Unveiling the Structural and Electronic Properties of the B-Nb₂O₅ Surfaces and Their Interaction with H₂O and H₂O₂
2018
Pinto, Mirele B. | Soares, Antonio Lenito | Quintão, Matheus Campos | Duarte, Hélio A. | De Abreu, Heitor A.
Niobium pentoxide (Nb₂O₅), also known as niobia, has been applied in several areas among others in heterogeneous catalysis. This is due to both its high acidity (Brönsted acid and Lewis acid sites) and its Lewis acid sites tolerant to water. The structure and morphology of these sites present tunable quantity and strength; however, little attention has been given to its polymorphic forms and reactivity. In this work, the surface properties of stoichiometric B phase (B-Nb₂O₅), including the cleavage surfaces, structural, energetic, and electronic properties, and chemical reactivity toward water (H₂O) and hydrogen peroxide (H₂O₂), by means of periodic density functional theory (DFT), have been studied through DFT calculations. An initial investigation was carried out to determine cleavage surface of the B-Nb₂O₅. Our results show that the B-Nb₂O₅ (010)-2 surface is the most stable (surface energy 0.52 J m–²) of the surfaces studied. Projected density of state (PDOS) analysis showed that the niobium atom is a Lewis acid site. When H₂O was adsorbed on the (010)-2 surface, the molecular adsorption was the most stable under Nb site. However, the results showed that both dissociative and molecular mechanisms must be present on the surface, although the dissociative one to a lesser extent. When H₂O₂ was adsorbed on the (010)-2 surface, the calculated adsorption energies showed that the preferred site for H₂O₂ adsorption is the Nb, with adsorption energy of 1.63 eV, which resulted in the formation of a hydrosuperoxo (HO₂ –) species. However, the HO₂ –, O₂ ²–, and H₂O₂ species may exist in equilibrium on the (010)-2 surface due to small difference between their adsorption energies (up to 0.14 eV).
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