Flame synthesis of supported titania photocatalytic layers

Flame synthesis is a widely implemented method for the industrial synthesis of nanocrystalline oxides and very reactive photocatalytic powders are currently produced using this method. However, for most applications (e.g. air and water treatment or industrial chemical transformations) the use of a powder photocatalyst poses several problems of process design and implementation, mainly because of the difficult final separation and catalyst recovery. The availability of supported photocatalyst is therefore very interesting. Several powder-supporting technologies are available but a number of associated drawbacks (e.g. process costs and activity depletion) is hindering their implementation. The present work addresses this issue, by flame synthesis of titania photocatalyst directly supported on alumina or stainless steel sheets. Materials and Methods: Titania films were produced by flame spray pyrolysis using an oxygen-assisted spray apparatus in order to inject the precursor coaxially with the pilot flame. Titanium tetraisopropoxide in ethanol (0.5 M) was used as liquid precursor. A constant 3 ml/min solution feed rate through the spray nozzle was obtained with a syringe pump. Titania nanoparticles were deposited by orthogonal impingement on a cooled substrate holder placed at 30 cm HAB (height above the burner). Cooling temperature was kept at about 100 °C to avoid water condensation on the holder. Stainless steel or alumina substrates were used with an exposure area of 5.2 cm2. A pneumatically operated mechanical shutter placed above the target substrate allows the control of the exposure time of the aerosol-generated powder deposition. After synthesis, titania films were properly annealed in order to improve film-substrate adhesion. The photocatalytic activity of titania films was evaluated by measuring the degradation of toluene in air at typical environment concentration. Results: Titania films supported on stainless steel and alumina substrates exhibit good photocatalytic activity for toluene degradation in air. The experimental data also suggest that post-deposition thermal annealing is very important in order to optimize the film-substrate adhesion. Discussion: These preliminary results demonstrate the potentials of flame synthesis in the direct production of photocatalytic titania films on solid substrates. Further studies are in progress in order to optimize the process and fully characterize the titania films obtained.