Surface Anion Vacancies Research Articles

Embedded-Cluster Study of Hydrogen Interaction with an Oxygen Vacancy at the Magnesium Oxide Surface

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.

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

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 3H), 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.

H2 chemisorption and consecutive UV stimulated surface reactions on nanostructured MgO

MgO nanoparticles obtained by chemical vapour deposition (CVD) were exposed to H2 and subsequently to UV irradiation and/or molecular oxygen at room temperature. A combined IR/EPR study reveals the role of low coordinated surface sites and anion vacancies in the diverse reaction steps. The hydride groups emerging from the initial H2 chemisorption processes (heterolytic splitting) play an active role in the consecutive reactions. They provide the electrons which are required for the UV induced formation of surface colour centres and for the production of superoxide anions (redox reaction). Both the colour centres and the superoxide anions are EPR active. The hydroxy groups resulting from H2 chemisorption do not actively participate in the consecutive reactions. Together with the OH groups formed in the course of colour centre formation they rather play the role of an observer. They undergo specific electronic interactions with both the colour centre and the superoxide anion which are IR inactive (or IR inaccessible) surface species. They may, however, be observed by IR spectroscopy via the specifically influenced OH stretching vibrations. This proves the intimate interplay between IR and EPR spectroscopy as applied to the surface processes under investigation. As a result, two paths were found for the three consecutive surface reaction steps: H2 chemisorption, colour centre formation and superoxide anion formation. In the first one a single, well defined surface area element is involved, namely a low coordinated ion pair, the cation of which is a constituent of an anion vacancy. In the second path a diffusion controlled intermediate step has to be adopted in which the electron required for the colour centre is transported by an H atom travelling from a hydride group to a remote anion vacancy. In either case there is clear experimental evidence that the finally resulting superoxide anions are complexed by the colour centre cations.

A Combined EPR and Quantum Chemical Approach to the Structure of Surface Fs+(H) Centers on MgO

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 of 25Mg 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.

Localization of excess electrons in cubic NanClm clusters.

Localization of excess electrons in sodium chloride clusters is studied using first-principles calculations based on the local-density approximation. In clusters with a rocksaltlike structure containing an anion vacancy, an excess electron was found to localize on an anion vacancy site, in analogy to the bulk F center, stabilizing the structure. Approximately the same degree of localization was found for surface, corner, and interior anion vacancies. For clusters without an anion vacancy, such as ${\mathrm{Na}}_{14}$${\mathrm{Cl}}_{13}$, the excess electron is found to be delocalized over the cluster surface. In contrast to previous theoretical predictions, no tendency is found for the excess electron to localize at a single sodium site, and localization-induced dissociation of a neutral alkali-metal atom is not expected to occur for an isolated cluster.

Surface species in CO 2 methanation over amorphous palladium/zirconia catalysts

The mechanism of methane formation over a palladium/zirconia catalyst prepared by wet impregnation of amorphous ZrO 2 has been investigated by diffuse reflectance FTIR spectroscopy. The production rate of surface formate, which represents the pivotal surface intermediate, is high when starting from gaseous CO 2 and H 2; a considerably lower rate is observed upon hydrogen exposure of a carbonate-covered catalyst surface. Reference compounds have been adsorbed to determine the CO stretching frequency of a potential formyl (HCO) surface intermediate, which apparently gives rise to a broad absorption around 1700 cm −1. This band is not observed under CO 2 hydrogenation conditions. Instead, a characteristic absorption is detected at 1760 cm −1, which is assigned to CO triply bound to the palladium particles. The latter species is correlated with surface formate, in an equilibrium that involves surface hydroxyl groups on the zirconia. No further surface species are detected under CO 2 hydrogenation conditions. A mechanism for the hydrogenation Of CO 2 to methane is proposed that accounts for these observations; the importance of surface hydroxyl groups and anion vacancies on the zirconia surface is emphasized.

Characterization of tungsten sulfide catalysts

Unsupported tungsten sulfide catalysts of high surface area (60 m 2/g) were prepared by in situ decomposition of ammonium tetrathiotungstate (ATT) in flowing helium at 450 °C. This catalyst (stoichiometry approx WS 2.2) was subjected to various pretreatments in both hydrogen and a mixture of 15% hydrogen sulfide in hydrogen at elevated temperatures (up to 450 °C). Temperature-programmed reduction (TPR) in hydrogen showed two peaks, indicating the presence of two species of sulfur with different binding energies. Surface areas and pore-size distributions of the catalysts were found to be relatively unaffected by treatment in hydrogen at 450 °C for 1 h, but treatment for 12 h caused the surface area to decrease from 60 to 35 m 2/g. Examination of catalyst samples under a scanning electron microscope showed that the large number of microcracks developed on decomposition of ATT were considerably reduced by a 12-h treatment in hydrogen at 450 °C. Low-temperature oxygen chemisorption (LTOC) was measured on tungsten sulfide subjected to pretreatment in hydrogen for different times and temperatures. Samples with the greatest sulfur loss showed the highest LTOC, suggesting that oxygen chemisorbs at surface anion vacancies (coordinatively unsaturated tungsten ions). Appreciable LTOC was generated only upon removal of sulfur corresponding to the second peak. The ratio of surface area to LTOC for a sample treated for 12 h in hydrogen led to a value of 170 A ̊ 2 O 2 molecule ( 85 A ̊ 2 O atom ). Catalyst activity for propylene hydrogenation at 100 and 150 °C was measured using a pulsed-flow microcatalytic reactor. Activity was found to increase with increasing LTOC.

Chemisorption/catalytic activity correlations for sulphided Ni/Mo/Al 2O 3 hydrodesulphurisation catalysts

Catalysts containing various amounts of Ni and Mo have been prepared by wet impregnation of γ-Al 2O 3. The activity of each catalyst for the hydrodesulphurisation of thiophene has been measured in a flow microreactor and compared with the activity of seven commercially available industrial catalysts. The sulphided catalysts have been characterized by determining the amount of O 2 adsorbed in pulse experiments at ambient temperature, by measuring the amount of O 2 adsorbed irreversibly in static experiments at 195 K, by measuring the amount of H 2S adsorbed irreversibly in static experiments at 650 K, and by determining the total amount of CO adsorbed at ambient temperature in static experiments. The results show that the quantity of O 2 adsorbed in pulse experiments does not reach a limit but increases continuously. It is concluded that the adsorption of O 2 under these conditions does not occur by attachment to a unique type of surface anion vacancy site. The amount of O 2 adsorbed at 195 K is much less than at ambient temperature and the low temperature experiments appear to provide a more reliable measure of the dispersion of the sulphide phase. For all Mo-containing catalysts the amount of O 2 adsorbed irreversibly at 195 K, the amount of H 2S adsorbed irreversibly at 650 K, and the amount of CO adsorbed at ambient temperature are comparable. It is suggested, therefore, that the adsorption site is the same for each adsorbate. From the stoichiometry of adsorption a model is developed in which it is proposed that there is 1 adsorption site for every 33 Mo atoms present. This adsorption site could correspond to the active site in the thiophene hydrodesulphurisation reaction. All attempts to correlate for the industrial catalysts the chemisorption data with the HDS activities have proved unsuccessful. It is concluded that similar correlations reported for unpromoted Mo/Al 2O 3 or Ni/Al 2O 3 catalysts do not apply in general to Ni/Mo/Al 2O 3 catalysts. Only in the case of a family of closely related catalysts were reasonable correlations obtained.

Interactions of acetone with MoO 3

Interactions of acetone and acetone/oxygen mixtures with MoO 3 were studied at 215–300 °C by pulse method. It has been shown that acetone is deoxygenated to propylene or oxidized to acetic acid, CO, and CO 2. Moreover a part of acetone is reversibly adsorbed and several minutes are necessary for acetone to be removed with a flow of carrier gas. As the surface is endowed with deoxygenating and oxidizing sites the desorbing acetone leaves the reactor mainly in the form of CO. These results agree with theoretical predictions based on the bond-strength model of active sites, according to which the sites of the first type (conversion of acetone) are localized on (100) face of MoO 3 and those of the second type (adsorption) on (001) and (101) planes. Both types of sites consist of surface anion vacancies and differ in the strength of bonding of oxygen. On the (100) face the bond strength is sufficiently large to break CO bond in acetone. On the other faces the interactions between acetone and surface are much weaker. In this context the application of acetone as poison of other reactions (e.g., conversion of methanol) is discussed.

Hydrodesulfurization of thiophene, benzothiophene, dibenzothiophene, and related compounds catalyzed by sulfided CoO$z.sbnd;MoO3/$gamma;-Al2O3: Low-pressure reactivity studies

Hydrodesulfurization experiments were carried out with a sulfided CoOMoO3γ-Al2O3 catalyst in a pulse microreactor operated at atmospheric pressure and temperatures of 350 to 450 °C. The reactants were hydrogen and pure sulfur-containing compounds (or pairs of compounds), including thiophene, benzothiophene, dibenzothiophene, several of their hydrogenated derivatives, and various methyl-substituted benzothiophenes and dibenzothiophenes. The aromatic compounds appeared to react with hydrogen by simple sulfur extrusion; for example, dibenzothiophene gave H2S + biphenyl in the absence of side products. The reactivities of thiophene, benzothiophene, and dibenzothiophene were roughly the same. Each hydrogenated compound (e.g., tetrahydrothiophene) was more reactive than the corresponding aromatic compound (e.g., thiophene). Methyl substituents on benzothiophene had almost no effect on reactivity, whereas methyl substituents on dibenzothiophene located at a distance from the S atom slightly increased the reactivity, and those in the 4-position or in the 4- and 6-positions significantly decreased the reactivity. In contrast to the observation of a near lack of dependence of low-pressure reactivity on the number of rings in the reactant, the literature shows that at high pressures the reactivity decreases with an increased number of rings. The pressure dependence of the structure-reactivity pattern is suggested to be an indication of relatively less surface coverage by the intrinsically more reactive compounds (e.g., thiophene) at low pressures but not at high pressures. The relative reactivities are also suggested to be influenced by differences in the structures of the catalyst at low and high hydrogen partial pressures, which may be related to the concentrations of surface anion vacancies and the nature of the adsorbed intermediates.

Electronic structure of the surface F centre in ionic crystals

Theoretical calculations for the F s centre (electron trapped at a surface anion vacancy) on the (001) surface of ionic crystals of the NaCl structure are presented. The one electron Schrödinger equation is solved numerically for the ground state and crystal field split excited state for various crystal potentials. The differences between the point ion potential and model potentials incorporating core repulsion are investigated, and surface relaxation, rumpling and electronic polarisation are also considered. Hyperfine constants for the ground state of MgO are in reasonable accord with experiment, but calculated transition energies do not seem to correspond to those observed in reflectance measurements, in accordance with Hartree-Fock cluster calculations on the same system.

Temperature effects on the hyperfine coupling of a surface centre

The ESR spectra of an electron trapped at a surface anion vacancy (F + centre) show temperature dependent hyperfine splitting constants. This temperature dependence is proportional to the absolute temperature and is much larger for surface centres than the equivalent bulk centres. The effect of lattice vibrations at the surface is discussed and the change in the hfs-constants is explained in terms of the variation of the mean square displacements of surface ions with temperature.

Electronic spectra of the surfaces of alkaline earth oxides

The surfaces of fine powder samples of MgO, CaO and SrO have been characterized by the diffuse reflectance technique over the wavelength range 200–2000 nm. Spectra due to F+s centres (electron in surface anion vacancy), SH centres (F+s centre with nearby —OH group) and adsorbed O–2 species have been identified. In powders which have been freed from surface contaminants fluorescences exist which can be quenched by O2, H2O or CO2. These fluorescences are interpreted as due to the presence of surface states which create an “effective” band gap at the oxide surfaces at significantly lower energies than the band gaps typical of the bulk oxides.

Paramagnetic defects associated with hydrogen adsorbed on the surface of magnesium and calcium oxides

The irradiation of MgO and CaO in the form of small particles (100–300Å) leads to the formation of surface paramagnetic centers (S centers) which will react with oxygen. Both S centers and new surface centers (S H centers) with g = 1.9998 are formed if the oxides are γ-irradiated in the presence of hydrogen or deuterium. The S H center shows a hyperfine coupling from one hydrogen nucleus, and both the S and S H centers show a hyperfine interaction with the Mg 25 nuclei of the lattice to give splitting constants of 25.5 and 10.2 gauss, respectively. These surface centers are believed to be intrinsic defects consisting of an electron trapped at a surface anion vacancy and provide a surface equivalent of the well-known bulk defects (F centers). The nearby proton in the S H center suggests that the hydrogen must react with surface oxygen ions during irradiation.

Chemisorption on some alkaline earth oxides. Part 1.—Surface centres and fast irreversible oxygen adsorption on irradiated MgO, CaO and SrO

A fast irreversible adsorption of oxygen at 20°C on γ- or neutron-irradiated MgO or CaO is correlated with observable surface paramagnetic centres (S centres) and colour centres. The S centre in MgO with gav= 2.0007 is believed to be an intrinsic defect consisting of an electron trapped at a surface anion vacancy. An additional oxygen-sensitive signal SH is related to the presence of some hydrogen contamination on the surface. The S centre in CaO, with g∥= 1.9992 and g⊥= 1.9969 after γ-irradiation to low doses, is similar in behaviour to that on MgO; however, at high γ-doses or after neutron irradiation a complex signal is produced. A surface centre with gav= 1.9798 and of low intensity has been detected on γ- and neutron-irradiated SrO, but this accounts for only a small fraction of the total oxygen adsorption on SrO. The formation of anion vacancies at the surfaces of unirradiated and irradiated oxides is discussed. These defects play a dual role in the irreversible adsorption of oxygen by providing a trap for an electron at the surface and a site capable of stabilizing the adsorbed oxygen species; other sites for oxygen adsorption are increasingly important in CaO and SrO and account for adsorption on the unirradiated oxides.