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In the present work, different ZrO2−TiO2mixed oxides, which are usually employed in photocatalytic processes and as catalytic support in heterogeneous catalysis, have been prepared via sol−gel synthesis or by wet impregnation. In the former case, as indicated by X-ray diffraction and Raman spectroscopy, solid solutions based on Ti ions diluted in the ZrO2matrix are formed in the range of Ti molar fraction from 0.01 to 0.15. These solids have interesting optical properties as the typical band gap transition of ZrO2undergoes a red shift proportional to the Ti loading which reach the remarkable value of 1.3 eV for the highest Ti concentration. Annealing under vacuum at various temperatures causes oxygen depletion with consequent reduction of the solid which shows up mainly in terms of formation of Ti3+reduced centers which are characterized by a typical broad EPR signal similar to that observed in reduced TiO2(anatase). The ability of the mixed oxides in stabilizing surface superoxide anions (O2•−) has also been investigated by EPR to obtain information both on the photoinduced charge separation process in the solids and on the state of thermally reduced systems. In the latter case the obtained evidence have some value to understand controversial problems concerning the TiO2reduced matrix.

Nitrogen-doped TiO 2materials were successfully prepared following three different preparation routes (sol−gel, mechanochemistry, and oxidation of TiN) and characterized by X-ray diffraction, electron microscopy, and various spectroscopic techniques. All samples absorb visible light, and the one obtained via sol−gel, showing the anatase structure, is the most active in the decomposition of organic compounds under visible light. Various nitrogen-containing species have been observed in the materials, whose presence and abundances depends on the preparative route. Ammonium NH 4+ions are residual of the synthesis using ammonium salts (sol−gel, mechanochemistry) and are quite easily eliminated, as shown by the parallel behavior of both NMR and XPS spectra. Cyanide CN −ions form at high temperature in parallel with the phase transition of the solid to rutile. Molecular nitric oxide forms in the case of materials exhibiting close porosity. The already reported bulk radical species, N b•, is the only paramagnetic center observed in all types of samples, and is responsible for the visible light sensitization of TiO 2. A mechanism for the formation of such a species in chemically prepared N-doped TiO 2materials is for the first time proposed based on the reduction of Nitric Oxide (NO) at oxygen vacancies

Fluorine-doped titanium dioxide was prepared via sol−gel synthesis and subsequent calcination in air. The presence of fluorine in the lattice induces the formation of reduced Ti 3+centers that localize the extra electron needed for charge compensation and are observed by electron paramagnetic resonance. Density functional theory calculations using hybrid functionals are in full agreement with such description. The extra electron is highly localized in a 3d orbital of titanium and lies a few tenths of an electron volt below the bottom of the conduction band. The preparation via sol−gel synthesis using aqueous solutions of fluorides also causes the formation of surface F −ions that substitute surface hydroxyl groups (OH −) without generating reduced centers.

A chemical model is proposed to explain the passivation of carriers upon etching and their reactivation by the presence of gases in p+-type mesoporous silicon (m-PS). The model is based on the results obtained by means of FTIR, EPR, NMR spectroscopies and ab-initio calculations. This chemical description accounts for the behaviour of mPS when contacted with NO2 or NH3. B atoms in subsurface location play a prominent role. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The present paper reports experimental evidence of the heterolytic dissociation of some saturated hydrocarbons (methane, ethane, propane, and cyclohexane) at the surface of high surface area CaO activated at high temperature. The reaction takes place at RT on particularly basic O^2- sites at the surface of CaO. The reaction has been followed via EPR spectroscopy in two distinct ways. The former one consists of monitoring the formation of superoxide radical anions (O2^-) which occurs when the hydrocarbon and molecular oxygen are coadsorbed at the surface via surface intermolecular electron transfer reaction. The latter one is based on ionization of the negative hydrocarbon fragment upon exposure to UV light with consequent formation of a surface trapped electron (a surface color center known as Fs(H)⁺) in which the electron is stabilized nearby an hydroxyl OH^- group produced by the hydrocarbon splitting. This latter OH^- group undergoes isotopic exchange transforming in OD^- when contacted with D2 or CD4.

The surface basic sites at the surface of calcium oxide have been explored following the interaction of nitric oxide with the solid by EPR. Various paramagnetic species are formed depending on the pressure of the gas contacted with the solid. In the initial steps of the interaction, a small amount of a NO3^2- species is formed probably because of the presence at the surface of “anomalous” peroxy or peroxo-like groups. For higher NO pressures, four types of NO2^2- surface species characterized by distinct values of the g and hyperfine tensors are formed by addition of the neutral molecule to basic O^2- sites of the surface. The process involves a considerable rearrangement of the spin distribution with respect to the NO molecule with a net decrease of the electron spin density on the N atom. The number of basic sites involved in this interaction is very low and amounts to a value in the range of 0.25−0.5% of the whole surface. These active sites are low coordination sites present at morphological defects of the polycrystalline solid. The concentration of the basic sites is 25 times greater than that of the corresponding sites on MgO thus confirming the higher overall basicity of CaO with respect to magnesium oxide. The surface activity shown by MgO and consisting in a low energy interaction of Lewis acid sites with the NO molecule is, on CaO, totally suppressed.

The interaction of nitric oxide with the surface of MgO at low temperature has been studied experimentally and theoretically. High-surface-area polycrystalline MgO prepared by chemical vapor deposition has been exposed to both ¹⁴NO and ¹⁵NO and the corresponding adsorption complexes have been monitored by means of electron paramagentic resonance (EPR) spectra. MgO(100) thin films grown on Mo(100) in UHV conditions have been exposed to ¹⁵NO and the adsorption products have been investigated by thermal desorption (TDS) and Fourier transform infrared (FTIR) spectroscopies. The structure and properties of NO/MgO(100) have been studied by cluster model density functional theory (DFT) calculations. The EPR data show that only 0.5% of the total NO deposited is in a paramagnetic state; out of the paramagnetic species, 98% are physisorbed and only 2% are chemisorbed. This is explained with the fact that on the terrace sites NO monomers interact very weakly and prefer to form dimers, (NO)2. Only at defect sites (low-coordinated cations) the interaction of NO monomers with the MgO surface is stronger and prevents the formation of the diamagnetic dimers. A small minority of chemisorbed species is formed only at low coordinated anions (steps, edges, or corners) or oxygen vacancies.

Evaporation of Cs atoms onto dehydrated polycrystalline MgO leads to the formation of surface color centers in correspondence of surface point defects. EPR spectroscopy has revealed that the adsorbed Cs atoms are partially ionized, and a fraction of the electron spin density is delocalized onto a surface oxygen vacancy or trap. The observed defect can thus be written as Cs^δ+(trap) ^δ- . These results give evidence of the preferential interaction of the metal atoms with specific surface defect sites in the early stages of the metal−support interaction. The reaction of these centers with molecular oxygen leads to bleaching of the surface with formation of the O2^- superoxide radical anion. A fraction of the adsorbed superoxide ions are adsorbed on “regular” Mg²⁺ sites while the remaining ones are adsorbed on top of Cs⁺ ions.

Upon nitrogen adsorption at low temperature onto the surface of magnesium oxide and calcium oxide containing FS⁺ centers (single electron trapped in a suitable surface vacancy), electron transfer occurs from the solid to the adsorbed molecule. About 90% of the total electron density is localized on the adsorbed molecule. The 11-electron N2^- radical anion so formed has been detected by electron paramagnetic resonance for both ¹⁴N2 and ¹⁵N2. The electron transfer is reversible and, when the pressure is lowered or the temperature increased, the N2 molecules desorb, regenerating the original FS⁺ center. The diatomic radical ion lies parallel to the surface, and the electron density is mainly confined in the πy* orbital. Theoretical calculations at the DFT level indicate that a small energy barrier separates the unbound FS⁺/N2 state from the bound FS²⁺/N2^- state. This explains the facile reversibility of the electron-transfer process. The calculated spin densities are in excellent agreement with those derived from the experiments. The results reported in this paper represent a new method for N2 activation over the basic alkaline earth oxides, which are also known to activate H2 by dissociative adsorption.

The local structure of V oxide/ZSM-5 catalysts prepared by the solid-state reaction of the ZSM-5 zeolites having different Si/Al ratios have been investigated by means of in situ photoluminescence spectroscopy in combination with other spectroscopic techniques such as UV−Vis, solid-state NMR, and ESR. It was found that vanadyl ions are present as tetrahedrally coordinated V⁵⁺ species bonded to terminal Si−OH groups and square pyramidally coordinated VO²⁺ species at the zeolite cationic sites for zeolites having a low Si/Al ratio (Si/Al = 20). On the other hand, for a high Si/Al ratio (Si/Al = 950), only the tetrahedrally coordinated V⁵⁺ species can be observed. UV irradiation of the V oxide/ZSM-5 catalyst in the presence of NO was found to lead to the photocatalytic decomposition of NO into N2, O2, and N2O. The increase of efficiency was found to be more remarkable on the V oxide/ZSM-5 catalyst with a high Si/Al ratio than with a low Si/Al ratio. The photoluminescence spectrum of the V oxide/ZSM-5 catalyst which can be attributed to the radiative decay process for the charge transfer excited triplet state of the tetrahedrally coordinated VO4 unit species was quenched by the addition of NO or propane, its extent depending on the pressure of these molecules. These results suggest that the charge transfer excited triplet state of the isolated tetrahedrally coordinated V⁵⁺ species plays a significant role not only in the photocatalytic decomposition of NO but also in its enhancement in the presence of propane.

The redox behavior of Cu/ZSM-5 zeolites prepared by ion exchange from Cu²⁺ aqueous solutions has been followed by a variety of spectroscopic techniques to provide a thorough picture of the so-called “self-reduction” of cupric ions, which occurs upon dehydration of the hydrated system at various temperatures, and of the reverse process of reoxidation as well. Conflicting hypotheses on both these processes are, in fact, present in the literature. The experimental techniques employed in this work are electron paramagnetic resonance (EPR), IR, and optical spectroscopies, extended X-ray absorption fine structure, and X-ray absorption near-edge structure. The early stages of dehydration (from room temperature to about 470 K) involve cupric ion migration and formation of EPR silent moieties but no reduction to Cu⁺. The onset of this latter phenomenon starts at 470 K and, in the range 470−670 K, involves the majority of copper ions present in the system. Rehydration of Cu⁺-containing samples does not cause direct Cu⁺ oxidation to Cu²⁺ but favor this latter process when O2 is used as oxidant. Oxidation of Cu⁺ to Cu²⁺ by molecular oxygen, in fact, does not take place at room temperature if O2 is contacted with the dehydrated material but easily occurs when oxygen is adsorbed on rehydrated samples at the same temperature.

Physisorption of N2 gas onto the surface of a metal oxide (MgO or CaO), containing paramagnetic trapped electron centers (FS⁺ color centers), leads to the formation of a paramagnetic species that, on the basis of its EPR spectrum and of the related spin-Hamiltonian parameters, is identified as a N2^- radical anion. The species in fact contains two nitrogen atoms and its g and A tensors are in agreement with what observed for the N2^- radical trapped in irradiated crystal of various azides. The surface N2^- species is formed by surface-to-molecule one-electron transfer, and its stability strictly parallels the stability of the physisorbed layer, the species formation being completely reversible and pressure dependent. When the N2 adlayer is desorbed, in fact, the N2^- spectrum vanishes and the original Fs⁺ spectrum is restored. Ab initio quantum chemical calculations on an embedded MgO cluster fully confirm the observed phenomenon indicating, in agreement with EPR analysis, the electron transfer of a large fraction of electron density into the π orbitals of the admolecule.

An embedded-cluster Hartree−Fock approximation is adopted for simulating the formation of Fs(H) color centers at the (001) surface of magnesium oxide. This process is assumed to take place in two steps at an isolated surface anion vacancy:  first, a hydrogen molecule is adsorbed dissociatively at the defect; second, following UV irradiation, a neutral hydrogen atom is removed and an electron remains trapped at the vacancy with a hydroxyl group nearby. According to the present calculations, the activation energy for the dissociation is appreciable (about 25 kcal/mol) and the products (a proton bound to a low-coordinated oxygen and a hydride ion above the vacancy) are considerably less stable than the reactants. The excitation of the adsorbed species owing to the UV irradiation is simulated by considering a singlet−triplet transition of the hydride−vacancy complex, which then dissociates into an H atom and a trapped lone electron. The electronic structure and the EPR parameters of the resulting paramagnetic state are explored. The theoretical results agree in many respects with the experimental data as concerns one of the forms of heterolitically dissociated hydrogen which are found at the defective MgO surface. However, from the viewpoint of the energetics, this model is untenable because that species is known to form irreversibly at room temperature with low activation energy.

EPR and ENDOR spectroscopy have been used to investigate a variety of trapped electron centers on the surface of polycrystalline MgO. The oxide was dehydrated under vacuum at different temperatures (673−1123 K) and UV irradiated under H2. The dehydration process results in the formation of surface anion vacancies, which subsequently act as excess electron traps (forming color centers). A variety of such color centers have been identified. At high activation temperatures (1123 K), surface FS⁺(H) color centers (type I) are formed, which have been assigned to an electron trapped by a specific anion vacancy. At slightly lower activation temperatures, a second FS⁺(H) color center (type II) predominates; this center has been assigned to an electron trapped in a higher coordinated surface vacancy. However, at low activation temperatures such that the oxide surface remains partially hydrated, different types of color centers are present. It is proposed that these centers arise from electron trapping at a surface cation−anion vacancy pair (tentatively assigned as PS^- centers). The mechanism by which the latter center is effectively reduced by a single electron is unclear. The distribution and abundance of these different trapped electron centers varies as a function of the dehydration temperature. The results show that the surface of polycrystalline MgO containing a variety of point defects changes dramatically depending on the pretreatment conditions.

The application of IR, Raman, UV–visible, EPR, XANES, EXAFS, and TPR techniques to the determination of the Fe3+local environment in Fe-silicalite is described and discussed. These methods give information concerning the structural changes of the zeolitic structure, upon thermal treatments and interaction with adsorbates. In particular the effect of template burning at 773 K or at 973 K on the local Fe3+environment has been discussed in detail. The reactivity of the zeolite towards NH3has also been considered.

The first steps of CO activation and hydrogenation reactions on Rh catalysts have been studied in order to identify possible intermediates in the reaction pathway to C1compounds. Rh catalysts were prepared by supporting Rh4(CO)12on ZrO2or on ZrO2after adsorption of Mo(CO)6. The Rh content on all catalysts was 1% (w/w) while the Mo/Rh atomic ratio ranged from 0.0 to 2.0. The supported catalysts were decomposed in H2at 523 K (LTR) and at 773 K (HTR). The samples were characterized by TEM, EPR, FTIR, and chemisorption of H2, O2, and CO. The catalytic activity was tested in the CO hydrogenation reaction at 493 K and 101 kPa. After the thermal treatment Mo was present on the surface as Mo(V) and Mo(VI). The total CO conversion increased with Mo/Rh ratio on LTR catalysts while it was unaffected on HTR catalysts. By increasing the Mo content the amount of CO2formed increased significantly. The CO carbon efficiency to methane and total hydrocarbon without CO2, did not depend on the Mo/Rh ratio for LTR catalysts while a constant decrease was observed for HTR catalysts. The carbon efficiency to methanol increased with the Mo content both on LTR and HTR catalysts, and that to ethanol changed with the Mo/Rh ratio and the activation temperature. Mo and Rh form a very complex interacting system: Mo inhibits the sintering of highly dispersed Rh obtained by Rh4(CO)12decomposition, and in the presence of hydrogen Rh promotes the reduction of MoO3with the formation of Mo bronzes. The environment of Rh particles is strongly modified by the presence of Mo as evidenced by modification of the IR spectrum of chemisorbed CO both in the linear and in the bridged Rh-CO region. On the basis of the experimental results the reaction paths to CO2, CH3OH, and CH4, and the effect of MoO3promoter are discussed in detail. The methane formation through different pathways involving a bicarbonate and/or a formate intermediate are presented.

CO and H2 are both chemisorbed by ZnO but H2 is more sensitive than CO to the surroundings of adsorption sites. When the two gases are coadsorbed, a strong interaction occurs, ascribed to the formation of a possible precursor in methanol synthesis.

Successive adsorption-desorption cycles of small doses of H2 on ZnO were followed by adsorption microcalorimetry. A progressive accelerating effect on H2 adsorption as the cycles proceed was evidenced, probably due to small increments in the irreversible species (H(II)) content of the solid.

Fs⁺(H) color centers at the surface of MgO have been studied using a combined EPR and quantum chemical approach. Fs⁺(H) are paramagnetic excess electrons centers where the unpaired electron is trapped in a surface anion vacancy. They are formed at the surface of thoroughly dehydrated MgO (1073K) upon UV irradiation under hydrogen in parallel with the formation of minor fractions of different color centers. The whole EPR spectrum resulting from irradiation has been analyzed by simulation of the experimental profile. Fs⁺(H) centers are characterized by an axial g tensor and display a hyperfine interaction with a hydrogen nucleus (belonging to an hydroxyl group stabilized nearby the vacancy) and two distinct families of²⁵Mg nuclei characterized by a large (10.5 G) and a small (0.7 G) hyperfine coupling constant, respectively. Both EPR and ab initio calculations on clusters of ions converge in indicating that the features of the centers are due to the polarization of the electron density by the positive charge of the hydrogen in the OH group toward two (or possibly three) equivalent magnesium ions of the vacancy close to the OH group itself. The formation mechanism of the centers is strictly analogous to that occurring upon contact of low ionization energy metals with the surface of MgO leading to the formation of another type of color centers. Partially hydrated MgO samples also give rise to another family of paramagnetic center based on electrons trapped in anion vacancies. This finding indicates that the structure of the partially hydroxylated oxide and the mechanism of its dehydration are still open questions.

The reactivity of copper-on-alumina catalysts in the reduction of NO by NH₃ + O₂ (SCR reaction) and oxidation of NH₃ to N₂ (NH₃ox reaction) as a function of the copper loading shows that (i) the specific rate of the SCR reaction per mole of copper decreases with increasing copper loading, whereas the NH₃ox reaction rate is nearly independent of the copper loading and (ii) the rate of the NH₃ox reaction is significantly higher in the absence of NO than that of the same reaction occurring during the SCR reaction. The catalysts were characterized by electron spin resonance (ESR), X-ray diffraction analysis, and UV-visible diffuse reflectance. The nature of the NO and NH₃ adsorbed species was studied using ESR and infrared (IR) spectroscopies, and IR studies of the reactive coadsorption of NO and NH₃ were carried out. The results obtained indicate that the above catalytic effects are due to the formation of nitrate species which block the reactivity of copper sites. The transformation of NO to N₂ via the intermediate formation of ammonium nitrate was shown, but the data suggest that this is only a secondary pathway of reaction responsible for the formation of N₂O as a byproduct. The nature of active copper species on the surface and the presence of competitive pathways of transformation of NO to N₂ are also discussed.Copyright 1995, 1999 Academic Press, Inc.

The r.t. interaction of oxygen with finely-divided copper in Cu/ZnO catalysts has been studied by gas-solid microcalorimetry. The oxidation is accompanied by the presence of a pseudo-equilibrium pressure, which increases as the reaction proceeds. Surface and several subsurface layers are involved, but a metallic core is always left.

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.

Superoxide radical ions were observed at the surface of an amorphous silica active in the catalytic ammoximation of cyclohexanone with molecular oxygen. They were generated by the contact of oxygen with a thermally activated sample. The presence of organic radicals on the spent catalyst, which probably played a role in the catalytic process, was also observed.

V₂O₅-WO₃/TiO₂ samples with compositions similar to those of commercial de-NO_x catalysts (WO₃ ~ 9% w/w, V₂O₅, < 3% w/w) are characterised by XRD, surface area and pore size distribution, Fourier transform infrared, laser Raman, UV-vis diffuse reflectance, electron paramagnetic resonance spectroscopies, and catalytic tests in the reduction of NOx by NH₃. The V₂O₅-WO₃/TiO₂ catalysts exhibit higher reactivity than the binary V₂O₅/TiO₂ and WO₃/TiO₂ samples with the same metal loading, and the temperature window for the SCR reaction is greatly widened. The catalysts consist of anatase TiO₂ and their morphological properties closely resemble that of WO₃/TiO₂ for (V + W) surface coverages lower than one. Monomeric vanadyls and wolframyls and polymeric W_wO_y groups are observed in the samples with low vanadia loadings that are apparently similar to those present on the surface of the binary oxide systems with comparable metal loadings. On increasing the vanadium loading, polyvanadate species are also formed. EPR, FTIR, FT-Raman, and UV-vis techniques indicate a strong electronic interaction between V and W oxide species at the surface of the TiO₂ support. This interaction leads to a higher reducibility of the ternary sample with respect to the corresponding binary ones. A synergism between V and W oxide surface species is suggested, which accounts for the high reactivity of the ternary samples in the SCR reaction. It is suggested that the higher reducibility of the samples, due to the electronic interactions between V and W and the TiO₂ support, is responsible for the higher reactivity of the ternary catalysts, particularly at low temperatures.Copyright 1995, 1999 Academic Press, Inc.

Cu2+ions supported on SiO2(Cm2+/SiO2) prepared by an ion-exchange method are reduced to Cu+when Cu2+/SiO2samples are evacuated at temperatures higher than 573 K Reduced Cu2+ions on SiO2(Cu+/SiO2catalyst) decomposes NO molecules photocatalytically and stoichiometrically into N2and O2at 275 K. The physicochemical and photochemical properties of copper ions anchored onto SiO2have been investigated by means of ESR and dynamic photoluminescence spectroscopies, as well as the analysis of photoreaction products. These results indicate that the excited state of the copper ions (Cu+species) plays a significant role in the photocatalytic decomposition of NO molecules and the photoreaction involves an electron transfer from the excited state of the Cu+ion into an anti-bonding π orbital of NO molecule within the lifetime of its excited state. Thus, the present results obtained with the Cu+/SiO2catalysts imply the possibility of their utilization as a potentially promising type of photocatalysts in gas-solid systems.

An alternative route for the preparation of Na43+ paramagnetic clusters in Y zeolites is described involving contact of the solid with a solution of sodium in liquid ammonia. Besides the EPR signal of the cluster a second signal, likely due to be a solvated electron in the sodalitic cage, is also observed.

The adsorption of NO onto copper-exchanged ZSM-5 zeolite has been studied by EPR spectroscopy. A distinct Cu⁺−NO species possessing an end-on η¹ bent structure has been identified. Its EPR spectrum with a well-defined superhyperfine coupling with^63,65Cu nuclei and hyperfine coupling with¹⁴N was interpreted in terms of completely anisotropic g and hyperfine tensors with noncoincident axes (monoclinic symmetry). The spin Hamiltonian parameters of this adduct have been analyzed in detail, leading to a semiquantitative molecular orbital correlation picture of the complex. The model developed shows that the unpaired electron resides mainly on the angularly coordinated NO and the copper superhyperfine structure arises from delocalization of the unpaired electron density onto Cu a‘ orbitals (3dz^{2, 3dxz, and 4s). The total spin density on copper is found to be equal to 0.2 and is shared among 3dz2 (0.079), 3dxz (0.021), and 4s (0.1) orbitals. The remaining part of the unpaired electron density is localized on nitrogen (0.55) and oxygen (0.25) atoms. The bonding interaction involves essentially an overlap of Cu 3dz2 and 3dxz with an antibonding 2p π* and a lone pair n orbital of the NO ligand as well as a 3dyz overlap with a second orthogonal NO 2p π* orbital. The appropriate molecular orbital diagram has been devised to account for the EPR data. The model is consistent with the observed magnetic properties of the investigated adducts and satisfactorily explains the previously unrecognized complex source of hyperfine couplings. The implications of this coordination mode on the possible molecular mechanism of the catalytic decomposition of NO over Cu/ZSM-5 catalysts are discussed.}

The influence of the hydrogen content and its evolution in the annealing treatments of the structural properties of hydrogenated amorphous silicon carbide (a-SiC:H) prepared by plasma enhanced chemical vapour deposition under different deposition conditions have been studied. The results indicate how strongly hydrogen affects the presence of defects in the amorphous network of the samples.

Various types of surface color centers at the surface of high surface area calcium oxide have been investigated by electron paramagnetic resonance. The defect centers, basically consisting in surface-trapped electrons, were generated either by UV irradiation of the solid in hydrogen (or deuterium) atmosphere or by addition to the oxide of low ionization energy metal (Mg, K) vapors. In the former case Fs(H)⁺ defects have been obtained and characterized by magnetic interaction between the trapped electron and the proton of a nearby surface hydroxyl. In the second case Fs⁺ centers and, in the case of the addition of potassium, a different center labeled Fs(K)⁺ and characterized by interaction of the unpaired electron and a single potassium nucleus have been observed. The reactivity of all centers with oxygen to form adsorbed superoxide radical anions has been also investigated. The structural features of the adsorbed superoxide ion have been determined in the case of the reaction between Fs(H)⁺ centers and O2, again on the basis of the observed magnetic interaction between the adsorbed moiety and a nearby hydoxyl group. The reported data are strictly analogous to earlier data on MgO so that the whole set of experimental results can be generalized and interpreted in terms of the global surface defective properties of the alkaline-earth metal oxides.

The characteristics of Cu ions loaded in different amounts on ZrO2have been studied as a function of the pretreatment conditions (in vacuo, in air, or pure O2, and after room temperature hydration) by EPR, FTIR of CO adsorption (at 77 K and 300 K), and UV-Vis-NIR diffuse reflectance spectroscopy. The present results indicate : (i) two families of CuIIcentres at the surface of oxidized samples; (ii) the easy reducibility of CuIIions to CuIions, and, eventually, to Cu0, especially in the presence of some surface contaminants and/or when starting from highly hydrated materials; (iii) CuIcentres are easily reoxidized to CuIIby the room temperature action of H2O, whereas the reoxidation of Cu0centres requires the action of O2at high temperatures; (iv) the surface situation typical of the starting Cu-ZrO2catalysts can be obtained again from reduced catalysts only by the joint action of O2at high temperatures (leading to CuO nanoparticles) and H2O at ambient temperature (redispersing the CuIIspecies). The stationary and nonstationary catalytic behaviour of Cu-ZrO2systems in the decomposition of N2O at 723 K depends strongly on the pretreatment conditions and can be correlated to the dispersion, oxidation, and hydration state of Cu surface species.

The results of EPR spectroscopy and FTIR spectroscopy of adsorbed CO show that non‐UHV vacuum or O2treatments in the range 650‐1100 K result in the formation of coordinatively unsaturated Zr3+centers at the surface of monoclinic ZrO2.

IR and EPR results of the low‐temp.

The results of EPR and adsorption calorimetry show that, mechanically ground quartz bears surface radicals originating from the homolytic cleavage of Si‐O‐Si bonds and subsequent reaction with atmospheric components.

The adsorption of oxygen on the CoO‐MgO surface is studied by EPR using 17O‐enriched oxygen.