Browsing by Subject "CoOx/ZrO2"

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    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.
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    Cobalt supported on zirconia and sulfated zirconia II. reactivity of adsorbed NOx compounds toward methane
    (Elsevier, 2004-04-25) Kantcheva, M.; Vakkasoglu, A. S.
    In order to evaluate the effect of sulfate ions in zirconia-supported cobalt catalysts on the reactivity of adsorbed NOx species toward methane, experiments involving the interaction of the hydrocarbon with NOx-free and NOx-precovered catalysts are performed. The nitrate species formed at room-temperature adsorption of NO/O-2 over the CoOx/ZrO2 catalysts are inert toward the methane in the 573-723 K temperature range. Over NOx-precovered CoOx/SO42--ZrO2 catalysts, oxidation of the hydrocarbon starts at 573-623 K with the participation of reactive nitro-nitrato species coordinated to cobalt sites. It is proposed that in the catalytic reduction of NO over the sulfated cobalt catalysts, the activation of methane occurs on cobalt sites and the products of the latter process-formate species and formic acid-are key intermediates capable of selectively reducing the nitro-nitrato species. (C) 2004 Elsevier Inc. All rights reserved.