PhotoCO2 · Photocatalytic reduction of carbon dioxide into fuels

Photocatalytic reduction of carbon dioxide into fuels

The photochemical synthesis of fuels from water and/or CO2 is a rapidly expanding research field that aims to store solar energy into a chemical fuel. This reaction requires the combination of a light harvesting unit and a catalytic component capable of performing the H+/CO2 reduction reaction. The immobilisation of molecular catalysts for solar fuels production is an attractive strategy to exploit electrocatalysts in a heterogeneous photocatalytic environment. In this project, I have investigated the interfacial electron transfer kinetics of hybrid systems composed of molecular catalysts for proton or CO2 reduction, immobilised onto nanostructured semiconductors. Thus, this work has pioneered the emerging research field of identifying the factors governing the efficiency of catalyst-semiconductor hybrid systems. The key findings of this project are summarised below. First of all, in order to prove the concept that electrocatalysts can receive electrons from appropriate photosensitisers in a heterogenised environment, nanostructured TiO2 was functionalised with a ruthenium dye and a molecular electrocatalyst for proton reduction (either a cobaloxime or a Ni DuBois type complex). These systems are capable of producing H2 in water and in the presence of a sacrificial electron donor, with a quantum yield up to 10%. My work has proven that the electron transfer from the photosensitiser to the molecular catalyst can be achieved either through an oxidative or reductive quenching of the dye, depending upon the system employed. I have also identified two main parameters affecting the kinetics of charge separation and recombination: the driving force of the electron transfer reaction, and the physical separation between the donor and acceptor moieties. The studies performed with proton reduction catalysts have been used as a model to characterise the more challenging CO2 conversion systems. We have demonstrated that by attaching a Ni-cyclam catalyst for CO2 reduction functionalised with carboxylic acid groups to a semiconductor allowing for a higher driving force, the electron transfer kinetics are accelerated. Studies with molecular catalysts for CO2 reduction also include a Re(bpy)(CO)3L attached onto the surface of TiO2. The immobilisation of the catalyst results in a 10-fold improvement in efficiency, due to an increase of the lifetime of the catalytic intermediates. Charge separation achieved with TiO2 functionalised with molecular catalysts was also compared to that of metal nanoparticles (including Pt, Ag, Au and Pd). Further charge separation studies were performed with a visible-light absorbing semiconductor (Cu2O) when forming an inorganic heterojunction with a charge acceptor (RuOx). The formation of the heterojunction resulted in a two-fold increased yield of long-lived electrons, correlating with the six-fold increase in the CO2 reduction yield.

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