1,720,972 research outputs found
Catalytic activity of molybdena/alumina catalysts for the hydrogen-deuterium equilibration at low temperature
No full textThe H2-D2 equilibration has been studied at low temperature (78 to 110 K) over reduced (H2 at 673 or 773 K) MoO3/gamma-alumina (Mo content 5% in atoms). From the e/Mo ratio (No. of H equivalents/No. of Mo atoms) it was possible to determine the surface composition with respect to Mon+ (n=4,5,6) and surface vacancies. It was therefore possible to correlate the catalytic activity for H2-D2 with specific surface configurations. The correlation plots (catalytic activity vs. surface composition) strongly suggest the presence of active sites cosisting of Mo4+...Mo4+ pairs or, possibly, vacancies pairs. A comparison with H2 adsorption at 78 K shows that the same sites and hydrogen forms are active in the case of H2 adsorption and H2-D2 equilibration. The relationship with the centres active for hydrogenation reactions is briefly discussed
X-RAY PHOTOELECTRON SPECTROSCOPIC AND THERMOGRAVIMETRIC STUDIES OF RHENIUM OXIDES SUPPORTED ON SILICA
No full textCopper cobalt hydroxysalts and oxysalts: bulk and surface characterization
No full textCopper-cobalt oxysalts of various Cu:Co atomic ratios (100:0. 92:8, 85:15, 77:23, 67:33, 15:85, 0:100) have been prepared by coprecipitation, at constant pH, from solutions of copper and cobalt nitrates added to a solution of NaHCO 3; the structure and chemical nature of the Cu-Co oxysalts have been investigated by several complementary techniques such as X-ray diffraction (XRD), magnetic susceptibility, reflectance spectroscopy and X-ray photoelectron spectroscopy (XPS) to obtain information both on their bulk and on their surface properties. In the range of Cu:Co atomic ratios from 100:0 to 67:33 the materials consist of Co-containing malachite, Cu 2-xCo xCO 3(OH) 2, while for Cu:Co equal to 0:100 and 15:85 the products are CoCO 3, spherocobaltite, and Cu-containing spherocobaltite Co 0.85Cu 0.15CO 3, respectively. From both reflectance spectroscopy and magnetic susceptibility measurements the presence of only Cu 2+ and Co 2+ species has been ascertained in all materials. Antiferromagnetic interactions are strong at high copper content and become weaker at increasing cobalt loading. The observed decrease in volume of the malachite monoclinic unit cell with incorporation of Co 2+ ions, in spite of the larger Co 2+ ion radius with respect to that of Cu 2+, indicates that distortion of the MO 6 polyhedra at the cation site of the malachite lattice is less when Cu 2+ is replaced by Co 2+. The presence of Cu 2+ in the spherocobaltite trigonal lattice shows a small cell volume decrease as expected from ionic radii difference. XPS has confirmed the presence of only Cu 2+ and Co 2+ species at the surface of the materials. It is also shown that the local nature of the chemical bond is essentially determined by the nature of the first-neighbour ligands. At higher copper content, where the materials are cobalt-containing malachite, the surface is enriched in cobalt
A STUDY OF ANOMALOUS TEMPERATURE-PROGRAMMED REDUCTION PROFILES OF CU2O, CUO, AND CUO-ZNO CATALYSTS
No full textA temperature-programmed reduction (TPR) study of CuO, Cu2O, and Cuo-ZnO catalysts, in the atomic range Cu:Zn = 92:8 to 67:33, is reported. In order to promote a correct experimental approach, much attention has been devoted to the choice of the experimental operating variables. Direct evidence is presented that the reduction profiles of the investigated specimens are markedly affected by artefacts when TPR measurements are carried out under conditions of an inadequate combination of initial amount of reducible species, total flow rate, initial hydrogen concentration, and heating rate. It is shown that under correct experimental conditions the reduction profile of each sample is characterized by a single and quite sharp reduction peak. In contrast to this, a dramatic change of shape occurs when an improper combination of the experimental operating variables is imposed: for both CuO and Cu2O an apparent double peak appears, whereas for the CuO-ZnO catalysts the reduction profiles are transformed into broad bands. In all cases, the reliability of the TPR results is completely lost. To explain the complex origin of these artefacts, it is suggested that both a special dynamic situation and activated H-2 adsorption-desorption phenomena coinciding with the chemical reduction are responsible for the observed perturbations. A simple kinetic analysis has been developed which enables a qualitative reproduction of the disturbed reduction profiles to be made. (C) 1994 Academic Press, Inc
Preparation and characterisation of cobalt?copper hydroxysalts and their oxide products of decomposition
No full textThree cobalt-copper hydroxysalts with chemical formulae Co(CO3)0.5(OH)1.0.0.11H2O, Co0.67Cu0.33(CO3)0.4(OH)1.2 and Co0.49Cu0.51(CO3)0.43(OH)1.14 have been obtained and characterised by powder X-ray diffraction (XRD), thermal analysis (TG, DTG and DTA), diffuse reflectance spectroscopy (DRS), magnetic susceptibility and X-ray photoelectron spectroscopy (XPS). The above precursors treated at 723 K for 24 h both in N2 and in air give several cobalt and copper oxides which were characterised by magnetic susceptibility, X-ray diffraction, X-ray photoelectron spectroscopy and diffuse reflectance. Depending on the Co:Cu atomic ratios and on the treatments with various gases, the products consist of oxides such as CoO, CuxCo1-xO, CuCoO2 and CuxCo3-xO4 for the samples calcined in N2, and Co3O4, CuxCo3-xO4 and CuO for those calcined in air
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A Highly Linear, Low Noise Continuous-Time Programmable Balancing Filter Used in ADSL Applications for Analog Echo Cancelling
No full text"Structural, Surface and Catalytic Properties of Bismuth Molybdovanadates containing foreign atoms. IV. Surface characterization and redox behaviour of iron-containing bismuth molybdovanadate catalysts by X-ray Photoelectron Spectroscopy"
No full textVariations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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