Pier Francesco Rossi

Characterization of the surface basicity of oxides by means of microcalorimetry and fourier transform infrared spectroscopy of adsorbed hexafluoroisopropanol

Abstract The adsorption of hexafluoroisopropanol on alumina, silica, titania, thoria, magnesia, and iron oxide has been studied by gravimetry, volumetric analysis, microcalorimetry, and Fourier transform infrared spectroscopy. On all ionic oxides the adsorption is completely dissociative up to near the saturation range, where small amounts of undissociated adsorbed species are formed. Coadsorption experiments indicate that adsorption is selective on basic sites. The heats released during adsorption on the predominant surface sites are different on the different oxides, and allow the following surface basic strength scale to be proposed: ThO2 ⩾ MgO > Fe2O3 > TiO2 > Al2O3 (973 K) > Al2O3 (773 K). On silica the adsorption mechanism is completely different because of the lack of basicity. The main adsorbed species is liquid-like, with a small amount of alkoxy species. The method seems reliable for characterization of the basicity of catalyst surfaces.

Microcalorimetric and FT-IR spectroscopic study of the adsorption of methanol on TiO2 (anatase)

The adsorption of methanol on TiO2 (anatase) activated under vacuum at different temperatures has been studied by adsorption microcalorimetry and FT-IR spectroscopy. Both dissociative and undissociative chemisorption of methanol molecules have been found corresponding to the evolution of very high adsorption heats (qdiff > 150 kJ mol−1). At higher coverages, hydrogen bonded and dimeric adsorbed species are formed corresponding to differential adsorption heats of 50–120 kJ mol−1.

FT-IR study of the brønsted acidity of phosphated and sulphated silica catalysts

Abstract The FT-IR spectroscopy of adsorbed basic molecules (piperidine, pyridine, dimethylether and acetonitrile) has been used to characterize the surface acidity of catalysts constituted by silica impregnated by sulphate and phosphate ions. Both samples, like pure silica, do not show surface Lewis acidity, under the given conditions. The presence of the oxo-anions instead cause the formation of Bronsted acid centers. The Bronsted acid strength of sulphated silica is stronger than that of phosphated silica. While POH groups are at least predominantly free from H-bonding on the activated surface, the SOH groups seem to be H-bonded leading to a broad, poorly characterized OH stretching absorption.

Coadsorption of methanol and carbon dioxide on alumina

A Fourier-transform infrared study, together with microcalorimetric measurements, have identified two types of surface interactions during the coadsorption of methanol and CO2 on alumina. Methanol displaces CO2 from hydrogen carbonates, while gaseous CO2 reacts with adsorbed methoxy groups, leading to adsorbed methyl carbonate-like species characterized by very strong bands at 1623, 1476 and 1370 cm–1.

Heat of water chemisorption on α-Al2O3 at 200–400 °C

The differential heats and the adsorption isotherms of water on α-Al2O3 have been measured at five different temperatures in the range 200–400 °C using a microcalorimetric–volumetric apparatus. The amounts of water adsorbed and integral heats decreased with increasing temperature. At all temperatures the differential heats decreased with increasing amount of chemisorbed water, suggesting an ordinary surface heterogeneity. At very low coverages, the heats of adsorption at all temperatures are > 200 kJ mol–1, at higher coverages they are < 200 kJ mol–1. Two types of adsorption were considered: dissociative adsorption and coordination of molecular water. We also carried out some thermokinetic investigations on heat emission as a function of time at increasing equilibrium water pressures at 200 °C and 400 °C and at different adsorption temperatures at 0.05 Torr equilibrium water pressure.

Infrared spectroscopic study of micronised geothite

Abstract Infrared spectra of micronised goethite between 4000 cm−1 and 20 cm−1 were studied. Stretching and bending vibrations of structural hydroxy-groups were assigned using data on deuterated samples and comparison with α-Fe2O3 obtained by thermal decomposition. The existence of two different kinds of surface hydroxyls was also confirmed and an interpretation of their nature proposed. Various measured low-frequency bands were assigned to vibrations of the structural FeO6 blocks.

Evolution of the surface of haematite prepared by thermal decomposition of goethite: A microcalorimetric study

Abstract α-Fe 2 O 3 samples have been prepared by thermal decomposition of crystalline α-FeOOH in air, and have been characterized by XRD and SEM analyses and surface area measurements. Three samples, decomposed at 673, 973 and 1373 K, respectively, have also been characterized by microcalorimetry following the adsorption at room temperature of water, n-butylamine and hexafluoroisopropanol. It has been concluded that the nature of the active surface sites of the haematite samples is not modified by thermal treatment; both rather strong acidic sites and moderately basic sites are retained. However, the local density is markedly changed, due to the poisoning effect of chemisorbed water present only on samples obtained by decomposition at the lower temperatures.

Heat-flow microcalorimetric study of the adsorptive properties of a slightly weathered bituminous coal

Abstract The adsorption of water, methanol and n-hexane on demineralized Colombian coal (fresh and exposed for 20 days to the atmosphere) has been investigated by means of heat-flow microcalorimetry. A sample of graphite has also been studied for comparison. The results have been interpreted considering the different activity of three kinds of regions of the coal surface: (i) inorganic region; (ii) oxidized organic region, containing carboxyl, carbonyl and/or phenol groups; (iii) graphite-like region. In the case of hexane adsorption, due to the weakness of the interaction, the differentiation of the two coals in relation to their slightly different oxidation state is particularly evident. It has been concluded that weathering causes a further slight increase of the oxidation state of the organic part of coal and of microporosity. Moreover, a stronger interaction of the organic part of coal with the residual inorganics after weathering is suggested.

Heat of chemisorption of 1-propanol on α-Al2O3 at 25-200°C : a microcalorimetric study

We have studied by means of differential microcalorimetry the adsorption process of 1-propanol on α-Al2O3 at the temperatures of 25, 50, 100, 150 and 200°C, respectively. Both amounts of adsorbed alcohol and heats released decrease as the temperature of adsorption increases. At very low coverage, the high value of differential heat shows a strong irreversible chemisorption on active sites (Lewis acid sites) (qdiff>200 kJ·mol−1). Moreover, we carried out some thermokinetic investigations on heat emission peaks at different coverage degree (different equilibrium pressure of 1-propanol vapour as a function of time) and at different temperatures of adsorption, at same coverage.ZusammenfassungMittels Differential-Mikrokalorimetrie wurde bei den Temperaturen 25, 50, 100, 150 und 200°C der Adsorptionsprozeß von 1-Propanol an α-Al2O3 untersucht. Sowohl die Menge adsorbierten Alkoholes als auch freigesetzter Wärme nimmt mit steigender Adsorptionstemperatur ab. Bei sehr geringfügiger Bedeckung zeigt der hohe Wert des Wärmeunterschiedes eine stark irreversible Chemisorption an aktiven Stellen (Lewis-Säure-Stellen) (qdiff> 200 kJ·mol−1). Weiterhin führten wir einige thermokinetische Untersuchungen des Wärmeemissionspeaks bei unterschiedlicher Bedeckung (unterschiedlicher Gleichgewichtsdruck von 1-Propanoldampf als Funktion der Zeit) sowie bei verschiedenen Temperaturen und gleicher Bedeckung durch.