Carbon aerogel supported first-row transition metal catalysts for methanolyis of ammonia borane
Ni-Ni3B, Co-B, and Co-Ni-B nanocomposites decorated on carbon aerogel were successfully synthesized by adopting the solution impregnation method. The nanocomposites were characterized by powder XRD, transmission electron microscopy and scanning electron microscopy . Although the as-prepared samples were found to be amorphous, upon annealing at higher temperatures, the catalysts turned crystalline and exhibited sharp and distinct peaks in the XRD pattern. The bright field TEM images and the high angle annular dark field images depicted that the metal nanocomposite particles were indeed dispersed on the carbon aerogel matrix which was further confirmed by TEM point and area EDS analysis and SEM area EDS analysis. When these catalysts were employed in the methanolysis of ammonia borane in different cat/AB wt% compositions, the catalytic activity was found to increase with increase in the concentration of the catalysts. Although the bare catalysts liberated three moles of H2 gas from AB, the supported catalysts evolved a maximum of 2.1 moles of H2 gas in the cat/AB = 0.5 by wt%. Moreover, catalytic activity of these catalysts was found to decrease in the order: Co- Co2B/CA< Ni-Ni3B/CA< Co-Ni-B/CA unlike the order observed in the case of bare catalysts. Recyclability of these catalysts was checked to determine the stability of these catalysts for longer runs. The Ni-Ni3B/CA and Co-Ni-B/CA catalysts showed considerable activity upto 8 cycles, whereas the Co-B/CA showed a significant drop in activity in the 4th cycle itself. Upon further characterization of the recovered catalysts upon recycling, it was found that the Co- B/CA catalyst got oxidised to Co3O4, while the Ni-Ni3B/CA and Co-Ni- B/CA catalysts remained partially oxidized and unoxidized respectively. Thus, the drop in the catalytic activity of the Co-B/ CA catalyst in comparison to the other catalysts during recyclability tests could be attributed to complete oxidation of the catalyst.