Non-traditional catalysis by plasmonic nanostructures

Litts, Chloe Anne

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https://hdl.handle.net/2142/132754 Copy

Description

Title

Non-traditional catalysis by plasmonic nanostructures

Author(s)

Litts, Chloe Anne

Issue Date

2025-11-11

Director of Research (if dissertation) or Advisor (if thesis)

Jain, Prashant K

Doctoral Committee Chair(s)

Jain, Prashant K

Committee Member(s)

• Mirica, Liviu M
• Olshansky, Lisa
• Yang, Hong

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• catalysis
• nanoparticles
• plasmonics
• photocatalysis
• photothermal effect
• metal oxides
• noble metals
• NHC
• N-heterocyclic carbene
• hydrogen production
• control experiments
• alkyne carboxylation

Abstract

Nanoscale catalysts are of growing interest in current research because of their tunable properties, optical responsiveness, and potential to drive sustainable chemical transformations. These systems include classical plasmonic noble metal nanoparticles and redox-active metal oxides with intriguing electronic properties, and these materials have been widely investigated for both thermocatalytic and photocatalytic applications, including CO2 reduction and selective hydrogenation. One appeal of these materials is their ability to interact with light, whether through plasmonic excitation, bandgap transitions, or defect-mediated absorption, raising questions about how light influences their catalytic behavior. However, the field remains plagued by insufficient control experiments and frequent misattribution of catalytic activity. This dissertation seeks to explore nanoscale catalysis carefully and to emphasize the need for careful control experiments, systematic benchmarking, and a clear understanding of the baseline material reactivity so that the impact of light on catalytic systems can be understood. In Chapter 2 of this thesis, the visible-light photocatalytic potential of oxygen deficient molybdenum oxide (MoO3-x) nanostructures was explored for energy relevant reactions such as nitrogen (N2) reduction and hydrogen (H2) evolution. Through careful control experiments, it was revealed that this material did not exhibit visible-light enhanced catalysis and the claim of MoO3-x being plasmonic was found to be questionable. Further investigation of several MoO3-x materials through systematic refractive index tests to evaluate its plasmonic properties revealed that MoO3-x does not appear to exhibit typical plasmonic behavior. The lack of plasmonic behavior coupled with its lack of visible-light photocatalytic activity suggests that plasmonic behavior may be essential for catalytic activity and highlights the need for more rigorous optical characterization before assigning plasmonic function to defective metal oxides. Chapter 3 shows a finding that emerged from control experiments in Chapter 2 and presents a cautionary tale about an unexpected source of H2 in the MoO3-x photocatalysis experiments. During control experiments, H2 was detected in a system containing only a vial, stir bar, and water, conditions under which H2 should not be produced. Further investigation revealed that the H2 resulted from a reaction between the metal core of the stir bar and residual aqua regia used to remove metal contaminants from the surface. Small cracks in the stir bar’s Teflon coating allowed aqua regia to penetrate and react with the core, producing H2, even hours after washing. This finding, published as a viewpoint in ACS Energy Letters, reminds the community of the importance of rigorous control experiments to ascertain genuine hydrogen production and alerts researchers to a previously overlooked source of spurious hydrogen evolution. Chapter 4 investigates the catalytic mechanisms of a hybrid catalyst consisting of silver nanoparticles (AgNPs) and a silver-N-heterocyclic carbene (Ag-NHC) complex in the carboxylation of alkynes. Phenylacetylene was used as the model alkyne. A systematic investigation was conducted to identify the roles played by the base (Cs2CO3), atmospheric conditions (CO2 saturated versus ambient), and each catalytic component, the Ag-NHC and AgNPs, both independently and in combination. The results showed that while carboxylation occurs in the presence of the Ag-NHC complex alone, the introduction of AgNPs leads to enhanced catalysis, which is attributed to the ability of the AgNP surface to activate phenylacetylene. In addition to stabilizing the AgNP, the NHC is also proposed to activate CO2 through the formation of an NHC-CO2 adduct. Experiments performed under light excitation exhibit higher yields; however, this photoenhancement was due to the photothermal effect. Atmospheric conditions dictated the pathway of the reaction. Under ambient conditions, the reaction favored a polymerization reaction, whereas a CO2-saturated atmosphere enabled carboxylation. Without the presence of CO2 activation via the Ag-NHC, the reaction did not proceed significantly. The base played a significant role in both pathways: higher yields were observed when it was present, which is attributed to the ability of the base to abstract protons from the phenylacetylene. This work sheds mechanistic light on the functioning of a hybrid nanocatalyst by uncovering the roles of each component and reaction conditions. Together, these chapters demonstrate the importance of mechanistic clarity, systematic experimental design, and detailed controls in the field of nanoscale catalysis. While the desirable properties from tunability to optical activity continue to inspire research in catalysis, this thesis shows that careful evaluation of baseline activity and ensuring reproducibility are vital to a true understanding of scientific phenomena in this field.

Graduation Semester

2025-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/132754

Copyright and License Information

Copyright 2025 Chloe Litts

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