Browsing by Subject "Reaction"

Now showing 1 - 3 of 3

First-principles investigation of heterogeneous electrocatalysis at the solid-liquid interface
(2020-12) Hartmann, Gregory Peter; Hwang, Gyeong S.; Henkelmen, Graeme; Ekerdt, John G; Keitz, Benjamin
Carbon-based metal-free electrocatalysts are promising alternatives to precious metals in energy storage and conversion applications providing comparable performance at much lower costs. Conventional studies in to their performance assume that these carbon electrodes are ideal non-polarizable electrodes, following the methodology used to describe metallic electrodes, to simplify the theoretical description of reaction overpotentials. This method has not provided a clear description of the active sites or the principles underlying this activity. This suggests that the conventional approach may not be adequate to distinguish the true catalytic performance of these materials. Graphene-like materials are to possesses a very small quantum, or electrode, capacitance, implying the chemical potential may vary substantially with charge in the system. This can, in turn, influence all the interactions occurring at the electrode/electrolyte interface. By introducing this concept to first-principles methodologies for studying interfacial reactions we can gain valuable insights in to the activity, identifying indicators of performance and design principles to optimize these carbon-based materials for catalysis. In this dissertation, we explore the catalytic performance of carbon-based metalfree electrodes in two parts. In Part I, we evaluate first-principles techniques used to describe the interface between graphene electrodes and aqueous solution, motivated by the inconclusive results of conventional approaches. Analysis demonstrates the influence of electrode capacitance over the predicted overpotentials. Additionally charge transfer is identified as key to the activation and adsorption of oxygen. Being a function of the substrate-intermediate-solvent interactions, we demonstrate that the solvent models must be carefully selected to provide the correct description of the Coulombic interactions. Part II focuses on describing the oxygen reduction performance of different types of activated graphene materials, motivated by the active sites identified in the previous section. The method outlined above is used to demonstrate a reaction mechanism for the oxygen reduction reaction occurring on topological defects. Important details governing the catalyst performance, including the role of pH and the high selectivity for the most efficient pathway, can be explained using this method. The work presented herein provides new insight into the electrochemical activity of graphene-like materials and future directions for catalyst development. We anticipate that the established methodology and analysis can be broadly applicable to other nanostructured materials and reaction chemistries for metal-free heterogeneous catalysis.
Oxygen scavenging styrene-butadiene-styrene block copolymer films for barrier applications
(2013-08) Tung, Kevin; Paul, Donald R.; Freeman, B. D. (Benny D.)
This dissertation discusses the oxidation behavior of reactive membranes that were produced by solution casting and by melt extrusion. These films, containing styrene-butadiene-styrene (SBS) block copolymer that undergoes catalytic oxidation, are of potential use as an oxygen scavenging polymer (OSP) for barrier applications. A thin film kinetic model was developed to ascertain reaction parameters that were used to describe thick film oxidation behavior. Ultimately complex structures containing these scavengers need to be produced via melt-extrusion. Therefore, processing conditions were established to ensure that melt-processed films have the same oxidation kinetics and capacity as those prepared by solution casting. Blends containing a non-reactive styrene phase and an oxygen-scavenging SBS phase were extruded and, by uptake and permeation experiments, their oxidation behaviors were monitored. The flux behavior and time lag extension as a function of oxygen pressure, film thickness, SBS scavenger and photoinitator contents were measured and compared to the theoretical model. The permeation behavior of the reactive blend films containing SBS showed that time lags can be extended via an oxidative mechanism and barrier properties be improved compared to traditional packaging membrane of native polystyrene.
Predictive modeling of optical enantiomeric excess determination assays for high-throughput asymmetric reaction screening
(2023-04-21) Howard, James Russell; Anslyn, Eric V., 1960-; Dalby, Kevin N; Hull, Kami L; Page, Zachariah A
High-throughput screening (HTS) of asymmetric transformations is vital to the development of modern pharmaceuticals and fine chemicals. Enantiomeric excess (ee) determination of asymmetric transformations is accelerated through the use optical techniques such as circular dichroism (CD)-based ee determination assays. However, the implementation of these assays requires calibration experiments using enantioenriched materials, which ultimately hinder the use of these assays in real-world applications. We report the prediction of calibration curves used in ee determination assays for chiral amines using a data-driven approach. By leveraging density functional theory-based chemical descriptors, we have developed a model that predicts calibration curves without performing prior calibration experiments. This calibration curve prediction method was applied to a multicomponent ortho-iminoboronic acid assembly. The ee values measured with the predicted calibration curves were within 10% of those measured with the experimental calibration curves. The generality of this approach was demonstrated using an octahedral Fe(II) complex for the ee determination of chiral amines. A diverse library of analytes was created to elucidate the electronic and steric factors which influence the CD response of the Fe(II) complex. After assessing the scope and limitations of the assay, we generated a model of calibration curves which could be used to determine the ee of unknown solutions with less than 6% error. This computational approach circumvents the need for chiral resolution to perform calibration experiments, which will ultimately accelerate reaction discovery and optimization.