Nb OH Research Articles

Regeneration of alkali poisoned TiO2-based catalyst by various acids in NO selective catalytic reduction with NH3

The K2O poisoned CuNbTi as a model catalyst (K-CuNbTi) was regenerated by various type and strength acids, among which H2SO4, C2H2O4 and H6O39SiW12 exhibited higher regeneration efficiencies. In contrast to H2O treatment, most potassium depositing on the surface of K-CuNbTi could be removed successfully for the acids regenerated catalysts. All the regenerated catalysts showed above 90% NOx conversion and 98% N2 selectivity in the temperature range of 300–350 °C under a high GHSV of 177,000 h−1. And the regenerated catalysts were characterized by different analytic techniques. Both of the surface chemical active oxygen species and the Cu+ species were obviously improved while KNbO3 was transformed into Nb = O and Nb–OH after regeneration process. Furthermore, the quantity and strength of surface acidity were enhanced and more acid sites were generated, especially Brønsted acid sites. The main acid sites of K-CuNbTi was Lewis acid sites while Brønsted acid sites became the dominating acid sites for the regenerated K-CuNbTi catalysts. Also, acid regeneration process promoted –NH2 active species formation on the catalyst. All above induced a better catalytic activity of regenerated catalyst, indicating that oxalic acid and silicotungstic acid could be the potential substitutes for sulfuric acid to regenerate alkali poisoning SCR catalysts.

Consideration on modeling of Nb sorption onto clay minerals

Abstract Sorption distribution coefficient (Kd) of niobium-94 on minerals are an important parameter in safety assessment of intermediate-depth disposal of waste from core internals etc. The Kd of Nb on clay minerals in Ca(ClO4)2 solutions were, however, not successfully modeled in a previous study. The high distribution coefficients of Nb on illite in Ca(ClO4)2 solutions were successfully reproduced by taking Ca–Nb–OH surface species into account. Solubility of Nb was studied in Ca(ClO4)2 solutions and the results were reproduced by taking an aqueous Ca–Nb–OH complex species, CaNb(OH)6 +, into account in addition to previously reported Nb(OH)6 − and Nb(OH)7 2−. Based on this aqueous speciation model, the Ca–Nb–OH surface species responsible for the sorption of Nb on illite in Ca(ClO4)2 solutions was presumed to be X_OCaNb(OH)6. Although uncertainties exist in the speciation of aqueous Ca–Nb–OH species, the result of this study proposed a possible mechanism for high distribution coefficient of Nb on illite in Ca(ClO4)2 solutions. The mechanism includes Ca–Nb–OH complex formation in aqueous, solid and surface phases.

Synthesis, characterization and correlation with the catalytic activity of efficient mesoporous niobia and mesoporous niobia–zirconia mixed oxide catalyst system

Mesoporous mixed oxide has always remained as a subject of interest due to diffusion relaxation and generation of new surface potential and acidity by mixing of different stoichiometries of metal oxide. To deliver more insight on niobia based catalyst, herein, we have synthesized mesoporous niobia and five different mole ratios of niobia–zirconia mixed oxide. The catalysts were characterized by XRD, N2-physisorption, FTIR, UV–Vis and NH3-TPD techniques. Less distorted NbO6 octahedra and NbOH or Nb–OH–Zr are found to act as Bronsted center responsible for reaction. The results established the role of Kung model to generate acidic sites in mesoporous mixed oxide.

Niobate nanoscroll composite with Fe2O3particles under moderate conditions: assembly and application research

Fe2O3nanoparticles are crosslinked to the niobate nanoscroll surface by association with terminal Nb–OH groups.

Group V metal substitution in silicate model zeolites: In search for the active site

Group V substituted zeolites were characterized using periodic DFT. It is found that the group V sites in zeolite materials become more hydrophobic and less reducible descending the column of the Period Table. The stable site with T O group for T = V, transforms into a site with T–OH group for T = Ta. For niobium both sites (Nb–OH and Nb O) are in competition with each other. The presence of Lewis acid base pairs is predicted in Nb and Ta dehydrated zeolites.

Local order structure and surface acidity properties of a Nb 2O 5/SiO 2 mixed oxide prepared by the sol–gel processing method

The sol–gel processing method was used as an alternative route to obtain Nb 2O 5 phase homogenously dispersed in the SiO 2 matrix, improving the thermal stability of the Brønsted acid sites, Nb–OH and Nb–OH–Si groups. The local niobium structure and the influence of the amount of niobia on the surface of the Nb 2O 5/SiO 2 system were studied by XAS and XPS, respectively. For the samples calcined at 423 and 873 K, the 3 d 5/2 BE values are at ca. 208.2 eV, indicating an ionic character for Nb(V) species in the SiO 2 matrix, probably associated to Si–O–Nb linkages. The features of Nb K-edge XANES spectra of samples show the absence of NbO species. The Nb K-edge EXAFS oscillations exhibit a shoulder at ca. 5.6 Å −1, which probably arises from Nb–O–Si. This fact corroborates the EXAFS simulation data of the second coordination shell, whose best fitting is achieved with three distances, two Nb–Nb lengths and one Nb–Si.

Local order structure and surface acidity properties of a Nb2O5/SiO2 mixed oxide prepared by the sol?gel processing method

The sol–gel processing method was used as an alternative route to obtain Nb 2 O 5 phase homogenously dispersed in the SiO 2 matrix, improving the thermal stability of the Brønsted acid sites, Nb–OH and Nb–OH–Si groups. The local niobium structure and the influence of the amount of niobia on the surface of the Nb 2 O 5 /SiO 2 system were studied by XAS and XPS, respectively. For the samples calcined at 423 and 873 K, the 3 d 5/2 BE values are at ca. 208.2 eV, indicating an ionic character for Nb(V) species in the SiO 2 matrix, probably associated to Si–O–Nb linkages. The features of Nb K-edge XANES spectra of samples show the absence of NbO species. The Nb K-edge EXAFS oscillations exhibit a shoulder at ca. 5.6 Å −1 , which probably arises from Nb–O–Si. This fact corroborates the EXAFS simulation data of the second coordination shell, whose best fitting is achieved with three distances, two Nb–Nb lengths and one Nb–Si.

Novel bioactive materials with different mechanical properties

Some ceramics, such as Bioglass ®, sintered hydroxyapatite, and glass-ceramic A-W, spontaneously bond to living bone. They are called bioactive materials and are already clinically used as important bone substitutes. However, compared with human cortical bone, they have lower fracture toughness and higher elastic moduli. Therefore, it is desirable to develop bioactive materials with improved mechanical properties. All the bioactive materials mentioned above form a bone-like apatite layer on their surfaces in the living body, and bond to bone through this apatite layer. The formation of bone-like apatite on artificial material is induced by functional groups, such as Si–OH, Ti–OH, Zr–OH, Nb–OH, Ta–OH, –COOH, and PO 4H 2. These groups have specific structures revealing negatively charge, and induce apatite formation via formations of an amorphous calcium compound, e.g., calcium silicate, calcium titanate, and amorphous calcium phosphate. These fundamental findings provide methods for preparing new bioactive materials with different mechanical properties. Tough bioactive materials can be prepared by the chemical treatment of metals and ceramics that have high fracture toughness, e.g., by the NaOH and heat treatments of titanium metal, titanium alloys, and tantalum metal, and by H 3PO 4 treatment of tetragonal zirconia. Soft bioactive materials can be synthesized by the sol–gel process, in which the bioactive silica or titania is polymerized with a flexible polymer, such as polydimethylsiloxane or polytetramethyloxide, at the molecular level to form an inorganic–organic nano-hybrid. The biomimetic process has been used to deposit nano-sized bone-like apatite on fine polymer fibers, which were textured into a three-dimensional knit framework. This strategy is expected to ultimately lead to bioactive composites that have a bone-like structure and, hence, bone-like mechanical properties.

Synthesis and structural characterization of three new compounds containing [Nb6Cl12]2+units as hydrated hydroxides

Conditions have been established for the preparation of three different hydrated chloroniobium cluster hydroxide compounds from alkaline solutions: triclinic [Nb6Cl12(OH)2(H2O)4]·4H2O 1 and two cubic compounds whose structures are uncertain with regard to the co-ordination of OH– ions, Nb6Cl12(OH)2(H2O)142 and Nb6Cl12(H2O)14(OMe)23. Based upon structural data 2 and 3 may contain either the cation [Nb6Cl12(H2O)6]2+ or the neutral cluster [Nb6Cl12(OH)2(H2O)4] containing hydroxo ligands. In 1 distinct distances of 2.147(5) for Nb–OH, and 2.250(5) and 2.289(5)A for Nb–OH2 were found. In 2 and 3 only distances of 2.19(1) and 2.193(7)A, respectively, were found for the Nb–O bonds of co-ordinated water (or hydroxide-water averaged). Compound 1 is triclinic, space group P1, a= 8.930(3), b= 9.195(2), c= 9.118(1)A, α= 104.37(2), β= 99.51(2) and γ= 116.99(2)°, U= 611.9(3)A3, Z= 1, R= 1, R= 0.036. Compound 2 is cubic, space group Fm3m, a= 14.59(2)A, U= 3105.7(1)A3, Z= 4, R= 0.028. Compound 3 is also cubic, space group Fm3m, a= 14.627(1)A, U= 3129.3(1)A3, Z= 4, R= 0.031. IR spectra of 1 show strong O–H and Nb–O stretching bands that reflect the presence of co-ordinated OH– groups. In 2 and 3 the IR spectra are less definitive about possible co-ordination of hydroxide.