Browsing by Subject "Catalyst"

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Open Access
Cyclodextrin functionalized nanofibers via electrospinning
(2014) Çelebioğlu, Aslı
Electrospinning is a commonly studied and widely applied technique for generating nanofibers, with a diameter ranging from several tens of nanometers to a few micrometers. The low-cost, simple set-up, relatively high production rate and reproducibility increase the interests on this method in both academia and industry. Electrospun nanofibers are produced from a broad range of materials with extremely high surface area, very light-weight, nano-porous features and distinct physical/mechanical properties. The general talk in this technique focuses on the production of nanofibers from polymer base materials. However, very recent studies demonstrated that, it is also possible to obtain nanofibers from non-polymeric systems. For this novel development in electrospinning researches, we have achieved to generate nanofibers from cyclodextrins (CD) without using a polymeric template. CD are cyclic oligosaccharides consisting of α-(1,4)-linked glucopyranose units. The truncated cone shape structure of CD provides a favorable place for various kinds of organic molecules to form non-covalent host-guest inclusion complexes (IC). The enhancements and progressing at the guest molecules property and situation, creating with the inclusion complexation, make CD applicable in variety of areas including filtration, pharmaceuticals, cosmetics, functional foods, textiles, analytic chemistry etc. In this thesis, we report on the electrospinning of CD nanofibers, represent their functionalization and potential applications. Firstly, we produced CD nanofibers from three different chemically modified CD types (hydroxypropyl-β-cyclodextrin (HPβCD), hydroxypropyl-γ-cyclodextrin (HPγCD) and methyl-β-cyclodextrin (MβCD)). Afterwards, the electrospinning of native CD (α-CD, β-CD and γ-CD) nanofibers was achieved. The molecular entrapment capability of CD nanofibers was shown by capturing toxic volatile organic compounds (VOCs) from the surrounding. As the next step, the polymer-free nanofibers were obtained from the cyclodextrin inclusion complexes (CD-IC) with antibacterial agent, vanillin and essential oils. Here, we have also indicated applicability of CD-IC nanofibers as a result of antibacterial test. The functionalization of the CD nanofibers was continued with the green and one-step synthesis of metal nanoparticles (Ag-NP, Au-NP and Pd-NP) incorporated nanofibers, in which CD were used as reducing, stabilizing agent and fiber template. Even, the antibacterial, SERS and catalyst potential of these CD based nanofibers were demonstrated for the related nanoparticles. Our research is expanded to a new stage by the production of insoluble poly-CD nanofibers. We have worked on different crosslinking agents to attain insoluble poly-CD nanofibers with uniform morphology. After the optimization of poly-CD nanofibers, the most durable polyCD nanowebs were selected for further analysis and evaluation of the filtration performance in liquid environment. Within poly-CD nanofibers, we have eliminated the solubility challenge of CD nanofibers that restrict their usage. So, we assume that, poly-CD nanofibers will lead-up to generation of new advances for practices of CD nanofibers. All studies showed that, the self-assembly and self-aggregation property of CD are the prior requirements for the electrospinnability of these small molecules. To conclude, very intriguing materials were obtained by integrating large surface area of nanofibers with specific host-guest inclusion complexation capability and non-toxic, biocompatible nature of the CD. Moreover, CD molecules, which are generally used in the powder form, were rendered into more applicable nanofibers form that will represent ease during their usage.
Open Access
Investigating the effect of catalysts in sodium-oxygen batteries
(2017-11) Tovini, Mohammad Fathi
The unique electrochemical and chemical features of sodium oxygen (Na-O2) batteries distinguish them from the lithium-oxygen (Li-O2) batteries. NaO2, which is the main discharge product, is unstable in the cell environment and its dissolution in the electrolyte triggers side products formation and charging potential increment. In the rst part of this thesis, RuO2 nanoparticles (NPs) dispersed on carbon nanotubes (CNTs) are used as a catalyst for Na-O2 batteries to elucidate the e ect of catalyst on this complex electrochemical system. RuO2/CNT contributes to the formation of a poorly crystalline and coating like NaO2 structure during oxygen reduction reaction (ORR) which is drastically di erent from the conventional micron sized cubic NaO2 crystals deposited on CNT. Our ndings demonstrate a competition among NaO2 and side products decompositions for RuO2/CNT during oxygen evolution reaction (OER). We believe that this is due to the lower stability of coating like NaO2 because of its non-crystalline nature and high electrode/electrolyte contact area. Although RuO2/CNT catalyzes the decomposition of side products at a lower potential (3.66 V) compared to CNT (4.03 V), it cannot actively contribute to the main electrochemical reaction of the cell during OER (NaO2→ Na+ + O2 + e{ ) due to the fast chemical decomposition of lm NaO2 to side products. Even though the long term e ect of RuO2 catalyst during cycling and resting tests seems to be positive in terms of lower overpotential, no bene ts of catalyst is observed for stability and e ciency of the cell for the rst cycles. Therefore, tuning the morphology and crystallinity of NaO2 by catalyst is detrimental for Na-O2 cell performance and it should be taken into account for the future applications. In the second part of this thesis, a 3D RuO2/Mn2O3/carbon nano ber (CNF) composite has been prepared as a bi-functional electrocatalyst towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in Na-O2 batteries. RuO2/Mn2O3/CNF exhibited higher speci c capacity (9352 mAh.gcarbon -1) than CNF (1395 mAh.gcarbon -1), Mn2O3/CNF (3108 mAh.gcarbon -1) and RuO2/CNF (4859 mAh.gcarbon -1), which is believed to be due to its higher active surface area than its counterparts and its unique morphology. Taking the bene t of RuO2 and Mn2O3 synergistic e ect, the decomposition of inevitable side products at the end of charge occurs at 3.838 V vs. Na/Na+ by using RuO2/Mn2O3/CNF, which is 388 mV more cathodic compared with CNF.
Open Access
Investigation of local structure effect and X-ray absorption characteristics (EXAFS) of Fe (Ti) K-edge on photocatalyst properties of SrTi (1-x)Fe xO (3-δ)
(2012) Ghaffari, M.; Liu, T.; Huang H.; Tan O.K.; Shannon, M.
In this study, the STF x photocatalyst powder was synthesized with a high temperature solid state reaction. The microstructures and surface of samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The electronic properties and local structure of the perovskite STF x (0 ≤ x ≤ 1) systems were probed by extended X-ray absorption fine structure (EXAFS) spectroscopy. The XPS results revealed that with increasing iron doping, the amount of Fe 3+ and Fe 4+ increased significantly. The X-ray absorption data are discussed in detail with respect to the Fe (Ti) K-edge. The substitution of iron by titanium increased the Ti (Fe)-O first shell disorder factors that can be explained by increasing the oxygen vacancies. Oxygen vacancies or defects act as electron traps, which could capture the photo induced electrons and thus could effectively inhibit the recombination of the photo induced electrons and holes. Moreover due to the substitution of Ti with Fe, lattice shrinkage was observed and the largest derivation from the Gaussian distribution in STF x was from those samples with x = 0.6 and x = 0.8. The substitution of iron by titanium increased the iron valence state, hence the formation of the Jahn-Teller Fe 4+ ion. With increasing iron dopant the [Ti(Fe)-O] ave decreased and bond length of [Ti-O] and the consequent [Ti-O-Ti] increased and this phenomenon affected the photocatalyst and photo degradation properties of material and also decreased its efficiency. © 2012 Elsevier B.V. All rights reserved.
Open Access
Mesoporous metallic rhodium nanoparticles
(Nature Publishing Group, 2017) Jiang B.; Li C.; Dag, Ö.; Abe, H.; Takei, T.; Imai, T.; Hossain, M. S. A.; Islam, M. T.; Wood, K.; Henzie, J.; Yamauchi, Y.
Mesoporous noble metals are an emerging class of cutting-edge nanostructured catalysts due to their abundant exposed active sites and highly accessible surfaces. Although various noble metal (e.g. Pt, Pd and Au) structures have been synthesized by hard- and soft-templating methods, mesoporous rhodium (Rh) nanoparticles have never been generated via chemical reduction, in part due to the relatively high surface energy of rhodium (Rh) metal. Here we describe a simple, scalable route to generate mesoporous Rh by chemical reduction on polymeric micelle templates [poly(ethylene oxide)-b-poly(methyl methacrylate) (PEO-b-PMMA)]. The mesoporous Rh nanoparticles exhibited a ∼1/42.6 times enhancement for the electrocatalytic oxidation of methanol compared to commercially available Rh catalyst. Surprisingly, the high surface area mesoporous structure of the Rh catalyst was thermally stable up to 400 °C. The combination of high surface area and thermal stability also enables superior catalytic activity for the remediation of nitric oxide (NO) in lean-burn exhaust containing high concentrations of O 2.
Open Access
Metal-free N-doped ultrafine carbon fibers from electrospun Polymers of Intrinsic microporosity (PIM-1) based fibers for oxygen reduction reaction
(Elsevier, 2020) Patil, Bhushan; Satılmış, Bekir; Uyar, Tamer
Synthesis of nitrogen-doped carbon fibers (CF) has been proved to be one of the most promising oxygen reduction reaction (ORR) catalysts which can replace the state-of-art Pt catalyst for non-noble metal-free light-weight devices. Polymers of Intrinsic Microporosity (PIM-1) is soluble in common organic solvents and can be tailored by functionalization owing to nitrile groups in the backbone. PIM-1 was functionalized to amide (hydrolyzed PIM-1), amine and amidoxime groups. The modified PIM-1s were electrospun into ultrafine fibers and pyrolyzed to obtain CF. The present article investigates the influence of different functional groups on the properties of PIM-1 based CF and their nitrogen-doping. Particularly, their ORR performance has been evaluated. Interestingly, CF from hydrolyzed PIM-1 have the highest pore volume with small pore size among the CF based on PIM-1, amine and amidoxime PIM-1. The amount of nitrogen-doping in these CF shows the trend according to the functional groups as PIM-1 > amine > amidoxime > amide. Among all these PIM-1 based CF; CF from hydrolyzed PIM-1 has the highest percentage of pyridinic and graphitic nitrogen, furthermore, electrocatalysis revealed that ORR processed through four-electron with the onset potential 985 mV vs. reversible hydrogen electrode (RHE) which is comparable with the standard Pt/C catalysts.
Open Access
Utilization of reducible mixed metal oxides as promoters for the enhancement of sulfur regeneration in nsr catalysts
(2016-07) Samast, Zehra Aybegüm
Pt functionalized binary, ternary, and quaternary oxides (e.g. Pt/BaO/CeO2/ZrO2/Al2O3) were synthesized by wetness impregnation method and characterized by X-ray Diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analysis, in-situ Fourier Transform Infrared (FTIR), and temperature programmed desorption (TPD) techniques. Effect of the synthesis sequence on the NOx storage capacity was investigated by synthesizing subsequently impregnated and co-impregnated ternary oxides. Influence of BaO loading on NOx uptake of quaternary oxides was examined by utilizing two different BaO loadings namely; 8 wt% and 20 wt% on co-impregnated ternary oxide, Pt10-10CeZrAl. Co-presence of CeO2-ZrO2 oxide domains leads to an increase in NOx storage. As BaO loading increases in quaternary oxides, thermal stabilities of nitrates and nitrites increase due to the formation of bulk/ionic nitrates. Although BaO impregnation on co-impregnated ternary oxides leads to a decrease in specific surface area (SSA) values due to sintering, NOx adsorption on BaO-functionalized quaternary oxides was found to be higher than the BaO deficient ternary oxides. Upon sulfur poisoning, formation of strongly bound bulk/ionic sulfate/sulfite functional groups on BaO containing catalysts result in a need for higher temperatures for complete sulfur regeneration. Comparison of the CeO2-ZrO2 promoted systems with that of the Pt/ 20 wt% Ba/Al2O3 conventional NOx Storage Reduction (NSR) catalyst suggests that ceria-zirconia promotion enhances the sulfur tolerance. In conclusion, in this study a new NSR catalyst namely, Pt20Ba10-10CeZrAl, which is promoted with reducible mixed metal oxides, was synthesized and characterized. This novel NSR catalyst formulation revealed favorable sulfur resistance with minor sacrifice in NOx storage ability.
Open Access
X-ray photoelectron spectroscopic and in-situ infrared investigation of a Ru/SiO2 catalyst
(1997) Sayan, Şafak
A 4 wt% Ru/Si02 catalyst which was previously prepared via an incipient wetness technique using a ruthenium nitrosyl-nitrate [Ru(N0)(N03)3] solution and a commercially available precursor (ruthenium nitrosyl-nitrate) were used in this study. The activation of the catalyst was investigated by using Infrared (IR) spectroscopy together with X-ray photoelectron spectroscopy (XPS). Special emphasis has been given to the study of formation of active species during annealing of the precursor and the catalyst for comparison purposes. The in-situ IR measurements performed on the catalyst suggested a possible metal and support interaction. XPS experiments revealed mixed oxidation states in the case of annealing of the precursor whereas annealing did not cause any change in the oxidation state of Ru present in the catalyst which lead to the conclusion that the influence of support and interactions between the metal and support prevented any reduction by annealing only. Carbon monoxide adsorption on the reduced catalyst followed by IR spectroscopy was performed to investigate the nature of active adsorption sites. The complexity of the spectrum of chemisorbed CO suggested the presence of small metal particles. The presence of Ru"^ centers as well as reduced Ru centers showed that the catalyst was not fully reduced under these conditions. Using this catalyst ammonia synthesis was achieved at 350 °C in a N2/H2 gas mixture (N2/H2 =3:1) for different reaction times. In addition to observed NH3 as reaction product, the results showed that NH^ surface species might represent intermediates in the ammonia synthesis reaction on Ru/Si02. Carbon monoxide adsorption on the catalyst after NH3 synthesis was performed to investigate the change in nature of active sites after ammonia production when compared with the reduced sample. Based on the experimental observations, participation of Ru° sites in ammonia synthesis was confirmed and a partial oxidation of the reduced Ru sites during synthesis was observed.
Open Access
XPS investigation of a Si-diode in operation
(Royal Society of Chemistry, 2012) Süzer, Şefik
X-ray photoelectron spectroscopy (XPS) is utilized to investigate a Si-diode during its operation under both forward and reverse bias. The technique traces chemical and location specified surface potential variations as shifts of the peak positions with respect to the magnitude as well as the polarity of the applied voltage bias, which enables one to separate the dopant dependent shifts from those of the chemical ones.