|The difference between treating organic compounds like benzene and treating nitrous oxides (NOx) and sulfur dioxide is that non- readily-volatile products such as nitric acid ions and sulfuric acid ions remain behind on the photocatalytic surface whereby decreasing photocatalytic performance (Fig. 2). Similarly, organic compounds that contain nitrogen or sulfur leave ions on the photocatalyst. For this reason, it is necessary to regenerate the photocatalyst by rinsing with water, which makes rain a potential candidate for outdoor applications. Another topic in the removal of NOx is the formation of NO2 under low light intensity or high humidity when nitric acid ions are saturated.|
In the early stages of research, the problem was dealt with by mixing in absorbents like activated carbon; utilizing a structure suited for NO2 absorption such as fine photocatalyst particulate, Macrodefect- Free cement or sol-gel paint, it has been possible to reduce the emission of NO2 gas. It is believed that the pore diameter and capacity, hydrophilicity/hydrophobicity and acid-base nature of the photocatalyst material have something to do with this.
|Fig. 4 : Environmental purification by photocatalyst (Use of sunlight: Passive purification, Use of artificial light: Active purification)|
Many local governments and other organizations are doing roadside evaluation tests of photocatalyst material used in passive purification systems for NOx. Osaka Prefecture did basic studies on the NOx purification performance of photocatalysts in 1996 and, in 1997, a prototype construction material was made with the cooperation of 6 companies amongst which were photocatalyst manufacturers. The prototype construction material was installed on National Rt. 43 (Dekijima Elementary School, Nishiyodogawa-ku, Osaka) and the purification performance of the product was studied along with the removal rate of air pollutants, the sustainability of the material's purification performance, the durability of the fabricated prototype and rain-washed run- off substances. It was confirmed that, even after 12 months of exposure, some of the deployed prototypes could maintain almost the initial NOx purification performance (purification of 20,000 m3 of air per hour per 1,000 m2) if provided sunlight of a certain intensity or greater and that, with further improvements, a practical photocatalyst construction material could be developed. This suggests that a material that fixes a photocatalyst could continually remove atmospheric NOx using only sunlight while also regenerating itself in the environment of installation. On these findings, Osaka Prefecture developed a new type of soundproof barrier using a photocatalyst and installed it along coastal prefectural roads (Surface area coated with photocatalyst: TL500 m x H2 m x Both road sides = Approx. 2,000 m2). Though the tests conducted by the various organizations differ, the daily removal rate was 0.5 - 1.5 mmol per m2. Moreover, across the 2-year test period, very little performance deterioration has been detected and the surface has remained clean despite its porous nature. In March last year, Fujita, Fujita Road, Taiheiyo Cement and Ishihara Sangyo/Ishihara Techno announced installation tests of a "Photo Road Construction Method" as an application for roads.
In order to verify the air purification effect of the photocatalyst material installed in a comparatively open environment (i.e., roadside, etc.) from the atmospheric concentration, it is necessary to install the photocatalyst material across a wide area. To understand the atmospheric purification effectiveness of the photocatalyst material based on data obtained in these past performance evaluation tests, numerical simulations were attempted with considerations going to roadside airflow, etc. It is urgent that an energy-saving, economically feasible purification system for removing NOx from ventilation gas be developed for installation in tunnel ventilation systems. For what regards indoor air purification systems, it is also important to confirm the effectiveness, making it necessary that test methods for evaluating the performance of the photocatalyst material and purification system be established. And, in order to materialize a compact purification system of smaller installation area, it is necessary to continue enhancing the performance of the photocatalyst material and composite materials.