Nature of GaO_x 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 71 Ga 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 29 Si 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 15 N 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–μ 2 -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–μ 2 -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).

Reversible transformation of sub-nanometer Ga-based clusters to isolated [4]Ga(4Si) sites creates active centers for propane dehydrogenation

“…Ga1–SiO 2–500 was synthesized using a previously reported method ( vide infra ), 21 and is denoted hereafter as Ga1 (500) . Exposure of Ga1 (500) to ambient air gave a material denoted as Ga1 (500-air) .…”

“…The resulting material was calcined, without exposure to ambient air, at 500 °C in a flow of synthetic air for 4 h (flow rate was 50 ml min −1 , heating ramp was 5 °C min −1 ) to yield Ga1 (500) that was handled and stored in a glovebox (H 2 O, O 2 < 1 ppm). 21…”

“…17–20 In our recent work, targeting at understanding the structure and surface properties of Ga-based films generated on a dehydroxylated silica support, we have observed that the deposition of one ALD cycle of trimethylgallium followed by calcination at 500 °C yields a submonolayer coverage of tetracoordinate and pentacoordinate Ga 3+ sites that contain, in their second coordination sphere, both Si and Ga atoms (that is, [4] Ga (Si/Ga) and [5] Ga (Si/Ga) , respectively). 21 Interestingly, [4] Ga (Si/Ga) sites provide strong Lewis acidity and their higher relative amount correlates with a higher activity in isobutane dehydrogenation. That being said, materials obtained using five and ten ALD cycles show a higher relative fraction of [5] Ga (Si/Ga) sites that are associated with mild Lewis acidity and are also less active in isobutane dehydrogenation and produce more coke.…”

“…sub-nanometer Ga x O y (OH) z clusters that feature no diffraction peaks and are not-discernable by high-resolution TEM. 21 Calcination at 650 °C leads to their conversion into [4] Ga (4Si) sites that are similar (both structurally and catalytically) to the isolated sites that are obtained by SOMC, i.e. , [(SiO) 3 Ga(XOSi)] sites (one strong LAS and one strong BAS).…”

71Ga NMR Signatures of Lewis and Brønsted Acid Sites in Gallium Silicates Evidenced and Deciphered upon Interaction with Probe Molecules

“…Gallium-containing materials are employed as catalysts in an array of industrially relevant reactions, including alkane dehydrogenation and dehydroaromatization, among others. − However, the role and the structures of Ga sites in these materials are often debated, with an array of oxidation states, geometries, and charge states invoked across the existing literature. − ,− This apparent ambiguity is due, in part, to the fact that direct insights into the distribution and geometry of Ga surface sites using conventional spectroscopic techniques remains challenging. One technique that has been widely used for understanding Ga-containing oxide materials is X-ray absorption spectroscopy (XAS), which can be used to provide insights into the coordination environment of Ga in materials.…”