Understanding Active Sites in Plasmonic Enhanced Catalysis

Catalysts are essential in producing chemicals and materials that support the global economy, but the standard catalytic process consumes vast resources to meet this demand. It has now been shown that significant portions of the required energy for these reactions can be eliminated by replacing or supplementing expensive heat and pressure with free or inexpensive light. Localized surface plasmon resonances enable the coupling of light into catalytic reactions to improve reaction rates and control reaction specificity through a process called plasmon enhanced catalysis. As an effect, plasmonic enhanced catalysis can lead to reaction rate enhancements of over 500 percent when compared to their thermal catalytic counterparts, alter the catalyst selectivity, and lead to increased catalyst lifetimes. This enables the production of chemicals with decreased demand to thermal energy and pressure. In turn, this decreases the capital cost for reactors and equipment, permitting a new optimization in the scale of chemical production facilities and energy demands. Currently, as an example, the catalytic production of ammonia requires 1-2 percent of global energy annually and is limited to large scale performance by highly industrialized nations.77 The use of plasmonic enhanced catalysis can drive down both capital and operating costs, while decreasing the environmental impact and expanding access to these vital chemical reactions. Plasmonic enhanced catalysis is a growing and thriving field due to its potential to alter current operations and downstream processing for various industries. Further growth of the field into far reaching applications is dependent on the answer to a seemingly basic question: What are the mechanisms driving the observed reaction rate improvements under light at the chemical active site? We and others have demonstrated that excited plasmonic nanoparticles generate heat and enable electron-transfer to improve catalytic processes. This dissertation details our recent work to expand our understanding of relevant plasmonic enhancement mechanisms. In more detail, we will discuss our works on untangling the nature and mechanisms of plasmonic enhanced catalysis by specifically focusing on the catalytic active site. This work is formulated by six chapters discussing various plasmonic catalytic reactions and associated systems, which share a common set of goals to advance the understanding of the mechanisms and implications of plasmonic catalysts while encouraging the use of well formulated catalytic systems for high-impact applications. Plasmonic catalysts have the potential to reshape the production of specialty and commodity chemicals by supporting new reaction pathways and allowing for decreased operating costs while increasing reaction efficiency. The above goals are obtained through a focus on the individual roles and interactions of thermal and electron-transfer effects in plasmonic catalysts. Upon light illumination, a plasmonic catalyst rapidly undergoes photothermal heating and can transfer high-energy electrons to local absorbed molecules. These two processes amplify reaction rates individually, and on a plasmonic catalyst are likely to be codependent. In this work, a variety of methods will be used to analyze these two effects to increase the understanding of the relationship between thermal and electron transfer processes. This dissertation contains introduction material (chapter 1), five chapters concerning our investigations to characterize active-site behavior in plasmonic enhanced catalysis, and three appendixes with further supporting information and detailed experimentation. The primary objectives of the second and third chapter focus on improving the characterization of thermal processes at the active site occurring under plasmonic excitation and how these methods can be used to better understand the non-thermal electron transfer process. In the second chapter, we use both on- and off-resonant plasmonic excitation to demonstrate the presence of nanoscale thermal gradients within a few nanometers of the surface of the plasmonic catalyst particle. These gradients underly a significant increase in the active site temperature over the bulk catalyst temperature. The subsequent chapter uses the temperature dependent reaction kinetics to accurately characterize this temperature difference between the active site and the catalyst bulk temperature. This chapter also outlines a highly relevant possible application for plasmonic catalysis and includes a mechanistic study into the cause of non-thermal electron transport in this system. The third chapter also demonstrates that plasmonic enhancement of chemical reactions can increase catalyst lifetimes. With the lessons learned from the previous chapters, chapter four uses active-site engineering to synthesize four catalysts with similar plasmonic metal nanoparticles, but with different active sites, to better understand their electron transfer processes. This work compares multiple methods to describe the relative contributions from thermal and non-thermal mechanisms in the plasmonically enhanced catalytic reaction, with a specific focus on the gold catalyzed oxidation of carbon monoxide in a hydrogen rich system. Here, we suggest a modified pathway of non-thermal electron transfer by observing the apparent electron transfer from the plasmonic gold to oxygen absorbed on the support or at the metal-support interface. Chapters five and six focus on alternative methods to understand plasmonic catalysis. Chapter five uses a plasmonic rhodium catalyst for the release of hydrogen from a liquid organic hydrogen carrier. This reaction, normally requiring high temperatures, is shown to be improved significantly with light incident on the plasmonic catalyst. By using a liquid phase reaction with increased thermal conductivity relative to the gas-phase, rapid stirring, and thermally regulated experimentation, we suggest the presence of an electron transfer to drive the reaction directly. Chapter six explores an alternative method to modify the LSPR energy in a plasmonic catalyst without altering the catalyst itself, which is performed by magnetic field modulation. The plasmonic gold nanoparticles analyzed in this work are non-magnetic, yet the oscillating electrons which form the LSPR’s can interact with a magnetic field. This project utilizes a magneto-spectrometer to characterize the plasmon absorption of gold nanorods and nanospheres in a magnetic field of varied strength. Results demonstrate the existence of an interaction between the LSPR energy and the magnetic field, which may be due to Lorentz forces acting on the LSPR. The modification of the LSPR energy, without altering the catalyst permanently, may result in new experiments probing the importance of the LSPR energy on hot electron transfer and enable further control of reaction specificity in plasmonically enhanced catalytic reactions. This dissertation outlines several notable recent findings in plasmonic catalysis with a focus on high-impact applications. Plasmonic catalysts open new avenues in catalysis, enabling the production of a wide range of chemicals with a reduced energy input and cost. These observations provide new insights for expanded applications of plasmon enhanced catalysis to decrease the energetic requirements of catalysis on a small to industrial scale. Future work in this field will be to determine suitable reactions for scaling to industrial proportions. We hope the mechanistic work provided in this document provides strong evidence for the rapid utilization of these processes in many global industries.