The Research On The Deposition Of Transition Metal Nanoparticles On The Surface Of Graphene And Its Applications In Catalyst

Hydrogen energy has become one of the most promising energies due to the fact that exhaustion of the traditional fossil energy will threaten our survival and development in a few decades. In exploring the application of hydrogen, the main challenge is to solve the problem of hydrogen storage. Compared to gaseous and liquid hydrogen storage methods, solid hydrogen storage has attracted considerable interest. Among various solid hydrogen storage materials, ammonia borane (NH3BH3, AB) is one of the most promising candidates due to its high hydrogen content, outstanding environmental stability and non-toxicity, which has stimulated substantial efforts to develop high efficient dehydrogenation catalysts.Transition metal nanoparticles (NPs) such as iron (Fe), cobalt (Co) and nickel (Ni) are quite attractive due to their advantages in cost, catalytic activity and recycling capability. However, their magnetism and strong aggregation propensity make it a challenge to develop high efficient dehydrogenation catalysts. In this study, we have utilized polyethyleneimine (PEI) to assist the deposition of transition metal NPs on graphene oxide (GO) and explored the application in catalytic dehydrogeneration of AB. Graphene is a two-dimensional platelet-like carbon film with the thickness of one carbon atom. Its specific surface area is as high as2630m2/g, an ideal candidate for supporting NP catalysts. PEI can effectively sequester the metal ions from the bulk solution, enabling numerous "nanoreactors" to be created on the graphene surface. With such PEI-GO composites as supports, we have achieved effective control to the size and morphology of transition metal NPs and demonstrated their remarkably improved catalytic properties.(1). Branched PEI contains a large amount of electron-rich amine groups, which enable them to be adsorbed or covalently linked to GO or reduced GO surfaces by physical and covalent interactions. Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) results show that PEI molecules can form flat, collapsed pancake-like adsorption layer structures on the GO surfaces. With the increase of the PEI concentration in PEI-GO suspensions, more PEI molecules are attached on the GO surface, and the PEI adsorption layers become thicker.(2). Fe-Ni NPs have been prepared by a simple co-reduction method of metal precursors with sodium borohydride (NaBH4) under ambient condition. The content of PEI attached on GO significantly affects the morphology and size of resulting bimetallic NPs. For the high PEI content, where most of the metal ions are confined within the attached PEI layer, the homogeneous nucleation and growth in the bulk solution are suppressed so that small, amorphous bimetallic NPs (～3nm in diameter) are formed. Compared to the Fe-Ni NPs directly deposited on GO, the PEI-decorated NPs on GO reveal a dehydrogenation rate of982ml/min/g at293K for the hydrolysis of AB, which is18times faster than that of the former and nearly comparable to that of the platinum catalyst deposited on carbon under the same condition.(3). A Co-B nanoparticles(NPs) catalyst with PEI-GO as the support has been synthesized by a simple method in which metal precursors were reduced by NaBH4under ambient condition. PEI plays a key role in controlling the size and morphology of Co-B NPs, by which most of the metal ions are confined within the attached PEI layer for the heterogeneous nucleation and growth. XRD and TEM results show that small, amorphous Co-B NPs with an average size-3nm are formed. The PEI-GO supported Co-B NPs exhibit high catalytic activity for dehydrogenation of AB due to their small particle size and good spatial distribution. The obtained hydrogen generation rate is up to1987ml/min/g at293K, higher than that of most of the reported Co-B based catalyst. The hydrolysis of AB catalyzed by PEI-GO/Co-B catalyst is nearly first-order with respect to the catalyst concentration and nearly zero-order reaction with respect to the NH3BH3concentration. Moreover, the activation energy for hydrolysis of AB reveals a very low value,16.65kJ/mol.