Noble metal-based nanomaterials with amorphous phase : preparation and catalytic applications

• Main Author: Cheng, Hongfei
• Other Authors: Zhang Hua
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/136770
• Institution: Nanyang Technological University

In amorphous phase, atoms are randomly arranged, which have lower average coordination number and more dangling bonds than those in crystalline phase, as a result, more active sites could be exposed for catalytic reactions. Up to date, different kinds of amorphous materials, such as metal oxides, transition metal chalcogenides and hydroxides, have been reported to exhibit superior catalytic performance compared to their crystalline counterparts. In catalytic reactions, noble metals have shown intrinsically high activity and excellent durability compared to the other kind of materials. However, it is challenging to prepare amorphous metal nanomaterials due to the strong interatomic metallic bonding. In addition to amorphous phase, recently, amorphous/crystalline heterophase is arising as another research hotspot. Similar to the widely reported heterostructures with different materials, which could combine the advantages of individual components, overcome their drawbacks, and generate new properties/functions, the heterophase nanostructures with homogeneous compositions could combine the properties of different phases and some unique properties might arise from the phase interfaces. However, so far the reported metal nanomaterials with amorphous/crystalline heterophase are prepared by physical methods, which have many drawbacks compared with wet-chemical methods. Hence, it is desirable to develop wet-chemical methods for preparing heterophase metal nanomaterials. Herein, this thesis focuses on the wet-chemical synthesis of amorphous and amorphous/crystalline heterophase noble metal-based nanomaterials. Furthermore, the phase-dependent physicochemical properties and catalytic applications of the as-prepared nanomaterials will be explored. Specifically, three research works will be discussed. First, a one-pot wet-chemical method is developed for synthesizing a series of amorphous/crystalline heterophase Pd nanosheets. The crystallinity of the Pd nanosheets can be controlled by fine-tuning the experimental conditions. These nanosheets are used as heterogeneous catalysts to study the chemoselectivity in hydrogenation of 4-nitrostyrene. The amorphous phase-dominant Pd nanosheets show an excellent chemoselectivity, while those crystalline phase-dominant Pd nanosheets exhibit a higher catalytic activity. This work might pave the way to preparing various heterophase nanostructures for promising applications. Second, the aforementioned wet-chemical method is extended to the synthesis of amorphous/crystalline heterophase PdCu nanosheets and the aging process of the synthesized PdCu nanosheets is systematically studied, during which their crystallinity increases, accompanied with changes in some physicochemical properties. As a proof-of-concept application, their aging effect on catalytic hydrogenation of 4-nitrostyrene is investigated. As a result, the amorphous phase-dominant nanosheets initially show excellent chemoselectivity. After aging for 14 days, their catalytic activity is greater than that of crystalline phase-dominant nanosheets. This work demonstrates the intriguing properties of heterophase nanostructures, providing a new platform for future studies on the regulation of functionalities and applications of nanomaterials by phase engineering. Third, a unique thiol molecule, namely bismuthiol I, is discovered, which can induce the transformation of Pd nanomaterials from face-centered cubic (fcc) phase into amorphous phase without destroying their integrity. This ligand-induced amorphization is realized by post-synthetic ligand exchange under ambient conditions and applicable to fcc Pd nanomaterials with different capping ligands. Importantly, the obtained amorphous Pd nanoparticles exhibit remarkably enhanced activity and excellent stability towards electrocatalytic hydrogen evolution in acidic solution. This work presents a facile and effective method for preparing amorphous Pd nanomaterials and demonstrates their promising electrocatalytic application.