1,721,274 research outputs found
Desorption study of NO and O-2 on Cu-ZSM-5
No full textA detailed temperature-programmed desorption (TPD) study on NO and O2 saturated Cu-ZSM-5 at different temperatures (300-723 K) has been performed. In the temperature range 373-723 K, the evolution of O2 and NO2 accompanying the desorption of NO from NO saturated Cu-ZSM-5 suggested the formation of nitrite/nitrate species. The amount of O2 adsorbed was very much lower than that of NO. The desorption profile of O2 after contacting Cu-ZSM-5 with O2 at 623 K showed a low temperature peak (369 K) confirming the spontaneous ability of O2 desorption from copper zeolite. Moreover, successive saturation cycles of NO followed by O2 and vice versa have been performed at various temperatures (298-623 K) to understand the modifications which the adsorption sites undergo when the two molecules NO and O2 are available together for adsorption on the catalyst sites. After each saturation cycle, a TPD profile was recorded following the evolution of NO, O2 and other NO(x) species. The competitive adsorption experiments revealed that, at 623 K, NO was not able to successfully compete with O2 for the adsorption sites, therefore the adsorption of NO at 623 K on O2 saturated catalyst was not completely restored. On the basis of the experimental work, an overall adsorption reaction scheme of NO on Cu-ZSM-5 was proposed
CuO based catalysts on modified acidic silica supports tested in the de-NOx reduction
No full textA series of dispersed CuO catalysts supported on modified silica supports with Al2O3 (SA), TiO2 (ST), and ZrO2 (SZ) were prepared optimising the adsorption method of copper deposition assisted by ultrasound treatment, already reported in a previous paper (S. Bennici, A. Gervasini, V. Ragaini, Ultrason. Sonochem. 10 (2003) 61). The obtained catalysts were characterized in their bulk (atomic absorption, x-ray diffraction, temperature programmed reduction) and surface (N2 adsorption, x-ray photoelectron spectroscopy, scanning electron microscopy) properties. The morphology of the finished materials was not deeply modified compared with that of the relevant supports. The employed complemented techniques evidenced a well dispersed CuO phase with a copper-support interaction on the most acidic supports (SA and SZ).
The catalyst performances were studied in the reaction of selective catalytic reduction of NOx with ethene in oxidizing atmosphere in a flow apparatus under variable times (0.360-0.072 s) and temperatures (200-450°C). The catalysts prepared on the most acidic supports (SA and SZ) were the most active and selective towards N2 formation. They showed a particular interesting activity in the reaction of NO2 reduction besides that of NO reduction
Characterization of the textural properties of metal loaded ZSM-5 zeolites
No full textThe textural properties of metal ion over-exchanged zeolites, Co-, Cu-, Ni-ZSM-5, were compared with those of the parent ZSM-5 by nitrogen adsorption measurements and photoelectron spectroscopy. The treatment of the adsorption isotherms permitted a thorough characterization of the microstructure of the samples. A first estimation of the microporous volume was made by the Dubinin-Radushkevich method. The "t-plot" method was used to determine the external surface and to obtain another estimation of the microporous volume. Effective micropore size distribution was obtained with the Horvath-Kawazoe approach. The Dubinin-Radushkevich method overestimated the microporous volume of the zeolites, in particular when the solid had an important external surface. It is shown that Cu-ZSM-5 had micropore size and micropore size distribution very close to those of ZSM-5, whereas Co-ZSM-5 and Ni-ZSM-5 had lower microporous volumes and larger external surfaces. The external to internal (microporous) surface ratios, rs, and the mesoporous to microporous volume ratios, rv, are proposed as practical parameters to classify the solids with respect to their microstructure. ZSM-5, a pure microporous solid, has low values of both rs and rv parameters. Ni-and Co-ZSM-5 have high rs ad rv parameters, indicative of some degree of mesoporous character. Cu-ZSM-5 has intermediate characteristics, typical for micro-mesoporous solid. XPS measurements confirmed that copper ions penetrated to a greater extent in the channels of ZSM-5 zeolite than cobalt and nickel ions, which merely deposited on the external surface of ZSM-5 zeolite as oxidic aggregates
HYDROGEN ADSORPTION AND DESORPTION ON ALUMINA SUPPORTED PLATINUM-MULTICOMPONENT CATALYSTS WITH A GAS-CHROMATOGRAPHIC PULSE TECHNIQUE
No full textA study of hydrogen chemisorption of multicomponent catalysts containing platinum as an active metal was performed by means of the pulse-flow chromatographic technique. Several series of adsorption experiments, performed at various constant intervals of time between hydrogen pulses, were carried out on each catalyst. During the experiments hydrogen desorption phenomena were observed. Hydrogen desorption was kinetically studied. The rate coefficients of the desorption reaction relevant to each catalyst were evaluated. Desorption has been discussed taking into account spillover effects. The occurrence of desorption phenomena gave rise to apparent adsorption parameters (i.e. platinum dispersion, platinum surface area), however proper parameters have been obtained by graphical extrapolation
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
Temperature Programmed Reduction/Oxidation (TPR/TPO) Methods
No full textThe redox properties of the metal oxides impart them peculiar catalytic activity which is exploited in
reactions of oxidation and reduction of high applicative importance. It is possible to measure the extent of
oxidation/reduction of given metal oxide by thermal methods which are become very popular: TPR and
TPO analyses. By successive experiments of reduction and oxidation (TPR-TPO cycles) it is possible to
control the reversible redox ability of a given oxide in view of its use as catalyst. The two methods are here
presented with explanation on some possibility of exploitation of kinetic study to derive quantitative
information on the reduction/oxidation of the oxide. Examples of selected metal oxides with wellestablished
redox properties which have been used in catalytic processes are shown
Fe2O3 Active phase for N2O decomposition: influence of the support nature on the properties and catalytic activity
No full textThermogravimetric 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)
Dispersion and surface states of copper catalysts by temperature-programmed-reduction of oxidized surfaces (s-TPR)
No full textSurface and sub-surface oxidation of dispersed copper phase by N2O adsorptive decomposition at controlled temperature followed by H2 temperature programmed reduction of the Cu2O surface layers formed (s-TPR) was performed on siliceous supported catalysts (ca. 6 wt.% Cu). The combined analysis permitted to measure the copper dispersion and to identify different surface copper species. Copper dispersion parameters were calculated from the H2-uptakes in the back-titration of the oxygen atoms fixed on the Cu particles by the s-TPR analysis. S-shaped curves were obtained plotting the H2 uptakes vs. N2O oxidation temperature, the change of slope could indicate the beginning of copper deep oxidation, ca. 70°C, that continued up to bulk oxidation at higher temperatures. Extrapolation of the H2-uptake to “zero-temperature” allowed calculating the “true” copper dispersion (DCu*) and related parameters. In addition, s-TPR provided qualitative and quantitative reduction profiles of the copper surface species. Besides Cu2O, formed by N2O oxidation of Cu(0) particles, copper species strongly interacting with support were clearly individuated as a function of the support nature
MICROCALORIMETRIC STUDY OF THE ACIDITY AND BASICITY OF METAL-OXIDE SURFACES
No full textThe adsorption of probe molecules such as NH3 and CO2 was studied on about 20 simple metallic oxides by use of microcalorimetry in order to determine the number and character of basic and acidic surface sites. Microcalorimetry allowed a simultaneous determination of the strength and energy distribution of the adsorption sites. The adsorption isotherms, the differential heats, the integral heats, and the energetic histograms are given for NH3 and CO2 adsorptions. Relationships between the average adsorption heats of NH3 and CO2 have been found as a function of the ionic character percentage and of the charge/radius ratio
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