Enhanced TiO2-Based Photocatalytic Volatile Organic Compound Decomposition Combined with Ultrasonic Atomization in the Co-Presence of Carbon Black and Heavy Metal Nanoparticles

Volatile organic compounds (VOCs) are representative indoor air pollutants that negatively affect the human body owing to their toxicity. One of the most promising methods for VOC removal is photocatalytic degradation using TiO2. In this study, the addition of carbon black (CB) and heavy metal nanoparticles (NPs) was investigated to improve the efficiency of a TiO2-based photocatalytic VOC decomposition system combined with ultrasonic atomization and ultraviolet irradiation, as described previously. The addition of CB and Ag NPs significantly improved the degradation efficiency. A comparison with other heavy metal nanoparticles and their respective roles are discussed.


Introduction
Volatile organic compounds (VOCs) are representative indoor air pollutants that negatively affect the human body owing to their toxicity and carcinogenicity.These pollutants are closely related to the sick building syndrome, including mucous membrane irritation, headaches, and fatigue [1,2].Photocatalytic degradation is one of the most promising methods for VOC removal [3][4][5][6].TiO 2 -photocatalyzed systems have been extensively studied because of their potential for operation under ultraviolet (UV, typically UV-A) irradiation without external heating.The presence of water molecules in the gas phase is effective in maintaining catalytic activity, and the formation of OH radicals is the key to achieving high degradation efficiency.However, hydrophobic VOCs such as toluene are less reactive than hydrophilic VOCs in TiO 2 -photocatalyzed systems.A combined ozone oxidation and photocatalytic degradation system has also been studied [7][8][9][10][11].Despite the effective degradation of gaseous organic compounds, these systems still suffer from several problems related to their strong dependence on relative humidity [12], the interference of intermediate degradation products with the degradation process [13,14], and catalyst deterioration owing to the adsorption of intermediates [13,15].In addition, the handling of ozone has some complexity due to its toxicity under high concentration conditions.Although a combined system using shorter wavelength UV-C has also been studied to generate OH radical effectively [16][17][18], the in situ formation of ozone is concerned.Therefore, developing other methods to effectively accelerate photocatalytic reactions is still desirable.
Sekiguchi et al. proposed a method for the decomposition of VOCs using mists containing TiO 2 particles generated by the ultrasonic (US) irradiation of a TiO 2 particle suspension [19,20].Ultrasonically generated mists range from nanometer to micrometer and allow the transportation of solid materials in the gas phase depending on the size of the mists [21,22].This method is applicable to the continuous decomposition of toluene as a model hydrophobic VOC by increasing the gas-liquid interphase owing to atomization.A possible application of this system is a humidifying air purifier with degradation functionality.In this system, ultrasonically generated mists react with gas-phase toluene to produce water-soluble intermediates that are dissolved in the TiO 2 suspension phase and then further decomposed.The incorporation of the water-soluble intermediate products into the droplet is the key to maintaining a stable photocatalytic decomposition rate.
In this study, we aimed to improve the degradation efficiency of the aforementioned systems by combining them with additives such as carbon black (CB) and heavy metal nanoparticles (NPs).Carbonaceous materials are well-known adsorbents for capturing hydrophobic organic molecules, possibly incorporating the intermediate product in a water phase [3], and heavy metal NPs have the potential to improve the photocatalytic efficiency of TiO 2 systems [23,24].The addition of both CB and Ag NPs was the best combination for improving the degradation efficiency in the gas and water phases.The roles of CB and Ag NPs are discussed.

Results and Discussion
Figure 1 shows the toluene degradation efficiency, E d , obtained in the absence (denoted as TiO 2 only) and presence of heavy metal NPs (denoted as Ag, Pt, Pd, and Au).The addition of a very small amount of metal NPs enhanced the E d from 40% to 48-60%.The E d values increased in the following order: Ag > Pd > Pt > Au > TiO 2 only.Next, CB was added to the TiO 2 suspension (denoted as CB) or the Ag-or Pd NP-containing suspension (denoted as Ag+CB and Pd+CB, respectively).In all cases, the addition of CB increased the E d by approximately 5% (from 40%, 60%, and 55% to 56%, 65%, and 59%, respectively; Figure 2).The addition of both heavy metal NPs and CB effectively improved the E d .The lower effectiveness of metal NPs in the co-presence of CB compared (approximately 5% increase) might be due to the lower toluene concentration at the high-level conversion region and resulting lower mass transfer.The XRD measurements of TiO 2 (as received) before and after the addition of Ag NPs and under UV irradiation or not were performed.In all cases, only the XRD pattern derived from anatase and rutile was observed (Figure S1).
A possible application of this system is a humidifying air purifier with degr functionality.In this system, ultrasonically generated mists react with gas-phase to produce water-soluble intermediates that are dissolved in the TiO2 suspensio and then further decomposed.The incorporation of the water-soluble inter products into the droplet is the key to maintaining a stable photocatalytic decom rate.
In this study, we aimed to improve the degradation efficiency of the aforeme systems by combining them with additives such as carbon black (CB) and heav nanoparticles (NPs).Carbonaceous materials are well-known adsorbents for ca hydrophobic organic molecules, possibly incorporating the intermediate prod water phase [3], and heavy metal NPs have the potential to improve the photoc efficiency of TiO2 systems [23,24].The addition of both CB and Ag NPs was combination for improving the degradation efficiency in the gas and water pha roles of CB and Ag NPs are discussed.

Results and Discussion
Figure 1 shows the toluene degradation efficiency, Ed, obtained in the (denoted as TiO2 only) and presence of heavy metal NPs (denoted as Ag, Pt, Pd, a The addition of a very small amount of metal NPs enhanced the Ed from 40% to The Ed values increased in the following order: Ag > Pd > Pt > Au > TiO2 only.N was added to the TiO2 suspension (denoted as CB) or the Ag-or Pd NP-con suspension (denoted as Ag+CB and Pd+CB, respectively).In all cases, the additio increased the Ed by approximately 5% (from 40%, 60%, and 55% to 56%, 65%, a respectively; Figure 2).The addition of both heavy metal NPs and CB effectively im the Ed.The lower effectiveness of metal NPs in the co-presence of CB co (approximately 5% increase) might be due to the lower toluene concentration at t level conversion region and resulting lower mass transfer.The XRD measurement (as received) before and after the addition of Ag NPs and under UV irradiation were performed.In all cases, only the XRD pattern derived from anatase and ru observed (Figure S1).To gain insight into the degradation reaction and the effects of CB and Ag NPs, the formation of water-soluble organic compounds (WSOCs) was monitored under four sets of reaction conditions: TiO 2 only, TiO 2 +CB, TiO 2 +Ag, and TiO 2 +CB+Ag.Toluene vapor was fed from 0 to 3 h while monitoring the concentration of the WSOCs, C WSOC .After 3 h of reaction, toluene feeding was stopped, while the C WSOC was continuously monitored.Figure 3 shows the time course of the C WSOC in the TiO 2 suspension with and without additives (CB or Ag).Without any additives (TiO 2 only), after 3 h, the C WSOC was approximately 0.9 mg-C/L and then slightly increased to approximately 1.0 mg-C/L after stopping toluene vapor feeding.In the presence of CB (TiO 2 +CB), after 3 h, the C WSOC was approximately twice as high as that without any additives (approximately 1.8 mg-C/L), and the C WSOC monotonically increased from approximately 1.8 to 2.4 mg-C/L despite the absence of toluene vapor feeding.Toluene was adsorbed on the CB during vapor feeding and then decomposed into water-soluble intermediates, increasing the C WSOC after stopping toluene feeding.When Ag NPs were added to the above two reaction systems (TiO 2 +Ag and TiO 2 +CB+Ag), the C WSOC decreased after stopping toluene vapor feeding, resulting in much lower C WSOC values (below 0.5 mg-C/L) compared to the cases in the absence of Ag (TiO 2 only and TiO 2 +CB).These results clearly show that Ag NPs accelerate the TiO 2 photocatalytic degradation of toluene, especially the successive decomposition of water-soluble intermediates into mineralized products.To gain insight into the degradation reaction and the effects of CB and Ag N formation of water-soluble organic compounds (WSOCs) was monitored under f of reaction conditions: TiO2 only, TiO2+CB, TiO2+Ag, and TiO2+CB+Ag.Toluene va fed from 0 to 3 h while monitoring the concentration of the WSOCs, CWSOC.Afte reaction, toluene feeding was stopped, while the CWSOC was continuously mo Figure 3 shows the time course of the CWSOC in the TiO2 suspension with and additives (CB or Ag).Without any additives (TiO2 only), after 3 h, the CW approximately 0.9 mg-C/L and then slightly increased to approximately 1.0 mg-C stopping toluene vapor feeding.In the presence of CB (TiO2+CB), after 3 h, the CW approximately twice as high as that without any additives (approximately 1.8 m and the CWSOC monotonically increased from approximately 1.8 to 2.4 mg-C/L des absence of toluene vapor feeding.Toluene was adsorbed on the CB during vapor and then decomposed into water-soluble intermediates, increasing the CWS stopping toluene feeding.When Ag NPs were added to the above two reaction (TiO2+Ag and TiO2+CB+Ag), the CWSOC decreased after stopping toluene vapor resulting in much lower CWSOC values (below 0.5 mg-C/L) compared to the case absence of Ag (TiO2 only and TiO2+CB).These results clearly show that Ag NPs ac the TiO2 photocatalytic degradation of toluene, especially the successive decom of water-soluble intermediates into mineralized products.To gain insight into the degradation reaction and the effects of CB and Ag formation of water-soluble organic compounds (WSOCs) was monitored under of reaction conditions: TiO2 only, TiO2+CB, TiO2+Ag, and TiO2+CB+Ag.Toluene v fed from 0 to 3 h while monitoring the concentration of the WSOCs, CWSOC.Aft reaction, toluene feeding was stopped, while the CWSOC was continuously mo Figure 3 shows the time course of the CWSOC in the TiO2 suspension with and additives (CB or Ag).Without any additives (TiO2 only), after 3 h, the CW approximately 0.9 mg-C/L and then slightly increased to approximately 1.0 mgstopping toluene vapor feeding.In the presence of CB (TiO2+CB), after 3 h, the C approximately twice as high as that without any additives (approximately 1.8 and the CWSOC monotonically increased from approximately 1.8 to 2.4 mg-C/L de absence of toluene vapor feeding.Toluene was adsorbed on the CB during vapo and then decomposed into water-soluble intermediates, increasing the CW stopping toluene feeding.When Ag NPs were added to the above two reaction (TiO2+Ag and TiO2+CB+Ag), the CWSOC decreased after stopping toluene vapor resulting in much lower CWSOC values (below 0.5 mg-C/L) compared to the cas absence of Ag (TiO2 only and TiO2+CB).These results clearly show that Ag NPs a the TiO2 photocatalytic degradation of toluene, especially the successive decom of water-soluble intermediates into mineralized products.We also calculated the toluene decomposition rate normalized by carbon numbers (R d ) during toluene vapor feeding (0-3 h), as well as the formation rate of WSOC (R g _ WSOC ) under toluene feeding (0-3 h) and after stopping the feeding (3-6 h).The R d and R g _ WSOC were compared for four reaction conditions: TiO 2 only, TiO 2 +CB, TiO 2 +CB+Ag, and TiO 2 +Ag (Figure 4).The addition of CB increased the R g _ WSOC .The ratio of the summed R g _ WSOC (determined by the summation of the values at 0-3 h and 3-6 h) to the R d was also calculated (denoted as f WSOC ).The f WSOC value for TiO 2 +CB was calculated as 98.8%, which is much higher than that for TiO 2 alone (63.3%), confirming that the TiO 2 suspension promotes the degradation of water-soluble intermediates, whereas the main role of CB is likely the adsorption of toluene rather than the promotion of decomposition reactions.The addition of Ag NPs to TiO 2 and TiO 2 +CB systems decreased the f WSOC from 63.3% to 11.6% and from 98.8% to 23.4%, respectively, which supports the view that Ag NPs enhanced the degradation of water-soluble intermediates to mineralized products.In conclusion, the enhanced degradation efficiency of the TiO 2 +CB+Ag system was ascribed to the combination of toluene adsorption by CB and the acceleration of the TiO 2 -photocatalyzed degradation of water-soluble intermediates.
Molecules 2024, 29, x FOR PEER REVIEW We also calculated the toluene decomposition rate normalized by carbon n (Rd) during toluene vapor feeding (0-3 h), as well as the formation rate of WSOC ( under toluene feeding (0-3 h) and after stopping the feeding (3-6 h).The Rd and were compared for four reaction conditions: TiO2 only, TiO2+CB, TiO2+CB+A TiO2+Ag (Figure 4).The addition of CB increased the Rg_WSOC.The ratio of the s Rg_WSOC (determined by the summation of the values at 0-3 h and 3-6 h) to the Rd w calculated (denoted as fWSOC).The fWSOC value for TiO2+CB was calculated as 98.8% is much higher than that for TiO2 alone (63.3%), confirming that the TiO2 sus promotes the degradation of water-soluble intermediates, whereas the main role likely the adsorption of toluene rather than the promotion of decomposition re The addition of Ag NPs to TiO2 and TiO2+CB systems decreased the fWSOC from 6 11.6% and from 98.8% to 23.4%, respectively, which supports the view that enhanced the degradation of water-soluble intermediates to mineralized prod conclusion, the enhanced degradation efficiency of the TiO2+CB+Ag system was to the combination of toluene adsorption by CB and the acceleration of th photocatalyzed degradation of water-soluble intermediates.To investigate the adsorption properties of the series of TiO2 suspensions, measurements were performed.To investigate the adsorption properties of the series of TiO 2 suspensions, UV-vis measurements were performed.Figure 5 shows the UV-vis spectra for a series of TiO 2 suspensions, including one containing Ag NPs, after US and UV irradiation.Regardless of the presence or absence of Ag NPs, US irradiation induced an increase in absorbance in the UV-vis spectra owing to the enhancement of dispersiveness.UV irradiation further increased the absorbance, especially for the TiO 2 suspension containing Ag NPs (TiO 2 +Ag), indicating that the oxidized surface of the Ag NPs was photoreduced by UV irradiation [25].The UV-vis spectra of the TiO 2 suspensions are shown in Figure S2.The addition of heavy metal NPs resulted in enhanced absorbance in the UV-vis spectra.The absorbance at 360 nm increased in the following order: Ag > Pd > Au > Pt > TiO 2 .This order is slightly different from that of the E d (Ag > Pd > Pt > Au > TiO 2 ).
We also performed X-ray photoelectron spectroscopy (XPS) measurements to study the surface chemical states of TiO 2 before and after UV irradiation.Figure 6a-d  XPS spectra for the Ti 2p region for a series of TiO 2 samples.The spectra for the O 1s are also shown in Figure S3.All the XPS spectra exhibited peaks at approximately 464 and 458 eV assignable to the Ti 2p 1/2 and 2p 3/2 states, respectively, which are derived from the Ti 4+ species on TiO 2 .For TiO 2 without metal NPs, the peak position and full width at half maximum (WFHM) values for Ti 2p 2/3 were nearly the same, regardless of the presence or absence of UV irradiation (0.741 and 0.740 eV for TiO 2 and TiO 2 -UV, respectively, Figure 7).In the case of the Ag NP-containing TiO 2 , the WFHM value increased from 0.744 to 0.763 eV upon UV irradiation, indicating that the Ti 2p 3/2 peak broadened.Ag NPs capture electrons generated from TiO 2 by UV irradiation and inhibit the recombination of electrons and holes [26].In addition, previous reports have found that the Ti 2p 3/2 peaks were broadened in the XPS spectra of Ag-loaded TiO 2 prepared by photoreduction because some of the surface Ti 4+ species were reduced to Ti 3+ by trapped electrons [25].In our case, the added Ag NPs were immobilized on the TiO 2 surface, promoting the reduction in the surface Ti 4+ species in a similar manner.We also performed X-ray photoelectron spectroscopy (XPS) measurements t the surface chemical states of TiO2 before and after UV irradiation.Figure 6a-d s XPS spectra for the Ti 2p region for a series of TiO2 samples.The spectra for the O also shown in Figure S3.All the XPS spectra exhibited peaks at approximately 464 eV assignable to the Ti 2p1/2 and 2p3/2 states, respectively, which are derived from species on TiO2.For TiO2 without metal NPs, the peak position and full width maximum (WFHM) values for Ti 2p2/3 were nearly the same, regardless of the pre absence of UV irradiation (0.741 and 0.740 eV for TiO2 and TiO2-UV, respectively 7).In the case of the Ag NP-containing TiO2, the WFHM value increased from 0.763 eV upon UV irradiation, indicating that the Ti 2p3/2 peak broadened.Ag NPs electrons generated from TiO2 by UV irradiation and inhibit the recombina electrons and holes [26].In addition, previous reports have found that the Ti 2p3 were broadened in the XPS spectra of Ag-loaded TiO2 prepared by photore because some of the surface Ti 4+ species were reduced to Ti 3+ by trapped electrons our case, the added Ag NPs were immobilized on the TiO2 surface, promot reduction in the surface Ti 4+ species in a similar manner.We also performed a similar XPS analysis for other metal (Pd, Au, and Pt) NPcontaining TiO 2 .A similar increase in the WFHM value was observed after UV irradiation, although the degree of increase was lower than that for Ag NPs (Figure S4).The WFHM values increased in the same order as the E d (i.e., Ag > Pd > Pt > Au, Figure 8).This result differs from the discussion of the UV-vis measurements (see above).To further investigate the correlation between the photocatalytic degradation performance and the surface chemical state, the WFHM value was plotted as a function of the toluene degradation rate, R d .Good correlation was found between the WFHM value and R d (Figure 9, R 2 = 0.97), indicating that capturing the electrons generated from TiO 2 by UV irradiation is the key to enhancing the degradation efficiency.Holes in TiO 2 generated by UV irradiation are known to react with H 2 O to give OH radicals, which further react with organic molecules to promote degradation reactions [27].The added Ag NPs effectively inhibited electron-hole recombination, serving as the most effective heavy metal NPs (Figure S5).The generated OH radicals effectively decompose toluene and WSOC intermediates captured by CB, resulting in the enhanced degradation efficiency.We also performed a similar XPS analysis for other metal (Pd, Au, and Pt) NPcontaining TiO2.A similar increase in the WFHM value was observed after UV irradiation, although the degree of increase was lower than that for Ag NPs (Figure S4).The WFHM values increased in the same order as the Ed (i.e., Ag > Pd > Pt > Au, Figure 8).This result differs from the discussion of the UV-vis measurements (see above).To further investigate the correlation between the photocatalytic degradation performance and the surface chemical state, the WFHM value was plotted as a function of the toluene degradation rate, Rd.Good correlation was found between the WFHM value and Rd (Figure 9, R 2 = 0.97),  We also performed a similar XPS analysis for other metal (Pd, Au, and containing TiO2.A similar increase in the WFHM value was observed after UV irr although the degree of increase was lower than that for Ag NPs (Figure S4).The values increased in the same order as the Ed (i.e., Ag > Pd > Pt > Au, Figure 8).Th differs from the discussion of the UV-vis measurements (see above).To further in the correlation between the photocatalytic degradation performance and the

Photocatalytic Degradation of Toluene
The experimental setup for the degradation of toluene (as a model VO ultrasonically generated mists containing TiO2 is shown in (Figure 10).The expe reactor, which consisted of poly(methyl methacrylate) resin (Figure S6), was e with an ultrasonic transducer (Honda Electronics, Toyohashi, Japan, HM Ultrasonically generated mists containing TiO2 were generated inside the reacto TiO2 suspension in deionized water was irradiated with 2.4 MHz ultrasoun Degussa P-25 TiO2 (Nippon Aerosil, Tokyo, Japan) was used in all the exp because the P-25 TiO2 particles can generate a sufficient amount of OH radica liquid phase [28].The crystal structure of the P-25 TiO2 particles was approxima anatase and 20% rutile, and the average particle diameter was approximately 30 submicrometer-sized aggregation of TiO2 was observed by scanning electron mi (SEM) (Figure S7).The surface area of TiO2, as measured with a BET surface known to react with H2O to give OH radicals, which further react with organic m to promote degradation reactions [27].The added Ag NPs effectively inhibited e hole recombination, serving as the most effective heavy metal NPs (Figure S generated OH radicals effectively decompose toluene and WSOC intermediates c by CB, resulting in the enhanced degradation efficiency.

Photocatalytic Degradation of Toluene
The experimental setup for the degradation of toluene (as a model VOC ultrasonically generated mists containing TiO2 is shown in (Figure 10).The expe reactor, which consisted of poly(methyl methacrylate) resin (Figure S6), was e with an ultrasonic transducer (Honda Electronics, Toyohashi, Japan, HM Ultrasonically generated mists containing TiO2 were generated inside the reactor TiO2 suspension in deionized water was irradiated with 2.4 MHz ultrasound Degussa P-25 TiO2 (Nippon Aerosil, Tokyo, Japan) was used in all the expe because the P-25 TiO2 particles can generate a sufficient amount of OH radica liquid phase [28].The crystal structure of the P-25 TiO2 particles was approximat anatase and 20% rutile, and the average particle diameter was approximately 30 submicrometer-sized aggregation of TiO2 was observed by scanning electron mic (SEM) (Figure S7).The surface area of TiO2, as measured with a BET surface a

Photocatalytic Degradation of Toluene
The experimental setup for the degradation of toluene (as a model VOC) using ultrasonically generated mists containing TiO 2 is shown in (Figure 10).The experimental reactor, which consisted of poly(methyl methacrylate) resin (Figure S6), was equipped with an ultrasonic transducer (Honda Electronics, Toyohashi, Japan, HM-303N).Ultrasonically generated mists containing TiO 2 were generated inside the reactor when a TiO 2 suspension in deionized water was irradiated with 2.4 MHz ultrasound waves.Degussa P-25 TiO 2 (Nippon Aerosil, Tokyo, Japan) was used in all the experiments because the P-25 TiO 2 particles can generate a sufficient amount of OH radicals in the liquid phase [28].The crystal structure of the P-25 TiO 2 particles was approximately 80% anatase and 20% rutile, and the average particle diameter was approximately 30 nm.The submicrometer-sized aggregation of TiO 2 was observed by scanning electron microscopy (SEM) (Figure S7).The surface area of TiO 2 , as measured with a BET surface analyzer (Micromeritics, Norcross, GA, USA, Flowsorb III-2305), was 50 m 2 g −1 .The TiO 2 concentration was fixed at 1.5 g L −1 .A black light blue (BLB) lamp with a maximum light intensity output of 365 nm (UV 365 ; Sankyo Denki, Hiratsuka, Japan, FL4BLB) was used as the light source.Detailed emission spectra of the lamps are presented elsewhere [19].The observed relative humidity was over 95%, which was outside of the measurement range for all conditions, indicating that the mist in the reactor could be maintained under stable conditions.
Molecules 2024, 29, x FOR PEER REVIEW 8 of 1 (Micromeritics, Norcross, GA, USA, Flowsorb III-2305), was 50 m 2 g −1 .The TiO concentration was fixed at 1.5 g L −1 .A black light blue (BLB) lamp with a maximum ligh intensity output of 365 nm (UV365; Sankyo Denki, Hiratsuka, Japan, FL4BLB) was used a the light source.Detailed emission spectra of the lamps are presented elsewhere [19].Th observed relative humidity was over 95%, which was outside of the measurement rang for all conditions, indicating that the mist in the reactor could be maintained under stabl conditions.Dry air containing 5 ppm of toluene vapor was introduced into the reactor at 3.0 L min −1 from the bottom side of the reactor.Before the reaction, TiO2 suspension wa irradiated by a US wave under black light irradiation at 35 °C to remove the pollutants on TiO2 surfaces and/or in aqueous phase.Toluene vapor was then introduced for 30 min to obtain a stable toluene concentration.After stabilizing the toluene concentration at 5 ppm the degradation reaction was started (t = 0 min).
In this study, CB (Mitsubishi Chemical Corporation, Tokyo, Japan, MA100) and/o heavy metal NPs, such as Ag (particle size: 5-30 nm), Pt (1-6 nm), Au (1-4 nm), and Pd (2-7 nm) (Renaissance Energy Research Co., Ltd., Osaka, Japan, as a dispersed solution) were introduced into the reaction system.The experimental schedule is shown in Figur S8.To compare the Ed after the addition of the additives with that before the addition o the additives, an experiment was first conducted using only a suspension of TiO2 particles and then a similar experiment was repeated with the addition of the additives.Th amounts of CB and metal NPs were 25 mg/L and 20 µmol/L, respectively.The Ed wa determined as follows:

Determination of the Amount of WSOCs
Ultrapure water treated by UV irradiation (ADVANTEC, Tokyo, Japan, RFU464CC was used as the suspension to reduce the TOC concentration (approximately 5 ppb).Th suspension (5 mL) in the reaction tank was collected from the sampling tube using syringe (TERUMO, Tokyo, Japan, SS-10SZP) through a cartridge filter (ADVANTEC 25HP045AN).The reaction conditions were the same as those for the photocatalyti degradation experiments.Dry air containing 5 ppm of toluene vapor was introduced into the reactor at 3.0 L min −1 from the bottom side of the reactor.Before the reaction, TiO 2 suspension was irradiated by a US wave under black light irradiation at 35 • C to remove the pollutants on TiO 2 surfaces and/or in aqueous phase.Toluene vapor was then introduced for 30 min to obtain a stable toluene concentration.After stabilizing the toluene concentration at 5 ppm, the degradation reaction was started (t = 0 min).
In this study, CB (Mitsubishi Chemical Corporation, Tokyo, Japan, MA100) and/or heavy metal NPs, such as Ag (particle size: 5-30 nm), Pt (1-6 nm), Au (1-4 nm), and Pd (2-7 nm) (Renaissance Energy Research Co., Ltd., Osaka, Japan, as a dispersed solution), were introduced into the reaction system.The experimental schedule is shown in Figure S8.To compare the E d after the addition of the additives with that before the addition of the additives, an experiment was first conducted using only a suspension of TiO 2 particles, and then a similar experiment was repeated with the addition of the additives.The amounts of CB and metal NPs were 25 mg/L and 20 µmol/L, respectively.The E d was determined as follows:

Determination of the Amount of WSOCs
Ultrapure water treated by UV irradiation (ADVANTEC, Tokyo, Japan, RFU464CC) was used as the suspension to reduce the TOC concentration (approximately 5 ppb).The suspension (5 mL) in the reaction tank was collected from the sampling tube using a syringe (TERUMO, Tokyo, Japan, SS-10SZP) through a cartridge filter (ADVANTEC; 25HP045AN).The reaction conditions were the same as those for the photocatalytic degradation experiments.
The suspension for WSOC concentration measurement was collected using a syringe through a cartridge filter every 1 h during UV irradiation for 6 h (t = 0-6 h).The air supply containing toluene was stopped at t = 3 h, whereas samples were collected every 1 h in the same manner until t = 6 h.As a pretreatment procedure, 5 mL of each water sample in the suspension was diluted to 50 mL with ultrapure water and analyzed using a TOC analyzer.When the experiments were conducted in the presence of additives, the experiments were first conducted with a TiO 2 particle suspension only.After confirming the stabilization of degradation efficiency, additives were added, and the experiments and sample collection were performed in the same manner.The schedule of these experiments is shown in Figure S9.
For comparison of the amount of carbon in the decomposed toluene with the amount of WSOCs present in the suspension as determined by a TOC meter, the R d [µg-C/s] and R g_WSOC [µg-C/s] were calculated using the following equations.
where M C7 , M toluene , C WSOC , and V denote the molar mass of seven carbons, molar mass of toluene, average WSOC concentration in the suspension at t = 0-3 h, and suspension volume, respectively.The f WSOC was then calculated from the R d and R g_WSOC as follows: × 100 (4)

XPS Analysis of the TiO 2 Suspension
Approximately 70 mL of the TiO 2 suspension was transferred from the photocatalytic reactor to a filter (ADVANTECH, GC-50) to separate the water by vacuum filtration.The obtained solid sample was dried in a desiccator for at least 24 h and used for XPS analysis.Heavy metal NP-containing TiO 2 samples were prepared in the same manner.For the UVirradiated samples, the TiO 2 suspension was irradiated with BLB lamp in the reactor, and UV irradiation was performed immediately before the XPS sample preparation (Figure S10).XPS spectra were measured on a JEOL (Tokyo, Japan) JPS-9030 spectrometer having a modified UHV chamber employing Mg Kα radiation.Charge correction was performed using the O 1s peak at 532.0 eV.

Conclusions
In conclusion, CB and Ag NPs are effective additives for the TiO 2 -photocatalytic decomposition of toluene under ultrasonic atomization.The R d was enhanced by a factor of approximately two in the co-presence of CB and Ag NPs.A detailed analysis of the WSOC concentration indicates that the roles of CB and Ag NPs in improving the degradation efficiency are different.CB effectively accumulated WSOC intermediates in the aqueous phase, whereas Ag NPs accelerated the decomposition of WSOCs.The XPS analysis of TiO 2 after the addition of different heavy metal NPs followed by UV irradiation revealed a reduction in the surface species, possibly owing to the electrons generated from TiO 2 by UV irradiation, as indicated by the increase in the WHFM of the Tip 2/3 peak.This value increased in the same order as the E d as follows: Ag > Pd > Pt > Au > TiO 2 (without NPs), implying that Ag NPs effectively capture the electrons derived from the UV irradiation of TiO 2 , thus suppressing the electron-hole recombination and improving the photocatalytic degradation efficiency.

Figure 1 .
Figure 1.Comparison of the toluene degradation efficiencies with and without the ad different types of heavy metal nanoparticles to the TiO2 suspension.

Figure 1 .
Figure 1.Comparison of the toluene degradation efficiencies with and without the addition of different types of heavy metal nanoparticles to the TiO 2 suspension.

Molecules 2024 ,Figure 2 .
Figure 2. Comparison of the toluene degradation efficiencies with and without additio and/or heavy metal (Ag or Pd) nanoparticles to the TiO2 suspension.

Figure 2 .
Figure 2. Comparison of the toluene degradation efficiencies with and without addition of CB and/or heavy metal (Ag or Pd) nanoparticles to the TiO 2 suspension.

Figure 2 .
Figure 2. Comparison of the toluene degradation efficiencies with and without additi and/or heavy metal (Ag or Pd) nanoparticles to the TiO2 suspension.

Figure 3 .
Figure 3. Change in the WSOC concentration in the TiO2 suspension with and without (CB or Ag).

Figure 3 .
Figure 3. Change in the WSOC concentration in the TiO 2 suspension with and without additives (CB or Ag).

Figure 4 .
Figure 4. Comparison of the decomposed carbon amounts in fed toluene and generated W the TiO2 suspension with and without additives.
Figure 5 shows the UV-vis spectra for a series suspensions, including one containing Ag NPs, after US and UV irradiation.Re of the presence or absence of Ag NPs, US irradiation induced an increase in absor the UV-vis spectra owing to the enhancement of dispersiveness.UV irradiation increased the absorbance, especially for the TiO2 suspension containing A (TiO2+Ag), indicating that the oxidized surface of the Ag NPs was photoreduced irradiation [25].The UV-vis spectra of the TiO2 suspensions are shown in Figure addition of heavy metal NPs resulted in enhanced absorbance in the UV-vis spec absorbance at 360 nm increased in the following order: Ag > Pd > Au > Pt > Ti order is slightly different from that of the Ed (Ag > Pd > Pt > Au > TiO2).

Figure 4 .
Figure 4. Comparison of the decomposed carbon amounts in fed toluene and generated WSOC in the TiO 2 suspension with and without additives.

Molecules 2024 ,Figure 5 .
Figure 5. UV-vis spectra of the TiO2 suspension in the presence or absence of Ag NP irradiation with US and/or UV.

Figure 5 .
Figure 5. UV-vis spectra of the TiO 2 suspension in the presence or absence of Ag NPs under irradiation with US and/or UV.

Figure 6 .
Figure 6.XPS spectrum of the Ti 2p peak for TiO2 suspension samples with and without Ag particles before and after UV irradiation Blue and red line shows raw and fitted date.(a) TiO2, (b) TiO2 after UV irradiation, (c) TiO2+Ag, and (d) TiO2+Ag after UV irradiation.

Figure 7 .
Figure 7.Comparison of the full width at half maximum (WFHM) values of the Ti 2p3/2 peak for TiO2 suspension samples with and without Ag particles before after UV irradiation.

Figure 6 .Figure 6 .
Figure 6.XPS spectrum of the Ti 2p peak for TiO 2 suspension samples with and without Ag particles before and after UV irradiation Blue and red line shows raw and fitted date.(a) TiO 2 , (b) TiO 2 after UV irradiation, (c) TiO 2 +Ag, and (d) TiO 2 +Ag after UV irradiation.

Figure 7 .
Figure 7.Comparison of the full width at half maximum (WFHM) values of the Ti 2p3/ TiO2 suspension samples with and without Ag particles before after UV irradiation.

Figure 7 .
Figure 7.Comparison of the full width at half maximum (WFHM) values of the Ti 2p 3/2 peak for TiO 2 suspension samples with and without Ag particles before after UV irradiation.
known to react with H2O to give OH radicals, which further react with organic m to promote degradation reactions[27].The added Ag NPs effectively inhibited hole recombination, serving as the most effective heavy metal NPs (Figure generated OH radicals effectively decompose toluene and WSOC intermediates by CB, resulting in the enhanced degradation efficiency.

Figure 8 .
Figure 8.Comparison of the WFHM values of the Ti 2p3/2 peak for samples of TiO2 suspen the addition of different types of heavy metal nanoparticles after UV irradiation.

Figure 9 .
Figure 9. Correlation between the degradation rate, Rd, and the WFHM values of the Ti for TiO2 suspension samples with the addition of different heavy metal NPs after UV irra

Figure 8 .
Figure 8.Comparison of the WFHM values of the Ti 2p 3/2 peak for samples of TiO 2 suspension with the addition of different types of heavy metal nanoparticles after UV irradiation.

Figure 8 .
Figure 8.Comparison of the WFHM values of the Ti 2p3/2 peak for samples of TiO2 suspen the addition of different types of heavy metal nanoparticles after UV irradiation.

Figure 9 .
Figure 9. Correlation between the degradation rate, Rd, and the WFHM values of the Ti 2 for TiO2 suspension samples with the addition of different heavy metal NPs after UV irrad

Figure 9 .
Figure 9. Correlation between the degradation rate, R d , and the WFHM values of the Ti 2p 3/2 peak for TiO 2 suspension samples with the addition of different heavy metal NPs after UV irradiation.

Figure 10 .
Figure 10.Setup for the photocatalytic decomposition of toluene vapor by TiO2-containing droplet generated by the ultrasonic atomization technique.

Figure 10 .
Figure 10.Setup for the photocatalytic decomposition of toluene vapor by TiO 2 -containing droplets generated by the ultrasonic atomization technique.