1,721,162 research outputs found
ACIDITY AND BASICITY OF METAL-OXIDE SURFACES .2. DETERMINATION BY CATALYTIC DECOMPOSITION OF ISOPROPANOL
No full textIn Part I of this series (Auroux, A., and Gervasini, A., J. Phys. Chem. 94, 6317 (1990)) a microcalorimetric investigation on the acid-base character of a large series of metal oxides was reported. A study concerning the acid-base catalytic properties of the same series of oxides is now reported together with a comparison with the previous results. The decomposition of isopropanol has been chosen as a test reaction to study the acid-base character of the catalytic sites of the oxides. For each oxide employed as catalyst, the rate coefficients as well as the Arrhenius parameters for both the dehydration and the dehydrogenation of isopropanol were determined. A relationship was found between the strength of the acid sites microcalorimetrically evaluated and the activation energy of the dehydration reaction, while the activation energy of the dehydrogenation reaction was independent of the strength of the catalytic sites. On the basis of the dehydration reaction, sets of oxides can be classified in scales of acid strength
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
THERMODYNAMICS OF ADSORBED MOLECULES FOR A NEW ACID-BASE TOPOCHEMISTRY OF ALUMINA
No full textIn the present study, two methods have been applied for characterizing the surface acidity and basicity of an amphoteric γ-alumina: a titration method using Hammett indicators and a gas phase adsorption method using basic (i.e. piperidine, diethylamine, n-butylamine, ammonia, pyridine, aniline), amphoteric (i.e. pyrrole, water, methylalcohol tert-butyl alcohol, acetonitrile), and acidic (i.e. m-cresol, carbon dioxide, acetic acid, sulfur dioxide) probe molecules by means of combined volumetry-calorimetry technique. The scales of acid-base strength obtained by the two methods and their reliability have been compared. Both the strength and number of acidic sites were related to the pKa of the probes. When using weaker probes (pKa ≤ 0) only the very strong acid sites are titrated. At pKa ≥ 5 a plateau is reached. Moreover, in order to evaluate more precisely the gas-solid interactions, the thermodynamic behaviour of the adsorbed probes has been studied, including the isotherm, the differential heat, the integral heat and the differential molar entropies of adsorption as a function of the coverage. Integral heat corresponding to the chemisorbed volume has been taken as a criterion to compare the acid strength of the sites. Besides, integral heat curves describe a parabola which curvature has been related to the strength of the adsorbate-adsorbent system. The values of the differential molar entropies of adsorbate confirm an immobile layer in the range 0-0.4 degree of coverage
Microcalorimetric investigation of the acidity and basicity of metal oxides
No full textThe acidity and basicity of about twenty simple oxides mainly used in catalysis and ceramic materials were investigated by adsorption microcalorimetry of basic and acidic gas probe molecules such as ammonia and carbon dioxide. The determination of the number, strength and site energy distribution of both acidic and basic sites in all the samples led to an improved description of the surfaces. The results allowed us to classify our samples in three main groups in relation to their acidic, basic or amphoteric character. Their acid-base behaviour was tentatively correlated to more general electronic and redox data given in the literature such as the electronegativity
Characterization of copper-exchanged ZSM-5 and ETS-10 catalysts with low and high degrees of exchange
No full textTwo different copper molecular sieve systems (Cu-ZSM-5 and Cu-ETS-10) have been prepared with different copper loadings from under- to overexchanged levels. The adsorption and surface properties have been studied using calorimetry of adsorbed NO, CO, C2H4 and NH3 probe molecules. The respective numbers and strengths of CO and NO adsorption sites on the surface, which are related to the ratio of Cu(I) to Cu(II), depend on the matrix type, location of the metal ions, and degree of Cu exchange. Cu(I) was the prominent copper species in Cu-ETS-10, and the number of Cu(I) species increased as the level of copper loading increased. In overexchanged Cu-ZSM-5, the Cu(I) sites were less prominent but more stable, as shown by temperature programmed reduction (TPR) experiments. The catalytic activity for NO reduction by C2H4 in oxygen-rich atmosphere appeared to depend on the Cu(II)/Cu(I) ratio, but also on the adsorption properties towards the NO reactive gas. The samples presenting the best NO adsorption properties are the most active in the NO-C2H4-O2 reaction. (C) 2000 Elsevier Science B.V. All rights reserved
Surface acidity of catalytic solids studied by base desorption : experimental and modelling approaches
No full textA thermogravimetric analyzer (TGA) was used to collect thermodesorption curves of 2-phenylethylamine (PEA) from acidic surfaces with the aim of determining the amount and distribution of the acid sites of the samples. Oxides widely used as active phase supports as well as catalytic phases were selected for this study: alumina, silica, silica-alumina, silica-zirconia, and silica-titania. The thermodesorption curves were collected at different heating rates (5 ≤ β (°C/min) ≤ 30) in inert atmosphere. The activation energies of PEA desorption from the acid sites were calculated from the dependence upon the heating rate β of the displacements of the observed desorption peaks (Tmax) as determined from the derivative of the TGA profiles (DTGA). For a more accurate kinetic study of the desorption phenomenon, a kinetic model based on parallel reactions of desorption, each one running with its own kinetic parameter in a temperature-dependent manner in accordance with Arrhenius's law, was applied to the experimental desorption data at the different heating rates. The quantitative acid site energy distribution was optimized for each sample, and kinetic parameters for each type of acid site were determined. The conclusions drawn from PEA thermodesorption were compared with the results obtained from the differential heat curves of ammonia adsorption measured at 80 °C in a volumetric-calorimetric line
Copper site energy distribution of de-NOx catalysts based on titanosilicate (ETS-10)
No full textThis study deals with copper-based catalysts prepared on a crystalline titanosilicate (ETS-10) matrix. Two samples prepared by ionic exchange, obtained by depositing different amounts of copper (6 and 11 wt%, corresponding to partial and total exchange, respectively), were selected for this study. Nitrogen monoxide was chosen as adsorbate to probe the surface properties of copper sites in terms of NO-Cu interaction energy. Microcalorimetric analysis of NO adsorption gave a direct measure of the NO-Cu energy of interaction and a view of the energy distribution of the copper sites from the differential and integral adsorption heat curves as a function of NO coverage. Initial heats of adsorption were around 110-130 kJ·mol-1 and fell down to 30-40 kJ·mol-1 at higher NO coverage. Volumetric isotherms of NO adsorption were measured at different temperatures (19-70 °C). A thermodynamic model was applied to the isotherms, making it possible to describe the NO-Cu site interaction energy by means of a function giving the number of Cu sites vs adsorption enthalpy, ΔaH. The high-loading copper sample had more energetic interaction with NO (-171 < ΔaH/kJ·mol-1 < -44), with a broader distribution, than the low-loading sample (-111 < ΔaH/kJ·mol-1 < -41). Parameters of adsorption, such as adsorption constant, molar entropy of adsorption, and half-coverage temperature at unit pressure (T°1/2), were obtained for each type of Cu site. The properties of the Cu sites were related with the activity measured in the reaction of selective catalytic reduction (SCR) of NO with ethylene
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
Study of the acidic character of modified oxide surfaces using the test of isopropanol decomposition
No full textThe evolution of the acid/base properties of a series of oxide supports (alumina, magnesia and silica) modified by increasing loadings of additive ions (Li+, Ni2+, and SO2-4) from 1 to 50% of the support surface coverage is reported using the catalytic test of isopropanol decomposition, studied as a function of the reaction temperature. The calculated kinetic parameters Ea, A, and ΔS≠ permit interpretation of the reaction mechanism with relation to the acidity/basicity of the modified surfaces. The series of alumina oxides, due to the amphoteric properties of the surfaces, decomposed isopropanol through an E2 mechanism leading to propene and di-isopropyl ether formation. The selectivity to the two products was dependent on the strength of the basicity (addition of lithium and nickel) or on the acidity (addition of sulfate) of the surfaces. Magnesia series oxides dehydrated isopropanol through an E1b mechanism due to the presence of very strong basic surfaces possessing some weak acid sites. The very weak amphoteric character of silica was strengthened by the loading of the three additives; the modified silica surfaces displayed enhanced decomposition activity with respect to pure silica
Site energy distribution of copper catalytic surfaces from volumetric data collected at various temperatures
No full textTwo catalysts prepared on a titanium silicate (ETS-10) matrix by loading 6 and 11 wt.% of copper were studied by employing nitrogen monoxide as adsorbate to probe the surface properties of copper centres. Volumetric isotherms of NO adsorption were collected at different temperatures (19-70°C). A thermodynamic model was applied to the isotherms in order to describe the behaviour of the NO-Cu system in terms of surface energy distribution of Cu sites. The higher loading copper sample had more energetic interaction with NO (-171 < ΔaH (kJ/mol) < -44), with higher heterogeneity, than the less loaded one (-111 < ΔaH (kJ/mol) < -41). The properties of the Cu sites were related with the activity measured in the SCR of NO with ethylene
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