403 research outputs found

    Flame-Formed Carbon Nanoparticles: Synthesis and characterization

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    Nanoparticles and nanostructured materials characterize an increasing research area, gaining strong attention from the scientific community in several fields. During the last decades, many and extraordinary technological advances have been obtained by nano-materials due to their physicochemical properties. In nature, at micro- and nano-scale, materials have existed for a long time before, but it is only through the advent of the technological era, and consequently, the development of nanotechnology, that they have come to the fore. There are several forms of nanoparticles: metal-based, organic-based or organic/inorganic combination and carbon-based ones. Carbon nanoparticles are the most widely studied as carbon is suitable and available raw material. Except for hydrogen, carbon has the most significant number of known compounds and is present on the planet in various forms: from carbon to light and heavy hydrocarbons. Carbon-based nanoparticles have shown a wide variety of structural arrangements that make them a great advantage as they are suitable for various purposes. Several techniques exist to cope with the production of the nano-size materials in both liquid and gas phase; examples are arc-discharge, laser ablation, chemical vapour deposition. The more the process allows to have a production (functional to specific final characteristics of the material) on a large scale and in an economical way, the more it is taken into consideration and studied. Among the various techniques, the use of flame and, therefore, combustion technology is increasingly taken into consideration. Traditionally, combustion is associated with the study of particulate matter and undesired products released into the atmosphere daily to understand the onset of their formation and reduce, if not abate, their emissions. Nevertheless, on the other hand, flame-formed carbon nanoparticles have been the subject of increasing interest in recent decades as a new procedure for synthesizing engineered nanoparticles. In order to obtain flame nanoparticles with desired characteristics and with the highest yield, it is necessary to have an in-depth knowledge of their formation process through the reaction system, the flame. It is necessary to delve into the chemical and physical details of the various steps of the mechanism that lead to the final product; pay attention to the inherent characteristics of the particles, such as size distribution, chemical composition, and physical characteristics. Moreover, depending on the final product to be obtained, flames can be modulated and varied in parameters such as temperature, residence time, mixing effect, and the fuel or additive structure. This PhD thesis focuses on studying and characterizing the carbon nanoparticles synthesized in the well-controlled combustion conditions of premixed fuel-rich flame, using a lab-scale reactor constituted by flat laminar ethylene/air premixed flame. The primary purpose of this activity has been to perform an experimental study on flame-formed carbon nanoparticles, with great attention on the still too unclear step of particle formation in flame, i.e. the nucleation. The first year of the PhD was primarily centred on the study and preliminary characterization of physicochemical evolution of flame-formed carbon nanoparticles. In order to produce different sizes of particles, carbon nanoparticles were collected at different distances from the flame front, i.e., the residence time in the flame was changed. Then, various techniques were used to characterize the produced particles. One of the first investigations was performed in the flame by the on-line differential mobility analyzer to study the particle size distribution. Subsequently, the analytical tools continued with ex-situ techniques such as Raman spectroscopy and Electron Paramagnetic Resonance, the former for chemical and structural information on particles modification and the latter to reveal and confirm the presence of radicals and to identify them. In this thesis, great attention was laid on the presence and role of radical species, above all, in the determining step of nucleation. For this reason, the research continued in the second year with a more detailed analysis of radical formation in the flame products mechanism and a more specific structural characterization of carbon nanoparticles. Indeed, a density functional theory study investigated some aspects related to the behaviour of radical molecules in flame in terms of dimerization and formation of cluster structures. Notably, the study was helpful in the differentiation between - and -radicals. Following the theoretical evaluation of the radical molecules, the question was raised about how such radicals could form, i.e., whether specific structural elements could facilitate their formation and, consequently, direct carbon particles' formation through a specific mechanism. This type of structural investigation was performed through the Proton Nuclear Resonance Spectroscopy ,1H-NMR; for the first time used in a system such as the one studied in this thesis work. Then, in the third and final year of this PhD research work, a comparative physicochemical evolution study in an aromatic fuel environment has been performed. The addition of an aromatic dopant, such as benzene, leads to some change in the flame and the particle formation in terms of particles size distribution, Raman features, and especially radical production, allowing to face up the same questions in such environment and to investigate the effect of aromatic fuel on the nature and the role of radicals in particle nucleation and growth

    Fundamental Studies of Humic Acid\u27s Influence on Pollutant Toxicity to Aquatic Organisms

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    The main purpose of the research presented in this dissertation was to further understand the intricate and convoluted interactions between natural organic material, biological entities, and pollutants. This was achieved by utilizing humic acids (HAs) from differing sources, chemically modified humic acid, two biological entities (model biomembranes and Artemia Franciscana), and three types of pollutants (cations, surfactants, and carbon nanotubes). Fluorescence spectroscopy and model biomembranes were used to measure the change in HA’s ability to interact with the biomembranes in the presence of cations. Three differently sourced HAs, chemical modified HAs, and a range of cations were studied to elucidate specific interactions that can occur in the environment. It was determined that the cations limited the ability of humic acids to interact with the biomembranes, which was attributed to humic acid conformation changes in the presence of cations, and the protection capacity increased as the softness of the cation increased. Artemia Franciscana (Artemia) was utilized as an analytic tool to determine the changes in toxicity of surfactants in the presence of humic acid. Artemia were exposed to three different surfactants, Triton X-100 (Tx-100), cetylpyridinium chloride (CPC), and sodium dodecyl sulfide (SDS), for both hatching studies and in vivo 31P NMR. It was determined by hatching assays that Tx-100 caused mortality after hatching while CPC and SDS inhibited hatching. 31P NMR corroborated these findings by showing an increase in phosphodiester bonds in saline water and in the Tx-100 exposure while there was no increase in the presence of the other two surfactants. HAs from three different sources were added to the surfactant exposures which showed that HAs played a mediation role in terms of toxicity and the extent of mediation was dependent on the type of HA and surfactant. Artemia was also utilized to measure the toxicity of carbon nanotubes under a variety of conditions. Both single-walled and multi-walled carbon nanotubes that were either in the presence of humic acid or had been sonicated were studied. Overall, there was no significant carbon nanotube toxicity to the Artemia

    Environmental Remediation Applications of Carbon Nanotube and Graphene Oxide: Adsorption and Catalysis

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    Environmental issues such as the wastewater have influenced each aspect of our lives. Coupling the existing remediation solutions with exploring new functional carbon nanomaterials (e.g. carbon nanotube, graphene oxide, graphene) by various perspectives shall open up a new venue to understand the environmental issues, phenomenon and find out the ways to get along with the nature. This review makes an attempt to provide an overview of potential environmental remediation solutions to the diverse challenges happening by using low-dimensional carbon nanomaterials and their composites as adsorbents, catalysts or catalysts support towards for the social sustainability.Comment: accepted review pape

    Highly Active Porous Catalysts Fabricated by Attachment of Palladium Nanoparticles on Hierarchical Carbon Structures

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    The effectiveness of metal-based catalysts can be significantly enhanced by increasing the available surface area relative to the volume through the creation of hierarchical nanostructures. The catalyst demonstrated here is palladium, which is a widely recognized heterogeneous catalyst suitable for a variety of industrial applications such as water purification, hydrogen storage, and electrochemical devices. In this study, a novel multi-scale supporting material developed in this group, has been used as support. It consists of micro-porous graphitic carbon with strongly attached carbon nanotubes. This can increase the surface area by orders of magnitude without increasing the size or weight while still maintaining structural integrity. This allows miniaturization of palladium catalysts structures that are lighter, smaller and more compact than conventional ones. Fabrication issues of these structures have been successfully addressed. Detailed micro-structural as well as spectroscopic analysis of the nanoparticles have been performed. Variations of palladium nanoparticles distribution with processing conditions, and the possible ways of controlling this distribution will be presented. Surface spectroscopic analysis indicates that these are zero-valent metallic palladium and do not degrade with time. The catalytic activity of palladium nanoparticles has been tested via bench-scale experiments for reductive dechlorination of carbon tetrachloride. It is seen that palladium functionalized carbon nanotubes is highly effective in the degradation of carbon tetrachloride and similar organic pollutants found commonly in drinking water sources. It was also demonstrated that palladium functionalized carbon nanotubes can be used repeatedly as the valence state of palladium does not change, and thus can be cost-effective. Future scope of these results and their connection to future device applications will be discussed

    Removal of diclofenac from aqueous solutions by adsorption on thermo‑plasma expanded graphite

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    The adsorption of diclofenac on thermo-plasma expanded graphite (a commercial product) from water solutions was investigated. The adsorbent material was characterized by SEM, TEM, BET, Raman and X-ray diffraction analyses. Typical diffractogram and Raman spectrum of graphitic material, dimension of 24.02 nm as crystallite dimension and a surface area of 47 m2 g−1 were obtained. The effect of pH on the adsorption capacity was evaluated in the range 1–7 and the adsorption mechanism was described by kinetic and isothermal studies. Pseudo-second order and Dubinin–Radushkevich models agreed with theoretical values of adsorption capacity (i.e. 400 and 433 mg g−1, respectively) and resulted to be the best fit for kinetics and isothermal experimental data. The thermodynamics of the process was evaluated by plotting the adsorption capacity/concentration ratio at the equilibrium as a function of different values of the multiplicative inverse of temperature. Moreover, the adsorbent regeneration was also investigated, comparing two different remediation techniques. Solvent washing performed with NaOH 0.2 M and thermo-treatment carried out by heating in an oven at 105 °C for 2 h and then at 200 °C for 4 h. The thermo-treatment was the best technique to regenerate the adsorbent, ensuring same performance after 4 cycles of use and regeneration

    Probing interfacial processes on carbon nanotubes and graphene surfaces

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    Ankara : The Department of Physics and the Graduate School of Engineering and Science of Bilkent University, 2012.Thesis (Master's) -- Bilkent University, 2012.Includes bibliographical references leaves 62-74.The surface of low-dimensional carbon (carbon nanotubes and graphene) has unique electronic properties due to the delocalized p-orbitals. Very high carrier mobility with nanoscale dimension make carbon nanotubes and graphene promising candidates for high performance electronics. Besides electronic properties, the delocalized orbitals have a strong tendency to adsorb aromatic molecules via p-electronic interactions. The strong non-covalent interactions between the graphitic surface and organic molecules provide a unique template for supramolecular chemistry and sensing applications. A comprehensive understanding of these forces at atomic and molecular level still remains a challenge. In this thesis, we have used carbon nanotube networks and graphene as model systems to understand molecular interactions on carbon surface. We have developed processes to integrate these model materials with sensitive and surface specific sensors, such as surface plasmon sensor and quartz crystal microbalance. In the first part of the thesis, we integrated surface plasmon resonance (SPR) sensors with networks of single-walled carbon nanotubes to study interactions between SWNT and organic molecules. In the second part, we probe interfacial processes on graphene surface by mass detection. We anticipate that the developed methods could provide a sensitive means of detecting fundamental interaction on carbon surfaces.Kakenov, NurbekM.S

    Characterizing the Adsorption-Bioavailability Relationship of PAHs Adsorbed to Carbon Nanomaterials in the Aquatic Environment

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    Concurrent with the high applicability of carbon nanomaterials (CNM) in a variety of fields and the potential use for pollution remediation, there is the inevitable release of CNMs into the environment. As a consequence of their unique physicochemical properties, CNMs entering the environment will interact with both abiotic and biotic factors. With CNM concentrations estimated to range from parts per billion to low parts per million and their high adsorption affinity for organic contaminants, there is significant concern that CNMs will act as œcontaminant transporters . Even though adsorption and desorption of contaminants from CNMs play a significant role in the ultimate fate of adsorbed compounds, currently there is little conclusive information characterizing the relationship between adsorption behavior and bioavailability of CNM-adsorbed contaminants. The goal of the present research was to establish a comprehensive understanding of the key mechanisms influencing bioavailability of CNM-adsorbed organic contaminants. To accomplish this, I utilized a systematic approach to characterize the influence of CNM morphology, contaminant physicochemical properties, and contaminant mixtures on the resulting bioavailability of the adsorbed compounds, where polycyclic aromatic hydrocarbons (PAHs) were selected as a model class of organic contaminants. Adsorption behavior of a suite of PAHs by suspended multi-walled carbon nanotubes (MWCNTs) and exfoliated graphene (GN) was characterized using batch adsorption isotherm techniques and fitting experimental data with established adsorption models. Bioavailability of CNM-adsorbed PAHs to Pimephales promelas (fathead minnow) was quantified using bile analysis via fluorescence spectroscopy. Multiple linear regression techniques were used to assess the influence of CNM type, PAH physicochemical characteristics, and concentration effects on adsorption of PAHs by MWCNTs as well as to model the relationship between adsorption behavior and the resulting bioavailability of MWCNT-adsorbed PAHs. While CNM structure and surface area differed, adsorption affinity was more influenced by PAH physicochemical characteristics. In particular, differences in adsorption of PAHs between MWCNT and GN became insignificant as hydrophobic and Ï€-Ï€ interactions with the particular PAHs increased. Similarly, bioavailability of CNM-adsorbed PAHs was less influenced by the type of CNM and more influenced by the PAHs physicochemical properties, particularly the size and morphology of the PAH molecules. A further investigation with a greater range of PAHs, showed that molecular morphology of small less hydrophobic PAHs was particularly influential on bioavailability when adsorbed to MWCNTs. Though adsorption of chemically similar PAHs was nearly identical in single-solute solutions, the resulting bioavailability was not the same and was attributed to differences in the PAH\u27s Ï€ electron system as a function of structure and aromatic makeup. Additionally, modeling the relationship between adsorption affinity (i.e. Log Kd) and resulting bioavailability of MWCNT-adsorbed PAHs, showed a direct correlation when Log Kd was greater than 2.5, where only the aqueous concentration of PAH remained bioavailable. However, lower adsorption affinity resulted in a variable amount of the MWCNT-adsorbed PAH remaining bioavailable in an unpredictable manner. The results of this work also indicated that there was a concentration effect influencing adsorption affinity and bioavailability. This was determined to largely be a function of molecular surface area coverage of MWCNT resulting in a change of the adsorption process from more heterogenous to more homogenous. Finally, adsorption of two pairs of chemically similar PAHs, (1) phenanthrene and anthracene and (2) fluoranthene and pyrene, in bi-solute mixtures confirmed that structural makeup of the molecule is signficantly influential on the adsorption-bioavailability relationship. PAHs that have increased contact with the surface of MWCNT, such as anthracene being linear to align with the curved surface of the tube or fluoranthene being more flexible to bend with the curved surface of the tube, outcompeted their chemically similar isoforms. Competitive interactions between PAHs at the surface of MWCNT decreased adsorption affinity of both PAHs within the bi-solute system thus increased bioavailability of the adsorbed PAHs. However, the effect of competition on PAH bioavailability appeared to be greater for less hydrophobic PAHs (i.e. phenanthrene and anthracene) compared with the more hydrophobic PAH pair (i.e. fluoranthene and pyrene). This was attributed to adsorption affinity of phenanthrene and anthracene dipping below Log Kd = 2.5 due to competitive interactions in a bi-solute system. Similar to the single solute studies, only when Log Kd \u3e 2.5 was bioavailability of adsorbed PAHs largely associated with just the aqueous concentration of PAH left in the system. Overall, the results of this work indicate that there is a correlation between bioavailability of CNM-adsorbed PAHs and observed adsorption behavior in aqueous systems, which is largely driven by the adsorbate\u27s physicochemical characteristics. Factors influencing CNM adsorption affinity of PAHs prior to organismal ingestion, such as concentration and competition, also influence bioavailability of the CNM-adsorbed PAHs in a similar manner. However, adsorption behavior of PAHs by CNMs in aqueous solution is not a perfect prediction of the resulting uptake of PAH into P. promelas bile, though my data does indicate an adsorption affinity threshold at which MWCNTs can significantly reduce bioavailability of the adsorbed PAHs. This work furthered our understanding in the factors that may predominantly influence the bioavailability of CNM-adsorbed organic contaminants and provided initial insight into the complex interactions that may occur after consumption on CNM-contaminant complexes that should be focused on in the future

    Metal organic frameworks as sorption media for volatile and semi-volatile organic compounds at ambient conditions

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    In this research, we investigated the sorptive behavior of a mixture of 14 volatile and semi-volatile organic compounds (four aromatic hydrocarbons (benzene, toluene, p-xylene, and styrene), six C-2-C-5 volatile fatty acids (VFAs), two phenols, and two indoles) against three metal-organic frameworks (MOFs), i.e., MOF-5, Eu-MOF, and MOF-199 at 5 to 10 mPa VOC partial pressures (25 degrees C). The selected MOFs exhibited the strongest affinity for semi-volatile (polar) VOC molecules (skatole), whereas the weakest affinity toward was volatile (non-polar) VOC molecules (i.e., benzene). Our experimental results were also supported through simulation analysis in which polar molecules were bound most strongly to MOF-199, reflecting the presence of strong interactions of Cu2+ with polar VOCs. In addition, the performance of selected MOFs was compared to three well-known commercial sorbents (Tenax TA, Carbopack X, and Carboxen 1000) under the same conditions. The estimated equilibrium adsorption capacity (mg.g(-1)) for the all target VOCs was in the order of; MOF-199 (71.7) > Carboxen-1000 (68.4) > Eu-MOF (27.9) > Carbopack X (24.3) > MOF-5 (12.7) > Tenax TA (10.6). Hopefully, outcome of this study are expected to open a new corridor to expand the practical application of MOFs for the treatment diverse VOC mixtures.This study was supported by a National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science, and Technology (MEST) (No. 2009-0093848). E Kwon also acknowledges the support made by a National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (No. 2914RA1A004893). The third author thanks SERB-DST, New Delhi for 'Young Scientist-Start up Research Grant (YSS/2015/001440)
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