UC San Diego

Carbon dioxide reduction has been an increasingly popular research in the renewable energy development as it can be used to store the solar energy in the form of chemical energy in liquid fuels, like gasoline and diesel. There are two main catalytic approaches to overcome the thermodynamically unfavored conversion of carbon dioxide (CO2) to carbon-based species, such as carbon monoxide and format: photochemical reduction using direct sunlight, and electrochemical reduction using electricity generated by solar panels.

In a typical photochemical system using rhenium or manganese bipyridine catalysts, previous work has been done on ligand modification to improve the quantum yield of carbon monoxide (CO) and other carbon species production. The work presented in this dissertation focuses on the structural modification of these catalysts to eliminate dimerization of manganese bipyridine catalyst upon first reduction and facilitate electron transfer from singly reduced photosensitizer to catalyst through non-covalent supramolecular assembly. In the former method, the bromide ligand of the manganese bipyridine catalyst (Mnbpy(CO)3Br) was replaced with a cyanide ligand (Mnbpy(CO)3CN) to reach an alternative reaction mechanism, in which disproportionation of two singly reduced manganese bipyridine catalyst occurs to give the active species without dimerization. In the latter method, electron transfer between the singly reduced photosensitizer and the catalyst is facilitated by the closer proximity of the two through non-covalent hydrogen bonding. Both method, unexpectedly, discovered the role of solvents in photocatalysis on product selectivity.

One of the biggest obstacles of electrochemical reduction of carbon dioxide in large-scale application is the immobilization of catalysts onto electrode surface. Most of the attachment methods in the literature face the issues of catalysts detachment and deactivation, and poor electrical contact between the catalyst and the electrode. A novel solvent-free synthetic method was invented to embed a top carbon dioxide electrocatalyst iron porphyrin into covalent organic frameworks. The COF-modified electrode demonstrated good activity for production of CO under electrocatalytic conditions in acetonitrile (MeCN) compared to the control electrode with only adsorbed iron porphyrins.