299 research outputs found

    Summaries of FY 1997 Research in the Chemical Sciences

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    The objective of this program is to expand, through support of basic research, knowledge of various areas of chemistry, physics and chemical engineering with a goal of contributing to new or improved processes for developing and using domestic energy resources in an efficient and environmentally sound manner. Each team of the Division of Chemical Sciences, Fundamental Interactions and Molecular Processes, is divided into programs that cover the various disciplines. Disciplinary areas where research is supported include atomic, molecular, and optical physics; physical, inorganic, and organic chemistry; chemical energy, chemical physics; photochemistry; radiation chemistry; analytical chemistry; separations science; heavy element chemistry; chemical engineering sciences; and advanced battery research. However, traditional disciplinary boundaries should not be considered barriers, and multi-disciplinary efforts are encouraged. In addition, the program supports several major scientific user facilities. The following summaries describe the programs

    Production of ion exchange membrane for hydrogen fuel cell

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    A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Doctor of Philosophy in Engineering. Johannesburg, 2017Among of the components of the fuel cell, the polymer electrolyte membrane is critical to the performance and life time of the cell. Over the years the mechanical properties of the membrane, water management have tended to limit its wide spread commercialization as an alternative source of the renewable energy for portable power units. Fuel cell continues to attract extensive research interest as potential source of renewable energy. This work focuses on the production of ionexchange membrane (IEM) for hydrogen fuel cell, using cheap and locally available starting materials. The polystyrene-butadiene rubber (SBR) of different styrene and butadiene compositions, have been explored for functionality in fuel cell application. The production process was conducted in three stages: the first stage involved hydrogenation process followed by sulfonation process. The second stage entailed the production of carbon nano-spheres for the blending in the hydrogenated sulfonated polystyrene-butadiene rubber. The blending was also done between hybrid nanoparticles and hydrogenated sulfonated polystyrenebutadiene rubber. The third stage was the casting in thin film of blended solutions employing the evaporative method and the use of casting tape machine technique. The thin film was later on characterized and tested in a single fuel cell stack. Controlled hydrogenation of SBR employing catalytic method was achieved with maximum degree of hydrogenation in the range of: 90 – 92% for SBR with 23.5% styrene content and for SBR 25% styrene content 76 – 80% for SBR with 40% styrene content and 82 – 92% for SBR with 52% styrene content. The optimum conditions of this process were obtained using the Design of Experiments. SBR was also hydrogenated using a photocatalytic method and the percentage of hydrogenation for all SBR compositions used was found in the range between 60 and 74%. The hydrogenation results using the catalyst were higher compared to those obtained with the photocatalytic method. Therefore they were used to develop the kinetic model for prediction of hydrogenation process. Langmuir – Hinshelwood models were reviewed in this project as they explain these heterogeneous catalytic processes. Data from the kinetic tests were fitted to Langmuir – Hinshelwood models and reaction constants were found in the range between 0.445 h-1 and 0.610 h-1 for the reaction temperature between 20 and 30°C. The hydrogenated SBR of different compositions were effectively sulfonated with chlorosulphonic acid employed as first sulfonating agent of concentrations 0.15, 0.175 and 0.25M for SBR 23.5 and 25% styrene content, for SBR 40% styrene content and for SBR 52% styrene content, respectively. The degree of sulfonation was found in the range between 56 and 72% depending on the rubber composition. Trimethylsilyl chlorosulfonate used as the second sulfonating agent was like wise attached to the same polymer back bone and the degree of sulfonation was between 59 and 74% depending on the rubber’s styrene content. Non-conductive carbon nanospheres (CNS) of uniform size of about 46 nm were produced employing the non-catalytic chemical vapour deposition method at 1000°C. Acetylene and argon were respectively used as carbon source and carrier gas, in a reactor of 16 mm in diameter. Successful blending of 4 wt% nanoparticles and hydrogenated sulfonated styrene butadiene solution was accomplished by magnetic stirring technique combined with ultrasonication at 60% amplitude. The blended solution was casted to produce a thin film membrane of 156 ÎŒm thickness. Further the tensile strength test of the membranes has shown an increase in Young’s Modulus by 72-120% for all the rubbers. This test was done using TA.XTplus, Texture Analyser machine. The water uptake increment was in the range of 20-27% and thermal stability in the range of 2-20% depending on the rubber composition. Purchased electrodes from FuelCellsEtc (USA), were pasted on both sides of the membranes by the means of hot press at 125oC for about 5 minutes at a pressure of 40 kPa. The Membrane Electrode Assembly (MEAs) fabricated were tested in the fuel cell stack. The highest power density of approximately 85mW/cm2 was obtained for 52% styrene nanocomposite membrane with 4% hybrid nanoparticles at the current density of 212.41mA/cm2 and the efficiency was between 41 and 43%. MEA fabricated with Nafion112 membrane was tested and yielded the open cell voltage of 0.79V, power density of about 77.34mW/cm2 and efficiency of 45%. Results obtained disclose that the MEA with nanocomposites based SBR 52% styrene composition yielded higher power density and higher voltage than the one with Nafion 112 which is one of the fuel cell membranes available on the market. The results obtained revealed that the nanocomposite membranes with 4% hybrid nanoparticles (CNS + SiO2) had higher voltage than the one with 4% CNS. These optimum conditions obtained in this work may be adopted for a typical continuous production of the membrane for hydrogen fuel cell.MT201

    A review of multiscale modeling of metal-catalyzed reactions: Mechanism development for complexity and emergent behavior

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    We review and provide a perspective on multiscale modeling of catalytic reactions with emphasis on mechanism development and application to complex and emergent systems. We start with an overview of length and time scales, objectives, and challenges in first-principles modeling of reactive systems. Subsequently, we review various methods that ensure thermodynamic consistency of mean-field microkinetic models. Next, we describe estimation of reaction rate constants via quantum mechanical and statistical-mechanical methods as well as semi-empirical methods. Among the latter, we discuss the bond-order conservation method for thermochemistry and activation energy estimation. In addition, we review the newly developed group-additivity method on adsorbate/metal systems and linear free energy or BrÞnsted-Evans-Polanyi (BEP) relations, and their parameterization using DFT calculations to generate databases of activation energies and reaction free energies. Linear scaling relations, which can enable transfer of reaction energetics among metals, are discussed. Computation-driven catalyst design is reviewed and a new platform for discovery of materials with emergent behavior is introduced. The effect of parameter uncertainty on catalyst design is discussed; it is shown that adsorbate-adsorbate interactions can profoundly impact materials design. Spatiotemporal averaging of microscopic events via the kinetic Monte Carlo method for realistic reaction mechanisms is discussed as an alternative to mean-field modeling. A hierarchical multiscale modeling strategy is proposed as a means of addressing (some of) the complexity of catalytic reactions. Structure-based microkinetic modeling is next reviewed to account for nanoparticle size and shape effects and structure sensitivity of catalytic reactions. It is hypothesized that catalysts with multiple sites of comparable activity can exhibit structure sensitivity that depends strongly on operating conditions. It is shown that two descriptor models are necessary to describe the thermochemistry of adsorbates on nanoparticles. Multiscale and accelerated methods for computing free energies in solution, while accounting explicitly for solvent effects in catalytic reactions, are briefly touched upon with the acid catalyzed dehydration of fructose in water as an example. The above methods are illustrated with several reactions, such as the CO oxidation on Au; the hydrogenation of ethylene and hydrogenolysis of ethane on Pt; the glycerol decomposition to syngas on Pt-based materials; the NH decomposition on single metals and bimetallics; and the dehydration of fructose in water. Finally, we provide a summary and outlook. © 2011 Elsevier Ltd

    Electron Microscopy Studies of Quantum Dot and Catalyst Nanomaterials

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    Materials with nanoscale dimensions often display unique optical, electronic and chemical properties that depend on their size, structure and composition. For instance, semiconductor nanocrystals, or quantum dots (QDs), exhibit quantum confinement effects which can lead to tunable electronic and optical properties. Similarly, nanoscopic metal structures ranging from nanoparticles to sub-nm clusters and even individual atoms can display excellent catalytic properties when dispersed on a suitable support, due to their high surface area-to-volume ratio and modified electronic properties. In order to develop meaningful structure-property correlations and optimize the performance of such nanomaterials, their structural and compositional features need to be carefully characterized by state-of-the-art aberration corrected analytical electron microscopy (AC-AEM).The first half of this dissertation concerns the study of semiconductor QDs prepared via a novel aqueous-phase biomineralization route that offers lower environmental impact and decreased economic costs compared to more traditional chemical synthesis routes. In particular, we show how the optical properties of these biosynthesized QDs have been understood and improved by a process of ‘microscopy informed nanomaterial design’. Firstly, cadmium sulfide QDs biosynthesized using an engineered bacterial strain of Stenotrophomonas maltophilia have been structurally and chemically analyzed by aberration corrected STEM. This has allowed us to establish a direct correlation between mean nanocrystal size and the particle growth time, which in turn affords us good control and tunability over the band gap energy and absorption/fluorescence behavior of the resultant QDs. Next, a single enzyme produced by the bacterium, namely cystathionine ?-lyase (smCSE), is shown to be responsible for both inducing CdS mineralization and templating nanocrystal growth. The production of size- and structure-controlled CdSe and CdSe-CdS core-shell QDs is also shown to be possible using this same enzyme. The palette of QD materials available from this biomineralization approach is then further expanded to cover other metal sulfide nanocrystals, such as PbS and PbS-CdS core-shell QDs, and CuInS2 (ternary) and (CuInZn)S2 (quaternary) alloy QDs. Detailed HR-TEM, STEM and XEDS measurements on these PbS- and CuInS2-based QD systems are described and then correlated to their functional properties. Finally, it is shown that this aqueous, room temperature biomineralization strategy can also be adapted to produce cerium-based oxide materials by employing a modified form of silicatein as the biosynthesis agent. In particular, AC-AEM analysis is used to validate the successful biomineralization of sub-3 nm fluorite-type CeO2 and CeO2-ZrO2 nanocrystals, which are amongst the smallest reported to date. Their catalytic properties and thermal stability are also explored in relation to the CO oxidation reaction.The second half of this dissertation presents three case studies which serve to demonstrate how the ability to visualize the structure and dispersion of metallic nano-catalysts by AC-AEM, when coupled with spectroscopic information obtained from other complementary techniques, can be used to advance our mechanistic understanding of how these catalysts operate. In the first example, HAADF-STEM analysis is coupled with in-situ X-ray absorption fine structure (XAFS) analysis to study a gold-on-carbon catalyst which is commercially used for the hydrochlorination of acetylene to produce vinyl chloride monomer. We unequivocally demonstrate that a mixture of atomically dispersed Au+ and Au3+ cations constitute the active centers in this Au/C catalyst system. In a second example, physically separate cobalt and platinum nanoparticles supported on either ?-Al2O3 or BaZrO3 are examined as CO2 methanation catalysts. Detailed HAADF-STEM analysis is employed to show how the Pt re-distributes during catalyst activation and ends up atomically decorating the more strongly anchored Co particles. Finally, structure sensitivity is demonstrated for nickel nanoparticles supported on SiO2 which were designed as catalysts for CO2 hydrogenation. A systematic series of Ni/SiO2 catalysts with different Ni loadings has been carefully characterized by HAADF-STEM imaging to obtain reliable Ni particle size distribution data which take into account corrections for metal oxidation effects. The same set of Ni/SiO2 materials were then also characterized by operando FT-IR and quick X-ray absorption spectroscopy. By correlating the complementary data obtained from these three techniques two distinct, particle size dependent pathways are identified for this CO2 hydrogenation reaction

    The unique reactions of surface bound H, bulk H, and gas phase H atoms with acetylene, ethylene and ethane on Ni(111)

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    Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 1999.Includes bibliographical references.by Kerstin Leigh Haug.Ph.D

    Mono and bi-alkyne functionalised polyethylene glycols : polymeric substrates in palladium catalysed oscillatory carbonylation reactions

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    Ph. D ThesisIn this work, the family of oscillatory carbonylation reactions was expanded to include mono and bi-functionalised polymeric substrates. As research around intelligent / stimuli responsive materials progress, it is increasingly important to expand capabilities of chemical oscillators, with a view of merging these two areas to achieve fully pulsatile polymeric materials, capable of autonomous volume changes over longer (i.e. weeks, months) periods of time. This study focused on mono alkyne functionalised polyethylene glycols (A-PEG2000) and bi-alkyne functionalised polyethylene glycols (A-PEG2000-A) as polymeric substrates in oscillatory carbonylation reactions. The work presented here systematically evaluates oscillatory and non-oscillatory reaction profiles recorded in experimental studies. The studies were designed to grasp the reaction processes and elucidate reaction mechanisms responsible for observed trends. Extensive studies were undertaken at different polymeric substrate, PdI2 and KI (originally added to aid PdI2 dissolution) concentrations. Across all studies, the concentration of the catalytic mixture consisting of PdI2 and KI in methanol ranged from 3 - 9 mM for KI and 15.1 - 60.4 ÎŒM for PdI2. Mono alkyne substrate concentrations ranged from 0.508 - 3.55 mM while the bi-alkyne substrate concentration ranged from 0.254 - 3.04 mM. The influence of methanol perturbation during the reaction and varying KI addition times was also investigated. Comparison of pH profiles for both substrates at the same molecular concentrations or same alkyne group concentrations was likewise assessed. At constant catalyst concentration (KI/PdI2), as the substrate concentration increased, the amplitudes and period of the pH oscillations increased in reactions with mono alkyne substrates. On increasing the bi-alkyne polymeric substrate concentrations, the size and amplitudes of pH oscillations varied, and was significantly dependent on the catalyst / substrate concentration. Increasing PdI2 concentration at constant KI and substrate concentrations increased [H+] generated via autocatalytic conversions of both alkyne functionalised polymer substrates. More [H+] was formed in reactions employing bi-alkyne substrates due to increased concentrations of alkyne groups (two alkyne ends per chain). Delaying KI addition times at constant PdI2 and mono alkyne substrate concentration shifted the reactions from oscillatory to non-oscillatory modes. Furthermore, increasing KI concentrations induced occurrence of oscillations in both substrates. Simple, complex, Canard and mixed mode oscillations, as well as other complex non-oscillatory features and pH transitions / offsets, were observed in some reactions as the mono alkyne and bi-alkyne polymer substrates and catalytic concentrations were altered. This work confirms the feasibility of pH oscillations with mono and bi-alkyne substrates in Pd-catalysed oxidative carboxylation reaction and opens new avenues in nonlinear chemical dynamics and intelligent polymeric materials. A merger of intelligent soft materials and this pH oscillator which exhibits extended oscillation duration in batch mode could potentially create self-oscillating regulatory devices for a range of applications including drug delivery and soft robotics.SAgE DTA Newcastl

    Research in the chemical sciences. Summaries of FY 1995

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    Chemical Kinetics

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    Chemical Kinetics relates to the rates of chemical reactions and factors such as concentration and temperature, which affects the rates of chemical reactions. Such studies are important in providing essential evidence as to the mechanisms of chemical processes. The book is designed to help the reader, particularly students and researchers of physical science, understand the chemical kinetics mechanics and chemical reactions. The selection of topics addressed and the examples, tables and graphs used to illustrate them are governed, to a large extent, by the fact that this book is aimed primarily at physical science (mainly chemistry) technologists. Undoubtedly, this book contains "must read" materials for students, engineers, and researchers working in the chemistry and chemical kinetics area. This book provides valuable insight into the mechanisms and chemical reactions. It is written in concise, self-explanatory and informative manner by a world class scientists in the field

    Soluble/MOF-Supported Palladium Single Atoms Catalyze the Ligand-, Additive-, and Solvent-Free Aerobic Oxidation of Benzyl Alcohols to Benzoic Acids

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    Metal single-atom catalysts (SACs) promise great rewards in terms of metal atom efficiency. However, the requirement of particular conditions and supports for their synthesis, together with the need of solvents and additives for catalytic implementation, often precludes their use under industrially viable conditions. Here, we show that palladium single atoms are spontaneously formed after dissolving tiny amounts of palladium salts in neat benzyl alcohols, to catalyze their direct aerobic oxidation to benzoic acids without ligands, additives, or solvents. With this result in hand, the gram-scale preparation and stabilization of Pd SACs within the functional channels of a novel methyl-cysteine-based metal-organic framework (MOF) was accomplished, to give a robust and crystalline solid catalyst fully characterized with the help of single-crystal X-ray diffraction (SCXRD). These results illustrate the advantages of metal speciation in ligand-free homogeneous organic reactions and the translation into solid catalysts for potential industrial implementation.This work was supported by the Ministero dell’Istruzione, dell’Università e della Ricerca (Italy) and the MINECO (Spain) (Projects PID2019−104778GB−I00, CTQ 2017–86735–P, RTC–2017–6331–5, Severo Ochoa program SEV–2016–0683 and Excellence Unit “Maria de Maeztu” CEX2019−000919−M). E.T. and M.M. thank MINECO and ITQ for the concession of a contract. D.A. acknowledges the financial support of the Fondazione CARIPLO/“Economia Circolare: ricerca per un futuro sostenibile” 2019, Project code: 2019–2090, MOCA and Diamond Light Source for awarded beamtime and provision of synchrotron radiation facilities and thanks Dr. Sarah Barnett and David Allan for their assistance at I19 beamline (Proposal No. MT18768-1). Thanks are also extended to the “2019 Post-doctoral Junior Leader-Retaining Fellowship, la Caixa Foundation (ID100010434 and fellowship code LCF/BQ/PR19/11700011” (J.F.-S.) and “La Caixa” scholarship (ID 100010434) LCF/BQ/DI19/11730029 (J.B.-S). E.P. acknowledges the financial support of the European Research Council under the European Union’s Horizon 2020 research and innovation programme/ERC Grant Agreement No 814804, MOF reactors. J.O.-M. acknowledges the Juan de la Cierva program for the concession of a contract (IJC2018-036514-I). We gratefully acknowledge to ALBA synchrotron for allocating beamtime and CLÆSS beamline staff for their technical support during our experiment. The computations were performed on the Tirant III cluster of the Servei d’Informàtica of the University of Valencia.Peer reviewe
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