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Scholarly Works (8 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2023)

The urgent global need for sustainable solutions has propelled the search for efficient catalytic systems. This thesis focuses on key reactions crucial for achieving a greener and more sustainable future. Employing computational techniques and theoretical models, the research investigates catalytic processes to unravel fundamental principles, guiding the design of advanced catalysts. The thesis leads to a comprehensive exploration of the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), partial oxidation of methane to methanol, and CO2 hydrogenation. By elucidating the structure, stability, and catalytic reactivity of diverse clusters and surfaces, the research aims to develop efficient and sustainable catalytic systems.Emphasizing accurate characterization of active site composition and understanding catalyst behavior under reaction conditions, the thesis employs first-principles density functional theory (DFT) calculations to determine optimal doping sites and structures for Sn atoms on the ii Sn-doped Indium Oxide (ITO) surface. The interaction between the ITO surface and Pt clusters is also explored, providing insights into the role of active sites in catalytic reactivity. Investigating small Pt clusters supported on ITO in HER, a size-dependent activity trend is uncovered, highlighting the catalytic potential of Pt hydride compounds. The inclusion of metastable isomers in microkinetic models emphasizes the significance of considering accessible isomer ensembles for accurate simulations of sub-nano supported cluster catalysts. Furthermore, the thesis unravels the intricate mechanism of OER, identifying the rate-determining step and changes in the oxidation state of Pt surface atoms (Pt-SA) during the catalytic cycle. Understanding electron transfer contributions from both Pt-SA and the ITO surface provides valuable insights for designing efficient OER catalysts. In the context of partial oxidation of methane to methanol, the research explores electrochemical methods for catalyst deposition, focusing on thin-film transition metal (oxy)hydroxides. The activity and selectivity of various catalysts are investigated, emphasizing control over kinetics and mass transfer for sustainable methanol production. Finally, the thesis investigates CO2 hydrogenation, a promising avenue for converting carbon dioxide into valuable chemicals. Analyzing the Zr3OxOHy (HCOO)z/Cu111 cluster under different reaction conditions, the research unveils the pivotal role of hydroxyls and HCOOs in shaping the reaction mechanism. Electronic insights guide the design of catalysts with optimal properties, potentially enhancing catalytic performance. By integrating computational modeling and experimental characterization techniques, this research enhances our understanding of critical catalytic reactions. The findings underscore the importance of active site composition, surface structures, and electronic properties in determining catalytic reactivity. The synergy between theory and experiment holds promising prospects for designing advanced catalysts with improved activity, selectivity, and stability.

    Cover page: Probing Active Sites and Their Evolution in Surface Catalysts: From Electrochemical to Thermal Environments
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2023)
    Single-atom catalysts (SACs) have attracted attention for their high catalytic activity and selectivity, but the nature of their active sites under realistic reaction conditions, involving various ligands, is not well-understood. In this study, we use density functional theory calculations and grand canonical basin hopping to theoretically investigate the active site for the oxygen evolution reaction (OER) on a single Pt atom supported on indium tin oxide, including the influence of the electrochemical potential. We show that the ligands on the Pt atom change from Pt-OH in the absence of electrochemical potential to PtO(OH)4 in electrochemical conditions. This change of the chemical state of Pt is associated with a decrease of 0.3 V for the OER overpotential. This highlights the importance of accurately identifying the nature of the active site under reaction conditions and the impact of adsorbates on the electrocatalytic activity. This theoretical investigation enhances our understanding of SACs for the OER.
    • 1 supplemental PDF
    Cover page: Elucidation of the Active Site for the Oxygen Evolution Reaction on a Single Pt Atom Supported on Indium Tin OxideCreative Commons 'BY-NC-ND' version 4.0 license
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2021)
    Supported single-atom and small cluster catalysts have become highly popular in heterogeneous catalysis. These catalysts can maximize the metal atom utilization while still showing superior catalytic performance. One of the main challenges in producing these small cluster catalysts is their low binding strength with the support, which causes these small clusters to sinter into larger nanoparticles. We have used first-principles simulations to study small Ptn(n: 1,2,3) clusters on indium oxide, tin doped indium oxide, and hydroxylated tin doped indium oxide. We report that the Ptncluster is stabilized in the presence of tin and that this is especially the case for Pt single atoms on the hydroxylated indium tin oxide support, which are anchored to the supportviathe hydroxyl group. On this support, the Pt single atoms become more stable than Pt2and Pt3clusters, hence decreasing sintering. These findings provide a promising way to design single-atom catalysts on electrically conducting supports for electrocatalytic applications and to better understand how functional groups on supports can increase the adhesion of cluster catalysts.
    • 1 supplemental PDF
    Cover page: Highly dispersed Pt atoms and clusters on hydroxylated indium tin oxide: a view from first-principles calculations
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2023)
    The growing concern over the escalating levels of anthropogenic CO2 emissions necessitates effective strategies for its conversion to valuable chemicals and fuels. In this research, we embark on a comprehensive investigation of the nature of zirconia on a copper inverse catalyst under the conditions of CO2 hydrogenation to methanol. We employ density functional theory calculations in combination with the Grand Canonical Basin Hopping method, enabling an exploration of the free energy surface including a variable amount of adsorbates within the relevant reaction conditions. Our focus centers on a model three-atom Zr cluster on a Cu(111) surface decorated with various OH, O, and formate ligands, noted Zr3Ox (OH)y (HCOO)z/Cu(111), revealing major changes in the active site induced by various reaction parameters such as the gas pressure, temperature, conversion levels, and CO2/H2 feed ratios. Through our analysis, we have unveiled insights into the dynamic behavior of the catalyst. Specifically, under reaction conditions, we observe a large number of composition and structures with similar free energy for the catalyst, with respect to changing the type, number, and binding sites of adsorbates, suggesting that the active site should be regarded as a statistical ensemble of diverse structures that interconvert.
      Cover page: Nature of Zirconia on a Copper Inverse Catalyst Under CO2 Hydrogenation ConditionsCreative Commons 'BY-NC-SA' version 4.0 license
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2023)
      A combination of density functional theory (DFT) and experiments with atomically size-selected Ptn clusters deposited on indium-tin oxide (ITO) electrodes was used to examine the effects of applied potential and Ptn size on the electrocatalytic activity of Ptn (n = 1, 4, 7, and 8) for the hydrogen evolution reaction (HER). Activity is found to be negligible for isolated Pt atoms on ITO, increasing rapidly with Ptn size such that Pt7/ITO and Pt8/ITO have roughly double the activity per Pt atom compared to atoms in the surface layer of polycrystalline Pt. Both the DFT and experiment find that hydrogen under-potential deposition (Hupd) results in Ptn/ITO (n = 4, 7, and 8) adsorbing ∼2H atoms/Pt atom at the HER threshold potential, equal to ca. double the Hupd observed for Pt bulk or nanoparticles. The cluster catalysts under electrocatalytic conditions are hence best described as a Pt hydride compound, significantly departing from a metallic Pt cluster. The exception is Pt1/ITO, where H adsorption at the HER threshold potential is energetically unfavorable. The theory combines global optimization with grand canonical approaches for the influence of potential, uncovering the fact that several metastable structures contribute to the HER, changing with the applied potential. It is hence critical to include reactions of the ensemble of energetically accessible PtnHx/ITO structures to correctly predict the activity vs Ptn size and applied potential. For the small clusters, spillover of Hads from the clusters to the ITO support is significant, resulting in a competing channel for loss of Hads, particularly at slow potential scan rates.
        • Article
        UCLA Previously Published Works (2022)
        A combination of density functional theory (DFT) and experiments with atomically size-selected Ptn clusters deposited on indium-tin oxide (ITO) electrodes was used to examine the effects of applied potential and Ptn size on the electrocatalytic activity of Ptn (n = 1, 4, 7, 8) for the hydrogen evolution reaction (HER). Activity is found to be negligible for isolated Pt atoms on ITO, increasing rapidly with Ptn size, such that Pt7/ITO and Pt8/ITO have roughly double the activity per Pt atom compared atoms in the surface layer of polycrystalline Pt. Both DFT and experiment find that hydrogen under-potential deposition (Hupd) results in Ptn/ITO (n = 4, 7, 8) adsorbing ~2 H atoms/Pt atom at the HER threshold potential, equal to ca. double the Hupd observed for bulk Pt or Pt nanoparticles. The cluster catalysts under electrocatalytic conditions are hence best described as a Pt hydride compound, significantly departing from a metallic Pt cluster. The exception is Pt1/ITO, where H adsorption at the HER threshold potential is energetically unfavorable. Theory combines global optimization with grand canonical approaches for the influence of potential, uncovering that several metastable structures contribute to HER, changing with the applied potential. It is hence critical to include reactions of the ensemble of energetically accessible PtnHx/ITO structures to correctly predict the activity vs. Ptn size and applied potential. For the small clusters, spillover of Hads from the clusters to the ITO support is significant, resulting in a competing channel for loss of Hads, particularly at slow potential scan rates.
          • Article
          • Peer Reviewed
          UCLA Previously Published Works (2024)
          Cu dissolution has been identified as the dominant process that causes cathode degradation and losses even under cathodic conditions involving methylamine. Despite extensive experimental research, our fundamental and theoretical understanding of the atomic-scale mechanism for Cu dissolution under electrochemical conditions, eventually coupled with surface restructuring processes, is limited. Here, driven by the observation that the working Cu electrode is corroded using mixtures of acetone and methylamine even under reductive potential conditions (-0.75 V vs. RHE), we employed Grand Canonical density functional theory to understand this dynamic process under potential from a microscopic perspective. We show that amine ligands in solution directly chemisorb on the electrode, coordinate with the metal center, and drive the rearrangement of the copper surface by extracting Cu as adatoms in low coordination positions, where other amine ligands can coordinate and stabilize a surface copper-ligand complex, finally forming a detached Cu-amine cationic complex in solution, even under negative potential conditions. Calculations predict that dissolution would occur for a potential of -1.1 V vs. RHE or above. Our work provides a fundamental understanding of Cu dissolution facilitated by surface restructuring in amine solutions under electroreduction conditions, which is required for the rational design of durable Cu-based cathodes for electrochemical amination or other amine involving reduction processes.
            Cover page: Origin of copper dissolution under electrocatalytic reduction conditions involving amines.
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2023)
            Hydrazine-assisted water electrolysis offers a feasible path for low-voltage green hydrogen production. Herein, the design and synthesis of ultrathin RhRu0.5 -alloy wavy nanowires as bifunctional electrocatalysts for both the anodic hydrazine oxidation reaction (HzOR) and the cathodic hydrogen evolution reaction (HER) is reported. It is shown that the RhRu0.5 -alloy wavy nanowires can achieve complete electrooxidation of hydrazine with a low overpotential and high mass activity, as well as improved performance for the HER. The resulting RhRu0.5 bifunctional electrocatalysts enable, high performance hydrazine-assisted water electrolysis delivering a current density of 100 mA cm-2 at an ultralow cell voltage of 54 mV and a high current density of 853 mA cm-2 at a cell voltage of 0.6 V. The RhRu0.5  electrocatalysts further demonstrate a stable operation at a high current density of 100 mA cm-2 for 80 hours of testing period with little irreversible degradation. The overall performance greatly exceeds that of the previously reported hydrazine-assisted water electrolyzers, offering a pathway for efficiently converting hazardous hydrazine into molecular hydrogen.
              Cover page: Bifunctional Ultrathin RhRu0.5‐Alloy Nanowire Electrocatalysts for Hydrazine‐Assisted Water SplittingCreative Commons 'BY-NC-ND' version 4.0 license
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