Formation Of Anion Vacancies Research Articles

Calculation of activation energies for defect movement during coincidence boundary migration in nickel oxide using computer simulation techniques

Two possible atomistic mechanisms of grain boundary migration in nickel oxide are examined for two coincidence boundaries using computer simulation techniques. In mechanism I, the boundary migrates by anion vacancy diffusion across it (calculated activation energy 0.64 eV in a (21̄1)/[011] boundary). In mechanism II, ions migrate across the boundary to add onto the opposite crystal lattice without the need for vacancies (activation energy 0.78 eV in a (12̄2)/[011] boundary and control by cation migration). The activation energy for an anion diffusion jump down a (21̄1)/[011] boundary is calculated as 1.57 eV. Anion vacancy supply down the boundary is therefore expected to be rate controlling for any migration mechanism requiring anion vacancies (i.e. mechanism I). There are few literature values for the grain growth activation energy in bulk NiO but values of ~0.9 eV are deduced from published data on grain sizes in NiO scales. 0.9 eV is low in comparison with literature values for a variety of ceramics. Comparison between 0.9 eV and the computer predicted activation energies for defect migration suggests that, unless the effective anion vacancy formation energy at a general grain boundary is considerably lower than expected, it is likely that anion vacancy formation is not required for boundary migration in NiO.

Formation of the active form of [formula omitted] catalyst in olefin metathesis

The formation, structure, and catalytic activity of Re 2 O 7 SiO 2 in olefin metathesis in the lowtemperature range (25–220 °C) has been investigated. It has been established that in precatalysis conditions two different oxide structures are formed. A mixed surface rhenium oxide-perrhenate phase, viz. ReO x · Re VIIO 4, containing Re(VII)( T d, Re(VI) and Re(IV) is formed upon aircalcining. A mesoperrhenate structure, viz. ReO x · Re VIIO 5, is formed on heating the dried samples in an inert gas flow. Electron Spin Resonance studies reveal that the activation (inert gas or vacuum treatment) leads to formation of anionic vacancies (F-centers) in the perrhenate phase. The catalytic activity in 1-butene metathesis has been investigated. The presence of a Re(VI)-mesoperrhenate structure has been found at the conditions of the maximum catalytic activity. The SiO 2 support exerts no stabilizing effect on this structure. Hence, it is unstable to moisture and easily reducible, which causes rapid catalyst deactivation. A complex active form including Re(VI), Re(VII), and an F-center (anionic vacancy) is suggested.

A Calculation of Vacancy Formation Energies in Strontium Oxide

AbstractA calculation of cation and anion vacancy formation energies in SrO is given after assessing three pair potentials, available in the literature, for their suitability. The calculation is done by a Born model including anharmonic potential effects and allows explicit relaxation of host ions about the defect. The vacancy formation energies are used to estimate a Schottky energy for SrO and this is compared with other available data on SrO and also several similarities of the present result with those of MgO found elsewhere are noted.

Structural Chemistry of Co-Mo-Alumnina Catalysts

Abstract The structural chemistry of Co-Mo-alumina hydrodesulfurization catalysts has been critically reviewed in this article. The location and nature of cobalt in the sulfided catalysts has been discussed. Small MoS2, crystallites (10 to 30 Å) that occur on the surface of a sulfided sample are unstable due to the high, net negative charge on the sulfur ions on their edges. They incorporate cations like Co2+ at the edges of the MoS2 sheet (not intercalated between the sheets) and attain stability. It is shown that for the range of crystallito sizes of MoS, in these systems, a maximum of one Co ion can be incorporated per two Mo ions in this manner. Cobalt thus lends structural stability to the MoS2, crystallites and suppresses the excessive formation of anion vacancies at crystallite edges. Molybdenum sulfide crystallites on sulfided Mo-A12O3, lacking this stabilizing influence of cobalt, lose excess sulfur, leading to the creation of a larger concentration of multiple anion vacancies at their edges. The presence of these strongly acidic multiple vacancies, however, leads to a lower structural stability of these crystallites. Moreover, these strongly acidic sites are also deactivated faster in the presence of unsaturated molecules (like diolefins, olefins, etc.), accounting for the fact that the superiority of Co-Mo-Al2O3 over Mo-Al2O3 manifests itself mainly in the steady-state activity [61–63]. The advantages of this model over those currently in vogue are the following: (1) It offers an explanation for the rather high cobalt concentration [Co/(Co + Mo)=0.31 necessary for optimum catalytic activity, (2) it accounts for the established accessibility of cobalt in the Co-MoS2 phase to probe moledules [40], and (3) the stabilization of the small MoS2 crystallites in sulfided Co-Mo-Al2O3 compared to Mo-Al2O3 with the consequent lower loss of sulfur ions from the former in flowing hydrogen (Table 3 and Ref. 28). The migration and sintering of MoS2 crystallites in sulfided Mo-2O3 but not co-Mo-Al2O3, observed by Okamoto et al. [21], is also in accord with the above picture of stabilization of small MoS2 crstallites by cobalt.

Chemical Diffusion in Nickel Oxide

The chemical diffusion coefficient was measured for undoped, single‐crystalline NiO at 900° to 1200°C and within an oxygen partial pressure of 10−5 to 0.21 atm. Electrical conductivity was used to monitor the reequilibration kinetics after the oxygen pressure was suddenly changed over the initially equilibrated NiO crystal. The chemical diffusion coefficient was calculated vs the reequilibration degree indicating the most stable range of investigations. The chemical diffusion coefficient value is virtually the same for the oxidation and reduction experiments, giving, respectively: Dchem.=(1.64 × 10−2) exp[‐(22,480±800)/RT] Dchem.=(9.68 × exp[‐(21,430±2600)/RT] It has been stated that chem is independent of the oxygen pressure and thus of oxide composition. The electrical conductivity depends on the oxygen partial pressure in the power (l/n) = (1/5.45), indicating that doubly ionized cation vacancies are the predominant defects. Deviation from the linear dependence of log α vs logpo2 was observed at <10−5 atm, indicating formation of anion vacancies of interstitials.

Ionic and Electronic Conductivity in Y 2 O 3‐Doped Monoclinic ZrO2

The electronic and ionic conductivities of were determined as a function of temperature, oxygen partial pressure, and trivalent dopant content. The experimental observations were in reasonable agreement with the proposed defect structure model. Ionic conductivity for undoped zirconia was attributed to doubly ionized oxygen interstitials at high oxygen pressures and to doubly ionized oxygen vacancies at the lower extreme of the oxygen partial pressure. Doped zirconia, contrary to the undoped sample, showed maximum ionic transport at intermediate oxygen pressures. The ionic conductivity, due to extrinsic dopant effects, increased by increasing the amount of dopant and its range extended over a wider oxygen partial pressure. The observed decrease in activation energy for ionic conductivity upon doping was attributed to the formation of anion vacancies whose concentration is directly proportional to the dopant content. The 1 mole per cent (m/o) sample showed predominant ionic transport.

Colour centres in electron irradiated mgo

Oxygen ion vacancies, which absorb light at 250 nm, are observed in 'pure' mgo after irradiation with 0.23-29 mev electrons. The threshold energy for anion vacancy formation is observed to occur at 0.33 mev; this corresponds to a displacement energy td=60 ev. Energy independent damage is observed below the threshold energy. Zero phonon lines are observed at wavelengths of 361.5, 1044.5 and 1342.5 nm in crystals irradiated at electron energies greater than 1.6 mev. Optical bleaching experiments suggest that these lines are associated with trapped electron centres. The energy dependence of the growth rate of the 1044.5 nm line confirms that intrinsic defect aggregates are involved, anion divacancies being the most probable.

Studies on Densification Behaviour and Phase Changes in TiO2-ZrO2and TiO2-Al2O3Composites Fired under Oxidizing and Reducing Atmospheres

X-ray diffraction studies and densification measurements were carried out on three compositions, 75% TiO2+25% ZrO2, 75% TiO2+25% Al2O3, and 100% TiO2, designated as TZ, TA and T respectively, heated under oxidizing and reducing atmospheres between 1200°C and 1400°C. The densification was always higher and the formation of the new phases took place with greater ease when the specimens were fired under reducing conditions. This was possible due to the formation of anionic vacancies in the TiO2 structure, which would enhance diffusion.

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.

The catalytic oxidation of 1-butene over bismuth molybdate catalystsII. Dependence of activity and selectivity on the catalyst composition

The dependence on the composition of the oxide catalyst of the rate and selectivity of the oxidative dehydrogenation of 1-butene to butadiene over Bi2O3 MoO3 catalysts has been investigated. The rate appears to be mainly determined by the Bi/Mo ratio and is optimal at Bi/Mo=1. This optimal composition is not connected with the presence of a particular compound in the binary oxide system: all compounds identified so far are either less active or not active at all. The activity pattern is complicated by a sintering reaction that differs in intensity in various ranges of the composition. The selectivity is mainly decided by the occurrence of the double-bond isomerization. This is more developed at lower temperatures and in the MoO3-rich compositions. It is considerably inhibited by a high-temperature pretreatment of the catalyst. The promoter action of Bi3+ is explained by the assumption of an enhanced tendency to dehydroxylation of the surface, thereby leading to the formation of anion vacancies that serve to accept the intermediate allylic structure. Formation of this structure is further assumed to be possible only if it becomes π bonded to Mo, thus explaining the advantage of the simultaneous presence of Bi and Mo.

Zwischengitterionen als wirksame störstellen in wärmeleitungsexperimenten an KCl- und KBr-einkristallen

A description of an experimental chamber is given in which optical absorption and thermal conductivity measurements, as well as U.V.- and X-irradiations, were performed in the temperature range from 1.8° to 22°K. With this arrangement, two photochemical reactions in KBr single crystals were investigated: (1) X-irradiation at 5°K produces essentially anion vacancies and probably interstitial ions as “anticenters”; (2) U.V.-irradiation of the system KBr+KH in the spectral range of the U-band bleaches this absorption and causes the formation of anion vacancies and the occupation of interstitial sites by H −-ions. With the first reaction, a strong increase of thermal resistivity was obtained which may be understood as an effect of phonon scattering by interstitial ions The order of magnitude of the phonon scattering cross-section was estimated to be 10 −15 cm 2 at 3.3°K. The thermal conductivity is still sensitive to a defect concentration of c ⩾ 10 −7. The stability of these interstitial ions was tested by annealing. The effects due to optical bleaching of the U-band are one order of magnitude smaller than the effects caused by X-irradiation of the pure crystals. Similar results for both reactions were obtained from KCl-single crystals. The temperature dependence of thermal resistivity of irradiated crystals deviates from that expected from a pure Rayleigh scattering.