Preparation Of Micronano-Transition Metal Composites For The Hydrolysis Of Ammonia Borane

Hydrogen as an efficient clean energy, has attracted widespread research concerns because its safty and friendship to the environment. Production, storage and transportation are the main points in the on-board application of hydrogen. So exploring novel hydrogen storage materials with high capacity remains a great challenge. Among many practical hydrogen storage materials, ammonia borane (NH3BH3, AB) is believed to be a prominent candidate owing to its low density, high hydrogen content (19.6wt%) and excellent stability in water. However, the hydrogen generation rate of AB in the absence of suitable catalysts is very slow, which seriously restricts its development as a promising hydrogen storage material. On the basis of the comprehensive overview of the process in AB as a hydrogen storage material, effects of transition metal catalysts and their composites on the catalytic hydrolysis of AB was systematically studied.(1) Triple-layered Ag@Co@Ni core-shell nanoparticles (NPs) containing a silver core, a cobalt inner shell, and a nickel outer shell were formed by an in situ chemical reduction method, and their growth mechanism and catalytic activities to the hydrolysis of AB were investigated. It was found that the growth mechanism for thetriple-layered Ag@Co@Ni core-shell NPs primarily takes advantage of the difference in reduction potentials of the three metal salts in the presence of the reducing agent AB and the relative magnetic permeability of different metals. It was found that the thickness of the double shells varied with different cobalt and nickel contents. Ag0.04@Co0.48@Ni0.48showed the most distinct core-shell structure, with a mean particle size of45-50nm. Compared with its bimetallic core-shell counterparts, this catalyst showed higher catalytic activity. The hydrolysis of AB catalyzed by Ag0.04@Co0.48@Ni0.48NPs completed within7min at298K, with a maximum hydrogen generation rate of2481.4mL·min-1·g-1. The activation energy was calculated to be39.37kJ·mol-1and the catalytic hydrolysis reaction was the first order with respect to the catalyst concentration. The synergetic interaction between Co and Ni in Ag0.04@Co0.48@Ni0.48NPs may play a critical role in the enhanced catalytic activity. Furthermore, cobalt-nickel double shells surrounding the silver core in the special triple-layered core-shell structure provided increasing amounts of active sites on the surface to facilitate the catalytic reaction.(2) Size controlled Ag0.04@Co0.48@Ni0.48core-shell NPs have been in situ synthesized by employing graphene (rGO) with different reduction degree as supports. The particle size of the NPs decreased with the increased reduction degree of rGO. Ag0.04@Co0.48@Ni0.48core-shell NPs supported on rGO reduced by high temperature method (rGO (H)) showed the smallest particle size (10-15nm). The number of C=O and C-O functional groups on the surface of rGO may play a major role in controlling the particle size. The strong steric hindrance effect of C=O resulted in the growth of large particles, while weak steric hindrance effect of C-O contributed to the formation of small particles. Therefore, the particle size of Ag0.04@Co0.48@Ni0.48NPs supported on rGO followed in this order:rGO reduced by solvothermal method (rGO (S))> rGO reduced by NaBH4(rGO (N))> rGO (H). The decrease in the particle size probably led to increasing active sites on the surface of catalysts. Thus, Ag0.04@Co0.48@Ni0.48/rGO (H) displayed the best catalytic activity. Kinetic studies demonstrated that the hydrolysis of AB catalyzed by Ag0.04@Co0.48@Ni0.48/rGO (H) NPs can complete within3.5min at298K, with a maximum hydrogen generation rate of4377.8mL·min-1·g-1. The catalytic hydrolysis reaction was the first order with respect to the catalyst concentration.(3) Non-noble Cu@FeCocore-shell NPs containing Cu core and FeCo shell have been successfully in situ synthesized via a facile chemical reduction method. The introduction of Cu core largely reduced the cost. It was verified that the synergetic interaction of the compositions contributed a lot to the enhanced catalytic activity of the catalysts. Among them, the Cu0.3@Fe0.1Co0.6NPs, with a particle size of10-15nm, showed the best catalytic activity. The hydrolysis of AB catalyzed by Cu0.3@Fe0.1Co0.6NPs can complete within3min at298K, with a maximum hydrogen generation rate of6674.2mL·min-1·g-1. It was found that the hydrolysis of AB catalyzed by Cu0.3@Fe0.1Co0.6NPs was the first order with respect to the catalyst concentration and the activation energy was calculated to be38.75kJ·mol-1. (4) Carbon aerogel (C) or rGO supported FeCo composite catalysts have been in situ synthesized by chemical reduction method and successfully employed in the hydrolysis of NH3BH3(AB) at room temperature. It was found that the contents of Fe and Co played a significant role in the catalytic activity. Among them, Fe0.3Co0.7showed the best catalytic activity. However, agglomeration emerged in Fe0.3Co0.7owning to the maganetic property of the NPs. Employing C or rGO as supports, the composites were well dispersed. The maximum loading of Fe0.3Co0.7on C and rGO can reach to40wt%and50wt%, respectively. Kinetic studies demonstrated that the composites exhibited excellent catalytic activity in the hydrolysis of AB, especially for the specimen of40wt%Fe0.3Co0.7/C and50wt%Fe0.3Co0.7/rGO. It was verified that the hydrolysis of AB catalyzed by40wt%Fe0.3Co0.7/C composite was completed within2min with a maximum hydrogen generation rate of13695.6mL·min-1·g-1at298K. In the presence of50wt%Fe0.3Co0.7/rGO, the catalytic hydrolysis of AB can rapidly complete within1min and its maximum hydrogen generation rate is as high as13915.7mL·min-1·g-1at298K. Their catalytic activities were greatly improved.