Elio Giamello

DRS UV-VIS AND EPR SPECTROSCOPY OF HYDROPEROXO AND SUPEROXO COMPLEXES IN TITANIUM SILICALITE

The most important spectroscopic features in the UV-Vis of framework and extraframework Ti(IV) in titanium silicalite are briefly summarized. The spectroscopic manifestations of the complexes formed by framework Ti(IV) in presence of H2O2 are reported. The formation of EPR active species is also considered.

The interaction of NO with copper ions in ZSM5: An EPR and IR investigation

The interaction of nitric oxide with copper ZSM5 zeolites at room temperature has been studied by EPR and FT-IR spectroscopy in the aim of investigating the surface intermediates involved in the decomposition of NO to N2 and O2. Particular care has been devoted to obtaining a catalyst in a well-defined oxidation state, i.e., with one of the two ionic forms of copper (Cu2+ or Cu+) clearly prevailing on the other. The interaction of NO with Cu2+/ZSM5 yields a reversibly adsorbed nitrosylic adduct easily desorbed upon pumping at 333 K. The species is diamagnetic and bears a partial positive charge. Cu+ is unstable under NO pressure and undergoes oxidation at room temperature. In the early stages of the interaction a paramagnetic nitrosyl of Cu+ is formed having a partial negative charge on NO. This species evolves with time (or NO pressure) towards a diamagnetic dinitrosyl capable of eliminating N2 with simultaneous oxidation of the site to Cu2+.

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.

Reactivity of V2O5-WO3/TiO2 catalysts in the selective catalytic reduction of nitric oxide by ammonia

Abstract The physico-chemical characteristics and the reactivity of sub-monolayer V2O5-WO3/TiO2 deNOx catalysts is investigated in this work by EPR, FT-IR and reactivity tests under transient conditions. EPR indicates that tetravalent vanadium ions both in magnetically isolated form and in clustered, magnetically interacting form are present over the TiO2 surface. The presence of tungsten oxide stabilizes the surface VIV and modifies the redox properties of V2O5/TiO2 samples. Ammonia adsorbs on the catalysts surface in the form of molecularly coordinated species and of ammonium ions. Upon heating, activation of ammonia via an amide species is apparent. V2O5-WO3/TiO2 catalysts exhibits higher activity than the binary V2O5/TiO2 and WO3/TiO2 reference sample. This is related to both higher redox properties and higher surface acidity of the ternary catalysts. Results suggest that the catalyst redox properties control the reactivity of the samples at low temperatures whereas the surface acidity plays an important role in the adsorption and activation of ammonia at high temperatures.

EPR Characterization and Reactivity of Surface-Localized Inorganic Radicals and Radical Ions

A review. The role of surface inorg. radicals and radical ions formed at the interface between a solid and a gas phase was discussed. The formation of such species is relevant to field such as electrochem., heterogeneous catalysts, photocatalysis and corrosion. The characterization of these radicals by ESR are given. [on SciFinder(R)]

Surface structure and reactivity of VTiO catalysts prepared by solid-state reaction 1. Formation of a VIV interacting layer

Abstract The solid-state interaction between V 2 O 5 and TiO 2 in the 700–800 K range of temperatures gives rise to the formation of V IV sites even in the absence of reducing agents. A V IV interacting layer covering the entire surface of TiO 2 anatase may be created in the absence of any indication of partial transformation to the rutile phase. The nature, amount, and distribution of these V IV sites are characterized by means of titration combined with selective extraction, reactivity measurements in o -xylene oxidation, evaluation of redox properties, and by XRD, XPS, and ESR analyses. The amount of V IV depends on the crystallographic nature (anatase or rutile) and surface area of the TiO 2 and on the conditions (temperature, time, and type of atmosphere) of the heat treatment. In the anatase sample the V IV sites can be reduced to V III , but not oxidized to V V due to the strong interaction with the titania surface. In rutile samples part of the V IV may be reduced to V III , but also oxidized to V V . The remaining V IV sites are present in solid solution in the rutile matrix and are not accessible to redox reagents. The model of a V IV -modified TiO 2 (anatase) surface is discussed with reference to the problem of surface diffusion of vanadium species on the anatase surface. In TiO 2 (rutile)-based samples, due to the competition of the migration of vanadium ions toward the bulk of the rutile with respect to surface diffusion, V 2 O 4 -like islands form that are coherently intergrown into the main rutile TiO 2 matrix.

Charge trapping in TiO2 polymorphs as seen by Electron Paramagnetic Resonance spectroscopy

Electron Paramagnetic Resonance (EPR) techniques have been employed to investigate charge carrier trapping in the two main TiO2 polymorphs, anatase and rutile, with particular attention to the features of electron trapping sites (formally Ti(3+) ions). The classic CW-EPR technique in this case provides signals based on the g tensor only. Nevertheless a systematic analysis of the signals obtained in the various cases (anatase and rutile, surface and bulk centers, regular and defective sites) has been performed providing useful guidelines on a field affected by some confusion. The problem of the localization of the electron spin density has been tackled by means of Pulse-EPR hyperfine techniques on samples appositely enriched with (17)O. This approach has led to evidence of a substantial difference, in terms of wavefunction localization between anatase (electrons trapped in regular lattice sites exhibiting delocalized electron density) and rutile (interstitial sites showing localized electron density).

Colour centres at the surface of alkali-earth oxides. A new hypothesis on the location of surface electron traps

Abstract Irradiation of highly dehydrated MgO by UV light in the presence of surface adsorbed hydrogen leads to the formation of particular types of surface colour centres indicated with F + S (H) (one-electron, paramagnetic) and F S (H) (two electrons, diamagnetic). F S centres are the surface counterparts of the well-known F colour centres formed in the bulk of ionic solids by high-energy irradiation or metal addition. In the particular case of F + S (H), the unpaired electron is in magnetic interaction with a nearby proton belonging to a hydroxyl group deriving from H 2 heterolytic dissociative chemisorption (H 2 →H + +H − ) and consequent H + stabilization on a surface oxide ion. The joint use of EPR, FT-IR and DR-UV–vis spectroscopies has allowed clarification of the mechanism of formation of these centres, which is based on the ionization of adsorbed hydryde groups by UV light and stabilization of the ionized electron into suitable positively charged surface electron traps. A fraction of these traps coincides with the site capable of stabilizing the hydride ion (in the form of bridged Mg 3 H), which is built up by an array of three Mg 2+ ions reproducing a (111) facelet of the oxide. The same site can be also seen as a O 2− 3c vacancy (3-coordinated surface anionic vacancy). Ab-initio quantum chemical calculations confirm the proposed assignment, which goes beyond the original model of 5-coordinated surface anion vacancies at the flat (100) MgO plane, which is thus left in favour of a new model that describes the surface electron traps as being localized in less coordinated regions of the surface.

A microcalorimetric method for the evaluation of copper surface area in CuZnO catalyst

Abstract The reaction of O2 and N2O with Cu, traditionally employed for the evaluation of copper surface area, has been investigated by adsorption microcalorimetry on some CuZnO catalysts with different copper loadings. The oxidation of copper at room temperature proceeds in different ways with the two gases, yielding surface and bulk oxidation with O2 and only a partial oxidation at the surface with N2O (θ = 0.3). The evaluation via oxygen adsorption at room temperature totally relies on a detailed investigation of the thermokinetics, where the accomplishment of the monolayer is marked by a sharp variation in the reaction kinetics. The heat of interaction of N2O with copper does not vary with coverage or with Cu loading. It is therefore suggested that the evaluation of the heat released upon contact of copper-containing catalysts with an excess of N2O can provide a precise and quick measurement of the copper surface area.

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.