Edoardo Garrone

Generation of superoxide ions at oxide surfaces

The superoxide radical anion O2- is both an important intermediate in heterogeneous catalytic oxidation and a useful probe for positive charges in ionic solids, such as metal oxides and zeolites. The paper illustrates the main circumstances under which stable superoxide anions are formed at surfaces: (i) direct surface–oxygen electron transfer; (ii) photoinduced electron transfer; (iii) surface intermolecular electron transfer; (iv) decomposition of hydrogen peroxide.

IR investigation of polymerization centres of the phillips catalyst

The CO-reduced Phillips catalyst (Cr(II) ions supported on silica) has been studied by IR spectroscopy. The following points have been examined: (i) CO/C2H4 interaction; (ii) vibrational features of the polymer; (iii) its reactivity towards oxygen; (iv) the effect of CO on the spectrum of the polymer and vice versa. The number of active sites is shown to increase during the polymerization and to reach some 10% of the total loading. The spectral features of the Cr ions involved in the active sites are consequently visible in IR: before polymerization they chemisorb CO more strongly and form stable CO-ethylene complexes; after polymerization they can still adsorb CO in a distinguishable form (stretching frequency at 2173 cm−1). The polymer chain does not show any terminal group such as methyl or vinyl: a cyclic structure is proposed. The chain may exist in either alkyl or alkylidenic form, in equilibrium with each other and differing by a proton reversibly transferred to a nearby oxygen. A model for the polymerization mechanism is advanced involving a metallacyclobutane intermediate, and accounting for the observed absence of scrambling among H atoms.

The chemistry of silica-supported Cr ions: A characterization of the reduced and oxidized forms of chromia/silica catalyst by calorimetry and ultraviolet-visible spectroscopy

CrO3SiO2 samples with low Cr loading (0.5% Cr by weight) have been prepared by impregnation. Formation of the oxidized surface phase, reduction, thermal deactivation and reoxidation have been studied. During the initial thermal treatment of the samples, two processes occur. These are anchorage of CrO3 onto the surface, yielding surface chromates and dichromates, stable toward temperature and second, degradation of nonanchored CrO3 to lower valence states, easily reoxidized at 823 K. Reduction with CO at 623 K yields a 98% population of CrII. Four species of such ions (with different electronic transitions) are present at the surface: two are reactive (CrA and CrB) and two less reactive (Crc1 and Crc2). Thermal deactivation converts CrA into Crc and does not affect CrB. A fraction of Crc (at least some 30%) is unavoidably present at the surface. In the most active samples CrA is about 45% and CrB about 25%. Reoxidation of CrA and CrB occurs with comparable reaction heats (480 and 425 kJ, respectively) via a nonactivated process. Crc oxidation is strongly activated and occurs under O2 pressure.

Ab initio study of the adducts of carbon monoxide with alkaline cations

The interaction between CO (either via the C or the O end) and the alkaline cations (Li+, Na+, K+, Rb+, and Cs+) has been studied by means of six ab initio methods, featuring the classical Hartree–Fock, the second order Mo/ller–Plesset treatment of electron correlation, one local density functional and two gradient‐corrected methods as well as a quadratic configuration interaction inclusive of single and double substitutions with a noniterative triples contribution to the energy. Basis sets adopted for CO, Li+, Na+, and K+ and the corresponding adducts are of triple‐ζ valence quality augmented with a double set of polarization functions (d on C and O; p on the cations). For Rb+ and Cs+, Hay–Wadt effective core potential basis sets have been adopted. Calculated features are the binding energy, the frequency and intensity of the CO stretch, the bending mode, the cation‐carbon (or oxygen) stretch, and the equilibrium geometry. Gradient‐corrected density functional methods yield results nearly as good as the ...

Two Coordination Modes of CO in Zeolites: A Temperature-Dependent Equilibrium

CO interacts with exchangeable cations M+ (gray spheres in the picture) of zeolites to form M+ ⋅⋅⋅CO and M+ ⋅⋅⋅OC species (C: black; O: white) which are in a temperature-dependent equilibrium. For Na-ZSM-5 (M+ =Na+ ) the difference in interaction energy amounts to 3.8 kJu2009mol-1 , as determined by means of variable-temperature FT-IR spectroscopy.

Infrared study of ZnO surface properties: II. H2CO interaction at room temperature

Abstract Carbon monoxide adsorption on hydrogen-covered surfaces gives rise to well-defined mixed CO-hydrogen structures in the adsorbed state. The centers adsorbing hydrogen and CO are formed by a triplet of exposed zinc ions and at least one reactive oxygen ion. These active groupings of ions are mutually interacting and grouped to form a bidimensional array which is similar to a patchy reconstructed 0001 face.

The chemistry of silica-supported chromium ions: Calorimetric and spectroscopic study of nitric oxide adsorption

At the surface of CO-reduced, low-loaded (0.5% by weight) chromia/silica samples, four kinds of divalent Cr ions are present. All react with NO eventually yielding linear dinitrosyl species. Differences are found as to the energetic and kinetic behaviour. The most exposed ions (A and B) readily form dinitrosyls, with NO infrared stretching vibrations at 1747–1865 and 1755–1880 cm−1, respectively. Less reactive Cr ions (designated C1 and C2) form at low coverage stable mononitrosyls (NO stretch at 1810–1815 cm−1), which change at high NO pressure into A and B dinitrosyls (C1 ions) or to new dinitrosyls (C2 ions: NO stretches at 1765–1987 cm−1). The different behaviour is explained assuming that A, B, and C ions are, before reaction, respectively 2-, 3-, and 4-coordinate to the surface.

FTIR investigation of the interaction at 77 K of diatomic molecular probes on MCM-22 zeolite

Abstract Three different Si(OH)Al sites have been singled out in MCM-22, absorbing at 3628, 3618 and 3585xa0cm −1 , tentatively assigned to hydroxyls located in supercages, 10-membered ring channels and six-membered rings, respectively. FTIR study of the adsorption of CO and N 2 at the nominal temperature of 77xa0K allowed the measurement of their acidity, which appears close to that measured for H-ZSM-5 by means of the same technique, and it is not modified by dealumination. Lewis sites are present in small amounts in vacuum-activated samples. After dealumination, no Lewis sites are present, being extraframework Al 3+ species coordinatively saturated by OH groups.

Variable‐temperature IR spectroscopic studies of CO adsorbed on Na‐ZSM‐5 and Na‐Y zeolites

CO interacts with extra‐framework alkali metal cations (M+=) of zeolites to form both M+⋯CO and M+⋯OC species. By using variable‐temperature FTIR spectroscopy, these C‐bonded and O‐bonded species were found to be in a temperature‐dependent equilibrium. For the same cation, the difference in interaction energy depends upon the zeolite framework. Thus, for the equilibrium process ZNa+⋯=CO ⇌ ZNa+⋯OC, where Z represents the zeolite framework, ΔH0 was found to take the values 3.8 and 2.4 kJ mol− for CO/Na‐ZSM‐5 and CO/Na‐Y, respectively. The C‐bonded species show always the highest cation–CO interaction energy.

An infrared spectroscopic characterization of the coordinative adsorption of carbon monoxide on TiO2

Abstract The Lewis coordinative adsorption of CO on some TiO 2 preparations was investigated by means of i.r. spectroscopy, gas volumetric dosing, and microcalorimetry. The adsorbed amounts, the number of i.r. bands formed and their spectral position were found to depend on the preparation route and on the presence of doping agents. Correlation plots are proposed between the extinction coefficients of CO bands and their frequency, as well as between CO frequencies and adsorption heats, and the present data are compared with data in the literature. The limits of applicability of the Beer-Lambert law to heterogeneous and scattering systems are discussed.