PHOTOCATALYTIC REACTIONS FOR ENERGY CONVERSION

General upward trends in fossil fuel consumption and CO2 emissions, along with the accepted belief that global chemistry substantially influences climate, require that scientific research provides efficient remedies and/or alternatives to the present scenario. Photocatalysis is often proposed as one of the most promising technique to achieve these purposes. This PhD thesis is mainly focused on the investigation of TiO2-based systems for the photocatalytic oxidation of formic acid in aqueous suspension, as well as for H2 production by methanol photo steam reforming in the gas phase. Two different approaches were adopted to minimize the drawbacks usually characterizing TiO2 photocatalysts: i) TiO2-WO3 mixed oxide photocatalysts were prepared with the aim of reducing the recombination rate of photopromoted electron/hole pairs. The superior photocatalytic performance of the mixed oxide system was mainly attributed to the positive effect induced by W in efficiently trapping the photopromoted electron from the conduction band of TiO2, ensuring extended charge carriers separation. Even better results were obtained upon the surface modification with Pt nanoparticles which, by virtue of the metal high work function, further enhanced e-/h+ separation. ii) surface modification of TiO2 with Au nanoparticles, possessing a Localized Surface Plasmon Resonance (LSPR) at λ = 530 nm was proved to be an efficient way to promote TiO2 photoactivity under visible light irradiation. By selecting three titania samples (i.e., a stoichiometric, nearly non defective TiO2, a N-doped TiO2 and a oxygen vacancy-rich TiO2), evidence of two different plasmonic photoactivity mechanisms was provided, with the so-called hot electron transfer promoting plasmonic photoactivity in the stoichiometric TiO2 and Plasmon Resonance Energy Transfer accounting for the observed plasmonic visible light photoactivity of doped samples. Being the abatement of CO2 through (photo)electrochemical reduction very challenging (E0(CO2/CO2-* = -2.14 V)), an alternative way has been studied: pyridinyl radicals (1-PyH*), photogenerated by irradiating a pyridine (Py) solution, were found to efficiently react with CO2 yielding a carbamic species (HPy-1-COOH), triggered by a stepwise mechanism where electron transfer from 1-PyH* precedes proton transfer. Formate (HCOO-) was also obtained, demonstrating that photoexcited pyridine does catalyze the 2e—reduction of CO2. Finally, Fenton oxidation of gaseous isoprene on the surface of aqueous Fe2+ droplets, yielding carboxylic acids, polyols and carbonyl compounds, detected in situ through ElectroSpray Ionization Mass Spectrometry, accounted for alternative routes for the conversion of organic gases into secondary organic aerosol, occurring under tropospheric conditions, and may be incorporated into present atmospheric chemistry models.