Photocatalytic Degradation Kinetics of Gaseous Formaldehyde Flow

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The photocatalytic degradation kinetics of gaseous formaldehyde flow using TiO2 nanowires HaiLong Dou, Dan Long, Xi Rao, Yongping Zhang, Yumei Qin, Feng Pan, and Kai Wu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06463 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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ACS Sustainable Chemistry & Engineering

The photocatalytic degradation kinetics of gaseous formaldehyde flow using TiO2 nanowires Hailong Dou, Dan Long, Xi Rao, and Yongping Zhang* Faculty of Materials and Energy, Southwest University, 2 Tiansheng Road, Beibei, Chongqing 400715, China Yumei Qin, Feng Pan, and Kai Wu* College of Chemistry and Molecular Engineering, Peking University, 202 Chengfu Road, Haidian, Beijing100871, China E-mail: [email protected] (Y.Z), and [email protected] (K.W.).

ABSTRACT: A high performance TiO2 nanowires photocatalyst was successfully prepared by a hydrothermal method to decompose gaseous formaldehyde into CO2 and H2O in a homemade tube reactor without secondary pollution under UV irradiation. The photocatalytic oxidization (PCO) kinetics fit well with the traditional Langmuir-Hinshelwood-Hougen-Watson (LHHW) model. Multiple parameters including formaldehyde concentration, flow rate, and light intensity were monitored online and proved to be key factors affecting the rate in the photocatalytic reactions. The crystallinity of photocatalyst and its surface reactive site density determined the

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adsorption equilibrium constant (KHCHO) of formaldehyde on TiO2. The experimental results show that the degradation kinetics of mobile gas-phase formaldehyde by TiO2 nanowires did not strictly conform to the first-order reaction kinetics, and its photocatalytic degradation rate increases with the increase of ultraviolet LED irradiation intensity. It takes only 8.6 minutes to completely degradate formaldehyde at a flow rate of 50 ml/min by 50 mg 700TiO2, and the reaction performance remains unchanged during the decomposing process of 1200 minutes. KEYWORDS:

Titanium

oxide

nanowires;

photocatalyst;

photocatalytic

oxidization;

formaldehyde; reaction kinetics

Introduction There existed a large number of organic pollutants in industrial production and human's everyday life in recent years, emanating great pollution to the atmospheres on which mankind depends. The main sources of air pollutants include toluene, benzene, formaldehyde (HCHO), second-hand smoke, and a series of toxic and harmful substances, such as CO, NOx, SO2.1 Among all those volatile organic compounds (VOC), formaldehyde is an irritant that produces allergic symptoms at low concentration, and a highly toxic and suspicious carcinogen recognized by the International Cancer Research Center (IAPC).2, 3 Formaldehyde is also the main source of indoor air pollution, and formaldehyde emission is considered as a major cause of "sick building syndrome". Excessive formaldehyde inhalation can cause human discomfort symptoms and aderse eents, such as sore throats and cough, choking sensation in chest, drowsiness, even leukemia, chromosome mutation and so on, endangering human health seriously.4, 5


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At present, much work has been done in air cleaning on removing the indoor HCHO pollutant included ventilation, adsorption6 and botanical air filtration.7, 8 However, most of these methods may cause secondary pollution and pollutant transfer, and pollution sources were not completely eliminated. The photocatalytic treatment of environmental pollutants is a green purify technology,9,

10, 11

and can achieve the real destruction of formaldehyde,12,

13, 14, 15

for

formaldehyde can be decomposed into CO2 and H2O. Limitation lies its low efficiency of the photocatalytic purify technology, thus developing high efficiency photocatalyst has a very promising prospect. Commonly used photocatalysts for treating polluted VOC gases included TiO2, ZnO, Fe2O3, WO3, g-C3N4, etc.16,

17, 18, 19, 20, 21

Titanium dioxide, as the most classical

photocatalyst, has been widely used in the treatment of low-concentration gaseous pollutant, the degradation of polluted water and the hydrogen generation via photocatalytic water splitting.22, 23, 24, 25

The photocatalytic efficiency of TiO2 is inhibited by its small surface area and wide band

gap (3.2 eV), limiting its application. There are several strategies to enhance the photocatalytic efficiency of titanium dioxide by enhancing the visible light adsorption and photocatalytic activity, such as tuning the electronic structure by doping non-metallic elements of C, N, P, S,26, 27, 28

and metal elements of V, Ag, Au, Pt, etc;29, 30, 31 changing the band structure of forming

composites with graphene, SnO2, CdS, BiVO4 and g-C3N4;32,

33, 34, 35, 36

and increasing the

specific surface area and the surface active sites by forming nanoscale structures, such as nanowires, nanosheets and hollow spheres.37,

38,

39

TiO2 nanowires exhibit excellent

photocatalytic responses under ultraviolet irradiation, and stable physical and chemical properties, non-toxicity, easy production with low-cost precursors. It is a promising issue to explore the possibility for enhancing the photocatalytic performance of formaldehyde degradation via ultraviolet irradiation by tuning the structure of TiO2 nanowires.


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In this paper, we reported the preparation of homogeneous TiO2 nanowires and its photodegradation of formaldehyde. TiO2 nanowires were synthesized by a hydrothermal method to enhance the specific surface area and the surface active sites in order to improve its photocatalytic efficiency. The test of photocatalytic degradation of formaldehyde is usually in a static environment with sampling and analysis at certain intervals. The real-time monitoring the photocatalytic degradation of formaldehyde by titanium dioxide has been rarely well-understood. In his work, a home-made instrument was designed to real-time monitor the photocatalytic degradation of low concentration mobile gas-phase formaldehyde by titanium dioxide nanowires. The reaction kinetics is of great significance for the practical application of the photocatalytic degradation of formaldehyde. Experimental details Synthesis of the TiO2 nanowires 3g titanium (IV) oxide (AeroxideTM P25) was dispersed in 300 ml 10 M NaOH solution under ultrasonic stirring, and the mixture was moved to 500 ml hydrothermal reactor and reacted continuously for 48 h at 165C. The white precipitate obtained by hydrothermal reaction was washed with HCl solution to pH neutral, then replaced Na+ with 0.1 M HCl solution for 24 h, and finally dried the precipitate to white titanate solid powder. The titanic acid powder was annealed at 600C, 650C and 700C for 90 min, with the heating rate of 3C/min. The obtained samples were labeled as 600TiO2, 650TiO2, and 700TiO2, respectively. Characterization


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The crystal structure was analyzed by X-ray diffractometer (XRD, Shimadzu XRD7000) with Cu K as irradiation (=0.15418nm), and transmission electron microscopy (TEM, Libra 200FE, Zeiss, Germany). The surface morphology was characterized by field emission scanning electron microscopy (FESEM, JSM-7800F10100). X-ray photoelectron spectroscopy (XPS) was carried out to analyze the chemical state and composition on a VG ESCALAB 250 spectrometer with Al Kα radiation (hν=1486.8 eV). The vibration state of the chemical bonds was analyzed by Fourier transform infrared spectroscopy (FTIR; Model Frontier, Perkin Elmer) using KBr pellets. The UV-Vis diffuse reflection spectra were obtained by a Agilent Cary 5000 UV-Vis-NIR spectrophotometer. The photoluminescence (PL) spectra were taken on a Hitachi F-7000 instrument with the xenon light as the excitation source. Nitrogen adsorption-desorption was conducted on a nitrogen adsorption apparatus at 77 K (Quadrasorbevo 2QDS-MP-30). The electrochemical test was carried out by AUTOLAB workstation (model PGSTAT 302N) in a three-electrode system. The 2 cm2 FTO conductive glass film was used as working electrode, the electrolyte was 0.25 M Na2SO4 solution, and the light source was 500 W xenon lamp with wavelength of 250-780 nm and intensity of 110 mW/cm2. Photocatalytic degradation of formaldehyde The photocatalytic degradation of mobile gas-phase formaldehyde was evaluated at room temperature by a home-made instrument, as indicated in Scheme 1. The catalyst was filled by slightly shaking in a 6 mm quartz tube with asbestos fiber sealed both ends, thus allows air and formaldehyde flow through easily. Before the degradation reaction, the flow rate of formaldehyde was well controlled, and the adsorption-desorption equilibrium was achieved. LED lamp of 365 nm was irradiated onto the surface of a quartz tube containing 50 mg


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photocatalyst connecting to the gas path of formaldehyde. The photocatalytic degradation of formaldehyde using TiO2 nanowires was determined by comparing the difference of formaldehyde concentration in the gas path before and after the degradation reaction. The photodecomposition processes were followed by a Shimadzu gas chromatography (GC-2018; molecular sieve columns TDX-01, TCD detector, and Ar carrier gas).

Scheme 1.Schematic diagram of home-made instrument for photodegrading mobile gaseous formaldehyde.

Results and discussion Structural characterization Figure 1 shows the XRD patterns of the TiO2 nanowires annealed at different temperatures. As a reference sample, the diffraction peaks for 600TiO2 are relatively weak, demonstrating the sample annealed at 600C has low crystallinity. The XRD patterns show that 650TiO2 and 700TiO2 samples have better crystallinity compared to 600TiO2 sample. Therefore, 650TiO2 and 700TiO2 samples were selected as the main catalysts to photodegrade formaldehyde. It is found


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that TiO2 nanowires exhibit several strong diffraction peaks located at 25.5, 48.2, 55.2, 62.9, corresponding to the (101), (200), (211), (204) planes of TiO2 with anatase phase structure (JCPDS 89-4921), which shows that the main crystal forms of 650TiO2 and 700TiO2 were the anatase phase. There appeared a diffraction peak located at 44.6, corresponding the characteristic plane (210) of rutile phase structure, and two other peaks corresponding to (002), and (301) plane of rutile phase (JCPDS21-1276) overlapped with anatase phase, indicating that the prepared titanium dioxide samples formed anatase-rutile heterojunction. Careful measurement shows that 700TiO2 has slightly better crystallinity than that of 650TiO2, since the full width at half maximum (FWHM) for the (101) plane is 0.23 and 0.26 for 700TiO2 and 650TiO2, respectively.

Figure 1. The XRD patterns of TiO2 nanowires annealed at different temperatures.

Figure 2(a) depicts the SEM image of the 700 TiO2 sample, indicating that the TiO2 nanowires appeared as a lathy nanowire structure, with diameters of 30-80 nm and length of 2-20 m.


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FESEM image in Figure 2 (b) indicates that the diameter of the nanowires is relatively uniform. Figure 2 (c) shows the TEM morphology of the TiO2 nanowire with an average diameter of about 50 nm. The high resolution TEM (Figure 2 (d)) showed the lattice fringes of TiO2 nanowires, and the inter planar spacing of 0.35 nm conforms to the (101) crystal plane of anatase phase, indicating that the sample is mainly composed of anatase TiO2, which is consistent with the XRD results.

Figure 2. (a, b) FESEM and (c, d) HRTEM images with different magnifications of 700TiO2 nanowires.


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Fourier transform infrared spectra in Figure 3 show the chemical bond vibration information of TiO2 nanowires. The absorption bands at 400-900 cm-1 are attributed to the stretching vibration of TiO and TiOTi.40 The absorption bands at 1080 cm-1 and 1140 cm-1 correspond to the COH stretching mode for species adsorbed on the surface of samples. The peaks at 1340 cm-1 and 1380 cm-1 attribute to the TiO stretching mode of TiO2. The broad bands at 1610 cm-1 and 3440 cm-1 are attributed to the stretching mode of hydroxyl groups (TiOH) adsorbed on the sample surface.37, 41 Moreover, the stretching vibration peaks of CO2 appeared at 2330 cm-1 and 2368 cm-1, are attributed to the adsorption of CO2 on the sample surface.37

Figure 3. FTIR spectra of TiO2 nanoswires annealed at different temperatures.

The XPS spectra (Fig. 4) were measured to analyze the oxidation state and the surface chemical composition of TiO2 nanowires. The high resolution spectra of Ti 2p in Fig. 4 (a) shows


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two symmetric peaks at 458.1 eV and 463.8 eV, which can be ascribed to the Ti 2p3/2 and 2p1/2 spin-orbital splitting photoelectrons, respectively. The binding energies for 650TiO2 and 700TiO2 are similar, and this is a typical characteristic of the formation of coordination between Ti(IV) and O atoms. The XPS spectra of O 1s in Fig. 4 (b) can be fitted into three peaks, and the binding energy of 529.3 eV, 530.8 eV and 531.8 eV can be ascribed to TiO (O1), adsorbed O2, and adsorbed water on the sample surface (O3), respectively. The intensity of TiO peak for 700TiO2 is slightly smaller than that of 650TiO2 in the O 1s XPS spectra, which may be due to the formation of more oxygen vacancies on the surface of 700TiO2. The peak at 529.3 eV of 700TiO2 shifts toward the lower binding energy position due to the formation of oxygen vacancies.

Figure 4. XPS high resolution spectra of Ti 2p (a), O 1s (b) of TiO2 nanowires annealed at different temperatures.


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To characterize the specific surface area of the samples, nitrogen adsorption-desorption isotherms were measured, as shown in Fig. 5 (a). The isotherms for the two samples are typical of type IV, suggesting the presence of mesopores. The BET specific surface area of 700TiO2 is 29.7 m2/g, slightly less than 34.3 m2/g of 650TiO2, whereas the BJH pore surface area of 700TiO2 is 61.5 m2/g, much larger than 40.7 m2/g of 650TiO2. It means that 700TiO2 can provide more active sites for the photocatalytic reaction, which is beneficial to enhance photocatalytic activity. This is consistent with the degradation reaction results. The corresponding mesoporous distribution of the sample is shown in Fig. 5 (b). The mesoporous size is mainly distributed in the range of 1-4 nm, manifested as a typical mesoporous material. The mesoporous surface of the material is helpful to increase the specific surface area of the material, thus enhance the photocatalytic activity.

Figure 5. Nitrogen adsorption-desorption isotherms (a), pore size distribution (b) of TiO2 nanowires annealed at different temperatures.


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UV-Vis absorption and PL spectra The UV-Vis absorbance spectra of the TiO2 nanowires were shown in Fig. 6 (a), which demonstrated that 700TiO2 has better optical absorption properties than 650TiO2, especially in the visible light region of 400-500 nm. P25 has no absorption band in the visible light region, while TiO2 nanowire has obvious visible absorption between 400 nm and 500 nm, which is beneficial to enhance the photocatalytic activity. The cut-off absorption edge of the TiO2 nanowires occurs at 394 nm, corresponding to the bandgap energy (Eg) of 3.14 eV, which is slightly smaller than the theoretical value of 3.2 eV. That shows that the TiO2 nanowires can be easily excited to promote photocatalytic reaction. Figure 6 (b) shows the PL spectra of the two samples, in which a broad PL band at approximately 347 nm is observed with the excitation wavelength of 290 nm. Generally, the lower PL intensity, the higher separation rate of the photogenerated electron-hole pairs. 700TiO2 shows a lower PL intensity than 650TiO2, which indicates that 700TiO2 has lower recombination rate of electron-hole pairs and is beneficial to enhance photocatalytic activity.42, 43, 44


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Figure 6.UV-Vis diffuse reflectance absorbance spectra (a), and PL spectra (b) of TiO2 nanowires annealed at different temperatures.

Electrochemical measurement Figure 7 (a) depicts the transient photocurrent response curves of TiO2 nanowires with several on-off cycles. It is clear that fast photocurrent response is observed, and without significant decrease after five cycles, indicating good reproducibility. The TiO2 nanowires were stable under UV light irradiation. Under the same bias voltage, the stable photocurrent value of 700TiO2 electrode is about 1.5 times higher than that of 650TiO2 electrode, which means that 700TiO2 electrode can produce more photoelectrons and efficient separation of electron-hole pairs.45 The electrochemical impedance spectra (EIS) of TiO2 nanowires are shown in Fig. 7(b). 700TiO2 exhibit smaller arc radius than that of 650TiO2 electrode, suggesting a less interfacial charge transfer resistance of working electrode and can effectively promote the transport and separation of photogenerated carriers in the photocatalytic reaction.45,

46, 47

Figures 7(c)-(d) show Mott-

Schottky curves of 650TiO2 and 700TiO2 electrodes, both exhibiting inverted S-shaped curves.


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The linear slope is positive, indicating that TiO2 is a typical n-type semiconductor. The slope of Mott-Schottky linear part of 650TiO2 electrode is larger than that of 700TiO2 electrode, indicating that the doping density of 650TiO2 electrode is lower than that of 700TiO2 electrode. The straight linear part of the Mott-Schottky curve of 650TiO2 electrode intersects with the xaxis at -0.46V (vs. Ag/AgCl), while the straight line part of the Mott-Schottky curve of 700TiO2 electrode intersects with the x-axis at -0.62V (vs. Ag/AgCl). M-S analysis shows that the flatband potential is -0.46 V, and -0.62 V for 650TiO2 and 700TiO2, respectively, indicating that 700TiO2 have stronger oxidizability compared with the 650TiO2 electrode.

Figure 7. Transient photocurrent responses (a), EIS spectra (b), and Mott–Schottky plots (c-d) of TiO2 nanowires annealed at different temperatures.


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Photocatalytic performance The photocatalytic performance of TiO2 nanowires were characterized by the degradation of mobile gas-phase formaldehyde at low concentration under 365 nm LED irradiation. The concentration of formaldehyde was fixed at 0.5 mg/m3 and the flow rate at 50 ml/min. The inlet and outlet of formaldehyde concentration were monitored to determine the formaldehyde degradation rate. Figure 8 (a) shows the degradation performance of TiO2 nanowires under 365 nm LED light (0.43 W). The catalyst reaches adsorption-desorption equilibrium within 100 min, and the outlet formaldehyde concentration decreases immediately after the lamp turned on, showing good photocatalytic degradation performance. However, the outlet formaldehyde concentration rises after a period of degradation and eventually reaches the equilibrium saturation. The reason is that 0.43 W lamp with low light power cannot degrade formaldehyde completely in time at the flow rate of 50 ml/min, and finally reaches the degradation equilibrium saturation. Figures 8 (b) and (c) showed the detailed diagrams of the corresponding degradation performance, as shown in Fig. 8(a). 650TiO2 decomposes formaldehyde completely within 19.2 min, while 700TiO2 only within 8.6 min. 700TiO2 demonstrated better degradation performance than 650TiO2. As a reference sample, the photocatalytic performance of commercial P-25 was shown in Fig. 8(b), showing very weak activity for formaldehyde decomposing. Figure 8 (d) shows the mass spectra of CO2 concentration in the gas path before and after degradation. The amount of CO2 in the gas path after formaldehyde degradation was about twice as much as that before degradation, which proves that formaldehyde was completely decomposed into CO2 under UV light by TiO2 nanowires.


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Figure 8. Degradation measurement of flowing gas-phase formaldehyde (a)-(c), mass spectrum of CO2 (d) of TiO2 nanoswires. PS: CH1- the intake formaldehyde concentration (Cin, fixed as 0.5mg/m3); CH2-the outlet formaldehyde concentration (Cout).

Figure 9 shows the stability test of 700TiO2, in which 700TiO2 accesses into the gas passageway at time 1 and reaches adsorption-desorption equilibrium at time 2. The degradation performance of formaldehyde under 365 nm UV irradiation (0.43 W) is measured first, showing formaldehyde cannot be completely degraded and the degradation equilibrium saturation is attained finally. The formaldehyde can be completely decomposed in real time by switching on the strong light of 365 nm UV light (7.9 W). The photocatalytic performance of formaldehyde


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degradation did not decrease after 12 cycles in 2600 min, indicating that the photocatalyst has good stability. TiO2 nanowires did not show obvious photochemical corrosion phenomena, and presented the catalyst could be used continuously.

Figure 9.Degradation measurement of flowing gas-phase formaldehyde stability test of TiO2. PS: CH1- the intake formaldehyde concentration (Cin, fixed as 0.5mg/m3); CH2-the outlet formaldehyde concentration (Cout); 1-TiO2 nanowires plug into the gas path; 2-the point of saturated adsorption of formaldehyde.

Kinetics analysis In order to estimate the kinetic rate of the photocatalytic degradation of mobile gas-phase formaldehyde, the concentration of formaldehyde was measured over time. The kinetic analysis was carried out for the experimental results of the formaldehyde degradation. The reaction mechanism involves adsorption/desorption of multiple gas molecules including HCHO, O2, H2O, and CO2 on the active sites, while the key reaction step to decompose formaldehyde is the


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interaction between adsorbed formaldehyde and adsorbed oxygen. The reaction process was proposed to progress as follows: K HCHO HCHO   HCHO

(1) (2)

K

O2 O2    O2

(3)

k HCHO  O2   CO2  H 2O

K

H 2O H 2O   H 2O  

(4) 

KO2

CO2   CO2  

(5)

Where  represents active site on the catalyst surface, k is the reaction rate constant (mgm3min-1).

KHCHO, KO2, KH2O, and KCO2 are the adsorption equilibrium constants for corresponding

compound (m3mg-1). The reaction between adsorbed HCHO and adsorbed O2 was proposed to be the rate determining step of the reaction process. The reaction rate could be expressed as:

r  k HCHOO2 (6) where HCHO and O2 are concentrations of adsorbed HCHO and adsorbed O2, which could be expressed based on Langmuir adsorption isotherm equation:

 HCHO 

O  2

K HCHO CHCHO (1  K HCHO CHCHO )

(7)

K O2 CO2 (1  K O2 CO2 )

(8)

Thus, the reaction rate can be expressed as:

r  k HCHOO2 

kK HCHO CHCHO K O2 CO2 (1  K HCHO CHCHO )(1  K O2 CO2 )

(9)


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where r is the reaction rate (mgm-3min-1), C is the concentration of reactant (mgm-3). Under the current experimental and practical condition, the O2 concentration was much higher than needed for the reaction. Then the reaction rate could be simplified as:

r  k HCHO 

kK HCHO CHCHO (1  K HCHO CHCHO )

(10)

The data generated in this study were fitted according to a general power law model resulting from Langmuir-Hinshelwood-Hougen-Watson (LHHW) model of reaction kinetics. When KC01, the LHHW formula can be transformed into a power function48, 49with a reaction rate expression of the form:

r

kK HCHO CHCHO dC  k HCHO   kC n dt 1  K HCHO CHCHO

(11)

where n is the reaction order, and k is the reaction rate constant with unit depends on the reaction order, n. Then,

dC  kdt Cn

(12)

C01 n  Ct1 n  kt 1 n

(13)

According to equation (13), the experimental results in Fig. 10 were fitted by the reaction kinetics. For 650TiO2 sample, the corresponding data showed that the linear fitting was the best when n was between 1.2 and 1.3. For 700TiO2 sample, the corresponding data showed that there was a good linear fitting when n was near 0.6. The results show that the degradation of formaldehyde in mobile gas phase by TiO2 nanowires was different from the first-order reaction kinetics reported in literatures15, 16, 50. The deviation from the first order reaction may be caused by the condition that KC0 is not much smaller than one.


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Figure 10. Fit of formaldehyde conversion data to a linearized n order reaction kinetics of 650TiO2 (a), and 700TiO2 (b).

In order to further explore the real-time formaldehyde degradation performance using TiO2 nanowires, formaldehyde degradation performance was tested under different flow rates and different light intensities, as shown in Figure 11. Fig. 11 (a) shows the formaldehyde removal performance of 650TiO2 under 365 nm UV light (1.2 W) at different flow rates, demonstrating that formaldehyde can be completely degraded in real time below the flow rate of 200 ml/min. When the flow rate exceeds 200 ml/min, formaldehyde cannot be completely removed at one time, reaching the degradation saturation. To explore the effect of light intensity, a 1.2 W UV lamp is switched to a weak light of 0.6 W, and the test results are shown in Fig. 11 (b). Fig. 11 (b) shows that 650TiO2 can only completely degrade the polluted gases with a flow rate of less than 50 ml/min in real time. With the increase of flow rate, the concentration of formaldehyde increases step by step, and finally the degradation reaches equilibrium saturation, indicating the degradation degree decreases continuously. The results show that the degradation of formaldehyde by TiO2 is related to the flow rate, and the photocatalyst has the rate related ACS Paragon Plus Environment

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degradation saturation. The degradation performance is also related to the intensity of lamp source. The higher the intensity of lamp source, the better the photocatalytic performance.

Figure 11.Variaton of formaldehyde concentration at different volumetric flow rate under 365 nm LED irradiation with power of 1.2 W (a) and 0.6 W (b) for 650TiO2 photocatalyst. PS: CH1Cin (fixed at 0.5mg/m3), CH2-Cout.

Finally, a tentative mechanism for photocatalytic degradation of formaldehyde was proposed by taking above-mentioned experimental results into consideration. High performance photocatalyst of TiO2 nanowires were successfully prepared by hydrothermal reaction. The photocatalytic degradation of formaldehyde is affected by its crystallinity, surface structure, which affect the adsorption equilibrium constants KHCHO. The TiO2 nanowires can decompose gaseous formaldehyde into CO2 and H2O under 365 nm UV lamp,11 as shown in Scheme 2, without secondary pollution, the reaction process is as follows: Photocatalyst → e- + h+

(14)

H2O → H+ + OH-

(15)

h+ + OH-→▪OH

(16)


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HCHO + ▪OH → HCO +H2O

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(17)

HCO + ▪OH → HCOOH

(18)

HCOOH + 2h+→ CO2 + 2H+

(19)

The kinetic simulation of photocatalytic degradation of formaldehyde deduces that the photocatalytic degradation of formaldehyde by titanium dioxide is not a first-order kinetic reaction, and the coverage of the active adsorption centers and the active adsorption centers of TiO2 nanowires on the catalyst surface has a great influence on the photocatalytic performance. By testing the relationship between the flow rate and formaldehyde concentration, it is concluded that the photocatalyst has a critical degradation point, and formaldehyde cannot be completely degraded in real time beyond the critical degradation value. The stronger the light intensity, the better the photocatalytic performance of TiO2 nanowires for formaldehyde degradation. The saturation point of photocatalytic degradation increases with the increase of light intensity. Generally speaking, the reaction rate constant was affected by many factors of isothermal adsorption and reaction process, such as light intensity, reactant concentration, gas flow rate, oxygen concentration, water vapor content and temperature.

Scheme 2. Schematic diagram of photocatalytic degradation of formaldehyde by TiO2 nanowires.


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Conclusion In conclusion, a facile hydrothermal approach was used to prepare TiO2 nanowires, whose crystallinity, surface oxygen vacancies, and reactive sites increase with the increase of annealing temperature. A home-made evaluation instrument was developed to monitor the photocatalytic degradation of mobile gas-phase formaldehyde real time. It is found that the formaldehyde cannot be completely degraded in real time beyond the critical flow rate. The stronger the light intensity, the better the formaldehyde degradation performance of TiO2 nanowires. The saturation point of photocatalytic degradation increases with the increase of light intensity. The reaction rate constant was affected by many factors of isothermal adsorption and reaction process, such as light intensity, reactant concentration, gas flow rate, oxygen concentration, water vapor content and temperature.

AUTHOR INFORMATION Corresponding Author * Author to whom correspondence should be addressed. E-mail: [email protected] (Y.Z), and [email protected] (K.W.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Conflicts of interest


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There are no conflicts to declare. ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 21173170 and 51801164).

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Graphical abstract:

The photocatalytic degradation of formaldehyde by TiO2 nanowires.


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Photocatalytic Degradation Kinetics of Gaseous Formaldehyde Flow

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