Structural, Electronic, and Thermodynamic Properties of the T and B Phases of Niobia: First-Principle Calculations

Abstract

Different polymorphs of Nb2O5 can be obtained depending on the pressure and temperature of calcination leading to different catalytic properties. Two polymorphs of niobia, T-Nb2O5 and B-Nb2O5, have been investigated by means of density functional/plane waves method. The equation of state predicted that B-Nb2O5 phase is more stable than the T-Nb2O5 at low temperature; however at high pressure both phases are stable. These results are in good agreement with the available experimental data. The calculated cohesive energies of 6.63 and 6.59 eV·atom–1 for the B-Nb2O5 and T-Nb2O5, respectively, also corroborate this conclusion, and it can be compared to the experimental value of 9.56 eV atom–1 estimated for the most thermodynamically stable phase. The topological analyses based on quantum theory of atoms in molecules (QTAIM) and electron localization function (ELF) were applied and reveal bonds with large ionic character for both phases. The B-Nb2O5 presented larger stiffness than T-Nb2O5, and the oxygen sites in the T-Nb2O5 are more compressible. The density of states comparison for both structures indicates that B-Nb2O5 has lower concentration of acid sites compared to T-Nb2O5. This result is consistent with the experimental observations that the concentration of Lewis acid sites decreases with the temperature.

Design of Nickel Supported on Water-Tolerant Nb2O5 Catalysts for the Hydrotreating of Lignin Streams Obtained from Lignin-First Biorefining

“…Generally, amorphous Nb 2 O 5 shows high surface acidity, which is related to its high specific surface area and number of surface defects (do Prado and Oliveira, 2017, Ziolek and Sobczak, 2017). However, amorphous Nb 2 O 5 is a fragile material, susceptible to changes by temperature and pressure (Pinto et al., 2017, Wojcieszak et al., 2006). Considering Nb 2 O 5 crystalline phases, they are formed by distorted octahedra (NbO 6 ), connected by edges and corners.…”

“…Considering Nb 2 O 5 crystalline phases, they are formed by distorted octahedra (NbO 6 ), connected by edges and corners. The distortion degree of NbO 6 octahedra depends on the polymorph structure (Nico et al., 2016, Pinto et al., 2017, Valencia-Balvín et al., 2014). This distortion leads to varied textural and structural stabilities as well as different surface acid properties and, therefore, impacts on catalytic properties.…”

Dual redox mediators accelerate the electrochemical kinetics of lithium-sulfur batteries

“…As shown in Fig. 2a, orthorhombic Nb 2 O 5 presents a unit cell with NbO 6 octahedra and NbO 7 pentagonal bipyramids connecting each other by sharing edges or vertexes 38,39 All the niobium cations are stacked layer by layer in the [001] direction forming a layered structure and the interlayer space is serving as fast Li + transport channel and storage space 38 . After lithiation, lithium cations occupy the interstices of NbO x polyhedrons (Fig.…”

Lewis Acid Catalysis of Nb2O5 for Reactions of Carboxylic Acid Derivatives in the Presence of Basic Inhibitors

“…According to the calcination temperature, the crystalline phases can be subdivided into the T and TT phases, which crystallize at low temperatures (400-600°C), the B and M phases, which crystallize at medium temperatures (600-900°C), and the H phase, which crystallizes at high temperature (> 1000°C). [74] Historically, the T-Nb 2 O 5 , M-Nb 2 O 5 , and H-Nb 2 O 5 phases were named according to the temperatures (low, medium, and high, respectively) at which they are obtained. [68] A diagram for the phase transitions of Nb 2 O 5 as a function of temperature is shown in Figure 1.…”

“…Nb 2 O 5 with these different structures can be synthesized by varying the calcination temperature, and the polymorphs are classified based on the temperature by which they have been obtained starting from amorphous niobium oxide. According to the calcination temperature, the crystalline phases can be subdivided into the T and TT phases, which crystallize at low temperatures (400–600 °C), the B and M phases, which crystallize at medium temperatures (600–900 °C), and the H phase, which crystallizes at high temperature (>1000 °C) . Historically, the T‐Nb 2 O 5 , M‐Nb 2 O 5 , and H‐Nb 2 O 5 phases were named according to the temperatures (low, medium, and high, respectively) at which they are obtained .…”