Bulk and Surface Transformations of Ga2O3 Nanoparticle Catalysts for Propane Dehydrogenation Induced by a H2 Treatment

Surface metal hydrides (M-H) are ubiquitous in heterogeneous catalytic reactions, while the detailed characterizations are frequently hindered by their high reactivity/low concentration, and the complicated surface structures of the host solids, especially in terms of practical solid catalysts. Herein, combining instant quenching capture and advanced solid-state NMR methodology, we report the first direct and unambiguous NMR evidence on the highly reactive surface gallium hydrides (Ga-H) over a practical Ga2O3 catalyst during direct H2 activation. The spectroscopic effects of 69Ga and 71Ga isotopes on the 1H NMR signal are clearly differentiated and clarified, allowing a concrete discrimination of the Ga-H signal from the hydroxyl crowd. Accompanied with quantitative and two-dimensional NMR spectroscopical methods, as well as density functional theory calculations, information on the site specification, structural configuration, and formation mechanism of the Ga-H species has been revealed, along with the H2 dissociation mechanism. More importantly, the successful spectroscopic identification and isolation of the surface Ga-H allow us to clearly reveal the critical but ubiquitous intermediate role of this species in catalytic reactions, such as propane dehydrogenation and CO2 hydrogenation reactions. The analytic approach presented in this work can be extended to other M-H analysis, and the insights will benefit the design of more efficient Ga-based catalysts.

Direct Detection of Reactive Gallium-Hydride Species on the Ga2O3 Surface via Solid-State NMR Spectroscopy.

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Atomic-scale changes of silica-supported catalysts with nanocrystalline or amorphous gallia phases: implications of hydrogen pretreatment on their selectivity for propane dehydrogenation

We synthesized Ga2O3 on γ-Al2O3 support with atomic layer deposition method (ALD) for the CO2-oxidative dehydrogenation of propane (CO2-ODHP). The Ga loading from 1 to 2.9 wt% was controlled by the number of ALD cycles from 1 to 3. For the comparison purposes, the equivalent catalysts were prepared with incipient wetness impregnation method. The catalysts were characterized by a variety of methods including temperature programmed desoprtion of NH3 (NH3-TPD), X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM) and energy dispersive spectroscopy mapping (EDS-Mapping). We identified the ALD catalysts more active and selective in CO2-ODHP reaction due to the higher surface total moderate acidity, Ga-O-Al linkages and Ga2O3 dispersion. The ALD-3C, with 2.9% Ga as the catalyst with the highest performance, gave 38% conversion and 82% selectivity to propylene after 120 min of CO2-ODHP reaction at 600 °C which was equal to 96% improvement in the propylene yield. 

Atomic layer deposition of Ga2O3 on γ-Al2O3 catalysts with higher interactions and improved activity and propylene selectivity in CO2-assisted oxidative dehydrogenation of propane

Gallia–alumina (Ga,Al)2O3(x : y) spinel-type solid solution nanoparticle catalysts for propane dehydrogenation (PDH) were prepared with four nominal Ga : Al atomic ratios (1 : 6, 1 : 3, 3 : 1, 1 : 0) using a colloidal synthesis approach. The structure, coordination environment and distribution of Ga and Al sites in these materials were investigated by X-ray diffraction, X-ray absorption spectroscopy (Ga K-edge) as well as 27Al and 71Ga solid state nuclear magnetic resonance. The surface acidity (Lewis or Bronsted) was probed using infrared spectroscopy with pyridine and 2,6-dimethylpyridine probe molecules, complemented by element-specific insights (Ga or Al) from dynamic nuclear polarization surface enhanced cross-polarization magic angle spinning 15N{27Al} and 15N{71Ga} J coupling mediated heteronuclear multiple quantum correlation NMR experiments using 15N-labelled pyridine as a probe molecule. The latter approach provides unique insights into the nature and relative strength of the surface acid sites as it allows to distinguish contributions from Al and Ga sites to the overall surface acidity of mixed (Ga,Al)2O3 oxides. Notably, we demonstrate that (Ga,Al)2O3 catalysts with a high Al content show a greater relative abundance of four-coordinated Ga sites and a greater relative fraction of weak/medium Ga-based surface Lewis acid sites, which correlates with superior propene selectivity, Ga-based activity, and stability in PDH (due to lower coking). In contrast, (Ga,Al)2O3 catalysts with a lower Al content feature a higher fraction of six-coordinated Ga sites, as well as more abundant Ga-based strong surface Lewis acid sites, which deactivate through coking. Overall, the results show that the relative abundance and strength of Ga-based surface Lewis acid sites can be tuned by optimizing the bulk Ga : Al atomic ratio, thus providing an effective measure for a rational control of the catalyst performance.

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Uncovering selective and active Ga surface sites in gallia–alumina mixed-oxide propane dehydrogenation catalysts by dynamic nuclear polarization surface enhanced NMR spectroscopy

Zhuoran Wang

Papers

Uncovering selective and active Ga surface sites in gallia–alumina mixed-oxide propane dehydrogenation catalysts by dynamic nuclear polarization surface enhanced NMR spectroscopy