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Open Access Publications from the University of California

Scholarly Works (10 results)

  • Thesis
  • Peer Reviewed
UC Irvine Electronic Theses and Dissertations (2023)

Catalysis remains an important field of research that drives economical approaches for various energy conversion applications from natural resources. Industrial catalyzed reactions occur at the surface of heterogeneous catalysts where many different active sites exist. Any atom not at the surface is not directly used to drive chemical reactions. To improve efficiency, catalytic nanomaterials have shrunk to the nano and atomic scale, increasing the surface area/volume ratio. Traditional characterization methods for nanomaterials provide a large scale picture of overall changes but do not exhibit a high degree of spatial information. To that end, transmission electron microscopy (TEM) may be used to probe materials down to the scale of individual atoms through both imaging and spectroscopy. Specialty holders allow for replication of benchtop reactions with which to investigate changes to nanomaterials in situ. Here, in situ TEM was used to gain insights into a variety of chemical processes including bond cleavage and surface reduction of different nanomaterials.First, we used in situ TEM to observe the contraction of the inter-metal center distances upon heating of a Zr based metal-organic framework before tracking the chemical bond changes with vibrational electron energy loss spectroscopy (EELS). The in situ results were corroborate with ex situ samples to validate the results. Then, in situ TEM and EELS was used to spatially map the Ce3+ aggregation on the surface of Pt loaded ceria nanoparticles. The localization of the Ce3+ had remained an unanswered question prior. The surface reduction was seen to proceed in an irreversible fashion which was attributed to orientated attachment, a precursor step to sintering. Finally, in situ TEM was performed in a liquid environment analyzing the growth of nanobubbles and the dissolution of octahedral core-shell nanoparticles. Both nanomaterials exhibited stages of structural changes that correspond to size stability and atom migration, respectively. As seen with the variety of nanomaterials observed, in situ TEM gives insight to the various chemical processes that occur when replicating benchtop reactions.

    Cover page: Insights into Chemical Processes Garnered Through In Situ Transmission Electron Microscopy
    • Article
    • Peer Reviewed
    UC Irvine Previously Published Works (2023)
    The addition of platinum-group metals (PGMs, e.g., Pt) to CeO2 is used in heterogeneous catalysis to promote the rate of redox surface reactions. Well-defined model system studies have shown that PGMs facilitate H2 dissociation, H-spillover onto CeO2 surfaces, and CeO2 surface reduction. However, it remains unclear how the heterogeneous structures and interfaces that exist on powder catalysts influence the mechanistic picture of PGM-promoted H2 reactions on CeO2 surfaces developed from model system studies. Here, controlled catalyst synthesis, temperature-programmed reduction (TPR), in situ infrared spectroscopy (IR), and in situ electron energy loss spectroscopy (EELS) were used to interrogate the mechanisms of how Pt nanoclusters and single atoms influence H2 reactions on high-surface area Pt/CeO2 powder catalysts. TPR showed that Pt promotes H2 consumption rates on Pt/CeO2 even when Pt exists on a small fraction of CeO2 particles, suggesting that H-spillover proceeds far from Pt-CeO2 interfaces and across CeO2-CeO2 particle interfaces. IR and EELS measurements provided evidence that Pt changes the mechanism of H2 activation and the rate limiting step for Ce3+, oxygen vacancy, and water formation as compared to pure CeO2. As a result, higher-saturation surface hydroxyl coverages can be achieved on Pt/CeO2 compared to pure CeO2. Further, Ce3+ formed by spillover-H from Pt is heterogeneously distributed and localized at and around interparticle CeO2-CeO2 boundaries, while activated H2 on pure CeO2 results in homogeneously distributed Ce3+. Ce3+ localization at and around CeO2-CeO2 boundaries for Pt/CeO2 is accompanied by surface reconstruction that enables faster rates of H2 consumption. This study reconciles the materials gap between model structures and powder catalysts for H2 reactions with Pt/CeO2 and highlights how the spatial heterogeneity of powder catalysts dictates the influence of Pt on H2 reactions at CeO2 surfaces.
      Cover page: How Pt Influences H2 Reactions on High Surface-Area Pt/CeO2 Powder Catalyst SurfacesCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Irvine Previously Published Works (2019)
      Control of reduction kinetics and nucleation processes is key in materials synthesis. However, understanding of the reduction dynamics in the initial stages is limited by the difficulty of imaging chemical reactions at the atomic scale; the chemical precursors are prone to reduction by the electron beams needed to achieve atomic resolution. Here, we study the reduction of a solid-state Pt precursor compound in an aberration-corrected transmission electron microscope by combining low-dose and in situ imaging. The beam-sensitive Pt precursor, K2PtCl4, is imaged at atomic resolution, enabling determination of individual (K, Pt, Cl) atoms. The transformation to Pt nanoclusters is captured in real time, showing a three-stage reaction including the breaking of the ionic bond, formation of PtCl2, and the reduction of the dual-valent Pt to Pt metal. Deciphering the atomic-scale transformation of chemicals in real time using combined low-dose and in situ imaging brings new possibility to study reaction kinetics in general.
        Cover page: Probing the dynamics of nanoparticle formation from a precursor at atomic resolution.
        • Article
        • Peer Reviewed
        UC Irvine Previously Published Works (2019)
        Control of reduction kinetics and nucleation processes is key in materials synthesis. However, understanding of the reduction dynamics in the initial stages is limited by the difficulty of imaging chemical reactions at the atomic scale; the chemical precursors are prone to reduction by the electron beams needed to achieve atomic resolution. Here, we study the reduction of a solid-state Pt precursor compound in an aberration-corrected transmission electron microscope by combining low-dose and in situ imaging. The beam-sensitive Pt precursor, K2PtCl4, is imaged at atomic resolution, enabling determination of individual (K, Pt, Cl) atoms. The transformation to Pt nanoclusters is captured in real time, showing a three-stage reaction including the breaking of the ionic bond, formation of PtCl2, and the reduction of the dual-valent Pt to Pt metal. Deciphering the atomic-scale transformation of chemicals in real time using combined low-dose and in situ imaging brings new possibility to study reaction kinetics in general.
          Cover page: Probing the dynamics of nanoparticle formation from a precursor at atomic resolutionCreative Commons 'BY' version 4.0 license
          • Article
          • Peer Reviewed
          UC Irvine Previously Published Works (2024)
          Single-atom catalysts (SACs) offer efficient metal utilization and distinct reactivity compared to supported metal nanoparticles. Structure-function relationships for SACs often assume that active sites have uniform coordination environments at particular binding sites on support surfaces. Here, we investigate the distribution of coordination environments of Pt SAs dispersed on shape-controlled anatase TiO2 supports specifically exposing (001) and (101) surfaces. Pt SAs on (101) are found on the surface, consistent with existing structural models, whereas those on (001) are beneath the surface after calcination. Pt SAs under (001) surfaces exhibit lower reactivity for CO oxidation than those on (101) surfaces due to their limited accessibility to gas phase species. Pt SAs deposited on commercial-TiO2 are found both at the surface and in the bulk, posing challenges to structure-function relationship development. This study highlights heterogeneity in SA coordination environments on oxide supports, emphasizing a previously overlooked consideration in the design of SACs.
            Cover page: Distribution of Pt single atom coordination environments on anatase TiO2 supports controls reactivity.
            • Article
            • Peer Reviewed
            UC Irvine Previously Published Works (2022)
            Point-of-care (POC) detection of herbicides is of great importance due to their impact on the environment and potential risks to human health. Here, we design a single-atomic site catalyst (SASC) with excellent peroxidase-like (POD-like) catalytic activity, which enhances the detection performance of corresponding lateral flow immunoassay (LFIA). The iron single-atomic site catalyst (Fe-SASC) is synthesized from hemin-doped ZIF-8, creating active sites that mimic the Fe active center coordination environment of natural enzyme and their functions. Due to its atomically dispersed iron active sites that result in maximum utilization of active metal atoms, the Fe-SASC exhibits superior POD-like activity, which has great potential to replace its natural counterparts. Also, the catalytic mechanism of Fe-SASC is systematically investigated. Utilizing its outstanding catalytic activity, the Fe-SASC is used as label to construct LFIA (Fe-SASC-LFIA) for herbicide detection. The 2,4-dichlorophenoxyacetic acid (2,4-D) is selected as a target here, since it is a commonly used herbicide as well as a biomarker for herbicide exposure evaluation. A linear detection range of 1-250 ng/mL with a low limit of detection (LOD) of 0.82 ng/mL has been achieved. Meanwhile, excellent specificity and selectivity towards 2,4-D have been obtained. The outstanding detection performance of the Fe-SASC-LFIA has also been demonstrated in the detection of human urine samples, indicating the practicability of this POC detection platform for analyzing the 2,4-D exposure level of a person. We believe this proposed Fe-SASC-LFIA has potential as a portable, rapid, and high-sensitive POC detection strategy for pesticide exposure evaluation.
              Cover page: Single-Atomic Site Catalyst Enhanced Lateral Flow Immunoassay for Point-of-Care Detection of HerbicideCreative Commons 'BY' version 4.0 license
              • Article
              • Peer Reviewed
              UC Irvine Previously Published Works (2024)
              Corrosion of electrocatalysts during electrochemical operations, such as low potential - high potential cyclic swapping, can cause significant performance degradation. However, the electrochemical corrosion dynamics, including structural changes, especially site and composition specific ones, and their correlation with electrochemical processes are hidden due to the insufficient spatial-temporal resolution characterization methods. Using electrochemical liquid cell transmission electron microscopy, we visualize the electrochemical corrosion of Pd@Pt core-shell octahedral nanoparticles towards a Pt nanoframe. The potential-dependent surface reconstruction during multiple continuous in-situ cyclic voltammetry with clear redox peaks is captured, revealing an etching and deposition process of Pd that results in internal Pd atoms being relocated to external surface, followed by subsequent preferential corrosion of Pt (111) terraces rather than the edges or corners, simultaneously capturing the structure evolution also allows to attribute the site-specific Pt and Pd atomic dynamics to individual oxidation and reduction events. This work provides profound insights into the surface reconstruction of nanoparticles during complex electrochemical processes.
                Cover page: Direct in-situ imaging of electrochemical corrosion of Pd-Pt core-shell electrocatalysts.
                • Article
                • Peer Reviewed
                UC San Diego Previously Published Works (2023)
                Designing cost-efffective electrocatalysts for the oxygen evolution reaction (OER) holds significant importance in the progression of clean energy generation and efficient energy storage technologies, such as water splitting and rechargeable metal-air batteries. In this work, an OER electrocatalyst is developed using Ni and Fe precursors in combination with different proportions of graphene oxide. The catalyst synthesis involved a rapid reduction process, facilitated by adding sodium borohydride, which successfully formed NiFe nanoparticle nests on graphene support (NiFe NNG). The incorporation of graphene support enhances the catalytic activity, electron transferability, and electrical conductivity of the NiFe-based catalyst. The NiFe NNG catalyst exhibits outstanding performance, characterized by a low overpotential of 292.3 mV and a Tafel slope of 48 mV dec-1 , achieved at a current density of 10 mA cm- 2 . Moreover, the catalyst exhibits remarkable stability over extended durations. The OER performance of NiFe NNG is on par with that of commercial IrO2 in alkaline media. Such superb OER catalytic performance can be attributed to the synergistic effect between the NiFe nanoparticle nests and graphene, which arises from their large surface area and outstanding intrinsic catalytic activity. The excellent electrochemical properties of NiFe NNG hold great promise for further applications in energy storage and conversion devices.
                  Cover page: NiFe Nanoparticle Nest Supported on Graphene as Electrocatalyst for Highly Efficient Oxygen Evolution Reaction
                  • Article
                  • Peer Reviewed
                  Chemical Sciences (2023)
                  Atomically dispersed catalysts such as single-atom catalysts have been shown to be effective in selectively oxidizing methane, promising a direct synthetic route to value-added oxygenates such as acetic acid or methanol. However, an important challenge of this approach has been that the loading of active sites by single-atom catalysts is low, leading to a low overall yield of the products. Here, we report an approach that can address this issue. It utilizes a metal-organic framework built with porphyrin as the linker, which provides high concentrations of binding sites to support atomically dispersed rhodium. It is shown that up to 5 wt% rhodium loading can be achieved with excellent dispersity. When used for acetic acid synthesis by methane oxidation, a new benchmark performance of 23.62 mmol·gcat-1·h-1 was measured. Furthermore, the catalyst exhibits a unique sensitivity to light, producing acetic acid (under illumination, up to 66.4% selectivity) or methanol (in the dark, up to 65.0% selectivity) under otherwise identical reaction conditions.
                    Cover page: Selective Formation of Acetic Acid and Methanol by Direct Methane Oxidation Using Rhodium Single-Atom Catalysts
                    • Article
                    • Peer Reviewed
                    UC Irvine Previously Published Works (2024)
                    Shape-controlled alloy nanoparticle catalysts have been shown to exhibit improved performance in the oxygen reduction reaction (ORR) in liquid half-cells. However, translating the success to catalyst layers in fuel cells faces challenges due to the more demanding operation conditions in membrane electrode assembly (MEA). Balancing durability and activity is crucial. Here, we developed a strategy that limits the atomic diffusion within surface layers, fostering the phase transition and shape retention during thermal treatment. This enables selective transformation of platinum-iron nanowire surfaces into intermetallic structures via atomic ordering at a low temperature. The catalysts exhibit enhanced MEA stability with 50% less Fe loss while maintaining high catalytic activity comparable to that in half-cells. Density functional calculations suggest that the ordered intermetallic surface stabilizes morphology against rapid corrosion and improves the ORR activity. The surface engineering through atomic ordering presents potential for practical application in fuel cells with shape-controlled Pt-based alloy catalysts.
                      Cover page: Highly stable and active catalyst in fuel cells through surface atomic ordering.
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