Electronic Structure of Oxygen Radicals on the Surface of VO_<i>x</i>/TiO₂ Catalysts and Their Role in Oxygen Isotopic Exchange

The electronic structure of oxygen radicals formed by adsorption of gas-phase oxygen on partially reduced sites of supported vanadium oxide catalyst V⁴⁺O_<i>x</i>/TiO₂ has been studied by periodic DFT. The unpaired electron density in the radicals is transferred from the paramagnetic V⁴⁺(3d¹) ion to the adsorbed oxygen atoms resulting in the formation of surface oxygen radicals: atomic O^–, superoxide O₂^–, and ozonide O₃^–. These radical species exhibit higher reactivity compared to the surface oxygen species stabilized on fully oxidized diamagnetic V⁵⁺(3d⁰) ions. Oxygen isotopic exchange over O^– radicals has been investigated by the climbing image nudged elastic band (CI-NEB) method. We show that molecular oxygen can exchange with the lattice oxygen of the surface paramagnetic radicals V⁵⁺O^– with low activation energy of about 14 kcal/mol, close to the value experimentally observed for some heterolytic R1 oxygen exchange reactions on vanadia catalysts. The obtained data suggest that O^– radicals formed as short-lived intermediates at elevated temperatures are likely to be the active sites of the oxygen exchange following the R1 mechanism. The properties of oxygen radicals and their possible role in catalytic oxidation processes taking place over bulk and supported metal oxide catalysts are discussed. It is suggested that oxygen radicals can be the active species in catalytic oxidation reactions

Molecular Mechanism of Oxygen Isotopic Exchange over Supported Vanadium Oxide Catalyst VO_<i>x</i>/TiO₂

Detailed molecular mechanisms of oxygen isotopic exchange over VO_<i>x</i>/TiO₂ catalyst following the R₀, R₁, and R₂ mechanisms were studied using periodic DFT analysis of possible pathways by the CI-NEB method. The electronic structures of surface VO_<i>x</i> species formed on the VO_<i>x</i>/TiO₂ model surface after interaction of molecular oxygen with fully oxidized OV⁵⁺–O–V⁵⁺O sites and reduced V³⁺–O–V³⁺ sites were analyzed. We found a number of metastable surface structures that are potential intermediates in the exchange reaction pathways. We present evidence that adsorption of two gas-phase oxygen molecules on a reduced V³⁺–O-V³⁺ site leads to the formation of a superoxide complex, followed by its transformation into a peroxide complex with low activation energy about <i>E</i>* = 0.04 eV (0.92 kcal/mol). Subsequent transformation of this surface superoxide-peroxide species follows the Langmuir–Hinshelwood mechanism without participation of lattice oxygen along the R₀ reaction pathway. We demonstrate that adsorption of molecular oxygen on fully oxidized OV⁵⁺–O–V⁵⁺O sites results in the formation of either monodentate V<(O₃) or bidentate V<(O₃)>V surface ozonide species. Their subsequent transformations result in oxygen isotopic exchange following the R₁ or R₂ mechanisms with the activation energies in the range of 1.44 to 1.64 eV for the R₁ mechanism and 1.81 eV for the R₂ one. These processes follow the Eley–Rideal mechanism with participation of one or two lattice oxygen atoms, correspondingly

Study of MgO transformation into MgF2 in the presence of CF2Cl2•

Alkaline-earth metal oxide aerogels prepared by sol–gel method followed by autoclave drying are nanocrystalline mesoporous materials with high reactivity. Bulk solid-state reaction of MgO aerogels with CF2Cl2 takes place after a long induction period, during which the active sites are accumulated on the surface of the nanoparticles. It was found that vanadium addition has a promoting effect on this reaction accelerating the process of the active sites formation. A method for characterization of electron-acceptor sites by electron spin resonance spectroscopy using perylene as the spin probe was developed. A good correlation was observed between the rate of the CF2Cl2 destructive sorption and the concentration of weak electron-acceptor sites. Simplified models of such sites were suggested. The acid sites on the modified MgO surface were supposed to be originated from separation of the charged fragments resulting in the surface polarization. Uncompensated oxygen substitution for chlorine and/or fluorine ions leads to appearance of Lewis acid sites while HCl/HF chemisorption results in Bronsted acid sites formation

Exploration of Optical, Redox, and Catalytic Properties of Vanadia-Mayenite Nanocomposites

The present paper continues the exploration of the physicochemical and catalytic properties of vanadia-mayenite composites. The samples were prepared by an impregnation of calcium aluminate Ca12Al14O33 (mayenite, C12A7) with a solution of vanadium precursor. Pure mayenite and V/C12A7 nanocomposites were characterized by Raman and diffuse reflectance UV–Vis spectroscopies. The reducibility of the samples was examined in temperature-programmed reduction experiments performed in a hydrogen atmosphere. The catalytic performance of vanadium-containing systems was studied in the non-oxidative dehydrogenation of ethane. As found, the low-loaded sample (5%V/C12A7 sample) contains vanadium predominantly in the form of Ca3(VO4)2, while for the 10%V/C12A7 sample, two types of calcium vanadates (Ca2V2O7 and Ca3(VO4)2) are registered. The presence of these phases defines the spectroscopic characteristics and the redox properties of nanocomposites. Both the samples, 5%V/C12A7 and 10%V/C12A7, exhibit comparable catalytic activity, which is mainly connected with the amount of the Ca3(VO4)2 phase. The uniqueness of the studied catalysts is their excellent tolerance toward coke formation under the reaction conditions

Evaluation of the air-dry mass of vetch-cereal grass mixtures according to their optimality in the ratio of neutral-detergent (NDF) and acid-detergent fiber (ADF)

On the experimental field an experiment was laid to study the productivity of two types of winter vetch: Hungarian vetch (Vicia pannonica Granz) varieties Orlan and Chernomorskaya and downy vetch (Vicia villosa op Roth) varieties Lugovskaya 2 and Glinkovskaya. The vetch was sown in a mixture with winter wheat (Triticum aestivum L.) variety Tanya. Various doses of mineral fertilizers. Phosphorus (superphosphate) and potassium (potassium chloride) were introduced during sowing, nitrogen (calcium nitrate) – as early spring feeding. As a control, we studied the variety of furry vetch Lugovskaya 2, since this is the most popular variety of winter vetch in the Krasnodar Territory. The soils of the experimental plots are represented by leached, low-humus heavy loamy powerful chernozem. Due to the limited feed intake, the diets of high-yielding cows are rich in highly digestible non-structural carbohydrates (ADF), which fermentation leads to the formation of a large amount of propionic acid in the rumen with a strong acidic effect and, conversely, NDF fermentation is slower and on a smaller scale. The variant winter wheat Tanya + winter vetch Lugovskaya 2 - is closer to the optimum in the second cut. The rest of the experimental options of vetchwheat and vetch-triticale grass mixtures also have a negative fiber balance, which is not acceptable for feeding high-producing dairy cattle