correlations), the structure of the ALD film as well as its interface with the silica support. A film of amorphous am-Al₂O₃ (42% of ^VAl and less than 5 % of ^VIAl) was observed, and the interface between the deposited alumina and the surface of the silica support was shown to be mainly built up of ^IVAl_(3Si), ^IVAl_(2Si) and ^VAl_(2Si) species. As the number of cycles increases, the surface was found to evolve from ^IV,VAlOSi environments towards ^IV,VAlO^IV,VAl environments. Both types of species can carry OH groups, forming thereby weak Brønsted acid sites. Strong Brønsted acid sites, formed by an ⁿAl species nearby a silanol SiOH group, are expected only for small number of ALD deposition cycles. ¹⁵N DNP SENS spectroscopy via ¹⁵N-labeled pyridine as a surface probe were used to identify the surface Brønsted and Lewis acid sites.
Overall, our work presents the extensive use of ssNMR technics in order to determine, in detail, the structure of a complex layer of aluminosilicates. Notably, we describe the structure of (i) the interface between silica and alumina and (ii) the surface sites as a function of the ALD layer thickness and provide a molecular view of the growth of an oxide layer.

Monu Kaushik

Appearances