Nature of GaOx Shells Grown on Silica by Atomic Layer Deposition

Abstract

Gallia-based shells with a thickness varying from a submonolayer to ca. 2.5 nm were prepared by atomic layer deposition (ALD) using trimethylgallium, ozone, and partially dehydroxylated silica, followed by calcination at 500 °C. Insight into the atomic-scale structure of these shells was obtained by high-field ⁷¹Ga solid-state nuclear magnetic resonance (NMR) experiments and the modeling of X-ray differential pair distribution function data, complemented by Ga K-edge X-ray absorption spectroscopy and ²⁹Si dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) studies. When applying one ALD cycle, the grown submonolayer contains mostly tetracoordinate Ga sites with Si atoms in the second coordination sphere (^[4]Ga_(Si)) and, according to ¹⁵N DNP SENS using pyridine as the probe molecule, both strong Lewis acid sites (LAS) and strong Brønsted acid sites (BAS), consistent with the formation of gallosilicate Ga-O-Si and Ga-μ²-OH-Si species. The shells obtained using five and ten ALD cycles display characteristics of amorphous gallia (GaO_x), i.e., an increased relative fraction of pentacoordinate sites (^[5]Ga_(Ga)), the presence of mild LAS, and a decreased relative abundance of strong BAS. The prepared Ga1-, Ga5-, and Ga10-SiO_2-500 materials catalyze the dehydrogenation of isobutane to isobutene, and their catalytic performance correlates with the relative abundance and strength of LAS and BAS, viz., Ga1-SiO_2-500, a material with a higher relative fraction of strong LAS, is more active and stable compared to Ga5- and Ga10-SiO_2-500. In contrast, related ALD-derived Al1-, Al5-, and Al10-SiO_2-500 materials do not catalyze the dehydrogenation of isobutane and this correlates with the lack of strong LAS in these materials that instead feature abundant strong BAS formed via the atomic-scale mixing of Al sites with silica, leading to Al-μ²-OH-Si sites. Our results suggest that ^[4]Ga_(Si) sites provide strong Lewis acidity and drive the dehydrogenation activity, while the appearance of ^[5]Ga_(Ga) sites with mild Lewis activity is associated with catalyst deactivation through coking. Overall, the atomic-level insights into the structure of the GaO_x-based materials prepared in this work provide a guide to design active Ga-based catalysts by a rational tailoring of Lewis and Brønsted acidity (nature, strength, and abundance).