Electrocatalytic and Photocatalytic CO"2" Reduction by Iron, Cobalt and Copper Compounds
Student thesis: Doctoral Thesis
Related Research Unit(s)
Molecular catalysis of carbon dioxide reduction using earth-abundant metal complexes as catalysts is a key challenge related to the production of useful products - the “solar fuels” - in which solar energy would be stored. A direct approach using sunlight energy as well as an indirect approach where sunlight is first converted into electricity can be used. In this thesis, a series of cobalt, iron and copper complexes were investigated as catalysts for CO2 reduction. There are four parts.
The first part describes the catalytic activity toward CO2 reduction by a CoII complex bearing the a pentadentate N5 ligand (2,13-dimethyl-3,6,9,12,18-pentaazabicyclo-[12.3.1]octadeca-1(18),2,12,14,16-pentaene). The cyclic voltammetry (CV) of the Co complex in a CO2-saturated solution ([CO2] = 0.23 M) shows strong catalytic currents at −1.5 V vs SCE. Electrolysis of the CoII complex at −1.5 V vs SCE led to the exclusive formation of CO with high faradaic yield of 41%. When using a photosensitizer under visible light excitation (λ > 460 nm, solvent acetonitrile) with the Co catalyst, CO with the turnover number (TON) of 270 could be obtained. A catalytic mechanism is suggested on the basis of experimental results.
The second part introduces the photocatalytic reduction of CO2 by [Fe(qpy)(OH2)2]2+ (qpy = 2,2′:6′,2″:6″,2‴-quaterpyridine). With Ru(bpy)32+ as the photosensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as the sacrificial reductant in CH3CN/triethanolamine solution under visible-light excitation (blue light-emitting diode), a turnover number (TON) for CO as high as > 3000 with up to 95% selectivity can be achieved for the Fe catalyst. More significantly, when Ru(bpy)32+ was replaced by the organic dye sensitizer purpurin, a TON of 1365 could be achieved in N,N-dimethylformamide (DMF) for the Fe catalyst.
The third part describes the use of [Cu(qpy)]2+ as the CO2 reduction catalyst. The Cu(II) quaterpyridine complex [Cu(qpy)]2+ is found to be a highly efficient and selective catalyst for visible-light driven CO2 reduction in CH3CN using [Ru(bpy)3]2+ as photosensitizer, BIH/TEOA as sacrificial reductant (white light-emitting diode). The photocatalytic reaction is greatly enhanced by the presence of H2O (1~3% v/v), and a TON of > 12,400 for CO production can be achieved. Results from Hg-poisoning and dynamic light scattering (DLS) experiments suggest that this photocatalysis is homogeneous. To the best of our knowledge [Cu(qpy)]2+ is also the first example of molecular Cu-based catalyst for the photoreduction of CO2.
In the last part the electro- and photocatalytic CO2 reduction performance by a triiron complex, [(µ3-O)(µ-OMe)Fe3(µ-PhenO)4(OMe)]ClO4 (PhenOH = 2-hydroxy-1,10-phenanthroline) is described. When electrolysis was carried out at -1.8 V for 2 h, CO with the faradaic yield of 21% could be obtained. However, when 2 mM of pyridine was introduced to the reaction system, the faradaic yield of CO increases to 32%. For the photocatalysis, CO with the TON of 288 and selectivity of 95% could be obtained using purpurin as photosensitizer. When the purpurin was replaced by Ru(bpy)3Cl2, CO with the TON of 370 and selectivity of 89% could be obtained.