A molecular noble metal-free system for efficient visible light-driven reduction of CO2 to CO

Abstract[edit | edit source]

The study presents a noble-metal-free molecular system for visible-light-driven CO₂ reduction to CO. The system is based on a pentadentate quinoline–pyridine ligand coordinated to iron (Fe), cobalt (Co), and nickel (Ni). The Fe complex demonstrates exceptional photocatalytic activity, achieving a turnover number (TON) of 544 and a CO selectivity of 99.3%, utilizing purpurin as a photosensitizer and BIH as a sacrificial electron donor. The Co and Ni complexes exhibit minimal CO production, attributed to their unfavorable redox potentials. The system's catalytic efficiency is examined through electrochemical and mechanistic studies.

Summary[edit | edit source]

The paper explores a photocatalytic CO₂-to-CO reduction system using iron, cobalt, and nickel complexes of a newly designed pentadentate ligand. The iron complex outperforms the cobalt and nickel counterparts, achieving high selectivity and efficiency. The catalyst is driven by a visible-light-active organic dye (purpurin) and BIH as an electron donor. Electrochemical studies indicate that the Fe(I)/Fe(0) redox potential is well-matched with the photosensitizer's reduction potential, enabling efficient electron transfer. Control experiments confirm that the iron complex is essential for CO production, and mechanistic insights suggest that a Fe(0) species is responsible for CO₂ activation.

Additional Remarks[edit | edit source]

• The study highlights the importance of designing catalysts with favorable redox potentials to enable efficient electron transfer in photocatalysis.
• The iron complex exhibits superior catalytic activity compared to Co and Ni due to its optimal Fe(I)/Fe(0) potential.
• The system maintains homogeneity throughout the reaction, as confirmed by mercury poisoning and dynamic light scattering (DLS) tests.
• The catalyst deactivates after 15 hours, likely due to product inhibition or irreversible changes in the coordination environment.

Content of the Published Article in Detail[edit | edit source]

• Introduction: Discusses the significance of solar-driven CO₂ reduction, previous work with noble-metal-based catalysts, and the need for earth-abundant alternatives.
• Catalyst Synthesis and Characterization: Details the synthesis of the pentadentate ligand (dqtpy) and its Fe, Co, and Ni complexes. Characterization techniques include X-ray crystallography, UV-vis spectroscopy, and electrochemical studies.
• Electrochemical Analysis: Evaluates redox potentials and their relevance to CO₂ reduction activity.
• Photocatalytic Studies: Investigates CO production efficiency using purpurin as a photosensitizer and BIH as an electron donor.
• Mechanistic Insights: Examines electron transfer pathways, catalyst stability, and the role of TFE as an additive.
• Conclusion: Summarizes key findings and suggests further optimization strategies.

Catalysts Tested in This Study[edit | edit source]

• Iron (Fe) complex: Demonstrates the highest activity with a CO TON of 544 and selectivity of 99.3%.

101019

• Cobalt (Co) complex: Poor CO production (TON = 8), attributed to an unfavorable redox potential.

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• Nickel (Ni) complex: Slightly better than Co (TON = 15) but still significantly lower than Fe.

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Photosensitizer[edit | edit source]

• Purpurin (PP) purpurin: A commercially available organic dye used to harvest visible light and transfer electrons to the catalyst.
• The dye undergoes reductive quenching by BIH, forming a PP²⁻ species that subsequently reduces the metal complex.

Investigation[edit | edit source]

	cat	cat conc [µM]	PS	PS conc [mM]	e-D	e-D conc [M]	.	.	solvent A	.	λexc [nm]	.	TON CO	TON H2	.
1.	Molecule:101019	50	purpurin	0.05	BIH	0.1			DMF		460		544	4		"%" is not declared as a valid unit of measurement for this property.
2.	Molecule:101019	0	purpurin	0.05	BIH	0.1			DMF		460		0	21		"%" is not declared as a valid unit of measurement for this property.
3.	Molecule:101019	50	purpurin	0	BIH	0.1			DMF		460		0	0		"%" is not declared as a valid unit of measurement for this property.
4.	Molecule:101019	50	purpurin	0	BIH	0			DMF		460		0	0		"%" is not declared as a valid unit of measurement for this property.

Investigation-Name: experimentsExportRefresh

Further Information[edit | edit source]

• Experimental Setup: CO₂-saturated DMF solutions containing catalyst (0.05 mM), purpurin (0.05 mM), BIH (0.1 M), and TFE (5%) were irradiated with blue LED light (460 nm).
• Key Parameters Measured: CO and H₂ production, turnover number (TON), selectivity, and catalyst stability.
• Control Experiments: Verified the necessity of all components (catalyst, photosensitizer, electron donor, and light) for efficient CO₂ reduction.
• Catalyst Stability: The reaction stops after 15 hours, likely due to catalyst deactivation.
• Catalyst Deactivation: The iron complex loses activity over time, possibly due to CO binding and irreversible reduction.
• Homogeneity of the Reaction: Mercury poisoning and DLS tests confirm the absence of nanoparticle formation, indicating a purely molecular catalytic process.

Sacrificial Electron Donor[edit | edit source]

• 1,3-Dimethyl-2-phenyl-2,3-dihydro-1H-benzimidazole BIH: Acts as a sacrificial electron donor, transferring electrons to purpurin, which then reduces the metal catalyst.
• Luminescence quenching experiments confirm that BIH efficiently reduces purpurin, forming PP²⁻, which subsequently reduces Fe(II) to Fe(0).

Additives[edit | edit source]

• 2,2,2-Trifluoroethanol (TFE) TFE: Enhances CO₂ reduction efficiency, likely by facilitating proton transfer and C–O bond cleavage.
• Solvent (DMF) DMF: Identified as the optimal reaction medium for achieving high TON and selectivity.

visible-light photocatalysis, CO2-to-CO reduction, iron molecular catalyst, cobalt complex, nickel complex, earth-abundant metals, purpurin photosensitizer, BIH sacrificial donor, pentadentate ligand, homogeneous catalysis, high turnover number, redox potential tuning, electrochemical characterization, mechanistic study, additive TFE, DMF solvent, solar fuels, sustainable chemistry, CO selectivity, molecular CO2 activation