Highly Efficient Photo-Degradation of Gaseous Organic Pollutants Catalyzed by Diatomite-Supported Titanium Dioxide

Volatile organic compounds (VOCs) are the most harmful contaminants that have been identified, most of which are gaseous organic pollutants. In this study, TiO2@diatomite catalysts with various loading amounts of TiO2 were fabricated using a facile solvothermal method with anhydrous ethanol as a solvent for the removal of VOCs. X-ray diffraction analysis revealed that TiO2 has an anatase phase and the introduction of diatomite has no negative effect. The catalysts were characterized using scanning electron microscopy and transmittance electron microscopy techniques. The results indicate that after introducing diatomite, TiO2 nanoparticles are mostly square-like and intact, and are uniformly immobilized in the diatomite. Finally, their photocatalytic performance was investigated using liquid ultraviolet spectrometry and gas chromatography-mass spectrometry. Among the catalysts tested, 0.35TiO2@diatomite (with a mass ratio of TiO2 to diatomite of 0.35) exhibited higher photocatalytic activity than the other samples, i.e., pure TiO2 and diatomite, and could effectively remove acetone and benzene, demonstrating its potential market application and practical significance.


Introduction
Volatile organic compounds (VOCs) are dominant components of indoor air pollutants. Mainly originating from decorative materials, such as carpets, paints, wallpapers, or PVC, they are also emitted or generated during human activities such as cooking or smoking tobacco [1][2][3][4]. Their harm to human health can thus not be ignored. The main problems resulting from VOCs have been reported by numerous researchers, such as Dai et al., who reported the characteristics and inhalation health risks of VOCs in newly renovated homes in Shanghai, China, and concluded that the concentrations of some VOCs present a mean cancer risk above the acceptable level [5]. Data on the respiratory effects of indoor air pollution among the elderly have been scarce. Therefore, Bentayeb et al. reported the effects of indoor air pollution on the respiratory health of the elderly, and found certain links between a variety of diseases, including respiratory diseases, and indoor air pollution [6]. In addition, Ye et al. estimated the acute cardiorespiratory effects of ambient volatile organic compounds, and their findings further support a link between incomplete combustion and cardiovascular health, as well as a link between atmospheric oxidation products and respiratory health [7]. The above studies have commonly concluded that VOCs are extremely harmful to human health. In addition, according to a survey conducted by the World Health Organization (WHO), the number of premature deaths Thus far, TiO 2 supported by diatomite has not been utilized in the degradation of gaseous organic pollutants. Herein, we report the successful application of well-designed TiO 2 @diatomite as a photo-catalytic agent for the removal of gaseous organic pollutants. In addition, diatomite was selected as a carrier, and diatomite loaded with different ratios of TiO 2 was used as a catalyst for the degradation of VOCs at room temperature under UV-Vis illumination. This catalyst is inexpensive and easy to prepare, and is thus a promising application for use in the field of interior decoration.  Figure 1 shows the XRD patterns of the as-prepared catalysts. Both TiO 2 and XTiO 2 @diatomite show the coexistence of an anatase phase with no additional peak, implying that no impurity appears in TiO 2 when TiO 2 is supported on diatomite. As indicated in Figure 1, the intensity of the peak at 25.27 • decreases with an increase in the amount of diatomite. The diffraction peaks at 25 [42][43][44]. The peaks at 21.8 • and 36.5 • are attributed to the SiO 2 features in diatomite [44]. The reflection peak of diatomite becomes relatively low compared with that of TiO 2 , and the pure diatomite peak is not too weak, as shown in Figure 2 below. Therefore, the introduction of diatomite cannot affect the crystal structure of the catalyst. This result is consistent with the scanning electron microscopy (SEM) and transmittance electron microscopy (TEM) characterizations. and easy to prepare, and is thus a promising application for use in the field of interior decoration.  Figure 1 shows the XRD patterns of the as-prepared catalysts. Both TiO2 and XTiO2@diatomite show the coexistence of an anatase phase with no additional peak, implying that no impurity appears in TiO2 when TiO2 is supported on diatomite. As indicated in Figure 1, the intensity of the peak at 25.27° decreases with an increase in the amount of diatomite. The diffraction peaks at 25.27°, 37.82°, 47.94°, 53.79°, 54.96°, 62.65°, 70.22°, and 75.08° correspond to the crystal faces (101), (004), (200), (105), (211), (204), (220), and (215) of anatase TiO2, respectively [42][43][44]. The peaks at 21.8° and 36.5° are attributed to the SiO2 features in diatomite [44]. The reflection peak of diatomite becomes relatively low compared with that of TiO2, and the pure diatomite peak is not too weak, as shown in Figure 2 below. Therefore, the introduction of diatomite cannot affect the crystal structure of the catalyst. This result is consistent with the scanning electron microscopy (SEM) and transmittance electron microscopy (TEM) characterizations.

SEM and TEM Investigations
SEM, TEM, and high-resolution transmission electron microscopy (HRTEM) images of the catalysts are shown in Figure 3. As can be seen from A1 and A2 in the figure, diatomite without the

SEM and TEM Investigations
SEM, TEM, and high-resolution transmission electron microscopy (HRTEM) images of the catalysts are shown in Figure 3. As can be seen from Figure 3(A1,A2) in the figure, diatomite without the loading of TiO 2 nanoparticles looks like a sunflower with a smooth surface and numerous pores. The SEM images of pure TiO 2 displayed in Figure 3(B1,B2) indicate that the TiO 2 nanoparticles are square-like and approximately 10-15 nm in size, corresponding to the results of the TEM images ( Figure 3(E1-E8)). As shown in Figure 3(C1-C6), the TiO 2 nanoparticles of 0.35TiO 2 @diatomite are not only uniformly loaded on the surface of the diatomite, but also adhered to the pores. This indicates that TiO 2 can be firmly and evenly loaded on the surface of the diatomite. Figure 3(C1-C6) also show that the size of the TiO 2 nanoparticles loaded on the surface of the diatomite is approximately 10-15 nm, which is congruent with the TEM results shown in Figure 3(E1-E8). In addition, the other load catalysts shown in Figure 3(D1-D6) were also analyzed. As indicated in D1 and D2, the load of 0.30TiO 2 @diatomite is quite small, and the surface of the diatomite is thus not completely filled with TiO 2 , as opposed to 0.40TiO 2 @diatomite (Figure 3(D3,D4)), 0.45TiO2@diatomite (Figure 3(D5,D6)), and 0.50TiO 2 @diatomite (Figure 3(D7,D8)). For the latter, as the load mass increases, the degree of aggregation increases significantly, and the aggregation of 0.50TiO 2 @diatomite thus becomes the most serious. There is no doubt that aggregation will cause a reduction in the photocatalytic efficiency. However, the 0.35TiO 2 @diatomite load mass was moderate and evenly distributed across the diatomite surface. Therefore, the catalyst of 0.35TiO 2 @diatomite may perform better than other catalysts in terms of the photocatalytic efficiency for the following two reasons: First, the proportion of TiO 2 is full on the surface of the diatomite. Second, TiO 2 is evenly distributed and relatively less clustered. In addition, as shown in Figure 3(E2), the lattice distance of TiO 2 is approximately 0.35 nm, corresponding to the (101) plane of anatase TiO 2 . As clearly shown above, the size of the TiO 2 nanoparticles loaded on the diatomite is nearly the same as that of pure TiO 2 , indicating that the introduction of diatomite has almost no influence on the size and morphology of the TiO 2 . From the HRTEM images shown in Figure 3(E1-E4), it can be clearly seen that the specific shape of TiO 2 is square-like. Catalysts 2020, 10, x FOR PEER REVIEW 9 of 19

UV-Vis Diffuse Reflectance Spectra
As shown in Figure 4, the UV-vis diffuse reflectance spectra of the catalysts were measured to study the optical properties of the samples [45]. Clearly, for pure TiO2, there is no absorption at above 400 nm. However, the absorption intensity at approximately 400 nm is stronger for XTiO2@diatomite than for pure TiO2 and diatomite, and the absorption (at above 400 nm) of 0.35TiO2@diatomite is the strongest among the XTiO2@diatomites considered. It can also be seen that the catalyst of TiO2@diatomite shows a slight red shift, suggesting that the catalyst of TiO2@diatomite can be excited by the visible light and thus an increase in the photocatalytic activity of the catalyst.
It can be seen from Figure 4 that pure XTiO2@diatomite achieves absorption at 200-400 nm, although, according to previous studies [46], pure SiO2 has no absorption at 200-400 nm. Therefore, the composite formed through a combination of the two with Si-O-Ti, which shows a red shift, may be attributed to the formation of this chemical bond, which reduces the energy and causes a slight red shift.

UV-Vis Diffuse Reflectance Spectra
As shown in Figure 4, the UV-vis diffuse reflectance spectra of the catalysts were measured to study the optical properties of the samples [45]. Clearly, for pure TiO 2 , there is no absorption at above 400 nm. However, the absorption intensity at approximately 400 nm is stronger for XTiO 2 @diatomite than for pure TiO 2 and diatomite, and the absorption (at above 400 nm) of 0.35TiO 2 @diatomite is the strongest among the XTiO 2 @diatomites considered. It can also be seen that the catalyst of TiO 2 @diatomite shows a slight red shift, suggesting that the catalyst of TiO 2 @diatomite can be excited by the visible light and thus an increase in the photocatalytic activity of the catalyst.

BET Analysis
The specific surface area and pore size distribution of the different catalysts were analyzed based on the nitrogen adsorption and desorption. The results are summarized in Table 2. The pure TiO2 nanoparticles have the largest specific surface area of approximately 115.74 m 2 g −1 . However, the specific surface area and pore volume of the pure diatomite are quite small, and thus the diatomite It can be seen from Figure 4 that pure XTiO 2 @diatomite achieves absorption at 200-400 nm, although, according to previous studies [46], pure SiO 2 has no absorption at 200-400 nm. Therefore, the composite formed through a combination of the two with Si-O-Ti, which shows a red shift, may be attributed to the formation of this chemical bond, which reduces the energy and causes a slight red shift.

BET Analysis
The specific surface area and pore size distribution of the different catalysts were analyzed based on the nitrogen adsorption and desorption. The results are summarized in Table 2. The pure TiO 2 nanoparticles have the largest specific surface area of approximately 115.74 m 2 g −1 . However, the specific surface area and pore volume of the pure diatomite are quite small, and thus the diatomite contributes less to the specific surface area of the XTiO 2 @diatomites. In other words, the specific surface areas of the catalysts were not related to the TiO 2 loadings. Therefore, the specific surface area and pore volume of the catalysts contribute little to their catalytic activity. As can be seen from Figure 5, the pure TiO 2 exhibits type-IV adsorption curves with hysteresis loops between the H1 and H2 types; the diatomite shows IUPAC type-II isotherms, indicating the presence of micropores in the diatomite [47]; and the TiO 2 @diatomite composites display type-IV isotherms with an overlap of the H2 and H3 hysteresis loops, which is related to the deposition of TiO 2 nanoparticles on the surface of the diatomite.

Photoluminescence (PL) Spectra
For the photocatalysts, apart from the surface area, the nanoparticle size, and the light absorption range, the electron-hole recombination property is also significant. PL can be used to evaluate the electron-hole recombination property. In other words, fluorescence will occur when electron-hole recombination arises [48]. Therefore, the rate of electron-hole recombination is inversely proportional

Photoluminescence (PL) Spectra
For the photocatalysts, apart from the surface area, the nanoparticle size, and the light absorption range, the electron-hole recombination property is also significant. PL can be used to evaluate the electron-hole recombination property. In other words, fluorescence will occur when electron-hole recombination arises [48]. Therefore, the rate of electron-hole recombination is inversely proportional to the fluorescence intensity, and thus a lower fluorescence intensity represents a lower electron and hole recombination rate and a higher photocatalytic activity [49,50]. As shown in Figure 6, the PL of different catalysts was measured at an excitation wavelength of 300 nm. The strongest peaks of different catalysts all appear within approximately 400 nm; however, their emission intensity varies significantly. For photocatalytic reactions, reducing the electron-hole recombination rate is an important goal. In other words, a low electron-hole recombination rate is favorable for a photocatalytic reaction. Among the catalysts applied, 0.35TiO 2 @diatomite has the lowest emission intensity, indicating that it has the lowest electron-hole recombination rate and thus possesses a better photocatalytic performance. The reduction of the luminescence intensity may be related to the Ti-O-Si band and ascribed to the increased electron capture centering on the surface of the TiO 2 nanoparticles, or an enhancement of the non-radiation-decay channels because of the SiO 2 around the TiO 2 nanoparticles.

Photocatalytic Performance of Different Catalysts
The TiO2 photocatalyst has a wide range of applications, such as air purification and sewage treatment. To evaluate the photocatalytic performance of different catalysts, a 5 W ultraviolet light source was used. Methylene blue (MB) was selected as the target pollutant to determine the optimal catalyst, and the optimal catalyst was then selected as the degradation catalyst for gaseous organic pollutants, such as acetone, benzene, methanol, and ethanol. Compared with pure TiO2, the XTiO2@diatomite composite shows a relatively larger adsorption capacity for MB in spite of its lower specific surface area and pore volume. As shown in Figure 7a, minor differences appeared in different catalysts. With the catalyst of 0.35TiO2@diatomite, the concentration of MB was nearly close to 0 within 90 min under UV light irradiation, and thus the degradation rate was faster than in the other similar photocatalysts. Clearly, 0.35TiO2@diatomite (recorded as the optimal catalyst) showed the highest degradation rate for the photocatalytic degradation of MB. The MB degradation efficiency of pure diatomite is shown in Figure 7a, in which the concentration of MB remains almost unchanged as the irradiation time increases, indicating that the photocatalytic activity of TiO2@diatomite is weak in the absence of TiO2, and that the photocatalytic activity of XTiO2@diatomite is better than that of Therefore, it can be concluded that the photocatalytic performance of 0.35TiO 2 @diatomite may be better than that of the other catalysts. This result is consistent with that obtained from the UV-vis diffuse reflectance spectra, confirming that 0.35TiO 2 @diatomite is the optimal catalyst.

Photocatalytic Performance of Different Catalysts
The TiO 2 photocatalyst has a wide range of applications, such as air purification and sewage treatment. To evaluate the photocatalytic performance of different catalysts, a 5 W ultraviolet light source was used. Methylene blue (MB) was selected as the target pollutant to determine the optimal catalyst, and the optimal catalyst was then selected as the degradation catalyst for gaseous organic pollutants, such as acetone, benzene, methanol, and ethanol. Compared with pure TiO 2 , the XTiO 2 @diatomite composite shows a relatively larger adsorption capacity for MB in spite of its lower specific surface area and pore volume. As shown in Figure 8a, minor differences appeared in different catalysts. With the catalyst of 0.35TiO 2 @diatomite, the concentration of MB was nearly close to 0 within 90 min under UV light irradiation, and thus the degradation rate was faster than in the other similar photocatalysts. Clearly, 0.35TiO 2 @diatomite (recorded as the optimal catalyst) showed the highest degradation rate for the photocatalytic degradation of MB. The MB degradation efficiency of pure diatomite is shown in Figure 8a, in which the concentration of MB remains almost unchanged as the irradiation time increases, indicating that the photocatalytic activity of TiO 2 @diatomite is weak in the absence of TiO 2 , and that the photocatalytic activity of XTiO 2 @diatomite is better than that of pure TiO 2 , suggesting a synergy effect between TiO 2 and diatomite (which may benefit from the formation of Si-O-Ti bonds). Because a Si-O-Ti bond can be formed when the surface of diatomite is covered with hydroxyl and silanol, this special structure allows TiO 2 to laboriously combine with diatomite, and further enhances its dispersibility, thereby significantly increasing its photocatalytic performance [46]. Among the types of XTiO 2 @diatomite applied, the photocatalytic activity of 0.35TiO 2 @diatomite was better than that of the other catalysts. In addition, the photocatalytic activity of TiO 2 @diatomite is not simply promoted by the increasing load ratio of TiO 2 to diatomite. By contrast, excessive loading amounts of TiO 2 can in turn reduce the photocatalytic capability. This was confirmed through 0.35TiO 2 @diatomite, which exhibited the highest performance. This can be explained by the fact that excessive TiO 2 causes an agglomeration of TiO 2 on the surface of the diatomite, and thus reduces the photocatalytic performance of TiO 2 @diatomite. This result agrees with the results obtained from the UV-vis diffuse reflectance spectra, PL spectra, and degree of aggregation shown on the SEM images. Therefore, 0.35TiO 2 @diatomite was selected for the degradation experiment of the following gaseous organic pollutants. Figure 8b,c show the degradation results of gaseous acetone and gaseous benzene, respectively. The gas concentration was controlled by adding 1 mL of saturated gas at room temperature to each of the six headspace bottles with three capacities (100, 200, and 500 mL), which were labeled as 100 mL(1), 100 mL(2), 200 mL(1), 200 mL(2), 500 mL(1), and 500 mL (2). In this experiment, bottles with the volumes shown in Figure 8c were used to degrade the gases, bottles of the same volume were used in repeated experiments three times, and the data were averaged to verify the accuracy. As can be seen from Figure 8b, under UV-Vis light irradiation, the optimal catalyst showed an excellent photocatalytic performance for gaseous acetone at different concentrations. In the experiment on the degradation of gaseous acetone, the concentration of gaseous acetone reached close to 0 in 90 min under UV-Vis light irradiation. This means that gaseous acetone can be completely decomposed in only 90 min at a fast rate, and slow decomposition can be completely achieved within 120 min. This is because the concentration of acetone is so high that it cannot be completely degraded within 90 min. In other words, after 90 min of irradiation, the degradation rate of 0.35TiO 2 @diatomite for gaseous acetone reaches up to 100%, which is greater than in most other studies [51]. From Figure 8c, it can be seen that gaseous benzene is degraded within 90 min. However, most of the gaseous benzene with different concentrations cannot be completely degraded. One possible reason for this is that the structure of a benzene molecule is extremely stable and difficult to break. Another possible reason is that the concentration of gaseous benzene is too high to be completely degraded. In this study, only two gaseous organic pollutants were investigated. In the future, we will study the degradation of the catalyst under other gaseous organic pollutants (such as methanol, ethanol, propanol, formaldehyde, and acetaldehyde) and summarize whether the effects on the polar and non-polar organics are identical.

Chemicals and Materials
Diatomite (Macklin, Shanghai, China), titanium tetrachloride (TiCl4, analytical reagent, Adamas, Beijing, China), absolute ethanol (analytical reagent, Tianjin Damao Chemical Reagents Factory, Tianjin, China), benzene (analytical reagent, Tianjin Damao Chemical Reagents Factory, Tianjin, China), acetone (analytical reagent, Tianjin Damao Chemical Reagents Factory, Tianjin, China), and deionized water were used for the synthesis of TiO2 and diatomite/TiO2. During the process of synthesizing diatomite/TiO2, the only difference from TiO2 is that a certain amount of diatomite is added to the synthetic process for TiO2. All the reagents listed were used as purchased and without further treatment.

Chemicals and Materials
Diatomite (Macklin, Shanghai, China), titanium tetrachloride (TiCl 4 , analytical reagent, Adamas, Beijing, China), absolute ethanol (analytical reagent, Tianjin Damao Chemical Reagents Factory, Tianjin, China), benzene (analytical reagent, Tianjin Damao Chemical Reagents Factory, Tianjin, China), acetone (analytical reagent, Tianjin Damao Chemical Reagents Factory, Tianjin, China), and deionized water were used for the synthesis of TiO 2 and diatomite/TiO 2 . During the process of synthesizing diatomite/TiO 2 , the only difference from TiO 2 is that a certain amount of diatomite is added to the synthetic process for TiO 2 . All the reagents listed were used as purchased and without further treatment.

Catalyst Preparation
The TiO 2 @diatomite was prepared using a facile solvothermal method applying absolute ethanol as a solvent. Pure ethanol (50 mL) was transferred into a three-necked flask (250 mL) placed in an ice bath and stirred for 30 min. Then, 0.5 mL of deionized water (0.5 mL) was added. After 5 min, TiCl 4 (2 mL) was added dropwise. The mixture was stirred until it became a transparent light-yellow solution. This solution was transferred to a dry Teflon-lined stainless-steel autoclave (100 mL) containing a certain amount of diatomite, and stirred evenly. The resulting mixture was then kept at 200 • C for 12 h in an oven. After cooling to room temperature, the white precipitates were filtered off, and washed with ethanol and deionized water several times until the pH of the filtrate reached approximately 7. The chlorine was also removed through the washing process. The collected precipitates were dried at 80 • C for 10 h in the oven to afford TiO 2 @diatomite. Detailed steps regarding the synthesis of TiO 2 @diatomite are also shown in Figure ??. For convenience, the catalysts are denoted as XTiO 2 @diatomite, where X represents the mass ratio of TiO 2 to diatomite (because diatomite cannot be expressed in moles). The pure TiO 2 was prepared according to the above procedure, except with the addition of diatomite steps.

Characterization
The catalysts were characterized based on X-ray diffraction patterns (XRD, Ultiam IV) recorded using Cu Kα radiation at a scan rate of 5°/min. Their microstructures were observed using fieldemission SEM (JEOL-JSM-7800F (Tokyo, Japan) with an energy dispersive spectrometer and S-4700 at an acceleration voltage of 20 KV), TEM (Hitachi-HT7700 (Tokyo, Japan) at an acceleration voltage of 120 KV and JEM-ARM-200F), and HRTEM (JEOL, JEM-ARM200F,Tokyo, Japan). The specific surface area and pore size distribution of the different catalysts were determined using a Brunauer-Emmett-Teller (BET) instrument with nitrogen adsorption at 77 K (Micrometrics ASAP 2020, Georgia, USA). In addition, the ultraviolet-visible spectrophotometer (Shimadzu UV-3600, Tokyo, Japan) was used to analyze the UV-vis diffuse reflectance spectra of the prepared catalysts. Moreover, the electron hole recombination rate was measured using a photoluminescence spectrometer (FL-7000, Hitachi, Tokyo, Japan) to evaluate the photocatalytic activity.

Evaluation of Photocatalytic Activity
The photocatalytic activities of different catalysts were initially evaluated based on the degradation of MB under UV-light irradiation at ambient temperature [52]. During this process, the catalyst (0.05 g) was dispersed in an MB solution (10 of MB (50 mg/mL, 30 mL)) in a Petri dish. A portion of MB solution was taken every 30 min for measurements using an ultraviolet-visible spectrophotometer. The degradation rate was determined by monitoring the change in concentration of the MB over time using an ultraviolet-visible spectrophotometer (Shimadzu UV-3600). The catalyst with the highest degradation rate was chosen for degradation of the different gaseous organic pollutants, including acetone and benzene. These degradation processes were carried out in sealed headspace bottles with different volumes (100, 200, and 500 mL). The concentration of gas was controlled by adding equal volumes of saturated gas to different volumes of sealed headspace bottles (to the best of our knowledge, at the same temperature, the concentration of saturated gas is constant). The specific experimental details are similar to those of the process of MB degradation described above [52], except that the MB solution was replaced with gaseous phase organic pollutants. The details of the experiment are as follows: A catalyst (0.05 g) was added to each of the headspace bottles, and 1 mL of saturated gaseous phase pollutants at room temperature was transferred to the sealed headspace bottles using a gas chromatography syringe with a capacity of 2.5 mL. In addition, the initial concentrations of gaseous acetone and benzene in the 100, 200, and 500 mL headspace bottles were approximately 2.95 × 10 −3 , 1.63 × 10 −3 , and 6.85 × 10 −4 mol/L, respectively. A xenon lamp (30 W)

Characterization
The catalysts were characterized based on X-ray diffraction patterns (XRD, Ultiam IV) recorded using Cu Kα radiation at a scan rate of 5 • /min. Their microstructures were observed using field-emission SEM (JEOL-JSM-7800F (Tokyo, Japan) with an energy dispersive spectrometer and S-4700 at an acceleration voltage of 20 KV), TEM (Hitachi-HT7700 (Tokyo, Japan) at an acceleration voltage of 120 KV and JEM-ARM-200F), and HRTEM (JEOL, JEM-ARM200F, Tokyo, Japan). The specific surface area and pore size distribution of the different catalysts were determined using a Brunauer-Emmett-Teller (BET) instrument with nitrogen adsorption at 77 K (Micrometrics ASAP 2020, Georgia, USA). In addition, the ultraviolet-visible spectrophotometer (Shimadzu UV-3600, Tokyo, Japan) was used to analyze the UV-vis diffuse reflectance spectra of the prepared catalysts. Moreover, the electron hole recombination rate was measured using a photoluminescence spectrometer (FL-7000, Hitachi, Tokyo, Japan) to evaluate the photocatalytic activity.

Evaluation of Photocatalytic Activity
The photocatalytic activities of different catalysts were initially evaluated based on the degradation of MB under UV-light irradiation at ambient temperature [52]. During this process, the catalyst (0.05 g) was dispersed in an MB solution (10 of MB (50 mg/mL, 30 mL)) in a Petri dish. A portion of MB solution was taken every 30 min for measurements using an ultraviolet-visible spectrophotometer. The degradation rate was determined by monitoring the change in concentration of the MB over time using an ultraviolet-visible spectrophotometer (Shimadzu UV-3600). The catalyst with the highest degradation rate was chosen for degradation of the different gaseous organic pollutants, including acetone and benzene. These degradation processes were carried out in sealed headspace bottles with different volumes (100, 200, and 500 mL). The concentration of gas was controlled by adding equal volumes of saturated gas to different volumes of sealed headspace bottles (to the best of our knowledge, at the same temperature, the concentration of saturated gas is constant). The specific experimental details are similar to those of the process of MB degradation described above [52], except that the MB solution was replaced with gaseous phase organic pollutants. The details of the experiment are as follows: A catalyst (0.05 g) was added to each of the headspace bottles, and 1 mL of saturated gaseous phase pollutants at room temperature was transferred to the sealed headspace bottles using a gas chromatography syringe with a capacity of 2.5 mL. In addition, the initial concentrations of gaseous acetone and benzene in the 100, 200, and 500 mL headspace bottles were approximately 2.95 × 10 −3 , 1.63 × 10 −3 , and 6.85 × 10 −4 mol/L, respectively. A xenon lamp (30 W) was adopted as a light source during the photocatalytic degradation process. The headspace bottle was placed under the light, and the bottle was directly irradiated. The whole process was degraded at room temperature. Throughout the degradation process, no heating and any agitation were used. Simultaneously, gases were taken every 30 min and the corresponding concentrations of organic gaseous pollutants were determined using gas chromatography-mass spectrometry (GC-MS). In addition, the degradation reaction of the organic gaseous pollutants was kept at ambient temperature. Four sealed headspace bottles of each gas were used for gaseous pollutant degradation. All reactions were repeated three times in parallel.

Conclusions
In conclusion, we successfully prepared a series of TiO 2 @diatomite catalysts using a facile solvothermal method applying anhydrous ethanol as a solvent for the removal of VOCs. The XRD patterns showed that TiO 2 in the catalyst is in an anatase phase, and the introduction of diatomite has no effect on the crystal structure of TiO 2 . SEM and TEM images indicated that the size of the TiO 2 nanoparticles is approximately 10-15 nm, and the morphologies of most of the TiO 2 nanoparticles are square-like. In addition, for the TiO 2 @diatomite, the TiO 2 nanoparticles are uniformly immobilized on the diatomite. Based on photocatalytic activity tests, 0.35TiO 2 @diatomite (with a TiO 2 to diatomite mass ratio of 0.35) displayed higher photocatalytic activity compared with that at other ratios, pure TiO 2 , and pure diatomite. This result is consistent with the characterization results of the UV-vis diffuse reflectance spectra and PL spectra. The photocatalytic performance and GC-MS test results showed that 0.35TiO 2 @diatomite can be used to effectively remove acetone and benzene. In addition, both TiO 2 and diatomite are low-cost, non-toxic, harmless, and stable. Therefore, a TiO 2 @diatomite catalyst could potentially be applied in the area of building construction and interior decoration.