1,721,011 research outputs found
Surface characterization and catalytic activity of CoxMg1 –xAl2O4 solid solutions. Oxidation of carbon monoxide by oxygen
No full textFormation of Co2+...(CO)n complexes on high surface area CoO–MgO, as investigated by electron spin resonance
No full textReactivity of some vanadium oxides: An EPR and XRD study
No full textV2O5, VO2 and V2O3 fresh samples and at different times after purchase or preparation (aged samples) were investigated by chemical analysis, redox treatments, XRD and EPR. The ageing process through a reaction with water and oxygen slowly oxidize crystalline VO2 and V2O3, leading to a quasi-amorphous phase with bariandite structure (V10O24 (.) 12H(2)O). The role of water is the progressive demolition of the compact structures and formation of hydrated phase. Kinetic study of VO2 oxidation by O-2 and O-2 + H2O mixture indicates that increasing the temperature LIP to 723 K the effect of water becomes less important. The reaction leads to partially oxidized products with decreasing water content: bariandite at room temperature, V3O7 (.) H2O at 383 K and V3O7 at 723 K. Kinetic investigation of V2O5 reduction by CO at 633-723 K showed that the reduction process proceeds trough the formation of V4+ and of electrons delocalized in the conduction band. © 2005 Elsevier Inc. All rights reserved
Manganese ions in the monoclinic, tetragonal and cubic phases of zirconia: an XRD and EPR study
No full textTHE DISPERSION OF CRV AND MOV ON THE SURFACE OF VARIOUS OXIDES, AS INVESTIGATED BY ESR SPECTROSCOPY
No full textSamples containing chromium (Cr-53-enriched or not) or molybdenum (Mo-95-enriched or not) were prepared by the equilibrium adsorption or impregnation methods at two different loadings (approximately 0.05 and 0.5%-1% by weight) using as supports ZrO2, SiO2, TiO2, Y2O3, Al2O3, Ga2O3 or La2O3. After thermal activation (heating with O2 at 773 K for the Cr samples followed by reduction with H-2 for the Mo samples), ESR signals from surface Cr(v) or Mo(v) species are observed. The spectra are satisfactorily resolved on ZrO2, SiO2, TiO2, Y2O3, whereas they are broad (and always structureless in the case of chromium) on Al2O3, Ga2O3 or La2O3, whether Cr-53-enriched (Mo-95-enriched) was used or not for the preparation of samples. Resolved spectra are assigned to mononuclear Cr(v) or Mo(v) surface species. Notably, the ESR spectrum of Mo-95(v) on Al2O3, in contrast to that of Cr-53(v) on the same matrix, shows partially resolved hyperfine structure which allows for the assignment of the spectrum to mononuclear Mo(v) species. The finding that the ESR signals of Cr(v) or Mo(v) on the surface of Al2O3Ga2O3 or La2O3 are much broader than on the other matrices is attributed to unresolved interaction with neighbouring Al-27, Ga-69, Ga-71 or La-139 (superhyperfine interaction, shf). The shf interaction is small on Y2O3 in view of the low nuclear spin and magnetic moment of the Y-89 nucleus and in fact resolved spectra are obtained on this matrix. Computer-calculated spectra assuming mononuclear Cr(v) or Mo(v) species interacting with two equivalent Al-27 nuclei (slightly anisotropic shf interaction 6 to 8.5 gauss) are in substantial agreement with experimental spectra
Electron spin resonance studies of the formation and thermal stability of oxygen radicals on CaO and some other oxides
No full textQuantitative EPR spectroscopy: comparison between primary standards and application to MgO-MnO and α-Al2O3-Cr2O3 solid solutions
No full textSelective catalytic reduction of N2O with CH4 on Ni-MOR: A comparison with Co-MOR and Fe-MOR catalysts
No full textThe selective catalytic reduction of N2O with CH4 in the presence of excess O-2 (CH4-SCRN2O) and CH4 + O-2 reaction were studied on Ni-MOR, Co-MOR and Fe-MOR, prepared from H-MOR or Na-MOR, by ionexchange or CVD. FTIR showed that transition metal ions (tmi) were well dispersed, mainly isolated with at least two coordinative vacancies. For CH4-SCRN2O, irrespective of Bronsted acid site and Na* amount, catalysts were active in the order Fe-MOR >Ni-MOR > Co-MOR. Using a methane-poor mixture, on all samples CH4-SCRN2O consisted of two nearly independent reactions: CH4 +N2O and CH4+ O-2. Using a methanerich mixture, CH4-SCRN2O consisted of a CH4+ N2O + O-2 reaction, whose stoichiometric ratios depended on the tmi. Whereas on Ni-MOR methane combustion occurred as side reaction, on Co-MOR and Fe-MOR no side reactions occurred. Accordingly, for CH4 + O-2 Co-MOR and Fe-MOR were poorly active, whereas Ni-MOR were highly active. We conclude that in CH4-SCRN2O a monoatomic oxygen form activates methane on Co-MOR and FeMOR catalysts, whereas both a monoatomic oxygen form and a molecular oxygen form activate methane on Ni-MOR. Being active for the CH4-SCRN2O and CH4-SCRNOx and forming CO only in traces, Ni-MOR are promising catalysts for the simultaneous SCR of N2O and NOx using CH4 as reducing agent. (C) 2013 Elsevier B.V. All rights reserved
Intrinsic and extrinsic paramagnetic centers in zirconia
No full textIn the literature two intrinsic paramagnetic centers have been identified by EPR in pure zirconia: Zr3+ ions, yielding an axial signal (g(parallel to) = 1.958 and g(perpendicular to) = 1.979) and electrons trapped in oxygen vacancies (V (o) over circle). In this study we investigated by quantitative-EPR pure and doped ZrO2 heated in air and redox treated at various temperatures. In pure and in doped zirconia the axial signal has similar spectral features, including the satellite line due to the most intense component of Cr-53 hyperfine structure, and the signal intensity yielded the same redox trend. In doped zirconia the concentration of the species yielding the axial signal was a constant fraction (about 50%) of the total chromium content. In pure zirconia samples, the concentration determined by EPR was also about 50% of the Cr impurity level determined by GFAAS. These findings attribute this signal to the extrinsic center, Cr5+ ion, arising from chromium impurity. In reduced zirconia a signal of Lorentzian shape (g = 2.002; DeltaH(pp) = 4-5 G) arises from the only intrinsic defect detected by EPR in zirconia bulk, namely V (o) over circle center. This signal could easily be confused with another having the same g value, but a different line width (DeltaH(pp) = 0.5-1 G) and line shape that forms in some vacuum-activated zirconia samples and arises from an extrinsic center, carbon paramagnetic impurity
EXCHANGE-COUPLED CRV AND MOV IONS ON THE SURFACE OF CROX ZRO2 AND MOOX ZRO2 - AN ELECTRON-PARAMAGNETIC RESONANCE STUDY
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