Abstract
In the past several decades, we have witnessed the rapid development of nanoscience and nanotechnology in chemistry, physics, and material science. The surface-to-volume ratio scales inversely with linear dimensions for a given material. Nanomaterials are three-dimensional solids composed of nanometer-sized grains or crystallites. They feature small dimensions that allow for more surface functionality. The unsaturated surface bonds and segregated energy levels lead to physical properties that are significantly different from their bulk counterparts. At the nanometer scale, the surfaces can significantly alter properties, thus leading to new effects such as surface plasmon resonance or size-dependent catalytic activities. These enhanced properties are highly desirable in energy and environmental-related fields.
Many synthesis approaches have been developed to generate high-quality nanoparticles of various geometric and compositional properties. Well-controlled synthesis of nanomaterials and nanoscale characterization enables us to unambiguously correlate the structural properties with the physical, chemical, and biological properties of nanomaterials, which form the core of nanoscience research. An essential requirement for nanomaterial synthesis is the homogeneity of the final particle sizes, shapes, as well as chemical compositions.
Heterogeneous catalysts play an essential role in every aspect of modern fuel production, pollution alleviation, commodity bulk production, etc. They are usually in the form of nanoparticles with high concentrations of active sites. Advances in nanoscience provide opportunities for constructing next-generation catalytic processes with high activities for energy-intense reactions (e.g., CO2 reduction), high selectivity to valuable products (e.g., acetylene hydrogenation), and extended lifespan. In order to achieve this goal, scientists are on constantly searching and developing novel synthetic methodologies based on theoretical approaches to produce highly stable, active catalysts. And this is the basic concept behind this thesis.
For nanomaterial synthesis, the d-band theory model guides the design and synthesis of alloy AgPd NPs, showing an optimized adsorption energy of intermediates and thus enhanced the CO2 conversion (Chapter 5) and dechlorination reactions (Chapter 6). The sizes and composition tunability is achieved via the OAm/OAc pairs. The successful construction of diluted Pd ensembles surrounded by Ag species leads to optimized intermediates adsorption. In Chapter 7, based on the strong coordination ability of OAm to Pt, in situ-generated CO and proton act as reductants, generating monodispersed Pt NPs of several different sizes. Such a well-controlled synthesis provides an unambiguous correlation between dehydrogenation activity and the availability of Pt surfaces. These synthesis methodologies are based on the classical nucleation theory. In Chapter 8, an unclassical nucleation process is adapted to construct high surface area porous CeO2 superparticles that show superior thermal stability. The formation mechanism also features an in situ reductant/structure-directing ligand generation, similar to the Pt NPs formation.
AgPd NPs serve as versatile nanomaterials for energy-related catalysis in electrocatalytic CO2 reduction and hydrodechlorination. The surface chemistry tuning is demonstrated the ligand effect and ensemble effect over AgPd NPs. Finally, in Chapter 8, two Pd/CeO2 featuring different Pd species were prepared, demonstrating enhanced acetylene thermos catalytic semi-hydrogenation activity.
