Microstructure and photocatalytic properties of nanostructured TiO2 and TiO2–Al coatings elaborated by HVOF spraying for the nitrogen oxides removal

This paper deals with the study of the photocatalytic behaviours of TiO2 and TiO2–10 wt.% Al coatings elaborated by high-velocity oxygen fuel (HVOF) spraying using spray-dried nanosized feedstock powders. In the HVOF flame, the powders were injected by two methods: internal injection, i.e. as in conventional HVOF process and external injection, i.e. outside the torch nozzle. Scanning electron microscopy and X-ray diffraction were employed to characterize the microstructure and the crystalline structure of the TiO2-based coatings. The photocatalytic activities were evaluated by the nitrogen oxides removal and compared with that of the initial powders. It was found that the amount of anatase in the coatings depends on the nature of the powder and also on the type of injection method. The TiO2 coatings, as well as, the TiO2–Al coatings elaborated using the external powder injection presented a better photocatalytic activity (the nitrogen oxides conversion rates were about 25–42% for NO and 14–18% for NOx) than those obtained by the conventional HVOF process (<5%). Besides, TiO2 coatings containing aluminium particles had an enhanced photocatalytic activity than those of un-doped titania deposits.