Click Dendrimer-Stabilized Nanocatalysts for Efficient Hydrogen Release upon Ammonia-Borane Hydrolysis

Ammonia-borane is one of the most convenient sources of H-2 upon hydrolysis under ambient conditions, but the reaction requires a good catalyst to become efficient. Here, H-2 production upon hydrolysis of ammonia-borane is catalyzed by late transition-metal nanoparticles (NPs). These NPs are stabilized by a first- or second-generation click dendrimer containing, respectively 27 and 81 terminal triethylene glycol termini and 9 resp. 27 1,2,3-triazole intradendritic ligands. No significant dendritic effects were observed, however. The noble-metal NPs are as expected much more efficient catalysts than the first-raw transition-metal NPs, and Rh and PtNPs are the most catalytically active NPs. In the presence of NaOH, however, the reactivity is boosted for all these click dendrimer-stabilized transition-metal NP catalysts except for the PtNPs. The optimized NaOH concentration is 0.3M NaOH per mol NH3BH3 for RhNPs. A TOF of up to 611 mol(H2) mol(catalyst)(-1) min(-1) for RhNPs at 20 degrees C is obtained, which is one of the best results among the literature. Interestingly, the reaction with D2O provides a kinetic isotope effect of k(D)/k(H)=2.8 suggesting that O-H bond cleavage of water occurs in the rate-limiting step. These experiments lead to a proposed mechanism for H-2 evolution using the ammonia-borane hydrolysis reaction.