Hydroxyapatite supported ruthenium(0) nanoparticles catalyst in hydrolytic dehydrogenation of ammonia borane: Insight to the nanoparticles formation and hydrogen evolution kinetics

Abstract

When a solution of ammonia borane is added to the suspension of ruthenium(III) ions supported on hydroxyapatite, both reduction of ruthenium(III) to ruthenium(0) nanoparticles and hydrogen release from the hydrolysis of ammonia borane occur concomitantly at room temperature. Using the hydrogen evolution from the hydrolysis of ammonia borane as reporter reaction provides valuable insights to the formation kinetics of ruthenium(0) nanoparticles. Thus, the rate constants for the slow nucleation and autocatalytic surface growth of ruthenium(0) nanoparticles could be obtained. Furthermore, the evaluation of rate constants at various temperatures provides the estimation of activation energies for both reactions; Ea=166±7kJ/mol for the nucleation and Ea=59±2kJ/mol for the autocatalytic surface growth of ruthenium(0) nanoparticles. The ruthenium(0) nanoparticles, in situ formed during the hydrolysis of ammonia borane and supported on hydroxyapatite, could be isolated from the reaction solution and characterized by a combination of advanced analytical techniques. The results show that (i) highly dispersed ruthenium(0) nanoparticles of 4.7±0.7nm size were formed on the surface of hydroxyapatite, (ii) they are highly active catalyst in the hydrolytic dehydrogenation of ammonia borane with a turnover frequency value of 137min−1 at 25.0±0.1°C, and (iii) they are long lived and reusable catalyst providing 87,000 turnovers for hydrogen generation from the hydrolysis of ammonia borane and preserving 92% of their initial catalytic activity even after the fifth run of hydrolysis of ammonia borane at 25.0±0.1°C. The results of kinetic study on the hydrogen generation from the hydrolysis of ammonia borane were also reported including the activation energy of 58±2kJ/mol for the hydrolytic dehydrogenation of ammonia borane.