preparation of hpa/bi2wo6 and its photocatalytic properties for denitrification

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY

Volume 42, Issue 8, Aug 2014 Online English edition of the Chinese language journal

Received: 22-May-2014; Revised: 24-Jun-2014. * Corresponding author. Tel: 18345995572, E-mail: [email protected]. Foundation item: Supported by the National Natural Science Foundation of China (50476091). Copyright 2014, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

RESEARCH PAPERCite this article as: J Fuel Chem Technol, 2014, 42(8), 978985

Preparation of HPA/Bi2WO6 and its photocatalytic properties for denitrification CHEN Ying1, XING Chen1, JI Sheng-lun1, LIANG Hong-bao2,* 1Provincial Key Laboratory of Oil & Gas Chemical Technology, Northeast Petroleum University, Daqing 163318, China; 2College of Mechanical Science and Engineering, Northeast Petroleum University, Daqing 163318, China

Abstract: The HPA/Bi2WO6 was prepared with Na2WO4·2H2O, Bi(NO3)3·5H2O and different surfactant templates. The prepared

sample was dried by the supercritical fluid drying (SCFD) method. It was characterized with XRD, FT-IR and SEM techniques and N2

sorption experiment. Its photocatalytic properties were evaluated with denitrification of nitrogen-containing simulated oil as model

reaction. It was shown that the sodium dodecyl sulfate (SDS) should be chosen as template. With this template, a highly dispersed,

crystalline and active photocatalyst was obtained. This catalyst had a larger specific surface area. The SCFD method effectively

decreased pore collapse and particles agglomeration degree, consequently increasing the specific surface area and improving the

catalytic performance. Immobilization of H3PW12O40 on Bi2WO6 increased the surface acid sites, and hence the photocatalytic activity.

When the H3PW12O40 loading was 10%, about 92.08% of nitrogen in simulated oil were removed by shining for 3 h with a xenon lamp

at the mcatalysts/msimulated oil ratio of 1:100.

Keywords: heteropoly acids; Bi2WO6; photocatalysis; supercritical fluid drying; denitrification

As a novel semiconductor photocatalyst, Aurivillius-type Bi2WO6 with a layer structure has a much narrower band gap width than TiO2

[1]. The dipole moment produced from the special structure of Bi2WO6 hinders the recombination of electrons and holes, increasing utilization efficiency of visible light[2]. Heteropoly acids (HPA) are a kind of acidic photocatalytic materials, which show high catalytic activity under UV light irradiation. However, it has a wide band gap and a HOMO-LUMO energy level of 3.1–4.6 eV, and can be easily dispersed in polar solvents. This significantly limits its application[3–6]. Immobilization of HPA on the semiconductor materials such as Bi2WO6 could make more excited electrons transfer to the HPA due to space potential difference originated from the band overlap effect. This decreases the recombination of electrons and holes and improves the light efficiency. As a good electron carrier, HPA could also transfer excited electrons from Bi2WO6 to dissolved oxygens through a “normal state-excited state-normal state” recycle process without changing itself. This further decreases the probability of electron and hole recombination and increases the light utilization efficiency[7,8]. The basic nitrogen-containing compounds in oil require stringent oil refinery conditions. NOx produced in the process of combustion causes a serious

environmental pollution[9]. HPA can readily attack on the basic nitrogen compounds in oil, greatly improving the denitrogenation performance of the catalyst. When evaporating the solvent, pore collapse and particle agglomeration would occur due to the surface tension effect, which would decrease the specific surface area of the catalyst. Nevertheless, this would be effectively alleviated by using the SCFD method[10]. 1 Experimental 1.1 Materials

Bi(NO3)35H2O, H4SiW12O40, H3PW12O40 and H3PMo12O40

were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. NaWO42H2O was purchased from Tianjin Dengfeng Chemical Reagent Factory. All chemicals were analytical grade and used without further purification. 1.2 Experimental process

1.2.1 Synthesis of Bi2WO6

CHEN Ying et al. / Journal of Fuel Chemistry and Technology, 2014, 42(8): 978985

Fig. 1 Photochemical reactor

In a typical process, 0.01 mol of Bi(NO3)3·5H2O and 0.005

mol of NaWO4·2H2O were dissolved in 40 mL of deionized water respectively. Then, the NaWO4·2H2O solution was added into the Bi(NO3)3·5H2O solution under vigorous magnetic stirring conditions. Subsequently, it was divided in two parts, and 0.5 g of SDS and PVP were respectively added to the two parts. After being stirred for 40 min, they were respectively sealed into two Teflon-lined stainless steel autoclaves, and heated at 160°C for 24 h. The product was filtered, washed with deionized water for several times and dried by the SCFD method. 1.2.2 Synthesis of HPA/Bi2WO6

Hydrothermal dispersion method: A certain amount of HAP

was dissolved in deionized water by stirring for 5 min. Then, the prepared Bi2WO6 was introduced. After further stirring for 20 min, the mixture was sealed into a Teflon-lined stainless steel autoclave and heated at 150°C for 22 h. Finally, the autoclave was cooled to room temperature, and the product was filtered, washed with deionized water and dried by the SCFD method. The resultant product was designated as HPA/Bi2WO6.

Incipient impregnation method: 0.001 mol of HPA was completely dissolved in quantitative amounts of deionized water. Then, Bi2WO6 was introduced. The obtained mixture was statically aged for 20 h at room temperature, and dried by the SCFD method. 1.2.3 Characterization

The crystalline structure of the catalyst was characterized

by powder X-ray diffractometer (XRD, DMAX-2000, graphite monochromatized Cu K radiation, = 0.15406 nm), the morphology of the catalyst was observed on a scanning electron microscope (SEM, ZEISS SIGMA) at an acceleration voltage of 7.0 kV. The specific surface area of the catalyst was calculated by the Brunauer-Emmett-Teller method (BET, NOVA-2000e). The UV-vis spectrum of the catalyst was

measured on a UV-vis spectrophotometer (UV-2550). 1.2.4 Determination of acid strength of immobilized HPA[11]

A quantitative amount of immobilized HAP was put in the

colorimetric cuvette. Then, solvent and indicator were introduced. The absorption peaks of the acidic and alkaline solutions were measured by UV-vis spectroscopy. Finally, the acid function value (H0) of HPA/Bi2WO6 was calculated by the following equation.

H0=pKa+log[B]/[BH+] (1) Where pKa is the equilibrium constant of the conjugate acid

of the indicator, [B] is the surface concentration of the basic indicator, and [BH+] is the surface concentration of conjugated acid.

Because the indicator molecules adsorbed on the HPA/Bi2WO6, the alkaline absorption peaks in the UV-vis spectrum decreased in intensity due to the decrease in the indicator concentration in the solution. In contrast, an increase in the intensity of the acid absorption peak was observed. Therefore, the acid strength of HPA/Bi2WO6 decreased with increasing H0

[12]. 1.2.5 Photocatalytic activity

The photocatalytic activity of HPA/Bi2WO6 was evaluated

by the denitrification degree of simulated oil (13% pyridine in light petroleum)[13]. In a typical operation, the catalyst, simulated oil and stirrer were added into the reactor which was shined by a 500-W xenon lamp. The photocatalytic set is shown in Figure 1.

At different irradiation time, about 8 mL of reaction mixture was taken out and centrifuged to remove the catalyst. The nitrogen content in the simulated oil was determined by the method for measuring basic nitrogen content in petroleum products (SH-T01629)[14]. The denitrification degree of the simulated oil was calculated by the equation of

wNB = (V1–V0)w×14×1000/m (2) Where V0 is the volume of perchlorate-ice acetic acid

standard solution consumed by the reference sample, V1 is the volume of perchlorate-ice acetic acid standard solution consumed by the measured sample, w is the mass concentration of perchlorate-ice acetic acid standard solution, m is the sample quality, and wNB is mass fraction of basic nitrogen. 2 Results and discussion 2.1 Characterization of Bi2WO6 prepared in the presence of different surfactants


Fig. 2 SEM images of Bi2WO6 (a) and its analogues prepared in the presence of PVP (b) and SDS (c)

2.1.1 SEM images

Figure 2 shows the SEM images of the Bi2WO6 prepared in the presence of different surfactants. Figure 2(a) shows the hydrothermally synthesized Bi2WO6 in the absence of surfactant. Its particles exhibit spherical shape with an average diameter of 2.5 m, which was composed of nanosheets. In contrast, the particles of Bi2WO6 prepared with PVP as template show a clew-like hollow sphere shape which has a shorter diameter of about 2 m (Figure 2(b)). The tighten

structure of clew-like particles decreased the specific surface area. In the case of Bi2WO6 prepared in the presence of SDS, the self-assembled spherical particles composed of nanoplates were observed (Figure 2(c)). The average diameter of these particles was about 2 m. Highly ordered nanoplate arrangement of these particles made the sample possess a lager specific surface area than the other two samples. This indicates that the sample synthesized in the presence of SDS would show a higher catalytic activity than the sample prepared with PVP as template.


Table 1 Surface areas of the samples prepared in the presence of

different surfactants

Sample Specific surface area A/(m2·g–1)

Bi2WO6 25.62

PVP-assisted Bi2WO6 21.23

SDS -assisted Bi2WO6 30.20

Fig. 3 XRD patterns of Bi2WO6 (a) and its analogues synthesized in

the presence of SDS (b) and PVP (c)

Fig. 4 XRD patterns of H3PW12O40/Bi2WO6 catalysts prepared by

the hydrothermal method (d) and the incipient impregnation method

(e), and Bi2WO6 (f)

2.1.2 XRD patterns

Figure 3 shows the XRD patterns of the prepared Bi2WO6

samples. The diffraction peaks are typical for Bi2WO6 (JCPDS No. 26-1044, a = 0.5480 nm, b = 0.5480 nm, c = 1.1500 nm). No other peak was observed, indicating that all the samples are pure Bi2WO6. Nevertheless, the intensity of the diffraction peaks of Bi2WO6 prepared in the presence of SDS was much more intense than those of the other two samples. This indicates that the SDS has a positive effect on the formation of Bi2WO6 structure. 2.2 XRD patterns of H3PW12O40/Bi2WO6 catalysts prepared by different methods

Figure 4 shows the XRD patterns of H3PW12O40/Bi2WO6 catalysts prepared by different methods. The diffraction peaks of both the H3PW12O40/Bi2WO6 catalysts could be easily indexed to the orthorhombic Bi2WO6 (JCPDS No.26-1044, a = 0.5480 nm, b = 0.5480 nm, c = 1.1500 nm), indicating that the orthorhombic structure of Bi2WO6 was not changed by immobilization of H3PW12O40 by these two methods. Regardless of this, the H3PW12O40/Bi2WO6 catalyst prepared by the hydrothermal method contains more impurities than that obtained by the incipient impregnation method. Moreover, the intensity of its diffraction peaks is much lower. This shows that the incipient impregnation method keeps the crystal structures of Bi2WO6 better than the hydrothermal method. 2.3 Characterization of the H3PW12O40/Bi2WO6 catalysts dried by the different methods

Figure 5 shows the SEM images of the catalysts dried by

the SCFD and the conventional methods respectively. The SCFD method gave much small amounts of surface defects. Although some surface defects present in the catalyst can inhibit the recombination of electrons and holes to certain extent, the presence of too much surface defects promotes it because they can act as the recombination sites, and consequently, decreasing the photocatalytic activity[15].

Figures 6 and 7 show the pore size distributions and N2 sorption isotherms of the samples dried by the conventional and SCFD methods, and the detailed results are listed in Table 2. The SCFD method can effectively alleviate the pore collapse occurred in the conventional drying process[16,17]. The BET specific surface area of the catalyst dried by the SCFD method is about 30.19 m2/g, which is larger than that (25.34 m2/g) of the sample dried by the conventional method. The adsorption isotherms of both the catalysts have a distinct hysteresis loop in the p/p0 of 0.5–0.9. They belong to the type IV isotherm in the IUPAC classification, which is characteristic of porous materials. The catalyst dried by the SCFD method has a smaller pore diameter than that dried by the conventional method (Figure 6). 2.4 Comparison of surface acid strength of the H3PW12O40/Bi2WO6 catalysts prepared by the different methods

The diffuse reflectance (DR) UV-vis spectra of the Bi2WO6

(synthesized in the presence of SDS) supported H4SiW12O40, H3PW12O40 and H3PMo12O40 are shown in Figure 8. The catalytic activity increased with increasing HPA acidity, which strengthens with the valence state of central atom. The H0 of different HPA decreased in the order: H4SiW12O40/Bi2WO6 > H3PMo12O40/Bi2WO6 > H3PW12O40/Bi2WO6 (Figure 8 and Table 3).


Fig. 5 SEM images of H3PW12O40/Bi2WO6 dried by the SCFD (a) and the conventional (b) methods

Fig. 6 Pore size distributions of the catalysts dried by the SCFD (a)

and the conventional methods (b)

Fig. 7 N2 sorption and desorption isotherms for S-catalyst and

T-catalyst

Table 2 Surface area of the catalyst by different drying methods

Drying method Specific surface area A/(m2·g–1) Average pore size d/nm

Conventional drying catalyst (T-catalyst) 25.34 4.23

Supercritical fluid drying catalyst (S-catalyst) 30.19 3.84

Fig. 8 UV-vis spectra of (a) H8SiW12O42/ Bi2WO6; (b)

H3PW12O40/Bi2WO6 and (c) H7PMo12O42/Bi2WO6

This shows that the H3PW12O40/Bi2WO6 has the strongest

acidity, being consistent with the strongest acidity of H3PW12O40 among the tree types of HPA[18].

2.5 DR UV-vis spectroscopy

Figure 9 shows the DR UV-vis spectra of

H3PW12O40/Bi2WO6 and Bi2WO6. A red-shift occurred to the adsorption peak of H3PW12O40/Bi2WO6, compared to that of Bi2WO6. This is due to the coupling effect between H3PW12O40 and Bi2WO6

[19]. The optical absorption property of the catalyst is recognized as a key factor for determining its photocatalytic activity. In the H3PW12O40/Bi2WO6 photocatalytic system, H3PW12O40 accelerates the transfer of the electrons from Bi2WO6 surface to O2 through a “normal state-excited state-normal state” recycle process, which changes the photocatalytic course and improves the photocatalytic efficiency[20], but does not alter its own structure. This effectively decreases the recombination of electrons and holes[21]. The photocatalytic mechanism of the HPA/Bi2WO6 is shown in Figure 10. 2.6 Effect of the type and concentration of HPA on the denitrogenation degree


Table 3 H0 of different heteropoly acids supported on SDS/Bi2WO6

Catalyst type Acid absorption Alkali absorption

H0 value λ/nm value λ/nm

H3PW12O40/SDS/ Bi2WO6 0.803 279 0.939 535 0.767

H8SiW12O42/SDS/ Bi2WO6 0.959 293 0.609 562 1.032

H7PMo12O42/SDS/Bi2WO6 0.786 302 0.903 559 0.774

Fig. 9 DR UV-vis spectra of (a) H3PW12O40/Bi2WO6 and (b)

Bi2WO6

Fig. 10 Photocatalytic mechanism of the HPA/Bi2WO6

Figure 11 shows the effect of the type and content of HPA

supported on the Bi2WO6 (synthesized with SDS) on the denitrogenation degree of simulated oil obtained under the following conditions: mcatalysts/msimulated oil ratio of 1/300, xenon lamp illumination time of 3 h. It is clear that the sample dried by the SCFD method showed higher denitrification degree than the sample dried by the conventional method regardless of the content of HPA. The denitrogenation degree of Bi2WO6 was 51.78%. It increased with increasing acid strength of these samples. Thus, H3PW12O40/Bi2WO6 exhibited the highest denitrogenation activity. The optimum loading of H3PW12O40 was 10%. Although a further increase in the H3PW12O40 loading increased the surface acidity, it affects the optical excitation performance of Bi2WO6, decreasing the denitrogenation degree. 2.7 Effect of catalyst/oil ratio on the denitrogenation degree

Fig. 11 Influence of the type and concentration of HPA on the

denitrogenation rate

: H3PW12O40/Bi2WO6 (S); : H7PMo12O42/Bi2WO6 (S);

: H8SiW12O42/Bi2WO6 (S); : H3PW12O40/Bi2WO6(T)

Fig. 12 Influence of catalyst-oil ratio on the denitrogenation rate

1:50; 1:100; 1: 200; 1:300; 1: 400; 0

Figure 12 shows the denitrogenation degrees obtained at different catalyst/oil ratios and the H3PW12O40 loading of 10%. In the blank batch, the nitrogen content in the simulated oil was kept almost constant irrespective of the shining time of the xenon lamp. However, after addition of catalyst, the denitrogenation degree increased with increasing catalyst amount and shining time of the xenon lamp. Under the conditions of mcatalysts/msimulated oil of 1:100 and shining time of 3 h, the denitrogenation degree reached 92.08%. A further increase in the catalyst amount and shining time has no positive effect. 3 Conclusions


The HPA/Bi2WO6 was hydrothermally synthesized in the presence of different surfactants. SDS as template more easily controlled the morphology of Bi2WO6 than PVP. It resulted in the formation of 3 dimensional (3D) flower-like hollow particles assembled by 2D nanosheets. The average size of these particles was about 2 m and the specific surface area of the prepared Bi2WO6 was 30.19 m2/g. Compared to H4SiW12O40 and H3PMo12O40, H3PW12O40 showed much stronger acidity. The coupling effect between H3PW12O40 and Bi2WO6 led to a red-shift of the adsorption peak. Compared to the conventional drying method, the SCFD method significantly decreased pore collapse and particle agglomeration degree and surface defects but increased the specific surface area. This greatly decreased the recombination of electrons and holes, and consequently improved the photocatalytic efficiency. The sample prepared under the optimal conditions showed a denitrogenation degree of 92.08% when the catalysts/simulated oil ratio was 1:100 and the shining time of the xenon lamp was 3 h. References

[1] Mcdowell N A, Knight K S, Lightfoot P. Unusual

high-temperature structural behaviour in ferroelectric Bi2WO6.

Chem-Eur J, 2006, 12(5): 1493–1499.

[2] Tang J W, Zou Z G, Ye J H. Photophysical and photocatalytic

properties of Bi2WO6. Catal Lett, 2004, 92(53): 256–270.

[3] Matsuda A, Daiko Y, Ishida T, Tadanaga K, Tatsumisago M.

Characterization of proton-conductive SiO2-H3PW12O40 composites

prepared by mechanochernical treatment. Solid State Ionics, 2007,

178(7/10): 709–712.

[4] Hori H, Hayakawa E, Koike K, Einaga H, Ibusuki T.

Decomposition of nonafluoropentanoic acid by heteropolyacid

photocatalyst H3PW12O40 in aqueous solution. J Mol Catal A: Chem,

2004, 211(1/2): 35–41.

[5] Gkika E, Troupis A, Hiskia A, Papaconstantinou E. Photocatalytic

reduction of chromium and oxidation of organics by

polyoxometalates. Appl Catal B: Environ, 2006, 62(1/2): 28–34.

[6] Kozhevnikov I V, Matveev K I. Homogeneous catalysts based on

heteropoly acids(review). Appl Catal, 1983, 5(2): 135–150.

[7] Feng Y, Wu Q S, Zhang G Y, Sun Y Q. Visible-light responsed

Bi2WO6 photocatalysts. Progress in Chemistry, 2012, 24(11):

2124–2131.

[8] Li S T, Wu C D, Yan Y S, Lv X M, Huo P W. Photodegradation

of organic contaminant by heteropoly acid. Progress in Chemistry,

2008, 20(5): 690–698.

[9] Liu L H, Liu S Q, Chai Y M, Liu C G. Preparation mechanism

and hydridenitrogenation performance of nickel phosphide catalyst.

Journal of Fuel Chemistry and Technology, 2013, 41(3): 335–340.

[10] Zhang J C, Li Q, Cao W L. Preparation of nanosized TiO2-ZnO

composite catalyst and its photocatalytic performance for degradation

of phenol. Chinese Journal of Catalysis, 2003, 24(11): 831–834.

[11] Fu X Z, Ding Z X, Su W Y, Li D Z. Structure of titania-based

solid superacids and their properties for photocatalytic oxidation.

Chinese Journal of Catalysis, 1999, 20(3): 321–325.

[12] Sun Y P, Zhao L, Zhao J L, Hou Y P, Chong F G, Liang Y.

Preparation and photocatalytic activity of heteropolyacid supported

on titanium dioxide photocatalysts. Journal of Chemical Engineering

of Chinese Universities, 2006, 20(4): 554–558.

[13] Chen Y, Xing C, Ji S L, Liang H B, Zhang H Y, Chen Y.

One-step preparation of H3PW12O40/Bi2WO6 nano-photocatalysts by

microwave liquid process and its photocatalysis denitrification

properties. Chemical Journal of Chinese Universities, 2014, 35(6):

1227–1285.

[14] SH/T 0162-1992, Method for the determination of basic nitrogen

in petroleum products.

[15] Jing L Q, Sun X J, Cai W M, Li X Q, Fu H G, Fan N Y.

Photoluminescence of Ce doped TiO2 nanoparticles and their

photocatalytic activity. Acta Chimica Sinica, 2003, 61(8):

1241–1245.

[16] Brown Z K, Fryer P J, Norton I T, Bridson R H. Drying of agar

gels using supercritical carbon dioxide. J Supercrit Fluid, 2010, 54(1):

89–95.

[17] Garcla-Gonzalez C A, Uy J J, Alnaief M, Smirnova I.

Preparation of tailor-made starch-based aerogel microspheres by the

emulsion-gelation method. Carbohyd Polym, 2012, 88(4):

1378–1386.

[18] Wang D S, Yan L, Wang X L. Supercritical carbon dioxide

drying. Journal of Molecular Catalysis(China), 2012, 26(4): 366–375.

[19] Li L, Ma Y, Cao Y Z, Ji Y, Guo Y X. Preparation of periodic

mesoporous H6P2W18/TiO2(Brij-76) composite and microwave

enhanced photocatalytic degradation of monochlorbenzene. Acta

Physico-Chimica Sinica, 2009, 25(7): 1461–1466.

[20] Tang J W, Zou Z G, Ye J H. Photophysical and photocatalytic

properties of AgInW2O8. J Phys Chem B, 2003, 107(51):

14265–14269.

[21] Ozer R R, Ferry J L. Investigation of the photocatalytic activity

of TiO2-polyoxometalate systems. Environ Sci Technol, 2001, 35(15):

3242–3246.

preparation of hpa/bi2wo6 and its photocatalytic properties for denitrification

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