Compact and effective photocatalytic air-purification unit by using of mercury-free excimer lamps with TiO₂ coated titanium mesh filter

Abstract

A photocatalyst–excimer–lamp hybrid reactor was investigated using a mercury-free xenon chloride excimer lamp wrapped with a TiO₂ modified titanium-mesh sheet (TMiP). Significant decomposition of 6 ppm of methylmercaptane gas has been achieved by the reactor in a 36 L box within 3 h.

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Since the discovery of photocatalytic water splitting by TiO₂ in 1967, the strong oxidation ability of TiO₂ has received growing attention.^1–3 TiO₂ generates hydroxyl radicals and superoxide ions by UV light irradiation. These are highly reactive with organic compounds. Recently, environmental contamination has become a serious problem in the world. Thus, the strong oxidation ability of TiO₂ is expected to resolve these problems. However, there are several limitations such as electron–hole recombination, low efficiency, and difficulty in decomposition of large amounts of pollutants. Moreover, use of mercury lamps, a typical UV-source for photocatalysis, should be avoided due to their environmental risk. Therefore, effective photocatalysis technology with a mercury-free UV-source is required. In the present work, a novel air-purification reactor was investigated by using an excimer lamp with a TiO₂ modified titanium-mesh sheet (TMiP™).⁴ TMiP provides excellent air pass through while maintaining a high level of surface contact. A xenon chloride excimer lamp generates 308 nm of UV light with high intensity and a large irradiation area. An important point is that TMiP in the reactor is used not only as a photocatalyst but also as one of the electrode of excimer lamp for generation of dielectric barrier discharge (outer electrode in Fig. 2). This structure makes sure of efficient UV irradiation onto the photocatalyst surface. The air purification efficiency of the reactor was examined by a high concentration methylmercaptane (CH₃SH) decomposition test.

An overview and a schematic view of the photocatalyst–excimer–lamp hybrid reactor are shown in Fig. 1 and 2, respectively. A xenon chloride filled quartz tube (10 mm i.d. × 55 mm length) was wrapped in a corrugated TMiP sheet. The fabrication method of TMiP was previously reported.⁴ The SEM image of TMiP is shown in the graphical abstract. The induction electrode is embedded in the quartz tube and the discharge electrode is a TMiP sheet connected to ground. A commercial electric power line of 50 Hz and 100 V was connected through a power supply. We used a voltage of 8 kVp-p, a frequency of 50 kHz, and a power of 5 W. When the AC high voltage is applied between the two electrodes, a dielectric barrier discharge occurs in the quartz tube which provided intense narrow band radiation at 308 nm from xenon chloride (XeCl*).⁵ UV intensity was measured using a UV RADIO METER UV-M03A with a UV-SN31 sensor head (ORC Manufacturing).

Fig. 1 A side view (left) and a top view (right) of photocatalyst–excimer–lamp hybrid air-purification unit.

Fig. 2 A schematic view of photocatalyst–excimer–lamp hybrid air-purification unit.

A schematic diagram of a total experimental system used for CH₃SH decomposition is shown in Fig. 3. The photocatalyst–excimer–lamp hybrid reactor and fan placed inside the test box (acryl sealed 36 L box). Air was blown on the reactor continuously. For continuous conditions, CH₃SH gas was introduced into the box by a heated permeation tube-based gas standards generator (Gastec Co., Ltd.) and was exhausted after the reaction. For batch conditions, CH₃SH gas flow was stopped at the concentration of 6 ppm. Time course of the concentration of CH₃SH was measured by a Kitagawa's detector tube (Komyo Rikagaku Kogyo K.K.). The experiments were carried out at room temperature and atmospheric pressure. After decomposition testing, TMiP was washed with MilliQ water for analysis of the decomposition products. An ion-chromatograph system (DX-120, DIONEX) with a separation column (IonPak AS12A, 4.0 mm i.d., 200 mm length, DIONEX) and a conductivity detector with a suppressor device (ASRS300, DIONEX) was used for the analysis.

Fig. 3 Schematic illustration of methylmercaptane decomposition test.

Fig. 4 shows the wavelength distribution for the emission from the excimer lamp and the absorptions of TMiP. Under the present conditions, the excimer lamp emitted mainly 308 nm light (Fig. 4, dashed line) originating from XeCl*.^6,7 The intensity of the UV light passed through the TMiP was 0.5 mW cm⁻². On the other hand, due to the wide band-gap of anatase TiO₂ photocatalysts (3.2 eV), TMiP mainly absorbs ultraviolet photons with a wavelength of less than 387 nm (Fig. 4, solid line).³ Therefore, emissions from the excimer lamp are in the range of the band gap of the TiO₂ catalyst on the TMiP surface. Moreover, TMiP did not show any destruction during use as an electrode and UV irradiation. This result indicates the strong adhesion of TiO₂ nanoparticles onto the TMiP surface.

Fig. 4 Wavelength distribution for the emission from the excimer lamp (dashed line) and the absorptions of TMiP (solid line).

Fig. 5 shows the time course of CH₃SH concentration in the test box under batch conditions. Under these conditions with the reactor turned off (open circles), the concentration was almost unchanged for 3 h. On the other hand, it can be seen that CH₃SH gas can be completely degraded within 3 h with the reactor turned on. The CH₃SH concentrations can be fitted with a pseudo-first-order kinetics given by the following equation, C = C_oexp(−k₁t). Where C_o is the initial CH₃SH concentration and k₁ is the observed rate constant. The values of the k₁ were calculated by exponential fitting of Fig. 5 to 1.4 h⁻¹. When the TiO₂ is irradiated with UV light from the excimer lamp, excitation of electrons into the conduction band takes place, resulting in formation of holes in the valance band. Both the holes and the electrons migrate to the TiO₂ surface, where they either recombine or react with adsorbed species such as H₂O and O₂. The holes oxidize adsorbed H₂O to ˙OH, which are the potential oxidants in photocatalysis, whereas the electrons reduce O₂ to O₂˙⁻³. CH₃SH could be decomposed to CO₂ and SO₄²⁻ by many possible pathways.⁸ In the present conditions, 8.2 mg L⁻¹ of SO₄²⁻ was detected in washing solution of TMiP after the decomposition test. Six ppm of CH₃SH in 36 L box could be converted to 8.5 mg L⁻¹ of SO₄²⁻. Therefore, CH₃SH was almost totally decomposed by the photocatalyst–excimer–lamp hybrid reactor. Interestingly, although it is thought that the fixation of SO₄²⁻ onto the TiO₂ surface will eventually affect the performance of the photocatalysis, the CH₃SH decomposition test under continuous conditions at 7 ppm showed that the photocatalyst–excimer–lamp hybrid reactor prevented the increase of the CH₃SH concentration for 30 h (Fig. 6). In the present study, the relative surface area of TMiP was 664 cm² cm⁻² (real surface area/mathematical surface area). Therefore, the photocatalyst–excimer–lamp hybrid reactor could decompose CH₃SH effectively and could fix a large amount of generated SO₄²⁻ with its large capacity.

Fig. 5 Time course of CH₃SH concentration with the photocatalyst–excimer–lamp hybrid reactor turned off (open circles) and turned on (solid circles) in test box under batch condition.

Fig. 6 Time course of CH₃SH concentration under continuous condition.

In conclusion, the removal of CH₃SH using a photocatalyst–excimer–lamp hybrid reactor was investigated. Much higher efficiency and continuous treatment without declining the efficiency was achieved. Although we used a simple and small reactor with conventional methods and conditions, for example, atmospheric pressure and a mercury-free UV-source, it would be attractive to develop a similar continuous-type air purification system for the practical treatment of highly CH₃SH contaminated environments, such as a refrigerator. Real odor samples such as kimchi were also prepared and successfully treated with the reactor in a refrigerator.

Notes and references

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