Nature of GaOx Shells Grown on Silica by Atomic Layer Deposition

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 (GaOx), 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-SiO2–500 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-SiO2–500, a material with a higher relative fraction of strong LAS, is more active and stable compared to Ga5- and Ga10-SiO2–500. In contrast, related ALD-derived Al1-, Al5-, and Al10-SiO2–500 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 GaOx-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).

The shell remains amorphous, for Ga10-SiO2−500, up to ca. 820 o C.   Note that small line shape distortions are seen between 0 kHz and 100 kHz in the 28.2 T experiment. . 71Ga MAS NMR experimental (black) and simulated (red) spectra of Ga10-SiO2−500 recorded at 20.0 T or 28.2 T. The various components are shown in different colors: [4] Ga (purple), [5] Ga (blue) and [6] Ga (yellow).

Figure S14 .
Figure S14.Comparison of the 71 Ga 20.0 T QPASS spectra of Ga10-SiO 2−500 (left) and Ga5-SiO 2−500 (right) showing the projection along the MAS dimension (blue) compared with the CPMG spectra (red).This experiment leads to the loss of intensities on the right and left side of the main peak, particularly for Ga5-SiO2−500.
S0 2 was fitted to 1 as obtained by fitting a β-Ga2O3 reference.CN stands for the coordination number.*Debye-Waller factors σ 2 were constrained to the same value for the Ga-Ga and Ga-Si paths.

Figure S26 .
Figure S26.PDF and dPDF of Ga1-, Ga5-and Ga10-SiO2−500.The traces in Figure S26 were obtained by subtraction of the PDF of SiO2−500 (black line in A-C) from the PDF of Ga1-(A), Ga5-(B) and Ga10-SiO2−500 (C).For the subtraction, all of the PDFs were normalized to the peak at ca. 1.5-1.6Å (due to the Si−O distance).

Figure S28 .Figure S29 .
Figure S28.(A)-(C) Peak fittings obtained with SrMise.*(D) Fitted positions of peaks a and b and their area ratio (b/a).* The quality of the peak fittings in SrMise is assessed using the Akaike information criterion (AIC), which is implemented in SrMise as AIC = χ 2 + 2k where χ 2 is the standard chi-square error (  2 = ∑   2 /  2 with residuals   and uncertainties   of the i-th data point) and k is the number of free parameters in the model.A lower value of AIC indicates a more plausible model, however only differences between the AIC of fittings of the same data are meaningful.3

Figure S30 .
Figure S30.RMC fits of Ga1-, Ga5-, Ga10-SiO2−500 and the partial contribution of Ga-O and Ga-Ga interatomic pairs without the introduction of any O-atom point defects.
*All samples were measured at ambient temperature.