The Synthesis Of Ir-based Noble Metal Nanocrystals And Their Catalytic Properties

Materials is the foundation of society development and provide continuous driving force for the development of social economy, material civilization and spiritual civilization. Because of the quantum size effect, small size effect and surface effect, materials at the nanoscale could display very interesting shape and size dependent physical and chemical properties which are different from their bulk counterparts. Noble metal nanomaterials have attracted great interest due to their preferable properties and potential applications in imaging, sensing, electronics, medicine and especially in the field of catalysis. Because the physical and chemical properties of nanocrystals could be widely tuned by tailoring the morphology, as a consequence, over the past years, synthesis of noble metal nanocrystals with a special morphology has been a hot research topics in the field of nanotechnology and nanoscience. Although amounts of noble metal nanocrystals with different shape and size has been prepared, the higher cost and limited catalytic activity and stability prevent them from practical application. In this thesis, our research mainly concentrates on the preparation of Ir based nanocrystals with improved catalytic performance by solvothermal method and the formation mechanism of these nanocrystals. In addition, the catalytic performance of the as-prepared nanocrytals are discussed in detail.First of all, we developed an efficient and simple method to synthesize branched Ir nanodendrites(NDs) with enhanced catalytic performance based on the oriented attachment mechanism. Transmission electron microscopy(TEM) images of the samples which were got out from the reaction solution at different time intervals indicate that the Ir NDs were formed via an imperfect oriented attachment formation mechanism. The shape and size of the Ir NDs could be hardly tuned by changing the precursor concentrations and reaction temperatures. However, the binding affinity of the ligands could drastically tune the shape and size of nanocrystals. Metal hydroxide and oxide-supported Ir NDs display improved activity for the catalytic oxidation of CO. Particularly, Fe(OH)x-supported Ir NDs with a 4 wt% Ir loading exhibit great CO oxidation catalytic performance and the full conversion temperature of CO is 120 oC. Moreover, in comparison with Ir nanoparticles(NPs) and Ir black, Ir NDs exhibit enhanced performance for the catalytic oxidation of ammonia. It is worth noting that the specific and mass activity of Ir NDs are 1.7 and 7 times higher than that of Ir NPs for the catalytic oxidation of ammonia. The improved catalytic performance of Ir NDs are derived not only from their specific surface area, but also from their rich active sites. The excellent catalytic performance of Ir NDs may open new ways for direct ammonia fuel cells and proton-exchange membrane fuel cells.Moreover, we demonstrate the synthesis of single-crystalline Cu-Ir polyhedral nanocages by a facile ―hot-injection‖ method with Cu NPs as a template based on a modified galvanic replacement mechanism. TEM images, localized surface plasmon resonance absorption spectra and the temporal evolution of the crystal structure and crystallinity reveal the temperal growth process from Cu NPs to Cu-Ir nanocages. Compared with solid Ir NPs, hollow porous nanostructure diminish amounts of buried nonfunctional Ir atoms, improving the using efficiency of Ir. Furthermore, alloying Ir with Cu not only enables the reduction of Ir loading but also the modification of the electron structure of Ir Ox, resulting in an improved catalytic performance. The as-synthesized Cu-Ir nanocages display improved catalytic performance toward the oxygen evolution reaction(OER) in 0.05 M H2SO4. Affording a current density of 10 m A/cm2, Cu1.11 Ir nanocages just need a overpotential of 286 m V for the OER, which is lower than that of Ir NDs(303 m V) and commercial Ir black(311 m V). For Cu1.11 Ir nanocages at an overpotential of 280 m V, the Ir-based mass activity can reach 73 m A/mg Ir, which is 2.65 times higher than that of Ir black(27.5 m A/mg Ir). Therefore, we expect that Cu1.11 Ir nanocages could be a promising candidate for acid polymer electrolyte membrane water electrolyzers.Finally, we present the synthesis of poly-crystalline Ni-Ir hollow porous nanocages with the same method as the synthesis of Cu1.11 Ir nanocages. TEM images and elemental mapping images indicate the hollow porous sturcture of Ni2.53 Ir nanocages. By tuning the molar ratio of precursors, core-shell NPs with a Ni core and a Ni-Ir alloy shell or hollow porous Ni-Ir nanocages with Ir branches on their surface can be obtained. In comparison with solid NPs, hollow porous bimetallic NPs possess high surface to volume ratio, large void space and synergetic effects, thus improving their catalytic activity. After voltammetric cycling in H2SO4 aqueous solution, Ni doped Ir Ox hollow porous nanostructures exhibit improved catalytic performance toward the OER in 0.05 M H2SO4, with a smaller Tafel slope of 46.6 m V/decade, a smaller overpotential of 302 m V affording a current density of 10 m A/cm2. Hence, the synthesized Ni2.53 Ir could be a promising OER catalyst for practical electrocatalytic water splitting systems.