1,721,019 research outputs found
Acidic character of metal loaded amorphous and crystalline silico-aluminas determined by XPS and adsorption calorimetry
Full text linkThe XPS technique in combination with microcalorimetry was used to picture the acid character of metal ion loaded zeolite and silica-alumina samples. Co, Cu, and Ni ions were loaded on ZSM-5 and SiO2-Al2O3 (about 1 mmol/g) by three different procedures: ion exchange, impregnation, and solid-state ion-exchange. The samples prepared by ion exchange of the two matrixes presented high values of the 2 p3/2 XPS bands of the metal ions, indicating the occurrence of charge transfer from the ions to the support matrix. This permitted the stabilization of the metal phases as isolated ions rather than as oxide clusters. Microcalorimetric experiments of ammonia adsorption were performed in order to determine the number, strength (i.e., adsorption enthalpy), and strength distribution of the acid sites on the samples and on the relevant matrixes. The N 1s XPS lines of ammonia adsorbed on the surfaces were decomposed into two component peaks, assigned to Brönsted (average value of BE, 402.2 eV) and Lewis (average value of BE, 400.4 eV) acid sites. The relative intensities of the two peak components were measured for the quantitative determination of Brönsted and Lewis acid site concentrations. Coupling the information from adsorption calorimetry and XPS spectroscopy of N 1s adsorbed lines, the absolute numbers of Brönsted and Lewis acid sites on each sample were determined. The two support matrixes were protonic acids (65 and 60% of Brönsted acid sites for ZSM-5 and SiO2-Al2O3, respectively), and the acidity of ZSM-5 was greater than that of SiO2-Al2O3 considering both the number of total acid sites and the acid strength. The presence of metal ions deposited on the two matrixes deeply changed the respective proportions of Brönsted and Lewis sites. A huge increase in the Lewis acid site population of the ZSM-5-based samples (70, 85, and 90% of Lewis sites for the samples containing Co, Cu, and Ni, respectively) and of the SiO2-Al2O3-based samples prepared by ion exchange (55, 60, and 70% of Lewis sites for the samples containing Co, Cu, and Ni, respectively) was observed
XPS study of the adsorption of SO2 and NH3 over supported tin dioxide catalysts used in de-NOx catalytic reaction
No full textAlumina and titania supported tin dioxide catalysts presenting various Sn loadings were prepared by impregnation. The acidity and basicity of the samples were determined by adsorption of ammonia and sulfur dioxide, respectively, using adsorption microcalorimetry and X-ray photoelectron spectroscopy. The surface reactivity of the samples and the chemical composition of the adsorbed species were determined as well as the heats of adsorption of the probe molecules. The catalysts were tested in the selective catalytic reduction of NO by C2H4. Both calorimetry and XPS experiments have shown that the SnO2/TiO2 series of samples was markedly more acidic than the SnO2/Al2O3 series, as the N/(Ti+Sn) molar ratios were noticeably higher than the corresponding N(Al+Sn) ratios. The number of acidic sites seemed to increase with the tin content when tin dioxide was well dispersed on the support. They are of Lewis type. It was shown that sulfur dioxide adsorption led to the formation of three types of species: SO2, sulfites, and sulfates. The basicity of the SnO2/Al2O3 series of samples was weaker than that of the alumina support and passed through a minimum around 12 wt % Sn. On the contrary, as the acidity, the basicity of the SnTi series did not seem dependent on the Sn concentration. This can be correlated to the bad dispersion of SnO2 on TiO2. In the NO reduction by C2H4 reaction, the turnover frequency mainly depends on the Sn dispersion, and the Sn centers are very active even at low amounts
Microcalorimetric studies of the surface reactivity of pollutant gas atmospheres on supported SnO2 and Ga2O3 catalysts
No full textThe surface properties of gallium oxide and tin dioxide supported on alumina or titania have been studied by adsorption microcalorimetry. The differential heats of adsorption of various pollutant adsorbates such as sulfur dioxide, nitrogen monoxide, nitrogen dioxide and also ammonia were measured on these catalytic surfaces. NH3, SO2, NO2 are strongly adsorbed while NO is only physisorbed. The supported Ga2O3 samples show a slight decrease in acidity as probed by ammonia adsorption, compared to alumina or titania. The addition of SnO2 decreases the number of strong acid sites but creates a few weak and medium strength acid sites on alumina and does not modify the acidity of titania. In all cases, the basicity, probed by SO2 adsorption, is very strongly affected by the deposition of Ga2O3 or SnO2. The differential heats of NO2 adsorption remain nearly constant on all samples. The heats of adsorption are discussed as a function of the coverage and of the amount of guest oxide
Influence of the Preparation Method on the Surface Characteristics and Activity of Boron-Nitride-Supported Noble Metal Catalysts
No full textIn this article, we report how variations in the preparation method of boron-nitride-supported noble metal catalysts may influence the surface characteristics of the active phase and consequently the potential applications as catalysts for oxidation reactions. The deposition and the dispersion of the active phase are strongly influenced by the preparation process and in particular by the protic or aprotic solvent used as the dispersing phase; in this study, benzene, glyme, water, tetrahydrofuran, diglyme, 2-propanol, and glycol have been investigated. Characterization techniques, such as Brunauer-Emmett- Teller, X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis, have been used to study the influence of the choice of a solvent phase on the particle size and dispersion of the metal deposited on the BN support. The modifications undergone by the support during the deposition of palladium in different solvents have also been studied. Through the use of the same deposition procedure, different noble metal coatings (Pt, Pd, Au, and Ag) have been prepared. The acidic and redox characteristics of the resulting samples were characterized by temperature-programmed reduction and adsorption microcalorimetry. The catalytic performances of these materials were tested in the total oxidation of methane in lean conditions ( excess oxygen and presence of water)
An in-depth study of supported In2O3 catalysts for the selective catalytic reduction of NOx: The influence of the oxide support
No full textThe influence of the oxide support (i.e., Al2O3, Nb2O5, SiO2, and TiO2) on the surface properties, reduction and oxidation properties, acid-base properties, and catalytic activity of supported indium oxide catalysts has been investigated by temperature-programmed reduction/oxidation, thermogravimetry coupled to differential scanning calorimetry, ammonia and sulfur dioxide adsorption calorimetry, and reduction of NO, by ethene in highly oxygen-rich atmosphere. Two series of In2O3-containing catalysts at low (approximate to 3 wt %) and at theoretical geometric monolayer (from 20 to 40 wt %) In2O3 Content were prepared and their properties were compared with unsupported In2O3 material. Supports able to disperse the In2O3 aggregates with high In stabilization gave rise to active catalytic systems. Among the studied oxide supports, Al2O3 and, to a lower extent, TiO2 were found to be the best supports for obtaining active de-NO, catalysts
Surface characteristics and activity in methanol partial oxidation of ald V2O5/SiO2 catalysts
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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
Surface characteristics and activity in selective oxidation of o-xylene in phtalic anhydride of ALD V2O5/SiO2 catalysts
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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