Browsing by Subject "Zirconia"
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Item Open Access Amorphous to tetragonal zirconia anostructures and evolution of valence and core regions(American Chemical Society, 2015) Vempati S.; Kayaci-Senirmak, F.; Ozgit-Akgun, C.; Bıyıklı, Necmi; Uyar, TamerIn this report, we study the evolution of valence band (VB) structure during a controlled amorphous to tetragonal transformation of ZrO2 core-shell nanostructures fabricated from electrospun nanofiber template (at 130, 200, and 250 °C). Shell-ZrO2 was formed with atomic layer deposition. X-ray diffraction and transmission electron microscopy are employed to unveil the transformation of amorphous to crystalline structure of ZrO2. O 1s core-level spectra indicated chemisorbed oxygen (OCh) of almost invariant fraction for the three samples. Zr 3s level suggested that the sample deposited at 130 °C has depicted a peak at relatively higher binding energy. Analyses on Zr 3d spectra indicated the presence of metallic-Zr (Zr+ζ, 0 ≤ |ζ| < 4), the fraction of which decreases with increasing template temperature. VB region is analyzed until ∼64 eV below the Fermi level (EF). The region close to EF depicted features that are dissimilar to the literature. This discrepancy is explained on the basis of the analyses from O 1s, Zr 3d, and Zr 4p levels including hybridization of orbitals from chemisorbed species. These levels were analyzed in terms of peak characteristics such as spectral position, area under the peak, etc. The results of this study would enhance the understanding of the evolution of various bands in the presence of OCh and changes to the crystallinity enabling the functionalities that are not available in the single-phase ZrO2.Item 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.Item Open Access Cubic-phase zirconia nano-island growth using atomic layer deposition and application in low-power charge-trapping nonvolatile-memory devices(Institute of Physics Publishing Ltd., 2017) El-Atab, N.; Ulusoy, T. G.; Ghobadi, A.; Suh, J.; Islam, R.; Okyay, Ali Kemal; Saraswat, K.; Nayfeh, A.The manipulation of matter at the nanoscale enables the generation of properties in a material that would otherwise be challenging or impossible to realize in the bulk state. Here, we demonstrate growth of zirconia nano-islands using atomic layer deposition on different substrate terminations. Transmission electron microscopy and Raman measurements indicate that the nano-islands consist of nano-crystallites of the cubic-crystalline phase, which results in a higher dielectric constant (κ ∼ 35) than the amorphous phase case (κ ∼ 20). X-ray photoelectron spectroscopy measurements show that a deep quantum well is formed in the Al2O3/ZrO2/Al2O3 system, which is substantially different to that in the bulk state of zirconia and is more favorable for memory application. Finally, a memory device with a ZrO2 nano-island charge-trapping layer is fabricated, and a wide memory window of 4.5 V is obtained at a low programming voltage of 5 V due to the large dielectric constant of the islands in addition to excellent endurance and retention characteristics.Item Open Access FT-IR spectroscopic investigation of the surface reaction of CH4 with NOx species adsorbed on Pd/WO3–ZrO2 catalyst(2007) Kantcheva, M.; Cayirtepe, I.The interaction of methane at various temperatures with NO x species formed by room temperature adsorption of NO + O2 mixture on tungstated zirconia (18.6 wt.% WO3) and palladium(II)-promoted tungstated zirconia (0.1 wt.% Pd) has been investigated using in situ FT-IR spectroscopy. A mechanism for the reduction of NO over the Pd-promoted tungstated zirconia is proposed, which involves a step consisting of thermal decomposition of the nitromethane to adsorbed NO and formates through the intermediacy of cis-methyl nitrite. The HCOO- formed acts as a reductant of the adsorbed NO producing nitrogen. © Springer Science+Business Media, LLC 2007.Item Open Access FTIR spectroscopic characterization of NOx species adsorbed on ZrO2 and ZrO2-SO42-(American Chemical Society, 2002) Kantcheva, M.; Ciftlikli, E. Z.The nature of the NOx species produced during the adsorption of NO at room temperature and during its coadsorption with oxygen on pure and sulfated zirconia has been investigated by means of in situ FTIR spectroscopy. The adsorption of NO on both samples occurs through disproportionation leading to the formation of nitrous acid; water molecules; nitro species; and anionic nitrosyls, NO-. A mechanism for the formation of these adsorption forms is proposed. The NO- species are stable on the surface of zirconia, whereas on the sulfated sample, they are readily oxidized by the SO42- groups. The process of NO disproportionation is favored by wet surfaces and occurs with participation of the tribridged (ZrO2) and terminal (ZrO2-SO42-) hydroxyl groups. Coadsorption of NO and O2 on pure zirconia leads to the formation of various kinds of nitrate species. The presence of sulfate ions reduces the amount of surface nitrates and decreases their thermal stability. An analysis of the combination bands of the nitrate species shows that this spectral region can be used for structural identification of bidentate and bridged nitrates.Item Open Access NOx storage and reduction pathways on zirconia and titania functionalized binary and ternary oxides as NOx storage and reduction (NSR) systems(Elsevier, 2014-08-01) Say, Z.; Tohumeken, M.; Ozensoy, E.Binary and ternary oxide materials, ZrO2/TiO2 (ZT) and Al2O3/ZrO2/TiO2 (AZT), as well as their Ptfunctionalized counterparts were synthesized and characterized via XRD, Raman spectroscopy, BET, in situ FTIR and TPD techniques. In the ZT system, a strong interaction between TiO2 and ZrO2 domains at high temperatures (>973K) resulted in the formation of a low specific surface area (i.e. 26 m2/g at 973K) ZT material containing a highly ordered crystalline ZrTiO4 phase. Incorporation of Al2O3 in the AZT structure renders the material highly resilient toward crystallization and ordering. Alumina acts as a diffusion barrier in the AZT structure, preventing the formation of ZrTiO4 and leading to a high specific surface area (i.e. 264 m2/g at 973K). NOx adsorption on the AZT system was found to be significantly greater than that of ZT, due to almost ten-fold greater SSA of the former surface. While Pt incorporation did not alter the type of the adsorbed nitrate species, it significantly boosted the NOx adsorption on both Pt/ZT and Pt/AZT systems. Thermal stability of nitrates was higher on the AZT compared to ZT, most likely due to the defective structure and the presence of coordinatively unsaturated sites on the former surface. Pt sites also facilitate the decomposition of nitrates in the absence of an external reducing agent by shifting the decomposition temperatures to lower values. Presence of Pt also enhances partial/complete NOx reduction in the absence of an external reducing agent and the formation of N2 and N2O. In the presence of H2(g), reduction of surface nitrates was completed at 623K on ZT, while this was achieved at 723K for AZT. Nitrate reduction over Pt/ZT and Pt/AZT via H2(g) under mild conditions initially leads to conversion of bridging nitrates into monodentate nitrates/nitrites and the formation of surface OH and NHx functionalities. N2O(g) was also continuously generated during the reduction process as an intermediate/byproduct.Item Open Access Observation of shear bond strength between zirconia core and silica based composite material; a finite element analysis(Scientific Society of the Silicate Industry, 2021-05-21) Soygun, Koray; Özer, Zafer; Kurtoğlu, Cem; Amirullah, M. Mame; Özbay, EkmelThe strength of all-ceramic restorations depends on not only properties of basic material but also used ceramic material, substructure and upper structure connection, the thickness of crown, design, and bonding technique of restoration. The purpose of this study is to analyze the shear stress between the zirconia core and silica-based ceramic by using the finite element method. Using 20-node structural solid elements, the shear stresses were calculated with the methods of shear test and Schmitz-Schulmeyer test through 3-dimensional finite element method. The commercial software Ansys (Ansys Version 11.0, ANSYS, Inc., Canonsburg, PA 15317, USA) was used to create a three-dimensional mesh. There is no significant difference between the test methods as a result of the force applied close to the bond interface. However, when moving away from the bond interface, it is understood that there is a failure in higher tensile forces. It was determined that there was no significant difference in the results when the force to be applied in both the Shear bond Test and Schmitz-Schulmeyer tests was performed the near interface of Zirconia core-Silica based veneer ceramicItem Open Access Routes of formation and composition of NOx complexes adsorbed on palladium-promoted tungstated zirconia(2006) Kantcheva, M.; Cayirtepe, I.Surface species obtained during the adsorption of NO and NO/O2 coadsorption at room temperature on Pd-free (WZ) and Pd-promoted tungstated zirconia (Pd/WZ) are identified by means of in situ FT-IR spectroscopy. The WZ and Pd/WZ samples have a tetragonal structure and contain randomly distributed mesoporous phase. Dispersed palladium(II) species are present in two different environments: (i) Pd2+ ions, which have only Zr4+ ions in their second coordination sphere and (ii) Pd2+ ions, which are linked to both zirconium and tungsten ions via oxygen bridges. On the Pd/WZ sample, NO undergoes oxidation to various NOx species depending on the temperature. The compounds formed at room-temperature oxidation are adsorbed N2O3 and products of its self-ionization, NO+ and NO2-. In this process W(VI) is involved, being reduced to W(IV). At high temperature N2O3 decomposes, restoring the WO species. Under these conditions, NO undergoes oxidation to NO2 by the Pd(II) ions, which are reduced to Pd(I). The nitrosyls of Pd(I) display high thermal stability and do not disappear upon evacuation at 623 K. During NO/O2 coadsorption on the Pd/WZ catalyst at room temperature, the amounts of surface nitrates and NO2/N2O4 formed in the gas phase are significantly lower than those observed under identical conditions in the presence of tungstated zirconia. It is concluded that promotion of tungstated zirconia with palladium(II) suppresses the oxidation of NO by molecular oxygen at room temperature due to the elimination of acidic protons involved in the process. © 2005 Elsevier B.V. All rights reserved.Item Open Access Spectroscopic characterization of tungstated zirconia prepared by equilibrium adsorption from hydrogen peroxide solutions of tungsten(VI) precursors(2007) Kantcheva, M.; Koz, C.Two series of WO x /ZrO2 samples are prepared by equilibrium adsorption from H2O2 solutions at pH 1.8 containing two different precursor anions, [W2O3(O 2)4(H2O)2]2- and [H 2W12O40]6-. The starting material is amorphous zirconium oxyhydroxide. The maximum W densities obtained are larger than that reported in the literature for systems synthesized by the same method using aqueous non-peroxide solutions. In the case of the metatungstate precursor, this increase is attributed to the generation of additional anchoring sites by interaction between the amorphous support and H2O 2. The high uptake achieved when the peroxo complex is used as a precursor is a result of both the ZrO x (OH)4-2x -H 2O2 interaction and low nuclearity of the adsorbing anion. The materials are characterized by XRD, DR-UV-vis, Micro-Raman and FT-IR spectroscopy. The surface acidities of samples with identical W loading prepared by equilibrium adsorption from the [H2W12O 40]6--H2O2 system and by impregnation with aqueous solution of ammonium metatungstate are investigated by FT-IR spectroscopy of CO adsorbed at 80 K. © Springer Science+Business Media, LLC 2007.