1,721,087 research outputs found
On the role of the calcination step in the preparation of active (superacid) sulfated zirconia catalysts
No full textStructural and Morphological Modifications of Sintering microcrystalline TiO2: XRD, HRTEM and FTIR study
No full textThe hydrated layer and the adsorption of carbon monoxide at the surface of titania (anatase)
No full textThe vacuum thermal dehydration of TiO2 has been investigated by FTIR spectroscopy and revealed that the surface hydrated layer is made up of hydroxyls and coordinated water mols. The thermal elimination of the two components of the surface hydrated layer gives rise to 2 different families of strong Lewis acid centers, at which CO adsorbs at ambient temp., yielding two σ-coordinated species characterized by different coverages, adsorption isotherms, νCO stretching frequencies and heats of adsorption. The properties of adsorbed CO are influenced, through inductive effects, by the nature and concn. of charge-releasing and/or withdrawing species present at the surface. On sulfate-contaminated TiO2 only one CO adsorbed species is obsd., consistent with the virtually substitution of surface hydroxyls with sulfate residues. The charge-withdrawing nature of the latter strongly affects the properties of adsorbed CO, in agreement with the proposed model
On the acid-catalyzed isomerization of light paraffins over a ZrO2/SO4 System: The effect of hydration
No full textOver a superacid ZrO2/SO4 system (ZS8: microcrystalline tetragonal, SO4 content ≈ 2.6 groups per nm2), the isomerization of n-butane proceeds rapidly and with high yield of i-butane at 423 K, if the catalyst is calcined in dry air at ≈ 823 K. The active catalyst typically shows: all surface sulfates in a highly covalent form, a high concentration of strong (aprotic) Lewis acid sites, and a fairly low (though never null) concentration of (protonic) Brønsted acid sites. A partial rehydration of the catalyst gradually converts the surface sulfates first into a less covalent form and then into an ionic form, decreases the concentration of strong Lewis sites, and increases (up to a factor of ≈ 10) the concentration of Brønsted sites, while the catalytic activity is gradually extinguished. Initial activity and initial surface features can be recovered by repeating the catalyst activation step. © 1994 Academic Press, Inc
Revisiting the use of 2,6-dimethyl-pyridine adsorption as a probe for the acidic properties of metal oxides
Full text link2,6-dimethylpyridine as an analytical tool to test the surface acidic properties of oxidic systems
No full textSpectroscopic study of anatase properties. Part 5. Surface modifications caused by potassium oxide addition
No full textIR spectroscopy was used to investigate the surface properties of anatase gels contg. ≤1% K2O, one of the additives most commonly used to give TiO2 pigmentary characteristics. K2O mostly collects at the surface of the material, as revealed by the spectral behavior of surface hydroxyls and of surface sulfate contaminants. During the thermal treatment leading to the pigmentary material, K2O progressively modifies TiO2 surface acidity, so that on the low-surface-area final product no Ti ions are revealed by suitable admols. (pyridine, CO), but only coordinatively unsatd. K ions, acting as weak Lewis centers. The surface modifications caused by K2O are slowly reversible on contact with H2O vapor
Surface characterization of hydrophobic zirconia-silica mixed oxides for clean oxidation catalysis
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