Facile synthesis by polyol method of a ruthenium catalyst supported on γ-Al2O3 for hydrolytic dehydrogenation of ammonia borane

Ruthenium (1.5 wt% Ru) supported on γ-Al2O3 was synthesized in the presence of ethylene glycol using ruthenium acetylacetonate as precursor. The catalyst was characterized by inductively coupled plasma (ICP) emission spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS). It was found that it consisted of Ru0 and RuO2 as spherical nanoparticles, with an average size of 4.2 nm, and aggregates deposited at the surface of the support. The catalyst was tested in hydrolysis of ammonia borane, NH3BH3. Experiments with variable amounts of catalyst (10–30 wt%), concentrations of the substrate (1.0–0.65 M), and temperatures (50–66 °C) were performed. The reaction was followed by volumetric (inverted burette) and spectroscopic (11B and 11B{1H} NMR) methods. A kinetic study was developed (apparent activation energy of 67 kJ mol−1; reaction orders vs. the concentration of NH3BH3 and ruthenium active sites of 0.4 and 1.0, respectively). Hydrogen generation rates up to 76.4 mL(H2) min−1 or 158.5 L(H2) min−1 g−1 (for the highest catalyst loading) were measured, which classify our catalyst among the most reactive ones. The NMR study showed a rapid NH3BH3 hydrolysis also at room temperature with initial formation of B(OH)4−, which besides would favor following equilibriums of formation of polyborates. Finally, after deactivation of the catalyst in the first run, the hydrolysis mechanism resulted substantially unmodified. These results are reported and discussed herein.

Hydrolysis of ammonia borane NH3BH3 catalyzed by 1.5 wt% Ru/γ-Al2O3 was followed by spectroscopic method (11B and 11B{1H} NMR). It was observed that NH3BH3 directly hydrolyzes into tetrahydroxyborate B(OH)4−, from which polyborates form post-hydrolysis.Figure optionsDownload high-quality image (83 K)Download as PowerPoint slideResearch highlights
► Hydrolysis of ammonia borane, NH3BH3, catalyzed by 1.5 wt% Ru/γ-Al2O3 obeys a power rate law where the apparent activation energy is 67 kJ mol−1, the reaction orders vs. the NH3BH3 concentration is 0.4 and the reaction orders vs. the concentration of ruthenium active sites is 1.0.
► Through 11B NMR analysis, it was observed that the initial product of the reaction is B(OH)4−, with no other intermediates being detected during the reaction.
► Through 11B NMR analysis, it was observed that the presence of B(OH)4− would favor equilibriums of formation, post-hydrolysis, of polyborates.
► The catalytic behavior of Ru/γ-Al2O3 is identical along the experimental pathway despite a deactivation after a first run, consistently with a heterogeneous catalysis following the Langmuir–Hinshelwood mechanism.