Supported Cobalt-based Nano-catalysts For Hydrolytic Dehyrogenation Of Ammonia Borane

Ammonia borane?NH₃?BH₃?has been considered as a potential hydrogen storage material,with merits of its high hydrogen storage gravimetric efficiency?with a theoretical value of 19.6 wt.%?,good-stability and controllable hydrolytic dehydrogenation performance.In order to get pure and high-speed hydrogen stream,catalyst with high catalytic activity,facile preparation techniques,low cost and long life is the key factor to regulate the hydrolysis reaction of ammonia borane.Co-based transition metal catalysts are widely used in catalytic hydrolysis reaction.But the disadvantages such as generally imperfect catalytic activity,poor hydrolysis dynamics performance and uncontrolled agglomeration,which leading to attenuate recyclability retention during hydrolysis reaction obstructed their practical applications.Herein,on the basis of comprehensive overview of the hydrolytic dehydrogenation performances of NH₃?BH₃,effects of the preparation methods,microstructure and carriers on the catalytic hydrolysis performances were systematically studied.By adding surfactant PVA and controlling the preparation process,we successfully synthesized waxberry-like Co and Co–Mo catalysts.Aslo,reduced graphene oxide sheets as the catalyst carrier were introduced to synthesize waxberry-like Co@rGO and Co–Mo@rGO composite catalyst as highly efficient catalysts for hydrolytic dehydrogenation of ammonia borane.Firstly,the Co@rGO exhibited waxberry-like structure and amorphous state,good dispersion,uniform particle size with 25 nm.The waxberry-like Co@rGO catalyst needed 10.8 min for hydrolytic dehydrogenation of NH₃?BH₃,with an initial turnover frequency?TOF?of 12.14 mol _H2·min^–1·mol _Co^–1 and the activation energy of 45.49 kJ·mol^–1.The work also included the kinetic studies on zero-order with respect to substrate concentration and first-order reaction with respect to catalyst concentration.Secondly,the introduction of Mo for preparing waxberry-like Co–Mo@rGO catalyst helped to form alloy catalyst with better structure,better dispersivity and smaller particle size.When the molar ratio of Co and Mo was 0.75:0.25,the bimetallic composite catalyst exhibited the superior activity with TOF value of 16.29mol_H2·min^–1·mol^–1_Co-Mo.The activation energy of the reaction was calculated to be 43.72kJ·mol^–1.Furthermore,the reusability tests revealed that the waxberry-like Co–Mo@rGO catalyst still showed good catalytic activity with 80.3%of their initial activity in five successive runs.The enhanced catalytic activities were due to the charge transfer interaction between graphene sheets and waxberry-like Co–Mo NPs,which was beneficial to improve the dispersion and stability of bimetallic NPs.Also,ligand effects on the formation of waxberry-like structure and amorphous state provided rich active site to further promote the catalytic activity.Ag–Co/PAMAM bimetallic composite catalyst has been synthesized by a facile co-complexation chemical reduction method.By exploiting the well-defined dendritic spatial construction of PAMAM dendrimers as NPs surface modification agent and stabilizer,PAMAM dendrimers were beneficial to realize controllable growth of Ag–Co bimetallic nanoparticles,which obviously decreased grain size to 5 nm and improved intergranular reunion with unconspicuous agglomeration and uniformly distribution.Besides,the synergistic effect between PAMAM dendrimers and bimetallic component facilitated the weakness of B–N bonds and promoted the formation of activated intermediate,thereby accelerating the catalytic reaction for the hydrolytic dehydrogenation of NH₃?BH₃.The Ag–Co/PAMAM catalysts showed the composition dependent catalytic activity in catalytic dehydrogenation of NH₃?BH₃.When the molar ratio of Ag and Co was 3:7,the catalyst exhibited the superior activity within 7.1 min and yielding an initial turnover frequency?TOF?as high as 15.84 mol _H2·min^–1·mol^–1_Ag–Co.The activation energy of the reaction was calculated to be 35.66 kJ·mol^–1 and the catalyst retained 60%of their initial activity with a complete release of hydrogen in five successive runs.A novel dendritic polymer modified reduced graphene oxide composite supported heterogeneous Ag_0.3Co_0.7 hydrolysis catalyst was synthesized.By mixing multifunctional poly?amidoamine??PAMAM?dendrimers with graphene oxide dispersion,positively charged PAMAM dendrimers easily assembled with negatively charged graphene oxide sheets through electrostatic interaction and hydrogen bonding interaction.We demonstrated that the composite architecture as catalyst support benefited to form well-dispersed and uniformly distributed Ag_0.3Co_0.7 NPs with average diameter of 5 nm.The synergistic interaction between the composite carrier and Ag_0.3Co_0.7 bimetallic components helped to promote the hydrolysis reaction and significantly improved the catalytic activity.By regulating the structure and loading percentage of composite carrier,it was found that when the ratio of PAMAM and GO is 5:1 and the loading percentage of composite carrier was 25%,the catalyst showed the optimal catalytic hydrolytic performance with optimal TOF value(19.79 mol_H2·min^–1·mol^–1_Ag-Co)and low apparent activation energy(34.21 kJ·mol^–1).Nanoporous graphene?PG?sheets with abundant nanoporous and defects on the sheets were fabricated via carbothermal metal oxide etching method.Then,PG supported Pt-Co composite catalyst was synthesized by coprecipitation liquid phase reduction method.Pt–Co@PG catalysts showed improved catalytic activity for hydrolytic dehydrogenation of NH₃?BH₃.The synthesized Pt–Co@PG with a loading anount of 30wt.%Pt–Co?atomic ratio 1:9?exhibited a superior TOF value of 461.17mol_H2·min^–1·mol_Pt^–1 and the activation energy?E_a?value of 32.79 kJ?mol^–1 for NH₃?BH₃hydrolysis.In addition,the Pt–Co@PG catalysts showed well-established reusability with81.2%of their initial catalytic activity after five runs of reactions within 2.5 min and TOF value of 374.47 mol_H2·min^–1·mol_Pt^–1,demonstrating that they had high durability.The nanopores structures not only acted as the anchoring site for stabilizing and dispersing the Pt–Co NPs,but also yielded a synergistic effect between nanoporous graphene and metal NPs on enhancing the charge transfer and improving catalytic activity and stability.