Browsing by Subject "Catalysts"

Now showing 1 - 6 of 6

Creating more effective functional materials: altering the electronics of conducting metallopolymers for different applications.
(2014-05) Raiford, Matthew Thomas; Holliday, Bradley J.; Humphrey, Simon M; Anslyn, Eric V; Jones, Richard A; Freeman, Benny D
Conducting metallopolymers possess attractive electronic properties for use in sensors, photoelectronic devices, catalysts, and other applications. Modification of the conducting polymer backbone, through chemical or electrochemical methods, enables control of catalytic, electronic, and optical properties of the metal via inductive modulation of the electron density. Understanding in detail the relationship between the metal and polymer backbone could lead to more effective metallopolymer materials. We hope to study this relationship by probing the band gaps, excited state energy levels, catalytic activity, and sensor function in four metallopolymer systems. Devices with sub-stochiometric ratios of Cu2ZnSnS4 NPs (CZTS: (Cu2Sn)1-xZn1/xS)(0≥x≥0.75)) grown in Cu(II) conducting metallopolymers were produced to study band gap tuning in hybrid materials. The valence and conductance bands of CZTS (x = 0.60) aligned with the HOMO/LUMO of the Cu(II) metallopolymer. Changing the alignment facilitated charge transfer in the hybrid material, leading to photovoltaic materials with efficiencies of ~0.1%. Chemoresistive ionophore sensors were developed by incorporating selective binding groups, such as thiourea, into conducting polymer backbones. Thiourea monomers and polymers showed high selectivity for Pb(II) ions over many competitive ions. XPS experiments demonstrated that reversible chelation of Pb(II) ions could be achieved through a simple uptake/rinse process. The conductivity of the thiourea polymer increased fifty-fold, from 7.75×10−2 S/cm2 to 3.5 S/cm2, after Pb(II) exposure. Sensitivity measurements indicated the sensors have limits of detection near 10−10 M. Highly conjugated ligands were synthesized to explore effective sensitization of visible and near-IR emitting lanthanides. (3,4-ethylenedioxy)thiophene was appended to dipyridophenazine and dipyridoquinoxaline to introduce a group that could be easily electropolymerized. These bi-functional ligands emitted from π-π* and an inter-ligand charge transfer excited states, and therefore, two distinct triplet states were observed. These separate energy pathways allowed for efficient sensitization of both visible (Tb(III), Eu(III), Dy(III)) and near-IR emitting (Nd(III), Yb(III), Er(III)) ions. Finally, we explored the oxidation of a rhodium-containing conducting metallopolymer and the subsequent effect on the activity of the metal center. Oxidation of the backbone led to ancillary ligand attenuation, allowing for control of the catalytically active species in the conducting metallopolymer. Rh(I,III) monomer and metallopolymer catalytic studies showed potential for new heterogenous/homogeneous hybrid catalysts.
Degradation mechanisms of Pt and Pt alloy nanocatalysts in proton exchange membrane fuel cells
(2017-05) Rasouli, SomayeSadat; Ferreira, Paulo J. (Paulo Jorge); Manthiram, Arumugam; Yu, Guihua; Nakashima, Naotoshi; Higashida, Kenji; Kongkanand, Anusorn
The goal of this PhD research is to fundamentally understand the degradation mechanisms and durability issues of Pt and Pt-alloy nanocatalysts in the cathode of proton exchange membrane fuel cells (PEMFCs). The primary tool for this research has been state-of-the-art transmission electron microscopy, including aberration-corrected TEM/STEM, in-situ TEM heating, 3D tomography, and Energy Dispersive Spectroscopy (EDS). In order to reveal the degradation mechanisms of nanocatalysts, both indirect and direct TEM methods were used. In the first part of this research, we performed post-mortem transmission electron microscopy (TEM) on the membrane electrode assembly (MEA) of PEMFCs. Using a thorough composition and morphological analysis of the catalysts after fuel cell cycling, we showed that the mechanisms proposed in the literature do not fully explain the degradation of the nanocatalysts. Accordingly, new mechanisms were proposed, namely: 1- Modified Ostwald ripening until adjacent particles make contact with each other and coalesce, 2-preferential deposition of single atoms and atomic clusters between two or more particles and consequently bridging between them. To evaluate these proposed mechanisms mentioned above, the second part of this work focused on determining the behavior of Pt and Pt-alloy nanoparticles during different stages of fuel cell cycling. The first challenge was to find a way to ensure that I was observing the exact same nanoparticles during the various stages of cycling. To accomplish this, we developed an experimental setup which replicates on a TEM grid the effect of voltage cycling on the cathode of an MEA. Using this approach, it was possible to track the behavior of a single nanoparticle at different stages of voltage cycling on the nano-atomic scale. Through these direct observations, we demonstrated that due to carbon corrosion the defects appear at the carbon/nanoparticle interface, which in turn result in particle migration and consequently coalescence. We also revealed the mass transfer mechanisms during the coalescence of nanoparticles. In addition, we revisited the commonly held view on the mechanism of particle dissolution and deposition. Thus, during the later stages of cycling, when the concentration of dissoluble Pt reaches a critical amount, single atoms and atomic clusters appear on the carbon support, which consequently move toward other particles and re-deposit on their surface. This dissolution happens preferentially at the corners and steps of the nanoparticle, while re-deposition occurs on {111} type planes. Contrary to the literature, it turned out that re-deposition is not necessarily an isotropic process as atomic clusters can deposit between two or more particles and bridge them. Furthermore, we investigated the atomic surface evolution and phase segregation of Pt3Co and PtNi nanoparticles under the effect of voltage through advanced spectroscopy technique such as EDS. While it is generally accepted in the literature that larger particles grow at the expense of smaller ones, this study showed that in case of alloys, deposition of Pt occurs on the surface of smaller particles rather than larger ones. This is due to the thicker Pt rich surfaces on the smaller particles, since the Pt rich surface act as nucleation sites for re-precipitation of Pt.
Development of Pd₃Co based catalysts for fuel cell applications and amine based solvents for CO₂ capture : a first principles based modelling of clean energy and clean air technology
(2014-12) Manogaran, Dhivya; Hwang, Gyeong S.; Mullins, C. B.
With the ever increasing environmental concerns in terms of the need for a vast improvement in clean energy and clean air technologies, this thesis focuses on analyzing the underlying principles that determine the activity of catalysts/sorbents for fuel cell applications and CO₂ capture using first principles based simulations with a view point to help fabricate efficient catalysts. We attempt to clarify the fuzzy concepts of existing surface-nearsurface interactions in Pd based electrocatalysts with particular attention to Pd₃Co alloy catalysts by presenting a thorough inter and intra-layer orbital analysis and bring forth the crucial role played by the surface-subsurface binding driven by the out of plane d-state interactions in determining the surface reactivity. We first decouple the effects induced by the different Pd-Pd and Pd-Co lattice parameters (lattice strain effect) from the hetero atom induced surface-subsurface interaction (we call it "interlayer ligand effect") and clearly demonstrate how enhanced surface-subsurface d [subscript xz+yz] interaction leads to an increased oxygen hydrogenation to H₂O in Pd₃Co based electrocatalysts. We then extend the concept of hetero atom induced surface-subsurface binding to a series of 3d transition metals and provide guidelines for the right choice of metals that may be potential ORR candidates. Finally, we describe the facet dependence and the effect of surface Au alloying on the surface reactivity of Pd₃Co electrocatalysts. In the second section of the thesis, we emphasize on the underlying principles of CO₂ capture by MEA and study the synergetic interplay of various factors that may lead to better CO₂ capture , also enabling efficient solvent regeneration. Though extensive studies are carried out on the most traditionally used alkanol amine MEA for CO₂ capture, there are several less studied aspects like the molecular orbital redistribution on CO₂ binding that decides the fate of the intermediate species and the role of water arrangement in assisting/hindering the progress of the reaction. We study the fundamental CO₂-amine interactions and highlight the crucial importance of alkanol-amine configuration, water arrangement and protonation/de-protonation tendencies at various basic sites in the development of the reaction. We then analyze the synergetic interplay between the inductive effect, the steric hindrance and the resonance in enhancing efficient CO₂ binding and allowing an alternative O-driven mechanism resulting into easy solvent regeneration. We believe that our efforts may help fabricate better catalysts and sorbents and help improve the existing clean energy and clean air technologies.
Model catalyst studies of the CO oxidation reaction on Titania supported gold nanoclusters
(2004) Stiehl, James Daniel; Mullins, C. B.
The chemical nature of gold has been determined to be much richer than previously thought. Recent discoveries that properly prepared gold catalysts (i.e. gold particle diameters in 2 – 5 nm size range) can catalyze the CO oxidation reaction at low temperature have spurned a renewed interest in the chemistry of gold. Despite the extensive research that has been performed regarding CO oxidation on Au based catalysts, many issues still remain unresolved. The origin of the particle size dependence of the reaction is not well understood. Also, details concerning the reaction mechanism, specifically identification of the active oxygen species, remain unresolved. In the following studies, ultra high vacuum, molecular beam, surface science techniques are used to study the CO oxidation reaction on titania supported gold nanoclusters (Au/TiO2). Using a radio frequency generated plasma-jet, it is possible to simultaneously populate the Au/TiO2 samples with atomically adsorbed and molecularly chemisorbed oxygen species, allowing for the opportunity to investigate the reactivity of each respective species. The reaction of CO with atomically adsorbed oxygen has been studied over a range of temperatures from 65 – 250 K as a function of gold coverage and oxygen coverage. The reaction is observed to be a strong function of both of the sample temperature and the oxygen coverage. The reaction is also relatively independent of the gold coverage on the sample, in contrast to findings for the reaction employing gas-phase reactants under moderate pressures. The formation and reactivity of molecularly chemisorbed oxygen on the samples following exposure to the plasma-jet was also investigated. Evidence is presented showing that some molecularly chemisorbed oxygen is formed as a result of recombination of impinging atoms on the model catalyst surface. Evidence is also presented showing that adsorption of an oxygen atom on the sample influences the chemisorption of molecular species from the gas phase. Finally, evidence is presented showing that the molecularly chemisorbed oxygen species can participate in the CO oxidation reaction at 77 K. This finding reveals a reaction channel for CO oxidation on Au/TiO2 model catalysts that does not require the dissociation of oxygen.
Noble metal-free cathode and electrolyte materials for low-cost, efficient Li-CO₂ batteries
(2020-05-13) Pipes, Robert Michel; Manthiram, Arumugam; Goodenough, John B; Yu, Guihua; Hwang, Gyeong S
IIn this dissertation, noble metal-free cathode and electrolyte materials are developed to improve the energy efficiency, capacity, and cycle life of lithium - carbon dioxide (Li-CO₂) batteries. These performance enhancements are achieved by reducing the overpotentials of the carbon dioxide reduction reaction (CDRR) during discharge and the carbon dioxide evolution reaction (CDER) during charge. In Chapter 1, an overview of the Li-CO₂ battery system is provided, including a description of Li-CO₂ electrochemistry, Li-CO₂ battery requirements and challenges, and a summary of prior noble metal-free catalysts reported to date. In Chapter 2, general experimental details are outlined, including the cell design, general materials, and characterization instruments used. In Chapter 3, a nanocomposite of anatase TiO₂ nanoparticles grown onto carbon nanotubes and mixed with carbon nanofibers (TiO₂-NP@CNT/CNF) is employed as a gas diffusion cathode (GDC) in Li-CO₂ batteries to improve CDRR kinetics and cycling stability. In Chapter 4, phenyl disulfide (PDS) is introduced as an Li-CO₂ battery electrolyte additive to allow for homogeneous CO₂ utilization. A reaction mechanism involving the formation of the intermediate S-phenyl carbonothioate (SPC⁻) is proposed and supported with experimental evidence. In Chapter 5, a composite of MoS₂ nanosheets grown onto multi-walled carbon nanotubes and mixed with single-walled carbon nanotubes (MoS₂-NS@MWNT/SWNT) is employed as an efficient Li-CO₂ GDC to reduce CDRR and CDER overpotentials. A mechanism is proposed in which the oxalate intermediate C₂O₄²⁻-Mo⁶⁺S₂⁴⁻ is formed, lowering the energy barrier to Li₂CO₃ decomposition on charge. Finally, in Chapter 6, freestanding vanadium nitride nanowires (VN-NW) are employed as a carbon-free Li-CO₂ GDC. The discharge product morphology is greatly improved compared to that of a control MWNT GDC, leading to reduced charge voltage and improved cycle life. Finally, a summary of all the work carried out in this dissertation and a brief perspective on future Li-CO₂ research is given in Chapter 7.
The surface chemistry of atomic oxygen pre-covered gold
(2008-05) Ojifinni, Rotimi Ayodele, 1975-; Mullins, C. B.
Gold used to be regarded as catalytically inert until about 20 years ago when it was shown that supported gold clusters < 5 nm in diameter exhibited some unique catalytic properties. Based on this revelation, several studies have demonstrated the feasibility of reactions previously thought of as impossible on gold. The ability of gold to oxidize CO below ambient temperatures at rates higher than conventional CO oxidation catalysts (Pd and Pt) has been shown to hold potentials for technological applications. Extensive past and on-going research are geared towards elucidating the mechanistic details of this reaction. The nature of the active sites, the effect of the supports and the effect of moisture are still debated in literature. I therefore present some experimental results supported with density functional theory calculations to shed additional light on some of the issues concerning gold catalysis in general, and low temperature CO oxidation in particular. Previous studies of the effect of moisture on oxide-supported gold reported that although water promotes CO oxidation on this surface by as much as two orders of magnitude, it is only a spectator molecule on the surface. I present here evidence for strong water-oxygen interactions when water is co-adsorbed with atomic oxygen on Au(111). Impinging a CO beam on the surface co-adsorbed with oxygen and water produces water-enhanced CO oxidation. Based on these results, I propose that CO reacts with hydroxyls formed from water-oxygen interactions to form CO₂, similar to a previous observation on Pt(111). Exposing a Au(111) surface pre-covered with ¹⁶O to isotopically labeled carbon dioxide (C¹⁸O₂) showed that ¹⁶O¹⁸O (m/e = 34) was produced from carbonate formation and decomposition. Estimates of reaction probability and activation energy gave ~ 10⁻⁴ - 10⁻⁵ and -0.15 eV respectively. The effect of annealing on the reactivity of oxygen pre-covered Au(111) was investigated using water, carbon monoxide and carbon dioxide as probe molecules. Precovering Au(111) with atomic oxygen followed by annealing resulted in surfaces that were less reactive towards water, CO and CO₂. Annealing is believed to stabilize the reactive metastable oxygen thereby increasing the barrier to reaction similar to what is reported on other surfaces.