Mechanistic Insights into Photochemical CO

Area of Science:

• Electrochemistry and Catalysis
• Computational Chemistry
• Materials Science

Background:

• Developing efficient catalysts for CO2 reduction is crucial for mitigating climate change.
• Iron porphyrin complexes show promise for electrochemical and photochemical CO2 conversion.
• Understanding reaction mechanisms is key to optimizing catalyst performance and selectivity.

Purpose of the Study:

• To investigate the photochemical CO2 reduction mechanism to methane using an iron tetraphenylporphyrin complex (Fe-p-TMA).
• To elucidate the factors governing product selectivity, specifically methane formation over other reduced products.
• To rationalize the role of the porphyrin ligand and iron center in the catalytic cycle.

Main Methods:

• Density Functional Theory (DFT) calculations were employed to model the reaction pathway.
• The study analyzed intermediate structures, transition states, and energy barriers.
• The mechanism of CO2 activation, protonation, and C-O bond cleavage was investigated.

Main Results:

• The catalyst Fe-p-TMA undergoes reduction and chloride dissociation, forming a reactive Fe(II) species coupled with a porphyrin diradical.
• CO2 is activated via nucleophilic attack, followed by proton transfer and C-O bond cleavage, leading to a key Fe(II)-CO intermediate.
• The pathway favors sequential reduction to methane, with the tetraphenylporphyrin ligand playing a redox-active role and the hydrogen evolution pathway having a higher energy barrier.

Conclusions:

• The iron tetraphenylporphyrin complex (Fe-p-TMA) effectively catalyzes the photochemical reduction of CO2 to CH4.
• The unique electronic structure of the catalyst, involving a redox-active ligand, directs selectivity towards methane.
• DFT calculations provide a detailed mechanistic understanding, explaining the observed product selectivity and ruling out formaldehyde, methanol, or formate as major products.