Dramatic synergy in CoPt nanocatalysts stabilized by "click" dendrimers for evolution of hydrogen from hydrolysis of ammonia borane

Abstract

Hydrolysis of ammonia borane (AB) is a very convenient source of H 2 , but this reaction needs catalytic activation to become practical under ambient conditions. Here this reaction is catalyzed by bimetallic late transition-metal nanoparticles (NPs) that are stabilized and activated by "click" dendrimers. Dendrimers 1 and 2 contain 27 or 81 triethylene glycol terminal groups and 9 or 27 1,2,3-triazole ligands, respectively, located on the dendritic tethers. A remarkable synergy between Pt and Co in the Pt-Co/"click" dendrimer nanocatalysts is revealed. These Pt-Co/"click" dendrimer catalysts are much more efficient for hydrolysis of AB than either "click" dendrimer-stabilized Co or Pt analogues alone. The best catalyst, Pt 1 Co 1 1, stabilized by the nonatriazole "click" dendrimer 1 achieves a turnover frequency number (TOF) of 303 mol H 2 mol cat -1 min -1 (606 mol H 2 mol Pt -1 min -1 ) at 20 ± 1 °C. The AB hydrolysis reaction catalyzed by Pt 1 Co 1 1 is boosted by NaOH, the TOF value reaching 476.2 mol H 2 mol cat -1 min -1 (952.4 mol H2 mol Pt -1 min -1 ), one of the very best results ever obtained for this reaction. The presence of ≥25% Pt in the CoPt nanoalloy provides a reaction rate higher than that obtained with the pure PtNP catalyst alone. The kinetics involves in particular a kinetic isotope effect k D k H of 2.46 obtained for the hydrolysis reaction with D 2 O, suggesting that an O-H bond of water is cleaved in the rate-determining step. Tandem reactions were carried out for the hydrogenation of styrene with hydrogen generated from the hydrolysis of AB. Performing this tandem reaction with D 2 O shows deuteration of the ethylbenzene products, confirming O-D bond cleavage and H/D scrambling on the bimetallic NP surface. Finally, a full reaction mechanism is proposed. This dramatic synergy type should also prove to be useful in a number of other catalytic systems.