Design of Nanostructured Ag Catalysts for Selective Heterogeneous Catalytic and Photocatalytic Oxidation Reactions.

Abstract

In this dissertation we have utilized a multi-faceted approach, combining quantum chemical calculations, well-controlled synthesis techniques and an arsenal of characterization techniques to design catalysts for important catalytic and photocatalytic oxidation reactions. Specific focus was placed on exploiting unique shape and size dependent properties of Ag nanoparticles to design new catalytic materials and entirely new reacting systems (coupling thermal and photonic stimuli) for the industrially relevant ethylene epoxidation reaction. We demonstrated novel routes for enhancing the selectivity and activity of Ag based epoxidation catalysts. The work presented in this dissertation provides two unique platforms for controlling and engineering the catalytic function of metallic nanoparticles. In the first example we utilize a combination of density functional theory calculations and shape and size controlled synthesis approaches to design highly selective (up to 85%) Ag catalysts for ethylene epoxidation. Our studies show that catalytic particles of controlled size and shape represent promising heterogeneous catalysts for selective production of chemicals and also act as a critical platform to study heterogeneous catalytic process and identify crucial factors that impact process selectivity. In the second example, it was demonstrated that plasmonic Ag nanostructures could couple thermal and solar energy to efficiently drive selective oxidation reactions at up to 100 K lower temperature than a pure thermal process. The results are important for a number of reasons: (i) This is the first observation of metallic nanoparticles performing steady-state photocatalytic reactions in the single and multi-excitation regimes under low intensity, continuous wave visible photon illumination, (ii) Plasmonic nanoparticles are unique in combing excellent thermocatalytic capabilities and strong interaction with UV and visible photons to effectively couple multiple energy stimuli to drive catalytic reactions and, (iii) The results suggest that since the reactions can be operated at lower temperatures, the long-term stability of catalysts and the product selectivity could be potentially enhanced. We provide a first-principles based model, which captures the effect of intensity and temperature on the process efficiency. These discoveries open the door to the design of new classes of (photo)catalytic materials, for energy efficient production of important chemicals and fuels.