Impact of Zr-doped-TiO2 photocatalyst on formaldehyde degradation

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Kinetics, Catalysis, and Reaction Engineering

Impact of Zr-doped-TiO2 photocatalyst on formaldehyde degradation by Na addition Baoqing Duan, Yukang Zhou, Chao Huang, Qiong Huang, yingwen chen, Haitao Xu, and Shubao Shen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03016 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 2, 2018

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Industrial & Engineering Chemistry Research

Impact of Zr-doped-TiO2 photocatalyst on formaldehyde degradation by Na addition Baoqing Duana, Yukang Zhoua, Chao Huanga, Qiong Huangb, Yingwen Chena, b *, Haitao Xuc, *, Shubao Shena a. State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing 210009, China b. Jiangsu Collaborative Innovation Center of Atmospheric Environment and Equipment Technologies, Jiangsu Key Laboratory of Atmospheric Environmental Monitoring & Pollution Control, Nanjing University of Information Science & Technology, Nanjing 210044, China c

School of Environmental Science and Engineering, Nanjing Tech University,

Nanjing 210009, China

* Corresponding author. Yingwen Chen, State Key Laboratory of Materials-Oriented Chemical Engineering, College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing 210009, China Address: No. 5 XinMoFan Road, Nanjing Tech University, Nanjing, 210009, P. R. China Email: [email protected] Tel: +86 25 58139922

Fax: +86 25 83587326

Haitao Xu, School of Environmental Science and Engineering, Nanjing Tech University, Nanjing 210009, China Address: No. 5 XinMoFan Road, Nanjing Tech University, Nanjing, 210009, P. R. China Email: [email protected] Tel: +86 25 58138855

Fax: +86 25 83582347

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Abstract: In this study, a series of Na-Zr/TiO2 photocatalysts with different Na contents were synthesized via the sol-gel process. The characterization of the materials showed that Na addition facilitates the maintenance of anatase and excess Na led to particle size increases because of the formation of Na2Ti3O7. The XPS results suggested that Na loading can increase the number of oxygen vacancies, leading to more abundant surface hydroxyl groups. The enhanced photocatalytic activities under visible light irradiation can be attributed to a decreasing band gap and increasing oxygen vacancies. The efficiency of the Na-Zr/TiO2 samples was investigated by the degradation of formaldehyde under visible light. A high degradation rate of formaldehyde of approximately 96.6% was obtained with 2.5 mol% Na added to the Zr/TiO2 samples.

Keywords: Zr/TiO2; Na; Photocatalyst; Formaldehyde;

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1. Introduction Photocatalysis has potential applications in indoor air pollution remediation because of its excellent activity, non-toxicity, and good thermal stability of catalysts1, 2. Recently, significant attention is being focused on the TiO2 semiconductor3 and its environmental protection applications are becoming increasingly common. TiO2 can maintain a favorable photocatalytic activity because of its ability to generate hole (h+)/electron (e-) pairs. These charge carriers can be transferred to the surface of TiO2, where they can react with water and oxygen in the air to produce hydroxyl radicals (OH.) and superoxide anions (O2-). These radicals exhibit strong oxidizability and can initiate various redox reactions with adsorbates. Thus, TiO2 is considered a promising photocatalyst. However, it is difficult to improve the quantum efficiency of TiO2 photocatalysts, because the photogenerated electrons and holes recombine easily, which is unfavorable for the redox reaction. To overcome this challenge, elemental doping has emerged as a promising technique4. Elemental doping broadens the range of the visible light response and limits the recombination of electron-hole pairs, improving the photocatalytic efficiency. Wang et al.5 reported that N,P co-doped TiO2/expanded graphite exhibited high removal rates of microcystin-LR. Transition metal doping of TiO2 can cause lattice defects and surface modification, changing its photocatalytic performance6. It has been reported that when a suitable amount of Zr4+ replaces Ti4+ in the TiO2 lattice, recombination of electrons and holes is inhibited by charge trapping, resulting in improved photocatalytic properties7, 8. In a previous study9, we successfully prepared a Zr0.08Ti0.92O2 photocatalyst by sol-gel processing, which exhibited enhanced photocatalytic performance. However, UV-vis spectroscopy indicated that the catalytic ability of Zr0.08Ti0.92O2 was sufficient only in the UV region, and failed to make full use of visible light. In light of these results, numerous studies have attempted to further improve the visible photocatalytic activity of the related materials. The promising and effective approach of doping TiO2 with various elements has received significant attention. Many potential strategies to improve the visible photocatalytic activity of TiO2 involve broadening its light 3



response and shifting the optical absorption from the ultraviolet to the visible light region, which has been achieved by doping with metals and nonmetals or coupling with semiconductors10. Liu et al.11 prepared Mo-N co-doped mesoporous TiO2 that exhibited a narrow band gap and intense light absorption in the visible region. The improved performance of TiO2 doped with suitable elements can be attributed to the synergistic effects among the doped elements. Recently, alkali-catalyzed materials have become an increasingly popular research topic, as alkali doping can enhance the efficiency of electron transfer as well as the production of free radicals12. In addition, ceramic photocatalysts containing alkaline titanate adopt a tunnel structure with excellent chemical stability and low metal content13. Li et al.14 reported that alkali metals are responsible for the dispersion of Pt species in Pt/TiO2 catalysts and improved photocatalytic activity towards formaldehyde oxidation. Thus, the role of alkali metals in photocatalysis has attracted increasing attention, but uncertainties remain regarding the photocatalytic performance of alkali metal-doped TiO2. Aubry et al.15 reported that Na+ would become a recombination center for electrons and holes, and hinder the formation of anatase. The effect of alkali metals on the photocatalytic properties remains unclear, and it is necessary to study the mechanism of alkali metal doping of TiO2. This study is regarding Zr-doped TiO2 with Na modification, which has not been reported to date. In this study, we explore the mechanism of Na addition and its effect on the photocatalytic activity of Zr-doped-TiO2 prepared by the sol-gel method. The doping and Na addition were performed to broaden the light response and shift the optical absorption of the prepared material from the ultraviolet to the visible light region. The photocatalytic characteristics of the as-synthesized Zr-TiO2 with Na addition (Na-Zr/TiO2) were evaluated by degrading low-concentration formaldehyde under visible light. 2. Experimental 2.1 Materials Tetra-n-butyl titanate (C16H36O4Ti, 98%), absolute ethyl alcohol (C2H6O, 99.7%), 4


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zirconium oxychloride octahydrate (ZrOCl2·8H2O, 99%), acetic acid (CH3COOH, 99.5%), P25 (TiO2, 99%) and sodium hydroxide (NaOH, 96%) were used in this study without further purification. 2.2 Catalyst preparation Solution A: Tetrabutyl titanate was dissolved in ethanol with vigorous stirring for 0.5 h to obtain a yellowish transparent sol. Solution B: The desired amount of zirconium oxychloride octahydrate was dissolved into acetic acid and added to absolute ethyl alcohol. Subsequently, sodium hydroxide (0 mol%, 1 mol%, 2 %, 2.5 mol%, 3 mol%, and 4 mol%) was added to the solution. Under vigorous stirring, solution B was added dropwise to solution A. Subsequently, a yellowish transparent solution was obtained under continuously stirring for 1 h. The obtained gel was then placed indoors under a normal air atmosphere to age naturally and then dried for 2 h at 80 °C. Finally, the catalyst precursor was calcined at 500 °C for 2.5 h in a muffle furnace to obtain Na-Zr/TiO2 photocatalysts with different Na-to-Ti ratios which are designated Na-Zr/TiO2(0%), Na-Zr/TiO2(1%), Na-Zr/TiO2(2%), Na-Zr/TiO2(2.5%), Na-Zr/TiO2(3%), and Na-Zr/TiO2(4%). 2.3 Photocatalyst characterization X-ray diffraction (XRD) patterns of the prepared Na-Zr/TiO2 photocatalysts were recorded using an X-ray diffractometer (D/max-RB, Rigaku, Japan) equipped with Cu Kα radiation (λ = 0.15406 nm) in the 2θ range of 5–80° at 4°/min. K is a dimensionless shape factor, λ is the X-ray wavelength, and θ is the Bragg angle. The standard diffraction patterns of anatase and rutile were used for comparison with the obtained XRD patterns. The morphology and lattice of the prepared Na-Zr/TiO2 were observed by transmission electron microscopy (TEM, JEM-2010 UHR, Japan). The ultraviolet-visible (UV-vis) spectra of the samples were recorded on a UV-vis spectrophotometer (Lambda 950, Perkin-Elmer, USA) with an integrating sphere attachment. The scanning range was between 200 and 800 nm and BaSO4 was used as a reference. The Brunauer-Emmett-Teller (BET) surface areas of the fabricated Na-Zr/TiO2 catalysts were determined by nitrogen adsorption using a JW-BK122W analyzer (JW-BK122W, China). The composition and electronic structures of the 5



samples were determined by X ray photoelectron spectroscopy (XPS, PHI 5000 Versa Probe, Japan) with monochromatic Al Kα radiation (1484.6 eV). C1s (284.6 eV binding energy) was used to calibrate the binding energy (BE) values. The metal element contents of catalysts were detected using inductively coupled plasma-optical emission spectroscopy (ICP-OES). The elemental distribution of sample was detected using energy dispersive spectroscopy (EDS). Raman spectroscopy was performed on a LabRam HR Evolution (JY HR800, France) instrument using He-Ne (632.8 nm) as a laser source. 2.4 Evaluation of photocatalytic activity The photocatalytic activities of the prepared Na-Zr/TiO2 photocatalysts were evaluated by the degradation of formaldehyde under visible light. The photocatalytic tests of the catalysts were conducted in a cube reactor with a volume of 216 L, which was composed of conventional light-transmissive glass. The amount of photocatalyst loaded for test was 0.4 g. As shown in Figure.1, a 25 W energy saving light was placed in the center of the reactor with the height of 30 cm from the bottom. A quantitative formaldehyde solution is injected into the glass reaction chamber through the injection needle, and the wind speed controller is used to diffuse the formaldehyde for a certain period of time to uniformly disperse the gas in the glass chamber. The initial concentration of formaldehyde ranged from 1.0 to 1.2 mg/m3, which was quantified by a formaldehyde meter (PPM-400ST Technology Ltd, SuZhou Stanford Ltd., England). The catalyst sample was loaded in a glass plate (diameter 10 cm) spreading evenly. The degradation efficiency of formaldehyde was calculated according to the following equation: η = (C0 ‒ Ct)/C0 × 100% where η is the removal efficiency, C0 is the initial concentration of formaldehyde, and Ct is the residual concentration of formaldehyde.

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Figure. 1. Scheme of experimental setup ( 1. Glass reactor, 2. Sampling hole, 3. Formaldehyde Meter, 4. Glass lid, 5. Remote controlled fan, 6. Sample plate, 7. Light source )

3 Results and discussion 3.1 XRD analysis

Figure. 2. XRD patterns of the Na-doped Zr/TiO2 (a) and the refined peaks in the range of 30–35°(b).

Figure.2a shows the XRD patterns of the Zr/TiO2 and Na-doped Zr/TiO2 with different Na+ dosages. The diffraction angle and crystal particle size were also calculated to explain the change of TiO2 lattice owing to the Zr and Na doping. After Zr doping, the diffraction angle reduced from 25.28° to 25.13°, and the crystal particle size became bigger from 11.8 nm to 28.2 nm. As for Na-Zr/TiO2(2.5%) sample, the diffraction angle of 25.21° and crystal particle size of 27.6 nm were obtained, all samples exhibited anatase structure and no impurity peaks were observed, indicating that Zr4+ and Na+ ions entered the TiO2 lattice. With further increasing Na content, the anatase peaks gradually weakened or even disappeared, indicating that excessive Na+ inhibits the formation of anatase. In addition, as shown in Figure.2b for the Na-Zr/TiO2(3%) and Na-Zr/TiO2(4%) samples, the characteristic peak of Na2Ti3O7 7



indicated that some of the Na+ cannot replace Ti4+ in the lattice sites. It should be noted that no Zr species was observed on the Na-Zr/TiO2, indicating that the Zr species had entered the TiO2 lattice. This was likely because the radius of Na+ (0.102 nm)16 is larger than that of Zr4+ (0.072 nm)17, allowing Zr4+ to more easily enter the TiO2 lattice. 3.2 TEM analysis

Figure. 3. TEM images of the Na-Zr/TiO2(2.5%) (a-b) and Na-Zr/TiO2(0%) (c-d) samples.

TEM images of Na-doped Zr/TiO2 nanoparticles after roasting at 500 °C are shown in Figure.3. The particle size of the Na-Zr/TiO2(2.5%) sample was approximately 16–35 nm, whereas the Na-Zr/TiO2(0%) sample had an average particle size of 30–60 nm, as shown in Figures. 3a and 3c. There were no obvious changes in the shape of the Na-Zr/TiO2(2.5%) or Na-Zr/TiO2(0%) particles, appearing only as a slight irregularity. Na-Zr/TiO2(2.5%) particles were more loosely oriented, which may be due to diffusion of the doped sodium18. In addition, clear lattice fringes of nanocrystals can be seen in Figures. 3b and 3d. The d spacing of the Na-Zr/TiO2(2.5%) and Na-Zr/TiO2(0%) plane were both 0.35 nm, which suggested the presence of the crystallographic plane of anatase TiO2 (101)19. 8


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3.3 XPS analysis

Figure. 4. XPS survey scan of Na-Zr/TiO2(2.5%) and Na-Zr/TiO2(0%) (a), Ti2p XPS spectrum(b), Zr3d XPS spectrum(c), and O1s XPS spectrum(d).

To explore the chemical states and surface of the as-prepared Na-Zr/TiO2(2.5%) and Na-Zr/TiO2(0%) samples, XPS was performed (Figure. 4). As shown in Figure. 4a, all samples contained Ti, Zr, O, and C, and the Na1s peak at 1073 eV was observed with Na addition. The peak at 284.6 eV arose from C–C bonding, which is due to the absorption of CO2 on the surface of the photocatalyst20, or possibly from the complete calcination of hydrocarbons associated with the XPS instrument21. The Ti2p profile spectra of the Na-Zr/TiO2 with different Na+ doping extents, is shown in Figure. 4b. The Ti2p spectra exhibited two major peaks of Na-Zr/TiO2(2.5%) at 458.25 and 463.80 eV, which arose from Ti4+2p3/2 and Ti4+2p1/2, and two shoulder peaks at 456.9 and 462.3 eV were ascribed to Ti3+2p3/2 and Ti3+2p1/2. In contrast, the Ti2p spectra of Na-Zr/TiO2(0%) only showed peaks at 458.5 and 464.2 eV corresponding to Ti4+2p3/2 9



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and Ti4+2p1/2, respectively. This indicates that the presence of Ti3+ caused the oxygen vacancies to maintain the electrostatic equilibrium of the crystal structure22, which is likely mechanism by which moderate Na addition could facilitate the formation of oxygen vacancies. Figure 4c shows the XPS spectra of the Zr3d core level of the Na-Zr/TiO2(2.5%) and Na-Zr/TiO2(0%) samples, where the peaks can be attributed to Zr3d5/2 and Zr3d3/2. After Na addition, the Zr4+ peak shifted towards a lower binding energy, which was likely caused by the interaction of Na+, Zr4+, and defect states formed in TiO2. The O1s XPS spectra of both samples are shown in Figure. 4d. The broad peak at 529.2 eV in the Na-Zr/TiO2(2.5%) spectrum and at 529.8 eV in the Na-Zr/TiO2(0%) spectrum can be attributed to Ti−O, while the other peak can be ascribed to surface −OH bonds. It was clear that the proportion of surface −OH peaks in Na-Zr/TiO2(2.5%) was higher than that of Na-Zr/TiO2(0%), due to the formation of oxygen-defective sites. As Na+ and Zr4+ entered the TiO2 lattice simultaneously, more oxygen vacancies formed, which can bind with hydrogen atoms forming hydroxyl surface groups23. 3.4 ICP-OES analysis Table 1. Element composition of samples Samples

ICP-OES

Zr/Ti molar ratio

Na/Ti molar ratio

Ti(wt.%)

Zr(wt.%)

Na(wt.%)

ICP-OES

XPS

ICP-OES

Na-Zr/TiO2(0%)

36.73

13.06

-

0.186

0.203

-

Na-Zr/TiO2(2.5%)

36.16

13.28

0.42

0.193

0.209

0.024

In this study, Na-Zr/TiO2(2.5%) showed the best photocatalytic activity. The ICP analysis was made, and the data were shown in Table 1. Zr-Ti ratio by ICP-OES was similar to the result calculated from XPS, and the Na-Ti molar ratio was determined to be 0.024, which was close to the stoichiometric molar ratio of 0.025. So the actual Na-Zr molar ratio could be calculated to be 0.124. The compared results demonstrated that the Na doping was fulfilled greatly in this study. 3.5 EDS analysis 10


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Figure. 5. Ti-element mapping(a), Zr-element mapping(b), Na-element mapping of Na-Zr/TiO2(2.5%)(c)

Table 2 . Element composition of Na-Zr/TiO2(2.5%)

sample

Ti(wt.%)

Zr(wt.%)

Na(wt.%)

Zr/Ti molar ratio

Na/Ti molar ratio

Na-Zr/TiO2(2.5%)

70.09

29.19

0.71

0.22

0.021

The EDS analysis for Na-Zr/TiO2(2.5%) sample was examined. The EDS element mapping shown in Figure.5.(a)-(c) demonstrated that Ti, Zr and Na are homogeneously dispersed in the detected sample. And the analysis data listed in Table2. illustrated Zr/Ti molar ratio of 0.22 was obtained, which was is in accordance with the results of ICP-OES and XPS analysis. 3.6 UV-vis analysis

Figure. 6. UV-Vis diffuse reflectance spectra of Na-Zr/TiO2 with different Na loadings (a) and 11



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Kubelka–Munk function plots for the band gap calculation of the Na-Zr/TiO2 photocatalyst samples (b).

UV-vis spectroscopy was used to investigate the optical absorption behavior of the Na-Zr/TiO2 samples. As shown in Figure. 6a, some differences in the optical absorption was observed for Na-Zr/TiO2 as a function of sodium content. The addition of Na to the Zr/TiO2 enhanced the visible absorbance behavior of the samples. Figure. 6b shows the calculated band gap energy of the prepared Na-Zr/TiO2 samples. The band gap is approximately 2.93 eV for Zr/TiO2, while a red shift towards longer wavelengths was detected upon Na incorporation into Zr/TiO2. The effect of Na on the Zr/TiO2 was characterized by a broad absorption at 2.87 to 2.68 eV. Because Na+ and Zr4+ entered the TiO2 lattice simultaneously, an impurity level between the conduction band (CB) and valance band was introduced. Meanwhile, it could be speculated that the position of CB decreased and VB position keep unchanged, which also were approved in other studies19,21. In addition, when the Na content was greater than 2.5 mol%, a slight blue shift was observed, likely due to the large band gap of sodium titanates24. 3.7 BET analysis Table 3. Characteristics of the Na-Zr/TiO2 samples with different Na loadings. Samples

Na content

BET surface

Pore volume

Pore size

(mol%)

area (m2/g)

(cm3/g)

(nm)

Na-Zr/TiO2(0%)

0

60.836

0.103

5.833

Na-Zr/TiO2(1%)

1

60.474

0.148

9.806

Na-Zr/TiO2(2%)

2

59.159

0.133

9.002

Na-Zr/TiO2(2.5%)

2.5

58.041

0.130

8.946

Na-Zr/TiO2(3%)

3

49.574

0.130

10.449

Na-Zr/TiO2(4%)

4

44.307

0.117

10.609

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Figure. 7. N2 adsorption-desorption isotherms of the prepared Na-Zr/TiO2 samples.

The nitrogen adsorption-desorption isotherms of the prepared Na-Zr/TiO2 samples are shown in Figure.7. It can be seen that all isotherms were type IV, indicating that the Na-Zr/TiO2 samples had a mesoporous structure21. The Zr/TiO2 sample showed a H2 type hysteresis loop, which can be attributed to capillary condensation in the mesopores25. However, after Na addition, the hysteresis loop changed to a H3 type due to particle agglomeration forming slit pores with non-uniform size26, 27. The physical characteristics of the Na-Zr/TiO2 photocatalysts, including BET surface area, pore volume, and pore size are listed in Table 3. The specific surface areas of the Na-Zr/TiO2 samples with the Na content of 1 mol% to 2.5 mol% were approximately 59±1 m2/g, which is similar to that of Zr-TiO2. However, this value decreased dramatically when the Na content was increased above 2.5 mol%, which may be due to the excess Na blocking the small pores, making it difficult for formaldehyde to diffuse to the active sites. This is likely the reason behind the poorer performance of the Na-Zr/TiO2(3%) and Na-Zr/TiO2(4%) photocatalysts. 3.8 Raman analysis

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Figure. 8. Raman spectra of the Na-Zr/TiO2(0%) and Na-Zr/TiO2(2.5%) samples.

Figure. 8 shows the Raman patterns of the Na-Zr/TiO2(0%) and Na-Zr/TiO2(2.5%) samples. There were no obvious changes in the peak positions between the Na-Zr/TiO2(0%) and Na-Zr/TiO2(2.5%) spectra. The Raman active mode centered around 135, 388, 513.5, and 633 cm-1 were designated as Eg(1), Eg(2), Eg(3), and Eg(4), respectively. Both samples exhibited bands arising from anatase TiO2, but the classic anatase peak is at 144 cm-1 28, 29. The Raman peak in the Na-Zr/TiO2 spectrum shifted to lower wave number, indicating that the addition of Na+ and Zr4+ to the TiO2 lattice resulted in lattice expansion30, in agreement with the XRD analysis. It should be noted that the intensity of the Eg(1) peak of Na-Zr/TiO2(2.5%) broadened and became more intense compared to that of the Na-Zr/TiO2(0%) samples, indicating that Na-Zr/TiO2(2.5%) was more crystalline31. 4 Photocatalytic performance of catalyst 4.1 Comparison of the photocatalytic activities of Na-Zr/TiO2 samples with different Na-to-Ti ratios and P25

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Figure. 9. Photocatalytic degradation of formaldehyde using the prepared Na-Zr/TiO2 samples

Before the photocatalytic degradation, the absorption-desorption balance was evaluated firstly. The test was lasted for 6h until the formaldehyde concentration came to be stable in box. The results were shown in Figure.9. The removal efficiencies by absorption fluctuated from 5.2% to 7.4%, which meant there was little effect on the comparison of photocatalytic capacity among different catalyst samples. For Zr/TiO2, 83% degradation of HCHO was achieved under visible light after 60 h, and the photocatalytic properties were improved after Na addition. Na-Zr/TiO2(2.5%) showed the best photocatalytic performance among the evaluated photocatalysts with the HCHO degradation rate reaching 95% within 60 h under visible light. Several underlying mechanisms led to the photocatalytic enhancement of Na-Zr/TiO2. The Na-Zr/TiO2(2.5%) sample has a larger specific surface area than the other catalysts, increasing the surface adsorption of HCHO. In addition, Na-Zr/TiO2(2.5%) is more crystalline, which can accelerate the separation of electrons and holes, resulting in improved quantum efficiency31. Appropriate amounts of Na played an important role in enhancing the number of oxygen vacancies22, which likely reduced the Zr/TiO2 band gap and improved visible light utilization. However, excessive amounts of Na inhibited the photocatalytic activity, as the Na-Zr/TiO2(4%) sample exhibited the worst photocatalytic efficiency. The sample of Na-Zr/TiO2(4%) reached its degradation limit after 24 hrs with a lower catalytic degradation efficiency. Seen from Figure 6, the blue shift of absorbance wavelength was observed for Na-Zr/TiO2(4%), indicating a higher band gap energy. Also, XRD exhibited that excess Na doping affected the anatase crystals formation as shown in Figure 2, and specific surface area decreased simultaneously. So the photocatalytic activity of Na-Zr/TiO2(4%) was not satisfied because of the microstructure changing. Addition, substrate and by-product could accumulate onto the catalyst surface to occupy the active sites.

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Figure. 10. Photocatalytic degradation of formaldehyde using the Na-Zr/TiO2 and P25 samples

As shown in Figure.10, the test for formaldehyde degradation without catalyst loading under visible light was done, and the final formaldehyde removal rate of 9.42% was obtained in 60h. In the same conditions, the Na-Zr/TiO2(0%), Na-Zr/TiO2(2.5%) and commercial P25 catalysts were loaded in the box respectively. After 60h operation, the stable formaldehyde degradation rates of 83%, 95% and 78% were observed corresponding to the Na-Zr/TiO2(0%), Na-Zr/TiO2(2.5%) and commercial P25, which illustrated the high capacity for formaldehyde degradation by Na doping. 4.2 Effect of calcination temperature on the Na-Zr/TiO2(2.5%) photocatalyst

Figure. 11. XRD patterns of the Na-Zr/TiO2(2.5%) sample calcined at 400 to 700 °C (a). Photocatalytic degradation of formaldehyde with photocatalysts calcined at different temperatures (b).

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Table 4. BET surface areas of Na-Zr/TiO2(2.5%) calcined at various temperatures Sample

400 °C

500 °C

550 °C

600 °C

700 °C

BET surface area

120.52

58.04

39.8

29.3

11.25

(m2/g)

To explore the influence of crystallinity on the performance of the photocatalysts, the Na-Zr/TiO2(2.5%) sample was calcined at temperatures ranging from 400 °C to 700 °C. As shown in Figure. 11(a), when the calcination temperature was 400 °C, the diffraction peak of anatase is weak. It is likely that some TiO2 is present in its amorphous phase. With increasing calcination temperature, the anatase peak gradually increases in intensity. When the calcination temperature was 600 °C, the diffraction peak of rutile phase began to appear and increased gradually with further increasing calcination temperature. As shown in Table 4, the maximum specific surface area was obtained for the Na-Zr/TiO2(2.5%)-400 samples (120.52 m2/g), indicating that most of the TiO2 in Na-Zr/TiO2(2.5%)-400 exists in the amorphous phase, consistent with the literature13. Additionally, the specific surface area of Na-Zr/TiO2(2.5%) gradually decreased. When the roast temperature was 500 °C, the sample exhibited better crystallinity and optimum specific surface area. From the results of the photocatalytic behavior, it was clear that the Na-Zr/TiO2(2.5%) sample calcined at 500 °C showed optimal photocatalytic activity. However, the photocatalytic activity began to decrease and catalytic efficiency was the worst at 700 °C, which agreed with the results of the XRD and BET analysis. 5 Conclusion A series of Na-Zr/TiO2 photocatalysts were successfully synthesized by the sol-gel method and their photocatalytic behavior was evaluated by the removal of formaldehyde under visible light. Proper amounts of sodium doping was beneficial for the formation of oxygen vacancies and generating surface hydroxyl groups. Analyses of XRD and Raman spectra revealed that all as-prepared samples exhibited a pure 17



Page 18 of 21

anatase phase. UV-vis analysis showed that the absorption edge of Zr/TiO2 was red shifted and the band gap decreased upon Na+ addition. The activity of the Na-Zr/TiO2 samples was closely related to the sodium content and was optimal at 2.5%. The highest formaldehyde photodegradation was achieved with Na-Zr/TiO2(2.5%) due to its excellent crystalline structure and high specific surface area. Under visible light, Na-Zr/TiO2(2.5%) exhibited high degradation of formaldehyde of approximately 96.6% within 60 h. Thus, the as-prepared Na-Zr/TiO2 can be used for the removal of indoor formaldehyde. Acknowledgments This research was financially supported by the Project of Science and Technology Department of Jiangsu Province (BE2016769), Natural Science Foundation of China (No. 51172107), Open fund by Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control (KHK1707), A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References (1) Lin M, Jwo C, Ho H, et al. Using Box Modeling to Determine Photodegradation Coefficients Describing the Removal of Gaseous Formaldehyde from Indoor Air[J]. AEROSOL AND AIR QUALITY RESEARCH, 2017,17(1):330-339. (2) Shi H, Zou Z. Photophysical and Photocatalytic Properties of ANbO(3) (A = Na, K) Photocatalysts[J]. JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 2012,73(6):788-792. (3) Safajou H, Khojasteh H, Salavati-Niasari M, et al. Enhanced Photocatalytic Degradation of Dyes Over graphene/Pd/TiO2 Nanocomposites: TiO2 Nanowires Versus TiO2 Nanoparticles.[J]. Journal of colloid and interface science, 2017,498:423-432. (4) Prabakar K, Takahashi T, Nezuka T, et al. Visible Light-Active Nitrogen-Doped TiO2 Thin Films Prepared by DC Magnetron Sputtering Used as a Photocatalyst[J]. RENEWABLE ENERGY, 2008,33(2):277-281. (5) Wang X, Wang X, Zhao J, et al. An Alternative to in Situ Photocatalytic Degradation of microcystin-LR by Worm-Like N, P Co-Doped TiO2/expanded Graphite by Carbon Layer (NPT-EGC)

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