Photo(electro)catalytic oxidation processes combining industrial pigments with semiconductors

The current energy crisis and the climate change issue call for an urgent action, in relation to the use of renewable sources of energy for the production of fuels and commodity chemicals. Among the available renewable sources, solar light is the one being abundant, free, widely distributed and inexhaustible. The energy that the Earth receives from the Sun would be in principle enough for powering the human activities for one year. Efficient capture and conversion of solar light into virtuous chemical processes is the goal of artificial photosynthesis(PSN). Taking inspiration from the natural process, the three main pillars of artificial PSN are light absorption / charge separation accumulation / conduction of redox processes. Most efforts have been dedicated to the water splitting reaction, where water is converted into its high energy constituents, H2 as the energy vector and O2. The technology in the field has rapidly grown in the last decade. More recently, artificial PSN strategies have targeted alternative processes, in particular for replacing the oxidative side of water splitting given the difficulty and high energetic barriers of this half reaction, and given the inutility of O2 byproduct. The development of materials for artificial PSN, and in particular for photochemical oxidation of organic substrates is the main goal of this Thesis, that will exploit the strategy of hybrid materials where organic dyes are interfaced with semiconductor(SC) nanomaterials. The designed materials have indeed the capabilities of: (i) efficiently absorbing the light in the visible region, due to the presence of the organic dye; (ii) promoting an efficient photoinduced dye/ SC charge separation; (iii) conducting the targeted redox reactions, possibly taking advantage of co-catalysts. We report photoanodes based on a quinacridone (QA) pigment vacuum sublimated in the form of nanoflakes over SC slides, for the photoelectrochemical activation of C-H bonds in organic compounds, being the first example of a dye sensitized photoanode for this application. The rational behind the reactivity is the ability of QA to operate through a photoinduced proton coupled electron transfer to the SC, generating a nitrogen centered radical capable of hydrogen atom abstraction from C-H bonds of the organic substrate. The key performance reactivity parameter is the BDFE of the N-H bond of the dye of 80.5 kcalmol-1, constituting a reactivity threshold towards the organic substrate. For scalability purposes, a device for artificial PSN applications should be wireless. Thus, combining the know on QA pigment, we developed dye sensitized nanoparticles (Nps) through a facile way and proposed them in wireless photocatalytic oxidations. We took the aerobic oxidation of TMB as a probe reaction to reveal the role of the QA dye onto the SC in the absorption of light and electron injection into TiO2, giving access to photocatalysis. Once the activity of QA sensitized Nps was proven, these were exploited in aerobic oxidation of glycerol with an aminoxyl radical cocatalyst. Oxidation of glycerol is indeed an interesting process given the large availability of glycerol and the high added value of its oxidation products. This PhD Thesis work paves the way towards sustainable chemical transformations employing light as a reactant, showing the potential of organic photocatalyst design. This research field is in rapid evolution and is expected to grow in the upcoming years; examples of emerging research lines in the recent literature landscape include reactivity in flow, multiphoton excitation, and the combination of light with electrochemistry. Objectives for the future include the development of new processes and an improved sustainability of existing ones; this should be done combining a fundamental understanding of the reaction mechanism and identification of active species.