A novel gas-phase photo-catalytic approach for H2 production from water and biowastes

The world economy growth and unavoidable depletion of fossil fuels have raised a social pressure to accelerate the transition towards a new sustainable economy based on clean and renewable energy sources. In the meantime, worldwide scientists are wondering about the strategies to be adopted to manage the transition period before moving entirely to a state where almost all the human needs will be met by sustainable resources. Hydrogen production by water photo-electrolysis and/or photo-reforming of waste organics represents a big challenge for implementing this goal. The research efforts are now intended for developing new catalytic materials capable to enhance the photo-efficiency of the process, but also for realizing photo-reactors of new design in order to maximize the photo-catalytic properties of those materials. In this context we have focused our attention on a novel gas-phase photo-catalytic approach to produce H2 from dilute aqueous organic waste solutions. A homemade photo-reactor, constructed in Pyrex and equipped with a quartz window, was used for the laboratory-scale experiments. A TiO2-based photo-active material, prepared as thin film, was suspended within the reactor headspace and irradiated perpendicularly by light coming from an external lamp simulating the solar spectrum. In this innovative configuration, the gas phase abounding in the organic component was generated directly in situ from the bottom of the reactor where an aqueous organic solution was kept boiling. A continuous flow of inert gas (10 ml/min) was fluxed into the boiling solution in order to facilitate the evaporation of the organic compounds and transfer outside the photo-generated H2 for the analysis by GC-TCD. There are many potential advantages of operating in gas phase with respect to the traditional slurry photo-chemical reactors: i) minimization of scattering phenomena, ii) good pattern of irradiation, iii) ability to transfer the power coming from sunlight to a conductive substrate, iv) potential scale-up for a delocalized production of energy. Moreover we will present a new technique of metal deposition carried out in gas-phase within the same reactor by photo-reduction. It is well known that decorating TiO2 with metal nano-particles improves solar light harvesting by shifting the band gap of TiO2 towards the visible part of the spectrum, as well as enhancing the photo-activity in terms of H2 production. Metal particle size and dispersion on TiO2 surface are of paramount importance to enhance both the solar photo-efficiency and -activity. Hence the adoption of a suited metal deposition technique is absolutely needed. Gas-phase photo-reduction method is based on the filling up of TiO2 micro-pores for capillarity with a solution of the metal precursor and following irradiation under a low inert gas flow. It allowed to obtain a good control of the metal particle dimension and size distribution by modulating the concentration of the precursor salt within the TiO2 micro-pores. We used Pt and Au/TiO2 but also non-noble metals Cu and Fe/TiO2 photo-catalysts gave us relatively good results in terms of H2 production and photo-current responses. This photo-catalytic approach may be used to treat aqueous organic waste streams coming from biomass processing (agro-food or agro-chemical), containing ethanol and other organics, which are too dilute to be used as feed for other processes.