Photocatalytic Degradation of Gaseous Acetone by Photocatalysts with Visible Light and their Potential Applications in Painting

Received: October 22, 2022
Revised: December 28, 2022
Accepted: January 26, 2023

 Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.

Cite this article:

Li, Y.H., Yang, S.H., Yuan, C.S., Shen, H., Hung, C.H. (2023). Photocatalytic Degradation of Gaseous Acetone by Photocatalysts with Visible Light and their Potential Applications in Painting. Aerosol Air Qual. Res. 23, 220358. https://doi.org/10.4209/aaqr.220358

HIGHLIGHTS

• VOCs can be degraded by visible light using paints made of Fe-TiO₂ photocatalysts.
• A maximum degradation was achieved by using 3% Fe/TiO₂ paint with visible light.
• Paint made of 3% Fe-TiO₂ photocatalyst was effective to control indoor/outdoor VOCs.

Keywords: Visible light irradiation, Painting, Photocatalysis, Volatile Organic Compounds (VOCs), Air pollution control

1 INTRODUCTION

Indoor air quality is crucial to protect human health for those who stay longer in indoors, particularly for the elderly and the children (Wang et al., 2019; Harb et al., 2018; Sheng et al., 2019). Artificial building materials used for decoration and daily necessities in indoor environments could likely emit volatile organic compounds (VOCs) and cause indoor air pollution (Rosário Filho et al., 2021; Huang et al., 2021), which was associated with an increase in health problem (Huang et al., 2021; Paterson et al., 2021). Therefore, in the past decades, many studies have investigated the potential techniques on the removal of indoor VOCs (Zheng et al., 2019; Ligotski et al., 2019; Zhang et al., 2019). Among these techniques, photocatalysis has been paid more attention on the removal of VOCs due to low cost, cleanliness, and chemical stability (Harb et al., 2018; Stucchi et al., 2018). Photocatalysts irradiated by ultraviolet (UV) light could produce radicals (i.e., OH×) with high oxidization potential, which can further degrade VOCs to harmless products such as CO₂ and H₂O (Wang et al., 2016).

The most commonly used photocatalyst is titanium dioxide (TiO₂) due to its high photocatalytic (PC) activity, high physical and chemical stability, and non-toxicity (Mohamed et al., 2012). However, the energy gap of TiO₂ is wide (3.2 eV), which can be solely stimulated by UV and near-UV lights. Therefore, some attempts on the modification of TiO₂ to absorb visible light have been conducted in the past (Wang et al., 2019; Gao et al., 2018). Doping metal or metallic oxides on TiO₂ is one of the most popular and promising methods for enhancing PC activity and visible light response. Transition metals (such as Zn and V) and noble metals (such as Pt, Au, and Ag) have been extensively used to modify TiO₂ in the previous studies (Aazam, 2014; Qamar et al., 2014). The doping of metals or metallic oxides on TiO₂ has been used to retard the recombination of electron and electron hole pairs, thus enhancing the PC degradation of VOCs.

Paints mixed with PC materials are considered to be effective in the removal of indoor VOCs (Monfared and Jamshidi, 2019; Enea et al., 2019). The categories of paints include anti-bacterial and anti-fungal paints, anti-fouling paints, harmful gas PC paints, and anti-rust paints (Sökmen et al., 2011; Kazuya et al., 2012; Guo et al., 2016). TiO₂ has been used as pigment in paint since 19^th century. With rising concern of indoor environment, especially COVID-19 in early of 2020, the antimicrobial and PC effects of TiO₂ was paid more attention. The paints contain photocatalysts of TiO₂ doped with novel metal became prevalent. These paints were also thought to be effect on the degradation of air pollutants. However, the PC activity was seriously affected by the conditions such as UV irradiation and the paint component (Baudys et al., 2021; Monteiro et al., 2015; Águia et al., 2011). Though the photocatalyst of TiO₂ modified with metal or metallic oxides was extensively investigated in photocatalysis, the practical usage of modified TiO₂ in paint was relatively less investigated. In the future, paints containing photocatalysts are certainly needed by the market to effectively decompose VOCs under the irradiation of visible light in indoor and outdoor environments.

TiO₂ and Ag-doped TiO₂ are usually discussed component in paint. However, TiO₂ was challenged to improve the PC reaction activity under visible light. NanoAg is classified under “extremely toxic” with L(E)C50 < 0.1 µg mL^–1 and expensive (Chen et al., 2022). Accordingly, this research aimed to produce a novel nano-sized visible-light induced photocatalyst component in paint. The environment friendly and hypotoxic element Fe was selected as the doping component. The cost of fabrication for Fe-doped TiO₂ was lower than that for Ag-doped TiO₂. Therefore, the photocatalyst was prepared by doping metal ions-Fe on TiO₂ to degrade VOCs (i.e., acetone). The VOC degradation efficiency of this novel photocatalysts and the paint made of it were compared with that of TiO₂ and Ag-doped TiO₂. The performance of self-produced nano-sized synthesized photocatalysts including TiO₂, Ag-doped TiO₂, and Fe-doped TiO₂ on the PC degradation of VOCs was initially evaluated in a self-designed batch PC reactor. The synthesized photocatalysts were further applied to produce PC paints and to investigate their effects on the PC degradation of VOCs in an environmental chamber.

 
2 MATERIALS AND METHODS

 
2.1 Preparation of Nano-sized Synthesized Photocatalysts and Paints

This study initially self-prepared nano-sized TiO₂, Fe-doped TiO₂, and Ag-doped TiO₂ by the sol-gel method. The photocatalysts were then coated on the frosted glasses (FGs). The details for the preparation process were shown in Supplementary Materials. The thickness of the photocatalyst coating was mainly controlled by the duration of the immersion process and FGs’ surface characteristics, which can be estimated by Eq. (1).

where T is the thickness of the photocatalyst, µm; W₂ is the weight of the FGs coated with photocatalyst, g; W₁ is the weight of the FGs without coating the photocatalyst, g; ρ_cat is the density of the photocatalyst, g cm^–3; and A_t is the surface area of the FGs, cm². 10⁴ is the unit conversion factor from cm to µm. In this study, the thickness of the photocatalyst coated on the FGs in this investigation was approximately 60 µm.

TiO₂ or synthesized photocatalysts was further mixed with acrylic resin and DI water at a certain ratio to prepare the paints. The paints were then brushed on the surface of the FGs (50 × 50 × 3 mm) to simulate the process of painting onto a building surface. The paints coated on the FGs were then dried at room temperature. The thickness of the dry paints coated on the FGs was about 150 µm which was estimated by Eq. (1).

In order to ascertain the characteristics of TiO₂, Fe-doped TiO₂, or Ag-doped TiO₂ photocatalysts, analysis on surface characterization was performed. The models of the analytical instruments were listed in Table S1.

 
2.2 Set-up of Photocatalytic Reaction System

The PC degradation experiments were carried out with gaseous acetone in a Pyrex glass cylindrical batch PC reactor with an inside diameter of 11 cm, a depth of 5 cm, and a total volume of 475 cm³ (see Fig. 1). The top of the PC reactor was sealed by a 15 cm diameter quartz glass cover with a Teflon septum between the cover and the PC reactor. The FGs coated with TiO₂, Fe-, or Ag-doped TiO₂ were illuminated by two fluorescent lamps (East Asia, FL-15D, 15W) situated over the top of the PC reactor. The distance between the lamps and the top of the PC reactor was about 4 cm. The spectrum wavelength of visible light irradiated from the fluorescent lamps mainly ranged from 400 to 600 nm with less radiation below 400 nm (see Fig. S1). The FGs coated with TiO₂, Fe-, or Ag-doped TiO₂ photocatalysts were laid on the top of a stainless-steel support situated in the center of the PC reactor. There are four holes on the outside wall of the PC reactor, which were used for acetone injection, air sampling, humidity, and air pressure control, respectively. A magnetic stirrer was used to completely mix the gases in the PC reactor and thus enhance the interaction of acetone over the surface of TiO₂, Fe-doped TiO₂, or Ag-doped TiO₂ photocatalysts.

Fig. 1. Schematic diagram of the batch PC reaction system.

 
2.3 Acetone Degradation with Nano-sized Synthesized Photocatalysts

The PC oxidation performance of TiO₂, Fe-, or Ag-doped TiO₂ photocatalysts coated on the FGs was investigated to decompose acetone under different experimental conditions. Prior to conducting the acetone degradation experiments, pre-experiments were initially performed to ensure the quality of the acetone degradation results. The pre-experiments including the adsorption and catalysis of acetone (without light irradiation) of TiO₂ and the photolysis of acetone without TiO₂ were performed at the reaction temperature of 25°C. The results of adsorption, photolysis, and catalysis of acetone were shown in Figs. S2 and S3, respectively. The pre-experimental results proved that acetone cannot be decomposed solely by adsorption, photolysis, or catalysis without light irradiation.

The PC reactor was firstly purged with high-purity nitrogen (N₂) gas to expel the residual acetone. The FGs coated with the photocatalysts were then placed in the center of the PC reactor, and air was then guided into it. All the inlets of the PC reactor were shut off until the air pressure in the PC reactor attained 1 atm. A fixed amount of 0.3 µL liquid phase acetone was injected into the PC reactor with a manual syringe bought from Agilent. It took about half an hour that all the acetone was wholely evaporated to the vapor phase and the systematic equilibrium was achieved.

The initial acetone concentration in the reactor was then detected and recorded as C₀. The lamps were turned on and the reaction time was then recorded as t = 0. The concentration of acetone was detected by a gas chromatograph with a flame ionization detector (GC-FID) (Aglent, HP 5890).

The degradation efficiency of acetone can be determined by Eq. (2)

where η is the degradation efficiency of acetone, %; C₀ is the initial concentration of acetone in the PC reactor, ppm; and C_e is the concentration of acetone when the variation of acetone concentration was steady state, ppm.

 
2.4 Acetone Degradation with Paints Made of Nano-sized Synthesized Photocatalysts in a Batch Photocatalytic Reactor

The degradation of acetone with paints made of TiO₂ or synthesized TiO₂ was further investigated. The FGs coated with various synthesized photocatalyst paints were situated in the center of the PC reactor described in Section 2.2. The procedure described in Section 2.3 was followed for the acetone degradation experiments. The experiments were performed under the following conditions: initial acetone concentration of 120 ± 5 ppm, reaction pressure of 1 atm, reaction temperature of 25°C, relative humidity of 5%. Each acetone degradation experiment was operated for a duration of 200 min to achieve the steady state.

 
2.5 Acetone Degradation with Paints Made of Nano-sized Synthesized Photocatalysts in an Environmental Chamber

The paints made of TiO₂, 1% Ag-doped TiO₂, or 3% Fe-doped TiO₂, respectively, were selected to compare their degradation efficiencies of acetone in an environmental chamber operating in the recirculation mode. The environmental chamber was made of stainless steel with a volume of 2.5 m³. A FG (300 × 300 × 3 mm) coated with paints made of different synthesized photocatalysts was situated in the center of the environmental chamber. The reaction temperature and relative humidity in the environmental chamber were set at 25°C and 50%, respectively, to simulate the typical indoor environments. Prior to conducting the acetone degradation experiments, the environmental chamber was initially purged with acetone free air, which was obtained by passing indoor air through an air clean system comprised of a high efficiency particulate air (HEPA) filter and an activated carbon cartridge. Acetone was injected into the environmental chamber awaiting for complete evaporation and its initial concentration was then measured (i.e., C₀ = 1.5 ppm). During the acetone degradation experiments, a set of two 15-W fluorescent lamps used as the source of visible light was then turned on. The concentrations of acetone in the environmental chamber were continuously measured in an interval of 5 min. Each test of the acetone degradation experiments was operated with a duration of 80 min.

 
2.6 Simulation of Acetone Degradation with the Paint Made of 3% Fe-doped TiO₂ in a Real Room

In order to evaluate the degradation efficiency of indoor VOCs using a paint made of 3% Fe-doped TiO₂, a simulated test was conducted in a real room. Assuming that VOCs are released gradually from indoor sources, the concentrations of VOCs in the room would increase proportionally with time at the initial stage. At steady state, an equilibration of VOC concentrations in the room was then achieved late on, which was attributed to a combination of emission, adsorption, desorption, and infiltration. Since it is difficult to derive an equation to describe the concentration of indoor VOCs with all the effects, some effects are ignored in this study. The effect of adsorption decreases indoor VOC concentration, while the effect of desorption is neglectable relative to the effect of adsorption. Thus, the combination effects of adsorption and infiltration could reduce the indoor VOC concentration. The simulation results would be enhanced if the effects of adsorption, desorption, and infiltration are ignored. Using power-rate equation to describe the variation of acetone reaction rate with its concentration, the VOC concentration in a real room can then be simplified by Eq. (3) (Luo and Lee, 1997):

where k_d is the release rate of the sources, mg m^–2 min^–1; S_d is the area of the sources, m²; k is the PC reaction rate constant determined from the experimental results attained in Section 2.5; C_r is the acetone concentration in the room, ppm; and n is the reaction order; V is the volume of the room, m³; m is the mass of the photocatalyst, g; n and k are empirical constants.

 
3 RESULTS AND DISCUSSION

 
3.1 Surface Characterization of Synthesized Photocatalysts

The surface characterization of the photocatalysts has influence on the PC reaction activity. Therefore, the surface characterization analysis of the synthesized photocatalysts was performed. The results were shown in the Supplementary Materials.

 
3.2 Photocatalytic Degradation Efficiency of Acetone with Synthesized Photocatalysts under the Irradiation of Visible Light in the Batch PC Reactor

The variation of acetone degradation efficiencies with different synthesized photocatalysts under the irradiation of visible light was investigated in a batch PC reactor. The acetone degradation experiments were performed under the initial acetone concentration of 120 ± 5 ppm, reaction temperature of 25°C, reaction pressure of 1 atm, and relative humidity of 5%.

 
3.2.1 Ag-doped TiO₂

Fig. 2 depicts the degradation efficiency of acetone under the irradiation of visible light using Ag-doped TiO₂ as photocatalysts. The degradation efficiencies of acetone using TiO₂, 1, 3, and 5% Ag-doped TiO₂ as photocatalysts were 32.2, 47.8, 28.7, and 17.9%, respectively. It showed that the degradation efficiency of acetone increased with low doping and then decreased with high doping amount of Ag⁺ to TiO₂. The highest degradation efficiency of 47.8% was obtained for 1% Ag-doped TiO₂. The results concurred with previous studies that Ag doping could retard the recombination of electron and electron hole pairs and deposited on TiO₂ surface as the electron-hole separation centers (Anandan et al., 2008), which made the adsorption of oxygen molecules much easier. Oxygen molecules could further capture e^– and yield the reactive oxidants, such as HO₂^– and O₂^–, which had the high potential to degrade organic compounds. Additionally, doping Ag⁺ to TiO₂ could increase the SSA of the synthesized photocatalysts, and also changed the state of electron and lead to the transformation of their charge, which would enhance the adsorptive capacity of acetone (Bin and Chen, 2011). As a result, the degradation efficiency of acetone could be upgraded with the doping of Ag⁺ to TiO₂.

Fig. 2. Variation of acetone concentration with reaction time using Ag-doped TiO₂ phtocatalysts in the batch PC reactor.

However, further increased amount of 3 and 5% Ag-doped TiO₂ could cause significant aggregation phenomena and thus decreased the SSA of Ag-doped TiO₂. As shown in Fig. S6 (a, b, and c), the crystallite diameter of Ag-TiO₂ increased with the amount of Ag doped. A cluster phenomenon was also observed for Ag-doped TiO₂. The similar aggregation phenomena were reported by other literatures (Suwanchawalit et al., 2012; Harifi and Montazer, 2014; Sobana et al., 2006). This could then lead to lower adsorptive capacity of acetone, resulting in lower degradation efficiency of acetone. Therefore, a suitable amount of Ag⁺ doped to TiO₂ could effectively enhance the PC degradation of acetone. This study revealed that the highest acetone degradation efficiency of 47.8% was observed for using 1% Ag-doped TiO₂ as a photocatalyst in a batch PC reactor.

 
3.2.2 Fe-doped TiO₂

The degradation efficiency of acetone under the irradiation of visible light using Fe-doped TiO₂ as photocatalysts is illustrated in Fig. 3. It showed that the degradation efficiencies of acetone using TiO₂, 1, 3, and 5% Fe-doped TiO₂ were 32.2, 53.7, 66.6, and 50.4% respectively. The degradation of acetone using Fe-doped TiO₂ was more efficiently than that of TiO₂ and Ag-doped TiO₂. The acetone degradation efficiency increased with low doping and then decreased with high doping amount of Fe³⁺ to TiO₂, with a peak acetone degradation efficiency of 66.6% for 3% Fe-doped TiO₂.

Fig. 3. Variation of acetone concentration with reaction time using Fe-doped TiO₂ phtocatalysts in the PC reactor.

Doping Fe³⁺ to TiO₂ could enhance the degradation of acetone under the irradiation of visible light, since the doping of Fe³⁺ could effectively enhance the UV-visible absorption intensity, and largely shifted the absorption band of TiO₂ toward the visible region, namely red-shift phenomenon (Umar and Abdul, 2008). Doping Fe³⁺ to TiO₂ could also promote the generation of electron-hole pairs (Eshaghi and Moradi, 2018) and further retard the recombination of electrons and electron holes (Zhou et al., 2005). Fe³⁺ doped to TiO₂ could be used as the medium of charge transmission across the interface. Fe³⁺ can further convert to either Fe²⁺ or Fe⁴⁺, assisting in effectively trapping electrons and holes. Because Fe³⁺ is relatively more stable than Fe²⁺ and Fe⁴⁺, Fe²⁺ and Fe⁴⁺ would easily release the trapped charge which would move to the surface and thus initiate the photocatalytic reactions.

Additionally, some physical characteristics, such as SSA, crystalline phase, and size of crystallite, could also affect the PC degradation of acetone. A larger SSA could enhance the adsorptive capacity of acetone. The analytical results of SSA listed in Table S4 showed that the SSA of Fe-doped TiO₂ became larger with the doping of Fe³⁺ to TiO₂. The crystalline phase of synthesized photocatalysts is another important factor that affects the PC reactions (Carp et al., 2004). As mentioned in the Section 3.1.1 (in the Supplementary Materials), most nano-sized synthesized photocatalysts prepared in this study had a crystalline phase of anatase. The formation rate of electron and electron hole pairs increased with the drop in photocatalyst size, especially for crystallite diameters below 10 nm (Zhou et al., 2006). However, Fe³⁺ might not only be the interfacial charge transfer but also the recombination center (Yu et al., 2009). As the mass content of Fe was higher than 3%, Fe³⁺ steadily acted as a recombination center, causing a reduction of PC activity.

 
3.2.3 Comparison of the photocatalytic degradation efficiency

The degradation of acetone using different photocatalysts under the irradiation of visible light was further compared. Overall, the degradation efficiencies of acetone were ordered as 3% Fe-doped TiO₂ > 1% Fe-doped TiO₂ > 5% Fe-doped TiO₂ > 1% Ag-doped TiO₂ > TiO₂ > 3% Ag-doped TiO₂ > 5% Ag-doped TiO₂. This implied that doping Fe³⁺ to TiO₂ could effectively enhance the PC activity of TiO₂. However, the PC activities for doping Ag⁺ strongly depend on the amount of Ag⁺ doped. Doping Ag to TiO₂ could promote the degradation of acetone as the mass content of Ag was below a critical value, which was about 1% as obtained in this investigation.

Doping Ag on the TiO₂ could construct the noble-metal TiO₂ heterojunctions, which significantly improved the photocatalytic activity of TiO₂ in visible-light region due to surface plasmon resonance (SPR) effect of noble metals (Jing et al., 2013; Yang et al., 2015) on the surfaces of noble metal, the SPR effect mainly arises from the collective oscillations of electrons. these energetic electrons transfer into the conduction band of the coupled semiconductor due to the Schottky barrier. A local electric field is then formed at the interface which favors the separation of photo-generated charges. However, for the Fe-doped TiO₂, the metal ion was implanted into the semiconductor bulk and modified the electronic structure of the semiconductor (Yamashita et al., 2002). The PC activity of Fe-TiO₂ was developed as a result of the ions of Fe introduced electron and holes trapping states near the conduction band bottom and valance band top of TiO₂ which enhanced the charge separation.

Additionally, the analytical results of EDS listed in Table S3 also confirmed these conclusions. The elemental contents of Ag for 1, 3, and 5% Ag-doped TiO₂ were 1.31, 2.98, and 5.23%, while those of Fe for 1, 3, and 5% Fe-doped TiO₂ were 0.92, 2.77, and 4.86%, respectively. The EDS results confirmed the existence and the percentages of Ag or Fe in the synthesized TiO₂ photocatalysts. The molar ratios of Ti to O (Ti:O) in 1, 3, and 5% Ag-doped TiO₂ were approximately 1:2, indicating that Ag-doped TiO₂ did not change the crystal structure of TiO₂. However, the molar ratios of Ti to O in 1, 3 and 5% Fe-doped TiO₂ were about 1:3, implying that Fe³⁺ was able to squeeze into the crystal structure of TiO₂ and replaced Ti⁴⁺ (Zhang et al., 2013).

 
3.3 Photocatalystic Degradation of Acetone with Paints Made of Synthesized Photocatalysts under the Irradiation of Visible Light in the Batch PC Reactor

The degradation of acetone by using paints made of synthesized photocatalysts prepared in this study was further investigated in the batch PC reactor. The experiments were performed to confirm the feasibility of paints used for the degradation of acetone in indoor environments. The paints were brushed on the surface of an FG. The FG brushed with paints was dried in room temperature and then situated in the batch PC reactor introduced in Section 2.2 to investigate the effect of the paints on acetone degradation.

 
3.3.1 Paints made of Ag-doped TiO₂

Fig. 4 depicts the variation of acetone concentration with reaction time using paints made of Ag-doped TiO₂ in the batch PC reactor. The degradation efficiencies of acetone using the paints made of TiO₂, 1, 3, and 5% Ag-doped TiO₂ were 18.0, 21.3, 17.4, and 8.7%, respectively. The degradation efficiency of acetone increased and then decreased with the amount of Ag⁺ doped to TiO₂. The maximum acetone degradation efficiency appeared for the paint made of 1% Ag-doped TiO₂. Though the PC activity of the paint made of 1% Ag-doped TiO₂ was better than that of the paint made of bare TiO₂, the PC degradation efficiency of acetone using paints was somehow lower than that of the related photocatalysts. Previous study reported that the particles of Ag-doped TiO₂ dispersed evenly in the slurry during the painting preparation process. However, some of the photocatalyst particles might be enclosed by the resin and turn invalid by the PC reaction, which might hinder the PC reaction effect (Caballero et al., 2010). Therefore, it is important to optimize a compromise ratio between the photocatalysts and the binder, which could maintain PC activity and paints’ stability. In addition to the PC degradation effect, the paints made of Ag-doped TiO₂ might present the sterilization effect. Although the PC degradation efficiency of VOCs might not be high compared to the related phtocatalysts, paints containing Ag have been investigated for antifungal and antibacterial agents in the past (Bellotti et al., 2015; Holtz et al., 2012).

Fig. 4. Variation of acetone concentration with reaction time using paints made of TiO₂ and 1–5% Ag-doped TiO₂ in the batch PC reactor.

 
3.3.2 Paints made of Fe-doped TiO₂

The variation of acetone concentration with reaction time using the paints made of Fe-doped TiO₂ in the batch PC reactor is illustrated in Fig. 5. The degradation efficiencies of acetone using the paints made of TiO₂, 1, 3, and 5% Fe-doped TiO₂ were 18.0, 27.6, 32.7, and 14.6%, respectively. Similarly, the acetone degradation efficiencies increased and then decreased with the amount of Fe³⁺ doped to TiO₂, with a peak degradation efficiency at 3% Fe-doped TiO₂. However, the acetone degradation efficiencies of the paints made of Fe-doped TiO₂ were lower than those of the related photocatalysts. This phenomenon was mainly attributed to the cover of resin on the surface of photocatalyst particles. The resin shrouding the surface of the photocatalyst could inhibit the absorption of incident light and thus deactivate the photocatalysts.

Fig. 5. Variation of acetone concentration with reaction time using paints made of TiO₂ and 1–5% Fe-doped TiO₂ in the batch PC reactor.

It was noted that the paints made of 3% Fe-doped TiO₂ had an optimal compromise ratio for the PC degradation of acetone. Overall, the acetone degradation efficiency of 32.7% for 3% Fe-doped TiO₂ was higher than 21.3% for 1% Ag-doped TiO₂. Moreover, the cost of materials used to prepare Fe-doped TiO₂ was cheaper than that for Ag-doped TiO₂. We concluded that doping Fe³⁺ could effectively reduce the energy gap and thus shift the absorption range to the visible light zone. Doping Fe³⁺ could also enhance the stability of the nano-sized TiO₂ cluster. This nature provided uniformity and stability during the painting preparation process (Saqlain et al., 2020). It concurred with previous study that doping Fe³⁺ to TiO₂ could enhance the adhesion of the photocatalysts (Dietrich et al., 2012).

 
3.3.3 Reusability of the paints made of different synthesized photocatalysts

Deactivation of the photocatalysts is one of the factors that limited the practical applications of photocatalysis. Multiple runs for the PC degradation of acetone were conducted to explore the stability of the paints made of different synthesized photocatlysts. The paints made of 1% Ag-TiO₂ and 3% Fe-TiO₂ were selected as the represent photocatalysts to achieve higher photodegradation efficiencies. The experiments were constantly conducted at the same operational conditions. Multiple results shown in Fig. 6 illustrated that the photoactivity of the paints maintained stable after three runs for approximately 6 hr per run.

Fig. 6. Multiple runs for acetone photocatlytic degradation using paints made of (a) 1% Ag-TiO₂; (b) 3% Fe-TiO₂ in the batch PC reactor.

 
3.4 Photocatalystic Degradation Efficiency of Acetone with Paints Made of Synthesized Photocatalysts under the Irradiation of Visible Light in the Environmental Chamber

To confirm the PC degradation effectiveness of organic compounds, the paints made of nano-sized synthesized photocatalysts were further tested to decompose acetone under the irradiation of visible light in the environmental chamber.

Fig. 7 depicts the variation of acetone concentration with reaction time using the paints made of TiO₂ and nano-sized synthesized photocatalysts. The concentration of acetone decreased almost linearly and achieved a constant level in 50 min. The degradation efficiencies of acetone using the paints made of TiO₂, 1% Ag-doped TiO₂, and 3% Fe-doped TiO₂ were 24.9, 32.4, and 46.2%, respectively. Moreover, the highest degradation efficiency of acetone appeared for the paint made of 3% Fe-doped TiO₂. In order to know the availability of the paint made of 3% Fe-TiO₂, the photodegradation efficiency was compared with previous published investigations. Though there are many investigations on the PC degradation of acetone, most of them were conducted under UV light as shown in Table 1. Among all the literatures, only the investigation published by Ho et al. (2019) was conducted under the light wavelength more than 380 nm. They achieved the removal efficiency of acetone was 46.7%, which was similar to the results of this investigation. Therefore, such paint could be effectively applied to the PC degradation of acetone. The paint made of 3% Fe-TiO₂ is thought to be the most suitable innovative photocatalytic paint for the degradation of indoor and outdoor organic pollutants due to higher degradation efficiency and cheaper raw materials.

Fig. 7. Variation of acetone concentration using paints made of synthesized phtocatalysts in an environmental chamber.

3.5 Acetone Degradation with Paint Made of 3% Fe-doped TiO₂ in a Simulated Room

3.5.1 Determination of reaction rate constant for acetone degradation using data from environmental chamber experiments

The experimental data attained from the environmental chamber with the paint made of 3% Fe-doped TiO₂ were used to determine the reaction-rate constant for acetone degradation. Because there is no continuous source of acetone in the environmental chamber, the k_d and S_d in Eq. (3) can be set as zero. The variation of acetone reaction rate with its concentration can then be derived by Eq. (4):

Eq. (4) can be further rearranged as shown below,

The value of n and k can be determined by linear plot of $In\left( \frac{V}{M}\frac{d_c}{d_t}\right)$ and ln(C_r). Experimental results attained from the environmental chamber with the paint made of 3% Fe-doped TiO₂ were substituted into Eq. (5). The simulation results of kinetic model with experimental results showed that n and k were 1.599 and 0.022, respectively. Therefore, Eq. (3) can be expressed by Eq. (6),

3.5.2 Simulation results of using the paint in a real room

Acetone is an indoor air pollutant that has been reported in precious studies (Sakai et al., 2017; Chan et al., 2018; Missia et al., 2010). Missia et al. (2010) conducted campaigns in five European cities (Milan, Copenhagen, Dublin, Athens and Nicosia) and found that the acetone concentration ranged from 2.8 to 308.8 µg m^-–3. Sakai et al. (2017) detected indoor acetone concentration as high as 133 µg m^–3 in Malaysia. Schieweck (2021) monitored the indoor air of prefabricated wooden house and got the highest acetone concentration of 522 µg m^–3. Acetone can be emitted from a variety of indoor sources including wooden materials, bleach, laundry detergent, floor glue, nail color remover, oil paint, and furniture polish, even from human breath (Schieweck, 2021; Caron et al., 2020). Acetone can be released continuously from indoor sources and its release rate varies with environmental parameters. The release rate of acetone was assumed as 3458 µg m^–2 h^–1, which was the highest value of acetone emission rate determined for the commercial particleboard (Caron et al., 2020). Though acetone can be released from different indoor sources, wooden materials can emit acetone continuously and get relatively higher acetone concentration (Missia et al., 2010). Assuming that the area of the acetone source was approximately 10 m² (wardrobe, 1.8 × 2.2 × 0.6 m; desk, 1.4 × 0.8 × 0.8 m), the wall and the ceiling of a room (4.2 × 3.6 × 2.8 m) were painted by paint made of 3% Fe-doped TiO₂ with the total painting area of 58.80 m². Considering the shading effect of furniture, 60 percent of the painting area (about 35.28 m²) was thought to be irradiated by visible light in the room. Thus, the indoor acetone concentration can be estimated by Eq. (6).

Based on the highest indoor acetone concentration of 522 µg m^–3 (Schieweck, 2021), the reduction rate of indoor acetone using the paint made of 3% Fe-doped TiO₂ calculated by Eq. (6) was 4.117 mg min^–1. The rate of acetone released from the sources was about 1.44 mg min^–1. Therefore, the amount of acetone reduced by the paint made of 3% Fe-doped TiO₂ was higher than the amount of acetone released from the source. It showed that the paint made of 3% Fe-doped TiO₂ should be effective to control the concentration of indoor acetone. However, the PC reaction rate was also influenced by other environmental factors such as the light irradiation intensity, which will be further investigated in the future work.

 
4 CONCLUSIONS

Self-prepared innovative paints made of synthesized photocatalysts were examined in this study in order to find an effective and cheap paint for VOC degradation. TiO₂, Ag-doped TiO₂, and Fe-doped TiO₂ were produced by sol-gel method and characterized for their surface properties. It showed that extra high intensity of anatase did exist in the synthesized photocatalysts. The light absorption of synthesized photocatalysts extended largely toward visible light, which indicating the band edge wavelength shifted to a longer wavelength with the doping of Ag⁺ and Fe³⁺ to TiO₂. Surface characterization analysis showed that the PC reactivity of 1% Fe-doped TiO₂, 3% Fe-doped TiO₂, 5% Fe-doped TiO₂, and 1% Ag-doped TiO₂ would be better than that of TiO₂. The degradation efficiencies of acetone with various synthesized photocatalysts were ordered as 3% Fe-doped TiO₂ > 1% Fe-doped TiO₂ > 5% Fe-doped TiO₂ > 1% Ag-doped TiO₂ > TiO₂ > 3% Ag-doped TiO₂ > 5% Ag-doped TiO₂. The highest acetone degradation efficiency of 32.7% was observed for the paint made of 3% Fe-doped TiO₂. Environmental chamber tests further indicated that acetone degradation efficiencies of 46.2% and 32.4% were obtained for the paints made of 3% Fe-doped TiO₂ and 1%Ag-doped TiO₂, respectively. The results implied that paint made of 3% Fe-doped TiO₂ is better than that made of Ag-doped TiO₂. The simulated results of paint made of 3% Fe-doped TiO₂ used in indoor environment confirmed that using the paint could effectively control indoor acetone pollution. Therefore, paint made of 3% Fe-doped TiO₂ is a suitable and cheap PC paint for the degradation of indoor and outdoor VOCs.

 
ACKNOWLEDGEMENTS

This work was financially supported by the Ministry of Science and Technology (MOST) of ROC (Taiwan) (grant number NSC102-2622-E-110-002-CC3), Yung-Chi Paint and Varnish Mfg. Co. Ltd., and the Hubei Provincial Natural Science Foundation of China (2020CFB862).

 
ADDITIONAL INFORMATION

Credit Authorship Contribution Statement

Yu-Hua Li: Conceptualization, Writing-Original Draft, Writing-Review & Editing. Sheng-Hua Yang: Investigation, Methodology. Chung-Shin Yuan: Funding acquisition, Project administration, Writing-Original Draft, Writing-Review & Editing. Huazhen Shen: Investigation. Chung-Hsuan Hung: Writing-Original Draft, Writing-Review & Editing.