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Open Access Publications from the University of California

Scholarly Works (13 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2018)

The subject of this dissertation focuses on the design and synthesis of new catalysts with well-defined structures and superior performance to meet the new challenges in heterogenous catalysis. The past decade has witness the development of nanoscience as well as the inorganic catalysts for industrial applications, however there are still fundamental challenges and practical need for catalysis. Specifically, it is desirable to have the ability to selectivity produce complex molecules from simple components. Another great challenge faced by the modern industry is being environmentally friendly, and going for a carbon neutral economy would require using CO2 as feedstock to produce valuable products. The work herein focuses on the design and synthesis of inorganic nanocrystal catalysts that address these challenges by achieving selective and sequential chemical reactions and conversion of CO2 to valuable products.

Chapter 1 introduces the development of heterogenous catalysis and the colloidal synthesis of metal nanoparticles catalysts with well-controlled structure. Tremendous efforts have been devoted to understanding the nucleation and growth process in the colloidal synthesis and developing new methods to produce metal nanoparticles with controlled sizes, shapes, composition. These well-defined catalytic system shows promising catalytic performance, which can be modulated by their structure (size, shape, compositions and the metal-oxide interfaces). The chapters hereafter explore the synthesis of new catalysts with controlled structures for catalysis.

Chapter 2 presents the design and synthesis of a three dimensional (3D) nanostructured catalysts CeO2-Pt@mSiO2 with dual metal-oxide interfaces to study the tandem hydroformylation reaction in gas phase, where CO and H2 produced by methanol decomposition (catalyzed by CeO2-Pt interface) were reacted with ethylene to selectively yield propyl aldehyde (catalyzed by Pt-SiO2 interface). With the stable core-shell architecture and well-defined metal-oxide interfaces, the origin of the high propyl aldehyde selectivity over ethane, the dominant byproduct in conventional hydroformylation, was revealed by in-depth mechanism study and attributed to the synergybetween the two sequential reactions and the altered elementary reaction steps of the tandem reaction compared to the single-step reaction. The effective production of aldehyde through the tandem hydroformylation was also observed on other light olefin system, such as propylene and 1-butene.

Chapter 3 expands the strategy of tandem catalysis into conversion of CO2 with hydrogen to value-added C2-C4 hydrocarbons, which is a major pursuit in clean energy research. Another well-defined 3D catalyst CeO2–Pt@mSiO2–Co was designed and synthesized, and CO2 was converted to C2-C4 hydrocarbons with 60% selectivity on this catalyst via reverse water gas shift reaction and subsequent Fischer–Tropsch process. In addition, the catalysts is stable and shows no obvious deactivation over 40 h. The successful production of C2−C4 hydrocarbons via a tandem process on a rationally designed, structurally well-defined catalyst demonstrates the power of sophisticated structure control in designing nanostructured catalysts for multiple-step chemical conversions.

Chapter 4 turns to electrochemistry and apply the precision in catalyst structural design to the development of electrocatalysts for CO2 reduction. Herein, atomic ordering of bimetallic nanoparticles were synthetically tuned, from disordered alloy to ordered intermetallic, and it showed that this atomic level control over nanocrystal catalysts could give significant performance benefits in electrochemical CO2 reduction to CO. Atomic-level structural investigations revealed the atomic gold layers over the intermetallic core to be sufficient for enhanced catalytic behavior, which is further supported by DFT analysis.

    Cover page: Rational Design and Synthesis of Inorganic Nanostructures for Tandem Catalysis and CO2 Conversion
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2020)
    The surface and interfaces of heterogeneous catalysts are essential to their performance as they are often considered to be active sites for catalytic reactions. With the development of nanoscience, the ability to tune surface and interface of nanostructures has provided a versatile tool for the development and optimization of a heterogeneous catalyst. In this Review, we present the surface and interface control of nanoparticle catalysts in the context of oxygen reduction reaction (ORR), electrochemical CO2 reduction reaction (CO2 RR), and tandem catalysis in three sections. In the first section, we start with the activity of ORR on the nanoscale surface and then focus on the approaches to optimize the performance of Pt-based catalyst including using alloying, core-shell structure, and high surface area open structures. In the section of CO2 RR, where the surface composition of the catalysts plays a dominant role, we cover its reaction fundamentals and the performance of different nanosized metal catalysts. For tandem catalysis, where adjacent catalytic interfaces in a single nanostructure catalyze sequential reactions, we describe its concept and principle, catalyst synthesis methodology, and application in different reactions.
      Cover page: Surface and Interface Control in Nanoparticle Catalysis
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2017)
      Conversion of carbon dioxide to C2-C4 hydrocarbons is a major pursuit in clean energy research. Despite tremendous efforts, the lack of well-defined catalysts in which the spatial arrangement of interfaces is precisely controlled hinders the development of more efficient catalysts and in-depth understanding of reaction mechanisms. Herein, we utilized the strategy of tandem catalysis to develop a well-defined nanostructured catalyst CeO2-Pt@mSiO2-Co for converting CO2 to C2-C4 hydrocarbons using two metal-oxide interfaces. C2-C4 hydrocarbons are found to be produced with high (60%) selectivity, which is speculated to be the result of the two-step tandem process uniquely allowed by this catalyst. Namely, the Pt/CeO2 interface converts CO2 and H2 to CO, and on the neighboring Co/mSiO2 interface yields C2-C4 hydrocarbons through a subsequent Fischer-Tropsch process. In addition, the catalysts show no obvious deactivation over 40 h. The successful production of C2-C4 hydrocarbons via a tandem process on a rationally designed, structurally well-defined catalyst demonstrates the power of sophisticated structure control in designing nanostructured catalysts for multiple-step chemical conversions.
        Cover page: Tandem Catalysis for CO2 Hydrogenation to C2–C4 Hydrocarbons
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2018)
        Pt-based alloys have shown great promise as cathodic catalysts for cost-effective proton-exchange membrane fuel cells. Post-synthesis treatment has been recognized as a critical step to improve the catalytic performance of Pt-based alloys. Here, we present the effects of catalyst processing on the catalytic behavior of Pt-Ni nanoframe electrocatalysts in oxygen reduction reaction. The Pt-Ni nanoframes were made by corroding the Ni-rich phase from solid rhombic dodecahedral particles. A total of three different corrosion procedures were compared. Among them, electrochemical corrosion led to the highest initial specific activity (1.35 mA cm-2 at 0.95 V versus reversible hydrogen electrode) by retaining more Ni in the nanoframes. However, the high activity gradually went down in a subsequent stability test due to continuous Ni loss and concomitant surface reconstruction. On the other hand, the best stability was achieved by a more-aggressive corrosion using oxidative nitric acid. Although the initial activity was compromised, this procedure imparted a less-defective surface, and thus, the specific activity dropped by only 7% over 30 000 potential cycles. These results indicate a delicate trade-off between the activity and stability of Pt-Ni nanoframe electrocatalysts. The obtained understanding of how to balance the activity-stability trade-off via catalyst processing can be generalized to other Pt-based alloys.
          Cover page: Effects of Catalyst Processing on the Activity and Stability of Pt–Ni Nanoframe Electrocatalysts
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2016)
          The concept of tandem catalysis, where sequential reactions catalyzed by different interfaces in single nanostructure give desirable product selectively, has previously been applied effectively in the production of propanal from methanol (via carbon monoxide and hydrogen) and ethylene via tandem hydroformylation. However, the underlying mechanism leading to enhanced product selectivity has remained elusive due to the lack of stable, well-defined catalyst suitable for in-depth comprehensive study. Accordingly, we present the design and synthesis of a three-dimensional (3D) catalyst CeO2-Pt@mSiO2 with well-defined metal-oxide interfaces and stable architecture and investigate the selective conversion of ethylene to propanal via tandem hydroformylation. The effective production of aldehyde through the tandem hydroformylation was also observed on propylene and 1-butene. A thorough study of the CeO2-Pt@mSiO2 under different reaction and control conditions reveals that the ethylene present for the hydroformylation step slows down initial methanol decomposition, preventing the accumulation of hydrogen (H2) and favoring propanal formation to achieve up to 80% selectivity. The selectivity is also promoted by the fact that the reaction intermediates produced from methanol decomposition are poised to directly undergo hydroformylation upon migration from one catalytic interface to another. This synergistic effect between the two sequential reactions and the corresponding altered reaction pathway, compared to the single-step reaction, constitute the key advantages of this tandem catalysis. Ultimately, this in-depth study unravels the principles of tandem catalysis related to hydroformylation and represents a key step toward the rational design of new heterogeneous catalysts.
            Cover page: Insights into the Mechanism of Tandem Alkene Hydroformylation over a Nanostructured Catalyst with Multiple Interfaces
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2018)
            Reducing the diameter of silver nanowires has been proven to be an effective way to improve their optoelectronic performance by lessening light attenuation. The state-of-the-art silver nanowires are typically around 20 nm in diameter. Herein we report a modified polyol synthesis of silver nanowires with average diameters as thin as 13 nm and aspect ratios up to 3000. The success of this synthesis is based on the employment of benzoin-derived radicals in the polyol approach and does not require high-pressure conditions. The strong reducing power of radicals allows the reduction of silver precursors to occur at relatively low temperatures, wherein the lateral growth of silver nanowires is restrained because of efficient surface passivation. The optoelectronic performance of as-prepared 13 nm silver nanowires presents a sheet resistance of 28 Ω sq-1 at a transmittance of 95% with a haze factor of ∼1.2%, comparable to that of commercial indium tin oxide (ITO).
              Cover page: Synthesis of Silver Nanowires with Reduced Diameters Using Benzoin-Derived Radicals to Make Transparent Conductors with High Transparency and Low Haze
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2017)
              Precise control of elemental configurations within multimetallic nanoparticles (NPs) could enable access to functional nanomaterials with significant performance benefits. This can be achieved down to the atomic level by the disorder-to-order transformation of individual NPs. Here, by systematically controlling the ordering degree, we show that the atomic ordering transformation, applied to AuCu NPs, activates them to perform as selective electrocatalysts for CO2 reduction. In contrast to the disordered alloy NP, which is catalytically active for hydrogen evolution, ordered AuCu NPs selectively converted CO2 to CO at faradaic efficiency reaching 80%. CO formation could be achieved with a reduction in overpotential of ∼200 mV, and catalytic turnover was enhanced by 3.2-fold. In comparison to those obtained with a pure gold catalyst, mass activities could be improved as well. Atomic-level structural investigations revealed three atomic gold layers over the intermetallic core to be sufficient for enhanced catalytic behavior, which is further supported by DFT analysis.
                Cover page: Electrochemical Activation of CO2 through Atomic Ordering Transformations of AuCu Nanoparticles
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2015)
                We enclose octahedral silver nanocrystals (Ag NCs) in metal-organic frameworks (MOFs) to make mesoscopic constructs O(h)-nano-Ag⊂MOF in which the interface between the Ag and the MOF is pristine and the MOF is ordered (crystalline) and oriented on the Ag NCs. This is achieved by atomic layer deposition of aluminum oxide on Ag NCs and addition of a tetra-topic porphyrin-based linker, 4,4',4″,4‴-(porphyrin-5,10,15,20-tetrayl)tetrabenzoic acid (H4TCPP), to react with alumina and make MOF [Al2(OH)2TCPP] enclosures around Ag NCs. Alumina thickness is precisely controlled from 0.1 to 3 nm, thus allowing control of the MOF thickness from 10 to 50 nm. Electron microscopy and grazing angle X-ray diffraction confirm the order and orientation of the MOF by virtue of the porphyrin units being perpendicular to the planes of the Ag. We use surface-enhanced Raman spectroscopy to directly track the metalation process on the porphyrin and map the distribution of the metalated and unmetalated linkers on a single-nanoparticle level.
                  Cover page: Mesoscopic Constructs of Ordered and Oriented Metal–Organic Frameworks on Plasmonic Silver Nanocrystals
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2020)
                  Promotion of C-C bonds is one of the key fundamental questions in the field of CO2 electroreduction. Much progress has occurred in developing bulk-derived Cu-based electrodes for CO2-to-multicarbons (CO2-to-C2+), especially in the widely studied class of high-surface-area "oxide-derived" copper. However, fundamental understanding into the structural characteristics responsible for efficient C-C formation is restricted by the intrinsic activity of these catalysts often being comparable to polycrystalline copper foil. By closely probing a Cu nanoparticle (NP) ensemble catalyst active for CO2-to-C2+, we show that bias-induced rapid fusion or "electrochemical scrambling" of Cu NPs creates disordered structures intrinsically active for low overpotential C2+ formation, exhibiting around sevenfold enhancement in C2+ turnover over crystalline Cu. Integrating ex situ, passivated ex situ, and in situ analyses reveals that the scrambled state exhibits several structural signatures: a distinct transition to single-crystal Cu2O cubes upon air exposure, low crystallinity upon passivation, and high mobility under bias. These findings suggest that disordered copper structures facilitate C-C bond formation from CO2 and that electrochemical nanocrystal scrambling is an avenue toward creating such catalysts.
                    Cover page: Electrochemically scrambled nanocrystals are catalytically active for CO2-to-multicarbons
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2018)
                    Surface condition plays an important role in the optical performance of semiconductor materials. As new types of semiconductors, the emerging metal-halide perovskites are promising for next-generation optoelectronic devices. We discover significantly improved light-emission efficiencies in lead halide perovskites due to surface oxygen passivation. The enhancement manifests close to 3 orders of magnitude as the perovskite dimensions decrease to the nanoscale, improving external quantum efficiencies from <0.02% to over 12%. Along with about a 4-fold increase in spontaneous carrier recombination lifetimes, we show that oxygen exposure enhances light emission by reducing the nonradiative recombination channel. Supported by X-ray surface characterization and theoretical modeling, we propose that excess lead atoms on the perovskite surface create deep-level trap states that can be passivated by oxygen adsorption.
                      Cover page: Giant Light-Emission Enhancement in Lead Halide Perovskites by Surface Oxygen Passivation
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