Nitrogen doped A2Nb4O11, process for preparation thereof, and method for degradation of organic pollutants

The present invention relates to nitrogen doped A2Nb4O11, which is represented by A2Nb4O11-xNx, to a process for the preparation thereof, and to a method for degradation of organic pollutants. The nitrogen doped A2Nb4O11 is a new photocatalyst for the photocatalytic degradation of organic pollutants in the waste water. The A2Nb4O11-xNx catalyst may be prepared by substituting some of the O elements of pure A2Nb4O11 with N elements, and a process for the preparation thereof comprises a step of doping of nitrogen with a nitrogen source through a solid state reaction. The new nitrogen doped A2Nb4O11 catalyst having a general formula A2Nb4O11-xNx has a narrower optical bandgap compared to pure A2Nb4O11, and therefore can be activated under the visible light range and it shows high efficiency in the degradation of organic pollutants.

TECHNICAL FIELD

The present invention relates to the field of photo-catalyst, particularly to nitrogen doped A2Nb4O11, process for preparation thereof, and method for degradation of organic pollutants.

STATE OF THE ART

We are interested in the use of non-layered niobate salts as photocatalysts as we anticipate that they would be more robust than the layered niobates and their protonated derivatives. K2Nb4O11is constructed from NbO6octahedra and has a tetragonal tungsten bronze (TB) crystal structure with triangle, quadrilateral and pentagonal tunnels. The pentagonal and quadrilateral tunnels are occupied by K cations and the triangle tunnels by Nb cations (M. Lundberg, M. Sundberg,J. Solid State Chem.,1986, 63, 216-230). It has been reported that Cu-doping of K2Nb4O11results in increased photocatalytic activity for the degradation of acid red G under UV irradiation (G. K. Zhang, X. Zou, J. Gong, F. He, H. Zhang, S. Ouvang, H. Liu,J. Molec. Catal. A: Chem.,2006, 255, 109-116).

SUMMARY OF THE INVENTION

The present invention provides a nitrogen doped photocatalyst which is denoted as A2Nb4O11—N, or represented by the following general formula (I)
A2Nb4O11-xNx(I)
wherein

A is selected from the elements of Group IA of the periodic table; and 0<x<1

According to one aspect of the present invention, A in the general formula (I) is Li, Na, K, Rb or Cs. Most preferably, A is K.

According to one aspect of the present invention, the compound of the general formula (I) of the present invention has a tetragonal tungsten bronze crystal structure.

According to one aspect of the present invention, the compound of the general formula (I) of the present invention may be used as a photocatalyst. Preferably, said photocatalyst can be activated under visible lights.

The present invention further provides a process for the preparation of the compound of the general formula (I) of the present invention, comprising the steps of:

1) surface acidification of A2Nb4O11, wherein A2Nb4O11is immerged in an acidic solution, filtered, washed and dried; and

2) nitrogen doping of A2Nb4O11to obtain A2Nb4O11-xNx, wherein A2Nb4O11obtained in step 1) is mixed with a nitrogen source and heated, the product is washed to remove residue nitrogen source adsorbed on the surface of the product and dried.

The A2Nb4O11-xNxcatalyst of the present invention is prepared by replacing some of the O elements in pure A2Nb4O11with N elements, and a process for the preparation thereof comprises a step of doping of nitrogen with a nitrogen source through a solid state reaction.

According to one aspect of the present invention, the nitrogen source in the above process may be an ammonium salt or a nitrogen-containing organic compound, such as ammonium carbonate or urea.

According to one aspect of the present invention, the acidic solution used in step 1) of the above process may be selected from the group consisting of hydrochloric acid, nitric acid, sulfuric acid, or phosphoric acid. Preferably, the acidic solution has a concentration of 1-10 mol/L.

According to one aspect of the present invention, in step 1), the ratio of the weight of A2Nb4O11to the volume of the acidic solution may be from 1 g:10 ml to 1 g:600 ml.

According to one aspect of the present invention, in step 1), the duration of the immersing may be 10-96 hours.

According to one aspect of the present invention, in step 1), the washing may be performed with distilled water; the drying may be performed under a temperature of 20-300° C., and the duration of the drying may be above 10 hours.

According to one aspect of the present invention, in step 2), the weight ratio of A2Nb4O11and the nitrogen source may be from 1:0.5 to 1:1.0.

According to one aspect of the present invention, in step 2), the heating may be performed under a temperature of 300-600° C.

According to one aspect of the present invention, in step 2), the duration of the heating may be 1-10 hours.

According to one aspect of the present invention, in step 2), the product may be washed with acetone and/or distilled water to remove residue nitrogen source, such as alkaline species, adsorbed on the surface of the product.

According to one aspect of the present invention, in step 2), the drying may be performed under a temperature of 20-300° C., and the duration of the drying may be 10-96 hours.

A2Nb4O11used in step 1) of the present invention has a tetragonal tungsten bronze crystal structure. It may be obtained commercially, or may be prepared according to a process known in the art, or may be prepared according a process wherein A2Nb4O11is prepared by heating a mixture of Nb2O5and A2CO3for several hours. In said process, the heating may be performed under a temperature of 800-1200° C.; the duration of the heating may be 8-50 hours; and the ratio of Nb2O5and A2CO3may be from 3:1 to 1:10.

The present invention further provides a method for degradation of organic pollutants, comprising contacting the organic pollutants with the compound of the general formula (I) of the present invention. As used herein, the term “organic pollutants” generally refers to organic substances which may cause adverse effects to human health and the environment. Preferably, the organic pollutants are those difficult to decompose in waste water. As used herein, the organic pollutants difficult to decompose are organic compounds which may be present in waste water for a long time without decomposition under ambient conditions, such as Orange G (OG) and bisphenol A (BPA).

It has been proven that the new catalyst having a general formula A2Nb4O11-xNxhas a narrower optical bandgap compared to pure A2Nb4O11, and therefore can be activated under the visible light range and it shows high efficiency in the degradation of organic pollutants, especially organic pollutants difficult to decompose. In addition, the process for the synthesis of the nitrogen doped A2Nb4O11is simple and can be performed on a large scale, and the process for nitrogen doping is less expensive than conventional sputtering ones. The present photocatalyst has the advantages of non-toxicity, chemical inertness, high stability under light irradiation, and high photo efficiency under visible light, and is therefore a superior photocatalyst.

DETAILED DESCRIPTION OF THE INVENTION

In order to prepare the nitrogen doped photocatalyst having the general formula A2Nb4O11-xNxto fulfill the object of the present invention, the process for the preparation thereof are exemplarily described with examples.

As the present invention may be embodied in several forms without departing from the spirit thereof, it should be understood that the embodiments of the present invention are not limited by any of the details of the description. Unless otherwise specified, it should be construed that all changes and modification of the embodiments of the present invention are within the scope as defined in the appended claims. Meanwhile, all the references cited in the present application are incorporated herein by reference in their entirety.

PREPARATION EXAMPLES

b) 1.0 g K2Nb4O11was immersed in 60 mL of 5 mol/L nitric acid solution for 48 hours, and then the product was filtered, washed with distilled water, and dried at 100° C. for 20 hours;

c) 10.0 g urea and 1.0 g K2Nb4O11were mixed and finely milled, and heated at 600° C. for 3 hours to provide a yellow product, which was washed with acetone to remove any residual alkaline species adsorbed on the surface of the product, and dried at 100° C. for 24 hours.

b) 1.0 g K2Nb4O11was immersed in 60 mL of 5 mol/L nitric acid solution for 48 hours, and then the product was filtered, washed with distilled water, and dried at 100° C. for 20 hours;

c) 10.0 g K2Nb4O11and 2.0 g urea were finely milled, and heated at 400° C. for 4 hours. The resulted yellow product was washed with acetone and distilled water, and dried at 70° C. for 24 hours.

b) 1.0 g K2Nb4O11was emerged immersed in 300 mL of 3 mol/L hydrochloric acid solution for 48 hours, and then the product was filtered, washed with distilled water, and dried at 100° C. for 20 hours;

c) 1.0 g K2Nb4O11and 10.0 g ammonium bicarbonate were finely milled, and heated at 400° C. for 4 hours. The resulted yellow product was washed with acetone and distilled water, and dried, at 70° C. for 24 hours.

b) 1.0 g Na2Nb4O11was immersed in 3060 mL of 3 mol/L nitric acid solution for 96 hours, and then the product was filtered, washed with distilled water, and dried at 300° C. for 10 hours;

c) 1.0 g Na2Nb4O11and 0.5 g urea were finely milled, and heated at 400° C. for 24 hours. The resulted yellow product was washed with acetone, and dried at 100° C. for 24 hours.

b) 1.0 g Na2Nb4O11was immersed in 60 mL of 5 mol/L nitric acid solution for 48 hours, and then the product was filtered, washed with distilled water, and dried at 100° C. for 24 hours;

c) 1.0 g Na2Nb4O11and 10.0 g urea were finely milled, and heated at 400° C., for 6 hours. The resulted yellow product washed with acetone, and dried at 100° C. for 24 hours.

b) 1.0 g Na2Nb4Oiiwas immersed in 60 mL of 5 mol/L nitric acid solution for 48 hours, and then the product was filtered, washed with distilled water, and dried at 100° C. for 20 hours;

c) 10.0 g urea and 1.0 g Na2Nb4O11were mixed and finely milled, and heated at 400° C. for 24 hours to provide a yellow product, which was washed with acetone to remove any residues adsorbed on the surface of the product, and dried at 100° C. for 24 hours.

TEST EXAMPLES

The nitrogen doped. K2Nb4O11used in the test Examples is K2Nb4O11—N prepared in Example 2.

1. Characterization of the Compound Having the General Formula (I)

The instruments for characterization include: powder X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), UV/Vis diffuse reflectance and photoluminescence spectroscopy (PL). The XRD analysis was performed on a Rigaku D-max X-ray diffractometer with Cu Kαirradiation (λ=1.5406 Å) at a scanning speed of 0.025°/sec over the scanning range of 20-70°. The morphologies were examined by a Philips XL30 environmental scanning electron microscope (ESEM) at an accelerating voltage of 10 kV. The surface analysis was done with a Leybold Heraeus-Shengyang SKL-12 electron spectrometer equipped with a VG CLAM 4 MCD electron energy analyzer, with Al—Kα as the excitation source. UV-Vis diffuse reflectance was performed on a Perkin Elmer Lambda 750 UV-Vis Spectrophotometer. Photoluminescence (PL) spectra were measured using a FluoroMax-3 spectrofluorimeter equipped with a pulsed xenon lamp as light source.

A 200 W xenon arc lamp (Newport, Model 71232) was used as the light source. OG or BPA aqueous solution (30 ml, 20 mg/L) and the photocatalyst (˜10 mg, nitrogen doped K2Nb4O11prepared in Example 2) were placed into a quartz tube reactor (12 mm in diameter and 200 mm in length) and the mixture was sonicated for 5 minutes to disperse the catalyst in the OG or BPA aqueous solution. The distance between the liquid surface and the light source was about 11 cm. Before the photoirradiation, the mixture was stirred in the dark for one hour so as to establish adsorption-desorption equilibrium on the surface of the catalyst for OG or BPA. The Infrared and UV light emitted from the Xe-lamp was filtered by a water jacket and a cutoff filter (Scott AG KV 399). Samples were collected at regular time intervals and centrifuged before Analysis. The concentrations of OG or BPA were measured with a Shimadzu UV-1700 UV-Vis spectro-photometer, wherein the OG or BPA concentration is proportional to its absorbance.

3. Results and Discussion

Powder X-ray diffraction (XRD) shows nearly identical patterns for K2Nb4O11and K2Nb4O11—N prepared in Example 2. Typical XRD patterns of K2Nb4O11and K2Nb4O11—N are shown inFIGS. 2A and 2B, respectively. For K2Nb4O11(FIG. 2A), all the diffraction peaks can be indexed as a tetragonal tungsten bronze structure (JCPDS 31-1059) with lattice constants of a=0.126 nm and c=0.398 nm. The XRD pattern of nitrogen doped K2Nb4O11nearly identical to that of undoped sample, as shown inFIG. 2B, indicating that there is no effect of nitrogen doping on the crystal structure of K2Nb4O11, which suggests that (loping occurs only on the surface.

The SEM photographs of K2Nb4O11and K2Nb4O11—N are shown inFIGS. 3A and 3B, respectively.FIG. 3Ashows that the sizes and shapes of particles are inhomogeneous and the surface is clean. However, when K2Nb4O11was heated together with urea at 400° C., the surface of the sample became flock-like, but the sizes and shapes of the particles were not significantly changed. The SEM and XRD results together indicate that nitrogen doping does not affect the morphology and crystal structure of K2Nb4O11, but affects the surface profile of the sample, as nitrogen doping occurs mainly on the surface of the sample,

The X-ray photoelectron spectroscopy (XPS) analysis is an important method to determine the composition and the chemical state of the elements. The XPS spectra of K2Nb4O11and K2Nb4O11—N in wide energy range are shown inFIG. 4. No significant contamination, besides carbon, is found in the spectra. The binding energy was determined by reference to C 1s line at 284.8 eV. In the whole energy range spectrum shown inFIG. 4, the elements K, Nb and O can be observed in K2Nb4O11and K2Nb4O11—N. However, N can only be seen in K2Nb4O11—N, indicating successful doping of N onto the surface of K2Nb4O11. The N concentration is calculated to be 3.9 atom % using the equation,

Where CNis the nitrogen concentration, INand Iiare the peak intensities of nitrogen and other elements, respectively; SNand Siare the relative sensitivity factors of nitrogen and other elements, respectively.

To further determine the chemical states of the elements Nb, O and N, core level XPS spectra of K2Nb4O11and K2Nb4O11—N are shown inFIG. 5. FIG.5-A1and B1show that the binding energies of Nb 3d5/2for K2Nb4O11and K2Nb4O11—N are 206.8 and 206.9 eV, respectively, which are consistent with the reported values (G. K. Zhang, Y. J. Hu, X. M. Ding, J. Zhou, J. W. Xie,J. Solid State Chem.,2008, 181, 2133-2138). The chemical shifts of the binding energies of Nb 3d5/2in these two materials are small. However, the full widths at half maximum (FWHM) of the Nb 3d5/2peaks are different. The FWHM of Nb 3d5/2for K2Nb4O11and K2Nb4O11—N is 1.7 and 1.9 eV, respectively. The broadening of the Nb 3d5/2peak indicates that the electron density on the Nb atoms in K2Nb4O11—N is higher than that in K2Nb4O11(T. Shishido, M. Oku, S. Okada, K. Kudou, J. Ye, T. Sasaki, Y. Watanabe, N. Toyota, H. Horiuchi, T. Fukuda,J. Alloy. Comp.,1998, 281, 196-201). The main O 1s peak at 530.8 and 530.6 eV inFIGS. 5A2and B2are assigned to lattice oxygen of K2Nb4O11and K2Nb4O11—N, respectively (A. Molak, E. Talik, M. Kruczek, M. Paluch, A. Ratuszna, Z. Ujma,Mater. Sci. and Engo. B,2006, 128, 16-24; S.-Y. Lai, Y. Qiu, S. Wang,J. Catal.,2006, 237, 303-313). A higher binding energy shoulder is found for both samples at about 532.6 eV, this is assigned to a mixture of surface hydroxyl and carbonate groups. Nitrogen is found only on K2Nb4O11—N and the core-level N1s XPS is shown in FIG.5-B3. The N1s spectrum is divided into two components with peak energies of 398.7 and 400.8 eV, respectively, which are assigned to be (N)i/(NO)Oand (NO)i/(NO2)O, respectively R. Asahi, T. Morikawa,Chem. Phys.,2007, 339, 57-63). (N)irepresents N in the interstitial space, (NO)Odenotes NO sitting at the site for the lattice O. Similarly, (NO)iand (NO2)Odesignate the interstitial NO and substitutional NO2for the lattice 0, respectively.

The light absorption of the samples can be measured with UV/Vis diffuse reflectance spectroscopy (M. A. Butler,J. Appl. Phys.,1997, 48, 1914-1920).FIG. 6shows the UV/Vis diffuse reflectance spectra of K2Nb4O11and K2Nb4O11—N. It is known that the optical absorption coefficient near the band edge follows the equation (ahv)2=A(hv−Eg), wherein a, h, v, Egand A are the absorption coefficient, Planck constant, light frequency, band gap, and a constant, respectively. From this equation, the band gaps can be calculated to be 3.27 eV and 3.12 eV for K2Nb4O11and K2Nb4O11—N, respectively. The red-shift of the absorption wavelength of K2Nb4O11—N compared with that of K2Nb4O11, indicating that nitrogen-doping has a narrowing effect on the band gap of the material.

Photoluminescence emission spectra of semiconductors are related to the transport/relaxation behavior of the photo-induced electrons and holes, and thus can be used to determine band gaps, and to detect impurities and defects (Y. C. Zhu, C. X. Ding,J. Solid State Chem.,1999, 145, 711-715). In order to study the effect of nitrogen doping on the band gap of K2Nb4O11, PL spectra are shown inFIG. 7. There is a peak at about 370 nm for both K2Nb4O11and K2Nb4O11—N, which is due to the band gap of K2Nb4O11crystals. On the other hand, K2Nb4O11—N has an additional broad emission peak from 380 to 600 nm (C. Yu, J. C. Yu,Catal. Lett.,2009, 129, 462-470; Y. Qiu, S. Yang,Advanced Functional Materials,2007, 17, 1345-1352; J. C. Yu, J. G. Yu, W. K. Ho, Z. T. Jiang, L. Z. Zhang,Chem. Mater.,2002, 14, 3808-3816), which confirms the effect of nitrogen doping on narrowing the band gap of K2Nb4O11—N.

3.6 Photocatalytic Degradation of Orange G by UV and Visible Light

The results of the photo-degradation of OG using K2Nb4O11and K2Nb4O11—N as photocatalysts are shown inFIG. 8. The degradation of OG was negligible after 4 h when K2Nb4O11was used as the photocatalyst. On the other hand, when K2Nb4O11—N was used, nearly 90% OG was degraded after 2 h of irradiation, indicating that nitrogen doping greatly enhances the photocatalytic activity of the K2Nb4O11. Control experiments show that both light and the photocatalyst are required for the degradation of OG. The degradation of OG by K2Nb4O11—N is only slightly less efficient than TiO2P25 when 330 nm cutoff filter is used. On the other hand, when 399 nm cutoff filter is used, the photocatalytic activity of K2Nb4O11—N is much higher than that of TiO2P25. As shown inFIG. 9, nearly 90% of OG is degraded over the K2Nb4O11—N after 12 h of photoirradiation when a 399 nm cutoff filter is used, while only 46% is degraded over TiO2P25.

FIG. 10shows the spectral changes of OG during irradiation using 399 nm cutoff filter with K2Nb4O11—N. The main absorption band of OG is at around 478 nm, which decreases with time upon irradiation, but the λmaxdoes not change, indicating that the photodegradation does not occur by a dye self-photosensitized oxidative mechanism (T. Wu, G. Liu, J. Zhao, H. Hidaka, N. Serpone,J. Phys. Chem. B,1998, 102, 5845-5851). Apart from the peak at 478 nm, the photodegradation of OG by K2Nb4O11—N also results in the disappearance of the peak at 330 nm, indicating that both the OG chromophores and the aromatic rings have been destroyed (X. Li, N. Kikugawa, J. Ye,Chem. Eur. J.,2009, 15, 3538-3545). Also the total organic carbon (TOC) value of the solution decreases by approximately 25% at 90% OG conversion after 12 h of irradiation (seeFIG. 11), indicating that OG is mainly degraded to aliphatic organic compounds and is only partially mineralized to CO2and/or CO.

To assess the stability of the photocatalyst, a sample of K2Nb4O11—N was aged under ambient conditions for six months and its photocatalytic activity was then tested. As shown inFIG. 12, the photocatalytic activity of the aged K2Nb4O11—N sample decreases by only about 6% compared with the freshly prepared sample, indicating that K2Nb4O11—N photocatalyst is reasonably stable when stored under ambient conditions.

3.7 Photocatalytic Degradation of BPA by Visible Light

In the absence of a photocatalyst, the concentration of BPA remained virtually unchanged even after 20 h of visible light irradiation (399 nm cutoff). Also, BPA was not degraded by the photocatalysts in the dark. However, upon visible light irradiation in the presence of K2Nb4O11—N, 90% of BPA was degraded after 6 h. This photoactivity is higher than that of Degussa TiO2P25, and is much higher than that of pure K2Nb4O11and Nb2O5. These results confirm that nitrogen doping greatly enhances the photoactivity of K2Nb4O11.

3.8 Mechanism for the Photocatalytic Activity of K2Nb4O11—N

Taking K2Nb4O11—N as an example, the band structure for the K2Nb4O11—N is proposed, as schematically shown inFIG. 14. In the K2Nb4O11—N photocatalyst, there exist isolated N 2p states above the valence-band maximum of K2Nb4O11, which give rise to the strong absorption enhancement in the visible region. Under visible light irradiation, electron and hole pairs would be generated between impurity N 2p states and the conduction band of Nb 4d (equation 1).
K2Nb4O11—N+visible lighth++e−CB(1)

The excited electrons e−cBin the conduction band would move to the surface and combine with surface-adsorbed oxygen to produce O2.−superoxide anion radicals. The O2.−radicals could then react with H2O to produce .OH radicals (equation 2), which are known to be one of the most oxidizing species. On the other hand, the reactive holes h′ would react with adsorbed OH−on the catalyst surface to also form .OH radicals (equations 3-4).
O2ads.−+2H2O→.OH−+H2O2(2)
H2OOH−ads+H+(3)
OHads−+h+→.OH  (4)

The .OH radicals would react with OG and BPA to produce H2O and CO2via various intermediates.

In order to provide more evidence to support the proposed mechanism, the effects of pH on the photocatalytic degradation of BPA by K2Nb4O11—N were also studied (seeFIG. 15). It was found that photocatalytic activity of K2Nb4O11—N increases when the solution pH decreases. This may be explained by the processes shown in equations 5-7, which occur in the presence of H+.
O2ads.−+H+→HO2ads.(5)
HO2ads.+H2O→.OHads+H2O2ads(6)
H2O2ads+eCB−→.OHads+OHads−(7)
These processes facilitate trapping of the electrons in the conduction band of K2Nb4O11—N which produces .OHads. This trapping mechanism retards the recombination of electron-hole pairs and allows a more efficient charge separation. Hence, the transfer of trapped electrons to dissolved oxygen in the solution would be enhanced and more holes and hydroxyl radicals would be available for the oxidation of BPA on the catalyst surface as well as in the solution phase. This pH effect supports our proposed mechanism for the photocatalytic activity of K2Nb4O11—N under visible light.

In the present invention, A2Nb4O11—N has been prepared, fully characterized and used for the photodegradation of OG and BPA. XRD and SEM show that the crystal structures of K2Nb4O11—N and K2Nb4O11are nearly identical, but the surface profile has been changed significantly due to the nitrogen doping. XPS and PL indicate that the nitrogen doping primarily occurs at the surface of K2Nb4O11, while UV/Vis diffuse reflectance data further reveal that nitrogen doping narrows the band gap of K2Nb4O11. The photocatalytic activity of the K2Nb4O11—N has been evaluated by photodegradation of OG and BPA under visible light irradiation. The results show that the photocatalytic activity of K2Nb4O11—N is significantly higher than that of pure K2Nb4O11and Degussa TiO2P25 under visible light irradiation, highlighting the importance of nitrogen doping of K2Nb4O11. Overall, we have for the first time prepared and characterized A2Nb4O11—N with high photocatalytic activity even with visible light illumination. Moreover, this photocatalyst is very stable (at least six months under ambient conditions), its preparation is simple and highly reproducible, and it is easy to separate from the solution by simple centrifugation.