Magnetically separable rhodium nanoparticles as catalysts for releasing hydrogen from the hydrolysis of ammonia borane

Abstract

Magnetically separable catalysts attract considerable attention in catalysis due to their facile separation from the reaction medium. This propensity is crucial for efficient multiple use of precious noble metal nanoparticles in catalysis. In fact, the isolation of catalysts from the reaction medium by filtration and washing results usually in the loss of huge amount of activity in the subsequent run of catalysis. Although many transition metal nanoparticle catalysts have been reported for the H₂ generation from the hydrolysis of ammonia borane, there is no study reporting the magnetically separable rhodium based catalysts for the hydrolytic dehydrogenation of ammonia borane. Here, we report the preparation of rhodium(0) nanoparticles supported on the surface of Fe₃O₄ and CoFe₂O₄ magnetic nanopowders as the first example of magnetically separable rhodium nanocatalysts. The resulting magnetically separable Rh⁰/Fe₃O₄ and Rh⁰/CoFe₂O₄ nanoparticles are highly active, long-lived and reusable catalysts in H₂ generation from the hydrolysis of ammonia borane providing a turnover frequency value of 273 and 720 min^-1, respectively, at 25.0 ± 0.1 °C. These magnetically separable catalysts show high reusability and long-term stability in the hydrolysis reaction. They retain their complete initial activity even after the 5th use releasing exactly 3.0 equivalent H₂ gas per mole of ammonia borane. The long-term stability tests show that Rh⁰/Fe₃O₄ and Rh⁰/CoFe₂O₄ nanoparticles provide a total turnover number of 125,000 and 245,000, respectively, in releasing H₂ from the hydrolysis of ammonia borane at room temperature. The long term stability and reusability of magnetically separable Rh⁰/Fe₃O₄ and Rh⁰/CoFe₂O₄ nanoparticles make them attractive catalysts for hydrogen generation in fuel cell applications.