Nanostructured Nb2O5 catalysts

NANO REVIEWS

Review Article

Nanostructured Nb2O5 catalysts

Yun Zhao1,2, Xiwen Zhou1, Lin Ye3 and Shik Chi Edman Tsang1*

1Department of Chemistry, University of Oxford, Oxford, United Kingdom; 2School of Chemical Engineering and the Environment, Beijing Institute of Technology, Beijing, P.R. China; 3Department of Chemistry, Fudan University, Shanghai, P.R. China

Received: 27 February 2012; Revised: 28 June 2012; Accepted: 1 July 2012; Published: 7 August 2012

Abstract

Niobium pentoxide (Nb2O5) has long been known to catalyze unique acid induced reactions, redox reductions and photo-catalytic reactions, etc. Recently, there have been significant advancements in tailoring the oxide materials with controlled structures and morphologies using nano-chemical synthesis by the help of surfactant or stabilizer for optimal catalytic performance. In this short review, we will particularly highlight these synthetic methods for preparation of Nb2O5 nanostructures, their potential applications in catalysis and their structure-activity relationships.

Keywords: niobium oxide; nanostructures; semi-conductor; crystal; facet; synthesis; catalysis



Yun Zhao is a visiting scholar at University of Oxford and an associate professor at Beijing Institute of Technology, China. She gained her PhD degree in 2002 at Beijing University of Chemical Technology and undertook the Postdoctoral Research during 2002 and 2004 at Tsinghua University. Her research interests are mainly on Nano-materials and Catalysis using one dimensional nanostructure such as carbon nanotubes, hydroxides and oxides. She has published more than 70 papers and patents in these areas.



Xiwen Zhou received his BSc degree from Beihang University (China) in 2008 and MSc degree from Imperial College London in 2009. Currently he is a PhD candidate in professor Tsang's group at University of Oxford. His research interest is mainly on nanocatalysts development for fuel production including green methanol production and H2 generation.



Lin Ye is a PhD student in the Department of Chemistry at Fudan University, China. She is working under the supervision of Professor Heyong He. Her research interests include synthesis and characterization of mesoporous transition metal oxides and their applications in heterogeneous catalysis.



Shik Chi Edman Tsang is a Professor of Chemistry and Head of Wolfson Catalysis Centre at the University of Oxford, UK. Before his academic appointment, he gained his PhD degree in 1991 at University of Reading and undertook the Royal Society Research Fellowship in 1995 at Oxford. His research interests are mainly on Nano-materials and Catalysis in energy and environment and he has published more than 200 papers, monographs and patents in these areas. Recently, he is actively developing some alternative clean technologies by hetergoeneous catalysis means. He won a number of academic awards such as IChemE-iAc Award from Institute of Chemical Engineers (Institute of Applied Catalysis, UK) and also an Outstanding Young Researcher Award from NSFC, China. He has recently been given the 2012 RSC Green Chemistry Award by the Royal Society of Chemistry, UK.

Nano Reviews 2012. © 2012 Yun Zhao et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution-Noncommercial 3.0 Unported License (http://creativecommons.org/licenses/by-nc/3.0/), permitting all non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Citation: Nano Reviews 2012, 3: 17631 - http://dx.doi.org/10.3402/nano.v3i0.17631

 

As a Group 5 transition element niobium-oxygen mainly exists in the form of stiochiometric oxides such as NbO, Nb2O3, NbO2 and Nb2O5 (1). Nb2O5 is the most well-known and studied oxide material amongst all these reported oxides. It gives a n-type semiconducting property with a band gap of about 3.4 eV, Nb2O5 has attracted a great deal of interests due to its many remarkable properties suitable for a wide range of applications such as for gas sensing, catalysis, electrochromics, photoelectrodes as well as in field-emission displays and microelectronics (26). In recent years replacement of stiochiometric reagents with solid catalysts in industrial chemical synthesis becomes a new environmental remediation technology. Nb2O5 shows a great promise in providing strong surface acidity and stability in aqueous medium for various acid-catalyzed reactions. In addition, pure or doped Nb2O5 are the focuses of growing attention for the photo-degradation of organic contaminants (7, 8). Particularly, the preparations of Nb2O5 with different nanostructures and morphologies by nano-chemical synthesis could enable detailed elucidation of structure-activity relationships.

Here we review some recent accomplishments in the synthesis and potential catalytic applications of the nanostructured Nb2O5.

Structures and properties of Nb2O5

The Nb2O5 exists in many polymorphic forms; TT-Nb2O5 (pseudohexagonal), T-Nb2O5 (orthorhombic) and H-Nb2O5 (monoclinic) are the most common phases (9, 10). H-phase is the most thermodynamic stable structure, while TT-phase is the least stable one. It is relatively easy to transform the TT-phase to the H-phase by appropriate heat treatment. Similarly, amorphous niobic acid can be converted into TT-Nb2O5 at lower temperature. TT-Nb2O5 changes almost continuously into T-Nb2O5 on being heated at 600–800°C. H-Nb2O5 arises from any other forms on heating in air to about 1,100°C.

As shown in Fig. 1, a unit cell of the pseudo-hexagonal structure of TT-Nb2O5 contains half of the formula equivalent, with a constitutional defect of an oxygen atom per unit cell. Each Nb atom is at the center of four, five or six oxygen atoms on the ab-plane and an Nb–O–Nb–O chain structure is along the c-axis. Thus, the oxygen deficiency leads to the distortion of these polyhedra. The T-Nb2O5 is built up with the orthorhombic unit cell. Each Nb atom is surrounded by six or seven oxygen atoms, making distorted octahedra or pentagonal bipyramids. These polyhedra are connected by edge- or corner-sharing in the ab-plane and by corner-sharing along the c-axis. The monoclinic lattice of H-Nb2O5 contains 3×4 and 3×5 blocks consisting of NbO6 octahedra. These blocks are coupled by edge-sharing with a shift of half a unit cell dimension along the c-axis. NbO6 units are joined up by corner-sharing with each other within a block (11). It is the richness in structural isotropy of Nb2O5 which is believed to give rise to different physicochemical properties including electronic, electrochromic, sensing, magnetic and electrical properties. In catalysis, surface acidity, redox and photocatalytic properties are also linked to these structures and intrinsic defects. For example, amorphous niobic acid shows strong Brønsted acidity due to the proton generation from water molecule on exposed Nb5+ (12). The acid strength H0= − 5.6 (13) is equivalent to ca. 70% H2SO4. Thus, the solid powder can be used as effective catalyst in aqueous media for esterification, olefin hydration and alcohol dehydration reactions. But the acid strength decreases with increasing temperature due to the dehydration of the niobic acid. Instead, variable surface Lewis acidities due to the exposed cations in proximity of oxygen vacant sites on the powder surface become more important at elevated temperature.

Fig 1
Fig. 1.   Structural schemes of Nb2O5 (a) TT-phase, (b) T-phase and (c) H-phase. : Nb atom; : O atom.

For the above structures and properties, the employment of Nb2O5 nanostructures exhibiting higher specific surface areas is more important in catalysis. In addition, the precise arrangement of atoms on facets, edges and corners at different sizes may be very different from the bulk, when used as catalysts they may give unique activity and selectivity for chemical reactions.

Synthesis of nanostructured Nb2O5

Nano-Nb2O5 can be obtained using many different synthetic methods. To get nanoparticles with no preferred shape, precipitation and sol–gel synthesis followed by calcination are often used. A precipitation method involving a reaction between a chemical precursor containing Nb5+ and aqueous ammonia represents the most simple preparation step. Zhou et al. (14) reported the synthesis of Nb2O5 nanoparticles by the decomposition of niobic acid obtained using a precipitation reaction of Nb-fluorocomplex and ammonia in aqueous solution. At the lower concentration, they found that the Nb2O5 nanoparticles show a uniform size of about 30–50 nm. Brayner et al. (15) prepared nano-Nb2O5 by calcination of niobia obtained by digestion from Nb ammonium complex [(NH4)H2[NbO(C2O4)3]·3(H2O)] in aqueous ammonia at pH 8.5. They showed that the as-prepared Nb2O5 after calcination at 400°C contains ∼4 nm particles. As shown in Fig. 2a, Nb2O5 nanoparticles of 20–50 nm diameter can be obtained by a precipitation of ammonium niobate oxylate hydrate with ammonia in water/ethanol at room temperature. In addition, Nb2O5 nanoparticles of 10–50 nm diameter (Fig. 2b) can be produced by the precipitation reaction of ammonium niobate oxylate hydrate with urea in water at 120°C.

Fig 2
Fig. 2.   TEM micrographs of Nb2O5 nanoparticles prepared using different conditions: (a) after ammonium niobate oxylate hydrate reaction with ammonia in water/ethanol at room temperature; (b) after ammonium niobate oxylate hydrate reaction with urea in water at 120°C.

For the preparation of supported nano-Nb2O5, Li and colleagues (16, 17) recently reported the preparation of a highly dispersed niobia on alumina through a chemical vapor deposition of niobium pentachloride on α-Al2O3 at 150°C, followed by hydrolysis with water vapor at 150°C and calcination. The same authors also prepared the same supported oxide through the impregnation of niobic acid pre-dissolved in aqueous solution of oxalic acid on α-Al2O3, followed by post-treatments with water vapor and calcination.

Sol–gel methods have a strong track record in the production of porous and high surface area oxide materials. Molecular precursors, mainly metal alkoxides, are generally used as starting materials. A macromolecular network is then obtained via hydrolysis and condensation (18). Ristić et al. (19) prepared Nb2O5 powders by the sol–gel procedure. Nb(OC2H5)5 dissolved in ethanol was (a) rapidly hydrolysed by adding a concentrated ammonia solution, or (b) slowly hydrolysed by adding a small amount of H2O. Thus the sizes of Nb2O5 were controlled at 26 and 30 nm, respectively. Uekawa et al. (20) reported the synthesis of highly crystallized Nb2O5 nanoparticles with a diameter of 4.5 nm based on a sol–gel route. The niobic acid precipitate was prepared using NbCl5, ethanol and NH3 aqueous solution as starting materials. The Nb2O5 nanoparticles were then obtained by heating a peroxo niobic acid sol prepared by peptization of the niobic acid precipitate with H2O2 aqueous solution.

In the past few years, more and more attention has been paid to one-dimensional Nb2O5, including nanotubes, nanowires, nanofibers, nanobelts and nanorods, due to their unique shape-dependent physical, chemical and optical properties. Many different routes are developed to synthesize the 1D Nb2O5. A structure-directing agent and hydrothermal/solvothermal technique are often employed.

Nanorods

Luo et al. (21) synthesized single crystalline Nb2O5 nanorods by a soft chemical process in an autoclave at 200°C for 3–30 days, in which the metal Nb powder and water were used as the starting materials. The synthesized nanorods are highly crystalline and their diameters are found to be ca. 50nm with the length up to several micrometers. George (22) prepared Nb2O5 nanorods via annealing Nb2O5@C core-shell nanorods, which was obtained from Nb(OEt)5 using RAPET (reaction under autogenic pressure at elevated temperatures) technique. The reaction of pentaethoxy niobate, Nb(OEt)5, at elevated temperature (800°C) under autogenic pressure provided a chemical route to niobium oxide nanorods coated with amorphous carbon. A vapor–solid process was assumed to control the formation of the one-dimensional nanostructures. Thus, the dissociation of Nb(OEt)5 at 800°C leads to an atomization of the precursor into carbon, hydrogen, oxygen, and perhaps niobium atoms. The niobium and oxygen atoms then react, and upon cooling form a rod-shaped Nb2O5 via the fast reactions of ether elimination and β-hydrogen transfer. The occurrence of these reactions, providing oxide nanoparticles in both solution and gas phase thermolysis of metal alkoxides. Since the process is kinetically controlled, carbon, having a slower solidification rate, forms the shell layer, and Nb2O5 has a much higher solidification rate than carbon for forming the core of the composite. As-prepared Nb2O5@C core-shell nanorods are annealed under air at 500°C for 3 h (removing the carbon coating), resulting in neat Nb2O5 nanorods. Li et al. (23, 24) reported a catalyst-free topochemical method using molten salt synthesis to synthesize rod-like single crystals of Nb2O5. The rod-like KNb3O8 which were first fabricated as the precursor by the molten salt method, were treated by proton exchange and heat treatment to yield the rod-like H-Nb2O5 single crystal. A pure TT-phase single crystalline Nb2O5 nanorods was prepared by Zhou et al. (25) using Nb–fluorocomplex and aqueous ammonia solution as starting materials in water/ethanol. The nanorods were about 100–200 nm in length and 20–30 nm in width. Nb2O5 nanorods with smaller aspect ratios were obtained while increasing the water/ethanol ratio. In addition, high quality Nb2O5 nanorods with different aspect ratios were generated by a simple solvothermal technique (26) using NbCl5, ethanol and cyclohexanol in an autoclave at 200–240°C for 8–90 h. As shown in Fig. 3, we synthesized TT-Nb2O5 nanorods with 5–20 nm diameter with 200–500 nm in length using oleic acid as a structure-directing agent and ammonium niobate oxylate hydrate as a starting material, trioctylamine as a solvent.

Fig 3
Fig. 3.   TEM micrograph of Nb2O5 nanorods.

Nanowires

Saito and Kudo (27) prepared the homogeneous TT phase niobia nanowires with diameters of 30–50 nm and lengths up to several micrometers through the calcination of niobium-based amorphous nanowires obtained by the reaction of water-soluble niobium oxo-oxalate complex (NH4)3[NbO(Ox)3]·H2O (Ox = oxalate) with trioctylamine (TOA) as a structure-directing agent. The growth of nanowire (NW) was proposed as follows: TOA coordinates to the niobium complex associated with dissociation of oxalate ligand under heat treatment below the decomposition temperature of oxalic acid, followed by self-assembly to each other via the hydrophobic interaction between methylene groups in TOA. The assembly then undergoes directional growth forming nanowire structures by heating above the decomposition temperature of oxalic acid to form Nb2O5-NW. Viet et al. (28) obtained the TT- and T-Nb2O5 nanofibers with an average diameter of 160 nm by combining the sol–gel chemistry and electrospinning technique. The solution for electrospinning was prepared from polyvinylpyrrolidone, ethanol, niobium ethoxide and acetic acid. Lim and Choi (29) described a simple thermal oxidation of niobium for the fabrication of niobium oxides, claiming that the niobium oxide nanowires can be prepared by controlling the concentration of oxygen in the environment. The annealing of Nb foils for the formation of nanostructures was carried out at various target temperatures from 600 to 1,000°C using a tube furnace. Dense Nb2O5 nanowires on a thin oxide film are produced if the annealing was performed in a restricted concentration of oxygen.

Nanobelts and nanotubes

Wei et al. (30) developed a simple synthetic route to prepare layered NH4Nb3O8 nanobelts in an autoclave at 170–200°C for 1–14 days using metallic Nb powder and urea as the starting materials. The as-obtained NH4Nb3O8 nanobelts were then converted to single-crystal Nb2O5 by thermal treatment in air. The lengths of the nanobelts ranged from several hundreds of nanometers to several tens of micrometers, and the thickness and width of nanobelts were about 15 and 60 nm, respectively. A possible mechanism for the formation of nanobelts was proposed as follows: First, Nb powder is reacted with the urea solution under hydrothermal conditions, and the layered structure NH4Nb3O8 is formed with interlayer spaces between the [NbO6] octahedral sheets. Secondly, the interactions between the layers of NH4Nb3O8 are weakened under hydrothermal conditions, and the layered structures are gradually exfoliated to form nanosheets. Since the nanosheets do not have an inversion symmetry, i.e. the layer is symmetric, and an intrinsic tension exists that might cause the edges of the nanosheets to roll up. Finally, the nanosheets are spitted to form nanobelts in order to release the strong stress and lower the total energy. Kobayashi et al. (31) reported the synthesis of T-Nb2O5 polycrystalline nanotubes through the thermal dehydration of H4Nb6O17 scrolls at 400–450°C. The nanoscroll-to-nanotube thermal transformation was particularly studied for H4Nb6O17·4.4H2O scrolls, prepared by exfoliation of K4Nb6O17. Yan and Xue (32) claimed a successful synthesis of monoclinic Nb2O5 nanotube arrays from pseudo-hexagonal Nb2O5 nanorods based on phase transformation accompanied by void formation.

Layered materials

The layered HxK1–xNb3O8 (x=0–1) strcuture as a new solid acid catalyst for selective hydration of ethylene oxide (EO) was prepared by calcining Nb2O5–K2CO3 mixture, followed by an ion-exchange in HNO3 solution (33). The KNb3O8 was initially prepared by molten salt method from a finely grinded mixture of Nb2O5 and K2CO3 with a slightly excessive amount (10%) of K2CO3 to the stoichiometric ratio. The calcination was held at 900°C for 1 h and then at 1,100°C for 5 h. The KNb3O8 product obtained was then washed with hot water to remove the residual K2CO3. The KNb3O8 was then used as the precursor to prepare HxK1–xNb3O8 by a proton exchange method. The KNb3O8 sample was stirred in a series of HNO3 solutions with different concentrations (0.2, 2.2, 3.8, 6.5, 10, 20 and 40 wt%) at solution/solid weight ratio of 60 for 48 h to produce HxK1–xNb3O8 with x of 0.11, 0.31, 0.35, 0.49, 0.71, 0.76 and 0.89, respectively.

Two- and three-dimensional porous materials

Two-dimensional (2D) hexagonal mesoporous niobium oxides have been first obtained by Ying and co-workers (34) and later by Stucky and co-workers (35) In the presence of triblock copolymer with elaborate addition of cation, a preparation of a highly ordered three-dimensional (3D) hexagonal mesoporous niobium oxide extended from 2D channel system was reported by Domen and co-workers (36). Following on the above works, Ye et al. (37) reported the synthesis of a well-ordered 3D cubic Ia3d mesoporous niobium oxide via a neutral templating route under controlled humidity. After removal of the template, the pore wall of mesoporous niobium oxide was reinforced by carbon-filling. Crystalline niobium oxide with an original mesoporous structure preserved was obtained after crystallization at 600°C in nitrogen and carbon removal at 400°C in oxygen (Fig. 4). During the preparation, humidity was found to be the main factor which affected the resulting structure. A relative humidity above 50% or below 30% led to the formation of worm-like or hexagonal mesoporous niobium oxides, respectively. Pyridine-IR study showed that the crystalline mesoporous niobium oxide exhibited medium to strong solid acidity.

Fig 4
Fig. 4.   (a) cubic amorphous Nb2O5 and (b) HRTEM image and ED pattern (inset) of cubic crystalline Nb2O5 (37).

Characterization

Crystal structures of bulk Nb2O5 synthesized are commonly studied by X-ray and electron diffraction techniques. Lattice parameters and average particle size are derived by fitting the observed XRD patterns to the respective crystal structures with a high degree of accuracy. The Brunauer, Emmett, and Teller (BET) surface area of Nb2O5 can be measured by a commercial surface area analyzer using nitrogen gas. Morphologies of the Nb2O5 can be observed by typical microscopies. It is however, when Nb2O5 reach nanometric size or below there were significant difficulties to characterize the materials. There are deviations in crystallographic lattice parameters from bulk structures (38) and gas adsorption response and stoichiometry can be different from the typical bulk materials (38). The structure of nanosize niobium oxides was extensively investigated by using extended X-ray absorption fine structure (EXAFS) spectroscopy and surface science techniques. The EXAFS spectroscopy provides structural information about a sample by way of the analysis of its X-ray absorption spectrum. It allows determining the chemical environment of a single element (e.g. Nb) in terms of the number and type of its neighbors (e.g. O), inter-atomic distances (e.g. Nb–O) and structural disorders. Attempts to correlate the acidic property to Nb–O, Nb–Nb distances and oxygen vacancies were made (38). XPS analysis can be used to characterize the stoichiometry and chemical states for Nb2O5 nanomaterials. The Nb/O ratio can be obtained after quantification analysis of the peak area. The chemical state of Nb can be inferred based on the binding energy of corresponding peak position. Diffuse reflectance UV-visible spectra can be used to measure the band-gap of Nb2O5 nanostructures. Under ultraviolet irradiation, electrons in the valence band absorb the photon energy and jump to the conduction band, leaving holes in the valence band. Light absorption is mainly determined by the band structure. The band-gaps of Nb2O5 nanostructures indicate the light absorption efficiency. Pyridine adsorption Infrared spectra can be used for determination of the amounts of acid sites over Nb2O5 nanostructures. IR bands at 1,550 and 1,450 cm−1 can be attributed to pyridinium ion and pyridine coordinated to Lewis acid site (12). Based on the peak area and the amount of Nb2O5, the acid amounts can be calculated. In addition, the uses of state-of-the art advance techniques including super-STEM, nano-diffraction, STM and atomic force microscope, field emission to reveal the nanostructures without much disrupting the atom arrangement in nano- or sub-nano oxides are crucially important.

Potential catalytic applications

Nb2O5 shows wide ranging potential applications in gas sensing, electrochromics, photoelectrodes as well as in field-emission displays and microelectronics. However apart from catalysis, there are little reports about potential uses of low dimensional Nb2O5 nanostructures in broad areas of interests. Only a few reports proposed the uses of these nanostructures as electrode materials. For example, Wei et al. (39) suggested the use of Nb2O5 nanobelts as electrode material for the lithium intercalation over a potential window of 3.0–1.2 V (vs. Li+/Li). Their cycling tests indicated that Nb2O5 nanobelts electrode shows a high reversible charge/discharge capacity, high rate capability and excellent cycling stability, making this material a good candidate as an electrode for high-power lithium ion battery. Viet et al. (28) evaluated the Nb2O5 nanofibers in different polymorphic forms for their application as cathode for lithium batteries. They found that the M-Nb2O5 exhibited the highest capacity and better capacity retention as compared to other phases.

On the other hand, low dimensional Nb2O5 nanostructures have been receiving increasing interests as solid catalysts for a wide range of chemical reactions. Zakzeski et al. (40) reported the preparation of benzoyl fluoride from benzotrifluoride catalyzed by nanostructure niobium oxide. Small Nb2O5 particles exhibited the highest activity with a conversion of 99.8% and a selectivity of 90% screened in the first 10 min of the reaction under mild conditions. A possible mechanism for the conversion of benzotrifluoride to benzoyl fluoride is presented in Scheme 1. In the first step, trifluoroacetic acid and benzotrifluoride coordinate to the niobium oxide catalyst. In the second step, a hydrogen bond is formed between the trifluoroacetic acid and the fluorine in the methyl group of the benzotrifluoride. Interaction of these species with the niobium oxide catalyst stabilizes them. In step 3, the C–F bond is broken, and HF is released into solution, where it then reacts with trifluoroacetic anhydride. Step 4 involves the formation of a species similar to an ester on the surface of the niobium catalyst. The catalyst assists in fluorine-oxygen transfer (step 5), leading to the formation of benzoyl fluoride and trifluoroacetyl fluoride bound to the surface of the niobium oxide. Molecular oxygen loosely coordinated to the catalyst is envisioned to facilitate the transfer of fluorine during this step, perhaps through the formation of FO2 as an intermediate. Finally, the benzoyl fluoride and trifluoroacetyl fluoride desorb from the surface of the catalyst. Benzoyl fluoride accumulates in the liquid phase, whereas trifluoroacetyl fluoride accumulates in the gas phase.

Fig 5
Scheme 1.  Catalytic conversion of benzotrifluoride to benzoyl fluoride over Nb2O5 nanocatalyst (40).

Srilatha et al. (41) reported the influence of chain length and the extent of the saturation on the esterification activity of long chain carboxylic acids over a supported Nb2O5 catalyst. The esterification reactivity was found to be inversely proportional to alkyl chain length of the acid. The activation energy for the fatty aid esterification increases with the extent of unsaturation fatty acid. Aranda et al. (42) claimed that the supported Nb2O5 catalyst gives better catalytic performance than zeolites for the esterification reaction of palm fatty acids and alcohol. Chai et al. (43) investigated a gas-phase dehydration of glycerol to produce acrolein over Nb2O5 catalysts. They reported that catalyst performance for this dehydration reaction is significantly affected by calcination temperature which is related to the surface acidity and crystallization of Nb2O5, as shown in Fig. 5.

Fig 6
Fig. 5.   Effect of the catalyst calcination temperature on (A) glycerol conversion and (B) acrolein selectivity at () TOS = 1–2 h and () TOS = 9–10 h (43).

Sun et al. (44) investigated the dehydration of methanol to dimethyl ether over Nb2O5 catalysts. Their results suggested that methanol is strongly dissociatively adsorbed on nano-Nb2O5 surface to form methoxy species and the dimethyl ether product is produced from the dehydration of the two methoxy species. Nano-size Nb2O5 clearly exhibited 100% selectivity towards the dimethyl ether product with a good stability and activity in the temperature range of 453–573 K without coke formation. Michalkiewicz et al. (45) carried out the selective methane oxidation to formaldehyde over T-Nb2O5, the mixture of M-Nb2O5 and TT-Nb2O5 as well as TT-Nb2O5. Using TT-Nb2O5 and M-Nb2O5 phases with a block type structure, they obtained higher formaldehyde selectivity (78% at 0.9 sec) than that of T-Nb2O5 (47% at 0.9 sec) the polymorphic form which does not have a block type structure. Zhang et al. (46) reported that Nb2O5 is the most active catalyst for the oxidation of cyclo-olefins to prepare 1,2-diols.

Highly dispersed niobia catalysts prepared by chemical vapor deposition have been claimed to be active for the synthesis of ethylene glycol from selective hydration of ethylene oxide (EO) (16). Similarly, the same group reported the niobium oxide impregnated on α-Al2O3 (17) promoted the conversion of EO from 34% in the non-catalytic process to >99% with keeping monoethylene glycol (MEG) selectivity at around 90% under H2O/EO ratio of 22. Moreover, the impregnated niobia catalyst demonstrated excellent stability with no apparent deactivation within 1,000 h of time-on-stream. Further study found that the acidity of the niobium oxide catalyst was weaker than that of bulk Nb2O5 phase, and the acidity decreased with calcination temperature. In terms of MEG yield, the suitable calcination temperature for α-Al2O3 supported Nb2O5 was found to be between 350 and 500°C. Calcination at 700°C or above caused the formation of TT-Nb2O5 which was ineffective to the hydration of EO. The layered HxK1–xNb3O8 (x=0–1) samples were also applied as new solid acid catalysts for the same reaction. The highest selectivity for monoethylene glycol (MEG) achieved was over 95% with the EO conversion of >99% under H2O/EO ratio of 8 at x of 0.7. It was revealed that a moderate acid strength and in situ self-exfoliation of HxK1–xNb3O8 sheets are crucial for the high MEG selectivity. This result clearly demonstrated that the best MEG selectivity is achieved within the mild pH range of 3.0–6.0. In view of insolubility in water of niobium oxide, the above catalysts show a great promise in the selective hydration of ethylene oxide (33).

Heterogeneous photocatalysis is one of the most intense studied technologies used for the removal of organic and inorganic contaminants and for hydrogen generation (47). The heterogeneous photocatalysis using semiconductors as catalysts is a promising process for applications in environmental remediation technology, because it shows considerably advantages over conventional technologies for organic pollutants degradation into innocuous final products (48). An important step of the photoreaction is the formation of electron-hole pairs, which needs energy to overcome the band gap between the valence and conduction bands. Band gap-illumination of a semiconductor generates electron–hole pairs, hot electrons promote to the conduction band and holes takes residence in the valence band. A portion of these electron hole pairs diffuses out to the surface of the crystal and participates in chemical reactions with electron donors and acceptors, resulting in photocatalysis (49). Nb2O5 presents a band gap value of about 3.4 eV, which is suitable for the photocatalysis at the moderate UV region. Therefore the investigation of Nb2O5 photocatalysts has now been receiving a noticeable attention.

Shishido et al. (50) demonstrated an effective organic solvents-free photooxidation of alcohols by using molecular oxygen with no additives over niobium oxide. They pointed out that this photooxidation of alcohol to a carbonyl compound proceeds selectively at low temperature over the Nb2O5 catalyst without the need of organic solvents and found that the rate-determining step is on the desorption of carbonyl compound. The mechanism for the solvents-free photooxidation of alcohols over Nb2O5 was given as follows (50): Alcohol is adsorbed onto Nb2O5 as an alcoholate species in the dark. The adsorbed alcoholate on the Nb2O5 is activated by transferring an electron to the conduction band reducing Nb5+ to Nb4+, and leaving a hole on the alcoholate. The formed alkenyl radical is converted to a carbonyl compound. The product is desorbed and then the reduced Nb4+ sites are reoxidized by the reaction with molecular oxygen. The rate-determining step of the photooxidation of alcohol over Nb2O5 is the process of desorption of the formed carbonyl compound.

Ohuchi et al. (51) also reported the photooxidation of 1-pentanol in liquid phase without solvents over niobium oxide in an O2-flowing batch system at the atmospheric pressure and room temperature. It was observed that niobium oxide exhibits the higher activity and highest selectivity to partial oxidation products among metal oxides studied. They found that the presence of Lewis acid sites promotes deep oxidation. Chen et al. (52) studied the photocatalytic water splitting reaction over Nb2O5. The mesoporous Nb2O5 catalysts showed a more superiority in photocatalytic activity than bulk Nb2O5 powder. Saito and Kudo (27) investigated the photocatalytic activities of TT-Nb2O5 nanowires for H2 or O2 evolution. It gave enhanced activities in the presence of sacrificial reagents, compared to spherical particles of conventional niobia with the same crystalline phase, as shown in Fig. 6.

Fig 7
Fig. 6.   O2 evolution from an aqueous silver nitrate solution (0.02 mol L−1) over TT-Nb2O5-NW () and TT-Nb2O5-B (). Conditions: catalyst, 0.1 g; light source, 300 W Xe lamp; reaction cell, top-irradiation cell with Pyrex glass window.

Prado et al. (7) investigated the application and reapplication of Nb2O5 in the photodegradation of indigo carmine dye, which is extensively used as textile coloring agent in dyeing of clothes. Almost 100% of dye degradation occurred at 90 min over nanostructure Nb2O5. This catalyst presented the best activity at pH < 4 with an ionic strength of 0.05 mol L−1. They showed that Nb2O5 can be recycled and re-applied in many photo-degradation steps, maintaining 85% of activity after the first 10 cycles of reaction to reach the steady state. However, the degradation rate of Nb2O5 was found to be much slower than that of TiO2 and ZnO. To enhance the photocatalytic efficiency, modified Nb2O5 were prepared by doping the structure with active metals, or fabricating it with various composites and mixed oxides as improved photocatalysts.

Ge et al. (53) found that carbon modified Nb2O5 nanostructures exhibit much better photocatalytic activity on degradation of RhB dye under visible light and are also capable of efficiently splitting water under visible light. Esteves et al. (48) reported the application of modified Nb2O5 in the photodegradation of methylene blue dye. Nb2O5 displays high photocatalytic activity to decompose the methylene blue dye. After modification (doping with W, Mo or H2O2 treatment) they reported that the activity of the Nb2O5 based catalyst can be further enhanced. Miyazaki et al. (54) examined photocatalytic activities of MnO2-loaded Nb2O5/carbon clusters composite material in the reduction of methylene blue. Nano-sized Nb2O5/carbon clusters composite material reduced the methylene blue under the irradiation of visible light (k>460 nm). The MnO2-loaded composite material could also decompose an aqueous silver nitrate solution by visible light irradiation and give O2 and Ag with a [O2]: [Ag] ratio of 1:4. Santana (55) evaluated the efficiency of photocatalytic vinasse treatment using solar radiation and found that Nb2O5–TiO2 was the most photoactive material.

Correlations between the crystal structure, surface properties and the catalytic activities

Nb2O5 is an n-type transition metal oxide semiconductor with strong surface acidity due to the presence of cations at high valency, it could be used as a catalyst for various photocatalytic and acid-catalyzed reactions. Therefore its unique structures must somehow correlate to catalytic and photocatalytic activities. However the structure-activity relationships have only been addressed by a few researchers. Onfroy et al. (56) investigated the relationship between surface acidity, catalytic activity, and the surface structure over niobium oxide supported on zirconia in detail. They reported for the surface Brønsted acidity is linked to catalytic activity for 2-propanol dehydration. A direct relationship was clearly observed between the rate of propene formation and the abundance of Brønsted acid sites. Analysis of the structure of Nb phase indicated that the catalytic activity and the Brønsted acidity are associated with polymeric NbOx species. Tanaka et al. (57) compared the difference in surface property and catalytic activity between amorphous and crystalline Nb2O5 with similar ordered mesoporous structure. The surface of amorphous sample was found to be hydrophilic and the surface OH groups were acidic, while the OH groups on crystalline mesoporous Nb2O5 were non-acidic and inside the pores were less hydrophilic. For oxidation of cyclohexene by an aqueous solution of H2O2, the high (95%) selectivity for 1,2-epoxycyclohexane was obtained at 40°C for 2 h in methanol over crystalline mesoporous Nb2O5 at 12% conversion. On the other hand, amorphous mesoporous Nb2O5 showed high (68%) selectivity for 1,2-cyclohexanediol in acetonitrile at 60°C for 2 h at 22% conversion. Our recent investigations (58) indicate that the crystalline TT phase Nb2O5 nanorod surface of higher surface acidity (Nb5+) give higher activity in photocatalytic decomposition of methylene blue, while less crystalline Nb2O5 nanosphere surface (associated with a degree of Nb4+ and oxygen defects) of lower surface acidity gave low activity.

Conclusion

Niobium oxide based materials are clearly effective catalysts in numerous catalytic applications including pollution abatement, selective oxidation, hydrocarbon conversion reactions, hydrogenation and hydrotreating, carbon monoxide hydrogenation, dehydration, hydration, photochemistry and electrochemistry and polymerization. The above reviews are only some selected examples. The number of references on using niobium oxide has been steadily increasing over the past decades, which reflects the growing interest in this area in catalysis. However, understanding of the structures and properties of this peculiar oxide does not seem to match up with the vigorous search of applications. Relatively little is known about the original of acidity, the redox behavior and photocatalytic properties that more detailed work is required in this area. In term of structure/morphology manipulation using nano-synthetic skills there have been a number of nice works in making one to three dimensional niobium oxides of defined size, structure and morphology. It is hoped that the employment of these as new model materials can reveal the fundamental and profound structure-activity (selectivity) relationships in elucidation of catalyst material interactions.

Conflict of interest and funding

There is no conflict of interest in the present study for any of the authors.

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*Shik Chi Edman Tsang
Wolfson Catalysis Centre
Inorganic Chemistry Laboratory
Department of Chemistry
University of Oxford
Oxford
OX1 3QR
United Kingdom
Email: edman.tsang@chem.ox.ac.uk

About The Authors

Edman Tsang

United Kingdom

Xiwen Zhou

United Kingdom

Lin Ye

China

Shik Chi Edman Tsang

United Kingdom

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