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Article

Trimesoyl Chloride-Melamine Copolymer-TiO2 Nanocomposites as High-Performance Visible-Light Photocatalysts for Volatile Organic Compound Degradation

by
Luqian Zhang
,
Chen Wang
*,
Jing Sun
and
Zhengkai An
School of Environmental Science and Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China
*
Author to whom correspondence should be addressed.
Catalysts 2020, 10(5), 575; https://doi.org/10.3390/catal10050575
Submission received: 6 April 2020 / Revised: 18 May 2020 / Accepted: 18 May 2020 / Published: 20 May 2020
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Abstract

:
Benzene is a typical volatile organic compound (VOC) and is found widely in industrial waste gases. In this study, trimesoyl chloride-melamine copolymer (TMP)-TiO2 nanocomposites with excellent photocatalytic efficiency in visible-light degradation of gas-phase benzene were synthesized via an in situ hydrothermal synthesis. The optimal conditions for TMP-TiO2 nanocomposite synthesis were determined by orthogonal experiments. The structural, physiochemical, and optoelectronic properties of the samples were studied by various analytical techniques. Ultraviolet-visible diffuse reflectance spectroscopy and surface photovoltage spectra showed that the positions of the light-absorbance edges of the TMP-TiO2 nanocomposites were sharply red-shifted to the visible region relative to those of unmodified TiO2. The most efficient TMP-TiO2 nanocomposite was used for photocatalytic oxidative degradation of gas-phase benzene (initial concentration 230 mg m−3) under visible-light irradiation (380–800 nm); the degradation rate was 100% within 180 min. Under the same reaction conditions, the degradation rates of unmodified TiO2 (hydrothermally synthesized TiO2) and commercial material Degussa P25 were 19% and 23.6%, respectively. This is because the Ti–O–N and Ti–O–C bonds in TMP-modified TiO2 reduce the band gap of TMP-TiO2. The amide bonds in the TMP decrease the TiO2 nanoparticle size and thus increased the specific surface area. The conjugated structures in the TMP provide abundant active sites for trapping photogenerated electrons and promote the separation and transfer of photogenerated electrons and holes.

Graphical Abstract

1. Introduction

Volatile organic compounds (VOCs) are a kind of important class of air pollutants and are usually found in the air of industrial areas [1]. VOCs are crucial precursors in the formation of fine particulate matter (PM2.5), ozone, and photochemical smog. They are harmful to the human body and have strong carcinogenic, teratogenic, and mutagenic properties [2,3]. Benzene is a typical VOC. It is highly toxic, is generated in large amounts from various sources, and is difficult to degrade naturally [4]. Benzene has been used by many researchers as a representative VOC pollutant. Because of increasing pollution by VOCs, it is important to develop techniques for separating or removing VOCs. The most common removal methods include adsorption, thermal burning, photocatalytic degradation, and plasma degradation [5,6,7].
Among these methods, photocatalytic oxidation of VOCs is considered to be one of the most promising environmentally friendly purification techniques because of its great performance, high stability, and cost effectiveness [8]. Among many catalysts, TiO2 has become the most commonly used photocatalyst because of its low price, photostability, and thorough degradation ability. However, its shortcomings such as the wide band gap for photocatalysis and the recombination of photogenerated electron-hole pairs have limited its applications.
Studies of photocatalytic techniques have therefore focused on developing methods for improving the electron-hole separation efficiency of TiO2 and expanding its response to visible light. Various strategies for modification of TiO2 photocatalysts to decrease the band gap, and enhance the electronic activity and quantum efficiency have been investigated. Estrellan and Gao et al. modified TiO2 by doping with metals and non-metals to decrease its band gap. However, the traditional doping modification process is complicated and the reaction conditions are uncertain. To improve the catalytic degradation efficiency of the catalyst under visible light, Mittal and Shayegan used noble metals and semiconductor oxides to construct heterojunctions or inorganic acids to further modify TiO2 to improve the electronic activity and quantum yield. These modification methods are expensive, toxic, and environmentally unfriendly [6,8,9]. Modification of TiO2 with polymers has caused widespread interest because the obtained catalyst has high efficiency, is non-toxic, environmentally benign, and the synthesis is convenient and cheap [10,11,12,13,14,15].
In this study, trimesoyl chloride-melamine copolymer (TMP)-modified anatase TiO2 materials were prepared by an in situ hydrothermal synthesis with tetrabutyl titanate as the precursor. The TMP-TiO2 nanocomposites were used in the photocatalytic oxidative degradation of gas-phase benzene under visible-light irradiation. The TMP-TiO2 nanocomposites were characterized by using XRD, Brunauer Emmett and Teller (BET), XPS, scanning electron microscopy (SEM), transmission electron microscopy (TEM), HR-TEM, UV-vis-DRS and surface photovoltage spectrum (SPS). Modification with TMP led to a 0.3-fold reduction in the particle size, better absorption in the visible-light range (400–800 nm), and increased the surface area ratio of the {001} crystal faces, which give strong oxidation. The photocatalytic efficiency of TMP-TiO2 in gas-phase benzene degradation was 100% in 180 min. These results show that the special backbone structure and large number of TMP functional groups enabled TiO2 to form Ti–O–C and Ti–O–N bonds, which decreased the TMP-TiO2 band gap. The conjugated structures in TMP can provide abundant active sites for trapping photogenerated electrons to promote the separation and transfer of photogenerated electrons and holes. In this study, high-efficiency, visible-light catalytic oxidation by TMP-TiO2 nanocomposites was achieved.

2. Results and Discussion

2.1. Optimal Conditions for TMP-TiO2 Synthesis

In order to analyze the influence of various experimental factors on the degradation efficiency of gaseous benzene, Minitab-17 was used to fit the experimental results in Table 1 below, and the regression equations of four factors and RE % were obtained:
RE (%) = RE/% = 36.85 + 17.55 A + 19.82 B − 31.72 C + 39.80 D − 3.735 A × A − 4.485 B × B + 8.850 C × C − 9.110 D × D.
As shown in Table 1, the variance analysis is carried out for the experimental regression model, and all the main factors of the model are examined, without considering the interaction between various factors. From the overall effect of the fitting, these models have a significant impact on the probability level of 5%, and the experimental error is small. Therefore, this regression equation can predict the photocatalytic degradation performance of the sample. According to the value of F and the signal-to-noise ratio (Figure 1) of degradation efficiency, the optimal synthesis conditions are: hydrolysis reaction temperature 80 °C, TMP: TiO2 precursor ratio 1:1, and then hydrothermal reaction at 180 °C for 8 h to prepare visible optical driven TMP-TiO2 nanocomposites (A2B2C3D2). The order of importance of the four control factors affecting degradation efficiency is: factor D > factor C > factor B > factor A.

2.2. Fourier-Transform Infrared (FTIR) and XPS Analysis

FTIR spectroscopy and XPS were used to investigate the interactions between the unmodified TiO2 and the TMP-TiO2 nanocomposites. The FTIR spectrum are shown in Figure 2. The FTIR spectrum of the unmodified TiO2 has peaks at 3360 and 1627 cm−1, which correspond to the stretching and bending vibrations peaks, respectively, of OH groups in physically adsorbed water and surface OH groups [16,17,18]. The peaks at 3220, 1718, 1535, and 1449 cm−1 in the TMP-TiO2 spectrum are not observed in the unmodified TiO2 spectrum and are respectively attributed to N–H stretching, C=O stretching, C–N–H bending vibrations, and COO-symmetric telescopic vibrations. All these new peaks indicate TMP adsorption on the TiO2 surface. The two absorption bands at 1249 and 1022 cm−1 are ascribed to Ti–O–C and Ti–O–N bonds, and suggest chemical bonding between the TMP and TiO2 in the TMP-TiO2 nanocomposites [19,20,21]. This was confirmed by XPS. The XP spectra (Figure 3a) show that the unmodified TiO2 contains only Ti, O, and C elements. The carbon arises from the uncertain hydrocarbons in the XPS instrument. The TMP-TiO2 nanocomposites (Figure 3a) contain Ti, O, N, and C elements, and the binding energies of Ti2p, O1s, N1s, and C1s are 460.7, 462.0, 401.9, and 284.8 eV, respectively. The total amount of nitrogen in the TMP-TiO2 nanocomposites was approximately 13.9 at %. In addition, we also recorded the high-resolution Ti2p XPS of the unmodified TiO2 and TMP-TiO2 nanocomposites (Figure 3b). The peaks in the TMP-TiO2 spectrum at 464.3 and 458.6 eV, which correspond to Ti2p1/2 and Ti2p3/2, are slightly transferred to higher binding energies compared with unmodified TiO2. These results indicate that the chemical environment of Ti in TMP-TiO2 has been altered by the formation of Ti–O–C and Ti–O–N bonds [22,23,24]. The O1s XP spectrum of TMP-TiO2 (Figure 3c) can be fitted by three peaks, corresponding to Ti–O (529.4 eV), O–H (531.2 eV), and N–O (532.7 eV) bonds, respectively. These results further confirm the presence of Ti–O–N bonds [25,26]. The C1s core levels (Figure 3d) of the TMP-TiO2 nanocomposites can be fitted by four peaks at binding energies of 285.0, 286.4, 288.3, and 289.2 eV, which are attributed to Ti–C–O, C–O, C–N, and O=C–O, respectively [14,27]. These results indicate that Ti–O–C bonds were formed in the TMP-TiO2 nanocomposites [28].

2.3. XRD Analysis

The effects of TMP on the structure of TiO2 synthesized by the hydrothermal method were studied by XRD; the patterns for the unmodified TiO2 and TMP-TiO2 nanocomposites are shown in Figure 4. The peaks at 25.3, 37.8, 48.2, 54.1, and 62.8 °C in the XRD pattern of the hydrothermally synthesized TiO2 are attributed to the (101), (004), (200), (105), and (205) reflections of anatase TiO2 (JCPDS No.84-1285). The rutile-phase diffraction peak at 28.2 °C (110) appeared in the XRD pattern of the TMP-TiO2 nanocomposites [29,30]. This shows that the crystalline structures of the unmodified TiO2 and the TMP-TiO2 nanocomposites consisted mainly of the anatase phase, with a little amount of the rutile phase which appeared in the TMP-TiO2. The rutile phase could be produced by interactions between the functional groups in TMP with hydroxyl groups on the TiO2 surface during hydrothermal synthesis. Modification with TMP broadened the anatase peaks, which indicates that the crystallinity of the TMP-TiO2 nanocomposites was lower than that of the unmodified TiO2 and the particle size decreased correspondingly. This could be caused by the effect of TMP amide bonds and indicates that modification with TMP could affect the crystalline structures during the hydrothermal synthesis of TiO2. Line-width analysis of the anatase (101) diffraction peak by the Scherrer formula showed that the estimated average crystallite sizes of the unmodified TiO2 and TMP-TiO2 nanocomposites are about 8.5 and 6.9 nm, respectively [31]. These results show that modification with TMP efficiently inhibits the TiO2 crystal size, which improves the photocatalytic activity. The photocatalytic activity of the anatase phase is higher than the rutile phase, but a mixed anatase–rutile phase could have a synergistic effect and inhibit electron-hole pair recombination. These results show that modification with TMP improves the photocatalytic activity.

2.4. BET Analysis

The nitrogen adsorption–desorption isotherm pore and the size distribution curves of unmodified TiO2 and TMP-TiO2 nanocomposites are shown in Figure 5, respectively. According to the classification of IUPAC, the unmodified TiO2 and TMP-TiO2 nanocomposites belong to the type IV isotherm with hysteresis loops, which indicates that the material has a porous like structure. The specific surface area of unmodified TiO2 and TMP-TiO2 are 37.44 and 67.29 m2g−1, and the average adsorption pore diameter is 16.9 and 9.7 nm, respectively. It can be seen that the specific surface area of TMP-TiO2 is higher than 55 m2g−1 of Degussa P25 TiO2 material [32]. Therefore, the high specific surface area of TMP-TiO2 is one of the reasons for its excellent photocatalytic degradation.

2.5. SEM and TEM Investigations

The morphologies and structures of the unmodified TiO2 and TMP-TiO2 nanocomposites were researched by SEM and TEM. The SEM image of the unmodified TiO2 in Figure 6a clearly shows that the particle size distribution of the unmodified TiO2 is not uniform and there is considerable aggregation. The SEM image in Figure 6b shows that the TMP-TiO2 particles are smaller than those of the unmodified TiO2 and better distributed. The TMP-TiO2 surface is rougher than that of unmodified TiO2, and the sample has a high degree of intraparticle porosity. The SEM images indicate that the TMP-TiO2 nanocomposites had excellent dispersion and intraparticle porosity compared with those of the unmodified TiO2.
The morphologies and crystal structures of the unmodified TiO2 and TMP-TiO2 nanocomposites were investigated by TEM. Figure 7a shows that the unmodified TiO2 particles have irregular shapes and are of non-uniform size. The TEM image of TMP-TiO2 in Figure 7c shows that the crystallite size of TMP-TiO2 is clearly smaller than that of the unmodified TiO2 and evenly distributed. Figure 7b,d shows the particle size histogram of TiO2 and TMP-TiO2, respectively. A random selection of 100 nanoparticles was made, and the crystallite sizes were determined by using ImageJ software. The crystallite sizes of the unmodified TiO2 is 6~13 nm, the average crystallite size is 9.71 nm, the crystallite sizes of TMP-TiO2 are 4~11 nm, and the average crystallite size is 6.84 nm. These values are consistent with those calculated by XRD through Scherrer formula. High-resolution TEM images are shown in Figure 7e. Three types of lattice are present, with lattice spacings of 0.19, 0.235, and 0.345 nm, consistent with the (200), (004), and (101) crystal planes of anatase TiO2 [24,30]. The corresponding selected-area electron diffraction (SAED) pattern (Figure 7f) confirms exposure of the (200), (004), and (101) crystal planes. In addition, according to previous reports, the (200) and (004) crystal planes both belong to the {001} crystal system, and their photooxidation activities are stronger than that of the (101) crystal plane [33,34]. This is one of the reasons for the strong catalytic activity of TMP-TiO2 in benzene degradation.

2.6. UV-Vis Diffuse Reflectance Spectra

Photo-absorption is one of the important factors in the performances of photocatalysts. Figure 8a shows that the unmodified TiO2 shows almost no absorption above 400 nm, whereas TMP-TiO2 shows obvious absorption in the visible-light region of 400–800 nm. In addition, the band gap energies of the unmodified TiO2 and TMP-TiO2 (Figure 8b), which were to be 3.05 and 2.30 EV, respectively, are shown using the Kubelka Munk function transformation. The chemical bonding between the hydrothermally synthesized TiO2 and TMP forms Ti–O–N and Ti–O–C bonds, which significantly reduces the band gap of TMP-TiO2 nanocomposite; thereby this could enhance its photocatalytic activity under visible-light irradiation [35].

2.7. SPS Analysis

In order to investigate the separation and recombination of photogenerated electrons and holes in TiO2 and TMP-TiO2 nanocomposites, the surface photovoltage spectra (SPS) were measured. As shown in Figure 9, we can clearly see that TiO2 has an obvious SPS response at 300–370nm, which is caused by the electron transition from the valence band to the conduction band of TiO2. In addition, we can see that the SPS response of the TMP-TiO2 nanocomposite is red-shifted to 550 nm, which is consistent with the UV-Vis results. This is attributed to the conjugated system in TMP trapping photogenerated electrons, which leads to the effective separation of photogenerated electron-hole pairs of TMP-TiO2.

2.8. Photocatalytic Efficiency

The CO2 gas production rates and benzene conversion rates with increasing irradiation time are shown in Figure 10. Under dark conditions, TMP-TiO2 reached adsorption saturation in 20 min, the removal rate of gaseous benzene was 13.2%, and CO2 was not produced. After 180 min of visible-light irradiation, the photodegradation rates of gaseous benzene by the unmodified TiO2 and P25 were only about 19% and 23.6%, respectively, and the produced CO2 contents were about 195 and 157 mg m−3, respectively. This shows that the photodegraded gaseous benzene cannot be effectively converted to CO2 and water when unmodified TiO2 and P25 are used as photocatalysts. TMP-TiO2 has better visible light photocatalytic performance. Gas-phase benzene was completely converted to CO2 and water under visible-light irradiation in 180 min; the amount of produced CO2 was 960 mg m−3 (Figure 10b). Moreover, the stability of visible light photocatalytic performance of the TMP-TiO2 nanocomposites were examined during four cycle experiments, and the results are shown in Figure 10c,d. At the same time, TMP-TiO2 after catalytic reaction was collected and washed by water and alcohol for many times, and then dried for characterization. We can see the FTIR, XPS, and XRD spectra before and after the photocatalytic reaction in Figure 11; the chemical bond composition and state of TMP-TiO2, and the crystal structure have not changed significantly, indicating that the TMP-TiO2 nanocomposites are well-recycled photocatalysts. The results indicate that the Ti–O–N and Ti–O–C bonds formed between TMP and TiO2 can improve the visible light absorption of TMP-TiO2 nanocomposites, and the conjugated system in TMP can provide a rich active site for capturing photogenerated electrons, which promotes the separation and transfer of photogenerated electrons and holes, and enhances their catalytic degradation activity.

2.9. Relationship between Physicochemical Properties and Photocatalytic Activity of TMP-TiO2 Nanocomposites

Scheme 1 shows a proposed mechanism of enhanced photocatalytic activity of the TMP-TiO2 nanocomposite photocatalysts under visible-light irradiation. The above results suggest that the special chemical structures of the TMP-TiO2 nanocomposites play crucial roles in improving the photocatalytic activity under visible-light irradiation. There are a lot of of C=C, hydroxyl, amidogen, carboxyl, and amide bonds on the surface of TMP [12,24,36]. Under high pressure and temperature, the amide bond and carboxyl group on the surface of TMP can form Ti–O–C and Ti–O–N bonds through chemical bond with the hydroxyl group on the TiO2 surface [21,22,23,37]. The amide bonds in the TMP cause the TiO2 nanoparticle size to decrease, and the relative surface area of the TiO2 nanomaterial increases. The Ti–O–N and Ti–O–C bonds between TiO2 and the TMP decrease the TiO2 band gap, and the conjugated system in the TMP provide abundant active sites for trapping photogenerated electrons; this promotes the separation and transfer of photogenerated electrons and holes [38]. The TMP-TiO2 nanocomposites therefore have high catalytic activity in benzene degradation under visible-light irradiation, and the mechanism of TiO2 modification by TMP is confirmed.

3. Materials and Methods

3.1. Chemicals and Materials

Melamine and benzene (AR) were purchased from the Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Absolute ethanol was purchased from the Tianjin FuYu Fine Chemical Co., Ltd. (Tianjin, China). Trimesoyl chloride was purchased from the Shanghai Aladdin Reagent Co., Ltd. (Shanghai, China). CuCl2 was purchased from the Xi Long Chemical Co., Ltd. (Shantou, China). A commercial TiO2 sample (P-25) was purchased from the Degussa Co., Ltd. (Steinheim, Germany). All these reagents were AR grade and could be used without purification, and all experiments used deionized water.

3.2. Preparation of Trimesoyl Chloride-Melamine Copolymer (TMP)

The TMP was synthesized according to the literature method [39,40,41]. Melamine (2.52 g) was added to a 100 mL three-necked, round-bottomed flask, which contained trimesoyl chloride (5.35 g) as both a reactant and the solvent. CuCl2 (0.1 g) was added as a catalyst. The mixture was refluxed at 95 °C with continuous stirring. After 6 h of reaction, the system was naturally cooled to room temperature. The resulting TMP was washed with a large amount of absolute ethanol and deionized water until the washing liquid was colorless. The TMP was collected by centrifugation and dried in a constant temperature blast furnace at 105 °C for 2 h.

3.3. Preparation of TMP-TiO2 Nanocomposites

Orthogonal experiment, factor design and signal-to-noise (S/N) ratio are used to optimally combine different selection factors to determine the most influential factors. The object of the research was to degrade gaseous benzene under visible-light irradiation by using the TMP-TiO2 nanocomposites as photocatalysts. The selected quality characteristic was therefore “the higher-the-better”, and the S/N ratio was determined from the following equation:
S N = − 10 log ( 1 n ∑ 1 y 2 )
where y is the measured value and n the number of measurements (in the present study n = 3).
Before the design experiment, we conducted a single factor experiment to prepare the visible light-driven TMP-TiO2 nanocomposites by an in situ hydrothermal method [42,43,44,45]. In the first part of the hydrolysis reaction, a molar ratio of 1:1 TMP and tetrabutyl titanate were added to a 100 mL three-necked round-bottom flask with anhydrous ethanol as the solvent, a certain amount of hydrochloric acid was added to adjust the pH of the reaction system to 3, and the mixture was heated at 80 °C for 4 h. In the second part of the hydrothermal reaction, the hydrolysis reaction product solution was transferred from the three-necked, round-bottomed flask to a Teflon autoclave. The autoclave was heated at 160 °C for 8 h, and then cooled naturally to room temperature. The products were collected by centrifugation and washed with a large amount of deionized water until the washing liquid was colorless. The products, i.e., the TMP-TiO2 nanocomposites, were dried at 105 °C for 2 h in an electro-thermos tatic blast oven. There were four controllable factors: hydrolysis reaction temperature, TMP:TiO2 precursor ratio, hydrothermal reaction temperature and hydrothermal reaction. We conducted a four factors three levels center combination design, and ran the experiment 3 times. The experimental factor level design and results are shown in Table 2 and Table 3.

3.4. Photocatalytic Activity Tests

The photocatalytic performances of the TMP-TiO2 nanocomposites, the hydrothermally synthesized TiO2 (unmodified TiO2), and P25 were determined from the decrease in the gaseous benzene concentration (initial concentration 230 mg m−3) at room temperature in an experimental reactor by using the experimental system designed in our laboratory.
A schematic diagram of the static experimental reactor is shown in Figure 12. The reactor was a 7.7 L cylindrical glass reactor. The outside of the reactor consisted of a water-bath sleeve to control the temperature of the catalytic reaction system. An internal 400 W halogen lamp (i.e., 38 mW cm–2) was used as a visible-light source (380–800 nm) and the light source was cooled by a cold trap. A cylindrical quartz catalyst carrier with a diameter of 11 cm and a height of 15 cm was placed outside the light source and a temperature sensor was placed on the quartz carrier. The reactor was kept completely sealed and high-purity air was used as the carrier gas in the degradation experiments. The concentration of gas-phase benzene was determined with a GC 2010 Plus system.
The weight of catalyst powder used in each set of experiments was kept at 0.5 g, dissolved using anhydrous ethanol, and uniformly applied to the quartz catalyst carrier. Prior to the experiments, the temperature of the water-bath sleeve was set at 25 °C, high-purity air was injected into the reactor, and then benzene (2 μL) was injected into the reactor by using a microsyringe. The light source was turned on for the photocatalytic degradation experiment, and samples (1 mL) were removed at a defined time interval (20 min). The gas-phase benzene content was determined by gas chromatography with nitrogen as the carrier gas.

3.5. Characterization

The morphologies of the unmodified TiO2 (the hydrothermally synthesized TiO2) and the TMP-TiO2 nanocomposites were investigated by transmission electron microscopy (TEM) (JEM2100, JEOL, Tokyo, Japan) and field-emission scanning electron microscopy (SEM) (Regulus8220, Hitachi, Tokyo, Japan). The unmodified TiO2 and TMP-TiO2 nanocomposites were examined by powder XRD (Shimadzu XRD-6100, Tokyo, Japan) with Cu Kα radiation. The chemical structures of the unmodified TiO2 and TMP-TiO2 nanocomposites were investigated by Fourier-transform infrared (FTIR) spectroscopy (Shimadzu IRAffinity-1s, Tokyo, Japan) at a resolution of 2 cm−1 and using KBr pellets. The surface compositions of the unmodified TiO2 and TMP-TiO2 nanocomposites were investigated by XPS (Thermo Fisher ESCALAB Xi+, Waltham, MA, United States). All binding energies were referenced to the C1s peak (284.8 eV) from adventitious carbon. UV-vis diffuse reflectance spectroscopy was performed with a UV spectrophotometer (Agilent CARY 300/PE lambda 750S, (Waltham, MA, United States) equipped with an integrating sphere attachment. The surface photovoltage spectrum (SPS) was measured by the built equipment. The powder sample was sandwiched between two ITO glass electrodes and placed in an atmosphere control vessel with a quartz window. The monochromatic light was obtained by using the light from the 500W Xenon lamp (CHF XQ 500W, Beijing, China) through the biprism monochromator (SBP3000, Beijing, China). A Geminiv2380 adsorption instrument was used, with N2 as adsorbate to record isotherms at 77 K. The specific surface area of TiO2 and TMP-TiO2 nanocomposites were calculated by using the Brunauer Emmett and Teller (BET) method from the adsorption desorption isotherms of nitrogen, and the pore sizes distribution of isotherm desorption branch were estimated.

4. Conclusions

In this study, we synthesized TMP-TiO2 nanocomposites by an in situ hydrothermal method and achieved efficient photocatalytic oxidation of gas-phase benzene under visible-light irradiation. The optimal conditions were a hydrolysis reaction, temperature set at 80 °C, a TMP:TiO2 precursor ratio 1:1, a hydrothermal reaction temperature of 180 °C, and a hydrothermal reaction time of 8 h. TEM images showed that the TMP-TiO2 nanocomposite prepared under the optimal conditions had a particle size of about 7 nm. A combination of Ti–O–N and Ti–O–C bonds in TMP-TiO2 was confirmed by FTIR spectroscopy and XPS. XRD patterns and high-resolution TEM images showed that the crystal structure of the TMP-TiO2 nanocomposites retained the anatase form and the {001} crystal system appeared. UV-vis diffuse reflectance spectroscopy and surface photovoltage spectra showed that the light-absorbance edge of the TMP-TiO2 nanocomposites clearly shifted to the visible-light region. The TMP-TiO2 nanocomposites completely degraded benzene gas under visible-light irradiation within 180 min; the photocatalytic activity was significantly higher than those of P25 and unmodified TiO2. Through the catalytic cycle experiments and XRD, XPS, FTIR characterization, TMP-TiO2 is a catalytic degradation material with excellent cycle stability.The TMP-TiO2 nanocomposites have a narrower band gap and smaller particle size, and the photogenerated electrons and holes are more effectively separated and transferred, compared with those in unmodified TiO2. This study shows that TMP-TiO2 nanocomposites are highly efficient visible-light- driven photocatalysts for gas-phase benzene degradation.

Author Contributions

Conceptualization, L.Z., C.W.; methodology, L.Z.; software, L.Z., J.S., Z.A.; formal analysis, L.Z.; data curation, L.Z., J.S.; writing—original draft preparation, L.Z.; writing—review and editing, L.Z., C.W.; visualization, L.Z., C.W., J.S.; supervision, C.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 10 00575 g001 550
Figure 1. Response graph for S/N ratios.
Figure 1. Response graph for S/N ratios.
Catalysts 10 00575 g001
Catalysts 10 00575 g002 550
Figure 2. Fourier transform infrared (FTIR) of unmodified TiO2 and trimesoyl chloride-melamine copolymer (TMP)-TiO2.
Figure 2. Fourier transform infrared (FTIR) of unmodified TiO2 and trimesoyl chloride-melamine copolymer (TMP)-TiO2.
Catalysts 10 00575 g002
Catalysts 10 00575 g003 550
Figure 3. (a) XPS of unmodified TiO2 and TMP-TiO2, (b) Ti2p XPS high resolution spectra of unmodified TiO2 and TMP-TiO2, (c) O1s XPS high resolution spectra of TMP-TiO2, and (d) C1s XPS high resolution spectra of TMP-TiO2.
Figure 3. (a) XPS of unmodified TiO2 and TMP-TiO2, (b) Ti2p XPS high resolution spectra of unmodified TiO2 and TMP-TiO2, (c) O1s XPS high resolution spectra of TMP-TiO2, and (d) C1s XPS high resolution spectra of TMP-TiO2.
Catalysts 10 00575 g003
Catalysts 10 00575 g004 550
Figure 4. XRD spectra of TiO2 and TMP-TiO2.
Figure 4. XRD spectra of TiO2 and TMP-TiO2.
Catalysts 10 00575 g004
Catalysts 10 00575 g005 550
Figure 5. N2 adsorption–desorption isotherm pore and the size distribution curves of unmodified TiO2 and TMP-TiO2 nanocomposites.
Figure 5. N2 adsorption–desorption isotherm pore and the size distribution curves of unmodified TiO2 and TMP-TiO2 nanocomposites.
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Catalysts 10 00575 g006 550
Figure 6. (a) Scanning electron microscopy (SEM) images of TiO2, and (b) SEM images of TMP-TiO2.
Figure 6. (a) Scanning electron microscopy (SEM) images of TiO2, and (b) SEM images of TMP-TiO2.
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Catalysts 10 00575 g007 550
Figure 7. (a) Transmission electron microscopy (TEM) images of TiO2. (b) particle size histogram of TiO2, (c) TEM images of TMP-TiO2, (d) particle size histogram of TMP-TiO2, (e) HR-TEM of TMP-TiO2, and (f) selected-area electron diffraction (SAED) pattern of TMP-TiO2.
Figure 7. (a) Transmission electron microscopy (TEM) images of TiO2. (b) particle size histogram of TiO2, (c) TEM images of TMP-TiO2, (d) particle size histogram of TMP-TiO2, (e) HR-TEM of TMP-TiO2, and (f) selected-area electron diffraction (SAED) pattern of TMP-TiO2.
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Catalysts 10 00575 g008 550
Figure 8. (a) UV-Vis DRS of TiO2 and TMP-TiO2, and (b) plots of transformed Kubelka-Munk function versus the energy of absorbed light.
Figure 8. (a) UV-Vis DRS of TiO2 and TMP-TiO2, and (b) plots of transformed Kubelka-Munk function versus the energy of absorbed light.
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Figure 9. Surface photovoltage spectrum (SPS) spectra of TiO2 and TMP-TiO2.
Figure 9. Surface photovoltage spectrum (SPS) spectra of TiO2 and TMP-TiO2.
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Figure 10. (a) Degradation rate of benzene gas concentration with visible light irradiation, (b) production rates for the CO2 gas, (c) cycling degradation rate of benzene on TMP-TiO2, and (d) CO2 produced with cycling runs.
Figure 10. (a) Degradation rate of benzene gas concentration with visible light irradiation, (b) production rates for the CO2 gas, (c) cycling degradation rate of benzene on TMP-TiO2, and (d) CO2 produced with cycling runs.
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Figure 11. (a) XRD spectra of TMP-TiO2 before and after photocatalytic reaction, (b) XPspectra of TMP-TiO2 before and after photocatalytic reaction, and (c) FTIR of TMP-TiO2 before and after photocatalytic reaction.
Figure 11. (a) XRD spectra of TMP-TiO2 before and after photocatalytic reaction, (b) XPspectra of TMP-TiO2 before and after photocatalytic reaction, and (c) FTIR of TMP-TiO2 before and after photocatalytic reaction.
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Catalysts 10 00575 sch001 550
Scheme 1. Proposed mechanism for enhancement of the photocatalytic activity of the TMP-TiO2 nanocomposite photocatalysts under visible-light irradiation.
Scheme 1. Proposed mechanism for enhancement of the photocatalytic activity of the TMP-TiO2 nanocomposite photocatalysts under visible-light irradiation.
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Catalysts 10 00575 g012 550
Figure 12. Benzene gas photocatalytic reactor.
Figure 12. Benzene gas photocatalytic reactor.
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Table
Table 1. Analysis variance of removal efficiency.
Table 1. Analysis variance of removal efficiency.
SourceDfAdj SSAdj MSF-ValueP-Value
Model81806.20225.776100.280.000Significant
A1113.17113.16550.260.000Significant
B1144.33144.33064.100.000Significant
C1369.53369.532164.130.000Significant
D1581.89581.892258.450.000Significant
A × A183.7083.70137.180.000Significant
B × B1120.69120.69153.610.000Significant
C × C1469.94469.935208.720.000Significant
D × D1497.95497.953221.170.000Significant
Error1840.532.251---
Total261846.73----
SR-sqR-sq (adj)R-sq(pred)
1.5004997.81%96.83%95.06%
Table
Table 2. Levels and codes of variables chosen for the experimental design.
Table 2. Levels and codes of variables chosen for the experimental design.
Coded LevelHydrolysis Reaction Temperature (°C)TMPs:TiO2 Precursor Ratio (-)Hydrothermal Reaction Temperature (°C)Hydrothermal Reaction Time (h)
−1601:21406
0801:11608
11002:118010
Table
Table 3. Experimental designs and results.
Table 3. Experimental designs and results.
RunCoded LevelsReal Values of ParametersResponse
ABCDA (°C)B (-)C (°C)D (h)Removal Efficiency (RE/%)Average S/N Ratio for RE
1−1−1−1−1601:2140672.8337.36466
2−1−1−1−1601:2140677.50
3−1−1−1−1601:2140671.16
4−1000601:1160886.5038.84016
5−1000601:1160887.30
6−1000601:1160888.70
7−1111602:11801092.6839.24549
8−1111602:11801090.23
9−1111602:11801092.13
100−101801:21601082.7338.24763
110−101801:21601082.35
120−101801:21601080.11
13001−1801:1180694.9139.45424
14001−1801:1180692.38
15001−1801:1180694.44
1601−10802:1140895.4139.68244
1701−10802:1140897.32
1801−10802:1140896.50
191−1101001:2180898.1939.90305
201−1101001:2180898.77
211−1101001:2180899.71
2210−111001:11401091.1439.19418
2310−111001:11401093.11
2410−111001:11401092.17
25110−11002:1160677.3537.80283
26110−11002:1160678.26
27110−11002:1160677.34

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Zhang, L.; Wang, C.; Sun, J.; An, Z. Trimesoyl Chloride-Melamine Copolymer-TiO2 Nanocomposites as High-Performance Visible-Light Photocatalysts for Volatile Organic Compound Degradation. Catalysts 2020, 10, 575. https://doi.org/10.3390/catal10050575

AMA Style

Zhang L, Wang C, Sun J, An Z. Trimesoyl Chloride-Melamine Copolymer-TiO2 Nanocomposites as High-Performance Visible-Light Photocatalysts for Volatile Organic Compound Degradation. Catalysts. 2020; 10(5):575. https://doi.org/10.3390/catal10050575

Chicago/Turabian Style

Zhang, Luqian, Chen Wang, Jing Sun, and Zhengkai An. 2020. "Trimesoyl Chloride-Melamine Copolymer-TiO2 Nanocomposites as High-Performance Visible-Light Photocatalysts for Volatile Organic Compound Degradation" Catalysts 10, no. 5: 575. https://doi.org/10.3390/catal10050575

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