Selectivity enhancement on heterogeneous electrocatalytic CO2 reduction : from modification of metallic electrodes to heterogenization of monofunctional molecular catalysts

• Main Author: Peng, Yuecheng
• Other Authors: Wang Xin (SCBE)
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Materials; Engineering::Chemical engineering
• Online Access: https://hdl.handle.net/10356/136767
• Institution: Nanyang Technological University

The work included in the thesis was aimed to improve the selectivity regarding the product distribution for CO2 electrocatalytic reduction performed on heterogeneous catalysts. Strategies for this purpose can be diverse. In the thesis, two directions were applied. On the one hand, surface morphology modifications can be done on metal electrode to regulate the active sites to raise the selectivity for certain product. On the other hand, creating monofunctional active sites originated from molecular catalysts has also been proven to be effective. First, a nanoporous structure was obtained on polycrystalline copper electrode through an alloy-dealloy process. Such modified copper electrode can selectively catalyze the production of C2H4 over CH4, compared to nonmodified copper electrode. Moreover, the Faradaic efficiency for ethylene can reach up to 35%. Probable explanation for the selectivity enhancement could be the exposure more crystalline orientation that favor ethylene production. Second, bipyridine carbonyl rhenium (I) complex are known to be active for electrocatalyzing CO2 reduction to CO. By grafting π-electron enriched groups like pyrene onto a molecule, a more stable heterogeneous attachment can be achieved on graphene support, realizing a catalytic CO2-to-CO conversion at 92% of efficiency on aprotic solvents. Third, an iron porphyrin complex with -NH2 pendant groups on the porphyrin ligand as a type of intramolecular H-bond donor presented superior selectivity and production rate towards CO2 reduction to CO after immobilized onto graphene as a heterogeneous catalyst in aqueous solution, with the faradaic efficiency of 99% for CO production. Electrochemical and theoretical studies revealed the reduced CO2 intermediate was stabilized by H-bond on Fe(0) site, accounting for better performance compared with analogues which lacked the intramolecular H-bond donor feature. In sum, morphological modification of catalysts with diverse active sites for CO2 electroreduction can benefit the selectivity towards certain product. By the way, construction composites with monofunctional active units while performing some molecular tuning can also boost the generation to specific product from CO2 electrocatalytic reduction.