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Open Access Publications from the University of California

Scholarly Works (12 results)

  • Article
  • Peer Reviewed
UCLA Previously Published Works (2019)
  • 1 supplemental PDF
Cover page: Surface Structure of Co3O4 (111) under Reactive Gas-Phase Environments
  • Article
  • Peer Reviewed
UCLA Previously Published Works (2019)
    Cover page: Correction to “The Surface Structure of Co3O4(111) under Reactive Gas-Phase Environments”
    • Thesis
    • Peer Reviewed
    UCLA Electronic Theses and Dissertations (2022)

    In the past decade, highly dispersed catalysts (HDCs) and more specifically single atom catalysts (SACs) have become popular in the field of heterogeneous catalysis. Although these catalysts efficiently utilize all the active metal in the catalyst, their structure and reactivity are inadequately understood. For the structure and stability of HDCs and SACs on metal oxides, the sensitivity of the very small clusters and single atoms to their surrounding gas/liquid environment results in many accessible states under reaction conditions, rendering simplistic structural models unrealistic. The unique chemical properties of the active sites by their lack of metal-metal coordination or poor mixing of electronic states with the support shift the free energy pathway of catalytic reactions, resulting in nontraditional kinetic behavior. In this dissertation, we use density functional theory (DFT), atomistic thermodynamics, and microkinetic modeling to study several HDCs and SACs to understand their structure, stability, and reactivity under realistic catalytic environments.DFT-based atomistic thermodynamics were performed to investigate the surface and structure of a highly dispersed Rh/CeO2 catalyst. We found that the interaction between Rh and the CO gas product induces a counterintuitive redispersion of Rh active sites during the steam reforming of methane, reflecting the sensitivity of the Rh single atoms to their surrounding environment. We further applied the atomistic thermodynamics approach in the simulation of the surface structure of a dual single-atom Mo1+Pd1/Co3O4 catalyst. We found that under reducing gas environments containing predominantly H2 and H2O, the Co3O4(111) surface becomes hydroxylated and Co-enriched. DFT-energetics-based microkinetic modeling of the anisole hydrodeoxygenation (HDO) reaction reveals that the hydroxylated and Co-enriched Co3O4(111) surface enables the cooperation between the Mo1 and Pd1 sites by allowing the shuttling of H through metastable H2O intermediates. Finally, we also applied DFT-energetics-based microkinetic modeling in the modeling of the hydrogenation of 1-hexyne and hydrogenation/isomerization of 1-hexene over a dilute Pd-in-Au catalyst. We found that the large barriers for H2 dissociation over Pd1 and for H exchange between individual Pd1 sites results in the high selectivity of the dilute Pd-in-Au catalyst for 1-hexene in the hydrogenation of 1-hexyne, as well as for 2-hexene in the hydrogenation/isomerization of 1-hexene.

      Cover page: Theoretical Modeling of Highly Dispersed Catalysts under Reactive Gas Environments
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2023)
      Despite the wide applications, the ab initio modeling of the ceria based catalyst is challenging. The partial occupation in the 4f orbitals creates a fundamental challenge for commonly used density functional theory (DFT) methods, including semilocal functionals with Hubbard U correction to force localization and hybrid functionals. In this work, we benchmark the random phase approximation (RPA) for ceria surface properties, including surface energy and hydrogenation energy, compared to the results utilizing the DFT + U approach or hybrid functionals. We show that, for the latter approaches, different surface properties require opposite directions of parameter tuning. This forms a dilemma for the parameter based DFT methods, as the improvement of a certain property by tuning parameters will inevitably lead to the worsening of other properties. Our results suggest that the parameter-free many-body perturbation theory methods exemplified by RPA are a promising strategy to escape the dilemma and provide highly accurate descriptions, which will allow us to better understand the catalytic reactions in ceria related systems.
      • 1 supplemental PDF
      Cover page: Toward more accurate surface properties of ceria using many-body perturbation theory.Creative Commons 'BY-NC-ND' version 4.0 license
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2019)
      The interaction of water with metal oxides controls their activity and stability in heterogeneous catalysis and electrocatalysis. In this work, we combine density functional theory calculations and infrared reflection absorption spectroscopy (IRAS) to identify the structural motifs formed upon interaction of water with an atomically defined Co3O4(111) surface. Three principal structures are observed: (i) strongly bound isolated OD, (ii) extended hydrogen-bonded OD/D2O structures, and (iii) a third structure which has not been reported to our knowledge. In this structure, surface Co2+ ions bind to three D2O molecules to form an octahedrally coordinated Co2+ with a "half hydration shell". We propose that this hydration structure represents an important intermediate in reorganization and dissolution on oxide surfaces which expose highly unsaturated surface cations.
        Cover page: Water on Oxide Surfaces: A Triaqua Surface Coordination Complex on Co3O4(111)
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2020)
        Dehydrogenation reactions are central for the production of functional molecules. Steam plays a pivotal role in ZnFe2O4-catalyzed 1-butene oxidative dehydrogenation (ODH). However, the essential effect of steam on this reaction is still unclear. Herein, we describe the structure-performance relationships of ZnFe2O4 in the presence/absence of steam by combined density functional theory and experimental studies. The catalytic performances of ZnFe2O4 under different reaction conditions were investigated. The ZnFe2O4(110) surface properties under reaction conditions and molecular reaction pathways were modeled. Free-energy profiles were calculated. We found that an oxygen-excess ZnFe2O4(110)-O termination, with an extra O atom bridging two Fe cations, is preferred under oxygen-rich conditions. However, both experimental and theoretical approaches indicate that this surface bicoordinated O is not stable at relatively high temperatures in the absence of steam, resulting in the reduction of surface Fe3+ cations to Fe2+ which are inactive in the 1-butene ODH reaction. It was found that steam interacts strongly with the ZnFe2O4(110)-O surface. Steam stabilizes the catalyst surface Fe3+ ions by converting bicoordinated O to more thermally stable hydroxyl groups. Surface hydroxyls are active sites for C-H bond cleavage in the 1-butene ODH reaction in the presence of steam. We propose that the role of steam elucidated here represents a general mode of steam influence in ODH reactions over oxide surfaces.
        • 1 supplemental PDF
        Cover page: Stabilizing Oxidative Dehydrogenation Active Sites at High Temperature with Steam: ZnFe2O4‑Catalyzed Oxidative Dehydrogenation of 1‑Butene to 1,3-Butadiene
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2022)
        • 1 supplemental PDF
        Cover page: Reaction product-driven restructuring and assisted stabilization of a highly dispersed Rh-on-ceria catalyst
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2020)
        The reaction of CO and O2 with submonolayer and multilayer CoOx films on Pt(111), to produce CO2, was investigated at room temperature in the mTorr pressure regime. Using operando ambient pressure X-ray photoelectron spectroscopy and high pressure scanning tunneling microscopy, as well as density functional theory calculations, we found that the presence of oxygen vacancies in partially oxidized CoOx films significantly enhances the CO oxidation activity to form CO2 upon exposure to mTorr pressures of CO at room temperature. In contrast, CoO films without O-vacancies are much less active for CO2 formation at RT, and CO only adsorbed in the form of carbonate species that are stable up to 260 °C. On submonolayer CoOx islands, the carbonates form preferentially at island edges, deactivating the edge sites for CO2 formation, even while the reaction proceeds inside the islands. These results provide a detailed understanding of CO oxidation pathways on systems where noble metals such as Pt interact with reducible oxides.
          Cover page: CO Oxidation Mechanisms on CoO x ‑Pt Thin Films
          • Article
          • Peer Reviewed
          LBL Publications (2023)
          Noble metals supported on reducible oxides, like CoOx and TiOx, exhibit superior activity in many chemical reactions, but the origin of the increased activity is not well understood. To answer this question we studied thin films of CoOx supported on an Au(111) single crystal surface as a model for the CO oxidation reaction. We show that three reaction regimes exist in response to chemical and topographic restructuring of the CoOx catalyst as a function of reactant gas phase CO/O2 stoichiometry and temperature. Under oxygen-lean conditions and moderate temperatures (≤150 °C), partially oxidized films (CoOx<1) containing Co0 were found to be efficient catalysts. In contrast, stoichiometric CoO films containing only Co2+ form carbonates in the presence of CO that poison the reaction below 300 °C. Under oxygen-rich conditions a more oxidized catalyst phase (CoOx>1) forms containing Co3+ species that are effective in a wide temperature range. Resonant photoemission spectroscopy (ResPES) revealed the unique role of Co3+ sites in catalyzing the CO oxidation. Density function theory (DFT) calculations provided deeper insights into the pathway and free energy barriers for the reactions on these oxide phases. These findings in this work highlight the versatility of catalysts and their evolution to form different active phases, both topological and chemically, in response to reaction conditions exposing a new paradigm in the catalyst structure during operation.
            Cover page: Elucidating the active phases of CoOx films on Au(111) in the CO oxidation reaction.
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2023)
            • 1 supplemental PDF
            Cover page: Identifying the Optimal Pd Ensemble Size in Dilute PdAu Alloy Nanomaterials for Benzaldehyde HydrogenationCreative Commons 'BY-NC-ND' version 4.0 license
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