Browsing by Author "Senftle, Thomas P"

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    Catalyst design for water treatment using ab initio simulation
    (2023-11-30) Chen, Yu; Senftle, Thomas P
    Shortage of clean water sources due to climate change, development of industrialization, and population growth is a concerning problem worldwide. Heterogenous catalysis is a promising strategy to reduce the concentration of undesirable substances during water treatment. In this thesis, I apply ab initio simulation to identify key material properties and fundamental reaction mechanisms that dictate catalyst performance for the treatment of two important water contaminants: nitrate and per-fluoroalkyl substances (PFAS). This insight in turn informs design strategies for designing better catalysts for these applications. Perfluorooctanoic acid (PFOA) is one of the most prevalent PFAS contaminants in surface and ground water. Working with experimentalist collaborators, we reported that hexagonal boron nitride (hBN) is a promising photocatalyst for PFOA degradation under UVC illumination, with an activity ~2x higher than TiO2. In my thesis, I applied density functional theory (DFT) in a grand canonical (GC) formalism (Bhati and Chen et al., J. Phys. Chem. C, 2020, 124, 49, 26625–26639) to determine the photo-catalytic mechanism responsible for PFOA degradation on hBN. (Chen et al. Environ. Sci. Technol., 2022, 56, 12, 8942–8952) I confirmed the favorability of the proposed photo-oxidation step of PFOA on the hBN surface: CnF2n+1COO− + h+ → CnF2n+1ꞏ + CO2. Furthermore, by investigating the electronic properties of hBN, I found that NB substitutional point defect introduces mid-gap states that enable the UVC light absorption and enhance charge carrier separation. Therefore, introducing more NB defects is a promising strategy to enhance the photocatalytic degradation performance of hBN. My work also helped to determine the role of surface hydrophobicity in promoting PFOA degradation, which is attributed both to stronger adsorption of the hydrophobic fluorinated tale of PFOA and the exclusion of water molecules that can scavenge photo-excited holes. (Wang and Chen et al., submitted) Thus, increasing surface hydrophobicity is another strategy for enhancing catalyst performance during PFOA degradation. Using this insight, we are now developing covalent organic framework (COF) catalysts with tunable functionality to tailor hydrophobic and electrostatic interactions, thus maximizing PFOA adsorption. Besides PFOA, nitrate is another pervasive surface and groundwater contaminant found worldwide. Nitrate anions are highly soluble and mobile, and can cause harmful health effects in humans, including diseases such as blue baby syndrome, cancer, etc. Investigating the reaction network of electrocatalytic nitrate reduction, we found Cu and Pd catalysts can play a synergistic role in nitrate removal. (Lim and Chen et al., ACS Catal. 2023, 13, 1, 87–98) Using DFT, we discerned how the electronic properties of the metal catalyst affect the nitrate reduction reaction mechanism, steering the product selectivity to either N2 or NH3. (Chen and Senftle, submitted) We propose that metals like Pd, with less-occupied and more-delocalized d orbital exhibit higher N2 selectivity due to adsorbate-adsorbate interactions that promote N–N bond formation over N–H bond formation. This insight sets the theoretical basis for the design of better Pd/Cu bimetallic catalysts for the selective disposal of nitrate from water.
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    DFT Investigation of PFAS Adsorption Configurations on Metal Surfaces
    (2023-04-25) Wang, AiShi; Senftle, Thomas P
    Per- and polyfluoroalkyl substances (PFASs) are a growing environmental concern. Efficient methods for degrading PFAS are urgently needed. In this thesis, we apply density functional theory (DFT) to better understand the molecular-scale process of PFAS degradation via catalytic hydrogenation on transition metal surfaces. We systematically investigate the adsorption configuration of PFAS molecules on catalyst surfaces composed of different metals. We find that the orientation of the PFAS molecule influences PFAS adsorption strength and removal rates, as the head group and tail group have a significantly different bonding strength to the surface. Adsorbate-adsorbate interactions between co-adsorbed PFAS molecules can promote PFAS adsorption through van der Waals forces among the tail groups of adjacent PFAS molecules, which can lead to the enhanced co-removal of differing PFAS molecules. Adsorbed hydrogen also influences the PFAS binding orientation by blocking the formation of metal-oxygen bonds between the surface and the PFAS head-group, thus promoting physisorption over chemisorption. Investigation of the hydrogenation mechanism responsible for activating C-F bonds shows that the reaction mechanism also is sensitive to the PFAS adsorption orientation. This mechanistic insight provides strategies for improving catalyst design to maximize PFAS uptake and degradation.
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    Multiscale Modeling of Electrochemical Interfaces
    (2022-04-21) Bhati, Manav; Senftle, Thomas P
    Interfaces are one of the most complicated yet significant junctions in materials where several sciences are at play simultaneously. Modeling such interfaces using computer simulations helps to decipher the interplay of these sciences at an atomic scale, which is difficult with experimental characterizations. Multiscale modeling techniques (described in Chapter 1) and the insights derived by implementing them are of high utility to both experimental and computational researchers. This work utilizes multiscale modeling approaches for a thorough investigation of several crucial interfaces that appear in a range of electrochemical applications. In Chapters 2 and 3, silicon/binder anodes in Li-ion batteries are studied using ReaxFF force field based molecular dynamics simulations. The strategies to develop the best binders for silicon anodes are established by a systematic investigation of several crucial binder properties. In Chapter 4, we developed an electronic grand-canonical formalism with density functional theory to model photo-catalytic reactions occurring at the semiconductor/electrolyte interfaces under constant potential. This is implemented on two reactions systems (hydrogen evolution on SiC and contaminant degradation on BN) to gain insights into key steps of each reaction mechanism on photo-catalytic surfaces. In Chapter 6, time-dependent density functional theory is utilized to investigate the nature of electronic excitations in less-explored yet experimentally-significant non-stoichiometric quantum dots that are ligated and solvated. The charge transfer phenomenon, which is usually absent in the stoichiometric quantum dots, seems to play a major role in influencing the optoelectronic properties of these non-stoichiometric quantum dots. Overall, this work presents several novel findings to assist a better design of electrochemical interfaces: establishes structure-property relationships to engineer best Si/binder interfaces for Li-ion batteries, develops modeling strategies for investigating photo-electrochemical reactions on semiconductor/electrolyte interfaces, and reveals charge transfer phenomenon in non-stoichiometric quantum dots and its impact on their optoelectronic properties.