Design And Controlled Synthesis Of Metal-based Nanocatalysts Towards Tunable Surface Charge State

Composite nanomaterials have attracted wide attention as they combine the functions of individual components as well as overcome the limitations of single-component materials in catalytic applications. As a variety of synthetic approaches have been developed to control the surface structures of metal nanocrystals, they are widely used in various catalytic systems. In the meantime, metal nanocrystals with specific structures provide excellent templates for the further growth of metals and semiconductors. Based on the synthesis of metal-based composite nanocatalysts, tunable interfacial effects together with interface control provide new opportunities for tailoring surface charge states. Therefore, the design of metal-based nanocatalysts is a promising approach to urther optimize the catalytic activities.In this dissertation, we developed aqueous-phase synthetic method for synthesizing Pd concave nanocubes, Pd-Pt bimetallic nanocubes, Pd-Pt bimetallic concave nanocubes, Ag-Cu2O composite structures, and Pd-TiO2 heterostructures. By synthesizing Pd concave structures at small sizes through oxidation etching, we explored the impact of high-index facets and specific area on the activity in electrocatalytic oxidation of formic acid. With the assistance of Ru species, the Pd-Pt bimetallic concave nanocubes with high-index facets were synthesized, which provided a platform to examine the polarization effect on catalytic performance by tuning surface charge density. The Ag-Cu2O composite structures with different interfacial lengths were designed and implemented in catalytic CO oxidation, in which the influence from the polarization effect and interfacial lengths was explored. The TiO2 amorphous thin films with different thickness were deposited on Pd nanocubes through an atomic layer deposition (ALD) technique, allowing to investigate the effect of interfacial effect on the catalytic efficiency of metal catalysts. The specific findings are listed below:1. We developed a controlled synthetic method for epitaxial metal growth. In this method, Pd nanocubes were used as the seeds whose specific sites were activated by oxidation etching so as to achieve the selective epitaxial growth of metal. For example, in the preparation of Pd concave nanocubes, the newly formed Pd atoms were selectively deposited on the edge and corner sites of the seeds to obtain highly active sites for catalytic reactions. In comparison with convention techniques, this method can avoid the growth of atoms on the undesired sites of the seeds, so that the final product well maintained the original size. Owing to the small size and high-index facets, the Pd concave nanocubes showed excellent catalytic activity in electrocatalytic oxidation of formic acid. In addition, this method can be extended to the growth systems of other metals. By depositing Pt on relatively cheaper Pd cubic seeds, the cost of raw materials can be greatly reduced while the high catalytic activity were maintained. The Pd-Pt bimetallic nanocubes with Pt only at 3.3% of the total mass can exhibit high catalytic performance in electrocatalytic oxygen reduction reaction (ORR). We further demonstrated that this method could achieve the controllable deposition of other materials by simply altering the surface state of different seeds.2. Pt has been recognized as a highly active catalyst for electrocatalytic ORR, because of its four-electron reaction mechanism and excellent acid resistance. However, the high cost of raw materials has limited the practical application of Pt in fuel cells. Compared with the low-index facets, the high-index ones exhibit superb catalytic performance due to the high density of low coordination atoms. As the catalytic active sites are often located on catalyst surface, it would be ideal that the interior materials of catalysts are replaced by cheaper metals to reduce material costs. In the meantime, the different work functions between the metals may induce a polarization effect to increase the charge density on active sites. In the synthesis, Pd nanocubes were used as the seeds, on which the selective growth of Pt at edge and corner sites was achieved through the underpotential deposition of Ru cations. As compared with the pure Pt with the same high-index facets, this catalyst exhibited obvious advantages in the electrocatalytic ORR. The interfacial polarization effect between Pd and Pt is the main factor for improving catalytic performance. The polarization led to the electron accumulation on Pt surface, which increased the charge density of Pt active sites and thus improved the activity of catalyst. This work provided a new way to reduce the cost of raw materials and enhance the catalytic activities.3. Heterogeneous catalysis often involves the transfer of charges from the surface of catalysts to the adsorbate. The efficiency of such a transfer is mainly dependent on the surface charge density of the catalysts. Therefore, we designed a unique solution-phase synthetic method for preparing the oxide-metal composite nanostructures with the exposed one-dimensional interfaces whose interfacial lengths could be precisely controlled. Due to the different work functions of the two materials, a polarization effect was induced at the Ag-CuO interface, thereby tailoring the charge state of CuO surface near the interface. Compared with the catalysts without exposed interface, this composite catalyst exhibited a lower apparent activation energy in the catalytic CO oxidation reaction, demonstrating superior catalytic activity. In addition, the number of active sites in the catalytic reaction can be tailored by increasing the interfacial lengths of Ag-CuO, so as to further improve the conversion rate of CO. Therefore, this work provided a new approach to optimize the catalytic performance by adjusting the interface of heterogeneous catalysts.4. Pd is an ideal catalyst that has been widely used in various catalytic reactions. However, Pd typically exhibits relatively low catalytic activity in CO oxidation, as CO is apt to be adsorbed on Pd to poison the surface for O2 activation. In our work, we revealed that this limitation could be overcome by integrating Pd with TiO2. The TiO2 was coated on Pd nanocubes with controllable thickness using an ALD method. Given the different work functions of TiO2 and Pd, the electrons in TiO2 semiconductor would flow towards Pd. With the electron density increased on Pd, the adsorption of CO to Pd was weakened while the oxygen activation could be facilitated. Meanwhile, the interface-confined sites at Pd-TiO2 may further enhance the oxygen activation. As the species adsorption and activation were strongly correlated with the electron density, the performance of Pd-TiO2 in CO oxidation turned out to depend on the TiO2 thickness, which determined the number of transferred electrons, within a certain range. This work provided a new strategy for enhancing catalytic performance through tailoring charge densities in hybrid catalysts.