Characterization of ceria-coated alumina carrier
TL;DR: In this paper, it was shown that different types of cerium oxide species are formed on the surface depending on the amount of CeO2 and temperature of calcination of the samples.
Abstract: CeO2–Al2O3 mixed oxides with different CeO2 loading (in the range of 0.5–12 wt.%) were prepared by wetness impregnation of alumina with aqueous solution of di-ammonium hexanitrate cerate (NH4)2[Ce(NO3)6]. The samples after calcination at 773 and 1073 K were characterized by different techniques, using X-ray diffraction (XRD), UV–VIS diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR). It is shown that different types of cerium oxide species are formed on the surface depending on the amount of CeO2 and temperature of calcination of the samples. XRD showed the formation of nanocrystallites of ceria on alumina surface when the amount of CeO2 is higher than 6 wt.%; at lower concentrations ceria was found to be amorphous. The ceria loading lower than 6 wt.% stabilizes the textural properties of alumina. At loading of 1 wt.% of CeO2 XPS spectra reveals the presence of a strong interaction between ceria and alumina leading to a formation of superficial CeAlO3-like phase. TPR results show that well dispersed CeO2 particles present on the surface of alumina form CeAlO3 at temperature of reduction in the range of 873–993 K, while for the reduction of CeO2 crystallites, a higher temperature of reduction of 1190 K is needed.
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TL;DR: A series of CeO2 promoted cobalt spinel catalysts were prepared by the co-precipitation method and tested for the decomposition of nitrous oxide (N2O) as mentioned in this paper.
Abstract: A series of CeO2 promoted cobalt spinel catalysts were prepared by the co-precipitation method and tested for the decomposition of nitrous oxide (N2O). Addition of CeO2 to Co3O4 led to an improvement in the catalytic activity for N2O decomposition. The catalyst was most active when the molar ratio of Ce/Co was around 0.05. Complete N2O conversion could be attained over the CoCeO.05 catalyst below 400 degrees C even in the presence of O-2, H2O or NO. Methods of XRD, FE-SEM, BET, XPS, H-2-TPR and O-2-TPD were used to characterize these catalysts. The analytical results indicated that the addition of CeO2 could increase the surface area Of Co3O4, and then improve the reduction of Co3+ to Co2+ by facilitating the desorption of adsorbed oxygen species, which is the rate-determining step of the N2O decomposition over cobalt spinel catalyst. We conclude that these effects, caused by the addition of CeO2, are responsible for the enhancement of catalytic activity Of Co3O4. (c) 2007 Elsevier B.V. All rights reserved.
434 citations
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TL;DR: In this paper, the acidity of catalysts containing Mg, Ce, La and Zr additives decreased with respect to that supported on bare Al 2 O 3 and the trend of metal dispersion was analyzed.
393 citations
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TL;DR: In this article, the effects of SO2 poisoning on the selective catalytic reduction of NO by NH3 over a Ce/TiO2 catalyst were studied and the analytical results indicate that there was no obvious change in the crystal structure of the different samples; however, the specific area decreased with SO 2 poisoning time, and sulfates were formed and preferentially diffused from the surface to a bulk phase during the poisoning process.
Abstract: The effects of SO2 on the selective catalytic reduction of NO by NH3 over a Ce/TiO2 catalyst were studied. Conversion of NO remained above 90% in the presence of 100 ppm SO2 at 350 °C for 48 h. However, when 180 ppm SO2 was added at 300 °C, NO conversion only remained above 90% during the first 12 h and then gradually decreased with time. Characterizations of fresh and SO2-poisoned Ce/TiO2 catalysts were carried out using Brunauer−Emmett−Teller method, ion chromatography, X-ray photoelectron spectroscopy, and X-ray diffraction. The analytical results indicate that there was no obvious change in the crystal structure of the different samples; however, the specific area decreased with SO2 poisoning time. Sulfates were formed and preferentially diffused from the surface to a bulk phase during the poisoning process. Temperature-programmed desorption and diffuse reflectance infrared Fourier-transform spectroscopy results show that in the presence of O2, SO2 could react with NH3 to form NH4HSO4, which is deposi...
367 citations
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TL;DR: In this paper, samples of cerium supported on titania with different Ce loadings have been prepared by an impregnation method and tested for the selective catalytic reduction of NO by NH 3 in the presence of excess oxygen.
296 citations
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TL;DR: In this paper, a single-step sol-gel method was used to obtain the best SCR activity and SO2 resistance regardless of the concentration of NO or SO2, while the strong interaction between ceria and titania as well as high concentration of amorphous or highly dispersed nano-crystalline ceria should be the reason for the excellent performance of the catalyst prepared by the single step sol−gel method.
250 citations
References
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TL;DR: The effective ionic radii of Shannon & Prewitt [Acta Cryst. (1969), B25, 925-945] are revised to include more unusual oxidation states and coordinations as mentioned in this paper.
Abstract: The effective ionic radii of Shannon & Prewitt [Acta Cryst. (1969), B25, 925-945] are revised to include more unusual oxidation states and coordinations. Revisions are based on new structural data, empirical bond strength-bond length relationships, and plots of (1) radii vs volume, (2) radii vs coordination number, and (3) radii vs oxidation state. Factors which affect radii additivity are polyhedral distortion, partial occupancy of cation sites, covalence, and metallic character. Mean Nb5+-O and Mo6+-O octahedral distances are linearly dependent on distortion. A decrease in cation occupancy increases mean Li+-O, Na+-O, and Ag+-O distances in a predictable manner. Covalence strongly shortens Fe2+-X, Co2+-X, Ni2+-X, Mn2+-X, Cu+-X, Ag+-X, and M-H- bonds as the electronegativity of X or M decreases. Smaller effects are seen for Zn2+-X, Cd2+-X, In2+-X, pb2+-X, and TI+-X. Bonds with delocalized electrons and therefore metallic character, e.g. Sm-S, V-S, and Re-O, are significantly shorter than similar bonds with localized electrons.
51,997 citations
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TL;DR: In this paper, high-resolution gold-valence-band photoemission spectra were obtained by the use of monochromatized k-ensuremath-alpha (kα) radiation and a single-crystal specimen.
Abstract: High-resolution gold-valence-band photoemission spectra were obtained by the use of monochromatized $\mathrm{Al} K\ensuremath{\alpha}$ radiation and a single-crystal specimen. After background and scattering corrections were made, the results were compared directly with broadened theoretical density-of-states functions. The following conclusions were drawn: (i) Relativistic band-structure calculations are required to fit the spectrum. (ii) Both the Korringa-Kohn-Rostoker calculation of Connolly and Johnson and the relativistic-augmented-plane-wave calculation by Christensen and Seraphin give density-of-states results that (after broadening) follow the experimental curve closely. (iii) Of the theoretical functions available to date, those with full Slater exchange agree best with experiment (perhaps because of a cancellation of errors). Fractional ($\frac{2}{3} or \frac{5}{6}$) exchange gives $d$ bands that are too wide. (iv) Eastman's 40.8-eV ultraviolet photoemission spectrum is similar to the x-ray spectrum, suggesting little dependence on photon energy above 40 eV. (v) Both (ii) and (iv) imply an absence of strong matrix-element modulation in the photoemission spectrum of gold.
5,238 citations
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TL;DR: A survey of the use of cerium oxide and CeO2-containing materials as oxidation and reduction catalysts is presented in this paper, with a special focus on catalytic interaction with small molecules such as hydrogen, carbon monoxide, oxygen, and nitric oxide.
Abstract: Over the past several years, cerium oxide and CeO2-containing materials have come under intense scrutiny as catalysts and as structural and electronic promoters of heterogeneous catalytic reactions. Recent developments regarding the characterization of ceria and CeO2-containing catalysts are critically reviewed with a special focus towards catalyst interaction with small molecules such as hydrogen, carbon monoxide, oxygen, and nitric oxide. Relevant catalytic and technological applications such as the use of ceria in automotive exhaust emission control and in the formulation of SO x reduction catalysts is described. A survey of the use of CeO2-containing materials as oxidation and reduction catalysts is also presented.
3,077 citations
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TL;DR: In this article, the authors measured the oxygen removal at various temperatures using TPR traces of unsupported or alumina-supported ceria and showed that the reduction of surface capping oxygen and bulk oxygen anions is associated with reduction of the shared oxygen anion at the interface.
1,726 citations
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01 Jan 1963
TL;DR: In the formalism of Newman-Penrose, a family of exact solutions of the Einsteirr-Maxwell equations of the type of Bertotti-Robinson is obtained with a cosmological term belonging to the degenerate type D in the algebraic classification of Petrov.
Abstract: It has been noted that the family of plane electromagnetic waves and the "electromagnetic universe" of Bertotti--Robinson exhaust the entire class of conformally flat Einsteirr-Maxwell spaces. In the formalism of Newman--Penrose a family of exact solutions of the Einstein--Maxwell equations of the type of Bertotti--Robinson is obtained with a cosmological term belonging to the degenerate type D in the algebraic classification of Petrov and describing the space--time generated by a covariantly constant, nonisotropic electromagnetic field.
1,380 citations