Browsing by Subject "Adsorption of NO and NO/O2"
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Item Open Access Characterization of LaMnAl11O19 by FT-IR spectroscopy of adsorbed NO and NO/O2(Elsevier, 2005-12-15) Kantcheva, M.; Agiral, A.; Samarskaya, O.; Stranzenbach, M.; Saruhan, B.The nature of the NOx species produced during the adsorption of NO at room temperature and during its coadsorption with oxygen on LaMnAl11O19 sample with magnetoplumbite structure obtained by a sol–gel process has been investigated by means of in situ FT-IR spectroscopy. The adsorption of NO leads to formation of anionic nitrosyls and/or cis-hyponitrite ions and reveals the presence of coordinatively unsaturated Mn3+ ions. Upon NO/O2 adsorption at room temperature various nitro–nitrato structures are observed. The nitro–nitrato species produced with the participation of electrophilic oxygen species decompose at 350 8C directly to N2 and O2. No NO decomposition is observed in absence of molecular oxygen. The adsorbed nitro–nitrato species are inert towards the interaction with methane and block the active sites (Mn3+ ions) for its oxidation. Noticeable oxidation of the methane on the NOx -precovered sample is observed at temperatures higher than 350 8C due to the liberation of the active sites as a result of decomposition of the surface nitro–nitrato species. Mechanism explaining the promoting effect of the molecular oxygen in the NO decomposition is proposed. (c)2005 Elsevier B.V. All rights reserved.Item Open Access Cobalt supported on zirconia and sulfated zirconia I. FT-IR spectroscopic characterization of the NOx species formed upon NO adsorption and NO/O2 coadsorption(Academic Press, 2004) Kantcheva, M.; Vakkasoglu, A. S.Cobalt catalysts are prepared by impregnating zirconia and sulfated zirconia using an aqueous solution of cobalt(II) acetate. XRD results show that the catalysts with 5 wt% cobalt loading contain a small amount of Co 3O4. Analysis of the FT-IR results on the adsorption of NO at room temperature reveals the formation of cobalt(II) mono- and dinitrosyls. It is shown that the nitrosyls formed on the sulfate-free catalyst with 5 wt% cobalt loading are unstable on prolonged contact with NO at room temperature due to the oxidation of adsorbed NO to NO2 - (nitro) and NO3 - species by cobalt(III) originating from the Co3O4 phase. This process does not occur in the case of the sulfated catalyst containing the same amount of cobalt, for which the existence of a Co3O4 phase is also detected. This experimental fact leads to the conclusion that the sulfate ions lower the reducibility of cobalt(III). Upon coadsorption of NO and O2 at room temperature on the samples studied, various kinds of surface nitrates are observed differing in the modes of their coordination. In the case of CoO x/SO4 2--ZrO2 catalysts, part of the bidentate nitrates transform to NO2 - (nitro) species after evacuation at 373 K. The nitro-nitrato species on the sulfated catalysts are characterized by a lower thermal stability than that of the nitrates on the CoOx/ZrO2 samples. © 2004 Elsevier Inc. All rights reserved.Item Open Access FT-IR Spectroscopic characterization of the intermediates in the selective catalytic reduction of NO with methane on Pd/ZrO(formula)-WO(formula) catalyst(Bilkent University, 2004) Çayırtepe, İlknurThis 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.Item Open Access FT-IR spectroscopic investigation of the reactivity of NOx species adsorbed on Cu2+/ZrO2 and CuSO4/ZrO2 catalysts toward decane(Elsevier, 2003-04-15) Kantcheva, M.The nature of the NOx species produced on NO adsorption and its co-adsorption with O-2 at room temperature on zirconia-supported copper(II) catalysts has been studied by means of in situ FT-IR spectroscopy. The samples were prepared by impregnation of zirconia with aqueous solutions of copper(II) nitrate and sulfate. The structural identification of the surface NOx complexes exhibiting absorptions in the fundamental nitro-nitrato region was performed by analyzing the combination bands of the nitrate species. In order to understand which factors control the selectivity of the catalysts in the catalytic reduction of NO by longer chain hydrocarbons, the stability of surface nitro-nitrato species and their reactivity toward adsorbed decane at various temperatures was investigated. The nitrates on the CuSO4/ZrO2 catalyst are characterized by significantly lower thermal stability than the nitro-nitrato species on the Cu2+/ZrO2 sample. The difference in the thermal stability of the NOx- species (x is 2 and 3) parallels their reactivity toward the adsorbed decane. The sulfate-free catalyst contains bidentate nitro species that are inert toward the hydrocarbon. The bidentate nitro species start to decompose to NO at temperatures higher than 523 K. In contrast, the nitrate species formed on the CuSO4/ZrO2 catalyst are able to oxidize the adsorbed decane completely at 523 K producing acetates, formates, adsorbed CO and isocyanate species. It is proposed that the presence of stable nitro species on the sulfate-free copper(II)-zirconia catalyst is associated with its non-selective behavior above 573 K in the reduction of NO with decane in an excess of oxygen reported in the literature. (C) 2002 Elsevier Science B.V. All rights reserved.Item Open Access Identification, stability, and reactivity of NOx species adsorbed on titania-supported manganese catalysts(Elsevier, 2001-12-10) Kantcheva, M.The nature of the NOx species obtained on NO adsorption and its coadsorption with O2 at room temperature on TiO2 and MnOx/TiO2 catalysts with two different manganese loadings has been studied by means ofin situFourier transform infrared spectroscopy. In order to obtain information about the potentials of titania-supported manganese materials as catalysts for selective catalytic reduction (SCR) of NO by hydrocarbons, the stability and reactivity of the adsorbed NOx species toward decane has been investigated. The adsorption of NO on the support and the catalysts leads to disproportionation of NO to anionic nitrosyl, NO−, its protonated form, NOH, and NO2. On coadsorption of NO and O2 at room temperature, various kinds of surface nitrates are observed differing in the mode of their coordination. The nitrates on the manganese-containing samples are characterized by significantly lower thermal stability than the NO− 3 species on the pure support, titania. The difference in the thermal stability of the nitrates parallels their reactivity toward the reducer (decane). The monodentate and bridged nitrates formed on the manganese catalysts studied are able to oxidize the adsorbed hydrocarbon at temperatures as low as 373 K. A mechanism for the interaction between the surface nitrates and the adsorbed decane is proposed in which the NO− 3 and NCO− species are considered as important intermediates leading to dinitrogen formation. The concentration of the reactive surface nitrates on the MnOx/TiO2 catalyst with manganese content corresponding to a monolayer is considerably greater than that on the sample with higher manganese loading. The former catalyst is promising for the SCR of NO by longer chain saturated hydrocarbons. °c 2001 Elsevier Science