Preparation, Characteristics And Photocatalytic Performance Of Nanosize TiO₂ Coated On Foam Metal Substrate

Photocatalysis, as an environmentally friendly technology, has attracted much attention and showed potential application in environmental protection, especially in the purification of indoor air pollution. However, the nano-size TiO₂ powder is easy to lose in gas-phase photocatalytic reaction and difficult to be collected after the reaction, which disturbs the recycle of photocatalyst and hampers the practical applications of nano-size TiO₂ powder. The immobilization of nano-size TiO₂ overcomes the difficulties in separation and recycle of photocatalyst and restrains aggregation and deactivation of the catalyst particles. The immobilized nano-size TiO₂ is an ideal material to design photocatalytic reactor by combining the functions of the photocatalyst and the support.The supports affect the activity of photocatalyst directly. The frequently-used supports for photocatalyst, such as glass ball, fiberglass, activated carbon, zeolite, ceramic foam, silica gel, etc., all have their limits. However, when foam metal is used as photocatalyst substrate, it has many advantages, such as good thermal stability and high mechanical strength. It can load the photocatalyst with stable activity dispersedly and make efficient utilization of photo energy. Especially, when foam metal is used as photocatalyst substrate, the uniform open-pore structure provides it excellent gas-dynamic properties to show significant advantages in gas-phase photocatalytic reactions.The immobilization technology resolves many problems such as difficulties in photocatalyst recycle, but it inevitably results in decrease of specific surface area and activity of nano-size photocatalyst. In order to improve the specific surface area of substrate and photocatalyst, increase the active sites number of loaded photocatalyst and enhance the photocatalytic activity, it is necessary to modify substrate and optimize loading methodology. The traditional high-temperature preparation method of TiO₂ needs high energy supply and has much influence on substrate. The specific surface area of photocatalyst is also decreased by high-temperature calcination. If TiO₂ with good crystallinity and high activity can be synthesized at low temperature, many technical problems in practical application will be solved. Moreover, the low quantum efficiency and limited absorption in UV light region of TiO₂ restrain its application in practice. Modification of TiO₂ is effective method to improve the photocatalytic activity and realize absorption of visible light, hence utilize the photo energy more efficiently. In this study, foam nickel was chosen as supports for TiO₂ photocatalysts. TiO₂ films loaded on foam nickel substrate with high photocatalytic activities were prepared by modifying foam nickel substrate and optimizing loading methodology. Coating mesoporous transition layers with large specific surface area contributed to improve the specific surface area of substrate and the photocatalytic activities and stabilities of composite photocatalysts. TiO₂ with large specific surface area and high photocatalytic activity was synthesized at low temperature, which reduced the influence of heating treatment on substrate. The enhancement of photocatalytic activities, the expansion of absorption to visible light region and the improvement of solar energy utilization on TiO₂ films have been successfully achieved through depositing noble metal Pt nano-particles on TiO₂ surface and doping transition metal ion Fe3+. The prepared samples have been characterized using analytical techniques of XRD, BET, TEM, EPR, XPS, SEM, etc. The loading techniques and photocatalytic activities of the composite photocatalysts loaded on foam nickel substrate have been investigated. The mechanism of the influence of substrate properties on photocatalytic activity of nano-size TiO₂ and the enhancement of TiO₂ photocatalytic activity by Pt loading and Fe3+ doping was discussed. Some pivotal technologies for practical applications were also discussed. The main content and innovation of this work are as follows:1. TiO₂ photocatalysts loaded on foam nickel substrate were prepared by sol-gel processes. The influence of pre-oxidation treatment of foam nickel substrate and number of TiO₂ coating cycles on the structure and activity of photocatalyst was investigated. The experimental results show that, after pre-oxidation treatment, the specific surface area of foam nickel increases with increasing treating temperature from 400 oC to 550 oC. When the foam nickel substrate is pre-oxidized at 550 oC, its specific surface area is enlarged by2 times comparing with the original foam nickel, so the amount of TiO₂ loading is improved. The nickel oxide formed on substrate surface holds back photo-generated electrons from transferring to the metal substrate, thus enhances the photocatalytic activity. The photocatalytic activity of TiO₂ loaded on pre-oxidized foam nickel is 90 % higher than that of TiO₂ loaded on foam nickel without pre-oxidation treatment. Repeated coating improves the amount of TiO₂ loaded on foam nickel, but the photocatalytic activity of TiO₂ cannot increase continuously by repeated coating. The optimum coating cycles for TiO₂ films is two. The photocatalytic activity of TiO₂ films coated for 2 cycles is about 13 % higher than that of TiO₂ films coated for 1 cycle. The Ni2+ ion transfers from the foam nickel substrate to TiO₂ films during the high-temperature treatment and results in the formation of nickel-doped TiO₂ films on foam nickel substrate. The absorption edges of nickel-doped TiO₂ films shift from UV region to visible light region (λ< 520 nm). The nickel-doped TiO₂ films display high photocatalytic activity under visible light irradiation.2. The TiO₂/SiO2, TiO₂/Al2O3 and TiO₂/Al2O3-SiO2 films as composite photocatalysts were loaded on foam nickel substrates by sol-gel processes. Mesoporous SiO2, Al2O3 and Al2O3-SiO2 films were loaded as transition layers. The specific surface area of substrate increases sharply by loading the transition layers. The maximal specific surface area of the substrates is 167.2 m2 g-1. The specific surface area of the composite photocatalyst is about 30 m2 g-1, which is as more than 30 times as that of TiO₂ films loaded on foam nickel without transition layers. The increment of active sites on loaded photocatalyst and the prevention of photo-generated electrons from transferring to the metal substrate caused by transition layers result in notable enhancement of photocatalytic activity and stability of the composite photocatalysts loaded on foam nickel substrate. Improving the acetaldehyde pre-absorption ability of composite photocatalysts will form an environment of higher pollutant concentration near the photocatalyst surface, and accelerate the photocatalytic reactions. But the improvement of pre-absorption ability of pollutants is not the necessary condition of photocatalytic activity enhancement. The increase of specific surface area is determinant of photocatalytic activity enhancement. The results show that, the degradation ratio of acetaldehyde on TiO₂ films without transition layers is 90 % after 360 min of UV light irradiation, while the degradation ratio of acetaldehyde on composite photocatalysts with transition layers is about 100 % after 60 min of UV light irradiation. After 5 consecutive runs, the photocatalytic activity of TiO₂ films without transition layers reduces by more than 50 %, while the photocatalytic activity of composite photocatalysts with transition layers only reduces by about 15 % after 10 consecutive runs.3. The nano-size TiO₂ (anatase and rutile) and TiO₂ (anatase and rutile) films loaded on foam nickel substrate were prepared by sol-gel processes under low temperature. The anatase or rutile TiO₂ crystal is formed by drying at room temperature through low-temperature-preparation method. Nano-size anatase TiO₂ with good crystallinity is obtained by heating at a relatively low temperature. The results show that, the photocatalytic activity of anatase TiO₂ loaded on foam nickel substrate prepared through low-temperature method is higher than that of rutile TiO₂ loaded on foam nickel substrate prepared through low-temperature method. When the preparation temperature is 240 oC, the degradation ratio of acetaldehyde on rutile TiO₂ photocatalyst is 40 % after 420 min of UV light irradiation, while the degradation ratio of acetaldehyde on anatase TiO₂ photocatalyst is nearly 100 % after 120 min of UV light irradiation.4. The Pt/TiO₂ photocatalysts were loaded on foam nickel substrate. The noble metal Pt nano-particles were deposited on TiO₂ surface through photodeposition method and heating reduction method. The results show that, the photocatalytic activity is enhanced greatly by Pt deposition, the acetaldehyde degradation rate on Pt/TiO₂ is as more than three times as that of TiO₂. In this study, the highest photocatalytic activity of Pt/TiO₂ is obtained by 0.3 wt % deposition of Pt. When Pt is deposited more than 0.3 wt %, the activity of Pt/TiO₂ decreases slightly. The Schottky barrier on Pt-TiO₂ interface generates from Pt deposition provides active electron trappers to inhibit the recombination of photo-generated electrons and holes. Hence the recombination rate is reduced and the activity of photocatalyst is enhanced. But when deposited on TiO₂ excessively, the Pt probably becomes the recombination sites of photo-generated electrons and holes, and then the recombination rate will increase. At the same time, the over-deposited metal Pt blocks the UV light irradiation and reduces the number of photo-generated electrons and holes on photocatalyst. As a result, the photocatalytic activity is depressed. Therefore, in order to improve the photocatalytic activity of TiO₂ efficiently, the Pt-doping concentration should be controlled in an optimum range.5. The Fe/TiO₂ (TiO₂ doped with transition-metal ion, Fe3+) photocatalysts were loaded on foam nickel substrate by sol-gel processes. The results show that, the absorption edge of TiO₂ shifts to longer wavelength gradually with increasing iron doping until reaches the maximum at 600 nm. The gaseous acetaldehyde adsorption ability of anatase TiO₂ and Fe/TiO₂ loaded on foam nickel does not change obviously after iron doping. In the present study, the photocatalytic activity of Fe/TiO₂ is improved with increasing iron-doping amount and the highest degradation rate of acetaldehyde is obtained by 0.3 wt % iron doping. When the iron-doping amount is more than 0.3 wt %, the activity of Fe/TiO₂ decreases slowly, which indicates that there is an optimum concentration for iron doping in TiO₂. The doped ferric ions in crystal lattice of TiO₂ act as capture traps. The redox processes of ferric ions trap the photo-generated electrons and holes simultaneously, so that the recombination of photo-generated electrons and holes is inhibited and the photocatalytic activity is promoted. When the concentration of doped ferric ions is too high, the distance between trapping sites for photo-generated electrons and holes decreases, the photo-generated electrons and holes will meet more trapping sites before they transfer to the surface, thus the doped ferric ions become indirect recombination sites of the photo-generated electrons and holes, the photocatalytic activity is depressed. TiO₂ films loaded on foam nickel substrates with high photocatalytic activities were prepared by pre-treating the substrate, coating TiO₂ films repeatedly, loading mesoporous transition layers and modifying TiO₂. Some of the developed series products of TiO₂ photocatalysts loaded on foam nickel substrates have been supplied on the market. The successful application of our study in the bronze storeroom of Shanghai Museum has also obtained good social and economic benefits.