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

Molecular Mechanism of the Formic Acid Decomposition on V₂O₅/TiO₂ Catalysts: A Periodic DFT Analysis

Molecular and dissociative forms of formic acid adsorption on the V₂O₅/TiO₂ model surface, possible intermediates, and transition states along of the dehydrogenation (HCOOH → CO₂ + H₂) and dehydration (HCOOH → CO + H₂O) pathways have been studied by the periodic density functional theory. The CI-NEB analysis of the reaction pathways showed that two types of molecular adsorbed HCOOH species initiate two completely different reaction channels. The first more stable adsorbed form is transformed into the surface formates, which decompose according to the “formate mechanism” to yield products of dehydrogenation, whereas the second weakly adsorbed molecular form decomposes, releasing CO and forming surface hydroxyls. Recombination of two surface hydroxyl groups V–OH to form adsorbed H₂O, followed by water desorption, completes the catalytic dehydration cycle without participation of the formate species. Comparison of the reaction pathways demonstrates that both dehydrogenation and dehydration of formic acid may occur over VO_<i>x</i>/TiO₂ model catalysts with the preferable dehydration pathway