1,721,722 research outputs found
On the kinetics of catalytic reduction of nitrogen oxide by alkenes in oxidizing atmosphere
No full textA kinetic model is presented which considers the catalytic reduction of NO to N2 by hydrocarbons in oxidizing atmosphere (NO-HC-O2) as the sum of two simultaneous competitive reactions: the red-ox reaction between NO and hydrocarbon (NO-HC) and the hydrocarbon oxidation (O2-HC). The model is developed for alkenes employed as reductants and noble metal catalysts, The ratio of the kinetic constants of the two reactions can be considered as a selectivity index in evaluating the ability of a catalyst to favor the desired NO reduction with respect to the undesired O2-HC oxidation
HALF-COVERAGE TEMPERATURE AT UNIT PRESSURE AS A CHARACTERISTIC PARAMETER OF CHEMISORPTION
No full textThe Half-Coverage Temperature at Unit Pressure is proposed as a new parameter of chemisorption, characteristic of the different types of sites of a heterogeneous surface. The temperature dependence of the adsorption equilibrium constant can be related to this new parameter
Thermogravimetric study of the kinetics of degradation of polypropylene with solid catalysts
No full textThe degradation of polypropylene in the presence of solid catalysts of different acidity was studied by thermogravimetric analysis (TGA) employing three different heating rates. The temperatures of onset, Ton, of maximum-rate, Tmax, as well as of end, Tend, of the degradation shifted to lower values as higher the acidity of the catalysts. A comparison among the catalytic activities has been performed on the basis of the apparent kinetic parameters, frequency factor and activation energy, obtained from the TG data. The maximum activity was observed for the more acidic catalysts (silica-aluminas)
CuOx sitting on titanium silicate (ETS-10): influence of copper loading on dispersion and redox properties in relation to de-NOx activity
No full textA series of copper-based catalysts prepared by dispersing the CuO phase on a titanium silicate (ETS-10) crystalline matrix were studied towards their de-NOx activity. The copper concentration ranged from 0.7 to 70 atomCu nm-2, corresponding to 3-18wt%. The activity of NO reduction with ethylene was related to morphological and chemical properties of the catalysts. The crystalline character of the catalysts possessing high internal surface and microporosity was preserved up to ca. 3 atomCu nm-2. At higher copper concentration, structure collapse was observed with formation of large aggregates of CuOx. Temperature-programmed reduction experiments showed two reduction peaks with maximum temperatures at ca. 470 and 560 K, for catalysts with copper concentration up to 3.5 atomCu nm-2. The two peaks corresponded to the reduction of highly dispersed and non-interacting CuOx species (470 K) and of crystalline CuOx species (560 K). Catalysts containing copper at higher concentration had only the high-temperature reduction peak, indicating the presence of large aggregates of CuOx. All the results collected seem basically consistent with a value of about 2.5-3 atomCu nm-2 for the maximum dispersion capacity of CuO on the ETS-10 matrix. The amount of copper deposited on ETS-10 affects the activity of catalysts towards NO reduction. The turnover frequencies per copper site calculated as a function of copper concentration showed a clear decreasing trend starting from 0.7 to 3.5 atomCu nm-2. Catalysts with higher copper concentration were completely inactive towards NO reduction
Effect of Methyl Substituents on the Reactivity of p-Bromonitrobenzene towards Sodium Toluene-p-thiolate in Dimethylformamide
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Riduzione catalitica di ossido di azoto con etilene su catalizzatori a base di rame. Un’interpretazione cinetica
No full textSurface properties of hihgly dispersed catalysts used in reactions of biomass exploitation
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Dispersed NbOx catalytic phases in silica matrixes : influence of Nb-concentration and preparative route
No full textNiobium oxide catalytic phase was dispersed in/over silica host structures by using different Nb sources (ammonium niobium oxalate complex and niobium pentaethoxide) and methodologies (coprecipitation, sol-gel, and impregnation). Three series of completely amorphous silica-niobia catalysts were obtained with 5, 15, 30, 45, and 60 mass % Nb2O5 by coprecipitation (aqueous route, from ammonium niobium oxalate) and sol-gel (organic route, from niobium pentaethoxide) and with 5, 10, and 20 mass % Nb2O5 by impregnation on a finite silica, from ammonium niobium oxalate. The surface and bulk catalyst properties of all the series of samples were studied by several physicochemical techniques [N2 adsorption/desorption, thermal gravimetric analysis (TGA), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflection spectra (UV-vis DRS), temperature-programmed reduction (TPR)], and the acid properties of the surfaces were investigated by base (2-phenylethylamine, PEA) titration in liquid (cyclohexane). Evidence from both XPS (3d Nb bands and O Is band) and UV-vis DRS (ligand-to-metal charge-transfer transitions (LMCT) from O2- to Nb5+ and edge energies) measurements showed that the Nb dispersion increased with Nb dilution in the host silica structures. The number of coordinated oxygens around the Nb center was 6, for the highest diluted samples, and converged to 2.5, indicating the progressive formation of Nb 2O5 nanodomains. As the Nb2O5 concentration decreased in the catalyst composition, the surface area values expanded and Nb dispersion increased, owing to Nb-O-Si linkage formation. The synthesis by sol-gel permitted accommodating a higher surface concentration of niobia than by the coprecipitation route
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