1,721,010 research outputs found
Use of overtone bands to monitor the state of the catalyst active phases during infrared studies of adsorption and catalytic reactions
No full textOvertone bands are observable in the i.r. spectra of mixed and supported oxide catalysts, and may be informative on the state of the active phases and their interactions with absorbates. In the cases of supported V2O5/TiO2, MoO3/Al2O3 and WO3/Al2O3 the behaviour of such bands indicates that single uncoupled MO bonds are exposed on the surface. In the case of bulk VPO catalysts they are informative on the presence of oxidized phases. © 1986
Fourier transform I.R. evidence of the formation of dioxymethylene species from formaldehyde adsorption on anatase and thoria
No full textHCHO adsorption on activated thoria produces, at 240 K, a species characterized as dioxymethylene; by heating at room temperature, it leads to formate and methoxy groups: the three species are observed by adsorption of HCHO on anatase at room temperature
FT-IR study of the effect of pretreatment on the surface properties of alumina produced by flame hydrolysis of aluminium trichloride
No full textA comparison of the behaviour of two samples of alumina obtained by pyrolysis of AlCl3 or thermal dehydration of boehmite has shown that the former contains chloride impurities that modify the spectra in the ν(OH) region as well as poison the strongest Lewis acid sites. The reactivity towards CO2, producing bicarbonates species, is also affected. Heat treatment in the presence of water partially eliminates Cl- and leads to a surface quite similar to that of the sample prepared by thermal dehydration of boehmite. © 1986
FT-IR characterization of the surface acidity of different titanium dioxide anatase preparations
No full textThe surface acidity of four different anatase preparations has been studied by FT-IR spectroscopy of adsorbed benzene, ammonia, pyridine, carbon monoxide and dioxide. Our results indicate that surface chemistry of anatase, in particular acidity, greatly depends on the preparation of the sample, both through the resulting morphology and the presence of surface impurities. Different morphologies may result in different types of Lewis acid sites. If pure, anatase does not show Brønsted acidity. The presence of silica impurities is responsible for an enhancement of Brønsted acidity on several samples, and may be evaluated by the intensity of an IR band situated near 3730 cm-1. Very strong Brønsted acidity is generated by the presence of sulfate impurities. © 1985
Microcalorimetric and Fourier transform infrared spectroscopic studies of methanol adsorption on Al2O3
No full textAdsorption microcalorimetry and infrared measurements using CH3OH, CD3OH, and CHD2OH show the existence of at least three steps during the adsorption of methanol on alumina activated at 773 K. At very low coverages, molecules coordinated on the strongest Lewis sites are evident, which easily transform into bridged methoxide species. With increasing coverage, another form, irreversibly adsorbed at room temperature but desorbed by evacuation at 373-473 K, becomes predominant. It is identified as undissociated methanol strongly hydrogen bonded on a cation-anion couple having a strong basic character. Finally, at high coverages a reversible form hydrogen bonded to basic sites is detected. © 1985 American Chemical Society
FT-IR and FT-FIR studies of vanadium, molybdenum and tungsten oxides supported on different carriers
No full textSurface Basicity of Mixed Oxides - Magnesium and Zinc Aluminates
No full textThe surface basicity of magnesium and zinc aluminates has been investigated by adsorption microcalorimetry, using hexafluoro-2-propanol as an acidic probe molecule. The data have been compared to those of the component oxides Al2O3, MgO, and ZnO as well as with two aluminate samples having an excess of bivalent ions with respect to the spinel stoichiometry. The surface properties of these materials have also been investigated by Fourier transform infrared spectroscopy of surface hydroxy groups and of adsorbed pyridine. Evidence is provided for a strong parallelism of the characteristics of the two stoichiometric spinels MgAl2O4 and ZnAl2O4. The strength and distribution of surface basic sites on these materials closely reproduce those of alumina. For stoichiometric materials the basicity scale is MgO > ZnO > MgAl2O4 congruent-to ZnAl2O4 congruent-to Al2O3. Nonstoichiometric aluminates having an excess of bivalent ion show an increased basicity, still lower than that of the corresponding bivalent cations oxides
Microcalorimetric and Ft-ir Spectroscopic Study of the Adsorption of Isopropyl-alcohol and Hexafluoroisopropyl Alcohol On Titanium-dioxide
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