Mechanistic Insights into Elevated Temperature Photocatalytic Processes via Combined Experimental and Theoretical Approach
Date of Award
Doctor of Philosophy
Craig Barnes, Bamim Khomami, Paul Frymier
Developing new catalytic technologies that allow for the production of fuels and building-block chemicals from CO2 and H2O photocatalytically is one of the greatest challenges of the 21st century. H2O and CO2 are difficult-to-activate molecules and their successful reduction is potentially kinetically limited. The studies presented herein are aimed at providing a detailed understanding of the reaction such that photocatalysts may be rationally selected and optimized. Our efforts also aim at developing photocatalysts and reactor design that allow utilization of full solar spectrum (UV-vis and IR), which could greatly enhance the overall quantum efficiency of the system. Elevated temperature is commonly avoided in photocatalysis due to the perception that high temperature leads to rapid exciton recombination and loss of quantum efficiency. Instead, enhanced catalytic activity is encountered in our study at autogenous temperatures (350°C+) produced through IR heating by employing a concentrated solar photoreaction (CSPR) approach.As we explore the photocatalytic synthesis of complex molecules, reaction mechanisms will involve more complex surface-bound intermediates of varying electronic character. This study highlights the interface between the classical electrochemical understanding of photocatalytic reactions where highly destabilized reaction intermediates are common and photocatalytic synthesis reactions where vibrational barriers may be contributing to active reaction pathways. Understanding the stability and electronic nature of these species on the catalyst surface may be crucial in dictating catalyst performance and selectivity. Our results have shed light onto several chemical and physical phenomena at the mechanistic level that drive elementary reaction steps on the surface. Some insights include the isolation of new H-transfer mechanisms and variation in surface chemistry under a range of experimental environments (variable temperature, variable chemical potential of reactants, kinetic isotope effect, etc.). Results indicate that photocatalyst with a high Debye temperature, robust bulk bonding, and elevated surface chemical reactivity could directly promote new reaction intermediates produced via thermal/vibrational routes that further enhance the selectivity towards hydrogenation. Identifying and understanding the effect of the stability of atomic H and how it is energetically driven through the reaction mechanism could dramatically enhance our ability to control selectivity in photocatalytic reaction mechanisms.
Poudyal, Samiksha, "Mechanistic Insights into Elevated Temperature Photocatalytic Processes via Combined Experimental and Theoretical Approach. " PhD diss., University of Tennessee, 2019.
Chapter 3 of this dissertation is a reprint of a manuscript published in Journal of Physical Chemistry under the title "Insights into Elevated-Temperature Photocatalytic Reduction of CO2 by H2O". Chapter 4 is a reprint of a manuscript titled "Photocatalytic CO2 Reduction by H2O: Insights from Modeling Electronically Relaxed Mechanisms", which has just been accepted to be published in Catalysis Science and Technology Journal and will hopefully be in print by the end of January, 2019. Chapter 5 is a partial reprint of a manuscript titled "The Nature of Surface-bound Atomic H and its Role in H-transfer Mechanisms and Catalyst Selectivity toward Hydrogenation vs. H2 Evolution", which has been submitted to Physical Review Letters and is currently under peer-review.