Study on the enhancement of photocatalytic environment purification through ubiquitous-red-clay loading
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Tungsten oxide (WO3) is regarded as a promising visible-light-sensitive photocatalyst, but its activity is not high. Further enhancement of its activity has been anticipated using techniques such as loading of a cocatalyst to apply the oxide to indoor environmental remediation; Pt has been reported as a good cocatalyst for WO3 photocatalysis. However, Pt is precious and expensive metal. Thus, in this study, we sought to find a ubiquitous cocatalyst and suitable photocatalyst system. As a result, this study revealed that loading a ubiquitous material of red-clay enhanced WO3 photocatalytic activity remarkably. As photocatalyst samples, mixtures consisting of the clay and WO3 with different weight ratios were prepared using a simple kneading method. Their photocatalytic activity was evaluated from decomposition of harmful organic contaminant, 2-propanol into CO2 under visible-light irradiation. The WO3 with 10% of the clay loading showed the highest activity among the samples and much higher activity than pure WO3. This higher activity might derive from the clay’s promotion of H2O2 decomposition and charge separation (holes and electrons). The H2O2 was generated from photocatalytic O2 reduction. This formation and accumulation on the pure WO3 surface led to decreased activity.
KeywordsTiO2 Remediation Zeolite Natural mineral Optical absorption
The global environment has been polluted by harmful substances of many kinds including volatile organic compounds (VOCs), causing damage to human health. It is extremely important to remove these harmful compounds and to purify the living environment. Purification by photocatalysis is an effective method to decompose harmful airborne organic contaminants [1, 2]. The decomposition mechanism is the following: when light is irradiated on a photocatalyst and the band gap is smaller than the light energy, the photocatalyst absorbs the light. Carriers (electrons and holes) are generated. The holes usually have strong oxidizing capability  to mineralize organic compounds, leading to production of much cleaner air.
Anatase-type TiO2 photocatalyst with low cost has been used for outdoor applications [4, 5, 6]. The semiconductor works as a photocatalyst under irradiation of UV light, a minor component of solar light. However, TiO2 is not useful for indoor applications because TiO2 can absorb only in the UV region. Furthermore, indoor lighting has very weak UV illumination. Visible-light-sensitive photocatalyst with high activity is promising for indoor air cleaning because indoor lighting can emit intense visible light. Consequently, the development of novel visible-light-sensitive photocatalysts and low-cost earth-abundant photocatalysts has been anticipated and explored [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24].
Yellow semiconductor WO3, with a band gap of 2.6–2.8 eV, is recognized as a promising candidate among visible-light-sensitive photocatalysts. Its potential at the bottom of the conduction band (CB) is more positive than that for one electron oxygen reduction . When WO3 is used as a photocatalyst, its photogenerated electrons cannot be consumed well. Pure WO3 is deactivated easily. Research related to loading of cocatalysts such as Pt, PtPb, Cu ion, and NaOH on WO3 has been undertaken to promote electron consumption and to increase its photocatalytic activity [25, 26, 27, 28, 29, 30, 31, 32]. However, Pt is a precious metal that cannot be used sufficiently because of its high cost and likely exhaustion of the Pt resources. Other reported cocatalysts are harmful, or the loading process is complex. A safe and abundant cocatalyst is still needed. Therefore, in this study, we tried to develop a novel mixture photocatalyst consisting of WO3 and ubiquitous cocatalyst; we also sought a simple method to prepare the mixture.
Apparently, soil is an abundant and ubiquitous material that might be a promising cocatalyst. From results of preliminary investigation, we selected red clay among soils. Results showed that red clay is a suitable cocatalyst. Moreover, red-clay-loaded WO3 exhibits remarkable photocatalytic activity.
2.1 Material preparation
Red-clay ball (Akadama soil; Tachikawa Heiwa Nouen Co. Ltd., Japan) preheated to 673 K was used for this study. The clay was crushed using a mortar and pestle to prepare fine particles. The crashed material and WO3 were mixed with proper mixing ratios using a mortar. Samples were obtained after the mixtures were dried at 343 K for 5 h. The weight ratios for the red clay and WO3 were, respectively, 1:100, 5:100, 10:100, and 50:100. Furthermore, percentage of loading weight in this study was obtained by dividing cocatalyst weight by WO3 weight and expressed as the divided amount per one hundred. For example, 1 wt% red clay-loaded WO3 was the sample with the ratio (1:100).
Samples were characterized using several analytical devices. Crystal structures were measured using an X-ray diffractometer (XRD, X’pert Pro; PANalytical B.V., Netherlands) with Cu Kα radiation. Optical absorption spectra were evaluated using a UV–Vis spectrophotometer (UV-2500PC; Shimadzu Corp., Japan). Reflectance spectra were first measured using BaSO4 as a reference. Then, the obtained data were converted into optical absorption spectra using Kubelka–Munk theory. Surface chemical states of the samples were measured using an X-ray photoelectron spectroscope (XPS, AXIS-HS; Kratos Analytical Ltd., UK) with monochromatic Al Kα radiation. Binding energy in the XPS spectra was calibrated using C 1s peak, of which the binding energy is 284.5 eV. The specific surface area was evaluated at 77 K using a surface analyzer (Gemini 2360; Micromeritics Co., USA) with Brunauer–Emmett–Teller (BET) method. The clay chemical composition was estimated using an inductively coupled plasma optical emission spectroscope (ICP-OES, ICPS-8100; Shimadzu Corp.).
Photocatalytic activity was evaluated at room temperature in a 500-mL cylindrical glass reactor . Photocatalytic decomposition of gaseous 2-propanol (IPA) into CO2 was selected as a model reaction because IPA is often used as a model of an organic gas in this field . Details of the photocatalytic evaluation procedures were the following: We spread powder photocatalyst with weight of 0.4 g uniformly on a Petri dish with 8.5 cm2 base area. Then, the dish was set on the center of the base in the reactor. After the inside atmosphere was replaced with pure air, concentrated IPA gas was injected to produce concentrations of IPA in the reactor of about 600–800 ppm. The reactor was kept in the dark until the adsorption–desorption equilibrium state was confirmed. Visible light (400 < λ < 530 nm) was irradiated using 300 W of Xe lamp equipped with Y-44, B390, HA-30 filters, and a water filter. The IPA concentration was measured using a gas chromatograph (GC-14B; Shimadzu Corp., Japan) with a flame-ionized detector (FID). The CO2 concentration was estimated using the chromatograph (GC-14B) with FID and a methanizer (TN-1; Shimadzu Corp.). The light intensity of the visible-light irradiation, set to about 1 mW cm−2, was measured using a spectroradiometer (UV-40; Ushio Inc., Japan). Maximum CO2 evolution rate was evaluated from the rate of zero order reaction [r = k (reaction rate constant), and C = k t] in the range where CO2 concentration linearly increased against time.
Reduction of O2 into H2O2 might occur during the photocatalytic IPA oxidation. The H2O2 generation on the sample was evaluated qualitatively using dimethylphenanthroline (DMP) method . After the reactor was kept in the dark for 10 min after visible-light irradiation, the sample was washed with distilled water. An aqueous solution possibly containing H2O2 can be prepared. Phosphate, DMP, and CuSO4 solutions were added to the aqueous solution. The color turns yellow if the solution contains H2O2. Its absorbance at 454 nm in wavelength was measured using UV–Vis to calculate the H2O2 amount.
3 Results and discussion
3.1 Crystal structure
Red clay has been used since ancient times for many applications such as bricks and horticulture soils. The red-clay ball (akadama) selected for this study is a volcanic product produced in eastern Japan. The clay is reportedly composed of quartz (SiO2), diaspore (AlOOH), hematite (Fe2O3), and goethite (FeOOH) [36, 37]. The clay components were ascertained using XRD in reference to these data.
3.2 Chemical composition
Weight ratios of metals to Si in red-clay ball
3.8 × 10−3
3.3 XPS analysis
3.4 Optical absorption spectra
The mixture can absorb visible light well, but its onset absorption edge was blue-shifted by 10 nm compared with that of pure WO3. Similar shifts were observed in other mixtures of NaOH-loaded WO3 and NaBiO3-loaded WO3, suggesting that it was not relevant to species of the loaded cocatalysts. Absorption spectra are reported to be affected by many factors including size , crystallinity , morphology , surface functional groups [30, 44], and grain boundary . Because its size, crystallinity, and morphology did not change by mixing, this blue shift might be related mainly to changes in the amounts of surface functional groups [30, 44] and grain boundary of WO3. The grain boundary may change because of stress from the mixing on a mortar.
3.5 Photocatalytic oxidation property
In photocatalytic oxidation of gaseous IPA, acetone is first generated as an intermediate in the gas phase . Then, acetone is oxidized further into CO2 via carboxylic acids such as formate (–COOH) and acetic acid (Figure S3) . Organic compounds of many kinds might be generated as intermediates. The final product (CO2) was monitored carefully for this evaluation of photocatalytic activity.
To confirm the Fe effect further, photocatalytic activity for 5 wt% of Fe3+-doped, zeolite-loaded WO3 was evaluated (Figure S4). The activity for the Fe-doped-zeolite-loaded WO3 also showed relatively high activity. These results showed that loading of a Fe3+-containing compound gave a positive effect for WO3 photocatalysis.
Surface area of the samples
Surface area (m2/g)
Mixtures of WO3 and red clay were prepared using a simple kneading method. The samples absorbed visible light well and showed photocatalytic activity under visible-light irradiation. The photocatalytic activity was evaluated from IPA decomposition into CO2. Relation between photocatalytic activity and mixing ratios (red clay to WO3) exhibited that 10 wt% of red-clay-loaded WO3 showed the highest activity. Furthermore, the activity of the mixture was higher than that of Pt-loaded WO3 with relatively high activity. This higher activity of the mixture should derive from good H2O2 consumption efficiency of the red clay. H2O2 that was generated from photocatalytic reduction of O2 on WO3 might accumulate on the WO3 surface and suppress WO3 photocatalytic activity.
Salient benefits of this mixture photocatalyst are its high activity and its usage of ubiquitous red clay as a cocatalyst. The clay is remarkably cheap and abundant. The mixture photocatalysts and preparation process are eco-friendly because no wastes, even wastewater, are produced during its preparation.
This work was partially supported by JSPS KAKENHI (15K05591 and 18K05207) and World Premier International Research Center Initiative (WPI Initiative) on Materials Nanoarchitectonics (MANA), Japan.
Compliance with ethical standards
Conflict of interest
The manuscript does not contain conflict of interest.
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