title: The Electro - and Photochemical Reduction of CO2 Mediated by Molecular Catalysts
Creators: name: Neri, G
Publisher: name: University of Liverpool Repository
Year: 2017

description: In this work, molecular complexes of transition metals have been synthesised and studied for their CO2 reduction ability, either in an electrochemical or photochemical system, with a focus on complexes of nickel with derivatives of the macrocycle cyclam (cyc, cyc = 1,4,8,11-tetraazacyclotetradecane) which are well-known for being extremely active CO2 reduction electrocatalysts in water. The cyc framework has been modified with functional groups suitable for binding to semiconductor oxides to obtain the new complexes Ni(cycC) and Ni(cycP) (cycC = 1,4,8,11-tetraazacyclotetradecane-6-carboxylic acid, cycP = {[(1,4,8,11-tetraazacyclotetradecan-1-yl)methyl]phosphonic acid}), and their electrochemical activity towards CO2 reduction in water has been evaluated. Modification of the ligand framework in the 6 position with a carboxylic acid does not change the CO2 reduction activity of the complex Ni(cycC) at pH 5, through the use of electrochemical and spectroscopic techniques it was found that there is a large increase in the CO2 reduction activity at pH 2, proposed to be due to the protonated carboxylic acid acting as an internal proton source. When Ni(cycC) was immobilised on TiO2 electrodes it was possible to measure the rate of photoinduced electron transfer by using μs-s transient absorption spectroscopy (TAS) under argon in the presence of a hole scavenger, however the carboxylic acid proved unstable under CO2. Ni(cycP) was synthesised to provide a stronger binding group to the surface. It was found that functionalisation on the 1 position affected the CO2 reduction activity in a negative way, however the complex was able to bind strongly to both TiO2 and ZrO2. ZrO2 nanoparticles modified with Ni(cycP) and a ruthenium dye were able to reduce CO2 to CO in water at pH = 4, with higher rates and turnover numbers compared to the components in solution, when illuminated with visible light. The improvement in activity for the heterogeneous photocatalyst was attributed to a faster electron transfer from the immobilised dye to the immobilised catalyst, calculated through detailed steady-state and transient spectroscopies, which prevented charge recombination. In collaboration with the University of Cambridge, Ni(cycP) has been immobilised on ZnSe quantum dots (QDs) and it has been proven to be an effective photocatalyst for CO2 reduction to CO in water. We have carried out a detailed ultrafast TAS study on suspensions of the modified QDs, and it has been found that in the presence of a hole scavenger, upon illumination the electrons are excited from the VB to the conduction band (CB), however they rapidly decay to trap states close to the CB to generate a long lived signal. When Ni(cycP) is present, faster decay of the trapped electron signal is observed, which is assigned to fast electronic transfer from the QDs to Ni(cycP). The knowledge of the mechanisms for CO2 reduction will allow rational design of better catalysts for CO2 reduction. In collaboration with the Rutherford Appleton Laboratories, we have designed an in situ spectroelectrochemical Sum Frequency Generation (SEC-SFG) technique using the ULTRA laser at the Central Laser facility. We have demonstrated the technique by analysing the redox behaviour of a well-known CO2 reduction catalyst, [Mn(bpy)(CO)3Br]. We were able to observe the redox species at the electrode surface as a Cyclic Voltammogram was carried out, and to propose the orientation of the species at the surface. Furthermore, the same technique has been applied to the study of the absorption mechanism of Ni(cycC) on the mercury surface, the first step in the catalytic cycle.