Browsing by Subject "In situ FT-IR spectroscopy"

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    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.
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    FT-IR Spectroscopic characterization of the intermediates in the selective catalytic reduction of NO with methane on Pd/ZrO(formula)-WO(formula) catalyst
    (2004) Çayırtepe, İlknur
    This work involves in situ FT-IR spectroscopic study of the routes of formation, composition and thermal stability of strongly bound NOx complexes on the surface of Pd/tungstated zirconia, and transformation of the surface NOx complexes in the presence of methane in order to elucidate the mechanism of selective catalytic reduction of NO with methane. Sol-gel polymer-template synthesis was chosen to obtain high surface area in the preparation of the tungstated zirconia used as support (WO3 nominal content of 18.6 wt %). The Pd(II) ions (0.1 wt%) have been deposited by impregnation. PXRD characterization shows that the support and the catalyst are tetragonal and contain mesoporous phase. The adsorption of NO at room temperature on the tungstated zirconia shows presence of coordinatively unsaturated Zr(IV) ions. The spectrum of NO adsorbed on palladium modified on tungstated zirconia reveals the existence of two types of Pd(II) sites. No exposed Zr(IV) ions are observed. The surface NOx species (N2O3, nitro and nitrito ions) on both samples are produced at room temperature by oxidation of NO with the W6+=O species. In the case of the Pd/tungstated zirconia, palladium(II) can oxidize NO to NO2 at 623 K. The adsorption of NO/O2 mixture at room temperature on the samples studied leads to formation of various kinds of surface nitrates characterized by different modes of coordination. The thermal stability of the nitrate species formed on both samples is comparable: They disappear after dynamic evacuation at 673 K. However, lower concentration of the surface nitrates on the Pd/tungstated zirconia compared to that on the tungstated zirconia indicates that in the former case the nitrates are coordinated to the support. The experimental results show that methane interacts differently with the NOx-precovered tungstated zirconia and Pd/tungstated zirconia although both materials are able to activate methane at the same temperature in absence of adsorbed NOx species. In the case of the tungstated zirconia the surface nitrates suppress the oxidation of methane, whereas the NOx-precovered Pd/tungstated zirconia catalyzes the formation of nitromethane. The latter compound is considered as a key intermediate in the selective catalytic reduction of NO with methane in excess oxygen. A mechanism, which involves direct activation of methane by the catalyst, leading to the products of the selective reduction (N2, CO2 and H2O) is proposed.
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    In situ FT-IR investigation of the reduction of NO with CO over Au/CeO2-Al2O3 catalyst in the presence and absence of H2
    (Elsevier, 2009) Kantcheva, M.; Samarskaya, O.; Ilieva, L.; Panteleo, G.; Venezia, A. M.; Andreeva, D.
    he NO + CO + H-2 reaction over CeO2, Au/CeO2 (3 wt% Au), Au/CeO2-Al2O3 (2.9 wt% Au, 20 wt% Al2O3) and CeO2-Al2O3 mixed support prepared by co-precipitation has been Studied by FT-IR spectroscopy at elevated temperatures. Formation of NCO species has been detected on all of the samples. The presence of metallic gold is not necessary for the generation of the isocyanates on ceria and the mixed ceria-alumina support. The NCO species are produced by a process involving the dissociation of NO on the oxygen vacancies of the support, followed by the reaction between N atoms lying oil the surface and CO molecules. Gold plays an important role in the modification of ceria leading to Ce3+ and oxygen vacancies formation, and causes significant lowering of the reduction temperature of CeO2 and CeO2-Al2O3 enhancing the reducibility of ceria surface layers. The role of H-2 is to keep the surface reduced during the course of the reaction. The onset temperature, at which the interaction between the surface isocyanates and No begins, is low (100 degrees C). This explains the high activity of the Au/CeO2-Al2O3 catalyst with 100% selectivity in the reduction of NO by CO at low temperature (200 degrees C) and in the presence of H-2 (C) 2008 Elsevier B.V. All rights reserved
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    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.