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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.

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 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.}