## 1 Introduction  
Various studies have explored the incorporation of photocatalytic technology to reduce the amount of particulate matter in air. Among the diverse functions of photocatalysts, their indoor and outdoor \"air purification\" function has rapidly gained attention owing to the recent worldwide outbreak of a novel, contagious respiratory disease, COVID-19. Photocatalysts can reduce or decompose nitrogen oxides (NO\({}_{2}\)), sulfur oxides, and volatile organic compounds (VOCs), which are precursors that generate particulate matter through the absorption of light (ultraviolet and visible rays). Owing to their photodegradation of harmful organic substances and bacteria, reduction of air pollutants, and photooxidation capabilities, photocatalysts can be applied to preserve the environment and combat viruses that affect daily life [1, 2]. Materials that can be used as photocatalysts include TiO\({}_{2}\), SiO\({}_{2}\), ZnO, WO\({}_{3}\), CdS, and ZrO\({}_{2}\), as well as perovskite-type composite metal oxides (e.g., SrTiO\({}_{3}\) and BaTiO\({}_{3}\)), which are known to exhibit photocatalytic effects [3, 4, 5, 6]. Among these, TiOis widely used as a photocatalyst because its physical and chemical stabilities do not change even when exposed to light. With its high oxidation and reduction power, it can decompose NO\({}_{x}\) (substances that are converted to fine dust) into nitric acid and VOCs into carbon dioxide (CO\({}_{2}\)) and water (H\({}_{2}\)O) [7,8]. Furthermore, TiO\({}_{2}\) is considered a safe and environmentally friendly material [9,10]. When treating pollutants using a TiO\({}_{2}\) photocatalyst, fine photocatalyst powder is difficult to separate from the suspension after reaction and suspended particles tend to agglomerate, so the fine particle suspension cannot be easily applied in a continuous flow process; rather, it can act as dust and TiO\({}_{2}\) powder must be recovered after the reaction is completed [11]. Therefore, to overcome the problem, research on TiO\({}_{2}\) thin-film manufacturing has been reported using thermal oxidation [12], sputtering [13, 14, 15], pyrolysis [16, 17], chemical vapor deposition (CVD) [18, 19, 20, 21], atomic layer deposition (ALD) [22, 23], and sol-gel methods [24, 25, 26, 27]. Among the aforementioned methods, sol-gel is one of the most important methods for the synthesis of various functional coating films and holds several advantages over other thin-film manufacturing techniques, such as low-temperature treatment, easy coating of large surfaces, and the possibility of forming porous thin films and homogeneous composite oxide thin films [28, 29]. In addition, several studies emphasizing the immobilization of TiO\({}_{2}\) particles or sols on a support are in progress, and the characteristics of ceramic materials as TiO\({}_{2}\) supports for the photocatalytic decomposition of VOCs have been widely emphasized [30, 31]. When TiO\({}_{2}\) nanoparticles are dispersed on the surfaces of ceramic materials, photocatalytic activity is improved due to the high specific surface area of TiO\({}_{2}\), high adsorption capacity, large pore volume, chemical stability, and excellent mechanical properties [32, 33]. Among such materials, alumina has been extensively used as a substrate due to its high strength, durability, cost-effectiveness, and versatility [34]. A porous support facilitating such an appropriate photo-interaction can be extremely advantageous in preventing oxidation reduction [35]. TiO\({}_{2}\) was coated on surface-treated porous alumina using a sol-gel method, heat-treated at a different firing temperature, and then a photocatalyst was inserted into the prepared functional photo reactor to perform a photo-oxidation test of acetaldehyde.