Enhanced Photocatalytic Performance of Bi 2 O 2 CO 3 Loaded Activated Carbon for Toluene Removal in Air

Featured Application: This work may provide potential applications in indoor air puriﬁcation for the removal of volatile organic compounds


Introduction
People usually stay indoors for the majority of the time (ca. 85~90%) during the day; thus receiving exposure to indoor air pollution from a variety of trace harmful gases such as volatile organic compounds (VOCs). Among the different VOCs, benzene, toluene, and xylene (BTX) can pose a risk of adverse health effects following long term exposure and may result in considerable harm to human health, with a high risk of developing asthma, cancers, heart disease, and lymphoma/leukemia, etc. [1][2][3][4]. Therefore, the efficient and economic disposal of BTX in the indoor environment is highly desired and has received considerable attention from researchers in recent years.
Adsorption technology is considered as a simple, economical, efficient, and environmentally friendly method for removing VOCs [5]. With adsorbents, no additional energy is needed because they are able to absorb VOCs by surface molecules at room temperature [6]. As a synthetic adsorbent, activated carbon (AC) is widely used for BTX removal due to its advantages of low costs, high adsorption capacity, high efficiency and multifunction [7][8][9][10][11][12][13]. Nevertheless, the process that AC removes gaseous pollutants is only a physical adsorption. If the active adsorption sites are saturated, generally AC can adsorb almost no more BTX and thus needs to be exchanged or regenerated. According to recent reports, AC can only act as a photocatalyst for the degradation of organic contaminants under UV irradiation [14][15][16][17][18]. Therefore, the introduction of narrow bandgap catalytic materials is highly necessary to expand the solar absorption spectrum to the visible light region [19,20].
Among all the visible light driven photocatalysts, layered structured Bi 2 O 2 CO 3 (BOC), one of the bismuth-containing Aurivillius compounds with alternative stacking of (Bi 2 O 2 ) 2+ thin layers separated by CO 3 2− groups [21][22][23], has been explored as a promising visible light active photocatalyst [24][25][26]. Furthermore, the layered structure can easily lead to the formation of 2D morphologies, such as sheet-/plate-like morphologies, which may result in excellent photocatalytic performance. In our previous work, we demonstrated the effective photocatalytic removal of toluene in air utilizing (002) oriented BOC nanosheets, which led to an almost complete degradation of toluene into CO 2 and H 2 O [27,28]. These results demonstrate the promising potential of utilizing layered BOC nanostructures as photocatalyst for removing VOCs. In this work, we aim to combine the advantages of both AC and BOC as a composite catalyst for the removal of toluene in air by fabricating AC modified with BOC nanomaterials. On one hand, the AC can adsorb toluene effectively due to its large surface area. On the other hand, BOC nanostructures can function as photocatalyst for the in-situ degradation of toluene.

Synthesis of Bi 2 O 2 CO 3
In a typical experiment, 2.325 g Bi 2 O 3 was dissolved in 450 mL 1 mol/L of HNO 3 solution under ultrasonication conditions. The obtained suspension was kept at 20 • C in a water bath for 1 h. Subsequently, 400 mL 0.6 mol/L Na 2 CO 3 solution was added into the above suspension at a speed of 30 mL/min, until a uniform solution with pH of about 7 was reached under magnetic stirring. Then, the as-obtained powder sample was centrifuged, washed with de-ionized water and ethyl alcohol for several times, and dried at 70 • C in an oven. Finally, the powdered Bi 2 O 2 CO 3 (labeled as BOC) sample was obtained.
The synthesis process of Bi 2 O 2 CO 3 nanomaterials follows Equations (1)- (6). CO 3 2+ is formed via a hydrolysis reaction process between Na 2 CO 3 2.3. Synthesis of BOC/AC Composites 2.325 g Bi 2 O 3 was dissolved in 450 mL 1 mol/L HNO 3 solution under ultrasonication condition. The obtained suspension was kept at 20 • C in a water bath. Then, 400 mL 0.6 mol/L Na 2 CO 3 solution was added into the above suspension at a speed of 30 mL/min, while stirring at a speed of 300 r/min. At the same time, different amounts (50 g, 100 g, Appl. Sci. 2022, 12, 12500 3 of 13 150 g, and 200 g) of honeycomb activated carbon (4~60 mesh, iodine value of 1050 mg/g) were added. The suspension was kept in 20 • C water bath for 6 h. Then, the product was washed with de-ionized water and ethyl alcohol several times. The as-obtained powder sample was centrifuged and dried at 70 • C in an oven. Finally, the BOC/AC composites (labeled as BOC/AC_50, BOC/AC_100, BOC/AC_150, and BOC/AC_200, respectively) were obtained.

Characterization
The X-ray diffraction (XRD) measurements were performed on a diffractometer (D8-Advance, Bruker, Karlsruhe, Germany) with monochromatized Cu Kα (λ = 1.54056 nm) radiation at a scanning speed of 0.15 • /s. The microstructure of the as-obtained samples was characterized using scanning electron microscopy (SEM, JSM-7001F, JEOL, Tokyo, Japan) operated at a voltage of 5 kV. The composition of chemical elements was performed using an X-ray fluorescence (XRF, Thermo Scientific, Niton, UK).
The N 2 adsorption/desorption measurements were performed on an automated gas sorption analyzer (AutosorbiQ2, Quantachrome, Boynton Beach, FL, USA) at P/Po = 0.99 and the specific surface areas were estimated using the Brunauer-Emmet-Teller (BET) method. Before analysis, all samples were dried at 90 • C for 1 h and degassed by vacuum at 200 • C for 24 h. To prepare the sample for photocatalytic toluene removal performance in air, 0.025 g BOC was dispersed into 150 mL de-ionized water under ultrasonication for 15 min. The suspension was filtered using a 0.1 µm nylon membrane through a vacuum pump. The photocatalytic performances of the BOC samples were evaluated by toluene removal in air using a gas phase photocatalytic testing system developed by our group (China Patent, NO.202210103256.X). The reactor is a stainless steel canister and the BOC membrane was placed at the bottom of the reactor. The gas product in the reactor was analyzed at regular time intervals, using a gas chromatograph (GC), equipped with two flame-ionization detectors (FID) [28]. Toluene analysis was performed with one FID loaded with an Rt-Q-Bond Plot column (30 m × 0.25 mm, film thickness 10 µm), while CO 2 analysis was performed by the other FID loaded with a packed column (TDX-01, 3 m × 3 mm) followed by a methanizer CO 2 concentration. The gas samples were fed to the GC online through an automatic gas sampling valve.

Photocatalytic Activity of BOC/AC Composites
The photocatalytic performances of the BOC/AC composites were investigated using a similar system to that described in Section 2.5.1. To imitate practical applications, the stainless steel canister was replaced by a 60 L Tedlar polyvinyl fluoride (PVF) pocket in a thermostatic test chamber and kept at a constant temperature (e.g., 20 • C). Then, 3 g BOC/AC particulates was placed on the dish and into the pocket. The incident light can pass through the PVF film to the BOC/AC surface. The structure is illustrated in Figure 1. 0.5 h to reach the adsorption/desorption equilibrium. A 300 W xenon lamp (CEL-HXF300, Beijing China Education AuLight Technology Co., Ltd., Beijing, China) was used as the simulated sunlight source. The gas product in the reactor was analyzed at regular time intervals using GC. In addition, the toluene gas in the pocket could not flow completely to the automatic gas sampling valve of the GC at 0 min because the pipe diameter of the valve is relatively small and the gas delivery pump has just started. Thus, the toluene concentration is low. This means that the first point (at t = 0) is not 100% for all studies.

Characterization and Properties of Bi2O2CO3
The crystal structure of the BOC nanomaterials prepared by chemical method was analyzed by XRD. As displayed in Figure 2, all diffraction peaks of the sample can be well indexed to the single phase of tetragonal Bi2O2CO3 (JCPDS 41-1488) with good crystallinity, without any other impurity peaks. Moreover, the average crystal size of the BOC nanosheets was estimated to be ca. 80 nm using the XRD patterns. To test the photocatalytic activity, the reaction reactor was first vacuumized by pump. Then, 60 L toluene gas (50.0 ppm in air) was injected as the target indoor pollutant. This required about 12 min. After that, the reaction reactor was kept in the dark for 0.5 h to reach the adsorption/desorption equilibrium. A 300 W xenon lamp (CEL-HXF300, Beijing China Education AuLight Technology Co., Ltd., Beijing, China) was used as the simulated sunlight source. The gas product in the reactor was analyzed at regular time intervals using GC. In addition, the toluene gas in the pocket could not flow completely to the automatic gas sampling valve of the GC at 0 min because the pipe diameter of the valve is relatively small and the gas delivery pump has just started. Thus, the toluene concentration is low. This means that the first point (at t = 0) is not 100% for all studies.

Characterization and Properties of Bi 2 O 2 CO 3
The crystal structure of the BOC nanomaterials prepared by chemical method was analyzed by XRD. As displayed in Figure 2, all diffraction peaks of the sample can be well indexed to the single phase of tetragonal Bi 2 O 2 CO 3 (JCPDS 41-1488) with good crystallinity, without any other impurity peaks. Moreover, the average crystal size of the BOC nanosheets was estimated to be ca. 80 nm using the XRD patterns. Appl. Sci. 2022, 12, x FOR PEER REVIEW 5 of 13 The morphology of the BOC sample was investigated by SEM. As shown in Figure  3, the BOC has a flower-like shape assembled by nanosheets during the crystallization process, with lateral sizes of several micrometers and a thickness of a few nanometers. Thus, the flower-like shape can provide more reactive sites owing to the unique layered structure of Bi2O2CO3 [29,30]. The photocatalytic activity of the BOC sample for removing gaseous toluene in air was investigated at a concentration of 50.0 ppm (450 mL) under irradiation by a Xe lamp as the light source. As displayed in Figure 4, the pristine BOC sample demonstrated an excellent photocatalytic efficiency for toluene removal with a degradation rate as high as 96.8% over 3 h. The morphology of the BOC sample was investigated by SEM. As shown in Figure 3, the BOC has a flower-like shape assembled by nanosheets during the crystallization process, with lateral sizes of several micrometers and a thickness of a few nanometers. Thus, the flower-like shape can provide more reactive sites owing to the unique layered structure of Bi 2 O 2 CO 3 [29,30]. The morphology of the BOC sample was investigated by SEM. As shown in Figure  3, the BOC has a flower-like shape assembled by nanosheets during the crystallization process, with lateral sizes of several micrometers and a thickness of a few nanometers. Thus, the flower-like shape can provide more reactive sites owing to the unique layered structure of Bi2O2CO3 [29,30]. The photocatalytic activity of the BOC sample for removing gaseous toluene in air was investigated at a concentration of 50.0 ppm (450 mL) under irradiation by a Xe lamp as the light source. As displayed in Figure 4, the pristine BOC sample demonstrated an excellent photocatalytic efficiency for toluene removal with a degradation rate as high as 96.8% over 3 h. The photocatalytic activity of the BOC sample for removing gaseous toluene in air was investigated at a concentration of 50.0 ppm (450 mL) under irradiation by a Xe lamp as the light source. As displayed in Figure 4, the pristine BOC sample demonstrated an excellent photocatalytic efficiency for toluene removal with a degradation rate as high as 96.8% over 3 h. Appl. Sci. 2022, 12, x FOR PEER REVIEW 6 of 13

Microstructure Analysis
The microstructures of the BOC/AC and AC samples were characterized by SEM. As shown in Figure 5, the BOC/AC_50 and BOC/AC_100 samples have flower-like BOC grown on the surface of AC, with a size of ca. 5 μm (Figure 5a,b). These results demonstrate the successful growth of BOC on the surface of AC via an in-situ crystallization process. The BOC/AC_150 and BOC/AC_200 have cylinder-like morphology with a length of ca. hundreds of nanometers (Figure 5c,d). Figure 5e,f show the morphologies of the pristine AC sample, which has a porous structure with high specific surface area and good adsorption ability [31][32][33].

Microstructure Analysis
The microstructures of the BOC/AC and AC samples were characterized by SEM. As shown in Figure 5, the BOC/AC_50 and BOC/AC_100 samples have flower-like BOC grown on the surface of AC, with a size of ca. 5 µm (Figure 5a,b). These results demonstrate the successful growth of BOC on the surface of AC via an in-situ crystallization process. The BOC/AC_150 and BOC/AC_200 have cylinder-like morphology with a length of ca. hundreds of nanometers (Figure 5c,d). Figure 5e,f show the morphologies of the pristine AC sample, which has a porous structure with high specific surface area and good adsorption ability [31][32][33].
The surface area, pore volume, and pore diameter results of the as-prepared samples are shown in Figure S1 and listed in Table 1. The BOC/AC_100 sample has the highest BET surface area among all BOC/AC and AC samples, possibly because of the flower-like BOC on the surface of the AC. This is highly beneficial for promoting the photocatalytic performance by providing an abundance of active reaction sites. All samples show similar pore volume and pore diameter (1.5 nm), which is due to the use of the same matrix of AC. For these composite materials, activated carbon has excellent adsorption capability for toluene gas and Bi 2 O 2 CO 3 photocatalysis can in situ mineralize the adsorbed toluene to CO 2 and H 2 O under light illumination.

Structure and Composition Analysis
The XRD patterns of the BOC/AC and AC are presented in Figure 6. All BOC/AC samples show similar XRD patterns (Figure 6a). In Figure 6b-d, the enlarged local XRD patterns clearly show that the peak intensity increased for BOC/AC_50 and BOC/AC_100 near 2θ = 30. The surface area, pore volume, and pore diameter results of the as-prepared samples are shown in Figure S1 and listed in Table 1. The BOC/AC_100 sample has the highest BET surface area among all BOC/AC and AC samples, possibly because of the flower-like BOC on the surface of the AC. This is highly beneficial for promoting the photocatalytic performance by providing an abundance of active reaction sites. All samples show similar pore volume and pore diameter (1.5 nm), which is due to the use of the same matrix of AC. For these composite materials, activated carbon has excellent adsorption capability for toluene gas and Bi2O2CO3 photocatalysis can in situ mineralize the adsorbed toluene to CO2 and H2O under light illumination.  near 2θ =30.2° and 32.7°, which are the characteristic peaks of Bi2O2CO3 (JCPDS: 41-1488), corresponding to the (013) and (110) facets, respectively. The energy dispersive spectroscopy (EDS) elemental mapping images (Figure 7) of BOC/AC_100 sample further proves the successful growth of Bi2O2CO3 on the surface of AC and the homogeneous elemental distribution of Bi, C, and O elements in the Bi2O2CO3 nanosheets. To quantify the composition of different chemical elements in the composite, XRF analysis was performed, which showed that the ratio of C: O: Bi was about 94.3: 4.0: 1.7 wt.%.

Photocatalytic Properties
The photocatalytic properties of the degradation of gaseous toluene (50.0 ppm) in air over the BOC/AC composites were investigated under light illumination. As shown in Figure 8a, all samples demonstrated good removal efficiency of more than 90%. Among

Photocatalytic Properties
The photocatalytic properties of the degradation of gaseous toluene (50.0 ppm) in air over the BOC/AC composites were investigated under light illumination. As shown in Figure 8a, all samples demonstrated good removal efficiency of more than 90%. Among the BOC/AC composites, BOC/AC_100 shows the optimal efficiency with a removal rate of up to 92.2% in 3 h. The performances are comparable and higher than previously reported typical composite photocatalysts such as N-TiO 2 /zeolite [34] and Fe 2 O 3 /In 2 O 3 composite [35], showing the promising potential application of BOC/AC composite catalysts. In previous works, other methods have been used for the removal of toluene from the air, such as biofiltration [36], post-plasma catalytic technology [37], and corona discharge [38]. Compared with those methods, which are usually expensive and complex, the present BOC/AC composite has the advantages of being green and low-cost. The products of toluene degradation were detected using GC. The results demonstrate that CO 2 is the main product (Figures 8b and S2) with the highest productivity of 82.5 mg/m 3 in 3 h for BOC/AC_100, and another main product should be H 2 O, on the basis that toluene is hydrocarbon. More reactive sites are provided for BOC/AC_100 due to the flower-like Bi 2 O 2 CO 3 on the surface of BOC/AC_50 and BOC/AC_100, which leads to enhanced photocatalytic performance. The photocatalytic process of the optimal sample (BOC/AC_100 composite) was investigated further. As shown in Figures 8c and S3, the toluene was completely removed in 7 h. Furthermore, the concentration of CO 2 increased significantly and reached 105.2 mg/m 3 (Figure 8d), which indicated the continuous degradation of toluene. However, the production rates of CO 2 were merely 38.3 mg/m 3 in 3 h and 55.4 mg/m 3 in 7 h for pristine AC. As we mentioned in the introduction, AC acts mainly as an adsorbent. Moreover, according to previous works, AC can also act as a photocatalyst for the degradation of organic contaminants. However, it is only active under UV irradiation with low catalytic efficiency. These results clearly show that the degradation rate of toluene is enhanced after the in-situ growth of BOC on the surface of the AC. This is attributed to the synergistic effect of the adsorption ability of AC and photocatalytic activity of both BOC and AC, demonstrating the advantage of the composite material.
The stability of BOC/AC composites (BOC/AC_100) for photocatalytic removal of toluene was evaluated by performing three experimental cycles under light illumination. As shown in Figure 8e, highly stable photocatalytic efficiency was maintained for BOC/AC_100, reaching as high as 90% for toluene removal after three cycles, which demonstrated the excellent recyclability of BOC/AC for photocatalytic toluene degradation.
In addition, we investigated the toluene removal efficiency using different amounts of BOC/AC_100 (1 g, 1.5 g, 2 g, and 3 g). As shown in Figure 8f, the photocatalytic activity was enhanced as the amount of catalyst increased, reaching the highest efficiency when 3 g of BOC/AC_100 was used.
A proposed photocatalytic reaction mechanism over the BOC/AC composites for the degradation of gaseous toluene under light irradiation can be explained as follows ( Figure 9). First, the honeycomb AC matrices adsorb and capture the toluene molecules. Then, under light illumination, the electrons are excited from the valence band (VB) to the conduction band (CB) in the surface of Bi 2 O 2 CO 3 loaded Activated Carbon, leaving holes in the VB. The AC can function as electron acceptors to reduce the recombination rate of electron-hole pairs. Consequently, the excited electrons on CB in Bi 2 O 2 CO 3 can be easily transferred to the surface of AC, thus promoting the separation and migration of photo-generated charge carriers.
mainly as an adsorbent. Moreover, according to previous works, AC can also act as a photocatalyst for the degradation of organic contaminants. However, it is only active under UV irradiation with low catalytic efficiency. These results clearly show that the degradation rate of toluene is enhanced after the in-situ growth of BOC on the surface of the AC. This is attributed to the synergistic effect of the adsorption ability of AC and photocatalytic activity of both BOC and AC, demonstrating the advantage of the composite material.   on the surface to active species of superoxide anions (•O 2− ). Consequently, the •O 2− active oxidizing species can oxidize toluene into intermediate products and finally into CO2 and H2O [39]. Meanwhile, the photo-generated holes (h + ) left on the BOC oxidize H2O on their surface into another highly active species of hydroxyl radicals (•OH). Finally, the produced •O 2− and •OH, with strong oxidizing ability, can efficiently degrade the intermediates into CO2 and H2O, which are not harmful.

Conclusions
In summary, a novel BOC/AC nanocomposite photocatalyst was fabricated by in-situ growth of flower-shaped BOC nanosheets on the surface of activated carbon Figure 9. Photocatalytic mechanism of toluene removal using BOC/AC composite.

Conclusions
In summary, a novel BOC/AC nanocomposite photocatalyst was fabricated by in-situ growth of flower-shaped BOC nanosheets on the surface of activated carbon through a facile chemical method. The as-synthesized BOC/AC nanocomposite demonstrated excellent removal efficiency for toluene, with more than 90% degradation in 3 h, which is comparable and even higher than reported catalysts. Moreover, CO 2 and H 2 O were confirmed as the major products. We also found that the catalytic activity was highly dependent on the loading content of BOC, with the highest performance obtained for 100 g loading. Furthermore, toluene could be completely removed after 7 h. Importantly, the photocatalytic efficiency remains constant after three cycles, demonstrating the good recyclability of the BOC/AC for photocatalytic toluene removal. The excellent photocatalytic degradation activity of the BOC/AC composite catalyst can be attributed to the synergistic effect of the adsorption ability of AC and the photocatalytic activity of both the BOC nanosheets and the AC. This work may provide useful guidance for indoor air purification, particularly for trace harmful gases such as VOCs.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/app122312500/s1, Figure S1: N2 adsorption− desorption isotherms (a) and pore size distributions (b) of AC and BOC/AC composites. Figure S2: Chromatograms of organic products at 3 h for BOC/AC_100 (a) and AC (b). Figure S3: Chromatograms of organic products at 7 h for BOC/AC_100 (a) and AC (b); Figure S4: Spectrum of the xenon lamp used.