Design and Synthesis of Metal Nanostructures for Plasmon-Enhanced Catalysis

The chemical industry depends on heterogeneous thermocatalytic processes to satisfy the ever-increasing demand for fuels and fertilizers. High temperatures and high pressures are generally required to accelerate chemical transformations and operate practical rates. These harsh conditions, however, lead to huge energy consumption and other side effects, such as the lifetime of catalysts and parasitic formation of by-products. Light is used as an alternative energy input to drive chemical reactions on semiconducting photocatalysts, but the slow reaction rates and insufficient control of product selectivity hinder wide adaptation of photocatalysis. Plasmonic metal nanoparticles have been recently proposed as a new type of catalysts with photoactivities. As already been widely used in thermocatalytic reactions, the strong light absorption capability from excitation of localized surface plasmon resonance (LSPR) of plasmonic catalysts could combine light and thermal energy to work cooperatively in enhancing rates of chemical reactions. This dissertation summarizes our efforts aiming to design plasmonic catalysts with high efficiency and high product selectivity. The catalytic properties of synthesized rhodium (Rh) and ruthenium (Ru) catalysts are investigated in two model reactions, carbon dioxide (CO2) hydrogenation and ammonia (NH3) synthesis.

Chapter 2 describes the development of slow-injection polyol methods to synthesize monodispersed Rh nanocubes with tunable size and resonant energy. The wide size tunability of slow-injection methods allows for the red-shift of resonant wavelength of small Rh nanostructures, which are in the deep ultraviolet (UV) region, to more accessible and practical near-UV and visible regions by increasing the size of Rh nanocubes.

Chapter 3 focuses on the product selectivity of plasmonic Rh nanocubes in CO2 hydrogenation. Rh nanocubes supported on aluminum oxide (Al2O3) nanoparticles equally produce methane (CH4) and carbon monoxide (CO) in pure thermal conditions. Under illumination of UV and blue light, the rate of CH4 production is significantly enhanced, and almost exclusive CH4 production is observed. This photo-selectivity can be attributed to selective activation of specific reaction intermediate by photo-generated hot electrons among competing reaction pathways.

Chapter 4 describes the effects of catalyst support and morphology of plasmonic Rh nanostructures on the catalytic activities in plasmon-enhanced CO2 hydrogenation. Significant improvements of reaction rates are observed by switching to reducible titanium oxide (TiO2) support and shrinking the size of Rh nanostructures. The enhancement of reaction rates by light can be partially attributed to local heating of catalyst bed.

Chapter 5 focuses on the catalytic activities of Ru-based catalysts for NH3 synthesis under light illumination. Photo-enhanced NH3 production, which highly depends on the size, support, and promoter of catalysts, is observed.

Chapter 6 discusses conclusion and future directions of this project. Molecular level insights of plasmon-enhanced catalysis are highly desired for both fundamental research and practical applications.