Innovative Photoreactors for unconventional sustainable processes

Introduction The development of new photoreactor configurations is a fundamental research topic for the transition from the lab-scale to the industrial application. Two main challenging processes were studied: i) the photocatalytic abatement of N-containing pollutants from waste water; ii) CO2 photoreduction to chemicals and fuels. The release of significant amounts of nitrates, nitrites and ammonia in waste waters, either deriving from industrial or agricultural activities, creates a big condern due to their health effects and their action as nutrients causing eutrophication. Their abatement through conventional methods is largely inefficient. In addition, a relatively new source of N-containing organic pollutants is related to the increasing use of drugs, which are more and more released in urban areas. A photocatalytic continuous or semibatch technology is here proposed in order to effectively remove such contaminants from water. The attention is mainly focused on photoreactor design. The same approach has been applied to another very challenging process, i.e. the photocatalytic reduction of CO2, which is here carried out in an innovative photoreactor operating at high pressure (up to 20 bar) [1,2]. Materials and Methods TiO2, either commercial or prepared through a flame pyrolysis process has been used as semiconductor. It has been added with Au, Ag, Pt or Pd with different loading. Characterisation of the materials has been carried ot by physisorption, XRD, DR-UV-Vis, FT-IR, TPR. The photocatalytic tests have been performed on two different types of photoreactors. The first is a continuously stirred photoreactor that can be either operated in batch, semibatch or continuous mode, with external irradiation. This has been employed at atmospheric pressure, with variable temperature and catalyst loading for the photocatalytic abatement of N-containing compounds. A completely different concept of photoreactor has been developed for the photoreduction of CO2, which operates up to 20 bar, 90°C either in batch or continuous mode. Results and Discussion In this research, we show case studies of novel concepts of photoreactors developed by our group [1,2]. In addition nanostructured photocatalysts were prepared and tested for the abatement of N-containing compounds, focusing on selectivity towards innocuous N2. Part of the photocatalysts have been prepared in nanosized form by using an innovative flame pyrolysis (FP) approach, able to synthesise in one step single or mixed oxide nanoparticles, characterized by homogeneous particle size and good phase purity. Particle size ranged between 5 and 60 nm, depending on synthesis conditions. Doping of the sample with different metals (0.1-0.5 wt%) was also investigated. The FP procedure proved an interesting method for the preparation of nanostructured Ti-based photocatalysts. Ca. 2 orders of magnitude higher activity with respect to the literature has been achieved during the photoreduction of CO2 at high pressure. Productivity as high as 102 mmol h−1kgcat−1for H2, 16537 mmol h−1kgcat−1 for formaldehyde and 2954.37 mmol h−1kgcat−1 for formic acid were achieved when using bare TiO2 at 7 bar. The addition of Au further improved the productivity and turned the products distribution towards CH4 and CH3OH. The FP semiconductors showed much more active also for the photooxidation of ammonia and the photoreduction of nitrates. Figure 1. Sketch of one of the photoreactors used in this work. Significance Photocatalysis is gaining increasing importance. The development of photoreactors and their optimization for each specific application is less addressed than the development of innovative materials. In some cases here discussed, practically relevant conversions can only be achieved by using unconventional operating conditions. References 1. Rossetti, I., Villa, A., Compagnoni, M., Prati, L., Ramis, G., Pirola, C., Bianchi, C.L., Wang, W., Wang, D., Catal. Sci. Technol. 5, 4481 (2015) 2. Galli, F., Compagnoni, M., Vitali, D., Pirola, C., Bianchi, C., Villa, A., Prati, L., Rossetti, I., Appl. Catal. B: Environmental, 200, 386 (2016)