Immobilized cobalt porphyrins for carbon dioxide electrochemical reduction in aqueous system
Electrocatalysis has attracted increasing attention as a sustainable pathway for utilization of carbon dioxide (CO2) to produce value added chemicals. Among various catalyst, cobalt porphyrins have long been employed for CO2 electrochemical reduction reaction (CO2ERR). However, their coordination environment, structure-activity relationship, interactions with supports have not been thoroughly studied. The goal of the thesis is to explore the methodologies for boosting the efficiency of cobalt porphyrins. In this context, the drop casting solvents, functional groups, supports, and immobilization methods were investigated, respectively, and their effects on catalytic performance were compared.
In the first part, the coordination environment and porphyrin structures were investigated. Cobalt tetraphenyl porphyrin (CoTPP) was immobilized on TiO2 nanotube (TNT) support via noncovalent drop casting. DMF, THF and pyridine were employed as solvents for drop casting to provide different coordination environment. As confirmed by spectrophotometric titration, pyridine forms a stronger coordination bond to CoTPP than DMF and THF thus leading to the highest efficiency among the drop-casting solvents. Moreover, the effect of porphyrin structure on CO2ERR was studied. It shows that the introduction of -COOMe group in CoTPP structure weakens the coordination bond between pyridine and CoTPP, resulting in a detrimental effect on CO2ERR.
To study the influence of supports on the catalytic performance, TiO2 was calcined at different temperatures and used as the support for immobilization of CoTPP via noncovalent drop casting. The crystalline phase of TiO2 and doping of TiO2 apparently affect CO2ERR. Anatase phase exhibits a higher activity and selectivity compared to rutile due to the enhanced conductivity which enables faster electron transfer between the support and CoTPP. As for dopants, the carbon doping in anatase TiO2 is proven to further enhance the conductivity, consequently resulting in the enhanced performance.
The above studies employed semiconductor TiO2 as the support for catalyst immobilization using a noncovalent approach. In contrast, covalent immobilization is advantageous as it ensures robust connection. In this thesis, carbon nanotubes (CNTs) were used as the support as they allow the formation of covalent bond. Covalent amide bond was formed between carboxyl functionalized CNTs and amino functionalized porphyrin CoTAP. Compared with noncovalent immobilization mode, covalent ligation leads to three times higher TOF and 13 % higher FEco. Tafel analysis reveal that covalent ligation not only promotes the electron transfer between CNTs and porphyrin but also improves the dispersion of CoTAP on CNTs.
Moreover, porphyrin was immobilized on CNTs via in situ polymerization. The ethynyl groups on peripheral ring of cobalt porphyrin (CoTEP) went through dimerization via hay coupling reaction and formed a covalent porphyrin framework on the surface of CNTs. The hybrid material (CoTEP@CNT) exhibited enhanced activity in CO2ERR. Compared with the physically mixed catalyst (CoTEP/CNT), CoTEP@CNT improved FE by 10 % and TOF by a factor of 1.3, which is one of the best polymerized porphyrins for CO2ERR. Characterization results reveal that the coating of CoTEP on CNTs can be improved by polymerization as a more uniform layer of porphyrin was formed on the sidewalls of CNTs.
In summary, the thesis has systematically studied the influence of various factors including drop-casting solvent, porphyrin structure, immobilization mode, and polymerization on the performance of cobalt porphyrin catalysts for CO2ERR. The underlying mechanisms on the CO2ERR were also investigated. The findings obtained from this thesis have great potential in the rational design of efficient catalyst/support system for CO2ERR.