Study on development of various Pt based nanostructured catalytic materials and their properties of catalytic reaction

Abstract

Nanoparticles are particles between 1 and 100 nm in size. It has distinct physical, optical, electromag-netic properties, which are different from those of bulk materials. Thus, nanoparticles research has received attention from many scientists over the past few decades because it is a wide variety of potential applications including fuel & solar cells, sensing & removal of hazardous materials, nanomedicine, optoelectronics, plas-monics, photovoltaics, and nanoelectronic devices fields. Among the various applications, i focus on the de-velopment of nanostructure catalytic material for the fuel cell application. Nowadays, a strategic develop-ment for both efficient and economical catalysts is required in fuel cell field. Shape and composition control of the metal NPs are a representative of alternative means to improve catalytic efficiency because the cata-lytic properties could depend on the shape and composition of the metal NPs. In this sense, the main purpose of this dissertation focused on the synthesis of shape and composition controlled metal NPs and study on their application for fuel cell. We have also studied the development of supporting material as another alter-native means to enhance catalytic performance. In chapter 2, dendritic Pt catalysts are one of superb candidates in the proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) due to high surface-area-to-volume ratio, plenty of adsorption site, and surface permeability. However, monometallic Pt catalyst suffers from poisoning asso-ciated with intermediates adsorbates, which limits methanol oxidation reaction (MOR) or oxygen reduction reaction (ORR) kinetics and long-term stability. To improve electocatalytic properties of Pt, morphology con-trol of the Pt particles and using Pt-based bimetallic alloy nanoparticles instead of pure Pt as the electrocata-lyst have been extensively studied for the past decade. In this regard, we developed a facile aqueous synthe-sis method for preparation of Au@Pt core-shell and Pd@Pt core-shell nanostructures based on Pt dendritic shell. As a results, prepared bimetallic core-shell nanostructures have exhibited higher electrocatalytic activity, stability, and durability than those of the monometallic Pt dendritic catalysts toward ORR and MOR because bimetallic core-shell nanostructures can be finely tuned by manipulating their core and shell structures. In this reason, core effect was investigated via synthesized Au@Pt core-shell nanostructures consisting of a dendritic Pt shell and structured Au cores (nanocubes, nanorods, and nanooctahedra) toward ORR. And also, we found that the electrocatalytic activity and stability of the prepared Pd@Pt core-shell nanostructures for the MOR were highly dependent on their Pt shell thicknesses. In chapter 3, we developed general synthetic route for preparation of highly multi-branched Pt and 3d-transition metal (M= Co, Ni, and Fe) alloy nanocrystals with controlled their composition (Pt: M = 8: 2). The introduction of OAm and ACX is crucial to determine well-defined highly multi-branched nanostructure. In addition, incorporating secondary metal affect the formation of many steps onto the surface of each branch. This understanding would be beneficial for preparation of highly multi-branch structures in the future. Prepared three types NCs showed significantly enhanced electrocatalytic performance compared with commercial Pt/C, which is mainly attributed to the fascinating properties of structure and composition. The HBPtCo alloy NCs exhibited the highest catalytic activity, stability, and improved CO tolerance among pre-pared catalysts on account of the modification in electronic structure, which correspond to the value calculated of d-band center. It is believed that these highly multi branched Pt-M alloy NCs open up new opportunities for fuel cell application. In chapter 4, to optimize the catalytic activity and stability of Pt-based catalysts for PEMFCs and DMFCs, a suitable support is required to well disperse these catalysts. Previous researchers have developed various supporting materials as replacing material of carbon support such as CNT, GO, RGO, Al2O3, TiO2, and CeO2. Among the various supporting materials, Cerium oxides (CeO2) have been successfully used as carriers to support noble metal NPs so as to further improve their catalytic activities and stabilities, because of the high capability of CeO2 in adsorbing CO or OH-species closely related to the redox process between the different possible oxidation states. Furthermore, catalytic properties of CeO2 NPs depend on their size or ex-posed facets on their surface of NPs. In this regard, we developed synthesis of the Pt-CeO2 hybrid nanostructure via preformed CeO2 NPs of different shapes sub-10 nm, and their catalytic performance for methanol oxidation.