Hydroxyapatite-nanosphere supported ruthenium(0) nanoparticle catalyst for hydrogen generation from ammonia-borane solution: kinetic studies for nanoparticle formation and hydrogen evolution

Abstract

The development of readily prepared effective heterogeneous catalysts for hydrogen generation from ammonia-borane (AB; NH₃BH₃) solution under mild conditions still remains a challenge in the field of “hydrogen economy”. In this study, we report our finding of an in situ generated, highly active ruthenium nanocatalyst for the dehydrogenation of ammonia-borane in water at room temperature. The new catalyst system consists of ruthenium(0) nanoparticles supported on nanohydroxyapatite (RuNPs@nano-HAp), and can be reproducibly prepared under in situ conditions from the ammonia-borane reduction of Ru³⁺ ions exchanged into nanohydroxyapatite (Ru³⁺@nano-HAp) during the hydrolytic dehydrogenation of ammonia-borane at 25 ± 0.1 °C. Nanohydroxyapatite-supported ruthenium(0) nanoparticles were characterized by a combination of advanced analytical techniques. The sum of their results shows the formation of well-dispersed ruthenium(0) nanoparticles with a mean diameter of 2.6 ± 0.6 nm on the surface of the nanospheres of hydroxyapatite by keeping the host matrix intact. The resulting RuNPs@nano-HAp are highly active catalyst in the hydrolytic dehydrogenation of ammonia-borane with an initial TOF value of 205 min⁻¹ by generating 3.0 equiv. of H₂ per mole of ammonia-borane at 25 ± 0.1 °C. Moreover, they are sufficiently stable to be isolated and bottled as solid materials, which can be reused as active catalyst under the identical conditions of first run. The work reported here also includes the following results: (i) monitoring the formation kinetics of the in situ generated RuNPs@nano-HAp by hydrogen generation from the hydrolytic dehydrogenation of ammonia-borane as the reporter reaction. The sigmoidal kinetics of catalyst formation and concomitant dehydrogenation fits well to the two-step, slow nucleation, followed by autocatalytic surface growth mechanism, P → Q (rate constant k₁) and P + Q → 2Q (rate constant k₂), in which P is Ru³⁺@nano-HAp and Q is the growing, catalytically active RuNPs@nano-HAp; (ii) the compilation of kinetic data for the RuNPs@nano-HAp catalyzed hydrolytic dehydrogenation of ammonia-borane depending on the temperature and catalyst concentration to determine the dependency of reaction rate on catalyst concentration and activation parameters (E_a, ΔH^#, and ΔS^#) of the reaction.