Abstract
Although a fabrication possibility of TiO2 coating phototcatalyst by a thermal spray process has been indicated, little has been known about the existence and distribution of anatase phase in the sprayed coating. It is necessary to observe precisely the microstructure of TiO2 coating photocatalyst for the improvement of the photocatalytic property of the coating. In this paper, to clarify the distribution of both rutile and anatase phases in the coating, microstructure of TiO2 coating was analyzed. Furthermore, the effect of adsorbent addition to TiO2 on the transformation temperature and NOX elimination property was also investigated. The sprayed particles passed away from the HVOF flame were collected in the water and were observed by the Laser-Raman spectroscopic analyses. In the relatively big particles in diameter, melting occurred only on the outer surface of the particle and the anatase phase was effectively retained at the inside of the particle. In the small particles, on the other hand, particles were fully melted and anatase phase completely transformed into rutile. Thus, the coating fabricated by HVOF spraying was constructed by both anatase phase region and anatase/rutile composite region. The transformation temperatures for TiO2, TiO2+Ca3 (PO4)2 and TiO2+Fe2O3 powder were 1173 K, 1273 K, and 1098 K, respectively. By the addition of the adsorbent to TiO2, transformation temperature increased in TiO2+Ca3 (PO4)2 system. This is beneficial for the retaining of anatase phase during and after the spraying. From the measurement results of NOX elimination property of the coating, the coating with adsorbents showed higher adsorption ability of NO2. It was revealed that the coating made by HVOF spraying has fairly high NOX elimination property among the other practical coatings, such as a solution spreading.
