Temperature-Dependent Degradation of Volatile Organic Compounds Using Ga₂O₃ Photocatalyst

Abstract

Volatile organic compounds (VOCs), including benzene, toluene, and formaldehyde, are hazardous air pollutants that require efficient and sustainable mitigation strategies. Photocatalytic degradation of VOCs offers a promising pathway; however, its performance is strongly influenced by multiple operational parameters. Here, we present a systematic investigation of toluene degradation under ultraviolet-C (UVC) irradiation across controlled temperatures using Ga₂O₃ as a photocatalyst. A comprehensive analysis revealed that elevated temperatures enhanced photocatalytic activity by accelerating chemical reaction rates. However, further temperature increases led to a decrease in performance due to a reduction in the reactant adsorption rate. An optimal operating temperature was identified, at which the balance between chemical reaction rates and reactant adsorption yields the highest degradation efficiency. These findings demonstrate Ga₂O₃ as a promising photocatalyst and provide fundamental insights into the temperature-dependent photocatalytic mechanisms governing VOC removal in practical environmental applications.

1. Introduction

Volatile organic compounds (VOCs), including benzene, toluene, and formaldehyde, are significant pollutants that pose serious risks to both human health and the environment [1]. VOCs are emitted from various sources, including industrial facilities, vehicle exhaust, indoor furnishings, solvents, and construction materials. Their high volatility and chemical reactivity facilitate widespread dispersion in the atmosphere, contributing to the formation of secondary pollutants, such as ozone and particulate matter [2]. Long-term exposure to VOCs has been reported to cause a variety of health issues, including respiratory disorders, neurological damage, and cancer-related problems [3,4,5]. For these reasons, regulatory agencies such as the World Health Organization (WHO), the U.S. Environmental Protection Agency (EPA), and the European Union have established increasingly stringent limits on VOC concentrations in indoor and outdoor environments [6,7]. Therefore, to achieve these standards, the development of efficient, sustainable, and cost-effective technologies for VOC removal is crucial for improving the air quality and public health.

Conventional approaches for VOC removal include thermal oxidation, condensation, adsorption using activated carbons or zeolites, and biofiltration methods. While these methods have been deployed in industrial and indoor applications, they suffer from critical limitations such as high energy consumption, generation of secondary pollutants, saturation and regeneration issues, and limited degradation efficiency for toxic or recalcitrant VOCs. To overcome these limitations, photocatalysis has emerged as a promising strategy for the efficient degradation of VOCs. Photocatalysts such as titanium dioxide (TiO₂), zinc oxide (ZnO), and gallium oxide (Ga₂O₃) generate electron-hole pairs under light irradiation [8,9,10,11,12]. These photogenerated charge carriers participate in redox reactions at the catalyst surface, leading to the oxidative decomposition of adsorbed VOC molecules into CO₂ and H₂O [13,14]. These materials possess wide bandgaps and high redox capabilities, which effectively suppress the recombination of photogenerated charge carriers, enhancing their catalytic efficiency. Among various photocatalysts, TiO₂ has been widely applied for VOC degradation for its photocatalytic activity and non-toxic properties [15,16,17]. However, TiO₂ suffers from intrinsic drawbacks, such as the recombination of photogenerated carriers and the accumulation of stable intermediates on its surface, which limit its long-term activity and efficiency [18]. In addition, ZnO offers a similar bandgap structure and facile synthesis. However, its photostability remains a challenge [19,20].

Recently, Ga₂O₃ has emerged as a promising alternative photocatalyst, attracting significant attention. With a wide bandgap around 4.8 eV, Ga₂O₃ provides a higher redox potential than TiO₂ and ZnO, enabling the efficient decomposition of stable aromatic hydrocarbons such as benzene and toluene [21,22,23]. Among its polymorphs, β-Ga₂O₃ demonstrated superior thermal stability and promising photocatalytic activity under ultraviolet-C (UVC) irradiation [24,25]. Furthermore, the porous surface morphology of Ga₂O₃ particles offers a large active surface area for VOC adsorption, enhancing photocatalytic efficiency. Previous studies indicate that β-Ga₂O₃ exhibits enhanced degradation efficiency for VOCs due to its high density of active sites and resistance to photocorrosion [26].

Despite these advantages, the photocatalytic performance of Ga₂O₃ can be further influenced by multiple operational parameters, including reactant concentration, wavelength of light, operating temperature, and catalyst loading. Previous studies have explored the photocatalytic activity of Ga₂O₃ and its structural advantages; however, the influence of operating temperature on VOC degradation efficiency has not been systematically addressed.

Therefore, in this study, we systematically investigated the effect of temperature on the photocatalytic degradation of toluene, a representative VOC, using β-Ga₂O₃ under UVC irradiation. By conducting controlled experiments across a defined temperature range, we elucidate the interplay between the competing effects of chemical reaction acceleration and adsorption reduction on overall photocatalytic performance. Through this mechanistic analysis, we identify the optimal operating temperature that maximizes degradation efficiency. Our findings advance beyond prior reports that largely focused on material synthesis or surface modifications, offering fundamental insights into the temperature–activity relationship of Ga₂O₃. Furthermore, this study provides practical guidelines for the rational design and optimization of photocatalytic systems for effective VOC removal in industrial and environmental applications.

2. Results and Discussion

The microstructure of the commercially available β-Ga₂O₃ powder was analyzed using field-emission scanning electron microscopy (SEM), as shown in Figure 1a. The SEM image revealed that the Ga₂O₃ photocatalyst consists of porous microrods with rough surfaces, a morphology that provides a large specific surface area and increases the number of active sites available for VOC adsorption and degradation [27,28,29]. The interconnected porous network further promotes gas adsorption and facilitates the diffusion of VOCs during the reaction, thereby accelerating the overall degradation process through improved mass transport [30,31]. The specific surface area was also measured by the Brunauer–Emmett–Teller (BET) method, yielding a value of 25.86 m²/g and an average mesopore size of 3.82 nm (Figure 1a). These values are in good agreement with previously reported data [26].

The particle size distribution of the β-Ga₂O₃ powder was evaluated using both SEM image analysis and particle size analysis (PSA). SEM image analysis of n = 30 particles revealed a mean size of 1.71 μm with a standard deviation of 0.84. PSA further confirmed a micrometer-scale particle distribution, with a D50 of 3.62 μm (Figure 1b). Combined with the porous microrod morphology, these features indicate that the photocatalyst possesses a large surface area and efficient light-scattering characteristics, thereby enhancing its overall photocatalytic activity.

The crystal structure of the β-Ga₂O₃ was analyzed using powder X-ray diffraction (XRD), as shown in Figure 1c. The XRD pattern confirmed the high crystallinity of the β-Ga₂O₃, with diffraction peaks matching the standard JCPDS reference pattern [32]. The absence of secondary phases indicates high phase purity, which is an important factor since mixed phases or amorphous domains can modify the bandgap and promote carrier recombination. β-Ga₂O₃ is particularly advantageous due to its superior thermodynamic stability, which prevents phase transformations that commonly occur in metastable γ-Ga₂O₃, thereby ensuring reliable photocatalytic performance under UVC irradiation.

The optical bandgap energy of the β-Ga₂O₃ were further investigated, as shown in Figure 1d. A sharp absorption edge was observed at 260–280 nm, corresponding to the wide bandgap of Ga₂O₃. The Tauc plot in the inset of Figure 1d estimated the optical bandgap of β-Ga₂O₃ to be approximately 4.48 eV, consistent with previously reported values [14]. This wide bandgap is advantageous for photocatalysis under UVC irradiation, as it provides a strong driving force for redox reactions and suppresses the recombination of photogenerated charge carriers, thereby enhancing overall photocatalytic efficiency. Taken together, these morphological, structural, and optical analyses demonstrate that β-Ga₂O₃, owing to its high crystallinity, wide bandgap, and large surface area, is a promising photocatalyst for VOC degradation under UVC irradiation.

The photocatalytic degradation of VOCs was evaluated in a designed reactor, as schematically illustrated in Figure 2a. Ga₂O₃ powder was uniformly distributed onto a mesh screen, which was positioned under a 254 nm UVC lamp to initiate photocatalysis [33,34,35,36]. Toluene was introduced into the system at an initial concentration of 8.0–8.5 mg/m³ at all temperature conditions. All measurements were conducted under steady-state conditions to ensure the accuracy and consistency of the results.

The photocatalytic degradation of VOCs was performed at a temperature range from 15 to 40 °C, with increments of 5 °C under UVC irradiation. A real-time toluene concentration was monitored using an air-quality analyzer, and the results were normalized to the total VOC values to enable a clear comparison of degradation performance at each temperature. As shown in Figure 2b, the photocatalytic toluene degradation efficiency increased with increasing temperature up to 30 °C, followed by a decline at 40 °C. To confirm that the observed VOC degradation originated from UVC-activated photocatalysis, a control experiment was conducted in the absence of UVC irradiation. No measurable change in toluene concentration was observed at any temperature, indicating that VOC degradation proceeded through photocatalytic activation.

This temperature-dependent profile arises from the interplay of two fundamental processes, where increased temperatures enhance chemical reaction rates while simultaneously suppressing reactant adsorption through reduced surface binding availability. Between 15 and 30 °C, the increased kinetic energy of photogenerated carriers and reactant molecules promotes faster redox reactions, outweighing the modest decline in adsorption capacity. However, at 40 °C, reduced adsorption of toluene molecules on the catalyst surface becomes the dominant limiting factor, thereby reducing the overall efficiency.

The photocatalytic kinetic data of toluene degradation were analyzed using the first order kinetics equation (Equation (1)), and the reaction rate constants (k) were calculated for each temperature as shown in Figure 3a,b. C and C₀ represent the toluene concentration at time t and the initial concentration, respectively [37,38].

−ln (C/C₀) = kt

(1)

The values of k increased steadily from 15 to 30 °C, reaching a maximum at 30 °C, and decreased at 40 °C. This observation highlights the existence of an optimal operating temperature that balances the rapid redox kinetics and sufficient reactant adsorption [39].

The Arrhenius relationship further supports these findings. A linear plot in the 15 to 30 °C range yields an apparent activation energy, revealing the intrinsic energy barrier for photocatalytic toluene degradation over Ga₂O₃. This indicates that the chemical reaction is the rate-limiting step, and thus the overall reaction rate follows the trend of increasing chemical reaction rate. Interestingly, the deviation from Arrhenius behavior beyond 30 °C signifies a shift in the rate-limiting step. While the chemical reaction dominates at lower temperatures, adsorption limitations prevail at higher temperatures. A similar mechanistic transition has also been reported in TiO₂-based VOC degradation systems, underscoring the importance of temperature in gas-phase photocatalysis [40,41].

In order to evaluate the stability of β-Ga₂O₃, five consecutive toluene degradation cycles were conducted under identical conditions. As shown in Figure 4a,b, the kinetic profiles and corresponding rate constants exhibited only a slight decrease with repeated use, confirming that the photocatalyst retained high activity across cycles. The structural stability of β-Ga₂O₃ was further verified by XRD analysis, as shown in Figure 4c. The diffraction peaks after repeated reaction closely matched those of pristine β-Ga₂O₃ without additional peaks or noticeable peak shifts, indicating that the crystal structure remains during the photocatalytic reaction. Together, these results demonstratethe structural stability and durability of β-Ga₂O₃ as a photocatalyst for VOC degradation. Additionally, toluene degradation using anatase-phase titanium dioxide (TiO₂) was performed under identical conditions at 30 °C for comparison with β-Ga₂O₃. (Figure 4d). The result showed a significantly lower reaction rate constant, underscoring the superior photocatalytic activity of β-Ga₂O₃. These results establish β-Ga₂O₃ as a highly effective photocatalyst with temperature-dependent activity.

To further rationalize this temperature-dependent behavior, a schematic illustration of the underlying mechanism is shown in Figure 5. This diagram highlights the interplay between chemical reaction rate and reactant adsorption. At low temperatures, strong adsorption ensures sufficient surface coverage of VOC molecules, but limited kinetic energy slows down photogenerated electron–hole driven redox reactions. At intermediate temperatures around 30 °C, optimal conditions are achieved. The adsorption of VOC molecules remains sufficiently strong while increased kinetic energy accelerates charge transfer and VOC oxidation pathways. However, at higher temperatures, weak reactant adsorption results in insufficient surface reactant coverage, reducing the effective utilization of the faster intrinsic reaction kinetics.

This mechanistic understanding identifies approximately 30 °C as the optimal temperature for photocatalytic toluene degradation on Ga₂O₃ and highlights the importance of balancing chemical reaction kinetics and reactant adsorption in designing efficient VOC degradation systems. These findings establish the temperature–activity relationship of Ga₂O₃ and offer practical insights for the rational design and optimization of photocatalytic VOC degradation systems.

3. Materials and Methods

3.1. Materials Characterization

Commercially available β-phase Ga₂O₃ (purity: 99.999%, particle size: 4.7 μm) was purchased from Taewon Scientific (Seoul, Republic of Korea). The morphology and microstructure of the Ga₂O₃ powder were analyzed using field-emission scanning electron microscopy (FE-SEM, JEOL JSM-7500F, Tokyo, Japan)). The crystal structure of the β-Ga₂O₃ was analysed using powder X-ray diffraction (XRD, D-8 Advance, Bruker, Bremen, Germany) with Cu Kα radiation (λ = 1.54178 Å). The 2θ range for XRD analysis was set to 20–60°, with a step size of 0.07°. The optical absorption property of β-Ga₂O₃ were measured using a diffuse reflectance spectroscopy (DRS, SHIMADZU, SolidSpec-3700, Kyoto, Japan). Particle size of the powder was determined using a particle size analyzer (PSA, Malvern Mastersizer 2000, Worcestershire, UK).

3.2. Photocatalytic Reactor Measurements

The photocatalytic experimental setup was designed using Ga₂O₃ powder as the photocatalyst, toluene as the target VOC, a VOC concentration detector, a UVC lamp as a light source, and an IR lamp as a heating source. A 4 W UVC lamp (UVG-11, Analytik Jena, Jena, Germany) emitting at a central wavelength of 254 nm and with an intensity of 312 μW cm⁻² was used as the irradiation source. The Ga₂O₃ powder was uniformly distributed onto a mesh screen, and toluene gas was supplied under UVC irradiation. The initial toluene gas concentration was adjusted within the range of 8.0 to 8.5 mg/m³, and steady-state conditions were achieved. Experiments were performed at temperatures ranging from 15 °C to 40 °C in 5 °C increments. Toluene concentrations were measured at 5-minute intervals using a KHALDER air quality analyzer. The photocatalytic degradation efficiency of Ga₂O₃ powder was determined by calculating the reaction rate constant k from the reaction rate kinetics equation.

4. Conclusions

In this study, we conducted a systematic investigation into the temperature-dependent photocatalytic degradation of toluene, a representative volatile organic compound, using Ga₂O₃ under UVC irradiation. The results clearly demonstrated that photocatalytic efficiency is strongly governed by the interplay between chemical reaction kinetics and reactant adsorption dynamics. The overall VOC degradation rate increased from 15 to 30 °C, reaching its maximum at 30 °C, and subsequently decreased at 40 °C. This non-linear behavior can be explained by the balance between two competing factors, the increasing chemical reaction and the decreasing reactant adsorption as the temperature increases. Consequently, 30 °C was identified as the optimal operational temperature for toluene degradation, at which photocatalytic activity is maximized. Our findings provide valuable mechanistic insights into the temperature–activity relationship of photocatalysis, offering a fundamental understanding for designing efficient VOC degradation systems for industrial and environmental applications.