Browsing by Subject "Catalytic activity"

Now showing 1 - 7 of 7

Open Access
Characterization of niobium-zirconium mixed oxide as a novel catalyst for selective catalytic reduction of NO x
(2009) Cayirtepe, I.; Naydenov, A.; Ivanov, G.; Kantcheva, M.
The performance of mixed niobium-zirconium oxide in the SCR of NO x with propene in excess oxygen has been studied. The mixed oxide is prepared by impregnation of hydrated zirconia with acidic solution (pH 0.5) of peroxoniobium(V) complex, [Nb2(O2)3] 4+, ensuring ZrO2:Nb2O5 mole ratio of 6:1. The calcined sample (denoted as 25NbZ-P) has the structure of Zr 6Nb2O17. According to the catalytic test, the conversion of NO x over the 25NbZ-P catalyst passes through a maximum at 220 °C. Based on the in situ FT-IR results, a reaction mechanism is proposed with nitroacetone and NCO species as the key reaction intermediates. The results of the investigation show that the catalytic properties of the Zr6Nb2O17 solid solution could be of interest regarding the development of low-temperature catalyst for the SCR of NO x with hydrocarbons. © 2009 Springer Science+Business Media, LLC.
Open Access
Effect of brush length of stabilizing grafted matrix on size and catalytic activity of metal nanoparticles
(Elsevier, 2020-06-01) Barsbay, Murat; Özgür, T. Ç.; Sütekin, S. D.; Güven, O.
The rationale of this work is to investigate the effect of chain length of stabilizing polymer brushes attached to silica surface on the formation and catalytic activity of metallic palladium nanoparticles (Pd NPs). Poly(vinyl pyrrolidone) (PVP) brushes forming a very thin shell were grafted on silica microparticles (PVP@SiO2) via RAFT mediated graft polymerization, thus controlling the molecular weights and structures of PVP grafts. Pd nanoparticles were formed in PVP stabilizing matrix by gamma-induced reduction of polymer-bound Pd(II) ions to yield Pd(0) decorated core-shell particles (PVP-PdNP@SiO2). SEC and TGA results indicated the formation of PVP brushes with different molecular weights on silica substrate. DLS and TEM measurements revealed that particle growth was sterically blocked by the increase in PVP brush length, thereby forming a greater number of small Pd nanoparticles rather than larger ones. PVP-PdNP@SiO2 samples with different PVP chain lengths and Pd sizes were evaluated for their catalytic activity and reusability in the reduction of 4-nitrophenol to 4-aminophenol. Although the nanoparticles formed in the presence of longer grafted chains are smaller, their leakage into the solution has been found to be more effectively prevented by these long grafts. Thus, PVP-PdNP@SiO2 samples with longer PVP grafts showed more stable catalytic activity in repeated reaction cycles. These findings are particularly important for heterogeneous catalysis systems in that they show the effect of the size of surface-bound polymeric stabilizers on metal NP formation and catalytic activity.
Open Access
Gold supported on ceria doped by Me3+ (Me = Al and Sm) for water gas shift reaction: Influence of dopant and preparation method
(2010) Andreeva, D.; Kantcheva, M.; Ivanov, I.; Ilieva, L.; Sobczak, J. W.; Lisowski, W.
Gold catalysts supported on ceria doped by Sm and Al were studied. The influence of the preparation method, as well as the nature of dopants on the structure, properties and WGS activity are investigated. The applied methods of preparation cause the modification of ceria in a different extent. In the sample prepared by co-precipitation (CP) and doped by Al, the vacancies are located within the bulk of ceria structure, whereas in the corresponding AuCeSmCP sample the vacancies are located most likely around Sm and the ceria structure seems to be better ordered than the Al doped ceria. There is no distinct correlation between the reducibility and WGS activity of the studied catalysts. The Au 4f XPS spectra of fresh samples reveal higher contribution of dispersed form of Au for Sm doped catalysts than for the corresponding Al doped samples. The Ce 3d XPS spectra disclose also a higher concentration of Ce3+ evaluated before the catalytic operation for Sm doped catalysts as compared with the Al doped fresh samples. The observations by "in situ" FT-IR spectroscopy agree well with the model of active sites and the, mechanism of the WGS reaction proposed recently by some of us. The amount of formate species observed on the AuCeSmCP is higher than that on the AuCeAlCP catalyst and parallels the catalytic activity. The higher concentration of active sites on the surface of the AuCeSmCP catalyst facilitates the dissociation of water.
Open Access
Preparation and characterization of niobia-containing solid acid
(2010) Çayırtepe, İlknur
The research in this work is directed towards (i) the development of effective methods for synthesis of new solid acids based on zirconia and niobia resulting in high concentration of acid sites and (ii) investigation of the potential of these materials as catalysts for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with hydrocarbons and alcohol transformations. A series of NbOx/ZrO2 and WOx/Nb2O5 samples have been obtained by the so called “peroxo route” using acidic H2O2 solutions as a reaction medium. The impregnation of hydrated zirconia with solutions of [Nb2(O2)3] 4+ ions allows the synthesis of Nb2O5-ZrO2 samples with niobium loading up to 30 wt % without formation of a seperate Nb2O5 phase. The presence of a single phase of Zr6Nb2O17 has been observed for the sample containing 25 wt% of Nb. The formation of Zr6Nb2O17 is favored by the partial solubility of hydrated zirconia in the H2O2 solution. Among the fully crystallized Nb2O5-based samples, the solid WO3-Nb2O5 solution containing 20 wt % of WO3 has the highest and strongest Brønsted acidity. Catalytic activity tests for the C3H6-SCR of NOx have shown that Pd-free and Pdpromoted niobia-based samples containing 20 wt% WO3 are inactive, whereas the Zr6Nb2O17 and 0.1Pd/ Zr6Nb2O17 samples catalyze this reaction. The conversion of NOx in the SCR of NOx with propene in excess oxygen over Zr6Nb2O17 passes through a maximum at 493 K. The mixed oxide displays good water tolerance and resistance toward SO2 poisoning. Based on the in situ FT-IR results, a reaction mechanism is proposed with nitroacetone and NCO species as the key reaction intermediates. The results of the investigation show that the catalytic properties of the new solid acid Zr6Nb2O17 could be of interest regarding the development of sulfur- and water-tolerant, low-temperature catalysts for the SCR of NOx with hydrocarbons. The Zr6Nb2O17 and Nb2O5-containing 20 % WO3 samples have potential as lowtemperature catalysts for oxidant-free or oxidative transformation of alcohols, respectively.
Open Access
Selective fluorescence sensing of biological thiols using a bodipy based bifunctional probe and the catalytic activity of short peptide amphiphile nanostructures : implications on the oring of life
(2013) Altay, Yiğit
Chemosensor development is an attractive field of modern chemistry and there exist large amount of contribution from all over the world. The biological importance of thiols triggered the development of sensors to differentiate especially cysteine (Cys), homocysteine (Hcy) and glutathione (GSH) which play key roles in biological systems. Concentration of those thiols results in number of diseases and their structural similarity complicates the differentiation. Optical probes especially fluorescent ones are widely employed for that purpose since it offers simplicity, sensitivity and low detection limits as well as real time analysis. BODIPY core is decorated with a Michael acceptor nitro-styrene group to covalent incorporation of thiols and with an aza-crown moiety to recognition of N-terminus of them. The work in this thesis is the first example in which one of them is separated from others or three of them separated from each other’s by chain length difference using fluorescence spectrometry. Formation of short peptides (2-4 aa residues) is considered to be likely under primordial conditions, following a number of scenarios. In this work, it is constructed a short peptide library limiting our choice of amino acids to those believed to be available at larger concentrations such as Gly, Ala, Asp and Cys. It is demonstrated that when acylated at the N-terminus, nanostructures of varying size and shapes were formed. Investigations on the catalytic activity of these nanostructures under different conditions are presented. The findings on the correlation of peptide structure and nanostructure formation and/or catalytic activity are presented.
Open Access
Structural, optical, electrical and electrocatalytic activity properties of luminescent organic carbon quantum dots
(Wiley-Blackwell, 2018) Karatutlu, Ali; Patil, Bhushan; Şeker, İ.; İstengir, S.; Bolat, A.; Yıldırım, O.; Sevgen, Y. N.; Bakış, Y.; Ortaç, Bülend; Yılmaz, Eda; Sapelkin, A.
Carbon is an essential element in human life and recently becoming technologically prominent due to the emerging field of “Carbononics”. We demonstrate organic carbon quantum dots (qdots) containing nitrile bonded (C≡N bond) d-glucose-like traces in various sizes obtained from wheat flour to be promising for imaging applications and to possess a relaxor ferroelectric property and an enhanced electrocatalytic activity that could reduce the cost of energy devices and simple to scale up for the commercialization. The secondary electron microscopy (SEM) imaging shows that the particle size of carbon qdots can be controlled via the sonication exposure time. Elemental analysis and vibrational spectroscopy results show that carbon qdots are sensitive to N2 gas in the atmosphere and could weaken its “carbogenic” property by making a stable C≡N bond at ambient atmosphere. Rietveld analysis and HR-TEM studies demonstrate that the structure of the C qdots was found to fit best with an acentric primitive orthorhombic lattice. The laser scanning confocal microscopy (LSCM) images show enhancement of the light emission when reducing the size and characteristic excitation wavelength-dependent light emission of C qdots. The photoluminescence and UV-Vis absorption spectroscopy techniques show surface dominant emission and absorption upon the nitrile bonding.
Open Access
Top-illuminated dye-sensitized solar cells with a room-temperature-processed ZnO photoanode on metal substrates and a Pt-coated Ga-doped ZnO counter electrode
(IOP Publishing, 2011-01-11) Kyaw, A. K. K.; Sun, X. W.; Zhao, J. L.; Wang, J. X.; Zhao, D. W.; Wei, X. F.; Liu, X. W.; Demir, Hilmi Volkan; Wu, T.
We report on top-illuminated, fluorine tin oxide/indium tin oxide-free (FTO/ITO-free), dye-sensitized solar cells (DSCs) using room-temperature- processed ZnO layers on metal substrates as the working electrodes and Pt-coated Ga-doped ZnO layers (GZO) as the counter electrodes. These top-illuminated DSCs with GZO render comparable efficiency to those employing commercial FTO counter electrodes. Despite a lower current density, the top-illuminated DSCs result in a higher fill factor than conventional DSCs due to a low ohmic loss at the electrode/semiconductor interface. The effect of metal substrate on the performance of the resulting top-illuminated DSCs is also studied by employing various metals with different work functions. Ti is shown to be a suitable metal to be used as the working electrode in the top-illuminated device architecture owing to its low ohmic loss at the electrode/semiconductor interface, minimum catalytic activity on redox reactions and high resistance to corrosion by liquid electrolytes.