Giuliana Magnacca

On the critical use of molar absorption coefficients for adsorbed species: the methanol/silica system

Abstract The ambient temperature adsorption of methanol vapour on two preparations of non-porous silica (Aerosil 50 and Aerosil 300) activated at two different temperatures (∼300 and 1073xa0K) has been studied by gas-volumetric and in situ transmission FTIR spectroscopic methods, in order to check the validity of equations of the Beer–Lambert type in the case of heterogeneous systems. The results indicate that, for both physically adsorbed and chemisorbed methanol, the calculated integral molar absorption coefficients are of the right order of magnitude (as compared with those typical of methanol in homogeneous phase), but the actual values vary much from one sample to another, depending on several factors. Among the factors, very important turn out to be: the specific surface area of the adsorbent, the sample thickness, the surface dehydration level of the adsorbing system and, most probably, the scattering properties of the adsorbent. In particular, the first two factors of variability imply that, when dealing with the spectral response of adsorbed species, an adequate normalization of the observed band intensities against sample surface area as well as against sample weight is difficult to achieve. The conclusion is that: (i) in the case of either physically adsorbed or chemisorbed species, the concept of molar absorption coefficient is virtually impossible to apply in general terms; (ii) the values calculated as “apparent” absorption coefficients cannot be transferred from one system to another, even if the e figures calculated for adsorbed species do fall in the range of e values determined for homogeneous systems.

Surface chemical functionalities in bioactive glasses. The gas/solid adsorption of acetonitrile

The room temperature adsorption of acetonitrile (ACN) on two sol–gel glassy systems for biomedical applications (58S and 77S; composition: SiO2–CaO–P2O5) was studied by in situ IR spectroscopy/adsorption microcalorimetry, and the influence of the chemical composition on the surface properties of the glasses was investigated. In order to highlight the role played by Ca and P species in modifying the surface properties of a silica matrix, some model systems were also investigated, namely: (i) a pure commercial amorphous silica; (ii) the same silica system, surface modified with dosed amounts of Ca and/or P species. Different effects were observed, depending on the metal (Ca) or non-metal (P) nature of the hetero-atoms. The contact of (perdeuterated) ACN with Ca surface species yielded a νCN IR band at ∼2185 cm−1, assigned to Ca2+u2006←u2006NCA coordinative interactions. The spectral position of the νCN band is independent of the sample origin (either bioactive glass or Ca-doped silica), whereas population and energy distribution of the species formed depend (strongly) on both sample origin and Ca loading. Surface acidic properties of all systems increase significantly with Ca content, as witnessed by both adsorbed amounts and adsorption enthalpies. The zero-coverage values of the latter quantity are quite high for the 58S and 77S bioactive glasses (∼95 and ∼80 kJ mol−1, respectively) and for medium–high Ca-loaded silicas (∼80 and >100 kJ mol−1 for 8 and 16% CaO, respectively). ACN adsorption enthalpy on surface SiOH groups is close to/lower than the enthalpy of ACN liquefaction (30 kJ mol−1). In the case of the highest Ca surface loading, the increased ionic nature of the system induces a significant surface reactivity, never observed in the case of bioactive glasses. By contrast, P species do not affect significantly the surface acid/base properties of all silica-based systems, especially when Ca species are present also (in excess).

Surface characterization of modified aluminas I. The Lewis acidity of Sm-Doped Al2O3

Abstract The surface behavior of an alumina catalyst (support) containing ≈3 wt% Sm 2 O 3 and fired at 773, 1273, and 1473 K has been investigated by IR spectroscopy and compared with the surface behavior of the corresponding undoped aluminas. It was observed that the addition of Sm, which at high temperatures stabilizes the transition (spinel) alumina phases, does not modify appreciably the spectral features of the surface hydrated layer (free surface OH groups), but modifies the Lewis surface acidity of the material. The ambient temperature adsorption of CO, pyridine, and CO 2 was investigated in some detail, and it was revealed that coordinatively unsaturated Sm centers progressively substitute, with increasing firing temperature, (all) the tetrahedrally coordinated Al centers in the surface layer. Spectral data indicate that the Lewis acidity of surface Sm 3+ sites is intermediate between that of tetrahedrally and octahedrally coordinated surface Al sites. Samples activated under nonoxidizing conditions tend to present at the surface (relatively stable) reduced Sm centers.

Surface characterization of modified aluminas. Part 6. The poisonous effect of lead

The presence of lead in three-way converters (TWC) is considered to be one of the most important causes of catalyst deactivation: its poisoning effect has been ascribed to the irreversible interaction with the catalytically active metal supported on the washcoat, leading to the formation of stable inert alloys. This paper considers the lethal poisonous effect played by lead under certain conditions (e.g., at the high temperatures easily reached by a n working catalyst) also on the alumina support (washcoat). The analysis was carried out on pure transition aluminas (A samples) and on Pb-containing alumina systems (APb samples). Structural and morphological studies were carried out by BET, XRD and (high-resolution) TEM, whereas surface features of pure and Pb-containing aluminas were analysed by in-situ FTIR spectroscopy. Data are reported for: (i) the adsorption of CO at room temperature (RT) (reveals strong Lewis acidity) and at 77 K (reveals total Lewis acidity); (ii) the RT adsorption of CO2 (reveals both Lewis acidity and, in the 1900–1200 cm−1 n spectral region, surface basicity). It could be concluded that the presence of Pb on transition alumina supports does not modify appreciably the surface area nor the range of existence of the various crystal phases (i.e., there are no physical effects), n but selectively eliminates both strong Lewis n acidic properties, and basic properties of the alumina support (surface chemical effects).

Some surface chemical features of Pt catalysts supported on Al2O3 and CeO2/Al2O3

Pt catalysts supported on alumina and ceria-alumina have been investigated by the adsorption/coadsorption of CO, H 2 O, and H 2 . Gas-volumetric and FTIR spectroscopic data indicate that, on pure alumina-supported systems, multiple-vacancy sites exist that tend to be eliminated in the presence of ceria; this is attributed to a strong Pt/CeO 2 interaction. In the presence of large amounts of CeO 2 , the catalysts are active in oxidation processes also in reducing conditions. If the alumina component of the support of Pt-containing catalysts undergoes the thermal γ → δ,θ-Al 2 O 3 phase transition, all the adsorptive capacity of Pt is lost.